Maartje N. Sevenster

Millimetre observations of infrared carbon stars - II. Mass loss rates and expansion velocities

Dust- and gas mass loss rates and distances are determined for a sample of about 330 infra-red carbon stars that probe a distance up to about 5.5 kpc. The dependence of the dust- and gas mass loss rates, and the expansion velocity upon galactic longitude (l) are studied. It is found that the expansion velocity significantly depends on l, but that the absolute bolometric magnitude, the dust mass loss rate and the gas-to-dust ratio depend on l marginally, if at all, and the gas mass loss rate does not depend on l. Beyond the solar circle, the expansion velocity (as well as the luminosity, dust-to-gas ratio, dust mass loss rate) is lower than inside the solar circle, as expected from the overall gradient in metallicity content of the Galaxy. Combining the average expansion velocity inside and beyond the solar circle with the theoretically predicted relation between expansion velocity and gas-to-dust ratio, we find that the metallicity gradient in the solar neighbourhood is about 0:034 dex/kpc, well within the quoted range of values in the literature.

OH-selected AGB and Post-AGB Objects. I. Infrared and Maser Properties

Using 766 compact objects found in a systematic survey of the Galactic plane in the 1612 MHz masing OH line, new light is cast on the IR properties of evolved stars on the thermally pulsing asymptotic giant branch (AGB) and beyond. The usual mid-IR selection criteria for post-AGB, based on IRAS colors, largely fail to distinguish early post-AGB stages. A two-color diagram from much narrower band MSX flux densities, with bimodal distributions, provides a better tool for doing this. Four mutually consistent selection criteria for OH-masing red proto–planetary nebulae 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 μm 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 color R21. The OH flux density is linearly correlated with the 60 μm flux density.

Millimetre observations of infrared carbon stars. I. The data

Millimetre observations of IRAS selected red carbon stars are presented. About 260 stars have been observed with SEST and IRAM in the CO (1-0) and CO (2-1) lines and partially in HCN (1-0) and SiO (3-2). An overall detection rate, in at least one line, of about 80% is achieved. The survey represents the second largest survey for AGB stars, and the largest ever for carbon stars. Two new detections in SiO (3-2) in carbon stars are reported. When available, the SiO/HCN and HCN/CO (1-0) line ratios are consistent with the ratios expected for carbon stars.

Anomalously Excited OH and Competition between Maser Transitions toward Centaurus A

The ground-state radio transitions of OH have been observed toward the nucleus of the nearby active galaxy Centaurus A, using the Australia Telescope Compact Array. The main lines at rest frequencies 1667 and 1665 MHz show molecular absorption at velocities similar to those observed for other molecular species at millimeter wavelengths, which indicates that the lines of sight at radio and millimeter wavelengths sample the same material. The satellite transitions at 1612 and 1720 MHz exhibit stimulated emission and absorption, including the first extragalactic detection of maser emission in the 1720 MHz line. A remarkable feature of the two satellite transitions is that they are conjugate for all velocity components, with one in emission and the other equally strong in absorption. This is explained as the competition of the two lines for the same inverting infrared radiation in the regime in which the infrared lines are just becoming optically thick. The transition from 1720 MHz to 1612 MHz maser emission depends critically on the OH column density and therefore yields direct information on the distribution of molecular gas.

A Shock-excited OH Maser in a Post-asymptotic Giant Branch Envelope?

We have observed a sample of OH 1612 MHz masing objects in all four OH ground-state transitions with the Australia Telescope Compact Array. One likely post-asymptotic giant branch (AGB) object is found to emit in the 1612, 1665, and 1720 MHz transitions. We discuss the evidence that this object may be an early post-AGB object and the possibility for such a circumstellar envelope to harbor a 1720 MHz maser. We argue that during a very brief period, just after the star has left the thermally pulsing phase of the AGB and the wind velocity starts to increase, post-AGB objects might show 1720 MHz emission. The best objects to search for such emission would be those that are masing at 1612 and 1665 MHz, but not at 1667 MHz nor in the 22 GHz H2O transition.We have observed a sample of OH 1612-MHz masing objects in all four OH ground--state transitions with the Australia Telescope Compact Array. One likely post-AGB object is found to emit in the 1612-MHz, 1665-MHz and 1720-MHz transitions. We discuss the evidence that this object may be an early post-AGB object and the possibility for such a circumstellar envelope to harbour a 1720-MHz maser. We argue that during a very brief period, just after the star has left the thermally-pulsing phase of the AGB and the wind velocity starts to increase, post-AGB objects might show \sath emission. The best objects to search for such emission would be those that are masing at 1612 MHz and 1665 MHz, but not at 1667 MHz nor in the 22-GHz H2O transition.

An Inner Ring and the Microlensing toward the Bulge

All current Bulge-Disk models for the inner Galaxy fall short of reproducing self-consistently the observed micro-lensing optical depth by a factor of two (

The ATCA/VLA OH 1612 MHz survey III. Observations of the Northern Galactic Plane ?

>2\sigma

OH-selected AGB and Post-AGB Objects. II. Blue versus Red Evolution Off the Asymptotic Giant Branch

). We show that the least mass-consuming way to increase the optical depth is to add density roughly half-way the observer and the highest micro-lensing-source density. We present evidence for the existence of such a density structure in the Galaxy: an inner ring, a standard feature of barred galaxies. Judging from data on similar rings in external galaxies, an inner ring can contribute more than 50% of a pure Bulge-Disk model to the micro-lensing optical depth. We may thus eliminate the need for a small viewing angle of the Bar. The influence of an inner ring on the event-duration distribution, for realistic viewing angles, would be to increase the fraction of long-duration events toward Baades window. The longest events are expected toward the negative-longitude tangent point at

Distribution functions for evolved stars in the inner galactic plane.

\ell\sim

ATCA/VLA OH 1612 MHz survey. III. (Sevenster+, 2001)

-22\degr . A properly sampled event-duration distribution toward this tangent point would provide essential information about viewing angle and elongation of the over-all density distribution in the inner Galaxy.All current bulge-disk models for the inner Galaxy fall short of reproducing self-consistently the observed microlensing optical depth by a factor of 2 (>2 σ). We show that the least mass-consuming way to increase the microlensing optical depth is to add density roughly halfway between the observer and the highest microlensing source density. We present evidence for the existence of such a density structure in the Galaxy: an inner ring, a standard feature of barred galaxies. Judging from data on similar rings in external galaxies, an inner ring can contribute more than 50% of a pure bulge-disk model to the microlensing optical depth. We may thus eliminate the need for a small viewing angle of the bar. The influence of an inner ring on the event duration distribution, for realistic viewing angles, would be to increase the fraction of long-duration events toward Baades window. The longest events are expected toward the negative-longitude tangent point at l ~ -22°. A properly sampled event duration distribution toward this tangent point would provide essential information about viewing angle and elongation of the overall density distribution in the inner Galaxy.