374 research outputs found
Emission from Very Small Grains and PAH Molecules in Monte Carlo Radiation Transfer Codes: Application to the Edge-On Disk of Gomez's Hamburger
We have modeled optical to far infrared images, photometry, and spectroscopy
of the object known as Gomez's Hamburger. We reproduce the images and spectrum
with an edge-on disk of mass 0.3M_sun and radius 1600AU, surrounding an A0 III
star at a distance of 280pc. Our mass estimate is in excellent agreement with
recent CO observations. However, our distance determination is more than an
order of magnitude smaller than previous analyses which inaccurately
interpreted the optical spectrum. To accurately model the infrared spectrum we
have extended our Monte Carlo radiation transfer codes to include emission from
polycyclic aromatic hydrocarbon (PAH) molecules and very small grains (VSG). We
do this using pre-computed PAH/VSG emissivity files for a wide range of values
of the mean intensity of the exciting radiation field. When Monte Carlo energy
packets are absorbed by PAHs/VSGs we reprocess them to other wavelengths by
sampling from the emissivity files, thus simulating the absorption and
re-emission process without reproducing lengthy computations of statistical
equilibrium, excitation and de-excitation in the complex many level molecules.
Using emissivity lookup tables in our Monte Carlo codes gives the flexibility
to use the latest grain physics calculations of PAH/VSG emissivity and opacity
that are being continually updated in the light of higher resolution infrared
spectra. We find our approach gives a good representation of the observed PAH
spectrum from the disk of Gomez's Hamburger. Our models also indicate the
PAHs/VSGs in the disk have a larger scaleheight than larger radiative
equilibrium grains, providing evidence for dust coagulation and settling to the
midplane.Comment: ApJ accepte
Forming the Dusty Ring in HR 4796A
We describe planetesimal accretion calculations for the dusty ring observed
in the nearby A0 star HR 4796A. Models with initial masses of 10-20 times the
minimum mass solar nebula produce a ring of width 7-15 AU and height 0.3-0.6 AU
at 70 AU in roughly 10 Myr. The ring has a radial optical depth of 1. These
results agree with limits derived from infrared images and from the excess
infrared luminosity.Comment: 6 pages, including 2 figures and 1 table; ApJ Letters, in pres
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
Relationships Among Occupation/Activity Patterns, Health and Stress Perceptions, and Life Orientation in Well Adults
Background: This study examined the relationships among activity choices, perceived health, stress, and life orientation (optimism-pessimism) in a general population of 675 healthy adults ranging in age from 18-91 years. The objective was to examine assumptions that occupational scientists and practitioners hold regarding the relationships among health factors and engagement in activities/occupations.
Method: The study used four self-report measures, including a customized activity card sort that asked participants about both healthy and unhealthy activity patterns. Responses were then compared with the participants’ perceptions of overall health, stress levels, and degrees of optimism and pessimism (life orientation).
Results: Major findings confirmed that being engaged in more activities overall is aligned with more optimally perceived health, positive life orientation, and lower stress. However, participation in unhealthy activities negatively affected overall health, stress perception, and life orientation. The study also confirmed that women tend to have higher perceived stress than men and that caring for others is associated with more positive health ratings.
Conclusion: The amount and type of activity participation appears to matter for even healthy individuals in terms of overall health, stress perception, and life orientation. This study confirms the importance of participating in a wide repertoire of activities and underscores the need for practitioners to ask clients about engagement in unhealthy activities as well as healthy ones
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