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NGC 7538 IRS1
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Title: A Comparative Astrochemical Study Of The High-Mass Protostellar Objects NGC 7538 IRS 9 and IRS 1
Authors: John C. Barentine, John H. Lacy

We report the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 and compare our observations to published data on the nearby object NGC 7538 IRS 1. Both objects originated in the same molecular cloud and appear to be at different points in their evolutionary histories, offering an unusual opportunity to study the temporal evolution of envelope chemistry in objects sharing a presumably identical starting composition. Observations were made with the Texas Echelon Cross Echelle Spectrograph (TEXES), a sensitive, high spectral resolution (R = {\lambda}/{\Delta}{\lambda} \simeq 100,000) mid-infrared grating spectrometer. Forty-six individual lines in vibrational modes of the molecules C2H2, CH4, HCN, NH3 and CO were detected, including two isotopologues (13CO, 12C18O) and one combination mode ({\nu}4 + {\nu}5 C2H2). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (\sim2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9.

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NGC 7538 S
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Title: From dusty filaments to massive stars: The case of NGC 7538 S
Authors: Raul Naranjo-Romero (CRyA-UNAM), Luis A. Zapata (CRyA-UNAM), Enrique Vazquez-Semadeni (CRyA-UNAM), Satoko Takahashi (ASIAA), Aina Palau (CSIC-IEEC), Peter Schilke (Universitat zu Koln)

We report on high-sensitivity and high-angular resolution archival Submillimeter Array (SMA) observations of the large (~15000 AU) putative circumstellar disk associated with the O-type protostar NGC 7538 S. Observations of the continuum resolve this putative circumstellar disk into five compact sources, with sizes ~ 3000 AU and masses ~10 solar masses. This confirm the results of recent millimetre observations made with CARMA/BIMA towards this object. However, we find that from most of these compact sources eject collimated bipolar outflows, revealed by our silicon monoxide (SiO {J}=5-4) observations and confirm that these sources have a (proto)stellar nature. All outflows are perpendicular to the large and rotating dusty structure. We propose therefore that, rather than being a single massive circumstellar disk, NGC 7538 S could be instead a large and massive contracting or rotating filament that is fragmenting at scales of 0.1 to 0.01 pc to form several B-type stars, via the standard process involving outflows and disks. As in recent high spatial resolution studies of dusty filaments, our observations also suggest that thermal pressure does not seem to be sufficient to support the filament, so that either additional support needs to be invoked, or else the filament must be in the process of collapsing. An SPH numerical simulation of the formation of a molecular cloud by converging warm neutral medium flows produces contracting filaments whose dimensions and spacings between the stars forming within them, as well as their column densities, strongly resemble those observed in the filament reported here.

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Posts: 131433
Date:
G111.80+0.58
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Title: Spitzer's mid-infrared view on an outer Galaxy Infrared Dark Cloud candidate toward NGC 7538
Authors: W. F. Frieswijk, M. Spaans, R. F. Shipman, D. Teyssier, S. J. Carey, A. G. G. M. Tielens

Infrared Dark Clouds (IRDCs) represent the earliest observed stages of clustered star formation, characterized by large column densities of cold and dense molecular material observed in silhouette against a bright background of mid-IR emission. Up to now, IRDCs were predominantly known toward the inner Galaxy where background infrared emission levels are high. We present Spitzer observations with the Infrared Camera Array toward object G111.80+0.58 (G111) in the outer Galactic Plane, located at a distance of ~3 kpc from us and ~10 kpc from the Galactic centre. Earlier results show that G111 is a massive, cold molecular clump very similar to IRDCs. The mid-IR Spitzer observations unambiguously detect object G111 in absorption. We have identified for the first time an IRDC in the outer Galaxy, which confirms the suggestion that cluster-forming clumps are present throughout the Galactic Plane. However, against a low mid-IR back ground such as the outer Galaxy it takes some effort to find them.

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Posts: 131433
Date:
NGC 7538 IRS1
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Title: Investigating the Nature of the Dust Emission around Massive Protostar
NGC 7538 IRS 1: Circumstellar Disk and Outflow?

Authors: James M. De Buizer- Gemini Observatory, and Vincent Minier - DAPNIA/DSM/CEA Centre d'Etudes de Saclay.

Researchers have obtained high resolution mid-infrared images of the high mass protostar NGC 7538 IRS 1 using Michelle on Gemini North and find that the circumstellar dust associated with this source is extended on both large and small scales.


A contour plot of the IRS 1, 2 and 3 region at 18.3 nm. These contours are overlaid on a colour image that is a 3-color composite made from 2MASS J (blue), H (green), and K (red) images.
The origin is the peak in the mid-infrared of IRS 1 located at R.A.(J2000)=23 13 45.36, Dec.(2000)=+61 28 10.6.


The large-scale mid-infrared emission is asymmetric about the peak of IRS 1, being more extended to the northwest than the southeast.
The position angle of the mid-infrared emission is similar to the position angle of the linearly distributed methanol masers at this location which are thought to trace a circumstellar disk.
However, this position angle is also very similar to that of the CO outflow in this region which appears to be centred on IRS 1.
They suggest that the large-scale extended mid-infrared emission is coming from dust heated on the walls of the outflow cavities near the source.
IRS 1 is also elongated in the mid-infrared on a smaller scale, and this elongation is near perpendicular to the axis of the CO outflow (and the linearly distributed methanol lasers).
Because of its orientation with respect to the outflow and its estimated size (Rdisk'450 AU at 11.7 nm), they propose that the small-scale elongation seen in the mid-infrared is a circumstellar disk that may be collimating the outflow from IRS 1.


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