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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-baryonic particle considered as a 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 with 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 macos2β=17.4-2.7+2.3 meV., Facultad de Ciencias Astronómicas y Geofísicas

ZZ Ceti stars form the most numerous group of degenerate variable stars. They are otherwise normal DA (H-rich atmospheres) white dwarfs that exhibit pulsations. Here, we present an asteroseismological analysis for 44 bright ZZ Ceti stars based on a new set of fully evolutionary DA white dwarf models characterized by detailed chemical profiles from the centre to the surface. One of our targets is the archetypal ZZ Ceti star G117-B15A, for which we obtain an asteroseismological model with an effective temperature and a surface gravity in excellent agreement with the spectroscopy. The asteroseismological analysis of a set of 44 ZZ Ceti stars has the potential to characterize the global properties of the class, in particular the thicknesses of the hydrogen envelope and the stellar masses. Our results support the belief that white dwarfs in the solar neighbourhood harbour a broad range of hydrogen-layer thickness., Facultad de Ciencias Astronómicas y Geofísicas

We put asteroseismological constraints on the internal rotation profile of theGWVir (PG1159- type) star PG 0122+200. To this end we employ a state-of-the-art asteroseismological model for this star and assess the expected frequency splittings induced by rotation adopting a forward approach in which we compare the theoretical frequency separations with the observed ones assuming different types of plausible internal rotation profiles. We also employ two asteroseismological inversion methods for the inversion of the rotation profile of PG 0122+200.We find evidence for differential rotation in this star.We demonstrate that the frequency splittings of the rotational multiplets exhibited by PG 0122+200 are compatible with a rotation profile in which the central regions are spinning about 2.4 times faster than the stellar surface.

Convective driving, the mechanism originally proposed by Brickhill for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and DBV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star’s convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the DBV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope; this data set was multiperiodic containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358’s convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star.

We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient DO white dwarfs, PG 1159 stars, and DB white dwarfs. White dwarf sequences are computed for a wide range of stellar masses and helium envelopes on the basis of a complete treatment of the evolutionary history of progenitors stars, including the core hydrogen and helium burning phases, the thermally pulsing asymptotic giant branch phase, and the born-again episode that is responsible for the hydrogen deficiency.We also provide colors and magnitudes for the newsequences for Teff < 40,000 K, where the NLTE effects are not dominant. These new calculations provide a homogeneous set of evolutionary tracks appropriate formass and age determinations for both PG1159 stars andDOwhite dwarfs. The calculations are extended down to an effective temperature of 7000 K. We applied these new tracks to redetermine stellar masses and ages of all known DO white dwarfs with spectroscopically determined effective temperatures and gravities, and compare them with previous results.We also compare for the first time consistent mass determinations for both DO and PG 1159 stars, and find a considerably higher mean mass for the DO white dwarfs. We discuss as well the chemical profile expected in the envelope of variable DB white dwarfs from the consideration of the evolutionary history of progenitor stars. Finally, we present tentative evidence for a different evolutionary channel, other than that involving the PG 1159 stars, for the formation of hot, hydrogen-deficient white dwarfs.

Context. PG 1159-035, a pre-white dwarf with Teff ≃ 140000 K, is the prototype of both two classes: the PG 1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. Analyzing the periods of pulsation, it is possible to measure the stellar mass, the rotational period and the inclination of the rotation axis, to estimate an upper limit for the magnetic field, and even to obtain information about the inner stratification of the star. Aims. We have three principal aims: to increase the number of detected and identified pulsation modes in PG 1159-035, study trapping of the star's pulsation modes, and to improve or constrain the determination of stellar parameters. Methods. We used all available WET photometric data from 1983, 1985, 1989. 1993 and 2002 to identify the pulsation periods. Results. We identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M/M⊙= 0.59 ±0.02 for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at rc/R* = 0.83 ± 0.05. From the multiplet splitting, we calculated the rotational period Prot = 1.3920 ± 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for ℓ = 1 modes and less than 50% for ℓ = 2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars., La lista completa de autores que integran el documento puede consultarse en el archivo., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

We used time-resolved ultraviolet spectroscopy obtained with the FOS and STIS spectrographs of the Hubble Space Telescope (HST), together with archival IUE observations to measure the effective temperature (Teff), surface gravity (log g) and distance (d) of the pulsating DB white dwarf GD 358 with unprecedented accuracy, and to show that the temperature did not change during the 1996 sforzando, when the star changed basically to a single mode pulsator. We also measured for the first time for a DBV the spherical harmonic degree (l) for two modes, with k = 8 and k = 9, which was only possible because the stellar light curve was dominated by a single mode in 1996. The independent spectra provide the following values: Teff = 24 100 ± 400K, log g = 7.91 ± 0.26 and d = 42.7 ± 2.5 pc. The ultraviolet spectroscopic distance is in better agreement with the seismological value, than the one derived by parallax.

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~ 1.1 M_sun) to theoretically crystallize while still inside the ZZ Ceti instability strip (T_eff ~ 12,000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the "mode trapping" caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized., Comment: 7 pages, 3 figures, 3 tables, accepted for Astronomy & Astrophysics

The results of a five-month monitoring campaign on the Seyfert 1.5 galaxy NGC 3227 are presented. Variability was detected in the continuum and in the broad emission lines. Cross correlations of the 4200 Å continuum light curve with the Hβ and He II λ4686 emission-line light curves indicate delays of 18 ± 5 and 16 ± 2 days, respectively, between the continuum variations and the response of the lines. We apply a maximum entropy method to solve for the transfer function that relates the Hβ and He II λ4686 lines and 4200 Å continuum variability and the result of this analysis suggests that there is a deficit of emission-line response due to gas along the line of sight to the continuum source for both lines. Using a composite off-nuclear spectrum, we synthesize the bulge stellar population, which is found to be mainly old (77% with age > 10 Gyr) with a metallicity twice the solar value. The synthesis also yields an internal color excess E(B−V) ≈0.04. The mean contribution of the stellar population to the inner 5" x 10" spectra during the campaign was ~40% .

We present narrow-band CCD images of the high-excitation Seyfert 2 galaxy ES0138-G01, centred on the emission tines [O III] λ5007, Hα+[N II] λλ6548, 6584 and adjacent continua. The continuum-subtracted [O III] image presents an elongated jet-like structure to the west of the nucleus which is only weakly visible in the Hα image, and which we conclude is ionized by the nuclear source. The ionization map shows high excitation at the nucleus and at the 'jet'. The continuum ratio map shows that this galaxy is very blue for its morphological type, consistent with the large contribution of a young stellar population up to 3 kpc from the nucleus. We also propose that ESO138 - GO1 is the hard X-ray source previously identified with the nearby galaxy NGC 6621.

The star formation history in the nucleus of late spiral galaxies is compared between a sample in a high galaxy density medium (HDS) and a control sample (CS) of isolated galaxies. We have observed 20 HDS and 18 CS galaxies from a larger list generated by the application of a group-finding algorithm to the SSRS survey. Using equivalent widths of absorption lines and the continuum distribution, we determined the nuclear stellar population types, from those dominated by old population to those containing star formation bursts of different ages and intensities. The HDS and CS stellar population type histograms are similar, suggesting that environmental influences, at least for the present sample, do not affect substantially the nuclear stellar population. However, the nuclear emission lines indicate that, in the BPT diagnostic diagrams, there is an excess of HDS galaxies located within or close to the AGN loci. For six HDS and two CS galaxies, it was possible to determine oxygen (0/H) and nitrogen (N/H) abundances. The samples present similar 0/H values, but in the CS galaxies the N/0 ratio is lower at equal galaxy luminosity.

Observações fotomítricas em UBV com multidiafragma, e observações espectrofotomítricas da região central das galãxias espirais são apresentadas para NGC 5530, 5643 e 6699, com núcleo normal; NGC 1097, 2997 e 5236, com núcleo peculiar; e NGC 1566, 3783 e IC4329A, com núcleo Seyfert. Um algoritmo matemãtico, uti lizando dados das larguras equivalentes das linhas de absorção, í desenvolvido para obter a composição estelar dos núcleos. Na solução do algoritmo são usados métodos de programação não linear vinculada. Os contínuos estelares e as observações UBV sio usados para obter valores independentes da absorção e do avermelhamento interno do núcleo. Os resultados teõricos permitem detectar três ciclos de formação de estrelas nos três tipos de nú cleos. O último destes ciclos é mais recente nos núcleos Seyfert, seguido dos peculiares e finalmente dos normais. As relações teõ ricas M/L e HCaII/KCaII, assim como a absorção interna, mostram diferenças entre os três tipos de núcleos. Multidiaphragm UBV photometry and spectrophotometry of the central region for spiral galaxies are given for NGC 5530, 5643 and 6699, with normal nucleus; NGC 1097, 2997 and 5236, with peculiar nucleus; and NGC 1566, 3783 and IC4329A, with Seyfert nucleus. A mathematical algorithm, to treat data of the equivalent width of the absorption lines, was developped in order to obtain the stellar composition of the nuclei. The solution of this problem was found by the method of constrained non-linear. programming. The spectral continua and the UBV observations were compared with the theoretical results to obtain the internal absorption and the reddening of the nuclei. The age of the last cicle of formation of stars, the internal absorption and the theoretical relations M/L and HCaII/KCaII, show differences among the three types of nuclei.