J/A+A/644/A45 R Crt and RT Vir Medicina spectra (Brand+, 2020)
Water vapour masers in long-period variable stars.
II. The semi-regular variables R Crt and RT Vir.
Brand J., Engels D., Winnberg A.
<Astron. Astrophys. 644, A45 (2020)>
=2020A&A...644A..45B 2020A&A...644A..45B (SIMBAD/NED BibCode)
ADC_Keywords: Masers ; Stars, variable ; Stars, giant ; Spectra, radio
Keywords: stars: AGB and post-AGB - masers
Abstract:
Water masers emitting at a radiofrequency of 22GHz are often found in
the circumstellar envelopes of evolved stars. We monitored the H2O
maser emission of a larger sample of evolved stars of different types
to study the maser properties as a function of stellar type.
We wish to understand the origin and evolution of the H2O masers in
circumstellar envelopes. In this paper, we take a closer look at R Crt
and RT Vir, two nearby (<250pc) semi-regular variable stars. The
findings complement our monitoring results for RX Boo and SV Peg, two
other semi-regular variable stars that we have discussed in a previous
paper.
Within the framework of the Medicina/Effelsberg H2O maser monitoring
programme, we observed the maser emission of R Crt and RT Vir for more
than two decades with single-dish telescopes. To get insights into the
distribution of maser spots in the circumstellar envelopes at
different times, to get an idea of their longevity, and, where
possible, to be able to link the phenomena seen in our observations to
maser locations within the envelopes, we collected interferometric
data for these stars, taken within the same period, from the
literature.
The H2O masers in R Crt and RT Vir exhibit brightness variations on
a variety of timescales. We confirm short-time variations of
individual features on timescales of months to up to 1.5 years, as
seen by previous monitoring programmes. Also decade-long variations of
the general brightness level, independent from individual features,
were seen in both stars. These long-term variations are attributed to
brightness variations occurring independently from each other in
selected velocity ranges and they are independent of the optical light
curve of the stars.
Expected drifts in velocity of individual features are usually masked
by the blending of other features with similar velocities. However, in
RT Vir, we found the exceptional case of a single feature with a
constant velocity over 7.5 years (<0.06km/s/yr).
We attribute the long-term brightness variations to the presence of
regions with higher-than-average density in the stellar wind and
hosting several clouds which emit maser radiation on short timescales.
These regions typically need ∼20 years to cross the H2O maser shell,
where the right conditions for exciting H2O masers are present.
Different clouds contained in such a region all move within a narrow
range of velocities, and so does their maser emission. This sometimes
gives the impression of longer-living features in single-dish spectra,
in spite of the short lifetimes of the individual components that lie
at their origin, thus, naturally explaining the longer timescales
observed. The constant velocity feature (11km/s) is likely to come
from a single maser cloud, which moved through about half of RT Vir's
H2O maser shell without changing its velocity. From this, we infer
that its path was located in the outer part of the H2O maser shell,
where RT Vir's stellar wind has, apparently, already reached its
terminal outflow velocity. This conclusion is independently
corroborated by the observation that the highest H2O maser outflow
velocity in RT Vir approaches the terminal outflow velocity, as given
by OH and CO observations. This is generally not observed in other
semi-regular variable stars. All four stars in our study are of
optical variability type SRb, indicating the absence of periodic
large-amplitude variations. Therefore, any likely responses of the
maser brightness to variations of the optical emission are masked by
the strong short-term maser fluctuations.
Description:
The FITS files are the spectra of the 22GHz water maser emission of
R Crt and RT Vir as shown in Figs. 13 and 14 in the paper. Flux
density versus Vlos (or frequency).
The observations were made with the 32-m Medicina antenna and the
100-m telescope at Effelsberg.
The spectra were originally in CLASS-format and were transformed to
FITS via the command 'fits write name.fits /mode spectrum' in CLASS.
CLASS is part of the Grenoble GILDAS software package.
The file names consist of the name of the star, followed by a running
number, and finally by the TJD (truncated Julian Date,
TJD=JD-2440000.5).
Objects:
--------------------------------------------
RA (2000) DE Designation(s)
--------------------------------------------
11 00 33.85 -18 19 29.6 R Crt = V* R Crt
13 02 37.98 +05 11 08.4 RT Vir = V* RT Vir
--------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 143 239 List of fits spectra
fits/* . 239 Individual fits spectra
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See also:
J/PASJ/53/517 : Water maser survey of late-type stars (Takaba+, 2001)
J/A+A/578/A119 : Water maser emission toward post-AGB and PN (Gomez+, 2015)
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 A6 --- Name Star name (R Crt or RV Vir)
8- 16 F9.5 deg RAdeg Right Ascension of center (J2000)
17- 25 F9.5 deg DEdeg Declination of center (J2000)
27- 29 I3 --- Nx Number of pixels along X-axis
31- 53 A23 "datime" Obs.date Observation date
55- 66 E12.7 Hz bFrep Lower value of relative frequency
68- 79 E12.7 Hz BFrep Upper value of relative frequency
81- 91 E11.6 Hz dFrep Frequency resolution
93- 94 I2 Kibyte size Size of FITS file
96-115 A20 --- FileName Name of FITS file, in subdirectory fits
117-143 A27 --- Title Title of the FITS file
--------------------------------------------------------------------------------
Acknowledgements:
Jan Brand, brand(at)ira.inaf.it
References:
Winnberg et al., Paper I 2008A&A...482..831W 2008A&A...482..831W (RX Boo and SV Peg)
(End) Patricia Vannier [CDS] 02-Nov-2020