Herschel/HIFI observations of red supergiants and yellow hypergiants

Abstract

Context. Red supergiant stars (RSGs) and yellow hypergiant stars (YHGs) are believed to be the high-mass counterparts of stars in the asymptotic giant branch (AGB) and early post-AGB phases. As such, they are scarcer and the properties and evolution of their envelopes are still poorly understood. Aims. We study the mass-loss in the post main-sequence evolution of massive stars, through the properties of their envelopes in the intermediate and warm gas layers. These are the regions where the acceleration of the gas takes place and the most recent mass-loss episodes can be seen. Methods. We used the HIFI instrument on-board the Herschel Space Observatory to observe sub-millimetre and far-infrared (FIR) transitions of CO, water, and their isotopologues in a sample of two RSGs (NML Cyg and Betelgeuse) and two YHGs (IRC+10420 and AFGL 2343) stars. We present an inventory of the detected lines and analyse the information revealed by their spectral profiles. A comparison of the line intensity and shape in various transitions is used to qualitatively derive a picture of the envelope physical structure. On the basis of the results presented in an earlier study, we model the CO and 13 CO emission in IRC+10420 and compare it to a set of lines ranging from the millimetre to the FIR. Results. Red supergiants have stronger high-excitation lines than the YHGs, indicating that they harbour dense and hot inner shells contributing to these transitions. Consequently, these high-J lines in RSGs originate from acceleration layers that have not yet reached the circumstellar terminal velocity and have narrower profiles than their flat-topped lower-J counterparts. The YHGs tend to lack this inner component, in line with the picture of detached, hollow envelopes derived from studies at longer wavelengths. NH 3 is only detected in two sources (NML Cyg and IRC+10420), which are also observed to be the strongest water-line emitters of the studied sample. In contrast, OH is detected in all sources and does not seem to correlate with the water line intensities. We show that the IRC+10420 model derived solely from millimetre low-J CO transitions is capable of reproducing the high-J transitions when the temperature in the inner shell is simply lowered by about 30%.

Detection of circumstellar nitric oxide

“…If N 2 is photodissociated faster than assumed in our chemical model (Bohlin et al 1978) the enhancement of nitrogen needed to fit the NO profiles would not be that high. Also, a detailed modeling of the line emission from H 2 O presented by Teyssier et al (2012) would help check that the abundance of this molecule is higher than what is assumed in the present work, which would result in lower nitrogen abundances. The model fitting is shown in Fig.…”

“…Recently, Teyssier et al (2012) have observed selected transitions of NH 3 , OH, H 2 O, CO, and 13 CO with HIFI towards this object. They also show that the model derived by Castro-Carrizo et al (2007) for low-J CO transitions was applicable, with minor changes, to higher excitation lines.…”

“…Many of the chemical reactions that synthesize molecules in interstellar clouds are also at work in CSEs around evolved stars, although one major difference in the chemistry of these two environments concerns the chemical composition of the starting bulk material, which in CSEs is given to a large extent by thermochemical equilibrium at the temperatures and pressures characteristic of the surroundings of the star. To gain insight into the synthesis of nitric oxide in IRC +10420, we carried out a chemical model based on the physical structure presented by Teyssier et al (2012), which reproduces the observations for low and high CO excitation lines. The mass-loss history derived for this object consisted of two mass ejection episodes from the star, which resulted in two detached shells (see Table 1).…”

“…To model the observed NO line profiles and to calculate the excitation of the transitions involved, we have used the molecular emission model of IRC +10420 by Teyssier et al (2012) restricted to the inner shell, using as radial abundance profile for NO that computed with the chemical model. As mentioned above, this model reproduces the characteristics of the molecular gas at high and low excitation regimes.…”

Circumstellar ammonia in oxygen-rich evolved stars

“…Bell et al 1980;Menten & Alcolea 1995), several (pre-)planetary nebulae (CRL 2688, OH 231.8+4.2, CRL 618) (Nguyen-Q-Rieu et al 1984;Morris et al 1987;Truong-Bach et al 1988, 1996Martin-Pintado & Bachiller 1992a,b;Martin-Pintado et al 1993, 1995, and the hypergiant IRC +10420 (Menten & Alcolea 1995). More recently, submillimetre observations with space observatories have detected rotational transitions of NH 3 from evolved stars of all chemical types, including the C-rich star IRC +10216 (C/O > 1; Hasegawa et al 2006;Schmidt et al 2016), several O-rich stars (C/O < 1; Menten et al 2010;Teyssier et al 2012), and the S-type star W Aquilae (C/O ≈ 1; Danilovich et al 2014).…”