Abstract
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
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