GSC 07396-00759 = V4046 Sgr C[D]: a Wide-separation Companion to the Close T Tauri Binary System V4046 Sgr AB
J. H. Kastner, G. G. Sacco, R. Montez Jr., D. P. Huenemoerder, H. Shi, E. Alecian, C. Argiroffi, M. Audard, J. Bouvier, F. Damiani, J.-F. Donati, S. G. Gregory, M. Güdel, G. A. J. Hussain, A. Maggio, T. Montmerle
aa r X i v : . [ a s t r o - ph . S R ] S e p GSC 07396–00759 = V4046 Sgr C[D]: a Wide-separationCompanion to the Close T Tauri Binary System V4046 Sgr AB
J. H. Kastner , G. G. Sacco , R. Montez Jr. , D. P. Huenemoerder , H. Shi , E. Alecian ,C. Argiroffi , , M. Audard , , J. Bouvier , F. Damiani , J.-F. Donati , S. G. Gregory , M.G¨udel , G. A. J. Hussain , A. Maggio , T. Montmerle ABSTRACT
We explore the possibility that GSC 07396–00759 (spectral type M1e) is awidely separated ( ∼ ′ , or projected separation ∼ ∼
12 Myr) classical T Tauri binary system V4046 Sgr AB, assuggested by the proximity and similar space motions of the two systems. If thetwo systems are equidistant and coeval, then GSC 07396–00759, like V4046 SgrAB, must be a spectroscopic binary with nearly equal-mass components, andV4046 Sgr must be at least ∼ Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester NY14623 USA ([email protected]) MIT, Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA02139, USA Observatoire de Paris, LESIA, 5, place Jules Janssen, F-92195 Meudon Principal Cedex, France Dip. di Fisica, Univ. di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy ISDC Data Center for Astrophysics, University of Geneva, Ch. d’Ecogia 16, CH-1290 Versoix, Switzer-land Observatoire de Gen´eve, University of Geneva, Ch. des Maillettes 51, 1290 Versoix, Switzerland UJF-Grenoble 1 / CNRS-INSU, Institut de Plan´etologie et d’Astrophysique de Grenoble (IPAG) UMR5274, Grenoble, F-38041, France IRAP-UMR 5277, CNRS & Univ. de Toulouse, 14 Av. E. Belin, F-31400 Toulouse, France California Institute of Technology, MC 249-17, Pasadena, CA 91125 USA University of Vienna, Department of Astronomy, T¨urkenschanzstrasse 17, 1180 Vienna, Austria ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei M¨unchen, Germany Institut d’Astrophysique de Paris, 98bis bd Arago, FR 75014 Paris, France T X ∼ K) X-ray spectrum, strongflaring, and relatively low-density plasma. These X-ray characteristics of GCS07396–00759 are indicative of a high level of coronal activity, consistent with itsapparent weak-lined T Tauri star status. Interactions between V4046 Sgr ABand GCS 07396–00759 when the two systems were more closely bound may beresponsible for (a) their dissolution ∼ yr ago, (b) the present tight, circularorbit of V4046 Sgr AB, and (c) the persistence of the gaseous circumbinary diskstill orbiting V4046 Sgr AB.
1. Introduction
Thanks to their proximity, individual members of the various nearby ( D < ∼
100 pc), young(age ∼ ∼
15 yr (for a recent review, seeZuckerman et al. 2011) provide readily accessible examples of, among other things, the lateevolution of protoplanetary disks (e.g., Akeson et al. 2011; Hughes et al. 2011, and referencestherein) and the early stages of evolution of exoplanetary systems (Marois et al. 2008) andhierarchical binary star systems (Kastner et al. 2008a). In their tests of a method to identifysuch comoving groups of young stars, Torres et al. (2006) established as a candidate memberof the β Pic Moving Group ( β PMG) the close binary classical T Tauri system V4046 Sgr( P ∼ . β PMG is estimated to be ∼
12 Myr (Zuckerman & Song 2004, and referencestherein), the twin members of this fascinating, short-period binary are both evidently stillaccreting (Stempels & Gahm 2004) from a relatively large and massive circumbinary disk ofgas and dust (Kastner et al. 2008b; Rodriguez et al. 2010).Torres et al. (2006, 2008) also identified, as another β PMG candidate, the star GSC07396–00759 (hereafter GSC0739, spectral type M1e or M1.5e; Riaz et al. 2006), on the basisof its proximity to V4046 Sgr (separation 2 . ′ ) and the similar radial velocities and simi-larly large photospheric Li abundances of the two systems (the Li λ β PMG members, it is reasonable to hypoth-esize that GSC0739 is a distant (projected separation ∼ β ∼
10% of the ( ∼
10 Myr) ages of these systems. Hence these objectsmay offer unique insight into the orbital evolution (and especially the dissolution) of younghierarchical binaries and, perhaps, the likelihood of, and conditions necessary for, formationof planets with circumbinary vs. circumstellar orbits.Here, we briefly evaluate the available data concerning the space motions of V4046 Sgrand GSC0730, and we analyze optical/IR photometry and serendipitous X-ray observationsof GSC0739 (including new XMM-Newton X-ray data obtained during the course of a coor-dinated observing campaign targeting V4046 Sgr; Argiroffi et al. 2011). We use the resultsto assess the likelihood that GSC0739 is indeed a distant companion to V4046 Sgr and tobetter ascertain the nature of the former. Finally, we consider the implications if the twosystems are (or were) physically bound.
2. Data, Analysis, and Results2.1. Proper motions and radial velocities
The UCAC3 catalog (Zacharias et al. 2010) proper motions (PMs) listed for V4046 Sgrand GSC0739 are ( µ α , µ δ ) = (+3 . ± . , − . ± .
3) mas yr − and (+1 . ± . , − . ± . − , respectively, indicating that (given the UCAC3 measurement errors) the PMs ofthe two systems are indistinguishable . The heliocentric radial velocities ( V helio ) of the twosystems are also very similar: Torres et al. (2006) list V helio = − . − for GSC0739, andCO radio line measurements yield V helio = − . ± . − for V4046 Sgr AB (Kastner et al.2008b; Rodriguez et al. 2010) . The identical PMs listed for the two systems by Torres et al. (2006, their Table 6) both correspond tothe Tycho catalog PM of V4046 Sgr AB, i.e., (+2 . ± . , − . ± .
3) mas yr − ; there is no Tycho cataloglisting for GSC0739. Recent results from high-resolution optical spectroscopy indicate that the systemic velocity of V4046Sgr AB may vary by ∼ . − (Donati et al. 2011). By matching Kurucz model atmospheres (Kurucz 1993) appropriate for late-type mainsequence stars (i.e., log g = 4 .
5) to optical/near-IR photometry available in SIMBAD forGSC0739 (Table 1), we deduce a photospheric effective temperature 3500 ±
100 K, consis-tent 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 toGSC0739 is 73 pc (i.e., that GSC0739 snd V4046 Sgr are equidistant), we determine abolometric luminosity L bol = 0 . L ⊙ (with an approximate uncertainty, dominated by theuncertainty in distance, of ∼ L bol = 0 . L ⊙ , following the method described inKenyon & Hartmann (1995).We used the foregoing results for GSC0739 and adopted the luminosity and temperatureresults reported for the individual components of V4046 Sgr AB by Donati et al. (2011) toplace these systems on theoretical pre-MS tracks (Fig. 1). The comparison illustrates thatthe 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 asa possible spectroscopic binary by Torres et al. (2006). The positions of the GSC0739 andV4046 Sgr AB systems with respect to the overlaid model evolutionary tracks indicate agesof 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 β PMG (seereview in Zuckerman & Song 2004). On the other hand, if GSC0739 is single, then Fig. 1indicates that its age is in the range ∼ β PMG member.
An off-axis Chandra/HETGS gratings spectrum of GSC0739 (total exposure 144.6 ks;OBSIDs 5422, 6265; PI: Herczeg) was serendipitously obtained in 2005 Aug. during anobservation targeting V4046 Sgr (G¨unther et al. 2006). The GSC0739 X-ray spectral dataare uncontaminated by V4046 Sgr AB, but the spectral resolution and the exposure of theGSC0739 source are somewhat compromised by its ( ∼ ′ ) off-axis position. We extracted andcombined the medium-energy gratings (MEG) and high-energy gratings (HEG) spectra forthe full ∼
145 ks exposure duration and generated a light curve covering the first (longer) ofthe two exposure segments; the resulting light curve and spectrum are displayed in Fig. 2. The SIMBAD database is maintained and operated at CDS, Strasbourg, France. .
014 counts s − .XMM European Photon Imaging Camera (EPIC) data were serendipitously obtainedfor GSC0739 in 2009 Sept., again during an observation (3 exposures of ∼
100 ks) targetingV4046 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 MOS detectors (the source also fell near the edge of theactive area of MOS1, but these data were only slightly compromised). We used the XMMScientific Analysis System (SAS version 10.0.0) to extract MOS1 and MOS2 light curvesfor the full ( ∼ ×
100 ks exposure (top panel of Fig. 3) as well asMOS1 and MOS2 CCD spectra and responses for each individual ∼
100 ks exposure interval(bottom panels of Fig. 3). Calibrations were performed using the current calibration files(CCF) from release note 271, 21-Dec-2010. Combined MOS1+MOS2 count rates are listedin Table 2.The Chandra and XMM light curves reveal strong X-ray flaring at GSC0739. An impul-sive flare with amplitude ∼
10 times the quiescent flux level and exponential decay is observedstarting at about 65 ks into the first ∼
100 ks segment of the Chandra observation (Fig. 2,top panel). A flare of similar shape (with amplitude ∼ ∼ ∼
100 ks exposure segment; a smaller flare (and several small count ratespikes) are also seen in the second and third XMM exposure segments (Fig. 3, top).Table 2 and Fig. 3 (bottom panels) summarize the results of fits of variable-abundance(see below) absorbed two-component thermal plasma models to spectra extracted fromthe three ∼
100 ks XMM/EPIC (MOS) exposure intervals. A similar model fit to Chan-dra/HETGS data (not shown) yields similar results — i.e., a prominent hard componentwith T X ∼
20 MK, and a weaker soft component with T X ∼ T X as theobservation progressed and the strong flare faded (the source may have suffered from photonpileup during the first exposure interval).Individual line intensities in the Chandra/HETGS data are not particularly well fit bythe foregoing simple (two-component) model, as expected given the continuum of plasmatemperatures that is likely present in the X-ray-emitting region. Hence, we performed global http://xmm.esa.int/sas and ATOMDBv2.0 ), such as 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 peaktemperature ( 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 (asafe assumption given the very modest absorption determined from the fits to XMM data;Table 2). To account for the instrumental off-axis PSF, we used a broadening equivalentto a Gaussian turbulent velocity of 600 km s − . The resulting best fit model is overlaid onthe Chandra/HETGS spectum in Fig. 2 (bottom panel); the best-fit model parameters are T peak = 11 . .
97 keV), α = 0 . β = − .
8, and an emission measure of 3 . × cm − .The best-fit abundance ratios (relative to solar) are Ne/O = 1 .
8, Mg/O = 0 .
27, Si/O = 0 . .
27 (the two-component plasma model fits to the XMM/MOS data also indicatean elevated Ne/O ratio and depressed Fe/O ratio). These Ne/O and Fe/O ratios are typical ofcoronally active stars (see review in Testa 2010). Analysis of X-ray gratings spectra of V4046Sgr AB likewise indicates enhanced Ne/O and low Fe/O, although the Ne/O enhancement ismore extreme (a factor ∼ ix line complex (at ∼ < ∼ cm − ) plasma. Fig. 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 dis-tribution (Table 2). These density- and temperature-sensitive Ne line ratios are consistentwith 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 diagnosticsalso stand in stark contrast to those of V4046 Sgr AB; the latter displays Ne (and O) lineratios indicative of a significant emission contribution from cooler ( ∼ ∼ cm − ; G¨unther et al. 2006, Argiroffi et al. 2011), as expected if muchof the X-ray emission (essentially, all of the soft component) is generated in accretion shocks(Sacco et al. 2010, and references therein). http://space.mit.edu/cxc/isis/
3. Discussion and Conclusions
The available proper motion and radial velocity data ( § § Based on its lack of H α emission (H α equivalent width of +3.1 ˚A; 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 prominenthard ( T X ∼ K) X-ray spectral component, strong X-ray flaring, and relatively low-densityX-ray-emitting plasma of V4046 Sgr C[D] ( § If the V4046 Sgr AB and C[D] systems are indeed bound, they are only weakly so, giventheir very large projected separation of ∼ D = 73 pc). Assuming thesystems are still bound, the orbital period of V4046 Sgr C[D] with respect to V4046 Sgr ABwould be of order ∼ yr, i.e., a significant fraction of their estimated ages. This systemwould therefore appear to be the most loosely bound hierarchical binary system known inthe β PMG. However, two similarly loosely bound binaries have been identified thus far in theTWA: TWA 11AB and TWA 11C, projected separation ∼ ∼ ∼ ∼ § ∼ ∼
25 Myr.Numerical simulations of hierarchical triple star systems demonstrate that the actionof Kozai oscillations, in combination with tidal friction, shrinks and circularizes the orbit ofthe inner binary (Fabrycky & Tremaine 2007). Although Fabrycky & Tremaine (2007) findthat the fraction of circularized, close (few-day period) binaries with tertiaries as distant asV4046 Sgr C[D] should be negligibly small, they also note that ∼
40% of hierarchical triplesystems are “lost” from their numerical sample, due to dissolution. Perhaps V4046 SgrAB and C[D] constitute such a (recently dissolved) hierarchical multiple system that, at aslightly earlier epoch, resembled closer, young hierarchical binaries (such as the ∼ ∼ yr ago and left behind a close binary with a nearly circular orbit(V4046 Sgr AB has an eccentricity e < .
01; Stempels & Gahm 2004).Given that V4046 Sgr has apparently well outlived the “canonical” few-Myr timescale forJovian planet formation (e.g., Currie et al. 2009, and references therein), one might furtherspeculate that the presence of V4046 Sgr C[D] may explain the longevity of the circumbi-nary disk around V4046 Sgr AB — just as the hierarchical natures of the young multiplesystems HD 98800 and Hen 3-600 may explain the persistence and observed properties ofthe 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 it remains to work out thedetails in all of these examples, in the specific case of V4046 Sgr it is possible that dynamicalinteractions with component(s) C[D] may have inhibited the formation of gas giant and icegiant planets in circumbinary orbits around components AB, thereby preserving the gaseous,circumbinary disk from which this close binary system is still accreting.
This research was supported by NASA Goddard Space Flight Center XMM-Newton GuestObserver Facility and NASA Astrophysics Data Analysis Program grants to RIT (NASAgrant numbers NNX09AT15G and NNX09AC96G, respectively). DPH was supported byNASA through the Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 forthe Chandra X-Ray Center and Science Instruments. MA acknowledges support from theSwiss National Science Foundation (grants PP002–110504 and PP00P2–130188). The au-thors thank B. Zuckerman for useful discussions, the anonymous referee for a thorough andinsightful review, and Emily Thompson (West Irondequoit [NY] High School) for contribu-tions to the catalog research reported herein.
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This preprint was prepared with the AAS L A TEX macros v5.2.
11 –Table 1:
GSC0739: Photometry a B V R I J H K14.14 12.78 12.13 10.64 9.44 (0.02) 8.77 (0.04) 8.54 (0.02)a) Photometric data compiled from SIMBAD (sources: Tycho and UCAC3 catalogs, and Torres et al. 2006).Photometric uncertainties available only for 2MASS data.
Table 2:
GSC0739: XMM X-ray spectral fit results a (obs. C b N H T EM c T EM c L x log L x /L bol interval) (s − ) (10 cm − ) (MK) (10 cm − ) (MK) (10 cm − ) (erg cm − s − )(1) 0.24 0.0–3.0 5.2–5.6 3.1–3.3 21–23 3.3–3.5 9 . × − . . × − . d . × − .
12 –Fig. 1.— The HR diagram positions of V4046 Sgr AB (circles; luminosity and temperaturedata from Donati et al. 2011) and GSC0739 (squares; see § M ⊙ (in intervals of 0.1 M ⊙ ,from right to left), while the isochrones (dashed lines) correspond to ages of 5, 10, 15, 25and 40 Myr (from top to bottom). 13 – . . . Time [ks] C oun t R a t e × − . C oun t s s − Å − − χ Wavelength (Å)
12 12.5 13 13.5 × − . .
12 12.5 13 13.5 × − . . N e X F e XX I F e XX F e XX N e I X F e X I X N e I X N e I X Fig. 2.—
Top:
Chandra/HETGS light curve of GSC0739, covering the longer ( ∼
100 ks)continuous observing segment of the two-segment 145 ks exposure. Black: source; grey:background.
Bottom:
Chandra/HETGS spectrum of GSC0739 (black, with grey error bars)overlaid with best-fit APED plasma model (red) in which the emission measure distributionis assumed to follow a power law (see text). The lower panel shows the residuals of the fit.The inset is a blowup of the 12.0–13.9 ˚A region, illustrating the strong emission lines of Ne x and Ne ix as well as weak lines of highly ionized Fe. 14 –Fig. 3.— Top:
XMM/EPIC MOS light curve of GSC0739. Binsize is 1500 s.
Bottom:
MOSspectra corresponding to the three ∼∼