HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line inventory of the envelope around the largest known star

Artikel i vetenskaplig tidskrift, 2013

Aims. The study of the molecular gas in the circumstellar envelopes of evolved stars is normally undertaken by observing lines of CO (and other species) in the millimetre-wave domain. In general, the excitation requirements of the observed lines are low at these wavelengths, and therefore these observations predominantly probe the cold outer envelope while studying the warm inner regions of the envelopes normally requires sub-millimetre (sub-mm) and far-infrared (FIR) observational data. Methods. To gain insight into the physical cond itions and kinematics of the warm (100–1000 K) gas around the red hyper-giant VY CMa, we performed sensitive high spectral resolution observations of molecular lines in the sub-mm / FIR using the HIFI instru- ment of the Herschel Space Observatory. We observed CO, H 2 O, and other molecular species, sampling excitation energies from a few tens to a few thousand K. These observations are part of the Herschel guaranteed time key program HIFISTARS. Results. We detected the J = 6–5, J = 10–9, and J = 16–15 lines of 12 CO and 13 CO at ∼ 100, 300, and 750 K above the ground state (and the 13 CO J = 9–8 line). These lines are crucial fo r improving the modelling of the internal layers of the envelope around VY CMa. We also detected 27 lines of H 2 O and its isotopomers, and 96 lines of species such as NH 3 ,SiO,SO,SO 2 HCN, OH and others, some of them originating from vibrationally excited levels. Three lines were not unambiguously assigned. Conclusions. Our observations confirm that VY CMa’s envelope must consist of two or more detached components. The molecular excitation in the outer layers is significantly lower than in the inner ones, resulting in strong self-absorbed profiles in molecular lines that are optically thick in this outer envelope, for instance, low-lying lines of H 2 O. Except for the most abundant species, CO and H 2 O, most of the molecular emission detected at these sub-mm / FIR wavelengths arise from the central parts of the envelope. The spectrum of VY CMa is very prominent in vibrationally excited lines, which are caused by the strong IR pumping present in the central regions. Compared with envelopes of other massive evolved stars, VY CMa’s emission is particularly strong in these vibrationally excited lines, as well as in the emission from less abundant species such as H 13 CN, SO, and NH 3 .