Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt..........r&link_type=abstract
PhD Thesis, Astrophysics Research Institute, Liverpool John Moores University, UK, 2001.
Astronomy and Astrophysics
Astrophysics
1
Scientific paper
The prevalence of CO outflows around young stellar objects (YSOs) suggests that mass-loss is a necessary ingredient of the star-formation process, and they are thought to provide a means to remove angular momentum from the accreting matter. Our understanding of molecular outflows from low-mass YSOs has come from a large number of studies performed during the last decade. However there have been relatively few similar detailed studies of high-mass YSO outflows, and it is still not clear whether outflows from high-mass YSOs are generated and entrained by the same mechanism as those from their low-mass counterparts. Studies of high-mass star-formation which attempt to extend the established correlations between outflow momentum-flux and source bolometric luminosity into the high-mass regime are further hindered by the problem of Malmquist bias, which we show to be prevalent in the existing studies of high-mass YSOs. We therefore selected a sample of known intermediate to high-mass YSOs with outflows with a range of luminosities but all located at a distance of 2 +/- 0.3 kpc. With this sample we are able to test the correlations between outflow dynamical properties and source properties free from biases due to source distance, and to investigate whether the other phenomena associated with low-mass YSO outflows are also common in high-mass flows. We present high-sensitivity 12CO maps of the 11 outflows from intermediate to high-mass YSOs, and discuss their morphology and dynamics. These data were also used to re-investigate the uncertainties involved in using spectral-line data to derive flow properties, particularly the importance of variations in optical depth and the problem of separating ambient cloud emssion from flow emission. We also present 13CO and C18O observations of the ``cores'' surrounding each YSO and discuss how the cloud core and outflow may be related. The primary conclusions of this work are: The correlation between outflow momentum flux (FrmCO ) and source bolometric luminosity (Lbol) is less well-established for high-mass YSOs than has previously been accepted. Outflows from high-mass YSOs show a much larger range (~1 to ~12) of γ, the slope of the mass-velocity relation than their low-mass counterparts, and show no Hubble-like law. They are also in general much more poorly collimated than outflows from low-mass YSOs. The correlation between outflow power and source bolometric luminosity may be purely due to an underlying relation between cloud core mass and flow mass, suggesting that the dynamical properties of molecular material may tell us nothing about the generation and entrainment of the flow. The choice of velocity cut-off between flow material and ambient cloud material dominates the uncertainty in masses derived from 12CO observations, and may increase the uncertainty in the derived flow mass by a factor of ~10 greater than estimates by previous studies which have not recognised this problem.
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