Marc T. Fontaine

FUSE Determination of Abundances in Long-Period Pulsating V1093 Her (PG 1716+426) Stars*

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of five V1093 Her (PG 1716+426) stars, the subgroup of hydrogen-rich subdwarf B stars that exhibit very low amplitude, long-period luminosity variations. Our primary aim is to investigate whether these stars display abundances which differ from those observed in the nonvariable sdB stars and also in the shorter period V361 Hya variables. For the light elements and for those beyond the iron peak, our abundances are consistent with the trends observed in earlier studies. For the important iron peak elements Ti, V, Cr, Mn, Fe, Co, and Ni, which are thought to be directly linked to the driving mechanism in both long-period and short-period variables, the abundances determined in the V1093 Her stars appear very homogeneous and exhibit mild enrichments (by factors of 2-5 for Cr, Mn, Fe, and Co) over the solar value. However, these abundances do not differ appreciably from those measured in a sample consisting of constant stars and one short-period pulsator. The implications of these findings for current models which involve both diffusion processes and stellar winds to account for the driving of nonradial pulsations in sdB stars are discussed.

Asteroseismological Studies of Long-Period Variable Subdwarf B Stars. II. Two-Color Photometry of PG 1338+481*

We present the results of an observational campaign for the long-period variable subdwarf B star PG 1338+481. Seven continuous weeks of observing time at the Steward Observatory 1.55 m Kuiper telescope on Mount Bigelow, Arizona,andthe1.3mMDMtelescopeatKittPeakrendered � 250hrofsimultaneousU/Rtimeseriesphotometry,as well as an extra � 70 hr of R-band–only data. The analysis of the combined light curves resulted in the extraction of 13 convincing periodicities in the 2100–7200 s range, with amplitudes up to � 0.3% and � 0.2% in the U and R, respectively. Comparing the ratios of amplitudes in the two wave bands to those predicted from theory suggests the presence of dipole modes, a notion that is further supported by the period spacing between the highest amplitude peaks. If confirmed, this poses a challenge to current nonadiabatic theory. At the quantitative level, we find that the distribution of the observed period spectrum is highly nonuniform and much sparser than that predicted from a representative model. We provide a possible interpretation in the text. The asteroseismological analysis attempted for PG 1338+481 on the basis of six observed periodicities believed to constitute consecutive dipole modes renders encouragingresults.Fixingtheeffectivetemperatureand surfacegravitytothespectroscopicestimates, wesuccessfully isolate just one family of optimal models that can reproduce the measured periods to better than 1%. While the stellar parametersthusinferredmustberegardedaspreliminary,theachievedfitbodeswellforfutureasteroseismicanalyses of long-period variable subdwarf B stars. Subject headingg stars: individual (PG 1338+481) — stars: interiors — stars: oscillations — subdwarfs

Analysis of the FUSE Spectrum of the Hot, Evolved Star GD 605*

Wepresentananalysisof theatmosphericpropertiesof theevolved,hydrogen-richobjectGD605usingFUSE,IUE, andopticalspectrainconjunctionwithnon-LTE(NLTE) modelatmospheresandsynthetic spectra.Wealsopresent an analysis of the interstellar medium along the line of sight toward this star. Our effective temperature determination relies onthe constraints on the ionizationbalance Oiv/Ovimposed bytheFUSEdata, whilethe surface gravity relies onamatchtotheBalmerlinesintheoptical spectrum.Our analysisyieldsTeA � 85;000 K,log g � 5:25,andahelium abundance close to the solar value. These parameters suggest that GD 605 is in a post-AGB evolutionary phase and belongstotheclassof hydrogen-richcentralstarof planetarynebulae,subclassO(H).Apartfromlinesof hydrogenand helium, about two dozen photospheric lines are observed in the FUSE data, which are dominated by the O vi kk1031.9 and1037.6transitions.Inaddition,wedetectlinesassociatedwiththefollowingions:Niv,Oiv,Ov,Siiv,Svi,Arvii,as well as Fe vii. Synthetic spectra based on NLTE line-blanketed model atmospheres reproduce most of the line profiles observed in what appears to be an atmosphere deficient in heavy elements. Our calculations do not fully reproduce the strength of the strongest ultraviolet lines seen, the O vi doublet, perhaps a sign that some contribution to this structure may arise in the interstellar medium or in a circumstellar environment. We discuss various scenarios to account for the absence of a visible nebula and the dearth of heavy elements in the atmosphere of GD 605. Subject headingg stars: abundances — stars: atmospheres — stars: individual (GD 605) — subdwarfs — techniques: spectroscopic Online material: color figures

FUSE Spectroscopy and Heavy Element Abundances in Hot, Hydrogen-Rich Subdwarfs

We have secured FUSE observations for a sample of hot, hydrogen-rich subdwarf B stars. The main objective of this project is to map in a more detailed way than has been done in the past pattern of heavy element abundances in sdB stars in order to understand the competing roles of gravitational settling, radiative element support, and mass loss in their photospheres. We present abundances analyses for a subsample of four objects and compare these results with predictions of the parameter-free version of the radiative levitation theory.