The Low-mass Members of the Ursa Major Association

The Ursa Major association (Eggen 1992) is a nearby (∼25 pc) collection of ten F2 to A5-type stars (King et al. 2003), aged 414 ± 23 Myr (Jones et al. 2015), distributed in a core mostly consisting of stars in the Big Dipper asterism, with a scatter in Galactic positions (XYZ) of about 1–3 pc. It is accompanied by a stream of 42 stars with a wider spectral types distribution and a XYZ scatter of 25–40 pc. The core is slightly better localized in Galactic velocities UVW with a scatter of 1–3  km s⁻¹, compared with 2.5–4  km s⁻¹ for the stream members. Despite it being the nearest open cluster to the Sun, its population of low-mass members is still missing (the stream contains 3 K-type stars and 1 M-type star), likely because they have been scattered over time. Even most recent surveys only identified two additional M-type candidate members (Gagné et al. 2018a). Preliminary surveys based on the kinematic part of the BANYAN Σ algorithm (Gagné et al. 2018b) did not allow us to uncover additional low-mass members in a larger XYZ region, suggesting they could have been scattered beyond recovery.

A recent study by Kounkel & Covey (2019) used a hierarchical clustering algorithm applied to Gaia DR2 data (Gaia Collaboration et al. 2018) to uncover over-densities in sky position, proper motion, and parallax within 1 kpc and 30° of the Galactic plane. They uncovered 1640 localized or extended over-densities, and estimated their isochronal ages. Because their method is based in direct observables rather than XYZUVW they recovered almost no over-densities within 70 pc, and they have thus not recovered the core or stream of Ursa Major.

We noticed that five of the Kounkel & Covey (2019) groups (Theia 906, 908, 1008, 1009 and 1091), totaling 1599 stars, have a UVW distribution similar to the Ursa Major core and stream, and extend spatially along two tails to almost 350 pc from the Sun (see Figure 1). Despite their wider UVW distributions (18, 33, and 11  km s⁻¹), they have non-dereddened and model-dependent isochronal ages consistent with Ursa Major (503–849 Myr). The wider spread especially in V could indicate that they are tidal tails well into the process of dissipation, but not beyond recovery as we previously hypothesized. The "Sirius supercluster" of Eggen (1992) displays a similar wide distribution in UVW space, and could be related despite its apparent contamination–Eggen (1992) noted that it consists of some stars aged ∼630 Myr and others ∼1 Gyr.

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The combined Gaia DR2 G versus G − G_RP color–magnitude diagram of the Theia groups discussed here seems consistent with a coeval, ∼500 Myr old stellar population: it ends abruptly at the upper main sequence (G − G_RP = 0), and its most massive member, HD 199713, is a B9 star (Wright et al. 2003), corresponding to a mass of ∼2.8 M_⊙ from Pecaut & Mamajek (2013). Other mass estimates for HD 199713 range from ${2.9}_{-0.4}^{+0.3}$ M_⊙(Anders et al. 2019) to 3.0 ± 0.4 M_⊙ (Stassun et al. 2019), and correspond a main sequence lifetime of 460–580 Myr (Choi et al. 2016). The low-mass region of the color–magnitude diagram contains 990 stars with G − G_RP between 1.0 and 1.4 mag, or spectral types M0–M5, and are not significantly over-luminous compared to the distribution of older field stars, consistent with the Ursa Major age. Both the Ursa Major stream of King et al. (2003) and the Sirius supercluster of Eggen (1992) have similar color–magnitude diagram sequences with consistent main-sequence turn-off points. One Sirius supercluster star is bluer, β Aur (spectral type A1, see Gray et al. 2003), but it is an eclipsing binary with component masses 2.35 ± 0.03 M_⊙ and 2.27 ± 0.03 M_⊙ (Belikov 1995), consistent with the age of Ursa Major.

The aforementioned Kounkel & Covey (2019) groups contain 20 white dwarfs: 16 are too old for Ursa Major based on a comparison with the Montréal C/O core cooling tracks⁴ (Fontaine et al. 2001). The local space densities of 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) suggest we could expect ∼520 main-sequence interlopers, i.e. a ∼33% contamination rate. Four white dwarfs (WD J235833.30+510815.54, WD J233848.60+404803.82, WD J232227.23+394456.36, and WD J121521.73−523646.04) have temperatures of 8000–24,000 K, masses of 1.00–1.27 M_⊙ (Gentile Fusillo et al. 2019), and total ages (main-sequence plus cooling) of 400–800 Myr, based on Gaia DR2 color–magnitude positions. More work is needed to disentangle the relations between the different groups discussed here, and to assess how the large structures recovered by Kounkel & Covey (2019), the Eggen (1992) Sirius supercluster, and the King et al. (2003) Ursa Major stream relate to the Ursa Major core.