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stellar evolution theory predicts the existence of massive o / ne white dwarfs as a result of carbon burning .
wds with masses exceeding 1m@xmath0 should be such o / ne wds , but there are other ways to produce massive wds , namely by binary merging or by mass accretion during close - binary evolution .
the question is : is there evidence for the existence of o / ne wds ?
a very useful overview about this topic has been given by weidemann ( 2003 ) at the last wd workshop at naples .
we refer to his paper for a detailed discussion and references concerning stellar evolution scenarios , which we only briefly summarize here .
o / ne wds should have masses of 11.3m@xmath0 .
below a core mass of 1m@xmath0 c burning can not ignite .
if the core mass exceeds 1.3m@xmath0 , an electron capture collapse will occur and probably result in a neutron star .
there is a debate as to what initial - mass range this 11.3m@xmath0 core - mass range corresponds .
depending on details in evolutionary models , the range of initial masses is between 911m@xmath0 or between 79m@xmath0 ( for solar metallicity ) . in any case , in order to stop the o / ne - core to grow ( by c burning ) beyond the 1.3m@xmath0 limit , the star must loose mass , either by wind mass loss or by envelope removal in close binary systems .
the wd mass distribution does show the existence of such massive wds , however , their origin is not clear .
it is possible that they are binary mergers .
if o / ne wds exist , then they have c / o envelopes and , thus , they are spectroscopically indiscernible from c / o wds . possible evidence , however , are observed neon overabundances in several cases : in neon novae , in some low - mass x - ray binaries , and in one individual object ( h1504 + 65 ) .
we will discuss here the lmxbs and h1504 + 65 . in the case of neon novae ,
only those very few objects with extreme neon overabundances ( over @xmath130 times solar , i.e.  3% mass fraction ) suggest o / ne wds , because up to 3% @xmath2ne is expected in c / o wds .    for the interpretation of ne overabundances in novae or lmxbs
it is important to consider gravitational settling of ne into the core of a c / o wd .
this problem is not yet solved theoretically .
it could well be , that ne overabundances observed in eroded wd cores are the result of this process and not the result of c burning .
spectroscopically , h1504 + 65 is an extreme member among the pg1159 stars , which are hot , h - deficient objects on or closely before the hot end of the wd cooling sequence .
they are probably the result of a late he - shell flash that causes ingestion and burning of h , and the exposition of he / c / o - rich intershell matter on the surface ( e.g.  werner 2001 ) .
h1504 + 65 is an extreme , and unique , pg1159 star because it is also he - deficient .
the first quantitative analysis revealed that the photosphere is mainly composed of c and o , by equal parts ( werner 1991 ) , and it has been discussed that h1504 + 65 is either a naked c / o core or that we see the c / o envelope of a o / ne wd . the latter possibility is supported by the fact that h1504 + 65 is the most massive known pg1159 star , although the spectroscopic mass determination suggests a mass slightly below 1m@xmath0 . from the discovery of ne lines in optical and euv spectra ( werner & wolff 1999 ) we found ne=25% .
this result is inconclusive concerning the nature of the star as a possible o / ne wd .
soft x - ray and fuv spectra of h1504 + 65 were taken by chandra / letg and fuse , respectively , and the following atmospheric and stellar parameters were derived ( werner et al .
2004 ) : @xmath3}\ ] ] @xmath4 @xmath5 estimated errors for abundances are : @xmath620% for mass fraction of c and o , and a factor of 3 for ne and mg .
one significant result is the detection of mg in high amounts , which seems to confirm the o / ne wd nature of h1504 + 65 .
however , a detailed discussion reveals that the abundance determinations of ne and mg still can not decide if h1504 + 65 exposes a c / o core or a c / o envelope . the final answer to this problem
was hoped to be accomplished by uv spectroscopy with hst . planned observations during cycle  13 aimed at the detection of
na which , if strongly overabundant , is a clear indicator for c burning .
unfortunately , the stis spectrograph aboard hst died before the observations were carried out .
it appears that this question will remain unanswered for many years to come .
this is disappointing , because h1504 + 65 is the only object for which we can hope to proof that single o / ne wds do really exist .
considering our mass determination , even if we stretch the result to the limit of our error bar , the mass of h1504 + 65 is slightly below 1m@xmath0 .
however , we point out that the evolutionary tracks which we used to derive the mass , are those of post - agb remnants that have lastly burned helium but  h1504 + 65 has no helium
. there are no appropriate evolutionary tracks available .
the main problem is that we have no idea what is the evolutionary history of this unique star : what event caused the helium - deficiency ?
we finally note that there is no indication that h1504 + 65 is in a binary system .
it could be that agb - like mass loss of single stars during carbon burning is not sufficient to stop the growth of the o / ne core before the onset of electron - capture collapse .
hence , close binary systems in which mass is transferred from the c - burning star onto a companion might be the only places where o / ne wds are created .
first evidence for the existence of such systems came from neon overabundances detected by line emission in ( low resolution ) asca spectra of the lmxb 4u1626 - 67 ( angelini et al .
evidence for neon - rich absorbing circumstellar material was found in chandra letg spectra of 4u0614 + 091 and three other lmxbs ( juett et al .
the detection of double - lined emissions in the chandra hetg spectrum of 4u1626 - 67 ( schulz et al .
2001 ) suggests neon and oxygen overabundances in the accretion disk of this system .    in total
there is now a group of five lmxbs , for which it is believed that they are ultracompact systems ( orbital periods below 1 hour , i.e.  separation of the order 1 light - s ) in which a neutron star or a black hole accretes matter from a low - mass ( @xmath7m@xmath0 ) degenerate star , the strongly ablated core of a wd .
mass transfer is driven by gravitational radiation .
the luminosity of these systems is dominated by the accretion disk . in the soft x - ray and uv spectral regions
we see the hot ( several 100000k ) innermost parts of the disk , which can be x - ray heated by the neutron star .
the optical light is dominated by cooler ( about 10000k ) , outer parts of the disk .
so we have the unique possibility to study the former wd interior composition by the determination of element abundances in the accretion disk .    for this purpose
we have obtained optical medium - resolution spectra of two of these systems ( 4u1626 - 67 and 4u0614 + 091 ) .
we have used the fors1 spectrograph attached to ut1 of eso s very large telescope to record long - slit spectra .
we used two grisms ( 600b and 600r ) and obtained spectra in the regions 34505900  and 52507450  with a resolution of about 5 .
the b magnitude of both targets is near 18.7 ; exposure times for both binaries were 87 and 58 minutes for the red and the blue spectra , respectively .
the observations were performed in service mode during nov .
2003 and mar .  2004 .
the spectra were reduced using standard iraf procedures .
they are presented in fig.1 , and show emission lines from cii / iii and oii / iii .
they lack hydrogen and helium lines , apparently confirming the h- and he - deficiency of the donor stars . in the case of 4u0614 + 091
we confirm the line identifications of nelemans et al .
( 2004 ) , who have obtained vlt / fors2 spectra with slightly higher resolution .
in addition to these data , we also make use of archival hst / uv data of 4u1626 - 67 , kindly provided in reduced form by homer et al .
the hst spectrum is shown in fig.2 .
it shows emission lines from civ , ov , and siiv .
it does not show heii  1640 .    we have begun the construction of accretion disk models to calculate synthetic spectra and report here on the current state of our work .
we use our accretion disk code acdc , which is described in detail by nagel et al .
in essence , it assumes a radial @xmath8-disk structure ( shakura & sunyaev 1973 ) .
then the disk is divided into concentric annuli . for each annulus
we solve the radiation transfer equation ( assuming plane - parallel geometry ) together with the nlte rate equations , plus energy- and hydrostatic equations , in order to calculate a detailed vertical structure .
the integrated disk spectrum is then obtained by co - adding the spectra from the individual annuli . in comparison with the observed uv / optical spectra we present here first model spectra for particular , representative disk regions .
4u1626 - 67 has an orbital period of 42  min ( see e.g.  chakrabarty 1998 , and references therein ) . the accretor is a 7.7s x - ray pulsar with a magnetic field strength of @xmath9 g . the donor s mass is smaller than 0.1m@xmath0 .
the system separation is 300000 km .
the inner disk edge is at the co - rotation radius , at r=6500 km from the pulsar , and the outer edge at the tidal truncation radius , near r=200000 km .
the mass transfer rate is roughly @xmath10m@xmath0/yr and the x - ray luminosity is about @xmath11erg / s .
fig.2 shows the uv spectrum of two disk models extending from r=650015000 km , seen under an inclination angle of 17@xmath12 .
they are irradiated by the neutron star , assuming the x - ray luminosity mentioned above and a blackbody energy distribution ( t@xmath13k ) .
the two models differ by their chemical composition .
one model is he - poor ( dominated by c , o , ne , mg ) , the other is he - rich with abundances typical for he - rich disks in am  cvn systems .
this comparison shows that neither model exhibits a clearly detectable heii  1640  line .
hence the lack of this line in the hst spectrum is no proof of he - deficiency .
the situation is perhaps more favorable in the optical region . in fig.1
we have plotted two model spectra from c / o dominated disk regions at r=70000 km , one without h and the other with trace h ( @xmath14 by number ) .
a strong h@xmath15 emission is seen in the second case , which is not present in the observed spectra .
although we have not yet performed this test with helium , we hope that we can derive a tight upper limit for the he abundance from the lack of hei lines .
the parameters of the system 4u0614 + 091 are poorly known .
no orbital period was measured . from x - ray bursts
it is assumed that the accretor is a neutron star . from the similarity of the x - ray spectral characteristics with 4u1626 - 67 , juett et al .
( 2001 ) suggest that this system is also ultracompact .
the optical spectrum displays a number of c and o lines , as already identified by nelemans et al .
the lack of hei and hi lines supports the suggestion of juett et al .
the two c / o dominated models shown in fig.1 were computed for this system .
irradiation by the central object is not taken into account .
they do show emission features , which qualitatively match some observed features .
it appears that the disk in this region is optically thin .
angelini , l. , white , n. e. , nagase , f. , et al .
1995 , apj , 449 , l41 chakrabarty , d. 1998 , apj , 492 , 342 homer , l. , anderson , s. f. , & wachter , s. 2002 , aj , 142 , 3348 juett , a. m. , psaltis , d. , & chakrabarty , d. 2001 , apj , 560 , l59 nagel , t. , dreizler , s. , rauch , t. , & werner , k. 2004 , a&a , in press nelemans , g. , jonker , p. g. , marsh , t. r. , & van der klis , m. 2004 , mnras , 348 , l7 schulz , n. s. , chakrabarty , d. , marshall , h. l. , et al .
2001 , apj , 563 , 941 shakura , n. i. , & sunyaev , r. a. 1973 , a&a , 24 , 337 weidemann , v. 2003 , in white dwarfs , eds .
d.de  martino , r.silvotti , j .-
e.solheim , r.kalytis , nato science series ii , 105 , 3 werner , k. 1991 , a&a , 251 , 147 werner , k. 2001 , in low mass wolf - rayet stars : origin and evolution , eds .
t.blcker , l.b.f.m.waters , a.a.zijlstra , ap&ss , 275 , 27 werner , k. , & wolff , b. 1999 , a&a , 347 , l9 werner , k. , rauch , t. , barstow , m. a. , & kruk , j. w. 2004 , a&a , 421 , 1169
we discuss the possibility to detect o / ne white dwarfs by evidence for ne overabundances . 
 the hottest known wd , h1504 + 65 , could be the only single wd for which we might be able to proof its o / ne nature directly . 
 apart from this , strong ne abundances are known or suspected only from binary systems , namely from a few novae , and from a handful of lmxbs . 
 we try to verify the hypothesis that the latter might host strongly ablated o / ne wd donors , by abundance analyses of the accretion disks in these systems . in any case , to conclude on o / ne wds just by strong ne overabundances is problematic , because ne enrichment also occurs by settling of this species into the core of c / o wds .