• DOI:10.1051/0004-6361/201321683
• Corpus ID: 55758451

Hifistars herschel/hifi observations of vy canis majoris : Molecular-line inventory of the envelope around the largest known star

@article{Alcolea2013HifistarsHO,
  title={Hifistars herschel/hifi observations of vy canis majoris : Molecular-line inventory of the envelope around the largest known star},
  author={Javier Alcolea and Valent{\'i}n Bujarrabal and Pere Planesas and David Teyssier and Jos{\'e} Cernicharo and Elvire De Beck and Leen Decin and Carsten Dominik and K. Justtanont and A. de Koter and A. P. Marston and Gary J. Melnick and Karl M. Menten and David A. Neufeld and H. Olofsson and M. R. Schmidt and Fredrik Schoier and Ryszard Szczerba and L. B. F. M. Waters},
  journal={Astronomy and Astrophysics},
  year={2013},
  volume={559},
  pages={1-25}
}

• J. Alcolea, V. Bujarrabal, +16 authors L. Waters
• Published 9 October 2013
• Physics
• Astronomy and Astrophysics

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
.