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Up: Search for young stars cloud


   
1 Introduction

The Corona Australis (CrA) molecular cloud complex (Dame et al. 1987) is one of the nearest regions of ongoing and/or recent intermediate- and low-mass star formation. The dark cloud near the emission line star R CrA (Knacke et al. 1973) is the densest cloud core with extinction up to $A_{V} \sim 45$ mag (Wilking et al. 1992). This cloud is also called FS 445-47 (Feitzinger & Stüwe 1984) and condensation A (Rossano 1978); Harju et al. (1993) resolved cloud A into five condensations, the star R CrA being located in A2. Cambrésy (1999) mapped the cloud using optical star counts. Between the stars R and T CrA, there is the reflection nebula NGC 6729; TY CrA and HD 176386 illuminate the nebula NGC 6726/6727. Several infrared (IR) surveys revealed a large population of embedded IR sources (Taylor & Storey 1984; Wilking et al. 1984, 1986, 1992, 1997), some of which are IR Class I objects, extremely young stars still deeply embedded in their dense circumstellar envelopes (Adams et al. 1987; André & Montmerle 1994). From the main-sequence contraction time of the early-type stars R, T, and TY CrA, the age of the cloud is estimated to be between $\le 1$ (Knacke et al. 1973) and 6 Myrs (Wilking et al. 1992).

The distance towards the CrA star forming region was estimated by Gaposchkin & Greenstein (1936) to be $150 \pm 50$ pc and later by Marraco & Rydgren (1981) to be $\sim 129$ pc (assuming R=4.5). The Hipparcos satellite tried to measure the parallax of the star R CrA and found $122 \pm 68$ mas, i.e. no reliable solution. Casey et al. (1998) estimated the distance towards the eclipsing double-lined spectroscopic binary TY CrA to be $129 \pm 11$ pc from their orbit solution.

Only a few low-mass pre-main sequence (PMS) stars, so-called T Tauri stars (TTS), associated with the CrA dark cloud had been found by H$\alpha $ and IR surveys (Knacke et al. 1973; Glass & Penston 1975; Marraco & Rydgren 1981; Wilking et al. 1984, 1986, 1992, 1997), all being classical TTS (cTTS) with IR excess and strong H$\alpha $ emission (see Table 1). Patten (1998) obtained optical photometry and spectroscopy of some more sources of, by then, unknown nature around R CrA, previously found by Knacke et al. (1973), Glass & Penston (1975), and Marraco & Rydgren (1981), and of some X-ray sources found in a pointed ROSAT observation. He classified some of them as new association members due to H$\alpha $ emission.

Table 1 gives a list of all previously known optically visible young stars in CrA, with their names, PMS types (Herbig Ae/Be or T Tauri stars), spectral types, H$\alpha $ and lithium equivalent widths, and some remarks, e.g. on radial velocity and binarity.


   
Table 1: Previously known or suspected optically visible young stars in CrA. PMS types are either cTTS or wTTS or intermediate-mass Herbig Ae/Be stars, one being of spectral type F0e. We also list spectral types, H$\alpha $ and lithium 6708 Å equivalent widths (negative when in emission) as well as radial velocities (RV in ${\rm km~s}^{-1}$), if available. Data for stars with CrAPMS designations are from Walter et al. (1997), for other stars with HBC number from the Herbig-Bell catalog (HBC, Herbig & Bell 1988), unless otherwise noted. At the end of the table, we also list four more late B-type stars, which might be associated with the CrA cloud. Some of the previously suspected TTS have been confirmed by our spectroscopy, but for MR81 H$\alpha $ 10 and Kn anon 2, we could not detect lithium
No. Designation GP75 Other name PMS Spec $W_{\lambda}({\rm H\alpha})$ $W_{\lambda }$(Li) Remarks
HBC   name   type type [Å] [Å]  

286

S CrA   Hen 3-1731 cTTS K6 -90.0 0.39e RV=0, 1.37 $^{\prime \prime}$ binary
287 TY CrA   CrAPMS 11 HeBe B9ea   yes triple, d=129 pcb
288 R CrA   CoD $-37^{\circ}13027$ HeAe A5e II     RV=-36.0
289 DG CrA   Hen 3-1734 cTTS K0e -77.9e 0.57e  
290 T CrA     HeFe F0e      
291 VV CrA   Hen 3-1736 cTTS K1e -72.0e    
673 MR81 H$\alpha $ 10       K 1.0 e noe non-TTSe,g
674 MR81 H$\alpha $ 6   CrAPMS 7 cTTS M1 -33.5 0.36  
675 Kn anon 2 j2     G0e 1.0e noe non-TTSe,g
676 CoD $-37^{\circ}13022$ i2 CrAPMS 1 wTTS K1 0.3 0.39 RV=-1.0
677 MR81 H$\alpha $ 2 i HaGr 1-100 cTTSe K8 -46.0e 0.47e  
678 V702 CrA a2 CrAPMS 2 wTTS G5 -0.7 0.28 RV=-1.2
679 CrAPMS 3 w   wTTS K2 -0.9 0.41 RV=-1.2, 4.5 $^{\prime \prime}$ pair
  CrAPMS 3/c     wTTSe M4e -6.8e 0.36e 4.5 $^{\prime \prime}$ pair
680 MR81 H$\alpha $ 14     wTTSe M3e -4.6e 0.64e TTSg
    e2     M3-5g em.g   TTSg
    f2     K4g     TTSg
    n   cTTSg   em.g    
  MR81 H$\alpha $ 12       M3-5g em.g   TTSg
  MR81 H$\alpha $ 13       M3-5g em.g   TTSg
  MR81 H$\alpha $ 15       M3-5g em.g   TTSg
  MR81 H$\alpha $ 16       M1g em.g   TTSg
  MR81 H$\alpha $ 17       M3-5g em.g   TTSg
  CrAPMS 4NW     wTTS M0.5 -1.1 0.45 RV=-2.2
  CrAPMS 4SE     wTTS G5 1.0 0.36 RV=-2.0
  CrAPMS 5     wTTS K5 -0.8 0.44 RV=-0.8
  MR81 H$\alpha $ 11NE   CrAPMS 6NE wTTS M3 -5.9 0.70 NE/SW 3 $^{\prime \prime}$ pair
  MR81 H$\alpha $ 11SW   CrAPMS 6SW wTTS M3.5 -9.8 0.44 NE/SW 3 $^{\prime \prime}$ pair
  CrAPMS 8 g2 Patten R9 wTTS M3 -3.9 0.57  
  CrAPMS 9     wTTS M2 -9.2 0.5  
  RXJ1855.1-3754   GSC 07916-00050 wTTSd K3d 1.6d 0.38d W(Li) $\simeq W$(Ca)d
  RXJ1857.7-3719   Patten R1c   M3-5g em.g   TTSg
  RXJ1858.9-3640   Patten R17c   M3-5g em.g   TTSg
  RXJ1859.7-3655   Patten R13a   M3g em.g   TTSg
  HR 7169 l HD 176269   B9 7.7e   $d \simeq 134$ pcf (*)
  HR 7170 k HD 176270   B8 7.0e   $d \simeq 77$ pcf (*)
  SAO 210888   HD 177076   B9.5     $d \simeq 185$ pcf
  HD 176386 p HIP 93425   B9     RV=7.3c, $d \simeq 136$ pcf, 4'' binaryc

References: (a) HBC, (b) Casey et al. 1998, (c) Simbad, (d) Neuhäuser et al. 1997, (e) this work, (f) Hipparcos, (g) Patten 1998.
Note: (*) Both HR 7169 and HR 7170 are spectroscopic binaries (Hoffleit 1982). With a separation of 13'' in 1982.66 (Torres 1985), this visual pair may be bound. The system was also detected by EO as the spatially unresolved source PMSCrA 10 (Walter et al. 1997).


With optical follow-up observations of previously unidentified X-ray sources detected with the Einstein Observatory (EO), Walter (1986) and Walter et al. (1997) found eleven new TTS members, namely CrAPMS 1 to 9, two of them (CrAPMS 4 and 6) being visual pairs consisting of two PMS stars (see Table 1). With only one exception (the cTTS CrAPMS 7), all of them are weak-emission line TTS[*] (wTTS).

In Table 1 we list all the previously known and suspected young stars in CrA, which are optically visible. Walter et al. (1997) could also establish the typical radial velocity of kinematic members of the CrA association: all the seven TTS, for which radial velocities are known, show velocities in the range of -2 to $0~{\rm km~s}^{-1}$ (heliocentric). Chen et al. (1997) also compiled a list of young stars in CrA and estimated their bolometric luminosities. We omit IR Class I sources and brown dwarf candidates in this paper, because they are too faint in X-rays to be detected in the ROSAT All-Sky Survey (RASS).

The early-type stars TY CrA, HR 7169, and HR 7170, all being spectroscopic binaries, were also detected by EO (Walter et al. 1997), but their X-ray emission may originate from late-type companions. While many of the optically visible TTS are known to be rather strong and variable X-ray emitters (e.g. Montmerle et al. 1983; Walter et al. 1988; Neuhäuser et al. 1995), it was surprising that a few IR Class I objects were also detected by ASCA and ROSAT X-ray observations (Koyama et al. 1996; Neuhäuser & Preibisch 1997). Wilking et al. (1997) also found five brown dwarf candidates, but they are not detected in deep ROSAT pointings (Neuhäuser et al. 1999).

Because there are several early-type stars in the CrA association, there should be much more than the $\sim 3$ dozen TTS listed in Table 1, if its initial mass function (IMF) is consistent with the Miller-Scalo IMF (Miller & Scalo 1979). From the spatial incompleteness of the EO observations and the X-ray variability of TTS, Walter et al. (1997) concluded that there should be $\sim 70$ TTS in CrA. From their IR survey, Wilking et al. (1997) estimated the number of the low-mass members to be 22 to 40 for an association age of $\sim 3$ Myrs. If star formation has been ongoing in CrA for more than $\sim 3$ Myrs, there should be even more PMS stars. Such older PMS stars, i.e. the post-TTS, should partly be found around the CrA dark cloud, because they had enough time to disperse out.

Optical follow-up observations of RASS sources in and around other star forming regions (Tau-Aur, Orion, Cha, $\rho$ Oph, Lup-Sco-Cen, etc.) revealed large populations of previously unknown PMS stars most of them being wTTS (see Neuhäuser 1997 for a review), identified as such with low- to intermediate resolution spectroscopy showing late spectral types, H$\alpha $ emission (or emission filling-in the absorption), and lithium 6708 Å absorption, a youth indicator. Because some of them were found even outside the star forming clouds, it was argued (Briceño et al. 1997) that many of these young stars are not PMS, but zero-age main-sequence (ZAMS) stars similar to the Pleiades, which also show strong X-ray emission, H$\alpha $ absorption, and lithium 6708 Å absorption. However, in the meantime, Covino et al. (1997), Neuhäuser et al. (1997), Wichmann et al. (1999), and Alcalá et al. (2000) have shown with high-resolution spectra that most of the previously claimed wTTS really are PMS stars, because they show more lithium than ZAMS stars of the same spectral type. Also, Neuhäuser & Brandner (1997) found that all 15 Li-rich stars found by ROSAT, which could be placed accurately into the H-R diagram using Hipparcos data, clearly are PMS stars. The young stars newly found outside of the clouds could either be ejected out of their parent cloud (Sterzik & Durisen 1995), or they could have formed locally in small cloud-lets which dispersed since then (Feigelson 1996; Mizuno et al. 1998). Many of the new ROSAT TTS in Taurus, Orion, and Lup-Sco-Cen are probably members of the Gould Belt (Guillout et al. 1998a,b), young stars still at least slightly above the ZAMS.

The CrA dark cloud is located $\sim 18^{\circ}$ below the galactic plane. According to Olano (1982, see also Fig. 1.10 in Pöppel 1997), the CrA and Chamaeleon clouds are not part of the Gould Belt nor of the Lindblad ring, because both CrA and Cha are far below the galactic plane, while the belt and the ring are both above the plane. In the cross-correlation of Tycho and RASS, the CrA association is seen as a small cluster of X-ray active stars around $l\simeq 0^{\circ}$ and $b \simeq -20 ^{\circ}$, see Fig. 3 in Guillout et al. (1998a) and Fig. 3 in Guillout et al. (1998b). The Gould Belt is above the galactic plane in this quadrant. Hence, neither CrA nor Cha are part of the belt, so that we should not expect to detect Gould Belt members in CrA. Many new TTS were discovered around the Cha clouds (Alcalá et al. 1995; Covino et al. 1997), so that we may expect to find many such TTS also here around the CrA dark cloud. Hence, we carried out an optical identification program to find more PMS in and around the CrA dark cloud among unidentified RASS sources[*]. In Sect. 2, we describe the X-ray data reduction and list all X-ray sources found with RASS (Table 2). Our spectra are presented in Sect. 3 together with lists of potential optical counterparts (Table 3). Then, in Sect. 4, we discuss the results of the spectroscopy including Table 4 with our new TTS. In Sect. 5 we list the available optical and IR photometry for the new TTS; the H-R diagram is shown and discussed in Sect. 6. Then, in Sect. 7, we present proper motions of some of our new PMS stars and discuss the 3D space motion of young stars in CrA. Finally, in Sect. 8, we estimate the completeness of our survey. We summarize our results in the last section.


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