Publications of the MPIfR Optical & Infrared Interferometry Group

R Cas

1) G. Weigelt, Y. Balega, K.-H. Hofmann and M. Scholz

First diffraction-limited speckle masking observations of the Mira variable R Cas with the 6m SAO telescope

Astronomy and Astrophysics, 316, L21-L24 (1996)

Abstract
We present the first diffraction-limited speckle masking observations of a Mira star with the Russian 6m SAO telescope. At 700 and 714nm wavelength a resolution of 30 milli-arcsec was achieved. The reconstructed true images are the images with the highest resolution obtained up to now in this wavelength region. The speckle interferograms of R Cas were recorded through interference filters with center wavelength/bandwidth of 714nm/6nm (strong TiO band absorption) and 700nm/6nm (moderate TiO absorption). The two reconstructed images show that the azimuthally averaged 700 and 714nm uniform-disk diameters of R Cas are 36mas +/- 2mas and 49mas +/- 2mas, respectively. In the 700nm and 714nm images the disks of R Cas are non-uniform and elongated along position angle 52 +/- 7 degrees and 57 +/- 7 degrees, respectively. The size (elliptical uniform disk fit) of the elongated disks is about 33.3mas x 38.7mas (axial ratio 0.86) in the 700nm pseudo-continuum filter and 42.3mas x 55.6mas (axial ratio 0.76) in the 714nm TiO absorption band filter. We discuss briefly theoretical aspects of these and previous observations.

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2) K.-H. Hofmann, Y.Y. Balega, M. Scholz, and G. Weigelt:

Multi-wavelength bispectrum speckle interferometry of R Cas and comparison of the observations with Mira star models

Astronomy and Astrophysics 353, 1016-1028 (2000)

Abstract. We present diffraction-limited (30mas) bispectrum speckle interferometry observations of the Mira star R Cas with the Russian 6m SAO telescope. The speckle interferograms were recorded through narrow-band interference filters with centre wavelength/bandwidth of 671nm/6nm and 714nm/6nm (strong TiO absorption band), 700nm/6nm (moderate TiO absorption), and 1045nm/9nm (continuum). The reconstructed images show that the average uniform-disk diameters of R Cas are 43.6mas ± 2.0mas at 671nm, 49.2mas ± 2.0mas at 714nm, 37.2mas ± 2.0mas at 700nm, and 29.9mas ± 3.0mas at 1045nm. In the 671nm, 714nm and 700nm images the disks of R Cas are non-uniform and elongated with position angles of the long axis of 52°, 57°, and 54°,and axis ratios of 0.70, 0.76 and 0.87, respectively. For example, at 671nm the size (elliptical uniform disk fit) of the elongated R Cas disk is 51.0mas x 35.6~mas. The 1045nm image shows no significant asymmetry.
We compare our observations with theoretical Mira star models and systematically check the capacity of monochromatic diameter ratios and linear diameters for discriminating between model representations of the observed star. Monochromatic tau=1 radii were derived from the observed visibilities by application of model-predicted center-to-limb variations of the intensity. Adopting the HIPPARCOS parallax we obtained a photospheric radius (Rosseland tau_Ross=1 radius) of R Cas of 377Rsol ± 60Rsol (32.9mas ± 3.3mas). The derived photospheric radius and the large period (~430 days) suggest first overtone pulsation for R Cas. We also determined the effective temperature of R Cas at near-minimum phase to 1880K ± 130K and propose Teff~1900K for spectral type M10 in the temperature calibration of very late M giant spectral classes.

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3) K.-H. Hofmann, Y.Y. Balega, M. Scholz, and G. Weigelt:

Multi-wavelength bispectrum speckle interferometry of R Leo and comparison with Mira star models

Astronomy and Astrophysics 376, 518-531 (2001)

Abstract. We present diffraction-limited (30mas resolution) bispectrum speckle interferometry of the Mira star R Leo with the 6m SAO telescope. The speckle interferograms were recorded through narrow-band interference filters with centre wavelength/bandwidth of 673nm/8nm (strong TiO absorption band), 656nm/10nm, 699nm/6nm and 781nm/14nm (moderate TiO absorption), 754nm/6nm (weak TiO absorption), and 1045nm/9nm (continuum). The reconstructed images show that the average uniform-disk diameters of R Leo are 60.6mas±3.0mas at 656nm, 75.6mas±3.7mas at 673nm, 52.5mas±2.5mas at 699nm, 48.7mas±2.3mas at 754nm, 55.0mas±2.7mas at 781nm, and 37.9mas±4.0mas at 1045nm. In all six observed wavelength bands the shape of R Leo shows no significant asymmetry. We compare our observations with Mira star models and check the ability of monochromatic linear diameters for discriminating between model representations of the observed star. Monochromatic tau_lambda=1 radii were derived from the observed visibilities by application of model-predicted center-to-limb variations of the intensity. Adopting the HIPPARCOS parallax we derived from the 1045nm-observation a photospheric radius (Rosseland tau_Ross=1 radius) of R Leo of 417 Rsol±97Rsol (19.2mas±2.0mas) indicating pulsation in the first-overtone mode. From JHKL photometry and the angular photospheric radius an effective temperature of 2590±180K at near maximum phase was obtained.

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