Rosa M. Torres

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

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

VLBA DETERMINATION OF THE DISTANCE TO NEARBY STAR-FORMING REGIONS. III. HP TAU/G2 AND THE THREE-DIMENSIONAL STRUCTURE OF TAURUS

Using multiepoch Very Long Baseline Array (VLBA) observations, we have measured the trigonometric parallax of the weak-line T Tauri star HP Tau/G2 in Taurus. The best fit yields a distance of 161.2 ± 0.9 pc, suggesting that the eastern portion of Taurus (where HP Tau/G2 is located) corresponds to the far side of the complex. Previous VLBA observations have shown that T Tau, to the south of the complex, is at an intermediate distance of about 147 pc, whereas the region around L1495 corresponds to the near side at roughly 130 pc. Our observations of only four sources are still too coarse to enable a reliable determination of the three-dimensional structure of the entire Taurus star-forming complex. They do demonstrate, however, that VLBA observations of multiple sources in a given star-forming region have the potential not only to provide a very accurate estimate of its mean distance, but also to reveal its internal structure. The proper motion measurements obtained simultaneously with the parallax allowed us to study the kinematics of the young stars in Taurus. Combining the four observations available so far, we estimate the peculiar velocity of Taurus to be about 10.6 km s–1 almost completely in a direction parallel to the Galactic plane. Using our improved distance measurement, we have refined the determination of the position on the H-R diagram of HP Tau/G2, and of two other members of the HP Tau group (HP Tau itself and HP Tau/G3). Most pre-main-sequence evolutionary models predict significantly discrepant ages (by 5 Myr) for those three stars—expected to be coeval. Only in the models of Palla & Stahler do they fall on a single isochrone (at 3 Myr).

VLBA Determination of the Distance to Nearby Star-forming Regions. I. The Distance to T Tauri with 0.4% Accuracy

In this article, we present the results of a series of 12 3.6 cm radio continuum observations of T Tau Sb, one of the companions of the famous young stellar object T Tauri. The data were collected roughly every 2 months between 2003 September and 2005 July with the Very Long Baseline Array (VLBA). Thanks to the remarkably accurate astrometry delivered by the VLBA, the absolute position of T Tau Sb could be measured with a precision typically better than about 100 μas at each of the 12 observed epochs. The trajectory of T Tau Sb on the plane of the sky could therefore be traced very precisely and was modeled as the superposition of the trigonometric parallax of the source and an accelerated proper motion. The best fit yields a distance to T Tau Sb of 147.6 ± 0.6 pc. The observed positions of T Tau Sb are in good agreement with recent infrared measurements, but they seem to favor a somewhat longer orbital period than that recently reported by Duchene and coworkers for the T Tau Sa/T Tau Sb system.

VLBA determination of the distance to nearby star-forming regions II. Hubble 4 and HDE 283572 in Taurus

The nonthermal 3.6 cm radio continuum emission from the naked T Tauri stars Hubble 4 and HDE 283572 in Taurus has been observed with the Very Long Baseline Array (VLBA) at six epochs between 2004 September and 2005 December with a typical separation between successive observations of 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 uniform proper motion. The best fits yield distances to Hubble 4 and HDE 283572 of -->132.8 ± 0.5 and -->128.5 ± 0.6 pc, respectively. Combining these results with the other two existing VLBI distance determinations in Taurus, we estimate the mean distance to the Taurus association to be 137 pc with a dispersion (most probably reflecting the depth of the complex) of about 20 pc.

Multiepoch VLBA Observations of T Tauri South

In this Letter, we present a series of seven observations of the compact, nonthermal radio source associated with T Tauri South made with the Very Long Baseline Array (VLBA) over the course of 1 year. The emission is found to be composed of a compact structure most certainly originating from the magnetosphere of an underlying pre-main-sequence star and a low brightness extension that may result from reconnection flares at the star-disk interface. The accuracy of the absolute astrometry offered by the VLBA allows us to make very precise determinations of the trigonometric parallax and proper motion of T Tau South. The proper motion derived from our VLBA observations agrees with that measured with the Very Large Array over a similar period to better than 2 mas yr-1, and it is fully consistent with the infrared proper motion of T Tau Sb, the pre-main-sequence M star with which the radio source has traditionally been associated. The parallax, π = 7.07 ± 0.14 mas, corresponds to a distance of 141.5 pc.

The Gould's Belt Very Large Array Survey. I. The Ophiuchus Complex

We present large-scale ( 2000 square arcminutes), deep ( 20 Jy), high-resolution ( 1 00 ) radio observations of the Ophiuchus star-forming complex obtained with the Karl G. Jansky Very Large Array at = 4 and 6 cm. In total, 189 sources were detected, 56 of them associated with known young stellar sources, and 4 with known extragalactic objects; the other 129 remain unclassied, but most of them are most probably background quasars. The vast majority of the young stars detected at radio wavelengths have spectral types K or M, although we also detect 4 objects of A/F/B types and 2 brown dwarf candidates. At least half of these young stars are non-thermal (gyrosynchrotron) sources, with active coronas characterized by high levels of variability, negative spectral indices, and (in some cases) signicant circular polarization. As

Tidal forces as a regulator of star formation in Taurus

Only a few molecular clouds in the solar neighbourhood exhibit the formation of only low-mass stars. Traditionally, these clouds have been assumed to be supported against more vigorous collapse by magnetic fields. The existence of strong magnetic fields in molecular clouds, however, poses serious problems for the formation of stars and of the clouds themselves. In this Letter, we review the three-dimensional structure and kinematics of Taurus – the archetype of a region forming only low-mass stars – as well as its orientation within the Milky Way. We conclude that the particularly low star formation efficiency in Taurus may naturally be explained by tidal forces from the Galaxy, with no need for magnetic regulation or stellar feedback.

VLBA Observations of Strong Anisotripic Radio Scattering Toward the Orion Nebula

We present observations of VLBA 20, a radio source found towards the edge of the Orion Nebula Cluster (ONC). Nonthermal emission dominates the spectral energy distribution of this object from radio to mid-infrared regime, suggesting that VLBA 20 is extragalactic. This source is heavily scattered in the radio regime. Very Long Baseline Array observations resolve it to ~34x19 mas at 5 GHz, and the wavelength dependence of the scattering disk is consistent with

VLBA determinations of the distances to nearby star-forming regions

\nu^{-2}

Milli-arcsecond binaries

at other frequencies. The origin of the scattering is most likely the ionized X-ray emitting gas from the winds of the most massive stars of the ONC. The scattering is highly anisotropic, with the axis ratio of 2:1, higher than what is typically observed towards other sources.