Publications of the MPIfR
Optical & Infrared
Interferometry Group
Temporal variations
of the outer atmosphere and
the dust shell of the carbon-rich Mira variable V Oph probed with
VLTI/MIDI
Ohnaka, K., Driebe, T., Weigelt, G.,
Wittkowski, M.
Astronomy & Astrophysics, vol.466, Issue 3,
pg.1099-1110 (2007)
Abstract
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 MIDI observations were
carried out at three different phases 0.18, 0.49, and 0.65, using three
different baselines configurations (UT2-UT4, UT1-UT4, and UT2-UT3) with
projected baseline lengths of 42-124 m. The wavelength dependence of
the uniform-disk diameters obtained at all epochs is characterized by a
roughly constant region between 8 and 10 micron with a slight dip
centered at ~9.5 micron and a gradual increase longward of 10 micron.
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 Rstar) at phases 0.18 and 0.65 than at phase
0.49 (~1.4 Rstar) 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 Rstar, and the
total optical depths at phases 0.18 and 0.65 are higher than that at
phase 0.49. Our MIDI observations and modeling indicate that
carbon-rich Miras also have extended layers of polyatomic molecules as
previously confirmed in oxygen-rich Miras.
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