arXiv subjects

J. W. Kruk

Publications and source records attributed to J. W. Kruk.

At least 55 records · Page 3Linked to original sources

arXiv2007

Elemental Abundances in PG1159 Stars

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric elemental abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. PG1159 stars appear to be the direct progeny of [WC] stars.

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arXiv2007

KPD0005+5106: Hottest DO White Dwarf Much Hotter Than Assumed

KPD0005+5106 is the hottest known helium-rich white dwarf. We have identified NeVIII lines in UV and optical spectra and conclude that it is significantly hotter than previously thought, namely Teff=200,000 K instead of 120,000 K. This is a possible explanation for the observed hard X-ray emission as being of photospheric origin. Concerning its evolutionary state, we suggest that KPD0005+5106 is not a descendant of a PG1159 star but more probably related to the O(He) stars and RCrB stars.

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arXiv2007

Identification of NeVIII lines in H-deficient (pre-) white dwarfs: a new tool to constrain the temperature of the hottest stars

For the first time, we have identified NeVIII absorption lines in far-UV spectra of the hottest known (Teff>150,000 K) hydrogen-deficient (pre-) white dwarfs of spectral type PG1159. They are of photospheric origin and can be matched by synthetic non-LTE line profiles. We also show that a number of UV and optical emission lines in these stars can be explained as being photospheric NeVIII features and not, as hitherto suspected, as ultrahigh ionised OVIII lines created along shock-zones in the stellar wind. Consequently, we argue that the long-standing identification of the same emission lines in hot [WR]-type central stars as being due to ultrahigh-ionised species (OVII-VIII, CV-VI) must be revised. These lines can be entirely attributed to thermally excited species (NeVII-VIII, NV, OVI). Photospheric NeVIII lines are also identified in the hottest known He-rich white dwarf KPD0005+5106 some of which were also attributed to OVIII previously. This is a surprise because it must be concluded that KPD0005+5106 is much hotter (Teff=200,000 K) than hitherto assumed (Teff=120,000 K). This is confirmed by a re-assessment of the HeII line spectrum. We speculate that the temperature is high enough to explain the mysterious, hard X-ray emission (1 keV) as being of photospheric origin.

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arXiv2007

High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216

LS V +4621 is the DAO-type central star of the planetary nebula Sh 2-216. We perform a comprehensive spectral analysis of high-resolution, high-S/N ultraviolet observations obtained with FUSE and STIS aboard the HST as well as the optical spectrum of LS V +4621 by means of state-of-the-art NLTE model-atmosphere techniques in order to compare its photospheric properties to theoretical predictions from stellar evolution theory as well as from diffusion calculations. From the N IV - NV, O IV - O VI, Si IV - Si V, and Fe V - Fe VII ionization equilibria, we determined an effective temperature of 95 +/- 2 kK with high precision. The surface gravity is log g = 6.9 +/- 0.2. An unexplained discrepancy appears between the spectroscopic distance d = 224 +46/-58 pc and the parallax distance d = 129 +6/-5 pc of LS V +4621. For the first time, we have identified Mg IV and Ar VI absorption lines in the spectrum of a hydrogen-rich central star and determined the Mg and Ar abundances as well as the individual abundances of iron-group elements (Cr, Mn, Fe, Co, and Ni). With the realistic treatment of metal opacities up to the iron group in the model-atmosphere calculations, the so-called Balmer-line problem (found in models that neglect metal-line blanketing) vanishes. Spectral analysis by means of NLTE model atmospheres has presently arrived at a high level of sophistication, which is now hampered largely by the lack of reliable atomic data and accurate line-broadening tables. Strong efforts should be made to improve upon this situation.

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arXiv2007

CalFUSE v3: A Data-Reduction Pipeline for the Far Ultraviolet Spectroscopic Explorer

Since its launch in 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) has made over 4600 observations of some 2500 individual targets. The data are reduced by the Principal Investigator team at the Johns Hopkins University and archived at the Multimission Archive at Space Telescope (MAST). The data-reduction software package, called CalFUSE, has evolved considerably over the lifetime of the mission. The entire FUSE data set has recently been reprocessed with CalFUSE v3.2, the latest version of this software. This paper describes CalFUSE v3.2, the instrument calibrations upon which it is based, and the format of the resulting calibrated data files.

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arXiv2007

Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs

We report the first discovery of argon in hot evolved stars and white dwarfs. We have identified the ArVII 1063.55A line in some of the hottest known (Teff=95000-110000 K) central stars of planetary nebulae and (pre-) white dwarfs of various spectral type. We determine the argon abundance and compare it to theoretical predictions from stellar evolution theory as well as from diffusion calculations. We analyze high-resolution spectra taken with the Far Ultraviolet Spectroscopic Explorer. We use non-LTE line-blanketed model atmospheres and perform line-formation calculations to compute synthetic argon line profiles. We find a solar argon abundance in the H-rich central star NGC1360 and in the H-deficient PG1159 star PG1424+535. This confirms stellar evolution modeling that predicts that the argon abundance remains almost unaffected by nucleosynthesis. For the DAO-type central star NGC7293 and the hot DA white dwarfs PG0948+534 and REJ1738+669 we find argon abundances that are up to three orders of magnitude smaller than predictions of calculations assuming equilibrium of radiative levitation and gravitational settling. For the hot DO white dwarf PG1034+001 the theoretical overprediction amounts to one dex. Our results confirm predictions from stellar nucleosynthesis calculations for the argon abundance in AGB stars. The argon abundance found in hot white dwarfs, however, is another drastic example that the current state of equilibrium theory for trace elements fails to explain the observations quantitatively.

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arXiv2006

High-resolution ultraviolet spectroscopy of PG1159-035 with HST and FUSE

PG1159-035 is the prototype of the PG1159 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 PG1159 stars reveals insight into stellar evolution and nucleosynthesis during AGB and post-AGB phases. We perform a quantitative spectral analysis of PG1159-035 focusing on the abundance determination of trace elements. We have taken high-resolution ultraviolet spectra of PG1159-035 with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. They are analysed with non-LTE line blanketed model atmospheres. We confirm the high effective temperature with high precision (Teff=140,000+/-5000 K) and the surface gravity of logg=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 PG1159 stars. We redetermine the nitrogen abundance and find it to be lower by one dex compared to previous analyses. The sulfur depletion is in contradiction with current models of AGB star intershell nucleosynthesis. The iron deficiency confirms similar results for other PG1159 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 PG1159 stars weakens the role of nitrogen as a distinctive feature of pulsators and non-pulsators in the GW Vir instability strip.

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arXiv2006

HST and FUSE Spectroscopy of the DAO-type Central Star LS V+4621

The DAO-type white dwarf LS V+4621 is the hydrogen-rich central star of the possible planetary nebula Sh 2-216. We have taken high-resolution, high-S/N ultraviolet spectra with STIS aboard the HST and FUSE in order to constrain its photospheric parameters. A detailed spectral analysis by means of state-of-the-art NLTE model-atmosphere techniques is presented which includes the determination the individual abundances of iron-group elements.

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arXiv2006

FUSE Spectroscopy of the DAO-type Central Star LS V+4621: Looking for the Photosphere in the Sea of Interstellar Absorption

The far-ultraviolet spectrum of the DAO White Dwarf LS V+4621, the exciting star of the possible planetary nebula Sh 2-216,is strongly contaminated by absorption features from the interstellar medium (ISM). For an ongoing spectral analysis, we aim to extract the pure photospheric spectrum in order to identify and model metal lines of species which are not detectable in the near-ultraviolet wavelength range. We have modeled the interstellar absorption precisely and considered it for the simulation of the FUSE (Far Ultraviolet Spectroscopic Explorer) observation. A state-of-the-art NLTE model-atmosphere spectrum which includes 16 elements is combined with the ISM absorption and then compared with the FUSE spectrum.

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arXiv2006

AGB star intershell abundances inferred from analyses of extremely hot H-deficient post-AGB stars

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars.

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arXiv2006

NLTE spectral analysis of GW Vir pulsators

GW Vir variables are the pulsating members in the spectroscopic class of PG 1159 stars. In order to understand the characteristic differences between pulsating and non-pulsating PG 1159 stars, we analyse FUSE spectra of eleven objects, of which six are pulsating, by means of state-of-the-art NLTE model atmospheres. The numerous metal lines in the FUV spectra of these stars allow a precise determination of the photospheric parameters. We present here preliminary results of our analysis.

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arXiv2005

Light and heavy metal abundances in hot central stars of planetary nebulae

We present new results from our spectral analyses of very hot central stars achieved since the last IAU Symposium on planetary nebulae held in Canberra 2001. The analyses are mainly based on UV and far-UV spectroscopy performed with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer but also on ground-based observations performed at the Very Large Telescope and other observatories. We report on temperature, gravity, and abundance determinations for the CNO elements of hydrogen-rich central stars. In many hydrogen-deficient central stars (spectral type PG1159) we discovered particular neon and fluorine lines, which are observed for the very first time in any astrophysical object. Their analysis strongly confirms the idea that these stars exhibit intershell matter as a consequence of a late helium-shell flash.

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arXiv2004

Fluorine in extremely hot post-AGB stars

We have discovered lines of highly ionized fluorine (Fv and Fvi) in the far-UV spectra of extremely hot (Teff=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.

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arXiv2004

HST and FUSE Spectroscopy of Hot Hydrogen-Rich Central Stars of Planetary Nebulae

High-resolution UV spectra, obtained with HST and FUSE, enable us to analyse hot hydrogen-rich central stars in detail. Up to now, optical hydrogen and helium lines have been used to derive temperature and surface gravity. Those lines, however, are rather insensitive; in particular, neutral helium lines have completely vanished in the hottest central stars. Therefore, we have concentrated on ionization balances of metals, which have a rich line spectrum in the UV, to establish a new temperature scale for our sample. Furthermore, we have determined abundances of light metals, which had been poorly known before. They show considerable variation from star to star. We present results of quantitative spectral analyses performed with non-LTE model atmospheres.

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arXiv2004

Iron Abundance in Hydrogen-Rich Central Stars of Planetary Nebulae

We report on an on-going analysis of high-resolution UV spectra of hot hydrogen-rich central stars of planetary nebulae (CSPN), obtained with the Hubble Space Telescope and FUSE. Since UV spectra of many CSPN are dominated by Fe and Ni lines, we intend to use them as temperature indicators to check the CSPN temperature scale we have derived earlier from CNO ionization balances. Furthermore, the observed line strengths of heavy metals show large variations between different objects suggesting a possible spread in abundances. We will determine abundances of iron group elements by quantitative spectral analyses with non-LTE model atmospheres.

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arXiv2004

FUSE spectroscopy of PG1159 Stars

PG1159 stars are hot hydrogen-deficient post-AGB stars with effective temperatures within a range from 75000 K up to 200000 K. These stars are probably the result of a late helium-shell flash that had occurred during their first descent from the AGB. The lack of hydrogen is caused by flash-induced envelope mixing and burning of H in deeper regions. Now the former intershell matter is seen on the surface of the stars. Hence the stellar atmospheres show metal abundances drastically different from the solar values. Our sample comprises ten PG1159 stars with effective temperatures between 85000 K and 170000 K. We present first results of our spectral analysis based on FUV spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).

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arXiv2004

Insight into AGB and post-AGB stellar evolution with FUSE

FUSE spectroscopy has proved that extremely hot hydrogen-deficient post-AGB stars (PG1159 stars) display matter on their surface that usually remains hidden in the region between the H- and He-burning shells of the former AGB star. Hence, the spectral analysis of PG1159 stars allows to study directly the chemistry of this intershell region which is the outcome of complicated burning and mixing processes during AGB evolution. Detailed abundance determinations provide constraints for these processes which are still poorly understood. With FUSE we have discovered high neon and fluorine overabundances. There is also a significant iron deficiency, which may be caused by s-process neutron capture transforming iron into heavier elements.

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arXiv2004

Identification of neon in FUSE and VLT spectra of extremely hot hydrogen-deficient (pre-) white dwarfs

One of the strongest absorption lines observed in far-ultraviolet FUSE spectra of many PG1159 stars remained unidentified up to now. We show that this line, located at 973.3A, stems from NeVII. We also present new optical high-resolution spectra of PG1159 stars, obtained with the ESO VLT, which cover the NeVII 3644A line and a newly identified NeVII multiplet in the 3850-3910A region. We compare the observed neon lines with NLTE models and conclude a substantial neon overabundance in a number of objects. Although a detailed analysis is still to be performed in order to compare quantitatively the abundances with evolutionary theory predictions, this corroborates the idea that the PG1159 stars and their immediate progenitors, the [WC]-type nuclei of planetary nebulae, display intershell matter of their precursor AGB stars. Possibly as the consequence of a late He-shell flash, H-deficient and (s-processed) Fe-depleted matter, that is strongly enriched by 3-alpha-processed elements (C, O, Ne), is dredged up to the surface. Hence, a detailed study of the element abundance patterns in these peculiar stars gives the unique possibility to probe mixing and nucleosynthesis processes in the precursor AGB stars.

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