Orbital and Evolutionary Constraints on the Planet Hosting Binary GJ 86 from the Hubble Space Telescope

0.99 MB·English

by  J. Farihi

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed15January2013 (MNLATEXstylefilev2.2)
Orbital and Evolutionary Constraints on the Planet
Hosting Binary GJ 86 from the Hubble Space Telescope
J. Farihi1,2⋆†, Howard E. Bond3,4‡, P. Dufour5, N. Haghighipour6, G. H. Schaefer7,
J. B. Holberg8 M. A. Barstow2, M. R. Burleigh2
3 1Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA
1 2Department of Physics & Astronomy, Universityof Leicester, LeicesterLE1 7RH
0 3Space Telescope Science Institute, Baltimore, MD 21218, USA
2 4Department of Astronomy & Astrophysics, Pennsylvania State University, UniversityPark, PA 16802, USA
5D´epartement de Physique, Universit´e de Montr´eal, Montr´eal, QCH3C 3J7, Canada
n
6Institute for Astronomy, University of Hawaii-Manoa, Honolulu, HI 96822, USA
a
J 7CHARA Array of Georgia State University,Mount Wilson Observatory, CA 91023, USA
8Lunar and Planetary Laboratory, Universityof Arizona, Tucson AZ 85721, USA
2
1

]
R
S ABSTRACT
.
h
p This paper presents new observations of the planet-hosting, visual binary GJ86
- (HR637) using the Hubble Space Telescope. Ultraviolet and optical imaging with
o WFC3 confirms the stellar companion is a degenerate star and indicates the binary
r semimajoraxis is largerthan previous estimates,with a&28AU. OpticalSTIS spec-
t
s troscopyofthe secondaryrevealsahelium-richwhite dwarfwithC absorptionbands
a 2
andT =8180K,thusmakingthebinarysystemrathersimilartoProcyon.Basedon
[ eff
the10.8pcdistance,thecompanionhas0.59M⊙ anddescendedfromamain-sequence
3 A star of 1.9M⊙ with an original orbital separation a&14AU. If the giant planet is
v coplanar with the binary, the mass of GJ86Ab is between 4.4 and 4.7 M .
Jup
1
The similarity of GJ86 and Procyon prompted a re-analysis of the white dwarf
0
in the latter system, with the tentative conclusion that Procyonhosts a planetesimal
6
population. The periastron distance in Procyon is 20% smaller than in αCenAB,
5
but the metal-enriched atmosphere of ProcyonB indicates that the planet formation
.
2 process minimally attained 25km bodies, if not small planets as in αCen.
1
2 Key words: binaries: visual— stars: individual (GJ86A, GJ86B)— planetary
1 systems— white dwarfs
:
v
i
X
r
a 1 INTRODUCTION tems are GJ86 (HR637, HD13445) (Queloz et al. 2000),
γCephei (Hatzeset al. 2003), HD41004 (Zuckeret al.
About 20% of known extrasolar planets orbit one compo-
2004), HD196885 (Correia et al. 2008), and HD17605
nentofastellarbinary(Raghavan et al.2006;Haghighipour
(Muterspaugh et al. 2010). All five of these binaries are
2006).Bothobservationandtheoryindicatethatthemajor-
thought to have a . 30AU and a giant planet orbiting
ity of these planets in binaries are similar to those orbiting
the primary star. However, only in GJ86 is the stellar sec-
single stars, owing to the wide (a>100AU) stellar separa-
ondary a white dwarf rather than a second main-sequence
tions(Desidera & Barbieri2007;Haghighipour2006).How-
star, and the Jovian planet thus orbits the originally less
ever, there are a growing number of systems which present
massivecomponent.Furthermore,theinitial binary separa-
a major challenge to planet-formation modelers, because
tion was smaller, making GJ86 a challenging environment
their giant planets orbit one component within closer bi-
in which to form planets.
nary systems. In order of discovery, these planetary sys-
Our view of planet formation in binaries has changed
significantly in the past 17 years. Simulations of binary
⋆ E-mail:[email protected] stars with a < 40AU failed to retain sufficient circum-
stellar material to form planets around one star, and thus
† STFCErnestRutherfordFellow
‡ Currentaddress:9615LabradorLane,Cockeysville,MD21030, precluded planet birth by either the core accretion or disk-
USA instabilitymechanisms(Nelson2000;Artymowicz & Lubow
(cid:13)c 0000RAS
2 J. Farihi et al.
Table 1.HSTCycle19ObservationsofGJ86B Table 2.FluxDensities&EffectiveWavelengths forGJ86Ba
Instrument UTDate Filter/Grating Exposures Filter λbeff Fλ/10−14 Fν/10−26
(s) (˚A) (ergs−1cm−2˚A−1) (ergs−1cm−2Hz−1)
WFC3UVIS 2012Mar31 F225W 1.0×16 F225W 2385 3.49 6.62
F225W 20.0×4 F275W 2722 4.04 9.99
F275W 10.0×4 F336W 3355 3.93 14.76
F336W 5.0×4 F390W 3908 3.40 17.34
F390W 4.0×4 F438W 4315 3.02 18.75
F438W 4.0×4 F555W 5237 2.18 19.91
F555W 2.0×4 F625W 6183 1.56 19.87
F625W 2.0×4 F814W 7899 0.80 16.74
F814W 30.0×4
a ThefluxofGJ86Awasmeasuredtobe31.4mJyinF225W.
STISCCD 2012May29 G430L 100.0×3 b Effective wavelengths were derived by convolving the adopted
G430M 600.0×4
G750M 400.0×3 stellarmodelinFλ withtheWFC3filtertransmissioncurves.
infrared makes this difficult but possible with adaptive op-
tics,yetwhitedwarfsdonotexhibitspectralfeaturesinthis
1994; see Prato & Weinberger 2010 for a detailed review).
wavelengthregion.ThusanopticalstudyofGJ86Bisneces-
Yet observers have imaged favorable planet-forming envi-
sary and a Hubble Space Telescope (HST) program was ini-
ronments around the components of close binary stars, im-
tiated to achieve this goal. In §2 are described the imaging
plying planet formation in these systems may be as com-
and spectroscopic observations, while §3 describes the at-
mon as around single stars (Akeson et al. 1998; Mathieu
mospheric modeling, derivation of current (and progenitor)
1994). For example, the two well-separated disks in the bi-
stellar parameters and binary orbit constraints. New static
nary system L1551 (Rodriguez et al. 1998) indicate that,
limits on the GJ86 planetary system are calculated based
despite disk truncation, it is still possible for both compo-
on the HST observations and possible similarities with the
nents to retain a significant amount of their original cir-
Procyon system are discussed in §4.
cumstellar material (0.03−0.06M⊙) in disks with consid-
erableradii(r≈10AU).Thesediskmassesarecomparable
totheminimum-massmodelof theprimordial Solar nebula
2 OBSERVATIONS & DATA REDUCTION
(Hayashi1981;Weidenschilling1977)suggestingthatplanet
formation in these environments may proceed as in disks Due to orbital motion observed between 2000 and 2005
around single stars. (Lagrange et al.2006;Mugrauer & Neuh¨auser2005),itwas
TheJovianplanet1 orbitingGJ86A(K0V)wasdiscov- expected that precise spatial offsets might be necessary for
ered via precision radial-velocity monitoring (Queloz et al. proper placement of the faint companion within the spec-
2000), revealing a 15.8d orbit (0.11AU separation) with troscopic aperture. Thus, WFC3 imaging was executed for
msini = 4MJup. At the time of discovery, an additional both photometric and astrometric purposes, prior to STIS
long-term radial velocity drift was reported, suggesting the spectroscopy.
presence of a more distant, unseen stellar companion. Sub-
sequently, the distant companion was directly detected at
a projected separation of 1′.′7, but 9mag fainter than the 2.1 WFC3 Imaging
primary in the K band (Els et al. 2001). The companion,
GJ86 was imaged on 2012 March 31 using the Wide Field
GJ86B,waslaterimagedbyMugrauer & Neuh¨auser(2005)
Camera 3 (WFC3) in the UVIS channel with a 512×512
andLagrange et al.(2006)whosedataclearlyrevealorbital
(UVIS2-C512C-SUB) subarray. The imaging sequence took
motion; between 2000 and 2005 the angular separation of
place over a single orbit and began with a series of 1s ex-
the pair changed by 0′.′25 and the position angle changed
posures in the F225W filter. In this short exposure time,
by 17◦. GJ86B cannot be a low-mass star or brown dwarf:
the flux from the V = 6.2mag, T ≈ 5200K primary was
eff
a substellar mass cannot be reconciled with the observed
expected to leave an unsaturated, linear response on the
radial-velocitydriftofGJ86A,andfollow-upmethane-band
CCD.ThissequenceemployedthedefaultUVISditherpat-
photometry showed GJ86B to have a near-IR color index
tern with four points in a parallelogram, and was repeated
near zero. Moreover, K-band spectroscopy of GJ86B re-
fourtimesforatotalof16exposures.Bothprimaryandsec-
vealsanessentiallyRayleigh-Jeanscontinuumsource.Thus,
ondarystarwerewell-detected inthisfirstsetof exposures,
GJ86B can only be a white dwarf (Lagrange et al. 2006;
and in the linear response regime with high signal-to-noise
Mugrauer & Neuh¨auser2005).
(S/N) >500. This set of images was used to robustly mea-
In order to ideally constrain the formation and evolu-
sure the separation of the binary, where the secondary star
tion of the giant planet orbiting GJ86A, it is necessary to
was found at offset:
obtain the orbit and component masses of the binary sys-
tem. The 3300:1 contrast ratio between components in the α=2′.′351(2) θ=88◦.96(4) (J2000) Date: 2012.2468
Immediately following the short exposure sequence, a
1 TheplanetisoftenreferredtoincorrectlyasGJ86b,whilethe single ditherpatternof four deeperexposureswas executed
correctdesignationisGJ86Ab in each of eight filters (see Table 1 and Figure 1). At the
(cid:13)c 0000RAS,MNRAS000,000–000
Constraints on Planet Hosting Binary GJ86 3
Figure 1. Images of GJ86 in the eight specified ultraviolet and optical filters using WFC3 UVIS. The images are oriented so that
upward is 17◦.60 east of north, and each frame is 8′′ on a side. The bottom-right image is a composite RGB frame using the F225W
(blue),F275W(green),andF336W(red)filters.AseriesofshortexposuresatF225Wwereobtainedwithanunsaturatedprimary,and
thesedatawereusedfortherelativeastrometryofthepair.
phase II design stage, the brightness of the companion was sured in an r =7pixel aperture radius and corrected using
poorly constrained and the longer exposures were to insure theencircledenergydataavailableintheWFC3Instrument
high S/N photometry (up to thelimit imposed by thePSF Handbook version 3.0 (Dressel et al. 2010). The aperture
wings of the primary) at all wavelengths. Fortunately, even correctionswerederivedbyinterpolatingthetabularvalues
without stray light correction, the dither-combined image at the effective wavelength of the best fitting atmospheric
setsyieldedS/N>500forGJ86Bshortwardof5500˚A,and model (see §3) in each bandpass, and the process was iter-
S/N>200beyond.Thus,therelativelyfaintcompanionwas ated until there were no significant changes in the photom-
mostly unaffected by theprimary. etry or stellar model parameters; in practice this occurred
afterasinglerepetition.ThebackgroundfluxofGJ86Awas
Photometry was performed on multi-drizzled images
produced by the STScI software and data pipeline OPUS subtractedbyexaminingpointsopposite, in xand y on the
2012.1 and CALWF3 2.6.2. The flux of GJ86B was mea- subarray, from the bright primary. This ambient light cor-
(cid:13)c 0000RAS,MNRAS000,000–000
4 J. Farihi et al.
Figure 2.Combined-frameSTISCCDspectral imagesofGJ86Btakeninthreegratings,withwavelength increasingtotheright.The
ambientlightoftheprimaryisrelativelymild,eveninredlight.Despiterejectionroutinesthatdrasticallyreducedthenumberofcosmic
rays, hot and cold pixels present in the single exposures, some artifacts remain in the combined 2D images, especially at the shortest
wavelength setting.
rectionresultedinfluxmeasurementchangeslessthan1.2% background.Theflux-calibratedandnormalizedSTISspec-
for the five filters below 5000˚A, but rose to around 4% for traaredisplayedinFigures3and4.TheS/Nperpixelwas
thethreelonger wavelength filters.Themeasured fluxesfor determinedfromcontinuumregionswithineachofthespec-
GJ86B are listed in Table 2. tratobeclose to85,88, and53in theG430L, G430M, and
G750M gratings respectively.
2.2 STIS Spectroscopy
3 STELLAR & BINARY PARAMETERS
Observations with the Space Telescope Imaging Spectro-
graph(STIS)wereexecutedon2012May29,beginningwith TheupperspectrumshowninFigure4displaysthedistinc-
anacquisitionsequenceofGJ86A,followedbyanoffsetand tive and broad C2 Swan bands, indicating that GJ86B has
acquisition/peak-upofGJ86BbasedontheWFC3astrom- a DQ spectral classification. This class of white dwarfs is
etry. Examination of the acquisition and peak-up images characterized by helium-dominated atmospheres with trace
shows they were successful. Spectroscopy was performed carbon, likely dredged from the core (Pelletier et al. 1986).
with the G430L, G430M, and G750M gratings as detailed NeitherCaiiKnorHαweredetectedintherelatively deep
inTable1,usingdefaultSTISditherpatternsalongtheslit, and higher resolution G430M and G750M spectra. Follow-
and achieving average resolutions of 6.8, 0.7, and 1.4˚A, re- ing convention the star has the white dwarf designation
spectively. WD0208−510 (McCook & Sion 1999).
Theindividualspectralimages werefirstshiftedontoa
common frame as indicated by the image header astrome-
3.1 Atmospheric Modeling
try and using integer pixel shifts. Next, the shifted images
were combined into a single 2D frame using mscombine in StellarparametersforGJ86Bwerederivedintwoways,us-
theIRAFSTSDASsoftwarepackage.Priortothisstep,the ingmodelatmospheregridsappropriateforDQwhitedwarfs
spectralimagescontainedasignificantnumberofbadpixels (seeDufouret al.2005 fordetails onthemodelsandfitting
and cosmic rays, especially in theG430M observations. Re- techniques). First, the effective temperature and solid an-
jection of these unwanted pixels was performed using ccd- gle were estimated from fitting the broad baseline WFC3
clip at 4σ, although this could not completely remove the photometry. These parameters were then combined with
accumulated artifacts. the Hipparcos parallax of 92.74mas (van Leeuwen 2007;
Figure 2 shows the final, combined spectral images for Perryman et al.1997)todeterminethestellarradius,which
each of the grating settings. Although stray light from the was then converted into mass using theoretical mass-radius
primaryisapparentinalltheimages,andmostreadilyseen relationships (Fontaine et al. 2001). However, the shape of
inthered,thesignalfromGJ86Bwasessentiallyunaffected. theenergydistribution–andthusthederivedeffectivetem-
Thecontrastbetweenthepeaksignalsfrom thesciencetar- perature–isslightlysensitivetotheamountofphotospheric
getandtheprimarydiffractionspikeswas17:1atthelowest carbon. Model fits to the C2 Swan bands in the G430L
intheG750Mobservations,andatleasttwiceashighinthe STISspectrumwereusedtomeasurethecarbonabundance,
region of thescience spectrum. with T and log g fixed at the values obtained from the
eff
Lastly, 1D spectra were extracted using x1d, and ap- WFC3photometry.Theeffectivetemperatureandsolid an-
propriate sky subtraction to correct for the relatively mild gle were re-derived using C/He fixed to the spectroscopic
(cid:13)c 0000RAS,MNRAS000,000–000
Constraints on Planet Hosting Binary GJ86 5
Figure 3.Flux-calibratedWFC3 photometry andSTIS spectra
ofGJ86BforeachoftheconfigurationsspecifiedinTable1.Fig-
ure4plotsthespectraingreaterdetail.
Table 3.Current&ProgenitorStellarParametersforGJ86AB
GJ86B/WD0208−510
WhiteDwarfRemnant:
SpT DQ6
Veff (mag) 13.2
Teff (K) 8180±120
logg (cms−2) 8.02±0.02
Mass(M⊙) 0.59±0.01
Radius(R⊙) 0.01245±0.0015
CoolingAge(Gyr) 1.25±0.05
log(C/He) −4.8±0.2 Figure 4. From top to bottom are normalized STIS spectra of
log(H/He) <−4.3 GJ86Bobtained withtheG430L, G430M,andG750Mgratings.
log(Ca/He) <−11.8 TheonlydetectedspectralfeaturesaretheC2Swanbands.Upper
limitsoncalciumandhydrogenwerederivedfromtheCaiiKand
Main-SequenceProgenitor: Hαregions(Table3).
SpT A5V
Mass(M⊙) 1.9±0.1
Lifetime(Gyr) 1.4±0.2 parameters is significant and most likely caused by the two
shortest wavelength fluxes.Indeed,the F225W and F275W
GJ86A/HR637/HD13445
bandpasses span a region influenced by at least one strong
SpT K0V Ci line which models tend to over-predict (in the core or
V (mag) 6.1 wings).AnexampleofthiswasobservedbyProvencal et al.
Teff (K) 5200 (2002)intheSTISultravioletspectrumofProcyonB,where
Mass(M⊙) 0.80 the strength of the 2478˚Aline is much weaker (by nearly a
Age(Gyr) 2.5 factorof 30inC/He) thanpredicted bymodels. Themodel
shortcomings are understood to be a consequence of the
Note. Parameters listed for GJ86A are values representa- high density environments in cool helium-rich white dwarf
tive of those found in the literature (Ghezzi etal. 2010; photospheres; the classical Van der Waal broadening treat-
vanBelle&vonBraun 2009; Mamajek&Hillenbrand 2008;
ment within the impact approximation, traditionally used
Valenti&Fischer 2005; Saffeetal. 2005; Ribasetal. 2003;
inDQmodelatmospheres,isnotappropriatefortheselines
Flynn&Morell1997).
(Koester et al. 1982).
Ifoneignoresthetwoultravioletfluxesaffectedbythese
value.Thisprocedurewasiterateduntilallparameterscon- strong Ci lines, and uses the remaining six WFC3 photo-
verged, with a good fit obtained at T = 8420 ± 90K, metric data points, then T =8210±180K, log(C/He)=
eff eff
log(C/He)−4.58±0.15. −4.76 ± 0.15 is obtained, in agreement with the param-
Second,becausetheSTISdataareflux-calibratedwith eters from fitting the spectrum alone. For all stellar and
a high degree of confidence, the effective temperature and evolutionary parameters, the weighted average of the pho-
carbon abundance can be simultaneously obtained from tometric (ignoring F225W and F275W) and spectroscopic
the G430L spectrum alone, yielding T = 8145±180K, effectivetemperatures,andthenon-weightedaverageofthe
eff
log(C/He)=−4.82±0.15.Thedifferenceinresultingmodel carbon abundances are thus adopted and listed in Table 3
(cid:13)c 0000RAS,MNRAS000,000–000
6 J. Farihi et al.
Figure 5. Comparison between the observed and modeled stellar fluxes. In the left-hand panel, the model fluxes agree well with the
WFC3photometryinallbandpassesexceptF225WandF275W(see§3.1).Intheright-handpanelthemodelspectrumreproducesboth
the optical continuum and C2 bands in the STIS G430L data, but indicates astrong Ci2478˚A feature that under-predicts the stellar
fluxinthetwoultravioletfilters.Allspectralfeaturesareduetoatomicormolecularcarbon.
(T = 8180±120K and log(C/He) = −4.8±0.2). Upper 4 BINARY & PLANETARY EVOLUTION
eff
limit hydrogen and calcium abundances were also derived
4.1 Total System Age
and are listed in Table 3, while Figure 5 shows the best
fittingmodelcomparedtothephotometryandG430Lspec- The white dwarf cooling age places a firm lower limit
trum. All the derived stellar parameters are listed in Table of 1.3Gyr on the lifetime of the system (Fontaine et al.
3. 2001). For the total age, one must add the hydrogen-
burning lifetime of the progenitor, which should be in
the range 1.2 − 1.6Gyr for stars of mass 1.8 − 2.0M⊙.
Thus, the evolved companion suggests a total system
age of 2.65 ± 0.25Gyr. Age estimates for the primary
include 2.0 − 2.9Gyr (Saffeet al. 2005) and 2.4Gyr
(Mamajek & Hillenbrand 2008) based on activity-age rela-
3.2 Current Binary Orbit
tions, and these broadly agree with that derived for the
Theepoch2012.2WFC3imagingprovidesanewpointalong secondary. Mamajek & Hillenbrand (2008) prefers an age
thebinaryorbitofGJ86,andisshowninFigure6together of 3.7Gyr based on activity-rotation-age metrics, but this
with previous astrometric measurements (Lagrange et al. value is significantly older than expected for the total life-
2006;Els et al.2001).Unfortunately,onequarterofthefull time of thewhite dwarf and its progenitor.
orbit has not yet been observed, and hence the orbital pa-
rameterscanonlybelooselyconstrained.Forallorbitalcal-
4.2 The Main-Sequence Progenitor System
culations, MA = 0.8M⊙ and MB = 0.6M⊙ are adopted
for simplicity; the actual component masses are not better To constrain formation scenarios for the giant planet or-
constrained at present. biting GJ86A, the binary orbit and component masses
Using the new astrometric datum, a statistical anal- when both stars were on the main sequence is needed. Us-
ysis of orbits that fit the observations was performed. A ing initial-to-final mass relations with some empirical con-
Monte Carlo simulation of 10000 orbital solutions was cre- straints at the low-mass end of progenitor and remnant
atedwithperiodsfrom20to500yr,eccentricitiesfrom0.00 masses (Williams et al. 2009; Kalirai et al. 2008), GJ86B
to 0.99, and with time of periastron passage from 1800 to likely descended from a main-sequence, A-type star with a
2200. From these solutions, the 100 orbits with the best mass near 1.9M⊙. With both stars on the main sequence,
χ2 values, and with total masses within the limited range the semimajor axis of the binary was smaller by a factor
Mtot =1.4±0.1M⊙ wereselected.Theseorbitsareplotted (Jeans 1924)
in Figure 6, and the resulting range in orbital parameters
is listed in Table 4. As can be seen, only the system incli- a0 = MB+MA (1)
nation is significantly constrained at present. The solutions a MB0 +MA
with low eccentricities tend to have relatively short orbital This ratio is not very sensitive to the range of al-
periods,whereasthelongerperiodsolutionsrequirehighec- lowed masses, and yields a0 = 0.52a for MA =
centricities. Notably, the orbital solution with the shortest 0.80M⊙, MB = 0.59M⊙, MB0 = 1.86M⊙. A canoni-
period yields a semimajor axis a =27.8AU with eccentric- cal mass value has been assumed for the primary, but
ity e=0.10, and is significantly larger than the representa- is consistent with isochrones and various literature es-
tive solution with a = 18.4AU adopted by Lagrange et al. timates (van Belle & von Braun 2009; Valenti& Fischer
(2006). 2005; Ribaset al. 2003; Flynn & Morell 1997). From the
(cid:13)c 0000RAS,MNRAS000,000–000
Constraints on Planet Hosting Binary GJ86 7
Table 4.OrbitalConstraintsfromMonteCarloSimulations
Parameter Range
P 120−481yr
T0 1933−2067
e 0.00−0.61
a 2′.′57−6′.′47
27.8−69.8AU
i 114◦.7−122◦.6
Ω 63◦.7−76◦.1
ω 0◦−358◦
Note.Theupperrangeofperiodsandsemimajoraxesresultfrom
thesimulationinputPmax=500yr,buttheseorbitsrequirehigh
eccentricities e>0.5andarethuslesslikely.
have their orbit expanded due to mass loss, by a factor
of MB0/MB ≈ 3.2. However, the critical radius for stabil-
ity also increases due to mass loss, but by a factor smaller
than (MB+MA)/(MB0 +MA)≈1.9, and therefore a nar-
row range of stable, planetesimal orbits are possible for the
Figure 6. The blue data points are the astrometric offsets de- white dwarf if a &30AU currently. It is worth noting that
terminedhereusingWFC3(leftmostpoint), together withthose thesecalculations arepurelystatic, and thusignorethedy-
reported byLagrangeetal.(2006)and Elsetal.(2001). Anini- namiceffectsofmasslosswhichmayhavesignificantimpact
tial sample of 10000 orbital solutions were generated through onstableorbits(Veras & Tout2012;Veras et al.2011).Cir-
a Monte Carlo search, and overplotted in black are the 100 or- cumbinaryorbitsbeyond80AUarealsostableinthecurrent
bits with the best χ2 values, within the restricted mass range
andformerbinaryconfigurationsfora≈30AUande<0.1.
Mtot=1.4±0.1M⊙. Thus, one of the science goals of the HST observing
program was to search for evidence of planetesimals in
this system, via atmospheric metal pollution in the white
current binary orbit simulations, a lower limit on the ini-
dwarf. As discussed in §2.2, calcium absorption was not
tial semimajor axis is obtained a0 > 14.5AU. Even at this
detected in the STIS data, and a strict upper limit of
lowerlimit,the0.11AUplanetaryorbitofGJ86Abisstable
log(Ca/He) < −11.8 was derived for GJ86B. This corre-
for binary eccentricities e < 0.9 (Holman & Wiegert 1999;
Rabl& Dvorak 1988). spondstoMCa <3.2×1017ginthestellarconvectionzone,
or an upper limit to the total accreted heavy element mass
of MZ . 2.0×1019g for material that is 1.6% calcium by
4.3 Planetesimals in GJ86 massasinthebulkEarth(All`egre et al.1995).Becausethe
Caii K line is the most sensitive tracer of metal pollution
Although the class of detached binary stars with a white atopticalwavelengthsforwhitedwarfsofthiseffectivetem-
dwarfandamain-sequencecomponentofspectraltypeKor perature (Koester et al. 2005; Zuckerman et al. 2003), this
earlier arereferred toasSirius-typebinaries (Holberg et al. large cometary-sized mass is the strongest possible limit to
2008),GJ86isalsonotablyProcyon-like.Table5liststheat- theobservable planetesimal population.
mospheric andstellar parameters for thesetwowhite dwarf
companion systems, demonstrating their broad similarities.
Because GJ86 is known to host a planetary system,
4.4 A Failed Planetary System at Procyon?
with at least one giant planet orbiting the primary, it may
alsosupportaplanetesimalpopulation.Thesesurvivingmi- Interestingly, and also using STIS, Provencal et al. (2002)
norbodiescould begravitationally perturbedintoclose en- foundthatProcyonBexhibitsastrongMgiiresonanceline
counterswith thewhitedwarf, where theywould betidally at 2800˚A, a few Fe lines in the near-ultraviolet, and Caii
shreddedandeventuallyaccreted,pollutingtheatmosphere H and K absorption in the optical. While Procyon is not
with heavy elements (Jura 2003), as observed at nearly 30 known to host any planets or planetesimal debris (via pre-
singlewhitedwarfs(G¨ansicke et al.2012;Farihi et al.2012; cision radial velocity, transits, astrometry, direct imaging,
Kilic et al. 2012; Debes et al. 2012). In a binary such as or infrared excess), the white dwarf is externally polluted
GJ86, stable planetesimal belts can persist as circumstel- with at least three heavy elements that could have origi-
lar rings at either star, or a circumbinary disk. nated in a minor planetary body (Farihi et al. 2010). Us-
One region that remains stable over both the current ingthepublishedmagnesiumabundance,ProcyonBhasac-
and former binary configurations and e < 0.1 is within cretedatleastMMg =5.1×1018gwithinthepastfew107yr,
roughly 3AU of the K star primary. Stable circumstellar where 2.8Myr is a single sinking timescale for this element
regions for the white dwarf are complicated by competing atthestellareffectivetemperature(Koester2009).Forma-
processes: while the region within 2AU will be depleted of terial with a chondritic or Earth-like elemental abundance,
small bodies by direct engulfment and tides during the gi- this is equivalent to a total accreted heavy element mass of
ant phases (Mustill & Villaver 2012), surviving objects will atleast3−6×1019g(Lodders & Fegley1998;All`egre et al.
(cid:13)c 0000RAS,MNRAS000,000–000
8 J. Farihi et al.
Table 5.ComparisonofGJ86B&ProcyonB circumstellar region at ProcyonB that remains stable over
thelifetime of the binary.Thus, a circumbinary population
Parameter GJ86B ProcyonBa of planetesimals is the most likely source of the metals ob-
served in ProcyonB. Observations with Herschel PACS at
SpT DQ6.2 DQZ6.5 160µm have not detected a far-infrared excess at Procyon,
Teff (K) 8180 7740 andlimitthefractionaldustluminositytoL/L∗ <5×10−7
Mass(M⊙) 0.59 0.60 overthe30−300AUregion(G.Kennedy2012,privatecom-
log(H/He) <−4.3 <−3.7
munication).Thislimit isonlyafewtimesgreaterthanthe
log(C/He) −4.8 −5.5
log(Mg/He)b ... −10.4 fractional dust luminosity of the Kuiper belt (Booth et al.
log(Ca/He) <−11.8 −11.8 2009; Stern 1996), and for similar particles would result in
anupperdustmasslimitofroughlyanEarthmassforPro-
PrimarySpT K0V F5IV cyon.
a(AU) >28 15.2
Onthemainsequence,Procyonshouldhavehadasemi-
e ... 0.41
majoraxisnear9AU.Evenfore=0.1thisimpliesthatdisk
material available for planet building would be confined to
a AtmosphericparametersfromProvencal etal.(2002).
b TheSTISspectraofGJ86BdonotcovertheMgii2800˚Ares- within 2.0AU of ProcyonA and 2.4AU of ProcyonB, and
thuscompletelywithintheirrespectivesnowlines.Thus,gi-
onancelines.
ant planet formation at either component of the Procyon
systemislikelyprecluded,andlikelyalsoforthea>55AU
1995), and equivalent to a 25−35km diameter object for stable circumbinary environment.Only theinnerregions of
typical asteroid densities. ProcyonA appear capable of forming small, solid planets
Other possible sources for the atmospheric metals are (asin αCenAB;Dumusqueet al. 2012),that persist to the
theinterstellarmedium,orthestellarwindfrom ProcyonA presentday,assumingthebinaryeccentricitywasnotlarger
(Provencal et al. 2002). In both of these cases the accreted whenbothstarswereonthemainsequence.Thus,thePro-
matter would have solar abundances (log(Mg/H)=−4.45, cyon system may represent a case of failed or truncated
log(Mg/He) = −3.35; Lodders 2003) and imply time- planetformation,wherelargeplanetesimalswereformedbut
averagedaccretionratesofM˙H =8.0×108gs−1 andM˙He = furthergrowth was prohibited.
2.6×108gs−1. Iftheheavyelements havebeen continually
capturedfromProcyonA,thenhydrogenandheliumshould
have accumulated in the white dwarf atmosphere over its
1.4Gyr cooling age (Fontaineet al. 2001). During this pe-
riod 3.6×1025gof hydrogenand1.1×1025gofhelium will
havebeenaddedtothe2.1×1028gheliumconvectionzone,
5 SUMMARY & OUTLOOK
yielding a current abundance log(H/He) =−2.2. However,
the upper limit derived from the lack of Hα absorption is High-contrast optical imaging and low-resolution spec-
log(H/He)<−3.7andinconsistentwithstellarwindaccre- troscopy with HST have unambiguously characterized the
tion. stellar companion to the planet-host star GJ86A. The sec-
If one ignores the issue of hydrogen accumulation, the ondary is a relatively cool white dwarf with a helium-
mass loss rate from ProcyonA that is necessary to supply dominatedatmosphereandmolecularabsorptionbandsdue
the magnesium in the white dwarf at the observed time- to trace carbon, which is almost certainly dredged from
averagedrateis1.8×10−9M⊙yr−1foraBondi-Hoyleaccre- the core. Neither Hα nor Caii K are detected in deeper,
tionflow(Debes2006)flowand9.1×10−8M⊙yr−1forgrav- medium-resolution spectroscopy, the latter placing modest
itational (Eddington) accretion. These mass-loss rates are limits on thepresence of scattered planetesimal material.
between 5 and 7orders of magnitude larger than that mea- The binary separation of GJ86 has continued to in-
sured for the Sun and αCen (Wood et al. 2001; Withbroe crease since last observed in 2005, and the actual orbit de-
1989). Therefore, wind capture can be ruled out with con- viatesfrompriorestimates.Whileone-quarterorbithasnot
fidence. Similar arguments can be used to discount the ac- yet been observed, Monte Carlo simulations constrain the
cretion of interstellar matter. First, hydrogen accumulates semimajor axis to a > 27.8AU, and the system inclination
whereasmetals sink,and thusthehydrogen-deficientatmo- within 114◦.7 6 i 6 122◦.6. For planetary-binary coplanar
sphere of ProcyonB is at odds with the accretion of solar orbits,theseresultsimplythemassofGJ86Abliesbetween
abundancematerialformorethan2%ofitscoolinglifetime. 4.4 and 4.7MJup.
Second,molecularclouddensitiesarerequiredtoaccountfor Futureimagingisnecessarytoobserveapastronanddy-
the observed metals (Farihi et al. 2010), but Procyon has namicallyconstrainthecomponentmasses.Thiscanbedone
beenmovingwithin thelocal, r∼100pc,ISM-poorBubble with adaptiveopticsin thenear-infrared,butsuperior data
(Redfield & Linsky 2008) for at least 2.6Myr. will come from follow up imaging and spectroscopy with
Theremainingpossibilityistheaccretionofoneormore HST in the near-ultraviolet. In this relatively low-contrast
planetesimals. Accounting for both the current and former wavelength regime, there will be narrow absorption lines
binaryconfigurations, stableorbital regions atProcyon can fromCi,andpossiblyMgiiwhichisastrongabsorptionfea-
be either circumbinary beyond 55.4AU or circumprimary tureinProcyonB.Suchspectrallinescanprovideabaseline
within 2.3AU (Holman & Wiegert 1999), although bodies radial velocity and gravitational redshift for the secondary;
inthelatterregionwouldbedynamicallyunlikelytobeper- these would more efficiently characterize the current and
turbed towards the surface of the white dwarf. There is no former binary configurations than near-infrared data.
(cid:13)c 0000RAS,MNRAS000,000–000
Constraints on Planet Hosting Binary GJ86 9
ACKNOWLEDGMENTS Jeans J. H. 1924, MNRAS,85, 2
Jura M. 2003, ApJ, 584, L91
The authors thank an anonymous referee for a timely and
KaliraiJ.S.,HansenB.M.S.,KelsonD.D.,ReitzelD.B.,
thorough report. J. Farihi thanks J. Provencal for helpful
Rich R.M., RicherH. B. 2008, ApJ, 676, 594
feedbackontheavailablespectraofProcyonB,aswellasG.
Kilic M.Patterson A.J., Barber S.,Leggett S.K.,Dufour
KennedyandM.Wyattforausefulsummaryoftheinfrared
P.2012, MNRAS,419, L59
excessanddustlimitsinthissystem.Thisworkisbasedon
Koester D. 2009, A&A,498, 517
observationsmadewiththeHubbleSpaceTelescopewhichis
Koester D., Rollenhagen K., Napiwotzki R., Voss B.,
operated by the Association of Universities for Research in
Christlieb N., Homeier D., Reimers D. 2005a, A&A, 432,
Astronomy under NASA contract NAS 5-26555. These ob-
1025
servations are associated with program 12548. Support for
KoesterD.,WeidemannV.,ZeidlerE.M.1982,A&A,116,
Program number 102548 was provided by NASA through
147
grant HST-GO-12548 from the Space Telescope Science In-
LagrangeA.M.,BeustH.,UdryS.,ChauvinG.,MayorM.
stitute.J.Farihigratefully acknowledges thesupportofthe
2006, A&A,459, 955
STFCviaanErnestRutherfordFellowshipandasaPDRA.
Lodders K. 2003, ApJ, 591, 1220
N. Haghighipour acknowledges support from NASA grants
LoddersK.,FegleyB.1998,ThePlanetaryScientist’sCom-
EXOBNNX09AN05GandHST-GO-12548.06-A.J.Holberg
panion (New York:Oxford University Press)
acknowledges support from NSF Grant AST-1008845. M.
Mamajek E. E., Hillenbrand L. A. 2008, ApJ, 687, 1264
Barstow and M. Burleigh acknowledge support from the
Mathieu R. D.1994, ARA&A,32, 465
STFC.
McCook G. P., Sion E. M. 1999, ApJS,121, 1
Mugrauer M., Neuh¨auserR. 2005, MNRAS,361, L15
Mustill A.J., Villaver E. 2012, ApJ, 761, 121
Muterspaugh M. W., et al. 2010, AJ, 140, 1657
REFERENCES
Nelson A. F. 2000, ApJ, 537, L65
Akeson R. L., Koerner D. W., Jensen E. L. N. 1998, ApJ, PelletierC.,FontaineG.,Wesemael,F.,MichaudG.,Weg-
505, 358 nerG. 1986, ApJ,307, 242
All`egreC.J.,PoirierJ.P.,HumlerE.,HofmannA.W.1995, Perryman M. A.C., et al. 1997, A&A,323, L49
Earth Planetary Sci. Letters, 4, 515 Prato L., Weinberger A. J. 2010, Planets in Binary Star
Artymowicz P., Lubow S. H.1994, ApJ,421, 651 Systems,ed. N. Haghighipour, Springer, New York,1
Booth M., Wyatt M. C., Morbidelli A., Moro-Mart´ın A., ProvencalJ.L.,ShipmanH.L.,KoesterD.,Wesemael,F.,
Levison H.2009, MNRAS,399, 385 Bergeron P. 2002, ApJ,568, 324
Correia A.C. M., et al. 2008, A&A,479, 271 Queloz D., et al. 2000, A&A,354, 99
Debes J. H. 2006, ApJ, 652, 636 Rabl G., Dvorak,R. 1988, A&A,191, 385
Debes J. H., Kilic M., Faedi, F., Shkolnik E. L., Lopez- RaghavanD.,HenryT.J.,Mason B.D.,SubasavageJ.P.,
Morales M., Weinberger A. J., Slesnick C., West R. G. JaoW.C.,BeaulieuT.D.,HamblyN.C.2006,ApJ,646,
2012, ApJ, 754, 59 523
Desidera S., Barbieri M. 2007, A&A,462, 345 Redfield S., Linsky J. L. 2008, ApJ,673, 283
Dressel L., et al. 2010, Wide Field Camera 3 Instrument Ribas I., Solano E., Masana E., Gim´enez A. 2003, A&A,
Handbook,Version 3.0, (Baltimore: STScI) 411, L501
Dufour P., Bergeron P., Fontaine G. 2005, ApJ, 627, 404 Rodriguez L. F., et al. 1998, Nature, 395, 355
DumusqueX., et al. 2012, Nature, 491, 207 Saffe C., G´omez M., Chavero C. 2005, A&A,443, 609
Els S. G., Sterzik M. F., Marchis, F., Pantin E., Endl M., Stern S. A.1996, A&A,310, 999
Ku¨rster M. 2001, A&A,370, L1 Valenti J. A., Fischer D. A.2005, ApJS,159, 141
Farihi J., Barstow M. A., Redfield S., Dufour P., Hambly van Belle G. T., von Braun K.2009, ApJ, 694, 1085
N.C. 2010, MNRAS,404, 2123 van Leeuwen F. 2007, A&A,474, 653
FarihiJ., G¨ansickeB. T., SteeleP.R.,Girven J.,Burleigh Veras D., Tout C. A.2012, MNRAS,422, 1648
M. R., Breedt E., Koester D.2012, MNRAS,421, 1635 VerasD.,WyattM. C., Mustill A.J., Bonsor A.,Eldridge
Flynn C., Morell O. 1997, MNRAS,286, 617 J. J. 2011, MNRAS,417, 2104
Fontaine G., Brassard P., Bergeron P. 2001, PASP, 113, Weidenschilling S.J. 1977, ApSS51, 153
409 Williams K.A., Bolte M., Koester D.2009, ApJ, 693, 355
G¨ansicke B. T., Koester D., Farihi J., Girven J., Parsons Withbroe G. L. 1989, ApJ, 337, L49
S.G., Breedt E. 2012, MNRAS,424, 333 Wood B. E., Linsky J. L., Mu¨ller H. R., Zank G. P. 2001,
GhezziL.,CunhaK.,SmithV.V.,deArau´joF.X.,Schuler ApJ,547, L49
S.C., dela Reza R.2010, ApJ,720, 1290 Zucker S., Mazeh T., Santos N. C., Udry S., Mayor M.
Haghighipour N.2006, ApJ,644, 543 2004, A&A,426, 695
HatzesA.P.,CochranW.D.,EndlM.,McArthurB.,Paul- Zuckerman B. Koester D., Reid I. N., Hu¨nsch M. 2003,
son D. B., Walker G. A. H., Campbell B., Yang S. 2003, ApJ,596, 477
ApJ,599, 1383
Hayashi C. 1981, Prog. Theor. Phys. Suppl.,70, 35
HolbergJ.B.,SionE.M.,OswaltT.,McCookG.P.,Foran
S., SubasavageJ. P. 2008, AJ, 135, 1225
Holman M. J., Wiegert P. A.1999, AJ, 117, 621
(cid:13)c 0000RAS,MNRAS000,000–000