arXiv subjects

Ted von Hippel

Publications and source records attributed to Ted von Hippel.

At least 73 records · Page 4Linked to original sources

arXiv2005

Faint Blue Objects in the Hubble Deep Field South Revealed: White Dwarfs, Subdwarfs, and Quasars

We explore the nature of the faint blue objects in the Hubble Deep Field South. We have derived proper motions for the point sources in the Hubble Deep Field South using a 3 year baseline. Combining our proper motion measurements with spectral energy distribution fitting enabled us to identify 4 quasars and 42 stars, including 3 white dwarf candidates. Two of these white dwarf candidates, HDFS 1444 and 895, are found to display significant proper motion, 21.1 $\pm$ 7.9 mas/yr and 34.9 $\pm$ 8.0 mas/yr, and are consistent with being thick disk or halo white dwarfs located at ~2 kpc. The other faint blue objects analyzed by Mendez & Minniti do not show any significant proper motion and are inconsistent with being halo white dwarfs; they do not contribute to the Galactic dark matter. The observed population of stars and white dwarfs is consistent with standard Galactic models.

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arXiv2005

Measuring the evolution of the most stable optical clock G 117-B15A

We report our measurement of the rate of change of period with time dP/dt for the 215 s periodicity in the pulsating white dwarf G 117-B15A, the most stable optical clock known. After 31 years of observations, we have finally obtained a 4 sigma measurement dP/dt_observed = (4.27 +/- 0.80) x 10^{-15} s/s. Taking into account the proper-motion effect of dP/dt_pm = (7.0 +/- 2.0) x 10^{-16} s/s, we obtain a rate of change of period with time of dP/dt = (3.57 +/- 0.82) x 10^{-15} s/s. This value is consistent with the cooling rate in our white dwarf models only for cores of C or C/O. With the refinement of the models, the observed rate of period change can be used to accurately measure the ratio of C/O in the core of the white dwarf.

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arXiv2005

Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main Sequence Stars

We present deep VI photometry of stars in the globular cluster M5 (NGC 5904) based on images taken with the Hubble Space Telescope. The resulting color-magnitude diagram reaches below V ~ 27 mag, revealing the upper 2-3 magnitudes of the white dwarf cooling sequence, and main sequence stars eight magnitudes and more below the turn-off. We fit the main sequence to subdwarfs of known parallax to obtain a true distance modulus of (m-M)_0 = 14.45 +/- 0.11 mag. A second distance estimate based on fitting the cluster white dwarf sequence to field white dwarfs with known parallax yielded (m-M)_0 = 14.67 +/- 0.18 mag. We couple our distance estimates with extensive photometry of the cluster's RR Lyrae variables to provide a calibration of the RR Lyrae absolute magnitude yielding M_V(RR) = 0.42 +/- 0.10 mag at [Fe/H] = -1.11 dex. We provide another luminosity calibration in the form of reddening-free Wasenheit functions. Comparison of our calibrations with predictions based on recent models combining stellar evolution and pulsation theories shows encouraging agreement. (Abridged)

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arXiv2005

Cool White Dwarfs in the Sloan Digital Sky Survey

A reduced proper motion diagram utilizing Sloan Digital Sky Survey (SDSS) photometry and astrometry and USNO-B plate astrometry is used to separate cool white dwarf candidates from metal-weak, high-velocity main sequence Population II stars (subdwarfs) in the SDSS Data Release 2 imaging area. Follow-up spectroscopy using the Hobby-Eberly Telescope, the MMT, and the McDonald 2.7m Telescope is used to demonstrate that the white dwarf and subdwarf loci separate cleanly in the reduced proper motion diagram, and that the contamination by subdwarfs is small near the cool white dwarf locus. This enables large statistically complete samples of white dwarfs, particularly the poorly understood cool white dwarfs, to be created from the SDSS imaging survey, with important implications for white dwarf luminosity function studies. SDSS photometry for our sample of cool white dwarfs is compared to current white dwarf models.

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arXiv2005

From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main Sequence Stellar Evolution in Open Clusters

I explore the current ability of both white dwarf cooling theory and main sequence stellar evolution theory to accurately determine stellar population ages by comparing ages derived using both techniques for open clusters ranging from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence evolutionary ages over the entire age range currently available for study. I also find that directly comparing main sequence turn-off ages to white dwarf ages is only weakly sensitive to realistic levels of errors in cluster distance, metallicity, and reddening. Additional detailed comparisons between white dwarf and main sequence ages have tremendous potential to refine and calibrate both of these important clocks, and I present new simulations of promising open cluster targets. The most demanding requirement for these white dwarf studies are very deep (V > 25-28) cluster observations made necessary by the faintness of the oldest white dwarfs.

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arXiv2004

The Initial-Final Mass Relationship: Spectroscopy of White Dwarfs in NGC 2099 (M37)

We present new observations of very faint white dwarfs (WDs) in the rich open star cluster NGC 2099 (M37). Following deep, wide field imaging of the cluster using CFHT, we have now obtained spectroscopic observations of candidate WDs using both GMOS on Gemini and LRIS on Keck. Of our 24 WD candidates (all fainter than V = 22.4), 21 are spectroscopically confirmed to be bona fide WDs, 4-5 of which are most likely field objects. Fitting 18 of the 21 WD spectra with model atmospheres, we find that most WDs in this cluster are quite massive (0.7-0.9 Msun), as expected given the cluster's young age (650 Myr) and, hence, high turnoff mass (~2.4 Msun). We determine a new initial-final mass relationship and almost double the number of existing data points from previous studies. The results indicate that stars with initial masses between 2.8 and 3.4 Msun lose 70-75% of their mass through stellar evolution. For the first time, we find some evidence of a metallicity dependence on the initial-final mass relationship.

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arXiv2004

Photometric Identification of Cool White Dwarfs

We investigate the use of a narrow-band DDO51 filter for photometric identification of cool white dwarfs. We report photometric observations of 30 known cool white dwarfs with temperatures ranging from 10,000 K down to very cool temperatures (<3500 K). Follow-up spectroscopic observations of a sample of objects selected using this filter and our photometric observations show that DDO51 filter photometry can help select cool white dwarf candidates for follow-up multi--object spectroscopy by rejecting 65% of main sequence stars with the same broad--band colors as the cool white dwarfs. This technique is not selective enough to efficiently feed single--object spectrographs. We present the white dwarf cooling sequence using this filter. Our observations show that very cool white dwarfs form a sequence in the r-DDO vs. r-z color--color diagram and demonstrate that significant improvements are needed in white dwarf model atmospheres.

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arXiv2004

Re-defining the Empirical ZZ Ceti Instability Strip

We use the new ZZ Ceti stars (hydrogen atmosphere white dwarf variables; DAVs) discovered within the Sloan Digital Sky Survey (Mukadam et al. 2004) to re-define the empirical ZZ Ceti instability strip. This is the first time since the discovery of white dwarf variables in 1968 that we have a homogeneous set of spectra acquired using the same instrument on the same telescope, and with consistent data reductions, for a statistically significant sample of ZZ Ceti stars. The homogeneity of the spectra reduces the scatter in the spectroscopic temperatures and we find a narrow instability strip of width ~950K, from 10850--11800K. We question the purity of the DAV instability strip as we find several non-variables within. We present our best fit for the red edge and our constraint for the blue edge of the instability strip, determined using a statistical approach.

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arXiv2004

Proper Motion Objects in the Hubble Deep Field

Using the deepest and finest resolution images of the Universe acquired with the Hubble Space Telescope and a similar image taken 7 years later for the Great Observatories Origins Deep Survey, we have derived proper motions for the point sources in the Hubble Deep Field--North. Two faint blue objects,HDF2234 and HDF3072, are found to display significant proper motion, 10.0 $\pm$ 2.5 and 15.5 $\pm$ 3.8 mas yr$^{-1}$. Photometric distances and tangential velocities for these stars are consistent with disk white dwarfs located at $\sim$ 500 pc. The faint blue objects analyzed by Ibata et al. (1999) and Mendez & Minniti (2000) do not show any significant proper motion; they are not halo white dwarfs and they do not contribute to the Galactic dark matter. These objects are likely to be distant AGN.

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arXiv2003

Intergalactic Globular Clusters

We confirm and extend our previous detection of a population of intergalactic globular clusters in Abell 1185, and report the first discovery of an intergalactic globular cluster in the nearby Virgo cluster of galaxies. The numbers, colors and luminosities of these objects can place constraints on their origin, which in turn may yield new insights to the evolution of galaxies in dense environments.

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arXiv2003

Using Optical and Near-IR Photometry to Test MACHO Lens Candidates

We obtained new VLT/ISAAC H-band observations for five MACHO LMC source stars and adjacent LMC field regions. After combining our near-IR photometry with HST/PC BVRI optical photometry, we compared the MACHO objects to the adjacent field stars in a variety of color-magnitude and color-color diagrams. These diagnostic diagrams were chosen to be sensitive to our hypothesis that at least some of the MACHO lenses were foreground Galactic disk or thick disk M dwarfs. For the five lensed objects we studied, our hypothesis could be ruled out for main sequence lens masses >= 0.1 Mo for distances out to 4 kpc. On the other hand, the fact that LMC-MACHO-5, an object not in our study, has been recently found to have just such a foreground lens, highlights that the remainder of the LMC MACHO objects should be searched for the signature of their lenses using our photometric technique, or via near-IR spectroscopy. We also constructed diagnostic color-color diagrams sensitive to determining reddening for the individual MACHO source stars and found that these five objects did not show evidence for significant additional reddening. At least these five MACHO objects are thus also inconsistent with the LMC self-lensing hypothesis.

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arXiv2002

Automated Stellar Spectral Classification and Parameterization for the Masses

Stellar spectroscopic classification has been successfully automated by a number of groups. Automated classification and parameterization work best when applied to a homogeneous data set, and thus these techniques primarily have been developed for and applied to large surveys. While most ongoing large spectroscopic surveys target extragalactic objects, many stellar spectra have been and will be obtained. We briefly summarize past work on automated classification and parameterization, with emphasis on the work done in our group. Accurate automated classification in the spectral type domain and parameterization in the temperature domain have been relatively easy. Automated parameterization in the metallicity domain, formally outside the MK system, has also been effective. Due to the subtle effects on the spectrum, automated classification in the luminosity domain has been somewhat more difficult, but still successful. In order to extend the use of automated techniques beyond a few surveys, we present our current efforts at building a web-based automated stellar spectroscopic classification and parameterization machine. Our proposed machinery would provide users with MK classifications as well as the astrophysical parameters of effective temperature, surface gravity, mean abundance, abundance anomalies, and microturbulence.

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arXiv2002

WIYN Open Cluster Study XI: WIYN 3.5m Deep Photometry of M35 (NGC 2168)

We present deep BVI observations of the core of M35 and a nearby comparison field obtained at the WIYN 3.5m telescope under excellent seeing. These observations display the lower main sequence in BV and VI CMDs down to V = 23.3 and 24.6, respectively. At these faint magnitudes background field stars are far more numerous than the cluster stars, yet by using a smoothing technique and CMD density distribution subtraction we recover the cluster fiducial main sequence and luminosity function to V = 24.6. We find the location of the main sequence in these CMDs to be consistent with earlier work on other open clusters, specifically NGC 188, NGC 2420, and NGC 2477. We compare these open cluster fiducial sequences to stellar models by Baraffe et al. (1998), Siess et al. (2000), Girardi et al. (2000), and Yi et al. (2001) and find that the models are too blue in both B-V and V-I for stars below ~0.4 Mo. M35 contains stars to the limit of the extracted main sequence, at M ~ 0.10-0.15 Mo, suggesting that M35 may harbor a large number of brown dwarfs, which should be easy targets for near-IR instrumentation on 8-10m telescopes. We also identify a new candidate white dwarf in M35 at V = 21.36 +- 0.01. Depending on which WD models are used to interpret this cluster candidate, it is either a very high mass WD (1.05 +- 0.05 Mo) somewhat older (0.19-0.26 Gyr, 3-4 sigma) than our best isochrone age (150 Myr), or it is a modestly massive WD (0.67-0.78 Mo) much too old (0.42-0.83 Gyr) to belong to the cluster.

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arXiv2002

Metal Abundances and Kinematics of Bright Metal-Poor Giants Selected from the LSE Survey: Implications for the Metal-Weak Thick Disk

We report medium-resolution (1-2 A) spectroscopy and broadband (UBV) photometry for a sample of 39 bright stars (the majority of which are likely to be giants) selected as metal-deficient candidates from an objective-prism survey concentrating on Galactic latitudes below |b| = 30 deg, the LSE survey of Drilling & Bergeron. Although the primary purpose of the LSE survey was to select OB stars (hence the concentration on low latitudes), the small number of bright metal-deficient giant candidates noted during this survey provide interesting information on the metal-weak thick disk (MWTD) population. The kinematics of the LSE giants indicate the presence of a rapidly rotating population, even at quite low metallicity. We consider the distribution of orbital eccentricity of the LSE giants as a function of [Fe/H], and conclude that the local fraction (i.e., within 1 kpc from the Sun) of metal-poor stars that might be associated with the MWTD is on the order of 30%-40% at abundances below [Fe/H] = -1.0. Contrary to recent analyses of previous (much larger) samples of non-kinematically selected metal-poor stars, we find that this relatively high fraction of local metal-poor stars associated with the MWTD may extend to metallicities below [Fe/H] = -1.6, much lower than had been considered before. We identify a subsample of 11 LSE stars that are very likely to be members of the MWTD, based on their derived kinematics; the lowest metallicity among these stars is [Fe/H] = -2.35. Implications of these results for the origin of the MWTD and for the formation of the Galaxy are considered. (abridged)

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arXiv2001

Three-Dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks

We explore the application of artificial neural networks (ANNs) for the estimation of atmospheric parameters (Teff, logg, and [Fe/H]) for Galactic F- and G-type stars. The ANNs are fed with medium-resolution (~ 1-2 A) non flux-calibrated spectroscopic observations. From a sample of 279 stars with previous high-resolution determinations of metallicity, and a set of (external) estimates of temperature and surface gravity, our ANNs are able to predict Teff with an accuracy of ~ 135-150 K over the range 4250 <= Teff <= 6500 K, logg with an accuracy of ~ 0.25-0.30 dex over the range 1.0 <= logg <= 5.0 dex, and [Fe/H] with an accuracy ~ 0.15-0.20 dex over the range -4.0 <= [Fe/H] <= +0.3. Such accuracies are competitive with the results obtained by fine analysis of high-resolution spectra. It is noteworthy that the ANNs are able to obtain these results without consideration of photometric information for these stars. We have also explored the impact of the signal-to-noise ratio (S/N) on the behavior of ANNs, and conclude that, when analyzed with ANNs trained on spectra of commensurate S/N, it is possible to extract physical parameter estimates of similar accuracy with stellar spectra having S/N as low as 13. Taken together, these results indicate that the ANN approach should be of primary importance for use in present and future large-scale spectroscopic surveys.

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arXiv2000

The White Dwarf Cooling Age of the Open Cluster NGC 2420

We have used deep HST WFPC2 observations of two fields in NGC 2420 to produce a cluster CMD down to V = 27. After imposing morphological selection criteria we find eight candidate white dwarfs in NGC 2420. Our completeness estimates indicate that we have found the terminus of the WD cooling sequence. We argue that the cluster distance modulus is likely to be close to 12.10 with E(B-V) = 0.04. With these parameters we find a white dwarf cooling age for NGC 2420 of 2.0 +- 0.20 (1 sigma) Gyrs. The 0.20 Gyr uncertainty includes errors in the photometry, sequence fitting, precursor time scales, and theoretical white dwarf cooling time scales. Comparing the cluster white dwarf cooling age to ages derived from stellar isochrone fitting we find a preference for ages derived from models incorporating convective overshoot.

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arXiv1999

(Teff,log g,[Fe/H]) Classification of Low-Resolution Stellar Spectra using Artificial Neural Networks

New generation large-aperture telescopes, multi-object spectrographs, and large format detectors are making it possible to acquire very large samples of stellar spectra rapidly. In this context, traditional star-by-star spectroscopic analysis are no longer practical. New tools are required that are capable of extracting quickly and with reasonable accuracy important basic stellar parameters coded in the spectra. Recent analyses of Artificial Neural Networks (ANNs) applied to the classification of astronomical spectra have demonstrated the ability of this concept to derive estimates of temperature and luminosity. We have adapted the back-propagation ANN technique developed by von Hippel et al. (1994) to predict effective temperatures, gravities and overall metallicities from spectra with resolving power ~ 2000 and low signal-to-noise ratio. We show that ANN techniques are very effective in executing a three-parameter (Teff,log g,[Fe/H]) stellar classification. The preliminary results show that the technique is even capable of identifying outliers from the training sample.

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arXiv1998

WIYN Open Cluster Study 1: Deep Photometry of NGC 188

We have employed precise V and I photometry of NGC 188 at WIYN to explore the cluster luminosity function (LF) and study the cluster white dwarfs (WDs). Our photometry is offset by V = 0.052 (fainter) from Sandage (1962) and Eggen & Sandage (1969). All published photometry for the past three decades have been based on these two calibrations, which are in error by 0.05 +- 0.01. We employ the Pinsonneault etal (1998) fiducial main sequence to derive a cluster distance modulus of 11.43 +- 0.08. We report observations that are >= 50% complete to V = 24.6 and find that the cluster central-field LF peaks at M_I ~ 3 to 4. This is unlike the solar neighborhood LF and unlike the LFs of dynamically unevolved portions of open and globular clusters, which rise continuously until M_I ~ 9.5. Although we find that >= 50% of the unresolved cluster objects are multiple systems, their presence cannot account for the shape of the NGC 188 LF. For theoretical reasons (Terlevich 1987; Vesperini & Heggie 1997) having to do with the survivability of NGC 188 we believe the cluster is highly dynamically evolved and that the missing low luminosity stars are either in the cluster outskirts or have left the cluster altogether. We identify nine candidate WDs, of which we expect three to six are bona fide cluster WDs. The luminosities of the faintest likely WD indicates an age (Bergeron, Wesemael, & Beauchamp 1995) of 1.14 +- 0.09 Gyrs. This 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 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. (abridged)

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