Michael P. Egan

A Population of Cold Cores in the Galactic Plane

Recent observations by the Midcourse Space Experiment (MSX) have revealed the presence of compact objects seen in absorption against bright mid-infrared emission from the Galactic plane. Examination of MSX and IRAS images of these objects reveal that they are dark from 7 to 100 μm. We find ~2000 clouds in a 1° × 180° scan along the Galactic equator. The data suggest these objects are dense (n > 105 cm−3), cold (T<20 K) cores, without accompanying envelopes.

The Physical Properties of the Midcourse Space Experiment Galactic Infrared-dark Clouds

The SPIRIT III infrared telescope on the Midcourse Space Experiment (MSX) satellite has provided an unprecedented view of the mid-infrared emission (8-25 μm) of the Galactic plane. An initial analysis of images from MSX Galactic plane survey data reveals dark clouds seen in silhouette against the bright emission from the Galactic plane (Egan et al.). These clouds have mid-infrared extinctions in excess of 2 mag at 8 μm. We probed the physical properties of 10 of these MSX dark clouds using millimeter-wave molecular rotational lines as an indicator of dense molecular gas. All 10 clouds were detected in millimeter spectral lines of H2CO, which confirms the presence of dense gas. The distances to these clouds range from 1 to 8 kiloparsecs and their diameters from 0.4 to 15.0 pc. Excitation analysis of the observed lines indicates that the clouds are cold (T 105 cm-3]. Some of the clouds have nearby H II regions, H2O masers, and other tracers of star formation at comparable spectral line velocities; however, only one cloud contains embedded centimeter or infrared sources. The lack of mid- to far-infrared emission associated with these clouds suggests that they are not currently forming high-mass stars. If star formation is present in these clouds, it is clearly protostellar class 0 or earlier.

Submillimeter Observations of Midcourse Space Experiment Galactic Infrared-Dark Clouds

We present 850 and 450 μm continuum images of infrared-dark clouds (IRDCs) taken with the Submillimeter Common-User Bolometer Array (SCUBA) submillimeter camera at the James Clerk Maxwell Telescope. The IRDCs are large (1-10 pc diameter) molecular cores with gas densities ~106 cm-3 and temperatures ≈15 K. We detected strong submillimeter sources with peak flux densities of ≈1 Jy beam-1 at 850 μm in all eight clouds that were observed. The submillimeter emission generally lies within the envelope of the mid-infrared extinction where dense gas has been detected using H2CO as a tracer. The dust temperatures in the bright, compact sources are calculated to lie in the range 10-25 K. The masses of these sources are estimated to be in the range of several tens up to about a thousand solar masses. The corresponding gas column densities range over an order of magnitude, up to about 1023 cm-2. Several of the sources are detected in emission at both 850 and 8 μm. Two of the sources have HCO+ line profiles characteristic of molecular infall. It is likely that the bright, compact sources seen in the SCUBA images are in various early stages of star formation, from preprotostellar cores to class I objects.

Large-Scale Structure of the Carina Nebula

Observations obtained with the Midcourse Space Experiment (MSX) satellite reveal for the first time the complex mid-infrared morphology of the entire Carina Nebula (NGC 3372). On the largest size scale of approximately 100 pc, the thermal infrared emission from the giant H ii region delineates one coherent structure: a (somewhat distorted) bipolar nebula with the major axis perpendicular to the Galactic plane. The Carina Nebula is usually described as an evolved H ii region that is no longer actively forming stars, clearing away the last vestiges of its natal molecular cloud. However, the MSX observations presented here reveal numerous embedded infrared sources that are good candidates for sites of current star formation. Several compact infrared sources are located at the heads of dust pillars or in dark globules behind ionization fronts. Because their morphology suggests a strong interaction with the peculiar collection of massive stars in the nebula, we speculate that these new infrared sources may be sites of triggered star formation in NGC 3372.

Mid-Infrared Spectroscopy of Carbon Stars in the Small Magellanic Cloud

We have observed a sample of 36 objects in the Small Magellanic Cloud (SMC) with the Infrared Spectrometer on the Spitzer Space Telescope. Nineteen of these sources are carbon stars. An examination of the near- and mid-infrared photometry shows that the carbon-rich and oxygen-rich dust sources follow two easily separated sequences. A comparison of the spectra of the 19 carbon stars in the SMC to spectra from the Infrared Space Observatory (ISO) of carbon stars in the Galaxy reveals significant differences. The absorption bands at 7.5 and 13.7 μm due to C2H2 are stronger in the SMC sample, and the SiC dust emission feature at 11.3 μm is weaker. Our measurements of the MgS dust emission feature at 26-30 μm are less conclusive, but this feature appears to be weaker in the SMC sample as well. All of these results are consistent with the lower metallicity in the SMC. The lower abundance of SiC grains in the SMC may result in less efficient carbon-rich dust production, which could explain the excess C2H2 gas seen in the spectra. The sources in the SMC with the strongest SiC dust emission tend to have redder infrared colors than the other sources in the sample, which implies more amorphous carbon, and they also tend to show stronger MgS dust emission. The weakest SiC emission features tend to be shifted to the blue; these spectra may arise from low-density shells with large SiC grains.

MSX, 2MASS, and the Large Magellanic Cloud: A Combined Near- and Mid-Infrared View

The Large Magellanic Cloud (LMC) has been observed by the Midcourse Space Experiment (MSX) in the mid-infrared and the Two Micron All Sky Survey (2MASS) in the near-infrared. We have performed a cross-correlation of the 1806 MSX catalog sources and nearly 1.4 million 2MASS cataloged point and extended sources and find 1664 matches. Using the available color information, we identify a number of stellar populations and nebulae, including main-sequence stars, giant stars, red supergiants, carbon- and oxygen-rich asymptotic giant branch (AGB) stars, planetary nebulae, H II regions, and other dusty objects likely associated with early-type stars. A total of 731 of these sources have no previous identification. We compile a listing of all objects, which includes photometry and astrometry. The 8.3 μm MSX sensitivity is the limiting factor for object detection: only the brighter red objects, specifically the red supergiants, AGB stars, planetary nebulae, and H II regions, are detected in the LMC. The remaining objects are likely in the Galactic foreground. The spatial distribution of the infrared LMC sources may contribute to understanding stellar formation and evolution and the overall galactic evolution. We demonstrate that a combined mid- and near-infrared photometric baseline provides a powerful means of identifying new objects in the LMC for future ground-based and space-based follow-up observations.

MSX, 2MASS, and the LMC: A Combined Near and Mid Infrared View

The Large Magellanic Cloud (LMC) has been observed by the Midcourse Space Experiment (MSX) in the mid-infrared and the Two Micron All Sky Survey (2MASS) in the near-infrared. We have performed a cross-correlation of the 1806 MSX catalog sources and nearly 1.4 million 2MASS cataloged point and extended sources and find 1664 matches. Using the available color information, we identify a number of stellar populations and nebulae, including main-sequence stars, giant stars, red supergiants, carbon- and oxygen-rich asymptotic giant branch (AGB) stars, planetary nebulae, H II regions, and other dusty objects likely associated with early-type stars. A total of 731 of these sources have no previous identification. We compile a listing of all objects, which includes photometry and astrometry. The 8.3 μm MSX sensitivity is the limiting factor for object detection: only the brighter red objects, specifically the red supergiants, AGB stars, planetary nebulae, and H II regions, are detected in the LMC. The remaining objects are likely in the Galactic foreground. The spatial distribution of the infrared LMC sources may contribute to understanding stellar formation and evolution and the overall galactic evolution. We demonstrate that a combined mid- and near-infrared photometric baseline provides a powerful means of identifying new objects in the LMC for future ground-based and space-based follow-up observations.

Dusty ring nebulae around new candidate Luminous Blue Variables

We report on the discovery of a further two ring nebulae in the Midcourse Space Experiment (MSX) Galactic Plane Survey; G24.73+0.69 and G26.47+0.02. Morphologically, both appear similar to the nebulae found around the Luminous Blue Variable (LBV) candidates G79.29+0.46 and Wra 17-96. A central, unresolved point source was identified in both cases - positional coincidence with the star StRS 237 was found for G26.47+0.02, while no optical counterpart could be identified for G24.73+0.69. However, subsequent near IR broadband imaging of the G24.73+0.69 field identified a very red - (J − K) ∼ 2 mag - stellar counterpart to the central object. Near-IR spectroscopy of both objects reveal rich emission line spectra dom- inated by H  ,H e and low excitation metals, suggesting classification as luminous B supergiants and revealing a striking superficial similarity to the other MSX ring sources and known LBVs. We utilised a NLTE model atomsphere code to model the K band spectra and near-IR spectral energy distributions of the central stars in order to determine their physical parameters. Adopting a distance, d = 5.2 kpc to G24.73+0.69 yields a temperature, T = 12 kK, luminosity, log (L/L� ) = 5.6 and mass loss rate, u M = 1 × 10 −5 Myr −1 . G26.47+0.02 appears to be a more extreme object; adopting d = 6.5 kpc results in T = 17 kK, log (L/L� ) = 6.0 and u M = 9 × 10 −5 Myr −1 , placing it at the Humphreys-Davidson limit for massive stellar objects. Analysis of the spatially resolved mid-IR fluxes of both objects reveal extended periods of enhanced mass loss, resulting in comparatively low mass nebulae, with chemistries dominated by O-rich dust (with a population of small Fe grains existing co-spatially with the silicate dust). Comparison to the other MSX ring nebulae sources reveals a homogeneous group of objects, with both stellar and nebular properties consistent with known LBVs. With both spectroscopic and/or photometric variability observed for those sources with multiepoch observations, we propose a close affinity between both classes of object and suggest that long term monitoring of the MSX sources will reveal them to be bona fide LBVs.

A Spitzer Space Telescope Infrared Spectrograph Spectral Atlas of Luminous 8 μm Sources in the Large Magellanic Cloud

We present an atlas of Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of highly luminous, compact, mid-IR sources in the Large Magellanic Cloud. Sources were selected on the basis of IR colors and 8 μm (Midcourse Space Experiment) fluxes indicative of highly evolved, intermediate-to-high-mass stars with current or recent mass loss at large rates. We determine the chemistry of the circumstellar envelope from the mid-IR continuum and spectral features and classify the spectral types of the stars. In the sample of 60 sources, we find 21 red supergiants (RSGs), 16 C-rich asymptotic giant branch (AGB) stars, 11 H II regions, 4 likely O-rich AGB stars, 4 Galactic O-rich AGB stars, 2 OH/IR stars, and 2 B[e] supergiants with peculiar IR spectra. We find that the overwhelming majority of the sample AGB stars (with typical IR luminosities of ~104 L⊙) have C-rich envelopes, while the O-rich objects are predominantly luminous RSGs with LIR ~ 105 L⊙. For both classes of evolved star (C-rich AGB stars and RSGs), we use the near-to-mid-IR spectral energy distributions to determine mean bolometric corrections to the stellar K-band flux densities. For carbon stars, the bolometric corrections depend on the IR color, whereas for RSGs, the bolometric correction is independent of IR color. Our results reveal that objects previously classified as planetary nebulae on the basis of IR colors are, in fact, compact H II regions with very red IRS spectra that include strong atomic recombination lines and polycyclic aromatic hydrocarbon emission features. We demonstrate that the IRS spectral classes in our sample separate clearly in IR color-color diagrams that use combinations of Two Micron All Sky Survey data and synthetic Spitzer Infrared Array Camera and Multiband Imaging Photometer for Spitzer fluxes derived from the IRS spectra. On this basis we suggest diagnostics to identify and classify, with high confidence levels, IR-luminous evolved stars and compact H II regions in nearby galaxies using Spitzer and near-IR photometry.We present an atlas of Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of highly luminous, compact mid-infrared sources in the Large Magellanic Cloud. Sources were selected on the basis of infrared colors and 8 micron (MSX) fluxes indicative of highly evolved, intermediate- to high-mass stars with current or recent mass loss at large rates. We determine the chemistry of the circumstellar envelope from the mid-IR continuum and spectral features and classify the spectral types of the stars. In the sample of 60 sources, we find 21 Red Supergiants (RSGs), 16 C-rich Asymptotic Giant Branch (AGB) stars, 11 HII regions, 4 likely O-rich AGB stars, 4 Galactic O-rich AGB stars, 2 OH/IR stars, and 2 B[e] supergiants with peculiar IR spectra. We find that the overwhelming majority of the sample AGB stars (with typical IR luminosities ~1.0E4 L_sun) have C-rich envelopes, while the O-rich objects are predominantly luminous RSGs with L_IR ~ 1.0E5 L_sun. We determine mean bolometric corrections to the stellar K-band flux densities and find that for carbon stars, the bolometric corrections depend on the infrared color, whereas for RSGs, the bolometric correction is independent of IR color. Our results reveal that objects previously classified as PNe on the basis of IR colors are in fact compact HII regions with very red IRS spectra that include strong atomic recombination lines and PAH emission features. We demonstrate that the IRS spectral classes in our sample separate clearly in infrared color-color diagrams that use combinations of 2MASS data and synthetic IRAC/MIPS fluxes derived from the IRS spectra. On this basis, we suggest diagnostics to identify and classify, with high confidence levels, IR-luminous evolved stars and HII regions in nearby galaxies using Spitzer and near-infrared photometry.

An Infrared Ring Nebula around MSX5C G358.5391+00.1305: The True Nature of Suspected Planetary Nebula Wray 17-96 Determined via Direct Imaging and Spectroscopy

The Midcourse Space Experiment (MSX) Galactic plane survey discovered a nearly perfectly circular ring nebula around the suspected planetary nebula Wray 17-96. Using near-IR spectral typing and modeling of the mid-IR nebula, we find that Wray 17-96 is more likely a candidate to be a luminous blue variable (LBV) surrounded by a large spherical ejecta shell. It is very similar to the G79.29+0.46 LBV candidate in Cygnus and the Pistol Star. The K-band spectrum and the mid-IR data indicate a stellar temperature of 13,000 K. The most likely distance to the source is 4.5 kpc, leading to a luminosity of 1.8 × 106 L☉. We suggest that the nebula consists of multiple shells and that an evolution from oxygen-rich to carbon-rich chemistry may be indicated.