High angular resolution study of the earliest stages of star formation

Resumen

In this thesis, we present an observational, high angular resolution study of the earliest stages of the star formation process, with a special emphasis in the phenomena of jets and protoplanetary disks, This study has been performed mainly from observations carried out with the radio interferometers Very Large Array (VLA), Combined Array for Research in Millimeter-wave Astronomy (CARMA), and Sub-Millimeter Array (SMA). We have studied five star-forming regions: two regions of low-mass star formation (L723 and HL/XZ Tau), one region of intermediate-mass star formation (NGC 2071), and two regions of massive star formation (W 75N and HH 80-81). We carried out VLA high angular resolution continuum observations at 3.6 cm and 7 mm towards the core of the L723 multipolar outflow. We found a multiple system of four radio sources suspected to be young stellar objects (YSOs) in a region of only 1200 AU in extent. Two of these objects (VLA 2A and VLA 2B) form a close binary system of YSOs with a separation of ~90 AU. We propose that the multipolar CO molecular outflow observed in L723 could result from the superposition of at least three independent bipolar outflows, driven by three different YSOs. Our observations suggest that VLA 2A is associated with an ionized radio jet, and that it is the driving source of the system of Herbig-Haro (HH) objects, as well as of one of the bipolar CO outflows. We propose VLA 2B as the driving source of the second CO outflow. Finally, the third CO outflow, seems to be a ''fossil'' outflow whose exciting source has not been very active in the recent past. We observed the dust emission at 7 mm associated with the young star HL Tau. This emission seems to be arising in a clumpy disk with radius of ~25 AU. We found a density gap in the disk at a radius of ~10-15 AU (similar to the radius of the orbit of Saturn) that may be the signpost of a growing protoplanet. Our observations also show that the young star XZ Tau, known to be a binary with 42 AU separation, is actually a triple star system. We propose that the remarkable ejection of gas from the XZ Tau system observed with the HST may be related to a periastron passage of our newly discovered close binary system. We studied the centimeter (free-free) and millimeter (dust) emission of the IR sources at the center of the NGC 2071 star-forming region. The centimeter emission of source IRS 1 presents an elongated morphology with protuberances at both edges. We interpret this source as a radio jet with strong interactions with the ambient medium or, alternatively, as two radio jets emerging from a close binary. Our highest angular resolution image of the core of the emission shows a hint of a double source, suggesting that a binary system could be present. The centimeter emission of source IRS 3 in NGC 2071 traces an ionized radio jet, while the millimeter emission traces a circumstellar disk of dust oriented perpendicular to the jet. The orientation of the millimeter emission coincides with that of the maser spots previously observed by Torrelles et al., that were also interpreted as tracing a circumstellar accretion disk. A modelling of the dust emission as an accretion disk allowed us to constrain the radius and accretion rate of the disk and the mass of the central star. We studied the nature and molecular environment of the radio sources in the W 75N region. Our results strongly support that source VLA 3 is a radio jet associated with a YSO. We detect important changes in total flux density, morphology, and position in the source Bc, suggesting that it is not tracing an independent star but actually is a radio HH object powered by VLA 3. If our interpretation is correct, this is one of the brightest radio HH objects known so far. Our observations of the large-scale molecular ammonia emission show that the star formation in W 75 N could be triggered by the collision of two filamentary molecular clouds. We observed the driving source of the HH 80-81 jet with the VLA and the SMA. We detected SO emission that suggests the presence of molecular gas orbiting around the driving source of the jet. From these observations, we estimate a lower limit to the central binding mass of 17 M_sun. We interpreted the 7 mm emission associated with the driving source of the HH 80-81 jet as a combination of free-free emission from the radio jet, and thermal dust emission from a perpendicular, circumstellar disk. After substraction of the free-free emission, we successfully modeled the 7 mm emission as arising from an accretion disk with a radius of 420 AU, an accretion rate of 7x10^{-6} M_sun/yr, and a mass of 14 M_sun, orbiting around a 10 M_sun protostar. We performed sensitive observations at 6 cm wavelength of the HH 80-81 jet revealing that the emission in the jet lobes, at ~0.5 pc from the driving source, is linearly polarized. The detection of linearly polarized emission confirms its synchrotron nature, implying the presence of relativistic electrons and a magnetic field associated with the radio jet. Following procedures similar to those used for extragalactic radio jets, that are characterized by strong synchrotron emission, we inferred the structure and strength of the magnetic field in the HH 80-81 jet. Our discovery of synchrotron radiation in a jet from a YSO represents an important step in the unification of the collimated outflow phenomena observed in many astrophysical contexts.