Minor Planet Evidence for Water in the Rocky Debris of a Disrupted Extrasolar MInor Planet

DOI: 10.1126/science.1239447, 218 (2013);342 Science

et al.J. FarihiMinor PlanetEvidence for Water in the Rocky Debris of a Disrupted Extrasolar

This copy is for your personal, non-commercial use only.

clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others

  here.following the guidelines

can be obtained byPermission to republish or repurpose articles or portions of articles

  ): October 11, 2013 www.sciencemag.org (this information is current as of

The following resources related to this article are available online at

http://www.sciencemag.org/content/342/6155/218.full.htmlversion of this article at:

including high-resolution figures, can be found in the onlineUpdated information and services,

http://www.sciencemag.org/content/suppl/2013/10/09/342.6155.218.DC1.html can be found at: Supporting Online Material

http://www.sciencemag.org/content/342/6155/218.full.html#ref-list-1, 5 of which can be accessed free:cites 35 articlesThis article

http://www.sciencemag.org/cgi/collection/astronomyAstronomy

subject collections:This article appears in the following

registered trademark of AAAS. is aScience2013 by the American Association for the Advancement of Science; all rights reserved. The title

CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience

on

Oct

ober

11,

201

3w

ww

.sci

ence

mag

.org

Dow

nloa

ded

from

o

n O

ctob

er 1

1, 2

013

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

on

Oct

ober

11,

201

3w

ww

.sci

ence

mag

.org

Dow

nloa

ded

from

o

n O

ctob

er 1

1, 2

013

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

and morphometric measurements of the tissuesin the developing mouse gut (Fig. 6C). Usingthese measurements as inputs in our model suf-fices to quantitatively predict the formation ofvilli (supplementary materials, Fig. 6D, and movieS3). Compared with the chick, where the endo-derm is more than 10 times stiffer than the ad-jacent mesenchyme, the mouse endoderm is onlyabout 1.5 times as stiff as the mesenchyme (fig.S3). Our simulations show that the soft endodermin mouse is essential for the initial folding that oc-curs in endoderm alone and for the direct formationof an array of previllous bumps, rather than zig-zags, which are qualitatively similar to sulcus for-mation on biaxially compressed gel surfaces thatlack a stiff top layer (24). The spacing of bumpsand, consequently, the spacing of villi are compa-rable to the thickness of the whole endoderm-mesenchyme composite (Fig. 6C), similar to chick.

The process of villification occurs before thedifferentiation of the gut endoderm into variousepithelial cell types (25–27) and well before thepostnatal process of crypt formation. In vitro cul-ture of intestinal stem cells results in the forma-tion of intestinal organoids that reproduce cryptstructure (28). These organoids consist of aninner epithelium with villuslike cell types andoutwardly projecting cryptlike structures. How-ever, no morphological structures are present inthese in vitro cultures resembling the physicalvilli. These results suggest that crypt formationlikely does not require the same muscle-drivencompression that is necessary for villi to form.

Additionally, further study is needed to un-derstand whether structural differences in thelumen of different regions of the gut are attrib-utable to distinctions in the parameters we havemeasured. For example, the short, wide villi thatcoat large longitudinal folds of the chick colonmay be attributable to the thicker muscle layersof the colon. Consistent with the muscle playing

such a role, studies have shown that transposi-tion of a ring containing all radial layers of thecolon into regions of the small intestine preservevilli morphology (29).

Our previous work provided a mechanicalbasis for the diversity of macroscopic loopingpatterns of the gut based on geometry, differen-tial growth, and tissue mechanics (30), and ourpresent results demonstrate that the same phys-ical principles drive morphological variation onthe luminal surface of the gut. Further, we seethat relatively minor changes in the geometry,growth, and physical properties of the develop-ing tissue in the guts of various species cansubstantially alter both the process and the formof villus patterning. A deep understanding of howpatterns vary requires us to combine our knowl-edge of biophysical mechanisms with the geneticcontrol of cell proliferation and growth; indeedthis variation can occur in an organism as a func-tion of its diet, across species, and over evolu-tionary time scales via natural selection.

References and Notes1. V. A. McLin, S. J. Henning, M. Jamrich, Gastroenterology

136, 2074–2091 (2009).2. T. K. Noah, B. Donahue, N. F. Shroyer, Exp. Cell Res. 317,

2702–2710 (2011).3. W. J. Krause, Anat. Histol. Embryol. 40, 352–359

(2011).4. J. W. McAvoy, K. E. Dixon, J. Anat. 125, 155–169 (1978).5. S. Ferri, L. C. U. Junqueira, L. F. Medeiros, L. O. Mederios,

J. Anat. 121, 291–301 (1976).6. D. R. Burgess, Embryol Exp. Morph. 34, 723–740 (1975).7. W. His, Anatomie Menschlicher Embryonen (Vogel,

Leipzig, Germany, 1880).8. D. E. Moulton, A. Goriely, J. Mech. Phys. Solids 59,

525–537 (2011).9. L. Bell, L. Williams, Anat. Embryol. 165, 437–455 (1982).10. M. Kurahashi et al., Neurogastroenterol. Motil. 20,

521–531 (2008).11. K. Fukuda, Y. Tanigawa, G. Fujii, S. Yasugi, S. Hirohashi,

Development 125, 3535–3542 (1998).12. H. Benabdallah, D. Messaoudi, K. Gharzouli, Pharmacol.

Res. 57, 132–141 (2008).

13. N. Harada, Y. Chijiiwa, T. Misawa, M. Yoshinaga,H. Nawata, Life Sci. 51, 1381–1387 (1992).

14. M. L. Lovett, C. M. Cannizzaro, G. Vunjak-Novakovic,D. L. Kaplan, Biomaterials 29, 4650–4657 (2008).

15. N. Bowden, S. Brittain, A. G. Evans, J. W. Hutchinson,G. W. Whitesides, Nature 393, 146–149 (1998).

16. L. Mahadevan, S. Rica, Science 307, 1740 (2005).17. B. Audoly, A. Boudaoud, J. Mech. Phys. Solids 56,

2444–2458 (2008).18. E. Hannezo, J. Prost, J.-F. Joanny, Phys. Rev. Lett. 107,

078104 (2011).19. M. Ben Amar, F. Jia, Proc. Natl. Acad. Sci. U.S.A. 110,

10525–10530 (2013).20. R. Sbarbati, J. Anat. 135, 477–499 (1982).21. K. D. Walton et al., Proc. Natl. Acad. Sci. U.S.A. 109,

15817–15822 (2012).22. A. Sukegawa et al., Development 127, 1971–1980 (2000).23. M. Ramalho-Santos, D. A. Melton, A. P. McMahon,

Development 127, 2763–2772 (2000).24. T. Tallinen, J. S. Biggins, L. Mahadevan, Phys. Rev. Lett.

110, 024302 (2013).25. M. Dauça et al., Int. J. Dev. Biol. 34, 205–218 (1990).26. Z. Uni, A. Smirnov, D. Sklan, Poult. Sci. 82, 320–327

(2003).27. F. T. Bellware, T. W. Betz, J. Embryol. Exp. Morphol. 24,

335–355 (1970).28. T. Sato et al., Nature 459, 262–265 (2009).29. W. H. St. Clair, C. A. Stahlberg, J. W. Osborne, Virchows

Arch. B Cell Pathol. Incl. Mol. Pathol. 47, 27–33 (1984).30. T. Savin et al., Nature 476, 57–62 (2011).

Acknowledgments: We thank M. Kirschner for providingXenopus tadpoles and O. Pourquie for providing snake embryos.D.L.K. and Tufts University hold a series of patents that cover theprocessing of silk into material structures, including those usedin the research reported here. T.T. acknowledges the Academy ofFinland for support. Computations were run at CSC–IT Centerfor Science, Finland. C.J.T. acknowledges the support of a grantfrom NIH RO1 HD047360. L.M. acknowledges the support ofthe MacArthur Foundation.

Supplementary Materialswww.sciencemag.org/content/342/6155/212/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S11Movies S1 to S3

8 April 2013; accepted 13 August 2013Published online 29 August 2013;10.1126/science.1238842

REPORTS

Evidence for Water in the Rocky Debrisof a Disrupted Extrasolar Minor PlanetJ. Farihi,1* B. T. Gänsicke,2 D. Koester3

The existence of water in extrasolar planetary systems is of great interest because it constrains thepotential for habitable planets and life. We have identified a circumstellar disk that resultedfrom the destruction of a water-rich and rocky extrasolar minor planet. The parent body formedand evolved around a star somewhat more massive than the Sun, and the debris now closely orbitsthe white dwarf remnant of the star. The stellar atmosphere is polluted with metals accretedfrom the disk, including oxygen in excess of that expected for oxide minerals, indicating that theparent body was originally composed of 26% water by mass. This finding demonstrates thatwater-bearing planetesimals exist around A- and F-type stars that end their lives as white dwarfs.

The enormous recent progress in the dis-covery of exoplanetary systems provides agrowing understanding of their frequency

and nature, but our knowledge is still limited inmany respects. There is now observational evi-dence of rocky exoplanets (1, 2), and the mass

and radius (and hence density) of these planetscan be calculated from transit depth and radialvelocity amplitude; however, estimates of theirbulk composition remain degenerate and model-dependent. Transit spectroscopy offers some in-formation on giant exoplanet atmospheres (3), andplanetesimal debris disks often reveal the signa-ture of emitting dust and gas species (4), yet bothtechniques only scratch the surface of planets, as-teroids, and comets. Interestingly, white dwarfs—the Earth-sized embers of stars like the Sun—offera unique window onto terrestrial exoplanetary sys-tems: These stellar remnants can distill entire

1Institute of Astronomy, University of Cambridge, CambridgeCB3 0HA, UK. 2Department of Physics, University of Warwick,Coventry CV5 7AL, UK. 3Institut für Theoretische Physik undAstrophysik, University of Kiel, 24098 Kiel, Germany.

*Corresponding author. E-mail: [email protected]

11 OCTOBER 2013 VOL 342 SCIENCE www.sciencemag.org218

planetesimals into their constituent elements,thus providing the bulk chemical composition forthe building blocks of solid exoplanets.

Owing to high surface gravities, any atmo-spheric heavy elements sink rapidly as whitedwarfs cool below 25,000 K (5), leaving be-hind only hydrogen and helium in their outer-most layers—a prediction that is corroboratedby observation (6). Those white dwarfs with rockyplanetary system remnants can become con-taminated by the accretion of small, but spec-troscopically detectable, amounts of metals (7).Heavy element absorption lines in cool whitedwarfs are a telltale of external pollution, oftenimplying either ongoing mass accretion ratesabove 108 g s−1 (8) or large asteroid-sized massesof metals within the convection zone of thestar (9).

In recent years, metal-rich dust (10, 11) and gas(12) disks, likely produced by the tidal disruptionof a large asteroid (13), have been observed tobe closely orbiting 30 cool white dwarfs [e.g.,(14–19)] and provide a ready explanation forthe metal absorption features seen in their atmo-spheres (20). The circumstellar material beinggradually accreted by the white dwarf can bedirectly observed in the stellar photosphere toreveal its elemental abundances (21). These plan-etary system remnants offer empirical insightinto the assembly and chemistry of terrestrial exo-planets that is unavailable for any exoplanet or-biting a main-sequence star.

Until now, no white dwarf has shown re-liable evidence for the accretion of water-rich,rocky planetary material. Unambiguous signa-tures of icy asteroids at white dwarfs shouldinclude (i) atmospheric metal pollution rich inrefractory elements; (ii) trace oxygen in excessof that expected for metal oxides; (iii) circum-stellar debris from which these elements are ac-creted; and, where applicable, (iv) trace hydrogen(in a helium-dominated atmosphere) sufficientto account for the excess oxygen as H2O. Thepresence of a circumstellar disk signals that ac-cretion is ongoing, identifies the source material,and enables a confident quantitative assessmentof the accreted elemental abundances, which in

turn allows a calculation of the water fraction ofthe disrupted parent body.

Themetal-enrichedwhite dwarfs GD 362 andGD16 both have circumstellar disks and relativelylarge trace hydrogen abundances in helium-dominated atmospheres (22), but as yet no as-sessment of photospheric oxygen is available(21, 23). These two stars have effective temper-atures below 12,000 K, and their trace hydrogencould potentially be the result of helium dredge-up in a previously hydrogen-rich atmosphere (24).The warmer, metal-lined white dwarfs GD 61and GD 378 have photospheric oxygen (25), butthe accretion history of GD 378 is unconstrained(i.e., it does not have a detectable disk), andwithout this information, the atmospheric oxygencould be consistent with that contained in dry min-erals common in the inner solar system (26). Inthe case of GD 61, elemental abundance uncer-tainties have previously prevented a formally sig-nificant detection of oxygen excess (27).

We used the Cosmic Origins Spectrograph(COS) onboard the Hubble Space Telescope toobtain ultraviolet spectroscopy of the white dwarfGD 61, and, together with supporting ground-based observations, we derived detections or lim-its for all the major rock-forming elements (O,Mg, Al, Si, Ca, Fe). These data permit a con-fident evaluation of the total oxygen fractionpresent in common silicates within the parentbody of the infalling material, and we identifiedexcess oxygen attributable to H2O as follows.(i) The observed carbon deficiency indicates thatthis element has no impact on the total oxygenbudget, even if every atom is delivered as CO2.(ii) The elements Mg, Al, Si, and Ca are as-

sumed to be carried as MgO, Al2O3, SiO2, andCaO at the measured or upper-limit abundance.(iii) The remaining oxygen exceeds that whichcan be bound in FeO, and the debris is interpretedto be water-rich. By this reasoning, we found oxy-gen in excess of that expected for anhydrous min-erals in the material at an H2O mass fraction of0.26 (Table 1 and Fig. 1).

Because we have assumed the maximum al-lowed FeO, and because some fraction of metal-lic iron is possible, the inferred water fraction ofthe debris is actually bound between 0.26 and0.28. Although this makes little difference in thecase of GD 61, where the parent body materialappears distinctlymantle-like (27), there are at leasttwo cases where metallic iron is a major (andeven dominant) mass carrier within the parentbodies of circumstellar debris observed at whitedwarfs (28). Overall, these data strongly suggestthat the material observed in and around pollutedwhite dwarfs had an origin in relatively massiveand differentiated planetary bodies.

We have assumed a steady state between ac-cretion and diffusion in GD 61. However, a typ-ical metal sinking time scale for this star is 105

years, and thus the infalling disk material couldpotentially be in an early phase of accretion wherematerial accumulates in the outer layers, priorto appreciable sinking (27). In this early-phasescenario, the oxygen excess and water fractionwould increase relative to those derived fromthe steady-state assumption, and hencewe confi-dently conclude that the debris around GD 61originated in a water-rich parent body. Althoughthe lifetimes of disks at white dwarfs are notrobustly constrained, the best estimates imply

Table 1. Oxide and water mass fractions inthe planetary debris at GD 61. We adopt thesteady-state values, which assume accretion-diffusionequilibrium.

Oxygen carrier Steady state Early phase

CO2 <0.002 <0.002MgO 0.17 0.18Al2O3 <0.02 <0.02SiO2 0.32 0.27CaO 0.02 0.01FeO* 0.05 0.02Excess 0.42 0.50H2O in debris 0.26 0.33*All iron is assumed to be contained in FeO; some metallic Fewill modestly increase the excess oxygen.

Fig. 1. Oxygen budget in GD 61 and terrestrial bodies. The first two columns are the early phase(EP) and steady-state (SS) fractions of oxygen carried by all the major rock-forming elements in GD 61,assuming that all iron is carried as FeO. Additional columns show the oxide compositions of the bulksilicate (crust plus mantle) Earth, Moon, Mars, and Vesta (35). Their totals do not reach 1.0 because traceoxides have been omitted. The overall chemistry of GD 61 is consistent with a body composed almostentirely of silicates, and thus appears relatively mantle-like but with substantial water. In contrast, Earth isrelatively water-poor and contains approximately 0.023% H2O (1.4 × 1024 g).

www.sciencemag.org SCIENCE VOL 342 11 OCTOBER 2013 219

REPORTS

that the chance of catching GD 61 in an earlyphase is less than 1% (17, 29–31).

The helium-rich nature of GD 61 permits anassessment of its trace hydrogen content andtotal asteroid mass for a single parent body. Thetotal metal mass within the stellar convectionzone is 1.3 × 1021 g, roughly equivalent to thatof an asteroid 90 km in diameter. However, be-cause metals continuously sink, it is expectedthat the destroyed parent body was substantiallymore massive, unless the star is being observedshortly after the disruption event. In contrast, hy-drogen floats and accumulates, and thus placesan upper limit on the total mass of accreted water-rich debris. If all the trace hydrogen were deliv-ered as H2O from a single planetesimal, the totalaccreted water mass would be 5.2 × 1022 g, and a26% H2O mass fraction would imply a parentbody mass of 2 × 1023 g, which is similar to thatof the main-belt asteroid 4 Vesta (32).

These data imply that water in planetesi-mals can survive post–main sequence evolution.One possibility is that solid or liquid water isretained beneath the surface of a sufficiently large(diameter >100 km) parent body (26), and isthus protected from heating and vaporizationby the outermost layers. Upon shattering duringa close approach with a white dwarf, any ex-posed water ice (and volatiles) should rapidlysublimate but will eventually fall onto the star;the feeble luminosities of white dwarfs are in-capable of removing even light gases by radia-tion pressure (31). Another possibility is that asubstantial mass of water is contained in hydratedminerals (e.g., phyllosilicates), as observed in main-belt asteroids via spectroscopy and inferred fromthe analysis of meteorites (33). In this case, theH2O equivalent is not removed until much highertemperatures are attained, and such water-bearingasteroids may remain essentially unaffected bythe giant phases of the host star.

The white dwarf GD 61 contains the unmis-takable signature of a rocky minor planet anal-ogous to the asteroid 1 Ceres in water content(34) and probably analogous to Vesta in mass.The absence of detectable carbon indicates thatthe parent body of the circumstellar debris wasnot an icy planetesimal analogous to comets, butwas instead similar in overall composition toasteroids in the outer main belt. This exoplan-etary system originated around an early A-typestar that formed large planetesimals similar tothose in the inner solar system that were thebuilding blocks for Earth and other terrestrialplanets.

References and Notes1. N. M. Batalha et al., Astrophys. J. 729, 27 (2011).2. F. Fressin et al., Nature 482, 195–198 (2012).3. D. K. Sing et al., Mon. Not. R. Astron. Soc. 416, 1443–1455

(2011).4. C. M. Lisse et al., Astrophys. J. 747, 93 (2012).5. D. Koester, Astron. Astrophys. 498, 517–525 (2009).6. B. Zuckerman, D. Koester, I. N. Reid, M. Hünsch,

Astrophys. J. 596, 477–495 (2003).7. Astronomers use the term “metal” when referring to

elements heavier than helium.

8. D. Koester, D. Wilken, Astron. Astrophys. 453, 1051–1057(2006).

9. J. Farihi, M. A. Barstow, S. Redfield, P. Dufour,N. C. Hambly, Mon. Not. R. Astron. Soc. 404, 2123 (2010).

10. M. Jura, J. Farihi, B. Zuckerman, Astron. J. 137,3191–3197 (2009).

11. W. T. Reach et al., Astrophys. J. 635, L161–L164(2005).

12. B. T. Gänsicke, T. R. Marsh, J. Southworth, A. Rebassa-Mansergas,Science 314, 1908–1910 (2006).

13. J. H. Debes, K. J. Walsh, C. Stark, Astrophys. J. 747, 148(2012).

14. J. Farihi et al., Mon. Not. R. Astron. Soc. 421, 1635–1643(2012).

15. J. Farihi, M. Jura, J. E. Lee, B. Zuckerman, Astrophys. J.714, 1386–1397 (2010).

16. S. Xu, M. Jura, Astrophys. J. 745, 88 (2012).17. J. Girven et al., Astrophys. J. 749, 154 (2012).18. J. Farihi, M. Jura, B. Zuckerman, Astrophys. J. 694,

805–819 (2009).19. M. Jura, J. Farihi, B. Zuckerman, Astrophys. J. 663,

1285–1290 (2007).20. M. Jura, Astrophys. J. 584, L91–L94 (2003).21. B. Zuckerman, D. Koester, C. Melis, B. M. S. Hansen,

M. Jura, Astrophys. J. 671, 872–877 (2007).22. M. Jura, M. Muno, J. Farihi, B. Zuckerman, Astrophys. J.

699, 1473–1479 (2009).23. D. Koester, R. Napiwotzki, B. Voss, D. Homeier,

D. Reimers, Astron. Astrophys. 439, 317–321 (2005).24. P. E. Tremblay, P. Bergeron, Astrophys. J. 672, 1144–1152

(2008).25. S. Desharnais, F. Wesemael, P. Chayer, J. W. Kruk,

R. A. Saffer, Astrophys. J. 672, 540–552 (2008).26. M. Jura, S. Xu, Astron. J. 140, 1129–1136 (2010).27. J. Farihi et al., Astrophys. J. 728, L8 (2011).28. B. T. Gänsicke et al., Mon. Not. R. Astron. Soc. 424,

333–347 (2012).29. B. Klein, M. Jura, D. Koester, B. Zuckerman, C. Melis,

Astrophys. J. 709, 950–962 (2010).30. M. Jura, Astron. J. 135, 1785–1792 (2008).

31. J. Farihi, B. Zuckerman, E. E. Becklin, Astrophys. J. 674,431–446 (2008).

32. C. T. Russell et al., Science 336, 684–686 (2012).33. A. S. Rivkin, E. S. Howell, F. Vilas, L. A. Lebofsky, in Asteroids

III, W. F. Bottke Jr., A. Cellino, P. Paolicchi, R. P. Binzel, Eds.(Univ. of Arizona Press, Tucson, AZ, 2002), pp. 235–253.

34. P. C. Thomas et al., Nature 437, 224–226 (2005).35. C. Visscher, B. Fegley Jr., Astrophys. J. 767, L12 (2013).

Acknowledgments: This work is based on observationsmade with the Hubble Space Telescope, which is operatedby the Association of Universities for Research in Astronomyunder NASA contract NAS 5-26555. These observations areassociated with program programs 12169 and 12474. Someof the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnershipamong the California Institute of Technology, the Universityof California, and NASA. The Observatory was made possibleby the generous financial support of the W. M. KeckFoundation. J.F. acknowledges support from the UK Scienceand Technology Facilities Council in the form of an ErnestRutherford Fellowship (ST/ J003344/1). The research leadingto these results has received funding from the European ResearchCouncil under the European Union’s Seventh FrameworkProgramme (FP/2007-2013)/ERC Grant Agreement no. 267697(WDTracer). B.T.G. was supported in part by the UK Science andTechnology Facilities Council (ST/I001719/1). Keck telescope timefor program 2011B-0554 was granted by NOAO through theTelescope System Instrumentation Program, funded by NSF.

Supplementary Materialswww.sciencemag.org/content/342/6155/218/suppl/DC1Materials and MethodsFig. S1Tables S1 and S2References (36, 37)

22 April 2013; accepted 15 August 201310.1126/science.1239447

Femtosecond Visualizationof Lattice Dynamics inShock-Compressed MatterD. Milathianaki,1* S. Boutet,1 G. J. Williams,1 A. Higginbotham,2 D. Ratner,1

A. E. Gleason,3 M. Messerschmidt,1 M. M. Seibert,1,4 D. C. Swift,5 P. Hering,1

J. Robinson,1 W. E. White,1 J. S. Wark2

The ultrafast evolution of microstructure is key to understanding high-pressure and strain-ratephenomena. However, the visualization of lattice dynamics at scales commensurate with thoseof atomistic simulations has been challenging. Here, we report femtosecond x-ray diffractionmeasurements unveiling the response of copper to laser shock-compression at peak normal elasticstresses of ~73 gigapascals (GPa) and strain rates of 109 per second. We capture the evolutionof the lattice from a one-dimensional (1D) elastic to a 3D plastically relaxed state within a few tensof picoseconds, after reaching shear stresses of 18 GPa. Our in situ high-precision measurement ofmaterial strength at spatial (<1 micrometer) and temporal (<50 picoseconds) scales providesa direct comparison with multimillion-atom molecular dynamics simulations.

The distinct properties of materials at high-pressure and/or strain-rate conditions leadto a broad range of phenomena in fields

such as high-energy-density physics (1), Earthand planetary sciences (2, 3), aerospace engi-neering (4), and materials science (5, 6). For thelatter, a predictive understanding and controlof mechanical properties, enabled by the di-

rect comparison of experiments with large-scaleatomistic simulations, is the ultimate goal. Where-as the bulk material behavior can be inferredby macroscopic measurements (7, 8), key infor-mation on the mechanical properties requiresknowledge of the physics embedded at thelattice level. Such knowledge has traditionallybeen obtained via nanosecond-resolution x-ray

11 OCTOBER 2013 VOL 342 SCIENCE www.sciencemag.org220

REPORTS

www.sciencemag.org/content/342/6155/218/suppl/DC1

Supplementary Materials for

Evidence for Water in the Rocky Debris of a Disrupted Extrasolar Minor

Planet

J. Farihi,* B. T. Gänsicke, D. Koester

*Corresponding author. E-mail: [email protected]

Published 11 October 2013, Science 342, 218 (2013)

DOI: 10.1126/science.1239447

This PDF file includes:

Materials and Methods

Fig. S1

Tables S1 and S2

References

Supporting Online Material for

Evidence for Water in the Rocky Debris

of Disrupted Extrasolar Minor Planets

J. Farihi1,4∗, B. T. Gansicke2, D. Koester3

1Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, UK2Department of Physics, University of Warwick, Coventry CV5 7AL, UK

3Institut fur Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany4STFC Ernest Rutherford Fellow

∗To whom correspondence should be addressed; E-mail: [email protected]

We describe here in detail the observations and analyses supporting the main paper, specifically

the spectroscopy of the metal-enriched white dwarf atmosphere and the analytical link to the

elemental abundances of the infalling planetary debris.

1 Summary of the Observations and Datasets

GD 61 exhibits infrared excess consistent with circumstellar dust orbiting within its Roche limit

(26), and bears the unambiguous signature of debris accretion via its metal-polluted atmosphere.

The white dwarf was observed with the Cosmic Origins Spectrograph (COS) during Hubble

Space Telescope Cycle 19 on 2012 January 28. The ultraviolet spectra were obtained with a

total exposure time of 1600 s (split between two FP-POS positions) using the G130M grating

and a central wavelength setting at 1291 A, covering 1130−1435 A at R ≈ 18 000. The COS

data were processed and calibrated with CALCOS 2.15.6, and are shown in Figure S1. Optical

1

spectroscopy of GD61 was obtained on 2011 October 24 with the Keck II Telescope and the

Echelle Spectrograph and Imager (36, ESI) in echelle mode, effectively covering 3900−9200 Å

at R 13 000. The spectra were obtained in a series of 16 exposures of 900 s each, for a total

exposure time of 4 hr, and reduced using standard tasks in IRAF1.

2 Derivation of Photospheric and Debris Abundances

Elemental abundances for GD61 were derived from the COS and ESI data by fitting white dwarf

atmospheric models (37) to the observed spectra. For these calculations, Teff = 17 280 K and

log g = 8.20 are adopted, based on a published analysis of low-resolution optical spectra (24).

The resulting photospheric abundances and upper limits are listed in Table S1 together with

previous measurements from the Far Ultraviolet Spectroscopic Explorer (24, FUSE) and Keck I

HIRES (26). Notably, all heavy element abundances agree well, despite being derived using

separate instruments and with multiple absorption lines across distinct wavelength regimes.

The transformation between the heavy element abundances in the white dwarf

atmospheres and those within the infalling planetary debris are calculated assuming a steady

state balance between accretion and diffusion. An early (or build-up) phase of accretion is

theoretically possible in GD61, but this is unlikely (see main paper). Importantly, in this case an

early phase would imply a larger oxygen excess and H2O fraction, and therefore the more

conservative, and most probable, assumption is made.

For white dwarfs with significant convection zones like GD61, the atmospheric mass

fraction Xz of heavy element z is related to its accretion rate zM via

z cvz zz tX MM (S1)

___________________________________

1IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the

Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science

Foundation.

2

where tz is the sinking timescale for the element and Mcvz is the mass of the stellar convection

zone. The mass fraction is determined from the model atmosphere fits and the sinking timescale

is known from white dwarf diffusion calculations (5). In essence, Equation S1 states that the

accretion rate of element z equals its rate of depletion as it settles below the mixing layer. The

ratio of two heavy elements within the debris (and hence parent body) is either the ratio of their

respective accretion rates in the steady state, or the ratio of their atmospheric mass fractions in

the early phase, and related by

Mz1

Mz2

=Xz1

Xz2

×tz2tz1

(S2)

Table S2 lists the relevant quantities of GD 61 for the key elements that determine the total

oxygen budget of the debris. The steady state metal abundances relative to oxygen are taken

from the fourth column. The sinking timescales for GD 61 have been updated following a

correction in the theoretical calculations2, and they are somewhat different than those presented

in a previous analysis (26). Notably, this correction has strengthened the case for an oxygen

excess in GD 61.

3 Evaluation of Oxygen Excess and Uncertainties

The method for calculating the overall oxygen budget is as follows. We begin with the columns

in Table S2, and in particular the identify the total oxygen budget with: 1) its mass accretion

rate for the steady state or 2) its mass within the stellar convection zone for the early phase.

We calculate the fraction of oxygen that can be absorbed as CO2 based on the upper limit for

carbon, and subtract this from the total available. Next, we perform a similar calculation for the

mass of oxygen in MgO, Al2O3, Si2, CaO, and FeO based on their detections or upper limits,

again subtracting these from the budget. After accounting for all the major oxygen carriers, any

remaining mass is considered excess.

2http://www1.astrophysik.uni-kiel.de/~koester/astrophysics/astrophysics.html

3

The collective data for GD 61 is robust and comprehensive, comprising four instruments

with each probing distinct wavelength regions and containing multiple transitions for each ele-

ment from the far-ultraviolet to the red optical region. The uncertainties in the metal abundances

of this white dwarf are given as 3σ adopted values in the last column of Table S1. Using a brute

force approach, all 128 possible combinations of abundance values are calculated for C, O, Mg,

Al, Si, Ca, Fe where the abundance values N(X)/N(He) take on each of the values x ± δx.

Evaluating all possible permutations, the dispersion in the resulting oxygen excesses values

(0.068) results in a 6.1σ confidence for the case of steady state accretion.

References and Notes

1. N. M. Batalha, et al., Astrophys. J. 729, 27 (2011)

2. F. Fressin, et al., Nature 482, 195 (2012)

3. D. K. Sing, et al., Mon. Not. R. Ast. Soc. 416, 1443 (2011)

4. C. M. Lisse, et al., Astrophys. J. 747, 93 (2012)

5. D. Koester, Astron. Astrophys. 498, 517 (2009)

6. B. Zuckerman, D. Koester, I. N.Reid, M. Hunsch, Astrophys. J. 596, 477 (2003)

7. D. Koester, D. Wilken, Astron. Astrophys. 453, 1051 (2006)

8. J. Farihi, M. A. Barstow, S. Redfield, P. Dufour, N. C. Hambly, Mon. Not. R. Ast. Soc. 404,

2123 (2010)

9. M. Jura, J. Farihi, B. Zuckerman, Astron. J. 137, 3191 (2009)

10. W. T. Reach, M. J. Kuchner, T. von Hippel, A. Burrows, F. Mullally, M. Kilic, D. E. Winget,

Astrophys. J. 635, L161 (2005)

4

Figure S1. The normalized COS spectra of GD 61 (grey), together with the best fitting model

spectra (red). Interstellar absorption features are indicated by vertical grey dashed lines, and

are blueshifted with respect to the photospheric features by 40 km s−1. Geocoronal airglow of

O I at 1302.2, 1304.9, and 1306.0 A can contaminate COS spectra to some degree, and typical

airglow line profiles are shown in the middle panel scaled to an arbitrary flux.

7

Table S1. Elemental Abundances N(X)/N(He) in GD 61

Ultraviolet Optical

Element COS FUSE ESI HIRES Adopted

Detections:

H −3.70 (0.10) −4.00 (0.10) −3.98 (0.10) −3.89 (0.15)

O −6.00 (0.15) −5.80 (0.20) −5.75 (0.20) −5.95 (0.13)

Mg −6.50 (0.30) −6.74 (0.10) −6.65 (0.18) −6.69 (0.14)

Si −6.82 (0.12) −6.70 (0.20) −6.85 (0.10) −6.85 (0.09) −6.82 (0.11)

S −8.00 (0.20) −8.00 (0.20)

Ca −7.77 (0.06) −7.90 (0.19) −7.90 (0.19)

Fe −7.60 (0.30) −7.60 (0.20) −7.60 (0.20)

Upper limits:

C −9.10 −8.80

N −8.00

Na −6.80

P −8.70

Al −7.80 −7.20

Ti −8.60

Sc −8.20

Cr −8.00

Fe −7.50

Ni −8.80

8

Table S2. Atmospheric and Debris Properties for Key Trace Elements in GD 61

Early Phase Steady State

Element tdiff XzMcvza Mz

(105 yr) (1021g) (108 g s−1)

H ∞ 5.755

C 1.730 < 0.001 < 0.001O 1.706 0.802 1.489

Mg 1.808 0.222 0.389

Al 1.735 < 0.019 < 0.035

Si 1.438 0.190 0.419

S 0.952 0.014 0.048

Ca 0.782 0.023 0.091

Fe 0.855 0.063 0.232

Total Z 1.332 2.704

Note. The metal-to-metal ratios within the planetary debris for the early phase and steady state

regimes are derived directly from the values in the third and fourth columns respectively.

aThe third column is the mass of each element residing in the convection zone of GD 61, and

their total (excluding hydrogen) represents a minimum mass for the parent body due to the

continual sinking of metals.

9

References and Notes

1. N. M. Batalha, W. J. Borucki, S. T. Bryson, L. A. Buchhave, D. A. Caldwell, J. Christensen-

Dalsgaard, D. Ciardi, E. W. Dunham, F. Fressin, T. N. Gautier, R. L. Gilliland, M. R.

Haas, S. B. Howell, J. M. Jenkins, H. Kjeldsen, D. G. Koch, D. W. Latham, J. J. Lissauer,

G. W. Marcy, J. F. Rowe, D. D. Sasselov, S. Seager, J. H. Steffen, G. Torres, G. S. Basri,

T. M. Brown, D. Charbonneau, J. Christiansen, B. Clarke, W. D. Cochran, A. Dupree, D.

C. Fabrycky, D. Fischer, E. B. Ford, J. Fortney, F. R. Girouard, M. J. Holman, J.

Johnson, H. Isaacson, T. C. Klaus, P. Machalek, A. V. Moorehead, R. C. Morehead, D.

Ragozzine, P. Tenenbaum, J. Twicken, S. Quinn, J. VanCleve, L. M. Walkowicz, W. F.

Welsh, E. Devore, A. Gould, Kepler’s first rocky planet: Kepler-10b. Astrophys. J. 729,

27 (2011). doi:10.1088/0004-637X/729/1/27

2. F. Fressin, G. Torres, J. F. Rowe, D. Charbonneau, L. A. Rogers, S. Ballard, N. M. Batalha,

W. J. Borucki, S. T. Bryson, L. A. Buchhave, D. R. Ciardi, J. M. Désert, C. D. Dressing,

D. C. Fabrycky, E. B. Ford, T. N. Gautier 3rd, C. E. Henze, M. J. Holman, A. Howard, S.

B. Howell, J. M. Jenkins, D. G. Koch, D. W. Latham, J. J. Lissauer, G. W. Marcy, S. N.

Quinn, D. Ragozzine, D. D. Sasselov, S. Seager, T. Barclay, F. Mullally, S. E. Seader, M.

Still, J. D. Twicken, S. E. Thompson, K. Uddin, Two Earth-sized planets orbiting Kepler-

20. Nature 482, 195–198 (2012). doi:10.1038/nature10780 Medline

3. D. K. Sing, F. Pont, S. Aigrain, D. Charbonneau, J.-M. Désert, N. Gibson, R. Gilliland, W.

Hayek, G. Henry, H. Knutson, A. L. des Etangs, T. Mazeh, A. Shporer, Hubble Space

Telescope transmission spectroscopy of the exoplanet HD 189733b: High-altitude

atmospheric haze in the optical and near-ultraviolet with STIS. Mon. Not. R. Astron. Soc.

416, 1443–1455 (2011). doi:10.1111/j.1365-2966.2011.19142.x

4. C. M. Lisse, M. C. Wyatt, C. H. Chen, A. Morlok, D. M. Watson, P. Manoj, P. Sheehan, T. M.

Currie, P. Thebault, M. L. Sitko, Spitzer evidence for a late-heavy bombardment and the

formation of ureilites in η Corvi at ∼1 Gyr. Astrophys. J. 747, 93 (2012).

doi:10.1088/0004-637X/747/2/93

5. D. Koester, Accretion and diffusion in white dwarfs. Astron. Astrophys. 498, 517–525 (2009).

doi:10.1051/0004-6361/200811468

6. B. Zuckerman, D. Koester, I. N. Reid, M. Hünsch, Metal lines in DA white dwarfs. Astrophys.

J. 596, 477–495 (2003). doi:10.1086/377492

7. Astronomers use the term ―metal‖ when referring to elements heavier than helium.

8. D. Koester, D. Wilken, The accretion-diffusion scenario for metals in cool white dwarfs.

Astron. Astrophys. 453, 1051–1057 (2006). doi:10.1051/0004-6361:20064843

9. J. Farihi, M. A. Barstow, S. Redfield, P. Dufour, N. C. Hambly, Mon. Not. R. Astron. Soc.

404, 2123 (2010).

10. M. Jura, J. Farihi, B. Zuckerman, Six white dwarfs with circumstellar silicates. Astron. J.

137, 3191–3197 (2009). doi:10.1088/0004-6256/137/2/3191

11. W. T. Reach, M. J. Kuchner, T. von Hippel, A. Burrows, F. Mullally, M. Kilic, D. E. Winget,

The dust cloud around the white dwarf G29-38. Astrophys. J. 635, L161–L164 (2005).

doi:10.1086/499561

12. B. T. Gänsicke, T. R. Marsh, J. Southworth, A. Rebassa-Mansergas, A gaseous metal disk

around a white dwarf. Science 314, 1908–1910 (2006). doi:10.1126/science.1135033

Medline

13. J. H. Debes, K. J. Walsh, C. Stark, The link between planetary systems, dusty white dwarfs,

and metal-polluted white dwarfs. Astrophys. J. 747, 148 (2012). doi:10.1088/0004-

637X/747/2/148

14. J. Farihi, B. T. Gänsicke, P. R. Steele, J. Girven, M. R. Burleigh, E. Breedt, D. Koester, A

trio of metal-rich dust and gas discs found orbiting candidate white dwarfs with K -band

excess. Mon. Not. R. Astron. Soc. 421, 1635–1643 (2012). doi:10.1111/j.1365-

2966.2012.20421.x

15. J. Farihi, M. Jura, J. E. Lee, B. Zuckerman, Strengthening the case for asteroidal accretion:

Evidence for subtle and diverse disks at white dwarfs. Astrophys. J. 714, 1386–1397

(2010). doi:10.1088/0004-637X/714/2/1386

16. S. Xu, M. Jura, Spitzer observations of white dwarfs: The missing planetary debris around

DZ stars. Astrophys. J. 745, 88 (2012). doi:10.1088/0004-637X/745/1/88

17. J. Girven, C. S. Brinkworth, J. Farihi, B. T. Gänsicke, D. W. Hoard, T. R. Marsh, D. Koester,

Constraints on the lifetimes of disks resulting from tidally destroyed rocky planetary

bodies. Astrophys. J. 749, 154 (2012). doi:10.1088/0004-637X/749/2/154

18. J. Farihi, M. Jura, B. Zuckerman, Infrared signatures of disrupted minor planets at white

dwarfs. Astrophys. J. 694, 805–819 (2009). doi:10.1088/0004-637X/694/2/805

19. M. Jura, J. Farihi, B. Zuckerman, Externally polluted white dwarfs with dust disks.

Astrophys. J. 663, 1285–1290 (2007). doi:10.1086/518767

20. M. Jura, A tidally disrupted asteroid around the white dwarf G29-38. Astrophys. J. 584, L91–

L94 (2003). doi:10.1086/374036

21. B. Zuckerman, D. Koester, C. Melis, B. M. S. Hansen, M. Jura, The chemical composition of

an extrasolar minor planet. Astrophys. J. 671, 872–877 (2007). doi:10.1086/522223

22. M. Jura, M. Muno, J. Farihi, B. Zuckerman, X-ray and infrared observations of two

externally polluted white dwarfs. Astrophys. J. 699, 1473–1479 (2009).

doi:10.1088/0004-637X/699/2/1473

23. D. Koester, R. Napiwotzki, B. Voss, D. Homeier, D. Reimers, HS 0146+1847—a DAZB

white dwarf of very unusual composition. Astron. Astrophys. 439, 317–321 (2005).

doi:10.1051/0004-6361:20053058

24. P. E. Tremblay, P. Bergeron, The ratio of helium‐ to hydrogen‐atmosphere white dwarfs:

Direct evidence for convective mixing. Astrophys. J. 672, 1144–1152 (2008).

doi:10.1086/524134

25. S. Desharnais, F. Wesemael, P. Chayer, J. W. Kruk, R. A. Saffer, FUSE observations of

heavy elements in the photospheres of cool DB white dwarfs. Astrophys. J. 672, 540–552

(2008). doi:10.1086/523699

26. M. Jura, S. Xu, The survival of water within extrasolar minor planets. Astron. J. 140, 1129–

1136 (2010). doi:10.1088/0004-6256/140/5/1129

27. J. Farihi, C. S. Brinkworth, B. T. Gänsicke, T. R. Marsh, J. Girven, D. W. Hoard, B. Klein,

D. Koester, Possible signs of water and differentiation in a rocky exoplanetary body.

Astrophys. J. 728, L8 (2011). doi:10.1088/2041-8205/728/1/L8

28. B. T. Gänsicke, D. Koester, J. Farihi, J. Girven, S. G. Parsons, E. Breedt, The chemical

diversity of exo-terrestrial planetary debris around white dwarfs. Mon. Not. R. Astron.

Soc. 424, 333–347 (2012). doi:10.1111/j.1365-2966.2012.21201.x

29. B. Klein, M. Jura, D. Koester, B. Zuckerman, C. Melis, Chemical abundances in the

externally polluted white dwarf GD 40: Evidence of a rocky extrasolar minor planet.

Astrophys. J. 709, 950–962 (2010). doi:10.1088/0004-637X/709/2/950

30. M. Jura, Pollution of single white dwarfs by accretion of many small asteroids. Astron. J.

135, 1785–1792 (2008). doi:10.1088/0004-6256/135/5/1785

31. J. Farihi, B. Zuckerman, E. E. Becklin, Spitzer IRAC observations of white dwarfs. I. Warm

dust at metal-rich degenerates. Astrophys. J. 674, 431–446 (2008). doi:10.1086/521715

32. C. T. Russell, C. A. Raymond, A. Coradini, H. Y. McSween, M. T. Zuber, A. Nathues, M. C.

De Sanctis, R. Jaumann, A. S. Konopliv, F. Preusker, S. W. Asmar, R. S. Park, R.

Gaskell, H. U. Keller, S. Mottola, T. Roatsch, J. E. Scully, D. E. Smith, P. Tricarico, M.

J. Toplis, U. R. Christensen, W. C. Feldman, D. J. Lawrence, T. J. McCoy, T. H.

Prettyman, R. C. Reedy, M. E. Sykes, T. N. Titus, Dawn at Vesta: Testing the

protoplanetary paradigm. Science 336, 684–686 (2012). doi:10.1126/science.1219381

Medline

33. A. S. Rivkin, E. S. Howell, F. Vilas, L. A. Lebofsky, in Asteroids III, W. F. Bottke Jr., A.

Cellino, P. Paolicchi, R. P. Binzel, Eds. (Univ. of Arizona Press, Tucson, 2002), pp. 235–

253.

34. P. C. Thomas, J. W. Parker, L. A. McFadden, C. T. Russell, S. A. Stern, M. V. Sykes, E. F.

Young, Differentiation of the asteroid Ceres as revealed by its shape. Nature 437, 224–

226 (2005). doi:10.1038/nature03938 Medline

35. C. Visscher, B. Fegley Jr., Chemistry of impact-generated silicate melt-vapor debris disks.

Astrophys. J. 767, L12 (2013). doi:10.1088/2041-8205/767/1/L12

36. A. I. Sheinis, M. Bolte, H. W. Epps, R. I. Kibrick, J. S. Miller, M. V. Radovan, B. C.

Bigelow, B. M. Sutin, ESI, a new Keck Observatory echellette spectrograph and imager.

Proc. Astron. Soc. Pac. 114, 851–865 (2002). doi:10.1086/341706

37. D. Koester, Mem. Soc. Astron. Ital. 81, 921 (2010).