Mid-infrared interferometry of the Mira variable RR Sco with the VLTI MIDI instrument
Thomas Driebe (Max-Planck Institute for Radio Astronomy)
Keiichi Ohnaka (Max-Planck Institute for Radio Astronomy)
Gerd Weigelt (Max-Planck Institute for Radio Astronomy)
We present the results of the first interferometric observations of the Mira
variable RR Sco with the MID-infrared Interferometer (MIDI) coupled to the
European Southern Observatory's Very Large Telescope Interferometer (VLTI),
together with K-band observations using VLTI VINCI. The observations were
carried out in June 2003, using unit telescopes UT1 and UT3, as part of the
Science Demonstration Time program. Projected baseline lengths ranged from 73 to
102m, and a spectral resolution of 30 was employed in the observations.The
uniform-disk diameter was found to be 18 mas between 8 and 10 micron, while it
gradually increases longward of 10 micron to reach 24 mas at 13 micron. The
uniform-disk diameter between 8 and 13 micron is significantly larger than the
K-band uniform-disk diameter of 10.2 ± 0.5 mas measured with VLTI VINCI
three weeks after the MIDI observations with projected baseline lengths of
≈ 15--16 m. Our model calculations show that optically thick emission
from a warm molecular envelope consisting of water and SiO can cause the
apparent diameter to be much larger than the continuum diameter. We find that a
warm molecular envelope model extending to ≈ 2.2 stellar radii can
reproduce the observed uniform-disk diameters between 8 and 10 micron, and the
observed increase of the uniform-disk diameter longward of 10 micron can be
explained by an optically thin dust shell consisting of silicate and corundum
grains.