Astrometric confirmation of young low-mass binaries and multiple systems in the Chamaeleon star-forming regions

Abstract

Context. The star-forming regions in Chamaeleon (Cha) are one of the nearest (distance ∼ 165 pc) and youngest (age ∼ 2 Myr) conglomerates of recently formed stars and the ideal target for population studies of star formation. Aims. We investigate a total of 16 Cha targets that have been suggested, but not confirmed, to be binaries or multiple systems in previous literature. Methods. We used the adaptive optics instrument Naos-Conica (NACO) at the Very Large Telescope Unit Telescope (UT) 4 / YEPUN of the Paranal Observatory, at 2-5 different epochs, in order to obtain relative and absolute astrometric measurements, as well as differential photometry in the J, H, and K band. On the basis of known proper motions and these observations, we analyse the astrometric results in our proper motion diagram (PMD: angular separation / position angle versus time), to eliminate possible (non-moving) background stars, establish co-moving binaries and multiples, and search for curvature as indications for orbital motion. Results. All previously suggested close components are co-moving and no background stars are found. The angular separations range between 0.07 and 9 arcsec, corresponding to projected distances between the components of 6-845 AU. Thirteen stars are at least binaries and the remaining three (RX J0919.4-7738, RX J0952.7-7933, VW Cha) are confirmed high-order multiple systems with up to four components. In 13 cases, we found significant slopes in the PMDs, which are compatible with orbital motion whose periods (estimated from the observed gradients in the position angles) range from 60 to 550 years. However, in only four cases there are indications of a curved orbit, the ultimate proof of a gravitational bond. Conclusions. A statistical study based on the 2MASS catalogue confirms the high probability of all 16 stellar systems being gravitationally bound. Most of the secondary components are well above the mass limit of hydrogen burning stars (0.08 M ), and have masses twice as high as this value or more. Massive primary components appear to avoid the simultaneous formation of equal-mass secondary components, while extremely low-mass secondary components are hard to find for both high and low mass primaries owing to the much higher dynamic range and the faintness of the secondaries.

HD 80606: searching for the chemical signature of planet formation

“…This is supported both by observations of binaries in star-forming regions (e.g. Reipurth et al 2007;Vogt et al 2012;King et al 2012) and by numerical models of binary formation (e.g. Reipurth & Mikkola 2012;Kratter 2011).…”

A direct imaging search for close stellar and sub‐stellar companions to young nearby stars

“…The multiplicity fraction with stellar companions (detected also by direct imaging, i.e. in a similar range of separations and magnitude difference as in our study) ranges typically between 28 and 36 %, as shown in the star-forming regions Lupus (Ghez et al 1997), ScorpiusCentaurus (Köhler et al 2000), Ophiuchus (Ratzka et al 2005), Corona Australis (Neuhäuser et al 2000b;Köhler et al 2008) and Chamaeleon (Lafrenière et al 2008;Vogt et al 2012;Schmidt et al 2013). In our sample of a total of 28 stellar systems there are ten stellar binaries (as summarized in Table 5 plus the two wide binaries HD 13246 AB and HD 33802 AB, whose components were observed individually in this study) and one stellar triple system, hence our multiplicity fraction is 11/28 = 39 % (±19 %), similar to the typical values for star-forming regions.…”

“…All targets were observed in the K s -band with typical total exposure times of 20 to 30 minutes per target, composed of many short integrations of only 0.347 s, the shortest possible integration time (DIT) with NACO in full frame readout-mode, in order to minimize saturation effects, induced by the bright target stars. In order to remove the bright sky-background in the infrared the jitter-technique from the eclipse pipeline (Devillard 1997 Our observing and reduction strategy, the calibrationprocedures and the astrometric data analysis, applied in general by our working group, are described in more detail by Vogt et al (2012). Some additional information referring to the particular observing runs discussed here can be found in Mugrauer et al (2010).…”

ζ1 + ζ2 Reticuli binary system: a puzzling chromospheric activity pattern

“…Samples of binary systems were included into stellar activity studies and also used to obtain activity-rotationage calibrations. Observational data (Reipurth et al 2007;Vogt et al 2012;King et al 2012) and numerical simulations (e.g. Reipurth & Mikkola 2012) support the idea that most of binary stars are formed from a common molecular cloud (i.e., coeval stars), therefore it is expected that components of binary systems present similar properties such as age and chemical composition.…”