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N. Fominn (Arecibo Observatory), Hong Tat, Yervant Terzian (Space Sciences, Cornell University), B. M. Lewis (Arecibo Observatory), Arecibo Collaboration, Space Sciences Collaboration
High-latitude OH/IR stars have a narrow range of progenitor mass and IR color, but a large 2-18\,km/s range of expansion velocities. While the OH and water masers are well known for the subset A of those in the Arecibo sky, with 0 < {\delta} < 38\deg, only a few previously had detected SiO masers. We report now on a systematic 43\,GHz survey for SiO masers using the Haystack antenna, to a 3{\sigma } sensitivity of {~}3\,Jy.
We detect 41 of the 115 \vert b \vert > 10\deg stars searched without extant detections; 22 of which are in A. Most of those with 25 {\mu }m fluxes, S(25) > 12\,Jy are detected, most with S(25) < 6\,Jy are not. The overall detection rate for A, after adding in previous detections, is 64% (51/80). Since the great majority of nondetections are weak IR sources, we conclude that most normal OH/IR stars host SiO masers.
Our nondetections are made more interesting though by inter-comparison with 22\,GHz results, where 90% of A have detected water masers. Joint water and SiO nondetections include the three proto planetary nebulae [PPN], 18095+2704, 19386+0153 & 20531+2905. This is expected as PPN no longer have much mass-loss. However, the only other SiO nondetections from A with S(25) > 12\,Jy (02547+1106, 20444+0540, 20547+0247) in fact have S(25) > 28\,Jy, which should make them easy SiO detections, are also WATER nondetections. These sources, unlike the PPN, have normal IR colors for high-latitude OH/IR stars. They may therefore be objects with fossil superwind shells surrounding a low mass-loss Mira, near the conclusion of its abrupt luminosity evolution after a thermal pulse. Their high 10% (3/30) frequency is additional evidence for the transient nature of superwind shells about these stars.