PG 1159 Stars Research Articles

An asteroseismic test of diffusion theory in white dwarfs

The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still taking place, allowing asteroseismic tests of the theory. We have obtained dual-site observations of the pulsating DB star CBS114, to complement existing observations of the slightly cooler star GD358. We recover the 7 independent pulsation modes that were previously known, and we discover 4 new ones to provide additional constraints on the models. We perform objective global fitting of our updated double-layered envelope models to both sets of observations, leading to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG1159 stars.

On the Nature of Stochastic Excited Oscillations: Subdwarf B Star PG 1605+072 as a Case Study

It is well known that the near discontinuities of the background state thermodynamic quantities, like those encountered in the upper layers of some stars, reflect acoustic-gravity waves and thereby partially contain them within the stellar interior. These reflections help to establish resonant cavities in which oscillations of discrete characteristic frequencies propagate. These oscillations, when excited, produce variations on the light curve. We present an investigation of the observed light curves of variable stars using a theoretical time series calculated on the assumption that the observed light curve is the superposition of sinusoidal waves of a given frequency, each one representing a stellar pulsation. By analyzing different simulated light curves, we show that the nature of the excitation of pulsating stars can be gathered from the time dependence of the observed mode's amplitudes, in spite of the limited information contained in some observational data. This analysis is based on the temporal variation of the oscillation amplitudes during several consecutive nights. It is expected that if an oscillation is excited and damped by a mechanism in stochastic equilibrium, the probability distribution of its amplitude, A, will be well defined. An interesting property is that the ratio of the standard deviation σA over the mean value μA is of the order of 0.52. Furthermore, we show from observational light curves that there is no stochastic mechanism exciting the oscillations in the subdwarf B star PG 1605+072. This new independent observational result solidifies the case that in subdwarf B stars, pulsations are originated by the excitation of intrinsically overstable modes.

Fluorine in extremely hot post-AGB stars: Evidence for nucleosynthesis

We have discovered lines of highly ionized fluorine ($\ion{F}{v}$ and $\ion{F}{vi}$) in the far-UV spectra of extremely hot ($T_\mathrm{eff}$ = 85 000–150 000 K) post-AGB stars. Our sample comprises H-rich central stars of planetary nebulae as well as H-deficient PG1159 stars. We performed non-LTE calculations and find strong F overabundances (up to 10 -4 by mass, i.e., 250 times solar) in a number of PG1159 stars, while F is essentially solar in the H-rich stars. Since PG1159 stars are believed to exhibit intershell matter of the preceding AGB phase on their surface, their chemical analyses allow for a direct insight into nucleosynthesis processes during the AGB phase. The high F abundances in PG1159 stars confirm the conclusion from abundance determinations in giants, that F is synthesized in AGB stars and that the F enrichment in the intershell must be very high.

Identification of a DO white dwarf and a PG 1159 star in the ESO SN Ia progenitor survey (SPY)

We present high-resolution VLT spectra of a new helium-rich DO white dwarf and a new PG1159 star, which we identified in the ESO SPY survey. We performed NLTE model atmosphere analyses and found that the PG1159 star is a low-gravity, extremely hot (Teff=160,000K, logg=6) star, having a carbon-helium dominated atmosphere with considerable amounts of oxygen and neon (He=38%, C=54%, O=6%, Ne=2% by mass). It is located within the planetary nebula nuclei instability strip, hence, future searches for an associated PN as well as for stellar pulsations might be successful. The DO white dwarf is a unique object. From the relative strength of neutral and ionized helium lines we found Teff approx. 60,000K, however, the HeII lines are extraordinarily strong and cannot be fitted by any model. Similar problems were encountered with hot subdwarfs and white dwarfs showing signatures of a super-hot wind. The reason is unknown.

Radial-velocity and line-profile variations in the sdBV star PG 1325+101

In this paper we present time-series low-resolution spectroscopy of the pulsating sdB star PG 1325+101, obtained with the Nordic Optical Telescope. We have cross-correlated the spectra and Fourier analysed the variations in radial velocity and the width of the cross-correlation function. From both resulting periodograms we easily recover the known main pulsation period. Further analysis of the spectra, after binning them in phase according to the pulsation period, clearly shows evidence for line-profile variations consistent with a radial pulsation mode. The variations show up in all Balmer and several helium lines sampled by the spectra, and can be understood in terms of temperature, gravity and radial-velocity variations. To the best of our knowledge PG 1325+101 is the faintest object in which line-profile variations due to pulsational instability have been detected so far.

The double-layered chemical structure in DB white dwarfs

We study the structure and evolution of white dwarf stars with helium-rich atmospheres (DB) in a self-consistent way with the predictions of time-dependent element diffusion. Our treatment of diffusion includes gravitational settling and chemical and thermal diffusion. OPAL radiative opacities for arbitrary metallicity and carbon-and oxygen-rich compositions are employed. Emphasis is placed on the evolution of the diffusion-modeled double-layered chemical structure. This structure, which is characterized by a pure helium envelope atop an intermediate remnant shell rich in helium, carbon and oxygen, is expected for pulsating DB white dwarfs, assuming that they are descendants of hydrogen-deficient PG1159 post-AGB stars. We find that, depending on the stellar mass, if DB white dwarf progenitors are formed with a helium content smaller than \approx 10^-3 M_*, a single-layered configuration is expected to emerge during the DB pulsation instability strip. We also explore the consequences of diffusively evolving chemical stratifications on the adiabatic pulsational properties of our DB white dwarf models. In this context, we find that the evolving shape of the chemical profile translates into a distinct behaviour of the theoretical period distribution as compared with the case in which the shape of the profile is assumed to be fixed during the evolution across the instability strip. Finally, we extend the scope of the calculations to the domain of the helium-rich carbon-contaminated DQ white dwarfs. In particular, we speculate that DQ white dwarfs with low detected carbon abundances would not be descendants of the PG1159 stars.

On the discovery of an enormous ionized halo around the hot DO white dwarf PG 1034+001

The discovery of the largest known planetary nebula on the sky surrounding the DO white dwarf PG 1034+001 with an apparent diameter of about 2°, corresponding to a linear diameter of 3.5-7.0 pc at the likely distance of 100-200 pc, has been reported by Hewett et al. (2003). A careful inspection of available sky survey data has now shown that this planetary nebula, Hewett 1, is surrounded by an elliptical emission shell with an apparent diameter of 6° x 9° (16.2 +6.1 -4.5 x 24.3 +9.1 -6.8 pc at d = 155 +58 -43 pc). A further emission structure, detected northeast of the central star, may indicate another shell with a size of 10° x 16°. From presently available observational data we do not have indications revealing whether the emission arises from material ejected from PG 1034+001 or from ionized ambient ISM. Improved proper motion data combined with radial velocity and distance data from the literature have enabled us to derive a Galactic orbit for the central star PG 1034+001. Its thin disk orbit and the morphology of the first halo suggest that the nebula is in an advanced stage of interaction with the interstellar medium.

Radial velocity and line-profile variations in the sdBV star PG 1325+101

In this report we present preliminary results from time-series low-resolution spectroscopy of the pulsating sdB star PG 1325+101 obtained with the Nordic Optical Telescope. We have cross-correlated the spectra and Fourier analysed the resulting radial velocities. From the resulting periodograms we easily recover the known main pulsation period. Further analysis of the spectra, after binning them in phase according to the pulsation period, clearly shows evidence for line-profile variations consistent with a radial pulsation mode. The variations show up in all Balmer lines and several Helium lines sampled by the spectra, and can be understood in terms of temperature, gravity and radial velocity variations. To the best of our knowledge PG 1325+101 is the faintest object in which line-profile variation due to pulsational instability has been detected so far.

MSST observations of the pulsating sdB star PG 1605+072

We present the first results from the MultiSite Spectroscopic Telescope (MSST) observations of the sdBV star PG 1605+072. Pulsating sdB stars (also known as EC 14026 stars) offer the chance to gain new insights into the formation and evolution of extreme Horizontal Branch stars using the tools of asteroseismology. PG 1605+072 is an outstanding object in its class, with the richest frequency spectrum, the longest periods, and the largest variations. The MSST campaign took place in May/June 2002 immediately following the Whole Earth Telescope Xcov22 run, which observed PG 1605+072 as an alternate target. We will first give an overview of the project and its feasibility, after which we will present the massive data set, made up of 399 h of photometry and 151 h of spectroscopy. The overall aims of the project are to examine light/velocity amplitude ratios and phase differences, changes in equivalent width/line index, and λ-dependence of photometric amplitudes, and to use these properties for mode identification.

Multisite spectroscopic and photometric observations of the pulsating sdB star PG 1605+072

AbstractWe present the first results from the MultiSite Spectroscopic Telescope (MSST) observations of the sdBV star PG 1605+072. Pulsating sdB stars (V361 Hya stars) offer the chance to gain new insights into the formation and evolution of extreme Horizontal Branch stars using the tools of asteroseismology. PG 1605+072 is an outstanding object in its class, with the richest frequency spectrum, the longest periods, and the largest variations. The MSST campaign took place in 2002 May/June and we present here the massive data set, made up of 399 hr of photometry and 151 hr of spectroscopy. The overall aims of the project are to examine light/velocity amplitude ratios and phase differences, changes in equivalent width/line index, and λ-dependence of photometric amplitudes, and to use these properties for mode identification.

Preliminary results of the multisite time-series campaign on the sdB pulsator PG 1325+101

We present the preliminary results of more than 160 h of time-series photometry on the subdwarf B pulsating star PG 1325+101 (Teff= 34 500 K log g= 6.1), obtained in March-April 2003 at nine different sites. The amplitude spectrum confirms the presence of the four frequencies found in the discovery run at the NOT in July 2001 (but with significantly lower amplitudes in two of them) and also shows at least two new low amplitude signals. Moreover, there is no sign of frequency splitting near the main peak at 7256 μHz, suggesting that this mode might be actually radial as indicated by the time-resolved spectroscopy (Ostensen and Telting, 2004).

A multi-site campaign on the long period variable subdwarf b star PG 1627+017

We report on the outcome of the first major multi-site campaign on a long-period variable subdwarf B star. The target PG 1627+017 was observed for a total of 334 h during April/May/June 2003 from the lynchpin observatory at Mt. Bigelow, Arizona, with important contributions coming from co-observations at SAAO, Calar Alto and Siding Spring. Preliminary analysis indicates the presence of 8–10 probable periods in the range ∼4500 to ∼8900 s with relative amplitudes between 0.1 and 0.5%;. The range over which the periods are found is in qualitative agreement with theoretical predictions and the number of periods found shows potential for an in-depth asteroseismological analysis. Multi-colour observations show that the relative amplitudes of pulsation are larger in the U-band than in the R, which again is in agreement with theory. However, more multi-colour observations are needed to be able to make statements at the quantitative level.

Time-series spectroscopy of pulsating sdB stars -- III.Line indices of PG 1605+072

We present the detection and analysis of line index variations in the pulsating sdB star PG 1605+072. We have found a strong dependence of line index amplitude on Balmer line order, with high-order Balmer line amplitudes up to 10 times larger than Hβ. Using a simple model, we have found that the line index may not only be dependent on temperature, as is usually assumed for oscillating stars, but also on surface gravity. This information will provide another set of observables that can be used for mode identification of sdBs.

PG 1605+072 in Wet XCov22: Support for the Multi Site Spectroscopic Telescope

AbstractThe Multi-site spectroscopic telescope is a virtual instrument and the name of a collaboration that opens up a new observational window by combining continuous observations of spectroscopic variations and simultaneous photometric monitoring. This constitutes an enormous observational effort, but in return promises to finally provide access to a mode identification for and an asteroseismological analysis of the pulsating sdB star PG 1605+072. Multi-Site Spectroscopic Telescope observations for this object have been secured during a large coordinated campaign in May and June of the year 2002. The frequency resolution and coverage of the photometric time series has been noticeably enhanced by a significant contribution from the Whole Earth Telescope, which was used to observe PG 1605+072 as an alternate target during the WET XCov22 campaign, also conducted in May 2002. This paper briefly outlines the motivation for the MSST project and tries to give a first assessment of the overall quality of the data obtained, with a focus on the Whole Earth Telescope observations.

PG 1613+426: A new sdB pulsator

We report the detection of short period oscillations in the hot subdwarf B (sdB) star PG 1613+426 from time-series photometry carried out with the 91-cm Cassegrain telescope of the Catania Astrophysical Observatory. This star, which is brighter than the average of the presently known sdB pulsators, with $B = 14.14$ mag, has $T_{\rm eff}=34\,400~{\rm K}$ and $\log g = 5.97$, its position is near the hot end of the sdB instability strip, and it is a pulsator with a well observed peak in the power spectrum at $144.18\pm 0.06\,\rm s$. This star seems to be well suited for high precision measurements, which could detect a possible multi-mode pulsation behaviour.

Asteroseismology of Pulsating sdB Stars Observed at Catania Astrophysical Observatory

AbstractWe present analysis of the data collected during three years of photometric campaigns at the 91 cm Cassegrain telescope of the Catania Astrophysical Observatory to observe the pulsating subdwarf B star PG 1336-018. For this eclipsing binary system, we obtained a precise determination of the orbital period which turns out to be P = 0.10101599±0.00000002 days. Time-series photometry reveals pulsation periods near 174 and 184 seconds. We also discuss a preliminary analysis of the data obtained in 2002 for the pulsating subdwarf PG 1325+101.

Time-series spectroscopy of pulsating sdB stars - II. Velocity analysis of PG 1605+072

We present the analysis of time-resolved spectroscopy of the pulsating subdwarf B (sdB) star PG 1605+072. From our main observing run of 16 nights we have detected velocity variations at five frequencies that correspond to those found in photometry. Based on these data, there appears to be change in amplitude of the dominant modes over about a year. However, when we include extra observations to improve the frequency resolution, we find that some of the frequencies are split into two or three. Simulations suggest that the apparent amplitude variation can be at least partially explained by a series of very closely spaced frequencies around the two strongest modes. By combining observations taken over ∼300 d we conclude that some of the closely spaced modes are caused by one mode the amplitude of which is varying, however this frequency is still within ∼1 μHz of an apparently stable frequency. Because of this kind of complexity and uncertainty we advise caution when trying to identify oscillation modes in this star.

FUSEobservations of PG1342444: new insights into the nature of the hottest DA white dwarfs

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the very hot (T e f f 60 000 K) DA white dwarf PG1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-A O V I lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called O V I central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement (T e f f 67 000 and 54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the C I I I multiplet near 1176 A, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191-B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study.

PG 1325+101 and PG 2303+019: Two new large amplitude subdwarf B pulsators

We report the detection of short period oscillations in the sdB stars PG 1325+101 (B = 13.8) and PG 2303+019 (alias HS 2303+0152, B = 16.0) from time-series photometry made at the Nordic Optical Telescope of a sample of 21 candidates. Both stars are multi-mode pulsators with at least three distinct periods in the range 100-140 s, and relatively large amplitudes up to 2.6 and 1.6% respectively. Moreover PG 1325+101 shows the shortest pulsation period ever registered among sdBV stars, 68.9 s, which corresponds to the first harmonic of the main signal. Following previous temperature and gravity determinations for PG 1325+101 (T e f f = 34500 K, log g = 6.1) and our NLTE model atmosphere analysis for PG 2303+019 (T e f f = 35 200 K, log g = 5.7), both stars are well inside the theoretical sdBV instability strip.

Boron Abundances in B‐Type Stars: A Test of Rotational Depletion during Main‐Sequence Evolution

Boron abundances have been derived for seven main-sequence B-type stars from Hubble Space Telescope STIS spectra around the B III λ2066 line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but is clearly depleted. In the other four stars, boron is undetectable, implying depletions of 1-2 dex. Three of these four stars are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only rotationally induced mixing predicts that boron depletions are unaccompanied by nitrogen enrichments. The inferred rate of boron depletion from our observations is in good agreement with these predictions. Other boron-depleted nitrogen-normal stars are identified from the literature. In addition, several boron-depleted nitrogen-rich stars are identified, and while all fall on the boron-nitrogen trend predicted by rotationally induced mixing, a majority have nitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron group (Cr, Mn, Fe, and Ni) abundances. The seven B-type stars have near-solar iron group abundances, as expected for young stars in the solar neighborhood. We have also analyzed the halo B-type star PG 0832+676. We find [Fe/H] = -0.88 ± 0.10, and the absence of the B III line gives the upper limit [B/H] < -2.5. These and other published abundances are used to infer the star's evolutionary status as a post-asymptotic giant branch star.