All-sky Co-moving Recovery Of Nearby Young Members (ACRONYM). II. The β Pictoris Moving Group^∗

The discoveries of young moving groups (YMGs) throughout the past two decades have provided samples of young nearby stars (e.g., Kastner et al. 1997; Webb et al. 1999; Torres et al. 2000; Zuckerman & Webb 2000; Zuckerman & Song 2004; Shkolnik et al. 2012; Malo et al. 2013, 2014a; Kraus et al. 2014; Aller et al. 2016) for studies of star formation, stellar evolution, and activity-rotation-age relations. They also serve as prime targets for direct-imaging searches for circumstellar disks, close stellar and brown dwarf (BD) companions, and young exoplanets. These groups most likely formed in coeval, cospatial, and comoving molecular clouds, spatially dispersed over time, and are now linked through their common space motion and indications of youth. Two of the youngest such groups are the 8–10 Myr (Barrado y Navascués 2006; Bell et al. 2015) TW Hya Association (TWA; Webb et al. 1999; Zuckerman et al. 2001b), named for the isolated T Tauri star, and the 20–25 Myr (Binks & Jeffries 2014; Malo et al. 2014a; Mamajek & Bell 2014; Bell et al. 2015) β Pictoris moving group (BPMG; Zuckerman et al. 2001a; Zuckerman & Song 2004; Torres et al. 2006, 2008; Schlieder et al. 2010, 2012a, 2012b), named for its debris disk and exoplanet-hosting hot star.

Surveys for young and active stars have revealed $\approx 10$ additional YMGs with ages between 8 and 300 Myr, filling a critical age gap between the very young star-forming regions and the old field population (Mamajek 2016). The close proximity of these YMG members to Earth ($\lesssim 100$ pc) is also beneficial for studies in need of high angular resolution, such as circumstellar disk and planet imaging, and sensitivity to low-mass stellar and substellar companions. At these young ages, disks and planets are also at their brightest and more easily detectable, e.g., the well-studied AU Mic debris disk (Liu 2004; Boccaletti et al. 2015) and substellar companions (Delorme et al. 2012; Bowler et al. 2013, 2015b).

In this paper, we present the All-sky Co-moving Recovery Of Nearby Young Members (ACRONYM) of the BPMG, the second in our series of YMG member searches, after our first paper on the 40 Myr Tuc-Hor YMG (Kraus et al. 2014). Early literature values of the age of the BPMG were $\approx 12\,\mathrm{Myr}$ for over a decade (Table 1 of Mamajek & Bell 2014). More recent studies, including lithium and isochrone analyses, have converged to an age of 23 ± 3 Myr (Binks & Jeffries 2014; Bell et al. 2015). By 23 Myr, the stars will likely have already formed their giant planets and are in the process of forming terrestrial planets (Lissauer 1987; Pollack et al. 1996; Kenyon & Bromley 2006). Currently, there are two directly imaged planets known around BPMG members: an ≈7 M_Jup planet around its namesake A0 star, β Pic (Lagrange et al. 2010), and a 2.5 M_Jup planet around the F0 star 51 Eridani (Macintosh et al. 2015). There is also one known free-floating planetary mass, the L dwarf PSO J318.5338–22.8603 (Liu et al. 2013; Allers et al. 2016). Smaller planets have not yet been found around such young stars, primarily due to the detection limitations of direct-imaging instruments and radial velocity (RV) searches. We focus our search on new low-mass members (K7–M9; ${M}_{* }\lesssim 0.7\,{M}_{\odot }$) around which most planets form and contrast ratios are more favorable for directly imaging planets and low-mass companions.

Prior to this publication, there were 146 systems confirmed as BPMG members, consisting of 16 systems with A- or F-star primaries, 33 with G or K primaries, 80 M-star (M0–M6) systems, and 14 BDs (≥M7), yielding a 61% M-star fraction for the AFGKM stars. (See the Appendix for a complete census.⁸ ) Yet, with M dwarfs making up 75% of the stellar mass function in the field (Bochanski et al. 2010), we expected that ∼75 of the low-mass members of the BPMG had yet to be discovered. In this paper, we report the confirmation of 66 low-mass BPMG members, 41 of which are new discoveries (37 M stars and four BDs), increasing the known M-star fraction to 69%.

Our photometric selection process to identify BPMG candidates and the spectroscopic analysis needed for confirmation of new members are nearly identical to those used in our search for new Tuc-Hor moving group members (Kraus et al. 2014). To summarize, we combined astrometry and photometry from USNO-B1.0 (Monet et al. 2003), 2MASS (Skrutskie et al. 2006), SDSS DR9 (Ahn et al. 2012), DENIS (Epchtein et al. 1994), and UCAC3 (Zacharias et al. 2010). These data yielded proper motions from the astrometry and spectral types (SpTs) and bolometric fluxes from spectral energy distribution (SED) fitting of the photometry for all sources in those catalogs. To concentrate our search on the most probable candidates, we narrowed our input sample using two spatial cuts. First, to avoid crowding and its effects on the inferred stellar properties, we neglected sources near the galactic plane and center ($| b| \lt 5^\circ$, or $| b| \lt 10^\circ$ and $310^\circ \lt l\lt 50^\circ$). Second, to focus our consideration on the locus of known BPMG members (Figure 1), we disregarded targets with $7h\lt \alpha \lt 18h$.

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Nearly all of the sources that passed our two selection criteria have proper motions or color–magnitude diagram positions inconsistent with membership in the BPMG, so they were winnowed to find the small number of bona fide members. We first computed the kinematic distance modulus (DM_kin) that would minimize the difference between a source's observed proper motion and the expected proper motion for a BPMG member at that position on the sky. The magnitude of this discrepancy is reported as ${{\rm{\Delta }}}_{\mathrm{PM}}$. We identified 4486 sources for which the observed and expected proper motions agreed within 3σ (i.e., ${{\rm{\Delta }}}_{\mathrm{PM}}/{\sigma }_{\mu }\lt 3$) or the total discrepancy was ${{\rm{\Delta }}}_{\mathrm{PM}}\lt 10$ mas yr⁻¹ and the inferred kinematic distance was $d\leqslant 65$ pc (${\mathrm{DM}}_{\mathrm{kin}}\leqslant 4.0$). We then estimated the height above the main sequence by comparing the kinematic distance modulus to the spectrophotometric distance modulus, requiring $0.5\leqslant ({\mathrm{DM}}_{\mathrm{kin}}\mbox{--}{\mathrm{DM}}_{\mathrm{phot}})\leqslant 4.0$ to select pre-main-sequence stars. The HR diagram criterion reduced our target list to 660 photometric + kinematic candidates. Finally, we narrowed our focus to the 104 candidates with SpTs of K7–M9. The SpT for each star was measured from the SED fitting process (SpT_SED) described by Kraus et al. (2014). In that paper, we showed that the SpT_SED values are consistent with SpTs measured from optical spectra for K7 ≤ SpT ≤ M5.5. Candidates with SpT_SED later than M5 were targeted with IR spectroscopy (Section 3) to search for indications of low gravity. Those candidates that appeared to be low-gravity and all those with SpT_SED < M5 were observed with optical spectroscopy. When possible, we measured the spectroscopic SpTs (SpT_spec) from the IR and/or optical, which are usually consistent to within one subclass for stars later than K7. In a few cases, poor seeing did not allow for a reliable narrowband TiO index to be measured (Shkolnik et al. 2009).

Several studies have demonstrated that new members of YMGs also can be identified using ultraviolet photometry from GALEX (Findeisen & Hillenbrand 2010; Rodriguez et al. 2011, 2013; Shkolnik et al. 2011). A test of this selection process in our Tuc-Hor search (Kraus et al. 2014) showed that applying a GALEX selection criterion to candidates is indeed more efficient, leading to a higher confirmation rate for spectroscopic follow-up. However, GALEX only provides coverage of two-thirds of the sky (excluding the galactic plane), so ≈33% of the unknown members would not be confirmed. Our photometric + kinematic selection procedure is unbiased toward stellar activity. Although the process requires the collection of more spectra of candidates, it will ultimately discover 50% more members with a small reduction in efficiency, i.e., 78% with GALEX preselection compared to ≈67% without for the Tuc-Hor sample.

We list the 104 late-type candidate members of the BPMG in Table 1 with their relevant photometric and kinematic information, as well as a log of the spectroscopic follow-up observations.

Table 1.  Selection Criteria and Observations of Candidate BPMG Members

2MASS J	RUSNOB	Ks	μ	SpTSED	mbol	DMphot	${{\rm{\Delta }}}_{\mathrm{PM}}$	DMkin	IR Spec.a	Opt. Spec.	UT date
	(mag)	(mag)	(mas yr−1)		(mag)	(mag)	(mas yr−1)	(mag)			(yyyymmdd)
00140580–3245599	18.86	12.03	(73, −46.3) ± 9.1	M7.7 ± 0.2	15.02 ± 0.02	2.02 ± 0.07	10.2	3.4	SXD	⋯	⋯
00164976+4515417	14.68	9.85	(62.5, −22.1) ± 3.5	M4.5 ± 0.3	12.61 ± 0.03	2.14 ± 0.25	9.6	4.0	⋯	Keck/HIRES	20120710
00172353–6645124	11.51	7.70	(104.3, −13.5) ± 1.0	M2.6 ± 0.4	10.31 ± 0.02	1.48 ± 0.24	2.4	2.8	⋯	Magellan/MIKE	20101229
00193931+1951050	14.39	10.08	(72.2, −49.7) ± 3.5	M4.5 ± 0.1	12.82 ± 0.02	2.36 ± 0.10	2.9	3.5	SXD	Magellan/MIKE	20101231
00194303+1951117	14.19	9.86	(70.6, −50.9) ± 3.5	M4.7 ± 0.1	12.64 ± 0.02	1.96 ± 0.10	4.9	3.5	SXD	Magellan/MIKE	20101231
00233468+2014282	9.97	7.34	(61.4, −38.3) ± 1.9	K7.6 ± 0.2	9.65 ± 0.04	2.34 ± 0.11	1.5	3.9	⋯	Keck/HIRES	20120710
00274534–0806046	16.46	10.61	(97.3, −66.9) ± 3.3	M6.6 ± 0.1	13.55 ± 0.02	1.10 ± 0.06	6.6	2.8	SXD	Keck/HIRES	20120710
00281434–3227556	13.97	9.28	(108.0, −42.6) ± 2.9	M4.3 ± 0.4	12.01 ± 0.04	1.76 ± 0.34	5.7	2.7	⋯	Keck/HIRES	20120710
00413538–5621127	18.06	10.86	(109.4, −52.9) ± 17.9	M7.9 ± 0.1	13.93 ± 0.02	0.84 ± 0.05	33.6	2.5	⋯	Magellan/MIKE	20101231
00482667–1847204	14.84	9.86	(74.9, −44.7) ± 2.3	M4.6 ± 0.2	12.64 ± 0.02	2.07 ± 0.20	4.3	3.4	⋯	Magellan/MIKE	20101231
00501752+0837341	13.44	8.90	(63.9, −34.5) ± 2.8	M3.9 ± 0.1	11.57 ± 0.02	1.74 ± 0.10	7.6	3.9	⋯	Magellan/MIKE	20101230
00570256–1425174	17.05	11.77	(57.6, −36.5) ± 4.6	M5.8 ± 0.3	14.63 ± 0.02	2.78 ± 0.30	3.5	3.9	SXD	⋯	⋯
01071194–1935359	11.11	7.25	(63.1, −39.8) ± 1.2	M2.8 ± 0.3	9.87 ± 0.02	0.91 ± 0.19	6.8	3.6	⋯	Magellan/MIKE	20101230
01303534+2008393	14.92	10.19	(56, −43.9) ± 5.2	M4.7 ± 0.3	12.98 ± 0.04	2.30 ± 0.24	1.9	3.9	⋯	Keck/HIRES	20120710
01351393–0712517	12.49	8.08	(97.7, −51.5) ± 4.2	M4.1 ± 0.1	10.79 ± 0.02	0.76 ± 0.10	8.9	2.8	SXD	Magellan/MIKE	20101230
01373940+1835332	9.93	6.72	(68.0, −47.0) ± 0.9	M1.5 ± 1.1	9.20 ± 0.09	0.99 ± 0.40	7.2	3.6	⋯	Magellan/MIKE	20101230
02175601+1225266	13.48	9.08	(51.2, −42.1) ± 4.6	M2.2 ± 0.2	11.69 ± 0.02	3.12 ± 0.09	4.2	4.0	⋯	Keck/HIRES	20120710
02232663+2244069	10.51	7.35	(99.0, −114.4) ± 2.0	M1.6 ± 0.9	9.88 ± 0.05	1.63 ± 0.43	8.5	2.2	⋯	Keck/HIRES	20120710
02241739+2031513	16.95	11.62	(52.7, −40.7) ± 3.2	M5.8 ± 0.2	14.45 ± 0.02	2.60 ± 0.18	9.5	4.0	SXD	Magellan/MIKE	20101230
02335984–1811525	13.04	9.22	(51.1, −23.5) ± 2.4	M2.9 ± 0.4	11.87 ± 0.03	2.85 ± 0.24	2.9	3.8	⋯	Keck/HIRES	20120710
02450826–0708120	14.37	9.95	(39.2, −38.3) ± 3.3	M4.1 ± 0.2	12.67 ± 0.01	2.64 ± 0.17	7.3	4.0	⋯	Magellan/MIKE	20101229
02485260–3404246	12.74	8.40	(89.0, −23.8) ± 1.4	M3.9 ± 0.4	11.09 ± 0.03	1.25 ± 0.30	7.7	2.4	SXD	Magellan/MIKE	20101229
02495639–0557352	16.81	11.06	(44.6, −35.0) ± 4.1	M5.9 ± 0.3	13.92 ± 0.02	1.97 ± 0.26	0.8	3.9	SXD	Magellan/MIKE	20101231
03255277–3601161	15.07	10.62	(34.7, −3.7) ± 3.1	M4.5 ± 0.1	13.36 ± 0.03	2.89 ± 0.09	1.6	4.0	⋯	Magellan/MIKE	20101229
03363144–2619578	15.44	9.76	(79.0, −25.5) ± 3.3	M6.1 ± 0.2	12.65 ± 0.02	0.53 ± 0.16	2.4	2.1	SXD	Magellan/MIKE	20101229
03370343–3042318	18.38	12.37	(31.9, −7.7) ± 4.7	M6.4 ± 0.2	15.24 ± 0.02	2.92 ± 0.13	1.2	4.0	SXD	Magellan/MIKE	20110615
03393700+4531160	13.22	9.09	(37.8, −74.7) ± 2.9	M3.5 ± 0.1	11.74 ± 0.02	2.23 ± 0.10	1.0	3.6	⋯	Keck/HIRES	20120710
03550477–1032415	19.08	11.98	(52.6, −38.4) ± 6.8	M8.9 ± 0.4	15.12 ± 0.02	1.72 ± 0.10	8.4	2.9	SXD	Keck/HIRES	20131024
04023239–0242335	11.47	7.55	(41.2, −38.9) ± 3.8	M2.6 ± 0.9	10.19 ± 0.05	1.36 ± 0.60	8.7	3.5	⋯	Magellan/MIKE	20101229
04023328–0242161	11.99	8.19	(36.0, −38.7) ± 5.4	M1.6 ± 1.2	10.74 ± 0.07	2.49 ± 0.69	4.7	3.6	⋯	Magellan/MIKE	20101229
04232720+1115174	16.50	11.23	(26.3, −46.7) ± 4.3	M5.7 ± 0.4	14.07 ± 0.02	2.33 ± 0.40	4.2	4.0	SXD	Keck/HIRES	20131024
05015665+0108429	12.08	7.68	(31.8, −90.4) ± 3.3	M3.6 ± 0.2	10.34 ± 0.02	0.75 ± 0.17	2.5	2.1	⋯	Magellan/MIKE	20101229
05061292+0439272	12.60	8.07	(32.7, −91.8) ± 3.5	M3.8 ± 0.2	10.75 ± 0.02	0.99 ± 0.12	7.4	2.3	⋯	Magellan/MIKE	20101230
05071137+1430013	14.69	9.66	(18.6, −72.1) ± 3.4	M4.9 ± 0.3	12.51 ± 0.03	1.61 ± 0.25	2.6	3.2	⋯	⋯	⋯
05115901+1728481	14.09	9.84	(13.0, −57.6) ± 4.4	M4.0 ± 0.3	12.53 ± 0.03	2.61 ± 0.26	2.1	3.8	⋯	Magellan/MIKE	20101230
05241914–1601153	12.55	7.81	(18.0, −35.4) ± 3.5	M4.9 ± 0.2	10.62 ± 0.02	−0.28 ± 0.21	5.2	2.6	⋯	Magellan/MIKE	20101229
05363846+1117487	11.38	7.41	(−3.8, −61.2) ± 3.2	M3.2 ± 0.4	10.07 ± 0.03	0.81 ± 0.30	8.5	3.5	⋯	Magellan/MIKE	20101230
05432597–6630096	⋯	10.86	(−12.4, 66.1) ± 11.0	M6.0 ± 0.2	13.77 ± 0.02	1.71 ± 0.19	15.7	2.9	⋯	Magellan/MIKE	20101230
06352229–5737349	10.25	7.15	(−17.6, 51.3) ± 0.9	M1.0 ± 0.2	9.66 ± 0.02	1.69 ± 0.07	2.2	2.8	⋯	Magellan/MIKE	20101229
06451593–1622030	17.95	11.93	(−15.9, −24.7) ± 4.5	M6.5 ± 0.4	14.80 ± 0.03	2.42 ± 0.20	1.3	3.6	⋯	Magellan/MIKE	20101231
08173943–8243298	10.50	6.59	(−81.9, 102.6) ± 1.2	M3.0 ± 0.3	9.23 ± 0.04	0.14 ± 0.14	3.0	2.1	⋯	Magellan/MIKE	20110614
13215631–1052098	⋯	8.62	(−71.6, −49.8) ± 3.4	M4.2 ± 0.3	11.33 ± 0.04	1.2 ± 0.25	3.3	3.5	⋯	Magellan/MIKE	20110614
15063505–3639297	15.84	11.11	(−42.3, −38.2) ± 10.6	M4.8 ± 0.2	13.94 ± 0.02	3.14 ± 0.20	13.2	4.5	⋯	Magellan/MIKE	20101231
18011345+0948379	13.66	9.37	(−4.4, −28.3) ± 3.1	M4.0 ± 0.3	12.10 ± 0.03	2.18 ± 0.26	7.1	3.9	SXD	Keck/HIRES	20120710
18055491–5704307	12.85	8.63	(−0.1, −72) ± 6.4	M3.6 ± 0.6	11.32 ± 0.04	1.74 ± 0.48	4.3	3.9	⋯	Magellan/MIKE	20120902
18090694–7613239	14.18	8.99	(11.2, −152.8) ± 3.0	M5.6 ± 0.5	11.81 ± 0.02	0.17 ± 0.46	1.1	2.2	⋯	Magellan/MIKE	20120902
18092970–5430532	14.52	9.12	(6.8, −103.7) ± 2.8	M5.6 ± 0.4	11.95 ± 0.03	0.31 ± 0.33	2.0	3.1	⋯	Magellan/MIKE	20120902
18151564–4927472	12.22	8.04	(8.3, −71.5) ± 1.6	M3.0 ± 0.4	10.69 ± 0.02	1.60 ± 0.25	2.0	3.9	⋯	Magellan/MIKE	20110614
18211526+1610078	10.68	7.48	(10.9, −19.3) ± 3.5	M1.2 ± 0.4	10.04 ± 0.03	1.97 ± 0.15	3.0	3.9	⋯	Keck/HIRES	20120710
18224692+6723001	18.83	12.35	(10.7, 43.2) ± 6.7	M7.2 ± 0.2	15.33 ± 0.02	2.55 ± 0.09	1.8	3.8	SXD	⋯	⋯
18420483–5554126	14.47	9.85	(12.9, −74.0) ± 5.2	M4.4 ± 0.3	12.57 ± 0.03	2.21 ± 0.25	2.0	3.8	⋯	Magellan/MIKE	20110614
18420694–5554254	12.66	8.58	(11.2, −81.4) ± 4.0	M3.3 ± 0.4	11.26 ± 0.03	1.92 ± 0.31	4.5	3.7	⋯	Magellan/MIKE	20110614
18435838–3559096	14.38	10.33	(20.9, −65.9) ± 6.5	M4.5 ± 0.9	13.10 ± 0.06	2.63 ± 0.77	6.9	3.9	⋯	Keck/HIRES	20120710
18471351–2808558	15.94	10.83	(8.6, −69.0) ± 4.1	M5.1 ± 0.3	13.69 ± 0.02	2.57 ± 0.27	7.8	3.8	SXD	Keck/HIRES	20120710
18504473–2353389	⋯	11.46	(9.7, −78.0) ± 16.9	M6.0 ± 0.8	14.31 ± 0.04	2.25 ± 0.69	10.6	3.4	SXD	⋯	⋯
18550451+4259510	13.54	8.92	(44.4, 21.6) ± 3.4	M4.2 ± 0.3	11.65 ± 0.03	1.51 ± 0.25	8.0	2.1	⋯	Keck/HIRES	20120710
19033299–3847058	14.67	9.73	(33.3, −117.5) ± 2.3	M5.2 ± 0.3	12.59 ± 0.03	1.37 ± 0.25	0.3	2.7	SXD	Keck/HIRES	20120710
19082110+2129364	13.00	8.87	(16.2, −21.9) ± 4.3	M3.7 ± 0.4	11.55 ± 0.03	1.88 ± 0.28	8.2	3.8	SXD	Keck/HIRES	20120710
19082195–1603249	16.77	11.40	(20.6, −50.9) ± 3.6	M6.8 ± 0.1	14.19 ± 0.04	1.62 ± 0.10	1.1	4.0	SXD	Keck/HIRES	20120710
19103951+2436150	8.85	5.77	(18.3, −14.1) ± 1.0	M1.8 ± 0.2	8.36 ± 0.03	0.01 ± 0.06	3.8	4.0	⋯	Keck/HIRES	20120710
19153643+2920117	11.05	7.70	(29.5, −3.1) ± 16.2	M2.0 ± 0.2	10.29 ± 0.03	1.85 ± 0.15	4.4	3.4	⋯	Keck/HIRES	20120710
19234442+2629030	11.10	7.88	(37.8, −25.2) ± 4.3	M2.4 ± 0.2	10.49 ± 0.03	1.79 ± 0.15	9.7	2.8	⋯	Keck/HIRES	20120710
19242697+2716351	12.26	8.82	(24.7, −6.0) ± 3.2	M2.6 ± 0.4	11.46 ± 0.03	2.63 ± 0.21	2.6	4.0	⋯	Magellan/MIKE	20110614
19243494–3442392	13.27	8.78	(23.2, −72.1) ± 1.8	M4.6 ± 0.2	11.55 ± 0.04	0.98 ± 0.13	4.0	3.7	⋯	Magellan/MIKE	20110614
19260075–5331269	13.16	8.68	(34.1, −88.1) ± 2.0	M4.2 ± 0.3	11.43 ± 0.03	1.29 ± 0.24	3.0	3.4	⋯	Magellan/MIKE	20120902
19275844+3309439	10.47	7.34	(38.4, −1.4) ± 21	M1.9 ± 0.2	9.97 ± 0.03	1.58 ± 0.07	1.3	3.0	⋯	Keck/HIRES	20120710
19300396–2939322	13.61	9.25	(22.8, −59.4) ± 3.6	M4.2 ± 0.1	11.96 ± 0.02	1.83 ± 0.10	2.0	4.0	⋯	Keck/HIRES	20120710
19355595–2846343	⋯	12.71	(26.6, −58.9) ± 5.1	L1.0 ± 0.45b	15.90 ± 0.03	2.02 ± 0.10	0.7	4.0	SXDc	Keck/HIRES	20131024
19410521+3806521	12.19	8.24	(31.8, 1.0) ± 3.3	M2.2 ± 0.4	10.86 ± 0.03	2.29 ± 0.22	2.5	3.7	⋯	Keck/HIRES	20120710
19472175+3954072	12.19	10.43	(26.9, 25.7) ± 17.5	M7.5 ± 0.3d	13.51 ± 0.03	0.60 ± 0.11	21.2	3.9	SXD	Magellan/MIKE	20110615
19533169–0707001	14.97	9.93	(39.1, −43.9) ± 2.8	M5.0 ± 0.3	12.75 ± 0.02	1.74 ± 0.25	7.1	3.8	SXD	⋯	⋯
19560294–3207186	12.59	8.11	(32.6, −61.0) ± 1.5	M3.7 ± 0.4	10.82 ± 0.04	1.14 ± 0.31	0.6	3.9	⋯	Keck/HIRES	20120710
19571814+6242573	17.58	11.99	(35.8, 21.4) ± 3.8	M6.5 ± 0.9	14.88 ± 0.04	2.5 ± 0.74	8.0	4.0	SXD	⋯	⋯
19572094+6242559	19.38	12.56	(38.5, 24.2) ± 6.4	M7.3 ± 0.2	15.57 ± 0.02	2.74 ± 0.08	7.7	3.8	SXD	⋯	⋯
20013718–3313139	11.62	8.24	(27.1, −60.9) ± 2.2	M1.0 ± 0.3	10.75 ± 0.03	2.78 ± 0.10	5.3	4.0	⋯	Magellan/MIKE	20110614
20034245+1432215	10.77	7.26	(20.9, −39.3) ± 15.0	M2.8 ± 0.2	9.95 ± 0.03	1.00 ± 0.09	19.3	4.0	⋯	Keck/HIRES	20120710
20083784–2545256	14.94	10.07	(28.8, −60.4) ± 2.8	M4.7 ± 0.2	12.84 ± 0.02	2.15 ± 0.21	6.9	3.9	⋯	Keck/HIRES	20120710
20085368–3519486	12.71	8.32	(48.8, −80.1) ± 1.3	M3.6 ± 0.3	10.95 ± 0.02	1.36 ± 0.21	2.8	3.3	SXD	Keck/HIRES	20120710
20100002–2801410	12.16	7.73	(40.4, −62.7) ± 0.9	M4.1 ± 0.2	10.46 ± 0.04	0.43 ± 0.13	2.2	3.7	⋯	Magellan/MIKE	20110614
20200623–3433203	17.13	12.00	(36.1, −59.7) ± 7.2	M6.1 ± 0.7	14.88 ± 0.03	2.76 ± 0.68	1.6	3.9	SXD	⋯	⋯
20285054+2141119	13.46	9.13	(53.2, −31.4) ± 4.9	M4.1 ± 0.2	11.81 ± 0.03	1.79 ± 0.14	5.8	3.2	SXD	Keck/HIRES	20120710
20333759–2556521	14.02	8.88	(51.8, −76.8) ± 1.5	M4.6 ± 0.1	11.65 ± 0.02	1.07 ± 0.10	3.5	3.3	SXD	Magellan/MIKE	20110614
20385687–4118285	15.95	10.74	(49.9, −52.7) ± 8.4	M6.2 ± 0.2	13.63 ± 0.03	1.44 ± 0.12	11.1	3.9	⋯	Magellan/MIKE	20120902
20390476–4117390	14.23	9.98	(48.4, −72.4) ± 7.8	M4.2 ± 0.1	12.68 ± 0.03	2.55 ± 0.10	1.6	3.5	⋯	Magellan/MIKE	20120902
20434114–2433534	12.02	7.76	(56.2, −72.0) ± 1.3	M3.5 ± 0.1	10.41 ± 0.02	0.91 ± 0.08	0.4	3.3	SXD	⋯	⋯
21100461–1920302	12.45	7.55	(85.1, −96.4) ± 2.1	M4.2 ± 0.2	10.29 ± 0.02	0.15 ± 0.21	5.1	2.6	SXD	Magellan/MIKE	20110614
21100535–1919573	11.09	7.20	(88.6, −92.5) ± 1.2	M3.0 ± 0.2	9.86 ± 0.02	0.77 ± 0.11	2.7	2.6	SXD	Magellan/MIKE	20110614
21103147–2710578	13.72	9.41	(70.6, −72.3) ± 2.3	M4.4 ± 0.2	12.16 ± 0.02	1.81 ± 0.21	4.9	3.1	SXD	Magellan/MIKE	20110614
21183375+3014346	10.83	7.79	(58.9, −22.5) ± 1.4	M1.8 ± 0.8	10.33 ± 0.06	1.99 ± 0.29	4.9	3.4	⋯	Keck/HIRES	20120710
21195985+3039467	15.86	10.65	(41.7, −20.1) ± 3.4	M5.1 ± 0.4	13.51 ± 0.02	2.39 ± 0.41	7.7	4.1	SXD	⋯	⋯
21200779–1645475	13.73	9.30	(62.1, −54.3) ± 2.6	M4.3 ± 0.1	12.01 ± 0.03	1.76 ± 0.09	5.0	3.5	⋯	Keck/HIRES	20120710
21374019+0137137	12.52	7.88	(80.0, −57.6) ± 3.4	M4.0 ± 0.3	10.59 ± 0.02	0.67 ± 0.23	1.6	3.0	SXD	Magellan/MIKE	20110614
21384755+0504518	14.20	9.87	(50.4, −40.2) ± 3.8	M3.9 ± 0.1	12.55 ± 0.02	2.71 ± 0.10	4.5	3.9	SXD	Magellan/MIKE	20110614
22010456+2413016	19.59	12.07	(51.4, −24.2) ± 6.3	M7.9 ± 0.2	15.12 ± 0.02	2.04 ± 0.08	1.8	4.0	SXD	⋯	⋯
22085034+1144131	13.45	9.04	(89.8, −50.8) ± 3.2	M4.4 ± 0.1	11.77 ± 0.02	1.42 ± 0.11	1.7	2.9	⋯	Keck/HIRES	20120710
22334687–2950101	18.52	11.97	(54.7, −46.2) ± 4.8	M7.4 ± 0.2	14.95 ± 0.03	2.08 ± 0.05	4.2	4.0	SXD	Magellan/MIKE	20110615
22440873–5413183	12.38	8.47	(70.9, −60.1) ± 8.7	M2.8 ± 0.4	11.09 ± 0.03	2.13 ± 0.25	13.1	3.3	⋯	Magellan/MIKE	20101231
22500768–3213155	16.07	11.58	(69.4, −64.8) ± 11	M6.3 ± 0.3	14.42 ± 0.03	2.17 ± 0.18	13.5	3.4	SXD	⋯	⋯
23010610+4002360	18.77	12.28	(73.4, −19.0) ± 4.4	M7.3 ± 0.2	15.23 ± 0.02	2.40 ± 0.09	6.7	3.5	SXD	Keck/HIRES	20120710
23224604–0343438	15.08	10.48	(63.8, −41.2) ± 3.5	M4.6 ± 0.1	13.24 ± 0.03	2.66 ± 0.09	0.7	3.8	⋯	Magellan/MIKE	20101230
23301129–0237227	14.23	9.77	(100.2, −69.7) ± 3.4	M5.6 ± 0.2	12.62 ± 0.02	0.98 ± 0.15	5.0	2.8	SXD	⋯	⋯
23314492–0244395	13.31	8.67	(93.6, −66.6) ± 3.4	M4.5 ± 0.3	11.42 ± 0.03	0.95 ± 0.25	6.1	2.9	SXD	Magellan/MIKE	20101231
23323085–1215513	9.83	6.57	(137.9, −81) ± 1.0	M2.4 ± 0.2	9.17 ± 0.03	0.47 ± 0.09	5.7	2.2	⋯	Magellan/MIKE	20101231
23355015–3401477	16.51	10.76	(88.1, −55.6) ± 10.0	M6.2 ± 0.5	13.63 ± 0.03	1.44 ± 0.37	5.5	3.1	SXD	Magellan/MIKE	20110615

Notes.

^aIRTF's short cross-dispersed (SXD) mode: 0.8–2.4 μm. See Table 2 for log of observations. ^bNIR SpT indicates M9 (AL13). ^cThis object was observed with SXD by (AL13). ^dOptical spectrum appears as early-type SB2, not an M star.

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After photometric candidate selection, spectroscopic observations are necessary to assess youth. Spectroscopic youth indicators include features of low gravity, strong Hα emission, and Li absorption. For our faintest and latest SpT candidates, we first vetted for signs of low gravity with lower-resolution near-infrared (NIR) spectra before collecting high-resolution optical spectra.

We obtained NIR spectra of the 43 candidates with SED-fit SpTs ≳ M4 using SpeX, the facility spectrograph of the NASA Infrared Telescope Facility (IRTF) on MaunaKea. We searched for signatures of low gravity in their spectra and remeasured their SpTs to confirm those determined by SED fitting using the method of (Allers & Liu 2013, hereafter AL13). Our NIR and SED SpTs agree well (to within one subclass). Spectra were reduced using Spextool⁹ (Cushing et al. 2004), which includes a correction for telluric absorption (Vacca et al. 2004).

We classified our spectra following the method of AL13. We determined gravity-insensitive NIR SpTs using visual comparison with spectral standards from Kirkpatrick et al. (2010), as well as the spectral indices from AL13. We then calculated the AL13 gravity-sensitive indices for each spectrum and determined gravity classifications. The results are listed in Table 2. Fifteen targets had NIR SpTs earlier than M5, for which the AL13 gravity classification cannot be applied. Of the remaining 27 targets with SpT ≥ M5, 17 candidates (64%) exhibited signs of youth, i.e., intermediate (INT-G) or very low (VL-G) gravity. These targets were then observed with high-resolution optical spectroscopy as described below to measure Hα, Li, and RVs. Other than three with ambiguous designations, all were confirmed as BPMG members. Our SpeX NIR spectra are displayed in Figure 2.

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Table 2.  NIR Spectroscopy Log

Object	UT date	Slit Width	sec z	${N}_{\exp }\times t({\rm{s}})$	S/N	${\mathrm{SpT}}_{\mathrm{NIR}}$ a	Gravity Typea
2MASS J	(YYYY-MM-DD)	('')			(J, H, K)
00140580–3245599	2010 Nov 14	0.8	2.23	8 × 90.0	132, 118, 134	M7	fld-g
00193931+1951050	2011 Jul 07	0.3	1.21	10 × 30.0	44, 49, 42	M4	⋯
00194303+1951117	2011 Jul 07	0.3	1.18	8 × 45.0	68, 74, 64	M4	⋯
00274534–0806046	2017 Jan 05	0.8	1.19	12 × 29.7	47, 60, 68	M6	fld-g
00570256–1425174	2010 Nov 11	0.8	1.22	16 × 60.0	125, 117, 93	M6	fld-g
01351393–0712517	2010 Nov 15	0.3	1.12	4 × 30.0	79, 57, 38	M4	⋯
02241739+2031513	2010 Oct 05	0.8	1.03	6 × 60.0	106, 111, 104	M6	int-g
02485260–3404246	2010 Nov 15	0.3	1.69	6 × 60.0	73, 49, 30	M5	int-g
02495639–0557352	2010 Oct 05	0.8	1.17	8 × 60.0	191, 200, 194	M6	vl-g
03363144–2619578	2010 Oct 05	0.8	1.48	6 × 60.0	301, 312, 296	M6	vl-g
03370343–3042318	2010 Oct 05	0.8	1.66	8 × 60.0	41, 40, 40	M6	fld-g
03550477–1032415	2010 Nov 11	0.8	1.23	16 × 60.0	116, 122, 112	M8	int-g
04232720+1115174	2010 Nov 11	0.8	1.21	16 × 60.0	214, 214, 190	M5	vl-g
18011345+0948379	2012 Jul 05	0.3	1.05	8 × 30.0	104, 108, 90	M5	int-g
18224692+6723001	2011 Jul 21	0.8	1.74	10 × 120.0	83, 79, 76	M7	fld-g
18471351–2808558	2011 Jul 07	0.8	1.52	8 × 120.0	95, 91, 86	M6	vl-g
18504473–2353389	2011 Jul 07	0.8	1.38	8 × 120.0	67, 72, 59	<M4	⋯
19033299–3847058	2011 Jul 07	0.8	1.92	8 × 120.0	155, 148, 131	<M4	⋯
19082110+2129364	2012 Jul 05	0.3	1.01	16 × 120.0	19, 24, 21	<M4	⋯
19082195–1603249	2011 Jul 07	0.8	1.26	10 × 120.0	112, 106, 97	M6	vl-g
19472175+3954072	2011 Jul 07	0.8	1.08	10 × 60.0	173, 187, 151	<M4	⋯
19533169–0707001	2010 Nov 18	0.5	1.61	6 × 60.0	245, 230, 190	M4	⋯
19571814+6242573	2010 Nov 18	0.5	1.51	6 × 120.0	135, 141, 139	M5	fld-g
19572094+6242559	2010 Nov 18	0.5	1.55	6 × 120.0	84, 88, 89	M6	fld-g
20085368–3519486	2012 Jul 05	0.3	1.76	8 × 15.0	80, 81, 62	M4	⋯
20200623–3433203	2010 Nov 14	0.8	2.09	8 × 90.0	157, 141, 144	M5	fld-g
20285054+2141119	2012 Jul 05	0.3	1.01	6 × 30.0	72, 74, 60	<M4	⋯
20333759–2556521	2011 Jul 08	0.3	1.46	10 × 60.0	123, 130, 114	M5	int-g
20434114–2433534	2011 Jul 08	0.3	1.44	8 × 30.0	104, 121, 107	M4	⋯
21100461–1920302	2011 Jul 08	0.3	1.35	8 × 30.0	105, 103, 85	M5	vl-g
21100535–1919573	2011 Jul 08	0.3	1.37	8 × 30.0	163, 194, 172	<M4	⋯
21103147–2710578	2011 Jul 08	0.3	1.49	8 × 60.0	48, 53, 49	M5	int-g
21195985+3039467	2010 Jul 17	0.8	1.33	6 × 60.0	146, 116, 81	M5	fld-g
21374019+0137137	2011 Jul 08	0.3	1.11	8 × 30.0	117, 123, 104	M5	int-g
21384755+0504518	2011 Jul 08	0.3	1.12	8 × 60.0	59, 66, 60	M4	⋯
22010456+2413016	2011 Jul 08	0.8	1.08	8 × 120.0	32, 35, 36	M8	fld-g
22334687–2950101	2010 Jul 17	0.8	1.60	8 × 60.0	118, 123, 122	M7	vl-g
22500768–3213155	2010 Nov 14	0.8	1.64	8 × 90.0	189, 172, 155	M4	⋯
23010610+4002360	2011 Jul 07	0.8	1.17	12 × 120.0	65, 62, 61	M7	vl-g
22500768–3213155	2010 Oct 05	0.5	1.74	6 × 90.0	263, 278, 262	M5	fld-g
23314492–0244395	2011 Jul 08	0.3	1.08	8 × 60.0	165, 168, 144	M5	int-g
23355015–3401477	2010 Nov 11	0.8	1.70	16 × 60.0	157, 105, 80	M6	vl-g

Note.

^aStars with SpTs earlier than M4 are outside the valid range of the AL13 NIR spectral-typing method. Uncertainty in the IR SpTs is ±1 subclass, as discussed in AL13. Stars with SpTs earlier than M5 cannot be classified with the AL13 NIR gravity classification.

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High-resolution spectra provide the RV necessary to calculate the space velocity (UVW) using the distance and proper motions and thus to kinematically match the target to the moving group. With kinematic false membership rates of ∼50% cumulatively for all YMGs and 6% for the BPMG (e.g., Pompéia et al. 2011; Shkolnik et al. 2012), both kinematics and independent spectral youth indicators are necessary to confirm a target as a bona fide YMG member. By combining all these data, we designate each candidate in Table 3 with a membership status of "N" for a nonmember when the kinematics or youth indicators conflict with membership, "Y" for confirmed members, and "Y?" for likely members whose RVs differ from the predicted values due to binarity in the system. Our assessment procedure is summarized in Figure 3. In this study, we use the kinematic distance derived by assuming that a candidate is a group member and thus only report the UVWs for those that we assess as indeed being members.

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Table 3.  Results of Optical Spectroscopy and Membership Assessments

Name	SpT	SpT	RVa	ΔRVb	Binary?	$\mathrm{EW}[\mathrm{Li}]$ d	$\mathrm{EW}[{\rm{H}}\alpha ]$ d	UVWe	XYZe	RV, Hα, Lif	BPMG	Prev.
	Spec	Source (ref)	(km s−1)	(km s−1)	(Ref)c	(Å)	(Å)	(km s−1)	(pc)	Assessment	Memb.?	Ref.g
00140580–3245599	M7	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
00164976+4515417	M4.1	Opt (B)	−12.05 ± 0.64	−5.5	VB (1)	<0.05	−7.29	(−9.4, −20.7, −5.2)	(−27, 53.9, −18.6)	Y? Y?	Y?
00172353–6645124	M2.5	Opt (C)	9.50 ± 0.90	−1.58	⋯	<0.05	−4	(−11.1, −15.2, −8.2)	(14.4, −18.3, −27.8)	YY?	Y	1
00193931+1951050	M4	IR (A)	−6.98 ± 0.37	−5.25	VB	0.12	−6.77	(−8, −19.7, −6)	(−14.3, 34.1, −33.8)	YYY	Y?
00194303+1951117	M4	IR (A)	−6.66 ± 0.26	−4.93	VB	<0.05	−5.13	(−7.6, −19.4, −6.4)	(−14.3, 34.1, −33.8)	YY?	Y
00233468+2014282	K7.5	Opt (B)	−2.17 ± 0.59	−0.64	VB	0.36	−0.18	(−10.1, −16.2, −8.5)	(−18.3, 40.8, −40.5)	YYY	Y	2
00274534–0806046	M5.6	Opt (B)	−3.61 ± 1.02	−7.97	⋯	0.65	−5.53	(−8.3, −18.9, −1.9)	(−3.3, 11.9, −34.1)	NYY	Y?	3
00281434–3227556	M4.8	Opt (B)	6.79 ± 2.66	−1.62	VB (2)	0.38	−10.63	(−11, −15.3, −7.7)	(4.3, −1.2, −34.4)	YYY	Y
00413538–5621127	M7.5	IR (D)	6.59 ± 1.82	−4.86	VB (3, 4)	1.34	−34.14	(−8.4, −17.2, −2.6)	(9, −12.6, −27.6)	YYY	Y
00482667–1847204	⋯		7.21 ± 0.68	−0.54	⋯	0.53	−6.57	(−9.4, −16.8, −8.9)	(−3.3, 6.2, −47.4)	YYY	Y
00501752+0837341	M4.5	Opt (E)	2.15 ± 2.00	−0.64	SB2	0.15	−4.83	(−11.7, −15.6, −7.6)	(−18.9, 29.7, −48.9)	YYY	Y
00570256–1425174	M6	IR (A)	⋯	⋯	VB (1)	⋯	⋯	⋯	⋯	???	N
01071194–1935359	M1.0	Opt (C)	7.90 ± 0.49	−1.29	⋯	0.32	−1.57	(−8.3, −16.6, −8.2)	(−6.6, 3.9, −51.9)	YYY	Y	4
01303534+2008393	M4.7	Opt (B)	−2.76 ± 0.85	−5.9	⋯	<0.05	−8.14	(−7, −18.7, −4.9)	(−32, 31.6, −40.2)	Y? Y?	Y?
01351393–0712517	M4	IR (A)	13.24 ± 5.21	4.12	SB2 (2)	<0.05	−6.45	(−12.9, −14.8, −12.5)	(−12.3, 6.4, −33.5)	YY?	Y?h	5, 6
01373940+1835332	⋯		0.73 ± 1.91	−3.23	⋯	0.46	−0.87	(−9.6, −17.4, −5.8)	(−28.6, 25.7, −35.8)	YYY	Y	7
02175601+1225266	M3.1	Opt (B)	9.20 ± 0.66	1.26	⋯	<0.05	−5.13	(−11.9, −15.8, −9.4)	(−39.8, 19.7, −44.8)	YY?	Y	6
02232663+2244069	M0	Opt (B)	12.96 ± 0.37	7.2	⋯	<0.05	−0.89	(−14.3, −12.5, −14)	(−19.2, 11.5, −15.9)	NY?	Ni	7
02241739+2031513	M6	IR (A)	8.62 ± 1.22	2.26	⋯	0.68	−10.65	(−13.5, −14.9, −8.4)	(−43.7, 24.7, −38.2)	YYY	Y
02335984–1811525	M2.9	Opt (B)	11.99 ± 1.17	−2.49	VB (5)	<0.05	−4.56	(−9.8, −15.5, −6.7)	(−23.4, −7.4, −52)	YY?	Y
02450826–0708120	⋯		11.36 ± 2.29	−2.14	⋯	0.46	−8.33	(−7.2, −16.6, −8.6)	(−34.9, −1, −52.6)	YYY	Y
02485260–3404246	M5	IR (A)	14.81 ± 0.49	−1.74	SB1 (2)	<0.05	−6.96	(−8.9, −16.2, −7.6)	(−7.5, −11, −27.1)	YY?	Y?h
02495639–0557352	M6	IR (A)	14.42 ± 0.44	0.86	⋯	0.59	−11.61	(−10.6, −16.2, −10)	(−34.8, −0.8, −49.2)	YYY	Y
03255277–3601161	⋯		16.61 ± 2.41	−1.52	⋯	<0.05	−6.21	(−9.2, −15.5, −8)	(−18.7, −29.8, −52.4)	YY?	Y
03363144–2619578	M6	IR (A)	16.65 ± 2.72	−1.54	⋯	0.33	−11.18	(−9.6, −15.2, −8)	(−11.8, −10.4, −21.1)	YYY	Y
03370343–3042318	M6	IR (A)	11.86 ± 3.47	−6.56	⋯	0.56	−10.33	(−7.2, −13, −4.1)	(−24.6, −28, −50.9)	Y? YY	Y?
03393700+4531160	M3.5	Opt (B)	7.70 ± 2.00	5.25	SB2	<0.05	−4.56	(−14.6, −13.4, −10.2)	(−45.6, 25, −7.2)	YY?	Y?h
03550477–1032415	M8	IR (A)	16.96 ± 0.71	−0.33	⋯	0.92	−7.02	(−11, −15.5, −8.1)	(−25.6, −9.8, −26.3)	YYY	Y
04023239–0242335	⋯		26.82 ± 0.28	10.53	⋯	<0.05	0.5	(−19.4, −18.1, −14.2)	(−38.3, −9, −31)	NN?	N
04023328–0242161	⋯		25.59 ± 0.67	9.31	⋯	<0.05	0.52	(−18, −17.6, −14.1)	(−40.1, −9.5, −32.5)	NN?	N
04232720+1115174	M5	IR (A)	14.30 ± 0.45	0.35	⋯	0.58	−4.05	(−10.9, −16.6, −8.4)	(−56.7, −3.4, −27.5)	YYY	Y
05015665+0108429	M4.0	Opt (F)	18.64 ± 0.51	1.55	⋯	<0.05	−4.87	(−11.6, −16.4, −9.6)	(−22.9, −7.7, −10.5)	YY?	Y?j
05061292+0439272	M4.0	Opt (G)	18.8 ± 2.4l	2.38	⋯	0.27l	−6.18l	(−12.4, −17.2, −9.3)	(−25.9, −7.4, −10.3)	YYY	Y
05071137+1430013	M5.5	IR (D)	⋯	⋯	VB	⋯	⋯	⋯	⋯	???	?
05115901+1728481	⋯		75.25 ± 0.45	61.95	⋯	<0.05	0.29	(−70.4, −21.6, −22.2)	(−55.9, −5.3, −12.7)	NN?	N
05241914–1601153	M4.5	Opt (C)	22.73 ± 1.42	2.72	VB (5)	<0.05	−8.14	(−12.3, −17.7, −9.7)	(−23.3, −18.4, −14.7)	YY?	Y	1
05363846+1117487	M4.0	Opt (G)	20.70 ± 0.36	5.54	⋯	<0.05	−2.48	(−15.3, −16.3, −12)	(−47.7, −11.9, −9.5)	Y? Y?	Y?
05432597–6630096	⋯		261.02 ± 0.93	243.7	⋯	<0.05	0	(13.1, −223.4, −139.7)	(3.6, −32.2, −19.8)	NN?	N
06352229–5737349	⋯		30.90 ± 0.86l	12.14	⋯	⋯	⋯	(−10.7, −27.3, −14)	(−1.8, −32.9, −15.2)	N??	N
06451593–1622030	⋯		−8.63 ± 0.16	−28.32	SB3	<0.05	−5.61	(9.2, 4.3, −4.8)	(−35.5, −37.9, −8)	NY?	N
08173943–8243298	M3.5	Opt (C)	16.86 ± 1.09	3.96	⋯	<0.05	−5.9	(−8.4, −19.4, −10.6)	(10.3, −21.6, −10.8)	YY?	Y	8
13215631–1052098	M4.5	Opt (C)	−4.01 ± 2.15	0.6	⋯	<0.05	−5.58	(−10.5, −16.3, −8.4)	(22.1, −22.2, 39.1)	YY?	Y
15063505–3639297	⋯		0.03 ± 1.81	4.06	⋯	0.78	−12.55	(−8.9, −18.8, −3.6)	(65.8, −36.4, 25.5)	YYY	Y
18011345+0948379	M4.8	Opt (B)	−22.88 ± 1.73	−3.52	⋯	0.15	−11.4	(−12.7, −19, −8.4)	(46.9, 34.2, 16.2)	YYY	Y
18055491–5704307	M2.6	Opt (H)	−0.61 ± 0.36	−0.38	⋯	<0.05	−6.11	(−10.1, −16.1, −7.9)	(53, −22.8, −17.3)	YY?	Y
18090694–7613239	M3.4	Opt (H)	6.98 ± 0.38	0.33	⋯	0.6	−8.27	(−9.9, −16.2, −9.5)	(18.6, −16.9, −11.1)	YYY	Y
18092970–5430532	M4.6	Opt (H)	−1.97 ± 0.85	−0.82	⋯	0.56	−8.18	(−10.7, −15.4, −8.8)	(37.5, −14.1, −11.6)	YYY	Y
18151564–4927472	M3.0	Opt (C)	−1.08 ± 3.72	1.88	⋯	<0.05	−5.55	(−8.3, −16.1, −10.1)	(56.2, −15.5, −15.4)	YY?	Y	8
18211526+1610078	M0.5	Opt (B)	−0.70 ± 0.74	19.3	⋯	<0.05	1.37	(3, −2.3, −5.1)	(42, 40.9, 14.4)	NN?	N
18224692+6723001	M7	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
18420483–5554126	M4.5	Opt (C)	0.43 ± 0.85	0.86	⋯	0.39	−6.26	(−9.1, −16, −9.1)	(50.4, −18.6, −20.5)	YYY	Y	9
18420694–5554254	M3.5	Opt (C)	1.17 ± 0.17	1.6	⋯	<0.05	−7.84	(−8.7, −17.5, −9.1)	(48.2, −17.8, −19.7)	YY?	Y	1
18435838–3559096	M4.5	Opt (B)	−5.19 ± 2.12	2.21	⋯	0.59	−16.62	(−8.3, −14.9, −11.3)	(58.5, −0.6, −14.8)	YYY	Y
18471351–2808558	M6	IR (A)	−7.46 ± 1.65	2.43	⋯	0.64	−7.53	(−7.2, −17.2, −8.3)	(55.9, 6.9, −11.5)	YYY	Y
18504473–2353389	⋯		⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
18550451+4259510	M4.1	Opt (B)	−28.39 ± 0.38	−8.29	⋯	<0.05	−3.99	(−11.9, −23.5, −12.6)	(7.4, 24, 7.9)	NY?	N
19033299–3847058	M4.3	Opt (B)	11.09 ± 0.24	17.22	⋯	<0.05	0.3	(6.2, −16.7, −14.8)	(32.8, −1, −11.2)	NN?	N
19082110+2129364	K7.5	Opt (B)	4.85 ± 0.81	24.52	⋯	<0.05	1.63	(5.9, 2.5, −6)	(33.6, 46.2, 6.1)	NN?	N
19082195–1603249	M5.4	Opt (B)	−9.09 ± 0.76	3.9	⋯	0.53	−7.96	(−6.6, −14.4, −10.2)	(58.1, 21.5, −11.9)	YYY	Y
19103951+2436150	K8.5	Opt (B)	−34.38 ± 0.97	−14.56	⋯	<0.05	1.52	(−17, −28.8, −11)	(34.1, 52.6, 7.7)	NN?	N
19153643+2920117	M0.0	Opt (B)	−7.52 ± 0.68	12.38	⋯	<0.05	1.37	(−4.7, −5.1, −6.4)	(22.4, 41.8, 6.8)	NN?	N
19234442+2629030	K7.8	Opt (B)	20.32 ± 1.03	39.9	⋯	<0.05	1.31	(11.1, 17.8, −5.8)	(18, 31.3, 3.3)	NN?	N
19242697+2716351	⋯		18.33 ± 0.81	37.93	⋯	<0.05	1.26	(7.6, 17.5, −5.5)	(30.6, 54.9, 6)	NN?	N
19243494–3442392	M4.0	Opt (C)	−4.49 ± 0.32	2.52	⋯	<0.05	−13.84	(−7.4, −16.5, −9.4)	(51.1, 3.4, −20.1)	YY?	Y	8
19260075–5331269	M2.2	Opt (H)	−1.33 ± 2.51	−0.67	SB2	0.15	−2.9	(−11, −15.9, −9.5)	(41.2, −11.8, −21.4)	YYY	Y
19275844+3309439	K8.0	Opt (B)	−32.22 ± 1.01	−12.56	⋯	0.06	1.74	(−14.6, −27.7, −9.8)	(15.8, 36.2, 5.2)	NNY?	N
19300396–2939322	M3.9	Opt (B)	−5.17 ± 0.95	3.29	⋯	<0.05	−6.17	(−6.8, −15.7, −9.9)	(58.2, 9.6, −22.4)	YY?	Y
19355595–2846343	M9	IR (A)	−5.08 ± 0.88	3.48	⋯	<0.05	−16.8	(−7, −15.5, −10.6)	(57.6, 10.9, −23.3)	YY?	Y	10
19410521+3806521	M3.7	Opt (B)	1.38 ± 0.50	20.66	⋯	<0.05	1.77	(−3.3, 3.5, −6.9)	(16.7, 51.9, 7.2)	NN?	N
19472175+3954072	⋯		−35.12 ± 2.00	−16.05	SB2	<0.05	0.39	(−18.1, −30.8, −6.9)	(16.2, 57.6, 7.7)	NN?	N
19533169–0707001	M4	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
19560294–3207186	M3.7	Opt (B)	−2.56 ± 0.69	4.33	SB2 (6)	<0.05	−6.48	(−6.3, −15.3, −11.3)	(53.2, 8.2, −27.3)	YY?	Y?h	2, 11
19571814+6242573	M5	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
19572094+6242559	M6	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
20013718–3313139	M1.0	Opt (C)	−3.71 ± 0.29	2.66	⋯	0.13	−1.69	(−7.1, −16.7, −9.4)	(55, 7.6, −29.9)	YYY	Y	4
20034245+1432215	M1.0	Opt (B)	−15.60 ± 0.73	1.58	⋯	<0.05	1.47	(−4.9, −17.7, −8.3)	(36.4, 50.7, −9.6)	YN?	N
20083784–2545256	M4.7	Opt (B)	−5.74 ± 1.57	2.58	⋯	0.22	−5.5	(−6.9, −16.5, −9.2)	(51.2, 15.2, −27.9)	YYY	Y
20085368–3519486	M4.0	Opt (B)	−3.73 ± 0.25	1.73	⋯	<0.05	−6.7	(−8.9, −15.5, −10.5)	(39.2, 4.1, −23.1)	YY?	Y
20100002–2801410	M3.0	Opt (C)	−8.56 ± 0.44	−0.94	SB2	<0.05	−9.52	(−11.2, −15.8, −9.1)	(46.8, 11.8, −26.3)	YY?	Y?h	1
20200623–3433203	M5	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
20285054+2141119	M2.5	Opt (B)	−3.25 ± 0.36	13.61	⋯	<0.05	−2.23	(−3.2, −4.5, −12)	(19.1, 38.6, −7.5)	NY?	N
20333759–2556521	M5	IR (A)	−8.48 ± 0.52	−1.27	⋯	0.47	−9.44	(−10.8, −17, −8.5)	(36.4, 12, −24.9)	YYY	Y	1
20385687–4118285	M4.3	Opt (H)	−1.32 ± 1.62	1.2	⋯	<0.05	−1.12	(−10.4, −13.6, −11.6)	(48.1, 0.1, −36.4)	YN?	N
20390476–4117390	M3.3	Opt (H)	4.09 ± 0.53	6.61	⋯	<0.05	−6.73	(−4.6, −16.1, −12.9)	(39.9, 0.1, −30.2)	NY?	N
20434114–2433534	M4	IR (A)	−5.0 ± 0.6l	2.12	VB (7)	<0.05l	−5.24l	(−8.5, −15.4, −10.5)	(35.1, 13.2, −26.1)	YY?	Y	1
21100461–1920302	M5	IR (A)	−3.97 ± 1.85	3.14	⋯	<0.05	−6.68	(−7.6, −15.5, −11.3)	(22.5, 12.6, −20.8)	YY?	Y	9
21100535–1919573	M2.0	Opt (C)	−5.58 ± 0.19	1.53	⋯	<0.05	−2.83	(−9.3, −15.6, −10.5)	(22.5, 12.6, −20.8)	YY?	Y	1
21103147–2710578	M5	IR (A)	−4.21 ± 0.33	0.85	VB? (5, 8)	0.51	−10.23	(−9.9, −14.9, −9.9)	(29.5, 10.4, −27.5)	YYY	Y	8
21183375+3014346	M0.0	Opt (B)	−22.47 ± 0.81	−7.37	⋯	<0.05	−2.66	(−10.6, −23.4, −7.8)	(10, 45.5, −11)	NY?	N
21195985+3039467	M5	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
21200779–1645475	M3.9	Opt (B)	−5.10 ± 0.62	2.07	⋯	<0.05	−3.86	(−9.6, −14.3, −10.8)	(31.9, 21.1, −32.4)	YY?	Y
21374019+0137137	M5	IR (A)	−15.1 ± 5.0	−5.03	VB (9)	<0.05	−10.11	(−12.6, −19.4, −6.5)	(17.9, 27.1, −23)	YY?	Y	12
21384755+0504518	M4	IR (A)	−16.14 ± 1.11	−5.5	SB3	<0.05	−4.83	(−11.4, −20.8, −6.5)	(25.1, 43.7, −33.2)	YY?	Y?h
22010456+2413016	M8	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
22085034+1144131	M4.3	Opt (B)	−9.26 ± 1.45	0.51	⋯	<0.05	−4.96	(−10.2, −15.5, −9.5)	(9.6, 29.8, −21.6)	YY?	Y
22334687–2950101	M7	IR (A)	−1.94 ± 0.30	−2.41	⋯	0.65	−22.18	(−10.6, −17.4, −7)	(30, 10.7, −54.5)	YYY	Y
22440873–5413183	M4.0	Opt (C)	−0.20 ± 0.62	−6.28	⋯	<0.05	−6.82	(−11.8, −16.1, −2.5)	(24.1, −11.3, −37.1)	NY?	N
22500768–3213155	M4	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
23010610+4002360	M7	IR (A)	−16.87 ± 0.42	−6.73	⋯	0.62	−8.88	(−9.7, −22, −5.8)	(−9.1, 46.7, −15.6)	NYY	Y?
23224604–0343438	⋯		−11.12 ± 1.88	−9.66	⋯	<0.05	−15.62	(−11.1, −20.8, −0.9)	(6.8, 29.3, −49)	NY?	N
23301129–0237227	M5	IR (A)	⋯	⋯	⋯	⋯	⋯	⋯	⋯	???	N
23314492–0244395	M5	IR (A)	−5.44 ± 0.19	−4.47	⋯	<0.05	−14.22	(−9.4, −18.6, −5.5)	(2.9, 19.3, −32.6)	YY?	Y	1
23323085–1215513	M0.0	Opt (C)	1.38 ± 0.37	0.42	⋯	0.16	−1.53	(−10.6, −15.4, −9.5)	(4.2, 10.2, −25.2)	YYY	Y	13
23355015–3401477	M6	IR (A)	5.9 ± 0.78	0.62	⋯	0.62	−8.42	(−9.2, −16.8, −9.7)	(12.6, 1.2, −39.8)	YYY	Y

Notes. (A)–(H): For the few targets for which we were unable to get reliable SpT_spec, we include literature values (SpTs in italics). Sources and references for these are as follows. A: methods of AL13, this work; B: narrow TiO-band index defined by Shkolnik et al. (2009), this work; C: TiO5-band index from Riaz et al. (2006); D: Gagné et al. (2015b); E: TiO-band index from Alonso-Floriano et al. (2015b); F: Schlieder et al. (2012a); G: Lépine et al. (2013); H: spectral fitting following the method of Kraus et al. (2014), this work.

^aSystemic RVs are reported for the spectroscopic binaries (SBs). ^bΔRV = RV_obs−RV_kin. ^c Literature references for binarity are as follows. 1: Nicholson (2015); 2: Malo et al. (2014a); 3: Liu et al. (2010); 4: Reiners et al. (2010); 5: Bergfors et al. (2010); 6: Elliott et al. (2014); 7: Shkolnik et al. (2009); 8: Janson et al. (2014); 9: Bowler et al. (2015a). ^dUncertainties in the EW of Li are typically less than 0.05 Å and in Hα are 0.1 Å. ^eUVWs and XYZs are not reported for nonmembers, as the kinematic distance used in the calculations of these values is meaningless if they are not actual members. ^fYes (Y), no (N), or uncertain (?) membership assessments of the RV, Hα emission, and Li absorption. ^gThe first references to list these targets as BPMG members. 1: Malo et al. (2013); 2: Lépine & Simon (2009); 3: Gagné et al. (2015a); 4: Kiss et al. (2011); 5: Shkolnik et al. (2012); 6: Binks & Jeffries (2016); 7: Schlieder et al. (2010); 8: Malo et al. (2014a); 9: Elliott et al. (2016); 10: Liu et al. (2016); 11: Elliott et al. (2014); 12: Schlieder et al. (2012a); 13: Torres et al. (2008). ^hThese SBs have system RVs consistent with the BPMG, strong Hα emission, and inconclusive Li observations. To avoid biasing the sample away from SBs, we designate them "Y?." These systems should be followed up for full orbital solutions and/or parallaxes to better understand their ages. ⁱSchlieder et al. (2010) reported an RV of 10.4 ± 2.0 km s⁻¹ and a ΔRV of 3.8 km s⁻¹ and thus considered this a likely BPMG member. ^jBinks & Jeffries (2016) reported Hα in absorption and thus rejected this star as a BPMG member. ^kNIR SpT indicates M9 (AL13). ^lRV, Hα, and/or Li data from the literature: 05061292+0439272 (Alcala et al. 1996; Alcalá et al. 2000; Lépine et al. 2013); 06352229–5737349 (Kordopatis et al. 2013); 20434114–2433534 (Shkolnik et al. 2012, 2009).

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We acquired high-resolution optical spectra of our BPMG candidates, excluding those with IR spectra indicating field surface gravities, over seven nights with the Magellan Inamori Kyocera Echelle (MIKE) optical echelle spectrograph on the Clay telescope at the Magellan Observatory and two nights with Keck's High Resolution Echelle Spectrometer (HIRES; Tables 1 and 3).

The MIKE data were collected with the 05 slit with a corresponding spectral resolution of ≈35,000 across the 4900–10000 Å range of the red chip. The data were reduced using the facility pipeline (Kelson 2003).¹⁰ Each stellar exposure was bias-subtracted and flat-fielded. After extraction of each order, the one-dimensional spectra were wavelength-calibrated with a ThAr arc lamp, which was taken after every stellar exposure to limit temperature differences and spectral drifts between the lamp and the target exposures. To correct for any remaining instrumental drifts, we used the telluric O₂ A band (7620–7660 Å) to align the MIKE spectra to <40 m s⁻¹. We then corrected for the heliocentric motion of the Earth. The final spectra are of moderate signal-to-noise ratio (S/N; $\approx 35$ pixel^–1 at 7000 Å).

We used the 0861 slit with HIRES to give a spectral resolution of ≈58,000. The detector is a mosaic of three 2048 × 4096 15 μm pixel CCDs spanning 4900–9300 Å and designated as the blue, green, and red chips. We used the GG475 filter with the red cross-disperser to maximize the throughput near the peak of an M dwarf SED. The HIRES data were reduced using the facility pipeline MAKEE¹¹ written by T. Barlow. The MAKEE pipeline performs bias subtraction, flat-field correction, spectral extraction, wavelength calibration using the ThAr spectra, and heliocentric velocity correction.

On each night, we observed RV standards with SpTs spanning those of our BPMG candidates. The standards were taken from Nidever et al. (2002), with updated RVs (if available) from Chubak et al. (2012). Each night, spectra were also taken of an A0V standard star for telluric line correction. The spectroscopically derived SpTs (SpT_spec) are listed in Table 3. For the late-type members, we list the NIR SpT. Optical SpTs were done with either TiO-band indices or spectral line fitting following Shkolnik et al. (2009) and Kraus et al. (2014), respectively. In some cases, due to poor continuum fitting, we include published spectroscopic SpTs.

4.1. RVs for Kinematic Matches to the BPMG

Since our targets are relatively red, we limited our RV measurements to the reddest orders where the S/N is the highest. We cross-correlated each order between 7000 and 9000 Å (excluding those orders with strong telluric absorption) using the IRAF routine FXCOR (Fitzpatrick 1993). We measured the RVs from the Gaussian peak fitted to the cross-correlation function of each order and adopted the average RV of all orders with their standard deviation as the measurement uncertainty. The MIKE and HIRES data provide RV measurements to better than 1 km s⁻¹ in almost all cases.

The target RVs are listed in Table 3, along with the difference in measured and predicted RV, assuming the star is a BPMG kinematic match: ΔRV = RV_obs–RV_kin. With an RV dispersion of known BPMG members of 1.8 km s⁻¹ (Mamajek & Bell 2014), we designate a star as a kinematic match if $| {\rm{\Delta }}\mathrm{RV}| \lt 5.4$ km s⁻¹, a 3σ deviation from a perfect match (Figure 4).

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The high resolution of the optical spectra also allows for the identification of double- or triple-lined spectroscopic binaries (SBs). Ten of our targets are SBs, consisting of seven SB2s and two SB3s plus one SB1 (2MASS J02485260–3404246; SpT_SED = M3.9), which appears to be RV variable when compared to literature RV values. Of the 10 SBs we identify (Table 3),¹² eight have systemic RVs consistent with those of BPMG members.

4.2. Hα Emission and Lithium Absorption as Youth Indicators

Nearly all young stars with SpT ≥ M0 exhibit Balmer emission, most notably Hα. Stauffer et al. (1997) demonstrated a clear demarcation between older M dwarfs and those younger than the $\approx 50\,\mathrm{Myr}$ old clusters IC 2602 (Dobbie et al. 2010) and IC 2391 (Barrado y Navascués et al. 2004). Exceptionally strong Hα emission can also indicate that a young star still hosts a gas-rich protoplanetary disk, where accreting gas is falling along magnetic field lines from the circumstellar disk onto the star (e.g., Barrado y Navascués & Martín 2003). None of our targets show signs of accretion, which is not unexpected at this age.

We apply the Stauffer et al. (1997) Hα criterion to our sample to identify those with emission levels consistent with stars younger than $\approx 50\,\mathrm{Myr}$. The Hα equivalent widths (EWs) for our sample are plotted in Figure 5 (left), identifying the stars we confirmed as BPMG members (as prescribed in Section 5 and Figure 3). The cases with strong Hα emission that we do not identify as BPMG members have $| {\rm{\Delta }}\mathrm{RV}| \gt 5.4$ km s⁻¹.

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Lithium absorption at 6708 Å is another spectroscopic age indicator in the optical spectrum of pre-main-sequence stars. Stars with M0 < SpT < M5 require 20–100 Myr to deplete their primordial lithium (Chabrier et al. 1996), with depletion timescales increasing for later SpTs. The earlier SpTs therefore allow for tighter age constraints. For objects with SpT ≥ M6, the detection of lithium sets an upper limit of $\approx 100\,\mathrm{Myr}$ on the star's age (Chabrier et al. 1996; Stauffer et al. 1998). We use the detection of lithium as a solid indication of youth. However, the lack of lithium absorption in the spectra of early Ms is not confirmation of old age because the depletion timescales are short, providing an age limit comparable to the age of the BPMG. We measured lithium EWs or set limits¹³ for all of our targets for which we acquired optical spectra (Figure 5, right). An analysis of the lithium depletion boundary (LDB) and corresponding age for the BPMG is presented in Section 5.1.

Three of our candidates have strong Hα in emission and Li absorption yet do not have RVs consistent with BPMG membership. We have designated these as possible members (with "Y?"); they may be SB1s in which an unseen companion is causing the RV deviation. Alternatively, these are young field interlopers into our sample.

The methodology of the ACRONYM of the BPMG is summarized in Figure 3, which traces the prescription of our candidate selection and confirmation. We combine proper-motion limits, suggestions of pre-main-sequence luminosity through photometric distance constraints, spectroscopic youth indicators such as low gravity, Li, and Hα, and RVs for kinematic matching.

Of our 104 BPMG candidates with follow-up IR and/or optical spectroscopy, we confirm 66 member systems, recovering 25 previously reported members and identifying 41 new ones. Nineteen of the 66 members have spectroscopic or visual companions. All of our candidates are listed with their final membership status in Table 3. Fourteen of the 66 members have "Y?" status. Ten of the candidates are SBs with system RVs consistent with the BPMG and strong Hα emission; two of these have strong Li absorption, confirming youth. Eight of the SBs have inconclusive Li observations, but, to avoid biasing the sample against SBs, we designate these as "Y?." These systems should be followed up for full orbital solutions and/or parallaxes to better understand their membership status.

Our confirmation yield for BPMG systems is 66 of 104 candidates (63%), a little lower than the 67% confirmation rate we achieved in our Tuc-Hor study (Kraus et al. 2014). The slightly lower yield results from the extension of our search to positions further north than those of known BPMG members. Though this extension decreases our spectroscopic yield, it allows us to determine the northern boundary of the BPMG (Figure 1).

The space velocities (UVW) and positions (XYZ) of confirmed members were calculated using the positions, RVs, proper motions, and kinematic distances. They are compared to the YMGs in Figure 6 and listed in Table 3. The few outliers in these figures are the known or suspected binaries. Of the kinematic matches to the BPMG, only two are clearly not members, implying a 3% false-membership rate based on kinematics alone. This number is slightly less than that calculated by Shkolnik et al. (2012), who measured the UVWs of all the nearby old M dwarfs and found that 6% have kinematic matches to the BPMG but are clearly old. Our process for photometrically selecting pre-main-sequence stars improves our ability to use kinematics to identify new candidate members.

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To present the new mass function for the BPMG, we converted the SpT_SED for each target at an assumed age of 23 Myr to mass using the SpT–T_eff relation from Herczeg & Hillenbrand (2014) for SpTs of F5 and later and from Pecaut & Mamajek (2013) for stars earlier than F5. We then used the mass–T_eff relation from Baraffe et al. (2015) for stars with masses $\leqslant 1.4$ ${M}_{\odot }$ and the latest PARSEC models of Chen et al. (2015) for masses >1.4 ${M}_{\odot }$.

In Figure 7, we compare the mass histogram for the BPMG before and after our confirmation of the new low-mass members. The new additions add 1, 2, 7, 17, and 10 objects, respectively, in the lowest five mass bins. There are also 14 literature substellar objects (to which we add four more) for which we did not attempt to estimate the masses, as they have SpTs later than M7 and the models for them have not yet been empirically calibrated.

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5.1. Lithium Depletion and the Age of the BPMG

Low-mass stars are gradually depleted of lithium as convection carries it deep into the stellar interior, where fusion converts it to helium. Lithium burning proceeds most rapidly for early-M stars (e.g., Figure 5, right panel), while stars of higher and lower mass burn lithium at correspondingly slower rates. This transition from lithium-rich to lithium-depleted is particularly rapid at the lower end of the depleted range, leading to a narrow LDB that serves as a sensitive probe of age. The BPMG is one of the youngest nearby moving groups for which lithium depletion of early M dwarfs has been observed (e.g., Mentuch et al. 2008), leading to a lithium depletion age of $\tau =24\pm 4\,\mathrm{Myr}$ (Binks & Jeffries 2016). The paucity of known M3–M5 members of the BPMG has led to uncertainty in the location and width of the LDB and, hence, the moving group's age, but our newly identified BPMG members now densely sample this mass range.

In Figure 8, we plot the SpT, absolute M_K magnitude, and lithium EW of all of the K7–M6 members of the BPMG, as projected onto the three corresponding two-dimensional planes. The BPMG members without lithium are clearly clustered among the M1–M4 stars, with most ≤M0 and ≥M4 stars bearing at least some lithium. To provide context for the pattern of lithium depletion, we also show the isochronal tracks of Baraffe et al. (2015) in each plane, spanning ages of 15–50 Myr. We convert the isochrones into observational space by adopting their estimate of M_K and applying the SpT–T_eff temperature scale of Herczeg & Hillenbrand (2014) and the A_Li–$\mathrm{EW}[\mathrm{Li}]$ curves of growth of Palla et al. (2007).

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Our results clearly demonstrate the tension between theory and observations for age determinations of young stars. In the HR diagram, nearly all sample members fall above the 15 Myr isochrone, implying a very young age of $\tau \sim 10\,\mathrm{Myr}$. However, the cluster sequence in M_K versus $\mathrm{EW}[\mathrm{Li}]$ implies an older age of $\tau \sim 25\,\mathrm{Myr}$, and the corresponding sequence in SpT_SED versus $\mathrm{EW}[\mathrm{Li}]$ implies an even older age of $\tau \gtrsim 50\,\mathrm{Myr}$. HR diagram ages are now broadly considered to underestimate the true age of stars (e.g., Naylor 2009; Pecaut et al. 2012), most likely due to theoretical overestimation of T_eff for a given mass (e.g., Kraus et al. 2015). However, theory and observations seem to broadly agree on the relation between luminosity and mass for M stars at 10–50 Myr (Kraus et al. 2015; Montet et al. 2015; Nielsen et al. 2016; Rizzuto et al. 2016). The model-derived ages in our three observational planes are consistent with these patterns, so we hereafter refine our analysis to the ($\mathrm{EW}[\mathrm{Li}]$–M_K) plane of Figure 8.

The observed sequence broadly agrees with the results of Binks & Jeffries (2016), who estimated the LDB age to be $\tau =21\pm 4$ Myr from the models of Baraffe et al. (2015) or $\tau =24\pm 4\,\mathrm{Myr}$ from the magnetized models of Feiden (2016). Most of our observed lithium-bearing stars do indeed fall between the 20 and 25 Myr isochrones. However, Binks & Jeffries (2016) identified the LDB primary via a discrete gap between the lithium-bearing and lithium-depleted members, while our more dense sequence also demonstrates for the first time that there is a finite width in SpT to the LDB. There is no clear gap in luminosity where there are no members with lithium, while lithium-depleted members extend as faint as M_K = 6.6. If we isolate the analysis to ≥M2 stars and identify the boundary to be the M_K where there are an equal number of lithium-depleted and lithium-rich members, then we find a boundary location of M_K = 5.6 mag, with eight stars in each group. We find that 68% of the interlopers fall in the range of $5.2\,\mathrm{mag}\lt {M}_{K}\lt 6.0$ mag, so the corresponding uncertainty is ±0.4 mag. The corresponding age from the Baraffe et al. (2015) models is $\tau =22\pm 6\,\mathrm{Myr}$.

Unresolved binarity could explain some extension of the lithium-bearing population upward, but it is difficult to explain the faintness of some lithium-depleted members; even the faintest lithium-depleted member (2MASS J03255277–3601161) shows excellent agreement with BPMG membership in all other aspects. The observations could indicate a genuine age spread, as has been suggested for AB Dor (López-Santiago et al. 2006) and Taurus–Auriga/32 Ori (Kraus et al. 2017). However, it also might result from variations in lithium-burning rates due to other stellar parameters. Lithium abundances have long been known to correlate with rotation for more massive stars (e.g., Soderblom et al. 1993), including in the BPMG (Messina et al. 2016), and substantial variations in lithium abundance have also been seen in populations both younger (Upper Sco; Rizzuto et al. 2015) and older (Pleiades; Somers & Stassun 2017). Convective mixing could be correlated with rotation at varying levels of subtlety, from direct impact on the convective mixing length (Bouvier 2008) to magnetic activity suppressing convection (Chabrier et al. 2007; Feiden 2016) or, even more subtly, to delayed rise in the core temperature (Baraffe & Chabrier 2010; Somers & Pinsonneault 2014). There is no straightforward way to disentangle these effects until purely geometric ages from astrometric tracebacks become feasible. However, our measured width for the LDB (${M}_{K}=5.6\pm 0.4$ mag) is 3 times broader than the corresponding boundary we measured in Tuc-Hor (${M}_{K}=7.12\pm 0.16$) via identical techniques. Either the astrophysics broadening the sequence change between 20 and 40 Myr or the BPMG may indeed have a substantially larger age spread than Tuc-Hor.

Finally, we find two outliers for which membership should be further inspected:

• 1.  
  The outlier 2MASS J03363144–2619578 (SpT_SED = M6.1; M_K = 7.66) appears to be partially depleted in lithium ($\mathrm{EW}[\mathrm{Li}]=330$ mÅ), even though it should be undepleted according to both SpT and M_K. It has also been suggested as a candidate member of the Tuc-Hor moving group (Rodriguez et al. 2013; Gagné et al. 2015a), and its observed RV (${v}_{\mathrm{rad}}=16.7\pm 2.7$ km s⁻¹) is only marginally more consistent with expectations for the BPMG (${v}_{\mathrm{rad}}=18.2$ km s⁻¹) than for Tuc-Hor (${v}_{\mathrm{rad}}=13.8$ km s⁻¹). The lithium abundance suggests that the older age of Tuc-Hor is more appropriate, but its membership will ultimately be confirmed by parallax; it should fall at d = 25 pc if in the BPMG or at d = 50 pc if in Tuc-Hor.
• 2.  
  The outlier 2MASS J02365171–5203036 (=GSC 08056–00482) was proposed by Zuckerman & Song (2004) to be an M3 member of Tuc-Hor, but Elliott et al. (2014) subsequently classified it as an M2 member of the BPMG, and Malo et al. (2014a) used their BANYAN formalism to estimate a most likely membership in Columba, albeit with a significant probability for the BPMG and a small probability for Tuc-Hor. This star sits low on the cluster sequence for the BPMG, but its high lithium abundance for its SpT ($\mathrm{EW}[\mathrm{Li}]\,=380\,{\rm{m}}\mathring{\rm{A}} ;$ Torres et al. 2006) would be inconsistent with Tuc-Hor or Columba membership for M2–M3 stars (e.g., Kraus et al. 2014) and is high even for the BPMG. Membership in the BPMG seems most likely, but a parallax would clarify the question of membership for this star.

In this paper, we present the results from our efficient photometric + kinematic selection process, which identified 104 low-mass BPMG candidates. Follow-up IR observations of the latest SpTs (SpT > M5) were collected with IRTF/SpeX to search for low-gravity objects, an indication of youth. We observed the young ones of these, plus all the earlier SpT candidates, with the high-resolution optical spectrographs at the Magellan and Keck telescopes. These data provided the RVs needed to confirm or reject stars comoving with the BPMG and youth indicators such as Li absorption and Hα emission.

Prior to our work, there were 94 known member systems with primary masses less than 0.7 ${M}_{\odot }$. In the Appendix, we compile the current census, with which we were able to test the efficiency and false-membership assignments using our selection and confirmation criteria. The addition of 41 new systems from this work represents a 44% increase in low-mass BPMG membership and a new sample around which to look for directly imaged planets. The four new BPMG members with SpTs ≥ M7 and likely less than 0.07 ${M}_{\odot }$ represent a 29% increase from the 14 already known. They are: 2MASS J00413538–5621127, 2MASS J03550477–1032415, 2MASS J22334687–2950101, and 2MASS J23010610+4002360. If the number of the BPMG's AFGK stars is complete, then the fraction of M stars in the BPMG is now 69%, near the expected value of 75%. However, the new IMF for the BPMG clearly shows a deficit of 0.2–0.3 ${M}_{\odot }$ stars by a factor of ∼2. In this case, we expect that the AFGK census of the BPMG is also incomplete, probably due to biases of searches toward the nearest of stars. Future surveys for AFGK and M SpT members should extend out to at least 100 pc for the BPMG.

E.S. thanks J. Teske for useful discussions and appreciates support from the HST grant HST-AR-13911.002-A and NASA/Habitable Worlds grant NNX16AB62G. K.A. acknowledges support by a NASA Keck PI Data Award (#1496879), administered by the NASA Exoplanet Science Institute. We also thank Dr. Eric Mamajek for a speedy and helpful referee's report. This research has made use of the VizieR catalog access tool, CDS, Strasbourg, France (Ochsenbein et al. 2000), and the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX13AC07G and by other grants and contracts.

To complement our search for new members, we compile the most comprehensive list of the 146 objects that have been reported in the literature as likely members of the BPMG in Table 4. This list includes every object that has been asserted to be a member, as well as any probabilistic candidates that have $\gt 50 \%$ membership probability (e.g., Malo et al. 2013). Past stars considered members that were later refuted by more data are not included (Binks & Jeffries 2016; Elliott et al. 2016; Liu et al. 2016.) We also exclude any purported members from traceback analyses (Ortega et al. 2002; Song et al. 2003; Makarov 2007), as recent revisions to the age of the BPMG cast doubt on tracebacks conducted with the previous (younger) age (Mamajek & Bell 2014). In compiling our census, we include every object that has a distinct entry in the 2MASS Point Source Catalog (Cutri et al. 2003) but do not include secondary components of close multiple systems. The effective angular separation cutoff is ≈4''–5''. We list the previously reported members in Table 4 along with all available data used for membership assessments: SpTs, proper motions, RVs, Hα EWs, Li EWs, and surface gravity assessments. We also compile parallaxes for 77 of the objects in the literature census. Parallaxes are not currently available for any sources in our own survey. For easier use of the complete census, we have appended the new BPMG members from this work to the bottom of Table 4.