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

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 H 2 O 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 H 2 O and CO lines, and present detections of several possible ν 2 = 1 vibrationally excited water lines. We also identify blended lines of OH, one unblended and several blended lines of NH 3 , and one possible detection of H 3 O + . 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 +210 −130 K, and lower limit on the water column density of (7.0 ± 1.2) × 10 19 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 H 2 O column densities are an order of magnitude lower (as are the mass loss rates).

Fundamental properties and atmospheric structure of the red supergiant VY Canis Majoris based on VLTI/AMBER spectro-interferometry

“…The decrease of the visibility indicates an increased contribution from extended intensity at these molecular bands. The presence of molecular layers in the extended atmospheres of red supergiants is consistent with earlier observations of red supergiants such as μ Cep and Betelgeuse (Perrin et al 2004(Perrin et al , 2005Tsuji 2006) and with the detection of warm water layers in the ISO and Herschel spectra of VY CMa (Polehampton et al 2010;Royer et al 2010).…”

PACS and SPIRE spectroscopy of the red supergiant VY CMa

“…A&A 518, L145 (2010) For the sake of clarity, PACS band B2A is not included, as it matches the wavelength range of PACS-B3A. The ISO-LWS spectrum is overplotted for comparison (Polehampton et al 2010). For PACS, the spectrum of the central spaxel is displayed, after application of the point source correction (version 2.0).…”

“…Figure 1 shows the full PACS and SPIRE spectrum of VY CMa. Phillips et al (1992) and Polehampton et al (2010), but due to the high density of lines, detailed modeling is needed to give a definitive answer (Matsuura et al 2010, in prep.). Both low-excitation (e.g., CO J = 4-3, at 55 K) and high-excitation (e.g., o-H 2 O (16 8,9 -15 9,6 ) at 5500 K) are found, allowing us to study the molecular envelope from the outermost layers down to the dust formation Line peak intensity, expressed in Jy, for the o-H 2 O (7 1,6 -6 2,5 ) line at 66.09 μm, in all PACS observations.…”

“…By this means, VY CMa has been shown to harbour a rich, water-dominated chemistry (e.g. Neufeld et al 1999;Polehampton et al 2010). The unprecedented wavelength coverage and spectral resolution of the Herschel instruments will undoubtedly push our understanding of the mass-loss related phenomena a step further.…”

Spectral Line Surveys of Evolved Stars

“…Observations of oxygen-rich evolved stars such as VY Canis Majoris, IK Tau, TX Cam, RX Boo, IRC +10011, and R Cas have also been carried out by Polehampton et al (2010) in the far-infrared region of the spectrum using the LWS instrument on board ISO.…”