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On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae

Gorlova, Nadya and Lobel, Alex and Burgasser, Adam J. and Rieke, George H. and Ilyin, Ilya and Stauffer, John R. (2006) On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae. Astrophysical Journal, 651 (2). pp. 1130-1150. ISSN 0004-637X. doi:10.1086/507590.

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We report on multiepoch optical and near-infrared spectroscopy around the first-overtone rovibrational band of CO in the pulsating yellow hypergiant ρ Cas, one of the most massive stars in the Galaxy and a candidate SN II progenitor. We argue that the double cores of the CO absorption lines, which have previously been attributed to separate circumstellar shells expelled during its recurrent outbursts, result in fact from a superposition of a wide absorption line and a narrow central emission line. The CO line doubling returns over subsequent pulsation cycles, where the superposed line emission assumes its largest intensity near phases of maximum light. We find that the morphology and behavior of the CO band closely resemble the remarkable "line-splitting phenomenon" also observed in optical low-excitation atomic lines. Based on radiative transport calculations, we present a simplified model of the near-infrared CO emission emerging from cooler atmospheric layers in the immediate vicinity of the photosphere. We speculate that the kinetic temperature minimum in our model results from a periodic pulsation-driven shock wave. We further discuss a number of alternative explanations for the origin of the ubiquitous emission-line spectrum, possibly due to a quasi-chromosphere or a steady shock wave at the interface of a fast expanding wind and the interstellar medium. We present a number of interesting spectroscopic similarities between ρ Cas and other types of cool variable supergiants, such as the RV Tau and R CrB stars. We further propose a possibly common mechanism for the enigmatic outburst behavior of these luminous pulsating cool stars.

Item Type:Article
Related URLs:
URLURL TypeDescription
Burgasser, Adam J.0000-0002-6523-9536
Rieke, George H.0000-0003-2303-6519
Stauffer, John R.0000-0003-3595-7382
Additional Information:© 2006 American Astronomical Society. Received 2005 October 4; accepted 2006 July 7. We would like to thank all the observers who helped us to obtain the NIR spectra. We also thank the staff at the Vatican Observatory, the Steward Observatory, and the CorMASS group for providing observing time and support during N. G.’s visits to VATT. The Service Mode Observing group at UKIRT is thanked for their timely observations, help with retrieving archival data, and advice on the data reduction. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the UK Particle Physics and Astronomy Research Council. We thank the AAVSO international database maintainers, with contributions from many observers worldwide to the light curve of Cas. Special thanks to K. Hinkle for drawing attention to the FTS KPNO spectra of Cas and HR 8752. J. Muzerolle and the anonymous referee are acknowledged for helpful comments on the manuscript. N. G. and G. R. were supported by a JPL/Caltech Spitzer grant under contract 1255094. A. L. acknowledges partial financial support from FUSE grants GI-D107 and GI-E068 by Johns Hopkins University.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
JPL/Caltech Spitzer1255094
Johns Hopkins UniversityGI-D107
Johns Hopkins UniversityGI-E068
Subject Keywords:infrared: stars; line: profiles; shock waves; stars: atmospheres; stars: oscillations; supergiants
Issue or Number:2
Record Number:CaltechAUTHORS:20110408-113845890
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Official Citation:On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae Nadya Gorlova et al. 2006 ApJ 651 1130
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:23254
Deposited On:10 Apr 2011 00:53
Last Modified:09 Nov 2021 16:12

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