We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000 K and 50000 K. Through a least-squares minimization scheme, we have analyzed the spectra of 114 magnetic DAs from the Sloan Digital Survey (SDSS; 95 previously published plus 14 newly discovered within SDSS

Euchner, Fabian

Hirsch, Heiko

Jordan, Stefan

Külebi, Baybars

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Cosmic Magnetic Fields:From Planets, to Stars and GalaxiesProceedings IAU Symposium No. 259, 2008K.G. Strassmeier, A.G. Kosovichev & J.E Beckman, eds.c© 2009 International Astronomical Uniondoi:10.1017/S1743921309030750Analysis of the hydrogen-rich magneticWhite Dwarfs in the SDSSBaybars Ku¨lebi1, Stefan Jordan1, Fabian Euchner2, and Heiko Hirsch31Astronomisches Rechen-Institut, Mo¨nchhofstrasse 12-14, D-69120 Heidelberg, Germany2Swiss Seismological Service, ETH Zu¨rich, Sonneggstrasse 5, CH-8092 Zu¨rich, Switzerland3Dr.-Remeis-Sternwarte, Bamberg, Sternwartestrasse 7, D-96049 BambergAbstract. We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with magneticﬁeld strengths between 1 MG and 1000 MG for temperatures between 7000 K and 50000 K.Through a least-squares minimization scheme, we have analyzed the spectra of 114 magneticDAs from the Sloan Digital Survey (SDSS; 95 previously published plus 14 newly discoveredwithin SDSS).Keywords. Stars: white dwarfs – magnetic ﬁelds1. IntroductionWhite dwarfs with magnetic ﬁelds between 104 and 109 G are thought to representmore than 10% of the total population of white dwarfs (Liebert et al. 2003). In this workwe present the re-analysis of the 95 DA Magnetic White Dwarfs (MWDs) discovered bySchmidt et al. (2003) and Vanlandingham et al. (2005), plus the analysis of 19 additionalobjects from SDSS up to Data Release 6 (9583 deg2; http://www.sdss.org/dr6/).Schmidt et al. (2003) and Vanlandingham et al. (2005) determined the ﬁeld strengthsand the inclinations of magnetic dipoles by comparing the observed spectra visually withmodel spectra calculated using a simpliﬁed radiation transfer code (Latter et al. 1987).Their and our results are compared on Fig. 1.2. AnalysisThe magnetic ﬁeld geometry of the MWDs was determined with a modiﬁed versionof the code developed by Euchner et al. (2002). This code calculates the total ﬂux (andcircular polarization) spectra for an arbitrary magnetic ﬁeld topology by adding up ap-propriately weighted model spectra for a large number of surface elements. Magnetic ﬁeldgeometries are accounted for by a centered or an oﬀset dipoles in this work. The modelparameters are the magnetic dipole ﬁeld strength Bd , the eﬀective temperature Teﬀ , theinclination of the dipole axis i, and an oﬀset along the magnetic dipole axis zoﬀ , if oﬀ-set dipoles are used. The observed spectra are ﬁtted using an evolutionary algorithm(Rechenberg 1994) with a least-squares quality function and all of our ﬁts resulted inreduced χ2 values between 0.8 and 3.0 except for some high-ﬁeld objects.3. DiscussionIn many cases oﬀset dipole models resulted in signiﬁcantly better ﬁts than the modelswith centered dipoles. In particular some MWDs with high ﬁeld strengths (> 50MG),where the spectra become very sensitive to the details of the magnetic ﬁeld geometry,are not accounted for by our simple models.379at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921309030750Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 14:55:27, subject to the Cambridge Core terms of use, available380 B. Ku¨lebi et al.SDSS has nearly tripled the number of MWDs; therefore the completeness of the totalknown MWD population is strongly aﬀected by the selection biases of SDSS (priorityselection of the spectroscopic targeting; Stoughton et al. 2002).High-ﬁeld MWDs are thought to be remnants of magnetic Ap and Bp stars. If ﬂuxconservation is assumed, population synthesis models predict the majority of MWDs’polar ﬁeld strengths to be in the interval 50–500MG (Wickramasinghe & Ferrario 2005).On the pther hand in our sample objects with magnetic ﬁeld strengths lower than 50MG are more numerous than the objects with higher magnetic ﬁeld strengths (see Fig. 2),partly due to the selection biases. Nevertheless our result is consistent with previous onesand supports the hypothesis that magnetic fossil ﬁelds from Ap/Bp stars alone are notsuﬃcient to produce the observed number of MWDs (Wickramasinghe & Ferrario 2005).Possible progenitor populations are A and B stars with magnetic ﬁeld strengths below100 G or magnetic F stars (Schmidt et al. 2003), which both are currently unobserved.Figure 1. Comparison of dipole magnetic ﬁeldﬁt values in this work versus Schmidt et al.(2003), Vanlandingham et al. (2005).Figure 2. Histogram of magnetic whitedwarfs in equal intervals of log B. Graycolumns represent the number of all DAMWDs and black shades represent the thecontribution of SDSS to DA MWDs.ReferencesEuchner, F., Jordan, S., Beuermann, K., Ga¨nsicke, B. T., & Hessman, F. V. 2002, A&A 390,633Latter, W. B., Schmidt, G. D., & Green, R. F. 1987, ApJ 320, 308Liebert, J., Bergeron, P., & Holberg, J. B. 2003, AJ 125, 348Rechenberg, I. 1994, Werkstatt Bionik und Evolutionstechnik No. 1 (Stuttgart: frommann-holzboog)Schmidt, G. D., Harris, H. C., Liebert, J., et al. 2003, ApJ 595, 1101Stoughton, C., Lupton, R. H., Bernardi, M., et al. 2002, AJ 123, 485Vanlandingham, K. M., Schmidt, G. D., Eisenstein, D. J., et al. 2005, AJ 130, 734Wickramasinghe, D. T. & Ferrario, L. 2005, MNRAS 356, 1576at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921309030750Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 14:55:27, subject to the Cambridge Core terms of use, available

Analysis of the hydrogen-rich magnetic White Dwarfs in the SDSS

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