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Abundance studies of sdB stars using UV echelle HST/STIS spectroscopy

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S. J. O'Toole, U. Heber

Astrophysics · Submitted 2 Mar 2006 · Published version · 26 references

Pith's verdict: A careful UV abundance dataset for five sdB stars and an honest null result, with the non-pulsator classification as the real weak point.

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arXiv IDastro-ph/0603069 (version 1)
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CitationS. J. O'Toole, U. Heber. "Abundance studies of sdB stars using UV echelle HST/STIS spectroscopy." arXiv:astro-ph/0603069 (2006). https://pith.science/paper/GMXDFI7L
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The paper

The paper claims

Five hot subdwarf B stars, two pulsators and three non-pulsators, show no abundance signature that separates pulsating from non-pulsating behavior.

The reading

This paper tests a specific hypothesis: that the pulsations of hot subdwarf B (sdB) stars are tied to their surface abundances of iron-group elements, since theory identifies iron-group opacity as the pulsation driver. The authors use high-resolution ultraviolet echelle spectra from the Space Telescope Imaging Spectrograph on the Hubble Space Telescope to measure abundances of 25 elements in five sdB stars, two pulsators and three non-pulsators. They find no clear correlation between pulsation and metal abundance; when the two non-pulsators are compared with each other, their abundance patterns differ as much as either differs from a pulsator. The paper concludes that surface abundance alone cannot be the discriminating factor and that another, still unknown parameter separates pulsating from non-pulsating sdB stars. If correct, this means spectroscopic abundance surveys cannot identify which sdB stars will pulsate, and the iron-opacity driving models need an additional ingredient.

What carries the argument

The load-bearing tool is high-resolution ultraviolet echelle spectroscopy with the Space Telescope Imaging Spectrograph in the near- and far-UV, which opens the iron-group lines (V, Cr, Mn, Fe, Co, Ni) that are inaccessible from the ground. Abundances are derived with LTE curve-of-growth measurements and spectrum synthesis using metal line-blanketed LTE model atmospheres, a synthesis code, published oscillator strengths, wavelengths, and damping constants, and published resonance-line data for the heaviest species. The internal check that carries the argument is the agreement of several ionization stages of the same element (for example, Fe iii/iv and Ni iii/iv agree to about 0.1 dex): the paper treats this agreement as evidence that systematic errors in abundances and effective temperature are small. The comparison structure then organizes the stars into pulsator/non-pulsator pairs matched in effective temperature, and relative abundances ($X/\mathrm{Fe}$) are used to isolate pattern differences from overall metallicity.

Extended reading notes

Core claim

On the paper's own terms, the central result is a null result: two pulsating sdB stars and three non-pulsating sdB stars with similar effective temperatures and surface gravities show no consistent abundance difference in the iron-group elements that are supposed to drive the pulsations. In the cool pair (Feige 48 and CPD -64 481), the iron-group patterns match closely and no pulsation-related difference appears. In the hot pair PG 1219+534 and CD -24 731, large absolute differences largely vanish once the difference in iron content is accounted for, except for nickel and tin. But the third hot comparison, PG 1219+534 versus Feige 66, shows patterns that stay dissimilar even after correcting for a factor-of-seven iron difference, and the two non-pulsators Feige 66 and CD -24 731 differ from each other just as much. Because two non-variable stars of similar temperature and gravity cannot be matched to each other, the pulsator/non-pulsator comparison is rendered arbitrary, and the paper concludes that another, as yet unknown, discriminating factor must exist beyond surface metal abundance.

Load-bearing premise

The comparison assumes that LTE model atmospheres with solar or scaled-solar opacity distribution functions yield iron-group abundances accurate enough to resolve differences between stars, even though nickel and manganese are often supersolar and some resonance lines show unmodelled non-LTE effects.

Editorial extensions

If this is right

  • Surface iron-group abundances cannot separate pulsators from non-pulsators, so the pulsation-driving mechanism requires an additional parameter beyond photospheric composition.
  • Supersolar-metallicity model atmospheres reconcile the Balmer-line and helium-ionisation temperatures for PG 1219+534 and CD -24 731, pointing to opacity-sampled models as the next step in sdB atmosphere modelling.
  • The apparent anti-correlation between iron abundance and the abundance of heavier elements relative to iron, if confirmed, provides a new constraint on diffusion models combining radiative levitation, gravitational settling, and weak winds.
  • The strong enhancements of gallium, germanium, tin, and lead in the hotter stars, together with the disappearance of Si iv above about 32,000 K, support the idea of a fractionated weak stellar wind that selectively removes lighter elements while heavy elements remain.
  • The abundance table for 25 elements across five stars gives diffusion theory a concrete multi-element dataset to reproduce, not just iron.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If surface abundances do not discriminate, the pulsation decision may be set by interior structure, rotation, magnetic fields, or evolutionary age rather than photospheric composition; asteroseismic modelling of mode frequencies could test this without new abundance data.
  • The combined opacity of iron plus nickel may be a better pulsation driver than iron alone; a targeted test would measure Fe and Ni in additional cool sdB pairs to see whether the Fe+Ni opacity sum separates the classes even when iron alone does not.
  • The large abundance scatter between two non-pulsators of similar temperature and gravity suggests that binary history, not just pulsation status, shapes sdB surface composition; a sample split by binary period and companion type could separate these effects.
  • Reanalysing the same five stars with opacity-sampled, non-LTE models could either confirm the null result or reveal that part of the apparent inter-star abundance differences is an artifact of the scaled opacity-distribution-function treatment.
Original abstract

Aims: We test the hypothesis that the pulsations in sdB stars are correlated with the surface abundances of iron-group elements. Any correlation might explain why, when given two spectroscopically similar stars, one will pulsate while the other will not. Methods: We have obtained high-resolution ultraviolet spectra two pulsating and three non-pulsating sdB stars using the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. We determined abundances for 25 elements including the iron group and even heavier elements such as tin and lead using LTE curve-of-growth and spectrum synthesis techniques. Results: We find no clear correlation between pulsations and metal abundances, and we comment on the resulting implications, including whether it is possible to determine the difference between a pulsating and a non-pulsating sdB spectroscopically. In addition to the main goal of our observations, we have also investigated the effect of supersolar metallicity on fundamental parameter determination, possible trends with iron abundance, and the hypothesis that weak winds may be selectively removing elements from the stellar envelopes. These effects provide challenges to stellar atmosphere modelling and diffusion models for sdB stars.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 5,686 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.77 · human participants 0.01 · controlled comparison 0.70 · randomized assignment 0.02 · states quantitative result 0.26 · compares with prior results 0.12 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Aims: We test the hypothesis that the pulsations in sdB stars are correlated with the surface abundances of iron-group elements. Any correlation might explain why, when given two spectroscopically similar stars, one will pulsate while the other will not. Methods: We have obtained high-resolution ultraviolet spectra two pulsating and three non-pulsating sdB stars using the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. We determined abundances for 25 elements including the iron group and even heavier elements such as tin and lead using LTE curve-of-growth and spectrum synthesis techniques. Results: We find no clear correlation between pulsations and metal abundances, and we comment on the resulting implications, including whether it is possible to determine the difference between a pulsating and a non-pulsating sdB spectroscopically. In addition to the main goal of our observations, we have also investigated the effect of supersolar metallicity on fundamental parameter determination, possible trends with iron abundance, and the hypothesis that weak winds may be selectively removing elements from the stellar envelopes. These effects provide challenges to stellar atmosphere modelling and diffusion models for sdB stars.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Aims: We test the hypothesis that the pulsations in sdB stars are correlated with the surface abundances of iron-group elements. contribution 0.54 0.20 0.03 speculative 0.43
s1 Any correlation might explain why, when given two spectroscopically similar stars, one will pulsate while the other will not. background 0.78 0.06 0.02 speculative 0.69
s2 Methods: We have obtained high-resolution ultraviolet spectra two pulsating and three non-pulsating sdB stars using the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. method 1.00 0.97 0.01 asserted 0.99
s3 We determined abundances for 25 elements including the iron group and even heavier elements such as tin and lead using LTE curve-of-growth and spectrum synthesis techniques. method 0.99 0.97 0.02 asserted 1.00
s4 Results: We find no clear correlation between pulsations and metal abundances, and we comment on the resulting implications, including whether it is possible to determine the difference between a pulsating and a non-pulsating sdB spectroscopically. result 0.99 0.97 0.94 asserted 0.99
s5 In addition to the main goal of our observations, we have also investigated the effect of supersolar metallicity on fundamental parameter determination, possible trends with iron abundance, and the hypothesis that weak winds may be selectively removing elements from the stellar envelopes. contribution 0.99 0.89 0.04 asserted 0.72
s6 These effects provide challenges to stellar atmosphere modelling and diffusion models for sdB stars. interpretation 0.98 0.28 0.03 asserted 0.99

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 1.00.

What was examined, and what was not

sources read
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Figures
Figure 1. Line profile fit for PG 1219+534 using solar metal￾icity models (top panel) and metal-rich models (10 times solar, bottom panels). The Balmer lines and He ii at 4686 ˚A match simultaneously when using the metal rich models (see text). For CD −24◦731 we found the same mismatch of the helium ionisation equilibrium as for PG 1219+534 when solar metalicity models were used (see
Figure 3. Comparison of a 15 ˚A wide section of the near￾and far-UV spectrum of PG 1219+534. In the NUV it is possible to measure the continuum level, while in the FUV it must be estimated. Note also the higher flux in the FUV. by no means perfect, we have endeavoured to identify as many spectral lines as possible, and then from these we have chosen suitable lines to measure equivalent widths or carry out spectrum fitting. Our method is outlined below, beginning with a brief discussion of the problems encoun￾tered with continuum definition. 4.1. Defining the continuum In each of our spectra,…
Figure 4. Spectrum fit to the C iii multiplet at 1174- 1177 ˚Afor Feige 48. The model is shown as the thick line. Other strong lines due to Criv and Fe iv are marked. although during our analysis we found that often more accurate oscillator strengths and wavelengths are available in other databases (e.g. Hirata & Horaguchi (1995)). In particular we found that several Co lines had poor term identifications in the Kurucz list, Fe oscillator strengths were often out of date, while oscillator strengths for Ti iii were completely inconsistent with our observations. To determine which lines in our…
Figure 6. A region of spectrum showing blended iron-group lines for Feige 48, PG 1219+534 and Feige 66. The line at ∼1858.2˚A is unidentified. in Feige 66 and one in PG 1219+534; the other stars have only upper limits. In the case of calcium, we were able to measure lines of Ca iii for Feige 66, CD −24◦731 and PG 1219+534. In the cooler pair the Ca lines are either very weak or blended, making measurements difficult. 4.3.4. The iron group Lines of the iron group elements often lie at similar wave￾lengths, making it difficult to measure equivalent widths, and necessitating an analysis by…
Figure 7. Determination of microturbulent velocity of CD −24◦731 and PG 1219+534using Criii lines. We find ξ = 5+2 −1 km s−1 for the former, while the latter is consis￾tent with ξ = 0 km s−1 . many wavelengths given in the Kurucz list are not accu￾rate (note that the Hirata & Horaguchi (1995) list uses the same wavelengths). We have examined the list of Uylings & Raassen (1997), however they only provide im￾proved oscillator strengths, and also use the Kurucz wave￾lengths. Iron: In all spectra there are many Fe iii and Fe iv lines measurable. In the two hottest objects in our sample, Fe v l
Figure 8. Spectrum fit to the C iv resonance lines at 1550˚A for Feige 66. The model is shown as the thick line. The line cores of the model do not match the observations, and the abundance is set around 0.8 dex lower than that derived from C iii lines. See text for details. only available for Cu iii. There are no published oscillator strengths of Cu iv. Zinc: Zn iii and iv lines are visible; however, the latter ion has no published oscillator strengths. 4.3.5. Gallium, Germanium, Tin and Lead The discovery of lines of Ga, Ge, Sn and Pb was made by O’Toole (2004). Lines of Ge iv and Sn iv…
Figure 9. Abundances measured for our five targets. Magenta symbols represent values determined using singly ionised lines, green represents doubly ionised lines, red represents triply ionised lines, blue denotes quadruply ionised lines and cyan represents upper limits. Note the generally excellent agreement between different ionisation stages
Figure 10. Spectrum fit to the Ge iv resonance lines at 1189 ˚A for Feige 66. The model is shown as the thick line. 5.3.3. Gallium, Germanium, Tin and Lead The heavy metals Ga, Ge, Sn and Pb are all enriched with respect to the Sun in all stars, reaching as high as 2.9 dex for Ga in PG 1219+534 or 2.75 dex for Pb in Feige 66. An example of a fit of the Ge IV 1189 ˚A line is shown in
Figure 11. Trends in heavy element abundances with iron abundance. ter all. At the time of Baschek et al. study, the quality of atomic data was lower than it is at present (although in many cases it is still insufficient or of low accuracy), so their errors are very large, particularly for the iron group. Recently, Edelmann et al. (2006) has carried out an analysis of CPD −64◦481 and CD −24◦731 based on high￾resolution optical echelle spectra. For both stars their abundances are consistent with those determined here. Finally, we can also compare any apparent trends seen here with the work of…
Figure 12. Comparison of iron-group abundances for our pulsator/non-pulsator pairs (F48 = Feige 48; CPD = CPD −64◦481; PG = PG 1219+534; CD = CD −24◦731; F66 = Feige 66). The dotted lines denote equal abundances and the difference in iron abundance. The differences between PG 1219+534, CD −24◦731 and Feige 66 are quite large – particularly for Fe, Ni and Mn – while the pairs PG 1219+534 & CD −24◦731 and Feige 48 & CPD −64◦481 show quite similar abundances, in particular when the differences in iron abundance are accounted for (see text). Mn – which have a significant opacity contribution – ar

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References
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4 arXiv papers cite this in 6 sentences

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Papers whose main result relates to this one's

Found among the papers closest to this one by title and main claim. Pith's classifier compared each pair's main results and a second model family confirmed the relation; checked against two further families, shown conflicts agreed 118 times in 120 and shown same results 218 times in 239. The quoted sentence is the other paper's main claim.

States the same result 1 paper

Papers citing it · 3 sentences without a confident label

Uses its data or values 1 paper
  • In order to fix a suitable chemical composition for our grid of model atmospheres, we relied on four abundance studies that have been done for Feige 48: [ref] (Keck/$HIRES$), [ref] ($FUSE$), [ref] ($HST/STIS$) and more recently [ref] (Keck/$HIRES$).

    Updates on the Pulsating SdB Star Feige 48 through Spectroscopy · Atmospheric Parameters for Feige 48 · 0.81

Compares with it 1 paper
Cites it as background 1 paper
  • In He-poor sdB stars, diffusion leads to strong helium depletion[Diffusion-only models fail to reproduce the observed helium abundances in sdB stars, implying the presence of additional atmospheric mixing processes, such as radiatively driven stellar winds [ref] or an as-yet unidentified form of turbulence [ref] . ] [ref] and pronounced heavy-element overabundances, in some cases reaching 3 dex above solar [ref] .

    Evidence for neutron capture in heavy-metal hot subdwarfs: Far-UV spectroscopy of EC22536-5304 and LSIV-14 116 · Diffusion · 0.88

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