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Register a new userHigh velocity water maser emission from the Post-AGB star OH 009.1-0.4
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Observations of H$_2$O masers towards the post-AGB star and water fountain source OH 009.1--0.4 were made as part of HOPS (The H$_2$O southern galactic Plane Survey), with the Mopra radiotelescope. Together with followup observations using the Australia Telescope Compact Array (ATCA), we have identified H$_2$O maser emission over a velocity spread of nearly 400km/s (--109 to +289km/s). This velocity spread appears to be the largest of any known maser source in our Galaxy. High resolution observations with the ATCA indicate the maser emission is confined to a region $0farcs3 times 0farcs3$ and shows weak evidence for a separation of the red- and blueshifted maser spots. We are unable to determine if the water fountain is projected along the line of sight, or is inclined, but either way OH 009.1--0.4 is an interesting source, worthy of followup observations.
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Water maser emission at 22 GHz is a useful probe to study the transition between the nearly spherical mass-loss in the AGB to a collimated one in the post-AGB phase. In their turn, collimated jets in the post-AGB phase could determine the shape of planetary nebulae (PNe) once photoionization starts. We intend to find new cases of post-AGB stars and PNe with water maser emission, including water fountains or water-maser-emitting PNe. We observed water maser emission in a sample of 133 objects, with a significant fraction being post-AGB and young PN candidate sources with strong obscuration. We detected this emission in 15 of them, of which seven are reported here for the first time. We identified three water fountain candidates: IRAS 17291-2147, with a total velocity spread of ~96 km/s in its water maser components and two sources (IRAS 17021-3109 and IRAS 17348-2906) that show water maser emission outside the velocity range covered by OH masers. We have also identified IRAS 17393-2727 as a possible new water-maser-emitting PN. The detection rate is higher in obscured objects (14%) than in those with optical counterparts (7%), consistent with previous results. Water maser emission seems to be common in objects that are bipolar in the near-IR (43% detection rate). The water maser spectra of water fountain candidates like IRAS 17291-2147 show significantly less maser components than others (e.g., IRAS 18113-2503). We speculate that most post-AGBs may show water maser emission with wide enough velocity spread (> 100 km/s) when observed with enough sensitivity and/or for long enough periods of time. Therefore, it may be necessary to single out a special group of water fountains, probably defined by their high maser luminosities. We also suggest that the presence of both water and OH masers in a PN is a better tracer of its youth, rather than the presence of just one of these species.
(Pseudo) radiative pumprate of OH 1612 MHz masers is defined for a sample of 44 OH/IR sources (infrared sources with OH 1612 MHz maser), irrespective of the real maser pumping mechanisms. The correlation between the (pseudo) maser pumprates and the evolutionary status of the maser sources reveals that the radiative pumprates of stellar OH masers are nearly fixed, which agrees with the theoretical prediction for radiatively pumped OH maser. The (pseudo) radiative pumprates of interstellar OH masers are not only very small but also varying broadly over two orders of magnitude, which is argued to be the manifestation of varying number of quiet absorbing OH cloudlets and/or various OH maser pumping mechanisms and/or competitive gain between mainline and 1612 MHz OH masers and/or anisotropy of the maser emission. The radiative pumprates of post-AGB OH masers very possibly decrease with increasing IRAS C32 color indices and distribute in an interim region between the stellar and interstellar OH masers in the pumprate-color diagram.
We intended to study the incidence and characteristics of water masers in the envelopes of stars in the post-AGB and PN evolutionary stages. We have used the 64-m antenna in Parkes (Australia) to search for water maser emission at 22 GHz, towards a sample of 74 sources with IRAS colours characteristic of post-AGB stars and PNe, at declination $< -32 deg$. In our sample, 39% of the sources are PNe or PNe candidates, and 50% are post-AGB stars or post-AGB candidates. We have detected four new water masers, all of them in optically obscured sources: three in PNe candidates (IRAS 12405-6219, IRAS 15103-5754, and IRAS 16333-4807); and one in a post-AGB candidate (IRAS 13500-6106). The PN candidate IRAS 15103-5754 has water fountain characteristics, and it could be the first PN of this class found. We confirm the tendency suggested in Paper I that the presence of water masers in the post-AGB phase is favoured in obscured sources with massive envelopes. We propose an evolutionary scenario for water masers in the post-AGB and PNe stages, in which ``water fountain masers could develop during post-AGB and early PN stages. Later PNe would show lower velocity maser emission, both along jets and close to the central objects, with only the central masers remaining in more evolved PNe.
We present results of a high resolution survey of OH masers in Galactic Star Forming Regions in order to study the maser emission and establish a list of suitable candidates for higher resolution instruments follow up. We used the Very Long Baseline Array (VLBA) to observe the 1665, 1667, 1612 and 1720 MHz OH maser transitions within 41 regions. These are the first high resolution observations for most of the sources. For all the transitions 30 sites of maser emission were detected, 4 of the sources have new detections, and approximately 40% of the sources in the sample exhibit highly compact structure. Finally we consider that the spectrum observed in W75N shows the early stage of a long period OH maser flare in the 1665 MHz line, the first of its kind.
Aims: To investigate the first high resolution optical spectrum of the B-type star, LS III +52 24, identified as the optical counterpart of the hot post-AGB candidate IRAS 22023+5249 (I22023). Methods: We carried out detailed identifications of the observed absorption and emission features in the high resolution spectrum (4290 - 9015 A) of I22023 obtained with the Utrecht Echelle Spectrograph on the 4.2m William Herschel Telescope. Using Kuruczs WIDTH9 program and the spectrum synthesis code, SYNSPEC, we determined the atmospheric parameters and abundances. The photospheric abundances were derived under the LTE approximation. The NEBULAR package under IRAF was used to estimate the electron temperature (T_e) and the electron density (N_e) from the [N II] and [S II] lines. Results: We estimated T_eff=24000 K, log g=3.0, xi_t=7 kms^{-1}. The derived CNO abundances suggest an evolved star with C/O < 1. P-Cygni profiles of hydrogen and helium indicate ongoing post-AGB mass loss. The presence of [N II] and [S II] lines and the non-detection of [O III] indicate that photoionisation has just started. The derived nebular parameters T_e=7000 K, N_e=1.2X10^{4} cm^{-3} are comparable to those measured in young, compact planetary nebulae (PNe). The nebular expansion velocity was estimated to be 17.5 kms^{-1}. Conclusions: The observed spectral features, large heliocentric radial velocity (-148.31 +/- 0.60 kms^{-1}), atmospheric parameters and chemical composition indicate that I22023, at a distance of 1.95 kpc, is an evolved post-AGB star belonging to the old disk population. The nebular parameters suggest that the central star may be evolving into a compact, young PN, similar to Hen3-1357.
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