Variable Water Maser Line in ZCMa
ATel #177; M. Teodorani (IRA/CNR, Italy)
on 13 Aug 2003; 07:34 UT
Credential Certification: Massimo Teodorani, Ph.D. (mteodorani@ira.cnr.it)
Subjects: Radio, Variables
M. Teodorani, Istituto di Radioastronomia, Consiglio Nazionale delle Ricerche, Bologna, reports that in April 16, 2003 a strong 22-GHz water-maser line was detected in the FUor-type protostar Z Canis Majoris. The Medicina 32-m radio telescope was used in connection with a high-resolution spectrum analyzer (8192 channels, bandwidth = 8 MHz). The beam size was 2'. The measured antenna temperature was 396 ± 26 mK, giving an integrated flux of 16.46 ± 2.7 Jy km/s. The measured FHWM = 41 KHz, corresponds to a typical maser feature. The line shows a velocity shift of -13.38 km/s with respect to the systemic velocity (Vsys = +28 km/s). The absence of any doubling of the line peak, which rules out an origin of the maser region in the Keplerian accretion disk, and the negative velocity suggests that the line is formed in a shock front produced in the approaching part of the bipolar outflow (Teodorani et al., 1997, A.Ap.S., 126, 91). The water maser line shows to be strongly variable on a yearly time-scale. Previous observations carried out by the author in Medicina showed that in May 31 and June 10, 2002, when no substantial weekly variations were recorded, the average line integrated flux was 2.02 ± 0.11 Jy km/s and the velocity shift was -13.31 km/s. It is suggested that the drastic yearly flux variability is caused by a cyclical replenishment effect of matter in the bipolar outflow, driven by ejections from the core of the accretion disc. The steadiness of visual magnitude around 10.23 ± 0.2 mag in 2002 and 2003 (courtesy: AAVSO data) and the present low-luminosity phase of ZCMa, shows that variable matter ejection with the consequent formation of maser regions characterized by different density phases, occurs in the quiescent phases too.