Showing papers on "Infrared dark cloud published in 2007"

Cold Dark Clouds: The Initial Conditions for Star Formation

Abstract: Cold dark clouds are nearby members of the densest and coldest phase in the Galactic interstellar medium, and represent the most accessible sites where stars like our Sun are currently being born. In this review we discuss recent progress in their study, including the newly discovered IR dark clouds that are likely precursors to stellar clusters. At large scales, dark clouds present filamentary mass distributions with motions dominated by supersonic turbulence. At small, subparsec scales, a population of subsonic starless cores provides a unique glimpse of the conditions prior to stellar birth. Recent studies of starless cores reveal a combination of simple physical properties together with a complex chemical structure dominated by the freeze-out of molecules onto cold dust grains. Elucidating this combined structure is both an observational and theoretical challenge whose solution will bring us closer to understanding how molecular gas condenses to form stars.

The Detection of Protostellar Condensations in Infrared Dark Cloud Cores

Abstract: Infrared dark clouds (IRDCs) are a distinct class of interstellar molecular cloud identified as dark extinction features against the bright mid-infrared Galactic background. Here we present high angular resolution millimeter continuum images obtained with the IRAM Plateau de Bure Interferometer toward four high-mass (200-1800 M?) IRDC cores that show evidence for active high-mass star formation (M > 8 M?). We detect twelve bright (>7 mJy), compact (2'', 0.024 pc) condensations toward these cores. Two of the cores (G024.60+00.08 MM1 and G024.60+00.08 MM2) are resolved into multiple protostellar condensations, while one core (G022.35+00.41 MM1) shows two condensations. The remaining core (G024.33+00.11 MM1) contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that this is a hot molecular core associated with an early stage in the formation of a high-mass protostar. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar (Mgas > 8 M?), and three cores are also forming lower mass protostars (Mgas ~ 2-5 M?). A comparison of the ratios of the gas masses (MG) to the Jeans masses (MJ) for IRDCs, cores, and condensations, provides broad support for the idea of hierarchical fragmentation. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.

The Protostar in the massive Infrared Dark Cloud IRDC18223-3

Abstract: At the onset of high-mass star formation, accreting protostars are deeply embedded in massive cores made of gas and dust. Their spectral energy distribution is still dominated by the cold dust and rises steeply from near-to far-infrared wavelengths. The young massive star-forming region IRDC18223-3 is a prototypical Infrared-Dark-Cloud with a compact mm continuum core that shows no protostellar emission below 8mum. However, based on outflow tracers, early star formation activity was previously inferred for this region. Here, we present recent Spitzer observations from the MIPSGAL survey that identify the central protostellar object for the first time at 24 and 70mum. Combining the mid- to far-infrared data with previous mm continuum observations and the upper limits below 8mum, one can infer physical properties of the central source. At least two components with constant gas mass M and dust temperature T are necessary: one cold component (~15K and ~576M_sun) that contains most of the mass and luminosity, and one warmer component (>=51K and >=0.01M_sun) to explain the 24mum data. The integrated luminosity of ~177L_sun can be used to constrain additional parameters of the embedded protostar from the turbulent core accretion model for massive star formation. The data of IRDC18223-3 are consistent with a massive gas core harboring a low-mass protostellar seed of still less than half a solar mass with high accretion rates of the order 10^-4M_sun/yr. In the framework of this model, the embedded protostar is destined to become a massive star at the end of its formation processes.

The protostar in the massive infrared dark cloud irdc 18223-3

Abstract: At the onset of high-mass star formation, accreting protostars are deeply embedded in massive cores made of gas and dust. Their spectral energy distribution is still dominated by the cold dust and rises steeply from near- to far-infrared wavelengths. The young massive star-forming region IRDC 18223-3 is a prototypical infrared dark cloud with a compact millimeter continuum core that shows no protostellar emission below 8 μm. However, based on outflow tracers, early star formation activity was previously inferred for this region. Here we present recent Spitzer observations from the MIPSGAL survey that identify the central protostellar object for the first time at 24 and 70 μm. Combining the mid- to far-infrared data with previous millimeter continuum observations and the upper limits below 8 μm, one can infer the physical properties of the central source. At least two components with constant gas mass M and dust temperature T are necessary: one cold component (~15 K and ~576 M☉) that contains most of the mass and luminosity, and one warmer component (≥51 K and ≥0.01 M☉) to explain the 24 μm data. The integrated luminosity of ~177 L☉ can be used to constrain additional parameters of the embedded protostar from the turbulent core accretion model for massive star formation. The data of IRDC 18223-3 are consistent with a massive gas core harboring a low-mass protostellar seed of still less than half a solar mass with high accretion rates of the order 10-4 M☉ yr-1. In the framework of this model, the embedded protostar is destined to become a massive star at the end of its formation processes.

Physical characteristics of a dark cloud in an early stage of star formation toward NGC 7538 : An outer Galaxy infrared dark cloud?

Abstract: Context. In the inner parts of the Galaxy the Infrared Dark Clouds (IRDCs) are presently believed to be the progenitors of massive stars and star clusters. Many of them are predominantly devoid of active star formation and for now they represent the earliest observed stages of massive star formation. Their Outer Galaxy counterparts, if present, are not easily identified because of a low or absent mid-IR b ackground. Aims. We characterize the ambient conditions by determining physical parameters in the Outer Galaxy IRDC candidate G111.80+0.58, a relatively quiescent molecular core complex in the vicinity of active star forming regions such as NGC 7538 and S159. Methods. We conduct molecular line observations on a number of dense cores in G111.80+0.58. We analyze the data in terms of excitation temperature, column and volume density, mass and stability. Results. The temperatures we find (15 ‐ 20 K) are higher than expected fr om only cosmic ray heating, but are comparable to those found in massive cores, such as IRDCs. Star forming activity could be present in some cores, as indicated by the presence of warm gas (NH3, 13 CO self-absorption) and Young Stellar Object candidates. The observed super-thermal line-widths are typical for star fo rming regions. The velocity dispersion is consistent with a turbulent energy cascade over the observed size scales of the complex. We do not find a corr elation between the gas temperature and the line-width. The LTE masses we derive are much larger than the thermal Jeans mass. Therefore, fragmentation is expected and may have occurred already, in which case the observed lines represent the combined emission of multiple unresolved components. Conclusions. We conclude that G111.80+0.58 is a molecular core complex with bulk properties very similar to IRDCs in an early, but not pristine, star forming state. The individual cores are clos e to virial equilibrium and some contain suffi cient material to form massive stars and star clusters. The ambient conditions suggest that turbule nce is involved in supporting the cores against gravitation al collapse, at least down to the observed sizes. Additional high resolution data are necessary to resolve and analyze the smaller scale properties.

The Spitzer Gould Belt Survey of Large Nearby Interstellar Clouds: Discovery of a Dense Embedded Cluster in the Serpens-Aquila Rift

Abstract: We report the discovery of a nearby, embedded cluster of young stellar objects, associated filamentary infrared dark cloud, and 4.5 micron shock emission knots from outflows detected in Spitzer/IRAC mid-infrared imaging of the Serpens-Aquila Rift obtained as part of the Spitzer Gould Belt Legacy Survey. We also present radial velocity measurements of the region from molecular line observations obtained with the Submillimeter Array (SMA) that suggest the cluster is co-moving with the Serpens Main embedded cluster 3 degrees to the north. We therefore assign it the same distance, 260 pc. The core of the new cluster, which we call Serpens South, is composed of an unusually large fraction of protostars (77%) at high mean surface density (>430 pc^-2) and short median nearest neighbor spacing (3700 AU). We perform basic cluster structure characterization using nearest neighbor surface density mapping of the YSOs and compare our findings to other known clusters with equivalent analyses available in the literature.

NH3 Observations of the Infrared Dark Cloud G28.34+0.06

Abstract: We present observations of the h3 (J,K) = (1,1) and (2,2) inversion transitions toward the infrared dark cloud G28.34+0.06, using the Very Large Array. Strong NH3 emission is found to coincide well with the infrared absorption feature in this cloud. The northern region of G28.34+0.06 is dominated by a compact clump (P2) with a high rotation temperature (29 K), large line width (4.3 km s$^{-1}$), and is associated with strong water maser (240 Jy) and a 24 $\mu$m point source with far IR luminosity of $10^3$ \lsun. We infer that P2 has embedded massive protostars although it lies in the 8 $\mu$m absorption region. The southern region has filamentary structures. The rotation temperature in the southern region decreases with the increase of the integrated NH3 intensity, which indicates an absence of strong internal heating in these clumps. In addition, the compact core P1 in the south has small line width (1.2 km s$^{-1}$) surrounded by extended emission with larger line width (1.8 km s$^{-1}$), which suggests a dissipation of turbulence in the dense part of the cloud. Thus, we suggest that P1 is at a much earlier evolutionary stage than P2, possibly at a stage that begins to form a cluster with massive stars.