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

Abstract

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.

The chemistry of planet-forming regions is not interstellar

“…For this paper we chose RNO 90, a young solar-mass star (G5) with strong water vapor emission in the mid-infrared. RNO 90 is located near (but not inside) the LDN 43 dark cloud roughly 10 degrees from the well-known ρ Ophiuchus young stellar cluster at a distance generally assumed to be that of the cluster of 125 pc 23,24 . The disk is optically thick in the infrared at all radii, and there have been no detections of structural "complications" as seen in more evolved disks, such as TW Hya 25 , although RNO 90 is also not as thoroughly observed.…”

The Gould Belt

“…Radio observations of water masers at 22.2 GHz [68] are being carried out in the Japanese program VERA (VLBI Exploration of Radio Astronomy). The trigonometric parallaxes of a number of low-mass nearby stars associated with the Gould belt have been determined as part of a separate program for the VLBI observations of radio stars in the continuum [69,70]. At present, the trigonometric parallaxes of more than 100 maser galactic sources have been measured by various groups of radio astronomers [71].…”

First stars of theρOphiuchi dark cloud

“…A range of distances to the cloud are reported in literature (Wilking et al 2008), spanning the range 120 pc to 145 pc; here we will use the distance of 120 pc, obtained from VLBI measurements by Loinard et al (2008) and valid for the northern part of the cloud where ρ Ophiuchi is located. The uncertainty on the distance can produce a systematic error of the X-ray luminosities of ∼0.16 dex.…”