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3 Optical follow-up spectroscopy

Out of the 206 X-ray sources, 89 have one or several nearby ($\le 40$ arcsec) potential optical counterparts (brighter than V=16 mag) in Simbad and/or GSC, including four previously known PMS stars (namely CrAPMS 1, 2, 3, and TY CrA) and four previously known older stars. In addition, there is one optically faint neutron star (V=25.7 mag, see footnote 2) and three extra-galactic sources (see Table 3). To identify a large fraction of the remaining 81 unidentified X-ray sources, we performed low-resolution spectroscopy of 148 stars, which are potential optical counterparts to 56 of those 81 X-ray sources.


   
Table 3: Optical counterparts. For each X-ray source listed in Table 2, we list here the optical counterpart from Simbad, GSC, or NED, unless no counterpart is found within 40 arcsec around the X-ray position (in all but four cases, these counterparts are the closest ones). We list running number as in Table 2, optical position, offset between X-ray and optical position (in arcsec), optical magnitude (V in mag), and the (log of the) X-ray to optical flux ratio. In the Col. ID, we list the nature of the counterpart, i.e. y for new young star listed in Table 4, z for zero-age main-sequence stars (i.e. low lithium), d for dKe or dMe stars, e for extra-galactic, n for neither of the above (mostly old stars), p for previously known TTS or non-TTS (not observed optically except TY CrA); counterparts without any entry in the Col. ID have not been observed optically. Finally, some remarks are given as found in Simbad (like proper motion PM as $[\mu _{\alpha} \cdot \cos \delta,\mu _{\delta}]$ in milli arc seconds per year and radial velocity RV in ${\rm km~s}^{-1}$). or from our optical follow-up observations. Data on optical counterparts are taken from Simbad or NED, if remarks are listed, otherwise from GSC. Some GSC counterparts appear on several GSC plates which may have different colors and different filters; we have always chosen the closest counterpart. For stars with V mag given with colons, positions and magnitudes are estimated from the DSS charts, because the identified counterpart is different from the relevant Simbad/GSC/NED counterpart
No. Optical position $\Delta$ V $\log$ ID Remarks
Table 2 $\alpha _{2000}$ $\delta _{2000}$ $[^{\prime \prime}]$ mag $f_{\rm X}/f_{V}$    
1 18:35:06.2 -34:04:02.8 28 13.9 -1.15    
3 18:35:46.6 -32:59:31.2 12 9.8 -1.24 p globular cluster NGC 6652
8 18:36:39.5 -34:51:25.0 16 13.0 -1.45 y  
9 18:37:17.6 -34:42:42.2 8 11.3 -2.31 n  
11 18:38:20.2 -35:23:37.2 5 12.2 -2.26 d dKe star
12 18:39:04.9 -37:05:27.8 22 13.1 -1.30 n  
13 18:39:05.3 -37:26:21.8 16 10.9 -2.35 y  
17 18:40:37.6 -37:28:18.1 28 11.2 -2.27 d dKe star
19 18:40:53.3 -35:46:44.6 22 14.5: -0.84 y  
23 18:41:48.6 -35:25:43.6 14 9.7 -2.44 y  
27 18:42:58.0 -35:32:42.9 10 12.2 -1.73 y  
30 18:44:21.9 -35:41:43.6 29 11.3 -1.81 y  
31 18:44:31.1 -37:23:34.3 26 13.1 -1.13 y  
35 18:45:09.3 -33:24:03.9 26 12.3 -2.18 n  
36 18:45:34.8 -37:50:19.6 7 9.2 -2.45 y HD 173148, G5 V, PM=[4.7,-32.0]
38 18:46:43.9 -36:04:52.2 13 12.0 -2.14   IRAS 18433-3608
39 18:46:45.6 -36:36:18.1 8 10.3 -2.45 y  
42 18:47:14.0 -37:09:48.3 6 12.1 -2.09    
45 18:47:44.6 -40:24:22.2 34 5.2 -4.18 n $\mu$ CrA, G5.5 I, PM=[24.6,-18.6], RV=-18.2, d=120 pc
48 18:48:35.8 -34:58:20.4 38 13.3 -1.73    
53 18:52:17.3 -37:00:12.0 14 13.9 -0.98 y IRAS 18489-3703
54 18:52:24.8 -37:30:35.6 10 12.5 -1.57 d dMe star
55 18:53:06.0 -36:10:22.8 28 9.6 -2.47 y HD 174656, G6 IV, PM=[-1.2,-34.0]
59 18:54:29.0 -37:39:04.5 26 11.8 -2.23    
63 18:56:37.3 -37:54:26.9 26 25.7 +4.77 p neutron star RXJ1856.5-3754
64 18:56:44.0 -35:45:31.9 10 13.0 -2.08 y  
65 18:56:49.2 -40:21:07.0 34 14.9 -0.92    
68 18:57:34.1 -37:32:32.3 25 15.0: -1.02 y  
73 18:58:43.4 -37:06:26.5 7 4.9 -4.86 p $\epsilon$ CrA, F2 V, W UMa-type, d=30 pc
77 19:00:49.5 -34:52:49.2 33 8.4 -3.61   HD 176247, G1 V, PM=[23.4,-24.0]
80 19:01:09.5 -36:47:51.7 11 12.7 -1.97 y VSS VIII-27
81 19:01:26.7 -40:22:34.0 24 13.6 -1.37 n  
82 19:01:28.7 -34:22:35.5 25 8.2 -3.29 y HD 176383, F5 V, PM=[9.9,-46.8]
83 19:01:34.9 -37:00:55.8 7 11.3 -2.03 p CrAPMS 1, K1 IV, wTTS
84 19:01:40.8 -36:52:34.2 30 9.5 -2.79 p TY CrA, Herbig Ae/Be PMS star
85 19:01:40.5 -36:44:31.9 27 13.0: -1.56 y VSS VIII-26
87 19:02:01.9 -37:07:43.2 14 10.4 -2.57 p CrAPMS 2, G5 IV, wTTS, PM=[0.0,-33.0]
89 19:02:22.1 -36:55:40.8 2 13.8 -1.60 p CrAPMS 3, K2 IV, wTTS
90 19:02:22.7 -39:22:21.9 11 14.2 -1.46    
91 19:02:25.9 -36:17:39.0 32     e galaxy CGMW 4-4634
93 19:02:43.6 -34:19:00.4 35 13.7 -1.75    
95 19:03:00.6 -40:09:16.8 33 13.4 -1.35 n  
97 19:03:58.4 -38:04:01.0 70 13.1 -2.04 z  
102 19:04:38.8 -40:48:15.4 29 11.4 -2.39    
108 19:06:24.8 -37:03:41.7 7 5.0 -5.37 n $\gamma$ CrA, F8 V, PM=[95,-274], RV=-51.0, d=21 pc
110 19:06:52.5 -37:48:37.6 16 6.2 -5.26 n HR 7232, G5 IV, d=17 pc
111 19:07:50.4 -39:23:32.2 9 14.2 -0.84 z  


Table 3. continued
No. Optical position $\Delta$ V $\log$ ID Remarks
  $\alpha _{2000}$ $\delta _{2000}$ $[^{\prime \prime}]$ mag $f_{\rm X}/f_{V}$    
112 19:09:39.8 -39:49:38.4 16 6.5 -4.19 n HR 7255, K1 III, PM=[-4.9,-21.7], d=107 pc
114 19:10:47.8 -38:54:34.7 19 7.6 -3.92 n HD 178558, F5 V, PM=[26.7,-33.5], d=61 pc
115 19:11:34.7 -34:35:09.1 10 10.7 -2.37 d dKe star
116 19:11:47.1 -36:41:42.9 7 13.5 -2.09 d dMe star
117 19:12:35.8 -34:31:31.8 14 11.4 -2.38 n  
119 19:13:10.5 -36:21:46.0 20 12.9 -1.98    
122 19:13:44.8 -33:04:06.6 20 14.1 -1.23    
123 19:13:51.8 -33:48:21.4 38 14.6 -1.13    
127 19:15:32.5 -35:28:49.2 23 14.4 -1.23    
128 19:15:46.7 -33:22:06.3 7 12.3 -2.17 z see (1)
130 19:16:13.8 -35:48:11.7 11 14.0 -1.28 d dKe star
135 19:17:23.8 -37:56:50.4 8 9.9 -2.37 y SAO 211129, K2, PM=[3.5,-31.0]
138 19:18:12.4 -38:23:04.2 5 8.6 -2.28 n HD 180445, G8 V, PM=[99.5,-93.0], d=42 pc
139 19:18:27.0 -39:13:01.4 37 13.8 -1.64    
141 19:19:31.0 -36:39:30.7 12 7.2 -4.80 n HD 180802, F7 V, PM=[36.4,-95.4], d=49 pc
142 19:19:54.7 -39:25:10.1 13 9.2 -3.34 n HD 180863, G8/K0 III, PM=[10.4,-126.0]
145 19:21:01.0 -33:20:28.7 34 14.1 -1.38    
147 19:21:29.7 -34:59:00.6 11 6.5 -3.32 z HR 7330, G1.5 V, d=21 pc, see (2)
151 19:23:20.0 -36:58:31.0 16 11.0 -2.83    
154 19:23:53.0 -40:36:56.5 10 4.0 -5.05 p $\alpha $ Sgr, B8 V, PM=[32.7,-120.8], RV=-0.7, d=52 pc
155 19:24:07.5 -33:33:30.0 36 15.6 -0.81    
158 19:24:34.9 -34:42:37.9 11 15.3 -0.97 d dMe star
165 19:27:26.7 -38:46:39.4 32 15.6 -0.51 n  
166 19:27:56.6 -39:54:39.4 30 13.1 -2.66    
167 19:28:05.5 -40:50:04.1 7 8.2 -3.31 p HD 182776, K2.5 III, RS CVn-type, d=240 pc
169 19:28:12.4 -39:14:56.5 20 15.3 -1.24    
171 19:28:31.9 -35:07:58.8 3 8.7 -2.60 z HD 182928, G5 V, PM=[-16.3,-14.4], d=234 pc, see (1)
172 19:28:48.7 -36:46:19.5 35 12.5 -2.01 n  
174 19:29:13.1 -40:55:15.0 34 13.7 -2.47 n  
175 19:29:35.0 -40:18:39.2 22 12.9 -1.48 z IRAS 19261-4024, see (1)
176 19:29:51.1 -37:02:16.7 21 9.1 -3.74 n HD 183198, G6 V, PM=[-22.2,-14.7], d=46 pc
177 19:29:57.8 -33:38:16.6 5 13.1 -1.76    
178 19:30:01.6 -33:02:43.1 22 12.6 -2.73    
179 19:30:38.3 -35:26:21.8 36 13.4 -1.54 n  
181 19:31:22.9 -40:08:19.8 37 12.2 -2.40 n  
183 19:31:38.7 -33:54:43.2 16 17.0 +0.56 e galaxy PKS 1928-340
187 19:32:43.8 -34:32:14.4 10 14.1 -1.52 n  
189 19:32:54.1 -35:54:33.7 25 14.9 -1.28    
190 19:33:17.8 -33:37:43.0 14 15.4 -0.93 n  
191 19:33:19.6 -38:12:13.6 19 12.0: -2.26 z  
193 19:33:40.4 -34:53:32.0 15 12.5 -1.92    
195 19:34:32.6 -36:21:11.6 31 13.4 -1.80 z  
197 19:34:46.6 -38:05:15.2 9 8.9 -3.03 n HD 184189, M2 III, 19 $^{\prime \prime}$ binary
199 19:36:01.5 -33:25:42.9 31 14.8 -1.38    
200 19:36:04.3 -40:02:58.6 37 12.6 -2.27 z  
202 19:37:16.3 -39:58:01.4 56 19.0 0.80 e quasar PKS 1933-400

Notes: (1) Maybe weak lithium (or noise), complex H$\alpha $, maybe double-lined, could be ZAMS, RS CVn-type, dKe/dMe, or a cool Algol. (2) Our detection of weak lithium with $W({\lambda}$(Li) $~= 0.19 \simeq W_{\lambda}$(Ca) confirms the classification of HR 7330 as member of the $\sim$ 200 Myr old nearby Castor moving group by Barrado y Navascues (1998).


Low-dispersion spectra were taken with the Boller & Chivens spectrograph of the ESO 1.5 m telescope on La Silla in twelve nights, namely 1995 July 16/17 to 21/22 and 1996 July 20/21 to 25/26. The wavelength range is 4500 to 6850 Å and the spectral resolution is $\sim 2.5$ Å. For technical details and data reduction, we refer to the relevant part of Sect. 2.2 in Walter et al. (1997).

In addition to those potential RASS source counterparts, we also observed a number of previously known or suspected young stars in CrA, because their lithium line strength was not known, see Table 1 for those data; most of these spectra were taken with the Boller & Chivens spectrograph. However, the stars HBC 673, 675, 677, and CrAPMS 3/c were observed with the ESO-3.5 m-NTT[*]. Here, we observed in the red medium-dispersion mode (EMMI red arm, CCD # 36, grating 6) at a resolution of 5500 in the wavelength range from 6160 to 7740 Å. While we could not detect lithium in HBC 673 and 675, we confirmed HBC 677 to be a cTTS and CrAPMS 3/c to be a wTTS (see Table 1).


 \begin{figure}
\par\includegraphics[angle=-90,width=18cm,clip]{ds1875_f1.ps}
\end{figure} Figure 1: Spatial distribution of RASS sources in CrA. We show all 206 RASS sources as dots superimposed on three contour levels of the IRAS 100  $\mu {\rm m}$ map. RASS sources for which we did optical follow-up observations are circled (Table 3). Our new wTTS are shown by filled circles, new cTTS by filled diamonds (Table 4). Previously known PMS stars are indicated by plusses (Table 1). The clustering in the center is around the R CrA dark cloud. We also indicate by capital letters the condensations identified by Rossano (1978). Both new cTTS are located outside the main clouds, but near small cloud-lets. The galactic parallel $b=-15^{\circ }$ is indicated in the upper right corner


 \begin{figure}
\par\includegraphics[width=16cm,clip]{ds1875f2.ps}
\end{figure} Figure 2: Finding charts for the new PMS stars. We show $5' \times 5'$ DSS charts (north is up, east to the left) for all new PMS stars, with one exception, namely RXJ1917.4-3756, which is easy to identify as (by far) the brightest star in its field


 \begin{figure}
\par\includegraphics[angle=90]{ds1875_f3.eps}
\end{figure} Figure 3: High-resolution optical spectra of the new TTS. The spectra of new TTS observed with the CTIO 4-m echelle. The left panel shows a 250 Å region including H$\alpha $; the right panel shows 30 Å including the Li I $\lambda $6707 Å and Ca I $\lambda $6717 Å lines. The left hand panels are scaled from 0 to 120% of the maximum flux, with ticks at the 50 and 100% levels. The right hand panels are scaled to show the shallow absorption profiles of the rapid rotators. In the right hand panels the ticks lie at 20% of the continuum level, with the top of the plot at the 120% level. If only a single tick is shown, the bottom of the panel is 80% of the maximum flux


 \begin{figure}
\par\includegraphics[angle=-90,width=18cm,clip]{ds1875_f4.ps}
\end{figure} Figure 4: Low-resolution optical spectra of the other new TTS. Spectra obtained at the ESO-1.52 m of those new TTS not shown in Fig. 3 as well as for MR81 H$\alpha $ 14, also found to be wTTS. The dotted line shows the location of the lithium 6708 Å line

For most of the RASS counterparts with detected lithium (in low-resolution spectra), we then took high-resolution spectra to confirm their youth. The high dispersion spectra were obtained with the CTIO 4m telescope and echelle spectrograph on 14 to 17 July 1998. We used the 226-3 cross disperser and the 31.6 l/mm echelle with the red optics. We used the SITe 2K #6 CCD detector, at a gain of 5, corresponding to about 1 e- per ADU and a read noise of about 3 e-. We used the GG385 filter for order sorting. We used a 150 $\mu$m (1 arcsec) slit and decker #9 (3.3 arcsec) for the stellar observations. The seeing was generally 1 to 1.5 arcsec, and there were some clouds on 3 of the 4 nights. The spectra cover the range from roughly 4400 Å through 7500 Å at a resolution of 25000. We obtained projector flat images to flatten the spectra. A Th-Ar comparison source was observed before and after each telescope slew. Each stellar observation was made in three parts to facilitate cosmic ray removal. Initial reductions were undertaken at CTIO, using the IRAF DOECSLIT package. We corrected for bias, extracted the orders, divided by the flats, and solved for the dispersion. The data were rebinned to a linear wavelength scale in each order. We removed the global background (the scattered light correction) but did not attempt to subtract the local background. The data were further reduced using IDL. We flattened the spectra in each order to remove any residual curvature left from the original flat division. We trimmed the ends of the orders. We then filtered the three individual spectra of each object to remove cosmic rays, and coadded the spectra. We determine radial velocities by cross-correlating the spectra against the sky spectra. We expect an internal precision of about 1 km s-1, except for the targets with poorer S/N.


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