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Register a new userInfrared interferometric observations of young stellar objects
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We present infrared observations of four young stellar objects using the Palomar Testbed Interferometer (PTI). For three of the sources, T Tau, MWC 147 and SU Aur, the 2.2 micron emission is resolved at PTIs nominal fringe spacing of 4 milliarcsec (mas), while the emission region of AB Aur is over-resolved on this scale. We fit the observations with simple circumstellar material distributions and compare our data to the predictions of accretion disk models inferred from spectral energy distributions. We find that the infrared emission region is tenths of AU in size for T Tau and SU Aur and ~1 AU for MWC 147.
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We discuss VLTI AMBER and MIDI interferometry in addition to single-dish Subaru observations of massive young stellar objects. The observations probe linear size scales between 10 to 1000 AU for the average distance of our sources.
The very inner structure of massive young stellar objects (YSOs) is difficult to trace. With conventional observational methods we identify structures still several hundreds of AU in size. However, the (proto-)stellar growth takes place at the innermost regions (<100 AU) where the actual mass transfer onto the forming high-mass star occurs. We present results from our programme toward massive YSOs at the VLTI, utilising the two-element interferometer MIDI. To date, we observed 10 well-known massive YSOs down to scales of 20 mas (typically corresponding to 20 - 40 AU for our targets) in the 8-13 micron region. We clearly resolve these objects which results in low visibilities and sizes in the order of 30-50 mas. For two objects, we show results of our modelling. We demonstrate that the MIDI data can reveal decisive structure information for massive YSOs. They are often pivotal in order to resolve ambiguities still immanent in model parameters derived from sole SED fitting.
Optical and near-infrared variability is a well-known property of young stellar objects. However, a growing number of recent studies claim that a considerable fraction of them also exhibit mid-infrared flux changes. With the aim of studying and interpreting variability on a decadal timescale, here we present a mid-infrared spectral atlas containing observations of 68 low- and intermediate mass young stellar objects. The atlas consists of 2.5-11.6 um low-resolution spectra obtained with the ISOPHOT-S instrument on-board the Infrared Space Observatory (ISO) between 1996 and 1998, as well as 5.2-14.5 um low-resolution spectra obtained with the IRS instrument on-board the Spitzer Space Telescope between 2004 and 2007. The observations were retrieved from the ISO and Spitzer archives and were post-processed interactively by our own routines. For those 47 objects where multi-epoch spectra were available, we analyze mid-infrared spectral variability on annual and/or decadal timescales. We identify 37 variable candidate sources. Many stars show wavelength-independent flux changes, possibly due to variable accretion rate. In several systems, all exhibiting 10 um silicate emission, the variability of the 6-8 um continuum and the silicate feature exhibit different amplitudes. A possible explanation is variable shadowing of the silicate emitting region by an inner disk structure of changing height or extra silicate emission from dust clouds in the disk atmosphere. Our results suggest that mid-infrared variability, in particular the wavelength-dependent changes, are more ubiquitous than was known before. Interpreting this variability is a new possibility to explore the structure of the disk and its dynamical processes.
The study of the chemical evolution of gas and dust from pre-stellar dense cores to circumstellar disks around young stars forms an essential part of understanding star- and planet formation. Throughout the collapse- and protostellar phases, simple and complex molecules are formed, many of which deplete onto cold grains and are eventually incorporated into the icy planetesimals of new solar systems. Tracing this chemical evolution provides a wealth of information, not only about the chemical processing in primitive solar nebulae, but also about physical processes which occur in the immediate surroundings of young stellar objects (YSOs). Here we review the chemical processes which occur in the protostellar environment, and models and observations of the chemical structure of the various stages of star formation. We briefly discuss the way in which molecular abundances are derived from observations, and conclude with two examples: the low- to intermediate mass YSOs in Serpens, and the massive YSOs in W3.
The results of a near-infrared (J H K LP) imaging linear polarimetry survey of 20 young stellar objects (YSOs) in rho Ophiuchi are presented. The majority of the sources are unresolved, with K-band polarizations, P_K < 6 per cent. Several objects are associated with extended reflection nebulae. These objects have centrosymmetric vector patterns with polarization discs over their cores; maximum polarizations of P_K > 20 per cent are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K-band core polarizations >6 per cent are only observed in Class I YSOs. A 3D Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the rho Oph YSOs with extended nebulae. A rho proportional to r^(-1.5) power law for the density is applied throughout the envelopes. The large-scale centrosymmetric polarization structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned non-spherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually <10 per cent, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes <=1.05 microns.
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