Brown Dwarfs, Low-Mass Stars, and Directly Imaged Exoplanets in the Era of Gaia

Conveners: Jackie Faherty, American Museum of Natural History; John Bochanski, Rider University; Jonathan Gagne, Carnegie DTM

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Astrometric surveys of increasing precision are responsible for our understanding of the structure of the Milky Way. For instance, early studies by Kapteyn 1905 and Eggen 1965 identified large kinematically coherent associations of stars through ground based kinematic studies of common proper motion and parallax. Those associations were resolved into smaller age-coherent groups with the progression to milliarcsecond parallaxes and proper motions with the Tycho and Hipparcos surveys (Perryman et al. 1997, Hog et al. 2000).

We are now on the precipice of a new era in the kinematic understanding of the Milky Way as we are moving from milli- to microarcsecond astrometry with the Gaia data release catalogs. It is no understatement to say that Gaia will revolutionize our understanding of the kinematic structure of the Milky Way galaxy.  Importantly it will drastically update our astrometric understanding of low-mass stars.  For instance, Hipparcos only had parallaxes for 10% of stars within 30pc, and 2% of all those within 100 pc.  Gaia will give us parallaxes for all stars within  50pc, down to the substellar boundary (0.072 Msol) and for all stars above 0.12 Msol within 150pc (through M5--the peak of the mass function).  In April 2018 Gaia will make public the second data release, containing 1.5 billion parallaxes and six million radial velocities. This will be a 10,000 fold increase over what Hipparcos supplied and at a precision up to 100 times better. Leading up to Cool Stars 20 (as well as for many years to follow) we will see a giant leap forward in the membership lists as well as fine-detailed kinematic, velocity, and spatial resolutions of all clusters, star forming regions, and moving groups within a few hundred parsecs from the Sun. Moreover, we will see the identification and refinement of co-moving pairs as well as hierarchical systems (triples, quadruples, etc.) that will be the seed for understanding co-evolving structures in the Galaxy.


Coeval associations of stars, such as those found in star forming regions, clusters, and kinematically coherent moving groups, are unique testbeds for stellar and Galactic investigations (Zuckerman & Song 2004; Torres et al. 2008). Within a few hundred parsecs of the Sun, there are numerous associations ranging in age from a few megayears (e.g. Rho Ophiuchus, 0.5-2 Myr, Wilking et al. 2008; Taurus, 1-2 Myr, Daemgen et al. 2015) through hundreds of megayears (e.g. Tucana Horologium, 45 Myr; AB Doradus, 150 Myr, Bell et al. 2015; Pleiades, 112 Myr, Dahm 2015; Hyades, 750 Myr, Brandt & Huang 2015). In depth studies of the closest associations to the Sun (< 200 pc) have revealed that they harbor large numbers of low-mass stars, brown dwarfs, and even objects whose mass falls below the deuterium burning boundary (so called free-floating planetary-mass objects; Gagne et al. 2015; Faherty et al. 2016). Moreover, given that moving groups harbor the closest young stars to the Sun, they are also the targeting ground for directly imaged exoplanets. Associations such as Tucana Horologium, TW Hya, and AB Doradus contain isolated objects that range in mass from a few solar masses down to a few Jupiter masses (Gagne et al. 2017, Faherty in prep) as well as stars with planetary-mass companions. Observations of these associations enable investigations of the mass function, kinematics, and spatial distribution across the full range of objects generated through star formation processes in different isolated groups at young (1-2 Myr), medium (30-50 Myr), and early adult (100-700 Myr) ages.

Interestingly, the origin and evolution of many of the nearby associations remains a mystery. Nearby moving groups such as AB Doradus and  Pictoris have overlapping kinematics and are widely distributed across the sky. Associations such as Tucana Horologium and TW Hya have bottom-heavy mass functions with a surprisingly large number of low-mass stars, brown dwarfs and free-floating planetary-mass objects (see e. g. Gagne et al. 2015, 2017; Faherty et al. 2016). Many associations do not trace back into a singular point which would be indicative of a singular moment for the collapse of a nascent cloud but rather appear to be filamentary in origin (e.g. Donaldson et al. 2016, Riedel et al. 2017).

In this splinter session we will highlight the work being done to refine cluster membership lists, determine mass functions, and ground the important relations for higher mass stars that might host giant exoplanets, low-mass stars, brown dwarfs, and free floating planetary-mass sources. We will also highlight the work ongoing to uncover co-moving, low-mass stars and brown dwarfs in Gaia and investigate telling parameters of co-evolution.


For the 180 minute allotted time slot, we will have two 80 minute sessions with a 20 minute break in the middle.  We will also reserve at least 10 minutes for a poster pop/lightning talk for anyone with a relevant poster on the topic.  With the remaining  150 minutes of content, we will host 9 speakers giving 15 minute talks (12+3).  We will reserve 15 minutes for a general discussion with the audience about the most important Gaia studies that are ongoing for low-mass stars, brown dwarfs, and directly imaged exoplanets.


If you are interested in speaking in this session or presenting a poster pop/lightning talk please submit to our google form.