High-energy processes in low-mass protostars – an X-ray to radio multi-wavelength perspective

Abstract

High-energy processes in protostars remain poorly understood. Only after the recently finished Chandra Orion Ultra-deep Project (COUP), statistically significant information on X-ray emission from Young Stellar Objects (YSOs) has been obtained. For an understanding of the mechanisms responsible for the X-ray emission, multi-wavelength correlations of flares, especially in the radio regime, are necessary and have become an active field of research. Magnetic fields cause several high-energy phenomena in the coronae of YSOs mainly due to magnetic reconnection which are observable in a wide wavelength range from nonthermal centimetric radio emission to X-rays. In this work, these processes were probed using a variety of very different methods which can be grouped into three major topics: <br /><strong>X-ray to radio multi-wavelength variability of Young Stellar Objects</strong> The Coronet cluster in the nearby R~CrA dark cloud offers the rare opportunity to study a compact cluster of several very young protostars which are detected at radio- and X-ray wavelengths. Initially, a study focusing separately on X-ray and radio variability of these sources was conducted. Subsequently, in August 2005, the same region was studied in the first simultaneous X-ray, radio, near-infrared, and optical monitoring campaign of YSOs. Several observatories were observing simultaneously, namely the Chandra X-ray Observatory, the VLA, as well as telescopes with optical and near-infrared detectors in Chile and South Africa. Remarkable intra-band variability but no clearly correlated variability was found. This most importantly suggests that there is no direct link between the X-ray and optical/infrared emission and supports the notion that accretion is not an important source for the X-ray emission of these YSOs. Combining the Chandra X-ray data collected in the course of the multi-wavelength campaign with previous archival data, one of the deepest X-ray datasets ever obtained of a star-forming region is obtained and discussed.<br /><strong>Radio emission from protostars</strong> Looking for compact nonthermal centimetric radio emission, high-sensitivity Very-Long-Baseline Interferometry (VLBI) observations of four nearby protostars were carried out, yielding the currently most sensitive data of such sources. Weak compact emission was found in the VLBI data of the class~0/I binary YLW15 VLA2, constraining the size of its corona to sub-AU scales. Since this source is part of a binary system with observed orbital motion, further VLBI observations will allow to quickly determine the orbit very accurately. The observed sources apparently were showing quiescent radio emission on the larger scales probed by the Very Large Array (VLA). Until now, only very few radio flares of YSOs have been observed in detail. In further work, two such examples are presented and analyzed: a flaring, deeply embedded protostar in Orion and a flaring binary T~Tauri system whose activity is due to inter-binary coronal interaction.<br /><strong>The earliest stages: Magnetic fields in molecular clouds</strong> While mapping molecular clouds in polarized dust continuum emission has become a standard technique, the potentially more powerful technique using the "Goldreich-Kylafis" effect has been only rarely used until now. This effect predicts weakly linearly polarized molecular line emission under certain circumstances. By choosing different transitions, it is possible to probe the magnetic field direction in different regions in a molecular cloud core, and additionally one gets information along the line of sight for optically thin emission lines. The XPOL correlation polarimeter at the IRAM 30m telescope was used in a search for such linearly polarized emission in several bright molecular transition lines towards prominent star-forming regions. The combined effects of instrumental polarization and extended emission were simulated for a thorough interpretation of the results. In one case, the observed polarization exceeds the simulated instrumental value.