Publication Date:
2013-12-06
Description:
We present an analysis of the far-infrared and submillimetre molecular emission-line spectrum of the luminous M-supergiant VY CMa, observed with the Spectral and Photometric Imaging Receiver (SPIRE) and Photodetector Array Camera and Spectrometer for Herschel spectrometers aboard the Herschel Space Observatory . Over 260 emission lines were detected in the 190–650 μm SPIRE Fourier Transform Spectrometer spectra, with one-third of the observed lines being attributable to H 2 O. Other detected species include CO, 13 CO, H $_2^{18}$ O, SiO, HCN, SO, SO 2 , CS, H 2 S and NH 3 . Our model fits to the observed 12 CO and 13 CO line intensities yield a 12 C/ 13 C ratio of 5.6 ± 1.8, consistent with measurements of this ratio for other M-supergiants, but significantly lower than previously estimated for VY CMa from observations of lower- J lines. The spectral line energy distribution for 20 SiO rotational lines shows two temperature components: a hot component at ~1000 K, which we attribute to the stellar atmosphere and inner wind, plus a cooler ~200 K component, which we attribute to an origin in the outer circumstellar envelope. We fit the line fluxes of 12 CO, 13 CO, H 2 O and SiO, using the smmol non-local thermodynamic equilibrium (LTE) line transfer code, with a mass-loss rate of 1.85 10 –4 M yr –1 between 9 R * and 350 R * . We also fit the observed line fluxes of 12 CO, 13 CO, H 2 O and SiO with smmol non-LTE line radiative transfer code, along with a mass-loss rate of 1.85 10 –4 M yr –1 . To fit the high rotational lines of CO and H 2 O, the model required a rather flat temperature distribution inside the dust condensation radius, attributed to the high H 2 O opacity. Beyond the dust condensation radius the gas temperature is fitted best by an r –0.5 radial dependence, consistent with the coolant lines becoming optically thin. Our H 2 O emission-line fits are consistent with an ortho:para ratio of 3 in the outflow.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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