Astronomy & Astrophysics

A&A 466, 1099-1110 (2007)
DOI: 10.1051/0004-6361:20066803

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

K. Ohnaka¹, T. Driebe¹, G. Weigelt¹, and M. Wittkowski²

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: kohnaka@mpifr-bonn.mpg.de
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

(Received 23 November 2006 / Accepted 30 January 2007)

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 C₂H₂ (+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 $\mu$ m with a slight dip centered at ~9.5 $\mu$ m and a gradual increase longward of 10 $\mu$ m. 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 C₂H₂ layers and an optically thin dust shell. Our modeling suggests that the C₂H₂ layers around V Oph are more extended (~1.7-1.8 $R_{\star}$ ) at phases 0.18 and 0.65 than at phase 0.49 (~1.4 $R_{\star}$ ) and that the C₂H₂ 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 $R_{\star}$ , and the total optical depths of $\mbox{$\tau_ \! \approx \! 0.6$ -0.9 ( $\tau_{11.3~\mu{\rm m}} \! \approx \! 0.003$ 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, $\mbox{$\tau_ \! \approx \! 0.3$ ( $\tau_{11.3~\mu{\rm m}} \! \approx \! 0.001$ 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 C₂H₂ layers and the dust shell, and consequently, this masks the size variation of the photosphere. Also, the observed weakness of the mid-infrared C₂H₂ absorption in carbon-rich Miras can be explained by the emission from the extended C₂H₂ layers and the dust shell.

Key words: infrared: stars -- techniques: interferometric -- stars: circumstellar matter -- stars: carbon -- stars: AGB and post-AGB -- stars: individual: V Oph

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