Lifetime of OH masers at the tip of the asymptotic giant branch
aa r X i v : . [ a s t r o - ph ] O c t Astronomy&Astrophysicsmanuscript no. 8250text c (cid:13)
ESO 2018October 23, 2018
Lifetime of OH masers at the tip of the asymptotic giant branch
D. Engels and F. Jim´enez-Esteban ,⋆ Hamburger Sternwarte, Gojenbergsweg 112, D–21029 Hamburg, Germanye-mail: [email protected]
Received ???; accepted ???
ABSTRACT
Context.
A large fraction of otherwise similar asymptotic giant branch stars (AGB) do not show OH maser emission. As shownrecently, 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 havesince lost their masers.
Results.
We redetected all 114 OH masers. The minimum lifetime inferred is 2800 years (1 σ ). This maser lifetime applies to AGBstars with strong mass loss leading to very red infrared colors. The velocities and mean flux density levels have not changed sincetheir discovery. As the minimum lifetime is of the same order as the wind crossing time, strong variations in the mass-loss processa ff ecting the excitation conditions on timescales of ≈ Key words.
OH masers – Stars: AGB and post-AGB – circumstellar matter
1. Introduction
Molecular OH maser emission at 1612 MHz is frequently ob-served from stars approaching the tip of their evolutionary trackon the asymptotic giant branch (AGB). These stars are in gen-eral long-period variables with pulsation periods > ≈ − M ⊙ yr − the dust envelope becomesopaque for visible light. Classically these ‘invisible’ stars werecalled OH / IR stars, but nowadays this term is often used in gen-eral reference to OH emitting AGB stars selected in the infrared(see review by Habing 1996).Interestingly, a large fraction ( ≈ / IRstars, does not exhibit a detectable OH 1612 MHz maser (Lewis1992). These infrared sources were baptized by Lewis as ‘OH / IRstar color mimics’. Some of them do exhibit mainline OH (at1665 or 1667 MHz) and / or 22 GHz H O masers (Lewis &Engels 1995), corroborating their cousinhood with OH / IR stars.OH / IR stars are therefore only part of the population of evolvedand obscured AGB stars with oxygen-rich chemistry.The reasons for the absence of 1612 MHz OH masers ina large fraction of oxygen-rich AGB stars are not known. Themaser photons are emitted by a transition between hyperfine lev-els of the groundstate of OH, which are inverted with the helpof pump photons at λ =
35 and 53 µ m, emitted from dust in theCSEs. A requirement for excitation is su ffi cient OH column den-sity, which might be low in mimics due to the destruction of OHby the interstellar UV field. This e ff ect cannot account for mim-ics in general, as mimics are also present at higher galactic lati-tudes, where the influence of the UV field is low. In addition, the Send o ff print requests to : D. Engels ⋆ Present address: Observatorio Astronomico Nacional, Apartado112, E–28803 Alcala de Henares, Spain presence of a hot white dwarf companion leading to photodis-sociation of OH is not generally able to suppress the OH maser(Howe & Rawlings 1994). A further requirement is velocity co-herence over large distances to allow amplification. Turbulencemay disrupt this coherence in the case of mimics.An alternative explanation is the assumption that the OHmaser is only present temporarily on the AGB and thereforethese stars may change between an OH / IR status and that of amimic. This explanation has been triggered by recent observa-tions showing the fading of the OH maser in IRAS 18455 + / IR stars12 years after their first detection (Lewis 2002). The high rateof ‘dead’ OH / IR stars among a sub-sample of 112 OH / IR starswith relatively blue colors led Lewis to conclude that the mean1612 MHz emission life is in the range 100–400 years.To test this lifetime we reobserved another sample of N >
100 OH / IR stars, which was drawn from the first surveys forOH maser emission prior to 1980. With a di ff erence of almost30 years between the two observations several masers were ex-pected to have disappeared.
2. Observations
The sample of OH / IR stars was mainly taken from the compi-lation of Baud et al. (1981), which contains N =
114 OH masersources located at 10 ◦ ≤ l ≤ ◦ , | b |≤ . ◦ . Their source,OH 57.5 + + =
116 objects and is listed in Table 1. Thecompilation of Baud et al. contains only masers that were dis-covered before 1978.To ensure that a non-detection would not be caused by inac-curate coordinates, we compiled the literature for the best avail-
D. Engels and F. Jim´enez-Esteban: Lifetime of OH masers at the tip of the AGB able radio coordinates and coordinates for their infrared counter-parts. The original coordinates could have been wrong by severalarc minutes, and not for all sources improved radio coordinateswere later obtained by follow-up observations. Infrared counter-parts (IRAS, MSX) were found for N =
113 OH maser sources,while there are no convincing counterparts for OH 18.3 + + − + + = . ′ + + . ′
3. OH 42.8 − ≈ ′ . IRAS 19108 + . ′
4. The IRAS source is weak andwas not detected by MSX. We also suspected that OH 39.6 + − / IR stars. Nevertheless, all threesources were reobserved.The observations were made during two nights 2005, August2-4 with the E ff elsberg radio telescope. As frontend we used the1.3-1.7 GHz HEMT receiver and as backend an 8192 channelautocorrelator. The correlator was split into four segments, ofwhich three were centered on the two OH main line frequen-cies 1665.401 and 1667.359 MHz and one on the OH satel-lite line frequency 1612.231 MHz. The fourth segment was notused because of technical problems. In this paper we focus onthe results of the 1612 MHz observations, for which the re-ceiver passed left circular polarization. We chose a bandwidthof 1.25 MHz, yielding a velocity resolution of 0.11 km s − anda velocity coverage of ± − , centered on the mean ve-locity of the two OH maser lines, as given by Baud et al. (1981).The beamwidth was 7 . ′
8. The coordinates to which the telescopewas pointed are given in Table 1. If not otherwise stated, theywere taken from the MSX6C catalog. The errors of all coordi-nates are smaller than a few arcsec, and therefore much smallerthan the beamwidth. System temperatures were ≈
25 K at zenith.We observed in position switch mode with an integration timeof 6 minutes (ON + OFF), yielding a typical sensitivity of 0.25 Jy(3 σ ). Fig. 1.
Deviations of measured radial velocities v ⋆ of the OH / IRstars observed from radial velocities v lit given by Baud et al.(1981). The data reduction was made within CLASS and includedremoval of the baseline and flux calibration. The calibrationwas made against 3C286 and 3C48 adopting flux densities of13 . ± .
05 Jy and 14 . ± .
05 Jy, respectively (Ott et al. 1994).No gain curve was applied as the flux calibrators were observedseveral times during the nights and showed no elevation depen-dent variations over the 20 − ◦ elevation range observed. Fig. 2. −
3. Results
We recovered the masers for all 113 OH maser sources with in-frared counterparts. In general, velocities and flux densities werecompatible with the values given by Baud et al. (1981). Table 1lists the ends v b and v r of the full velocity range, over whichemission was detected, and the integrated flux densities SI b andSI r . We adopted the midpoint of the velocity range v b − v r as ra-dial velocity v ⋆ of the star. The integrated flux densities SI b andSI r were determined by integrating the calibrated spectra overthe two halves of the velocity range v ⋆ − v b and v r − v ⋆ .Figure 1 shows a comparison of the velocities between ourdata and the values listed by Baud et al. (1981). Within the errors( ± − ) there is good agreement between the velocities. In afew sources, weak emission was found outside the main peaks,which led to radial velocities deviating by as much as 7 km s − from the previous values. OH 24.7 − − − ≈
111 km s − was clearly detected, while amarginal feature at ≈
142 was considered as the second line. Thenew spectrum shows that the second line is at the lower velocityof 62 km s − . In OH 20.4 − ∗ becausethe blue emission peak was corrupted by interference.Figure 3 shows a comparison of the integrated flux densities.The scatter is largely within the margins expected from the largeamplitude variations due to the pulsations of the star. Typicalvariations have a factor of about 2.5 (Herman & Habing 1985).Mixed results were obtained for the three maser sourceswithout IRAS / MSX counterparts. For OH 18.3 + − (3 Jy), which possibly belongs to theinterstellar maser line at +
11 km s − , discussed by Baud et al.(1979). For OH 39.6 + ′ x15 ′ region around the position of the two nearest IRAS sources, afterno OH masers were detected at the IRAS positions themselves. . Engels and F. Jim´enez-Esteban: Lifetime of OH masers at the tip of the AGB 3 Fig. 3.
Comparison of measured integrated flux densities SI = SI b + SI r of the OH / IR stars observed with flux densities S lit givenby Baud et al. (1981). The units are 10 − Watt m − . The dashedlines delimit the deviations expected due to flux variations by afactor 2.5.No maser could be found down to 0.8 Jy (3 σ ). OH 42.8 − α =
19h 13m 36.4s( ± δ = + ◦ ′ ′′ ( ± ′ ). The position is still too coarseto allow an unambiguous identification with an infrared source.The most likely candidate is IRAS 19112 + ′ away. Fig. 4. − / IRstars of Baud et al. (1981), that we did not redetect at all, or notat the expected velocities, are OH 18.3 + + / IR stars, while for the third, OH 42.8 − / IR star is retained. We further summarize that allOH / IR stars in the sample (N = + OH 42.8 − ∆ t =
28 years as the minimum time passedbetween discovery and redetection in 2005.
4. Lifetime of OH Masers
The restrictions on the lifetime of OH masers following from theredetection of all bona-fide OH / IR stars, can be accessed usinga basic law of combinatorics.We will assume that stars enter at random times into thephase where they support maser emission. We assume also thatthe mean lifetime of an OH maser in a CSE is T years, and aknown OH maser is revisited after ∆ t years and has disappeared.Assuming ∆ t < T , the probability to disappear is ∆ t / T and theprobability to detect a maser again is (1 − ∆ t / T ).If a sample of n OH masers is revisited and all are redetected,the accumulated probability P n that no maser disappeared within ∆ t years is P n = − ∆ tT ! n (1)The probability to detect the disappearance of a single maseris P n = n · ∆ tT ! · − ∆ tT ! n − (2)where the factor n allows for the fact that any of the n masersmight have been a ff ected.More general, the probability to detect the disappearance of m masers among n stars after ∆ t years is P mn = n ! m !( n − m )! · ∆ tT ! m · − ∆ tT ! n − m (3)This general equation (a standard relation in combinatorics) in-cludes the case that no maser ( m =
0) disappeared, or that allof them are extinguished ( m = n ), e.g., the valid range for m is0 ≤ m ≤ n . Fig. 5.
Probabilities P mn that out of n =
114 OH masers, m maserswith a lifetime T will have disappeared after ∆ t =
28 years.In Fig. 5 we plot the probabilities derived from Eq. (3) forthe case n =
114 stars, m = − ∆ t = T up to 20 000 years. The curves tell us for T =
15 000 years, for example, that the probability to redetectall masers is 81%, and that there is a 17% chance of observ-ing one star with an extinguished maser. Even the non-detectionof two masers is non-negligible: 2%. For smaller lifetimes, say T < m ≤ D. Engels and F. Jim´enez-Esteban: Lifetime of OH masers at the tip of the AGB
Table 2.
Lower lifetime limits T min at di ff erent significance lev-els for 1612 MHz OH masers based on observations in thispaper. P mn is the probability for finding exactly m extinguishedmaser among n stars reobserved after ∆ t years. n m ∆ t T min P mn Comments[yr] [yr] [%]114 0 28 550 0.2 3 σ This paper114 0 28 1050 4.6 2 σ
114 0 28 2800 31.8 1 σ approach 100%, because now the disappearance of m > T in which the proba-bility of finding at least one maser extinguished exceeds 68.2%,can be excluded at the 1 σ level. The probability of finding atleast one extinguished maser is (1 − P ), which is di ff erentfrom the probability P (Eq. 2) for finding exactly one extin-guished maser. For our sample, probabilities (1 − P ) > . P < .
318 to detect an extinguished maser are obtained forlifetimes T < ∼ T min = σ level.Lower lifetime limits with higher significance levels are given inTable 2. Lifetimes of T <
400 years, as derived by Lewis (2002),have P < .
01 and can be excluded for our sample.
5. Discussion
The high rate of extinguished masers in the sample studied byLewis (2002) (Arecibo sample) is at odds with our results. Therate is also astonishing given that previous monitoring programsdid not witness similar maser luminosity fading processes overperiods of several years. On the other hand, a disappearance ofmasers in our sample would not have been too surprising. Forone part of the sample, drastic mass-loss variations due to theonset of a ‘superwind’ a few hundred years ago was postulated(Justtanont et al. 2006), and another part of the sample belongs tothe group of ‘non-variable OH / IR stars’, for which a loss of theirmasers within a couple of thousand years is predicted (Engels2002).
Before discussing possible causes due to the di ff erent sampleproperties, we will have a look at the restrictions on lifetimesfor the Arecibo sample using the equations of the previous sec-tion. Lewis (2002) concluded that after ∆ t =
14 years, from n =
112 stars, five masers will have disappeared. The af-fected stars are the IRAS objects 15060 + + + + + T = − , + + + =
314 years. We will ask here the probability of finding m or more extin-guished masers for a given lifetime. This allows the determina-tion of upper limits for the OH maser lifetimes in the Arecibosample at di ff erent significance levels. For the purpose of thiscalculation, we will use m = + ∆ t =
12 years.The probability of finding at least m masers extinguishedamong n stars after ∆ t years is Q mn = n X i = m P in (4)For m =
1, the case discussed in the previous section, one finds Q n = n X i = P in = n X i = P in − P n = − P n (5)The probability Q is plotted as a function of T in Fig. 6.This probability drops below 31.2% at T =
476 years, meaningthe exclusion of longer lifetimes at the 1 σ -level. Lifetimes T > ∼ σ ) (Table 3). Fig. 6.
Probabilities Q that out of n =
112 OH masers, m ≥ T will have disappeared after ∆ t = T max atthe 1 σ and 2 σ -level significance.Thus, the high rate of extinguished masers in the Arecibosample indeed points to rather short lifetimes, while in oursample, similar short lifetimes are unlikely. However, as Lewis(2002) already pointed out, the two samples cannot be compareddirectly, because the stars with extinguished masers mostly be-long to a population of OH / IR stars with low main-sequencemasses, blue IRAS colors, periods P <
700 days, and envelopeexpansion velocities v e <
12 km s − , which have to be distin-guished from the classical OH / IR stars observed here. The lat-ter have larger progenitor masses, redder IRAS colors, periods1000–2000 days, and v e >
12 km s − . The classical OH / IR starshave larger mass-loss rates and probably create CSEs, provid-ing an environment more favorable for OH maser emission. Thehigher stability of their masers may then be responsible for thelonger lifetimes compared to those hosted by the less dense en-velopes of the bluer OH / IR stars. . Engels and F. Jim´enez-Esteban: Lifetime of OH masers at the tip of the AGB 5
Table 3.
Upper lifetime limits T max at di ff erent significance lev-els for 1612 MHz OH masers, based on observations of theArecibo sample. Q mn is the probability for finding at least m ex-tinguished masers among n stars reobserved after ∆ t years. n m ∆ t T max Q mn Comments[yr] [yr] [%]112 4 12 2410 0.2 3 σ Arecibo112 4 12 1005 4.6 2 σ sample112 4 12 476 31.2 1 σ In line with our results, thus far, no extinction of an OH maserhas been reported from monitoring programs. The longest wascarried out over 15 years by Etoka & Le Squeren (2000), butcontained only 7 Mira variables, which are IRAS sources withblue colors. A larger number of OH / IR stars (n =
37) was coveredby van Langevelde et al. (1993), however the monitoring timewas only 3 years. 1 σ lower limits T min ≈
100 years for thesesamples do not give useful lifetime constraints. A larger sam-ple containing 60 OH / IR stars was monitored over 10 years byHerman & Habing (1985) and van Langevelde et al. (1990), butit is a subsample of the present one studied here, and thereforegives no independent information.
Drastic changes of the excitation conditions for OH masers ontimescales of hundreds of years is implied by the two windregime, invoked by Justtanont et al. (1996) to model the CSEof the classical OH / IR star OH 26.5 + / IR star and part of our sample. Their model consistedof an inner ‘superwind’ and an outer more tenuous AGB wind(hosting the OH maser). The transition to the 550 times highermass-loss rates of the ‘superwind’ lasted less than 150 yearsand started ≈
200 years ago. Support for a generalization of thismodel to OH / IR stars has commonly been obtained from obser-vations of water-ice (Justtanont et al. 2006). Water-ice is formedin the outer envelopes at radii > cm, and the di ff erence be-tween OH / IR stars with or without a water-ice absorption bandat 3.1 µ m might be related to the question, if the high dust den-sities of the ‘superwind’ have already reached this region or not.Given the short timescales involved, one expects that, at least inone star, the transition region between both winds has passed itsOH maser shell during the last 30 years. However, no evidencecomes from the flux level of the OH masers or their velocitiesthat the excitation conditions of any of them (encompassing mostof the stars studied by Justtanont et al.) have been changed dueto a several hundred-fold increase of the density or the numberof pump photons in their surroundings. For part of our sample, a lifetime of the OH masers of less thana few thousand years is predicted due to the dissipation of theCSE during transition to the post-AGB phase. Stars in this phaseare distinguished by their non-variability and are called ‘non-variable OH / IR stars’, with non-variability defined as absenceof large-amplitude variations typical for Mira stars. The non-variable OH / IR stars are thought to have stopped their pulsa-tions and the associated strong mass loss, and have therefore de- veloped a hollow shell, which continues to expand. Inside, thedensities will have dropped sharply and it takes < ∼ − and a OH masershell radius of several 10 cm. The drop in density will ulti-mately extinguish the OH masers. Support for this scenario wasgiven by Engels (2002), who reported the disappearance of theH O maser of OH 17.7-2.0 (a present sample member) withina decade. This extinction of the maser was attributed to the ex-pected drop of densities in the H O maser zone, after which theinner boundary of the hollow shell passed. As H O masers inOH / IR stars are typically located ≈
10 times closer to the starthan OH masers, the extinction of the OH maser in OH 17.7-2.0is expected to happen in the coming centuries. Also, the dis-appearance of the OH maser in IRAS 18455 + ≈ / IRstars is >
820 years (1 σ ). The re-detection of all OH masers istherefore still compatible with the classification of these stars astransition objects. On the other hand, Gray et al. (2005) modeledthe decline of the OH maser emission after detachment of theCSE and found that the masers disappear even before the innerborder of the detached envelope has reached the OH maser shell,because they depend on pumping photons that emerge from thedust further inside. Therefore, the decay of the masers starts im-mediately after detachment and is finished in their models within <
100 years. To maintain the classification of the non-variableOH / IR stars as post-AGB stars, it is therefore mandatory to as-sume that the mass loss does not stop abruptly with the cessationof the large-amplitude variations at the end of the AGB evolu-tion. The mass-loss rates more likely decline gradually over atime range of several thousand years.
Because of the rarity of masers in the post-AGB phase, masersin general will disappear with the end of AGB evolution. TheOH / IR stars with bluer envelopes are however genuine AGBstars and therefore, the disappearance of their masers requiresa di ff erent explanation. The traditional assumption is that thetimescales for which mass-loss rates change in response to evo-lutionary changes in the AGB (the intermittent thermal pulses)are much longer than the wind crossing times of a few thou-sand years through the CSE. Prior to departure from the AGB,there are only the phases during and after a thermal pulse inwhich shorter timescales are prevalent. Lewis (2002) thereforeassociates the observed short lifetimes of the OH masers in thelower-mass stars with a brief evolutionary phase after thermalpulses of duration ≈
500 years, when mass-loss rates are su ffi -ciently high to drive a wind able to host masers. The decline ofthe masers would then be a result of rapidly declining mass-lossrates.We observed the infrared counterparts of most OH masersin the Arecibo sample (Jim´enez-Esteban et al. 2005) and mon-itored them for several years, including the five stars discussedby Lewis (2002). IRAS 18455 + + = U Equ) is also non-variable; thisis known as a peculiar star surrounded by an edge-on disk or
D. Engels and F. Jim´enez-Esteban: Lifetime of OH masers at the tip of the AGB torus (Barnbaum et al. 1996), instead of a radial symmetric CSE.Thus, this star may not be representative of OH / IR stars in gen-eral. IRAS 15060 + + + M ⊙ star spends on the thermal pulsing AGB is of the order of 5 · years (Vassiliadis & Wood 1993), with at least 10% of this timehaving su ffi ciently high mass-loss rates to be able to sustain amaser. With a OH maser detection rate of 60% (Lewis 1992)among IRAS selected AGB star samples, the expected lifetimesof OH masers are > ∼
30 000 years, and therefore it is unlikelythat the (temporary) disappearance of the masers in the bluer en-velopes is linked to evolution. The reappearance of the OH maserin IRAS 19479 + ff ect.The lack of evidence for major changes of the infrared prop-erties 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 asalternatives. One might envisage modulations of the mass-lossprocess on timescales of hundreds of years, which might su ffi ceto a ff ect the excitation conditions significantly. Such modula-tions of the envelope structure were found in studies of dust scat-tered light at distances up to > ∼ cm from the stars (Mauron& Huggins 2006). At such a range of distances, the history ofthe mass-loss process can be studied over ≈
10 000 years. Theyfind in the case of IRC + ff erent lifetimes de-rived from the Arecibo sample and from the sample studiedhere, point to a di ff erent susceptibility of their masers to the in-ferred changes of the excitation conditions. The masers in clas-sical OH / IR stars, as studied in this paper, would be more robustagainst extinction than the ones in bluer envelopes. The inabil-ity to distinguish OH / IR stars and ‘OH / IR star mimics’ by waysother than by their masers would then be explained by the in-stability of the OH masers, which turn on or o ff in response tovariations of the envelope structure. The timescales of such vari-ations would be of the order of < ∼ > ∼ / IRstars. Any sample of oxygen-rich AGB stars would then show,at di ff erent times, a di ff erent set of stars exhibiting OH maseremission.
6. Conclusions
We find that the lifetime of OH masers in classical OH / IR starsis > / IR stars are intransition to the post-AGB stage, a sudden decline of the mass-loss rates at the end of AGB evolution is ruled out. The previ-ous observed disappearance of OH masers in stars with bluerenvelopes is probably not associated with major changes in themass-loss process. The susceptibility of masers to smaller vari-ances in their environment may lead to OH masers as transientphenomena on the AGB. This gives a natural explanation for thedetection of OH masers in only a part of any AGB star samplewith otherwise similar properties.
Acknowledgements.
We thank B.M. Lewis for information on the most recentobservations of the OH maser emission in several ‘dead OH / IR stars’. The com-ments of the referee H. Habing are acknowledged. This research has made use ofthe SIMBAD database, operated at CDS, Strasbourg, France. The observationswere made with the E ff elsberg 100-m telescope operated by the Max-Planck-Institut f¨ur Radioastronomie (MPIfR). References
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Table 1. OH / IR star sample and 1612 MHz OH maser properties.Coordinates: a) Bowers & de Jong (1983); b) IRAS; c) Bowers et al. (1981); d) this paper; otherwise MSX6C.
Object Coordinates (2000) v b SI b v r SI r Comments[km s − ] [Jy*km s − ] [km s − ] [Jy*km s − ]OH 10.5 + a
17 51 48.8 −
17 42 30 − − + −
18 41 10 111.2 1.7 145.2 0.7OH 11.5 + −
18 52 57 10.1 23.1 75.1 130.9OH 12.3 − −
18 27 49 21.4 9.7 51.4 11.8OH 12.8 − −
18 15 02 − − + −
17 26 34 15.1 3.6 37.1 0.8OH 12.8 − −
18 47 11 − + −
15 03 42 − − + −
16 55 31 − − −
15 34 39 − − + −
14 39 54 − + −
14 55 15 − − −
15 03 22 − + −
14 16 19 28.3 1.0 66.5 0.7OH 17.0 − −
14 10 35 32.2 6.6 68.1 4.9OH 17.1 − −
14 42 26 − − −
14 28 19 157.2 1.3 184.2 1.7OH 17.4 − −
13 55 50 10.5 12.0 47.2 6.9OH 17.7 − −
14 28 58 46.4 52.2 75.1 131.0OH 18.2 + −
12 55 22 111.9 1.8 138.4 2.4OH 18.3 + a
18 23 50.8 −
12 50 38 9.8 – – – interstellarOH 18.3 + −
12 47 42 31.3 8.5 64.3 10.3OH 18.5 + −
12 08 09 164.6 10.4 187.8 9.4OH 18.7 + −
11 53 00 − + −
12 26 14 − − −
12 37 55 29.9 8.4 68.3 12.7OH 19.5 + −
10 02 50 28.5 3.3 62.6 4.6OH 20.2 − −
11 16 10 8.5 10.2 44.3 8.8OH 20.3 − −
11 56 45 − − −
11 15 54 – – 60.3 –OH 20.4 + −
10 27 21 19.4 0.8 57.0 0.7OH 20.6 + −
10 46 54 69.5 2.6 112.2 2.7OH 20.7 + −
10 50 52 116.9 20.3 155.7 14.2OH 20.8 − −
11 09 54 30.3 0.9 64.9 2.6OH 20.8 + −
09 18 31 11.2 3.7 41.5 4.4OH 21.5 + −
09 58 15 95.2 24.6 135.6 18.0OH 21.9 + −
09 39 50 65.8 4.8 105.9 3.6OH 22.1 − −
09 57 36 102.4 6.9 131.1 3.1OH 22.3 − −
10 42 35 97.5 1.2 126.9 1.4OH 23.1 − −
08 58 02 18.7 12.8 50.6 11.7OH 23.7 + −
07 36 50 − + −
08 04 01 93.3 3.5 118.0 9.0OH 23.8 − −
08 41 12 32.4 2.9 66.8 7.8OH 24.3 + −
07 27 58 39.3 1.8 75.8 2.4OH 24.5 + −
07 19 23 − − − −
07 18 17 60.3 5.3 110.8 7.7OH 24.7 + −
07 18 57 39.3 5.1 73.3 4.8OH 24.7 + −
07 13 11 19.8 12.8 65.0 14.8OH 24.7 − −
08 05 00 77.0 3.1 106.3 3.1OH 25.1 − −
07 09 54 128.7 4.8 156.2 4.1OH 25.5 − −
06 44 50 8.4 8.3 77.0 14.7OH 25.5 + −
06 27 24 21.4 4.7 56.2 3.1OH 26.2 − −
06 15 01 48.4 23.9 95.3 20.3OH 26.3 + −
05 49 10 − − − −
06 40 33 13.9 9.6 44.9 17.3OH 26.4 − −
07 13 47 − − + −
05 23 59 10.5 139.4 43.5 237.9OH 27.0 − −
05 21 07 86.0 5.2 117.7 7.0
D. Engels and F. Jim´enez-Esteban: Lifetime of OH masers at the tip of the AGB
Table 1. continued.
Object Coordinates (2000) v b SI b v r SI r Comments[km s − ] [Jy*km s − ] [km s − ] [Jy*km s − ]OH 27.2 + −
05 02 39 71.6 1.5 113.4 2.0OH 27.3 + −
04 57 11 36.3 38.8 64.5 19.7OH 27.5 − −
05 09 18 92.1 7.1 121.2 9.1OH 27.8 − −
05 11 08 67.8 2.6 102.2 2.8OH 28.5 − −
03 55 55 92.9 13.8 122.0 13.8OH 28.7 − −
04 00 46 27.6 8.5 65.1 4.8OH 29.4 − −
03 29 31 106.1 17.0 140.1 16.3OH 30.1 − −
02 35 36 31.4 16.8 69.5 16.4OH 30.1 − −
02 50 29 76.8 65.8 121.2 79.9OH 30.7 + −
01 46 43 47.8 10.0 85.2 7.6OH 31.0 + −
01 45 11 26.7 0.2 41.7 12.9OH 31.0 − −
01 48 30 110.0 45.8 140.0 3.9OH 31.5 − −
01 17 02 19.4 3.3 50.9 2.8OH 31.7 − −
01 26 47 64.8 1.3 92.8 2.1OH 32.0 − −
01 03 52 54.0 24.5 98.2 13.8OH 32.1 + −
00 17 14 123.8 1.6 150.8 2.8OH 32.8 − −
00 14 11 42.9 55.8 78.5 26.8OH 33.4 − +
00 27 30 43.2 4.3 78.3 6.2OH 34.7 + +
01 56 19 10.2 4.2 44.6 5.1OH 34.9 + +
02 03 48 52.8 2.1 84.9 1.4OH 35.2 − +
00 48 51 21.3 21.8 73.8 15.8OH 35.6 − +
02 12 18 63.0 30.2 93.0 21.1OH 36.4 + +
03 06 53 82.2 3.3 122.4 2.9OH 36.9 + +
04 02 32 − − − +
03 20 16 73.0 10.7 103.9 9.1OH 37.7 − +
03 40 58 99.2 1.3 123.9 0.6OH 39.6 + +
06 21 05 no detection IRAS 18584 + + b
19 00 24.6 +
06 16 31 no detection IRAS 18579 + + +
06 42 55 2.1 75.6 38.0 129.7OH 39.9 − +
06 13 16 132.9 10.6 165.0 11.3OH 40.1 + +
07 30 28 26.0 1.1 67.8 1.5OH 42.3 − +
08 16 34 41.4 22.7 77.6 41.8OH 42.6 + +
08 37 49 34.1 5.6 71.8 4.8OH 42.8 − d
19 13 36.4 +
08 22 39 − − ≈ ′ OH 43.6 − +
09 18 24 58.8 1.6 86.3 1.4OH 43.8 + +
09 51 50 − − +
09 15 45 37.8 0.8 65.5 1.1OH 43.9 + +
10 14 33 32.4 2.0 67.4 1.5OH 44.8 − +
09 27 57 − − + +
11 10 35 16.5 14.9 53.7 14.2OH 51.8 − +
16 37 25 − + +
18 10 09 − − +
18 13 10 − + +
19 50 41 12.5 14.8 44.2 9.3OH 57.5 + +
22 35 17 − − + +
22 33 42 30.1 2.2 46.3 4.8OH 63.9 − +
27 07 45 − + +
29 13 01 − − − +
28 23 10 − − − +
29 04 07 − − − +
35 45 42 − + +
39 07 00 − − − +
40 06 58 − − +
42 48 12 − − + +
44 58 07 − − + +
59 51 20 − − + +
62 26 53 − − + c
03 25 08.4 +
65 32 07 − − + c
03 33 30.5 +
60 20 09 − − . Engels and F. Jim´enez-Esteban: Lifetime of OH masers at the tip of the AGB 9 List of Objects ‘IRAS 18455 + + + + − + + − + − − − − − + + − + − + + − + + + + + + + + + + ++