Journal ArticleDOI
Infrared polarization images of star-forming regions. I. The ubiquity of bipolar structure
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In this article, the authors used polarization images of several star-forming regions with mass outflows (GSS 30, S255, GL 5180, GL 2591, GGD 27, and NGC 7538) to establish the universality of bipolarity and of shell or cavity structure in the IRN consistent with that of CO outflow.Abstract:
The inefficiency of the stellar formation process leads rather generally to high residual dust densities, and so to the existence of infrared reflection nebulosity (IRN), in regions of star formation. Polarization images of several star-forming regions with mass outflows (GSS 30, S255, GL 5180, GL 2591, GGD 27, and NGC 7538) presented here: (1) establish the universality of bipolarity and of shell or cavity structure in the IRN consistent with that of CO outflow; (2) identify the source of the mass outflow in each case; (3) show that the opening angle near this central source is large; and (4) demonstrate several instances of multiple shells, probably arising from episodic mass loss. Astrometry of 2.2-micron sources with arcsecond accuracy identifies the illuminating source of each IRN uniquely with a compact H II region or a bright IR source. The polarization images provide strong evidence for large-scale dust toroids around each of these sources. The density and mass of these disks are estimated from the extinction through the disk.read more
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Journal ArticleDOI
The Impact of the Massive Young Star GL 2591 on Its Circumstellar Material: Temperature, Density, and Velocity Structure
Floris van der Tak,Ewine F. van Dishoeck,Neal J. Evans,Neal J. Evans,Eric J. Bakker,Eric J. Bakker,Geoffrey A. Blake +6 more
Abstract: 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 300 K), in which high-temperature reactions are affecting abundances.
Journal ArticleDOI
Optical Spectroscopy of Embedded Young Stars in the Taurus-Auriga Molecular Cloud
TL;DR: In this article, the first optical spectroscopic survey of Class I sources in the Taurus-Auriga dark cloud is described, and it is shown that forbidden emission lines are stronger and more common in Class I than in T Tauri stars.
Journal ArticleDOI
VLBI observations of 6.7 and 12.2 GHz methanol masers toward high mass star-forming regions - II. Tracing massive protostars
TL;DR: In this paper, the absolute positions of methanol maser sites were compared with those of typical tracers of young massive stellar objects, such as ultra-compact Hii regions, outflows and hot molecular cores.
Journal ArticleDOI
The Impact of the Massive Young Star GL 2591 on its Circumstellar Material: Temperature, Density and Velocity Structure
Floris van der Tak,Ewine F. van Dishoeck,Neal J. Evans,Neal J. Evans,Eric J. Bakker,Eric J. Bakker,Geoffrey A. Blake +6 more
TL;DR: In this article, the temperature, density and kinematics of the gas and dust surrounding the luminous young stellar object GL~2591 are investigated on scales as small as 100 AU, probed by 4.7 micron absorption spectroscopy, to over 60,000 AU.
Journal ArticleDOI
Near-Infrared Imaging Polarimetry of Embedded Young Stars in the Taurus-Auriga Molecular Cloud
TL;DR: In this paper, a near-infrared imaging polarimetry of 21 embedded protostars in the Taurus-Auriga molecular cloud is described, and an analytic scattering model for the near-IR colors and polarizations of embedded objects is developed.