High-resolution IR and radio observations of AGB stars

Abstract

Aims. We present the results of observations with interferometers of a sample of pulsating asymptotic giant branch (AGB) stars in the infrared and at radio wavelengths. The goal of these observations is to explore the extended stellar atmospheres and to establish links between the spatial scales of molecular envelopes and of the dust shell. This is the key to better understand the process of dust formation and therefore of mass loss. Methods. We used the ESO VLTI/MIDI interferometer in the N band, the Keck Interferometer in the K band, and NRAO VLBA observations of SiO masers at 7 mm wavelength of a sample of AGB stars: U Ari, W Cnc, RX Tau, RT Tau, RT Aql, S Ser, and V Mon. The various instruments probe different altitudes of the atmosphere of the AGB stars. They are sensitive to regions below the silicate dust condensation distance and provide the opportunity of finding hints about how dust and its precursors form in the extended atmosphere of an AGB star. The K-band observations are sensitive to water and carbon-monoxyde vapors. Unfortunately, we were only able to observe S Ser in this wavelength range. Results. We find a ratio of 2.2 between the molecular envelope radius and the photospheric size, which is consistent with previous results. The N-band observations are mostly sensitive to vapors of SiO and water and to dust (alumina and silicate). The silicate dust shell is fully resolved, and no precise parameters can be deduced from the N-band observations other than a spatial extension of at least 12-16 R for our sample. The sizes found for the SiO region are consistent with the radii of the SiO maser rings provided by the VLBA observations. The sizes of the alumina and water vapor regions are systematically found to be larger. There is clear evidence that SiO is absent from regions farther from the star where silicate dust condenses. Conclusions. These observations support a possible scenario in which SiO is adsorbed by species such as corundum. An alternative explanation could be that SiO has chemically disappeared at this range of distances.

Resolving the extended atmosphere and the inner wind of Mira (oCeti) with long ALMA baselines

“…If we use our adopted value of 12.3 mas for R , then the inner dust formation radius would be ∼4−5 R . Compared to the recent model of Perrin et al (2015), this range is significantly smaller than the silicate formation radii, which is at least 12 R , but is consistent with the radii of corundum formation.…”

“…Using the hydrodynamical model from Ireland et al (2004a,b), Gray et al (2009) have modelled SiO maser emission in Mira variables and found that the presence of Al 2 O 3 dust may either enhance or suppress SiO maser emission. From interferometric observations of various Mira variables at near-infrared (2.2 µm), mid-infrared (8−13 µm), and radio (43 GHz, 7 mm) wavelengths, Perrin et al (2015) have fitted the visibility data with models similar to those of Perrin et al (2004;stellar photosphere + detached shell of finite width) and found that Al 2 O 3 dust predominantly forms between 3 R and 4.5 R , while silicate dust forms in 12−16 R , which is significantly beyond the radius of SiO maser emission and the silicate dust formation radius derived from previous observations (e.g. Danchi et al 1994).…”

“…Hence, SiO emits rotational emission up to a radius of ∼50 R , far beyond the radius of the molecular layer probed by infrared interferometers. Perrin et al (2015), assuming that the mid-infrared N-band visibilities between 7.80 µm and 9.70 µm are the only signature of gas-phase SiO emission, concluded that the SiO can only be found in gas phase within 3 R . We suggest that this discrepancy is due to the excitation effect of SiO molecules.…”

Evidence for localized onset of episodic mass loss in Mira

“…This could be consistent with localization within the intermediate layer feeding the extended molecular layers or with that material itself, with coherent structure lasting over several pulsational periods, but not indefinitely consistent with the observed shell expansion velocity ≈few km s −1 - Hinkle & Lebzelter (2015). It is reasonable to find that the cause is absorption by dust located in or near the extended molecular layers as the dust condensation radius is expected in this region, as discussed in, for example, Reid & Menten (1997), Perrin et al (2004Perrin et al ( , 2015, and Wittkowski et al (2007). In the next section, an estimate of the mass of the clump with respect to the net mass loss rate leads to another plausible explanation for a location near or somewhat interior to this region, which Tsuji (2000) describe as a MOLsphere.…”

“…Al 2 O 3 is a good candidate for dust formation at high temperature in the molecular shell of Mira stars (see e.g. Perrin et al 2015). But the condensation of Al 2 O 3 at distances and concentrations implied by observations requires high transparency of the grains in the visual and near-IR region to avoid destruction by radiative heating (Höfner et al 2016).…”