The non-thermal 3.6 cm radio continuum emission from the young stars S1 and DoAr21 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 to 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 DoAr21 of 116.9 ^ { +7.2 } _ { -6.4 } pc and 121.9 ^ { +5.8 } _ { -5.3 } pc , respectively .
Combining these results , we estimate the mean distance to the Ophiuchus core to be 120.0 ^ { +4.5 } _ { -4.2 } pc , a value consistent with several recent indirect determinations , but with a significantly improved accuracy of 4 % .
Both S1 and DoAr21 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 .