2M1155−79 (= T CHAMAELEONTIS B): A LOW-MASS, WIDE-SEPARATION COMPANION TO THE NEARBY, "OLD" T TAURI STAR T CHAMAELEONTIS

To search for candidate comoving, wide-separation companions to T Cha, we made use of the VizieR⁵ table browsing and source matching utilities available in TOPCAT.⁶ We first compiled a list of all 2MASS⁷ sources within 20' of T Cha that have K magnitudes and J−K colors within the range expected for low-mass (late K or M-type) stars within ∼100 pc, i.e., K ⩽ 12 and 0.8 ⩽ J − K ⩽ 1.2. We then used the UCAC3 (Zacharias et al. 2010) PM catalog to select the subset of these 2MASS sources whose measured PMs lie within ±15 mas yr⁻¹ of each of the PM components of T Cha. This resulted in a list of six comoving companion candidates to T Cha. Of these candidates, the closest PM match to T Cha is 2MASS 11550485−7919108 (hereafter 2M1155−79); its UCAC3 PM is indistinguishable from that of T Cha, given the respective measurement uncertainties (Table 1). With an angular separation of 636, this candidate is also the closest in proximity to T Cha. The other five objects with PMs similar to T Cha (and 2M1155−79) all have RA PM components |μ_α| ⩽ 32 mas yr⁻¹; these are most likely field stars unrelated to either the Cha Association or the background Chamaeleon cloud T Tauri star population (given that three of the five are undetected in X-rays; see Section 2.2 and Terranegra et al. 1999).

Low-mass pre-MS stars are luminous X-ray sources, with typical X-ray luminosities (relative to bolometric) in the range L_X/L_bol  ∼ 10⁻⁴–10⁻³ (e.g., Kastner et al. 1997). Hence, to confirm that 2M1155−79 is young, we examined an XMM-Newton observation, obtained on UT 2009 March 16, that targeted (and detected) T Cha (Güdel et al. 2010). The observation duration was 11.5 ks; the effective exposure times with the European Photon Imaging Camera (EPIC) pn, MOS1, and MOS2 detectors were 3.3, 10.6, and 10.2 ks, respectively (the useful pn exposure time being limited due to intervals of high background). The merged XMM/EPIC 0.5–2.0 keV image is displayed in Figure 1, overlaid with the positions and PM vectors of T Cha and the half-dozen bright, red 2MASS stars that have similar UCAC3 PMs (selected as described in Section 2.1). Of these seven stars with similar PMs, only T Cha and 2M1155−79 are detected as X-ray sources (note, however, that two of these PM-selected stars do not fall within the XMM field of view).

Zoom In Zoom Out Reset image size

The two other luminous X-ray sources in the field, which have UCAC3 catalog PMs that are less than half those of T Cha and 2M1155−79, correspond to 2MASS 11583429−7913175 (= RXJ 1158.5−7913; hereafter RXJ1158.5) and 2MASS 11581646−7931082 (hereafter 2M1158−79). The former is a weak-lined T Tauri star (wTTS) of spectral type K3, previously identified by Alcala et al. (1995) as being associated with the Chamaeleon cloud complex. The Cha clouds and their associated TTS population lie ∼160 pc from Earth (Whittet et al. 1997)—i.e., ∼60 pc behind the Cha Association—consistent with the small PM of RXJ1158.5 relative to Cha member T Cha (Terranegra et al. 1999). Meanwhile, 2M1158−79 has no previous references listed in SIMBAD.⁸ Given its JHK magnitudes and colors relative to those of RXJ1158.5 (Table 1), and the similarity of their PMs, 2M1158−79 is therefore most likely a previously unidentified, lower-mass (early M) member of the Cha cloud wTTS population. PMs are unavailable for potential optical counterparts to the other, weaker X-ray sources in Figure 1.

We used the XMM Scientific Analysis System (SAS⁹ version 10.0.0) to extract pn, MOS1, and MOS2 CCD spectra and responses for the three bright sources that surround T Cha in the EPIC image (spectral fitting results for T Cha itself were presented in Güdel et al. 2010). Calibrations were performed using the current calibration files from release note 271, 2010 December 21. Background-subtracted total counts and count rates for each source, as obtained from the pn, MOS1, and MOS2 spectral extractions, are listed in Table 1 (the anomalously low pn count rate of 2M1155−79 is the result of its position on a bad row of that detector). We summarize the results of simultaneous fits of an absorbed single-component thermal plasma emission model to the three (pn, MOS1, MOS2) EPIC spectra of 2M1155−79 and 2M1158−79 and an absorbed two-component plasma model to the EPIC spectra of RXJ1158.5 in Table 1, and we display the spectra overlaid with these best-fit models in Figure 2 (a second, lower-temperature component is necessary to model the low-energy regions of the EPIC spectra of RXJ1158.5). The X-ray properties of 2M1155−79 inferred from spectral fitting are similar to those of the background (Cha cloud) wTTS RXJ1158.5 (and are typical of wTTS more generally), consistent with a wTTS classification for 2M1155−79 (Section 3.2). The best-fit model for 2M1158−79 (whose EPIC spectra are evidently somewhat harder than those of 2M1155−79 and RXJ1158.5; Figure 2) indicates a high plasma temperature and very large L_X/L_bol ratio, suggesting it may have been undergoing a strong flare during the short exposure targeting T Cha.

Zoom In Zoom Out Reset image size

Spectra of T Cha and 2M1155−79 covering the Hα and Li i λ6708 spectral region at resolution ∼7000 were obtained with the Australian National University's Siding Spring Observatory (SSO) 2.3 m telescope and WiFeS spectrometer (Dopita et al. 2007) on 2011 October 12 (Figure 3). Exposure times were 2 × 600 s and 300 s for 2M1155−79 and T Cha, respectively, and the stars were observed at air mass ∼2.5. Spectra were reduced as described in Riedel et al. (2011); absolute flux calibration was not performed. In the resulting, normalized spectra, both stars display Hα strongly in emission and show strong Li λ6708 absorption lines (Hα and Li λ6708 line equivalent widths are reported in Table 1). The photospheric absorption features in the spectrum of 2M1155−79 are evidently a close match to those of the M3 field dwarf GJ 752A = HD 180617 (Henry et al. 1994); see Figure 3 (left). Hence, we adopt M3 as the spectral type of 2M1155−79 for purposes of the discussion in Section 3.

Zoom In Zoom Out Reset image size

Li is rapidly depleted in low-mass pre-MS stars, the λ6708 photospheric absorption line becoming difficult to detect in nearby, young mid-M stars (even in high-resolution spectra) by the time such stars are of age > 10 Myr (Yee & Jensen 2010). Hence, the detection of Li absorption in the SSO WiFeS spectrum of 2M1155−79 (Section 2.3), combined with its strong X-ray emission (Section 2.2) and Hα emission (Section 2.2), confirms that this star is young. More specifically, the large λ6708 Li i EW we measure in the spectrum of 2M1155−79 is comparable to those of mid-M stars in the ∼8 Myr old TW Hya Association (TWA; Rodriguez et al. 2011) and the ∼7 Myr old Cha group (even though 2M1155−79 is among the lowest-mass Cha members known; Torres et al. 2008). In contrast, its Li absorption line EW is a factor at least ∼3 larger than those of M stars (of all spectral subtypes) in the ∼12 Myr old β Pic Moving Group (Yee & Jensen 2010). Hence, its Li absorption line strength alone places an upper limit of ∼10 Myr on the age of 2M1155−79.

Adopting the data and methods in Kenyon & Hartmann (1995), the 2MASS magnitudes and M3 spectral type of 2M1155−79 suggest an effective temperature of 3400 ± 100 K (where the error corresponds to an uncertainty of one spectral subclass) and—assuming 2M1155−79, like T Cha, lies at D = 109 pc (Torres et al. 2008) but, unlike T Cha, suffers negligible reddening (given its low N_H; see below)—a bolometric luminosity log L_bol/L_☉ = −1.16. The pre-MS evolutionary tracks of Siess et al. (2000), which are appropriate given the age and mass range of interest here (e.g., Lawson & Feigelson 2001), then indicate an age ∼ 10 Myr and mass just under ∼0.3 M_☉ (Figure 4). The former is consistent with the upper limit on the age of 2M1155−79 imposed by its strong Li absorption. In light of their similar PMs (Section 2.1; see also Terranegra et al. 1999) and the fact that (adopting D = 109 pc) the two stars fall near the same (age 10 Myr) theoretical isochrone in Figure 4—an isochrone that is, in turn, very similar to the estimated (7 Myr) age of the Cha group with which T Cha is kinematically associated (Torres et al. 2008, and references therein)—it appears that T Cha and 2M1155−79 are equidistant, coeval, and comoving. The latter star is therefore most likely a newly identified member of the Cha Association.

Zoom In Zoom Out Reset image size

Furthermore, given the low surface density of known members of the Cha group (∼0.18 deg⁻²; Torres et al. 2008), the Poisson probability P that T Cha and 2M1155−79 represent a chance alignment of two (otherwise unrelated) Cha Association stars within the (∼0.2 deg²) XMM/EPIC field is vanishingly small (P ≈ 6 × 10⁻⁴). We therefore assert that these two stars very likely comprise a very wide (projected separation ∼38 kAU) binary system; hence, hereafter, we refer to 2M1155−79 as T Cha B.

The Hα emission line strength of T Cha displayed in our spectrum (Table 1 and Figure 3, center) lies near the middle of the (wide) range previously measured for this star, and is only marginally consistent with "classical" (as opposed to weak-lined) T Tauri star status (the T Cha Hα line has also occasionally been observed in absorption; Torres et al. 2008; Schisano et al. 2009). The Hα line width at 10% of peak intensity at the time of our observations, W₁₀  = 475 km s⁻¹, would place T Cha among the weakly accreting T Tauri stars—with an inferred accretion rate of dM/dt ∼ 3 × 10⁻⁹ M_☉ yr⁻¹—given the empirical relationship between dM/dt and W₁₀ described in Natta et al. (2004).

In contrast, we measure W₁₀  = 149 km s⁻¹ for T Cha B (Figure 3, right), suggesting dM/dt < 10⁻¹¹ M_☉ yr⁻¹—consistent with the wTTS classification for this star one would determine from its Hα emission-line EW (Table 1; e.g., Barrado y Navascués & Martín 2003). Furthermore, whereas T Cha A displays a strong mid- to far-infrared excess in 2MASS and Wide-field Infrared Survey Explorer¹⁰ data (e.g., K − W3 ∼ 2.3, compared with K − W3 ∼ 0.5 for the wTTS RXJ1158.5; Table 1) and is evidently subject to varying degrees of obscuration by its dusty circumstellar disk (Schisano et al. 2009), T Cha B displays only a modest mid-infrared excess (K − W3 ∼ 0.8)—although the star evidently has a substantial ∼20 μm flux excess (as does the background wTTS 2M1158−79; Table 1), suggesting a significant mass of cool circumstellar dust. The presence of disk gas around T Cha A—and the apparent absence of such intervening disk gas, along the line of sight to T Cha B—is also evident in the large contrast between their respective X-ray absorbing columns (Table 1).