Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...440..728i&link_type=abstract
Astrophysical Journal v.440, p.728
Astronomy and Astrophysics
Astronomy
15
Stars: Circumstellar Matter, Ism: Molecules, Masers, Polarization, Stars: Carbon
Scientific paper
We have investigated H12CN υ = 0 J = 1-0 emission from six optical carbon stars. All of them show unusual components of narrow velocity widths superposed on the broad component, except RY Dra. The spike pattern does not agree with that expected from the hyperfine structure of HCN in LTE. All the spikes show strong time variability in intensity, with timescales of the order of months in the case of W Ori and Y CVn. Linear polarization probably exists in the spikes of the two stars. We have also monitored Y CVn in the H13CN υ = 0 J = 1-0 line simultaneously with the H12CN. The prominent spike previously reported by us has been absent during the present observation period. Furthermore, there is no relation between the variations of the two emission lines.
We have concluded that the spike components found in W Ori and Y CVn are the ground state masers in the H12CN line. The same is probably true for the other three stars. Emission due to the F = 2-1 hyperfine component is most likely amplified. The masers are preferentially seen in carbon stars with relatively low rates of mass loss of around 10-7 Msun yr-1. Their frequency of occurrence is at most 50% among optical carbon stars which show a broad emission component and have a mid-infrared color 6f log (12F25/25F12) less than -0.77.
The broad components as well show some peculiar features. Brightness temperatures for all the six stars become extremely high if we adopt HCN envelope sizes found in the literature. UU Aur has shown a considerable change in the intensity as well as in the line shape. The anomalous enhancement in the bluer part in Y Tau may be related to an asymmetry in the innermost region of the envelope. Some modifications are needed in the current understanding of HCN emission from optical carbon stars. In addition, RY Dra seems to posses a very HCN-rich envelope compared to Y CVn and shows a blueshifted absorption-like dip which is unusual for optical carbon stars.
Izumiura Hideyuki
Tsuji Takashi
Ukita Nobuharu
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