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The Low-Mass Members of the Ursa Major Association

We present a set of 5 over-densities in spatial-kinematic space previously identified by Kounkel&Covey (2019) that may constitute extended tidal tails associated with the nearby, 400-Myr old Ursa Major association. This sample of 1599 stars have a main-sequence turnoff point consistent with the age of Ursa Major, and might contain its missing low-mass members. It includes 4 candidate white dwarfs with properties that seem consistent with a total age of about 400 Myr, but we estimate from older white dwarf interlopers that about 33% of the sample may be contaminated by random field stars.

The aforementioned Kounkel & Covey (2019) groups contain 20 white dwarfs based on their position in the Gaia DR2 color-magnitude diagram: 16 of them are clearly too old for bearing any relation to Ursa Major based on a comparison with the Montréal C/O core cooling tracks 1 . Further, all of these contaminant white dwarfs are located within 200 pc, suggesting that the more distant ones have simply not been uncovered because of their faintness. Combining this number with local number space densities for white dwarfs (4.49 ± 0.38 × 10 −3 objects pc −3 ; Hollands et al. 2018) and main-sequence stars (98.4 ± 6.8 × 10 −3 objects pc −3 ; Kirkpatrick et al. 2012) suggests we might expect a total of ∼520 main-sequence false-positive members, or a contamination rate of about ∼33%. We note that most of the white dwarf interlopers are located in Theia 906 (6 interlopers) and Theia 908 (also 6 interlopers), indicating they may be more contaminated -consistent with their wider U V W distributions. The remaining are in Theia 1008 (3 interlopers) and Theia 1091 (1 interloper). Four additional white dwarfs (WDJ235833.30+510815.54, WDJ233848.60+404803.82, WDJ232227.23+394456.36, and WDJ121521.73−523646.04) have temperatures in the range 8000 K-24000 K, masses in the range 1.00-1.27 M (Gentile Fusillo et al. 2019), and total (main-sequence plus cooling) ages in the range 400-800 Myr, based on non-dereddened Gaia DR2 color-magnitude positions.
Most of the members in these candidate tidal tails are distributed between −20 to +20°in Galactic longitude, indicating that there might not be many candidates left to be recovered beyond the −30 to +30°range that was surveyed by Kounkel & Covey (2019). Stars at the upper main sequence of the color-magnitude diagram have a range of distances from 73 pc to 330 pc, compared to 67-330 pc for the full sample: this indicates no obvious distancedependent contamination in their sample of lower-mass stars. It can however be expected that more members may be left to uncover within 70 pc of the Sun, where the Kounkel & Covey (2019) algorithm was less sensitive. We have observed no clear correlation between the spatial XY Z and kinematic U V W distribution of members, meaning that any search for nearby members will likely require using a very wide net in U V W space, and may suffer a high rate of contamination despite the unusual kinematics of Ursa Major compared with the Solar neighborhood. This also explains why our previous searches based on the U V W distribution of the core members failed to uncover anything.
More work is clearly needed to disentangle the relations between the different groups discussed here, and to assess whether the large structures recovered by Kounkel & Covey (2019) are really related to Ursa Major, and where the Eggen (1992) Sirius supercluster, and the King et al. (2003) Ursa Major stream, fit into that picture. It is likely, for example, that some members at the extremes of the U V W distributions of Theia 906 and 908 suffer from low-quality measurements or correspond to contaminants. Using metrics of stellar activity and rotation periods from the TESS mission (Ricker et al. 2015) will be useful to corroborate membership although only for the nearest members (e.g., see Curtis et al. 2019), and a more detailed study of individual members will be useful to further investigate the relation between the Kounkel & Covey (2019) groups discussed here and Ursa Major. Gaia DR3 will be useful in this investigation as it will likely contain radial velocity measurements for several stars discussed here. It will also be interesting to determine why Ursa Major might have much larger tidal tails with a wider space velocity spread than the similarly-aged Coma Ber (Silaj & Landstreet 2014) which tidal tails were recently discovered by Tang et al. (2019). Studying the four white dwarfs discussed above in more details will also be useful to better age-date this population.
We would like to thank Eric E. Mamajek and Andrew W. Mann for useful comments and discussion. This research made use of data from the ESA mission Gaia, processed by the Gaia Data Processing and Analysis Consortium whose funding has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
1 Computed similarly as the pure C core models described in Fontaine et al. (2001), and available at http://www.astro.umontreal.ca/ bergeron/CoolingModels/  (2019) groups that may constitute tidal tails to the Ursa Major group. They are located between ∼70 pc and ∼330 pc, and have space velocities similar to Ursa Major although more scattered, especially in the direction of Galactic rotation (V ). Ellipses represent the multivariate Gaussian models of BANYAN Σ, and indicate the loci of members for IC 2391, Platais 8, IC 2602, Lower Centaurus Crux (LCC), Upper Centaurus Lupus (UCL), Upper Scorpius (USCO), the Taurus-Auriga star-forming region (TAU), the Pleiades association (PLE), the Hyades association (HYA), the AB Doradus moving group (ABDMG), the Ursa Major association (UMA), and the Coma Berenices association (CBER). For more details on these models and associations, see Gagné et al. (2018c). Data behind figures is available as online-only material.