Carbon-rich (DQ) white dwarfs in the Sloan Digital Sky Survey

Among the spectroscopically identified white dwarfs, a fraction smaller than 2% have spectra dominated by carbon lines, mainly molecular C2, but also in a smaller group by CI and CII lines. These are together called DQ white dwarfs. We want to derive atmospheric parameters Teff,log g, and carbon abundances for a large sample of these stars and discuss implications for their spectral evolution. Sloan Digital Sky Survey spectra and ugriz photometry were used, together with GAIA Data Release 2 parallaxes and G band photometry. These were fitted to synthetic spectra and theoretical photometry derived from model atmospheres. We found that the DQs hotter than Teff ~10000 K have masses ~0.4 Msun larger than the cooler ones, which have masses typical for the majority of white dwarfs, ~0.6 Msun. A significant fraction of the hotter objects with Teff > 14500 K have atmospheres dominated by carbon.Comment: Accepted by Astronomy & Astrophysic

The temporal changes of the pulsational periods of the pre-white dwarf PG 1159-035

PG 1159-035, a pre-white dwarf with T=140000 K, is the prototype of the PG1159 spectroscopic class and the DOV pulsating class. Changes in the star cause variations in its oscillation periods. The measurement of temporal change in the oscillation periods, dP/dt, allows us to estimate directly rates of stellar evolutionary changes, such as the cooling rate and the envelope contraction rate, providing a way to test and refine evolutionary models for pre-white dwarf pulsating stars. We measured 27 pulsation modes period changes. The periods varied at rates of between 1 and 100 ms/yr, and several can be directly measured with a relative standard uncertainty below 10%. For the 516.0 s mode (the highest in amplitude) in particular, not only the value of dP/dt can be measured directly with a relative standard uncertainty of 2%, but the second order period change, d(dP/dt)/dt, can also be calculated reliably. By using the (O-C) method we refined the dP/dt and estimated the d(dP/dt)/dt for six other pulsation periods. As a first application, we calculated the change in the PG 1559-035 rotation period, dP_rot/dt = -2.13*10^{-6} s/s, the envelope contraction rate dR/dt = -2.2*10^{-13} solar radius/s, and the cooling rante dT/dt = -1.42*10^{-3} K/s.Comment: 8 pages; 2 figures; 2 tables; appendix with 2 table

Pulsating White Dwarfs

The Sloan Digital Sky Survey has allowed us to increase the number of known white dwarfs by a factor of five and consequently the number of known pulsating white dwarfs also by a factor of five. It has also led to the discovery of new types of variable white dwarfs, as the variable hot DQs, and the pulsating Extremely Low Mass white dwarfs. With the Kepler Mission, it has been possible to discover new phenomena, the outbursts present in a few pulsating white dwarfs.Comment: 10 pages, 2 figures, Wide-field variability surveys: a 21st-century perspective, 22nd Los Alamos Stellar Pulsation Conference Series meeting, San Pedro de Atacama, Chile, Nov. 28 - Dec. 2, 201

Seismology Of White Dwarfs: The ZZ Ceti Stars

We calculate an extensive adiabatic model grid for pulsating white dwarfs with H dominated atmospheres, the ZZ Ceti stars. We developed a new approach for asteroseismology, using the relative observed amplitudes as weights, and compared the computed modes with the observed ones for the class of ZZ Ceti stars. We measure the H layer mass for 83 stars and found an average of M(H)/M(*) = 10(-6.3), which is thinner than the predicted value of M(H)/M(*) = 10(-4). Our results indicate that the stars lose more mass during their evolution than previously expected.Astronom

Panchromatic fits to the Globular Cluster NGC 6366

We present panchromatic isochrone fits to the color magnitude data of the globular cluster NGC 6366, based on HST ACS/WFC and SOAR photometric data. Before performing the isochrone fits, we corrected the photometric data for differential reddening and calculated the mean ridge line of the color magnitude diagrams. We compared the isochrones of Dartmouth Stellar Evolution Database and PAdova and TRieste Stellar Evolution Code (with microscopic diffusion starting on the main sequence). Based on previous determinations of the metallicity of this cluster we test it from [Fe/H]=-1.00 to [Fe/H]= -0.50, and the age from 9 to 13 Gyrs. The uncertainties do not decrease when we fit simultaneous colors. We also find that the Dartmouth Stellar Evolution Database isochrones have a better fit in the sub giant branch and low main sequence than the PAdova and TRieste Stellar Evolution Code. Considering the most recent spectroscopic determination of the metallicity ([Fe/H]= -0.67), we find E(B-V)=0.69+/-0.02, (m-M)_V=15.02+/-0.07 and 11+/-2 Gyr for NGC 6366.Comment: 4 pages, 2 figures. Proceedings of the conference "Reading the book of globular clusters with the lens of stellar evolution", to be published in Memorie della Societ\'a Astronomica Italian

Low mass variable stars in the globular cluster NGC 6397

We have conducted a photometric survey of the globular cluster NGC 6397 in a search for variable stars. We obtained ~11h of time-resolved photometric images with one ne European Southern Observatory-Very Large Telescope using the FOcal Reducer and low dispersion Spectrograph imager distributed over two consecutive nights. We analyzed 8391 light curves of stars brighter than magnitude 23 with the 465 nm-filter, and we identified 412 variable stars, reaching ~ 4.8 +- 0.2 per cent of variability with timescales between 0.004 and 2d, with amplitudes variation greater than +- 0.2 mag.Comment: 9 figures, complementary dat

The age-metallicity dependence for white dwarfs

We present a theoretical study on the metallicity dependence of the initial$-$to$-$final mass relation and its influence on white dwarf age determinations. We compute a grid of evolutionary sequences from the main sequence to $\sim 3\, 000$ K on the white dwarf cooling curve, passing through all intermediate stages. During the thermally-pulsing asymptotic giant branch no third dredge-up episodes are considered and thus the photospheric C/O ratio is below unity for sequences with metallicities larger than $Z=0.0001$. We consider initial metallicities from $Z=0.0001$ to $Z=0.04$, accounting for stellar populations in the galactic disk and halo, with initial masses below $\sim 3M_{\odot}$. We found a clear dependence of the shape of the initial$-$to$-$final mass relation with the progenitor metallicity, where metal rich progenitors result in less massive white dwarf remnants, due to an enhancement of the mass loss rates associated to high metallicity values. By comparing our theoretical computations with semi empirical data from globular and old open clusters, we found that the observed intrinsic mass spread can be accounted for by a set of initial$-$to$-$final mass relations characterized by different metallicity values. Also, we confirm that the lifetime spent before the white dwarf stage increases with metallicity. Finally, we estimate the mean mass at the top of the white dwarf cooling curve for three globular clusters NGC 6397, M4 and 47 Tuc, around $0.53 M_{\odot}$, characteristic of old stellar populations. However, we found different values for the progenitor mass, lower for the metal poor cluster, NGC 6397, and larger for the younger and metal rich cluster 47 Tuc, as expected from the metallicity dependence of the initial$-$to$-$final mass relation.Comment: Accepted for publication in MNRA

Unified line profiles for hydrogen perturbed by collisions with protons: satellites and asymmetries

We present new calculations of unified line profiles for hydrogen perturbed by collisions with protons. We report on new calculations of the potential energies and dipole moments which allow the evaluation of profiles for the lines of the Lyman series up to Lyman$\delta$ and the Balmer series up to Balmer10. Unified calculations only existed for the lines Lyman$\alpha$ to Lyman$\gamma$ and Balmer$\alpha$ including the H$_2^+$ quasi-molecule. These data are available as online material accompanying this paper and should be included in atmosphere models, in place of the Stark effect of protons, since the quasi-molecular contributions cause not only satellites, but large asymmetries that are unaccounted for in models that assume Stark broadening of electrons and protons are equal.Comment: 13 pages, 25 figures. Accepted for publication in MNRA