arXiv subjects

W. Landsman

Publications and source records attributed to W. Landsman.

22 records · Page 2Linked to original sources

arXiv1998

The Hot Stars of Old Open Clusters: M67, NGC 188 and NGC6791

We analyze ultraviolet (~1500 A) images of the old open clusters M67, NGC 188, and NGC 6791 obtained with Ultraviolet Imaging Telescope (UIT) during the second flight of the Astro observatory in March 1995. Twenty stars are detected on the UIT image of M67, including 11 blue stragglers, seven white dwarf candidates, and the yellow giant -- white dwarf binary S1040. The ultraviolet photometry of the blue stragglers F90 (S975) and F131 (S1082) suggests that these stars have hot subluminous companions. We present a semi-empirical integrated ultraviolet spectrum of M67, and show that the blue stragglers dominate the integrated spectrum at wavelengths shorter than 2600 A. The number of white dwarfs in M67 is roughly consistent with the number expected from white dwarf cooling models. Eight candidate sdB/sdO stars are detected in NGC 6791, and two are detected in NGC 188. The luminosity range 1.10 < log L/Lsun < 1.27, derived from the ultraviolet photometry of the six sdB candidates, is consistent with theoretical models of metal-rich hot horizontal branch (HB) stars. The fraction of hot HB stars in both NGC 6791 and NGC 188 is about 30%, implying that the integrated spectra of both clusters should show a UV turnup at least as strong as that observed in any elliptical galaxy.

astro-ph↗

arXiv1998

Hot UV bright stars in globular clusters

We have obtained medium-resolution spectra of seven UV-bright stars discovered on images of four southern globular clusters obtained with the Ultraviolet Imaging Telescope (UIT). Effective temperatures, surface gravities and helium abundances are derived from LTE and non-LTE model atmosphere fits. Three of the stars have sdO spectra, including M4-Y453 (Teff = 58800 K, log g = 5.15), NGC 6723-III60 (Teff = 40600 K, log g = 4.46) and NGC 6752-B2004 (Teff = 37000 K, log g = 5.25). All seven stars lie along either post-extended horizontal branch (EHB) or post-early AGB evolutionary tracks. The post-early AGB stars show solar helium abundances, while the post-EHB stars are helium deficient, similar to their EHB progenitors.

astro-ph↗

arXiv1997

UV Spectral Dating of Stars and Galaxies

An echelle spectrogram (R = 30,000) of the 2300-3100 A region in the ultraviolet spectrum of the F8V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spectra of similar stars according to age and metal content. In particular, the spectral break at 2640 A is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time since the last major star formation episode in the z=1.55 early-type galaxy LBDS 53W091, whose rest frame mid-ultraviolet spectrum, observed with the Keck Telescope, is dominated by the flux from similar stars that are at or near the main-sequence turnoff in that system (Spinrad et al. 1997). Our result, 1 Gyr if the flux-dominating stellar population has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate and thereby relaxes constraints on cosmological parameters that were implied by the earlier work.

astro-ph↗

arXiv1997

S1040 in M67: A Post-Mass-Transfer Binary with a Helium-Core White Dwarf

We have obtained spectra of the yellow giant S1040 in the open cluster M67 using the Goddard High-Resolution Spectrograph (GHRS) and the Faint Object Spectrograph on the Hubble Space Telescope. S1040 is a single-lined spectroscopic binary with a 42.8d period that occupies a "red straggler" position in the M67 color-magnitude diagram (CMD), 0.2 mag blueward of the giant branch. A detection of S1040 at 1620 A with the Ultraviolet Imaging Telescope provided evidence that the secondary is a hot white dwarf, and thus that the anomalous location of S1040 in the CMD is likely due to a prior episode of mass-transfer. Our GHRS spectrum shows a broad Ly alpha absorption profile that confirms the white dwarf identification of the S1040 secondary. A model atmosphere fit to the GHRS spectrum yields Teff = 16,160 K, log g = 6.7, and a mass of 0.22 Msun, for an assumed cluster distance of 820 pc and reddening of E(B-V) = 0.02. The unusually low mass derived for the white dwarf implies that it must have a helium core, and that a mass-transfer episode must have begun while the progenitor was on the lower giant branch. We construct a plausible mass-transfer history for S1040 in which it originated as a short (~2d) period binary, and evolved through a blue straggler phase to reach its current state.

astro-ph↗