Regions Of Molecular Clouds Research Articles

Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, and HCO+ observations of the galactic star-forming region NGC 6334 FIR II are presented, complemented by [C i] 3PP0 and 3PP1 data from the Atacama Pathfinder Experiment (APEX 12-m telescope). Embedded in the extended molecular cloud and associated with the H ii region NGC 6334–D, there is a molecular “void”. [C i] correlates well with 13CO and other molecular lines and shows no rim brightening relative to molecular cloud regions farther off the void. While an interpretation in terms of a highly clumped cloud morphology is possible, with photon dominated regions (PDRs) reaching deep into the cloud, the data do not provide any direct evidence for a close association of [C i] with PDRs. Kinetic temperatures are ~40-50 K in the molecular cloud and 200 K toward the void. CO and [C i] excitation temperatures are similar. A comparison of molecular and atomic fine structure line emission with the far infrared and radio continuum as well as the distribution of 2.2m H2 emission indicates that the well-evolved H ii region expands into a medium that is homogeneous on pc-scales. If the H2 emission is predominantly shock excited, both the expanding ionization front (classified as subsonic, “D-type”) and the associated shock front farther out (traced by H2) can be identified, observationally confirming for the first time a classical scenario that is predicted by evolutionary models of Hii regions. Integrated line intensity ratios of the observed molecules are determined, implying a mean C18O/C17O abundance ratio of 4.13 ± 0.13 that reflects the 18O/17O isotope ratio. This ratio is consistent with values determined in nearby clouds. Right at the edge of the void, however, the oxygen isotope ratio might be smaller.

ABSTRACTStars form in the densest, coldest, most quiescent regions of molecular clouds. Molecules provide the only probes that can reveal the dynamics, physics, chemistry, and evolution of these regions, but our understanding of the molecular inventory of sources and how this is related to their physical state and evolution is rudimentary and incomplete. The Spectral Legacy Survey (SLS) is one of seven surveys recently approved by the James Clerk Maxwell Telescope (JCMT) Board of Directors. Beginning in 2007, the SLS will produce a spectral imaging survey of the content and distribution of all the molecules detected in the 345 GHz atmospheric window (between 332 and 373 GHz) toward a sample of five sources. Our intended targets are a low‐mass core (NGC 1333 IRAS 4), three high‐mass cores spanning a range of star‐forming environments and evolutionary states (W49, AFGL 2591, and IRAS 20126), and a photodissociation region (the Orion Bar). The SLS will use the unique spectral imaging capabilities of HARP‐B/ACSIS (Heterodyne Array Receiver Programme B/Auto‐Correlation Spectrometer and Imaging System) to study the molecular inventory and the physical structure of these objects, which span different evolutionary stages and physical environments and to probe their evolution during the star formation process. As its name suggests, the SLS will provide a lasting data legacy from the JCMT that is intended to benefit the entire astronomical community. As such, the entire data set (including calibrated spectral data cubes, maps of molecular emission, line identifications, and calculations of the gas temperature and column density) will be publicly available.

Interferometric observations of high-mass regions in interstellar molecular clouds have revealed hot molecular cores that have substantial column densities of large, partly hydrogen-saturated molecules. Many of these molecules are of interest to biology and thus are labeled "biomolecules." Because the clouds containing these molecules provide the material for star formation, they may provide insight into presolar nebular chemistry, and the biomolecules may provide information about the potential of the associated interstellar chemistry for seeding newly formed planets with prebiotic organic chemistry. In this overview, events are outlined that led to the current interferometric array observations. Clues that connect this interstellar hot core chemistry to the solar system can be found in the cometary detection of methyl formate and the interferometric maps of cometary methanol. Major obstacles to understanding hot core chemistry remain because chemical models are not well developed and interferometric observations have not been very sensitive. Differentiation in the molecular isomers glycolaldehdye, methyl formate, and acetic acid has been observed, but not explained. The extended source structure for certain sugars, aldehydes, and alcohols may require nonthermal formation mechanisms such as shock heating of grains. Major advances in understanding the formation chemistry of hot core species can come from observations with the next generation of sensitive, high-resolution arrays.

Abstract We have conducted deep near-infrared surveys of the Sh-2 255, W3 Main and NGC 7538 massive star forming regions using simultaneous observations of the JHKs-band with the near-infrared camera SIRIUS on the UH 88-inch telescope and with SUBARU. The near-infrared surveys cover a total area of ~ 72 arcmin2 of three regions with 10-σ limiting magnitudes of ~ 19.5, 18.4 and 17.3 in J, H and Ks-band, respectively. Based on the color-color and color-magnitude diagrams and their clustering properties, the candidate young stellar objects are identified and their luminosity functions are constructed in Sh-2 255, W3 Main and NGC 7538 star forming regions. A large number of previously unreported red sources (H-K > 2) have also been detected around these regions. We argue that these red stars are most probably pre-main-sequence stars with intrinsic color excesses. The detected young stellar objects show a clear clustering pattern in each region: the Class I-like sources are mostly clustered in molecular cloud region, while the Class II-like sources are in or around more evolved optical HII regions. We find that the slopes of the Ks-band luminosity functions of Sh-2 255, W3 Main and NGC 7538 are lower than the typical values reported for the young embedded clusters, and their stellar populations are primarily composed of low mass pre-main-sequence stars. From the slopes of the Ks-band luminosity functions, we infer that Sh-2 255, W3 Main and NGC 7538 star forming regions are rather young (age ≤ 1 Myr).

Abstract We perform 3D MHD simulations of cluster formation in turbulent magnetized dense molecular clumps, taking into account the effect of protostellar outflows. Our simulation shows that initial interstellar turbulence decays quickly as several authors already pointed out. When stars form, protostellar outflows generate and maintain supersonic turbulence that have a power-law energy spectrum of Ek ~ k−2, which is somewhat steeper than those of driven MHD turbulence simulations. Protostellar outflows suppress global star formation, although they can sometimes trigger local star formation by dynamical compression of pre-existing cores. Magnetic field retards star formation by slowing down overall contraction. Interplay of protostellar outflows and magnetic field generates large-amplitude Alfven and MHD waves that transform outflow motions into turbulent motions efficiently. Cluster forming clumps tend to be in dynamical equilibrium mainly due to dynamical support by protostellar outflow-driven turbulence (hereafter, protostellar turbulence).

Context. The M 16 nebula is a relatively nearby Hii region, powered by O stars from the open cluster NGC 6611, which borders to a Giant Molecular Cloud. Radiation from these hot stars has sculpted columns of dense obscuring material on a few arcmin scales. The interface between these pillars and the hot ionised medium provides a textbook example of a Photodissociation Region (PDR). Aims. To constrain the physical conditions of the atomic and molecular material with submillimeter spectroscopic observations. Methods. We used the APEX submillimeter telescope to map a ${\sim} 3'\times3'$ region in the CO $J= 3$–2, 4–3 and 7–6 rotational lines, and a subregion in atomic carbon lines. We also observed C 18 O(3–2) and CO(7–6) with longer integrations on five peaks found in the CO(3–2) map. The large scale structure of the pillars is derived from the molecular lines' emission distribution. We estimate the magnitude of the velocity gradient at the tips of the pillars and use LVG modelling to constrain their densities and temperatures. Excitation temperatures and carbon column densities are derived from the atomic carbon lines. Results. The atomic carbon lines are optically thin and excitation temperatures are of order 60 K to 100 K, well consistent with observations of other Hii region-molecular cloud interfaces. We derive somewhat lower temperatures from the CO line ratios, of order 40 K. The Ci/CO ratio is around 0.1 at the fingers tips.

Diffuse far-ultraviolet (FUV; 1370-1670 A) flux from the Taurus molecular cloud region has been observed with the SPEAR/FIMS imaging spectrograph. An FUV continuum map of the Taurus region, similar to the visual extinction maps, shows a distinct cloud core and halo region. The dense cloud core, where the visual extinction Av > 1.5, obscures the background diffuse FUV radiation, while scattered FUV radiation is seen in and beyond the halo region, where Av < 1.5. The total intensity of H2 fluorescence in the cloud halo is I = 6.5 × 104 photons cm-2 s-1 sr-1 in the 1370-1670 A wavelength band. A synthetic model of the H2 fluorescent emission fits the present observation best with a hydrogen density nH = 50 cm-3, H2 column density N(H2) = 0.8 × 1020 cm-2, and incident FUV intensity IUV = 0.2. H2 fluorescence is not seen in the core, presumably because the required radiation flux to induce fluorescence is unable to penetrate the core region.

Chandra ACIS-I data of the molecular cloud and H II region complex NGC 6334 were analyzed. The hard X-ray clumps detected with ASCA (Sekimoto and coworkers) were resolved into 792 point sources. After removing the point sources, an extended X-ray emission component was detected over a 5 × 9 pc2 region, with the 0.5-8 keV absorption-corrected luminosity of 2 × 1033 ergs s-1. The contribution from faint point sources to this extended emission was estimated as at most ~20%, suggesting that most of the emission is diffuse in nature. The X-ray spectrum of the diffuse emission was observed to vary from place to place. In tenuous molecular cloud regions with hydrogen column density of (0.5-1) × 1022 cm-2, the spectrum can be represented by a thermal plasma model with temperatures of several keV. The spectrum in dense cloud cores exhibits harder continuum, together with higher absorption of more than ~3 × 1022 cm-2. In some of such highly obscured regions, the spectra show extremely hard continua equivalent to a photon index of ~1, and favor a nonthermal interpretation. These results are discussed in the context of thermal and nonthermal emission, both powered by fast stellar winds from embedded young early-type stars through shock transitions.

We present high-resolution (~5'') maps of the J = 1–0 transitions of 13CO and C18O toward the nucleus of NGC 6946 made with the Owens Valley Millimeter Array. The images are compared with existing 12CO (1–0) maps to investigate localized changes in gas properties across the nucleus. As compared with 12CO, both 13CO and C18O are more confined to the central ring of molecular gas associated with the nuclear star formation; that is, 12CO is stronger relative to 13CO and C18O away from the nucleus and along the spiral arms. The 12CO (1–0)/13CO (1–0) line ratio reaches very high values of greater than 40. We attribute the relative 13CO weakness to a rapid change in the interstellar medium (ISM) from dense star-forming cores in a central ring to diffuse, low-density molecular gas in and behind the molecular arms. This change is abrupt, occurring in less than a beam size (90 pc), about the size of a giant molecular cloud. Column densities determined from 13CO (1–0), C18O (1–0), and 1.4 mm dust continuum all indicate that the standard Galactic conversion factor, XCO, overestimates the amount of molecular gas in NGC 6946 by factors of ~3–5 toward the central ring and potentially even more so in the diffuse gas away from the central starburst. We suggest that the nuclear bar acts to create coherent regions of molecular clouds with distinct and different physical conditions. The 12CO (1–0)/13CO (1–0) line ratio in galactic nuclei can be a signpost of a dynamically evolving ISM.

We have collected one-dimensional raster-scan observations of the active star-forming region Sharpless 171 (S171), a typical H  region-molecular cloud complex, with the three spectrometers (LWS, SWS, and PHT-S) on board ISO. We have detected 8 far-infrared fine-structure lines, (O  )5 2µm, (N  )5 7µm, (O  )6 3µm, (O  )8 8µm, (N ) 122 µm, (O ) 146 µm, (C ) 158 µm, and (Si  )3 5µm together with the far-infrared continuum and the H2 pure rotation transition (J = 5-3) line at 9.66 µm. The physical properties of each of the three phases detected, highly-ionized, lowly-ionized and neutral, are investi- gated through the far-infrared line and continuum emission. Toward the molecular region, strong (O ) 146 µm emission was observed and the (O  )6 3µm to 146 µm line ratio was found to be too small (∼5) compared to the values predicted by current photodissociation region (PDR) models. We examine possible mechanisms to account for the small line ratio and conclude that the absorption of the (O  )6 3µ ma nd the (C) 158 µm emission by overlapping PDRs along the line of sight can account for the observations and that the (O ) 146 µm emission is the best diagnostic line for PDRs. We propose a method to estimate the effect of overlapping clouds using the far-infrared continuum intensity and derive the physical properties of the PDR. The (Si  )3 5µm emission is quite strong at almost all the observed positions. The correlation with (N ) 122 µm suggests that the (Si ) emission originates mostly from the ionized gas. The (Si  )3 5µ mt o (N) 122 µm ratio indicates that silicon of 30% of the solar abundance must be in the diffuse ionized gas, suggesting that efficient dust destruction is undergoing in the ionized region.

The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in absorption against the diffuse 8 μm Galactic background and in emission from cold dust at 850 μm. A detailed comparison of the dust properties at these two wavelengths reveals that standard models for the diffuse interstellar dust in the Galaxy are not consistent with the observations. The ratio of absorption coefficients within the cloud is κ8/κ850 ≤ 1010, which is well below that expected for the diffuse interstellar medium where κ8/κ850 ~ 1700. This may be due to the formation of ice mantles on the dust and grain coagulation, both of which are expected within dense regions of molecular clouds. The 850 μm emission probes the underlying radial structure of the filament. The profile is well represented by a marginally resolved central region and a steeply falling envelope, with Σ(r) ∝ r-α, where α ≥ 3, indicating that G11.11-0.12 is the first observed filament with a profile similar to that of a nonmagnetic isothermal cylinder.

We present the results of a deep search for the molecular hydrogen shock fronts associated with young stellar outflows in the giant molecular cloud and massive star-forming region W51. A total of 14 outflows were identified by comparing images in the H and K bands and in a narrowband filter centered on the H2 1-0 S(1) line at 2.122 μm. A few of the newly discovered outflows were subsequently imaged at higher spatial resolution in the S(1) filter; one outflow was also imaged in the 1.644 μm emission line of [Fe II]. For two of the outflows, high-resolution echelle spectroscopy in the H2 1-0 S(1) line was obtained using NIRSPEC at Keck. For one outflow additional high-resolution spectra were obtained in the [Fe II] line and in Brγ. The largest and best-studied outflow shock front shows a remarkably broad [Fe II] line, an unusual high-velocity component in Brγ, and comparably narrow line widths in the H2 1-0 S(1) line. A scenario involving high-velocity shocks and UV excitation of preshock material is used to explain these spectra.

We present measurements of the emission from the centers of fifteen spiral galaxies in the 3 P 1 - 3 P 0 [CI] fine-structure transition at 492 GHz. Observed galaxy centers range from quiescent to starburst to active. The intensities of neutral carbon, the J = 2-1 transition of 1 3 CO and the J = 4-3 transition of 1 2 CO are compared in matched beams. Most galaxy centers emit more strongly in [CI] than in 1 3 CO, completely unlike the situation pertaining to Galactic molecular cloud regions. [CI] intensities are lower than, but nevertheless comparable to J = 4-3 1 2 CO intensities, again rather different from Galactic sources. The ratio of [CI] to 1 3 CO increases with the central [CI] luminosity of a galaxy; it is lowest for quiescent and mild starburst centers, and highest for strong starburst centers and active nuclei. Comparison with radiative transfer model calculations shows that most observed galaxy centers have neutral carbon abundances close to, or exceeding, carbon monoxide abundances, rather independent from the assumed model gas parameters. The same models suggest that the emission from neutral carbon and carbon monoxide, if assumed to originate in the same volumes, arises from a warm and dense gas rather than a hot and tenuous, or a cold and very dense gas. The observed [CI] intensities together with literature [CII] line and far-infrared continuum data likewise suggest that a significant fraction of the emission originates in medium-density gas (n = 10 3 -10 4 cm - 3 ), subjected to radiation fields of various strengths.

Results of observations of the H2O maser in S269 carried out from October 1980 to February 2001 on the 22-m telescope (RT-22) of the Pushchino Radio Astronomy Observatory are presented. During the monitoring of S269, variability of the integrated flux of the maser emission with a cyclic character and an average period of 5.7 years was observed. This may be connected with cyclic activity of the central star during its formation. Emission at radial velocities of 4–7 km/s was detected. Thus, the H2O maser emission in S269 extends from 4 to 22 km/s, and is concentrated in three radial-velocity intervals: 4–7, 11–14, and 14–22 km/s. In some time intervals, the main group of emission features (14–22 km/s) had a triplet structure. The central velocity of the total spectrum is close to the velocity of the CO molecular cloud and HII region, differing from it by an amount that is within the probable dispersion of the turbulent gas velocities in the core of the CO molecular cloud. A radial-velocity drift of the component at VLSR≈20 km/s with a period of ≈26 years has been detected. This drift is likely due to turbulent (vortical) motions of material.

We have investigated the gaseous and solid state molecular composition of dense interstellar material that periodically experiences processing in the shock waves associated with ongoing star formation. Our motivation is to confront these models with the stringent abundance constraints on CO2, H2O and O2, in both gas and solid phases, that have been set by ISO and SWAS. We also compare our results with the chemical composition of dark molecular clouds as determined by ground-based telescopes. Beginning with the simplest possible model needed to study molecular cloud gas-grain chemistry, we only include additional processes where they are clearly required to satisfy one or more of the ISO-SWAS constraints. When CO, N2 and atoms of N, C and S are efficiently desorbed from grains, a chemical quasi-steady-state develops after about one million years. We find that accretion of CO2 and H2O cannot explain the ISO observations; as with previous models, accretion and reaction of oxygen atoms are necessary although a high O atom abundance can still be derived from the CO that remains in the gas. The observational constraints on solid and gaseous molecular oxygen are both met in this model. However, we find that we cannot explain the lowest abundances seen by SWAS or the highest atomic carbon abundances found in molecular clouds; additional chemical processes are required and possible candidates are given. One prediction of models of this type is that there should be some regions of molecular clouds which contain high gas phase abundances of H2O, O2 and NO. A further consequence, we find, is that interstellar grain mantles could be rich in NH2OH and NO2. The search for these regions, as well as NH2OH and NO2 in ices and in hot cores, is an important further test of this scenario. The model can give good agreement with observations of simple molecules in dark molecular clouds such as TMC-1 and L134N. Despite the fact that S atoms are assumed to be continously desorbed from grain surfaces, we find that the sulphur chemistry independently experiences an "accretion catastrophe" . The S-bearing molecular abundances cease to lie within the observed range after about years and this indicates that there may be at least two efficient surface desorption mechanisms operating in dark clouds -one quasi-continous and the other operating more sporadically on this time-scale. We suggest that mantle removal on short time-scales is mediated by clump dynamics, and by the effects of star formation on longer time-scales. The applicability of this type of dynamical-chemical model for molecular cloud evolution is discussed and comparison is made with other models of dark cloud chemistry.

We investigate through dedicated numerical simulations the evolution of the soft X-ray absorption properties of a cloud surrounding a gamma-ray burst source. We show that the absorption properties of the material are strongly modified by the ionization induced by the intense burst flux. We derive the temporal evolution of the measured column density as a function of the density and size of the absorbing medium. Even if their statistical significance is not extremely compelling, we find that the detection in several bursts of variable absorption during the gamma-ray phase can be accounted for if these bursts are associated to overdense regions in molecular clouds with properties similar to those of star formation globules. We fit our model variable column density to the data of GRB 980329 and GRB 780506, showing that with this method the size, density and density distribution of the material surrounding a burst can be measured.

We present the first coherent dynamical study of the cloud fragmentation phase, collapse, and early stellar evolution of a solar mass star. We determine young star properties as the consequence of the parent cloud evolution. Mass, luminosity, and effective temperature in the first million years of the proto-Sun result from gravitational fragmentation of a molecular cloud region that produces a cluster of prestellar clumps. We calculate the global dynamical behavior of the cloud using isothermal three-dimensional hydrodynamics and follow the evolution of individual protostars in detail using a one-dimensional radiation hydrodynamic system of equations that comprises a correct standard solar model solution, as a limiting case. We calculate the pre-main-sequence (PMS) evolutionary tracks of a solar mass star in a dense stellar cluster environment and compare it to one that forms in isolation. Up to an age of 950,000 yr, differences in the accretion history lead to significantly different temperature and luminosity evolution. As accretion fades and the stars approach their final masses, the two dynamic PMS tracks converge. After that, the contraction of the quasi-hydrostatic stellar interiors dominate the overall stellar properties and proceed in very similar ways. Hence, the position of a star in the Hertzsprung-Russell diagram becomes a function of age and mass only. However, our quantitative description of cloud fragmentation, star formation, and early stellar evolution predicts substantial corrections to the classical, i.e., hydrostatic and initially fully convective models: at an age of 1 million yr, the proto-Sun is twice as bright and 500 K hotter than according to calculations that neglect the star formation process.

We analyze the effects of molecular depletion on the thermal balance of well-shielded, quiescent dark cloud cores. Recent observations of the significant depletion of molecules from the gas phase onto grain surfaces in dark clouds suggest the possibility that the gas-phase cooling in these regions is greatly reduced and consequently that gas kinetic temperatures might be increased. We reexamine cooling and heating processes in light of possible molecular depletion, including the effect of coupling between the gas and the grains. At densities ≤103.5 cm-3, the gas temperature can be significantly increased by the depletion of coolant species without significantly affecting the dust temperature because of the relatively weak gas-dust coupling. At higher densities, this coupling becomes sufficiently rapid to overwhelm the effect of the reduced gas-phase cooling, and depletion has little effect on the gas temperature while raising the dust temperature 1 K. The result is that depletion at densities ≥104.5 cm-3 can proceed without being evident as an enhanced gas temperature or without self-limiting due to an increase in the dust temperature increasing the desorption rate. This is consistent with observations of depletion in cold, dense regions of quiescent molecular clouds. It also suggests that depletion in moderate density regions can increase the thermal gas pressure, effectively enhancing the confinement of denser portions of molecular clouds and possibly accelerating the collapse of cloud cores.

Turbulent fragmentation determines where and when protostellar cores form, and how they contract and grow in mass from the surrounding cloud material. This process is investigated, using numerical models of self-gravitating molecular cloud dynamics. Molecular cloud regions without turbulent driving sources, or where turbulence is driven on large scales, exhibit rapid and efficient star formation in a clustered mode, whereas interstellar turbulence that carries most energy on small scales results in isolated star formation with low efficiency. The clump mass spectrum of shock-generated density fluctuations in pure hydrodynamic, supersonic turbulence is not well fit by a power law, and it is too steep at the high-mass end to be in agreement with the observational data. When gravity is included in the turbulence models, local collapse occurs, and the spectrum extends towards larger masses as clumps merge together, a power-law description dN/dM ~ M^nu becomes possible with slope nu < -2. In the case of pure gravitational contraction, i.e. in regions without turbulent support, the clump mass spectrum is shallower with nu = -3/2. The mass spectrum of protostellar cores in regions without turbulent support and where turbulence is replenished on large-scales, however, is well described by a log-normal or by multiple power laws, similar to the stellar IMF at low and intermediate masses. In the case of small-scale turbulence, the core mass spectrum is too flat compared to the IMF for all masses.

Using hydrodynamic simulations, we investigate the time evolution and fragmentation of regions within molecular clouds that have lost their turbulent support, leading to gravitational contraction. The initial density distributions are described by random Gaussian fluctuations with varying slopes ν of the power spectrum P(k) k-ν, covering the range from flat (ν = 0) to very steep (ν = 3) spectra. We consider molecular cloud volumes containing different masses relative to the average Jeans mass MJ, from 1MJ to 222MJ. This parameter study extends a previous detailed analysis of systems with, initially, P(k) k-2 and mass 222MJ. The dynamical evolution of the simulated molecular cloud regions is insensitive to the slope of the initial density fluctuation spectrum. The system evolves into a complex network of intersecting filaments and collapsing clumps, leading to the formation of a compact cluster of accreting and interacting embedded protostellar cores. The cluster builds up as a bound entity but dissolves later due to collisional effects. In all simulations, the mass spectrum of collapsed cores is very broad, has approximately log-normal shape, and peaks roughly at the average Jeans mass. This supports the hypothesis that the average Jeans mass is the main parameter determining the peak in the stellar spectrum and suggests that the interplay between self-gravity on the one side and thermal and turbulent pressure on the other side is the dominant process that regulates the formation of stellar clusters.