Water vapor and silicon monoxide maser observations in the protoplanetary nebula OH 231.8+4.2

Abstract

Context. OH 231.8+4.2 is a well studied preplanetary nebula (pPN) around a binary stellar system that shows a remarkable bipolar outflow. Aims. To study the structure and kinematics of the inner 10-80 AU nebular regions probed by SiO and H 2 O maser emission, where the agents of wind collimation are expected to operate, in order to gain insights into the, yet poorly known, processes responsible for the shaping of bipolar pPNe. Methods. We performed high-resolution observations of the H 2 O 6 1,6 -5 2,3 and 28 SiO v = 2, J = 1-0 maser emissions with the Very Long Baseline Array. The absolute position of both emission distributions were recovered using the phase referencing technique, and accurately registered in HST optical images. Results. Maps of both masers were produced and compared. H 2 O maser clumps are found to be distributed in two areas of 20 mas in size spatially displaced by ∼60 milli-arcs along an axis oriented nearly north-south. SiO masers are tentatively found to be placed between the two H 2 O maser emitting regions, probably indicating the position of the Mira component of the system. Conclusions. The SiO maser emission traces an inner equatorial component with a diameter of 12 AU, probably a disk rotating around the M-type star. Outwards, we detect in the H 2 O data a pair of polar caps, separated by 80 AU. We believe that the inner regions of the nebula probably have been altered by the presence of the companion, leading to an equator-to-pole density contrast that may explain the lack of H 2 O masers and strong SiO maser emission in the denser, equatorial regions.

VERA observations of SiO maser emission from R Aquarii

“…Therefore, we accurately and quantitatively confirm the conclusion by Desmurs et al (2000): both maser lines come from similar circumstellar regions, but their spots are only coincident in a small fraction of the cases. Sometimes (see e.g.…”

“…Previous similar results have been proposed by several authors from standard VLBI experiments not including absolute coordinate measurements (e.g. Desmurs et al 2000;Matsumoto et al 2008). But, because of the lack of absolute astrometry, the comparison of the positions of both lines in those observations was difficult and the results were uncertain and controversial.…”

“…However, some observational features remain controversial. For instance, the spots of the maser transitions v = 1 and v = 2 J = 1−0 often occupy similar regions, but are very rarely coincident (Desmurs et al 2000). This property is not understood well and has been used as an argument for radiative or collisional pumping mechanisms (see details in the extensive discussion by Desmurs et al 2000, and in Sect.…”

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

“…TX Cam; Kemball et al 2009). Based on previous observations of the SiO maser region of TX Cam (Kemball & Diamond 1997;Desmurs et al 2000), Kemball et al (2009) conclude that the tangential polarization morphology could be an inter cycle property of the SiO maser emission toward this star. Instead of tangential polarization morphology, the EVPA vectors could indicate a bipolar magnetic field morphology for the SiO maser region of OH 44.8-2.3 (Fig.…”

“…Cotton et al 2008;Desmurs et al 2007;Imai et al 2005), and as far as we can tell, no high-resolution VLBI observation has been performed to image SiO masers in OH/IR stars. These objects have larger CSEs and much longer periods up to 2000 days than do Mira variables (Herman & Habing 1985b).…”