A VLBA Survey of Radio Stars in the Orion Nebula Cluster. II. Astrometry

Abstract

From Very Long Baseline Array (VLBA) observations we previously identified a population of 123 young stellar systems with nonthermal radio emission toward the core of the Orion Nebula Cluster (ONC). We find optical sources in the Gaia DR2 catalog for 34 of them within 0. 2 of the radio positions. Most of the radio sources are likely companions of Gaia detections. However, there are 11 stars whose VLBA position differ from Gaia by < 1.6 mas, and some of these radio sources probably are the direct counterparts of the optical stars. We are able to obtain radio proper motions for 23 stars. Combining the stellar proper motions derived from the VLBA and Gaia DR2 data we find the global motion and velocity dispersion of the ONC to be (µ α * , µ δ )=(1.20± 0.09, 0.18 ± 0.09) mas yr −1 and (σ µα * , σ µ δ )=(0.84 ± 0.09, 1.30 ± 0.09) mas yr −1 . Finally, we looked for ordered motions by estimating the means of scalar and vectorial products, which results in v •r = −0.61 ± 1.00 km s −1 and v ×r = 0.57 ± 0.95 km s −1 . These do not show indications that the young stellar cluster is in expansion, contraction or rotation.

The Life and Times of Giant Molecular Clouds

“…In this scenario almost all stellar velocity vectors should point radially towards or away from a dense, bound cluster that is the collapse center, while in a scenario where collapse is local, much less organisation is expected. Recent Gaia observations have largely failed to detect the radial signature expected for global collapse (Kounkel et al 2018;Ward and Kruijssen 2018;Kuhn et al 2019;Dzib et al 2021). On the contrary, observations of 109 OB associations shows that they are not the relic of expanding clusters but were formed with low levels of expansion, tracing the fractal structure of their parent GMC (Ward et al 2020a).…”

Non-thermal radio continuum emission from young nearby stars

“…This statistical anomaly could therefore hint at the presence of additional still unknown close companions and thus an even higher close companion fraction. This suspicion is supported by the findings of Forbrich et al (2021) and Dzib et al (2021), who carried out a VLBA survey for nonthermal emission toward 556 compact radio sources previously identified in a deep VLA survey of the Orion Nebula Cluster (ONC; Forbrich et al 2016Forbrich et al , 2017, of which they detected 123 sources with the VLBA. Of the 34 VLBA radio sources that are associated with GDR2-listed stars within 0.…”

“…The radio-bright nearby young stars identified here provide an interesting sample for future astrometric studies using VLBI arrays aimed at searching for hitherto unknown tight binary components or even exoplanets. The promising potential of such an approach, for both identifying hitherto unknown companions and obtaining high-precision astrometry, has been demonstrated recently by Forbrich et al (2021) and Dzib et al (2021). But, these studies also showed that, owing to the strongly variable nature of the coronal gyro-synchrotron emission, not all VLA-detected sources might be detectable later with the VLBA.…”

“…ativistic, seems to be the most common situation, but maser-amplified cyclotron emission (where the electrons are non-relativistic) and synchrotron radiation (associated with highly relativistic electrons) have been suggested in a few cases (Dzib et al 2010;Deller et al 2013). Regardless of the exact emission mechanism, the emission is normally confined to regions extending at most a few stellar radii (Smith et al 2003;Massi et al 2006;Torres et al 2012), making high resolution Very Long Baseline Interferometry (VLBI) observations of coronal radio emission a powerful tool to study the astrometry of low-mass young stars (e.g., Ortiz-León et al 2017a;Dzib et al 2021). Furthermore, milli-arcsecond resolution VLBI observations would efficiently filter out any possible thermal free-free contribution to the radio fluxes, since even faint detections will have corresponding brightness temperatures of several 10 6 K (Forbrich et al 2021).…”

“…Of prime interest are of course the position predictions for the stars in question at the time of the respective VLA observations, which we compare in the following to the actually observed VLA detections. The position predictions for the other epochs/missions are only used to assess the consistency of the multi-epoch astrometry for the respective star and to identify stars with generally troublesome astrometry, which could indicate, e.g., unresolved multiplicity (see, e.g., Dzib et al 2021;Forbrich et al 2021).…”