A new mid-infrared map of the BN/KL region using the Keck telescope

We present a new mid-infrared (12.5micron) map of the BN/KL high-mass star-forming complex in Orion using the LWS instrument at Keck I. Despite poor weather we achieved nearly diffraction-limited images (FWHM = 0.38'') over a roughly 25'' X 25'' region centered on IRc2 down to a flux limit of ~250 mJy. Many of the known infrared (IR) sources in the region break up into smaller sub-components. We have also detected 6 new mid-IR sources. Nearly all of the sources are resolved in our mosaic. The near-IR source ''n'' is slightly elongated in the mid-IR along a NW--SE axis and perfectly bisects the double-peaked radio source ''L''. Source n has been identified as a candidate for powering the large IR luminosity of the BN/KL region (L = 10^5 L_sun). We postulate that the 12 micron emission arises in a circumstellar disk surrounding source n. The morphology of the mid-IR emission and the Orion ''hot core'' (as seen in NH_3 emission), along with the location of water and OH masers, is very suggestive of a bipolar cavity centered on source n and aligned with the rotation axis of the hypothetical circumstellar disk. IRc2, once thought to be the dominant energy source for the BN/KL region, clearly breaks into 4 sub-sources in our mosaic, as seen previously at 3.8 -- 5.0 micron. The anti-correlation of mid-IR emission and NH_3 emission from the nearby hot core indicates that the IRc2 sources are roughly coincident (or behind) the dense hot core. The nature of IRc2 is not clear: neither self-luminous sources (embedded protostars) nor external heating by source I can be definitively ruled out. We also report the discovery of a new arc-like feature SW of the BN object, and some curious morphology surrounding near-IR source ''t".Comment: To appear in The Astronomical Journal, July 2004 (16 pages, 7 figures

Spectral classification of the brightest objects in the galactic star forming region W40

We present high S/N, moderate resolution near-infrared spectra, as well as 10 micron imaging, for the brightest members of the central stellar cluster in the W40 HII region, obtained using the SpeX and MIRSI instruments at NASA's Infrared Telescope Facility. Using these observations combined with archival Spitzer Space Telescope data, we have determined the spectral classifications, extinction, distances, and spectral energy distributions for the brightest members of the cluster. Of the eight objects observed, we identify four main sequence (MS) OB stars, two Herbig Ae/Be stars, and two low-mass young stellar objects. Strong HeI absorption at 1.083 micron in the MS star spectra strongly suggests that at least some of these sources are in fact close binaries. Two out of the four MS stars also show significant infrared excesses typical of circumstellar disks. Extinctions and distances were determined for each MS star by fitting model stellar atmospheres to the SEDs. We estimate a distance to the cluster of between 455 and 535 pc, which agrees well with earlier (but far less precise) distance estimates. We conclude that the late-O star we identify is the dominant source of LyC luminosity needed to power the W40 HII region and is the likely source of the stellar wind that has blown a large (~4 pc) pinched-waist bubble observed in wide field mid-IR images. We also suggest that 3.6 cm radio emission observed from some of the sources in the cluster is likely not due to emission from ultra-compact HII regions, as suggested in other work, due to size constraints based on our derived distance to the cluster. Finally, we also present a discussion of the curious source IRS 3A, which has a very strong mid-IR excess (despite its B3 MS classification) and appears to be embedded in a dusty envelope roughly 2700 AU in size.Comment: Accepted for publication in The Astronomical Journal. 29 pages, 10 figure

First science results from SOFIA/FORCAST: The mid-infrared view of the compact HII region W3A

The massive star forming region W3 was observed with the faint object infrared camera for the SOFIA telescope (FORCAST) as part of the Short Science program. The 6.4, 6.6, 7.7, 19.7, 24.2, 31.5 and 37.1 \um bandpasses were used to observe the emission of Polycyclic Aromatic Hydrocarbon (PAH) molecules, Very Small Grains and Big Grains. Optical depth and color temperature maps of W3A show that IRS2 has blown a bubble devoid of gas and dust of $\sim$0.05 pc radius. It is embedded in a dusty shell of ionized gas that contributes 40% of the total 24 \um emission of W3A. This dust component is mostly heated by far ultraviolet, rather than trapped Ly$\alpha$ photons. This shell is itself surrounded by a thin ($\sim$0.01 pc) photodissociation region where PAHs show intense emission. The infrared spectral energy distribution (SED) of three different zones located at 8, 20 and 25\arcsec from IRS2, show that the peak of the SED shifts towards longer wavelengths, when moving away from the star. Adopting the stellar radiation field for these three positions, DUSTEM model fits to these SEDs yield a dust-to-gas mass ratio in the ionized gas similar to that in the diffuse ISM. However, the ratio of the IR-to-UV opacity of the dust in the ionized shell is increased by a factor $\simeq$3 compared to the diffuse ISM.Comment: Accepted for publication in ApJ letters; 13 pages, 3 figures 1 tabl

First Science Observations with SOFIA/FORCAST: Properties of Intermediate-Luminosity Protostars and Circumstellar Disks in OMC-2

We examine eight young stellar objects in the OMC-2 star forming region based on observations from the SOFIA/FORCAST early science phase, the Spitzer Space Telescope, the Herschel Space Observatory, 2MASS, APEX, and other results in the literature. We show the spectral energy distributions of these objects from near-infrared to millimeter wavelengths, and compare the SEDs with those of sheet collapse models of protostars and circumstellar disks. Four of the objects can be modelled as protostars with infalling envelopes, two as young stars surrounded by disks, and the remaining two objects have double-peaked SEDs. We model the double-peaked sources as binaries containing a young star with a disk and a protostar. The six most luminous sources are found in a dense group within a 0.15 x 0.25 pc region; these sources have luminosities ranging from 300 L_sun to 20 L_sun. The most embedded source (OMC-2 FIR 4) can be fit by a class 0 protostar model having a luminosity of ~50 L_sun and mass infall rate of ~10^-4 solar masses per year.Comment: Accepted by ApJ Letter

The translucent molecular clouds toward hd 154368 .1. Extinction, abundances, and depletions

We report the results of a comprehensive analysis of a line of sight observed using the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST), toward HD 154368, an 09.5 Iab star located about 800 pc away. The line of sight intersects translucent interstellar cloud material, having a color excess E(B - V) = 0.82 and known high molecular abundances from ground-based data. The HST observations consist of high-resolution spectra obtained with grating ECH-B at wavelengths longward of 1800 Angstrom, and moderate-resolution spectra from grating G160M at shorter wavelengths. We observed some 19 wavelength settings, covering the positions of numerous atomic and molecular transitions. We also incorporate IUE and ground-based data in the study, the latter coming from several observatories at visible and millimeter wavelengths. In this paper we report on the general goals and results of the study, with emphasis on the atomic abundances and depletions. We find that the column densities of most species are slightly higher, relative to the adopted total gas column density, than in some thinner clouds in which most of the absorption arises in a single dense component. Consequently the depletions of elements from the gas phase onto the dust are less than in ''typical'' diffuse clouds such as the one toward zeta Oph. Most of the gas toward HD 154368 resides in two main clouds, centered near -3.26 and -20.95 km s(-1) (heliocentric). Our profile analyses show that the depletions in these two clouds are similar. We discuss the possible relationship of this result to the ultraviolet extinction curve toward HD 154368, derived from IUE spectra