Astron. Astrophys. 319, 607-616 (1997)

1. Introduction

SiO maser emission can be used to investigate the physical conditions in the innermost layers of the circumstellar shells around oxygen-rich late-type stars (see, e.g., Cernicharo et al. 1994 or the review by Baudry 1993). These regions are hidden in the thermal emission of molecular lines, while the maser nature of the SiO v =1 emission allows us to probe some characteristics of the immediate photospheric environment. Recently, Cernicharo et al. (1994) observed in R Leo a v =1, J =2 1 SiO emission at velocities larger than the terminal velocity () of the expanding circumstellar CO gas. Cernicharo et al. also made lunar occultation observations of the SiO maser emission in this object founding (i) a rather compact rotating structure corresponding to the bulk of the SiO line profile; and (ii) two extended lobes, or opposite regions in the plane of the sky, associated with the blue and red wing SiO emission near the terminal velocity of the circumstellar envelope. On the other hand, recent VLBI observations of the v =1, J =1 0 line of SiO have been made in the late-type stars U Her, TX Cam, W Hya, and VX Sgr (Diamond et al. 1994; Miyoshi et al. 1994; Greenhill et al. 1995). The SiO emission in the three first objects can be explained as arising from shell/ring-like structures. VX Sgr shows, however, an asymmetric distribution of the SiO emission which leads Greenhill et al. to suggest the existence of an asymmetry in the mass-loss or in the stellar atmosphere. The comparison of the regions where the SiO and the circumstellar dust emission arise indicates that the former originates between the star photosphere and the outer edge of the shell where dust is formed.

Although single-dish telescope observations of SiO maser emission lack the angular resolution of VLBI, they offer a sensitivity large enough to reveal details of the emission at high velocities, and hence of the velocity field in the innermost layers of the circumstellar envelope. These regions are of particular interest as dust is formed there and the velocity field is a tracer of the dynamical processes occurring in the region prior to the acceleration of the molecular envelope. In this paper we present sensitive observations of the v=1 J=2-1 SiO maser emission toward 37 O-rich evolved stars, together with the thermal emission of the J =2 1 line of CO. The objects have been selected to cover a large variety of long period variables, including Miras, semiregulars, supergiants, and OH/IR stars. They are listed in Table 1.

Table 1. Stellar sample and observed line parameters for January 1994 observations

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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