Jill Rathborne

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey is a new survey of Galactic 13CO J = 1 → 0 emission. The survey used the SEQUOIA multipixel array on the Five College Radio Astronomy Observatory 14 m telescope to cover a longitude range of l = 18°-557 and a latitude range of |b| 40°. At the velocity resolution of 0.21 km s-1, the typical rms sensitivity is σ(T) ~ 0.13 K. The survey comprises a total of 1,993,522 spectra. We show integrated intensity images (zeroth moment maps), channel maps, position-velocity diagrams, and an average spectrum of the completed survey data set. We also discuss the telescope and instrumental parameters, the observing modes, the data reduction processes, and the emission and noise characteristics of the data set. The Galactic Ring Survey data are available to the community online or in DVD form by request.

Infrared Dark Clouds: Precursors to Star Clusters

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 Characterization and Galactic Distribution of Infrared Dark Clouds

Using 13CO J = 1 ? 0 molecular line emission from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS), we have established kinematic distances to 313 infrared dark clouds (IRDCs) by matching the morphology of the molecular line emission in distinct velocity channels to their mid-infrared extinction. The Galactic distribution of IRDCs shows an enhancement toward the Galaxys most massive and active star-forming structure, the so-called 5 kpc ring. IRDCs have typical sizes of ~5 pc, peak column densities of ~1022 cm-2, LTE masses of ~5 ? 103 M?, and volume-averaged H2 densities of ~2 ? 103 cm-3. The similarity of these properties to those of molecular clumps associated with active star formation suggests that IRDCs represent the densest clumps within giant molecular clouds where clusters may eventually form.

A catalog of msx infrared dark cloud candidates

We use 8.3 um 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 um 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 sigma 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 infrared dark clouds. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the clouds average extinction, are found by iteratively fitting 2-dimensional elliptical Gaussians 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.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 clouds 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.

The Early Stages of Star Formation in Infrared Dark Clouds: Characterizing the Core Dust Properties

Identified as extinction features against the bright Galactic mid-infrared background, infrared dark clouds (IRDCs) are thought to harbor the very earliest stages of star and cluster formation. In order to better characterize the properties of their embedded cores, we have obtained new 24 μm, 60-100 μm, and submillimeter continuum data toward a sample of 38 IRDCs. The 24 μm Spitzer images reveal that while the IRDCs remain dark, many of the cores are associated with bright 24 μm emission sources, which suggests that they contain one or more embedded protostars. Combining the 24 μm, 60-100 μm, and submillimeter continuum data, we have constructed broadband spectral energy distributions (SEDs) for 157 of the cores within these IRDCs and, using simple graybody fits to the SEDs, have estimated their dust temperatures, emissivities, opacities, bolometric luminosities, masses, and densities. Based on their Spitzer/Infrared Array Camera 3-8 μm colors and the presence of 24 μm point-source emission, we have separated cores that harbor active, high-mass star formation from cores that are quiescent. The active protostellar cores typically have warmer dust temperatures and higher bolometric luminosities than the more quiescent, perhaps pre-protostellar, cores. Because the mass distributions of the populations are similar, however, we speculate that the active and quiescent cores may represent different evolutionary stages of the same underlying population of cores. Although we cannot rule out low-mass star formation in the quiescent cores, the most massive of them are excellent candidates for the high-mass starless core phase, the very earliest in the formation of a high-mass star.

Massive Protostars in the Infrared Dark Cloud MSXDC G034.43+00.24

We present a multiwavelength study of the infrared dark cloud MSXDC G034.43+00.24. Dust emission, traced by millimeter/submmillimeter images obtained with the IRAM, JCMT, and CSO telescopes, reveals three compact cores within this infrared dark cloud with masses of 170-800 M☉ and sizes <0.5 pc. Spitzer 3.6-8.0 μm images show slightly extended emission toward these cores, with a spectral enhancement at 4.5 μm that probably arises from shocked H2. In addition, the broad line widths (ΔV ~ 10 km s-1) of HCN (4-3) and CS (3-2) and the detection of SiO (2-1), observed with the JCMT and IRAM telescopes, also indicate active star formation. Spitzer 24 μm images reveal that each of these cores contains a bright, unresolved continuum source; these sources are most likely embedded protostars. Their millimeter-to-mid-IR continuum spectral energy distributions reveal very high luminosities, 9000-32,000 L☉. Because such large luminosities cannot arise from low-mass protostars, MSXDC G034.43+00.24 is actively forming massive (~10 M☉) stars.

Water masers associated with infrared dark cloud cores

We present a survey of the 616-523 H2O maser transition toward a sample of 140 compact cores in infrared dark clouds using the Very Large Array. Strong (>1 Jy) H2O maser emission was found associated with 17 cores, indicative of star formation in these cores. We infer that the cores with H2O masers have embedded protostars. Cores associated with maser emission have masses of 12 to 2 × 103 M☉, similar to the mass range in the entire sample. The H2O maser detection rate (12%) toward the compact, cold cores is much lower than that toward high-mass protostellar objects and ultracompact H II regions. The detection rate of H2O masers is significantly higher for higher mass cores than for lower mass cores. We suggest that some of the most massive infrared dark cloud cores without H2O maser emission are at an evolutionary stage earlier than the protostellar phases. They are prime candidates for high-mass starless cores.

The giant pillars of the Carina Nebula

Results are presented from a multi-wavelength study of the giant pillars within the Carina Nebula. Using near-IR data from 2MASS, mid-IR data from MSX, 843 MHz radio continuum maps from the MOST and molecular line and continuum observations from the SEST, we investigate the nature of the pillars and search for evidence of ongoing star formation within them. Photodissociation regions (PDRs) exist across the whole nebula and trace the giant pillars, as well as many ridges, filaments and condensations (A v > 7 mag). Morphological similarities between emission features at 21 μm and 843MHz adjacent to the PDRs, suggests that the molecular material has been carved by the intense stellar winds and UV radiation from the nearby massive stars. In addition, star forming cores are found at the tips of several of the pillars. Using a stellar density distribution, several candidate embedded clusters are also found. One is clearly seen in the 2MASS images and is located within a dense core (G287.84-0.82). A search for massive young stellar objects and compact H II regions using mid-IR colour criteria, reveal twelve candidates across the complex. Grey-body fits to SEDs for four of these objects are suggestive of OB-stars. We find that massive star formation in the Carina Nebula is occurring across the whole complex and confirm it has been continuous over the past 3 Myrs.

A Catalog of Midcourse Space Experiment Infrared Dark Cloud Candidates

We use 8.3 um 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 um 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 sigma 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 infrared dark clouds. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the clouds average extinction, are found by iteratively fitting 2-dimensional elliptical Gaussians 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.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 clouds 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.

The formation and early evolution of young massive clusters

We review the formation and early evolution of the most massive and dense young stellar clusters, focusing on the role they can play in our understanding of star and planet formation as a whole. Young massive cluster (YMC) progenitor clouds in the Galactic Center can accumulate to a high enough density without forming stars that the initial protostellar densities are close to the final stellar density. For this to hold in the disk, the time scale to accumulate the gas to such high densities must be much shorter than the star formation timescale. Otherwise the gas begins forming stars while it is being accumulated to high density. The distinction between the formation regimes in the two environments is consistent with the predictions of environmentally-dependent density thresholds for star formation. This implies that stars in YMCs of similar total mass and radius can have formed at widely different initial protostellar densities. The fact that no systematic variations in fundamental properties are observed between YMCs in the disk and Galactic Center suggests stellar mass assembly is not strongly affected by the initial protostellar density. We review recent theoretical advances and summarize the debate on three key open questions: the initial (proto)stellar distribution, infant (im)mortality and age spreads within YMCs. We conclude: the initial protostellar distribution is likely hierarchical; YMCs likely experienced a formation history that was dominated by gas exhaustion rather than gas expulsion; YMCs are dynamically stable from a young age; and YMCs have age spreads much smaller than their mean age. Finally, we show that it is plausible that metal-rich globular clusters may have formed in a similar way to YMCs in nearby galaxies. In summary, the study of YMC formation bridges star/planet formation in the solar neighborhood to the oldest structures in the local Universe. [abridged]