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12. N. Zwahlen et al., Astron. Astrophys. 425, L45–L48(2004).
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17. E. Høg et al., Astron. Astrophys. 355, L27–L30(2000).
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21. M. A. Giannuzzi, Astron. Astrophys. 293, 360 (1995).
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Netherlands (1983).

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27. S. Röser, E. Schilbach, A new assessment of the kinematic
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ACKN OWLED GMEN TS

We thank the National Radio Astronomy Observatory, Green Bank
Telescope, Arecibo Observatory, and Effelsberg Telescope staff
who coordinated, conducted, and correlated observations for thisproject. All data presented in this paper are maintained in the
National Radio Astronomy Observatory archive. The National Radio
Astronomy Observatory is a facility of the National Science
Foundation operated under cooperative agreement by Associated
Universities, Inc. This work made use of the Swinburne University

CIRCUMSTELLAR DISKS

Large impacts around a solar-analog
star in the era of terrestrial
planet formation
Huan Y. A. Meng,1 Kate Y. L. Su,2 George H. Rieke,1,2 David J. Stevenson,3 Peter Plavchan,4,5
Wiphu Rujopakarn,2,6,7 Carey M. Lisse,8 Saran Poshyachinda,9 Daniel E. Reichart10
The final assembly of terrestrial planets occurs via massive collisions, which can launch
copious clouds of dust that are warmed by the star and glow in the infrared. We report
the real-time detection of a debris-producing impact in the terrestrial planet zone
around a 35-million-year-old solar-analog star. We observed a substantial brightening of
the debris disk at a wavelength of 3 to 5 micrometers, followed by a decay over a
year, with quasi-periodic modulations of the disk flux. The behavior is consistent withthe occurrence of a violent impact that produced vapor out of which a thick cloud ofsilicate spherules condensed that were then ground into dust by collisions. These results
demonstrate how the time domain can become a new dimension for the study ofterrestrial planet formation.

C

ircumstellar disks are where planetary systems form and evolve. Gas-rich and optically thick protoplanetary disks are born
together with young stars but dissipate
within a few million years (My), setting
the time scale for gas-giant planet formation (1).
Dusty, optically thin debris disks then emerge,
sustained by the fragmentation of colliding planetesimals (2). They are warmed by their stars andlight up in the infrared (IR), allowing them to be
detected over the entire lifetimes of main-sequencestars. Consequently, debris disks are ideal tools
in which to search for phases occurring in otherplanetary systems that are analogous to major
events in the evolution of the solar system, suchas the formation of terrestrial planets. Dynamical
simulations and meteoritics indicate that the end
stage of terrestrial planet formation, from ~30 to~100 My (3, 4), is marked by frequent large impacts, up to the scale of the one leading to theformation of the Moon (5–7). Such events can
produce a huge amount of dust, which may dramatically increase the IR emission from the debris disk (8).
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of Technology software correlator, developed as part of theAustralian Major National Research Facilities Programme andoperated under license. C.M. acknowledges financial support
from the U.S. National Science Foundation through awards
AST-1313428 and AST-1003318, from a Lawrence Livermore
National Laboratory minigrant to the University ofCalifornia–Los Angeles, and from the Spitzer Science Center
Visiting Graduate Student Program. G.C.B. acknowledges
support from the Academica Sinica Institute for Astronomy
and Astrophysics.

ID8 (2MASS J08090250-4858172) is a young
solar-analog star, with spectral type G6V andsolar metallicity, in the 35-My-old open starcluster NGC 2547 (9). It emits strongly in the IR,with a fractional disk luminosity of Ldisk/L∗ = 3.2 ×
10−2 (L∗, stellar luminosity) (10). The mid-IR
spectrum of ID8 (observed in 2007) shows strong
crystalline silicate features between 8 and 30 mm,
indicative of very fine dust particles (Fig. 1); models of the observed spectral energy distribution(SED) require submicrometer-sized amorphoussilicate dust particles as well (10). Particles areblown out by radiation pressure and are lost to
the system if the ratio of radiation to gravitational forces is larger than 0.5. For ID8, thecritical radius (11) below which nonporous silicate grains are lost is ~0.5 mm. The tiniest particles in the disk are smaller than this limit,
suggesting ongoing dust replenishment in thesystem.
The disk emission was recently found to varyon a yearly time scale (8). Collisional cascades
among planetesimals sustain most debris disks.
However, variations this rapid cannot be sup-SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/345/6200/1029/suppl/DC1
Supplementary Text
Tables S1 to S5
References (28–33)
15 May 2014; accepted 23 July 201410.1126/science.1256101

ported by this means, because the cascade timescales for significant variations in dust production are at least a few hundred orbits (8). To explore the origin of the variations, we have used
the Infrared Array Camera [IRAC (12)] onboard
the Spitzer Space Telescope to monitor the ID8
system. The observations extended from 25 May
2012 to 23 August 2013, providing a total timebaseline of 454 days, with a 157-day gap between
visibility windows. At the same time, intensive
optical monitoring of ID8 was obtained from theground in the V and Cousins I bands, where wefound that the output of the star is stable within~1.5% root mean square (supplementary text).
Because the stellar contribution is effectively
constant in time, we fitted its spectrum and removed it from the total (star + disk) IR fluxes toobtain the light curves of the debris disk, as shownin Fig. 2. This revealed an average disk color of[3.6] − [4.5] = 1.00, corresponding to a blackbody
temperature of 730 K, which is consistent withthe temperatures found in previous analyses ofthe mid-IR spectroscopy (10). We calculated the expected color trend of the entire ID8 system forpossible causes of the disk variations and compared it with our data. The result is not conclusive
within the errors, but suggests that a combinationof changing dust temperature and dust-emitting
1

Lunar and Planetary Laboratory and Department ofPlanetary Sciences, University of Arizona, 1629 East
University Boulevard, Tucson, AZ 85721, USA. 2Steward
Observatory and Department of Astronomy, University of
Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA.
3
Division of Geological and Planetary Sciences, California
Institute of Technology, MC 170-25, 1200 East California
Boulevard, Pasadena, CA 91125, USA. 4NASA Exoplanet
Science Institute, California Institute of Technology, MC
100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA.
5
Missouri State University, 901 South National Avenue,
Springfield, MO 65897, USA. 6Department of Physics, Facultyof Science, Chulalongkorn University, 254 Phayathai Road,
Pathumwan, Bangkok 10330, Thailand. 7Kavli Institute for the
Physics and Mathematics of the Universe (WPI), Todai
Institute for Advanced Study, University of Tokyo, 5-1-5
Kashiwanoha, Kashiwa, 277-8583, Japan. 8Space
Department, Applied Physics Laboratory, Johns Hopkins
University, 11100 Johns Hopkins Road, Laurel, MD 20723,
USA. 9National Astronomical Research Institute of Thailand(Public Organization), Ministry of Science and Technology,
191 Siriphanich Building, Huay Kaew Road, Muang District,
Chiang Mai 50200, Thailand. 10Department of Physics andAstronomy, Campus Box 3255, University of North Carolina
at Chapel Hill, Chapel Hill, NC 27599, USA.

sciencemag.org SCIENCERE S EAR CH | R E P O R T S

area, or in area alone rather than purely in temperature, may be responsible for the disk variations (supplementary text).

In the following analysis, we focus on the
4.5-mm data where we have observations fromboth years, and the disk is measured at a higher

signal-to-noise ratio. The data at 3.6 mm show
consistent behavior, although at lower signal-tonoise ratio. In 2012, the disk flux density stayed

Fig. 1. Spectrum of the ID8 system. The photometric errors are smaller than the symbol size andare not shown; errors of the Spitzer/IRS spectrum
are indicated as vertical lines. Red crosses at 24 mmshow the three epochs of Spitzer/MIPS observations that led to the discovery of disk variability (8).
The ranges of the new measurements at 3.6 and4.5 mm in this work are plotted as blue vertical lines.
CTIO,Cerro Tololo Inter-American Observatory; 2MASS,
2 Micron All Sky Survey; MIPS, Multiband Imaging
Photometer for Spitzer; IRS, Infrared Spectrograph;
WISE, Wide-field Infrared Survey Explorer.

Fig. 2. Light curvesof the debris diskof ID8 at 3.6 and4.5 mm, assuming noerror in photosphere
subtraction. The gap
between Barycentric
Modified Julian Date
(BMJD) 56155 and56311 occurred when
ID8 was outside of theSpitzer visibility
windows.

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R ES E A RC H | R E PO R TS

near 2.2 milli-Jansky (mJy), despite ~10% variations. However, at the start of 2013 it had brightened to above 3.0 mJy, indicating a significant
increase in the amount of dust, probably froma new impact before 2013. This elevated level
then decayed throughout 2013. An exponential
fit suggests a decay time scale of ~370 days at both
wavelengths. This is too fast to be reconciled withdecades-long collisional cascades (8) and is too
slow for the direct radiative blowout of tiny particles, which should take <30 days at the orbit
derived below for the disk.
To better understand the impact, we estimate thedisk mass based on a fit to the entire mid-IR spectrum, which is dominated by small grains. Because
the new debris in the disk has not reached equilib-

rium in a full collisional cascade, we could not
make the conventional assumption of a power-law
size distribution. For emitting grains of 0.5 mm inradius, we found a disk mass of 1.1 × 1019 kg, which
is a lower limit because it ignores larger particles.
An independent estimate assuming a power-law
grain size distribution up to 1 mm obtains an identical mass estimate (10). This mass, if the grains werecompacted into a solid body, is equivalent to an~180-km-diameter asteroid (of density 3700 kg m−3).
Given the estimated mass and particle size, theID8 disk may be optically thick, in which significant mass could be obscured and unseen.
Considering the decay in 2013, and assuming
it applies to the full spectrum, we estimated themass loss rate to be at least 1011 kg s−1. The spec-

Fig. 3. Analysis of the 4.5-mm time series of the ID8 disk. (A) Periodogram of the detrended 2013
light curve.The vertical dashed lines represent the identified periods P1 and P2, which are determined after
removing the strongest frequency in each iteration (22) and thus appear slightly offset from the raw peak
positions. (B) Detrended 4.5-mm data (black diamonds) with the composite of two sine waves of P1 and P2(gray curve). Because their real waveforms are probably nonsinusoidal, the comparison is meaningful only
for the general timing of the highs and lows.

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trum was obtained in 2007; however, the Spitzer
and WISE photometric points (Fig. 1) imply thatthe mid-IR spectrum had a similar shape at leastfrom January 2004 through November 2010. Thatis, it appears that the small grains, with a net volume of an ~180-km-diameter asteroid, were lostfrom the system and would have to be replenished on a decadal (or shorter) time scale tomaintain the mid-IR spectrum. Though arisingfrom different physical processes, the mass loss
rate is five to seven orders of magnitude greaterthan the dust mass loss rates of comets Hale-Bopp(13) and Halley (14), which are among the dustiest
comets known in the solar system, and is threeorders of magnitude greater than that of theevaporating planet KIC 12557548b (15).
The variability of the IR emission of ID8 ismuch too fast to arise in a conventional debris
disk sustained by a collisional cascade (8). We
hypothesize that the impact responsible for theincreased disk emission in 2013 involved twolarge bodies and was sufficiently violent to yielda silica-rich vapor plume. Glassy silicate spherules
will condense from the vapor with diverse forms(16), consistent with the presence of amorphoussilicates in the SED model (10). In this case, there
may have been temporal spectral features after
the new impact, but they would have been missed
because we do not have a new mid-IR spectrum in
2013. The condensation process has been modeled in (17), which shows that the typical spherule
size depends sensitively on the circumstances of
the impact, particularly its velocity, but ranges
from about 10 mm to 1 mm. The condensates areproduced quickly over several hours. Initially, thecloud of spherules will not radiate efficiently inthe mid-IR, because the total surface area of all thespherules is small. However, they will break eachother down through collisions, which generate theobserved micrometer-sized or smaller particles asdaughter products. The IR output will rise as themass is distributed into many small grains, making
the consequences of the initial impact visible as anincrease in the IR emission.
Because the size distribution of condensate
spherules is strongly peaked around the average(17), we treat them as being equal in size. Then arough estimate of the time scale for destroying
them (and hence for the decay of the debris cloud)can be obtained by attributing all the disk mass tothem immediately after the impact and assumingthat they are removed by breakdown in a collisionalcascade with eventual ejection, when their daughter particles are submicrometer in size, by radiationpressure. We found that different models for thedestruction rate of such spherules give consistenttime scales (18, 19). To order of magnitude, therange of decay time scales as a function of spherule
size from 10 to 1000 mm is 100 days to 10 years. The
observed decay time of the ID8 disk corresponds toa spherule size of ~100 mm, which corresponds toan impact velocity of 15 to 18 km s−1 [for a bodywith a diameter of 100 to 1000 km impacting onan even larger one (17)]. Thus, the 1-year exponential decay time scale is a natural result for a system
where seed grains have condensed from a vapor
cloud and are destroying themselves throughsciencemag.org SCIENCERE S EAR CH | R E P O R T S

collisions. An analysis with some similarities toours for the bright debris disk of HD 172555(20) found that dust created in a hypervelocity
impact will have a size slope of ~ –4, in agreementwith the fits of (10) to the IR spectrum of ID8.
After the exponential decay is removed fromthe data (“detrending”), the light curves at both
wavelengths appear to be quasi-periodic. The
regular recovery of the disk flux and lack of extraordinary stellar activity essentially eliminate
coronal mass ejection (21) as a possible driver ofthe disk variability. We employed the SigSpec algorithm (22) to search for complex patterns in
the detrended, post-impact 2013 light curve. The
analysis identified two significant frequencies withcomparable amplitudes, whose periods are P1 =25.4 T 1.1 days and P2 = 34.0 T 1.5 days (Fig. 3A)
and are sufficient to qualitatively reproduce mostof the observed light curve features (Fig. 3B).
The quoted uncertainties (23) do not account forsystematic effects due to the detrending and thus
are lower limits to the real errors. Other peaks withlonger periods in the periodogram are aliases or
possibly reflect long-term deviation from the exponential decay. These artifacts make it difficult
to determine whether there are weak real signals
near those frequencies.
We now describe the most plausible interpretation of this light curve that we have found.
The two identified periods have a peak-to-peakamplitude of ~6 × 10−3 in fractional luminosity,
which provides a critical constraint for models of
the ID8 disk. In terms of sky coverage at the disk
distance inferred from the IR SED, such an amplitude requires the disappearance and reappearance every ~30 days of the equivalent of an opaque,stellar-facing “dust panel” of radius ~110 Jupiterradii. One possibility is that the disk flux periodicity arises from recurring geometry that changes
the amount of dust that we can see. At the timeof the impact, fragments get a range of kick velocities when escaping into interplanetary space.
This will cause Keplerian shear of the cloud (24),leading to an expanding debris concentration
along the original orbit (supplementary text). If
the ID8 planetary system is roughly edge-on, thelongest dimension of the concentration will beparallel to our line of sight at the greatest elongations and orthogonal to the line of sight nearconjunctions to the star. This would cause theoptical depth of the debris to vary within anorbital period, in a range on the order of 1 to 10
according to the estimated disk mass and particle sizes. Our numerical simulations of such dust
concentrations on moderately eccentric orbits areable to produce periodic light curves with strong
overtones. P2 and P1 should have a 3:2 ratio ifthey are the first- and second-order overtones ofa fundamental, which is consistent with the measurements within the expected larger errors (<2sor better). In this case, the genuine period shouldbe 70.8 T 5.2 days (lower-limit errors), a value
where it may have been submerged in the periodogram artifacts. This period corresponds to asemimajor axis of ~0.33 astronomical units, whichis consistent with the temperature and distancesuggested by the spectral models (10).
SCIENCE sciencemag.org

Despite the peculiarities of ID8, it is not aunique system. In 2012 and 2013, we monitored
four other “extreme debris disks” (with disk fractional luminosity ≥10−2) around solar-like stars
with ages of 10 to 120 My. Various degrees of IR
variations were detected in all of them. The
specific characteristics of ID8 in the time domain,
including the yearly exponential decay, additional more rapid weekly to monthly changes, andcolor variations, are also seen in other systems.
This opens up the time domain as a new dimension for the study of terrestrial planet formationand collisions outside the solar system. The variability of many extreme debris disks in the eraof the final buildup of terrestrial planets may
provide new possibilities for understanding theearly solar system and the formation of habitable
planets (25).
RE FERENCES AND NOTES

1. R. Helled et al., in Protostars and Planets VI, H. Beuther,R. Klessen, C. Dullemond, T. Henning, Eds. (Univ. of Arizona
Press, Tucson, AZ, 2014), in press; available at http://arxiv.org/
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AC KNOWLED GME NTS

H.Y.A.M, K.Y.L.S., and G.H.R. thank R. Malhotra and A. Gáspár for
valuable discussions. This work is based on observations madewith the Spitzer Space Telescope, which is operated by the Jet
Propulsion Laboratory (JPL), California Institute of Technology,
under a contract with NASA. Support for this work was provided
by NASA through an award issued by JPL/Caltech and by NASA
grant NNX13AE74G. All data are publicly available through theNASA/IPAC Infrared Science Archive.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/345/6200/1032/suppl/DC1
Supplementary Text
Figs. S1 to S4
References (28–45)
23 April 2014; accepted 15 July 201410.1126/science.1255153

SUPERFLUIDITY

A mixture of Bose and Fermi superf luids
I. Ferrier-Barbut,* M. Delehaye, S. Laurent, A. T. Grier,† M. Pierce,
B. S. Rem,‡ F. Chevy, C. Salomon
Superconductivity and superfluidity of fermionic and bosonic systems are remarkablemany-body quantum phenomena. In liquid helium and dilute gases, Bose and Fermi
superfluidity has been observed separately, but producing a mixture in which both the fermionicand the bosonic components are superfluid is challenging. Here we report on the observationof such a mixture with dilute gases of two lithium isotopes, lithium-6 and lithium-7. We probe
the collective dynamics of this system by exciting center-of-mass oscillations that exhibit
extremely low damping below a certain critical velocity. Using high-precision spectroscopyof these modes, we observe coherent energy exchange and measure the coupling between
the two superfluids. Our observations can be captured theoretically using a sum-rule
approach that we interpret in terms of two coupled oscillators.

In recent years, ultracold atoms have emerged
as a unique tool to engineer and study quantum
many-body systems. Examples include weaklyinteracting Bose-Einstein condensates (1, 2),
two-dimensional gases (3), and the superfluidMott insulator transition (4) in the case of bosonicatoms, and the crossover between Bose-Einsteincondensation (BEC) and fermionic superfluidity
described by the the theory of Bardeen, Cooper,
and Schrieffer (BCS) for fermionic atoms (5). Mix-tures of Bose-Einstein condensates were produced
shortly after the observation of BEC (2), and a
BEC mixed with a single-spin state Fermi sea
was originally observed in (6, 7). However, realizing
a mixture in which both fermionic and bosonicspecies are superfluid has been experimentallychallenging. This has also been a long-sought goalin liquid helium, where superfluidity was achieved
separately in both bosonic 4He and fermionic 3He.
The double superfluid should undergo a transition
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