Albert A. Zijlstra

Stellar Populations of the Dwarf Irregular Galaxy WLM

We have obtained V-band photometry for 8000 stars, and I-band photometry for 16000 stars in the field of the dwarf irregular galaxy WLM, a member of the Local Group. From the VI color-magnitude diagram we infer metallicities and ages for the stellar populations in the main body and in the halo of WLM. The youngest stars in the galaxy have an age of {approximately}10{sup 7} Gyr. For the oldest stars we derive an age of {gt}10{sup 10} Gyr and a metallicity of [Fe/H]={minus}1.45{plus_minus}0.2. We construct a deep luminosity function, obtaining an accurate distance modulus m{minus}M=24.75{plus_minus}0.1 for this galaxy based on the I-magnitude of the RGB tip and adopting E(V{minus}I)=0.03. We establish the presence of an extended, slightly flattened halo consisting of Population II stars. Cluster D of Ables & Ables (1977, ApJS, 34, 245) is confirmed as the only globular cluster present in the main body of this galaxy. There are no other clusters brighter than I=19.5. The B{minus}V color gradient observed by Ables & Ables (1977) is shown to be caused by a stellar-population transition, and not a metallicity gradient or dust extinction. {copyright} {ital 1997 American Astronomical Society.}

Discovery of a Bow Shock around Vela X-1

We report the discovery of a symmetric bow shock around the well-known high-mass X-ray binary (HMXB) Vela X-1. Wind bow shocks are a ubiquitous phenomenon around OB-runaway stars, but now such a structure is found around an HMXB. The presence of a bow shock indicates that the system has a high (supersonic) velocity with respect to the interstellar medium. From the symmetry of the bow shock, the direction of motion and, moreover, the origin and age of the system can be derived. Our observation supports Blaauws scenario for the formation of an OB-runaway star by the supernova explosion of the binary companion.

Dwarf Galaxies Also Have Stellar Halos

We present evidence for the existence of an old stellar halo in the dwarf irregular galaxy WLM, an isolated member of the Local Group. The halo consists of Population II stars, with low metallicities and age ≥1010 yr. The finding of a halo in a dwarf irregular galaxy argues for a generic mode of galaxy formation that requires a halo in the presence of a disk, regardless of galaxy size. This implies that formation mechanisms are similar along the spiral Hubble sequence, and favors the scenarios where the formation takes place during the original collapse and accretion of the protogalactic gas clouds. Halo formation also appears not necessarily to be related to the presence of a bulge, or a nucleus, since WLM lacks both of these components.

Chemical abundances of planetary nebulae in the Sagittarius dwarf elliptical galaxy

Spectrophotometry and imaging of the two planetary nebulae He 2-436 and Wray 16-423, recently discovered to be in the Sagittarius dwarf elliptical galaxy, are presented. Wray 16-423 is a high-excitation planetary nebula (PN) with a hot central star. In contrast, He 2-436 is a high-density nebula with a cooler central star and evidence of local dust, the extinction of which exceeds that for Wray 16-423 by EB-V = 0.28. The extinction to Wray 16-423 (EB-V = 0.14) is consistent with the line-of-sight extinction to the Sagittarius Dwarf. Both PNs show Wolf-Rayet features in their spectra, although the lines are weak in Wray 16-423. Images in [O III] and Hα + [N II], although affected by poor seeing, yield a diameter of 12 for Wray 16-423 after deconvolution, while He 2-436 was unresolved. He 2-436 has a luminosity about twice that of Wray 16-423, and its size and high density suggest a younger nebula. In order to reconcile the differing luminosity and nebular properties of the two nebulae with similar-age progenitor stars, it is suggested that they are on He-burning tracks An abundance analysis is presented for both PNs using empirical abundance determinations. The abundance pattern is very similar in both nebulae, and both show an oxygen depletion of -0.4 dex with respect to the mean oxygen abundance of Galactic planetary nebulae and [O/H] = -0.6. The Sagittarius PN progenitor stars are representative of the higher metallicity tail of the Sagittarius population. The pattern of abundance depletion is similar to that in the only other planetary nebula in a dwarf galaxy companion of the Milky Way, that in Fornax, for which new spectra are presented. However, the abundances are larger than for Galactic halo PNs, suggesting a later formation age. The oxygen abundance of the Sagittarius galaxy deduced from its PN shows similarities with that of dwarf ellipticals around M31, advancing the notion that this galaxy was a dwarf elliptical before its interaction with the Milky Way.

The planetary nebula BD +30°3639: the infrared spectrum during post-AGB stellar evolution

We present a radiative-transfer calculation which reproduces the infrared spectrum of the planetary nebula BD~+30

Evolution and Mass Loss of AGB Stars in the Small Magellanic Cloud

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Obscured AGB stars in the Magellanic clouds. I. IRAS Candidates

3639. We calculate the transfer process through absorption and scattering in a spherical-symmetric multi-grain dust shell. The emission of transiently heated particles is taken into account, as well as polycyclic aromatic hydrocarbons. We obtain an acceptable fit to most of the spectrum, including the PAH infrared bands. At submillimetre wavelengths the observed emission is larger than the model predicts, indicating that large dust conglomerates (``f{}luffy grains) may be needed as an additional constituent. The fit favours a distance of

Mass-loss rates and luminosity functions of dust-enshrouded AGB stars and red-supergiants in the LMC

ge 2 ,

Radial velocities of planetary nebulae towards the Galactic bulge

kpc, which implies that BD~+30

Obscured Asymptotic Giant Branch stars in the Magellanic Clouds

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