James Di Francesco

An Extinction Threshold for Protostellar Cores in Ophiuchus

We have observed continuum emission at λ = 850 μm over ~4 deg2 of the Ophiuchus star-forming cloud using the Submillimeter Common-User Bolometric Array on the James Clerk Maxwell Telescope, producing a submillimeter continuum map 20 times larger than previous Ophiuchus surveys. Our sensitivity is 40 mJy beam-1, a factor of ~2 less sensitive than earlier maps. Using an automated identification algorithm, we detect 100 candidate objects. Only two new objects are detected outside the boundary of previous maps, despite the much wider area surveyed. We compare the submillimeter continuum map with a map of visual extinction across the Ophiuchus cloud derived using a combination of Two Micron All Sky Survey and R-band data. The total mass in submillimeter objects is ≈50 M☉ compared with ≈2000 M☉ in observed cloud mass estimated from the extinction. The submillimeter objects represent only 2.5% of the cloud mass. A clear association is seen between the locations of detected submillimeter objects and high visual extinction, with no objects detected at AV 15, by some mechanism (e.g., loss of nonthermal support).

A High-Resolution Study of the Slowly Contracting, Starless Core L1544

We present interferometric observations of N2H+ (1-0) in the starless, dense core L1544 in Taurus. Redshifted self-absorption, indicative of inward motions, is found toward the center of an elongated core. The data are fit by a nonspherical model consisting of two isothermal, rotating, centrally condensed layers. Through a hybrid global-individual fit to the spectra, we map the variation of infall speed at scales ~1400 AU and find values ~0.08 km s-1 around the core center. The inward motions are small in comparison to thermal, rotational, and gravitational speeds but are large enough to suggest that L1544 is very close to forming a star.

A Catalog of Young Stellar Groups and Clusters within 1 Kiloparsec of the Sun

We present a catalog of near-infrared surveys of young ( a few 106 yr) stellar groups and clusters within 1 kpc from the Sun, based on an extensive search of the literature from the past ten years. We find 143 surveys from 69 published articles, covering 73 different regions. The number distribution of stars in a region has a median of 28 and a mean of 100. About 80% of the stars are in clusters with at least 100 members. By a rough classification of the groups and clusters based on the number of their associated stars, we show that most of the stars form in large clusters. The spatial distribution of cataloged regions in the Galactic plane shows a relative lack of observed stellar groups and clusters in the range 270° < l < 60° of Galactic longitude, reflecting our location between the Local and Sagittarius arms. This compilation is intended as a useful resource for future studies of nearby young regions of multiple star formation.

THE MASS DISTRIBUTION OF STARLESS AND PROTOSTELLAR CORES IN GOULD BELT CLOUDS

Using data from the SCUBA Legacy Catalogue (850 μm) and Spitzer Space Telescope (3.6-70 μm), we explore dense cores in the Ophiuchus, Taurus, Perseus, Serpens, and Orion molecular clouds. We develop a new method to discriminate submillimeter cores found by Submillimeter Common-User Bolometer Array (SCUBA) as starless or protostellar, using point source photometry from Spitzer wide field surveys. First, we identify infrared sources with red colors associated with embedded young stellar objects (YSOs). Second, we compare the positions of these YSO candidates to our submillimeter cores. With these identifications, we construct new, self-consistent starless and protostellar core mass functions (CMFs) for the five clouds. We find best-fit slopes to the high-mass end of the CMFs of –1.26 ± 0.20, –1.22 ± 0.06, –0.95 ± 0.20, and –1.67 ± 0.72 for Ophiuchus, Taurus, Perseus, and Orion, respectively. Broadly, these slopes are each consistent with the –1.35 power-law slope of the Salpeter initial mass function at higher masses, but suggest some differences. We examine a variety of trends between these CMF shapes and their parent cloud properties, potentially finding a correlation between the high-mass slope and core temperature. We also find a trend between core mass and effective size, but we are very limited by sensitivity. We make similar comparisons between core mass and size with visual extinction (for A_V ≥ 3) and find no obvious trends. We also predict the numbers and mass distributions of cores that future surveys with SCUBA-2 may detect in each of these clouds.

A High-resolution Study of the H I-H2 Transition across the Perseus Molecular Cloud

To investigate the fundamental principles of H2 formation in a giant molecular cloud, we derive the H I and H2 surface density (?H I and ?H2) images of the Perseus molecular cloud on sub-pc scales (~0.4?pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the ?H2 distribution across Perseus. We find a relatively uniform ?H I ~ 6-8 M ??pc?2 for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between ?H2/?H I and ?H I + ?H2. The transition between the H I- and H2-dominated regions occurs at N(H I) + 2N(H2) ~ (8-14)?? 1020?cm?2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of CO-dark gas, f DG ~ 0.3. While significant spatial variations of f DG are found, we do not find a clear correlation with the mean V-band extinction.

Quiescent Dense Gas in Protostellar Clusters: The Ophiuchus A Core

We present combined BIMA interferometer and IRAM 30 m telescope data of N2H+ 1-0 line emission across the nearby dense star-forming core Ophiuchus A at high linear resolution (e.g., ~1000 AU). Six maxima of integrated line intensity are detected, which we designate Oph A-N1 through N6. The N4 and N5 maxima coincide with the starless continuum objects SM1 and SM2, respectively, but the other maxima do not coincide with previously identified objects. In contrast, relatively little N2H+ 1-0 emission coincides with the starless object SM2 and the Class 0 protostar VLA 1623. The FWHM of the N2H+ 1-0 line, ΔV, varies by a factor of ~5 across Oph A. Values of ΔV < 0.3 km s-1 are found in 14 locations in Oph A, but only that associated with N6 is both well defined spatially and larger than the beam size. Centroid velocities of the line, VLSR, vary relatively little, having an rms of only ~0.17 km s-1. Small-scale VLSR gradients of less than 0.5 km s-1 over ~0.01 pc are found near SM1, SM1N, and SM2 but not N6. The low N2H+ abundances of SM2 or VLA 1623 relative to SM1, SM1N, or N6 may reflect relatively greater amounts of N2 adsorption onto dust grains in their colder and probably denser interiors. The low ΔV of N6, i.e., 0.193 km s-1 FWHM, is only marginally larger than the FWHM expected from thermal motions alone, suggesting that turbulent motions in the Oph A core have been reduced dramatically at this location. The nondetection of N6 in previous thermal continuum maps suggests that interesting sites possibly related to star formation may be overlooked in such data.

The Spitzer Survey of Interstellar Clouds in the Gould Belt. II. The Cepheus Flare Observed with IRAC and MIPS

We present Spitzer Infrared Array Camera (IRAC; ~2 deg^2) and Multiband Imaging Photometer for Spitzer (MIPS; ~8 deg^2) observations of the Cepheus Flare, which is associated with the Gould Belt, at an approximate distance of ~300 pc. Around 6500 sources are detected in all four IRAC bands, of which ~900 have MIPS 24 μm detections. We identify 133 young stellar object (YSO) candidates using color-magnitude diagram techniques, and a large number of the YSO candidates are associated with the NGC 7023 reflection nebula. Cross-identifications were made with the Guide Star Catalog II and the IRAS Faint Source Catalog, and spectral energy distributions (SEDs) were constructed. SED modeling was conducted to estimate the degree of infrared excess. It was found that a large majority of disks were optically thick accreting disks, suggesting that there has been little disk evolution in these sources. Nearest-neighbor clustering analysis identified four small protostellar groups (L1228, L1228N, L1251A, and L1251B) with 5-15 members each and the larger NGC 7023 association with 32 YSO members. The star-formation efficiency for cores with clusters of protostars and for those without clusters was found to be ~8% and ~1%, respectively. The cores L1155, L1241, and L1247 are confirmed to be starless down to our luminosity limit of L _(bol) = 0.06 L ⊙.

The Spitzer Survey of Interstellar Clouds in the Gould Belt. I. IC 5146 Observed With IRAC and MIPS

We present observations of two areas totalling 0.57 deg^2 in the IC 5146 star-forming region at 3.6, 4.5, 5.8, 8.0, 24, and 70 μm observed with the Spitzer Space Telescope. We reexamine the issue of the distance to this cloud and conclude a value of 950 ± 80 pc is most likely. We compare source counts, colors, and magnitudes in our observed region to a subset of the SWIRE data that was processed through our pipeline. We identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, many of which were previously unknown. We compare the colors of these YSOs to the models of Robitaille et al. and perform simple fits to the SEDs to estimate properties of the circumstellar disks likely to surround the Class II and III sources. We also compare the mid-IR disk excesses to Hα emission-line data where available. We present a quantitative description of the degree of clustering, estimate the star formation efficiency, and discuss the fraction of YSOs in the region with disks relative to an estimate of the diskless YSO population. Finally, we compare the YSO distribution to the cold dust distribution mapped by SCUBA and briefly describe the diffuse emission likely due to PAHs associated with the H II region.

From Filamentary Networks to Dense Cores in Molecular Clouds: Toward a New Paradigm for Star Formation

Recent studies of the nearest star-forming clouds of the Galaxy at submillimeter wavelengths with the Herschel Space Observatory have provided us with unprecedented images of the initial and boundary conditions of the star formation process. The Herschel results emphasize the role of interstellar filaments in the star formation process and connect remarkably well with nearly a decades worth of numerical simulations and theory that have consistently shown that the ISM should be highly filamentary on all scales and star formation is intimately related to self-gravitating filaments. In this review, we trace how the apparent complexity of cloud structure and star formation is governed by relatively simple universal processes - from filamentary clumps to galactic scales. We emphasize two crucial and complementary aspects: (i) the key observational results obtained with Herschel over the past three years, along with relevant new results obtained from the ground on the kinematics of interstellar structures, and (ii) the key existing theoretical models and the many numerical simulations of interstellar cloud structure and star formation. We then synthesize a comprehensive physical picture that arises from the confrontation of these observations and simulations.

A FIRST LOOK AT THE AURIGA-CALIFORNIA GIANT MOLECULAR CLOUD WITH HERSCHEL * AND THE CSO: CENSUS OF THE YOUNG STELLAR OBJECTS AND THE DENSE GAS

We have mapped the Auriga/California molecular cloud with the Herschel PACS and SPIRE cameras and the Bolocam 1.1 mm camera on the Caltech Submillimeter Observatory with the eventual goal of quantifying the star formation and cloud structure in this giant molecular cloud (GMC) that is comparable in size and mass to the Orion GMC, but which appears to be forming far fewer stars. We have tabulated 60 compact 70/160 μm sources that are likely pre-main-sequence objects and correlated those with Spitzer and WISE mid-IR sources. At 1.1 mm, we find 18 cold, compact sources and discuss their properties. The most important result from this part of our study is that we find a modest number of additional compact young objects beyond those identified at shorter wavelengths with Spitzer. We also describe the dust column density and temperature structure derived from our photometric maps. The column density peaks at a few × 10 22 cm −2 (NH2) and is distributed in a clear filamentary structure along which nearly all of the pre-main-sequence objects are found. We compare the young stellar object surface density to the gas column density and find a strong nonlinear correlation between them. The dust temperature in the densest parts of the filaments drops to ∼10 K from values ∼14–15 K in the low-density parts of the cloud. We also derive the cumulative mass fraction and probability density function of material in the cloud, which we compare with similar data on other star-forming clouds.