Browsing by Subject "g29-38"
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Item 2006 Whole Earth Telescope Observations Of GD358: A New Look At The Prototype DBV(2009-03) Provencal, J. L.; Montgomery, Michael H.; Kanaan, A.; Shipman, H. L.; Childers, D.; Baran, A.; Kepler, S. O.; Reed, M.; Zhou, A.; Eggen, J.; Watson, T. K.; Winget, D. E.; Thompson, Susan E.; Riaz, B.; Nitta, Atsuko; Kleinman, S. J.; Crowe, R.; Slivkoff, J.; Sherard, P.; Purves, N.; Binder, P.; Knight, R.; Kim, S. L.; Chen, W. P.; Yang, M.; Lin, H. C.; Lin, C. C.; Chen, C. W.; Jiang, X. J.; Sergeev, A. V.; Mkrtichian, D.; Andreev, M.; Janulis, R.; Siwak, M.; Zola, S.; Koziel, D.; Stachowski, G.; Paparo, M.; Bognar, Z.; Handler, G.; Lorenz, D.; Steininger, B.; Beck, P.; Nagel, T.; Kusterer, D.; Hoffman, A.; Reiff, E.; Kowalski, R.; Vauclair, G.; Charpinet, S.; Chevreton, M.; Solheim, J. E.; Pakstiene, E.; Fraga, L.; Dalessio, J.; Montgomery, Michael H.; Winget, D. E.We report on the analysis of 436.1 hr of nearly continuous high-speed photometry on the pulsating db white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 and 4000 mu Hz, with the dominant peak at 1234 mu Hz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l = 1 modes in the limit of slow rotation. The high-k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high-k multiplets is highly suspect. The k = 9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high-k modes. Analysis of the k = 9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 sforzando event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.Item Constraining the Surface Inhomogeneity and Settling Times of Metals on Accreting White Dwarfs(2008-10) Montgomery, M. H.; Thompson, S. E.; von Hippel, Ted; Montgomery, M. H.; von Hippel, T.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 lead to constraints on the gravitational settling times.Item Precision Asteroseismology Of The Pulsating White Dwarf Gd 1212 Using A Two-Wheel-Controlled Kepler Spacecraft(2014-07) Hermes, J. J.; Charpinet, S.; Barclay, Thomas; Pakstiene, E.; Mullally, Fergal; Kawaler, Steven D.; Bloemen, S.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, Michael H.; Van Grootel, V.; Huber, Daniel; Still, Martin; Howell, Steve B.; Caldwell, Douglas A.; Haas, Michael R.; Bryson, Stephen T.; Winget, D. E.; Montgomery, Michael H.We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more than 11.5 days of space-based photometry obtained during an engineering test of the two-reaction-wheel-controlled Kepler spacecraft. We detect at least 19 independent pulsation modes, ranging from 828.2-1220.8 s, and at least 17 nonlinear combination frequencies of those independent pulsations. Our longest uninterrupted light curve, 9.0 days in length, evidences coherent difference frequencies at periods inaccessible from the ground, up to 14.5 hr, the longest-period signals ever detected in a pulsating WD. These results mark some of the first science to come from a two-wheel-controlled Kepler spacecraft, proving the capability for unprecedented discoveries afforded by extending Kepler observations to the ecliptic.Item Pulsational Mapping Of Calcium Across The Surface Of A White Dwarf(2010-05) Thompson, Susan E.; Montgomery, Michael H.; von Hippel, Ted; Nitta, Atsuko; Dalessio, J.; Provencal, J.; Strickland, W.; Holtzman, Jon A.; Mukadam, Anjum; Sullivan, D.; Nagel, T.; Koziel-Wierzbowska, D.; Kundera, T.; Zola, S.; Winiarski, M.; Drozdz, M.; Kuligowska, E.; Ogloza, W.; Bognar, Z.; Handler, G.; Kanaan, A.; Ribeira, T.; Rosen, R.; Reichart, D.; Haislip, J.; Barlow, B. N.; Dunlap, B. H.; Ivarsen, K.; LaCluyze, A.; Mullally, Fergal; Montgomery, Michael H.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 model for polar accretion of calcium and rules out equatorial accretion.Item A Second Case of Outbursts in A Pulsating White Dwarf Observed By Kepler(2015-09) Hermes, J. J.; Montgomery, M. H.; Bell, Keaton J.; Chote, P.; Gansicke, B. T.; Kawaler, Steven D.; Clemens, J. Christopher; Dunlap, B. H.; Winget, D. E.; Armstrong, D. J.; Montgomery, M. H.; Bell, Keaton J.; Winget, D. E.We present observations of a new phenomenon in pulsating white dwarf stars: large-amplitude outbursts at timescales much longer than the pulsation periods. The cool (T-eff = 11,060 K), hydrogen-atmosphere pulsating white dwarf PG 1149+057 was observed nearly continuously for more than 78.8 day by the extended Kepler mission in K2 Campaign 1. The target showed 10 outburst events, recurring roughly every 8 day and lasting roughly 15 hr, with maximum flux excursions up to 45% in the Kepler bandpass. We demonstrate that the outbursts affect the pulsations and therefore must come from the white dwarf. Additionally, we argue that these events are not magnetic reconnection flares, and are most likely connected to the stellar pulsations and the relatively deep surface convection zone. PG 1149+057 is now the second cool pulsating white dwarf to show this outburst phenomenon, after the first variable white dwarf observed in the Kepler mission, KIC 4552982. Both stars have the same effective temperature, within the uncertainties, and are among the coolest known pulsating white dwarfs of typical mass. These outbursts provide fresh observational insight into the red edge of the DAV instability strip and the eventual cessation of pulsations in cool white dwarfs.