A VLT/NACO Survey for Triple Systems Among Visual Pre-Main-Sequence Binaries

Abstract

We present preliminary results concerning a survey for triple systems among Pre-Main Sequence (PMS) binaries. Among the 37 systems of our sample, 7 are found to be triple and 6 quadruple. Accounting for two additional companions known from other studies, this corresponds to a degree of multiplicity among binaries (number of triples or quadruples divided by the number of systems) of 35±10%. 1 Motivations If one considers a wide 2 arcsec Pre-Main Sequence (PMS) binary (corresponding to ∼ 300AU at 150 pc, the distance of the nearest star forming regions, such as Taurus, Ophiuchus, Lupus, etc.), then a close (∼ 0.1-0.2 arcsec) companion could still exist since such a hierarchical system should be dynamically stable. We started searching the sample of wide PMS binaries of Reipurth & Zinnecker [9] to discover additional close companions using the Adaptive Optics near-infrared instrument NAOS/CONICA (NACO) attached to the telescope UT4/Yepun of the Very Large Telescope (VLT). 2 Observations and Reduction Observations were carried out in service mode from October 22 2002 until March 26 2003. Each object has been observed through three narrow-band filters (Brγ (2.166μm), H2 (2.122μm) and FeII (1.644μm)) with three dithered exposures per filter. Single exposure integration times vary from typically 0.5 s to 30 s giving a magnitude limit for the deepest exposures of K∼ 15. Data were reduced in the usual way : sky subtraction, flat-fielding, bad-pixels and cosmics corrections. 3 Preliminary Results 3.1 Multiple Systems As a preliminary result of our survey, among the 37 observed systems (mostly in Taurus) 7 were found to be triples and 6 to be quadruples (see Fig. 1 and Fig. 2). ESO Symposia: Science with Adaptic Optics, pp. 182–188, 2005. © Springer-Verlag Berlin Heidelberg 2005 184 S. Correia et al. Fig. 1. Triples systems detected in our VLT/NACO survey. North is up, east is left. Fig. 2. Quadruple systems detected in our VLT/NACO survey. North is up, east is left. A VLT/NACO Survey for PMS Triple 185 Table 1. Field star density near our binary objects as estimated from 2MASS. System Cloud distance density Radius(density=1) [pc] [star/arcsec] [arcsec] ESO Hα 283.... Circinus 700 1.67 10−3 13.8 PHα 30 Gum Neb. 450 1.39 10−3 15.1 CGHα 5/6........ Gum Neb. 450 4.78 10−4 25.8 VW Cha Cha I 160 5.33 10−4 24.4 Sz 30 Cha I 160 3.89 10−4 28.6 Sz 41 Cha I 160 4.78 10−4 25.8 LkHα 336 L1622 460 1.56 10−4 45.2 J 4872 Taurus 142 2.44 10−4 36.1 UX Tau Taurus 142 1.78 10−4 42.3 UZ Tau Taurus 142 3.11 10−4 32.0 FV Tau Taurus 142 1.56 10−4 45.2 GG Tau Taurus 142 1.44 10−4 46.9 LkHα 262/263... MBM 12 275 1.22 10−4 51.0 Two of the triples (CGHα 5/6 and PHα 30) and one of the quadruple systems (ESO Hα 283) are new detections. CGHα 6 was known from [9] to form a binary with the 11.1 arcsec north-western companion CGHα 5. Here we resolved for the first time CGHα 6 as a close 0.5 arcsec binary, making it a hierarchical triple. PHα 30 is a 0.67 arcsec binary with a wide companion found in our survey at 11.9 arcsec separation. ESO Hα 283 appears to be a possible quadruple system composed of the original 2.08 arcsec binary detected by [9] plus a third (\c1) and a faint fourth (\c2) component located 2.57 arcsec SE and 9.3 arcsec SW of the primary, respectively. 3.2 Chance Projections In order to infer the proportion of systems which are gravitationally bound and those which are only the result of chance projection, we used two approaches. The first one is a statistical approach which consists in estimating the field star density for each system. For that purpose we compiled the number of 2MASS objects with K≤15 in a 5x5 arcmin field surrounding each target. Since we did not attempt to distinguish between cloud members and background/foreground stars, the result is therefore an upper-limit on the field star density. Table 1 gives the estimated field star density in stars/arcsec2 and the radius (in arcsec) of the projected surface which corresponds to unit density. Two systems (ESO Hα 283 and PHα 30) show a non-negligible probability of chance projections within the NACO FOV (either 13.6×13.6 or 27.8×27.8 arcsec for the S13 and S27 cameras respectively). The second approach is an attempt to determine the nature of the new or so far unconfirmed components through the use of both a color-color J-H/H-K diagram and a H-K/Mk color-magnitude diagram. 2MASS combined magnitudes or single-component magnitudes for the well-resolved (≥ 2 arcsec) companions are 186 S. Correia et al. 0.0 0.5 1.0 1.5 H-K -0.5 0.0 0.5 1.0 1.5 2.0 2.5