Daniel T. Jaffe

A CS J = 5 → 4 Mapping Survey Toward High-Mass Star-forming Cores Associated with Water Masers

We have mapped 63 regions forming high-mass stars in CS J = 5 → 4 using the CSO. The CS peak position was observed in C34S J = 5 → 4 toward 57 cores and in 13CS J = 5 → 4 toward the nine brightest cores. The sample is a subset of a sample originally selected toward water masers; the selection on maser sources should favor sources in an early stage of evolution. The cores are located in the first and second Galactic quadrants with an average distance of 5.3 ± 3.7 kpc and were well detected with a median peak signal-to-noise ratio in the integrated intensity of 40. The integrated intensity of CS J = 5 → 4 correlates very well with the dust continuum emission at 350 μm. For 57 sufficiently isolated cores, a well-defined angular size (FWHM) was determined. The core radius (RCS), aspect ratio [(a/b)obs], virial mass (Mvir), surface density (Σ), and the luminosity in the CS J = 5 → 4 line (L(CS54)) are calculated. The distributions of size, virial mass, surface density, and luminosity are all peaked with a few cores skewed toward much larger values than the mean. The median values, μ1/2, are as follows: μ1/2 (RCS) = 0.32 pc, μ1/2 ((a/b)obs) = 1.20, μ1/2 (Mvir) = 920 M⊙, μ1/2 (Σ) = 0.60 g cm-2, μ1/2 (L(CS54)) = 1.9 × 10-2 L⊙, and μ1/2 (Lbol/Mvir) = 165 (L/M)⊙. We find a weak correlation between C34S line width and size, consistent with Δv ~ R0.3. The line widths are much higher than would be predicted by the usual relations between line width and size determined from regions of lower mass. These regions are very turbulent. The derived virial mass agrees within a factor of 2-3 with mass estimates from dust emission at 350 μm after corrections for the density structure are accounted for. The resulting cumulative mass spectrum of cores above 1000 M⊙ can be approximated by a power law with a slope of about -0.9, steeper than that of clouds measured with tracers of lower density gas and close to that for the total masses of stars in OB associations. The median turbulent pressures are comparable to those in UCH II regions, and the pressures at small radii are similar to those in hypercompact H II regions (P/k ~ 1010 K cm-3). The filling factors for dense gas are substantial, and the median abundance of CS is about 10-9. The ratio of bolometric luminosity to virial mass is much higher than the value found for molecular clouds as a whole, and the correlation of luminosity with mass is tighter.

Young, Low-Mass Brown Dwarfs with Mid-Infrared Excesses

We have combined new I, J, H, and Ks imaging of portions of the Chamaeleon II, Lupus I, and Ophiuchus star-forming clouds with 3.6-24 ?m imaging from the Spitzer legacy program From Molecular Cores to Planet-Forming Disks to identify a sample of 19 young stars, brown dwarfs, and sub-brown dwarfs showing mid-infrared excess emission. The resulting sample includes sources with luminosities of 0.5 > log(L*/L?) > -3.1. Six of the more luminous sources in our sample have been previously identified by other surveys for young stars and brown dwarfs. Five of the sources in our sample have nominal masses that are at or below the deuterium-burning limit (12MJ). Over three decades in luminosity, our sources have an approximately constant ratio of excess to stellar luminosity. We compare our observed spectral energy distributions (SEDs) to theoretical models of a central source with a passive irradiated circumstellar disk and test the effects of disk inclination, disk flaring, and the size of the inner disk hole on the strength/shape of the excess. The observed SEDs of all but one of our sources are well fitted by models of flared and/or flat disks.

Stellar Properties of Pre-Main-Sequence Stars from High-Resolution Near-Infrared Spectra

We present high-resolution (R = 50,000) spectra at 2.2 ?m of 16 young stars in the ??Ophiuchi dark cloud. Photospheric features are detected in the spectra of 11 of these sources, all Class II young stellar objects. The five featureless spectra consist of two Class I, two Class I.5, and one Class II. One star, GSS 29, is identified as a spectroscopic binary based on radial velocity variations. The radial velocities for the remaining sample are consistent with 12CO and H2CO gas velocities and further confirm the membership of the sources in the ??Oph cluster. For the 10 spectroscopically single Class II sources, we measure effective temperatures, continuum veiling, and v sin i rotation from the shapes and strengths of atomic photospheric lines by comparison with spectral synthesis models at 2.2 ?m. We measure surface gravities in two stars from the integrated line flux ratio of the 12CO line region at 2.3 ?m and the Na I line region at 2.2 ?m. Although the majority (8/10) of the Class II stars have similar effective temperatures (3530 ? 100 K), they exhibit a large spread in bolometric luminosities (a factor of ~8), as derived from near-IR photometry. In the two stars for which we have surface gravity measurements from spectroscopy, the photometrically derived luminosities are systematically higher than the spectroscopic luminosities. The spread in the photometrically derived luminosities in our other sources suggests either a large spread in stellar ages or nonphotospheric emission in the J band since anomalous and significant veiling at J has been observed in other T Tauri stars. Our spectroscopic luminosities result in older ages on the Hertzsprung-Russell diagram than are suggested by photometry at J or K. Most of our sources show a larger amount of continuum excess (FK ex) than stellar flux at 2.2 ?m (FK*), substantially higher in many cases (rK ? FK ex/FK* = 0.3?4.5). The derived veiling values at K (rK) appear correlated with mid-IR disk luminosity and with Brackett ? equivalent width, corrected for veiling. The derived v sin i rotation is substantial (12?39 km s-1), but systematically less than the rotation measured in Class I.5 (flat) and Class I sources from other studies in Ophiuchus. In four stars (Class I and I.5 sources), the absence of any photospheric lines is likely due to large continuum excess and/or rapid rotation if the stars have late-type photospheres.

Mass flows in cometary ultracompact H II regions

High spectral and spatial resolution, mid-infrared fine-structure line observations toward two ultracompact H II (UC H II) regions (G29.96-0.02 and Mon R2) allow us to study the structure and kinematics of cometary UC H II regions. In our earlier study of Mon R2, we showed that highly organized mass motions accounted for most of the velocity structure in that UC H II region. In this work, we show that the kinematics in both Mon R2 and G29.96 are consistent with motion along an approximately paraboloidal shell. We model the velocity structure seen in our mapping data and test the stellar wind bow shock model for such paraboloidal-like flows. The observations and the simulation indicate that the ram pressures of the stellar wind and ambient interstellar medium cause the accumulated mass in the bow shock to flow along the surface of the shock. A relaxation code reproduces the mass flows velocity structure as derived by the analytical solution. It further predicts that the pressure gradient along the flow can accelerate ionized gas to a speed higher than that of the moving star. In the original bow shock model, the star speed relative to the ambient medium was considered to be the exit speed of ionized gas in the shell.

A Spectroscopic Technique for Measuring Stellar Properties of Pre-Main-Sequence Stars

We describe a technique for deriving effective temperatures, surface gravities, rotation velocities, and radial velocities from high-resolution near-IR spectra. The technique matches the observed near-IR spectra to spectra synthesized from model atmospheres. Our analysis is geared toward characterizing heavily reddened pre–main-sequence stars, but the technique also has potential applications in characterizing main-sequence and post–main-sequence stars when these lie behind thick clouds of interstellar dust. For the pre–main-sequence stars, we use the same matching process to measure the amount of excess near-IR emission (which may arise in the protostellar disks) in addition to the other stellar parameters. The information derived from high-resolution spectra comes from line shapes and the relative line strengths of closely spaced lines. The values for the stellar parameters we derive are therefore independent of those derived from low-resolution spectroscopy and photometry. The new method offers the promise of improved accuracy in placing young stellar objects on evolutionary model tracks. Tests with an artificial noisy spectrum with typical stellar parameters and a signal-to-noise ratio of 50 indicate a 1 σ error of 100 K in Teff, 2 km s-1 in v sin i, and 0.13 in continuum veiling for an input veiling of 1. If the flux ratio between the sum of the Na, Sc, and Si lines at 2.2 μm and the (2–0) 12CO band head at 2.3 μm is known to an accuracy of 10%, the errors in our best-fit value for log g will be Δ log g = 0.1–0.2. We discuss the possible systematic effects on our determination of the stellar parameters and evaluate the accuracy of the results derivable from high-resolution spectra. In the context of this evaluation, we quantitatively explore the degeneracy between temperature and gravity that has bedeviled efforts to type young stellar objects using low-resolution spectra. The analysis of high-resolution near-IR spectra of MK standards shows that the technique yields very accurate values for the effective temperature. The greatest uncertainty in comparing our results with optical spectral typing of MK standards is in the spectral type–to–effective temperature conversion for the standards themselves. Even including this uncertainty, the 1 σ difference between the optical and infrared temperatures for dwarfs at 3000–5800 K is only 140 K. In a companion paper, we present an analysis of heavily extincted young stellar objects in the ρ Ophiuchi molecular cloud.

High-resolution observations of the near-infrared emission from NGC 6822 Hubble V

We have observed Hubble V, the brightest H II region complex in the dwarf irregular galaxy NGC 6822, at near-infrared (near-IR; 1.8-2.4 μm) wavelengths. The line emission maps of Hubble V show the typical structure of a photodissociation region (PDR) where an ionized core, traced by compact He i emission (2.0587 urn) and Bry emission (2.1661 μm), is surrounded by an outer layer traced by molecular hydrogen (H 2 ) emission. The measured line ratios of H 2 2-1 S(1) (2.2477 μm)/1-0S(1) (2.1218 μm) from 0.2 to 0.6 and the unshifted and unresolved line profiles suggest that the H 2 emission originates purely from a PDR. We find no evidence for shock activity. By comparing the H 2 results with a PDR model, we conclude that Hubble V includes dense (10 4.5 cm -3 ) and warm PDRs. In this environment, most of the H 2 molecules are excited by far-ultraviolet photons (with a field strength of 10 2-4 times that of the average interstellar field), although collisional processes de-excite H 2 and contribute significantly to the excitation of the first vibrational level. We expect that Hubble V is in the early stage of molecular cloud dissolution.