A12CO,13CO, and C18O Survey of Infrared Dark Clouds

Infrared dark clouds (IRDCs) are believed to host the earliest stages of high-mass star and cluster formation. Because O stars typically travel short distances over their lifetimes, if IRDCs host the earliest stages of high-mass star formation then these cold, dense molecular clouds should be located in or near the spiral arms in the Galaxy. The Galactic distribution of a large sample of IRDCs should therefore provide information on Galactic structure. Moreover, determination of distances enables mass and luminosity calculations. We have observed a large sample of IRDC candidates in the first Galactic quadrant in the dense gas tracer CS (2-1) using the Mopra telescope in order to determine kinematic distances from the molecular line velocities. We find that the IRDCs are concentrated around a Galactocentric distance of ~4.5 kpc, agreeing with the results of Simon et al. This distribution is consistent with the location of the Scutum-Centaurus spiral arm. The group of IRDCs near the Sun in the first quadrant detected in 13CO (1-0) in Simon et al. is not detected in the CS data. This discrepancy arises from the differences in the critical densities between the 13CO (1-0) and CS (2-1) lines. We determine that the Midcourse Space Experiment selected IRDCs are not a homogeneous population, and 13CO (1-0) traces a population of IRDCs with lower column densities and lower 1.1 mm flux densities in addition to more dense IRDCs detected in CS. Masses of the first quadrant IRDCs are calculated from 13CO (1-0) maps. We find a strong peak in the Galactocentric IRDC mass surface density distribution at R Gal ~ 4.5 kpc.

We present a study of the molecular cloud content and star formation activity in the far outer Galaxy at Galactocentric radii greater than 13.5 kpc. The properties of star-forming regions associated with far outer Galaxy clouds are examined within a 60 deg2 area toward the second Galactic quadrant based on the FCRAO CO Survey of the Outer Galaxy, the IRAS Point Source Catalog, and follow-up 13CO(J = 1-0) and K'-band near-infrared imaging. This region contains 63 far outer Galaxy molecular clouds; the most massive clouds have molecular masses of ~104 M?. The K'-band imaging of 10 IRAS point sources associated with the far outer Galaxy clouds resulted in the detection of 11 stellar clusters with Galactocentric distances between 13.5 and 17.3 kpc. Several of these clusters are comparable to the well-studied clusters found within 1 kpc of the Sun in terms of the number of stars. We have also examined a much larger sample of far outer Galaxy molecular clouds using the entire FCRAO CO survey. The mass spectrum of 246 far outer Galaxy clouds found within a 300 deg2 area has a power-law slope of -1.88, similar to, although slightly steeper than that found for molecular clouds inside the solar circle. Global measures of the star formation activity, as traced by the ratio of far-infrared luminosity to molecular cloud mass, indicate that these far outer Galaxy clouds are equally active sites of massive star formation as molecular clouds associated with the W3/W4/W5 region and clouds found in the inner Galaxy. Therefore, despite the different environment expected in the far outer Galaxy, the cloud mass spectrum and star formation activity per unit mass are similar to that found throughout the Galaxy. Finally, based on Two Micron All Sky Survey data, we identify 31 additional candidate far outer Galaxy star-forming regions within the larger survey area.

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.

Sometimes, early star formation can be found in cold and dense molecular clouds, such as an infrared dark cloud. Considering that star formation often occurs in clusters, H II regions may be triggering a new generation of star formation, so we can search for the initial stage of massive star formation around H II regions. Based on the above, this work introduces one method to search for the initial stage of massive star formation around H II regions. Towards one section of the H II region G18.2–0.3, multiwavelength observations are carried out to investigate its physical properties. Through analysis, we find three potential initial stages of massive star formation, suggesting that it is feasible to use in searching for the initial stage of massive star formation around H II regions.

We analyze both HCN J = 1–0 and HNC J = 1–0 line profiles to study the inflow motions in different evolutionary stages of massive star formation: 54 infrared dark clouds (IRDCs), 69 high-mass protostellar objects (HMPOs), and 54 ultra-compact H ii regions (UCHIIs). Inflow asymmetry in the HCN spectra seems to be prevalent throughout all the three evolutionary phases, with IRDCs showing the largest excess in the blue profile. In the case of the HNC spectra, the prevalence of blue sources does not appear, apart from for IRDCs. We suggest that this line is not appropriate to trace the inflow motion in the evolved stages of massive star formation, because the abundance of HNC decreases at high temperatures. This result highlights the importance of considering chemistry in dynamics studies of massive star-forming regions. The fact that the IRDCs show the highest blue excess in both transitions indicates that the most active inflow occurs in the early phase of star formation, i.e., in the IRDC phase rather than in the later phases. However, mass is still inflowing onto some UCHIIs. We also find that the absorption dips of the HNC spectra in six out of seven blue sources are redshifted relative to their systemic velocities. These redshifted absorption dips may indicate global collapse candidates, although mapping observations with better resolution are needed to examine this feature in more detail.

It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs) likely represent the birth sites massive stars. Therefore, this class of objects gets increasing attention. To enlarge the sample of well-characterised IRDCs in the southern hemisphere, we have set up a program to study the gas and dust of southern IRDCs. The present paper aims at characterizing the continuuum properties of this sample of objects. We cross-correlated 1.2 mm continuum data from SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between emission sources at millimeter wavelengths and the IRDCs we see at 8 $\mu$m in absorption against the bright PAH background. Analysing the dust emission and extinction leads to a determination of masses and column densities, which are important quantities in characterizing the initial conditions of massive star formation. The total masses of the IRDCs were found to range from 150 to 1150 $\rm M_\odot$ (emission data) and from 300 to 1750 $\rm M_\odot$ (extinction data). We derived peak column densities between 0.9 and 4.6 $\times 10^{22}$ cm$^{-2}$ (emission data) and 2.1 and 5.4 $\times 10^{22}$ cm$^{-2}$ (extinction data). We demonstrate that the extinction method fails for very high extinction values (and column densities) beyond A$_{\rm V}$ values of roughly 75 mag according to the Weingartner & Draine (2001) extinction relation $R_{\rm V} = 5.5$ model B. The derived column densities, taking into account the spatial resolution effects, are beyond the column density threshold of 3.0 $\times 10^{23}$ cm$^{-2}$ required by theoretical considerations for massive star formation. We conclude that the values for column densities derived for the selected IRDC sample make these objects excellent candidates for objects in the earliest stages of massive star formation.

Infrared Dark Clouds (IRDCs) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background. Millimeter continuum maps toward 38 IRDCs reveal extended cold dust emission to be associated with each of the IRDCs. IRDCs range in morphology from filamentary to compact and have masses of 120 to 16,000 Msun, with a median mass of ~940 Msun. Each IRDC contains at least one compact (<=0.5 pc) dust core and most show multiple cores. We find 140 cold millimeter cores unassociated with MSX 8um emission. The core masses range from 10 to 2,100 Msun, with a median mass of ~120 Msun. The slope of the IRDC core mass spectrum (alpha ~ 2.1 +/- 0.4) is similar to that of the stellar IMF. Assuming that each core will form a single star, the majority of the cores will form OB stars. IRDC cores have similar sizes, masses, and densities as hot cores associated with individual, young high-mass stars, but they are much colder. We therefore suggest that IRDC represent an earlier evolutionary phase in high-mass star formation. In addition, because IRDCs contain many compact cores, and have the same sizes and masses as molecular clumps associated with young clusters, we suggest that IRDCs are the cold precursors to star clusters. Indeed, an estimate of the star formation rate within molecular clumps with similar properties to IRDCs (~2 Msun/yr) is comparable to the global star formation rate in the Galaxy, supporting the idea that all stars may form in such clumps.

CS (2-1) measurements toward a large sample of fourth Galactic quadrant infrared dark clouds (IRDCs) were made with the Australia Telescope National Facility Mopra telescope in order to establish their kinematic distances and Galactic distribution. Due to its large critical density, CS unambiguously separates the dense IRDCs from more diffuse giant molecular clouds. The fourth-quadrant IRDCs show a pronounced peak in their radial galactocentric distribution at R = 6 kpc. The first-quadrant IRDC distribution (traced by 13CO emission) also shows a peak, but at a galactocentric radius of R = 5 kpc rather than 6 kpc. This disparity in the peak galactocentric radius suggests that IRDCs trace a spiral arm which lies closer to the Sun in the fourth quadrant. Indeed, the deduced IRDC distribution matches the location of the Scutum-Centaurus arm in Milky Way models dominated by two spiral arms. Since, in external galaxies, OB stars form primarily in spiral arms, the association of IRDCs with a Milky Way spiral arm supports the idea that high-mass stars form in IRDCs. The first-quadrant IRDC distribution also reveals a second peak near the solar circle, possibly due to the fact that 13CO could trace somewhat lower density IRDCs. The reliability of the MSX IRDC catalog by Simon and coworkers is estimated by using the CS detection rate of IRDC candidates. The overall reliability is at least 58%, and increases to near 100% for high contrasts, Galactic longitudes within ~30° of the Galactic center, and large mid-IR backgrounds. A significant fraction of our IRDC sample (14%) showed two CS velocity components, which probably represent two distinct IRDCs along the same line of sight.

Ever since their discovery, infrared dark clouds (IRDCs) are generally considered to be the sites just at the onset of high-mass (HM) star formation. In recent years, it has been realized that not all IRDCs harbor HM young stellar objects (YSOs). Only those IRDCs satisfying a certain mass–size criterion, or equivalently above a certain threshold density, are found to contain HMYSOs. In all cases, IRDCs provide ideal conditions for the formation of stellar clusters. In this paper, we study the massive stellar content of IRDCs to readdress the relation between IRDCs and HM star formation. For this purpose, we have identified all IRDCs associated with a sample of 12 Galactic molecular clouds (MCs). The selected MCs have been the target of a systematic search for YSOs in an earlier study. The cataloged positions of YSOs have been used to search all YSOs embedded in each identified IRDC. In total, we have found 834 YSOs in 128 IRDCs. The sample of IRDCs have mean surface densities of 319 M ⊙ pc − 2 , mean mass of 1062 M ⊙ , and a mass function power-law slope −1.8, which are similar to the corresponding properties for the full sample of IRDCs and resulting physical properties in previous studies. We find that all those IRDCs containing at least one intermediate to HM young star satisfy the often-used mass–size criterion for forming HM stars. However, not all IRDCs satisfying the mass–size criterion contain HM stars. We find that the often-used mass–size criterion corresponds to 35% probability of an IRDC forming a massive star. Twenty-five (20%) of the IRDCs are potential sites of stellar clusters of mass more than 100 M ⊙ .

Infrared dark clouds (IRDCs) are cold, dense structures that are likely representative of the initial conditions of star formation. Many studies of IRDCs employ CO to investigate cloud dynamics, but CO can be highly depleted from the gas phase in IRDCs, which affects its fidelity as tracer. The CO depletion process is also of great interest in astrochemistry because CO ice in dust grain mantles provides the raw material for the formation of complex organic molecules. We study CO depletion towards four IRDCs to investigate its correlation with the H_2 number density and dust temperature, calculated from Herschel far-infrared images. We used ^ ̊m CO : J=1̊ightarrow0 and $2̊ightarrow1$ maps to measure the CO depletion factor, f_D, across IRDCs G23.46-00.53, G24.49-00.70, G24.94-00.15, and G25.16-00.28. We also considered a normalised CO depletion factor, f_D^ which takes a value of unity, that is, no depletion, in the outer lower-density and warmer regions of the clouds. We then investigated the dependence of f_D and f_D^ on the gas density, n_̊m H, and dust temperature, T_̊m dust. The CO depletion rises as the density increases and reaches maximum values of f_D^ in some regions with n_ ̊m H ≳3 ̊m cm although with significant scatter at a given density. We find a tighter, less scattered relation of f_D^ with temperature that rapidly rise for temperatures łesssim18:K. We propose a functional form f_D^ ̊m exp (T_0/ T_ ̊m dust -T_1 ) with T_0≃4:K and T_1≃12:K to reproduce this behaviour. We conclude that CO is strongly depleted from the gas phase in cold, dense regions of IRDCs. This means that if it is not accounted for, CO depletion can lead to an underestimation of the total cloud masses based on CO line fluxes by factors up to ∼5. These results indicate a dominant role for thermal desorption in setting near equilibrium abundances of gas-phase CO in IRDCs and provide important constraints for astrochemical models and the chemodynamical history of gas in the early stages of star formation.

We study Giant Molecular Cloud (GMC) environments surrounding 10 Infrared Dark Clouds (IRDCs), using $^{13}$CO(1-0) emission from the Galactic Ring Survey. We measure physical properties of these IRDCs/GMCs on a range of scales extending to radii, R, of 30 pc. By comparing different methods for defining cloud boundaries and for deriving mass surface densities and velocity dispersions, we settle on a preferred "CE,$\tau$,G" method of "Connected Extraction" in position-velocity space plus Gaussian fitting to opacity-corrected line profiles for velocity dispersion and mass estimation. We examine how cloud definition affects measurements of the magnitude and direction of line-of-sight velocity gradients and velocity dispersions, including associated dependencies on size scale. CE,$\tau$,G-defined GMCs show velocity dispersion versus size relations $\sigma\propto{s}^{1/2}$, which are consistent with the large-scale gradients being caused by turbulence. However, IRDCs have velocity dispersions that are moderately enhanced above those predicted by this scaling relation. We examine the dynamical state of the clouds finding mean virial parameters $\bar{\alpha}_{\rm{vir}}\simeq 1.0$ for GMCs and 1.6 for IRDCs, broadly consistent with models of magnetized virialized pressure-confined polytropic clouds, but potentially indicating that IRDCs have more disturbed kinematics. CE,$\tau$,G-defined clouds exhibit a tight correlation of $\sigma/R^{1/2}\propto\Sigma^n$, with $n\simeq0.7$ for GMCs and 1.3 for IRDCs (c.f., a value of 0.5 expected for a population of virialized clouds). We conclude that while GMCs show evidence for virialization over a range of scales, IRDCs may be moderately super virial. Alternatively, IRDCs could be virialized but have systematically different $^{13}$CO gas phase abundances, i.e., due to freeze-out, affecting mass estimations.

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 background.

Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveals many compact (<0.5 pc) and massive (10-2100 M ⊙ ) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. 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 sites of massive star formation in molecular clouds are investigated by comparing high-resolution radio surveys of molecular and ionized gas emission in the Milky Way. CO emission maps from the Massachusetts-Stony Brook survey of the first Galactic quadrant are used to locate, in l, b, and v, the molecular clouds associated with radio recombination-line H II regions. It is found that the radio H II regions are typically associated with giant molecular clouds (GMCs) with diameters of 20-60 pc and virial masses of 100,000 to a million solar masses. The radio H II regions appear preferentially concentrated toward the centers of the GMCs, contrary to the 'blister' picture of massive star formation on cloud surfaces.

We use 8.3 μm mid-infrared images acquired with the Midcourse Space Experiment satellite to identify and catalog infrared dark clouds (IRDCs) in the first and fourth quadrants of the Galactic plane. Because IRDCs are seen as dark extinction features against the diffuse Galactic infrared background, we identify them by first determining a model background from the 8.3 μm images and then searching for regions of high decremental contrast with respect to this background. IRDC candidates in our catalog are defined by contiguous regions bounded by closed contours of a 2 σ decremental contrast threshold. Although most of the identified IRDCs are actual cold dark clouds, some as yet unknown fraction may be spurious identifications. For large high-contrast clouds, we estimate the reliability to be 82%. Low-contrast clouds should have lower reliabilities. Verification of the reality of individual clouds will require additional data. We identify 10,931 candidate IRDCs. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the cloud's average extinction, are found by iteratively fitting two-dimensional elliptical Gaussian functions to the contrast peaks. We identify 12,774 cores. The catalog contains the position, angular size, orientation, area, peak contrast, peak contrast signal-to-noise, and integrated contrast of the candidate IRDCs and their cores. The distribution of IRDCs closely follows the Galactic diffuse mid-infrared background and peaks toward prominent star-forming regions, spiral arm tangents, and the so-called 5 kpc Galactic molecular ring.