Non-radial G-mode Pulsations Research Articles

A pulsating white dwarf in an eclipsing binary

White dwarfs are the burnt out cores of Sun-like stars and are the final fate of 97% of all stars in our Galaxy. The internal structure and composition of white dwarfs are hidden by their high gravities, which causes all elements, apart from the lightest ones, to settle out of their atmospheres. The most direct method to probe the inner structure of stars and white dwarfs in detail is via asteroseismology. Here we present the first known pulsating white dwarf in an eclipsing binary system, enabling us to place extremely precise constraints on the mass and radius of the white dwarf from the light curve, independent of the pulsations. This 0.325M$_\odot$ white dwarf --- one member of SDSS J115219.99+024814.4 --- will serve as a powerful benchmark to constrain empirically the core composition of low-mass stellar remnants and investigate the effects of close binary evolution on the internal structure of white dwarfs.

On the evolutionary status and pulsations of the recently discovered blue large-amplitude pulsators (BLAPs)

Abstract The blue large-amplitude pulsators (BLAPs) constitute a new class of pulsating stars. They are hot stars with effective temperatures of ∼30 000 K and surface gravities of log g ∼ 4.9, that pulsate with periods in the range 20−40 min. Until now, their origin and evolutionary state, as well as the nature of their pulsations, were not been unveiled. In this paper, we propose that the BLAPs are the hot counterpart of the already known pulsating pre-extremely low mass (pre-ELM) white dwarf (WD) stars, that are He-core low-mass stars resulting from interacting binary evolution. Using fully evolutionary sequences, we show that the BLAPs are well represented by pre-ELM WD models with high effective temperature and stellar masses ∼0.34 M⊙. From the analysis of their pulsational properties, we find that the observed variabilities can be explained by high-order non-radial g-mode pulsations or, in the case of the shortest periods, also by low-order radial modes, including the fundamental radial mode. The theoretical modes with periods in the observed range are unstable due to the κ mechanism associated with the Z-bump in the opacity at log T ∼ 5.25.

Radial-velocity measurements of the pulsating zirconium star: LS IV−14°116*

The helium-rich hot subdwarf LS IV -14 116 shows remarkably high surface abundances of zirconium, yttrium, strontium, and germanium, indicative of strong chemical stratification in the photosphere. It also shows photometric behaviour indicative of non-radial g-mode pulsations, despite having surface properties inconsistent with any known pulsational instability zone. We have conducted a search for radial velocity variability. This has demonstrated that at least one photometric period is observable in several absorption lines as a radial velocity variation with a semi-amplitude in excess of 5 km s$^{-1}$. A correlation between line strength and pulsation amplitude provides evidence that the photosphere pulsates differentially. The ratio of light to velocity amplitude is too small to permit the largest amplitude oscillation to be radial.

A new class of pulsating white dwarf of extremely low mass: the fourth and fifth members

We report the discovery of two new pulsating extremely low-mass (ELM) white dwarfs (WDs), SDSS J161431.28+191219.4 (hereafter J1614) and SDSS J222859.93+362359.6 (hereafter J2228). Both WDs have masses <0.25 Msun and thus likely harbor helium cores. Spectral fits indicate these are the two coolest pulsating WDs ever found. J1614 has Teff = 8880 +/- 170 K and log g = 6.66 +/- 0.14, which corresponds to a roughly 0.19 Msun WD. J2228 is considerably cooler, with a Teff = 7870 +/- 120 K and log g = 6.03 +/- 0.08, which corresponds to a roughly 0.16 Msun WD, making it the coolest and lowest-mass pulsating WD known. There are multiple ELM WDs with effective temperatures between the warmest and coolest known ELM pulsators that do not pulsate to observable amplitudes, which questions the purity of the instability strip for low-mass WDs. In contrast to the CO-core ZZ Ceti stars, which are believed to represent a stage in the evolution of all such WDs, ELM WDs may not all evolve as a simple cooling sequence through an instability strip. Both stars exhibit long-period variability (1184-6235 s) consistent with non-radial g-mode pulsations. Although ELM WDs are preferentially found in close binary systems, both J1614 and J2228 do not exhibit significant radial-velocity variability, and are perhaps in low-inclination systems or have low-mass companions. These are the fourth and fifth pulsating ELM WDs known, all of which have hydrogen-dominated atmospheres, establishing these objects as a new class of pulsating WD.

DISCOVERY OF AN ULTRAMASSIVE PULSATING WHITE DWARF

We announce the discovery of the most massive pulsating hydrogen-atmosphere (DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) = 9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425-595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains SNe Ia progenitor systems.

SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF

We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1?m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1?m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M = 0.88 ? 0.02 M ? and T eff = 12, 100 ? 140 K. These values are consistent with those derived from the optical spectra and photometric colors.

ON THE CHALLENGING VARIABILITY OF LS IV-14°116: PULSATIONAL INSTABILITIES EXCITED BY THE ϵ-MECHANISM

We investigate the pulsation driving mechanism responsible for the long-period photometric variations observed in LS IV-14{\deg}116, a subdwarf B star showing a He-enriched atmospheric composition. To this end, we perform detailed nonadiabatic pulsation computations over fully evolutionary post He-core-flash stellar structure models, appropriate for hot subdwarf stars at evolutionary phases previous to the He-core burning stage. We found that the variability of LS IV-14{\deg}116 can be attributed to nonradial g-mode pulsations excited by the {\epsilon}-mechanism acting in the He-burning shells that appear before the star settles on the He-core burning stage. Even more interestingly, our results show that LS IV-14{\deg}116 could be the first known pulsating star in which the {\epsilon}-mechanism of mode excitation is operating. Last but not least, we find that the period range of destabilized modes is sensitive to the exact location of the burning shell, something that might help to distinguish between the different evolutionary scenarios proposed for the formation of this star.

Discovery of Two New Accreting Pulsating White Dwarf Stars

We report the discovery of two new accreting pulsating white dwarf stars among the cataclysmic variables of the Sloan Digital Sky Survey: SDSS J074531.91+453829.5 and SDSS J091945.10+085710.0. We observe high-amplitude nonsinusoidal variations of 4.5%-7% at a period close to 1230 s in the optical light curves of SDSS J074531.91+453829.5 and a low-amplitude variation of 0.7%-1.6% near 260 s in the light curves of SDSS J091945.10+085710.0. We infer that these optical variations are a consequence of nonradial g-mode pulsations in the accreting primary white dwarfs of these cataclysmic variables. However, we cannot rule out the remote possibility that the 260 s period could be the spin period of the accreting white dwarf SDSS J091945.10+085710.0. We also uncovered a nonvariable, SDSS J171145.08+301320.0, during our search; our two observing runs exclude any pulsation-related periodicities in the range 85-1400 s with an amplitude >= 0.5%. This discovery paper brings the total number of known accreting white dwarf pulsators to 11.

MOSTDetectsg‐Modes in the Late‐Type Be Star β Canis Minoris (B8 Ve)

The Microvariability and Oscillations of Stars (MOST) satellite has detected low-amplitude light variations (?m ~ 1 mmag) in the Be star ? CMi (B8 Ve). The observations lasted 41 days and the variations have typical periods ~0.3 days. We demonstrate that the dominant frequencies are consistent with prograde high-order g-modes of m = -1 excited by the Fe bump of opacity in an intermediate-mass (?3.5 M?) star with a nearly critical rotation period of 0.38 days. This is the first detection of nonradial g-mode pulsations in a Be star later than B6 leading to the possibility that pulsations are excited in all classical Be stars.

High-resolution ultraviolet spectroscopy of PG 1159-035 with HST and FUSE

Context. PG 1159-035 is the prototype of the PG 1159 spectral class which consists of extremely hot hydrogen-deficient (pre-) white dwarfs. It is also the prototype of the GW Vir variables, which are non-radial g-mode pulsators. The study of PG 1159 stars reveals insight into stellar evolution and nucleosynthesis during AGB and post-AGB phases. Aims. We perform a quantitative spectral analysis of PG 1159-035 focusing on the abundance determination of trace elements. Methods. We have taken high-resolution ultraviolet spectra of PG 1159-035 with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. They are analysed with non-LTE line blanketed model atmospheres. Results. We confirm the high effective temperature with high precision (T eff = 140000 ± 5000 K) and the surface gravity of log g = 7. For the first time we assess the abundances of silicon, phosphorus, sulfur, and iron. Silicon is about solar. For phosphorus we find an upper limit of solar abundance. A surprisingly strong depletion of sulfur (2% solar) is discovered. Iron is not detected, suggesting an upper limit of 30% solar. This coincides with the Fe deficiency found in other PG 1159 stars. We redetermine the nitrogen abundance and find it to be lower by one dex compared to previous analyses. Conclusions. The sulfur depletion is in contradiction with current models of AGB star intershell nucleosynthesis. The iron deficiency confirms similar results for other PG 1159 stars and is explained by the conversion of iron into heavier elements by n-capture in the s-processing environment of the precursor AGB star. However, the extent of the iron depletion is stronger than predicted by evolutionary models. The relatively low nitrogen abundance compared to other pulsating PG 1159 stars weakens the role of nitrogen as a distinctive feature of pulsators and non-pulsators in the GW Vir instability strip.

Abell 43, a second pulsating “hybrid-PG 1159” star

We report observations of the planetary nebula nucleus Abell 43, obtained at the 2.5 m Nordic Optical Telescope, which show that it is a pulsator. Abell 43, a “hybrid-PG 1159” type star, is the second pulsator of this class, after HS 2324+3944. From the limited data set acquired, we find that Abell 43 exhibits at least two periods of 2600 s and 3035 s, the longest ones observed up to now in PG 1159 and “hybrid-PG 1159” pulsators. This strongly suggests that the variations are due to non-radial g-mode pulsations and cannot be a consequence of binarity. This discovery raises puzzling questions regarding the excitation mechanism in this H rich, C and O poor “hybrid-PG 1159” since the C and O abundances are too low to trigger the instability through the κ-mechanism induced by the partial ionization of C and O, a mechanism invoked to explain the instability in the PG 1159 stars and in the previously known “hybrid-PG 1159” pulsator HS 2324+3944.

Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209

We performed time-series photometry of the PG 1159-type star HE 1429-1209, which was recently discovered in the ESO SPY survey. We show that the star is a low-amplitude (≈0.05 mag) non-radial g-mode pulsator with a period of 919 s. HE 1429-1209 is among the hottest known post-AGB stars ( K) and, together with the known pulsator RX J2117.1+3412, it defines empirically the blue edge of the GW Vir instability strip in the HRD at high luminosities.

Hubble Space TelescopeObservations of Ultraviolet Oscillations in WZ Sagittae During the Decline from Outburst1

We present a time series analysis of Hubble Space Telescope observations of WZ Sge obtained in 2001 September, October, November, and December as WZ Sge declined from its 2001 July superoutburst. Previous analysis of these data showed the temperature of the white dwarf decreased from ~29,000 to ~18,000 K. In this study we binned the spectra over wavelength to yield ultraviolet light curves at each epoch that were then analyzed for the presence of the well-known 27.87 and 28.96 s oscillations. We detect the 29 s periodicity at all four epochs, but the 28 s periodicity is absent. The origin of these oscillations has been debated since their discovery in the 1970s, and competing hypotheses are based on either white dwarf nonradial g-mode pulsations or magnetically channelled accretion onto a rotating white dwarf. By analogy with the ZZ Ceti stars, we argue that the nonradial g-mode pulsation model demands a strong dependence of pulse period on the white dwarf's temperature. However, these observations show that the 29 s oscillation is independent of the white dwarf's temperature. Thus, we reject the white dwarf nonradial g-mode pulsation hypothesis as the sole origin of the oscillations. It remains unclear if magnetically funneled accretion onto a rapidly rotating white dwarf (or belt on the white dwarf) is responsible for producing the oscillations. We also report the detection of a quasi-periodic oscillations (QPO) with period ~18 s in the September light curve. The amplitudes of the 29 s oscillation and the QPO vary erratically on short timescales and are not correlated with the mean system brightness nor with each other.

The asteroseismological potential of the pulsating DB white dwarf stars CBS 114 and PG 1456+103

We have acquired 65 h of single-site time-resolved CCD photometry of the pulsating DB white dwarf star (DBV) CBS 114 and 62 h of two-site high-speed CCD photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103 is complicated and variable on time-scales of approximately 1 week and could only partly be deciphered with our measurements. The modes of CBS 114 are more stable in time and we were able to arrive at a frequency solution somewhat affected by aliasing, but still satisfactory, involving seven independent modes and two combination frequencies. These frequencies also explain the discovery data of the star, taken 13 yr earlier. We find a mean period spacing of 37.1 ± 0.7 s significant at the 98 per cent level between the independent modes of CBS 114 and argue that they are caused by non-radial g-mode pulsations of spherical degree l= 1. We performed a global search for asteroseismological models of CBS 114 using a genetic algorithm, and we examined the susceptibility of the results to the uncertainties of the observational frequency determinations and mode identifications (we could not provide m values). The families of possible solutions are identified correctly even without knowledge of m. Our best-fitting model suggests Teff= 21 000 K, M*= 0.730 M⊙ and log(MHe/M*) =−6.66, XO= 0.61. The latter value of the central oxygen mass fraction implies a rate for the 12C(α,γ)16O nuclear reaction near S300= 180 keV b, consistent with laboratory measurements.

The B-type variable HD 131120 modelled by rotational modulation

We present ground-based multi-colour Geneva photometry and high-resolution spectra of the variable B-type star HD 131120. All sets of data reveal a monoperiodic star with a period of 1.569 days. We compare the variability of the star with a non-radial g-mode pulsation model and with a rotational modulation model. The latter model appears to be the best explanation for the observed line-prole variations of the star. Moreover, contrary to the pulsational model, it is able to reproduce the behaviour of the rst three moments of the Si and He lines as well as the large variability of their equivalent width. We therefore remove the star from the list of new Slowly Pulsating B stars found from Hipparcos data.

Discovery and asteroseismological analysis of a new pulsating DB white dwarf star, PG 2246+121

We report 36.6 h of time-resolved CCD photometry of the DB white dwarf star PG 2246+121 and the discovery that it is a new pulsating variable. Analysis of our compact single-site data set allowed the detection of three mode multiplets, two triplets at 256 and 329 s, respectively, and one doublet at 286 s. The frequency splitting within those structures is exactly the same within the length and accuracy of our data set. We argue that these multiplets are the result of non-radial g-mode pulsations, most probably of spherical degree l=1, which then yields a formal stellar rotation period of 2.00±0.12 d. We suggest that the excited modes are three consecutive radial overtones of order 3–7, most likely k=4,5,6. This discovery's impact on the understanding of pulsating DB white dwarfs is discussed.

Optical variability of the B-type star HD 105382: Pulsation or rotation?

We present ground-based multi-colour Geneva photometry and high-resolution high S/N spectra of the variable B-type star HD 105382. We provide evidence that this star is not a Be star, as indicated in the literature. The monoperiodic variability found in the Hipparcos data is confirmed in our ground-based follow-up observations. All existing data give rise to the detection of the period of 1.295 days. We try to interpret the variability of the star in terms of a non-radial g-mode pulsation model and of a rotational modulation model. None of these two is able to explain the observed line-profile variations in full detail.

AN O VIII SEQUENCE FOR PLANETARY NEBULA NUCLEI

ABSTRACT The detection of O VIII emission lines in ultraviolet and optical spectra of high-excitation planetary nebula central stars suggests an "O VIII Sequence" for these object. Emission lines of O VII are also detected. Seven out of ten central stars for these planetaries are known to be nonradial g-mode pulsators. The stars form a narrow strip when plotted on the log L*/Lsun - log Teff H-R diagram.

Probing the Extraordinary Ends of Ordinary Stars: White Dwarf Seismology with the Whole Earth Telescope

AbstractDuring the final evolution of most stars, they shed their outer skin and expose their core of the hot ashes of nuclear burning. As these hot and very dense cores cool into white dwarf stars, they go through episodes of multiperiodic, nonradial g-mode pulsation. The tools of stellar seismology allow us to use the pulsation spectra as powerful probes into the deep interiors of these stars. Progress in white dwarf seismology has required significant international cooperation, since another consequence of the complex pulsations of these stars is decoding the true pulsation frequencies requires a coordinated global effort involving high-speed photometric observations. Through one such effort, the Whole Earth Telescope project, we have located subsurface composition changes, detected differential rotation and magnetic fields, and measured fundamental quantities such as stellar mass, luminosity, and distance to extraordinary accuracy.

Observation of a variable, ZZ Ceti white dwarf: GD154

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K–11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100–1000 seconds.