We present the first public version (v0.2) of the open-source and community-developed Python package, Astropy. This package provides core astronomy-related functionality to the community, including support for domain-specific file formats such as Fle
xible Image Transport System (FITS) files, Virtual Observatory (VO) tables, and common ASCII table formats, unit and physical quantity
We present a self-consistent three-dimensional Monte-Carlo radiative transfer model of the stellar and dust emission in the Milky-Way, and have computed synthetic observations of the 3.6 to 100 microns emission in the Galactic mid-plane. In order to
compare the model to observations, we use the GLIMPSE, MIPSGAL, and IRAS surveys to construct total emission spectra, as well as longitude and latitude profiles for the emission. The distribution of stars and dust is taken from the SKY model, and the dust emissivities includes an approximation of the emission from polycyclic aromatic hydrocarbons in addition to thermal emission. The model emission is in broad agreement with the observations, but a few modifications are needed to obtain a good fit. Firstly, by adjusting the model to include two major and two minor spiral arms rather than four equal spiral arms, the fit to the longitude profiles for |l|>30 degrees can be improved. Secondly, introducing a deficit in the dust distribution in the inner Galaxy results in a better fit to the shape of the IRAS longitude profiles at 60 and 100 microns. With these modifications, the model fits the observed profiles well, although it systematically under-estimates the 5.8 and 8.0 microns fluxes. One way to resolve this discrepancy is to increase the abundance of PAH molecules by 50% compared to the original model, although we note that changes to the dust distribution or radiation field may provide alternative solutions. Finally, we use the model to quantify which stellar populations contribute the most to the heating of different dust types, and which stellar populations and dust types contribute the most to the emission at different wavelengths.
HYPERION is a new three-dimensional dust continuum Monte-Carlo radiative transfer code that is designed to be as generic as possible, allowing radiative transfer to be computed through a variety of three-dimensional grids. The main part of the code i
s problem-independent, and only requires an arbitrary three-dimensional density structure, dust properties, the position and properties of the illuminating sources, and parameters controlling the running and output of the code. HYPERION is parallelized, and is shown to scale well to thousands of processes. Two common benchmark models for protoplanetary disks were computed, and the results are found to be in excellent agreement with those from other codes. Finally, to demonstrate the capabilities of the code, dust temperatures, SEDs, and synthetic multi-wavelength images were computed for a dynamical simulation of a low-mass star formation region. HYPERION is being actively developed to include new features, and is publicly available (http://www.hyperion-rt.org).
Min et al. (2009) presented two complementary techniques that use the diffusion approximation to allow efficient Monte-Carlo radiation transfer in very optically thick regions: a modified random walk and a partial diffusion approximation. In this not
e, I show that the calculations required for the modified random walk method can be significantly simplified. In particular, the diffusion coefficient and the mass absorption coefficients required for the modified random walk are in fact the same as the standard diffusion coefficient and the Planck mean mass absorption coefficient.
We present initial results from a population synthesis model aimed at determining the star formation rate of the Milky-Way. We find that a total star formation rate of 0.68 to 1.45 Msun/yr is able to reproduce the observed number of young stellar obj
ects in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the 3D Galactic distributions of YSOs and interstellar dust, and using model SEDs to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, Halpha, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the star formation rate estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic star formation rate, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.
We present a highly reliable flux-limited census of 18,949 point sources in the Galactic mid-plane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope GLIMPSE I and II surveys of 274 deg^2 of
the Galactic mid-plane, and consist mostly of high- and intermediate- mass young stellar objects (YSOs) and asymptotic giant branch (AGB) stars. The selection criteria were carefully chosen to minimize the effects of position- dependent sensitivity, saturation, and confusion. The distribution of sources on the sky and their location in IRAC and MIPS 24 microns color-magnitude and color-color space are presented. Using this large sample, we find that YSOs and AGB stars can be mostly separated by simple color-magnitude selection criteria into approximately 50-70% of YSOs and 30-50% of AGB stars. Planetary nebulae and background galaxies together represent at most 2-3% of all the red sources. 1,004 red sources in the GLIMPSE II region, mostly AGB stars with high mass- loss rates, show significant (>0.3 mag) variability at 4.5 and/or 8.0 microns. With over 11,000 likely YSOs and over 7,000 likely AGB stars, this is to date the largest uniform census of AGB stars and high- and intermediate mass YSOs in the Milky-Way Galaxy.
In this contribution, I review the applications and potential limitations of the spectral energy distribution fitting tool that I have developed, with a strong emphasis on the limits to which this tool can be used to improve our understanding of mass
ive star formation. I discuss why our current grid of models cannot be used to distinguish between the several competing theories of massive star formation. I also discuss stellar mass determinations, artificial correlations between parameters in the grid of models, multiplicity, confusion, dust assumptions, and unique fits. I briefly review the improvements we intend to carry out for our next grid of models, which will eliminate many of these limitations. Finally, I show examples of applications of this tool to massive young stars.
We present a list of 552 sources with suspected variability, based on a comparison of mid-infrared photometry from the GLIMPSE I and MSX surveys, which were carried out nearly a decade apart. We were careful to address issues such as the difference i
n resolution and sensitivity between the two surveys, as well as the differences in the spectral responses of the instruments. We selected only sources where the IRAC 8.0 and MSX 8.28 micron fluxes differ by more than a factor of two, in order to minimize contamination from sources where the difference in fluxes at 8 micron is due to a strong 10 micron silicate feature. We present a subset of 40 sources for which additional evidence suggests variability, using 2MASS and MIPSGAL data. Based on a comparison with the variability flags in the IRAS and MSX Point-Source Catalogs we estimate that at least a quarter of the 552 sources, and at least half of the 40 sources are truly variable. In addition, we tentatively confirm the variability of one source using multi-epoch IRAS LRS spectra. We suggest that most of the sources in our list are likely to be Asymptotic Giant Branch stars.
