A Preliminary VLBA Distance to the Core of Ophiuchus, with an Accuracy of 4%

Abstract

ABSTRACTThe non-thermal 3.6 cm radio continuum emission from the young stars S1and DoAr21 in the core of Ophiuchus, has been observed with the Very LongBaseline Array (VLBA) at 6 and 7 epochs, respectively, between June 2005 andAugust 2006. The typical separation between successive observations was 2 to 3months. Thanks to the remarkably accurate astrometry delivered by the VLBA,the trajectory described by both stars on the plane of the sky could be tracedvery precisely, and modeled as the superposition of their trigonometric parallaxand a uniform proper motion. The best fits yield distances to S1 and DoAr21of 116.9 +7.2−6.4 pc and 121.9 +5.8−5.3 pc, respectively. Combining these results, we es-timate the mean distance to the Ophiuchus core to be 120.0 +4.5−4.2 pc, a valueconsistent with several recent indirect determinations, but with a significantlyimproved accuracy of 4%. Both S1 and DoAr21 happen to be members of tightbinary systems, but our observations are not frequent enough to properly derivethe corresponding orbital parameters. This could be done with additional data,however, and would result in a significantly improved accuracy on the distancedetermination.Subject headings: Astrometry — Stars: individual (S1, DoAr21) — Radiationmechanisms: non-thermal — Magnetic fields — stars: formation — Binaries:general1. IntroductionOphiuchus is one of the most active regions of star-formation within a few hundred par-secs of the Sun (e.g. Lada & Lada 2003). It has played an important role in the development