Solar Abundances of Rock Forming Elements, Extreme Oxygen and Hydrogen in a Young Polluted White Dwarf
J. Farihi, D. Koester, B. Zuckerman, L. Vican, B. T. Gänsicke, N. Smith, G. Walth, E. Breedt
aa r X i v : . [ a s t r o - ph . E P ] A ug Mon. Not. R. Astron. Soc. , 000–000 (0000) Printed 27 August 2018 (MN L A TEX style file v2.2)
Solar Abundances of Rock Forming Elements, Extreme Oxygen andHydrogen in a Young Polluted White Dwarf
J. Farihi ⋆ †, D. Koester , B. Zuckerman , L. Vican , B. T. G¨ansicke , N. Smith ,G. Walth , E. Breedt Department of Physics and Astronomy, University College London, London WC1E 6BT Institut f¨ur Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany Department of Physics and Astronomy, University of California, Los Angeles CA 90095, USA Department of Physics, University of Warwick, Coventry CV4 7AL Steward Observatory, University of Arizona, Tucson AZ 85721, USA
ABSTRACT
The T eff =
20 800 K white dwarf WD 1536 +
520 is shown to have broadly solar abundances ofthe major rock forming elements O, Mg, Al, Si, Ca, and Fe, together with a strong relative de-pletion in the volatile elements C and S. In addition to the highest metal abundances observedto date, including log (O/He) = − .
4, the helium-dominated atmosphere has an exceptionalhydrogen abundance at log (H/He) = − .
7. Within the uncertainties, the metal-to-metal ratiosare consistent with the accretion of an H O-rich and rocky parent body, an interpretation sup-ported by the anomalously high trace hydrogen. The mixed atmosphere yields unusually shortdiffusion timescales for a helium atmosphere white dwarf, of no more than a few hundred yr,and equivalent to those in a much cooler, hydrogen-rich star. The overall heavy element abun-dances of the disrupted parent body deviate modestly from a bulk Earth pattern, and suggestthe deposition of some core-like material. The total inferred accretion rate is 4 . × g s − ,and at least 4 times higher than any white dwarf with a comparable diffusion timescale. No-tably, when accretion is exhausted in this system, both metals and hydrogen will becomeundetectable within roughly 300 Myr, thus supporting a scenario where the trace hydrogen isrelated to the ongoing accretion of planetary debris. Key words: circumstellar matter— stars: abundances— stars: individual (WD 1536 + A decade of observational and theoretical studies by many as-tronomers has shown that, over a wide range of effective stellartemperatures, the presence of heavy elements in white dwarf at-mospheres is evidence for orbiting planetary systems (Farihi 2016;Vanderburg et al. 2015; Jura & Young 2014; Veras et al. 2015).With this relatively recent shift in paradigm, the discovery of theprototype, metal-lined white dwarf by van Maanen (1917) nearlya century ago – while not a planet detection itself, but the signa-ture of accreted planetary debris – is arguably the first astronomi-cal evidence of the presence of planetary systems around other stars(Zuckerman 2015).According to all dynamical models that deliver sufficient plan-etesimal masses into the innermost system where it can be ac-creted, each exoplanetary system hosted by a metal-enriched whitedwarf must harbor at least a belt of minor bodies and one ma- ⋆ E-mail: [email protected]† STFC Ernest Rutherford Fellow jor planet (Frewen & Hansen 2014; Veras et al. 2013; Debes et al.2012; Bonsor et al. 2011). The gravitational field of the planet(s)can perturb the orbits of the planetesimals onto orbits passingnear the white dwarf so that they are tidally disrupted.
Spitzer and complementary ground-based observations have established afirm connection between the atmospheric heavy elements in whitedwarfs and the presence of dust and gas within the tidal radius ofthe star (Farihi et al. 2009; von Hippel et al. 2007; Jura et al. 2007;G¨ansicke et al. 2006).Because the metal-to-metal sinking timescales vary by nomore than a factor of a few, the relative, steady-state abundances ofthe accreted planetary debris can be analytically linked to those ob-served in the polluted atmosphere (Koester 2009), thus making thestellar surface an effective mirror of planetesimal composition. Thefirst detailed abundance study of any metal-enriched white dwarfwas carried out for the current record holder for number (16) ofdetected heavy elements, GD 362, demonstrating that the debriswas broadly terrestrial-like (Zuckerman et al. 2007). Since then, thebroad pattern of bulk, Earth-like compositions has been seen – es-pecially with ultraviolet
HST observations – in several more stars c (cid:13) J. Farihi et al.
Figure 1.
In the left panel is a portion of the HIRES spectrum, with wavelength given in vacuum and showing a strong triplet of Al II . The right panel shows aportion of the spectra obtained at the MMT, containing telluric features, H a , two He I , lines, and metal lines of O I , Mg II , and Si II . Unusually, the white dwarfexhibits strong lines of both H and He I , indicating a mixed atmospheric composition. with five or more heavy elements (O, Mg, Si, Ca, Fe) that indicatemelting and differentiation among extrasolar, rocky planetesimals,and a diversity of overall compositions similar to different classesof Solar System meteorites (Xu et al. 2013; G¨ansicke et al. 2012).Importantly, while most polluted white dwarfs appear to becontaminated by debris from parent bodies that were relativelypoor in H O and other volatiles (Jura & Xu 2012), there is at leastone case where substantial H O can be confirmed in an other-wise volatile- and carbon-poor planetesimal. The debris orbitingand polluting the atmosphere of GD 61 originated in a rocky minorplanet roughly the size of Vesta and containing approximately 26%water by mass (Farihi et al. 2013b). Another polluted white dwarfwith a substantial oxygen excess is SDSS J124231.07+522626.6,where the parent body likely had an even higher water content(Raddi et al. 2015). Such water-rich asteroids are important aspotential building blocks of habitable planetary surfaces, espe-cially if most small and rocky planets form dry as did the Earth(Morbidelli et al. 2000).This paper reports the identification and analysis of H, O, Mg,Al, Si, Ca, Ti, Cr, and Fe in the helium atmosphere white dwarfWD 1536 + O. Sec-tion 2 presents spectroscopic observations from several facilitiesthat resulted in the detection of all the major rock forming ele-ments, and strong upper limits on key volatiles. The atmosphericmodeling is discussed in Section 3, along with the determination ofstellar parameters, and elemental abundances within the star andthe disrupted parent body. The paper explores the so-far uniqueproperties of this star as something of a transition object betweenhelium- and hydrogen-rich, polluted white dwarfs, with the conclu-sions presented in Section 4.
WD 1536 +
520 was first identified in the Second Byurakan SkySurvey (SBS 1536 + I , weakerlines of H) white dwarf from a low resolution, R ≈
400 spec-trum. It was spectroscopically observed as part the Sloan DigitalSky Survey in 2002 (SDSS 153725.71 + R ≈ ugriz photometry alone results in a tempera-ture estimate of 22 000 K (Girven et al. 2011), the presence of thesemetal absorption features in a modest resolution spectrum is re-markable – at similar T eff and irrespective of atmospheric composi-tion, the detection of atmospheric metals in white dwarfs typicallyrequires powerful, high-resolution spectroscopy with Keck or theVLT (Koester et al. 2005). The star has an infrared excess detectedby WISE (Debes et al. 2011b; Barber et al. 2014) at 3.4 and 4.6 µ m,where the data are consistent with passively heated debris orbitingwithin the Roche limit, similar to roughly 40 other metal-enrichedwhite dwarfs accreting from analogous disks (Farihi 2016).Follow up observations were obtained in 2014 April with theMMT using the Blue Channel Spectrograph. Spectra were takenthrough a 1 ′′ slit with the 832 l mm − grating in first and secondorder, covering 6200 − − I II , and Si II .Additional, medium-resolution spectra were taken in 2014July using the double arm ISIS spectrograph on the WHT. Simul-taneous blue and red spectra were taken through a 1 ′′ slit using theR1200B and R1200R gratings, with the 5300 ˚A dichroic, resultingin two spectra covering 4500 − II and Si II features in wavelength regions not covered by theMMT dataset.Lastly, high-resolution observations carried out in 2015 Aprilwith the HIRESb spectrograph on Keck I. The setup was identi- c (cid:13) , 000–000 xtreme O and H in a Polluted White Dwarf Table 1.
Stellar Parameters for WD 1536 + g (AB mag) 17.06 d (pc) 217 ± T eff (K) 20800 ± g (cm s − ) 7 . ± . M ⊙ ) 0 . ± . − . ± . ( M cvz / M ) − . + − Table 2.
Lines Used for Abundance Determinations and Upper LimitsIon Vacuum Wavelength ( ˚A)C II I II I II II II II II II II II cal to that described in (Zuckerman et al. 2011), covering the range3130–5940 ˚A. The blue cross disperser was combined with a 1 . ′′ R ≈ IRAF and
MAKEE . This dataset revealsmultiple lines of Mg II , Al II , Si II , Ca II , Ti II , Cr II , and Fe II , a por-tion of which is shown in Figure 1.All spectra were reduced in the standard fashion, by average-combining each spectrum after extraction, using variance weight-ing for sky subtraction and rejection of bad pixels and cosmic rays. The multiple spectral datasets were analyzed together using whitedwarf atmospheric models, where the input physics is detailed inKoester (2010). The final stellar parameters were based on spectralfits to the latest SDSS spectrum, obtained with the BOSS spec-trograph. We calculated a 3-dimensional model grid in T eff , log g ,and [H/He], keeping the latter fixed while fitting the first two pa-rameters. This is a more stable procedure than fitting for all threeparameters, since the effect of [He/H] and log g on the spectrumis much smaller than that of the temperature. The results indicatethe best fit is near 20 800 K, which was confirmed by repeating asimilar fit with log g kept fixed and fitting for T eff and [H/He]. Thefinal stellar parameters are given in Table 1.For the determination of abundances and upper limits, allavailable spectra (Keck, MMT, SDSS, WHT) were used with themethod of line profile fitting. Table 2 lists all the individual ionsand wavelengths used for this purpose. After a good fit was ap-proximated, models were re-calculated with ± . Table 3.
Abundances, Masses, and Accretion Rates for Trace Elements[Z/He] Early Phase Steady StateElement [Z/He] – t diff X z M cvz ˙ M z [Z/H] ⊙ (yr) (10 g) (10 g s − )H − . ¥ − . . . − . − .
09 302 1.276 1.329Mg − . + .
39 166 0.424 0.804Al − . + .
16 141 0.023 0.050Si − . + .
14 122 0.269 0.692P − . . . − . . . − . + .
38 146 0.042 0.091Ti − . + .
24 126 0.001 0.003Cr − . + .
43 103 0.012 0.037Fe − . + .
03 97 0.354 1.148 S Note . Errors in abundance determinations are typically 0 . − . . × g of helium and 4 . × g of hydro-gen. Due to their continual sinking, the mass of heavy elements withinthe convection zone represents a minimum mass for the parent body. Themetal-to-metal ratios within the planetary debris for the early phase andsteady state regimes are derived directly from the values in the fifth andsixth columns respectively. The diffusion timescales are a sensitive func-tion of T eff within the range of acceptable temperatures for WD 1536, andthus contribute some additional uncertainty to the derived abundance ratios. where visual inspection of each line determined the abundance andan error estimate. In the case of multiple lines the final abundancewas determined as a weighted average. Repeating the analysis with T eff and log g varied within the adopted errors, the systematic errorswere found to be approximately 0 .
20 dex, but in the same direc-tion for all elements. Relative abundance errors are 0 .
05 dex. Thechanges of the convection zone and diffusion timescales contribute0 .
10 dex; and thus the total systematic error in relative abundancesis 0 .
12 dex.
The abundances, relative to helium, of all trace elements are givenTable 3, together with diffusion timescales for each species. Thethird column compares the atmospheric, heavy element abundancesin the white dwarf (relative to He) in units of solar values (rel-ative to H; Lodders 2003), demonstrating that WD 1536 nomi-nally exceeds the solar values for nearly all detected elements.These absolute abundances surpass the previous record holderSDSS J073842.56+183509.6 (Dufour et al. 2010) by a factor of 3–10, and GD 362 by over an order of magnitude (Xu et al. 2013).Also calculated are the mass of each element present in thephotosphere of the star, which is equivalent to the mass fractionof a given element X z , multiplied by the mass of the convectionzone M cvz . If WD 1536 is in an early phase of accretion, whereless than a single diffusion timescale has expired since the onset ofatmospheric pollution, then the metal-to-metal abundances of theinfalling debris are exactly mirrored by those in the atmosphereand given in the fifth column. If instead pollution has been ongoingfor at least 5 diffusion timescales (Koester 2009), then the systemis in a steady-state balance between accretion and diffusion and theabundance ratios are reflected in the sixth column. For all detected c (cid:13) , 000–000 J. Farihi et al.
Figure 2.
Derived number abundances for the planetary debris pollutingthe outer layers of WD 1536. Shown in blue and green respectively, are theearly phase and steady state abundances of each heavy element relative toSi, divided by the same ratio as determined for the bulk Earth (All`egre et al.2001). A typical uncertainty in the derived ratios is 0.12 dex, and errorbars of this size are overplotted. Also plotted are the same ratios for CIchondrites and the Sun (Lodders 2003), demonstrating that these composi-tions can be confidently ruled out for WD 1536 based on ground-based dataalone. At face value, the disrupted parent body appears broadly similar tothe bulk Earth, with notably high chromium. elements but oxygen, the metal-to-metal ratios show little variationbetween the early phase and steady state solutions.In Figure 2 are plotted both the early phase and steady stateabundances of heavy elements, relative to silicon and normalizedto the bulk Earth values from All`egre et al. (2001). As can beseen, the debris orbiting and polluting WD 1536 is bulk Earth-likein the major rock forming elements to within a factor of aroundtwo. There is a notable enhancement in chromium, yet an apparentdeficit in phosphorous. This two-fold deviation in opposite direc-tions is difficult to reconcile, as both chromium and phosphorousare siderophiles with similar condensation temperatures (Lodders2003). Similar enhancements in chromium have been seen inthe white dwarfs PG 0843+516 and GALEX J193156.8+011745(G¨ansicke et al. 2012) – together with bulk Earth or higher phos-phorous abundances, as expected – but are otherwise not commonlyseen in polluted white dwarfs (Xu et al. 2014; Jura et al. 2012). Be-cause phosphorous has only been detected in white dwarfs at ul-traviolet wavelengths, the upper limit derived for WD 1536 fromoptical data may be uncertain. With this caveat, the data are consis-tent with the accretion of substantial core-like material. O The total oxygen budget can be evaluated by accounting for all theexpected oxides originating in planetary solids (Farihi et al. 2013b;Klein et al. 2010). In the early phase and steady state scenarios,oxygen is first assumed to be carried exclusively by MgO, Al O ,SiO , CaO, and FeO within the debris. There are three possibleoutcomes from such an analysis.(i) Insufficient oxygen to account for metal oxides. This out-come can imply that iron was delivered not as FeO but substantiallyas metal. Table 4.
Oxide, Iron Metal, and Water Mass FractionsOxygen Carrier Early Phase Steady StateMgO 0.22 0.40Al O a O in debris: 0.25 –Fe in metal: 0.00 1.00 a Upper limit for FeO.
Note . The first five rows assumes oxygen is carried to maximum capacityby all the major rock forming elements, but in fact iron can also be in puremetal or iron-nickel alloy with no oxygen. The nominal oxygen budget inthe steady state is unphysical unless 100% of the total iron is carried asmetal, and the nominal O/Si and O/Mg ratios are marginally higher thantabulated. (ii) An oxygen budget as expected for oxides in planetary solids.In this case the debris is rocky and poor both in water ices andhydrated minerals resulting from aqueous alteration.(iii) Excess oxygen beyond that of anhydrous minerals alone. Inthis case, H O is the most likely source of the oxygen surplus.Carbon can confidently be ignored as an oxygen carrier for thefollowing reasons. First, carbon has been found to be significantlydepleted relative to solar and volatile-rich, cometary abundancepatterns in nearly all polluted white dwarfs where measurementsare available (Wilson et al. 2016; Koester et al. 2014; Farihi et al.2013a; Jura et al. 2012; G¨ansicke et al. 2012; Jura 2006). Second,CO and CO are no more than 5%–10% of the volatile con-tent of Solar System comets, which are dominated by water ice(Binzel et al. 2000). Third, for WD 1536 in particular, the upperlimit carbon abundance suggests that it cannot be a significantsource of excess oxygen.Table 4 evaluates the nominal oxygen budget for WD 1536 forboth an early phase and steady state accretion history. In the steadystate, there is insufficient oxygen to account for Mg, Al, Si, Ca,and Fe bound in oxides – only if 100% of the iron was delivered asmetal or alloy can the oxygen budget be considered balanced andphysical . In this case, the nominal oxygen abundance still requiresa modest, 5 −
10% increase to account for the other elements (Mg,Al, Si, Ca) that only form rocks, but such leeway is well withinthe uncertainties. This is another strong indication that the materialorbiting and polluting the white dwarf has a substantial core-likecomponent. Of the total iron mass present in the Earth, metallic Fein the core is thought to represent 87%, whereas Fe in the mantleand crust is only 13% (McDonough 2000), some of which is alsometal. Thus, the scenario where the iron in WD 1536 was containedessentially in pure metals or alloys is plausible. Within the derivedphotospheric abundance errors, a steady state solution without anyiron oxides would readily allow for solutions where the parent bodycontained water ice or hydrated minerals.While the range of allowed abundance ratios also permits solu-tions without any excess oxygen, the striking hydrogen abundancein WD 1536 must be considered, and which clearly favors a water-rich interpretation. While an early phase of accretion predicts anoxygen excess and thus the need for H O within the planetary de-bris, the heavy element settling times are relatively short, and thus c (cid:13) , 000–000 xtreme O and H in a Polluted White Dwarf Figure 3.
Accretion rate versus diffusion timescale for WD 1536 anda large sample of metal-enriched white dwarfs observed with
Spitzer ∼ koester;Koester 2009). The hydrogen-rich stars are shown as red filled and blackopen circles, while the helium-rich stars are shown as blue filled and greyopen circles; filled symbols correspond to the detection of infrared excess.Within each atmospheric class, left to right represents decreasing T eff . Re-markably, WD 1536 sits in a region that is otherwise exclusively occupiedby stars with hydrogen atmospheres. G166-58 is the coolest white dwarfwith a hydrogen-rich atmosphere and an infrared excess. catching the star in this phase is less likely. If disks last for at least10 yr (Girven et al. 2012), then the probability that WD 1536 isnot yet in a steady-state phase of accretion is less than 1%. Thetotal hydrogen mass within the stellar atmosphere is 4 . × g,and could have been delivered by an asteroid with total mass a fewto several times 10 g and which was 5–10% H O by mass. Thiswould be consistent with the lower mass limit of 4 . × g fromthe heavy elements alone.While uncertain, the totality of data discussed in this sectionfavors the deposition of H O onto the stellar surface and carriedby the parent body whose debris now orbits the star. In the nextsection, the anomalously high trace hydrogen abundance is shownto be transient, thus strengthening this interpretation.
The mass of the convection zone in WD 1536 is tiny – 10 timessmaller than those within the bulk of known polluted white dwarfswith helium atmospheres. There are two reasons for this. First, the T eff and 60 Myr cooling age mean the star is experiencing the earlystages of convection zone growth (Paquette et al. 1986). In fact,with T eff > Spitzer , as a func-tion of their inferred accretion rates and sinking timescales basedon Ca II detections (Bergfors et al. 2014). WD 1536 lies above threehydrogen-rich stars whose disks have been detected in the in- Figure 4.
Relatively cool, helium atmosphere white dwarfs with de-tected hydrogen absorption. Light blue open circles are nearby stars fromBergeron et al. (2011) and Voss et al. (2007), while the grey dots are amore distant and abundant sample from Koester & Kepler (2015). Filledpurple circles are stars with strong Ca II from Dufour et al. (2007), andfilled dark blue circles are Figure 3 stars with matching criteria. The up-per panel demonstrates the anomalous nature of WD 1536, whose H/Heratio is over an order of magnitude higher than any comparable object andsecond only to GD 362 (Zuckerman et al. 2007). Overplotted in the lowerpanel are tracks demonstrating the directly proportional change in H/He dueto the growth of the convection zone over time in pure helium atmospherestars (Koester 2009), shown for five orders of magnitude of initial [H/He] = . , , − . , − . , − .
0. These calculations assume complete mixing ofthe outer layers. frared (Farihi et al. 2010b; von Hippel et al. 2007), and has an evenshorter sinking timescale than G166-58 (Farihi et al. 2008). Thisso-far unique position for a helium-dominated star strongly sug-gests 1) it is accreting at a high rate in a steady state, and 2) thatthe older, cooler stars with similar sinking timescales do not oftenexperience similarly high rates of accretion. If this interpretation iscorrect, it would support a decreasing trend of planetary dynamicalactivity in the post-main sequence, as measured by the mass influxof planetesimals towards the white dwarf host, consistent with the-oretical predictions (Mustill et al. 2014; Veras et al. 2013)Figure 4 highlights the exceptional H/He in WD 1536. The up-per panel plots samples of helium-rich white dwarfs with trace hy-drogen detected directly through Balmer absorption features (typi- c (cid:13) , 000–000 J. Farihi et al. cally only H a ) . Interestingly, a substantial fraction of the plottedstars are also polluted with heavy elements, although a strong biasis present at T eff . I absorption rapidly becomes too weak to detect in low- and medium-resolution spectra, whereas strong Ca II absorption can indicate ahelium-rich atmosphere (Dufour et al. 2007). At the warmer end ofthe temperatures shown in Figure 4, the bias towards metal detec-tion is not an issue. Caution should be used when viewing Figure4; the plotted stars do not represent an evolutionary sequence, andselection biases play a large role. That being said, the cooler starswith substantial hydrogen are either born with substantially moremassive reservoirs than can currently be inferred in earlier evolu-tionary stages, or accrete H-rich planetary material. Assuming complete mixing of the outer stellar layers, the lowerpanel of Fig 4 plots tracks of constant hydrogen mass withinotherwise-pure helium atmosphere white dwarfs, as a function oftemperature (Koester 2009). In this simple model where no strat-ification occurs between hydrogen and helium, the observationalsignature of most fixed masses of hydrogen at T eff ≈ Thus in the absence of continued accretion WD 1536 willhave both its metals and trace hydrogen wiped clean from itsphotosphere . Without the influence of ongoing, external pollu-tion, the heavy elements will completely sink beneath the photo-sphere within a few 10 years at most. But over longer timescales,the remarkably high abundance of atmospheric hydrogen will bedrowned by the deepening helium convection zone. By the timeWD 1536 has cooled to 15 000 K in 140 Myr, the mass of the con-vection zone will have grown by 4 orders of magnitude and exhibita trace hydrogen abundance log(H/He) = − .
7. At this stage, thestar will either appear as a fairly average helium-rich white dwarf,where hydrogen is difficult or impossible to detect in modest res-olution spectra due to its apparent faintness relative to the nearbysamples shown in Fig 4. When the star has cooled to 12 000 K af-ter another 190 Myr, it will certainly not have detectable hydrogen.The unavoidable conclusion is that this white dwarf is being wit-nessed at a special time, in a transient phase, and the hydrogenis related to the orbiting planetary debris, and thus water is likelypresent.Considering that WD 1536 may have only accreted ∼ gof hydrogen onto its atmosphere and log(H/He) = − .
7, it canbe seen that if a significantly larger and water-rich parent body WD 1536 displays strong Balmer lines up to and including H d . had been deposited, then hydrogen could (temporarily) have be-come the dominant atmospheric constituent. For example, the dis-rupted asteroids polluting GD 61 or SDSS 1242 might have deliv-ered ∼ g or ∼ g of hydrogen respectively, resulting inabundances of log(H/He) ∼ ∼ +
2. In both casesWD 1536 would temporarily appear as a hydrogen-rich star despitebeing dominated by the underlying helium. Therefore, the accretionof water-rich planetary debris has the potential to have an observ-able effect on H/He white dwarf spectral evolution.
The young, helium-atmosphere, white dwarf WD 1536 exhibits thehighest abundances of heavy elements yet seen among pollutedhosts of evolved planetary systems. In addition to the broadly solarabundances of the major rock forming elements O, Mg, Al, Si, Ca,and Fe, this star also has a remarkably high trace hydrogen abun-dance of log(H/He) = − .
7. Considering the 1) abundance patternof heavy elements, 2) the anomalously high trace hydrogen, and 3)the transient detectability of both the metals and the hydrogen, themost realistic conclusion is that the parent body whose debris isboth orbiting and polluting WD 1536 contained traces of H O.The thinness of the convection zone is a result of relativeyouth and relatively high mass of trace hydrogen within a helium-dominated atmosphere. Due to these combined facts, the outer lay-ers of WD 1536 essentially behave as a hydrogen-rich white dwarf,with metal sinking timescales of only a few hundred years at most,hence supporting a steady state interpretation of the metal abun-dances. If these are indeed in a steady state, then WD 1536 has thehighest instantaneous accretion rate yet observed among pollutedwhite dwarfs.
ACKNOWLEDGMENTS
J. Farihi thanks S. Desch for feedback on a draft manuscript. Theauthors acknowledge both the MMT and WHT (Service programSW2014a39) for the expedient use of their Directors’ time, with-out which these results would not have been possible, and ananonymous reviewer for feedback that improved the quality ofthe manuscript. J. Farihi gratefully acknowledges the support ofthe STFC via an Ernest Rutherford Fellowship. This research wassupported in part by a NASA grant to UCLA, and by an NSFpre-doctoral fellowship to L. Vican. The research leading to theseresults has received funding from the ERC under the EuropeanUnion’s 7 th Framework Programme n. 320964 (WDTracer).
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