Light Curve Patterns and Seismology of a White Dwarf with Complex Pulsation

The ZZ Ceti star KUV 02464+3239 was observed over a whole season at the mountain station of Konkoly Observatory. A rigorous frequency analysis revealed 6 certain periods between 619 and 1250 seconds, with no shorter period modes present. We use the observed periods, published effective temperature and surface gravity, along with the model grid code of Bischoff-Kim, Montgomery and Winget (2008) to perform a seismological analysis. We find acceptable model fits with masses between 0.60 and 0.70 M_Sun. The hydrogen layer mass of the acceptable models are almost always between 10^-4 and 10^-6 M_*. In addition to our seismological results, we also show our analysis of individual light curve segments. Considering the non-sinusoidal shape of the light curve and the Fourier spectra of segments showing large amplitude variations, the importance of non-linear effects in the pulsation is clearly seen.Comment: 5 pages, 6 figures, in "Stellar Pulsation: Challenges for Theory and Observation", Eds. J. Guzik and P. A. Bradley, AIP

Characterizing the pulsations of the ZZ Ceti star KUV 02464+3239

We present the results on period search and modeling of the cool DAV star KUV 02464+3239. Our observations resolved the multiperiodic pulsational behaviour of the star. In agreement with its position near the red edge of the DAV instability strip, it shows large amplitude, long period pulsation modes, and has a strongly non-sinusoidal light curve. We determined 6 frequencies as normal modes and revealed remarkable short-term amplitude variations. A rigorous test was performed for the possible source of amplitude variation: beating of modes, effect of noise, unresolved frequencies or rotational triplets. Among the best-fit models resulting from a grid search, we selected 3 that gave l=1 solutions for the largest amplitude modes. These models had masses of 0.645, 0.650 and 0.680 M_Sun. The 3 `favoured' models have M_H between 2.5x10^-5 - 6.3x10^-6 M_* and give 14.2 - 14.8 mas seismological parallax. The 0.645 M_Sun (11400 K) model also matches the spectroscopic log g and T_eff within 1 sigma. We investigated the possibility of mode trapping and concluded that while it can explain high amplitude modes, it is not required.Comment: 11 pages, 8 figures, accepted for publication in MNRA

A New Timescale for Period Change in the Pulsating DA White Dwarf WD 0111+0018

We report the most rapid rate of period change measured to date for a pulsating DA (hydrogen atmosphere) white dwarf (WD), observed in the 292.9 s mode of WD 0111+0018. The observed period change, faster than 10^{-12} s/s, exceeds by more than two orders of magnitude the expected rate from cooling alone for this class of slow and simply evolving pulsating WDs. This result indicates the presence of an additional timescale for period evolution in these pulsating objects. We also measure the rates of period change of nonlinear combination frequencies and show that they share the evolutionary characteristics of their parent modes, confirming that these combination frequencies are not independent modes but rather artifacts of some nonlinear distortion in the outer layers of the star.Comment: 10 pages, 6 figures, accepted for publication in The Astrophysical Journa

New chemical profiles for the asteroseismology of ZZ Ceti stars

We compute new chemical profiles for the core and envelope of white dwarfs appropriate for pulsational studies of ZZ Ceti stars. These profiles are extracted from the complete evolution of progenitor stars, evolved through the main sequence and the thermally-pulsing asymptotic giant branch (AGB) stages, and from time-dependent element diffusion during white dwarf evolution. We discuss the importance of the initial-final mass relationship for the white dwarf carbon-oxygen composition. In particular, we find that the central oxygen abundance may be underestimated by about 15% if the white dwarf mass is assumed to be the hydrogen-free core mass before the first thermal pulse. We also discuss the importance for the chemical profiles expected in the outermost layers of ZZ Ceti stars of the computation of the thermally-pulsing AGB phase and of the phase in which element diffusion is relevant. We find a strong dependence of the outer layer chemical stratification on the stellar mass. In particular, in the less massive models, the double-layered structure in the helium layer built up during the thermally-pulsing AGB phase is not removed by diffusion by the time the ZZ Ceti stage is reached. Finally, we perform adiabatic pulsation calculations and discuss the implications of our new chemical profiles for the pulsational properties of ZZ Ceti stars. We find that the whole $g-$mode period spectrum and the mode-trapping properties of these pulsating white dwarfs as derived from our new chemical profiles are substantially different from those based on chemical profiles widely used in existing asteroseismological studies. Thus, we expect the asteroseismological models derived from our chemical profiles to be significantly different from those found thus far.Comment: 12 pages, 11 figures, 1 table. To be published in Ap

Asteroseismology of the Kepler V777 Her variable white dwarf with fully evolutionary models

DBV stars are pulsating white dwarfs with atmospheres rich in He. Asteroseismology of DBV stars can provide valuable clues about the origin, structure and evolution of hydrogen-deficient white dwarfs, and may allow to study neutrino and axion physics. Recently, a new DBV star, KIC 8626021, has been discovered in the field of the \emph{Kepler} spacecraft. It is expected that further monitoring of this star in the next years will enable astronomers to determine its detailed asteroseismic profile. We perform an asteroseismological analysis of KIC 8626021 on the basis of fully evolutionary DB white-dwarf models. We employ a complete set of evolutionary DB white-dwarf structures covering a wide range of effective temperatures and stellar masses. They have been obtained on the basis of a complete treatment of the evolutionary history of progenitors stars. We compute g-mode adiabatic pulsation periods for this set of models and compare them with the pulsation properties exhibited by KIC 8626021. On the basis of the mean period spacing of the star, we found that the stellar mass should be substantially larger than spectroscopy indicates. From period-to-period fits we found an asteroseismological model characterized by an effective temperature much higher than the spectroscopic estimate. In agreement with a recent asteroseismological analysis of this star by other authors, we conclude that KIC 8626021 is located near the blue edge of the DBV instability strip, contrarily to spectroscopic predictions. We also conclude that the mass of KIC 8626021 should be substantially larger than thought.Comment: 7 pages, 5 figures, 3 tables. To be published in Astronomy and Astrophysic

The rate of cooling of the pulsating white dwarf star G117−-B15A: a new asteroseismological inference of the axion mass

We employ a state-of-the-art asteroseismological model of G117-B15A, the archetype of the H-rich atmosphere (DA) white dwarf pulsators (also known as DAV or ZZ Ceti variables), and use the most recently measured value of the rate of period change for the dominant mode of this pulsating star to derive a new constraint on the mass of axion, the still conjectural non-barionic particle considered as candidate for dark matter of the Universe. Assuming that G117-B15A is truly represented by our asteroseismological model, and in particular, that the period of the dominant mode is associated to a pulsation g-mode trapped in the H envelope, we find strong indications of the existence of extra cooling in this star, compatible with emission of axions of mass m_a \cos^2 \beta = 17.4^{+2.3}_{-2.7} meV.Comment: 9 pages, 5 figures and 3 tables. Accepted for publication in MNRA

The First Three Rungs of the Cosmological Distance Ladder

It is straightforward to determine the size of the Earth and the distance to the Moon without making use of a telescope. The methods have been known since the 3rd century BC. However, few amateur or professional astronomers have worked this out from data they themselves have taken. Here we use a gnomon to determine the latitude and longitude of South Bend, Indiana, and College Station, Texas, and determine a value of the radius of the Earth of 6290 km, only 1.4 percent smaller than the true value. We use the method of Aristarchus and the size of the Earth's shadow during the lunar eclipse of 2011 June 15 to derive an estimate of the distance to the Moon (62.3 R_Earth), some 3.3 percent greater than the true mean value. We use measurements of the angular motion of the Moon against the background stars over the course of two nights, using a simple cross staff device, to estimate the Moon's distance at perigee and apogee. Finally, we use simultaneous CCD observations of asteroid 1996 HW1 obtained with small telescopes in Socorro, New Mexico, and Ojai, California, to derive a value of the Astronomical Unit of (1.59 +/- 0.19) X 10^8 km, about 6 percent too large. The data and methods presented here can easily become part of a beginning astronomy lab class.Comment: 34 pages, 11 figures, accepted for publication in American Journal of Physic

Asteroseismology of the Kepler V777 Herculis variable white dwarf with fully evolutionary models

Context. DBV stars are pulsating white dwarfs with atmospheres rich in He. Asteroseismology of DBV stars can provide valuable clues about the origin, structure and evolution of hydrogen-deficient white dwarfs, and may allow to study neutrino and axion physics. Recently, a new DBV star, KIC 8626021, has been discovered in the field of the Kepler spacecraft. It is expected that further monitoring of this star in the next years will enable astronomers to determine its detailed asteroseismic profile. Aims. We perform an asteroseismological analysis of KIC 8626021 on the basis of fully evolutionary DB white-dwarf models. Methods. We employ a complete set of evolutionary DB white-dwarf structures covering a wide range of effective temperatures and stellar masses. They have been obtained on the basis of a complete treatment of the evolutionary history of progenitors stars. We compute g-mode adiabatic pulsation periods for this set of models and compare them with the pulsation properties exhibited by KIC 8626021. Results. On the basis of the mean period spacing of the star, we found that the stellar mass should be substantially larger than spectroscopy indicates. From period-to-period fits we found an asteroseismological model characterized by an effective temperature much higher than the spectroscopic estimate. Conclusions. In agreement with a recent asteroseismological analysis of this star by other authors, we conclude that KIC 8626021 is located near the blue edge of the DBV instability strip, contrarily to spectroscopic predictions. We also conclude that the mass of KIC 8626021 should be substantially larger than thought.Facultad de Ciencias Astronómicas y Geofísica