Issue |
A&A
Volume 378, Number 2, November I 2001
|
|
---|---|---|
Page(s) | 522 - 538 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20011184 | |
Published online | 15 November 2001 |
Monitoring of long term behaviour of OH masers in semiregular variables: R Crt, W Hya and RT Vir*
1
Toruń Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87100 Toruń, Poland
2
ARPEGES, Observatoire de Paris, 92195 Meudon, France
3
GRAAL, Université de Montpellier II, 34095 Montpellier, France
Corresponding author: M. Szymczak, msz@astro.uni.torun.pl
Received:
25
June
2001
Accepted:
8
August
2001
We present and interpret the results of a long-term OH variability
study of three semiregular stars, one type a (SRa), W Hya, and two
type b (SRb), R Crt and RT Vir.
The 1665 and 1667 MHz OH masers of the three semiregulars were
observed at intervals in the period 1982 January-1995 December
using the Nançay radio telescope, and we searched for 1612 MHz
emission.
The OH maser profiles of the studied stars significantly deviated
from a standard double-peaked profile. The timescale of profile changes
in the two SRb stars R Crt and RT Vir was as short as a month.
The OH profiles of the SRa star W Hya were much more stable but since
November 1986 a very blue-shifted feature appeared at 1667 MHz.
Our phase-lag measurements suggest that this feature comes from
a detached OH shell of radius ~ cm.
Faint 1612 MHz emission was found in W Hya only.
Weak emission at velocities very close to the
systemic velocity usually appeared during some intervals of high maser
activity in R Crt and RT Vir and was almost always present in W Hya.
For R Crt we estimated that this tangential emission disappeared when
the kinetic temperature in the OH maser regions dropped below 150-200 K.
For a few features, line narrowing and re-broadening were observed
on timescales of 90-200 days. The linewidth was inversely proportional
to the peak flux density, suggesting unsaturated amplification.
Cyclic variations in the integrated flux density were observed in all
the three stars. The OH variability curves were generally characterised
by large amplitude (
) variations over 400-800 days
superimposed with 100-300-day variations of
.
Only the measured OH period of W Hya, of
days, was
in agreement with the optical period. The two SRb stars exhibited
multi-periodic OH variability including with two statistically
significant periods.
The behaviour of their red- and blue-shifted emission was less
correlated than in W Hya. The ratios of the flux densities at 1667 MHz
to that at 1665 MHz in all the three stars were about 2 at epochs of high
OH activity and usually increased during weak maser emission.
Long term behaviour of the OH masers from W Hya resembled that of standard
OH Miras, while that of R Crt and RT Vir suggested thin and
clumpy envelopes where unsaturated emission was sustained in some clouds.
Key words: masers / stars: variables: general / stars: AGB and post-AGB / stars: circumstellar matter / stars: individual: R Crt, RT Vir, W Hya
© ESO, 2001
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