Search for radiative pumping lines of OH masers: I. The 34.6um absorption line towards 1612 MHz OH maser sources

The 1612 MHz hydroxyl maser in circumstellar envelopes has long been thought to be pumped by 34.6um photons. Only recently, the Infrared Space Observatory has made possible spectroscopic observations which enable the direct confirmation of this pumping mechanism in a few cases. To look for the presence of this pumping line, we have searched the Infrared Space Observatory Data Archive and found 178 spectra with data around 34.6um for 87 galactic 1612MHz masers. The analysis performed showed that the noise level and the spectral resolution of the spectra are the most important factors affecting the detection of the 34.6um absorption line. Only 5 objects from the sample (3 red supergiants and 2 galactic center sources) are found to show clear 34.6um absorption (all of them already known) while two additional objects only tentatively show this line. The 3 supergiants show similar pump rates and their masers might be purely radiatively pumped. The pump rates of OH masers in late type stars are found to be about 0.05, only 1/5 of the theoretical value of 0.25 derived by Elitzur (1992). We have also found 16 maser sources which, according to the analysis assuming Elitzur's pump rate, should show the 34.6 $\mu$m absorption line but do not. These non-detections can be tentatively explained by far-infrared photon pumping, clumpy nature of the OH masing region or a limb-filling emission effect in the OH shell.Comment: 11 pages, 8 figures, 3 table

A method for comparing discrete kinematic data and N-body simulations

This paper describes a method for quantitatively comparing an N-body model with a sample of discrete kinematic data. The comparison has two stages: (i) finding the optimum scaling and orientation of the model relative to the data; and (ii) calculating a goodness of fit, and hence assessing the plausibility of the model in vew of the data. The method derives from considering the data and model both as samples from some underlying binned distribution function, and applying probability theory arguments. As an example, I consider a published N-body model for the Galactic bulge and disc, and fictitious l,b,v measurements, and recover (with error estimates) the spatial and velocity scales of the model and the orientation of the bar. The fictitious data are actually derived from the model by assuming the mass scale and the solar position, but their size and extent mimics a recent survey of OH/IR stars. The results indicate that mass of the bulge and our viewing angle of the bar are usefully estimable from current surveys.Comment: To appear in A

OH 12.8-0.9: A New Water-Fountain Source

We present observational evidence that the OH/IR star OH 12.8-0.9 is the fourth in a class of objects previously dubbed "water-fountain" sources. Using the Very Long Baseline Array, we produced the first images of the water maser emission associated with OH 12.8-0.9. We find that the masers are located in two compact regions with an angular separation of ~109 mas on the sky. The axis of separation between the two maser regions is at a position angle of 1.5 deg. East of North with the blue-shifted (-80.5 to -85.5 km/s) masers located to the North and the red-shifted (-32.0 to -35.5 km/s) masers to the South. In addition, we find that the blue- and red-shifted masers are distributed along arc-like structures ~10-12 mas across oriented roughly perpendicular to the separation axis. The morphology exhibited by the water masers is suggestive of an axisymmetric wind with the masers tracing bow shocks formed as the wind impacts the ambient medium. This bipolar jet-like structure is typical of the three other confirmed water-fountain sources. When combined with the previously observed spectral characteristics of OH 12.8-0.9, the observed spatio-kinematic structure of the water masers provides strong evidence that OH 12.8-0.9 is indeed a member of the water-fountain class.Comment: 12 pages, 2 figures (1 color), accepted for publication in the Ap J Letter

Equilibrium Models for two Samples of OH/IR Stars

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Bipolar outflows in OH/IR stars

We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of ten OH/IR stars is observed at high angular resolution, including bipolar nebulae (OH231.8+4.2), bright post-AGB stars (HD 101584) and reflection nebulae (e.g. Roberts 22). The IRAS colour--colour diagram separates the sample into different types of objects. One group may contain the progenitors to the (few) extreme bipolar planetary nebulae. Two objects show colours and chemistry very similar to the planetary nebulae with late IR-[WC] stars. One object is a confirmed close binary. A model is presented consisting of an outer AGB wind which is swept up by a faster post-AGB wind, with either wind being non-spherically symetric. The interface of the two winds is shown to exhibit a linear relation between velocity and distance from the star. The OH data confirms the predicted linear velocity gradients, and reveals torus-like, uniformly expanding components. All sources are discussed in detail using optical/HST images where available. ISO data for Roberts 22 reveal a chemical dichotomy, with both crystalline silicates and PAHs features being present. IRAS 16342-3814 shows a dense torus; HST data shows four point-like sources located symmetrically around the nebula, near the outer edge of the dense torus. Lifetimes for the bipolar OH/IR stars are shown to be in excess of 10^4 yr, longer than normal post-AGB timescales. This suggests that the disks are near-stationary. We suggest that accretion from such a disk slows down the post-AGB evolution. Such a process could explain the link between the long-lived bipolar nebular geometry and the retarded star.Comment: 31 pages, LaTeX using mn.sty. MNRAS, accepted for publicatio

OH-selected AGB and post-AGB objects I.Infrared and maser properties

Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz OH line, new light is cast on the infrared properties of evolved stars on the TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS colours, largely fail to distinguish early post-AGB stages. A two-colour diagram from narrower-band MSX flux densities, with bimodal distributions, provides a better tool to do the latter. Four mutually consistent selection criteria for OH-masing red PPNe are given, as well as two for early post-AGB masers and one for all post--AGB masers, including the earliest ones. All these criteria miss a group of blue, high-outflow post-AGB sources with 60-mum excess; these will be discussed in detail in Paper II. The majority of post-AGB sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly low outflow velocities, although the fractions of single peaks and irregular spectra may vary with age and mass. The OH flux density shows a fairly regular relation with the stellar flux and the envelope optical depth, with the maser efficiency increasing with IRAS colour R21. The OH flux density is linearly correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted

Near-IR variability properties of a selected sample of AGB stars

We present the results of a near-infrared monitoring programme of a selected sample of stars, initially suspected to be Mira variables and OH/IR stars, covering more than a decade of observations. The objects monitored cover the typical range of IRAS colours shown by O-rich stars on the Asymptotic Giant Branch and show a surprisingly large diversity of variability properties. 16 objects are confirmed as large-amplitude variables. Periods between 360 and 1800 days and typical amplitudes from 1 to 2 magnitudes could be determined for nine of them. In three light curves we find a systematic decrease of the mean brightness, two light curves show pronounced asymmetry. One source, IRAS 07222-2005, shows infrared colours typical of Mira variables but pulsates with a much longer period (approx. 1200 days) than a normal Mira. Two objects are ither close to (IRAS 03293+6010) or probably in (IRAS 18299-1705) the post-AGB phase. In IRAS 16029-3041 we found a systematic increase of the H-K colour of approximately 1 magnitude, which we interpret as evidence of a recent episode of enhanced mass loss. IRAS 18576+0341, a heavily obscured Luminous Blue Variable was also monitored. The star showed a continued decrease of brightness over a period of 7 years (1995 - 2002).Comment: 9 pages + 3 appendix, 36 figures, photometry table, accepted in Astronomy & Astrophysic

HE 3-1475 AND ITS JETS

We present spectra and high-resolution images taken with HST, the NTT, the VLA, and the MPIA/ESO 2.2m of the emission-line star He 3-1475 which we suggest is a post-AGB star. The star is at the origin of a 15-arcsec-long structure containing symmetrically opposing bright knots. The knots have radial velocities of about 500 km/s from the center of He 3-1475 to the ends of the jets. HST snapshots show that the core of He 3-1475 is unipolar with a star at the SE end and the nebula fanning out toward the NW. VLA observations show the presence of OH masers, which are positioned parallel to the optical jets. A model is proposed that accounts for all of the observational data. This unusual object may link the OH/IR stars having extreme outflow velocities with highly bipolar planetary nebulae.Comment: 14 pages, uu-encoded postcript. 6 figures available on request from Matt Bobrowski ([email protected]

Near-IR spectroscopy of planetary nebulae precursors

We present near-IR spectroscopy of a sample of 30 IRAS sources recently identified as late AGB stars, post-AGB stars or early PNe. The spectra obtained are centered at various wavelengths covering the molecular hydrogen v=1-0 S(1) and v=2-1 S(1) emission lines, the recombination lines of hydrogen Br-gamma, Pf-gamma and Br-alpha, and the CO[v=2-0] first overtone bandhead at 2.294 microns. As a result of these observations we have increased from 4 to 13 the total number of proto-PNe detected in molecular hydrogen. When the molecular hydrogen is fluorescence-excited the detection rate is found to be directly correlated with the evolutionary stage of the central star, rather than with the nebular morphology. In contrast, shocked-excited molecular hydrogen is detected only in strongly bipolar proto-PNe, sometimes even at an early stage in the post-AGB phase. The strong correlation of shocked-excited molecular hydrogen emission with bipolarity found confirms the result previously reported by Kastner et al. (1996) in evolved PNe. However, our results show that this correlation does not exist in the case of fluorescence-excited molecular hydrogen.Comment: Accepted for publication in A&

Evidence of triggered star formation in G327.3-0.6. Dust-continuum mapping of an infrared dark cloud with P-ArT\'eMiS

Aims. Expanding HII regions and propagating shocks are common in the environment of young high-mass star-forming complexes. They can compress a pre-existing molecular cloud and trigger the formation of dense cores. We investigate whether these phenomena can explain the formation of high-mass protostars within an infrared dark cloud located at the position of G327.3-0.6 in the Galactic plane, in between two large infrared bubbles and two HII regions. Methods: The region of G327.3-0.6 was imaged at 450 ? m with the CEA P-ArT\'eMiS bolometer array on the Atacama Pathfinder EXperiment telescope in Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were used in this study. Results: Ten massive cores were detected in the P-ArT\'eMiS image, embedded within the infrared dark cloud seen in absorption at both 8 and 24 ?m. Their luminosities and masses indicate that they form high-mass stars. The kinematical study of the region suggests that the infrared bubbles expand toward the infrared dark cloud. Conclusions: Under the influence of expanding bubbles, star formation occurs in the infrared dark areas at the border of HII regions and infrared bubbles.Comment: 4 page