The molecular and dusty composition of Betelgeuse's inner circumstellar environment

Abstract

Context. The study of the atmosphere of red supergiant stars in general and of Betelgeuse (α Orionis) in particular is of prime importance to understand dust formation and how mass is lost to the interstellar medium in evolved massive stars. Aims. A molecular shell, the MOLsphere (Tsuji 2000a, ApJ, 538, 801), in the atmosphere of Betelgeuse has been proposed to account for the near-and mid-infrared spectroscopic observations of Betelgeuse. The goal is to further test this hypothesis and to identify some of the molecules in this MOLsphere. Methods. We report on measurements taken with the mid-infrared two-telescope beam combiner of the VLTI, MIDI, operated between 7.5 and 13.5 µm. The data are compared to a simple geometric model of a photosphere surrounded by a warm absorbing and emitting shell. Physical characteristics of the shell are derived: size, temperature and optical depth. The chemical constituents are determined with an analysis consistent with available infrared spectra and interferometric data. Results. The MIDI data are well modeled with a geometrically thin shell whose radius varies from 1.31 to 1.43 R across the N band with a typical temperature of 1550 K. We are able to account for the measured optical depth of the shell in the N band, the ISO-SWS spectrum and K and L band interferometric data with a shell whose inner and outer radii are given by the above range and with the following species and densities: H 2 O (7.1 ± 4.7 × 10 19 cm −2 ), SiO (4.0 ± 1.1 × 10 20 cm −2 ), Al 2 O 3 (2.4 ± 0.5 × 10 15 cm −2 ). Conclusions. These results confirm the MOLsphere model. We bring evidence for more constituents and for the presence of species participating in the formation of dust grains in the atmosphere of the star, i.e. well below the distance at which the dust shell is detected. We believe these results bring key elements to the understanding of mass loss in Betelgeuse and red supergiants in general and bring support to the dust-driven scenario.

The mid-infrared diameter of W Hydrae

“…Hron et al 1998;) AGB stars, as well as for RSG stars (e.g. Perrin et al 2007). These layers were introduced earlier to explain spectroscopic observations (e.g.…”

“…e.g. for Mira stars Ohnaka et al 2005;, and for RSG stars Perrin et al 2007;and Verhoelst et al 2009). The radiative importance of dust in an extended molecular shell depends on the local temperature, the time dependent growth rate, and the wavelengthdependent optical properties.…”

The dust condensation sequence in red supergiant stars

“…Concerning the first problem, a significant SiO layer just above the photosphere, which deepens the absorption band, could be present since such a layer is detected around α Ori with MIDI interferometry by Perrin et al (2007), although the SiO bandhead does not correspond well to the observed residuals around 8 μm. The current dust model may also not yet contain the correct silicate dust composition, shape, and grain size.…”

“…Our model assumes the same inner radius for all dust species, but the discrepancy observed here may indicate that the continuous opacity source is located even closer to the base of the wind, right above the photosphere. The detection of alumina 0.5 R above the photosphere of Betelgeuse (Verhoelst et al 2006;Perrin et al 2007) indicates that dust condensation so close to the photosphere is possible.…”

The inhomogeneous submillimeter atmosphere of Betelgeuse

“…Obviously, there is not a monotonic decrease in the mean gas temperature of the extended atmosphere from the photosphere outwards. Indeed, modeling of infrared molecular lines from RSGs has already provided evidence for the presence of cool (1500−2000 K) gas between 1.3 and 1.5 R (Tsuji 2000;Perrin et al 2004Perrin et al , 2007Ohnaka 2004;Montargès et al 2014). However, these models are not sensitive to hotter plasma that may be co-located with the cool gas.…”