Temporal variations of the outer atmosphere and the dust shell of the carbon-rich Mira variable V Ophiuchi probed with VLTI/MIDI

Abstract

Aims. We present the first multi-epoch N-band spectro-interferometric observations of the carbon-rich Mira variable V Oph using MIDI at the ESO's Very Large Telescope Interferometer. Our aim is to study temporal variations of physical properties of the outer atmosphere and the circumstellar dust shell based on spectrally-dispersed N-band visibilities over the C 2 H 2 (+HCN) features and the dust emission. Methods. Our MIDI observations were carried out at three different phases 0.18, 0.49, and 0.65, with three different baselines (projected baseline lengths of 42-124 m) using four 8.2 m Unit Telescopes (UT2-UT4, UT1-UT4, and UT2-UT3 baseline configurations). Results. The wavelength dependence of the uniform-disk diameters obtained at all epochs is characterized by a roughly constant region between 8 and 10 µm with a slight dip centered at ∼9.5 µm and a gradual increase longward of 10 µm. These N-band angular sizes are significantly larger than the estimated photospheric size of V Oph. The angular sizes observed at different epochs reveal that the object appears smaller at phase 0.49 (minimum light) with uniform-disk diameters of ∼5-12 mas than at phases 0.18 (∼12-20 mas) and 0.65 (∼9-15 mas). We interpret these results with a model consisting of optically thick C 2 H 2 layers and an optically thin dust shell. Our modeling suggests that the C 2 H 2 layers around V Oph are more extended (∼1.7-1.8 R ) at phases 0.18 and 0.65 than at phase 0.49 (∼1.4 R ) and that the C 2 H 2 column densities appear to be the smallest at phase 0.49. We also find that the dust shell consists of amorphous carbon and SiC with an inner radius of ∼2.5 R , and the total optical depths of τ V ≈ 0.6-0.9 (τ 11.3 µm ≈ 0.003 and 0.004 for amorphous carbon and SiC, respectively) found at phases 0.18 and 0.65 are higher than the value obtained at phase 0.49, τ V ≈ 0.3 (τ 11.3 µm ≈ 0.001 and 0.002 for amorphous carbon and SiC, respectively). Conclusions. Our MIDI observations and modeling indicate that carbon-rich Miras also have extended layers of polyatomic molecules as previously confirmed in oxygen-rich Miras. The temporal variation of the N-band angular size is largely governed by the variations of the opacity and the geometrical extension of the C 2 H 2 layers and the dust shell, and consequently, this masks the size variation of the photosphere. Also, the observed weakness of the mid-infrared C 2 H 2 absorption in carbon-rich Miras can be explained by the emission from the extended C 2 H 2 layers and the dust shell.

The mid-infrared diameter of W Hydrae

“…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.…”

Dust and molecular shells in asymptotic giant branch stars

“…The FDD diameter as function of wavelength (Fig. 4h) shows a strong increase of 30% shortward of 9 μm which could be related to the presence of molecular layers of C 2 H 2 and HCN (see Ohnaka et al 2007;Sacuto et al 2011). These molecular layers would then lie at about 2 photospheric radii.…”

“…The only observations with MIDI of classical C-rich stars were conducted by Ohnaka et al (2007) for V Oph and Sacuto et al (2011) for R Scl. Ohnaka et al (2007) modeled V Oph with a dust shell, consisting of AMC (85%) and SiC (15%), with a multi-dimensional Monte Carlo code (Ohnaka et al 2006), and added a polyatomic molecular layer, consisting of C 2 H 2 and HCN, where the opacities of these molecules were calculated with an appropriate band model assuming LTE.…”

“…Ohnaka et al (2007) modeled V Oph with a dust shell, consisting of AMC (85%) and SiC (15%), with a multi-dimensional Monte Carlo code (Ohnaka et al 2006), and added a polyatomic molecular layer, consisting of C 2 H 2 and HCN, where the opacities of these molecules were calculated with an appropriate band model assuming LTE. Sacuto et al (2011) applied a self-consistent dynamic model atmosphere to their data (Höfner et al 2003;Mattsson et al 2008Mattsson et al , 2010.…”

Modelling the atmosphere of the carbon-rich Mira RU Virginis

“…So far, interferometric variability was observed only for one star (V Oph, Ohnaka et al 2007); therefore, we decided to analyse the data all at the same time. Eggen (1975).…”