Stars, Interstellar Medium and the Galaxy | Open Access Articles

Thirty-Five New Pulsating Da White Dwarf Stars, Anjum S. Mukadam, Ted Von Hippel, Et Al.

Ted von Hippel

We present 35 new pulsating DA (hydrogen atmosphere) white dwarf stars discovered from the Sloan Digital Sky Survey (SDSS) and the Hamburg Quasar Survey (HQS). We have acquired high-speed time series photometry of preselected DA white dwarfs with a prime focus CCD photometer on the 2.1 m telescope at McDonald Observatory over 15 months. We selected these stars on the basis of prior photometric and spectroscopic observations by the SDSS and HQS. For the homogeneous SDSS sample, we achieve a success rate of 80% for finding new variables at a detection threshold of 0.1%-0.3%. With 35 newly discovered DA variable …

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Measuring The Evolution Of The Most Stable Optical Clock G 117-B15a, S. O. Kepler, Ted Von Hippel, Et Al.

Ted von Hippel

We report our measurement of the rate of change of period with time () 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 σ measurement observed = (4.27 ± 0.80) × 10-15 s s-1. Taking into account the proper-motion effect of proper = (7.0 ± 2.0) × 10-16s s-1, we obtain a rate of change of period with time of = (3.57 ± 0.82) × 10-15 s s-1. This value is consistent with the cooling rate in our white dwarf …

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Redefining The Empirical Zz Ceti Instability Strip, Anjum S. Mukadam, Ted Von Hippel, Et Al.

Ted von Hippel

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 redefine 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 ~950 K, from …

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Pulsational Mapping Of Calcium Across The Surface Of A White Dwarf, Susan E. Thompson, Ted Von Hippel, Et Al.

Ted von Hippel

We constrain the distribution of calcium across the surface of the white dwarf star G29-38 by combining time-series spectroscopy from Gemini-North with global time-series photometry from the Whole Earth Telescope. G29-38 is actively accreting metals from a known debris disk. Since the metals sink significantly faster than they mix across the surface, any inhomogeneity in the accretion process will appear as an inhomogeneity of the metals on the surface of the star. We measure the flux amplitudes and the calcium equivalent width amplitudes for two large pulsations excited on G29-38 in 2008. The ratio of these amplitudes best fits a …

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Constraining The Evolution Of Zz Ceti, Anjum S. Mukadam, S. O. Kepler, D. E. Nather, M. Kilic, F. Mullally, T. Von Hippel, S. J. Kleinman, A. Nitta, J. A. Guzik, P. A. Bradley, J. Matthews, K. Sekiguchi, D. J. Sullivan, T. Sullivan, R. R. Shobbrook, P. Birch, X. J. Jiang, D. W. Xu, S. Joshi, B. N. Ashoka, P. Ibbetson, E. Leibowitz, E. O. Ofek, E. G. Meištas, R. Janulis, R. D. Ališauskas, R. Kalytis, G. Handler, D. Kilkenny, D. O'Donoghue, D. W. Kurtz, M. Müller, P. Moskalik, W. Ogloza, S. Zola, J. Krzesiński, F. Johnannessen, J. M. Gonzalez-Perez, J. E. Solheim, R. Silvotti, S. Bernabei, G. Vauclair, N. Dolez, J. N. Fu, M. Chevreton, M. Manteiga, O. Suárez, A. Ulla, M. S. Cunha, T. S. Metcalfe, A. Kanaan, L. Fraga, A. F. M. Costa, O. Giovannini, G. Fontaine, P. Bergeron, M. S. O'Brien, D. Sanwal, M. A. Wood, T. J. Ahrens, N. Silvestri, E. W. Klumpe, S. D. Kawaler, R. Riddle, M. D. Reed, T. K. Watson

Ted von Hippel

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt ≤ (5:5 ± 1:9) x 10^-15 s s^-¹, after correcting for proper motion. Our results are consistent with previous Ṗ values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is …

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Constraining The Surface Inhomogeneity And Settling Times Of Metals On Accreting White Dwarfs, Michael H. Montgomery, S.E. Thompson, Ted Von Hippel

Ted von Hippel

Due to the short settling times of metals in DA white dwarf atmospheres, any white dwarfs with photospheric metals must be actively accreting. It is therefore natural to expect that the metals may not be deposited uniformly on the surface of the star. We present calculations showing how the temperature variations associated with white dwarf pulsations lead to an observable diagnostic of the surface metal distribution, and we show what constraints current data sets are able to provide. We also investigate the effect that time-variable accretion has on the metal abundances of different species, and we show how this can …

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Full-Text Articles in Stars, Interstellar Medium and the Galaxy

Thirty-Five New Pulsating Da White Dwarf Stars, Anjum S. Mukadam, Ted Von Hippel, Et Al.

Ted von Hippel

Measuring The Evolution Of The Most Stable Optical Clock G 117-B15a, S. O. Kepler, Ted Von Hippel, Et Al.

Ted von Hippel

Redefining The Empirical Zz Ceti Instability Strip, Anjum S. Mukadam, Ted Von Hippel, Et Al.

Ted von Hippel

Pulsational Mapping Of Calcium Across The Surface Of A White Dwarf, Susan E. Thompson, Ted Von Hippel, Et Al.

Ted von Hippel

Ted von Hippel

Constraining The Surface Inhomogeneity And Settling Times Of Metals On Accreting White Dwarfs, Michael H. Montgomery, S.E. Thompson, Ted Von Hippel

Ted von Hippel