Other
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt..........v&link_type=abstract
PhD Thesis, University of Amsterdam, 1999
Other
17
Shock Waves, Circumstellar Matter, Stars: Formation, Stars: Pre-Main Sequence, Stars: Variables, Ism: Jets And Outflows, Infrared: Stars, Stars: V351 Ori, Afgl 490, Iras 03260+3111, L1489 Irs, L1551 Irs5, Iras 12496-7650, Ggd 27, Lkhalpha 225, Sh2-106, Afgl 2591, Cep A, Elias 3-1, T Tau, Hd 97048, Hd 97300, Hr 5999, R Cra, T Cra, Ww Vul, Bd+40 4124, Lkhalpha 224, Hd 200775
Scientific paper
In my dissertation I present the results of optical, infrared and submm studies of the circumstellar environment of young stellar objects, mostly of intermediate mass. Both the circumstellar dust and gas are studied, using results from the Hipparcos mission, the Infrared Space Observatory (ISO) and several ground-based telescopes. The basic stellar properties of a sample of Herbig Ae/Be stars (HAeBes) were derived using the astrometric data from the Hipparcos mission. The basic conclusion from this study is that most HAeBes in our sample must be pre-main sequence stars. An evolutionary scenario for the dissipation of dust around Herbig Ae/Be stars is outlined, based on the new stellar parameters derived from the Hipparcos astrometry. Using the photometry obtained by Hipparcos, I also study the circumstellar dust through its effect on the light of the central star as it moves in and out of our line of sight. I show that such an effect can only be seen towards Herbig Ae/Be systems with a central star of spectral type A0 or later, which is explained as being due to the evolutionary effect that Herbig Be stars are not optically visible while still contracting towards the zero-age main sequence. I also looked in more detail at the thermal emission from dust in the disks of two Herbig Ae systems, AB Aur and HD 163296, using ISO spectroscopy and new VLA data. In both these systems, significant grain growth has already occurred. However, the degree in which this has happened, as well as the degree in which the dust has crystallized, differs greatly between these two systems of identical mass and age. Clearly other parameters than just stellar mass and age influence the dissipation speed and degree of processing of the dust in a circumstellar disk. Another way to study circumstellar dust is through submillimeter photometry. I have followed this approach to study the dust in the star forming region associated with the Herbig stars R and T CrA, using 450 and 850 micron maps obtained with SCUBA at the JCMT. Extended emission is present throughout the region at both 450 and 850 microns. The SCUBA maps do not show an enhanced intensity at the positions of the Herbig Ae/Be stars R CrA and T CrA. Six point-like submm sources were detected in the R CrA region, of which four have not been detected before. Two of these could not be identified with a near-infrared source, making them valid new candidates for Class 0 sources. The remainder of my dissertation is devoted to ISO spectroscopy of star forming regions. I first look in some detail at a number of individual objects (S106, Cep A East, BD+40o 4124 region, T Tau). The differences found in the emission line and the solid-state spectra of these objects cannot be explained in terms of differences in orientation and mass, but must reflect an evolution of the circumstellar material. The line of sight towards the embedded YSO LkHα 225 in the BD+40° 4124 region was found to have a CO2 gas/solid ratio that is more than a factor 100 higher than that found in any other line of sight so far and may contain the most evolved molecular core known to date. After these studies of individual systems, I focus on a larger sample, consisting of 10 embedded YSOs and 11 Herbig Ae/Be stars, to study the gas in the circumstellar environment in a more systematic way. I conclude that the infrared atomic fine structure lines and the infrared molecular emission lines are in general dominated by emission from photodissociation regions (PDRs) and/or shocks. The distinction between PDRs and shocks can be made with relative ease using infrared spectroscopy using the presence of [S i] emission (indicative of the presence of a shock), strong [C ii] emission (PDR) and PAH emission (PDR). Based on these results I suggest an evolutionary scenario in which the circumstellar material around a young star changes from being heated mechanically by shocks into heated by radiation from the central star through a PDR as the star clears its surroundings. Electronic version available at: http://zon.wins.uva.nl/~mario/ or http://www.astro.uva.nl/~mario/publist.html Printed copies available on request.
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