Spectroscopic Confirmation of an M6 Dwarf Companion to the Nearby Star BD-08 2582

BD-08 2582 is a K7/M0 dwarf roughly 15 pc from the Sun that has been the subject of over 60 years of observational study, (Stedman & Vyssotsky 1956), including dedicated binary surveys (Ward-Duong et al. 2015). Despite this, its companion BD-08 2582B, included in the Washington Double Star catalog (Mason et al. 2001) with the citation "B. A. Skiff, private communication," has been largely overlooked. Reylé (2018) identified this source in Gaia DR2 (Gaia Collaboration 2018), reporting a color-based classification of M7, but did not associate the source with BD-08 2582A. Marginal astrometric evidence of a secondary (σ = 1.9) was reported on the basis of Hipparcos–Gaia proper motion acceleration, with a minimum separation of 1.3 au and a highly uncertain mass estimate of ${0.27}_{-0.23}^{+0.24}$ M_⊙ (Kervella et al. 2019).

BD-08 2582B has been detected in multiple photometric and astrometric catalogs. The most recent Gaia EDR3 release (Gaia Collaboration et al. 2020) shows that the two sources have equivalent parallactic distances (A: 14.498 ± 0.004 pc; B: 14.491 ± 0.008 pc) and similar—but not identical—proper motions (A: μ = [−304.318 ± 0.018, 209.456 ± 0.011] mas yr⁻¹, B: μ = [−308.995 ± 0.037, 215.369 ± 0.025] mas yr⁻¹). The separation is measured to be ρ = 841931 ± 000003 at position angle θ = 26428414 ± 000013 (primary to secondary) at reference epoch 2016.0 (Julian Date JD 2457388.500), with a corresponding projected separation of 122.00 ± 0.07 au and a relative G-band magnitude of 5.7494 ± 0.0007. The system was also resolved in 2MASS (Skrutskie et al. 2006) with a similar separation ρ = 823 ± 008 and position angle θ = 2640 ± 06 at JD 2451185.833, and with ΔK_s  = 4.00 ± 0.03. It has also been resolved in PanSTARRS (Flewelling et al. 2020) with DR2 detection data providing a somewhat different separation ρ = 77690 ± 00025 and position angle θ = 247429 ± 0024 at JD 2456734.359, possibly due to saturation of the primary. The optical-infrared color of BD-08 2582B, r − K_s  = 5.86 ± 0.02, is consistent with an M6 dwarf (Best et al. 2018).

Here, we report resolved red optical spectroscopy of BD-08 2582A and B. We observed the system with the KAST spectrograph on the Shane 3 m telescope on 2019 December 9 (UT). The two sources were observed sequentially using the 2'' slit while guiding on the primary, with two exposures in the red channel (6300–9000 Å) totaling 60 s for BD-08 2582A and 1200 s for BD-08 2582B. The G2 V star HD 76151 was observed for telluric absorption calibration and Hiltner 600 for flux calibration (Hamuy et al. 1994) Spectral data were reduced using the kastredux code ⁵ using default settings. Figure 1 shows the resolved and normalized spectra of both components compared to the best-fitting SDSS spectral standards from Bochanski et al. (2007) and Kesseli et al. (2017). BD-08 2582A is intermediate between K7 and M0 (it appears as both spectral types in the literature), while BD-08 2582B is well-matched to an M6 template, just slightly earlier than the M7 photometric classification from Reylé (2018). We measured the Lépine et al. (2007) metallicity index ζ = 1.019 ± 0.007, indicating solar metallicity, consistent with the primary ([Fe/H] = 0.03 ± 0.011; Gaidos et al. 2014). BD-08 2582B also exhibits Hα emission with an equivalent width of −2.89 ± 0.04 Å. Using a χ continuum correction factor from Douglas et al. (2014) yields $\mathrm{log}{L}_{{\rm{H}}\alpha }/{L}_{\mathrm{bol}}=(5.0\pm 1.0)\times {10}^{-5}$, in line with the average activity level of nearby M6 dwarfs (West et al. 2015), and places BD-08 2582B just below the saturation regime with Ro = P_rot/τ_conv ≲ 0.5 (Newton et al. 2017). We see no evidence of Li I absorption in the spectra of either source.

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