Varoujan Gorjian

Bio: Varoujan Gorjian is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Galaxy & Planet. The author has an hindex of 30, co-authored 115 publications receiving 4220 citations. Previous affiliations of Varoujan Gorjian include Jet Propulsion Laboratory.

Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) I: Overview and Initial Results

Abstract: We are performing a uniform and unbiased, ~7x7 degrees imaging survey of the Large Magellanic Cloud (LMC), using the IRAC and MIPS instruments on board the Spitzer Space Telescope in order to survey the agents of a galaxy's evolution (SAGE), the interstellar medium (ISM) and stars in the LMC. The detection of diffuse ISM with column densities >1.2x10^21 H cm^-2 permits detailed studies of dust processes in the ISM. SAGE's point source sensitivity enables a complete census of newly formed stars with masses >3 solar masses that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass loss rates >1x10^-8 solar masses per year will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by three months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are non-proprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data with a special focus on the N79 and N83 region. The SAGE epoch 1 point source catalog has ~4 million sources. The point source counts are highest for the IRAC 3.6 microns band and decrease dramatically towards longer wavelengths consistent with the fact that stars dominate the point source catalogs and that the dusty objects, e.g. young stellar objects and dusty evolved stars that detected at the longer wavelengths, are rare in comparison. We outline a strategy for identifying foreground MW stars, that may comprise as much as 18% of the source list, and background galaxies, that may comprise ~12% of the source list.

Host Galaxies, Clustering, Eddington Ratios, and Evolution of Radio, X-Ray, and Infrared-selected AGNs.

Abstract: We explore the connection between different classes of active galactic nuclei (AGNs) and the evolution of their host galaxies, by deriving host galaxy properties, clustering, and Eddington ratios of AGNs selected in the radio, X-ray, and infrared (IR) wavebands. We study a sample of 585 AGNs at 0.25 10^(–2). We interpret these results in terms of a simple model of AGN and galaxy evolution, whereby a "quasar" phase and the growth of the stellar bulge occurs when a galaxy's dark matter halo reaches a critical mass between ~10^(12) and 10^(13)M_☉ . After this event, star formation ceases and AGN accretion shifts from radiatively efficient (optical- and IR-bright) to radiatively inefficient (optically faint, radio-bright) modes.

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

Abstract: 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.

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

Abstract: 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 sq. deg.) including the Body, Wing, and Tail in 7 bands from 3.6 to 160 micron using the IRAC and MIPS instruments on the Spitzer Space Telescope. The data were reduced, mosaicked, and the point sources 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 micron/24 micron ratio is somewhat lower and the 70 micron/160 micron ratio is somewhat higher than the average. The global infrared spectral energy distribution shows that the SMC has ~3X lower aromatic emission/PAH (polycyclic aromatic hydrocarbon) abundances compared to 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 spectral energy distribution (SED) of HII/star formation regions is compared to the equivalent Large Magellanic Cloud average HII/star formation region SED. These preliminary results are expanded in detail in companion papers.

Spitzer SAGE Survey of the Large Magellanic Cloud. II. Evolved Stars and Infrared Color-Magnitude Diagrams

Abstract: Color-magnitude diagrams (CMDs) are presented for the Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution) survey of the Large Magellanic Cloud (LMC). IRAC and MIPS 24 mu m epoch 1 data are presented. These data represent the deepest, widest mid-infrared CMDs of their kind ever produced in the LMC. Combined with the Two Micron All Sky Survey, the diagrams are used to delineate the evolved stellar populations in the LMC, as well as Galactic foreground and extragalactic background populations. Some 32,000 evolved stars brighter than the tip of the red giant branch are identified. Of these, approximately 17,500 are classified as oxygen-rich, 7000 as carbon-rich, and another 1200 as "extreme'' asymptotic giant branch (AGB) stars. Brighter members of the latter group have been called "obscured'' AGB stars in the literature owing to their dusty circumstellar envelopes. A large number (1200) of luminous oxygen-rich AGB stars/M supergiants are also identified. Finally, there is strong evidence from the 24 mu m MIPS channel that previously unexplored, lower luminosity oxygen-rich AGB stars contribute significantly to the mass-loss budget of the LMC (1200 such sources are identified).

The Infrared Array Camera (IRAC) for the Spitzer Space Telescope

Abstract: The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 � m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 � m; 4.5 and 8 � m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques

Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

Evolution of asymptotic giant branch stars II. Optical to far-infrared isochrones with improved TP-AGB models

Abstract: We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly-improved treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from Girardi et al. (2000, AA the bell-shaped sequences in the Hertzsprung-Russell (HR) diagram for stars with hot-bottom burning; the changes of pulsation mode between fundamental and first overtone; the sudden changes of mean mass-loss rates as the surface chemistry changes from M- to C-type; etc. Theoretical isochrones are then converted to about 20 different photometric systems - including traditional ground-based systems, and those of recent major wide-field surveys such as SDSS, OGLE, DENIS, 2MASS, UKIDSS, etc., - by means of synthetic photometry applied to an updated library of stellar spectra, suitably extended to include C-type stars. Finally, we correct the predicted photometry for the effect of circumstellar dust during the mass-losing stages of the AGB evolution, which allows us to improve the results for the optical-to-infrared systems, and to simulate mid- and far-IR systems such as those of Spitzer and AKARI. We illustrate the most striking properties of these isochrones by means of basic comparisons with observational data for the Milky Way disc and the Magellanic Clouds. Access to the data is provided both via a web repository of static tables (http://stev.oapd.inaf.it/ dustyAGB07 and CDS), and via an interactive web interface (http://stev.oapd. inaf. it/cmd), which provides tables for any intermediate value of age and metallicity, for several photometric systems, and for different choices of dust properties.

GLIMPSE. I. An SIRTF Legacy Project to Map the Inner Galaxy

Abstract: The Galactic Legacy Infrared Mid‐Plane Survey Extraordinaire (GLIMPSE), a Space Infrared Telescope Facility (SIRTF) Legacy Science Program, will be a fully sampled, confusion‐limited infrared survey of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} ewcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\frac{2}{3}$ \end{document} of the inner Galactic disk with a pixel resolution of ∼1 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \u...

Toward Understanding Massive Star Formation

Abstract: Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation: ▪ monolithic collapse in isolated cores ▪ competitive accretion in a protocluster environment ▪ stellar collisions and mergers in very dense systems We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass ...