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Multiplicity of Young Stars in and Around R Coronae Australis A&A 488, 997–1006 (2008) Astronomy DOI: 10.1051/0004-6361:200809897 & c ESO 2008 Astrophysics Multiplicity of young stars in and around R Coronae Australis R. Köhler1,2,R.Neuhäuser3,S.Krämer3,Ch.Leinert2,T.Ott4, and A. Eckart5 1 ZAH Landessternwarte, Königstuhl, 69117 Heidelberg, Germany e-mail: [email protected] 2 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany 3 Astrophysikalisches Institut und Universitäts-Sternwarte, Friedrich-Schiller-Universität Jena, Schillergässchen 2-3, 07745 Jena, Germany 4 MPI für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany 5 I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany Received 3 April 2008 / Accepted 25 June 2008 ABSTRACT Context. In star-forming regions like Taurus-Auriga, it has been found that most young stars are born as multiples, which theories of star formation should definitely take into account. The R CrA star-forming region has a small dark cloud with quite a number of protostars, T Tauri stars, and some Herbig Ae/Be stars, plus a number of weak-line T Tauri stars around the cloud found by ROSAT follow-up observations. Aims. We would like to detect multiples among the young stars in and around the R CrA cloud in order to investigate multiplicity in this region. Methods. We performed interferometric and imaging observations with the speckle camera SHARP I at the ESO 3.5 m NTT and adaptive optics observation with ADONIS at the ESO 3.6 m telescope, all in the near-infrared bands JHK obtained in the years 1995, 2000, and 2001. Results. We found 13 new binaries among the young stars in CrA between 0.13 arcsec (the diffraction limit) and 6 arcsec (set as an upper separation limit to avoid contamination by chance alignments). While most multiples in CrA are binaries, there are also one quadruple (TY CrA), and one triple (HR 7170) which may form a quintuple together with the binary HR 7169. One of the newly detected companions with a large magnitude difference found near the M3-5 type T Tauri star [MR 81] Hα 17 could be a brown dwarf or an infrared companion with an edge-on disk. Among seven Herbig Ae/Be stars in CrA, six are multiple. Conclusions. The multiplicity frequency in CrA is as high as in similar star forming regions. By comparing with the period distribution of main-sequence stars and extrapolating to separations not probed in this survey, we conclude that the companion-star frequency is (95 ± 23)%; i.e. the average number of companions per primary is 0.95. Key words. stars: pre-main-sequence – stars: binaries: close – infrared: stars – instrumentation: high angular resolution – surveys 1. Introduction number compared to other star-forming regions. In addition, the brown dwarf member Denis1859 is a known binary (Bouy et al. The star-forming region Corona Australis (southern crown), ab- 2004). breviated CrA or R CrA, is today known as one of the nearest re- / Therefore, we performed this new homogeneous multiplic- gions of ongoing and or recent intermediate- and low-mass star ity survey of most CrA members with the infrared speckle cam- formation. The dark cloud around R CrA has an extinction of up era SHARP I and the infrared adaptive optics (AO) instrument to AV ∼ 45 mag. The age ranges between ≤1 Myr for the pro- ± ADONIS. We explain our observations and the data reduction in tostars to 9 4 Myr for some T Tauri stars (James et al. 2006). Sect. 2, list all results in Sect. 3, compare our observations of See Neuhäuser & Forbrich (2008) for a recent review. multiples with previous observations in Sect. 4, and conclude in Casey et al. (1998) determined the distance towards the Sect. 5. eclipsing double-lined spectroscopic binary (SB) TY CrA to be 129 ± 11 pc from their orbit solution. Because this is consis- tent with other earlier estimates (see discussion in Neuhäuser & 2. Observations and data reduction Forbrich 2008) and, at the same time, the best current estimate, we use ∼130 pc as the distance for the young stars in CrA. We have observed the sample of 49 optically visible young mem- Neuhäuser & Forbrich (2008) give a list of all known op- bers of the R CrA association listed in Table 2. This list is tically visible members compiled from various sources. Only compiled from Glass & Penston (1975), Marraco & Rydgren a few multiples have been published among the CrA mem- (1981), Herbig & Bell (1988), Walter et al. (1997), as well bers so far; see Table 1 for a listing. Hence, while six out of as Neuhäuser (1997)andNeuhäuser et al. (2000). seven Herbig Ae/Be stars are known or suspected to be multiple, The majority of our targets (Table 2) were observed with only five T Tauri stars (TTS) were known to be multiple, a low the speckle interferometry method during two observing runs in July 1995 and July 2001 at the European Southern Observatory Based on observations obtained at the European Southern (ESO) 3.5 m New Technology Telescope (NTT) on La Silla, Observatory, La Silla, proposal numbers 55.E-0968, 65.I-0086, and Chile. We used the SHARP I camera (System for High 67.C-0213. Angular Resolution Pictures) of the Max-Planck-Institut for Article published by EDP Sciences 998 R. Köhler et al.: Multiplicity of young stars in and around R Coronae Australis Table 1. Previously known multiples among optically visible young stars in and around CrA (c, d). Designation Separation Position Brightness ratio (a) Epoch Reference [arcsec] Angle [◦] JH Kday/month/year S CrA 1 135 04/07/1942 (1) 1 134 (15) 1.41 ± 0.06 157 ± 20.3 ± 0.2 (b) 12/04/1995 (2) 1.37 ± 0.02 147 ± 2 1981.25 (3) 1.3 149 April 1991 (4) 1.346 ± 0.009 144.5 ± 0.225/08/1996 (5) 1.34 ± 0.04 147 0.45 0.41 0.39 06-08/06/1987 (6) 0.49 ± 0.03 0.49 ± 0.04 0.52 ± 0.05 01/07/1996 (7) TY CrA B-C eclipsing spectroscopic binary (13, 14) A-BC 4.1 138 (15) A-D 0.294 ± 0.007 188.5 ± 131/03/2002 (8) R CrA spectro-astrometric binary according to Takami et al. 2003, but no binarity detected in Bailey (1998) TCrA ≥0.076 ± 0.005 273.0 ± 1.426/08/1996 (5) ≥0.140 ± 0.009 277.7 ± 1.326/06/1997 (5) VV CrA 2.1 ± 0.148± 20.96 ± 0.02 (b) 12/04/1995 (2) 2.08 ± 0.025 46.5 ± 0.12 1993/1994 (9) 1.9 44 April 1991 (4) 2.10 44 0.98 0.26 0.12 06-08/06/1987 (6) 0.1 ± 0.02 0.3 ± 0.03 0.95 ± 0.06 01/07/1996 (7) CrAPMS 3 4.5 58 April 1991 (4) 4.66 58 0.12 0.13 0.15 06-08/06/1987 (6) 0.19 (10) 0.14 ± 0.01 0.15 ± 0.01 0.17 ± 0.02 01/07/1996 (7) CrAPMS 6 3.71 207.26 0.95 0.95 0.89 1997 (10) RXJ1857.5-3732 ≤4 (11) DENIS-PJ 0.065 ± 0.001 279.2 ± 0.124/09/2002 (12) 185950-370632.9 0.057 ± 0.0005 279.1 ± 0.124/09/2002 (12) 0.066 ± 0.003 283.8 ± 1.212/09/2000 (12) 0.059 ± 0.003 271.8 ± 1.212/09/2000 (12) HR 7169 spectroscopic binary (WDS, Worley & Douglass 1997), 13 arc sec CPM pair with HR 7170 HR 7170 spectroscopic binary (WDS, Worley & Douglass 1997), 13 arc sec CPM pair with HR 7169 HD 176386 ∼4 ∼137 (16) Remarks: (a) Always ≤1 here by definition. (b) For consistency, we give here the reciprocal value of the flux ratio given by Ghez et al. (c) RXJ1846.7-3636 was listed as ∼8 binary in (11), i.e. with separation above the 6 upper limit used here, hence regarded as two separate stars here. (d) The very low-mass star or brown dwarf CrA 444 may also be a binary (Lopez Marti et al. 2005). References: (1) Joy & van Biesbroeck (1944), (2) Ghez et al. (1997), (3) Baier et al. (1985), (4) Reipurth & Zinnecker (1993), (5) Bailey (1998), (6) Chelli et al. (1995), (7) Prato et al. (2003), (8) Chauvin et al. (2003), (9) Ageorges et al. (1997), (10) Walter et al. (1997), (11) Neuhäuser et al. (2000), (12) Bouy et al. (2004), (13) Gaposchkin & Greenstein (1936), (14) Casey et al. (1998), (15) Proust et al. (1981) (16) Dommanget & Nys (2002). Extraterrestrial Physics (Hofmann et al. 1992). All observations used. The targets were observed at 4 different positions on the were done in the K-band at 2.2 μm; some of the stars, where a detector. In total, 120–240 frames were recorded to obtain a to- companion candidate was detected, were also observed in the tal integration time of 240 s for each target. Both SHARP I and H-andJ-bands at 1.2 μmand1.6 μm, respectively. For most ADONIS/SHARP II were used in a configuration with a field- targets, integration times of 0.5 s per frame were used. For the of-view of about 12 × 12. bright stars R and T CrA, the time was reduced to 0.2 s to avoid The detectors of the SHARP cameras use separate read-out saturating the detector. On each target, 500 frames were taken electronics for the four quadrants, which leads to discontinuities in 1995, or 600 frames in 2001, giving a total integration time at the quadrant borders.
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