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

The nonthermal 3.6 cm radio continuum emission from the young stars S1 and DoAr 21 in the core of Ophiuchus has been observed with the Very Long Baseline Array (VLBA) at 6 and 7 epochs, respectively, between June 2005 and August 2006. The typical separation between successive observations was 2-3 months. Thanks to the remarkably accurate astrometry delivered by the VLBA, the trajectory described by both stars on the plane of the sky could be traced very precisely, and modeled as the superposition of their trigonometric parallax and a uniform proper motion. The best fits yield distances to S1 and DoAr 21 of 116.9−6.4+7.2 and 121.9−5.3+5.8 pc, respectively. Combining these results, we estimate the mean distance to the Ophiuchus core to be 120.0−4.2+4.5 pc, a value consistent with several recent indirect determinations, but with a significantly improved accuracy of 4%. Both S1 and DoAr 21 happen to be members of tight binary systems, but our observations are not frequent enough to properly derive the corresponding orbital parameters. This could be done with additional data, however, and would result in a significantly improved accuracy on the distance determination.