van der Tak, Floris F. S. and van Dishoeck, Ewine F. and Evans, Neal J., II and Bakker, Eric J. and Blake, Geoffrey A. (1999) The Impact of the Massive Young Star GL 2591 on Its Circumstellar Material: Temperature, Density, and Velocity Structure. Astrophysical Journal, 522 (2). pp. 991-1010. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120914-085348736
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The temperature, density, and kinematics of the gas and dust surrounding the luminous (2 × 10^4 L_☉) young stellar object GL 2591 are investigated on scales as small as ~100 AU, probed by 4.7 μm absorption spectroscopy, to over 60,000 AU, probed by single-dish submillimeter spectroscopy. These two scales are connected by interferometric 86-115 and 226 GHz images of size 30,000 AU and resolution 2000 AU in continuum and molecular lines. The data are used to constrain the physical structure of the envelope and investigate the influence of the young star on its immediate surroundings. The infrared spectra at λ/Δλ ≈ 40,000 indicate an LSR velocity of the ^(13)CO rovibrational lines of -5.7 ± 1.0 km s^(-1), consistent with the velocity of the rotational lines of CO. In infrared absorption, the ^(12)CO lines show wings out to much higher velocities, ≈-200 km s^(-1), than are seen in the rotational emission lines, which have a total width of ≈75 km s^(-1). This difference suggests that the outflow seen in rotational lines consists of envelope gas entrained by the ionized jet seen in Brγ and [S II] emission. The outflowing gas is warm, T > 100 K, since it is brighter in CO J = 6 → 5 than in lower-J CO transitions. The dust temperature due to heating by the young star has been calculated self-consistently as a function of radius for a power-law density distribution n = n_0r^(-α), with α = 1-2. The temperature is enhanced over the optically thin relation (T ~ r^(-0.4)) inside a radius of 2000 AU, and reaches 120 K at r ≾ 1500 AU from the star, at which point ice mantles should have evaporated. The corresponding dust emission can match the observed λ ≥ 50 μm continuum spectrum for a wide range of dust optical properties and values of α. However, consistency with the C^(17)O line emission requires a large dust opacity in the submillimeter, providing evidence for grain coagulation. The 10-20 μm emission is better matched using bare grains than using ice-coated grains, consistent with evaporation of the ice mantles in the warm inner part of the envelope. Throughout the envelope, the gas kinetic temperature as measured by H_2CO line ratios closely follows the dust temperature. The values of α and n_0 have been constrained by modeling emission lines of CS, HCN, and HCO^+ over a large range of critical densities. The best fit is obtained for α = 1.25 ± 0.25 and n_0 = (3.5 ± 1) × 10^4 cm^(-3) at r = 30,000 AU, yielding an envelope mass of (42 ± 10) M_☉ inside that radius. The derived value of α suggests that part of the envelope is in free-fall collapse onto the star. Abundances in the extended envelope are 5 × 10^(-9) for CS, 2 × 10^(-9) for H_2CO, 2 × 10^(-8) for HCN, and 1 × 10^(-8) for HCO^+. The strong near-infrared continuum emission, the Brγ line flux, and our analysis of the emission-line profiles suggest small deviations from spherical symmetry, likely an evacuated outflow cavity directed nearly along the line of sight. The A_V ≈ 30 toward the central star is a factor of 3 lower than in the best-fit spherical model. Compared to this envelope model, the Owens Valley Radio Observatory (OVRO) continuum data show excess thermal emission, probably from dust. The dust may reside in an optically thick, compact structure, with diameter ≾30 AU and temperature ≳1000 K, or the density gradient may steepen inside 1000 AU. In contrast, the HCN line emission seen by OVRO can be satisfactorily modeled as the innermost part of the power-law envelope, with no increase in HCN abundance on scales where the ice mantles should have been evaporated. The region of hot, dense gas and enhanced HCN abundance (~10^(-6)) observed with the Infrared Space Observatory therefore cannot be accommodated as an extension of the power-law envelope. Instead, it appears to be a compact region (r < 175 AU, where T > 300 K), in which high-temperature reactions are affecting abundances.
|Additional Information:||© 1999 American Astronomical Society. Received 1998 August 10; accepted 1999 April 23. The authors are grateful to Michiel Hogerheijde for useful discussions and for his eff�orts on the Monte Carlo radiative transfer code, to Mario van den Ancker and Göran Sandell for providing their unpublished JCMT/ UKT14 data and for reducing the SCUBA data, to Craig Kulesa for communicating his Phoenix results, and to an anonymous referee whose comments helped to improve this paper. They also would like to thank the sta�ffs of the OVRO, CSO, JCMT, NRAO 12 m, and NOAO 2.1 m telescopes for their support, especially Remo Tilanus and Fred Baas at the JCMT and Ken Hinkle, Dick Joyce, and Je� Valenti at the NOAO 2.1 m. Byron Mattingly carried out the NRAO 12 m observations. F. v. d. T. is grateful to the Leids Sterrewacht Fonds, the Stimuleringsfonds Internationale Betrekkingen of the Netherlands Organisation for Scientific Research (NWO), and the Leids Kerkhoven-Bosscha Fonds for travel support. Astrochemistry in Leiden is supported by grant 781-76-015 from the Netherlands Foundation for Research in Astronomy. N. J. E. acknowledges support from NSF Grant AST 93-17567 and the Randall Professorship. G. A. B. gratefully acknowledges support provided by NASA grants NAG 5-4383 and NAG 5-3733.|
|Subject Keywords:||accretion, accretion disks; circumstellar matter; infrared: stars; stars: individual (AFGL 2591); stars: pre-main sequence; submillimeter|
|Official Citation:||The Impact of the Massive Young Star GL 2591 on Its Circumstellar Material: Temperature, Density, and Velocity Structure Floris F. S. van der Tak et al. 1999 ApJ 522 991|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||14 Sep 2012 16:54|
|Last Modified:||26 Dec 2012 16:10|
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