122 research outputs found
Interpreting Spectral Energy Distributions from Young Stellar Objects. I. A grid of 200,000 YSO model SEDs
We present a grid of radiation transfer models of axisymmetric young stellar
objects (YSOs), covering a wide range of stellar masses (from 0.1Msun to
50Msun) and evolutionary stages (from the early envelope infall stage to the
late disk-only stage). The grid consists of 20,000 YSO models, with spectral
energy distributions (SEDs) and polarization spectra computed at ten viewing
angles for each model, resulting in a total of 200,000 SEDs. [...]. These
models are publicly available on a dedicated WWW server:
http://www.astro.wisc.edu/protostars/ . In this paper we summarize the main
features of our models, as well as the range of parameters explored. [...]. We
examine the dependence of the spectral indices of the model SEDs on envelope
accretion rate and disk mass. In addition, we show variations of spectral
indices with stellar temperature, disk inner radius, and disk flaring power for
a subset of disk-only models. We also examine how changing the wavelength range
of data used to calculate spectral indices affects their values. We show sample
color-color plots of the entire grid as well as simulated clusters at various
distances with typical {\it Spitzer Space Telescope} sensitivities. We find
that young embedded sources generally occupy a large region of color-color
space due to inclination and stellar temperature effects. Disk sources occupy a
smaller region of color-color space, but overlap substantially with the region
occupied by embedded sources, especially in the near- and mid-IR. We identify
regions in color-color space where our models indicate that only sources at a
given evolutionary stage should lie. [...].Comment: 69 pages, 28 figures, Accepted for publication in ApJS. Preprint with
full resolution figures available at http://www.astro.wisc.edu/protostars
2-D and 3-D Radiation Transfer Models of High-Mass Star Formation
2-D and 3-D radiation transfer models of forming stars generally produce
bluer 1-10 micron colors than 1-D models of the same evolutionary state and
envelope mass. Therefore, 1-D models of the shortwave radiation will generally
estimate a lower envelope mass and later evolutionary state than
multidimensional models. 1-D models are probably reasonable for very young
sources, or longwave analysis (wavelengths > 100 microns). In our 3-D models of
high-mass stars in clumpy molecular clouds, we find no correlation between the
depth of the 10 micron silicate feature and the longwave (> 100 micron) SED
(which sets the envelope mass), even when the average optical extinction of the
envelope is >100 magnitudes. This is in agreement with the observations of
Faison et al. (1998) of several UltraCompact HII (UCHII) regions, suggesting
that many of these sources are more evolved than embedded protostars.
We have calculated a large grid of 2-D models and find substantial overlap
between different evolutionary states in the mid-IR color-color diagrams. We
have developed a model fitter to work in conjunction with the grid to analyze
large datasets. This grid and fitter will be expanded and tested in 2005 and
released to the public in 2006.Comment: 10 pages, 8 figures, to appear in the proceedings of IAU Symp 227,
Massive Star Birth: A Crossroads of Astrophysics, (Cesaroni R., Churchwell
E., Felli M., Walmsley C. editors
OVI, NV and CIV in the Galactic Halo: II. Velocity-Resolved Observations with Hubble and FUSE
We present a survey of NV and OVI (and where available CIV) in the Galactic
halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Hubble Space Telescope (HST) along 34 sightlines. These ions are usually
produced in nonequilibrium processes such as shocks, evaporative interfaces, or
rapidly cooling gas, and thus trace the dynamics of the interstellar medium.
Searching for global trends in integrated and velocity-resolved column density
ratios, we find large variations in most measures, with some evidence for a
systematic trend of higher ionization (lower NV/OVI column density ratio) at
larger positive line-of-sight velocities. The slopes of log[N(NV)/N(OVI)] per
unit velocity range from -0.015 to +0.005, with a mean of
-0.0032+/-0.0022(r)+/-0.0014(sys) dex/(km/s). We compare this dataset with
models of velocity-resolved high-ion signatures of several common physical
structures. The dispersion of the ratios, OVI/NV/CIV, supports the growing
belief that no single model can account for hot halo gas, and in fact some
models predict much stronger trends than are observed. It is important to
understand the signatures of different physical structures to interpret
specific lines of sight and future global surveys.Comment: ApJ in press 43 pages, 22 fig
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