Showing papers on "Infrared dark cloud published in 2009"

MIPSGAL: A Survey of the Inner Galactic Plane at 24 and 70 μm

Abstract: MIPSGAL is a 278 deg^2 survey of the inner Galactic plane using the Multiband Infrared Photometer for Spitzer aboard the Spitzer Space Telescope. The survey field was imaged in two passbands, 24 and 70 μm with resolutions of 6″ and 18″, respectively. The survey was designed to provide a uniform, well-calibrated and well-characterized data set for general inquiry of the inner Galactic plane and as a longer-wavelength complement to the shorter-wavelength Spitzer survey of the Galactic plane: Galactic Plane Infrared Mapping Survey Extraordinaire. The primary science drivers of the current survey are to identify all high-mass (M > 5 M⊙) protostars in the inner Galactic disk and to probe the distribution, energetics, and properties of interstellar dust in the Galactic disk. The observations were planned to minimize data artifacts due to image latents at 24 μm and to provide full coverage at 70 μm. Observations at ecliptic latitudes within 15° of the ecliptic plane were taken at multiple epochs to help reject asteroids. The data for the survey were collected in three epochs, 2005 September–October, 2006 April, and 2006 October with all of the data available to the public. The estimated point-source sensitivities of the survey are 2 and 75 mJy (3 σ) at 24 and 70 μm, respectively. Additional data processing was needed to mitigate image artifacts due to bright sources at 24 μm and detector responsivity variations at 70 μm due to the large dynamic range of the Galactic plane. Enhanced data products including artifact-mitigated mosaics and point-source catalogs are being produced with the 24 μm mosaics already publicly available from the NASA/IPAC Infrared Science Archive. Some preliminary results using the enhanced data products are described.

The initial conditions of stellar protocluster formation - I. A catalogue of Spitzer dark clouds

Abstract: Context. The majority of stars form in clusters. Therefore a comprehensive view of star formation requires understanding the initial conditions for cluster formation. Aims. The goal of our study is to shed light on the physical properties of infrared dark clouds (IRDCs) and the role they play in the formation of stellar clusters. This article, the first of a series dedicated to the study of IRDCs, describes techniques developed to establish a complete catalogue of Spitzer IRDCs in the Galaxy. Methods. We have analysed Spitzer GLIMPSE and MIPSGAL data to identify a complete sample of IRDCs in the region of Galactic longitude and latitude 10° < |l|< 65° and |b| < 1°. From the 8 m observations we have constructed opacity maps and used a newly developed extraction algorithm to identify structures above a column density of 1 1022 cm-2. The 24 m data are then used to characterize the star formation activity of each extracted cloud. Results. A total of 11 303 clouds have been extracted. A comparison with the existing MSX based catalogue of IRDCs shows that 80% of these Spitzer dark clouds were previously unknown. The algorithm also extracts ~20 000 to 50 000 fragments within these clouds, depending on detection threshold used. A first look at the MIPSGAL data indicates that between 20% and 68% of these IRDCs show 24 m point-like association. This new database provides an important resource for future studies aiming to understand the initial conditions of star formation in the Galaxy.

Mid-infrared extinction mapping of infrared dark clouds: probing the initial conditions for massive stars and star clusters

Abstract: Infrared dark clouds (IRDCs) are cold, dense regions of giant molecular clouds that are opaque at wavelengths ~10 μm or more and thus appear dark against the diffuse Galactic background emission. They are thought to be the progenitors of massive stars and star clusters. We use 8 μm imaging data from Spitzer Galactic Legacy Mid-Plane Survey Extraordinaire to make extinction maps of 10 IRDCs, selected to be relatively nearby and massive. The extinction mapping technique requires construction of a model of the Galactic IR background intensity behind the cloud, which is achieved by correcting for foreground emission and then interpolating from the surrounding regions. The correction for foreground emission can be quite large, up to ~50% for clouds at ~5 kpc distance, thus restricting the utility of this technique to relatively nearby clouds. We investigate three methods for the interpolation, finding systematic differences at about the 10% level, which, for fiducial dust models, corresponds to a mass surface density Σ = 0.013 g cm-2, above which we conclude that this extinction mapping technique attains validity. We examine the probability distribution function of Σ in IRDCs. From a qualitative comparison with numerical simulations of astrophysical turbulence, many clouds appear to have relatively narrow distributions suggesting relatively low (less than five) Mach numbers and/or dynamically strong magnetic fields. Given cloud kinematic distances, we derive cloud masses. Rathborne, Jackson, and Simon identified cores within the clouds and measured their masses via millimeter dust emission. For 43 cores, we compare these mass estimates with those derived from our extinction mapping, finding good agreement: typically factors of 2 difference for individual cores and an average systematic offset of 10% for the adopted fiducial assumptions of each method. We find tentative evidence for a systematic variation of these mass ratios as a function of core density, which is consistent with models of ice mantle formation on dust grains and subsequent grain growth by coagulation, and/or with a temperature decrease in the densest cores.

The "Nessie" Nebula: Cluster Formation in a Filamentary Infrared Dark Cloud

Abstract: The "Nessie" Nebula is a filamentary infrared dark cloud (IRDC) with a large aspect ratio of over 150:1 (15 × 001 or 80 pc × 0.5 pc at a kinematic distance of 3.1 kpc). Maps of HNC (1-0) emission, a tracer of dense molecular gas, made with the Australia Telescope National Facility Mopra telescope, show an excellent morphological match to the mid-IR extinction. Moreover, because the molecular line emission from the entire nebula has the same radial velocity to within ±3.4 km s–1, the nebula is a single, coherent cloud and not the chance alignment of multiple unrelated clouds along the line of sight. The Nessie Nebula contains a number of compact, dense molecular cores which have a characteristic projected spacing of ~4.5 pc along the filament. The theory of gravitationally bound gaseous cylinders predicts the existence of such cores, which, due to the "sausage" or "varicose" fluid instability, fragment from the cylinder at a characteristic length scale. If turbulent pressure dominates over thermal pressure in Nessie, then the observed core spacing matches theoretical predictions. We speculate that the formation of high-mass stars and massive star clusters arises from the fragmentation of filamentary IRDCs caused by the "sausage" fluid instability that leads to the formation of massive, dense molecular cores. The filamentary molecular gas clouds often found near high-mass star-forming regions (e.g., Orion, NGC 6334, etc.) may represent a later stage of IRDC evolution.

Rotational structure and outflow in the infrared dark cloud 18223-3

Abstract: Aims. We examine an Infrared Dark Cloud (IRDC) at high spatial resolution as a means to study rotation, outflow, and infall at the onset of massive star formation. Methods. The IRDC 18223-3 was observed at 1.1 mm and 1.3 mm with the Submillimeter Array (SMA) and follow-up short spacing information was obtained with the IRAM 30m telescope. Additional data were taken at 3 mm with the IRAM Plateau de Bure interferometer (PdBI). Results. Submillimeter Array observations combined with IRAM 30 m data in 12 CO(2-1) reveal the outflow orientation in the IRDC 18223-3 region, and PdBI 3 mm observations confirm this orientation in other molecular species. The implication of the outflow's presence is that an accretion disk is feeding it, so using line data for high density tracers such as C 18 O, N 2 H + , and CH 3 OH, we looked for indications of a velocity gradient perpendicular to the outflow direction. Surprisingly, this gradient turns out to be most apparent in CH 3 0H. The large size (28 000 AU) of the flattened rotating object detected indicates that this velocity gradient cannot be due solely to a disk, but rather from inward spiraling gas within which a Keplerian disk likely exists. The rotational signatures can be modeled via rotationally infalling gas. From the outflow parameters, we derive properties of the source such as an outflow dynamical age of ~37 000 years, outflow mass of ~ 13 M ⊙ , and outflow energy of ~1.7 x 10 46 erg. While the outflow mass and energy are clearly consistent with a high-mass star forming region, the outflow dynamical age indicates a slightly more evolved evolutionary stage than previous spectral energy distribution (SED) modeling indicates. Conclusions. The orientation of the molecular outflow associated with IRDC 18223-3 is in the northwest-southeast direction and velocity gradients orthogonal to the outflow reveal a large rotating structure likely harboring an accretion disk within. We also present a model of the observed methanol velocity gradient. The calculated outflow properties reveal that this is truly a massive star in the making. These data present evidence for one of the youngest known outflow/infall/disk systems in massive star formation. A tentative evolutionary picture for massive disks is discussed.

Infrared dark cloud cores in the SCUBA Legacy Catalogue

Abstract: We present an investigation of candidate Infrared Dark Cloud cores as identified by Simon et al. (2006) located within the SCUBA Legacy Catalogue. After applying a uniform noise cut to the Catalogue data we identify 154 Infrared Dark Cloud cores that were detected at 850 µm and 51 cores that were not. We derive column densities for each core from their 8 µm extinction and find that the IRDCs detected at 850 µm have higher column densities (a mean of 1.7×10 22 cm 2 ) compared to those cores not detected at 850 µm (a mean of 1.0×10 22 cm 2 ). Combined with sensitivity estimates, we suggest that the cores not detected at 850 µm are low mass, low column density and low temperature cores that are below the sensitivity limit of SCUBA at 850 µm. For a subsample of the cores detected at 850 µm those contained within the MIPSGAL area) we find that two thirds are associated with 24 µm sources. Cores not associated with 24 µm emission are either “starless” IRDC cores that perhaps have yet to form stars, or contain low mass YSOs below the MIPSGAL detection limit. We see that those “starless” IRDC cores and the IRDC cores associated with 24 µm emission are drawn from the same column density population and are of similar mass. If we then assume the cores without 24 µm embedded sources are at an earlier evolutionary stage to cores with embedded objects we derive a statistical lifetime for the quiescent phase of a few 10 3 –10 4 years. Finally, we make conservative predictions for the number of observed IRDCs that will be observed by the Apex Telescope Galactic Plane Survey (ATLASGAL), the Herschel Infrared Galactic Plane Survey (Hi-GAL), the JCMT Galactic Plane Survey (JPS) and the SCUBA-2 “All Sky” Survey (SASSy).

Rotational Structure and Outflow in the Infrared Dark Cloud 18223-3

Abstract: We examine an Infrared Dark Cloud (IRDC) at high spatial resolution as a means to study rotation, outflow, and infall at the onset of massive star formation. Submillimeter Array observations combined with IRAM 30 meter data in 12CO(2--1) reveal the outflow orientation in the IRDC 18223-3 region, and PdBI 3 mm observations confirm this orientation in other molecular species. The implication of the outflow's presence is that an accretion disk is feeding it, so using high density tracers such as C18O, N2H+, and CH3OH, we looked for indications of a velocity gradient perpendicular to the outflow direction. Surprisingly, this gradient turns out to be most apparent in CH3OH. The large size (28,000 AU) of the flattened rotating object detected indicates that this velocity gradient cannot be due solely to a disk, but rather from inward spiraling gas within which a Keplerian disk likely exists. From the outflow parameters, we derive properties of the source such as an outflow dynamical age of ~37,000 years, outflow mass of ~13 M_sun, and outflow energy of ~1.7 x 10^46 erg. While the outflow mass and energy are clearly consistent with a high-mass star forming region, the outflow dynamical age indicates a slightly more evolved evolutionary stage than previous spectral energy distribution (SED) modeling indicates. The calculated outflow properties reveal that this is truly a massive star in the making. We also present a model of the observed methanol velocity gradient. The rotational signatures can be modeled via rotationally infalling gas. These data present evidence for one of the youngest known outflow/infall/disk systems in massive star formation. A tentative evolutionary picture for massive disks is discussed.

Evidence of triggered star formation in G327.3-0.6 - Dust-continuum mapping of an infrared dark cloud with P-ArTéMiS

Abstract: Aims. Expanding HII regions and propagating shocks are common in the environment of young high-mass star-forming complexes. They can compress a pre-existing molecular cloud and trigger the formation of dense cores. We investigate whether these phenomena can explain the formation of high-mass protostars within an infrared dark cloud located at the position of G327.3-0.6 in the Galactic plane, in between two large infrared bubbles and two HII regions. Methods. The region of G327.3-0.6 was imaged at 450 μm with the CEA P-ArTeMiS bolometer array on the Atacama Pathfinder EXperiment telescope in Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were used in this study. Results. Ten massive cores were detected in the P-ArTeMiS image, embedded within the infrared dark cloud seen in absorption at both 8a nd 24μm. Their luminosities and masses indicate that they form high-mass stars. The kinematical study of the region suggests that the infrared bubbles expand toward the infrared dark cloud. Conclusions. Under the influence of expanding bubbles, star formation occurs in the infrared dark areas at the border of HII regions and infrared bubbles.

Evidence of triggered star formation in G327.3-0.6. Dust-continuum mapping of an infrared dark cloud with P-ArT\'eMiS

Abstract: Aims. Expanding HII regions and propagating shocks are common in the environment of young high-mass star-forming complexes. They can compress a pre-existing molecular cloud and trigger the formation of dense cores. We investigate whether these phenomena can explain the formation of high-mass protostars within an infrared dark cloud located at the position of G327.3-0.6 in the Galactic plane, in between two large infrared bubbles and two HII regions. Methods: The region of G327.3-0.6 was imaged at 450 ? m with the CEA P-ArT\'eMiS bolometer array on the Atacama Pathfinder EXperiment telescope in Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were used in this study. Results: Ten massive cores were detected in the P-ArT\'eMiS image, embedded within the infrared dark cloud seen in absorption at both 8 and 24 ?m. Their luminosities and masses indicate that they form high-mass stars. The kinematical study of the region suggests that the infrared bubbles expand toward the infrared dark cloud. Conclusions: Under the influence of expanding bubbles, star formation occurs in the infrared dark areas at the border of HII regions and infrared bubbles.

Atomic carbon in an infrared dark cloud

Abstract: Infrared dark clouds (IRDCs) are potential sites of massive star formation, dark in the near-infrared, but in many cases already with indications of active star-formation from far-infrared and submm observations. They are an ideal test bed to study the role of internal and external heating on the structure of the molecular cloud material.