The Mira variable S Orionis: relationships between the photosphere, molecular layer, dust shell, and SiO maser shell at 4 epochs

Abstract

Aims. We present the first multi-epoch study that includes concurrent mid-infrared and radio interferometry of an oxygen-rich Mira star. Results. The modeling of our MIDI data results in phase-dependent continuum photospheric angular diameters of 9.0 ± 0.3 mas (phase 0.42), 7.9 ± 0.1 mas (0.55), 9.7 ± 0.1 mas (1.16), and 9.5 ± 0.4 mas (1.27). The dust shell can best be modeled with Al 2 O 3 grains using phase-dependent inner boundary radii between 1.8 and 2.4 photospheric radii. The dust shell appears to be more compact with greater optical depth near visual minimum (τ V ∼ 2.5), and more extended with lower optical depth after visual maximum (τ V ∼ 1.5). The ratios of the 43.1 GHz/42.8 GHz SiO maser ring radii to the photospheric radii are 2.2 ± 0.3/2.1 ± 0.2 (phase 0.44), 2.4 ± 0.3/2.3 ± 0.4 (0.55), and 2.1 ± 0.3/1.9 ± 0.2 (1.15). The maser spots mark the region of the molecular atmospheric layers just beyond the steepest decrease in the mid-infrared model intensity profile. Their velocity structure indicates a radial gas expansion. Conclusions. S Ori shows significant phase-dependences of photospheric radii and dust shell parameters. Al 2 O 3 dust grains and SiO maser spots form at relatively small radii of ∼1.8−2.4 photospheric radii. Our results suggest increased mass loss and dust formation close to the surface near the minimum visual phase, when Al 2 O 3 dust grains are co-located with the molecular gas and the SiO maser shells, and a more expanded dust shell after visual maximum. Silicon does not appear to be bound in dust, as our data show no sign of silicate grains.

The mid-infrared diameter of W Hydrae

“…This is very similar to findings by Weiner et al (2003), Perrin et al (2004) and Tatebe et al (2006) for similar stars. In particular, the diameter behavior throughout the N-band is comparable with observations of the O-rich Mira stars RR Sco and S Ori by Ohnaka et al (2005) and Wittkowski et al (2007), respectively. W Hya is therefore described by an analogous model.…”

“…comm. ; preliminary results for R Car, R Cnc and Z Pup) for MIDI AT observations, by Wittkowski et al (2007) (O-rich Mira S Ori), and Ohnaka et al (2007) (C-rich Mira V Oph) for MIDI Unit Telescope (UT) observations, and by Karovicova (private communication; preliminary result for RR Aql) for both AT and UT observations. Since the photometric accuracy of MIDI used with the ATs is very low, these values should be used with caution.…”

“…1e in Ohnaka et al (2005) and Figs. 2-5d in Wittkowski et al (2007) leads to the idea that, in all three stars, the same constituents and mechanisms are responsible for this appearance.…”

High-resolution IR and radio observations of AGB stars

“…However, Cotton et al (2004) found a fairly small scatter of the ratio of the SiO maser ring diameter to the apparent size at 2.2 µm around a value of approximately 2. This was confirmed by observations of S Orionis by Wittkowski et al (2007). The relation of the SiO maser ring diameter to the inner size of the dust shell remains unclear.…”

What causes the large extensions of red supergiant atmospheres?

“…However, the detailed dust formation, condensation sequence, and massloss mechanisms are also not yet fully understood in the case of oxygen-rich AGB stars (cf., e.g., Karovicova et al 2013, for a recent discussion). Recent indications based on a polarimetric aperture masking technique (Norris et al 2012) and on mid-infrared interferometry (Wittkowski et al 2007;Karovicova et al 2011Karovicova et al , 2013 point toward transparent dust grains forming already at relatively small radii of about 1.5 stellar radii. These may be grains of amorphous Al 2 O 3 and/or magnesiumrich iron-free ("forsterite") silicates, while iron-rich silicates form at larger radii.…”