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Register a new userVLBA determinations of the distances to nearby star-forming regions
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Authors
Laurent Loinard
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Using phase-referenced multi-epoch Very Long Baseline Array observations, we have measured the trigonometric parallax of several young stars in the Taurus and Ophiuchus star-forming regions with unprecedented accuracy. The mean distance to the Taurus complex was found to be about 140 pc, and its depth around 20 pc, comparable to the linear extent of Taurus on the plane of the sky. In Ophiuchus, 4 sources were observed so far. Two of them were found to be at about 160 pc (the distance traditionally attributed to Ophiuchus), while the other 2 are at about 120 pc. Since the entire Ophiuchus complex is only a few parsecs across, this difference is unlikely to reflect the depth of the region. Instead, we argue that two physically unrelated sites of star-formation are located along the line of sight toward Ophiuchus.
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The determination of accurate distances to star-forming regions are discussed in the broader historical context of astronomical distance measurements. We summarize recent results for regions within 1 kpc and present perspectives for the near and more distance future.
The LkH$alpha$ 101 cluster takes its name from its more massive member, the LkH$alpha$ 101star, which is an $sim15$ M$_odot$ star whose true nature is still unknown. The distance to the LkH$alpha$ 101 cluster has been controversial for the last few decades, with estimated values ranging from 160 to 800 pc. We have observed members and candidate members of the LkH$alpha$ 101 cluster with signs of magnetic activity, using the Very Long Baseline Array, in order to measure their trigonometric parallax and, thus, obtain a direct measurement of their distances. A young star member, LkH$alpha$ 101 VLA J043001.15+351724.6, was detected at four epochs as a single radio source. The best fit to its displacement on the plane of the sky yields a distance of 535$pm$29 pc. We argue that this is the distance to the LkH$alpha$ 101 cluster.
In this article, we present the results of a series of twelve 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 two months between September 2003 and July 2005 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 micro-arcseconds at each of the twelve observed epochs. The trajectory of T Tau Sb on the plane of the sky could, therefore, be traced very precisely, and 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 seem to favor a somewhat longer orbital period than that recently reported by Duchene et al. (2006) for the T Tau Sa/T Tau Sb system.
The non-thermal 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 6 epochs between September 2004 and December 2005 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.
Multi-epoch radio-interferometric observations of young stellar objects can be used to measure their displacement over the celestial sphere with a level of accuracy that currently cannot be attained at any other wavelength. In particular, the accuracy achieved using carefully calibrated, phase-referenced observations with Very Long Baseline Interferometers such as NRAOs Very Long Baseline Array is better than 50 micro-arcseconds. This is sufficient to measure the trigonometric parallax and the proper motion of any radio-emitting young star within several hundred parsecs of the Sun with an accuracy better than a few percent. Using that technique, the mean distances to Taurus, Ophiuchus, Perseus and Orion have already been measured to unprecedented accuracy. With improved telescopes and equipment, the distance to all star-forming regions within 1 kpc of the Sun and beyond, as well as their internal structure and dynamics could be determined. This would significantly improve our ability to compare the observational properties of young stellar objects with theoretical predictions, and would have a major impact on our understanding of low-mass star-formation.
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