arXiv subjects

B. G. Castanheira

Publications and source records attributed to B. G. Castanheira.

16 recordsLinked to original sources

arXiv2021

Effects of Magnetic Fields in Hot White Dwarfs

In this work, we study the effects of temperature on magnetic white dwarfs. We model their interior as a nuclei lattice surrounded by a relativistic free Fermi gas of electrons, accounting for effects from temperature, Landau levels and anomalous magnetic moment. We find that, at low densities (corresponding to the outer regions of star), both temperature and magnetic field effects play an important role in the calculation of microscopic thermodynamical quantities. To study macroscopic stellar structures within a general-relativistic approach, we solve numerically the coupled Einstein's-Maxwell's equations for fixed entropy per particle configurations and discuss how temperature affects stellar magnetic field profiles, masses and radii.

astro-ph.SR↗

arXiv2018

GD358: three decades of observations for the in-depth asteroseismology of a DBV star

We report on the analysis of 34 years of photometric observations of the pulsating helium atmosphere white dwarf GD358. The complete data set includes archival data from 1982-2006, and 1195.2 hours of new observations from 2007- 2016. From this data set, we extract 15 frequencies representing g-mode pulsation modes, adding 4 modes to the 11 modes known previously. We present evidence that these 15 modes are ell = 1 modes, 13 of which belong to a consecutive sequence in radial overtone k. We perform a detailed asteroseismic analysis using models that include parameterized, complex carbon and oxygen core composition profiles to fit the periods. Recent spectroscopic analyses place GD358 near the red edge of the DBV instability strip, at 24,000 plus or minus 500 K and a log g of 7.8 plus or minus 0.08 dex. The surface gravity translates to a mass range of 0.455 to 0.540 solar masses. Our best fit model has a temperature of 23,650 K and a mass of 0.5706 solar masses. That is slightly more massive than suggested by most the recent spectroscopy. We find a pure helium layer mass of 10^-5.50, consistent with the result of previous studies and the outward diffusion of helium over time.

astro-ph.SR↗

arXiv2018

The McDonald Observatory search for pulsating sdA stars: asteroseismic support for multiple populations

Context. The nature of the recently identified "sdA" spectroscopic class of star is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravities intermediate to main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors, or blue stragglers in the halo. Aims. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by any sdA stars would contribute importantly to our developing understanding of this class. Methods. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signatures from four new pulsators. Results. Out of 23 sdA stars observed, we clearly detect stellar pulsations in seven. Dominant pulsation periods range from 4.6 minutes to 12.3 hours, with most on ~hour timescales. We argue specific classifications for some of the new variables, identifying both compact and likely main sequence dwarf pulsators, along with a candidate low-mass RR Lyrae star. Conclusions. With dominant pulsation periods spanning orders of magnitude, the pulsational evidence supports the emerging narrative that the sdA class consists of multiple stellar populations. Since multiple types of sdA exhibit stellar pulsations, follow-up asteroseismic analysis can be used to probe the precise evolutionary natures and stellar structures of these individual subpopulations.

astro-ph.SR↗

arXiv2018

The sdA problem - II. Photometric and Spectroscopic Follow-up

Subdwarf A star (sdA) is a spectral classification given to objects showing H-rich spectra and sub-main sequence surface gravities, but effective temperature lower than the zero-age horizontal branch. Their evolutionary origin is an enigma. In this work, we discuss the results of follow-up observations of selected sdAs. We obtained time resolved spectroscopy for 24 objects, and time-series photometry for another 19 objects. For two targets, we report both spectroscopy and photometry observations. We confirm seven objects to be new extremely-low mass white dwarfs (ELMs), one of which is a known eclipsing star. We also find the eighth member of the pulsating ELM class.

astro-ph.SR↗

arXiv2016

Pruning The ELM Survey: Characterizing Candidate Low-Mass White Dwarfs Through Photometric Variability

We assess the photometric variability of nine stars with spectroscopic Teff and log(g) values from the ELM Survey that locate them near the empirical extremely low-mass (ELM) white dwarf instability strip. We discover three new pulsating stars: SDSS J135512.34+195645.4, SDSS J173521.69+213440.6 and SDSS J213907.42+222708.9. However, these are among the few ELM Survey objects that do not show radial velocity variations to confirm the binary nature expected of helium-core white dwarfs. The dominant 4.31-hr pulsation in SDSS J135512.34+195645.4 far exceeds the theoretical cutoff for surface reflection in a white dwarf, and this target is likely a high-amplitude delta Scuti pulsator with an overestimated surface gravity. We estimate the probability to be less than 0.0008 that the lack of measured radial velocity variations in four of eight other pulsating candidate ELM white dwarfs could be due to low orbital inclination. Two other targets exhibit variability as photometric binaries. Partial coverage of the 19.342-hr orbit of WD J030818.19+514011.5 reveals deep eclipses that imply a primary radius > 0.4 solar radii--too large to be consistent with an ELM white dwarf. The only object for which our time series photometry adds support to the ELM white dwarf classification is SDSS J105435.78-212155.9, with consistent signatures of Doppler beaming and ellipsoidal variations. We interpret that the ELM Survey contains multiple false positives from another stellar population at Teff < 9000 K, possibly related to the sdA stars recently reported from SDSS spectra.

astro-ph.SR↗

arXiv2014

Discovery of binarity, spectroscopic frequency analysis, and mode identification of the delta Sct star 4CVn

More than 40 years of ground-based photometric observations of the delta Sct star 4CVn revealed 18 independent oscillation frequencies, including radial as well as non-radial p-modes of low spherical degree l<=2. From 2008 to 2011, more than 2000 spectra were obtained at the 2.1-m Otto-Struve telescope at the McDonald Observatory. We present the analysis of the line-profile variations, based on the Fourier-parameter fit method, detected in the absorption lines of 4CVn, which carry clear signatures of the pulsations. From a non-sinusoidal, periodic variation of the radial velocities, we discovered that 4CVn is an eccentric binary system, with an orbital period Porb = 124.44 +/- 0.03 d and an eccentricity e = 0.311 +/- 0.003. We firmly detect 20 oscillation frequencies, 9 of which are previously unseen in photometric data, and attempt mode identification for the two dominant modes, f1 = 7.3764 c/d and f2 = 5.8496 c/d, and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, vsini ~ 106.7 km/s, translates to a rotation rate of veq/vcrit >= 33%. This relatively high rotation rate hampers unique mode identification, since higher-order effects of rotation are not included in the current methodology. We conclude that, in order to achieve unambiguous mode identification for 4CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.

astro-ph.SR↗

arXiv2010

New developments of the ZZ Ceti instability strip: The discovery of eleven new variables

Using the SOAR 4.1 m telescope, we report on the discovery of low amplitude pulsations for three stars previously reported as Not-Observed-to-Vary (NOV) by Mukadam et al. (2004) and Mullally et al. (2005), which are inside the ZZ Ceti instability strip. With the two pulsators discovered by Castanheira et al. (2007), we have now found variability in a total of five stars previously reported as NOVs. We also report the variability of eight new pulsating stars, not previously observed, bringing the total number of known ZZ Ceti stars to 148. In addition, we lowered the detection limit for ten NOVs located near the edges of the ZZ Ceti instability strip. Our results are consistent with a pure mass dependent ZZ Ceti instability strip.

astro-ph.SR↗

arXiv2007

Seismological studies of ZZ Ceti stars. I. The model grid and the application to individual stars

We calculate and explore an extensive adiabatic model grid for pulsating white dwarfs with H dominated atmospheres, the ZZ Ceti stars. We also compared the computed modes with the observed ones for five ZZ Ceti stars that are a representative sample of the whole class of pulsators. We describe our new approach for seismological studies, using the relative observed amplitudes to give weights for the periods in the fit and the external mass and temperature determinations as a guide. Our seismological study is clear evidence that seismology is indeed a powerful tool in the study of stellar structure and evolution.

astro-ph↗

arXiv2007

White Dwarf Mass Distribution in the SDSS

We determined masses for the 7167 DA and 507 DB white dwarf stars classified as single and non-magnetic in data release four of the Sloan Digital Sky Survey (SDSS). We obtained revised Teff and log g determinations for the most massive stars by fitting the SDSS optical spectra with a synthetic spectra grid derived from model atmospheres extending to log g=10.0. We also calculate radii from evolutionary models and create volume-corrected mass distributions for our DA and DB samples. The mean mass for the DA stars brighter than g=19 and hotter than Teff=12000K is M(DA)= 0.593+/-0.016M(Sun). For the 150 DBs brighter than g=19 and hotter than Teff=16000K, we find M(DB)=0.711+/-0.009 M(Sun). It appears the mean mass for DB white dwarf stars may be significantly larger than that for DAs. We also report the highest mass white dwarf stars ever found, up to 1.33 M(Sun).

astro-ph↗

arXiv2006

Towards a pure ZZ Ceti instability strip

We have observed again two stars inside the ZZ Ceti instability strip that were previously classified as not-observed-to-vary (NOV) by Mukadam et al. (2004) and found them to be low-amplitude variables. Some evidence points to a pure ZZ Ceti instability strip; other evidence contests it. The two stars previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic effective temperatures that place them inside the ZZ Ceti instability strip, and they were "contaminating" the strip as constant stars, which could indicate that the instability strip was no longer a simple evolutionary stage. A pure instability strip indicates that pulsation is a normal phase which all DAs must go through. We used effective temperatures derived from SDSS optical spectra by comparing them with model atmospheres to look for pulsators through time-resolved photometry and stars previously classified as NOV. Our new results indicate, but do not prove, a pure instability strip, because there are still other NOV stars that need to be observed again. Additionally, we have discovered five other ZZ Ceti stars based on their effective temperatures.

astro-ph↗

arXiv2006

Gemini spectra of 12000K white dwarf stars

We report signal-to-noise ratio SNR ~ 100 optical spectra for four DA white dwarf stars acquired with the GMOS spectrograph of the 8m Gemini north telescope. These stars have 18 ~ 21) optical spectra acquired by the Sloan Digital Sky Survey through model atmosphere fitting are trustworthy. Our spectra range from 3800A to 6000A, therefore including H beta to H9. The H8 line was only marginally present in the SDSS spectra, but is crucial to determine the gravity. When we compare the values published by Kleinman et al. (2004) and Eisenstein et al. (2006) with our line-profile (LPT) fits, the average differences are: Delta Teff ~ 320 K, systematically lower in SDSS, and Delta log g ~ 0.24 dex, systematically larger in SDSS. The correlation between gravity and effective temperature can only be broken at wavelengths bluer than 3800 A. The uncertainties in Teff are 60% larger, and in log g larger by a factor of 4, than the Kleinman et al. (2004) and Eisenstein et al. (2006) internal uncertainties.

astro-ph↗

arXiv2006

Analysis of IUE spectra of helium-rich white dwarf stars

We studied the class of DB white dwarf stars, using re-calibrated UV spectra for thirty four DBs obtained with the IUE satellite. By comparing the observed energy distributions with model atmospheres, we simultaneously determine spectroscopic distances (d), effective temperature (Teff), and surface gravities (log g). Using parallax measurements and previous determinations of Teff and log g from optical spectra, we can study whether the atmospheres of eleven DB stars are consistent with pure He or have a small amount of H contamination. We also report on our observations of seventeen stars with Teff close to the DB instability strip through time series photometry and found them to be non variable within our detection limits.

astro-ph↗

arXiv2005

Discovery of eleven new ZZ Ceti stars

We report the discovery of eleven new ZZ Cetis using telescopes at OPD (Observatório do Pico dos Dias/LNA) in Brazil, the 4.1 m SOAR (Southern Astrophysical Research) telescope at Cerro Pachon, Chile, and the 2.1 m Otto Struve telescope at McDonald observatory. The candidates were selected from the SDSS (Sloan Digital Sky Survey) and SPY (ESO SN Ia progenitor survey), based on their Teff obtained from optical spectra fitting. This selection criterion yields the highest success rate of detecting new ZZ Cetis, above 90% in the Teff range from 12000 to 11000 K. We also report on a DA not observed to vary, with a Teff placing the star close to the blue edge of the instability strip. Among our new pulsators, one is a little bit cooler than this star for which pulsations were not detected. Our observations are an important constraint on the location of the blue edge of the ZZ Ceti instability strip.

astro-ph↗

arXiv2005

Discovery of fourteen new ZZ Cetis with SOAR

We report the discovery of fourteen new ZZ Cetis with the 4.1m Southern Astrophysical Research telescope, at Cerro Pachon, in Chile. The candidates were selected from the SDSS (Sloan Digital Sky Survey) DA white dwarf stars with Teff obtained from the optical spectra fit, inside the ZZ Ceti instability strip. Considering these stars are multi-periodic pulsators and the pulsations propagate to the nucleus of the star, they carry information on the structure of the star and evolution of the progenitors. The ZZ Cetis discovered till 2003 are mainly within 100 pc from the Sun, and probe only the solar vicinity. The recently discovered ones, and those reported here, may sample a distinct population as they were selected mainly perpendicular to the galactic disk and cover a distance up to ~400pc.

astro-ph↗

arXiv2005

Measuring the evolution of the most stable optical clock G 117-B15A

We report our measurement of the rate of change of period with time dP/dt for the 215 s periodicity in the pulsating white dwarf G 117-B15A, the most stable optical clock known. After 31 years of observations, we have finally obtained a 4 sigma measurement dP/dt_observed = (4.27 +/- 0.80) x 10^{-15} s/s. Taking into account the proper-motion effect of dP/dt_pm = (7.0 +/- 2.0) x 10^{-16} s/s, we obtain a rate of change of period with time of dP/dt = (3.57 +/- 0.82) x 10^{-15} s/s. This value is consistent with the cooling rate in our white dwarf models only for cores of C or C/O. With the refinement of the models, the observed rate of period change can be used to accurately measure the ratio of C/O in the core of the white dwarf.

astro-ph↗

arXiv2005

Eleven New DAVs from the Sloan Survey

We report the discovery of eleven new variable DA white dwarf (ZZ Ceti) stars. Candidates were selected by deriving temperatures from model fits to spectra obtained from the Sloan Digital Sky Survey. We also find objects whose temperatures and gravities indicate they lie within the instability strip for pulsation, but which were not observed to vary. Although the temperatures are based on relatively low S/N spectra, an impure strip is unexpected, and if confirmed suggests that our knowledge of the pulsation mechanism is incomplete. This work brings the total number of published variable DA white dwarf stars to 82.

astro-ph↗