The time variation in infrared water-vapour bands in mira variables

Matsuura, Mikako ORCID: https://orcid.org/0000-0002-5529-5593, Yamamura, Issei, Cami, J, Onaka, Takashi and Murakami, Hiroshi 2002. The time variation in infrared water-vapour bands in mira variables. Astronomy & Astrophysics 383 , pp. 972-986. 10.1051/0004-6361:20011796

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Abstract

The time variation in the water-vapour bands in oxygen-rich Mira variables has been investigated using multi-epoch ISO/SWS spectra of four Mira variables in the 2.5-4.0 mu m region. All four stars show H₂O bands in absorption around minimum in the visual light curve. At maximum, H₂O emission features appear in the ~ 3.5-4.0 mu m region, while the features at shorter wavelengths remain in absorption. These H₂O bands in the 2.5-4.0 mu m region originate from the extended atmosphere. The analysis has been carried out with a disk shape, slab geometry model. The observed H₂O bands are reproduced by two layers; a ``hot'' layer with an excitation temperature of 2000 K and a ``cool'' layer with an excitation temperature of 1000-1400 K. The column densities of the ``hot'' layer are 6*E<sup>20</sup>-3*E<sup>22</sup> cm<sup>-2</sup>, and exceed 3*E<sup>21</sup> cm<sup>-2</sup> when the features are observed in emission. The radii of the ``hot'' layer (R_hot) are ~ 1 R_* at visual minimum and 2 R_* at maximum, where R_* is a radius of background source of the model, in practical, the radius of a 3000 K black body. The ``cool'' layer has the column density (N<sub>cool</sub>) of 7*E<sup>20</sup>-5*E<sup>22</sup> cm<sup>-2</sup>, and is located at 2.5-4.0 R<sub>*</sub>. N<sub>cool</sub> depends on the object rather than the variability phase. The time variation of R<sub>hot</sub>/R<sub>*</sub> from 1 to 2 is attributed to the actual variation in the radius of the H<sub>2</sub>O layer, since the variation in R<sub>hot</sub> far exceeds the variation in the ``continuum'' stellar radius. A high H_2O density shell occurs near the surface of the star around minimum, and moves out with the stellar pulsation. This shell gradually fades away after maximum, and a new high H₂O density shell is formed in the inner region again at the next minimum. Due to large optical depth of H₂O, the near-infrared variability is dominated by the H₂O layer, and the L'-band flux correlates with the area of the H₂O shell. The infrared molecular bands trace the structure of the extended atmosphere and impose appreciable effects on near-infrared light curve of Mira variables. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE....

Item Type:	Article
Date Type:	Published Online
Status:	Published
Schools:	Physics and Astronomy
Publisher:	EDP Sciences
ISSN:	0004-6361
Date of Acceptance:	17 December 2001
Last Modified:	17 May 2024 14:45
URI:	https://orca.cardiff.ac.uk/id/eprint/168098

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