Helen Kirk

Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II.

We present a census of the population of deeply embedded young stellar objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the Cores to Disks (c2d) legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have applied a method developed for identifying embedded protostars in Perseus to these data sets and in this way construct a relatively unbiased sample of 27 candidate embedded protostars with envelopes more massive than our sensitivity limit (about 0.1 M?). As in Perseus, the mid-infrared sources are located close to the center of the SCUBA cores and the narrowness of the spatial distribution of mid-infrared sources around the peaks of the SCUBA cores suggests that no significant dispersion of the newly formed YSOs has occurred. Embedded YSOs are found in 35% of the SCUBA cores?fewer than in Perseus (58%). On the other hand the mid-infrared sources in Ophiuchus have less red mid-infrared colors, possibly indicating that they are less embedded. We apply a nearest neighbor surface density algorithm to define the substructure in each of the clouds and calculate characteristic numbers for each subregion?including masses, star formation efficiencies, fraction of embedded sources, etc. Generally the main clusters in Ophiuchus and Perseus (L1688, NGC 1333, and IC 348) are found to have higher star formation efficiencies than small groups such as B1, L1455, and L1448, which on the other hand are completely dominated by deeply embedded protostars. We discuss possible explanations for the differences between the regions in Perseus and Ophiuchus, such as different evolutionary timescales for the YSOs or differences, e.g., in the accretion in the two clouds.

Dynamics of Dense Cores in the Perseus Molecular Cloud

We survey the kinematics of over 150 candidate (and potentially star-forming) dense cores in the Perseus molecular cloud with pointed N2H+(1-0) and simultaneous C18O(2-1) observations. Our detection rate of N2H+ is 62%, rising to 84% for SCUBA-selected targets. In agreement with previous observations, we find that the dense N2H+ targets tend to display nearly thermal line widths, particularly those that appear to be starless (using Spitzer data), indicating that turbulent support on the small scales of molecular clouds is minimal. For those N2H+ targets that have an associated SCUBA dense core, we find that their internal motions are more than sufficient to provide support against the gravitational force on the cores. Comparison of the N2H+ integrated intensity and SCUBA flux reveals fractional N2H+ abundances between 10-10 and 10-9. We demonstrate that the relative motion of the dense N2H+ gas and the surrounding C18O gas is less than the sound speed in the vast majority of cases (~90%). The point-to-point motions we observe within larger extinction regions appear to be insufficient to provide support against gravity, although we sparsely sample these regions.

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).

The James Clerk Maxwell telescope legacy survey of nearby star-forming regions in the gould belt

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hr of observing time to be carried out from 2007 to 2009. In this survey, we will map with SCUBA-2 (Submillimetre Common-User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations, we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogs, over almost 700 deg(2) of sky. The resulting images will yield the first catalog of prestellar and protostellar sources selected by submillimeter continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain with the array receiver HARP (Heterodyne Array Receiver Program) CO maps, in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimeter. The images and source catalogs will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel, and JWST.

YOUNG STELLAR GROUPS AND THEIR MOST MASSIVE STARS

We analyze the masses and spatial distributions of 14 young stellar groups in Taurus, Lupus3, ChaI, and IC348. These nearby groups, which typically contain 20-40 members, have membership catalogs complete to {approx}0.02 M{sub sun}, and are sufficiently young that their locations should be similar to where they formed. These groups show five properties seen in clusters having many more stars and much greater surface density of stars: (1) a broad range of masses, (2) a concentration of the most massive star toward the center of the group, (3) an association of the most massive star with a high surface density of lower mass stars, (4) a correlation of the mass of the most massive star with the total mass of the group, and (5) the distribution of a large fraction of the mass in a small fraction of the stars.

The JCMT Gould Belt Survey: first results from the SCUBA-2 observations of the Ophiuchus molecular cloud and a virial analysis of its prestellar core population

In this paper, we present the first observations of the Ophiuchus molecular cloud performed as part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) with the SCUBA-2 instrument. We demonstrate methods for combining these data with previous HARP CO, Herschel, and IRAM N2H+ observations in order to accurately quantify the properties of the SCUBA-2 sources in Ophiuchus. We produce a catalogue of all of the sources found by SCUBA-2. We separate these into protostars and starless cores. We list all of the starless cores and perform a full virial analysis, including external pressure. This is the first time that external pressure has been included in this level of detail. We find that the majority of our cores are either bound or virialized. Gravitational energy and external pressure are on average of a similar order of magnitude, but with some variation from region to region. We find that cores in the Oph A region are gravitationally bound prestellar cores, while cores in the Oph C and E regions are pressure-confined. We determine that N2H+ is a good tracer of the bound material of prestellar cores, although we find some evidence for N2H+ freeze-out at the very highest core densities. We find that non-thermal linewidths decrease substantially between the gas traced by C18O and that traced by N2H+, indicating the dissipation of turbulence at higher densities. We find that the critical Bonnor–Ebert stability criterion is not a good indicator of the boundedness of our cores. We detect the pre-brown dwarf candidate Oph B-11 and find a flux density and mass consistent with previous work. We discuss regional variations in the nature of the cores and find further support for our previous hypothesis of a global evolutionary gradient across the cloud from south-west to north-east, indicating sequential star formation across the region.

DENSE GAS TRACERS IN PERSEUS: RELATING THE N2H+, NH3, AND DUST CONTINUUM PROPERTIES OF PRE- AND PROTOSTELLAR CORES

We investigate 35 prestellar cores and 36 protostellar cores in the Perseus molecular cloud. We find a very tight correlation between the physical parameters describing the N2H+ and NH3 gas. Both the velocity centroids and the line widths of N2H+ and NH3 correlate much better than either species correlates with CO, as expected if the nitrogen-bearing species are probing primarily the dense core gas where the CO has been depleted. We also find a tight correlation in the inferred abundance ratio between N2H+ and para-NH3 across all cores, with N(p-NH3)/N(N2H+) = 22 ± 10. We find a mild correlation between NH3 (and N2H+) column density and the (sub)millimeter dust continuum derived H2 column density for prestellar cores, N(p-NH3)/N(H2) ~10–8, but do not find a fixed ratio for protostellar cores. The observations suggest that in the Perseus molecular cloud the formation and destruction mechanisms for the two nitrogen-bearing species are similar, regardless of the physical conditions in the dense core gas. While the equivalence of N2H+ and NH3 as powerful tracers of dense gas is validated, the lack of correspondence between these species and the (sub)millimeter dust continuum observations for protostellar cores is disconcerting and presently unexplained.

The Dynamics of Dense Cores in the Perseus Molecular Cloud II: the Relationship between Dense Cores and the Cloud

We utilize the extensive data sets available for the Perseus molecular cloud to analyze the relationship between the kinematics of small-scale dense cores and the larger structures in which they are embedded. The kinematic measures presented here can be used in conjunction with those discussed in our previous work as strong observational constraints that numerical simulations (or analytic models) of star formation should match. We find that dense cores have small motions with respect to the {sup 13}CO gas, about one third of the {sup 13}CO velocity dispersion along the same line of sight. Within each extinction region, the core-to-core velocity dispersion is about half of the total ({sup 13}CO) velocity dispersion seen in the region. Large-scale velocity gradients account for roughly half of the total velocity dispersion in each region, similar to what is predicted from large-scale turbulent modes following a power spectrum of P(k) {proportional_to} k {sup -4}.

THE ROLE OF TURBULENCE AND MAGNETIC FIELDS IN SIMULATED FILAMENTARY STRUCTURE

We use numerical simulations of turbulent cluster-forming regions to study the nature of dense filamentary structures in star formation. Using four hydrodynamic and magnetohydrodynamic simulations chosen to match observations, we identify filaments in the resulting column density maps and analyze their properties. We calculate the radial column density profiles of the filaments every 0.05 Myr and fit the profiles with the modified isothermal and pressure confined isothermal cylinder models, finding reasonable fits for either model. The filaments formed in the simulations have similar radial column density profiles to those observed. Magnetic fields provide additional pressure support to the filaments, making ‘puffier’ filaments less prone to fragmentation than in the pure hydrodynamic case, which continue to condense at a slower rate. In the higher density simulations, the filaments grow faster through the increased importance of gravity. Not all of the filaments identified in the simulations will evolve to form stars: some expand and disperse. Given these different filament evolutionary paths, the trends in bulk filament width as a function of time, magnetic field strength, or density, are weak, and all cases are reasonably consistent with the finding of a constant filament width in different star-forming regions. In the simulations, the mean FWHM lies between 0.06 and 0.26 pc for all times and initial conditions, with most lying between 0.1 to 0.15 pc; the range in FWHMs are, however, larger than seen in typical Herschel analyses. Finally, the filaments display a wealth of substructure similar to the recent discovery of filament bundles in Taurus. Subject headings:

MULTIPLE OUTFLOWS AND PROTOSTARS IN BARNARD 1

Using optical (Hα and [S II]), near-IR (H2 and Ks), and submillimeter (850 and 450 μm) data, we have examined the region surrounding the Barnard 1 (B1) core and found a multitude of new shocks from protostellar outflows. We trace several flows, some of which are large, parsec-scale outflows with dynamic ages of order 104 yr, indicating that star formation has been taking place in B1 for at least that long. We can confidently identify eight protostars that are driving outflows. Of those eight protostars, one source, SMM 2 (SMM J033330+31095) is a new Class 0 source, giving B1 a total of three Class 0 protostars. Based on the number of shocks and protostars in this region, B1 appears to be a much more active region of star formation than previously thought. The number of shocks is comparable to or greater than those of other active star-forming regions in Perseus (e.g., IC 348, L1455, and L1448).