V L B I S T U D Y OF T H E SIO M A S E R E M I S S I O N A T 43 G H Z

I N T H E C I R C U M S T E L L A R E N V E L O P E O F R C A S

FRANCISCO COLOMER* Centro Astrondmico de Yebes. Apartado 148. E - 19080 Guadalajara. Spain

Abstract. We discuss the European effort in the study of millimeter-wave circumstellar masers with very long baseline interferometry. We show some results of the first observa- tions, and discuss present and future work.

Key words: Stars: Radio Interferometry - - Masers: SiO -- stars: individual: R Cas

We have under taken the s tudy of the silicon monoxide (SiO) maser emis- sion in the circumstellar envelopes of l a te - type stars with European VLBI telescopes since 1990. We have obtained the first detection of compact SiO maser features with interferometric baselines up to 1750 kin, providing fringe spacings as small as 0~.r0004 (Colomer et aI. 1992).

We observed the Mira star R Cas for 6 hours with the Onsala-Effelsberg baseline. The fringe-rate map we obtain in Figure 1 is consistent with pre- vious observations of this source (Lane et aI. 1982; McIntosh et al. 1989). We clearly distinguish four regions of emission, distributed within 40 mas. Ampli tude visibilities for this source yield Gaussian sizes in the range 1 - 2 mas (4 to 8 • 1012 cm at 270 pc), demonstrat ing the existence of s tructure at milliaxcsecond level.

We have performed a new set of observations with 14 VLBI telescopes (the VLBA telescopes in the USA, Onsala, Effelsberg, Yebes, and the IRAM 30-m on top of Pico Veleta) to be processed soon at the VLBA correlator. We expect to produce high quality results thanks to the much improved u -v coverage obtained.

A c k n o w l e d g e m e n t s

This project is being developed in cooperation with D.Graham, T.Krichbaum, A.Witzel, R.Booth, B.RSnngng, P.de-Vicente, A.Barcia, J.G6mez-GonzLlez, V.Bujarrabal, J.Alcolea, A.Baudry, and N.Brouillet. It would not have been possible without the expertise and help of the staff crews at all participating observatories.

* Previously at Onsala Space Observatory. S - 439 92 Onsala. Sweden

Astrophysics and Space Science 224: 445-446, 1995. © 1995 Kluwer Academic Publishers. Printed in Belgium.

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Fig. 1. Fringe-rate map of the SiO masers in RCas.

References

Colomer, F., et al. 1992, AedA 254, L17. Lane, A.P., 1982, Observations of SiO masers in the circumsteUar envelopes o] late-type

stars, PhD Thesis, University of Massachusetts. McIntosh, G.C., Predmore, C.R., Moran, J.M., Greenhill, L.J., Rogers, A.E.E., Barvains,

R., 1989, Ap J 337, 934.