Anneila I. Sargent

A survey for circumstellar disks around young stellar objects

Results are presented from a survey for 1.3 mm radiation toward 86 stars in the Taurus-Auriga dark clouds, including classical T Tauri stars, stars in T associations, and a few weak emission-line stars or naked T Tauri stars. The results show that 42 percent of the stars have detectable emission from small particles. The aggregate particle masses are found to be between 0.00001 and 0.01 solar mass, suggesting total disk masses between 0.001 and 1 solar mass. For several source between 1.3 and 2.7 mm, the spectral indices indicate that the particle emissivities are weaker functions of frequency, compared to the usual case of interstellar grains. Particle growth via adhesion in the dense disks is proposed to explain this result. The results show that disks more massive than the minimum mass of the protosolar system commonly accompany the birth of solar-mass stars, indicating that planetary systems are probably common in the Galaxy. 59 refs.

From Molecular Cores to Planet‐forming Disks: An SIRTF Legacy Program

Crucial steps in the formation of stars and planets can be studied only at mid‐ to far‐infrared wavelengths, where the Space Infrared Telescope (SIRTF) provides an unprecedented improvement in sensitivity. We will use all three SIRTF instruments (Infrared Array Camera [IRAC], Multiband Imaging Photometer for SIRTF [MIPS], and Infrared Spectrograph [IRS]) to observe sources that span the evolutionary sequence from molecular cores to protoplanetary disks, encompassing a wide range of cloud masses, stellar masses, and star‐forming environments. In addition to targeting about 150 known compact cores, we will survey with IRAC and MIPS (3.6–70 μm) the entire areas of five of the nearest large molecular clouds for new candidate protostars and substellar objects as faint as 0.001 solar luminosities. We will also observe with IRAC and MIPS about 190 systems likely to be in the early stages of planetary system formation (ages up to about 10 Myr), probing the evolution of the circumstellar dust, the raw material for planetary cores. Candidate planet‐forming disks as small as 0.1 lunar masses will be detectable. Spectroscopy with IRS of new objects found in the surveys and of a select group of known objects will add vital information on the changing chemical and physical conditions in the disks and envelopes. The resulting data products will include catalogs of thousands of previously unknown sources, multiwavelength maps of about 20 deg^2 of molecular clouds, photometry of about 190 known young stars, spectra of at least 170 sources, ancillary data from ground‐based telescopes, and new tools for analysis and modeling. These products will constitute the foundations for many follow‐up studies with ground‐based telescopes, as well as with SIRTF itself and other space missions such as SIM, JWST, Herschel, and TPF/Darwin.

Particle emissivity in circumstellar disks

Submillimeter continuum observations of 29 pre-main-sequence objects in Taurus and Orion are used to study the wavelength dependence of particle emission. These objects are mostly T Tauri stars whose long-wavelength emission is thought to originate in circumstellar disks. The flux densities imply power-law frequency distributions with spectral indices between 2 and 3 in almost all cases. If the emission is optically thin, the particle emissivities have power-law indices between −1 and 1; otherwise, these values are lower limits. It is argued that in most cases the emission is optically thin at wavelengths near 1 mm, so the measured incides should be close to the true values

A High-Resolution Study of Gas and Dust around Young Intermediate-Mass Stars: Evidence for Circumstellar Disks in Herbig Ae Systems

As part of a long-term program of observations to search for and characterize disks of gas and dust around intermediate-mass counterparts to solar-mass T Tauri stars, we have probed the environments of seven pre-main-sequence stars of spectral type Ae using millimeter-wave continuum and molecular line aperture synthesis imaging. In each case we identify a compact region of thermal continuum emission centered on the star. Upper limits to radii are in the range 200-300 AU for five members of our sample, and 680 AU for the distant source HD 245185. We identify an elongated continuum source around HD 163296, with a semimajor axis of 110 AU. Adopting relatively high values for dust grain opacities, we obtain minimum masses of circumstellar dust and gas in the range 0.005-0.034 M☉ for the seven sources, assuming that the observed continuum emission is optically thin. We detect molecular line emission from gas regions centered on four of the stars. Two of these regions are spatially resolved and are found to be elongated, with semimajor axes of 310 and 450 AU for HD 163296 and AB Aur, respectively. Ordered velocity gradients along the major axes of both of these structures point strongly to the presence of orbiting material in disklike configurations, and we argue that the nebular environments of our entire sample include substantial disk components.

Star Formation in Clusters: A Survey of Compact Millimeter-Wave Sources in the Serpens Core

We report the results of a millimeter interferometric survey of compact 3 mm continuum sources in the inner 55 ? 55 region of the Serpens core. We detect 32 discrete sources above 4.0 mJy beam-1, 21 of which are new detections at millimeter wavelengths. By comparing our data with published infrared surveys, we estimate that 26 sources are probably protostellar condensations and derive their mass by assuming optically thin thermal emission from dust grains. The mass spectrum of the clumps, dN/dM~M -->?2.1, is consistent with the stellar initial mass function, supporting the idea that the stellar masses in young clusters are determined by the fragmentation of turbulent cloud cores.

The Mass Distribution And Lifetime Of Prestellar Cores In Perseus, Serpens, And Ophiuchus

We present an unbiased census of starless cores in Perseus, Serpens, and Ophiuchus, assembled by comparing large-scale Bolocam 1.1 mm continuum emission maps with Spitzer c2d surveys. We use the c2d catalogs to separate 108 starless from 92 protostellar cores in the 1.1 mm core samples from Enoch and Young and their coworkers. A comparison of these populations reveals the initial conditions of the starless cores. Starless cores in Perseus have similar masses but larger sizes and lower densities on average than protostellar cores, with sizes that suggest density profiles substantially flatter than ρ∝r^-2. By contrast, starless cores in Serpens are compact and have lower masses than protostellar cores; future star formation will likely result in lower mass objects than the currently forming protostars. Comparison to dynamical masses estimated from the NH3 survey of Perseus cores by Rosolowsky and coworkers suggests that most of the starless cores are likely to be gravitationally bound, and thus prestellar. The combined prestellar core mass distribution includes 108 cores and has a slope of α = -2.3 ± 0.4 for M > 0.8 M☉. This slope is consistent with recent measurements of the stellar initial mass function, providing further evidence that stellar masses are directly linked to the core formation process. We place a lower limit on the core-to-star efficiency of 25%. There are approximately equal numbers of prestellar and protostellar cores in each cloud; thus the dense prestellar core lifetime must be similar to the lifetime of embedded protostars, or 4.5 x 10^5 yr, with a total uncertainty of a factor of 2. Such a short lifetime suggests a dynamic, rather than quasi-static, core evolution scenario, at least at the relatively high mean densities (n > 2 x 10^4 cm^-3) to which we are sensitive.

The evolution of outflow-envelope interactions in low-mass protostars

We present multiline and continuum observations of the circumstellar environment within 10to the 4th AU of a sample of protostars to investigate how the effects of outflows on their immediate environment change over time. 12CO (1–0) emission probes the high-velocity molecular outflows near the protostars and demonstrates that the outflow opening angle widens as the nascent star evolves. Maps of the 13CO (1–0) and HCO+ (1–0) outflow emission show that protostellar winds erode the circumstellar envelope through the entrainment of the outer envelope gas. The spatial and velocity distribution of the dense circumstellar envelope, as well as its mass, is traced by the C to the 18th O (1–0) emission and also displays evolutionary changes. We show that outflows are largely responsible for these changes and propose an empirical model for the evolution of outflow-envelope interactions. In addition, some of the outflows in our sample appear to affect the chemical composition of the surrounding environment, enhancing the HCO+ abundance. Overall, our results confirm that outflows play a major role in the star formation process through their strong physical and chemical impacts on the environments of the young protostars.

The Spitzer c2d Survey of Large, Nearby, Insterstellar Clouds. II. Serpens Observed with IRAC

We present maps of 0.89?deg2 of the Serpens dark cloud at 3.6, 4.5, 5.8, and 8.0??m observed with the Spitzer Space Telescope Infrared Array Camera (IRAC). We discuss in detail the data processing carried out by the c2d team on IRAC data. More than 100,000 compact sources have been extracted, but we confine most of our discussion to the most reliable subset of these sources. This includes those that are detected above 7?? in all four IRAC bands or those detected in the two shorter IRAC bands together with 2MASS. We estimate completeness limits for our survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets, a 0.10 deg2 set of low-extinction regions near the dark cloud and a 1 deg2 subset of the SWIRE Elais N1 data that was processed through our pipeline. We find that it is possible to identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, most of which were previously unknown. In addition to the dense area of new star formation known before in the core region (cluster A), we also find a moderately rich area to the south (cluster B). Our mapped area also includes the Herbig Ae star VV Ser, whose Spitzer images have been carefully modeled in a separate study. The extreme sensitivity of Spitzer IRAC allows us to search to very low luminosity limits for young substellar objects. The comparison of the Serpens region with the reference areas suggests that a population of infrared excess sources exists in Serpens at least down to luminosities of L ~ 10-3 L? and possibly lower.

Bolocam Survey for 1.1 mm Dust Continuum Emission in the c2d Legacy Clouds. II. Ophiuchus

We present a large-scale millimeter continuum map of the Ophiuchus molecular cloud. Nearly 11 deg2, including all of the area in the cloud with AV ≥ 3 mag, was mapped at 1.1 mm with Bolocam on the Caltech Submillimeter Observatory (CSO). By design, the map also covers the region mapped in the infrared with the Spitzer Space Telescope. We detect 44 definite sources, and a few likely sources are also seen along a filament in the eastern streamer. The map indicates that dense cores in Ophiuchus are very clustered and often found in filaments within the cloud. Most sources are round, as measured at the half-power point, but elongated when measured at lower contour levels, suggesting spherical sources lying within filaments. The masses, for an assumed dust temperature of 10 K, range from 0.24 to 3.9 M☉, with a mean value of 0.96 M☉. The total mass in distinct cores is 42 M☉, 0.5%-2% of the total cloud mass, and the total mass above 4 σ is about 80 M☉. The mean densities in the cores are quite high, with an average of 1.6 × 106 cm-3, suggesting short free-fall times. The core mass distribution can be fitted with a power law with slope α = 2.1 ± 0.3 for M > 0.5 M☉, similar to that found in other regions, but slightly shallower than that of some determinations of the local IMF. In agreement with previous studies, our survey shows that dense cores account for a very small fraction of the cloud volume and total mass. They are nearly all confined to regions with AV ≥ 9 mag, a lower threshold than found previously.

High-Resolution Studies of Gas and Dust Around Young Intermediate-Mass Stars. II. Observations of an Additional Sample of Herbig Ae Systems

In an earlier paper (Mannings & Sargent; Paper I), we presented evidence for disks of gas and dust associated with seven Herbig Ae stars, based on high-resolution interferometric millimeter-wave obser- vations of continuum and molecular line emission. These systems are simultaneously high-mass analogs of the approximately solar-mass T Tauri stars and the evolutionary precursors of the prototypical main- sequence debris-disk sources b Pic, a Lyr, and a PsA. Here we extend the original survey to include four additional Herbig Ae systems. We have also imaged two of the sources from Paper I at higher resolution. The new data are presented and analyzed, and are combined with the results from the earlier sample in order to address the properties of this class of circumstellar disk. Derived disk masses are indistinguishable from the masses of T Tauri disks. Although the combined sample is small, it seems likely that disk masses are essentially uncorrelated with stellar mass for premain-sequence stars of spec- tral type A0 and later. Subject headings: accretion, accretion diskscircumstellar matterstars: premain-sequence