Massive Young Stellar Objects in the Galactic Center. I. Spectroscopic Identification from Spitzer/IRS Observations

We present results from our spectroscopic study, using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, designed to identify massive young stellar objects (YSOs) in the Galactic Center (GC). Our sample of 107 YSO candidates was selected based on IRAC colors from the high spatial resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone (CMZ), which spans the central ~300 pc region of the Milky Way Galaxy. We obtained IRS spectra over 5um to 35um using both high- and low-resolution IRS modules. We spectroscopically identify massive YSOs by the presence of a 15.4um shoulder on the absorption profile of 15um CO2 ice, suggestive of CO2 ice mixed with CH3OH ice on grains. This 15.4um shoulder is clearly observed in 16 sources and possibly observed in an additional 19 sources. We show that 9 massive YSOs also reveal molecular gas-phase absorption from CO2, C2H2, and/or HCN, which traces warm and dense gas in YSOs. Our results provide the first spectroscopic census of the massive YSO population in the GC. We fit YSO models to the observed spectral energy distributions and find YSO masses of 8 - 23 Msun, which generally agree with the masses derived from observed radio continuum emission. We find that about 50% of photometrically identified YSOs are confirmed with our spectroscopic study. This implies a preliminary star formation rate of ~0.07 Msun/yr at the GC.Comment: Accepted for publication in Ap

The Spitzer Warm Mission Science Prospects

After exhaustion of its cryogen, the Spitzer Space telescope will still have a fully functioning two-channel mid-IR camera that will have sensitivities better than any other ground or space-based telescopes until the launch of JWST. This document provides a description of the expected capabilities of Spitzer during its warm mission phase, and provides brief descriptions of several possible very large science programs that could be conducted. This information is intended to serve as input to a wide ranging discussion of the warm mission science, leading up to the Warm Mission Workshop in June 2007

Simple hydrodynamical Simulations of the Circumnuclear Disk

The “circumnuclear disk” (CND) is a dense, clumpy, asymmetric ring‐like feature centered on Sgr A*. The outer edge of the CND is not distinct but the disk extends for more than 7 pc; the distinct inner edge, at a radius of ≃1.5 pc, surrounds the “mini‐spiral” of the HII region, Sgr A West. We present simple 3D hydrodynamical models of the formation and evolution of the CND from multiple selfgravitating infalling clouds and compare the results with recent observations. We assume the clouds are initially Bonner‐Ebert spheres, in equilibrium with a hot confining inter‐cloud medium. We include the gravitational potential due to the point‐mass of Sgr A* as well as the extended mass distribution of the underlying stellar population. We also include the effects of the ram pressure due to the stellar winds from the central cluster of early‐type stars. A single spherically symmetric cloud cannot reproduce the clumpy morphology of the CND; multiple clouds on diverse trajectories are required so that cloud‐cloud collisions can circularize the clouds' orbits while maintaining a clumpy morphology. Collisions also serve to compress the clouds, delaying tidal disruption while potentially hastening gravitational collapse. Low density clumps are disrupted before reaching the inner CND radius, forming short‐lived arcs. The outer parts of more massive clumps get tidally stripped, forming long‐lived low‐density wide‐angle arcs, while their cores potentially undergo gravitational collapse. The fine balance between resisting tidal disruption and preventing gravitational collapse implies that most if not all clumps are not stable for much more than an orbit. Thus, in order for the CND to be a long‐lived clumpy object, it must be continually fed by additional in‐falling clouds. Clouds that survive to small radii are likely to be the sites of present or future star formation. However, within a few parsecs of Sgr A*, the stellar winds decelerate any in‐falling cloud so that the wind‐cloud interface becomes Rayleigh‐Taylor unstable, potentially disrupting the cloud and inhibiting star formation

Point Sources from a Spitzer IRAC Survey of the Galactic Center

We have obtained Spitzer/IRAC observations of the central 2.0 x 1.4 degrees (~280 x 200 pc) of the Galaxy at 3.6-8.0 microns. A point source catalog of 1,065,565 objects is presented. The catalog includes magnitudes for the point sources at 3.6, 4.5, 5.8, and 8.0 microns, as well as JHK photometry from 2MASS. The point source catalog is confusion limited with average limits of 12.4, 12.1, 11.7, and 11.2 magnitudes for [3.6], [4.5], [5.8], and [8.0], respectively. We find that the confusion limits are spatially variable because of stellar surface density, background surface brightness level, and extinction variations across the survey region. The overall distribution of point source density with Galactic latitude and longitude is essentially constant, but structure does appear when sources of different magnitude ranges are selected. Bright stars show a steep decreasing gradient with Galactic latitude, and a slow decreasing gradient with Galactic longitude, with a peak at the position of the Galactic center. From IRAC color-magnitude and color-color diagrams, we conclude that most of the point sources in our catalog have IRAC magnitudes and colors characteristic of red giant and AGB stars.Comment: 44 pages, 13 figures, ApJS in pres

First Spectroscopic Identification of Massive Young Stellar Objects in the Galactic Center

We report the detection of several molecular gas-phase and ice absorption features in three photometrically-selected young stellar object (YSO) candidates in the central 280 pc of the Milky Way. Our spectra, obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, reveal gas- phase absorption from CO_2 (15.0 μm), C_2H_2 (13.7 μm) and HCN (14.0 μm). We attribute this absorption to warm, dense gas in massive YSOs. We also detect strong and broad 15 μm CO_2 ice absorption features, with a remarkable double- peaked structure. The prominent long-wavelength peak is due to CH_3OH-rich ice grains, and is similar to those found in other known massive YSOs. Our IRS observations demonstrate the youth of these objects, and provide the first spectroscopic identification of massive YSOs in the Galactic Center

Massive Young Stellar Objects in the Galactic Center

We present results from our spectroscopic study, using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, designed to identify massive young stellar objects (YSOs) in the Galactic Center (GC). Our sample of 107 YSO candidates was selected based on IRAC colors from the high spatial resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone (CMZ), which spans the central approximately 300 pc region of the Milky Way Galaxy. We obtained IRS spectra over 5 micron to 35 micron using both high- and low-resolution IRS modules. We spectroscopically identify massive YSOs by the presence of a 15.4 micron shoulder on the absorption profile of 15 micron CO2 ice, suggestive of CO2 ice mixed with CH30H ice on grains. This 15.4 micron shoulder is clearly observed in 16 sources and possibly observed in an additional 19 sources. We show that 9 massive YSOs also reveal molecular gas-phase absorption from C02, C2H2, and/or HCN, which traces warm and dense gas in YSOs. Our results provide the first spectroscopic census of the massive YSO population in the GC. We fit YSO models to the observed spectral energy distributions and find YSO masses of 8 - 23 solar Mass, which generally agree with the masses derived from observed radio continuum emission. We find that about 50% of photometrically identified YSOs are confirmed with our spectroscopic study. This implies a preliminary star formation rate of approximately 0.07 solar mass/yr at the GC

Spitzer Warm Mission Archive Science Opportunities

The rich data archive from the Spitzer cryogenic mission will be comprised of approximately 25 TB of data. A five-year warm mission would add an additional 15–20 TB. All of these data will be processed and archived to form homogeneous, reliable database to support research for decades after the end of the Spitzer mission. The SSC proposes a robust archival research program during the warm mission phase. A sampling of possible archival programs are described