July 29 - Aug 3 2018, Boston / Cambridge, USA

The "Cambridge Workshops of Cool Stars, Stellar Systems and the Sun" are held biennially and have evolved to be the premier conference series for cool star research.

Splinter sessions are selected. See the list!

SOC and LOC plan the following important dates:
• Early March - registration opens
• March 16 - deadline for contributed talks in the plenary session and financial support
• April 15 - contributed speakers announced
• May 1 - deadline for early registration
• June 15 - detailed splinter session programs online, deadline for poster abstracts and regular registration, deadline for hosting CS 21
• July 10 - deadline for late registration
• July 29: CIAO workshop / ExoPAG 18
• July 29 to Aug 3: conference
• Aug 4: Hack day

## Submit contributions

We accept submissions for plenary talks and poster contributions. Splinter sessions are organized independently and have their own deadlines and submission processes, see the list of splinter sessions for links to their webpages.

Our facilities can support about 500 attendees with talks or posters. Based on the previous Cool Stars meetings, we expect that every interested astronomer should have a chance to join in, but the only guarantee is to submit your abstract early by filling out the form below. (If you just want to be included in future announcement emails but do not wish to submit an abstract at this time, you can subscribe here.)

## Accepted contributions

Use the table below to view and search abstracts or jump directly to a listing of all talks or posters.

Type Author Title Authorlist Affiliations Abstract
Type Author Title Authorlist Affiliations Abstract

## Talks

### Early Results from the Transiting Exoplanet Survey Satellite (TESS)

science theme: Galactic Cartography in the Gaia Era
schedule: Sun, TBA (invited)

George Ricker; TESS Science Team
MIT

The Transiting Exoplanet Survey Satellite (TESS) will discover thousands of exoplanets in orbit around the brightest stars in the sky. In a two-year survey, TESS will monitor ~200,000 pre-selected bright stars for planetary transits in the solar neighborhood at a 2-minute cadence. The survey will identify planets ranging from Earth-sized to gas giants, around a wide range of stellar types and orbital distances. TESS will also provide full frame images (FFI) at a cadence of 30 minutes or less. These FFI will provide precise photometric information for every object within the 2300 square degree instantaneous field of view of the TESS cameras. In total, more than 30 million stars and galaxies brighter than magnitude I=16 will be precisely photometered during the two-year prime mission. In principle, the lunar-resonant TESS orbit will provide opportunities for an extended mission lasting more than a decade, with data rates of ~100 Mbits/s.

An extended survey by TESS of regions surrounding the North and South Ecliptic Poles will provide prime exoplanet targets for characterization with the James Webb Space Telescope, as well as other large ground-based and space-based telescopes of the future. The TESS legacy will be a catalog of the nearest and brightest main-sequence stars hosting transiting exoplanets, which should long endure as the most favorable targets for detailed future investigations.

The launch as a NASA Astrophysics Explorer is to take place in April 2018 from Cape Canaveral on a SpaceX Falcon 9 rocket. First light results from the TESS mission will be presented.

### 3D mapping of the Solar Neighbourhood with Gaia DR2

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA

Eleonora Zari; Anthony Brown
Leiden Observatory

Because of their relatively short lifetimes, O and B type stars are good tracers or recent star formation events in the Milky Way and in other galaxies. Bouy & Alves (2015) analysed the spatial density of OB stars within 500 pc from the Sun using the Hipparcos catalogue and reported the discovery of large scale stream-like structures.

We plan to use Gaia DR2 to expand Bouy & Alves study, aiming at: 1) tracing the spatial distribution of early-type stars in the Solar Neighbourhood, making use of Gaia DR2 precise astrometry; 2) producing an age map of the Solar Neighbourhood using Gaia photometry to determine stellar ages; 3) tracing the kinematics of the Solar Neighbourhood using Gaia proper motions and radial velocities. The interpretation of the global age map in conjunction with the 3D spatial and kinematic map will allow to draw a picture of the star formation history of the Solar Neighbourhood.

A few months after Gaia DR2, we will present our results from an initial exploration of Gaia DR2 data and show how this incredibly large amount of information will revolutionise our understanding of large-scale star formation processes.

### Metal-poor stars observed by Gaia-ESO and other large stellar spectroscopic surveys

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA

Rodolfo Smiljanic
Nicolaus Copernicus Astronomical Center, Poland

I will present an overview of the properties of about 1300 metal-poor stars (defined here as those with [Fe/H] $<$ -0.8 dex) observed by the Gaia-ESO Spectroscopic Survey so far (internal data release 5). Stars observed both with the UVES and Giraffe spectrographs are included in the sample. When possible, Gaia parallaxes and proper motions will be used to compute their kinematics and orbits. I will report on an analysis using machine learning techniques with the aim of disentangling the various stellar populations at the low metallicity regime, including the thick disk, the metal-weak thick disk, and the different halo components. Further, I will report on similar analyses using samples of metal-poor stars taken from public releases of other large spectroscopic surveys (e.g., RAVE and APOGEE). Finally, I will also highlight the need to improve the analysis of metal-poor stars in these surveys. For example, a comparison between observations and model isochrones in the $T_{\rm eff}$ vs. $\log$ $g$ plane shows that spectroscopic values of temperature and gravity tend to have important accuracy issues.

### Rotating Stars from Kepler Observed with Gaia DR2

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA

James R. A. Davenport (1,2); Ruth Angus (3); Kevin R. Covey (2); David Kipping (3); Marcel Agüeros (3)
(1) University of Washington; (2) Western Washington University; (3) Columbia University

The Kepler mission produced rotation period measurements for more than 30,000 stars, enabling for the first time statistical studies of the age distribution of field stars using "gyrochronolgy". Intriguingly, McQuillan et al. (2013, 2014) also discovered a bimodal distribution of rotation periods for nearby M and K dwarfs, suggesting a dearth of stars with ages of ~600 Myr. Using Gaia DR1, Davenport (2017) found this bimodality extended to the G dwarfs within ~300 pc as well, having previously been obscured due to subgiant contamination. Using parallaxes from Gaia DR2, we are now able to explore the bimodal rotation period distribution for all main sequence Kepler stars as a function of their distance. This new sample allows us to constrain the volumetric extent of this period bimodality, and thus explore the recent star formation history of the Milky Way disk in 3D.

### The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA

Megan Bedell
Center for Computational Astrophysics, Flatiron Institute

The compositions of stars are a critical diagnostic tool for many topics in astronomy such as the evolution of our Galaxy, the formation of planets, and the uniqueness of the Sun. Previous spectroscopic measurements indicate a large intrinsic variation in the elemental abundance patterns of stars with similar overall metal content. However, systematic errors arising from inaccuracies in stellar models are known to be a limiting factor in such studies, and thus it is uncertain to what extent the observed diversity of stellar abundance patterns is real. We have performed an analysis of 79 Sun-like stars within 100 parsecs and achieve abundance measurements of 30 elements with precisions of 2 percent. Systematic errors are minimized in this study by focusing on solar twin stars and performing a line-by-line differential analysis using high-resolution, high-signal-to-noise spectra. We resolve [X/Fe] abundance trends in galactic chemical evolution at precisions of $10^{-3}$ dex Gyr$^{-1}$ and reveal that stars with similar ages and metallicities have nearly identical abundance patterns. These results imply that exoplanets may exhibit much less compositional diversity than previously thought. We also discuss new insights from this data set on the dimensionality of chemical abundance space and prospects for chemical tagging studies.

### Physical properties of evolved Open Clusters in the Gaia era

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA

L. Casamiquela (1); C. Soubiran (1); H. Bouy (1); U. Heiter (2); P. Jofré (3); S. Blanco-Cuaresma (4)
(1) Laboratoire d'Astrophysique de Bordeaux -CNRS; (2) Uppsala Universitet; (3) Universidad Diego Portales; (4) Harvard-Smithsonian Center for Astrophysics

Evolved Open Clusters (OCs) are excellent tracers of the formation and evolution of the galaxy, as well as an ideal laboratory to test theories of star formation and evolution. In particular, nearby OCs are commonly used as benchmark objects to assess the determination of physical properties of field stars. We have designed a project to perform an in-depth study of the physical properties of a sample of nearby clusters: 13 OCs at 500 pc around the Sun. We determine shape, radii, extinction, galactic velocity, age and chemical composition, using recent data from Gaia DR2, asteroseismic information from K2, ground-based spectroscopic surveys and other complementary high resolution spectroscopic data. This project has many applications, including validation of stellar evolution theories and calibration of parametrization methods. Also it has the potential to answer questions: how do the structural properties correlate with the age of the OC and with the environment?, what is the formation mechanism of tidal tails?, what is the influence of density, age and galactic environment in the disruption process of OCs? We will present the first results of this project short after the release of Gaia DR2.

### The Dynamics of OB Associations

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA

Nicholas J. Wright
Keele University, UK

The formation and evolution of young star clusters and OB associations is fundamental to our understanding of the star formation process, the conditions faced by young binary and planetary systems, and the formation of long-lived open and globular clusters. Despite this our understanding of the physical processes that drive this evolution has been limited by the static nature of most observations. This is all changing thanks to a revolution in kinematic data quality from large-scale radial velocity surveys and new astrometric facilities such as Gaia. I will present new kinematic data for multiple OB associations from Gaia and ground-based astrometric surveys that show they have considerable kinematic substructure and no evidence for the radial expansion pattern predicted by theories such as residual gas expulsion. This means that, contrary to the standard view of OB associations as expanded star clusters, these systems could never have been dense star clusters in the past and were most likely born as extended and highly substructured groups of stars. This places strong constraints on the primordial clustering of young stars and the conditions faced by young planetary systems.

### Towards the six-dimensional view of the Orion Complex

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA

Marina Kounkel (1); Kevin Covey (1); Genaro Suarez (2); Carlos Román-Zúñiga (2); Jesus Hernandez (2); APOGEE collaboration
(1) Department of Physics and Astronomy, Western Washington University, 516 High Street, Bellingham, WA 98225, USA; (2) Instituto de Astronomıa, Universidad Nacional Autónoma de México, A.P. 70-264, 04510, Mexico, D.F., México

We present the analysis of spectroscopic and astrometric data from APOGEE-2 and \textit{Gaia} (DR1 and later DR2) that trace the structure as well as the star forming history within the Orion Complex. Applying a hierarchical clustering algorithm to the 6-dimensional data, we identify spatially and kinematically distinct groups of young stellar objects with ages ranging from 1 to 12 Myr. We find substructure in the massive clusters such as NGC 2024 and $\lambda$ Ori. We also reconstruct the population of the currently dissipated Orion C molecular cloud which formed $\sigma$ Ori, and is independent from Ori OB1ab region. With the current data, these structures are most distinct in the radial velocity space; with \textit{Gaia} DR2 it will be be possible to achieve a comparable resolution in the proper motions and parallax space, which will further improve on both the ability to identify subclusters and derive ages. These observations provide an unprecedented portrait of the structure and dynamics of the Orion Complex, a region that provides a critical case study for cluster assembly within a molecular cloud and the subsequent cluster dispersal.

### Status of the Gaia Mission and the Gaia DR2 Results

science theme: Galactic Cartography in the Gaia Era
schedule: Mon, TBA (invited)

F. Thévenin
Université de la Côte d'Azur, Observatoire de la Côte d'Azur, CNRS

The Gaia mission is in its fourth year of work, and is a key space mission devoted to astrometric measurements of stars. More than astrometric data will be delivered in the Gaia DR2, 3, 4 and the mission is now foreseen to be extended. For the Gaia DR2 one of the most important result is the parallaxes of 1.3 billion sources with accurate determinations. The use of them deserve some attention and careful read of the companion papers that explain in detail how to use the catalogue and what the limits are. On the 25 April the catalog will be online for all the community of astronomers to produce stellar astrophysics from young to very old stars, isolated or in clusters. The return on the physics of stars is vast and for sure will be a central part of the workshop. Inputs on the parallaxes and PM or first estimates of Teff and reddening of stars or radial velocities and more outputs will be presented and discussed.

### Accretion Dynamics in Pre-Main Sequence Binary Systems

science theme: Solar/Stellar Environments
schedule: Tue, TBA

B. Tofflemire (1); R. Mathieu (1); G. Herczeg (2); C. Johns-Krull (3); R. Akeson (4); D. Ciardi (4); D. Ardila (5)
(1) University of Wisconsin-Madison; (2) Kavli Institute for Astronomy & Astrophysics, Peking University; (3) Rice University; (4) IPAC; (5) JPL

Over the past thirty years, a detailed picture of star formation has emerged that highlights the significance of the interaction between a pre-main sequence star and its protoplanetary disk. This star-disk interaction has been extensively characterized in the case of single stars, revealing implications for pre-main sequence stellar evolution and planet formation. Many stars, however, form in binary or higher-order systems where orbital dynamics fundamentally alter this star-disk interaction. In short-period binaries, orbital resonances are predicted to carve out the center of the protoplanetary disk, leading to periodic accretion streams that bridge the gap between a circumbinary disk and the central stars. To test these predictions, we have conducted an intensive observational campaign combining multi-color photometry and high-resolution spectroscopy in time-series. Within these data we search for periodic trends in the accretion rate and in the velocity structures of accretion-tracing emission lines. I will present results highlighting the detection of periodic enhanced accretion events in two eccentric binaries (DQ Tau and TWA 3A) and evidence for preferential mass accretion onto the TWA 3A primary. Both results are presented in the context of recent hydrodynamic simulations of binary accretion.

### Irradiance Coronal Dimming and its Connection to CME Kinetics

science theme: Solar/Stellar Environments
schedule: Tue, TBA

James Paul Mason(1); Nick Arge(1); Barbara J. Thompson(1); David F. Webb(2); Thomas N. Woods(3)
(1) NASA Goddard Space Flight Center; (2) Boston College Institute for Scientific Research; (3) University of Colorado at Boulder Laboratory for Atmospheric and Space Research

When coronal mass ejections (CMEs) depart the corona, they leave behind a transient void. Such a region evacuated of plasma is known as coronal dimming and it contains information about the kinetics of the CME that produced it. The dimming can be so great that it reduces the overall energy output of the star in particular emission lines, i.e., dimming is observable in spectral irradiance. This should be generally true for magnetically active stars.

We use the Solar Dynamics Observatory (SDO) EUV Variability Experiment (EVE) data to search for and parameterize dimming. We search these light curves for dimming around $>$8,500 $\geq$C1 solar flares. In prior work, we have found that it is important to remove the gradual flare phase from dimming light curves in order to obtain slopes and magnitudes that are consistent with what can be obtained by spatially isolating flaring loops in spectral image data. Applying that method, we come to a total of ~13 million light curves in which to search for dimming. We parameterize each light curve in terms of magnitude, slope, and duration and correlate these with independently-derived CME speed and mass. Thus, we obtain a robust relationship between irradiance coronal dimming and CME kinetics.

Here, we briefly describe the feature detection and characterization algorithms developed and applied to the irradiance light curves. Machine learning techniques have been used for both this backend processing pipeline and to analyze the results. All of the code is open source python available on GitHub (github.com/jmason86/James-s-EVE-Dimming-Index-JEDI). We then discuss results on the comparison between our new catalog and the established Coordinated Data Analysis Workshops' CME Catalog.

This method may be capable of not only detecting CMEs from other stars, but estimating their kinetic energy and frequency of occurrence; information needed for assessing exoplanet habitability.

### Radio activity reloaded: Young Stellar Objects as seen by VLA, VLBA, and ALMA

science theme: Solar/Stellar Environments
schedule: Tue, TBA

Jan Forbrich
University of Hertfordshire

### Space Weather Beyond the Solar System

science theme: Solar/Stellar Environments
schedule: Tue, TBA

J. D. Alvarado-Gómez (1); J. J. Drake (1); G. A. J. Hussain (2); O. Cohen (3); J. Sanz-Forcada (4); B. Stelzer (5); J. F. Donati (6); E. M. Amazo-Gómez (7); C. Garraffo (1); S. P. Moschou (1)
(1) Harvard-Smithsonian Center for Astrophysics; (2) European Southern Observatory; (3) University of Massachusetts at Lowell; (4) Centro de Astrobiología (CSIC-INTA); (5) Eberhard Karls Universität; (6) CNRS-IRAP; (7) Max-Planck-Institut für Sonnensystemforschung

Stellar magnetic fields completely dominate the environment around late-type stars. They are responsible for driving the coronal high-energy radiation (EUV/X-rays), transient events such as flares and Coronal Mass Ejections (CMEs), and the development of stellar winds and astrospheres. The study of these phenomena is nowadays possible combining Zeeman-Doppler Imaging observations of stellar magnetic fields, with sophisticated 3D magneto-hydrodynamics models of the corona and wind structure of stars. I will briefly review some generalities of this data-driven methodology and discuss two specific applications in the context of cool main sequence stars: a) The suppression of CMEs in active stars by an overlying magnetic field, and b) The evolution of the space weather and habitability conditions through the magnetic cycle of a young Sun.

### Prominence formation and ejection in Cool Stars

science theme: Solar/Stellar Environments
schedule: Tue, TBA

Carolina Villarreal D'Angelo (1); Moira Jardine (1); Victor See (2)
(1) SUPA, School of Physics and Astronomy, University of St Andrews; (2) Departament of Physics and Astronomy, University of Exeter

Slingshot prominences are cool, mainly neutral clouds that are trapped in the coronae of magnetically active stars. They have been observed in single and binary G and K type stars for many years now, but they have only recently been detected in M dwarfs (Vida et al. 2016). They carry away both mass and angular momentum when they are ejected and the impact of this mass on any orbiting planets may be important for the evolution of exoplanetary atmospheres. We have recently modelled both the masses and ejection frequencies of such prominences and determined the contribution their ejection makes to the rate of loss of mass and angular momentum of the star. We have also studied the evolution of these type of prominences for a star with a mass equal to our Sun, employing a rotational evolution code (Jonhstone et al. 2015, Tu et al. 2015).

In this talk I will present our results and compare it with observed prominence masses and lifetimes for a range of stars. I will discuss the implication of the ejection of these prominences on exoplanets, and the influence that they may have had in our solar system.

### Detection of a Millimeter Flare from Proxima Centauri

science theme: Solar/Stellar Environments
schedule: Tue, TBA

Meredith MacGregor (1); Alycia Weinberger (1); David Wilner (2); Adam Kowalski (3); Steven Cranmer (3)
(1) Carnegie DTM; (2) Harvard-Smithsonian Center for Astrophysics; (3) University of Colorado Boulder

We present new analyses of ALMA 12-m and ACA observations at 233 GHz (1.3 mm) of the Proxima Centauri system with sensitivities of 9.5 and 47 $\mu$Jy beam$^{-1}$, respectively, taken from 2017 January 21 through 2017 April 25. These analyses reveal that the star underwent a significant flaring event during one of the ACA observations on 2017 March 24. The complete event lasted for approximately 1 minute and reached a peak flux density of $100\pm4$ mJy, nearly a factor of $1000\times$ brighter than the star's quiescent emission. At the flare peak, the continuum emission is characterized by a steeply falling spectral index with frequency, $F_\nu \propto \nu^\alpha$ with $\alpha = -1.77\pm0.45$, and a lower limit on the fractional linear polarization of $|Q/I| = 0.19\pm0.02$. Since the ACA observations do not show any quiescent excess emission, we conclude that there is no need to invoke the presence of a dust belt at $1-4$ AU. We also posit that the slight excess flux density of $101\pm9$ $\mu$Jy observed in the 12-m observations compared to the photospheric flux density of $74\pm4$ $\mu$Jy extrapolated from infrared wavelengths may be due to coronal heating from continual smaller flares, as is seen for AU Mic, another nearby, well-studied, M dwarf flare star. If this is true, then the need for warm dust at $\sim0.4$ AU is also removed.

### Full Sun Spectrally Resolved Soft X-ray Measurements from the Miniature X-ray Solar Spectrometer (MinXSS) CubeSats

science theme: Solar/Stellar Environments
schedule: Tue, TBA

Christopher S. Moore
Harvard-Smithsonian Center for Astrophysics

Detection of soft X-rays from the atmosphere of low mass stars like the Sun, provide direct information on coronal plasma at temperatures in excess of ~1 MK and is commonly used as a proxy for surface magnetic activity. There have been relatively few solar spectrally resolved measurements from 0.5-10. keV. The Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is the first solar science oriented CubeSat mission flown for the NASA Science Mission Directorate, and has provided full Sun measurements from 0.8-12 keV, with resolving power ~40 at 5.9 keV, at a nominal ~10 second time cadence. MinXSS measurements span over four orders of magnitude in photon flux and can be used to extract the spectral components of the quiet Sun, active regions and solar flares. Furthermore, elemental abundance variations among the various solar conditions yield information on plasma transport and heating processes in the corona. Finally, MinXSS solar measurements can insight in determining how complex spatial features map to full Sun integrated signatures, which is vital to understanding properties of other low mass stars.

### First X-ray detection of plasma motions in a stellar flare and in the associated CME

science theme: Solar/Stellar Environments
schedule: Tue, TBA

C. Argiroffi (1,2); F. Reale (1,2); J. J. Drake (3); A. Ciaravella (2); P. Testa (3); R. Bonito (2); M. Miceli (1,2); S. Orlando (2); G. Peres (1,2)
(1) DiFC, Università degli Studi di Palermo, Italy; (2) INAF - Osservatorio Astronomico di Palermo, Italy; (3) Smithsonian Astrophysical Observatory, Cambridge, MA, US

The solar corona has always been the starting point to understand all the magnetically-related phenomena occurring in stellar atmosphere. Stars however show activity levels up to $10^4$ higher than the Sun. Therefore, direct observations of the different magnetic phenomena in active stars are crucial to understand how they scale with the activity level. While global coronal properties are well studied, many other phenomena, among which flaring plasma motions and coronal mass ejections (CME), remain observationally unexplored in active stars. By performing time-resolved X-ray spectroscopy of a strong stellar flare, observed with Chandra/HETGS, we present here the direct and unambiguous evidence of Doppler shifts due to upward and downward motions of plasma within the flaring loop, and, unexpectedly, also of the subsequent CME expulsion. The observed motions within the flaring loop neatly agree with HD model predictions, indicating that the standard flare model holds also for flares $10^4$ more energetic than the most intense solar ones. Moreover, this first direct and clear observation of a stellar CME allows us to infer its physical properties, including mass ($\sim10^{20}$ g) and kinetic energy ($\sim10^{34}$ erg). These values provide fundamental constraints to extrapolate the properties of solar CME to that of active stars, and eventually to understand the potential effects of CME on exoplanetary systems.

### The Space Weather Environment that Stars Create

science theme: Solar/Stellar Environments
schedule: Tue, TBA (invited)

Rachel A. Osten
Space Telescope Science Institute, Johns Hopkins University

One of the central tenets of stellar astrophysics (and indeed, one of the rationales for the Cool Stars, Stellar Systems, and the Sun conference series) is the reciprocal utility in studying both the Sun and stars. That is, we can use the detailed knowledge gleaned about physical processes on the Sun and apply those findings to other stars. Conversely, the diversity of stars and their reach of parameter space broadens the conclusions based on a single stellar evolutionary path. The Sun's radiation, particles, and mass loss have long been known to impact the inner solar neighborhood, leading to detailed space weather studies in our solar system. The discovery of thousands of exoplanets broadens this study to consider the space weather environment that is experienced in other solar systems. The solar-stellar connection is absolutely necessary here. Recent stellar observations shed light on key differences with the middle-aged Sun. I will talk about recent results regarding extreme stellar flares and what this tells us about the scaling between solar and stellar flares and the underlying physics. I will motivate the need for a better understanding of unsteady and steady stellar mass loss, and highlight some recent results questioning the applicability of solar flare-coronal mass ejection scalings into the stellar regime. An improved understanding of how differences in stellar magnetospheric properties impact the environment around them is a necessary ingredient to put the Sun in context, and is especially important in considering the impact of a star's space weather environment.

### Non-LTE stellar parameters and abundances of metal-poor stars in the Galaxy

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

Rana Ezzeddine(1); Tatyana Sitnova(2); Lyudmilla Mashonkina(2); Anna Frebel(1)
(1) Massachusetts Institute of Technology (2)Institute of Astronomy, Russian Academy of Sciences

The chemical compositions of metal-poor stars provide important observational clues to the astrophysical objects that enriched the primordial gas with elements heavier than H and He. Accurate atmospheric parameters are a prerequisite to any precise abundance determination. Spectroscopically derived stellar parameters, however, can be inaccurate when assuming Local Thermodynamic Equilibrium (LTE) methods, especially for metal-poor stars. This can have important consequent effects on the derived abundances and interpretations. In this talk, I will present Non-LTE atmospheric stellar parameters of a sample of metal-poor stars in our Galaxy. I will show that departures from LTE in stellar parameters can grow up to ~1 dex in [Fe/H], 200 K in Teff and 0.6 dex in logg toward the lowest metallicities. I will reflect upon the effects that these differences can have on stellar population studies, especially in the era of large-scale surveys such as Gaia, APOGEE, RAVE and GALAH.

### Kepler/K2: mission update & overview of its cool star legacy dataset

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

J. Dotson (1); G. Barentsen (2); A. M. Cody (2); M. Gully-Santiago (2); C. Hedges(2)
(1) Kepler / K2 Project Scientist, NASA Ames Research Center; (2) Kepler Guest Observer Office / NASA Ames Research Center

The K2 survey has expanded the Kepler legacy by using the repurposed spacecraft to observe a selection of fields along the ecliptic plane. The K2 dataset includes high-precision 30-minute cadence of over 60,000 stars cooler than 4000K. K2 observations of cool stars are providing an unprecedented look at activity, flares, and rotation of cool stars. In addition, K2 has observed open and globular clusters at all ages, including very young (1-10 Myr, e.g. Taurus, Upper Sco, NGC 6530), moderately young (0.1-1 Gyr, e.g. M35, M44, Pleiades, Hyades), middle-aged (e.g. M67, Ruprecht 147, NGC 2158), and old globular clusters (e.g. M9, M19, Terzan 5). K2 observations of stellar clusters are exploring the rotation period-mass relationship to significantly lower masses than was previously possible, shedding light on the angular momentum budget and its dependence on mass and circumstellar disk properties, and illuminating the role of multiplicity in stellar angular momentum. I will review the cool stars and star clusters sampled by K2 across 18 fields so far, highlighting several characteristics, caveats, and unexplored uses of the public data set along the way. With fuel running out in 2018, I will discuss the closing Campaigns, as well as the data archive and TESS-compatible software tools the K2 mission intends to leave behind for posterity.

### The Sun as a star: the evolution of stellar activity during the main sequence

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

Diego Lorenzo-Oliveira (1); Fabrício C. Freitas (1); Jorge Meléndez (1); Megan Bedell (2,7); Iván Ramírez (3); Jacob L. Bean (2); Martin Asplund (4); Lorenzo Spina (1,8); Stefan Dreizler (5); Alan Alves-Brito (6); Luca Casagrande (4)
(1) Universidade de São Paulo, Departamento de Astronomia do IAG/USP, Brazil; (2) University of Chicago, Department of Astronomy and Astrophysics, USA; (3) Tacoma Community College, USA; (4) The Australian National University, Research School of Astronomy and Astrophysics, Australia; (5) Institut für Astrophysik, Universität Göttingen, Germany; (6) Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Brazil; (7) Center for Computational Astrophysics, Flatiron Institute, USA; (8) Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Australia

The magnetic activity of solar type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1-2 Gyr followed by a relatively inactive constant phase. We revisited the activity-age relation using time-series observations of a large sample of 82 solar twins whose precise isochronal ages and other important physical parameters have been determined. We measured the Ca II H and K activity indices using ~9000 HARPS spectra, to assess the relation between chromospheric activity and stellar age. The age-activity relation is statistically significant up to ages around 6-7 Gyr. We found evidence that, for the most homogenous set of old stars, the chromospheric activity indices seem to continue decreasing after the solar age towards the end of the main-sequence. Our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations effects. The Sun seems to have a normal activity level and variability for its age, being thus a normal Sun-like star.

### Optical high-resolution spectroscopy of 14 young $\alpha$-rich stars

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

T. Matsuno (1,2); D. Yong (3); W. Aoki (1,2); M. N. Ishigaki (4)
(1) Sokendai; (2) National Astronomical Observatory of Japan; (3) Australian National University; (4) University of Tokyo

Combination of asteroseismology and a large spectroscopic survey has revealed the existence of a peculiar stellar population: they look young from their mass but look old from their $\alpha$-element abundances (so-called young $\alpha$-rich stars). We obtained high quality optical spectra for 14 of them to confirm their reported abundances and to constrain their origin from unexplored elements. We first confirm that their $\alpha$-element abundances are consistent with belonging to the $\alpha$-rich population in the Galactic disk. Newly obtained abundances of neutron-capture elements are also similar to those of typical stars in the $\alpha$-rich population. Although one object has been shown to have high Li abundance, the other 13 objects do not show significant abundance anomalies and we conclude that they have the typical abundance pattern of old stars over all the measured elements. Together with the high frequency of radial velocity variation, we suggest they are results of stellar merger or mass transfer from red giants.

### Fixing UV Continuous Opacities and Model Spectra for Cool Stars

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

Jeff A. Valenti (1); Nikolai Piskunov (2)
(1) STScI; (2) Uppsala University

Opacity is a fundamental physical property and an essential ingredient in stellar models. Over the decades, observations of cool stars and the Sun have revealed a variety of opacity sources that were neglected in models. HST/STIS spectra of cool stars with accurately measured angular diameters reveal that model spectra in the near ultraviolet are too bright by as much as a factor of two over the entire wavelength range. At high spectral resolution it is clear that the model errors are in the "continuum" between strong lines, rather than the line cores. We infer that one or more continuous opacity sources are missing from some commonly used models. The dominant opacity source in the NUV turns out to be molecular dissociation of CH, NH, and OH. New photoionization cross sections for neutral metals also increase opacity. Predicted UV fluxes are now substantially better, but discrepancies remain. UV continuum fluxes impact atmospheric structure, abundance determinations for rare elements, stellar population synthesis, and photochemistry in exoplanet atmospheres.

### Are open clusters chemically homogeneous?

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

F. Liu (1); M. Asplund (2); D. Yong (2); S. Feltzing (1); J. Melendez (3); I. Ramirez (4)
(1) Lund University; (2) Australian National University; (3) University of Sao Paulo; (4) Tacoma Community College

The common wisdom is that all stars in an open cluster are expected to share the same chemical abundance pattern. This is a fundamental assumption of the concept of chemical tagging: the ability to identify which stars were born together even the stars are now dispersed throughout the Milky Way. However, recently we have found that this may in fact not be the case, which would have far-reaching implications. Our team has developed a differential analysis technique that has led the field of stellar chemical abundances to unprecedented precision, about 2 percent, a five-fold improvement over traditional analyses. Our study on the Hyades revealed that this open cluster is chemically inhomogeneous at the 0.02 dex level, which could be due to pollution of gas before complete mixing of the proto-cluster cloud. Our recent study on the M67 showed a clear signature of atomic diffusing to be about 0.1 dex through the evolution stage in this benchmark open cluster. Our results thus provide significant new constraints on the chemical composition of open clusters and a challenge to the current view of Galactic archeology. A window of opportunity is open to conduct high-precision chemical abundance studies of a large number of open clusters in near future, in order to investigate the intrinsic chemical inhomogeneity level as well as 'cluster-to-cluster' abundance differences.

### Modelling Brightness Variability of Sun-Like Stars

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

V. Witzke (1); A. I. Shapiro (1); S. K. Solanki (1, 2); N. A. Krivova (1)
(1) Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077, Göttingen, Germany; (2) School of Space Research, Kyung Hee University, Yongin, Gyeonggi, 446-701, Republic of Korea

Observations of Sun-like stars revealed stellar brightness variability on multiple time-scales, where variations on the time-scales of rotational periods and of magnetic activity cycles are induced by stellar magnetic features. The interest in understanding such variations is many-fold, for example to determine stellar properties, and because they are a limiting factor in exoplanet detection. Similar brightness changes occur on our Sun, where they can be observed in detail and have been extensively studied. Thus, models for solar brightness variations have been developed for decades and provide accurate agreement with the solar observations. Since stellar variability is based on the same concepts that were used in solar irradiance models, those can be extended to investigate Sun-like stars. In our approach we focus on stars with different fundamental stellar parameters, e.g. metallicities and effective temperatures. In order to obtain realistic contrasts for magnetic features on different stars we calculate emergent spectra with the ATLAS9 code by using three dimensional (3D) cubes of near-surface magneto convection, which are obtained with the MURaM code. In particular, we show that the solar metallicity value corresponds to a local minimum for the brightness variations, which allows to explain a long-standing puzzle: The observation of the anomalously low solar variability on the magnetic activity cycle time-scale in comparison to stellar brightness variability of Sun-like stars with the same magnetic activity as the Sun.

### The evolution of cool dwarf spin rates: Data, models, and tensions

science theme: Fundamental Properties of Cool Stars
schedule: Wed, TBA

Marcel Agüeros (1); Kevin Covey (2); Jason Curtis (1); Stephanie Douglas (3); Sean Matt (4)
(1) Columbia University; (2) Western Washington University; (3) Harvard-Smithsonian Center for Astrophysics; (4) University of Exeter

Stellar ages are notoriously difficult to measure accurately for low-mass stars, severely limiting our ability to address questions ranging from the evolutionary state of exoplanets to the chemical history of the Galaxy. Gyrochronology, which uses stellar rotation as a proxy for age, is a promising solution to this quandary. Unfortunately, however, theoretical calibrations of the age-rotation relation have historically been hampered by the lack of rotational measurements for large numbers of low-mass stars with a wide range of well-known ages. We are still far from being able to describe fully the evolution of rotation for low-mass stars, or from being able to use rotation measurements to estimate accurately the ages of isolated field stars. I will first summarize recent ground-based and space-based work to characterize the rotational behavior of G, K, and M dwarfs in open clusters ranging in age from 125 Myr (the Pleiades) to 3 Gyr (Ruprecht 147), and then compare these data to each other and to models for stellar spin-down in order to appraise our current understanding of the age-rotation relation.

### Stellar Ages and Galactic evolution: what have we learned from Asteroseismology

science theme:
schedule: Wed, TBA (invited)

S. Hekker
(1) Max Planck Institute for Solar system research; (2) Stellar Astrophysics Centre

With the advent of space telescopes MOST, CoRoT and Kepler/K2 asteroseismology has undergone a revolution. In this talk I will discuss what we have learned from asteroseismology of cool main-sequence stars, subgiants and red giants in terms of Stellar Ages and Galactic Evolution. I will also address future prospects of asteroseismology with the launch of TESS and Plato.

### Simulations of Flux Emergence in Cool Stars: What's Convection, Rotation, and Stellar Structure got to do with it?

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

Maria Weber (1,2)
(1) University of Chicago; (2) Adler Planetarium

Establishing the details of magnetic flux emergence plays a key role in deciphering stellar dynamos and starspot properties. Motivated by the fibril nature of solar surface magnetism, insight into the flux emergence process has been obtained by assuming the bundles of magnetic field giving rise to starspots consists partly of idealized, buoyantly rising thin flux tubes (TFTs). Here we present multiple sets of TFT simulations in rotating spherical shells of convection representative of cool stars. Our solar simulations reproduce sunspot observables such as low-latitude emergence, tilting action toward the equator following the Joy's Law trend, and a phenomenon akin to active longitudes. We comment on the effect of rotation and convective flows (both local and mean) on the subsequent evolution of rising flux tubes in Suns rotating at three and five times the solar rate. Typically, rapid rotation deflects the flux tubes poleward, while strong radial flows distort the flux tube and differential rotation supplies it with added torque. Such TFT simulations share similarities, and a few differences, with buoyant magnetic structures that have recently been realized self-consistently in a unique set of convective dynamo simulations. Finally, we compare our TFT models in partially convective stars to those of fully convective M dwarfs. In these simulations computed at the solar rotation rate, the expected starspot latitudes deviate from the solar trend, favoring significantly poleward latitudes unless the differential rotation is sufficiently prograde or the magnetic field is strongly super-equipartition. This work is a step toward linking magnetic flux emergence, convection, and dynamo action along the lower end of the main sequence.

### Exploring the Role of a Tachocline in M-Type Stars’ Magnetism

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

Connor Bice (1,3); Juri Toomre (1,3); Benjamin Brown (2,3)
(1) JILA; (2) LASP; (3) Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder

M-type stars are quickly stepping into the forefront as some of the best candidates in searches for habitable Earth-like exoplanets, and yet many M-stars exhibit extraordinary flaring events which would bombard otherwise habitable planets with ionizing radiation. Stars’ propensity for such flaring appears to be intimately tied to their rotation rates in a rotation-activity relation, with fast rotators (Ro $\leq$ 0.1) reaching a plateau in observational proxies for magnetic activity such as X-ray luminosity. Observers also find that the fraction of M-type stars which are magnetically active rapid-rotators transitions swiftly from roughly 10 % for main-sequence stars earlier (more massive) than spectral type M3.5 (0.35 M$_\odot$), to nearly 90 % for stars later (less massive) than M3.5. Suggestively, stars below 0.35 M$_\odot$ become fully convective and may no longer contain a tachocline, a shear layer revealed by helioseismology to lie at the base of the solar convection zone, and which is traditionally thought to be fundamental in organizing the solar dynamo. Using the spherical MHD code Rayleigh, we have compared the peak field strengths, topologies, and time-dependencies of the dynamos generated within slowly (Ro $\sim$ 0.4) and quickly (Ro $\sim$ 0.05) rotating stars of masses 0.3 M$_\odot$ and 0.375 M$_\odot$, finding some evidence for distinctive differences in the magnetism of stars containing tachoclines relative to their fully convective counterparts.

### What is Kepler really telling us about Starspots?

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

Gibor Basri
UC Berkeley

The signature of starspots in Kepler data is obvious and pervasive. There have been many papers discussing aspects of these lightcurves, including rotation period detection, dependence of amplitude on rotation, differential rotation signatures, and activity cycles. There are some subtleties of interpretation, however, that have received much less attention (but are relevant to these analyses). 1) as the lightcurves change, how much of that is due to differential rotation (spots changing phase with respect to each other) and how much is due to spot evolution (spots growing and decaying in various locations)? 2) given that Kepler only provides differential photometry, how can we properly convert light deficits to actual spot areas (what does the unspotted star look like)? 3) how much information is actually in the light curves, given that there are generally only one or two dips in intensity per rotation?

I discuss progress on these questions, utilizing a large search of parameter space with spot models. I also present a new rotation-activity relation: there is a strong correlation between a) the ratio of of the time spent by a lightcurve in single dip mode to that spent in double dip mode, and b) the rotation period of that star. I give a tentative interpretation of this new result.

### Imaging Active Stellar Surfaces with Photometric, Spectroscopic, and Interferometric Observations

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

Rachael Roettenbacher
Stockholm University

In the outer layers of cool stars, stellar magnetism stifles convection creating localized, dark starspots. Recently, studies have shown that the presence of starspots can lead to inaccurate estimates of stellar parameters and can obscure the signal of planets. In order to begin disentangling the signatures of stellar magnetism, we image active stellar surfaces with a three state-of-the-art techniques, including ground-breaking aperture synthesis imaging. We obtain interferometric data with sub-milliarcsecond resolution from the Michigan InfraRed Combiner (MIRC) at Georgia State University's Center for High Angular Resolution Astronomy (CHARA) Array. We characterize active RS CVn binary systems and compare the interferometric images with contemporaneous Doppler images from high-resolution spectra and light-curve inversion images. We observe wide-spread regions of suppressed convection on active RS CVn primary stars that would affect stellar parameter estimates and cannot be easily explained by dynamo theories. We extend this study by surveying spotted stars to understand how stellar magnetism changes across evolutionary states, impacts the evidence of companions and their characterization, and differs from the Sun for stars with large convective envelopes.

### Identifying the spectroscopic signatures of magnetic features on the surfaces of the Sun and Sun-like stars

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

Raphaëlle D. Haywood; the HARPS-N Solar Telescope Team
Harvard College Observatory

Magnetic activity features, such as faculae, spots, granulation, and flares leave an imprint in the optical spectrum of a star. Their spectroscopic signatures inform our knowledge of stellar magnetic fields, velocity flows and convection processes; they are also the main obstacle to detecting and characterising small planets orbiting Sun-like stars. The Sun is the only star whose surface can be imaged directly and at high spatial resolution, allowing us to identify the spectroscopic signatures of individual features directly. We use full-disk images from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/HMI) to reconstruct the disk-integrated radial-velocity of the Sun. We compare our reconstructed radial velocities to spectroscopic observations of sunlight reflected from the asteroid Vesta, and of the Sun seen as a distant, point-like star through the HARPS-N Solar Telescope. We find that the intrinsic radial-velocity variations of the Sun arise predominantly from suppression of convective blueshift by faculae, rather than spots. This is in agreement with our analysis of simultaneous spectroscopic and photometric observations of the Sun-like star CoRoT-7. We are now reconstructing the shape of the Fe I line at 6173A observed by SDO/HMI, which displays distortions induced by magnetic features, particularly faculae. Comparing these line-profile distortions with the cross-correlation functions of the HARPS-N spectra, we will identify a proxy for faculae, which will enable us to see faculae in observations of Sun-like stars.

### The Activity Cycle of HAT-P-11

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

Brett M. Morris (1); Leslie Hebb (2); Suzanne Hawley (1); James R. A. Davenport (1, 3); Eric Agol (1)
(1) University of Washington; (2) Hobart and William Smith Colleges; (3) Western Washington University

HAT-P-11 is an active K4 dwarf in the Kepler field. We have studied its starspots with Kepler short-cadence photometry, and its chromospheric emission with echelle spectroscopy from the ground. We take advantage of starspot occultations by its highly-misaligned planet to compare the spot size and latitude distributions to those of sunspots. We find that the spots of HAT-P-11 are distributed in latitude much like sunspots near solar activity maximum, with mean spot latitude of $16\pm1^o$. The majority of starspots of HAT-P-11 have physical sizes that closely resemble the sizes of sunspots at solar maximum, with occasional spots larger than the largest sunspots. We measure the mean spotted area coverage on HAT-P-11 is 3 per cent, roughly two orders of magnitude greater than the Sun’s. The similarities in spot distribution and radii are interesting given the star’s similar rotation period (29 d) but smaller mass ($0.8 M_\odot$) than the Sun. The chromospheric emission of HAT-P-11 is consistent with an $\sim10$ year activity cycle, which plateaued near maximum during the Kepler mission. In the cycle that we observed, the star seemed to spend more time near active maximum than minimum. We compare the log $R^\prime_{HK}$ normalized chromospheric emission index of HAT-P-11 with other stars. HAT-P-11 has unusually strong chromospheric emission compared to planet-hosting stars of similar effective temperature and rotation period, perhaps due to tides raised by its planet.

### Probing the surfaces of Sun-like stars using transiting planets and 3D magneto-hydrodynamical simulations

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

H. M. Cegla
University of Geneva

Our ability to spatially resolve and visually inspect the solar surface makes the Sun the best studied star to date. Even so, the intricacies of many surface phenomena are poorly understood, especially once we move beyond the Sun. Not only will this prevent us from confirming Earth-like planets, but it also limits many areas of stellar physics, such as dynamo theories. In this talk, I will present a new technique to use transiting planets as probes to spatially resolve stellar spectra. With this, we can inspect centre-to-limb variations in the local absorption line profiles shape and net velocity. In turn, this allows us to search for signatures of magnetic activity (e.g. magneto-convection, spots, faculae) and surface differential rotation (as well as determine the star-planet alignment). It also means that, for the first time, we can make detailed comparisons with 3D magnetohydrodynamical simulations of main-sequence stars other than the Sun. We have successfully applied this technique to a G, K, and M dwarf (WASP-8, HD189733, GJ436); for our brightest target, HD189733, we can detect significant differential rotation and confirm good agreement with MHD simulations. For the Sun, we can make more precise centre-to-limb comparisons, and examine the impact of the magnetic field on convective variations. I will also demonstrate how we can use the simulations to predict the relationship between convection-induced line profile shape variations and radial velocities.

### Enhanced stellar activity for slow antisolar differential rotation?

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA

Axel Brandenburg (1,2); Mark S. Giampapa (3)
(1) University of Colorado; (2) Nordita; (3) National Solar Observatory

High precision photometry of solar-like members of the open cluster M67 with Kepler/K2 data has recently revealed enhanced activity for stars with a large Rossby number, which is the ratio of rotation period to the convective turnover time. Contrary to the well established behavior for shorter rotation periods and smaller Rossby numbers, the chromospheric activity of the more slowly rotating stars of M67 was found to increase with increasing Rossby number. Such behavior has never been reported before, although it was theoretically predicted to emerge as a consequence of antisolar differential rotation (DR) for stars with Rossby numbers larger than that of the Sun, because in those models the absolute value of the DR was found to exceed that for solar-like DR. Using gyrochronological relations and an approximate age of 4 Gyr for the members of M67, we compare with computed rotation rates using just the B-V color. The resulting rotation--activity relation is found to be compatible with that obtained by employing the measured rotation rate. This provides additional support for the unconventional enhancement of activity at comparatively low rotation rates and the possible presence of antisolar differential rotation.

### How planets affect cool stars

science theme:
schedule: Thu, TBA (invited)

Katja Poppenhaeger
Queen's University Belfast

All cool stars display magnetic phenomena including flares, spots, and coronal high-energy emission, collectively called magnetic activity. Cool stars also spin down over time by shedding a magnetized wind which couples to the stellar magnetic field; these magnetic effects therefore fade over timescales of gigayears. Cool stars with exoplanets in close orbits may be a fundamental exception from the age-activity-rotation relationships which govern other cool stars. In analogy to close stellar binaries, a planet and its star are thought to interact tidally and magnetically. If the planetary orbit is shorter than the stellar rotation period, angular momentum transfer from the orbit to the stellar spin can take place. This can lead to a spin-up (or inhibited spin-down) of the host stars and a shrinking planetary orbit. In addition to this long-term evolution, planets have also been speculated to trigger stellar flares or other changes in the stellar atmosphere through magnetic interaction or accretion of evaporating exoplanetary material. Observational searches for such enhanced rotation or activity of Hot-Jupiter host stars are not straightforward due to the presence of selection effects and the stochastic nature of stellar activity itself; however, there has been progress over the past years which I will report on. I will also highlight the influence these effects can have on exoplanets themselves.

### Stellar magnetism: origins, effects, and enigmas

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA (invited)

Matthew Browning (1); Laura Currie (1); Lucia Duarte (1); Lewis Ireland (1); Felix Sainsbury-Martinez (1); Maria Weber (2,3)
(1) University of Exeter; (2) University of Chicago; (3) Adler Planetarium

All main-sequence stars transport some energy by convection, whether within a central core (as in massive stars), a convective envelope (as in the Sun), or throughout the interior (as in lower-mass stars). In many cases this convection, coupled to rotation and occurring in regions of high electrical conductivity, likely acts to build magnetic fields through dynamo action. The resulting magnetism leads to a host of observable effects, with the high-energy emission from a star, its rotational evolution, and perhaps even its overall structure all influenced (in certain cases) by the magnetic fields. I will review how the fields probably arise, and what they do, drawing partly on the results of large-scale numerical simulations that have sought (together with 1-D models and basic theory) to model stellar interiors with reasonable fidelity. I will argue that the theoretical models can provide some insight into how strong the fields can get, what spatial and temporal structure they might have, and how these things depend on parameters like the overall rotation rate or stellar mass - but will also highlight some enduring puzzles that have yet to be solved.

### The devil is in the details: Spicules, Jets, and the fine structure of the solar atmosphere

science theme: Solar/Stellar Magnetic Fields and Surface Structure
schedule: Thu, TBA (invited)

Gianna Cauzzi (1,2)
(1) National Solar Observatory, USA; (2) INAF-Arcetri, Italy

The proximity of our Sun allows us to observe the atmosphere of a star in exquisite details, with current facilities routinely reaching resolutions of few 100 km at the solar surface. These observations of the outer solar atmosphere reveal a plethora of magnetically driven, highly dynamic features such as spicules, jets or flare-related transients (just to name a few), many occurring at scales we can't yet fully resolve. While most of these features seem to interact with the coronal plasma, their actual role in providing mass and energy to the corona and the solar wind remains uncertain - a debate fueled by the difficulty in properly deriving their physical characteristics.

In this talk I will focus on state-of-the-art, high-resolution observations of the dynamic solar atmosphere, in particular the chromosphere and transition region, and on current efforts in interpreting and modelling the polarimetric data in terms of the underlying magnetic and thermodynamic structure. I will discuss the prospects for making significant progress in resolving some of these questions using the four-meter aperture Daniel K. Inouye Solar Telescope (DKIST), with first light expected in 2020.

### Weather Patterns on Exoplanet Analogues

science theme: Very Low Mass (VLM) Objects
schedule: Fri, TBA

Johanna Vos
University of Edinburgh

Photometric variability monitoring is sensitive to atmospheric features as they rotate in and out of view, allowing us to probe the presence of surface inhomogeneities caused by patchy clouds, hot spots and temperature fluctuations. Periodic variability has been detected in L and T brown dwarfs, and more recently in a small sample of free-floating, planetary-mass objects. These young, low-gravity objects share a striking resemblance with the directly-imaged planets and can be studied in far greater detail in the absence of a bright host star. The large amplitudes observed in this small sample of low-gravity objects suggests that variability may be enhanced for the exoplanet analogues. We have recently carried out the first large survey for weather patterns on exoplanet analogues. I will present the results of this survey and discuss what we have learned about the role of surface gravity in variability properties.

### Understanding planets hotter than cool stars: the MMT Exoplanet Atmosphere SURvEy (MEASURE)

science theme: Very Low Mass (VLM) Objects
schedule: Fri, TBA

J. L. Birkby
University of Amsterdam

High-resolution spectroscopy is a robust and powerful tool in substellar companion characterization. It uses changes in the Doppler shift of the faint companion to disentangle its spectrum from the glare of its host star. The technique is sensitive to the depth, shape, and position of the object’s spectral lines, and thus reveals information about its composition, atmospheric structure, mass, global wind patterns, and rotation. I will present the first results from MEASURE: the MMT Exoplanet Atmosphere SURvEy. This 40 night survey of 11 exoplanet atmospheres at R~30,000 is the largest homogenous high-resolution survey to date. Many of the planets in the survey have temperatures hotter than brown dwarfs, and some are even hotter than M-dwarfs. I will focus on how the survey will provide an accurate measurement of the temperature at which atmospheres transition into having thermal inversion layers. At high spectral resolution, these features are clearly detected via multiple strong emission lines from e.g. CO, and preliminary results indicate this occurs at hotter temperatures than previously expected. I will discuss the chemical species and processes responsible for this in exoplanet atmospheres and contrast this to processes occurring in the atmospheres of brown dwarfs and M-dwarfs. MEASURE not only provides a homogenous dataset to perform comparative exoplanetology, but provides complementary high-resolution spectra for substellar objects already observed with HST and Spitzer, allowing the next step in the detailed characterization of exoplanet atmospheres and their more massive substellar cousins.

### Young brown dwarfs and the planetary spin-mass relation: new insights from K2

science theme: Very Low Mass (VLM) Objects
schedule: Fri, TBA

Aleks Scholz
University of St Andrews

While brown dwarfs show great similarities with during their early evolution, their spin evolution is much more akin to that of giant planets. We have used high precision lightcurves from the K2 mission to measure new rotation periods for brown dwarfs in star forming regions, ranging from 1 to 10Myr in age and from 0.02 to 0.08 Msol in mass (published in Scholz et al. 2015, ApJ, and Scholz et al. 2018, ApJ, submitted). Based on these robust period samples, we detect a weak link between slow rotation and the presence of disks in brown dwarfs, for the first time with mid-infrared data. The analysis of periods vs. age shows, however, that disk braking is inefficient or short-lived. By and large, young brown dwarfs retain their angular momentum through the first Myr of their evolution, in stark contrast to low-mass stars. For the first time, we demonstrate that the rotation rates of young brown dwarfs, calculated forward to the age of the solar system, fit the spin-mass relation of planets, a trend that holds over at least six orders of magnitude in object mass, including those formed by core accretion, disk fragmentation, and core collapse. We argue that studies of rotation in young brown dwarfs and planetary-mass objects have potential to shed light on the change in formation scenario as a function of mass.

### High-Energy Radiation at the Substellar Boundary

science theme: Very Low Mass (VLM) Objects
schedule: Fri, TBA

K. Punzi (1); J. Kastner (1); D. Principe (2); B. Stelzer (3); U. Gorti (4); I. Pascucci (5); C. Argiroffi (6)
(1) Rochester Institute of Technology; (2) MIT Kavli Institute; (3) Eberhard Charles University; (4) SETI Institute; (5) Lunar and Planetary Laboratory, The University of Arizona; (6) INAF-Osservatorio Astronomico di Palermo

We have conducted a Cycle 18 Chandra Large Program survey of very cool members of the $\sim$8 Myr-old TW Hydra Association (TWA) to extend our preliminary study of the potential connections between M star disks and X-rays (Kastner et al. 2016, AJ, 152, 3) to the extreme low-mass end of the stellar initial mass function. Thus, we can further investigate the potential connection between the intense X-ray emission from young, low-mass stars and the lifetimes of their circumstellar planet-forming discs, as well as better constrain the age at which coronal activity declines for stellar masses approaching the H-burning limit of $\sim$0.08 M$_{\odot}$. This extended survey supports the conclusions found in the initial survey: there exists a trend of decreasing X-ray luminosity relative to bolometric luminosity (L$_{\rm X}$/L$_{\rm bol}$) with decreasing effective temperature (T$_{\rm eff}$) for TWA M stars. The earliest-type (M0-M2) stars appear overluminous, having log(L$_{\rm X}$/L$_{\rm bol}$) $\approx$ -3.0, but for spectral types M4 and later log(L$_{\rm X}$/L$_{\rm bol}$) decreases and its distribution broadens, with these mid- to late-M TWA stars appearing underluminous in X-rays compared to very young pre-main sequence stars of similar spectral type and luminosity. Additionally, the fraction of TWA stars that display evidence for residual primordial disk material sharply increases in this same (mid-M) spectral type regime. Thus, our data suggests that disk survival times may be longer for ultra-low-mass stars and brown dwarfs than for higher-mass M stars. These types of observations can provide key information about the X-ray radiation fields of M stars to constrain circumstellar disk and exoplanet formation and evolution models, allowing us to explore the realistic effects of M star X-rays on the survival of atmospheres of terrestrial planets.

### Cloud Formation on Brown Dwarfs and Hot Jupiters

science theme: Very Low Mass (VLM) Objects
schedule: Fri, TBA

Diana Powell (1); Xi Zhang (1); Peter Gao (2); Jonathan Fortney (1); Mark Marley (3); Vivien Parmentier (4)
(1) UC Santa Cruz; (2) UC Berkeley; (3) NASA Ames; (4) Le LAM, Laboratoire d'Astrophysique de Marseille

Clouds on brown dwarfs and extrasolar worlds are seemingly abundant and interfere with observations; however, little is known about their properties. In this talk I will present the first application of a bin-scheme microphysical and vertical transport model to determine the size distribution of cloud particles in the atmospheres of very low gravity brown dwarfs as well as hot Jupiters. We predict particle size distributions from first principles and investigate how observed cloud properties depend on the atmospheric thermal structure and vertical mixing. In particular, we investigate whether these objects’ interesting observational properties can be explained by clouds. For hot Jupiters, I will show that the predicted realistic size distributions are frequently bimodal and irregular in shape and that the cloud opacities are roughly constant across a broad wavelength range with the exception of features in the mid-infrared. I will also suggest that cloud opacities in emission may serve as sensitive tracers of the thermal state of a planet’s deep interior through the existence or lack of a cold trap in the deep atmosphere.

### Measuring the radius and mass of the smallest stars

science theme: Very Low Mass (VLM) Objects
schedule: Fri, TBA

Avi Shporer
MIT

Compared to the large number of planets and Sun-like stars with measured radius and mass, there are only a handful of small stars, from $\approx$0.2 M$_{\odot}$ down to brown dwarf mass, with these measured properties. This gap prevents testing theoretical radius-mass relations for the smallest stars. Furthermore, these stars are interesting also as transiting planet hosts, where an improved understanding of their radius and mass will lead to an improved measurement of the radius and mass of transiting planets such stars host. We are working to increase the sample size of the smallest stars with measured radius and mass. These objects are detected first as transiting planet candidates, where radial velocity follow-up identifies their true nature. This work has so far resulted in the discovery of three objects (Shporer et al. 2017, ApJL, 847, 18) and we are currently working on completing the orbit of several others.

### A New Generation of Substellar Atmosphere & Evolution Models

science theme: Very Low Mass (VLM) Objects
schedule: Fri, TBA (invited)

M. Marley (1); D.Saumon (2)
(1) NASA Ames Research Center; (2) Los Alamos National Laboratory

We present a new generation of substellar atmosphere and evolution models. These models, spanning 200 to 2400 K and masses from about 1 to 80 Jupiter masses for a range of metallicities and C/O ratios, are appropriate for interpreting spectra and photometry of brown dwarfs and directly imaged extrasolar giant planets. Notable improvements from past such models include updated opacities and cloud models. We find that the edge of the main sequence has moved down, compared to our previous models, from 0.075 to 0.072 $\,\rm M_\odot$. In addition to comparing the new models to various datasets, we will discuss how data retrieval methods and forward models working together can validate assumptions and lead to greater understanding than either approach alone.

## Posters

Posters will be listed here after the SOC has reviewed the submitted abstracts.