The ISO Long Wavelength Spectrometer line spectrum of VY Canis Majoris and other oxygen-rich evolved stars^*,**

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Space Science & Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK e-mail: edward.polehampton@stfc.ac.uk
³ Department of Physics, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta, T1J 1B1, Canada
⁴ Netherlands Institute for Space Research (SRON), Landleven 12, 9747 AD Groningen, The Netherlands
⁵ Department of Physics and Astronomy, The Open University, Milton Keynes, MK6 7AA, UK

Received: 16 September 2009
Accepted: 10 November 2009

Abstract

Context. The far-infrared spectra of circumstellar envelopes around various oxygen-rich stars were observed using the ISO Long Wavelength Spectrometer (LWS). These have been shown to be spectrally rich, particularly in water lines, indicating a high H₂O abundance.

Aims. We have examined high signal-to-noise ISO LWS observations of the luminous supergiant star, VY CMa, with the aim of identifying all of the spectral lines. By paying particular attention to water lines, we aim to separate the lines due to other species, in particular, to prepare for forthcoming observations that will cover the same spectral range using Herschel PACS and at higher spectral resolution using Herschel HIFI and SOFIA.

Methods. We have developed a fitting method to account for blended water lines using a simple weighting scheme to distribute the flux. We have used this fit to separate lines due to other species which cannot be assigned to water. We have applied this approach to several other stars which we compare with VY CMa.

Results. We present line fluxes for the unblended H₂O and CO lines, and present detections of several possible vibrationally excited water lines. We also identify blended lines of OH, one unblended and several blended lines of NH₃, and one possible detection of H₃O⁺.

Conclusions. The spectrum of VY CMa shows a detection of emission from virtually every water line up to 2000 K above the ground state, as well as many additional higher energy and some vibrationally excited lines. A simple rotation diagram analysis shows large scatter (probably due to some optically thick lines). The fit gives a rotational temperature of 670 K, and lower limit on the water column density of  cm^-2. We estimate a CO column density ~100 times lower, showing that water is the dominant oxygen carrier. The other stars that we examined have similar rotation temperatures, but their H₂O column densities are an order of magnitude lower (as are the mass loss rates).