Thomas P. Robitaille

Astropy: A community Python package for astronomy

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 flexible image transport system (FITS) files, Virtual Observatory (VO) tables, and common ASCII table formats, unit and physical quantity conversions, physical constants specific to astronomy, celestial coordinate and time transformations, world coordinate system (WCS) support, generalized containers for representing gridded as well as tabular data, and a framework for cosmological transformations and conversions. Significant functionality is under active development, such as a model fitting framework, VO client and server tools, and aperture and point spread function (PSF) photometry tools. The core development team is actively making additions and enhancements to the current code base, and we encourage anyone interested to participate in the development of future Astropy versions.

Interpreting Spectral Energy Distributions from Young Stellar Objects. II. Fitting Observed SEDs Using a Large Grid of Precomputed Models

We present a method to analyze the spectral energy distributions (SEDs) of young stellar objects (YSOs). Our approach is to fit data with precomputed two-dimensional (2D) radiation transfer models spanning a large region of parameter space. This allows us to determine not only a single set of physical parameter values but the entire range of values consistent with the multiwavelength observations of a given source. In this way we hope to avoid any overinterpretation when modeling a set of data. We have constructed spectral energy distributions from optical to submillimeter wavelengths, including new Spitzer IRAC and MIPS photometry, for 30 young and spatially resolved sources in the Taurus-Auriga star-forming region. We demonstrate fitting model SEDs to these sources and find that we correctly identify the evolutionary stage and physical parameters found from previous independent studies, such as disk mass, disk accretion rate, and stellar temperature. We also explore how fluxes at various wavelengths help to constrain physical parameters and show examples of degeneracies that can occur when fitting SEDs. A Web-based version of this tool is available to the community.

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 0.1 to 50 M☉) 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 10 viewing angles for each model, resulting in a total of 200,000 SEDs. We have made a careful assessment of the theoretical and observational constraints on the physical conditions of disks and envelopes in YSOs and have attempted to fully span the corresponding regions in parameter space. These models are publicly available on a dedicated Web server. In this paper we summarize the main features of our models, as well as the range of parameters explored. Having a large grid covering reasonable regions of parameter space allows us to shed light on many trends in near- and mid-IR observations of YSOs (such as changes in the spectral indices and colors of their SEDs), linking them with physical parameters (such as disk and infalling envelope parameters). In particular, 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 Spitzer 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. We find that, while near-IR (such as JHK) and mid-IR (such as IRAC) fluxes are useful in discriminating between stars and YSOs, and are useful for identifying very young sources, the addition of longer wavelength data such as MIPS 24 μm is extremely valuable for determining the evolutionary stage of YSOs.

The Spitzer/GLIMPSE Surveys: A New View of the Milky Way

A brief description is given of the GLIMPSE surveys, including the areas surveyed, sensitivity limits, and products. The primary motivations for this review are to describe some of the main scientific results enabled by the GLIMPSE surveys and to note potential future applications of the GLIMPSE catalogs and images. In particular, we discuss contributions to our understanding of star formation and early evolution, the interstellar medium, galactic structure, and evolved stars. Infrared dark clouds (IRDCs), young stellar objects (YSOs), and infrared bubbles/H II regions are discussed in some detail. A probable triggered star formation associated with expanding infrared bubbles is briefly mentioned. The distribution and morphologies of dust and polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium are discussed. Examples are shown from GLIMPSE images of bow shocks, pillars (elephant trunks), and instabilities in massive star-formation regions. The infrared extinction law of diffuse interstellar dust is discussed. The large-scale structure of the Galaxy has been traced by red-clump giants using the GLIMPSE point-source catalog to reveal the radius and orientation of the central bar, the stellar radial scale length, an obvious increase in star counts toward the tangency to the Scutum-Centaurus spiral arm, the lack of an obvious tangency from star counts toward the Sagittarius spiral arm, and a sharp increase in star counts toward the nuclear bulge. Recent results on evolved stars and some serendipitous discoveries are mentioned. More than 70 refereed papers have been published based on GLIMPSE data as of 2008 November.

The Present-Day Star Formation Rate of the Milky Way Determined from Spitzer-Detected Young Stellar Objects

We present initial results from a population synthesis model aimed at determining the star formation rate (SFR) of the Milky Way. We find that a total SFR of 0.68-1.45 M ? yr?1 is able to reproduce the observed number of young stellar objects (YSOs) in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the three-dimensional Galactic distributions of YSOs and interstellar dust, and using model spectral energy distributions 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, H?, 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 SFR estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic SFR, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.

Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). I. Overview

The Small Magellanic Cloud (SMC) provides a unique laboratory for the study of the lifecycle of dust given its low metallicity (~1/5 solar) and relative proximity (~60 kpc). This motivated the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud) Spitzer Legacy program with the specific goals of studying the amount and type of dust in the present interstellar medium, the sources of dust in the winds of evolved stars, and how much dust is consumed in star formation. This program mapped the full SMC (30 deg^2) including the body, wing, and tail in seven bands from 3.6 to 160 μm using IRAC and MIPS on the Spitzer Space Telescope. The data were reduced and mosaicked, and the point sources were measured using customized routines specific for large surveys. We have made the resulting mosaics and point-source catalogs available to the community. The infrared colors of the SMC are compared to those of other nearby galaxies and the 8 μm/24 μm ratio is somewhat lower than the average and the 70 μm/160 μm ratio is somewhat higher than the average. The global infrared spectral energy distribution (SED) shows that the SMC has approximately 1/3 the aromatic emission/polycyclic aromatic hydrocarbon abundance of most nearby galaxies. Infrared color-magnitude diagrams are given illustrating the distribution of different asymptotic giant branch stars and the locations of young stellar objects. Finally, the average SED of H II/star formation regions is compared to the equivalent Large Magellanic Cloud average H II/star formation region SED. These preliminary results will be expanded in detail in subsequent papers.

IR DUST BUBBLES. II. PROBING THE DETAILED STRUCTURE AND YOUNG MASSIVE STELLAR POPULATIONS OF GALACTIC H II REGIONS

We present an analysis of late-O/early-B-powered, parsec-sized bubbles and associated star-formation using 2MASS, GLIMPSE, MIPSGAL and MAGPIS surveys. Three bubbles were selected from the Churchwell et al. (2007) catalog. We confirm that the structure identified in Watson et al. (2008) holds in less energetic bubbles, i.e. a PDR, identified by 8 μm emission due to PAHs surrounds hot dust, identified by 24 μm emission and ionized gas, identified by 20 cm continuum. We estimate the dynamical age of two bubbles by comparing bubble sizes to numerical models of Hosokawa & Inutsuka (2006). We also identify and analyze candidate young stellar objects (YSOs) using SED fitting and identify sites of possible triggered star-formation. Lastly, we identify likely ionizing sources for two sources based on SED fitting. Subject headings: Spitzer, stars: formation, ISM: HII regions

INTRINSICALLY RED SOURCES OBSERVED BY SPITZER IN THE GALACTIC MIDPLANE

We present a highly reliable flux-limited census of 18,949 point sources in the Galactic midplane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) I and II surveys of 274 deg2 of the Galactic midplane, 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 the Infrared Array Camera and the Multiband Image Photometer for Spitzer 24 μm 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. 1004 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 μm. With over 11,000 likely YSOs and over 7000 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.

HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE): The Large Magellanic Cloud dust

The HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) of the Magellanic Clouds will use dust emission to investigate the life cycle of matter in both the Large and Small Magellanic Clouds (LMC and SMC). Using the Herschel Space Observatory’s PACS and SPIRE photometry cameras, we imaged a 2° × 8° strip through the LMC, at a position angle of ~22.5° as part of the science demonstration phase of the Herschel mission. We present the data in all 5 Herschel bands: PACS 100 and 160 μm and SPIRE 250, 350 and 500 μm. We present two dust models that both adequately fit the spectral energy distribution for the entire strip and both reveal that the SPIRE 500 μm emission is in excess of the models by ~6 to 17%. The SPIRE emission follows the distribution of the dust mass, which is derived from the model. The PAH-to-dust mass (f_(PAH)) image of the strip reveals a possible enhancement in the LMC bar in agreement with previous work. We compare the gas mass distribution derived from the HI 21 cm and CO J = 1−0 line emission maps to the dust mass map from the models and derive gas-to-dust mass ratios (GDRs). The dust model, which uses the standard graphite and silicate optical properties for Galactic dust, has a very low GDR = 65^(+15) _(−18) making it an unrealistic dust model for the LMC. Our second dust model, which uses amorphous carbon instead of graphite, has a flatter emissivity index in the submillimeter and results in a GDR = 287^_(+25)_(−42) that is more consistent with a GDR inferred from extinction.

HYPERION: an open-source parallelized three-dimensional dust continuum radiative transfer code

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 is problemindependent, 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 �� .