Gaia-based Isochronal, Kinematic, and Spatial Analysis of the Cha Association

We employed two quantitative criteria, kinematic offset (K_kin) and magnitude offset (ΔM), to help assess CA membership. These two metrics, listed in Table 2, correspond to those defined in M+13. Specifically, the kinematic offset is defined as

$$\begin{eqnarray}&&{K}_{\mathrm{kin}}=\sqrt{{\left(U-{U}_{0}\right)}^{2}+{\left(V-{V}_{0}\right)}^{2}+{\left(W-{W}_{0}\right)}^{2}},\end{eqnarray} \tag{ 1 }$$

where (U₀, V₀, W₀) is the mean space motion for CA as calculated from the 30 bona fide M+13 members used for the SLFR analysis (Section 3.1). We defined ΔM as

$$\begin{eqnarray}&&{\rm{\Delta }}M=M^{\prime} (c)-M,\end{eqnarray} \tag{ 2 }$$

where c is the color of the star, $M^{\prime} (c)$ is the absolute magnitude of the empirical isochrone at the star's color, and M is the measured absolute G magnitude of the star. Outliers in ΔM are those with ∣ΔM∣ ≥ 2σ_ΔM and in K_kin are ${K}_{\mathrm{kin}}\geqslant 2{\sigma }_{{K}_{\mathrm{kin}}}$, where σ_ΔM  = 0.629 mag and ${\sigma }_{{K}_{\mathrm{kin}}}=3.2$ km s⁻¹. These standard deviations are calculated for the M+13 bona fide membership list with usable data (30 stars), i.e., omitting M+13's (17) provisional and rejected members.

Table 2. Membership Quality Flags

Name	Kkin	${\sigma }_{{K}_{\mathrm{kin}}}$	ΔM	σΔM	Status	Notes
HD 8279	4.35	0.21	1.8911	0.0066	R	Previously rejected (Section 4.1.2)
CP-68 1388	2.0	1.0	−0.1975	0.0097	M
2MASS J11183572–7935548A	4.7	1.6	−1.200	0.033	M
RX J1123.2–7924	15.8	2.9	0.9509	0.0065	R	Previously rejected (Section 4.1.2)
HIP 55746	7.1	1.2	1.383	0.027	R	Previously rejected (Section 4.1.2)
RX J1137.4–7648	4.6	5.0	1.8790	0.0046	R	Previously rejected (Section 4.1.2)
2MASS J11404967–7459394	4.2	1.0	0.025	0.016	M
2MASS J11411722–7315369	⋯	⋯	1.093	0.012	P
2MASS J11432669–7804454	17.9	4.1	−0.83	0.24	R	Probable Cha Cloud (Section 4.1.2)
2MASS J11432968–7418377	⋯	⋯	0.8659	0.0090	P
RX J1147.7–7842	1.81	0.90	−0.6586	0.0097	M
RX J1149.8–7850	1.0	1.3	−0.0002	0.0097	M
RX J1150.9–7411	2.7	1.3	−0.892	0.042	M
2MASS J11550485–7919108	⋯	⋯	−0.059	0.015	M
T Cha	3.0	1.3	−7.72	0.13	M	Anomalous CMD position (Section 4.1.3)
RX J1158.5–7754B	1.3	2.0	0.2454	0.0105	M
RX J1158.5–7754A	3.6	1.8	−0.7542	0.0091	M
HD 104036	1.87	0.51	3.5594	0.0092	M
CXOU J115908.2–781232	1.22	0.25	0.774	0.015	M
RX J1159.7–7601	1.3	3.7	0.3029	0.0073	M
2MASS J12000269–7444068	⋯	⋯	0.799	0.012	P
HD 104237A	0.69	0.42	2.21	0.016	M
HD 104237D	⋯	⋯	−0.115	0.015	M
HD 104237E	⋯	⋯	−1.027	0.036	M
2MASS J12005517–7820296	3.4	1.3	−0.125	0.020	M
HD 104467	1.6	1.0	0.010	0.038	M
2MASS J12014343–7835472	5.93	0.65	4.168	0.045	M	Anomalous CMD position (Section 4.1.3)
USNO-B 120144.7–781926	0.83	1.11	0.064	0.022	M
CXOU J120152.8–781840	2.5	1.1	0.33	0.0163	M
RX J1202.1−7853	3.35	0.23	−0.279	0.013	M
RX J1202.8−7718	0.77	0.61	0.365	0.010	M
RX J1204.6−7731	3.6	2.0	−0.1695	0.0091	M
HD 105234	⋯	⋯	3.1047	0.0083	P
2MASS J12074597–7816064	1.5	1.9	1.029	0.014	R	Previously rejected (Section 4.1.2)
RX J1207.7−7953	1.01	0.70	0.3388	0.0087	M
HIP 59243	⋯	⋯	3.2697	0.0075	P
HD 105923	0.9	2.0	0.0231	0.0091	M
RX J1216.8–7753	1.3	1.9	0.304	0.012	M
RX J1219.7–7403	9.2	3.3	0.2485	0.0075	M
RX J1220.4–7407	2.1	1.4	−1.09	0.24	R	Probable Cha Cloud (Section 4.1.2)
2MASS J12203396–7135188	⋯	⋯	−0.216	0.012	P
2MASS J12203619–7353027	⋯	⋯	0.4854	0.0086	P
2MASS J12210499–7116493	2.6	2.5	0.0482	0.0080	M
2MASS J12220068–7001041	⋯	⋯	0.547	0.011	P
2MASS J12222238–7137040	⋯	⋯	0.248	0.014	P
2MASS J12224862–7410203	⋯	⋯	−0.822	0.021	P
2MASS J12255824–7551116	⋯	⋯	⋯	0.784	0.015	P
2MASS J12324805–7654237	⋯	⋯	0.08	0.18	P
2MASS J12332483–6848553	⋯	⋯	0.613	0.020	P
2MASS J12351540–7043079	⋯	⋯	0.473	0.014	P
RX J1239.4–7502	1.5	2.8	0.0672	0.0077	M
2MASS J12421315–6943484	⋯	⋯	0.235	0.019	P
RX J1243.1–7458	11.1	7.0	0.27	0.11	R	Previously rejected (Section 4.1.2)
2MASS J12425584–7034207	⋯	⋯	0.270	0.018	P
2MASS J12473611–7031135	⋯	⋯	0.5594	0.0073	P
CD-69 1055	2.5	1.9	0.1176	0.0087	M
MP Mus	2.32	0.21	−0.2265	0.0097	M

Note. Key for status column: M = member; P = provisional member; R = rejected.

Download table as:  ASCIITypeset image

Figure 2, like Figure 8 in M+13, illustrates the kinematic and magnitude offsets plotted against each other. The figure includes all stars from M+13 (i.e., M+13's bona fide, provisional, and rejected members) with Gaia parallaxes and G − G_RP colors. Stars with K_kin or ΔM values that place them within both 2σ boundaries, within errors, are henceforth considered to be high-probability members of CA. Stars outside these bounds were reconsidered for membership on a case by case basis. For instance, the two magnitude outliers beyond 1σ_K , HD 104036 and HD 104237A, are both A stars in the bluest region of the CMD. This region is not well fit by the empirical isochrone so the large ΔM is not enough to disqualify their membership. Additional cases, including stars whose DR2 parallaxes render their CA membership doubtful, are discussed in Sections 4.1.2 and 4.1.3.

Zoom In Zoom Out Reset image size

Figure 3 presents Gaia CMDs for the full sample of M+13 stars initially considered CA members, i.e., for the stars listed in Table 1 (left panel) and for the final membership list, which is presented and discussed in Section 4.1.5 (right panel). The empirical isochrone in both panels is that found for the original 30 star data set (i.e., is the same as the SLFR curve in Figure 1). In the following, we describe the reasoning behind retaining or rejecting the >2σ outliers plotted in Figure 2.

Zoom In Zoom Out Reset image size

We corroborate the rejected status of all of the eleven CA members rejected by M+13. Five of the M+13 rejects clearly maintain their nonmember status on the basis of the large Gaia-based kinematic and magnitude offsets determined here (Table 2). One rejected M+13 member with offsets <2σ, 2MASS J12074597–7816064, is unlikely to be a member given its position in the Gaia CMD (Figure 1). This star falls below our empirical isochrone, despite being a suspected spectroscopic binary (M+13). Its kinematic offset is also at the edge of acceptability (K_kin = 3.03 km s⁻¹). We hence exclude 2MASS J12074597–7816064 from our final CA membership list. Another five M+13 rejected stars now convincingly lose membership status based on Gaia parallaxes (2MASS J11334926–7618399, RX J1243.1–7458, RX J1150.4–7704, TYC 9238-612-1, and TYC 9420-676-1). VW Cha, which was previously rejected on the basis of a pre-Gaia distance estimate, was not reconsidered in this work because no Gaia parallax was reported in DR2.

Two stars previously considered bona fide CA members by M+13, 2MASS J11432669–7804454 and RX J1220.4–7407, have CMD positions consistent with young ages and are marginally consistent with CA membership (in terms of ΔM) given our SLFR analysis, although both were flagged as photometric binaries due to their elevation above the single-star locus. Kinematically, RX J1220.4–7407 lies within the distribution of CA members (K_kin = 2.11 km s⁻¹), while 2MASS J11432669–7804454 has a large kinematic offset (17.9 km s⁻¹). However, these stars have Gaia parallaxes of 5.54 and 6.71 mas, respectively, a factor of 1.5–2 smaller than the mean for CA (9.81 mas). Notably, both stars have high RUWE and astrometric_excess_noise values, suggesting that their parallax solutions may not be reliable. If their parallaxes are in fact accurate, their relatively large distances would indicate they are younger, background T Tauri stars associated with the Cha star-forming region (see, e.g., Kastner et al. 2012). Their positions above the single-star locus would be consistent with their belonging to this (generally younger) Cha cloud population. Given these uncertainties, we do not include these two stars in our final roster of bona fide CA members, although their status is worth revisiting in future Gaia data releases.

Of the six provisional members identified by M+13, two can be rejected outright based on their Gaia parallaxes: TYC 9414-191-1 (π = 1.56 mas) and CM Cha (π = 5.15 mas). TYC 9414-191-1 is likely a background star with proper motions similar to those of CA members, and CM Cha (like 2MASS J11432669–7804454 and RX J1220.4–7407) appears to belong to the more distant Cha cloud T Tauri star population. The other four M+13 provisional members are discussed in Section 4.1.4.

T Cha: T Cha was previously established as a kinematic member of CA (Torres et al. 2008). The kinematic analysis of T Cha is more complicated than other CA members, however, due to its variable RV; measurements range from ∼6 to ∼30 km s⁻¹ (Schisano et al. 2009). We adopt the RV measured by Guenther et al. (2007), 14.0 ± 1.3, which is near the median of the values reported by Schisano et al. (2009); with this RV, the UVW of T Cha (presented in Section 4.1.5) yield a K_kin of 3.0 ± 1.3 km s⁻¹, well within the 2σ region. Adopting a larger value for its RV (e.g., the Gaia DR2 RV, 25.52 ± 4.24) would make T Cha a kinematic outlier. However, its XYZ (52.7, −90.1, −31.4 pc) positions T Cha near the median of CA. We conclude that the Gaia DR2 astrometric data for T Cha support its membership in CA, although the variable nature of its RV still remains to be characterized (see discussion in Schisano et al. 2009).

The Gaia DR2 photometric data for T Cha (Figure 1) show it to be far redder and fainter than expected for an CA star of its spectral type and mass (G8 and 1.3 M_⊙, respectively; Schisano et al. 2009). In DR2, T Cha was flagged as having a suspect phot_bp_rp_excess_factor factor (i.e., E = 2.2), indicative of poor-quality photometry. This is perhaps due to the star's notable variable behavior: T Cha is known to exhibit variable extinction in the optical regime, as a consequence of its highly inclined, dusty disk (i = 73°;  Hendler et al. 2018). Schisano et al. (2009) found that T Cha shows a visual extinction (corrected for ISM extinction) of ∼0.5 mag on average, but with large excursions, sometimes reaching a maximum of ∼3 mag. Variable extinction in young stars with disks, such as that displayed by T Cha, has been hypothesized to result from a nested inner/outer disk structure wherein a warped inner disk causes quasi-periodic occultations of the photosphere (Alencar et al. 2010). Modeling of the disk around T Cha shows that instead, its variability could be caused by an asymmetric, puffed-up inner disk rim (Olofsson et al. 2013).

Based on its position in the Gaia CMD (Figure 3, right) and assuming a spectral type of G8 (Schisano et al. 2009)—which suggests an absolute G magnitude of 3.3, given the SLFR-derived empirical isochrone (Figure 1)—and applying a standard ISM reddening law (Cardelli et al. 1989), we infer an A_V of 6.0 mag at the time of Gaia observations. This A_V is a factor of ∼10 larger than the typical A_V reported by Schisano et al. (2009)—a surprising result, given that (as of DR2) Gaia data included nine viable visibility periods. Alternatively, if we were to accept the G − G_RP of T Cha at face value, then the star displays a color excess E(G − G_RP ) = 1.11, which would imply A_V  ∼ 3.5 assuming T Cha's obscuration follows the same (Cardelli et al. 1989) standard reddening law. This is still a factor of ∼5 larger than typical for T Cha, and is furthermore discrepant with A_V as determined from its spectral-type-based absolute magnitude. Thus, while T Cha's anomalous position in the CA CMD is likely due in part to obscuration by its disk, and suggests the disk dust exhibits non-ISM-like reddening, we caution that its position (low and red) in the CMD may also be a consequence of suspect Gaia photometry.

2MASS J12014343–7835472: This early-M star (M2.25; Luhman 2004), also known as CA 11, is notably underluminous for a star in this spectral type regime (ΔM ∼ 4 mag; Figure 1). Previous studies of CA have proposed that 2MASS J12014343–7835472 is orbited by a nearly edge-on disk (Luhman 2004), such that the cold, outer parts of the disk flare to block the stellar photospheric emission, which is then only detected in the form of scattering off the inner disk (Fang et al. 2013). The star also appears to be actively accreting disk material, given its large Hα equivalent width (Luhman 2004).

Although 2MASS J12014343–7835472 has a large K_kin value (5.7 km s⁻¹), this is mainly due to its low measured RV (11.44 ± 2.53 km s⁻¹); its Gaia DR2 proper motions are generally consistent with those of other CA members. Furthermore, its lithium absorption-line (λ6708) equivalent width is consistent with the young age of the CA (M+13). Its membership in CA is hence supported by the available data, provided its photospheric emission is indeed strongly attenuated by a nearly edge-on disk. Unlike T Cha, the $G-{G}_{{R}_{P}}$ color of 2MASS J12014343–7835472 appears consistent with its spectral type (Figure 1), suggesting that the occulting disk dust has a significant component of large grains even as the Gaia photometry is dominated by starlight scattered off the disk surface.

Provisional M+13 candidates: Two stars designated as provisional CA members in the M+13 CA study, RX J1202.8–7718 and CXOU J115908.2–781232, are upgraded here to bona fide members. RX J1202.8–7718 was identified as a kinematic member in M+13 but its status was uncertain, with the possibility remaining that it could belong to the Lower Centaurus Crux (LCC) subgroup of the Scorpius-Centaurus OB association. In our analysis, RX J1202.8–7718 has both low K_kin (0.31 km s⁻¹) and ΔM (0.39 mag), suggesting that it is unlikely to be an interloper from the LCC. Likewise, on the basis of its pre-Gaia proper motion and distance, M+13 suspected that CXOU J115908.2–781232 may be associated with the Cha I cloud. However, Gaia DR2 places the star at ∼105 pc, consistent with CA and much nearer to Earth than Cha I. Given its low K_kin (0.31 km s⁻¹) and ΔM (0.82 mag), we include CXOU J115908.2–781232 among our bona fide CA member list.

Two other M+13 provisional members, HD 105234 and HIP 59243, lack the requisite RVs to determine their UVW and, hence, kinematic offsets, and are thus also classed as provisional members in our final membership list. Therefore, of the six provisional members presented by M+13, two are rejected, two are upgraded to bona fide, and two maintain their provisional status.

Gagné & Faherty ( 2018 ) candidates: On the basis of a statistical analysis of Gaia DR2 data (utilizing BANYAN; Gagné et al. 2018c), GF18 identified a total of 15 stars that are candidate members of CA. These stars were designated as high-probability members (five stars), possible members (five), and low-probability members (five), based on BANYAN probabilities of membership in CA as well as in other young moving groups. Their XZ distribution (Figure 4, left panel) makes apparent that the GF18 high-probability members are found within the spatial locus of stars we designate as bona fide CA members—albeit outside the core CA group (centered at X ∼ 49 pc, Z ∼ −28 pc), because the GF18 study excluded stars with distances >100 pc, whereas the lower-probability members are found at the upper XZ periphery of the group.

Zoom In Zoom Out Reset image size

In contrast, there is no such clear distinction between the CMD positions of the three categories of GF18 candidates. All 15 stars lie along the single-star CMD locus of CA determined via our SLFR analysis, although most (13 of 15) lie below the empirical isochrone, by up to a magnitude (Figure 4, right panel). While this might cast doubt on their CA membership, we caution that most of the GF18 stars lie in a region of CMD space that is poorly sampled by kinematically verified CA members. Hence, the final membership status of all 15 stars is not easily assessed and must await measurements of, e.g., RVs and Li absorption-line strengths.

To assign spectral types to the provisional CA members gleaned from the GF18 study, as well as other provisional members (Section 4.1.5), we have determined an empirical relationship between spectral subtype and Gaia color for bona fide K- and M-type CA members. For this purpose, we selected those K and M stars from our final membership list (Section 4.1.5) for which spectral types were determined from optical spectroscopy and for which E(B − V) ≤ 0.05, based on data presented in M+13. The resulting empirical relationship is illustrated in Figure 5. The best-fit second-order polynomial shown in the Figure is given by

$$\begin{eqnarray}&&\mathrm{Sp}(c)=-12.78{c}^{2}+42.54c-29.26,\end{eqnarray} \tag{ 3 }$$

where Sp is the spectral index of the star, defined such that (...−1, 0, +1, ...) = (...K7, M0, M1, ...), and c is the G − G_RP color. Figure 5 demonstrates that Equation (3) provides an accuracy of roughly a subtype for K and M stars in the CA. The spectral types determined from this empirical color–subtype relationship for the provisional members from GF18 are, in most cases, 0.5–2.0 subtype later than those reported by GF18. Our determinations are likely to be more reliable, however, given that they are based on the Gaia colors of bona fide CA members of known spectral type, and the color–subtype relationship is known to be age-dependent (e.g., Pecaut & Mamajek 2013).

Zoom In Zoom Out Reset image size

Our final CA membership list, including spectral types, distances, Gaia photometry, and 2MASS photometry (Skrutskie et al. 2006), is presented in Table 3. Most CA candidates without RVs are here designated as provisional members—the exceptions being three systems lacking RVs that were classified as bona fide members by M+13, for which we find Gaia DR2-based XYZ and CMD positions that are consistent with membership. The provisional members are presented in Table 4. The latter list includes three new members revealed via a search for wide-separation companions (Table 5; see Section 4.2.2).

Table 3. Final Cha Association Membership List ^a

Name	SpT	Distance (pc) b	GBP	G	GRP	J	H	K	Multiplicity c	Disk? d
CP-68 1388	K1	${115.66}_{-0.38}^{+0.39}$	10.568	10.023	9.347	8.48	8.01	7.79	⋯	⋯
2MASS J11183572−7935548A	M4.5	${94.14}_{-1.30}^{+1.34}$	15.513	13.869	12.50	10.50	9.89	9.62	P	⋯
2MASS J11183572−7935548B*	⋯	${93.04}_{-3.35}^{+3.61}$	⋯	16.189	⋯	⋯	⋯	⋯	⋯	⋯
2MASS J11404967−7459394	M5.5	${95.68}_{-0.64}^{+0.64}$	18.833	16.429	14.983	12.68	12.15	11.77	⋯	⋯
RX J1147.7−7842	M3.5	${100.52}_{-0.39}^{+0.39}$	13.814	12.352	11.168	9.52	8.86	8.59	P	⋯
RX J1149.8−7850	M0	${100.28}_{-0.25}^{+0.25}$	13.0419	11.932	10.904	9.45	8.72	8.49	⋯	Y
RX 1150.9−7411	M3.7	${93.61}_{-1.56}^{+1.62}$	14.852	13.457	12.133	10.38	9.78	9.48	VP	⋯
2MASS J11550485−7919108	M3	${100.60}_{-0.59}^{+0.60}$	16.826	14.818	13.468	11.22	10.47	10.08	C	Y
T Cha	G8	${108.98}_{-0.97}^{+0.99}$	13.682	12.974	11.368	8.96	7.86	6.95	⋯	Y
RX J1158.5−7754B	M3	${102.92}_{-0.37}^{+0.37}$	14.629	13.202	12.031	10.34	9.72	9.44	⋯	⋯
RX J1158.5−7754A	K4	${104.47}_{-0.38}^{+0.39}$	10.688	9.980	9.191	8.22	7.56	7.40	VP	⋯
HD 104036	A7	${103.95}_{-0.41}^{+0.42}$	6.806	6.693	6.528	6.29	6.22	6.11	V	⋯
CXOU J115908.2−781232*	M4.75	${105.50}_{-0.68}^{+0.69}$	17.118	15.256	13.942	12.01	11.45	11.17	⋯	Y
Cha AaAbB	B9	111H	4.862	4.782	4.931	5.02	5.04	4.98	V	⋯
RX J1159.7−7601	K4	${99.21}_{-0.23}^{+0.23}$	11.491	10.809	10.040	9.14	8.47	8.30	⋯	⋯
HD 104237C	M/L	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯
HD 104237B	K/M	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯
HD 104237A	A7.75	${107.76}_{-0.67}^{+0.68}$	6.710	6.543	6.321	5.81	5.25	4.59	⋯	Y
HD 104237D	M3.5	${100.61}_{-0.62}^{+0.62}$	14.440	13.021	11.823	9.62	8.74	8.12	⋯	?
HD 104237E	K5.5	${101.52}_{-0.34}^{+0.35}$	12.653	11.838	10.675	≥9.10	≥8.25	7.49	P	Y
2MASS J12005517−7820296	M5.75	${102.39}_{-0.85}^{+0.87}$	17.865	15.583	14.182	11.96	11.40	11.01	S	Y
HD 104467	G3	${98.11}_{-1.56}^{+1.62}$	8.796	8.428	7.914	7.26	6.97	6.85	C	⋯
2MASS J12014343−7835472	M2.25	${104.334}_{-1.13}^{+1.16}$	18.205	17.115	15.949	14.36	13.38	12.81	⋯	Y
USNO-B 120144.7−781926	M5	${101.29}_{-0.64}^{+0.65}$	17.248	15.276	13.904	11.68	11.12	10.78	⋯	Y
CXOU J120152.8−781840	M4.75	${102.16}_{-0.70}^{+0.71}$	16.939	14.982	13.650	11.63	11.04	10.77	⋯	⋯
RX J1202.1−7853	M0	${99.36}_{-0.45}^{+0.46}$	12.547	11.510	10.504	9.215	8.46	8.31	V	⋯
RX J1202.8−7718*	M3.5	${103.52}_{-0.37}^{+0.37}$	14.795	13.375	12.199	10.51	9.83	9.59	⋯	⋯
RX J1204.6−7731	M3	${100.24}_{-0.33}^{+0.34}$	13.94	12.571	11.420	9.77	9.13	8.88	⋯	⋯
RX J1207.7−7953	M3.5	${99.30}_{-0.30}^{+0.30}$	14.793	13.340	12.154	10.43	9.76	9.57	⋯	⋯
HD 105923	G8	${105.78}_{-0.40}^{+0.41}$	9.245	8.837	8.300	7.67	7.31	7.18	VC	⋯
RX J1216.8−7753	M4	${101.27}_{-0.40}^{+0.40}$	14.236	12.921	11.789	10.09	9.47	9.24	⋯	⋯
RX J1219.7−7403	M0	${100.82}_{-0.27}^{+0.27}$	13.201	12.140	11.121	9.75	9.05	8.86	⋯	⋯
2MASS J12210499−7116493	K7	${98.92}_{-0.23}^{+0.24}$	12.044	11.203	10.316	9.09	8.42	8.24	⋯	⋯
RX J1239.4−7502	K3	${103.10}_{-0.28}^{+0.29}$	10.531	9.975	9.307	8.43	7.95	7.78	⋯	⋯
CD-69 1055	K0	${94.32}_{-0.26}^{+0.26}$	10.132	9.606	8.967	8.18	7.70	7.55	⋯	⋯
MP Mus	K1	${98.34}_{-0.30}^{+0.30}$	10.547	9.952	9.231	8.28	7.64	7.29	⋯	Y

Notes.

^a * = previously not considered a bona fide member (M+13). ^b Distances have been calculated using the inverse parallax method with zero-point corrections (Lindegren et al. 2018). ^c S = suspected spectroscopic binary (M+13); V = resolved binary (Briceño & Tokovinin 2017); P = possible photometric binary (this work); C = potential wide-separation companion (this work). ^d IR excess and/or other evidence indicative of presence of circumstellar disk (M+13 and references therein).

Download table as:  ASCIITypeset image

Table 4. Provisional Cha Association Membership List

Name	SpT a	Distance (pc) b	GBP	G	GRP	J	H	K	Multiplicity c	Disk? d
2MASS J11411722−7315369	M4.5	${95.51}_{-0.47}^{+0.47}$	17.382	15.478	14.155	12.16	11.56	11.31	⋯	⋯
2MASS J11432968−7418377	M3.5	${98.84}_{-0.38}^{+0.38}$	15.742	14.176	12.955	11.25	10.60	10.37	⋯	⋯
2MASS J11550336−7919147	M6	${101.26}_{-5.12}^{+5.70}$	20.476	19.925	18.189	15.85	15.03	≥ 12.64	⋯	Y
2MASS J12000269−7444068	M4	${98.80}_{+0.48}^{-0.48}$	15.959	14.324	13.081	11.37	10.67	10.42	⋯	⋯
2MASS J12011981−7859057	M5	${101.55}_{-0.77}^{+0.78}$	17.251	15.179	13.823	11.75	11.20	10.89	⋯	⋯
HD 105234	A9‡	${103.96}_{-0.37}^{+0.38}$	7.570	7.419	7.195	6.87	6.76	6.68	V	Y
HIP 59243	A6‡	${99.27}_{-0.32}^{+0.32}$	6.934	6.794	6.605	6.35	6.23	6.17	V	⋯
2MASS J12115619−7108143	M3	${105.28}_{-0.75}^{+0.76}$	14.530	13.021	11.828	10.09	9.51	9.24	⋯	⋯
2MASS J12203396−7135188	M4	${92.36}_{-0.48}^{+0.49}$	14.880	13.219	11.971	10.14	9.57	9.27	⋯	⋯
2MASS J12203619−7353027	M2.5	${99.40}_{-0.34}^{+0.35}$	14.485	13.141	11.998	10.41	9.71	9.48	⋯	⋯
2MASS J12220068−7001041	M4	${99.39}_{-0.47}^{+0.47}$	15.402	14.024	12.787	≥ 10.40	≥ 10.00	≥ 9.90	⋯	⋯
2MASS J12222238−7137040	M2	${94.61}_{-0.24}^{+0.25}$	13.769	12.530	11.425	9.90	9.27	8.99	⋯	⋯
2MASS J12224862−7410203	M5.5	${93.82}_{-0.78}^{+0.79}$	18.688	16.175	14.698	12.26	11.69	11.29	P	⋯
2MASS J12255824−7551116	M4.5	${98.98}_{-0.63}^{+0.64}$	17.324	15.366	14.034	12.02	11.49	11.19	⋯	⋯
2MASS J12324805−7654237	M3	${85.17}_{-6.11}^{+7.14}$	14.310	12.835	11.639	9.88	9.26	8.96	⋯	⋯
2MASS J12332483−6848553	M5	${98.86}_{-0.83}^{+0.85}$	17.829	15.712	14.344	12.28	11.68	11.37	⋯	⋯
2MASS J12351540−7043079	M5	${92.94}_{-0.56}^{+0.57}$	17.853	15.701	14.317	12.18	11.66	11.33	⋯	⋯
2MASS J12421315−6943484	M5	${98.85}_{-0.80}^{+0.82}$	17.167	15.144	13.789	11.76	11.15	10.87	⋯	⋯
2MASS J12425584−7034207	M5	${98.86}_{-0.75}^{+0.77}$	18.261	15.975	14.570	12.32	11.79	11.46	⋯	⋯
2MASS J12473611−7031135	M3	${99.35}_{-0.30}^{+0.31}$	14.628	13.269	12.119	10.50	9.86	9.71	⋯	⋯

Notes.

^a Spectral types determined. ^b Distances have been calculated using the inverse parallax method with zero-point corrections (Lindegren et al. 2018). ^c S = suspected spectroscopic binary (M+13); V = resolved binary (Briceño & Tokovinin 2017); P = possible photometric binary (this work). ^d IR excess and/or other evidence indicative of presence of circumstellar disk (M+13 and references therein).

Download table as:  ASCIITypeset image

Table 5. New Wide-separation Companions to CA Members ^a

Name	SpT	R.A.	Decl.	π	G	G − GRP	PMRA	PMDec
		(deg)	(deg)	(mas)	(mag)	(mag)	(mas yr−1)	(mas yr−1)
2MASS J11550485−7919108	M3	178.7692	−79.3198	9.886 ± 0.058	14.818	1.35	−41.18 ± 0.13	−4.34 ± 0.09
2MASS J11550336−7919147	M6	178.7628	−79.3208	9.82 ± 0.53	19.92	1.74	−39.74 ± 1.22	−4.66 ± 0.68
HD 104467	G3	180.4120	−78.9881	10.14 ± 0.17	8.428	0.515	−41.11 ± 0.26	−5.41 ± 0.26
2MASS J12011981−7859057	M5	180.3316	−78.9849	9.794 ± 0.075	15.179	1.3562	−41.99 ± 0.12	−5.35 ± 0.10
HD 105923	G8	182.9084	−71.1767	9.400 ± 0.036	8.837	0.537	−38.720 ± 0.060	−7.42 ± 0.05
2MASS J12115619−7108143	M3	182.9836	−71.1374	9.44 ± 0.68	13.021	1.1928	−38.64 ± 0.11	−8.11 ± 0.09

Note.

^a Candidate wide-separation comoving systems are listed as pairs, with the previously identified CA member listed first and its candidate wide-separation companion listed second.

Download table as:  ASCIITypeset image

For each star in Tables 3 and 4, we also indicate stellar multiplicity and the presence/absence of evidence of a circumstellar disk. Multiplicity is further subdivided into photometric binaries (P), visual binaries from Briceño & Tokovinin (2017; V), suspected spectroscopic binaries from M+13 (S), and candidate wide-separation companion systems (C). Binarity and multiplicity within the final membership list are further discussed in Section 4.2.

Table 6 lists the heliocentric space positions (XYZ) and velocities (UVW) of the final CA membership. The medians and (uncertainty-weighted) means of these spatial and kinematic coordinates, as calculated from the positions and velocities of bona fide members in Table 3, are tabulated in Table 7.

Figure 6 illustrates the individual and mean positions and velocities. Based on our final membership, the mean distance to the CA is 100.99 pc, with a standard deviation σ = 4.62 pc. The structure and spectral type distribution of the association are discussed in Section 5.2.

Zoom In Zoom Out Reset image size

On the basis of our SLFR analysis, we identify five photometric binary candidates, i.e., stars that lie above the single-star locus at positions consistent with their being double or perhaps triple systems: RX J1150.9−7411, RX J1158.5−775A, HD 104237E, RX J1147.7−7842, and 2MASS J11183572−7935548. These stars have ΔM values of −1.211, −0.670, −0.903, −0.767, and −1.038, respectively. Two of these systems, RX J1150.9−7411 and RX J1158.5−775A, have been resolved visually (separations 0875 and 0073, respectively; Köhler 2001; Briceño & Tokovinin 2017), but evidently, even the former is unresolved in Gaia DR2.

HD 104237E is already known to have a companion, HD 104237D (at separation 424;  Briceño & Tokovinin 2017), and would be a hierarchical triple system if HD 104237E is confirmed as a binary (see Section 4.2.3). The DR2-based binary candidacy of RX J1147.7−7842 is also novel. Both systems warrant spectroscopic follow-up to search for RV variability, since close binaries can have suspect Gaia DR2 astrometry (e.g., Kastner 2018). We note that Briceño & Tokovinin (2017) failed to detect companions at separations as small as ∼01 (∼10 au) for these two stars.

Briceño & Tokovinin (2017) found that the 2MASS J11183572−7935548 system (henceforth J1118AB) is a 092 separation binary consisting of an M4.5 primary and a lower-luminosity companion that may be related to the transition-disk nature of the object. The Gaia DR2 data confirm the angular separation (09) and establish that this corresponds to a projected physical separation of 85 au. The parallaxes for J1118AB agree, within the errors, confirming that they constitute a physical pair. Unfortunately, the only Gaia photometry for J1118B is in the G band, so we could not ascertain the CMD location of this companion. J1118A was flagged as a spectroscopic binary in M+13 and, via our SLFR method, we find it is also a candidate photometric binary. In light of the presence of the faint visual companion 2MASS 1118B (Briceño & Tokovinin 2017), it appears that 2MASS 1118AB is a possible hierarchical triple system.

We searched for potential wide-separation companions to all CA members retained after applying the criteria described in Section 4.1. Specifically, we searched the Gaia DR2 catalog for equidistant, comoving stars by querying the catalog for all Gaia sources within a 500'' radius of the position of each of these CA members and then reordering the resulting source list by parallax. This search radius corresponds to ∼50 kau, or ∼0.25 pc, at the mean distance to CA. While a search of CA member fields within this radius typically returns ∼2000 stars, once ordered by parallax, potential (equidistant) companions to the star originally queried rise to the top of the list and are hence conspicuous. All stars so identified have parallaxes and proper motions within a few percent of the star searched and so were accepted as wide-separation companion candidates.

Applying this method, we have identified three new candidate members of CA. Gaia DR2 data for these candidates are listed in Table 5. The faintest and reddest of these three, 2MASS J11550336−7919147, is described below. The other two comoving companion candidates, 2MASS J12011981−7859057 and 2MASS J12115619−7108143, are mid-M stars that fall along the empirical single-star isochrone, with ΔM ≤ 2σ_ΔM . These two candidate comoving companions have projected physical separations from their primaries of 5.7 kau and 17.7 kau, respectively. We estimated their spectral types (M5 and M3, respectively) from the empirical relationship between Gaia G − G_RP color and spectral type for CA members described in Section 4.1.4. Neither star displays evidence of a dusty disk in the form of an IR excess, i.e., both have 2MASS/WISE colors consistent with those of "diskless" young-M stars of similar spectral type (Pecaut & Mamajek 2013).

This comoving companion search also recovered multiple stars already considered CA members as components of possible wide binary systems. The RX J1158.5−7754 system was matched with HD 104036. Three stars, 2MASS J12005517−7820296, CXOU J120152.8−781840, and USNO-B 120144.7−781926, were identified as a three-component comoving system. We also identified CXOU J115908.2−781232, a provisional M+13 member, as another possible component of the HD 104237 multiple system (see Section 4.2.3).

2MASS J11550336−7919147: The third, newly identified wide-comoving companion, 2MASS J11550336−7919147 (henceforth 2MASS J1155−7919B), at an absolute G magnitude of ∼15 and G − G_RP  ∼ 1.75, is both the faintest and the reddest object thus far identified in CA (Dickson-Vandervelde et al. 2020). As described in Dickson-Vandervelde et al. (2020), we find 2MASS J1155−7919B is comoving with 2MASS 11550485−7919108 (hereafter 2MASS J1155−7919A). The star 2MASS J1155−7919A was itself initially thought to be a wide-separation comoving companion to T Cha Kastner et al. (2012), before Gaia DR2 data established that T Cha and J1155−7919A are neither equidistant nor precisely comoving (Kastner 2018). The 2MASS J1155−7919AB pair has a projected separation of 575, corresponding to a projected physical separation of 566 au. In Dickson-Vandervelde et al. (2020), we suggested that the position of 2MASS J1155−7919B at the extreme faint, red end of the single-star locus of the CA CMD reflected its likely status as a substellar object, with a bolometric luminosity ($\mathrm{log}{L}_{\mathrm{bol}}/{L}_{\odot }=-3.2$) that would imply its mass is a mere ∼10 M_Jup. This would make 2MASS J1155−7919B the lowest-mass CA member presently known—even less massive than WISEA J120037.79−784508.3, a recently identified brown dwarf candidate and possible CA member (Schutte et al. 2020) ⁵ However, a potential alternative model to explain the large absolute G magnitude and red color of 2MASS J1155−7919B is that the object is in fact a mid-M star—possibly a near-twin to host 2MASS J1155−7919A—that is obscured by a large column density of gray dust in a nearly edge-on disk. Under this interpretation, 2MASS J1155−7919B would be analogous to 2MASS J12014343−7835472 (=CA 11; 4.1.3), but even more highly obscured by its disk. Such an alternative explanation is motivated by the fact that 2MASS J1155−7919B is consistently 5 mag dimmer than its host in G, B_p , R_p , J, and H (Table 5), and that its spectral type, as obtained from its color (via the relationship in Figure 5), is M6. In a forthcoming paper (D. A. Dickson-Vandervelde et al. 2020, in preparation), we further explore these two possible scenarios for the nature of 2MASS J1155−7919B.

HD 104237 is a proposed quintuplet system within CA, consisting of a triple system dominated by the bright Herbig Ae/Be star HD 104237A and including the close binary HD 104237DE (Feigelson et al. 2003; Grady et al. 2004). Our wide-separation companion search (Section 4.2.2) also suggests that the previously identified CA member CXOU J115908.2−781232 is a possible additional companion to HD 104237A. We also flagged HD 104237E as a possible photometric binary, as noted in Section 4.2.1. While stellar components A, D, and E were all resolved within Gaia DR2, B and C were not, most likely being incorporated into the Gaia point-spread function of HD 104237A.

The majority of stars in the HD 104237 system lie near the spatial median of CA, the densest region of CA, and the system is well within the group's tidal radius (Section 5.2). However, the Gaia DR2 parallaxes of HD 104237D and E place these two stars 6–7 pc closer to Earth than HD 104237A. Indeed, during our wide-separation companion search, HD 104237D+E flagged each other, while HD 104237A only matched (in terms of parallax and proper motion tolerances) with the star CXOU J115908.2−781232. If HD 104237A+E is in fact a bound system, this would imply that the Gaia DR2 parallaxes for components D and E are spurious. On the other hand, as in the case of T Cha and J1155−7919A, the star originally designated T Cha B (Kastner et al. 2012; Kastner 2018), it is possible that the apparent D and E components of the HD 104237 system are in fact merely CA members that are seen in projection near the bright primary star. This would not be surprising, given the system's position near the median XYZ of CA. If the potential new components of HD 104237, CXOU J115908.2−781232 and HD 104237Eb, are, in fact, bound to HD 104237A, this could be a seven-star system. Alternatively, the bound components may in fact consist of a triple system comprising HD 104237Ea+Eb+D plus a quadruple system comprising HD 104237A+B+C and CXOU J115908.2−781232. Gaia DR3 should help resolve some of this uncertainty concerning the composition of the HD 104237 system by confirming and/or refining the DR2 parallaxes to its individual components.