The curious case of PDS 11: a nearby, >10 Myr old, classical T Tauri binary system
Blesson Mathew, P. Manoj, B. C. Bhatt, D. K. Sahu, G. Maheswar, S. Muneer
aa r X i v : . [ a s t r o - ph . S R ] M a r Draft version November 13, 2018
Preprint typeset using L A TEX style AASTeX6 v. 1.0
THE CURIOUS CASE OF PDS 11: A NEARBY, >
10 MYR OLD, CLASSICAL T TAURI BINARY SYSTEM
Blesson Mathew and P. Manoj Department of Astronomy and AstrophysicsTata Institute of Fundamental ResearchHomi Bhabha Road, Colaba, Mumbai 400005, India
B. C. Bhatt
Indian Institute of AstrophysicsKoramangala, Bangalore 560034, India
D. K. Sahu
Indian Institute of AstrophysicsKoramangala, Bangalore 560034, India
G. Maheswar
Aryabhatta Research Institute of Observational Sciences (ARIES)Nainital 263002, India
S. Muneer
Indian Institute of AstrophysicsKoramangala, Bangalore 560034, India [email protected] ABSTRACTWe present results of our study of PDS 11 binary system, which belongs to a rare class of isolated, highgalactic latitude T Tauri stars. Our spectroscopic analysis reveals that PDS 11 is a M2 − M2 binarysystem with both components showing similar H α emission strength. Both the components appear tobe accreting, and are classical T Tauri stars. The lithium doublet Li i λ i λ −
15 Myr for PDS 11A. Comparison with pre-main sequenceevolutionary models indicates that PDS 11A is a 0.4 M ⊙ T Tauri star at a distance of 114 −
131 pc.PDS 11 system does not appear to be associated with any known star forming regions or movinggroups. PDS 11 is a new addition, after TWA 30 and LDS 5606, to the interesting class of old, dusty,wide binary classical T Tauri systems in which both components are actively accreting.
Keywords: stars: pre-main sequence – stars: variables: T Tauri – (stars:) circumstellar matter –infrared: stars – (stars:) binaries: general INTRODUCTIONT Tauri stars are low-mass (K & M spectral types)young stars which are in their pre-main sequencephase of evolution (e.g., Joy 1945; Bertout 1989;Herczeg & Hillenbrand 2014). They are often associ-ated with cloud complexes such as Taurus, Orion and Ophiuchus (Herbig 1962; Herczeg & Hillenbrand 2014;Appenzeller & Mundt 1989). However, T Tauri starsare also found in isolated regions above the Galacticplane and far from any dark clouds (de la Reza et al.1989; McGehee 2013; Elliott et al. 2016). TW Hydraewas the first such ‘isolated T Tauri star’ identified,
B. Mathew et al.
Figure 1 . BV R color composite of PDS 11, with North upand East to the left. This 4 ′ × ′ composite image is con-structed from our observations with the HCT and is colorcoded as blue, green and red in B , V , R , respectively. Theimage is centered on PDS 11 binary system, with the north-eastern component being PDS 11A. which was later found to be part of an association ofabout three dozen members, known as TW Hydrae asso-ciation (TWA, Kastner et al. 1997; Zuckerman & Song2004; Mamajek 2016). They were not runaway starsfrom molecular clouds but formed in situ in the presentregion ∼
10 Myr ago, which is now devoid of molec-ular gas (Rucinski & Krautter 1983; Tachihara et al.2009). Since then several such nearby, young asso-ciations have been identified such as, the β Pictorismoving group, the AB Doradus moving group, the Tu-cana/Horologium association etc., within 100 pc of theSun (see Zuckerman & Song 2004; Torres et al. 2008;Mamajek 2016, for a review).In this paper we present a detailed analysis of thehigh galactic latitude (b = − ◦ ) binary T Tauri starsystem PDS 11 (GSC 04744-01367, IRAS 04451-0539).Gregorio-Hetem et al. (1992) have reported this to be abinary system with components PDS (PDS stands forPico dos Dias Survey) 11A and PDS 11B. PDS 11A isthe North-Eastern component of the binary (Figure 1).PDS 11A and PDS 11B are present in the Washingtondouble star (WDS) catalog, with the component magni-tudes being 14.76 and 15.34, respectively. The positionangle (PA) and separation were found to be 216 ◦ and8.8 ′′ , respectively (Mason et al. 2001). Our analysis in-dicate that PDS 11 is a young (10 −
15 Myr), nearby(114 −
131 pc) binary T Tauri system, where both thecomponents are possibly accreting. The paper is ar-ranged as follows. Section 2 describes the optical andnear-IR observations. The results from this study arepresented in Sect. 3. A detailed discussion of the keyresults is given in Sect. 4, and finally in Sect. 5 wepresent our conclusions. OBSERVATIONS AND DATA REDUCTION2.1.
Optical spectroscopy
Optical spectra of both the components of the PDS 11system in the wavelength range 3800 − mounted on the 2-m HimalayanChandra Telescope (HCT). The wavelength range wascovered using Grism 7 (blue region, 3800 − − ′′ wide and 11 ′ long slit provide aneffective resolving power of ∼
900 in blue region and ∼ . Further, the extracted spectra were wave-length calibrated and flux calibrated. The target wasagain observed on March 15 to confirm the spectral fea-tures observed. The log of observations is given in Table1. 2.2. Near-infrared spectroscopy
We also obtained near-IR spectra of PDS 11 withTIFR near-infrared spectrometer and imager (TIR-SPEC; Ninan et al. 2014) mounted on 2-m HCT. Thespectra were taken in Y (1.02 − µ m), J (1.21 − µ m), H (1.49 − µ m) and K (2.04 − µ m) bands,in combination with Grism and L3 slit (1.97 ′′ wide and300 ′′ long). The effective resolving power is around1200. The program stars PDS 11A and PDS 11B areco-aligned in the slit and care is taken while reducingthe spectra to extract them separately. A telluric stan-dard HIP34768 (A1V spectral type) at nearby airmasswas also observed. The observations were carried outin dithered mode. The log of infrared spectroscopicobservations is given in Table 1. Argon spectra wereobtained after the object spectra for wavelength cali-bration. The wavelength calibrated object spectrum isdivided with the the telluric spectrum whose hydrogenabsorption lines were removed. The resultant spectrumis multiplied with a blackbody spectrum of 9230 K, cor-responding to A1V spectral type of the telluric standard IRAF is distributed by the National Optical Astronomy Ob-servatories, which are operated by the Association of Universi-ties for Research in Astronomy, Inc., under cooperative agreementwith the National Science Foundation
DS 11: a nearby, >
10 Myr old CTTS binary system Y , J , H and K bands. Table 1 . Journal of spectroscopic observationsObject Date Optical InfraredExp.time (s) Exp.time (s)Gr7/167l Gr8/167l
Y J H K (1) (2) (3) (4) (5) (6) (7) (8)PDS 11A 2016 Feb. 13 1800 1800 . . . .2016 Feb. 14 . . 1000 1000 1000 10002016 Mar. 15 1800 1800 . . . .PDS 11B 2016 Feb. 13 1800 1800 . . . .2016 Feb. 14 . . 1000 1000 1000 9002016 Mar. 15 900 900 . . . .Feige 34 2016 Feb. 13 600 600 . . . .HIP 34768 2016 Feb. 14 . . 400 400 240 160
Optical photometry
We imaged the 10 ′ × ′ region centered on PDS 11 in BV R passbands (Bessell 1990) on 2016 March 19 us-ing HFOSC. The data reduction was carried out usingvarious packages available in IRAF. Aperture photome-try was performed on the program star and nearby fieldstars. The B , V , R magnitudes of the nearby field starswere obtained using available SDSS photometry, whichwere converted to Bessell system using the transforma-tion relations given in Lupton (2005) . The magnitudeof the program stars were calibrated differentially withrespect to the nearby field stars. RESULTS3.1.
Spectral analysis: optical and near-IR
The spectra of both PDS 11A (Figure 2) and PDS11B (Figure 3) look very similar due to the presenceof Balmer emission lines, from H α all the way upto H8 (8 − ii H & K emission lines and TiOabsorption bands, albeit with different line strengths.Gregorio-Hetem et al. (1992) reported an H α equivalentwidth (EW) of −
20 ˚A for PDS 11A and −
42 ˚A for PDS11B. The equivalent width measured from our spectraare −
23 ˚A and −
27 ˚A, which is quite different from theearlier measurements, particularly for PDS 11B. TheEW and full width at half maximum (FWHM) of theprominent spectral lines are given in Table 2. Figure 2 . Optical spectrum of PDS 11A. The spectrum isflux calibrated and is normalized at 5500 ˚A. Prominent spec-tral lines are marked.
We found [O i ] λ i ] λ i ] λ B. Mathew et al.
Figure 3 . Optical spectrum of PDS 11B. The spectrum isflux calibrated and is normalized at 5500 ˚A. Prominent spec-tral lines are marked. -400 -200 0 200 4000.70.80.911.1 -500 0 5000.60.811.26100 6200 6300 64000.60.811.2 6100 6200 6300 64000.60.811.2
Figure 4 . Li i λ i ] λ λ dence for the presence of He i λ λ i λ i λ i λ λ i λ i emission lines areformed from photoionization and subsequent recombi-nation in the accretion shock region, close to the stellarsurface (Zuckerman et al. 2014). Hence, the presenceof [O i ] λ i emission lines in the spectrum ofPDS 11B suggest that it belongs to the class of accretingT Tauri stars.The flux calibrated Y , J , H , K spectra of PDS 11Aand PDS 11B, normalized to band center values areshown in Figure 5. Since the signal to noise is low, thespectra is smoothed to 5 points for display purpose. Pa β is the only prominent spectral line found in PDS 11A,which is present in emission with an EW of 3 ˚A. Noprominent emission or absorption features are presentin PDS 11B. 3.2. Spectral type estimation
We estimated the spectral type of PDS 11A and PDS11B using TiO bands, the dominant molecular absorp-tion band in M-type stars. The spectral type has beenestimated from the TiO5 spectral index, which is de-fined as the ratio of mean flux value in 7126 − − − × TiO5 + 8.2. They suggested using this rela-tion as a reliable spectral type estimator in the range K7to M6.5 dwarfs. We have measured the flux values fromthe flux calibrated spectra obtained on 2016 February13 and found TiO5 to be 0.60 ± ± ± ± λ λ − L3 (Slesnick et al. 2006).
DS 11: a nearby, >
10 Myr old CTTS binary system Figure 5 . Near-Infrared Y , J , H , K , spectra of PDS 11A (left) and PDS 11B (right) Table 2 . EW and FWHM of the major spectral lines in PDS 11A and PDS 11B. EW ofemission lines are shown in negativeSpectral line Date of obs. PDS 11A PDS 11BEW (˚A) FWHM (˚A) EW (˚A) FWHM (˚A)(1) (2) (3) (4) (5) (6)Ca ii K 2016 Feb. 13 -23 ± ± ± ± ± ± ± ± ii H + H ǫ ± ± ± ± ± ± ± ± δ ± ± ± ± ± ± ± ± γ ± ± ± ± ± ± ± ± ± ± β ± ± ± ± ± ± ± ± i ± ± ± ± i (5890+5896) 2016 Feb. 13 5.2 ± ± ± ± ± ± ± ± i ]6300 2016 Feb. 13 . . -1.2 ± ± ± ± α ± ± ± ± ± ± ± ± i ± ± Table 2 continued
B. Mathew et al.
Table 2 (continued)
Spectral line Date of obs. PDS 11A PDS 11BEW (˚A) FWHM (˚A) EW (˚A) FWHM (˚A)(1) (2) (3) (4) (5) (6)2016 Mar. 15 0.40 ± ± i ± ± ± ± ± ± ± ± i ± ± ± ± ± ± ± ± H α : Accretion indicator Historically, the strength of the H α line has beenused to distinguish between accreting classical T Tauristars (CTTS) from weak-lined T Tauri stars (WTTS),where the H α emission is due to chromospheric activ-ity. An H α equivalent width EW(H α ) ∼
10 ˚A wasset as the discrimination boundary. However, be-cause of the ‘contrast effect’ of the photosphere, nounique EW(H α ) value distinguishes all CTTSs fromWTTSs, and several authors have proposed EW(H α )dividing line as function of spectral type (Mart´ın 1998;White & Basri 2003; Barrado y Navascu´es & Mart´ın2003). White & Basri (2003) prescribed empiricallydetermined maximum EW(H α ) values observed fornon-accreting T Tauri stars for different spectraltype ranges. Barrado y Navascu´es & Mart´ın (2003)proposed EW(H α ) values as a function of spec-tral type derived from the observed saturation limitfor the chromospheric activity at Log(L Hα /L bol ) = − α ) values of PDS 11A and PDS 11B,which are well above the lines depicting White & Basri(2003) and Barrado y Navascu´es & Mart´ın (2003) crite-ria. Thus, both PDS 11A and PDS 11B are accretingand are CTTSs.We computed the accretion rates for PDS 11A fromthe H α and H β line luminosities using the empiri-cal relations given by Herczeg & Hillenbrand (2008),Fang et al. (2009), and Ingleby et al. (2013). For thedistance range listed in Table 3 (114 −
131 pc) corre-sponding to the age range of 10 −
15 Myr, the accre-tion rates obtained are in the range of 4.2 × − − × − M ⊙ yr − with a median value of ∼ × − M ⊙ yr − . The accretion rates we obtain for PDS 11Ais significantly lower than that found for < Figure 6 . The EW(H α ) criterion which distin-guishes CTTSs from WTTSs as a function of spectraltype from White & Basri (2003) (dashed lines) andBarrado y Navascu´es & Mart´ın (2003) (solid line) is shown.Observed EW(H α ) values of PDS 11A (blue solid circle)and PDS 11B (red solid circle) from 2016 February 13 arealso shown. Kim et al. 2016). They are, however, quite similar tothe accretion rates found for M2 members of the 10 − i line luminosity and the L acc estimated fromthem for the PDS 11A are significantly higher thanthat expected from chromospheric activity. FollowingManara et al. (2013), the noise introduced in the esti-mated L acc due to chromospheric contamination (seeeqn. 2 in Manara et al. 2013) is < × − L ⊙ , whilethe accretion luminosity of PDS 11A is ∼ × − L ⊙ ,which is ∼ β and Br γ lines using the empirical relationsfrom Muzerolle et al. (1998), Calvet et al. (2004) andNatta et al. (2006). For PDS 11A, the expected Pa β EW is ∼ ∼ × − W cm − ) and the DS 11: a nearby, >
10 Myr old CTTS binary system γ EW is ∼ ∼ × − Wcm − ). From the observed spectra discussed in Sect.3.1 we found that Pa β EW is around 3 ˚A whereas noclear emission is present in Br γ , which agrees with theseestimates. Thus the strength of the observed H i lines inthe optical and near-IR spectra of PDS 11A is consistentwith the star accreting at a rate of ∼ − M ⊙ yr − .3.4. Age estimation of PDS 11A from Li i λ The Li i λ i λ i λ i λ i λ i λ i λ i λ i λ eff ) is 3490K; Pecaut & Mamajek 2013) at an age of 15 Myr. Thismeans that since Li i λ i λ i λ Figure 7 . The Li λ β Pic (24Myr, blue), Tucana (45 Myr, magenta), Columba (42 Myr,magenta), Carina (45 Myr, magenta), ǫ Cha (11 Myr, cyan),Octans (20 Myr, yellow), Argus (40 Myr, black), AB Dor(149 Myr, red). The ages of each of the moving groups istaken from the recent compilation by Bell et al. (2015). Theage of Argus and Octans association is not listed in Bell et al.(2015) and hence we used the age given in da Silva et al.(2009).
11A is located between 10 Myr old TW Hya associa-tion and 24 Myr old β Pic moving group. It may benoted there are no moving groups with Li i EW mea-surements of stars between 10 and 24 Myr. Hence, fromLDB method and the analysis of Li i EW distribution inmoving groups, we found that PDS 11A has an age of10 −
15 Myr, which will be used in further discussion.PDS 11 is not the first case of a T Tauri binary sys-tem where only one of the components show Li i λ i λ i λ i λ B. Mathew et al. accretion in young stars can increase the central tem-perature due to which lithium can get severely depleted(Chabrier et al. 1996).3.5.
Stellar parameters
From our photometry we estimated V and ( B − V )values of PDS 11A and PDS 11B as 14.75 ± V ),1.43 ± ± V ), 1.28 ± B − V ) color of PDS 11A andPDS 11B is 1.46, considering that both are M2 stars(Pecaut & Mamajek 2013). Our observed ( B − V ) col-ors are found to be bluer by 0.03 mag and 0.18 magthan the intrinsic values, for PDS 11A and PDS 11B,respectively. This has been noticed in previous studiesand could be caused by the lower gravity of pre-MS starswith respect to the dwarfs (Song et al. 2002). From theobserved ( B − V ) values, the color excess E ( B − V ) ofboth the stars is found to be negative and hence will beconsidered as zero from now on. This is understandablesince these are high Galactic latitude objects and hencesuffer little extinction.Comparison with 10 −
15 Myr isochrones fromBaraffe et al. (2015) for an M2 star (T eff = 3490 K)indicates a mass of 0.4 M ⊙ and luminosity in therange 0.089 − ⊙ (log L/L ⊙ = − − bol in the range of 7.37 − V magnitude (M V ) is obtained usingthe bolometric correction of -1.80 for M2 stars given inPecaut & Mamajek (2013). The observed V magnitude,m V = 14.75, and M V indicate a distance of 114 − i λ Table 3 . Stellar propertiesReference PDS 11A PDS 11B(1) (2) (3)Sp.type M1.7 ± ± eff (K) 3490 3490L bol (L ⊙ ) 0.067 − E ( B − V ) 0 0Distance (pc) 114 −
131 .Age (Myr) 10 −
15 .Mass (M ⊙ ) 0.4 .V r ( km s − ) 21 ± ± µ α (mas yr − ) 6.0 ± ± µ δ (mas yr − ) -1.4 ± ± Table 3 continued
Figure 8 . 2MASS ( J − H ) vs ( H − K ) color-color di-agram: PDS 11A and PDS 11B are shown in open bluetriangle and circle, respectively. The sample of CTTS areshown in black circles, WTTS in magenta squares and TDcandidates in green diamonds. Main sequence and giant se-quence, shown in solid and dotted lines, respectively, is fromKoornneef (1983), which is converted to 2MASS system us-ing the transformation relations from Carpenter (2001). TheCTTS location is from Meyer et al. (1997) and is shown indot-long dash line. The ( J − H ), ( H − K ) colors are red-dening corrected using the relation from Rieke & Lebofsky(1985). Table 3 (continued) Reference PDS 11A PDS 11B(1) (2) (3)
Note —References: 1 – Gregorio-Hetem et al.(1992), 2 – Whitelock et al. (1995), 3 – Qi et al.(2015)
Infrared excess
Infrared excess in the energy distribution is one ofthe defining criteria to identify T Tauri stars amongthe sample of low mass stars (Calvet & Gullbring 1998;Meyer et al. 1997). We made use of the archival 2MASSdata to estimate the near-IR ( J − H ), ( H − K ) col-ors and use them to assess near-IR excess in PDS 11Aand PDS 11B. Figure 8 shows the location of PDS 11A& 11B in the ( J − H ) − ( H − K ) color-color diagram.For assessing the nature of near-IR excess in PDS 11components, we also show CTTS, WTTS and transi-tional disk candidates in the figure. The 32 CTTSshown are of spectral types M0 − M3 from the Taurusstar forming region, identified from Furlan et al. (2006)
DS 11: a nearby, >
10 Myr old CTTS binary system − M3 from the Taurus andChamaeleon star forming regions (Furlan et al. 2011;Manoj et al. 2011). We have used the 2MASS colors of asample of 16 TD candidates in Taurus and ChamaeleonI, taken from Kim et al. (2013). It is immediately ev-ident from Figure 8 that PDS 11B shows considerableIR excess and is found to be on the CTTS locus. The( J − H ) , ( H − K ) colors of the object seems to behigher than the sample of CTTS used for this analysis.However, since the location do not contain T Tauri starsother than CTTS, it is pretty clear that PDS 11B belongto CTTS category. The near-IR excess of PDS 11A issimilar to the WTTS/TDs, suggesting significantly lesshot dust material around it. In order to see whetherour stars have any analogs in any of the moving groupin terms of near-IR colors, we have represented them in( J − H ) versus ( H − K ) color-color diagram. We have in-cluded stars in the spectral range M0 − M3, from knownmoving groups listed in Zuckerman & Song (2004) andTorres et al. (2008). As seen from Figure 9, almost allthe members of various moving groups are found to beclustered near the main sequence, similar to PDS 11A.The extreme type of IR excess seen in PDS 11B is gen-erally not seen in any other association members. Insummary, both PDS 11A and PDS 11B are found tolie on the CTTS locus, indicating the presence of warmcircumstellar dust around them. In addition to the near-IR excess, both the stars are accreting (Sect. 3.3) andshow veiling in the observed spectra (discussed in Sect.4.1), qualifying them as classical T Tauri stars. PDS11B show very high near-IR excess and lies at the ex-treme end of the CTTS locus. Since none of the knownCTTS display such high near-IR excess (see Figure 8), itis worth exploring the nature of PDS 11B from furtherstudies. 3.7.
Spectral Energy Distribution
The Spectral Energy Distribution (SED) of PDS 11Aand PDS 11B is constructed with the available photo-metric data given in Table 4. The SEDs are shown inFigures 10. We have used BT-Settl model atmospherescorresponding to the temperature (T eff ) and gravity(log g ) values of of PDS 11A and PDS 11B. Even thoughT eff of both the stars are 3490 K, we have taken theBT-Settl atmosphere for 3500 K, which is the closesttemperature for which stellar atmosphere is available.Generally, in the case of pre-MS stars, model atmo-spheres corresponding to log( g ) = 4.5 is used for SEDanalysis. We have verified this in the case of PDS 11A,which has a log( g ) value of 4.34 from the stellar modelsof Baraffe et al. (2015). Since BT-Settl model atmo-spheres corresponding to T eff = 3490 K and log( g ) =4.34 is unavailable, we have used the nearest combina- Figure 9 . 2MASS ( J − H ) vs ( H − K ) color-color diagram:PDS 11A and PDS 11B are shown in open blue triangleand circle, respectively. The sample of stars from movinggroups are shown in different colored filled circles; TW Hya(green), β Pic (blue), Tucana (black), Columba (cyan), ǫ Cha(magenta), Argus (red), AB Dor (yellow). ( J − H ) and ( H − K ) colors are not dereddened since color excess values ofmoving groups are not available. All the other sequences aresame as in Figure 8. -13 -12 -11 -10 -9 λ F λ ( e r g s - c m - ) PDS 11A λ ( µ m)10 -13 -12 -11 -10 -9 λ F λ ( e r g s - c m - ) PDS 11B
Figure 10 . Observed SEDs of PDS 11A and PDS 11B. Pho-tometry at all the epochs are plotted. For PDS 11A theyfall on top of each other, while PDS 11B show significantvariability. Also shown are the BT-Settl model for T eff =3500 K and log(g) = 4.5. The model is normalized to theobserved J-band flux. tion of T eff = 3500 K and log( g ) = 4.5 for SED analysis.PDS 11A do not show much of IR excess whereasPDS 11B shows considerable excess with the SED ris-0 B. Mathew et al. ing in near-IR itself (Figure 10). We found that mid-infrared magnitudes are available from
W ISE missionfor PDS 11B, but the beam size includes PDS 11A aswell. Comparison of J , H , K magnitudes of both starsfrom 2MASS and Whitelock et al. (1995) indicate thatmost of the excess emission is coming from PDS 11B.PDS 11B is about 2.2 mag brighter than PDS 11A in L band (Whitelock et al. 1995). PDS 11B shows con-siderably high ( J − H ) and ( H − K ) color excess ( ∼ Table 4 . Available dataReference Band PDS 11A PDS 11B(1) (2) (3) (4)This work B ± ± V ± ± R ± ± B V R I I ± ± J ± ± K ± ± J ± ± H ± ± K ± ± J ± ± H ± ± K ± ± L ± ± Note —References: DENIS – DENIS Consor-tium 2005, 2MASS – Cutri et al. (2003), G92 –Gregorio-Hetem et al. (1992), W95 – Whitelock et al.(1995); L mag given in SAAO system.4.
DISCUSSIONOur analysis so far suggest that both components ofthe PDS 11 system are of similar spectral type, M2.Both PDS 11A and PDS 11B show strong H α emission,confirming their CTTS status. While PDS 11B showstrong excess emission in the near-IR, PDS 11A show noor weak excess, indicating the that no hot dust is present close to the star. Intriguingly, PDS 11A show lithiumabsorption, suggesting that it is 10 −
15 Myr old; PDS11B do not show lithium absorption. Below we discussmore aspects on veiling and binarity of this system.4.1.
Veiling estimate from Ca i λ The presence of excess continuum emission, referredto as veiling, is often observed in classical T Tauri stars(Joy 1949; Johns-Krull & Valenti 2001). In the mag-netospheric accretion model, veiling is due to the dissi-pation of energy in the post-shock region at the base ofthe magnetic funnel, close to the stellar surface (Koenigl1991; Hartmann et al. 1994; Calvet & Gullbring 1998).Herczeg & Hillenbrand (2014) suggested a method to es-timate veiling in T Tauri stars. They found that mea-sured EW values of Ca i λ i ) = − x − x . Since both PDS 11A andPDS 11B are of M2 spectral type, ‘ x ’ corresponds to60 (Herczeg & Hillenbrand 2014). Hence the expectedCa i λ i ) measured on 2016February 13 agrees with the expected value, whereas itchanged to 5.6 ± i λ i ) value of 6.0 ± i λ i ) relation given in Herczeg & Hillenbrand(2014). We found that the spectral type of PDS 11Acould change from M2 to M1 for a change in EW(Ca i )from 6.8 to 5.6 ˚A. In the case of PDS 11B, the Ca i λ ∼ i λ i λ DS 11: a nearby, >
10 Myr old CTTS binary system i λ i ) relationgiven in Herczeg & Hillenbrand (2014). Thus, based onEW(Ca i ), the spectral type of PDS 11B was of M1 andPDS 11A of M2 type, for the epoch 2016 February 13.The age of PDS 11A is estimated in the range 10 − ∼
27 times higher inan M2 star like PDS 11A, when compared to M1 star likePDS 11B. Since we found Li i λ i ) expected for PDS 11Bis around 0.02 ˚A. This value is far below the detectionlimit of our instrument, which supports our propositionthat Li i λ Possible association with a moving group
We have used the Banyan II webtool to checkwhether our candidates are associated with any of thenearby kinematic groups. Banyan II is a Bayesian analy-sis tool which makes use of the position and space veloc-ities of the object to assess the match with the databaseof nearby ( <
100 pc) moving groups, younger than 100Myr (Gagn´e et al. 2014; Malo et al. 2013). The helio-centric radial velocity and proper motion of PDS 11Aand PDS 11B are taken from the literature and are givenin Table 3. From the analysis we found that PDS 11Aand PDS 11B are not associated with any known movinggroup. Banyan II analysis gives 100% probability thatPDS 11A and PDS 11B belong to young field population(Gagn´e et al. 2014). It is quite possible that our objectparameters may not match with that of any known as-sociation and hence has been ascribed to young fieldpopulation.4.3.
PDS 11: a wide binary classical T Tauri system?
Although the PDS 11 system has been treated as bi-nary in the literature (Gregorio-Hetem et al. 1992), it isyet to be demonstrated that PDS 11A & 11B are gravita-tionally bound. It has been identified as visual binariesin Washington Double Star catalog (Mason et al. 2001),but that do not guarantee them being gravitationallybound. Instead, we found that the proper motion inRA and Dec for both the stars are similar within uncer- tainties (Table 3). This suggests that PDS 11A and PDS11B form a binary system. Considering PDS 11 at a dis-tance of 114 −
131 pc, with a separation of 8.8 ′′ betweenthe components (Mason et al. 2001), the physical sepa-ration between PDS 11A and PDS 11B is ∼ − τ d ), τ d = 4 × (M/M ⊙ ) . , in terms of the mass ofthe parent star (M). Since the mass of PDS 11A andPDS 11B are estimated to be 0.4 M ⊙ , the disk lifetimeis around 2.0 Myr. Evidently it is quite puzzling howa disk which harbors enough gas and dust survive in10 −
15 Myr old system like PDS 11. T Tauri binary sys-tems with disks at ages older than the typical disk dis-sipation timescales have been reported in the literature.Most of them belong to nearby ( <
100 pc) young movinggroups. They are: 8 Myr old binary systems TW Hya(Teixeira et al. 2008), HR 4796 (Kastner et al. 2008),TWA 30 (Looper et al. 2010a,b), T Cha (Kastner et al.2012), 20 Myr old V4046 Sgr (Kastner et al. 2011), andLDS 5606 (Rodriguez et al. 2014). These objects belongto class of wide binaries, where the separation betweenthe components is in the range 1.7 kau (for LDS 5606,Rodriguez et al. 2014) to 41 kau (for TW Hya − TWA28 system, Teixeira et al. 2008). Among them, onlyTWA 30 and LDS 5606 are T Tauri binary systems inwhich both components are accreting. The binary com-ponents of TWA 30 system are separated by ∼ ∼ β Pic moving group (Rodriguez et al. 2014;Zuckerman et al. 2014).To summarize, PDS 11 is the third such system, afterTWA 30 and LDS 5606, which belong to the interestingclass of old, dusty, wide binary classical T Tauri systemsin which both components undergo active accretion. Itis quite possible that PDS 11A is a transition disk can-didate since it is accreting and lacks hot dust materialclose to the star (see Sect. 3.3 & 3.6). However, fur-ther studies are needed to confirm other transition diskproperties such as mid- and far-IR excess and the pres-ence of outer disk in PDS 11A. Also, further observa-tions are needed to assess whether the near-IR excess inPDS 11B is entirely due to circumstellar material or dueto the contribution from late-type companion. If con-firmed, this would be the first known example of a > B. Mathew et al.
Myr old binary system, where one of the componentsharbor a radially continuous full disk, while the other issurrounded by a disk with inner hole or gap. CONCLUSIONWe have analyzed the star/disk properties and derivedthe spectral type of the T Tauri binary system PDS11 from optical photometry, spectroscopy and infraredspectroscopic observations. Our analysis indicates thatPDS 11 is the new addition, after TWA 30 and LDS5606, to the interesting class of old, dusty, wide binaryclassical T Tauri systems in which both components areactively accreting. The main conclusions from this studyare listed below. • The spectral type of PDS 11A and PDS 11Bwere not known. We have classified both as M2-type making use of the TiO λ • PDS 11A and PDS 11B are found to have H α emis-sion strength of ∼
25 ˚A , which is higher than thethreshold value of chromospherically active stars.The median accretion rate derived from H α emis-sion line is around ∼ × − M ⊙ yr − for PDS11A. PDS 11B show very high near- and mid-infrared excess. The emission lines of [O i ] λ i λ −
15 Myr old system like PDS11A. • We found that PDS 11A is less than 15 Myrfrom age dating using lithium depletion bound-ary method. Further, from the comparison ofLi i λ − • Since Li i λ • From our analysis PDS 11 is identified as abinary system with component masses of 0.4 M ⊙ ,luminosity of 0.067 − ⊙ and at a distanceof 114 −
131 pc. • From the analysis with Banyan II webtool, PDS11A and PDS 11B were not identified as membersof any known moving group and hence is consid-ered as a young field binary system.ACKNOWLEDGMENTSWe would like to thank the staff at IAO, Hanle andits remote control station at CREST, Hosakote for theirhelp during the observation runs. This research usesthe SIMBAD astronomical data base service operatedat CDS, Strasbourg. This publication made use dataof 2MASS, which is a joint project of University ofMassachusetts and the Infrared Processing and Anal-ysis Centre/California Institute of Technology, fundedby the National Aeronautics and Space Administrationand the National Science Foundation.REFERENCES
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DS 11: a nearby, >
10 Myr old CTTS binary system13