arXiv subjects

John S. Carr

Publications and source records attributed to John S. Carr.

42 records · Page 3Linked to original sources

arXiv2003

Hot H2O Emission and Evidence for Turbulence in the Disk of a Young Star

We report on the detection and analysis of hot ro-vibrational H2O emission from SVS-13, a young stellar object previously known to have strong CO overtone bandhead emission. Modeling of the high-resolution infrared spectrum shows that the H2O emission is characterized by temperatures ~ 1500 K, significantly lower than the temperatures that characterize the CO bandhead emission. The widths for the H2O lines are also found to be smaller than those for the CO lines. We construct a disk model for the emission that reproduces the CO and H2O spectrum. In this model, the H2O lines originate at somewhat larger disk radii (<= 0.3 AU) than the CO overtone lines (<= 0.1 AU). We find that the H2O abundance is about a factor of 10 lower than the calculated chemical equilibrium abundance. Large, approximately transonic, local line broadening is required to fit the profile of the CO bandhead. If this velocity dispersion is identified with turbulence, it is of significant interest regarding the transport of angular momentum in disks. Large local broadening is also required in modeling CO overtone emission from other young stellar objects, suggesting that large turbulent velocities may be characteristic of the upper atmospheres of the inner disks of young stars.

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arXiv2001

Evidence for Residual Material in Accretion Disk Gaps: CO Fundamental Emission from the T Tauri Spectroscopic Binary DQ Tau

We present the discovery of CO fundamental ro-vibrational emission from the classical T Tauri spectroscopic binary DQ Tau. The high-resolution infrared echelle spectra reveal emission lines from both the v=1 and v=2 vibrational levels with line widths of roughly 70 km/s. The average CO excitation temperature is approximately 1200 K. We model the spectra as arising from gas in Keplerian rotation about the center-of-mass of the binary. The disk model requires gas with an average surface density of 5 x 10-4 g cm-2 that extends outward to 0.5 +/- 0.1 AU and inward to at least 0.1 AU from the center-of-mass. The radial extent for the emitting gas is close to the predicted size of the gap in the DQ Tau accretion disk that is expected to be dynamically cleared by the binary. We interpret these results, and previous modeling of DQ Tau's spectral energy distribution, as evidence for a small amount (~ 1e-10 Msun) of diffuse material residing within the optically-thin disk gap. Thus dynamical clearing has not been completely efficient in the DQ Tau binary. We suggest that the material is associated with a flow from the circumbinary disk which feeds the ongoing accretion at the stellar surfaces.

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arXiv2000

Stellar Iron Abundances at the Galactic Center

We present measurements of [Fe/H] for six M supergiant stars and three giant stars within 0.5 pc of the Galactic Center (GC) and one M supergiant star within 30 pc of the GC. The results are based on high-resolution (lambda / Delta lambda =40,000) K-band spectra, taken with CSHELL at the NASA Infrared Telescope Facility.We determine the iron abundance by detailed abundance analysis,performed with the spectral synthesis program MOOG.The mean [Fe/H] of the GC stars is determined to be near solar,[Fe/H] = +0.12 $\pm$ 0.22. Our analysis is a differential analysis, as we have observed and applied the same analysis technique to eleven cool, luminous stars in the solar neighborhood with similar temperatures and luminosities as the GC stars. The mean [Fe/H] of the solar neighborhood comparison stars, [Fe/H] = +0.03 $\pm$ 0.16, is similar to that of the GC stars. The width of the GC [Fe/H] distribution is found to be narrower than the width of the [Fe/H] distribution of Baade's Window in the bulge but consistent with the width of the [Fe/H] distribution of giant and supergiant stars in the solar neighborhood.

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arXiv1999

The First Stellar Abundance Measurements in the Galactic Center: the M Supergiant IRS 7

The first measurement of the photospheric abundances in a star at the Galactic Center are presented. A detailed abundance analysis of the Galactic Center M2 supergiant IRS 7 was carried out using high-resolution near-infrared echelle spectra. The Fe abundance for IRS 7 was found to be close to solar, [Fe/H] = -0.02 +/- 0.13, and nearly identical to the Fe abundances we obtained for the nearby M supergiants alpha Ori and VV Cep. Analysis of the first and second overtone lines of CO was used to derive an effective temperature of 3600 +/- 230 K, a microturbulent velocity of 3.0 +/- 0.3 km/s, and a carbon abundance log epsilon(C) = 7.78 +/- 0.13, or [C/H] = -0.77. In addition, we find a high depletion of 0.74 +/- 0.32 dex in O and an enhancement of 0.92 +/- 0.18 dex in N. These abundances are consistent with the dredge-up of CNO-cycle products but require deep mixing in excess of that predicted by standard models for red supergiants. In light of our measured solar Fe abundance for IRS 7, we discuss other indicators of metallicity at the Galactic Center, the interpretation of low-resolution near-infrared spectra of late-type giants and supergiants, including the need for caution in using such spectra as measures of metallicity, and the evolution of massive young stars at the Galactic Center. We suggest the possibility that rapid stellar rotation is common for stars formed under conditions in the Galactic Center, and that extra internal mixing induced by high rotation rates, rather than evolution at high metallicity, is the explanation for many of the unusual properties of the hot emission-line stars in the Galactic Center.

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arXiv1995

Medium Resolution Near Infrared (2.15micron--2.35micron) Spectroscopy of Late-type Main Sequence stars

We present an atlas of moderate resolution ($λ/Δλ\sim$1380) 2.15~\micron\ to 2.39~\micron\ spectra of 33 luminosity class V stars of spectral type F3 to M6 in the MK classification system. We find that the equivalent widths of several spectral features vary significantly with temperature and typically allow effective temperature determination accurate to approximately $\pm$350~K.

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arXiv1995

Observation of Infrared and Radio Lines of Molecules toward GL2591 and Comparison to Physical and Chemical Models

We have observed rovibrational transitions of acetylene and HCN near 13 microns in absorption toward GL2591. We also observed rotational lines of CS, HCN, H2CO, and HCO+. The combined data are analyzed in terms of models with a cloud envelope with density gradients and discrete regions of hot, dense gas, probably near the infrared source. The abundance of HCN is enhanced by a factor of 400 in the gas producing the infrared absorption, in agreement with chemical models which involve depletion of molecules onto grains and subsequent sublimation when temperatures are raised.

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