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D. T. Jaffe

Publications and source records attributed to D. T. Jaffe.

32 records · Page 2Linked to original sources
arXiv2004

R=100,000 Mid-IR Spectroscopy of UCHII Regions: High Resolution is Worth it!

Ultracompact HII regions are signposts of massive star formation and their properties provide diagnostics for the characteristics of very young O stars embedded in molecular clouds. While radio observations have given us a good picture of the morphology of these regions, they have not provided clear information about the kinematics. Using high spectral resolution observations of the 12.8 micron [NeII] line, it has been possible for the first time to trace the internal kinematics of several ultracompact HII regions. We find that the motions in the cometary ultracompact HII regions MonR2 and G29.96-0.02 are highly organized. The velocity patterns are consistent with parabolic ionized flows along a neutral boundary layer.

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arXiv2003

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

We present high resolution (R=50,000) spectra at 2.2 um of 16 young stars in the rho Ophiuchi dark cloud. Photospheric features are detected in the spectra of 11 of these sources, all Class II young stellar objects. In 10 of these sources, we measure effective temperatures, continuum veiling, and vsini rotation from the shapes and strengths of atomic photospheric lines by comparing to spectral synthesis models at 2.2 um. We measure surface gravities in 2 stars from the integrated line flux ratio of the 12CO line region at 2.3 um and the Na I line region at 2.2 um. Although the majority (8/10) of the Class II stars have similar effective temperatures (3530 K +/-100 K), they exhibit a large spread in bolometric luminosities (factor ~8), as derived from near-IR photometry. In the two stars where we have surface gravity measurements from spectroscopy, the photometrically derived luminosities are systematically higher than the spectroscopic luminosities. Our spectroscopic luminosities result in older ages on the H-R diagram than is suggested by photometry at J or K. Most of our sources show a substantially larger amount of continuum excess than stellar flux at 2.2 um. The derived veiling values at K appear correlated with mid-IR disk luminosity, and with Brackett gamma equivalent width, corrected for veiling. The derived vsini 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.

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arXiv2003

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. For pre-main sequence stars, we use the same matching process to also measure the amount of excess near-IR emission. 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. 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. The analysis of high resolution near-IR spectra of MK standards shows that the technique gives very accurate values for the effective temperature. The biggest 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 sigma difference between the optical and IR temperatures for 3000-5800 K dwarfs is only 140 K. In a companion paper (Doppmann, Jaffe, & White 2003), we present an analysis of heavily extincted young stellar objects rho Oph.

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arXiv2003

Kinematics of Ultracompact HII Regions Revealed: High Spectral and Spatial Resolution Mapping of the 12.8 micron [NeII] Line in Monoceros R2

We present the first results of a study of the kinematics and morphology of ultracompact HII regions using a new observational technique. We used very high spatial (1.5") and spectral (3.4 km/s)resolution observations of the [NeII] line at 12.8 microns to study the ionized gas in Monoceros R2. The [NeII] emission shows an HII region with highest emission measure in a ~24" shell. Line widths are as narrow as 8 km/s at some positions. In places where the lines are complex and broader, the additional width is most likely due to overlap of narrower features along the line of sight. The global kinematics suggest that the 24" shell is expanding at ~10 km/s. However, the spectral profiles toward the southeast side of the shell are not consistent with a simple expansion picture.

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arXiv2002

Looking for Pure Rotational H_2 Emission from Protoplanetary Disks

We report on a limited search for pure-rotational molecular hydrogen emission associated with young, pre-main-sequence stars. We looked for H_2 v=0 J = 3->1 and J = 4->2 emission in the mid-infrared using the Texas Echelon-Cross-Echelle Spectrograph (TEXES) at NASA's 3m Infrared Telescope Facility. The high spectral and spatial resolution of our observations lead to more stringent limits on narrow line emission close to the source than previously achieved. One star, AB Aur, shows a possible (2sigma) H_2 detection, but further observations are required to make a confident statement. Our non-detections suggest that a significant fraction, perhaps all, of previously reported H_2 emission towards these objects could be extended on scales of 5" or more.

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arXiv2001

TEXES: A Sensitive High-Resolution Grating Spectrograph for the Mid-Infrared

We discuss the design and performance of TEXES, the Texas Echelon Cross Echelle Spectrograph. TEXES is a mid-infrared (5-25 um) spectrograph with several operating modes: high-resolution, cross-dispersed with a resolving power of R=100,000, 0.5% spectral coverage, and a ~1.5 by 8" slit; medium-resolution long-slit with R~15,000, 0.5% coverage, and a ~1.5 by 45" slit; low-resolution long-slit with (delta lambda)~0.004 um, 0.25 um coverage, and a ~1.5 by 45" slit; and source acquisition imaging with 0.33" pixels and a 25 by 25" field of view on a 3m telescope. TEXES has been used at the McDonald Observatory 2.7m and the IRTF 3m telescopes, and has proven to be both sensitive and versatile.

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arXiv1999

Atomic Carbon Is a Temperature Probe in Dark Clouds

We have mapped the C I 3P1-3P0 line at 492 GHzin three molecular clouds immersed in weak ultraviolet radiation fields, TMC-1, L134N, and IC 5146. In all three clouds, the CI peak TA* ~ 1 K, with very small dispersion. The spatial C I distribution is extended and rather smooth. The J = 2-1 transitions of CO isotopomers were observed at the same angular resolution as C I. The C I peak TA* is typically a third of the peak TA* of 13CO J = 2-1, and the C I emission is usually more extended than emission in 13CO or C18O J=2-1. The C I linewidth is close to the 13CO J = 2-1 linewidth, larger than the C18O J = 2-1 line width, and smaller than the 12CO J = 2-1 linewidth. The uniformity of the C I peak TA* is remarkable for a line in the Wien portion of the Planck function and indicates a very uniform excitation temperature. This uniformity is best explained if the line if opaque and thermalized. If so, the CI line probes kinetic temperature in clouds exposed to low ultraviolet fluxes. This conclusion has significant implications for the thermal balance in such clouds. At Av ~ 2, these clouds have a remarkably constant temperature from place to place and from cloud to cloud (7.9+/-0.8 K). Photodissociation region models of clouds immersed in the mean interstellar radiation field tend to predict stronger lines than we see, but this may be an artifact of assumptions about the temperature.

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arXiv1999

Low Velocity Ionized Winds from Regions Around Young O Stars

We have observed seven ultracompact HII regions in hydrogen recombination lines in the millimeter band. Toward four of these regions, there is a high velocity (full width to half maximum 60-80 km/s) component in the line profiles. The high velocity gas accounts for 35-70% of the emission measure within the beam. We compare these objects to an additional seven similar sources we have found in the literature. The broad recombination line objects (BRLOs) make up about 30% of all sources in complexes containing ultracompact HII regions. Comparison of spectral line and continuum data implies that the BRLOs coincide with sources with rising spectral indices, >=0.4 up to 100 GHz. Both the number of BRLOs and their frequency of occurrence within HII region complexes, when coupled with their small size and large internal motions, mean that the apparent contradiction between the dynamical and population lifetimes for BRLOs is even more severe than for ultracompact HII regions. We evaluate a number of models for the origin of the broad recombination line emission. The lifetime, morphology, and rising spectral index of the sources argue for photo- evaporated disks as the cause for BRLOs. Existing models for such regions, however, do not account for the large amounts of gas observed at supersonic velocities.

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arXiv1998

Molecular Cloud Structure in the Magellanic Clouds: Effect of Metallicity

The chemical structure of neutral clouds in low metallicity environments is examined with particular emphasis on the H to H_2 and C+ to CO transitions. We observed near-IR H_2 lines and the CO J=1-0 line from 30 Doradus and N159/N160 in the Large Magellanic Cloud and from DEM S 16, DEM S 37, and LI-SMC 36 in the Small Magellanic Cloud. We find that the H_2 emission is UV-excited and that (weak) CO emission always exists (in our surveyed regions) toward positions where H_2 and [CII] emission have been detected. Using a PDR code and a radiative transfer code, we simulate the emission of line radiation from spherical clouds and from large planar clouds. Because the [CII] emission and H_2 emission arise on the surface of the cloud and the lines are optically thin, these lines are not affected by changes in the relative sizes of the neutral cloud and the CO bearing core, while the optically thick CO emission can be strongly affected. The sizes of clouds are estimated by measuring the deviation of CO emission strength from that predicted by a planar cloud model of a given size. The average cloud column density and therefore size increases as the metallicity decreases. Our result agrees with the photoionization regulated star formation theory by Mc Kee (1989).

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arXiv1996

Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers

We have observed 150 regions of massive star formation, selected originally by the presence of a water maser, in the J = 5-4, 3-2, and 2-1 transitions of CS, and 49 regions in the same transitions of C$^{34}$S. Over 90% of the 150 regions were detected in the J = 2-1 and 3-2 transitions of CS and 75% were detected in the J=5-4 transition. We have combined the data with the J = 7-6 data from our original survey (Plume et al. 1992) to determine the density by analyzing the excitation of the rotational levels. Using Large Velocity Gradient (LVG) models, we have determined densities and column densities for 71 of these regions. The gas densities are very high (the mean log of the density is 5.9), but much less than the critical density of the J=7-6 line. Small maps of 25 of the sources in the J = 5-4 line yield a mean diameter of 1.0 pc. The mean virial mass is 3800 solar masses. The mean ratio of bolometric luminosity to virial mass (L/M) is 190, about 50 times higher than estimates using CO emission, suggesting that star formation is much more efficient in the dense gas probed in this study. The gas depletion time for the dense gas is roughly 1.3 x 10^7 yr. We find no statistically significant linewidth--size or density--size relationships in our data. Instead, both linewidth and density are larger for a given size than would be predicted by the usual relationships. We find that the linewidth increases with density, the opposite of what would be predicted by the usual arguments. We estimate that the luminosity of our Galaxy (excluding the inner 400 pc) in the CS J = 5-4 transition is 15 to 23 L_sun, considerably less than the luminosity in this line within the central 100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to CS luminosity is higher in M82 than in any cloud in our sample.

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arXiv1996

Molecular Hydrogen Emission from the Inner 400 Parsecs of the Galaxy II. The UV-Excited H_2

We have observed near-IR H$_2$ line emission on large scales in the Galactic center. Paper I discussed our 400 pc long strip map and 50 pc map of the H_2 v=1-0 S(1) line. In this paper, we present observations of the higher vibrational lines, H_2 v=2-1 S(1) and v=3-2 S(3), at selected positions and conclude that strong far--UV radiations excites the H_2. We compare the H_2 v=1-0 S(1) emission to far-IR continuum emission and show that the ratio of these two quantities in the Galactic center equals the ratio seen in the starburst galaxies, M82 and NGC 253, and in ultraluminous infrared bright galaxies.

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arXiv1996

Molecular Hydrogen Emission from the Inner 400 Parsecs of the Galaxy

We have mapped the H_2 v=1-0 S(1) (λ=2.1215 \micron) emission line along a 400 pc long strip and in a 50 pc region in the Galactic center. There is H_2 emission throughout the surveyed region. The typical de-reddened (A_K = 2.5 mag) surface brightness, about 3 X 10^{-5} ergs/s/cm^2/sr, is similar to the surface brightness in large-scale photon-dominated regions in the galactic disk. We investigate two possible excitation mechanisms for the H_2 emission: UV-excitation by photons from OB stars, and shock waves, and conclude that UV-excitation is more likely. The total H_2 v=1-0 S(1) luminosity in the inner 400 pc region of the Galaxy is 8000 L_sun. The ratio of the H_2 to far-IR luminosity in the inner 400 pc of the Galaxy agrees with that in starburst galaxies and ultraluminous infrared bright galaxies.

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arXiv1993

Extended Co(7-6) Emission from Warm Gas in Orion

We mapped quiescent 807 GHz CO(7-6) emission from Orion along a strip in RA extending from 0.7 pc west to 1.2 pc east of Ori theta 1C. The lines arise in warm gas with T > 40 K. The line brightness temperature is > 160 K in the di- rection of theta 1C, more than twice the dust temperature, and still exceeds 35 K more than a parsec east of theta 1C. The lines are narrow, with a max- imum velocity width of 7 km s^-1 near theta 1C and decreasing to 1.5-3 km s^-1 at the map boundaries. The density of the emitting gas is > 10^4 cm^-3 and the column density exceeds 10^21 cm^-2. The correlation of the bright, narrow CO(7-6) lines with 158 micron [C II] emission suggests that over the entire region mapped, the narrow CO lines arise in warm photo-dissociation regions ex- cited by UV photons from the Trapezium cluster. Although the Trapezium stars lie in front of the Orion A molecular cloud, not all of the warm gas is at the cloud surface. To the east of theta 1C the CO(7-6) lines split into two velo- city components (also seen in 13CO(2-1) emission) which persist over several arcminutes. Since only one of these components can be on the surface, the other must arise from a dense, UV-illuminated clump or filament within the molecular cloud. Comparison of the quiescent CO(7-6) emission to CO(7-6) ob- served in a cross-map of the energetic Orion KL outflow shows that the lumino- sity of shock-excited CO(7-6) emission in Orion is only a few percent of the luminosity of the widespread quiescent CO(7-6) emission.

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