Herschel/HIFI observations of red supergiants and yellow hypergiants: I. Molecular inventory

Abstract

Red supergiant stars (RSGs) and yellow hypergiant stars (YHGs) are believed to be the high-mass counterparts of stars in the AGB and early post-AGB phases. 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. We used the HIFI instrument on-board the Herschel Space Observatory to observe sub-mm and 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. On the basis of the results presented in an earlier study, we model the CO and 13CO emission in IRC+10420 and compare it to a set of lines ranging from the mm, to the FIR. 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. NH3 is only detected in two sources (NML Cyg, 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 mm 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%.

14 pages, 13 figures, accepted in Astronomy and Astrophysics

References in corpus (7)

• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles I. Theoretical model -- Mass-loss history unravelled in VY CMa
• Atmospheric dynamics and the mass loss process in red supergiant stars
• The close circumstellar environment of Betelgeuse - II. Diffraction-limited spectro-imaging from 7.76 to 19.50 microns with VLT/VISIR
• The Distance and Size of the Red Hypergiant NML Cyg from VLBA and VLA Astrometry
• Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics
• The chemical composition of the circumstellar envelopes around yellow hypergiant stars
• The structure and chemistry of the massive shell around AFGL 2343: 29SiO and HCN as tracers of high-excitation regions

Cited by in corpus (12)

• 3D MHD astrospheres: applications to IRC-10414 and Betelgeuse
• A lambda 3mm and 1mm line survey toward the yellow hypergiant IRC +10420
• ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H${}_2$O and OH lines
• Circumstellar ammonia in oxygen-rich evolved stars
• The chemistry of dust formation in red supergiants
• Multiple components in the molecular outflow of the red supergiant NML Cyg
• To the problem of spectral mimicry of supergiants
• Analysis of stellar spectra with 3D and NLTE models
• A New Titanium Oxide Index in Visual Band
• Dense molecular clumps in the envelope of the yellow hypergiant IRC+10420
• Molecules and Chemistry in Red Supergiants
• Spectroscopic analysis of cool giants and supergiants