Dieter Engels

Lifetime of OH masers at the tip of the asymptotic giant branch

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 (1σ). 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.

An infrared study of galactic OH/IR stars. II. The "GLMP sample" of red oxygen-rich AGB stars

We present optical and near-infrared finding charts taken fr om the DSS and 2MASS surveys of 94 IRAS sources selected from the GLMP catalogue, and accurate astrometry (≈ 0.2 ′′ ) for most of them. Selection criteria were very red IRAS colours representative for OH/IR stars with optically thick circumstellar shells and the p resence of variability according to the IRAS variability index (VAR>50). The main photometric properties of the stars in this ‘GLMP sample’ are presented, discussed and compared with the correspondent properties of the ‘Arecibo sample’ of OH/IR stars studied nearlier. We find that 37% of the sample (N= 34) has no counterpart in the 2MASS, implying extremely high optical depths of their shells. Most of the sources identified in the 2MASS are faint (K>8) and are of very red colour in the near-infrared, as expected. The brightest 2MASS counterpart (K=5.3 mag) was found for IRAS 18299‐1705. Its blue colour H‐K=1.3 suggests that IRAS 18299‐1705 is a postAGB star. Few GLMP sources have faint but relatively blue counterparts. They might be misidentifed field stars or stars th at recently experienced a drop of their mass loss rates. The ‘GLMP sample’ in general is made of oxygen-rich AGB stars, which are highly obscured by their circumstellar shells. They bel ong to the same population as the reddest OH/IR stars in the ‘Arecibo sample’.

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. Sixteen objects are confirmed as large-amplitude variables. Periods between 360 and 1800 d and typical amplitudes 1 m < ∆K < 2 m could be determined for nine of them. In three light curves, we find a systematic decrease in the mean brightness, and two light curves show pronounced asymmetry. One source, IRAS 07222–2005, shows infrared colours typical of Mira variables, but it pulsates with a much longer period (≈1200 d ) than a normal Mira. Two objects are either close to (IRAS 03293+6010) or probably in (IRAS 18299–1705) the post-AGB phase. In IRAS 16029–3041 we found a systematic increase in the H − K colour of ≈1 m , 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 in brightness over a period of 7 years (1995–2002).

OH masers in the Milky Way and Local Group galaxies in the SKA era

The intense line emission of OH masers is a perfect tracer of regions where new stars are born aswell as of evolved stars, shedding large amounts of processed matter into the interstellar medium. From SKA deep surveys at 18 cm, where the maser lines from the ground-state of the OH molecule arise, we predict the discovery of more than 20000 sources of stellar and interstellar origin throughout the Galaxy. The study of this maser emission has many applications, including the determination of magnetic field strengths from polarisation measurements, studies of stellar kinematics using the precisely determined radial velocities, and distance determinations from VLBI astrometry. A new opportunity to study shocked gas in different galactic environments is expected to arise with the detection of lower luminosity masers. For the first time, larger numbers of OH masers will be detected in Local Group galaxies. New insights are expected in structure formation in galaxies by comparing maser populations in galaxies of different metallicity, as both their properties as well as their numbers depend on it. With the full capabilities of SKA, further maser transitions such as from excited OH and from methanol will be accessible, providing new tools to study the evolution of star-forming regions in particular.

Study of extremely reddened AGB stars in the Galactic bulge

Context. Extremely reddened AGB stars lose mass at high rates of >10^-5 Msun/yr. This is the very last stage of AGB evolution, in which stars in the mass range 2.0--4.0 Msun (for solar metallicity) should have been converted to C stars already. The extremely reddened AGB stars in the Galactic bulge are however predominantly O-rich, implying that they might be either low-mass stars or stars at the upper end of the AGB mass range. Aims. To determine the mass range of the most reddened AGB stars in the Galactic bulge. Methods. Using Virtual Observatory tools, we constructed spectral energy distributions of a sample of 37 evolved stars in the Galactic bulge with extremely red IRAS colours. We fitted DUSTY models to the observational data to infer the bolometric fluxes. Applying individual corrections for interstellar extinction and adopting a common distance, we determined luminosities and mass-loss rates, and inferred the progenitor mass range from comparisons with AGB evolutionary models. Results. The observed spectral energy distributions are consistent with a classification as reddened AGB stars, except for two stars, which are proto-planetary nebula candidates. For the AGB stars, we found luminosities in the range 3000--30,000 Lsun and mass-loss rates 10^-5--3x10^-4 Msun/yr. The corresponding mass range is 1.1--6.0 Msun assuming solar metallicity. Conclusions. Contrary to the predictions of the evolutionary models, the luminosity distribution is continuous, with many O-rich AGB stars in the mass range in which they should have been converted into C stars already. We suspect that bulge AGB stars have higher than solar metallicity and therefore may avoid the conversion to C-rich. The presence of low-mass stars in the sample shows that their termination of the AGB evolution also occurs during a final phase of very high mass-loss rate, leading to optically thick circumstellar shells.

On the Evolutionary Connection Between AGB Stars and PNe

The ‘O‐rich AGB sequence’ is a sequence of colours describing the location of O‐rich AGB stars in the IRAS two‐colour diagram [12]–[25] vs [25]–[60] (See Figure 1). We propose an evolutionary scenario for this sequence in which all stars, independent of their progenitor mass, start the AGB phase in the blue part of the ‘O‐rich AGB sequence’ and then evolve toward redder colors, although only the more massive stars would reach the very end of the ‘O‐rich AGB sequence’. The sources located in the blue part of the sequence are mainly Mira variables, whose mean period is increasing with the IRAS colours. Most of them will evolve into O‐rich Type II (and III) Planetary Nebulae. Part of the stars located in the red part of the sequence will change their chemical composition from O‐rich to C‐rich during their evolution in the AGB phase, and might evolve into C‐rich Type II Planetary Nebulae. Hot bottom burning may prevent the conversion to carbon stars of the rest of sources located in the red part of the sequen...

HS 0551+7241: A New Possible Magnetic Cataclysmic Variable in the Hamburg-CfA Bright Quasar Survey

We present an analysis of spectroscopic observations of a newly discovered cataclysmic variable, HS 0551+7241. In 1995, the stars brightness dropped by ΔB ~ 2.5 mag and HS 0551+7241 entered a low state lasting for ~2 yr. The Hα, Hβ, and He II λ4686 emission-line radial velocity curves show evidence of short ~50 minute fluctuations superposed on longer, ~4 hr, variations. We find similar modulations in the line fluxes and the line equivalent widths. The continuum magnitude light curves suggest that HS 0551+7241 may be an eclipsing system. If the 4 hour period is related to the orbital period, then the mass function is ~0.138 M⊙. Relatively strong He II λ4686 emission and short fluctuations may suggest that the binary is an intermediate polar. In that case, the ~50 minute variations would correspond to an Alfven radius of ~3 × 1010 cm.