euv spectroscopy in astrophysics: the role of compact objects

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Advances in Space Research 38 (2006) 1501–1508

EUV spectroscopy in astrophysics: The role of compact objects

K.S. Wood a, M.P. Kowalski a,*, R.G. Cruddace a, M.A. Barstow b

a Naval Research Laboratory, Code 7655, 4555 Overlook Avenue, SW, Washington, DC 20375, USAb Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

Received 30 September 2004; received in revised form 3 May 2005; accepted 20 February 2006

Abstract

The bulk of radiation from million-degree plasmas is emitted at EUV wavelengths. Such plasmas are ubiquitous in astrophysics, andexamples include the atmospheres of white dwarfs, accretion phenomena in cataclysmic variables (CVs) and some active galactic nuclei(AGN), the coronae of active stars, and the interstellar medium (ISM) of our own galaxy as well as of others. Internally, white dwarfs areformally analogous to neutron stars, being stellar configurations where the thermal contribution to support is secondary. Both stellartypes have various intrinsic and environmental parameters. Comparison of such analogous systems using scaled parameters can be fruit-ful. Source class characterization is mature enough that such analogies can be used to compare theoretical ideas across a wide dynamicrange in parameters, one example being theories of quasiperiodic oscillations. However, the white dwarf side of this program is limited bythe available photometry and spectroscopy at EUV wavelengths, where there exist critical spectral features that contain diagnostic infor-mation often not available at other wavelengths. Moreover, interstellar absorption makes EUV observations challenging. Results froman observation of the hot white dwarf G191-B2B are presented to demonstrate the promise of high-resolution EUV spectroscopy. Twotypes of CVs, exemplified by AM Her and EX Hya, are used to illustrate blending of spectroscopy and timing measurements. Dynamicaltimescales and envisioned performance parameters of next-generation EUV satellites (effective area >20 cm2, spectral resolution >10,000)make possible a new level of source modeling. The importance of the EUV cannot be overlooked given that observations are continuallybeing pushed to cosmological distances, where the spectral energy distributions of X-ray bright AGNs, for example, will have their max-ima redshifted into the EUV. Sometimes wrongly dismissed for limitations of small bandwidth or local view from optical depth limita-tions, the EUV is instead a gold mine of information bearing upon key issues in compact objects, but it is information that must be wonthrough the triple combination of high-spectral resolution, large area, and application of advanced theory.� 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Extreme ultraviolet; Astrophysical spectroscopy; White dwarfs

1. Introduction

White dwarfs were the original compact objects. Armedonly with pre-Fermi physics Eddington (1926) describedthe belated realization of the extremely small size of SiriusB, some 60 years after its discovery: ‘‘The mass . . . is foundfrom the double star orbit and is quite trustworthy. Theabsolute magnitude is 11.3, corresponding to a luminosity1/300 of that of the sun. The faintness would occasion nosurprise if this were a red star; but in 1914 W.S. Adams

0273-1177/$30 � 2006 COSPAR. Published by Elsevier Ltd. All rights reserv

doi:10.1016/j.asr.2006.02.043

* Corresponding author.E-mail address: [email protected] (M.P. Kowalski).

made the surprising discovery that the spectrum is that ofa white star not very different from Sirius itself. . . . we find. . . the radius is 18,800 km. Apparently, then we have a starof mass about equal to the sun and of radius much lessthan Uranus. The calculated density is 61,000 g cm�3, just

about a ton to the cubic inch. This argument has been known

for some years. . . I think it has generally been considered

proper to add the conclusion ‘which is absurd.’. . . I do notsee how a star, which has once got into this compressedcondition is ever going to get out of it. So far as we know,the close packing of material is only possible so long as thetemperature is great enough to ionize the material. Whenthe star cools down and regains the normal density ordi-narily associated with solids it must expand and do work

ed.

mailto:[email protected]

1502 K.S. Wood et al. / Advances in Space Research 38 (2006) 1501–1508

against gravity. The star will need energy in order to cool.

. . .’’From this start, white dwarfs have become keystones to

understanding stellar evolution in the Galaxy. Over 90% ofits stars evolve into white dwarfs. For temperatures of80,000–300,000 K, white dwarf spectra peak in the FUVand EUV, and optical data provide comparatively lessinformation. Photometric and spectral observations withEUVE (Extreme Ultraviolet Explorer) have produced awealth of information about their atmospheres, but resultswere limited by its modest effective area and spectralresolution.

2. White dwarf – neutron star correspondences

Nuclear burning has ceased in white dwarfs, except inthe atmosphere when the star is accreting. (Neverthelessthe nuclear burning that does occur in surface layers thatmake up only about 10�5 of the stellar mass accounts forclassical novae, which in turn contribute much or most ofthe 26Al in the Galaxy!) White dwarfs have a stable config-uration and balance gravity with electron Fermi pressure,so that there are �2 electrons per phase space volume h3.By analogy, one could have a star with �2 neutrons perh3, and this analogy was pursued theoretically (Oppenhei-mer and Volkoff, 1939). Yet the discovery of neutron starsin the 1960s managed to be once again a surprise, becauseto skeptics that analogy had seemed untrustworthy.

The white dwarfs and the neutron stars are now the twogreat families of Fermi-pressure-supported compactobjects. Because no suitable fermions exist beyond neu-trons, greater densities and relativistic degeneracy produceblack holes. Analogies and correspondences between thefamilies have proliferated in ways no one could have antic-ipated. Both neutron stars and white dwarfs exist in manycombinations of parameters such as spin period, magnetic

Table 1Correspondence between white dwarfs and neutron stars

White dwarf family

Support by electron Fermi pressure, R � 3000–12,000 kmMagnetic, isolated white dwarfRapidly rotating magnetic white dwarfPG1159Non-pulsing isolated white dwarf, hotNon-pulsing isolated white dwarf, coolSirius BU Gem, SS Cyg(not clear)Classical novaDwarf nova, Z CamLargest nova?Doppler-broadened disk linesShort binary period (AM CVn): gravitational radiationCV orbital period anomalies in ‘‘O–C’’DQ Her (intermediate polar)AM Her (polar)Type Ia Supernova

moment, and accretion environment. With few exceptions(and those exceptions are instructive), to each white dwarfobservational class or phenomenon, there corresponds ananalogous neutron star category or phenomenon. Oneneeds to scale the physics properly incorporating differenc-es in mass, radius and other parameters to recognize anddevelop the correspondences. One instructive example isthe correspondence between classical novae in white dwarfsand X-ray bursts in neutron stars. In each case, the com-pact object in a binary accretes material of roughly cosmicabundances, which settles into a surface layer until pressureat its base becomes critical and thermonuclear runawayensues. Historically, familiarity with classical novaebrought quick recognition of what was happening in (type1) X-ray bursts. In general, theoretical understanding ineither family should extrapolate properly to the other.Table 1 lists correspondences between the white dwarfand neutron star families.

Where symmetry of the correspondence is broken, thewhite dwarf side often seems more interesting: whitedwarfs, because of different initial conditions, have a widerrange of core compositions (He, CO, Ne, etc.) and hence awider range of final masses. Neutron stars initialize at mass1.4 Mx and usually remain near that value except whensubject to prolonged accretion; white dwarf initial massesrange over a factor of two. White dwarf spectral energy dis-tributions peak where many atomic transitions also occur,making their spectra more interesting and challenging espe-cially in the EUV. White dwarfs can have larger magneticmoments than neutron stars, e.g., AM Her is a stronger‘‘magnet’’ (in the sense of magnetic moment) than a magn-etar neutron star. Isolated white dwarfs may have ‘‘levitat-ed’’ atmospheres in which the effects of radiation pressurecan be pursued species by species. Their photospheres aremore accessible to observation. White dwarfs are numer-ous; there are plenty of nearby targets supporting appropri-

Neutron star family

Support by neutron Fermi pressure: R � 10 kmCrab pulsarMillisecond pulsars(not clear)Non-pulsing central star in SNRNo analog, too faint to have been seenNon-accreting binary neutron star, e.g., Be starLMXB, atollLMXB, Z, accreting near Eddington limitX-ray burst(various possibilities, including Rapid Burster)X-ray superburst?Doppler-broadened disk Fe lines –still uncertainShort binary period (X1820-303): gravitational radiation‘‘O–C’’ effects in EXO 0748-676HMXB, Her X-1, Cen X-3No analog, would be magnetar in binaryNeutron star – black hole by accretion from companion (rare)

Fig. 1. Observed (points with error bars) and theoretical (continuouscurves) white dwarf luminosity functions. The theoretical curves are fordifferent white dwarf cooling models. Curves are calculated for differentvalues of disk age in 2-Gyr steps.

K.S. Wood et al. / Advances in Space Research 38 (2006) 1501–1508 1503

ately detailed observation, to be compared with similarlydetailed numerical modeling. As another illustration, whitedwarfs are thought to be progenitors of the cosmologicallyimportant type Ia supernovae yet this widespread notionhas never been pursued to the point of successfully identi-fying the progenitors with a specific known stellar popula-tion. That unfinished business raises questions regardingutilization of the SNe as cosmological probes.

So much observational and theoretical work has beendone on neutron stars that more open questions exist onthe white dwarf side than on the neutron star side. As willbe seen, these questions have gone unanswered largely forlack of EUV data. It is not that white dwarfs can only beseen in this channel; it is rather that many crucial observa-tions have to be made there, and this in turn is primarilybecause of the temperatures characteristic of young whitedwarfs.

3. Spectral energy distributions

Neutron stars emit by various mechanisms, one of whichis thermal emission directly from the surface. The luminos-ity grows with temperature as T4 until the Eddington limitis reached, at which point the surface temperature is slight-ly over 2 keV. At this temperature, the associated radiationpressure will seriously retard any accretion, and if it some-how becomes hotter temporarily it can cause outflows. Thisis also the temperature where the surface becomes unstableto radiation, leading to levitation seen in X-ray bursts as anexpansion of the stellar radius. The analogous temperaturefor white dwarfs is

T � ð2 keV=kBÞ � ð0:6M�=1:4M�Þ1=4

� ð5500km=10kmÞ�1=2 ð1Þ

or T � (70 eV/kB). Levitation is possible as the white dwarfapproaches this temperature, which is essentially the max-imum possible, �800,000 K. The spectra of younger whitedwarfs and those with photospheres heated by accretionpeak in the EUV.

Cooling depends on the radius and hence mass and oncomposition, but the basic issue is how much heat can betransported through the interior and radiated by the sur-face. Fig. 1 (Wood, 1995) shows one way of comparingcooling models with observational data. Both the data(points with error bars) and the theoretical curves repre-sent white dwarf luminosity functions. The peak of thespectral energy distribution shifts from the EUV to theFUV as the white dwarf cools, but the process takes asignificant fraction of the Hubble time. Comparison withobservation is made by calculating the observed distribu-tion of stellar characteristics as a function of the popula-tion age. The white dwarf luminosity function for theGalaxy folds together the history with which normalstars have been deposited in the white dwarf stellargraveyard. There, they constitute an ensemble of thermalclocks, recording the times since endpoints of nuclear

burn were reached. This slow cooling evolves the spec-trum downward in frequency through a succession ofwavebands, making white dwarfs the ideal star for theworking out of all stellar evolution in the Galaxy. Mostvaluably, it represents fossilized information bearingupon the past history of production of compact objects.The theoretical curves shown in Fig. 1 are only the onesthat reasonably bracket the observations. A model for amuch younger population would produce too manybright white dwarfs and not enough faint ones. A modelfor a much older population would also fit badly, butwith the opposite signature.

The analogous neutron star cooling curve problem isalso now being pursued using Chandra observations ofneutron stars in SNR, a rather small sample. The twoproblems worked together should eventually confirmunderstanding of stellar evolution endpoints and evolu-tion of Galactic populations or else expose basic defectsin that picture. Both because there are fewer neutronstars and because their cooling models are complicatedby the possibility of significant non-photon contributionsto cooling, neutron stars are probably inferior to thewhite dwarfs for this purpose, but they do complementthe white dwarf information by providing the picturefor a different sample with higher initial mass, whereevolution goes more rapidly. Eventually, this researchwill clarify production rates for white dwarfs, neutronstars and black holes over the life of the Galaxy. Theserates are important to many issues in Galactic astrophys-ics. Finally, the crucial data points that provide absolutecalibration for the whole curve by linkage to contempo-

rary rates of white dwarf formation are the ones thatcome from young, hot white dwarfs, the ones whose spec-tral energy distributions peak in the EUV.


4. White dwarf atmospheres

Our understanding of physical mechanisms that deter-mine white dwarf evolution leaves major questions unan-swered. The emergence from the asymptotic giant branchof two groups of white dwarfs whose compositions aredominated by H or He is beginning to be understood,but the complex relationship between these branches anda demonstrable temperature gap in the cooling sequenceof the He-rich branch cannot be explained. Determinationof the photospheric He and heavy-element content pro-vides crucial information on the evolutionary history ofthese stars.

G191-B2B is one of the brightest and best studied of thehot, H-rich white dwarfs, lying near the top of their coolingsequence. It is one of a group of white dwarfs with temper-atures in excess of 50,000 K that contain significant quan-tities of heavy elements in their atmospheres, inparticular C, N, O, Si, S, P, Fe, and Ni (Bruhweiler andKondo, 1983; Sion et al., 1992; Vennes et al., 1992, 1996;Holberg et al., 1994). Such material strongly depressesthe EUV continuum, when compared to that of stars withpure H atmospheres. G191-B2B remains an important tar-get for EUV spectroscopy to determine the primary sourcesof opacity and to obtain a self-consistent model, with aneffective temperature, surface gravity and composition thatcan match the optical, FUV, and EUV observationssimultaneously.

Initial attempts to model its EUVE spectrum failed toreproduce either the observed flux level or the continuumshape (Barstow et al., 1996), which was believed to resultfrom inclusion of an insufficient number of Fe and Ni lines.Adding some 9 million predicted lines to the few thousandwith measured wavelengths did provide a self-consistentmodel able to reproduce the EUV, UV and optical spectra(Lanz et al., 1996). However, good agreement required asignificant quantity of He, either in the stellar photosphereor in an ionized interstellar–circumstellar component.Unfortunately, this inferred He contribution could not beresolved using EUVE from the many Fe and Ni lines.

More recently, it has been shown that photosphericheavy elements may not be distributed homogeneously(by depth) and that more complex stratified structures

Fig. 2. EUV spectrum of the white dwarf G191-B2B made with the J-PEX inbest-fit model is the solid line.

are needed to reconcile models and data at all wavelengths(Barstow et al., 1999). Important progress has also beenmade in incorporating radiative levitation and diffusionself-consistently into the models (Schuh et al., 2001), wherethe need for a He contribution is reduced but noteliminated.

We show in Fig. 2 the EUV spectrum of G191-B2B,made with the J-PEX high-resolution spectrometer, flownon a sounding rocket (Cruddace et al., 2002). The modelhad a homogeneous distribution of elements, and agree-ment between the model and the data is strikingly good.The broad features between 227 and 232 A are a charac-teristic of the overlapping series of interstellar He IIabsorption lines superposed on a continuum. Taken withthe strong depression in flux below 227 A, this is strongevidence for the presence of interstellar or circumstellarHe II along the line of sight. Conclusive proof isobtained when NHeII is set to zero, and the fit degradesmarkedly. The exposure was insufficient to detect anyphotospheric He line (e.g., 243 A) with high significance;however, when NHeII and nHe are fitted jointly the confi-dence contours support a positive detection of photo-spheric He. Surprisingly, models with stratifieddistributions have not produced better fits (Barstowet al., 2005). This example, based on only a few hundredseconds of data, marks in principle the beginning of anew epoch of EUV astronomy, one where high spectralresolution is used to disentangle source and ISM effectsand arrive at rigorous astrophysical results that areimportant for both those sources and the ISM. It illus-trates how even a small bandwidth (220–245 A) that hap-pens to be rich in features can, when expanded with highresolution, become a field where elusive prizes concerningcomposition details may be won.

5. Unification of spectral and temporal regimes: CVs

One may view white dwarfs as populating the intersec-tion of two astrophysics paradigms as represented inFig. 3. The additional EUV channel and the relatively long-er dynamical timescales combine so that white dwarfs canbe studied in ways unavailable for neutron stars. Thesemethodologies then overlap what can be done in laborato-

strument flown on a sounding rocket. The data are the error bars and the

Fig. 3. White dwarfs are at the intersection of two paradigms in astrophysics.


ry and coronal plasma research. There is great potentialhere, as yet somewhat unrealized because of the demandsof EUV work.

CVs, which are binaries consisting of a white dwarf anda companion in sufficient proximity to produce a high rateof mass transfer, are illustrative of this intersection. Exam-ining cases will show how timing and spectroscopy covercomplementary aspects of these variable objects. It will alsouncover a more negative point regarding unrealized com-pleteness of understanding: there has not emerged a suc-cessful theoretical unification of the information derivedfrom these two methodologies. For example, quasiperiodicoscillations (QPOs) are a well-studied phenomenon forwhich there is a body of theory developed by many groups,treating them as hydrodynamic modes produced in accre-tion. Spectroscopy measurements bear on the state of thevery same accreting plasma, its density, temperature, ioni-zation states, and ultimately geometries. The need for uni-fied treatment is evident but full challenge has not beentaken up. Communities of theorists have specialized inone or the other aspect. One can try to excuse this on theground that instruments delivering spectra have not usuallybeen able to detect the temporal effects and vice versa, butit is also partly because of substantial difficulties. Eachbody of information places strong demands on theory.

Those generalities apply to two distinct categories ofCVs. In the magnetic CVs (polars), the QPOs and associat-ed emission spectroscopy both pertain to regions nearpolar caps. In these binary systems there is no accretiondisk. In contrast, there are non-magnetic CVs where QPOsand emission phenomena are both associated with disks. Inboth cases, theory has treated timing and spectra separate-ly, despite their pertaining to the same flows. It may even-tually emerge that treatments that serve to unify one casealso help the other. Also, there is a cross-cutting theoreticalchallenge to be sought, this being links across the popula-tions – either linking accreting WD to NS and BH as inthe schema of Table 1 or more ambitiously and elusively,linking different CV classes, with and without disks. Thisdevelopment is also in early stages. Hence, to deal withthe state of the art in CVs we must treat magnetic and

non-magnetic cases separately, but making somewhat par-allel points about how the EUV can contribute.

5.1. Magnetic CVs

Consider first AM Her stars (polar), which have thelargest magnetic moments among compact stars. They lackaccretion disks because the magnetic field constrains flowall the way from the companion star to the compact object.The white dwarf and companion are locked in synchronismso that the spin and orbit periods are the same. QPO phe-nomena have long been known in the optical, predatingdiscovery of horizontal branch oscillations in neutronstars, the latter being the earliest NS QPOs discovered.AM Her stars have been modeled (in a succession of treat-ments done by several groups, and gradually improvingover time in fidelity of treatment) as a magnetically fun-neled supersonic flow impacting on the white dwarf sur-face, flowing through a standoff shock (Fig. 4) near thesurface. The shock may be unstable to an oscillatorybehavior, useful for diagnostic purposes. Vertical oscillato-ry movement of the shock front is thought to manifest itselfas a QPO in at least two components of the three-compo-nent spectrum, the cyclotron (optical) emission and the (X-ray) Bremsstrahlung. The optical oscillations are wellestablished but the predicted X-ray QPOs are faint andhave not yet been confirmed. Moreover, since they derivefrom the same accretion column the two kinds of oscilla-tions are predicted to be synchronized cycle by cycle, a factthat can in principle be brought out by cross-correlatingsimultaneous optical and X-ray data streams. Whetherthe third spectral component, a �40-eV black body fromthe polar cap, is also synchronized is at this time unclearfrom either the observational or the theoretical standpoint.Modeling of the shock is carried out primarily by means oftwo-temperature numerical codes with appropriate physicsfor radiation and electron–ion interactions (Wolff et al.,1989; Wood et al., 1992). Linear (analytical) analysis showsreduced ion–electron exchange induces the oscillationinstability, while non-linear (numerical) analysis shows thatthermal conduction cannot dampen the oscillations. These

Fig. 4. Schematic representation of a polar white dwarf. The accretion ismodeled as a flow strictly following rigid field lines, becoming supersonicand then striking the star to produce a standoff shock. R

*is the radius of

the white dwarf and h is a polar angle measured with respect to themagnetic axis.


models represent the mature development of the subjectand the papers cited may be used to trace back throughthe full history of this line of theory, which is not our pur-pose here. Rather, we now turn to the disconnect betweenthis timing work and the high-resolution EUV spectrosco-py on the same plasmas.

The same accretion geometry has been explored withcomplementary spectroscopic techniques, using EUV spec-tra obtained at various phases of the period. An exampleusing EUVE is the phase-resolved spectrum of AR UMa(an AM Her-type or polar system) obtained by Szkodyet al. (1999). These systems, then, are simultaneously richin both timing and spectroscopic information and eachbody of observational knowledge has spawned correspond-ing – yet independent – theory. Not only is the theoreticalsynthesis awaited, the observational picture also falls farshort of what one would like, namely simultaneity of cov-erage in optical, EUV, and X-rays, in which the temporalinformation reaches the known dynamical timescales (sec-onds) and samples the phases of the period, while the spec-troscopy harvests the plasma diagnostics from the lines, allsimultaneously. However, the EUVE mission has ended,CHIPS (Cosmic Hot Interstellar Plasma Spectrometer) isoptimized for diffuse emission, and while the Chandra res-olution is good at EUV wavelengths (R � 2000) its sensitiv-ity is low. No new missions are planned, but future EUV

spectral measurements (Kowalski et al., 2005) will requirehigh resolution (R � 10,000) and sufficient sensitivity toprovide adequate statistics on dynamical timescales,implying areas >20 cm2.

5.2. Non-magnetic CVs

Many QPO modes are known in accreting neutronstars, analogous in the sense of Table 1 to cataclysmicvariables. Rich phenomenology can occur in a singlesource, for example, Sco X-1. QPOs have provided thebest diagnostics for the low-mass X-ray binaries, wherecoherent periods are both rare and transient. Whitedwarf QPOs connect phenomenologically to NS QPOs,but EUV spectroscopy is the supplementary, powerfultool effectively available for the WD alone in determiningplasma conditions. The hope is to understand QPO phe-nomena thoroughly in white dwarfs and then export thatunderstanding to neutron stars. The first goal is tounderstand QPO frequencies and they vary with bright-ness, which correlates with accretion rate. A correlationbetween the low- and high-frequency QPO frequenciesexists from white dwarfs through neutron stars to blackholes. It was developed first observationally and thenexplored theoretically (Belloni et al., 2002; Mauche,2002a; Titarchuk and Wood, 2002; Woudt and Warner,2002). The empirical relation, fitted in a manner guidedby the theory, is mlow = 0.081 mhigh. In terms of the theo-ry, both QPOs are again hydrodynamic modes, the latterassociated somewhat straightforwardly with the Keplerfrequency and the former involving magnetoacousticeffects between the field and disk. Under this picture,Titarchuk and Wood (2002) derive the proportionalityconstant linking the two frequencies mMA = CMAmK,where the constant relates to geometry and the usualplasma beta factor as:

CMA ¼p

2 4p½ðf þ bÞ=ð1þ bÞ�1=2ðH=RoutÞ. ð2Þ

They find that H/Rout = 0.015 and b = 0.1. These are re-cent developments, but they show the enticing possibilityof a QPO phenomenon that actually bridges the gap be-tween white dwarfs and neutron stars.

Once again, we turn from the timing theory at a rep-resentative state of development and turn to the comple-mentary EUV spectroscopy. This field is in too great astate of flux for it to be the case that the timing andspectroscopy are invariably available on the sameobjects, but nonetheless a major mystery is emergingon the spectroscopic side. EUVE spectra of some dwarfnovae are rich in features, which are persistent but notwell understood (Mauche, 2002b). The intermediate polarEX Hya is a non-magnetic CV and also a strong EUVsource for which the spectral information is unusuallygood. Belle et al. (2002) shows the time-resolved spectrafrom EUVE. The number of significant line detections issmall and represent blends of Fe and other elements. The


first challenge here is to unravel these spectra, identifyingthe features with confidence, so that the problem mayyield to higher resolution.

6. Cosmological issues

The EUV viewpoint on compact objects continuesbeyond the Galaxy to extragalactic astrophysics and cos-mology. Some of this again involves white dwarfs indirect-ly, while other aspects involve AGN. First, type IaSupernovae are now accepted as the chief standard candleof the cosmic distance scale, which led to the original (andstill most direct) basis for inferences regarding the existenceof Dark Energy. Missions such as JDEM, the Joint DarkEnergy Mission (for which one candidate realization isSNAP, the Supernova Acceleration Probe), will observelarge numbers of these SNe, class by class, out toz = �1.7. The assumed type Ia progenitor is a carbon–ox-ygen core white dwarf, which has exceeded its Chandrase-khar limit at 1.4 Mx, either through accretion or merger ofa double-degenerate system. This implies the existence of apopulation of short period binary systems that will evolveinto type Ia events within a cosmological timescale (�Gyr).Unfortunately, in spite of several comprehensive searches,few clear examples of such systems have emerged. Prudencemay favor getting better observational understanding ofsuch precursor systems as a prerequisite to the develop-ment of such large missions. As the progenitors are likelyto be hot but scarce, deeper white dwarf EUV surveys withhigh-resolution spectroscopy followup may discovercandidates.

Second, much emphasis in NASA’s Strategic Plan andSEU/Origins roadmaps has been given to the investigationof the early universe. Sources at cosmological distanceshave spectral energy distributions redshifted, which thensets the bandwidth requirement for corresponding instru-mental designs. A striking illustration of this is the factthe James Webb Space Telescope (JWST) will follow upcosmological work initiated in the visible and near-UV

wavelengths using HST, but does so with an instrumentoptimized for infrared wavelengths – the UV science pur-sued to higher z moves to the IR. Similarly, X-ray sourcesat z > 5 (i.e., AGN) will find shifted into the ultrasoft X-ray/EUV a portion of their output that increases as z

increases, which is to say as they become more interestingcosmologically. This includes, for example, the spectralregions near K transitions of C, N, and O, and L-transi-tions of Fe at rest energies �1 keV. Although extragalacticmeasurements are complicated by interstellar absorption atwavelengths above 100 A, regions (e.g., the Lockman hole)of low column encompass sufficient solid angle to providereasonable numbers of extragalactic targets for instrumentswith sufficient sensitivity. Moreover, accretion processesongoing in distant X-ray AGNs are self-similar to thosein CVs. There has been continual borrowing of ideas andinsights from the Galactic accreting sources to the AGNand vice versa and one should anticipate that this will be

the case as high resolution EUV spectroscopy begins tocontribute significantly.

7. Conclusions

White dwarfs are a key component of the compact pop-ulations representing stellar evolution endpoints. Under-standing their phenomenology and sub-populations in anevolutionary context, particularly if pursued in conjunctionwith a similar effort on neutron stars, is central to the astro-physics of compact objects. In recent decades X-ray diag-nostics have been used to advance dramatically thecharacterization and understanding of the neutron starpopulation, but progress on white dwarfs has been slowedby lack of suitable photometry and spectroscopy at EUVwavelengths, despite the fact that they are comparativelynumerous so that there are outstanding white dwarf ‘‘lab-oratories’’ in the Solar neighborhood. It is necessary toachieve sufficient spectral resolution to permit study ofthe white dwarfs through the absorption and emission fea-tures of the ISM. This obstacle is rapidly being overcome.Suitable technologies have progressed to the point wherepractical photometers and spectrometers can be designedwith effective areas >20 cm2 and with spectral resolution�10,000 (Kowalski et al., 2003a,b).

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euv spectroscopy in astrophysics: the role of compact objects

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