The HARPS search for southern extrasolar planets: XXXIII. New multi-planet systems in the HARPS volume limited sample: a super-Earth and a Neptune in the habitable zone

1.2 MB·English

by  G. Lo Curto

Astronomy&Astrophysicsmanuscriptno.final˙arx (cid:13)c ESO2013
January16,2013
The HARPS search for southern extrasolar planets (cid:63)
XXXIII. New multi-planet systems in the HARPS volume limited sample:
a super-Earth and a Neptune in the habitable zone. (cid:63)(cid:63)
G.LoCurto1,M.Mayor2,W.Benz3,F.Bouchy2,5,G.He´brard5,6,C.Lovis2,C.Moutou4,D.Naef2,F.Pepe2,
D.Queloz2,N.C.Santos7,8,D.Segransan2,andS.Udry2
1 ESO-EuropeanSouthernObservatory,Karl-Schwarzschild-Strasse2,85748GarchingbeiMu¨nchen,Germany
2 ObservatoiredeGene`ve,Universite´deGene`ve,51ch.desMaillettes,1290Sauverny,Switzerland
3 PhysikalischesInstitutUniversita¨tBern,Sidlerstrasse5,3012Bern,Switzerland
4 Laboratoired’AstrophysiquedeMarseille,TraverseduSiphon,13376Marseille12,France
3
5 Institutd’AstrophysiquedeParis,UMR7095CNRS,Universite´Pierre&MarieCurie,98bisboulevardArago,75014Paris,France
1
6 ObservatoiredeHaute-Provence,CNRS/OAMP,04870Saint-Michel-l’Observatoire,France
0
7 CentrodeAstrofisica,UniversidadedoPorto,RuadasEstrelas,4150-762Porto,Portugal
2
8 DepartamentodeF´ısicaeAstronomia,FaculdadedeCieˆncias,UniversidadedoPorto,Portugal

n
a
J ABSTRACT

5
The vast diversity of planetary systems detected to date is defying our capability of understanding their formation and evolution.
1
Well-definedvolume-limitedsurveysarethebesttoolatourdisposaltotackletheproblem,viatheacquisitionofrobuststatisticsof

theorbitalelements.WeareusingtheHARPSspectrographtoconductoursurveyof≈850nearbysolar-typestars,andinthecourse
]
P of the past nine years we have monitored the radial velocity of HD103774, HD109271, and BD-061339. In this work we present
E thedetectionoffiveplanetsorbitingthesestars,withmsin(i)between0.6and7Neptunemasses,fourofwhichareintwomultiple
systems,comprisingonesuper-Earthandoneplanetwithinthehabitablezoneofalate-typedwarf.Althoughforstrategicreasons
.
h wechoseefficiencyoverprecisioninthissurvey,wehavethecapabilitytodetectplanetsdowntotheNeptuneandsuper-Earthmass
p range,aswellasmultiplesystems,providedthatenoughdatapointsaremadeavailable.
-
o Keywords.extra-solarplanets–multipleplanetarysystems–radialvelocity–HD103774–HD109271–BD-061339
r
t
s
a
[ 1. Introduction tioninducedontheparentstarbythesystemsrequiresasubstan-
tial observational effort both in terms of observing time and of

2 Aftermorethan15yearsofexo-planetsdetections(Mayor1995, themeasurementprecision.
v Schneider2012), we have witnessed an extreme diversity of
Our survey is using the HARPS spectrograph, which is
1 planetary systems, but we have not yet detected a system that
4 could be considered a twin to our own solar system, not even mounted full time at the 3.6m ESO telescope in La Silla
7 limited to the giant (and easier to detect) planets. We are left and is optimized for precise radial velocity measurements
2 (Mayor2003). To date, it has discovered more than 150 exo-
withthequestionofwhetheroursystem,andwithitourplanet,
. planets,mostofthem(throughthehigh-precisionprogram)with
1 arethenorm,orrathertheexceptioninplanetformationandevo-
massesinthesuper-Earthrange;thisdefinitelychangedourview
0 lution.Todate,onlyaboutonethirdofthedetectedplanetsare
3 foundinconfirmedmulti-planetsystems(Schneider2012).This onthefield.
1 fractionissteadilyrisingwiththeincreasingamountofavailable Since the start of HARPS operations we are monitoring
v: high-qualitydata,indicatingthatwearefightingagainstadetec- about 850 stars with moderate radial velocity precision (≈ 2−
i tion bias. Multi-planet systems are more challenging to detect 3ms−1),searchingforJupiterandNeptuneplanetsorbitingsolar-
X
because they require many data points to permit the identifica- type stars (LoCurto2010), while the HARPS high-precision
r tion of the individual signals and because, as is revealed from program(Mayor2003),whichroutinelyachievesprecisionbet-
a the data collected so far, often they involve low-mass planets terthan1ms−1,targetslowermassplanets,mostlyinthesuper-
in the super-Earth to Neptune mass range (e.g. LoCurto2010, Earth mass regime. The goals of our program are the acquisi-
He´brard2010, Lovis2011, Fischer2012). The combination of tion of accurate orbital elements of Jupiter and Neptune-mass
the low amplitude of these signals with the more complex mo- planetsinthesolarneighborhood(within57.5pc)andthesearch
forJupitertwins,whichmightrevealsolarsystemtwins.Ofthe
Sendoffprintrequeststo:G.LoCurto 43 planets discovered so far by this program, 17 have periods
longerthan1000days(Moutou2009,Naef2010,twomoreare
e-mail:[email protected]
(cid:63) Based on observations made with the HARPS instrument on the presented in a paper in preparation), but their eccentricities are
ESO 3.6 m telescope at La Silla (Chile), under the GTO program ID significantly higher than that of Jupiter. The program has de-
072.C-0488 and the regular programs: 085.C-0019, 087.C-0831 and tected three systems with two planets and one triple planetary
089.C-0732. system so far (He´brard2010, LoCurto2010). Out of the nine
(cid:63)(cid:63) RVdataareonlyavailableinelectronicformattheCDS planetsinmultiplesystemswehavediscoveredsofar,onlytwo
1
G.LoCurtoetal.:TheHARPSsearchforsouthernextrasolarplanets
have masses larger than or equal to Jupiter, which is consistent
withtheobservedtrendoflow-massplanetsbeingdetectedpri-
marilyinmultiplesystems.
In this paper we report three stars with multiple Keplerian-
likesignals.Insection2wediscussthestars’properties,andin
section3theorbitalsolutions.Insection4webrieflydiscussthe
encounteredsystems.
2. Thestars
The three stars belong to our volume-limited sample and
were selected to be low-activity, non-rotating, solar-type stars
on the main sequence (LoCurto2010). We have used the
magnitude estimates by Hog (2000), and the one by Koen
(2011) for the redder star BD-061339. They originate from Fig.1.SpectralfluxdensityforHD103774measuredinJy.The
the Tycho and HIPPARCOS catalogs, respectively (ESA-1997, solidlineisablackbodywiththetemperatureofthestar:6489K.
VanLeeuwen2007). The color indices and the parallaxes are Thearrowsindicatethatthethreedatapointsareonlyupperlim-
fromVanLeeuwen2007,andthebolometriccorrectionsareex- its.
tractedfromacalibrationtothedatapresentedbyFlower(1996),
withuncertaintiesgenerallybelow0.1mag. sented an improved photometric calibration for the metallicity.
WeadoptedtheirprescriptiontoestimatethemetallicityofBD-
061339, and used the dispersion around their calibration as an
2.1. HD103774
estimate of the uncertainty. This value is well above the mea-
The star HD103774 is a young, metal-rich, main-sequence star surementuncertaintycomputedviaerrorpropagationfromtheir
with spectral type F5V (Pickles2010, Sousa2011). Its average formula,andislimitedbythequalityofthecalibration.
activity level is low, but it varied considerably during the time Owingtotheredcolorofthestartheestimateoftheactivity
spanofourobservations(seefigure4),goingfromamore”ac- indexlogR(cid:48) isconsideredunreliableandnovalueisincluded
tive” state with log(R ) = −4.72 to a more ”quiet” state with intable1. HK
HK
log(R ) = −5.00 (see Noyes1984 for a definition of the ac-
HK
tivity indicator). We therefore expect to measure some level of
3. Orbitalsolutions
radialvelocityjitter.
Using the online data service VizieR, we found measure-
Westudiedtheradialvelocity(RV)variationofourtargetstars
mentsofthisstarinthemid-andfar-infrared,fromtheIRC(in-
via the analysis of the periodograms of the RV data following
strumentonboardtheAKARIsatellite,Ishihara2010)andIRAS
thegeneralizedLomb-Scargleperiodogrammethodoutlinedby
(Moshir1990)catalogs(seefigure1).Thedatapointsat25,60,
Zechmeister and Kuerster (2009) and via the use of genetic al-
and100µmareupperlimits.Thesolidlineinfigure1reproduces
gorithmasdescribedintheworkbySegransanetal.(2011).In
theblackbodycorrespondingtothestar’sphotospherictempera-
allcasesweverifiedwhethertheRVwascorrelatedwiththebi-
tureof6489K.Theuncertaintyonthetemperature(77K)would
sectorspanofthecrosscorrelationfunction(CCF),theactivity
not be noticed on the scale of this graph. The point at 12µm
indicator R (Noyes1984), and the FHWM of the CCF be-
HK
is deviating from the black body and might seem to indicate
fore claiming a companion. In addition to this, we also studied
aninfraredexcess.Circum-stellardustisindeedquitecommon
theperiodogramoftheseindicators.Whenattemptingafitwith
around early-type young stars. While the evidence for infrared
moredegreesoffreedom(e.g.,usingamodelwithmoreplanets
excess at 12 µm is only marginal (1 sigma) and calls for more
or adding a long-term drift to the model), we verified with an
precise measurements, no excess is measured up to 9 µm, giv-
F-testthatthenewfitbringsasignificantimprovementoverthe
ing an indication on the minimum size of the grains that could
fitwithfewerdegreesoffreedom.WerananF-testandobtained
possiblybepresentinthedisk.
a false-alarm probability (FAP) for which we used a threshold
of 10−4 to identify significant peaks. The orbital elements are
finallydisplayedintable2.
2.2. HD109271
This old and quiet G5V star does not show infrared excess
3.1. HD103774
(Ishihara2010, IRAS data are not available), and has a near-
solar metallicity (Sousa2011). Its basic parameters, as for the We acquired 103 radial velocity measurements for this star in
otherstars,areshownintable1. about7.5years.Theaverageuncertaintyis≈2.6ms−1,whilethe
standarddeviationoftheradialvelocityovertheentiredatasetis
≈ 26ms−1,aclearindicationofvariability.Theperiodogramof
2.3. BD-061339
theradialvelocities(Fig.2)indicatesanexcessofpowerat≈5.9
BD-061339isalate-typedwarf(spectraltypeK7V/M0V),with days,whilenoothersignificantpeakisvisible.Theone-yearand
no obvious infrared excess (Moshir1990, IRAS Faint Source thetwo-yearaliases(at5.79dand5.84d)arealsopresentinthe
RejectCatalog,Version2.0). periodogram,butathalfthepowerofthemainsignal.
Theestimationofthemetallicityinlate-typedwarfsiscom- Infigure3weshowtheKeplerianfitasafunctionofphase
plicated by the abundance of molecular bands of various metal togetherwiththedata.Theorbitisevident,althoughnoisy.
oxides in the spectra (e.g. TiO, VO). Neves et al. (2012) have Although the periodogram shows various structures
discussed various calibrations present in the literature and pre- (”bumps”at≈11d,13d,15d,470d,740d),theyarenotsignifi-
2
G.LoCurtoetal.:TheHARPSsearchforsouthernextrasolarplanets
Table1.Observedandinferredstellarparametersofthethreestarsofthisstudy.π:parallax,d:distance,M :absolutemagnitudein
V
V, Bol.corr.:bolometriccorrection, L:luminosity,T :effectivestellartemperature, M :stellarmass,logg:gravityatthestar’s
eff ∗
surface,vsini:projectedrotationalvelocityofthestar,logR(cid:48) :activityindicator(Noyes1984),P (logR(cid:48) ):star’srotationperiod
HK rot HK
estimatedviatheactivityindicator.
Parameter HD103774 HD109271 BD-061339
Hipparcosname HIP 58263 61300 27803
Spectraltype F5V G5V K7V/M0V
V [mag] 7.12±0.01 8.05±0.01 9.69±0.01 (Hog2000,Koen2010)
B−V [mag] 0.503±0.014 0.658±0.002 1.321±0.001 (VanLeeuwen2007)
π [mas] 18.03±0.52 16.21±0.75 49.23±1.65 (VanLeeuwen2007)
d [pc] 55±2 62±3 20±1
M [mag] 3.40±0.06 4.1±0.1 8.15±0.07
V
Bol.Corr. [mag] -0.013 -0.097 -0.782 (Flower1996)
L [L ] 3.5±0.3 2.0±0.3 0.095±0.01
(cid:12)
Teff [K] 6489±77 5783±62 4324±100 (Sousa2011,Ammons2006)
[Fe/H] [dex] 0.28±0.06 0.10±0.05 -0.14±0.17 (Sousa2011,Neves2012)
M [M ] 1.335±0.03 1.047±0.024 0.7 (Girardi2000)
∗ (cid:12)
logg [cgs] 4.26±0.13 4.28±0.10 4.74±0.02 (Girardi2000,Sousa2011)
age [Gyr] 1.05±0.64 7.3±1.2 4.4±4.0 (Girardi2000)
vsini [kms−1] 8.1 2.7 0.57 (Glebocki2005,CORAVEL)
logR(cid:48) -4.85±0.07 -4.99±0.08 -
HK
P (logR(cid:48) ) [days] 7 24 - (Noyes1984)
rot HK
. .
30
er 25
w
o
p 20
d
ze 15
ali
m 10
or
N 5
0
2. 5. 10.0 20. 50. 100.0 200. 500. 1000. 2000. 5000.
. Period [days] .
Fig.2.PeriodogramoftheRVdataforHD103774.Asignificant
peak is visible at ≈ 5.9 days. The topmost dashed line is the
10−4 false-alarm probability, the lower dotted lines correspond
to10−3,10−2,and10−1false-alarmprobabilities.
Fig.4.Variationoftheradialvelocity(upperpanel)andtheac-
. HD103774 harpsmean . tivityindex(lowerpanel)ofHD103774withtime.
40
20
Becausewehaveenoughdatapoints,weseparatelyanalyzed
s] 0 threegroupsofRVdatainthreedisjointJuliandatebins.From
m/ orbital fitting we have obtained the same periods for the three
RV [ −20 groups of data within the experimental uncertainty of ≈ 10−3
days. The same is true for the phase of the orbit, although in
−40 this case the uncertainties are larger (≈ 2 days). Therefore the
detectedperiodisconstantwithinthetimespanofthemeasure-
−60
ment,i.e.,7.5years.
−0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 The period of ≈ 5.9 days isquite close to the estimatedro-
f
tationperiodofthestar(Noyes1984)of≈7days.However,the
. .
lack of periodicity of the bisector span, together with the lack
Fig.3.KeplerianfitoftheradialvelocitydataofHD103774as ofcorrelationbetweenbisectorandradialvelocity,andthefact
afunctionoforbitalphase. thattheorbitalperiodisstrictlyconstantalongthe7.5yearsof
datagivesusconfidencethatthemostlikelycauseoftheradial
velocity variation with the period of ≈ 5.9 days is a low-mass
cant,andweseenocorrelationoftheradialvelocitieswiththe companion.
activity indicators S (Baliunas1995) or log(R ) (Pearson When looking at the residuals we still notice a significant
MW HK
coefficient < 0.1), nor with the bisectors or the FWHM. The scatter of the data. Moreover, the periodogram of the resid-
time series of the activity index log(R ) shows strong vari- uals shows an excess of power corresponding to a period of
HK
ations of the index with time (Fig. 4), indicating that the star ≈ 750 days, jointly with its one-year alias at ≈ 250 days. We
activitylevelmightincreaseconsiderably. significantly improved the overall fit to the data by fitting two
3
G.LoCurtoetal.:TheHARPSsearchforsouthernextrasolarplanets
. .
-4.70
er 25
w
-4.75 o 20
p
d
e 15
z
R)HK -4.80 Normali 1 50
(10 -4.85 0
g 2. 5. 10.0 20. 50. 100.0 200. 500. 1000. 2000. 5000.
o
l -4.90 .. Period [days] ..!
-4.95
er 20
w
-5.00 po 15
-40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 d
e
Residuals from single keplerian fit (m/s) aliz 10
m
Fig.5. Activity index log(R ) of HD103774 as a function of or 5
HK N
theresidualsfromthesingle-Keplerianfit.Astrongcorrelation
0
isevident.ThevalueofthePearsoncorrelationcoefficientis0.6. 2. 5. 10.0 20. 50. 100.0 200. 500. 1000. 2000. 5000.
.. Period [days] ..
Keplerians.However,wedetectedastrongcorrelationbetween
10
the radial velocity residuals to the single-Keplerian fit and the er
w
activity index log(R ) (Fig. 5) with the value of the Pearson o 8
HK p
correlationcoefficientwhichisequalto0.6,andfoundapeakin ed 6
z
the periodogram of the activity index corresponding to this pe- mali 4
riod.Forthesereasonsweattributethevariationoftheresiduals Nor 2
totheone-Keplerianfittostellarmagneticcycles(Santos2010).
0
2. 5. 10.0 20. 50. 100.0 200. 500. 1000. 2000. 5000.
Thehighlyvaryinglog(RHK)reachesvaluesashighas−4.7, . Period [days] .!
lendingfurthersupporttothesuggestionthattheactivityofthis
early-type, relatively young star could be responsible for the Fig.6. Periodograms of HD109271. From top to bottom: peri-
large χ2 and the scatter of the residuals around the one-planet odogram of the data, the residuals after the one-planet fit, and
fit. theresidualsafterthetwo-planetsfit.Thedashedlinemarksthe
10−4 FAP, the dotted lines, from top to bottom, the 10−3,10−2,
and10−1FAP.
3.2. HD109271
WemeasuredtheradialvelocityofthestarHD10927195times
in7.5years.TheRVvariesby6ms−1RMS,wellinexcessofthe of each other. When constraining the analysis to consider time
average RV error, which is below 1ms−1. The periodogram of spansshorterthanoneyearbutlongerthan200days,bothpeaks
theobserveddataisshowninthetoppaneloffigure6,andtwo disappear, and they reappear together only when the time span
peaks are clearly visible, one of which is above the 10−4 FAP is longer than ≈ 600 days, an indication that the signal could
level.WefitaKepleriantothissignalandobtainedagoodfitfor be the peak at ≈ 430 days, and the one at ≈ 198 days could
aperiodof≈7.85days. be the alias. The third signal, which could be associated to a
1.3 Neptune-mass planet on a 1 AU eccentric (e≈ 0.36) or-
After fitting the first period, looking at the periodogram of
bit, is very noisy and at the limit of detectability by our survey
the residuals (figure 6, middle panel) we notice a second peak,
(K1 < 2ms−1,< δRV >≈ 1ms−1). Although we cannot asso-
correspondingtothesecond-highestpeakintheperiodogramof
ciatethissignaltostellaractivity,weneedalargerdatasetand
the observations, above our FAP limit. This signal corresponds
toavariationwithaperiodof≈30.98days. a higher precision to eventually confirm it as a companion to
HD109271.
The phased radial velocity curves of the two planets result-
ing from a two-Keplerians (simultaneous) fit and after subtrac-
tion of the RV signal of the other planet are shown in figure 7.
3.3. BD-061339
Neither of the signals is correlated with the bisector, with the
activity index, or the FWHM of the CCF; we also verified that We collected 102 radial velocity measurements of the star BD-
theperiodogramsoftheseparametershavenopoweratperiods 061339 in about eight years . The average RV uncertainty is
corresponding to the identified orbital periods. Moreover, both 2.7 ms−1. Figure 8 shows the periodogram of the observations,
signals were identified using the entire data set and also using clearlyindicatinganexcessofpowerat≈125days,wellabove
three disjoint subgroups of the entire data set, indicating that the10−4FAPlevel.
theyarepresentduringthewholetimespanofourobservations. A Keplerian fit to the data allowed us to reconstruct an or-
Therefore we consider the origin of the two signals to be two bitwhoseRVvariationasfunctionoftheorbitalphaseisshown
low-masscompanionsorbitingthestarHD109271. inFigure9.Theorbitalfitisveryrobust,nocorrelationisseen
After the two-planets fit to the HD109271 RV data, the pe- betweentheradialvelocitiesandtheactivityindex,thebisector
riodogram of the residuals shows two signals above our FAP spanortheFWHMoftheCCF,andsubsamplesofthedatase-
threshold at P≈ 198d and P≈ 430d (figure 6, bottom panel). lectedinJuliandateshowaperiodicRVvariationwiththesame
Within the uncertainties, these periods are the one year alias periodwithintheuncertainties.
4
G.LoCurtoetal.:TheHARPSsearchforsouthernextrasolarplanets
. .
. HD109271 harpsmean .
15
er
5 w
o
p 10
RV [m/s] 0 malized 5
or
N
−5
0
2. 5. 10.0 20. 50. 100.0 200. 500. 1000. 2000. 5000.
−0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 . Period [days] .
φ
.. HD109271 harpsmean ..! Fig.10.PeriodogramoftheresidualstothefirstKeplerianfitto
the of BD-061339 RV data. A signal at P≈ 3.9 days is visible
5 justabovethe10−4FAPlevel.
V [m/s] 0
R
−5
−0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
φ
. .!
Fig.7.RadialvelocitycurvesofplanetsHD109271b(toppanel)
andHD109271c(bottompanel)asafunctionofphase.
. .
15
er
w
o
d p 10
e
z
mali 5 Fig.11. Evolution in time of the integrated flux in the Ca II H
or line of BD-061339. The star appears much less active during
N
thelastobservationcampaigns.Theredpoints(darker)represent
0
2. 5. 10.0 20. 50. 100.0 200. 500. 1000. 2000. 5000. theamplitudeoftheCaIIHre-emissionnormalizedat6540Å,
. Period [days] . while the green points (lighter) represent the S index. The
MW
latterone isheavily affectedby thelow S/Nin theblue sideof
Fig.8.PeriodogramoftheobservationsofBD-061339.Thepe-
riod at ≈ 125 days is clearly visible well above the 10−4 FAP thespectrum.
level.
. BD−061339 harpsmean . days due to the many periods covered through the entire ob-
servations campaign. Because of the quite red color of the star
10 (B−V = 1.321),theactivityindicatorlog(R )isnotreliable.
HK
In an attempt to study the activity of this evolved star, we ob-
5
servedtheevolutionofthere-emissionintheCaIIHandKlines
m/s] 0 withtime.Toquantifythere-emissionwenormalizedthespec-
V [ tratothecontinuum.AssuggestedbyMould(1976),theregion
R
−5 around6540ÅisonlyweaklyaffectedbyTiObands.Bydirect
inspectionweverifiedthatthecontributionoftheTiObandsat
−10 this wavelength for this star is indeed negligible, and that the
widthoftheH lineissuchthatitdoesnotaffecttheregionwe
α
−0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 plannedtousetoestimatethecontinuum.Thenormalizedinte-
f grated flux of the Ca II H line as function of time is shown in
. . figure11.Itdropsbyafactor≈ 2betweenJD2454000andJD
Fig.9. Orbital fit of the 125-day signal to the BD-061339 RV 2456000.Infigure11wealsoshowthetimedependenceofthe
data. SMW index: this index drops as well as a function of time, al-
thoughtheslopeissignificantlysmaller.TheS indexisnor-
MW
malized to the continuum much closer to the re-emission line,
TheperiodogramoftheresidualsshowsanexcessatP≈3.9 but the signal-to-noise ratio (S/N) is very low in this region,
dayswiththesignaljustabovethe10−4FAPlevel. andthesubstantialphotonnoisestronglyinfluencesthemeasure-
The peak in the periodogram at ≈ 3.9 days is well defined, ment.Forthisreasonweattemptedthenormalizationat6540Å.
andthefitwithatwo-planetsmodelconvergesandsignificantly The star is becoming less active in recent times. Figure 12
improvestheχ2.TheF-testgivesa99%probabilityofimprove- shows the re-emission in the Ca II H line in April 2007 (JD =
mentofatwo-Kepleriansimultaneousfitoveraone-Keplerian. 2454200)andSeptember2012(JD=2456200).Theintensityof
The uncertainty on the period from the fit is less than 10−3 thelinealmosthalvedduringthisperiod.
5
G.LoCurtoetal.:TheHARPSsearchforsouthernextrasolarplanets
100000
10000 1m/s limit 3m/s limit 10m/s limit
Time span limit (9 years)
1000
ys)
a
d
d ( 100
o
eri
P
10
1
All discovered exoplanets
Fig.12. Ca II H re-emission from two spectra of BD-061339 Solar system planets
HARPS planets from this program
acquiredinApril2007andSeptember2012.Theintensityofthe HARPS planets from this work
0.1
lineofthemorerecentspectrumisreducedalmostbyhalf. 1 10 M sin ( i1)0 0 (M ) 1000 10000
2 Earth
. .
BD−061339 harpsmean Fig.14.Mass-perioddistributionoftheRV-detectedplanets.The
crosses represent all detected planets, the filled triangles the
15
planets from this program, the asterisks the solar system plan-
ets,andthefilledcirclestheplanetspresentedinthiswork.
10
m/s] 5 activity, at other times detection of planets around these stars
V [ maybecomepossible.
R 0
−5
−10 4. Discussion
−0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
f We have identified five planetary signals with msin(i) going
from0.6to7timesthemassofNeptune.Assumingtheylieinthe
. .
sameregionoftheradius-massspaceastheconfirmedtransiting
Fig.13. Radial velocity as function of the orbital phase of the
planets(Schneider2012),wecanestimatetheirradiusandthere-
short-periodplanetofthestarBD-061339.
foretheirdensity.Withthisassumption,weobtainamaximum
densityforthesenewlydiscoveredplanetsthatiscomparableto
Neptune’s or Jupiter’s mean density, making them most likely
gaseousplanets.
Becauseofthevariationoftheactivitylevelofthestarwith
Observingthecurrentdetectionsinthemass-perioddiagram
time,wedividedourdatasetintotwosubsamples:onecontain-
(figure 14), we see that the planets presented in this paper are
ing data acquired before JD = 2454500 and the other being its closetothedetectionlimitofoursurvey,ofabout≈3ms−1,and
complement.Thesesubsamplescontainaboutthesamenumber
thattwoofthemfallinthelow-populatedregionofintermediate
of measurements, therefore our ability to identify the periodic periods, between ≈ 10 and ≈ 200 days. This region is indeed
variation induced by the potential second planet should be the
mostly occupied by planets detected in multiple planetary sys-
sameinbothofthem(thesamplingisroughlysimilar).Thenor-
tems. It makes us wonder whether the presence of more than
malized power at the peak in the periodogram goes from 3.9
oneplanetbreaksacertainsymmetrythatthenallowstheplanet
(arbitrary units) for observations taken before JD=2454500 to
tooccupyarangeofotherwise”forbidden”orbitalperiods.
10.1 for observations taken after this date, the power increases
The periods of the two Neptune-mass planets orbiting
foreachnewaddeddatapoint.Theperiodstaysconstantwithin
the star HD109271 are near a 1:4 resonance, but this would
one sigma in the two datasets. It is worth underlining that the
require a thorough dynamical analysis to be confirmed. The
uncertaintyontheperiodissmallalsoforthetwoseparatesub-
planetary system orbiting the star HD109271 is very similar to
samples:lessthan10−2days.
the HD69830 system (Lovis2006): the periods of the planets
Adding the fact that we see no correlation between the RV b and c differ only by 10% and 2% respectively, while the
residuals to the one-planet fit and the FWHM of the CCF or planet masses differ by less than a factor of two, which is an
the bisector span, nor do we see significant power in the peri- indication that there seem to be some ”preferred architectures”
odogramoftheseparametersaroundthe≈ 3.9daysperiod,we among planetary systems. Whether this is real or an effect of
gainconfidencethatthesecondsignal,whichisbecomingmore ourlimitedunderstandingoftherealityisanopenquestion.
evident as the star becomes less active, is indeed a planet. The
RVasafunctionoftheorbitalphaseisshowninfigure13. TheouterplanetofthesystemorbitingthestarBD-061339
This case is nice and instructive, and clearly shows how a is particularly interesting because it falls right in the middle of
planetary signal emerges from the noise of an active star when the habitable zone (HZ) of its star. Following the method out-
theactivityleveldrops.Italsoremindsusthat”active”starsare linedbyKane(2012),weestimatethehabitablezoneofthestar
not necessarily always active, and that if at some time they are toextendfrom0.5to0.9AU.Theorbitofplanetc,evenconsid-
inappropriate for a planet search because of their high level of eringitsrelativelyhigheccentricityof0.31,liesentirelywithin
6
G.LoCurtoetal.:TheHARPSsearchforsouthernextrasolarplanets
Table2.Orbitalparametersfortheplanets. P:period,T:timeatperiastronpassage,e:eccentricity,ω:argumentofperiastron, K:
semi-amplitudeoftheradialvelocitycurve,msini:projectedplanetarymass,a:semi-majororbitalaxis,V:averageradialvelocity,
N :numberofmeasurements,<δRV>:averageuncertaintyontheradialvelocitymeasurements,σ(O-C):rootmeansquareof
meas
theresidualsfromtheKeplerianfit.
Parameter HD103774b HD109271b HD109271c BD−061339b BD−061339c
P [days] 5.8881±0.0005 7.8543±0.0009 30.93±0.02 3.8728±0.0004 125.94±0.44
T [JD-2400000] 55675.4±2.0 55719.1±3.6 55733±8 55220.50±0.10 55265.2±17.6
e 0.09±0.04 0.25±0.08 0.15±0.09 0.0(fixed) 0.31±0.11
ω [deg] -42±64 -64±20 4±120 0.0(fixed) 41±55
K [ms−1] 34.3±1.8 5.6±0.5 4.9±0.4 4.4±0.6 9.1±2.9
msini [M ] 0.367±0.022 0.054±0.004 0.076±0.007 0.027±0.004 0.17±0.03
Jup
msini [M ] 116±7 17±1 24±2 8.5±1.3 53±8
Earth
a [AU] 0.070±0.001 0.079±0.001 0.196±0.003 0.0428±0.0007 0.435±0.007
V [kms−1] -3.049±0.003 -4.903±0.001 -4.903±0.001 23.625±0.003 23.625±0.003
N 102 87 87 102 102
meas
Timespan [days] 2734 2683 2683 2955 2955
<δRV> [ms−1] 2.6 0.9 0.9 2.7 2.7
σ(O-C) [ms−1] 11.43 2.05 2.05 4.3 4.3
χ2 15.7±0.6 4.6±0.3 4.6±0.3 2.47±0.23 2.47±0.23
red
theHZ.Dependingonthealbedo(intherange0.4-0.9)andon rence is generally low. Interestingly, most of the planets with
theatmosphericmodel(efficient/inefficientheattransport)the periodsinthisrangebelongtomultipleplanetarysystems.
surfacetemperatureoftheplanetcanbebetween0oC and100 Owingtothesmallamplitudeofthesignalsandtotheprox-
oC.Althoughsuggestive,thisisasimplisticapproachhowever, imityoftheiramplitudes,approximately100datapointspersys-
because it does not take into account that the albedo as well as temwerenecessarytoachievethecurrentdetections,whichsup-
thestellarfluxvarywithwavelength.Moreover,duetoitsmass, ports ample time coverage and high-cadence monitoring as the
theplanetislikelytobeagasplanet,makingconditionsforlife key elements to access some of the most interesting planetary
probablymoredifficult. detections.
Acknowledgements. WethankthestaffoftheLaSillaObservatoryatESOin
5. Conclusions Chilefortheirpassionateandprofessionalsupport.
This research has made use of the VizieR catalogue access tool, CDS,
Wehavepresentednewresultsfromournine-yearlow-precision Strasbourg,France.TheoriginaldescriptionoftheVizieRservicewaspublished
(2 − 3ms−1) survey of a volume-limited sample of the solar inA&AS143,23.
NCS acknowledges the support by the European Research
neighborhoodwithHARPS.Wedetectedfiveexo-planetsorbit- Council/European Community under the FP7 through Starting Grant
ing three low-activity, slowly rotating solar-type stars. Four of agreement number 239953, as well as from Fundac¸a˜o para a Cieˆncia e a
thedetectedplanetsareintwomulti-planetsystems.Theplanets Tecnologia (FCT) through program Cieˆncia2007 funded by FCT/MCTES
havemsin(i)rangingfrom0.6to7Neptunemasses,thelightest (Portugal) and POPH/FSE (EC), and in the form of grants reference
PTDC/CTE-AST/098528/2008andPTDC/CTE-AST/098604/2008.
beinginthesuper-Earthmassrange,underliningthatoursurvey,
WethanktherefereeMartinKu¨rsterforthehelpfulcommentswhichcon-
albeitlimitedinprecisionbecauseofastrategicalchoice,hasthe tributedtoimprovethispaper.
capabilitytodetectNeptune-massandinsomecasesevensuper-
Earthplanets.
The star BD-061339 hosts two planets: in the case that References
sin(i) = 1, they would be a super-Earth, possibly in a tidally
Ammons,S.M.,etal.,2006,ApJ,638,1004A
locked configuration due to its proximity to the host star, and Baliunas,S.L.etal.,1995,ApJ,438,269
a planet of ≈ three Neptune masses in the habitable zone. The ESA,1997,ESA-SP1200“TheHIPPARCOSandTYCHOcatalogue”
case presented by this star is particularly instructive because it Fischer,D.etal.,2012,ApJ,745,21
Flower,P.J.,1996,ApJ,469,355
shows how one of the signals became more clear with time, as
Girardi,L.etal.,2000,A&AS,141,371
thestellaractivitywasdiminishing.
GlebockiR,etal.,Vizieronlinedatacatalog,2005,ESASP-560,571
ThestarHD103774,hostingoneplanetwithmsin(i)roughly He´brard,G.etal.,2010,A&AS,512,A46
the mass of Neptune, has also shown signs of varying activity Hog,E.etal.,2000,A&A,355,L27
during the nine years of our monitoring campaign. A reminder Ishihara,D.etal.,2010,A&A,514A,1I
Kane,S.etal.,2012,PASP,124,914
that the stellar activity changes with time, and stars that at one
Koen,C.etal.,2010,MNRAS,403,1949K
timeareunsuitableforhigh-precisionRVmeasurements,might LoCurto,G.etal.,2010,A&A,512,A48
become more quiet at another time and offer the possibility to Lovis,C.etal.,2006,Nature,441,305
acquirepreciseRVdata. Lovis,C.etal.,2011,A&A,528,A112
Mayor,M.,Queloz,D.,1995,Nature,378,355
ThestarHD109271hoststwoplanetswithmsin(i)approxi-
Mayor,M.etal.,2003,TheMessenger,114,20
matelythemassofNeptuneandwithorbitalperiodsandmsin(i) Moshir,M.etal.,1990,IRASF.C,0M
very similar to planets b and c around the star HD698430 Mould,J.R.,1976,ApJ,207,535
(Lovis2006). This fact suggests that some planetary architec- Moutou,C.etal.,2009,A&A,496,513
turesmightbe”preferred”inNature. Naef,D.etal.,2007,A&A,470,721
Naef,D.etal.,2010,A&A,523,A15
Two out of the five planets we detected have periods in the
Neves,V.etal.,2012,A&A,538,A25
range between 10 and 200 days, a region where planet occur- Noyes,R.W.etal.,1984,ApJ,279,763
7
G.LoCurtoetal.:TheHARPSsearchforsouthernextrasolarplanets
Pickles,A.,DePagne,E.,2010,PASP,122,898,1437
Santos,N.C.etal.,2010,A&A,511,A54
Schneider,J.,http://exoplanet.eu,15October2012
Segransan,D.etal.,2011,A&A,535,A54
Sousa,S.etal.,2011,A&A,533A,141S
VanLeeuwen,F.etal.,2007,“ValidationofthenewHipparcosreduction”,2007,
A&A,474,653
Zechmeister,M.,Kuerster,M.,2009,A&A,496,577Z
8