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
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
(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
pc and later
by Marraco & Rydgren (1981) to be
pc (assuming R=4.5).
The Hipparcos satellite tried to measure the parallax of the
star R CrA and found
mas, i.e. no reliable solution.
Casey et al. (1998) estimated the distance towards the
eclipsing double-lined spectroscopic binary TY CrA
to be
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
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
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
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
and lithium equivalent widths, and some remarks, e.g. on radial
velocity and binarity.
No. | Designation | GP75 | Other name | PMS | Spec |
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Remarks |
HBC | name | type | type | [Å] | [Å] | |||
286 |
S CrA | Hen 3-1731 | cTTS | K6 | -90.0 | 0.39e | RV=0, 1.37
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|
287 | TY CrA | CrAPMS 11 | HeBe | B9ea | yes | triple, d=129 pcb | ||
288 | R CrA | CoD
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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![]() |
K | 1.0 e | noe | non-TTSe,g | |||
674 | MR81 H![]() |
CrAPMS 7 | cTTS | M1 | -33.5 | 0.36 | ||
675 | Kn anon 2 | j2 | G0e | 1.0e | noe | non-TTSe,g | ||
676 | CoD
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i2 | CrAPMS 1 | wTTS | K1 | 0.3 | 0.39 | RV=-1.0 |
677 | MR81 H![]() |
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
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|
CrAPMS 3/c | wTTSe | M4e | -6.8e | 0.36e | 4.5
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|||
680 | MR81 H![]() |
wTTSe | M3e | -4.6e | 0.64e | TTSg | ||
e2 | M3-5g | em.g | TTSg | |||||
f2 | K4g | TTSg | ||||||
n | cTTSg | em.g | ||||||
MR81 H![]() |
M3-5g | em.g | TTSg | |||||
MR81 H![]() |
M3-5g | em.g | TTSg | |||||
MR81 H![]() |
M3-5g | em.g | TTSg | |||||
MR81 H![]() |
M1g | em.g | TTSg | |||||
MR81 H![]() |
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![]() |
CrAPMS 6NE | wTTS | M3 | -5.9 | 0.70 | NE/SW 3
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||
MR81 H![]() |
CrAPMS 6SW | wTTS | M3.5 | -9.8 | 0.44 | NE/SW 3
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||
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) ![]() |
||
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 |
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|||
HR 7170 | k | HD 176270 | B8 | 7.0e |
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|||
SAO 210888 | HD 177076 | B9.5 |
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|||||
HD 176386 | p | HIP 93425 | B9 |
RV=7.3c,
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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
(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
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
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
Myrs.
If star formation has been ongoing in CrA for more than
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,
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
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
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
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
and
,
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.
Copyright The European Southern Observatory (ESO)