A&A 475, 941-948 (2007)
DOI: 10.1051/0004-6361:20078250
D. Engels -
F. Jiménez-Esteban
Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
Received 10 July 2007 / Accepted 20 September 2007
Abstract
Context. A large fraction of otherwise similar asymptotic giant branch stars (AGB) do not show OH maser emission. As shown recently, a restricted lifetime may give a natural explanation as to why only part of any sample emits maser emission at a given epoch.
Aims. We wish to probe the lifetime of 1612 MHz OH masers in circumstellar shells of AGB stars.
Methods. We reobserved a sample of OH/IR stars discovered more than 28 years ago to determine the number of stars that may have since lost their masers.
Results. We redetected all 114 OH masers. The minimum lifetime inferred is 2800 years (1). This maser lifetime applies to AGB stars with strong mass loss leading to very red infrared colors. The velocities and mean flux density levels have not changed since their discovery. As the minimum lifetime is of the same order as the wind crossing time, strong variations in the mass-loss process affecting the excitation conditions on timescales of
3000 years or less are unlikely.
Key words: stars: AGB and post-AGB - circumstellar matter - masers
Interestingly, a large fraction (40%) of any sample of IRAS
sources, selected to match the infrared colors of known OH/IR
stars, does not exhibit a detectable OH 1612 MHz maser (Lewis
1992). These infrared sources were baptized by Lewis as "OH/IR
star color mimics''. Some of them do exhibit mainline OH (at 1665 or
1667 MHz) and/or 22 GHz H2O masers (Lewis & Engels
1995), corroborating their cousinhood with OH/IR stars.
OH/IR stars are therefore only part of the population of evolved and
obscured AGB stars with oxygen-rich chemistry.
The reasons for the absence of 1612 MHz OH masers in a large fraction
of oxygen-rich AGB stars are not known. The maser photons are emitted
by a transition between hyperfine levels of the groundstate of OH,
which are inverted with the help of pump photons at
and
53
m, emitted from dust in the CSEs. A requirement for
excitation is sufficient OH column density, which might be low in
mimics due to the destruction of OH by the interstellar UV field. This
effect cannot account for mimics in general, as mimics are also
present at higher galactic latitudes, where the influence of the UV
field is low. In addition, the presence of a hot white dwarf companion
leading to photodissociation of OH is not generally able to suppress
the OH maser (Howe & Rawlings 1994). A further requirement
is velocity coherence over large distances to allow
amplification. Turbulence may disrupt this coherence in the case of
mimics.
An alternative explanation is the assumption that the OH maser is only present temporarily on the AGB and therefore these stars may change between an OH/IR status and that of a mimic. This explanation has been triggered by recent observations showing the fading of the OH maser in IRAS 18455+0448 over a time range of a decade (Lewis et al. 2001) and the absence of masers in four stars during a revisit of 328 OH/IR stars 12 years after their first detection (Lewis 2002). The high rate of "dead'' OH/IR stars among a sub-sample of 112 OH/IR stars with relatively blue colors led Lewis to conclude that the mean 1612 MHz emission life is in the range 100-400 years.
Table 1: OH/IR star sample and 1612 MHz OH maser properties.
To test this lifetime we reobserved another sample of N> 100 OH/IR stars, which was drawn from the first surveys for OH maser emission prior to 1980. With a difference of almost 30 years between the two observations several masers were expected to have disappeared.
To ensure that a non-detection would not be caused by inaccurate
coordinates, we compiled the literature for the best available radio
coordinates and coordinates for their infrared counterparts. The
original coordinates could have been wrong by several arc minutes, and
not for all sources improved radio coordinates were later obtained by
follow-up observations. Infrared counterparts (IRAS, MSX) were found
for N=113 OH maser sources, while there are no convincing counterparts
for OH 18.3+0.1, OH 39.6+0.9, and
OH 42.8-1.0. In the case of OH 18.3+0.1, the absence of an
infrared counterpart coincident with the accurate position given by
Bowers & de Jong (1983) confirms the suspicion of
Winnberg et al. (1981) that this maser is actually of
interstellar nature. OH 39.6+0.9 was discovered by Johansson et
al. (1977), which gave an error of rms =
for
its position. The source was never reobserved. The nearest IRAS
counterparts are IRAS 18578+0616 and 18584+0616,
with a distance of
.
OH 42.8-1.0 has also never
been reobserved since its discovery by Baud et
al. (1979). The positional error of the discovery position
is rms
3
.
IRAS 19108+0815 is the nearest IRAS
counterpart, with a distance of
.
The IRAS source is weak
and was not detected by MSX. We also suspected that OH 39.6+0.9 and
OH 42.8-1.0 are not OH/IR stars. Nevertheless, all three sources
were reobserved.
The observations were made during two nights 2005, August 2-4 with
the Effelsberg radio telescope. As frontend we used the 1.3-1.7 GHz
HEMT receiver and as backend an 8192 channel autocorrelator. The
correlator was split into four segments, of which three were centered
on the two OH main line frequencies 1665.401 and 1667.359 MHz and one
on the OH satellite line frequency 1612.231 MHz. The fourth segment
was not used because of technical problems. In this paper we focus on
the results of the 1612 MHz observations, for which the receiver
passed left circular polarization. We chose a bandwidth of 1.25 MHz,
yielding a velocity resolution of 0.11 km s-1 and a velocity coverage
of 112.5 km s-1, centered on the mean velocity of the two OH maser
lines, as given by Baud et al. (1981). The beamwidth was
.
The coordinates to which the telescope was pointed are
given in Table 1. If not otherwise stated, they were taken
from the MSX6C catalog. The errors of all coordinates are smaller than
a few arcsec, and therefore much smaller than the beamwidth. System
temperatures were
25 K at zenith. We observed in position
switch mode
with an integration time of 6 min (ON+OFF), yielding a typical
sensitivity of 0.25 Jy (3
).
The data reduction was made within CLASS and included removal of the
baseline and flux calibration. The calibration was made against 3C 286
and 3C 48 adopting flux densities of
Jy and
Jy, respectively (Ott et al. 1994). No gain
curve was applied as the flux calibrators were observed several times
during the nights and showed no elevation dependent variations over the
elevation range observed.
Figure 1 shows a comparison of the velocities between
our data and the values listed by Baud et al. (1981).
Within the errors (2 km s-1) there is good agreement between the
velocities. In a few sources, weak emission was found outside the main
peaks, which led to radial velocities deviating by as much as 7 km s-1 from the previous values. OH 24.7-0.1 and OH 20.4-0.3
are not included in the figure. In the case of
OH 24.7-0.1 (cf. Fig. 2), the velocities needed a
complete revision. In the discovery paper of this source (Johansson
et al. 1977), only the stronger of the two lines at
111 km s-1 was clearly detected, while a marginal feature at
142 was considered as the second line. The new spectrum shows
that the second line is at the lower velocity of 62 km s-1. In
OH 20.4-0.3 we could not measure v* because the blue
emission peak was corrupted by interference.
![]() |
Figure 1:
Deviations of measured radial velocities ![]() ![]() |
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Figure 2: 1612 MHz OH maser spectrum of OH 24.7-0.1. |
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![]() |
Figure 3:
Comparison of measured integrated flux densities
SI = SI
![]() ![]() |
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Figure 3 shows a comparison of the integrated flux densities. The scatter is largely within the margins expected from the large amplitude variations due to the pulsations of the star. Typical variations have a factor of about 2.5 (Herman & Habing 1985).
Mixed results were obtained for the three maser sources without
IRAS/MSX counterparts. For OH 18.3+0.1 we found a single line
at 9.8 km s-1 (3 Jy), which possibly belongs to the interstellar maser
line at +11 km s-1, discussed by Baud et al. (1979). For
OH 39.6+0.9 we searched for the maser in a 1515
region around the position of the two nearest IRAS
sources, after no OH masers were detected at the IRAS positions
themselves. No maser could be found down to 0.8 Jy
(3
). OH 42.8-1.0 was detected (Fig. 4). To improve the coordinates we tried to maximize the
signal. The improved position is
(
5.0s),
= +08
22
39
(
2
). The
position is still too coarse to allow an unambiguous identification
with an infrared source. The most likely candidate is
IRAS 19112+0816, which is approximately 1
away.
![]() |
Figure 4: 1612 MHz OH maser spectrum of OH 42.8-1.0. |
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To summarize, the only sources from the sample of OH/IR stars of Baud
et al. (1981), that we did not redetect at all, or not at
the expected velocities, are OH 18.3+0.1 and
OH 39.6+0.9. They are two of the three objects for which no
infrared counterpart was found. We therefore conclude that these two
sources are not OH/IR stars, while for the third,
OH 42.8-1.0, the classification as OH/IR star is
retained. We further summarize that all OH/IR stars in the sample (N=113 + OH 42.8-1.0) still possess an 1612 MHz OH maser. As these stars
were discovered prior to 1978, we adopt
years as the
minimum time passed between discovery and redetection in 2005.
We will assume that stars enter at random times into the phase where
they support maser emission. We assume also that the mean lifetime of
an OH maser in a CSE is T years, and a known OH maser is revisited
after
years and has disappeared. Assuming
,
the probability
to disappear is
and the probability to detect a
maser again
is
.
If a sample of n OH masers is revisited and all are redetected, the
accumulated probability P0n that no maser disappeared within
years is
![]() |
(1) |
More general, the probability to detect the disappearance of m masers among
n stars after
years is
![]() |
Figure 5:
Probabilities Pnm that out of n=114 OH masers, m masers with a lifetime T will
have disappeared after
![]() |
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In Fig. 5 we plot the probabilities derived from
Eq. (3) for the case n=114 stars, m=0-2 missing
masers, and
years for lifetimes T up to 20 000
years. The curves tell us for T=15 000 years, for example, that the
probability to redetect all masers is 81%, and that there is a 17%
chance of observing one star with an extinguished maser. Even
the non-detection of two masers is non-negligible: 2%. For smaller
lifetimes, say T<5000 years, the
cumulative probability for
does not approach
100%, because now the disappearance of m>2 masers is
not unlikely anymore.
Table 2:
Lower lifetime limits
at different significance levels
for 1612 MHz OH masers
based on observations in this paper. Pnm is the
probability for finding exactly m extinguished maser among
n stars reobserved after
years.
As expected, the probability to redetect all masers decreases
towards lower lifetimes.
There is no absolute lower lifetime limit, but it is possible
to define such limits in relation to significance levels.
For example, lifetimes T in which the probability of finding
at least one maser extinguished exceeds 68.2%, can be
excluded at the 1
level. The probability of finding
at least one extinguished maser is (
1 - P0114), which
is different from the probability P1141 (Eq. (2)) for finding exactly one extinguished maser.
For our sample, probabilities
(1 - P0114) >
0.682
or
P0114 < 0.318 to detect an extinguished maser are obtained
for lifetimes
years (Fig. 5).
As all masers were redetected,
years is a lower
limit of the lifetime at the 1
level. Lower lifetime limits with higher significance levels are given in
Table 2. Lifetimes of T<400 years, as derived by Lewis
(2002), have
P0114<0.01 and can be excluded for our
sample.
We will ask here the probability of finding m or more extinguished
masers for a given lifetime. This allows the determination of upper limits for
the OH maser lifetimes in the Arecibo sample at different significance
levels. For the purpose of this calculation, we will use m=4extinguished masers (omitting IRAS 15060+0947) and make an
adjustment to
years.
The probability of finding at least m masers extinguished among
n stars after
years is
![]() |
(5) |
![]() |
Figure 6:
Probabilities Q1124 that out of n=112 OH
masers, ![]() ![]() ![]() ![]() ![]() |
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Thus, the high rate of extinguished masers in the Arecibo sample
indeed points to rather short lifetimes, while in our sample,
similar short lifetimes are unlikely. However, as Lewis
(2002) already pointed out, the two samples cannot be
compared directly, because the stars with extinguished masers
mostly belong to a population of OH/IR stars with low main-sequence
masses, blue IRAS colors, periods P<700 days, and envelope
expansion velocities
km s-1, which have to be
distinguished from the classical OH/IR stars observed here.
The latter have larger progenitor masses, redder IRAS colors,
periods 1000-2000 days, and
km s-1. The classical OH/IR
stars have larger mass-loss rates and probably create CSEs,
providing an environment more favorable for OH maser emission. The
higher stability of their masers may then be responsible for the
longer lifetimes compared to those hosted by the less dense
envelopes of the bluer OH/IR stars.
Table 3:
Upper lifetime limits
at different significance levels
for 1612 MHz OH masers, based on
observations of the Arecibo sample. Qnm is the
probability for finding at least m extinguished masers among
n stars reobserved after
years.
Among our sample, 45 OH masers were monitored by Herman & Habing
(1985) and they found 13 (30%) of them with no or
very weak (irregular) variations. Assuming that this fraction of
non-variable objects is valid for our full sample, we find that the
lifetime of the OH masers in non-variable OH/IR stars is >820 years
(1
). The re-detection of all OH masers is therefore still
compatible with the classification of these stars as transition
objects. On the other hand, Gray et al. (2005) modeled the
decline of the OH maser emission after detachment of the CSE and found
that the masers disappear even before the inner border of the detached
envelope has reached the OH maser shell, because they depend on
pumping photons that emerge from the dust further inside. Therefore,
the decay of the masers starts immediately after detachment and is
finished in their models within <100 years. To maintain the
classification of the non-variable OH/IR stars as post-AGB stars, it
is therefore mandatory to assume that the mass loss does not stop
abruptly with the cessation of the large-amplitude variations at the
end of the AGB evolution. The mass-loss rates more likely decline
gradually over a time range of several thousand years.
We observed the infrared counterparts of most OH masers in the Arecibo
sample (Jiménez-Esteban et al. 2005) and monitored them
for several years, including the five stars discussed by Lewis
(2002). IRAS 18455+0448 is non-variable,
corroborating Lewis' conclusion that this star is already in the
post-AGB phase. IRAS 20547+0247 (=U Equ) is also
non-variable; this is known as a peculiar star surrounded by an
edge-on disk or torus (Barnbaum et al. 1996), instead of
a radial symmetric CSE. Thus, this star may not be representative of
OH/IR stars in general. IRAS 15060+0947, as discussed by
Lewis (2002), and the remaining two stars,
IRAS 19479+2111 and IRAS 19529+3634,
are large-amplitude variables. We did not find variations in the mean
magnitudes or colors during the monitor period 1999-2005 for any of
these stars, indicating that their mass-loss rates did not change
strongly. Thus, there is no independent evidence for a change in the
mass-loss process being a cause for the disappearance of the
masers. Furthermore, the typical time a 1
star spends on
the thermal pulsing AGB is of the order of
years
(Vassiliadis & Wood 1993), with at least 10% of this
time having sufficiently high mass-loss rates to be able to sustain a
maser. With a OH maser detection rate of 60% (Lewis 1992)
among IRAS selected AGB star samples, the expected lifetimes of OH
masers are
30 000 years, and therefore it is unlikely that the
(temporary) disappearance of the masers in the bluer envelopes is
linked to evolution. The reappearance of the OH maser in
IRAS 19479+2111 after a couple of years indicates that the
maser extinction in blue envelopes in general is not permanent, but
rather a temporary effect.
The lack of evidence for major changes of the infrared properties
precludes the lack of pumping photons as cause for the (temporary)
extinction of the OH masers in blue envelopes. Variations of density
or velocity coherence disruption remain as alternatives. One might
envisage modulations of the mass-loss process on timescales of
hundreds of years, which might suffice to affect the excitation
conditions significantly. Such modulations of the envelope structure
were found in studies of dust scattered light at distances up to
1021 cm from the stars (Mauron & Huggins 2006).
At such a range of distances, the history of the mass-loss process can be
studied over
10 000 years. They find in the case of
IRC+10216, a carbon star on the AGB, shells spaced at irregular
intervals corresponding to timescales of 200-800 years (Mauron &
Huggins 2000), showing the presence of modulation on the
required timescales. In hydrodynamical models for dust driven AGB
winds, Simis et al. (2001) found that such quasi-periodicity
develops naturally in the winds, if dust and gas are not perfectly
coupled. While the increase of densities probably associated with
the shells would improve the excitation conditions for OH masers,
an increase of turbulent motion would likely decrease the gain lengths.
If such modulations of the envelope structure are responsible for OH maser
extinction, then the different lifetimes derived from the Arecibo
sample and from the sample studied here, point to a different
susceptibility of their masers to the inferred changes of the
excitation conditions. The masers in classical OH/IR stars, as studied
in this paper, would be more robust against extinction than the ones
in bluer envelopes. The inability to distinguish OH/IR stars and
"OH/IR star mimics'' by ways other than by their masers would then be
explained by the instability of the OH masers, which turn on or off in
response to variations of the envelope structure. The timescales of such
variations would be of the order of 1000 years in the case of
stars with blue envelopes and
3000 years for the reddest OH/IR
stars. Any sample of oxygen-rich AGB stars would then show, at
different times, a different set of stars exhibiting OH maser emission.
Acknowledgements
We thank B. M. Lewis for information on the most recent observations of the OH maser emission in several ``dead OH/IR stars''. The comments of the referee H. Habing are acknowledged. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. The observations were made with the Effelsberg 100-m telescope operated by the Max-Planck-Institut für Radioastronomie (MPIfR).