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Water Vapour on Betelgeuse as Revealed by TEXES High-Resolution 12 Micron Spectra
Authors: N. Ryde, G. M. Harper, M. J. Richter, T. K. Greathouse, J. H. Lacy

The outer atmosphere of the M supergiant Betelgeuse is puzzling. Published observations of different kinds have shed light on different aspects of the atmosphere, but no unified picture has emerged. They have shown, for example, evidence of a water envelope (MOLsphere) that in some studies is found to be optically thick in the mid-infrared.
In this paper, researchers present high-resolution, mid-infrared spectra of Betelgeuse recorded with the TEXES spectrograph. The spectra clearly show absorption features of water vapour and OH.
They show that a spectrum based on a spherical, hydrostatic model photosphere with T_eff = 3600 K, an effective temperature often assumed for Betelgeuse, fails to model the observed lines.
Furthermore, they show that published MOLspheres scenarios are unable to explain their data. However, the researchers are able to model the observed spectrum reasonably well by adopting a cooler outer photospheric structure corresponding to T_mod = 3250 K.
The success of this model may indicate the observed mid-infrared lines are formed in cool photospheric surface regions.
Given the uncertainties of the temperature structure and the likely presence of inhomogeneities, they cannot rule out the possibility that their spectrum could be mostly photospheric, albeit non-classical.
The data put new, strong constraints on atmospheric models of Betelgeuse and they conclude that continued investigation requires consideration of non-classical model photospheres as well as possible effects of a MOLsphere.
They show that the mid-infrared water-vapour features have great diagnostic value for the environments of K and M (super-) giant star atmospheres.

Read More (PDF)

High-resolution spectra of Betelgeuse in the 12 µm region, here 809.75 . 813.0 cm.1(dots), are shown. The observations are corrected for the star’s velocity at the time of observation. The wavelengths are thus the laboratory wavelengths. Uppermost in the Figure, the sky spectrum is shown. The almost vertical lines show the ranges of the spectral orders. In this figure five orders are present. Overlaid on the observed Betelgeuse spectrum is a spectrum calculated on the basis of a classical, one-dimensional model atmosphere (Te_ = 3600 K) with, most importantly, OH and water-vapour lines, but also weak metal lines (full line). Only a subset of water-vapour lines in the wavelength region is included. The water-vapour lines included are those with accurately measured wavelengths. A few other water-vapour lines are marked with an asterisk. Shifted vertically downwards from the observed Betelgeuse spectra is shown first the sunspot spectrum of this area. Next below is shown a pure water-vapour spectrum, based on the NASA-Ames water-vapour line list for a Betelgeuse model.

Betelgeuse is a red supergiant star, and is one of the largest stars known.
Betelgeuse is called alpha Orionis even though it is fainter (visual) than Beta Orionis (Rigel). This is because Betelgeuse, a variable star, was misclassified.

Position(2000): RA: 5h55m00.00s DE:+07°00'00.0"

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