Analysis of Pulsating White Dwarf Star Light Curves

Analysis techniques are presented for extracting the frequencies contained in the light curves of pulsating white dwarf stars. In several surface temperature regimes, these astronomical objects are unstable to gravity mode pulsations which result in brightness variations corresponding to the periods of the excited modes. There is a rich array of possible periods with values ranging from about 100 to 1000 seconds. Mode periods present in the light curve are detected by undertaking a Fourier analysis of the time series light curve; theoretical models of the star can be refined with this information. The Fourier analysis needs to take into account such things as finite length, data gaps and the presence of noise

The post-outburst pulsations of the accreting white dwarf in the cataclysmic variable GW Librae

We present new time series photometry of the accreting pulsating white dwarf system GW Librae obtained in 2012 and 2013 at the University of Canterbury Mt John Observatory in New Zealand. Our 2012 data show the return of a ∼19 min periodicity that was previously detected in 2008. This pulsation mode was a dominant feature of our quality 2012 May data set, which consisted of six contiguous nights; a detailed analysis indicated a degree of frequency variability. We show by comparison with the previously identified pulsation modes that this periodicity is best explained as a new mode, and that the quasi-stability of the periods appears to be a general feature of the pulsations in these systems. We also find a previously unreported 3-h modulation period, which we believe to be related to the known two and four hour periods of so far unknown origin

Kant, Morality, and Hell

In this paper I argue that, although Kant argues that morality is independent of God (and hence, agrees with the Euthyphro), and rejects Divine Command Theory (or Theological Voluntarism), he believes that all moral duties are also the commands of God, who is a moral being, and who is morally required to punish those who transgress the moral law: "God’s justice is the precise allocation of punishments and rewards in accordance with men’s good or bad behavior." However, since we lack a strict proof of God's existence, we can still fulfill our duties from the motive of duty. if we did know that God exists, then this would undermine our pure moral motivation to do our duty, since we would have an even stronger interest in pleasing God through our good conduct. The effect of undermining our pure moral motivation would be to make us less eligible for divine reward, since God rewards us for doing our duty from the motive of duty

GW Librae: Still Hot Eight Years Post-Outburst

We report continued Hubble Space Telescope (HST) ultraviolet spectra and ground-based optical photometry and spectroscopy of GW Librae eight years after its largest known dwarf nova outburst in 2007. This represents the longest cooling timescale measured for any dwarf nova. The spectra reveal that the white dwarf still remains about 3000 K hotter than its quiescent value. Both ultraviolet and optical light curves show a short period of 364-373 s, similar to one of the non-radial pulsation periods present for years prior to the outburst, and with a similar large UV/optical amplitude ratio. A large modulation at a period of 2 h (also similar to that observed prior to outburst) is present in the optical data preceding and during the HST observations, but the satellite observation intervals did not cover the peaks of the optical modulation so it is not possible to determine its corresponding UV amplitude. The similarity of the short and long periods to quiescent values implies the pulsating, fast spinning white dwarf in GW Lib may finally be nearing its quiescent configuration.Comment: 6 figures, accepted in A

Multicolor Observations of a Planetary Transit of HD 209458

We present B, V, R, I and Z-band photometric observations of HD 209458 during the transit by its planetary companion on UT 1999 November 15 with the University of Hawaii 0.6m and 2.2m telescopes and the High Altitude Observatory STARE telescope. The detailed shape of the transit curve is predicted to vary with color due primarily to the color-dependent limb-darkening of the star, but potentially due as well to the effect of color-dependent opacity in the planetary atmosphere. We model the light curves and present refined values for the transit timing and orbital period, useful for planning future observations of the planetary transit. We also derive significantly improved measurements of the planetary radius, R_p = 1.55 +/- 0.10 R_Jup, stellar radius, R_s = 1.27 +/- 0.05 R_Sun, and orbital inclination, i = 85.9 +/- 0.5 deg. The derived planetary radius favors evolutionary models in which the planet has a low albedo.Comment: 6 pages, 3 figures, accepted for publication in ApJ Letter

New Pulsating DB White Dwarf Stars from the Sloan Digital Sky Survey

We are searching for new He atmosphere white dwarf pulsators (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars). Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities (Winget et al. 2004), a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. So far we have discovered nine new DBVs, doubling the number of previously known DBVs. Here we report the new pulsators' lightcurves and power spectra.Comment: 15 pages, 2 figures, 3 tables, ApJ accepte

Found: the progenitors of AM CVn and supernovae .Ia

We present optical and X-ray observations of two tidally distorted, extremely low-mass white dwarfs (WDs) with massive companions. There is no evidence of neutron stars in our Chandra and XMM observations of these objects. SDSS J075141.18−014120.9 (J0751) is an eclipsing double WD binary containing a 0.19 M⊙ WD with a 0.97 M⊙ companion in a 1.9 h orbit. J0751 becomes the fifth eclipsing double WD system currently known. SDSS J174140.49+652638.7 (J1741) is another binary containing a 0.17 M⊙ WD with an unseen M ≥ 1.11 M⊙ WD companion in a 1.5-h orbit. With a mass ratio of ≈0.1, J1741 will have stable mass transfer through an accretion disc and turn into an interacting AM Canum Venaticorum (AM CVn) system in the next ≈160 Myr. With a mass ratio of 0.2, J0751 is likely to follow a similar evolutionary path. These are the first known AM CVn progenitor binary systems and they provide important constraints on the initial conditions for AM CVn. Theoretical studies suggest that both J0751 and J1741 may create thermonuclear supernovae in ∼10^8 yr, either .Ia or Ia. Such explosions can account for ∼1 per cent of the Type Ia supernova rate

Measuring The Evolutionary Rate Of Cooling Of ZZ Ceti

We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 +/- 1.4) x 10(-15) s s(-1) employing the O - C method and (5.45 +/- 0.79) x 10(-15) s s(-1) using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 +/- 1.0) x 10(-15) s s(-1). After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 +/- 1.1) x 10(-15) s s(-1). This value is consistent within uncertainties with the measurement of (4.19 +/- 0.73) x 10(-15) s s(-1) for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.NSF AST-1008734, AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009Astronom

Measuring The Evolutionary Rate Of Cooling Of ZZ Ceti

We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 +/- 1.4) x 10(-15) s s(-1) employing the O - C method and (5.45 +/- 0.79) x 10(-15) s s(-1) using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 +/- 1.0) x 10(-15) s s(-1). After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 +/- 1.1) x 10(-15) s s(-1). This value is consistent within uncertainties with the measurement of (4.19 +/- 0.73) x 10(-15) s s(-1) for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.NSF AST-1008734, AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009Astronom