Ted von Hippel

Detection of intergalactic red-giant-branch stars in the Virgo cluster

It has been suspected for nearly 50 years that galaxy clusters contain a population of intergalactic stars ripped from the galaxies during cluster formation, or when the galactic orbits pass through the cluster centre. Observational support for theexistence of such a stellar population is provided both by measurements of the diffuse light in clusters, and by the recent detection of planetary nebulae with positions or velocities far removed from any observable cluster galaxy,. But estimates for the mass of the diffuse population and its distribution relative to the cluster galaxies are still highly uncertain. Here we report the direct detection of intergalactic stars in deep images of a blank field in the Virgo cluster. The data suggest that these stars form approximately one-tenth of the total stellar mass of the cluster. We observe a relatively homogeneous distribution of stars, with evidence of a slight gradient towards M87.

WIYN Open Cluster Study. I. Deep Photometry of NGC 188

We have employed precise and carefully calibrated V- and I-band photometry of NGC 188 at WIYN Observatory to explore the cluster luminosity function (LF) and study the cluster white dwarfs. Our photometry is offset by V = 0.052 (fainter) from that of Sandage and Eggen & Sandage. All published photometry for the past three decades has been based on these two calibrations, which are in error by 0.05 ± 0.01. We employ the Pinsonneault et al. fiducial open cluster main sequence to derive a distance modulus of 11.43 ± 0.08 and E(B-V) = 0.09, with the largest single source of error caused by uncertainty in the cluster metallicity. We report observations that are ≥50% complete along the main sequence to V = 24.6. We find that the NGC 188 central-field LF peaks at MI ≈ 3 to 4. This is unlike the solar neighborhood LF and unlike the LFs of dynamically unevolved portions of open and globular clusters, all of which typically rise continuously until MI ≈ 9.5. Although we find that ≥50% of the unresolved objects in this cluster are multiple systems with mass ratios ≥0.3, their presence cannot account for the shape of the NGC 188 LF. For theoretical reasons having to do with the long-term survivability of NGC 188, we believe the cluster is highly dynamically evolved and that the low-luminosity stars missing from the central cluster LF are either in the cluster outskirts or have left the cluster altogether. We identify nine candidate white dwarfs (WDs) in NGC 188, of which we expect at least three, and perhaps six, are bona fide cluster WDs. The luminosities of the faintest likely WD indicate an age of 1.14 ± 0.09 Gyr, where the error in age includes the cluster distance uncertainty and we assume the WD has a hydrogen atmosphere. This age is a lower limit to the cluster age, and observations probing to V = 27 or 28 will be necessary to find the faintest cluster WDs and independently determine the cluster age. While our lower age limit is not surprising for this ≈6 Gyr–old cluster, our result demonstrates the value of the WD age technique with its very low internal errors.

Physical parametrization of stellar spectra: the neural network approach

We present a technique which employs artificial neural networks to produce physical parameters for stellar spectra. A neural network is trained on a set of synthetic optical stellar spectra to give physical parameters (e.g. Teff, log g, [M/H]). The network is then used to produce physical parameters for real, observed spectra. Our neural networks are trained on a set of 155 synthetic spectra, generated using the spectrum program written by Gray (Gray & Corbally 1994, Gray & Arlt 1996). Once trained, the neural network is used to yield Teff for over 5000 B–K spectra extracted from a set of photographic objective prism plates (Bailer-Jones, Irwin & von Hippel 1997a). Using the MK classifications for these spectra assigned by Houk (1975, 1978, 1982, 1988), we have produced a temperature calibration of the MK system based on this set of 5000 spectra. It is demonstrated through the metallicity dependence of the derived temperature calibration that the neural networks are sensitive to the metallicity signature in the real spectra. With further work it is likely that neural networks will be able to yield reliable metallicity measurements for stellar spectra.

Contribution of White Dwarfs to Cluster Masses

I have undertaken a literature search through 1997 July 31 of white dwarfs (WDs) in open and globular clusters. I have tried to make a careful evaluation in each case of the likelihood that the object is a WD and that it is a cluster member. The results are presented for 13 open clusters and 11 globular clusters. Currently there are 36 single WDs and five WDs in binaries known among the open clusters, and 340 single WDs and 11 WDs in binaries known among the globular clusters. From these data, I have calculated WD mass fractions for four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one globular cluster (NGC 6121). I develop a simple model of cluster evolution that incorporates stellar evolution but not dynamical evolution to interpret the WD mass fractions. I augment the results of my simple model by turning to sophisticated N-body simulations incorporating stellar evolution. I find that even though these clusters undergo a range of degrees of kinematic evolution, from moderate (the Pleiades, NGC 2168, and NGC 3532) to strong (the Hyades and NGC 6121), the WD mass fraction is relatively insensitive to kinematic evolution and little changed from a model incorporating only stellar evolution with a Salpeter-like initial mass function. By comparing the cluster mass functions with that of the Galactic disk, and incorporating plausibility arguments for the mass function of the Galactic halo, I estimate the WD mass fraction in these two field populations. I assume the Galactic disk is ~10 Gyr old and that the Galactic halo is ~12 Gyr old, although the WD mass fraction is insensitive to age within this regime. I find that the Galactic halo should contain from 8%–9% (α = -2.35) to perhaps as much as 15%–17% (α = -2.0) of its stellar mass in the form of WDs. The Galactic disk WD mass fraction should be 6% to 7% (for a median stellar age of 5 to 7 Gyr and α = -2.35), consistent with the empirical estimates of 3% to 7%.

Stellar populations and the white dwarf mass function: Connections to supernova IA luminosities.

We discuss the luminosity function of SNe Ia under the assumption that recent evidence for dispersion in this standard candle is related to variations in the white dwarf mass function (WDMF) in the host galaxies. We develop a simple parameterization of the WDMF as a function of age of a stellar population and apply this to galaxies of different morphological types. We show that this simplified model is consistent with the observed WDMF of Bergeron et al. (1992) for the solar neighborhood. Our simple models predict that WDMF variations can produce a range of more than 1.8 mag in M

Neural Network Classification of Stellar Spectra

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Main-Sequence Masses and Radii from Gravitational Redshifts

(SN Ia), which is comparable to the observed value using the data of Phillips (1993) and van den Bergh (1996). We also predict a galaxy type dependence of M

Optical gamma-ray burst followup at Kitt Peak National Observatory

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Automated classification of stellar spectra — II. Two-dimensional classification with neural networks and principal components analysis

(SN Ia) under standard assumptions of the star formation history in these galaxies and show that M

Semi-automated extraction of digital objective prism spectra

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