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magnetic white dwarfs comprise @xmath15% of all white dwarfs and have surface field strengths in the range @xmath13@xmath210@xmath3 to 10@xmath4 gauss ( g ) .
the 65 isolated ( non - interacting binary ) cases known at the time of the review of wickramasinghe & ferrario ( 2000 ) cover most of the white dwarf spectral types ( eg . da , db , dq , dc ) , but have up to now not included any dz stars which show lines of heavy elements like ca , mg , na and fe .
this has restricted their use as astrophysical laboratories of the effects of strong magnetic fields on the light elements hydrogen , helium and molecular carbon .
the subject of this paper is the discovery of the first magnetic dz object , identified in the course of routine spectral classification of cool stars from the luyten half second ( 1979 , lhs ) proper motion catalog .
we present in @xmath52 the spectrum of this object , lhs  2534 ( wd  1221 - 023 , using the notation of mccook & sion 1999 ) .
this dwarf offers the first empirical data in an astrophysical setting of the zeeman effect on neutral na , mg , and both ionized and neutral ca .
the optical spectrum of the new magnetic white dwarf was obtained on 8 february 1998 using the low resolution imaging spectrograph ( lris , oke et al .
1994 ) at the 10 m w. m. keck observatory ( keck - ii ) on mauna kea , hawaii .
these observations were made as part of a service observing request .
the 300 g / mm grating blazed at 5000    was used with a one arc second slit to obtain spectra of 6  resolution covering @xmath6   . a single 900-second exposure was obtained , and the data are plotted in figure  1 .
in addition to being magnetic , lhs  2534 is one of the most heavily - blanketed of the known cool dz white dwarfs .
the temperature is evidently not too different from that of the sun , as the strongest features  3933 , 3968   , 4226   , 5175   , and 5892aa   are also among the strongest in the optical spectrum of the sun .
hydrogen , especially h@xmath7 , is not detected , so one may conclude the star has a helium - dominated atmosphere , like most dz stars .
monochromatic magnitudes for many cool white dwarfs were measured using the palomar multichannel spectrophotometer colors and published by oke ( 1974 ) and greenstein ( 1976 ) ; the colors b(4255   ) , g(4717   ) , v(5405   ) , r(6944   ) and i(8000   ) overlap the wavelength range of these spectra .
synthetic colors from the pure helium atmosphere models of bergeron , wesemael , & beauchamp ( 1995 ) may be compared .
the v - i slope is probably least affected by metallic absorption .
the measured value of + 0.24 from our spectrum compares with + 0.266 for a 6,000  k log g=8 atmosphere , and + 0.152 at 6,500  k. from this we may conclude that lhs  2534 has a @xmath8 near 6,000  k. the star is clearly warmer than the heavily - blanketed lp  701 - 29 ( dahn et al 1978 ) for which kapranidis & liebert ( 1986 ) estimated @xmath9  k. the v - i measurement of greenstein ( 1984 ) suggests @xmath14,800  k from the pure - helium models .
both g - r and especially b - v are substantially redder than the pure - he models predict .
likewise , lhs  2534 is cooler than the heavily - blanketed dz star g  165 - 7 , for which wehrse & liebert ( 1980 ) estimated 7,500  k and the v - i color ( greenstein 1984 ) suggests 7,100  k. perhaps the most similar of the well known dz stars is van  maanen  2 , at g - r = + 0.26 and v - i = + 0.13 .
bergeron , ruiz & leggett ( 1997 ) estimate 6,770  k from fitting a multi - color energy distribution of this star .
the zeeman effect on neutral sodium is a classic problem ( zeeman 1897 ) that is encountered here for the first time in regard to white dwarfs .
thus , we briefly summarize the situation .
sodium is isoelectronic with hydrogen , so magnetic effects involve only the single valence electron .
the d@xmath10 ( @xmath11 ) and d@xmath12 ( @xmath13 ) lines comprise a resonance doublet that couples the @xmath14 term with @xmath15 and @xmath16 , respectively . the feature is also seen in g  165 - 7 ( hintzen & strittmatter 1974 , wehrse & liebert 1980 ) , where the doublet splitting can not be resolved due to pressure broadening .    in a weak magnetic field
( @xmath17  g ) , the @xmath18 levels are each split into 2 sublevels , and the @xmath19 level splits into 5 according to @xmath20 , and the magnitude of the splitting is computed according to ls coupling .
ten distinct components result .
this is the regime normally encountered in solar observations ( e.g. , beckers 1969 ; caccin , gomez , & severino 1993 ) .
the spin and orbit decouple when the splitting due to the external magnetic field overwhelms that due to the fine - structure effect . in figure  1
, the feature appears as a strong triplet at @xmath215862 , 5892 , and 5924 . the observed splitting is not only considerably larger than the 6    fine - structure effect , but the pattern is centered near the mean wavelength of the nonmagnetic doublet .
hence , we conclude that the paschen - back approximation is appropriate and we analyze the feature as an ordinary linear zeeman triplet with an insignificant quadratic component .
the displacements of the @xmath22 components are then @xmath23  @xmath24 and @xmath2592  @xmath24 for @xmath265862 and @xmath265924 , respectively .
( it is customary in atomic spectroscopy to use wavenumber units . ) from the linear zeeman effect , the mean surface field ( cf .
garstang 1977 ) is computed according to @xmath27 which yields an average value for the two components of @xmath28  g = 1.92  mg . there are , in fact , direct laboratory measurements of the d lines which overlap this field strength and corroborate the accuracy of the linear approximation .
garn et al .
( 1966 ) reported splittings between the @xmath22 components that from 30    for a longitudinal field of 0.94  mg up to 163    at 5.1  mg .
our measurement of 1.92  mg is the mean surface field strength .
detailed modeling of the line profile , preferably supported with spectropolarimetric observations , is necessary to draw conclusions about the field geometry .
a dipolar geometry is usually an adequate approximation , though the pattern is often offset significantly from the center of the star .
time - dependent observations might determine if the star rotates , and allow the modeling of periodic changes in the geometric view .
both @xmath22 components and , to a lesser extent , the @xmath29 component should be circularly polarized , while linear polarization and polarization of the continuum should be small .
the subordinate triplet connects levels @xmath30 with @xmath31 for @xmath215167.3 , 5172.7 , 5183.6 , respectively . in lhs  2534
the region shows 4 principal components at wavelengths of @xmath15149   , 5180   , 5205   , and 5235   . modeling each component of the parent triplet as a simple zeeman triplet in a field of @xmath32  mg
indeed produces a complex with only 4 lines due to overlapping of some of the 9 components .
the short - wavelength edge matches that of the data , but the splitting between lines is somewhat less than observed and thus the feature does not extend sufficiently far to the red .
we take this as evidence that the linear zeeman approximation has broken down for this ion , where the fine - structure effect is comparable to the magnetic interaction .
we are aware of no computations of the behavior of in this intermediate regime .
the ion is isoelectronic with , but has spin - orbit splitting which even exceeds that of the features and results in the well - known `` h '' ( 3933   ) and `` k '' ( 3968   ) doublet components being well - resolved even at low spectral resolution in zero field . since the linear magnetic term in the hamiltonian is thus comparable to the spin - orbit term , the splitting at such a low field is more complicated still .
calculations have been published by kemic ( 1975 ) . at a field strength of 1.9  mg , the 10 zeeman components of the doublet group themselves into 3 features centered around @xmath33 , and comprised primarily of transitions from upper levels @xmath16 @xmath34 , @xmath16 @xmath35 , and @xmath15 @xmath36
these are heavily blended for the observed line widths , and together result in the broad depression centered near 3957   .
strong absorption features due to and many other heavy elements are prominent in the spectra of late type stars shortward of 4000   , and may also contribute in lhs  2534 .
finally , the strong neutral calcium resonance line at 4226    shows a complex structure , and any magnetic components are severely blended .
inr would like to thank fred chaffee for undertaking the keck service observations .
we are grateful to d.t .
wickramasinghe for helpful discussions .
jl acknowledges the hospitality of the institute for theoretical physics , u.c .
santa barbara , at which part of this work was completed .
the itp is supported in part by the national science foundation grant phy 94 - 07194 .
gds thanks the australian national university for hospitality and support during a sabbatical leave .
support for the study of magnetic stars and stellar systems at steward observatory is also provided by the nsf through grant ast 97 - 30792 to gds .
a spectroscopic survey of unstudied luyten half second proper motion stars has resulted in the discovery of an unusual new magnetic white dwarf . 
 lhs  2534 proves to be the first magnetic dz , showing zeeman - split and components , as well as and lines for which zeeman components are blended . 
 the splittings result in a mean surface field strength estimate of 1.92  mg . 
 apart from the magnetic field , lhs  2534 is one of the most heavily - blanketed and coolest dz white dwarfs at @xmath0  k.