GSC 07396–00759 = V4046 Sgr C[D]: A WIDE-SEPARATION COMPANION TO THE CLOSE T TAURI BINARY SYSTEM V4046 Sgr AB

The UCAC3 catalog (Zacharias et al. 2010) proper motions (PMs) listed for V4046 Sgr and GSC0739 are (μ_α, μ_δ) = (+ 3.3 ± 1.7, −52.0 ± 1.3) mas yr⁻¹ and (+1.1 ± 2.8, −40.0 ± 13.6) mas yr⁻¹, respectively, indicating that (given the UCAC3 measurement errors) the PMs of the two systems are indistinguishable.¹⁴ The heliocentric radial velocities (V_helio) of the two systems are also very similar: Torres et al. (2006) list V_helio = −5.7 km s⁻¹ for GSC0739, and CO radio line measurements yield V_helio = −6.2 ± 0.2 km s⁻¹ for V4046 Sgr AB (Kastner et al. 2008b; Rodriguez et al. 2010).¹⁵

By matching Kurucz model atmospheres (Kurucz 1993) appropriate for late-type main-sequence (MS) stars (i.e., log g = 4.5) to optical/near-IR photometry available in SIMBAD¹⁶ for GSC0739 (Table 1), we deduce a photospheric effective temperature of 3500 ± 100 K, consistent with the M1–1.5 spectral type previously determined for GSC0739 (Torres et al. 2006; Riaz et al. 2006). From the Kurucz model normalization, and assuming the distance to GSC0739 is 73 pc (i.e., GSC0739 and V4046 Sgr are equidistant), we determine a bolometric luminosity L_bol = 0.15 L_☉ (with an approximate uncertainty, dominated by the uncertainty in distance, of ∼25%). This estimate is consistent with that obtained from the J magnitude and V−K color of GSC0739, L_bol = 0.17 L_☉, following the method described in Kenyon & Hartmann (1995).

We used the foregoing results for GSC0739 and adopted the luminosity and temperature results reported for the individual components of V4046 Sgr AB by Donati et al. (2011) to place these systems on theoretical pre-MS tracks (Figure 1). The comparison illustrates that the two systems can be both equidistant and coeval only if GSC0739, like V4046 Sgr AB, consists of two components with nearly equal luminosities. Indeed, GSC0739 is flagged as a possible spectroscopic binary by Torres et al. (2006). The positions of the GSC0739 and V4046 Sgr AB systems with respect to the overlaid model evolutionary tracks indicate ages between ∼10 and ∼20 Myr, if GSC0739 is a binary. This is consistent with independent (kinematic and pre-MS evolutionary track) age estimates of ∼12 Myr for the βPMG (see review in Zuckerman & Song 2004). On the other hand, if GSC0739 is single, then Figure 1 indicates that its age is in the range ∼3–10 Myr, i.e., somewhat young for a βPMG member.

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An off-axis Chandra/HETGS gratings spectrum of GSC0739 (total exposure 144.6 ks; ObsIDs 5422, 6265; PI: Herczeg) was serendipitously obtained in 2005 August during an observation targeting V4046 Sgr (Günther et al. 2006). The GSC0739 X-ray spectral data are uncontaminated by V4046 Sgr AB, but the spectral resolution and the exposure of the GSC0739 source are somewhat compromised by its (∼3') off-axis position. We extracted and combined the medium-energy grating (MEG) and high-energy grating (HEG) spectra for the full ∼145 ks exposure duration and generated a light curve covering the first (longer) of the two exposure segments; the resulting light curve and spectrum are displayed in Figure 2. The total (HEG+MEG) count rate, averaged over the entire 144.6 ks exposure duration, was 0.014 counts s⁻¹.

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XMM/European Photon Imaging Camera (EPIC) data were serendipitously obtained for GSC0739 in 2009 September, again during an observation (three exposures of ∼100 ks) targeting V4046 Sgr (Argiroffi et al. 2011). As the source fell along a bad column of EPIC's pn detector, we only analyzed data from the two metal oxide semiconductor (MOS) detectors (the source also fell near the edge of the active area of MOS1, but these data were only slightly compromised). We used the XMM Scientific Analysis System (SAS¹⁷ version 10.0.0) to extract MOS1 and MOS2 light curves for the full (∼5 day) duration of the 3 × 100 ks exposure (top panel of Figure 3) as well as MOS1 and MOS2 CCD spectra and responses for each individual ∼100 ks exposure interval (bottom panels of Figure 3). Calibrations were performed using the current calibration files from release note 271, 2010 December 21. Combined MOS1+MOS2 count rates are listed in Table 2.

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The Chandra and XMM light curves reveal strong X-ray flaring at GSC0739. An impulsive flare with amplitude ∼10 times the quiescent flux level and exponential decay is observed starting at about 65 ks into the first ∼100 ks segment of the Chandra observation (Figure 2, top panel). A flare of similar shape (with amplitude ∼4 times quiescent) is observed near the beginning of the overall ∼5 day XMM exposure and its decay appears to last the entire duration of the first ∼100 ks exposure segment; a smaller flare and several small count rate spikes are also seen in the second and third XMM exposure segments (Figure 3, top).

Table 2 and Figure 3 (bottom panels) summarize the results of fits of variable-abundance (see below) absorbed two-component thermal plasma models to spectra extracted from the three ∼100 ks XMM/EPIC (MOS) exposure intervals. A similar model fit to Chandra/HETGS data (not shown) yields similar results—i.e., a prominent hard component with T_X ∼ 20 MK and a weaker soft component with T_X ∼ 5–7 MK—despite the different instrumentation; we conclude that, overall, the plasma physical conditions did not change markedly between 2005 and 2009. Nevertheless, the strong short-term variability of the source is apparent in the XMM fit results (Table 2), which indicate a steady decline in the emission measure of the hard component and, possibly, a slight decrease in T_X as the observation progressed and the strong flare faded (the source may have suffered from photon pileup during the first exposure interval).

Individual line intensities in the Chandra/HETGS data are not particularly well fit by the foregoing simple (two-component) model, as expected given the continuum of plasma temperatures that is likely present in the X-ray-emitting region. Hence, we performed global fitting of a broken power-law emission measure distribution model (using isis¹⁸ and ATOMDB v2.0¹⁹), such as that used by Rosner et al. (1978) or Peres et al. (2001) for modeling coronal loops, to the gratings data. The model is specified by an emission measure normalization, the peak temperature (T_peak), the power-law slopes below (α) and above (β) the peak temperature, and the elemental abundances. The interstellar absorption was assumed to be negligible (a safe assumption given the very modest absorption determined from the fits to XMM data; Table 2). To account for the instrumental off-axis line spread function, we used a broadening equivalent to a Gaussian turbulent velocity of 600 km s⁻¹. The resulting best-fit model is overlaid on the Chandra/HETGS spectrum in Figure 2 (bottom panel); the best-fit model parameters are T_peak = 11.2 MK (0.97 keV), α = 0.2, β = −1.8, and an emission measure of 3.3 × 10⁵² cm⁻³. The best-fit abundance ratios (relative to solar) are Ne/O = 1.8, Mg/O = 0.27, Si/O = 0.6, and Fe/O = 0.27 (the two-component plasma model fits to the XMM/MOS data also indicate an elevated Ne/O ratio and depressed Fe/O ratio). These Ne/O and Fe/O ratios are typical of coronally active stars (see review in Testa 2010). Analysis of X-ray gratings spectra of V4046 Sgr AB likewise indicates enhanced Ne/O and low Fe/O, although the Ne/O enhancement is more extreme (a factor ∼5 larger than solar; Günther et al. 2006) in the case of this (actively accreting) system.

Although the resolution of the Chandra/HETGS spectrum of GSC0739 suffers from degradation due to the off-axis position of the source, it is apparent that the forbidden to intercombination line ratio within the He-like Ne ix line complex (at ∼13.5 Å) is large (Figure 2, inset in the bottom panel), indicative of lower-density (≲10¹⁰ cm⁻³) plasma. Figure 2 (bottom panel) also illustrates the relatively large Ne x to Ne ix line ratios of GSC0739, reflecting the dominance of relatively hot (≳10 MK) plasma in its emission measure distribution (Table 2). These density- and temperature-sensitive Ne line ratios are consistent with coronal X-ray emission and, hence, a weak-lined T Tauri classification for this source (see, e.g., Kastner et al. 2004; Huenemoerder et al. 2007). Both Ne line spectral diagnostics also stand in stark contrast to those of V4046 Sgr AB; the latter displays Ne (and O) line ratios indicative of a significant emission contribution from cooler (∼3 MK) plasma at much higher density (∼10¹² cm⁻³; Günther et al. 2006; Argiroffi et al. 2011), as expected if much of the X-ray emission (essentially, all of the soft component) is generated in accretion shocks (Sacco et al. 2010 and references therein).

Based on its lack of Hα emission (Hα equivalent width of +3.1 Å; Riaz et al. 2006), V4046 Sgr C[D] appears to be best classified as a weak-lined T Tauri system (wTTS); i.e., unlike V4046 Sgr AB, the system shows no evidence for ongoing accretion. The prominent hard (T_X ∼ 10⁷ K) X-ray spectral component, strong X-ray flaring, and relatively low-density X-ray-emitting plasma of V4046 Sgr C[D] (Section 2.3)—all of which are indicative of a high level of coronal activity—are fully consistent with such a wTTS classification.

If the V4046 Sgr AB and C[D] systems are indeed bound, they are only weakly so, given their very large projected separation of ∼12,350 AU (assuming D = 73 pc). Assuming the systems are still bound, the orbital period of V4046 Sgr C[D] with respect to V4046 Sgr AB would be of order ∼10⁶ yr, i.e., a significant fraction of their estimated ages. This system would therefore appear to be the most loosely bound hierarchical binary system known in the βPMG. However, two similarly loosely bound binaries have been identified thus far in the TWA: TWA 11AB and TWA 11C, projected separation ∼13,500 AU (Kastner et al. 2008a), and TW Hya (TWA 1) and TWA 28, projected separation ∼41,000 AU (Teixeira et al. 2008). The separations of these nearby, young hierarchical multiple systems are comparable to those of the most loosely bound binaries in the solar neighborhood (the separation distribution of nearby field binaries appears to be truncated at ∼20,000 AU; Close et al. 1990). The (very low mass) TWA 30A and 30B star/disk systems, which are separated on the sky by ∼3400 AU (Looper et al. 2010), are also very weakly bound.

However, the analysis in Section 2.2 strongly implies that V4046 Sgr AB and the putative V4046 Sgr C[D] could only be coeval if the latter, like the former, is a nearly equal component binary—consistent with previous suspicions concerning this system (Torres et al. 2006). Furthermore, if the two systems indeed formed together, then the position of V4046 Sgr C[D] in Figure 1 appears to place a conservative lower limit of ∼8 Myr on the age of the V4046 Sgr multiple system, while—on the basis of this comparison of V4046 Sgr C[D] with pre-MS tracks alone, and given the present uncertainties in the stellar parameters—the system may be as old as ∼25 Myr.

Numerical simulations of hierarchical triple star systems demonstrate that the action of Kozai oscillations, in combination with tidal friction, shrinks and circularizes the orbit of the inner binary (Fabrycky & Tremaine 2007). Although Fabrycky & Tremaine (2007) find that the fraction of circularized, close (few-day period) binaries with tertiaries as distant as V4046 Sgr C[D] should be negligibly small, they also note that ∼40% of hierarchical triple systems are "lost" from their numerical sample due to dissolution. Perhaps V4046 Sgr AB and C[D] constitute such a (recently dissolved) hierarchical multiple system that, at a slightly earlier epoch, resembled closer, young hierarchical binaries (such as the ∼8 Myr old TWA systems HD 98800 and Hen 3-600). If so, the dissolution of V4046 Sgr AB and C[D] evidently occurred ∼10⁶ yr ago and left behind a close binary with a nearly circular orbit (V4046 Sgr AB has an eccentricity e < 0.01; Stempels & Gahm 2004).

Given that V4046 Sgr has apparently well outlived the "canonical" few-Myr timescale for Jovian planet formation (e.g., Currie et al. 2009 and references therein), one might further speculate that the presence of V4046 Sgr C[D] may explain the longevity of the circumbinary disk around V4046 Sgr AB—just as the hierarchical natures of the young multiple systems HD 98800 and Hen 3-600 may explain the persistence and observed properties of the circumbinary dust disk in each of these systems (see, e.g., Prato et al. 2001; Furlan et al. 2007; Andrews et al. 2010, and references therein). Although the details in all of these examples remain to be worked out, in the specific case of V4046 Sgr it is possible that dynamical interactions with component(s) C[D] may have inhibited the formation of gas giant and ice giant planets in circumbinary orbits around components AB, thereby preserving the gaseous, circumbinary disk from which this close binary system is still accreting.