Monthly Notices of the Royal Astronomical Society (Monthly Notices of the Royal Astronomical Society)

THE ROYAL ASTRONOMICAL SOCIETY (THE ROYAL ASTRONOMICAL SOCIETY)
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Time variations of H2O and SiO masers in the proto-Planetary Nebula OH 231.8+4.2

  • Jaeheon Kim (Shanghai Astronomical Observatory, Chinese Academy of Sciences) ;
  • S-H Cho (Korea Astronomy and Space Science Institute) ;
  • V Bujarrabal (Observatorio Astronómico Nacional (OAN-IGN)) ;
  • H Imai (Center for General Education, Institute for Comprehensive Education, Kagoshima University) ;
  • R Dodson (International Centre for Radio Astronomy Research, University of Western Australia) ;
  • D-H Yoon (Korea Astronomy and Space Science Institute) ;
  • B Zhang (Shanghai Astronomical Observatory, Chinese Academy of Sciences)
  • Received : 2018.12.25
  • Accepted : 2019.06.27
  • Published : 20190900
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Abstract

H2O (22 GHz) and SiO masers (43, 86, 129 GHz) in the bipolar proto-planetary nebula OH 231.8+4.2 were simultaneously monitored using the 21-m antennas of the Korean VLBI Network in 2009-2015. Both species exhibit periodic flux variations that correlate with the central star's optical light curve, with a phase delay of up to 0.15 for the maser flux variations with respect to the optical light curve. The flux densities of SiO v = 2, J = 1→0 and H2O masers decrease with time, implying that they may disappear in 10-20 yr. However, there seems to have been a transient episode of intense H2O maser emission around 2010. We also found a systematic behaviour in the velocity profiles of these masers. The velocities of the H2O maser components appear to be remarkably constant, suggesting ballistic motion for the bipolar outflow in this nebula. On the other hand, those of the SiO maser clumps show a systematic radial acceleration of the individual clumps, converging to the outflow velocity of the H2O maser clumps. Measuring the full widths at zero power of the detected lines, we estimated the expansion velocities of the compact bipolar outflow traced by H2O maser and SiO thermal line, and discussed the possibility of the expanding SiO maser region in the equatorial direction. All of our analyses support that the central host star of OH231.8 is close to the tip of the AGB phase and that the mass-loss rate recently started to decrease because of incipient post-AGB evolution.

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Acknowledgement

We are grateful to the anonymous referee of this paper for his/her helpful comments. The KVN observations are supported through the high-speed network connections among the KVN sites provided by the KREONET (Korea Research Environment Open Network), which is managed and operated by the KISTI (Korea Institute of Science and Technology Information). We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. This work has been supported by the Spanish MINECO, grant AYA2016-78994-P, and partly sponsored by the 100 Talents Project of the Chinese Academy of Sciences, the National Natural Science Foundation of China under grant 11673051.