ARTEMiS (Automated Robotic Terrestrial Exoplanet Microlensing Search) - A possible expert-system based cooperative effort to hunt for planets of Earth mass and below

0.34 MB·English

by  M. Dominik

Astron. Nachr./AN,No.,1–4(2007)/DOI
ARTEMiS (Automated Robotic Terrestrial Exoplanet Microlensing
Search) – A possible expert-system based cooperative effort to hunt for
planets of Earth mass and below
M. Dominik1,⋆,⋆⋆ K. Horne1, A. Allan2, N.J. Rattenbury3, Y. Tsapras4, C. Snodgrass5, M.F. Bode4,
M.J. Burgdorf4, S.N. Fraser4, E. Kerins3, C.J. Mottram4, I.A. Steele4, R.A. Street6, P.J. Wheatley7,
Ł.Wyrzykowski8,9
1 SUPA, University of St Andrews, School of Physics & Astronomy, North Haugh, St Andrews, KY16 9SS, United
8
Kingdom
0
2 SchoolofPhysics,UniversityofExeter,StockerRoad,ExeterEX44QL,UnitedKingdom
0
2 3 JodrellBankObservatory,Macclesfield,Cheshire,SK119DL,UnitedKingdom
4 AstrophysicsResearchInstitute,LiverpoolJohnMooresUniversity,TwelveQuaysHouse,EgertonWharf,Birkenhead,
n
CH411LD,UnitedKingdom
a 5 EuropeanSouthernObservatory(ESO),Casilla19001,SantiagodeChile,Chile
J
6 LasCumbresObservatoryGlobalTelescopesNetwork,6740BCortonaDr,Goleta,CA93117,UnitedStatesofAmerica
4 7 DepartmentofPhysics,UniversityofWarwick,Coventry,CV47AL,UnitedKingdom
1
8 InstituteofAstronomy,UniversityofCambridge,MadingleyRoad,CambridgeCB30HA,UnitedKingdom

] 9 WarsawUniversityAstronomicalObservatory,Al.Ujazdowskie4,00-478Warszawa,Poland
h
p
Thedatesofreceiptandacceptanceshouldbeinsertedlater
-
o
r
Thetechniqueofgravitationalmicrolensingiscurrentlyuniqueinitsabilitytoprovideasampleofterrestrialexoplan-
t
s etsaroundbothGalacticdiskandbulgestars,allowingtomeasuretheirabundanceanddeterminetheirdistributionwith
a
respecttomassandorbitalseparation.Thus,valuableinformationfortestingmodelsofplanetformationandorbitalmi-
[
grationisgathered,constitutinganimportantpieceinthepuzzlefortheexistenceoflifeformsthroughouttheUniverse.
1 Inordertoachievethesegoalsinreasonabletime,awell-coordinatedeffortinvolvinganetworkofeither2mor4×1m
v telescopesateachsiteisrequired.ItcouldleadtothefirstdetectionofanEarth-massplanetoutsidetheSolarsystem,and
2 evenplanetslessmassivethanEarthcouldbediscovered.FromApril2008,ARTEMiS(AutomatedRoboticTerrestrialEx-
6 oplanetMicrolensingSearch)isplannedtoprovideaplatformforathree-stepstrategyofsurvey,follow-up,andanomaly
1 monitoring.AsanexpertsystemembeddedineSTAR(e-ScienceTelescopesforAstronomicalResearch),ARTEMiSwill
2 giveadviceforfollow-upbasedonapriorityalgorithmthatselectstargetstobeobservedinordertomaximizetheex-
. pectednumberofplanetdetections,andwillalsoalertondeviationsfromordinarymicrolensinglightcurvesbymeans
1
oftheSIGNALMENanomalydetector.WhiletheuseoftheVOEvent(VirtualObservatoryEvent)protocolallowsadi-
0
rectinteractionwiththetelescopesthatarepartoftheHTN(HeterogeneousTelescopeNetworks)consortium,additional
8
interfacesprovidemeansofcommunicationwithallexistingmicrolensingcampaignsthatrelyonhumanobservers.The
0
successofdiscoveringaplanetbymicrolensingcriticallydependsontheavailabilityofatelescopeinasuitablelocationat
:
v therighttime,whichcanmeanwithin10min.Toencouragefollow-upobservations,microlensingcampaignsaretherefore
i releasingphotometricdatainrealtime.Onongoingplanetaryanomalies,world-wideeffortsarebeingundertakentomake
X surethatsufficientdataareobtained,sincethereisnosecondchance.Real-timemodellingofferstheopportunityoflive
r discoveryofextra-solarplanets,therebyproviding“Sciencelivetoyourhome”.
a
(cid:13)c 2007WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
1 Detecting planets by microlensing ofitslightbythegravitationalfield oftheforegroundstar,
constitutingagravitationalmicrolensingevent.
IfanobservedstarintheGalacticbulgeatdistanceDShap- Ifmoreoveraplanetorbitingtheforeground’lens’star
pens to be aligned with a foregroundstar with mass M at happens to be separated by an angle δ ∼ θE, it can re-
distanceDLwithinanangleθ <∼θE,where vealits existence by meansof a shortdistortionto the ob-
servedlightcurve(Mao&Paczyn´ski1991),dependingon
θE =r4Gc2M DL−1−DS−1 (1) istisdmesatshsrleaestointhgebrectuwrereenntscelvaeimraslh(Bouornsdanetdasle.v2e0ra0l4d;aUyds.aBlsek-i
(cid:0) (cid:1)
etal.2005;Gouldetal.2006),microlensingledtothedis-
istheangularEinsteinradius,itexhibitsatransientbright-
coveryofOGLE-2005-BLG-390Lb,thefirstcoolrocky/icy
ening(Einstein1936)overaboutamonthduetothebending
exoplaneteverfound(Beaulieuetal.2006;Dominik,Horne
&Bode2006).
⋆ RoyalSocietyUniversityResearchFellow
⋆⋆ Correspondingauthor:e-mail:[email protected] Microlensingobservationscanprovidesamplesofplan-
ets orbiting stars in two distinct populations, namely the
(cid:13)c 2007WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
2 M.Dominiketal.:ARTEMiS–AnexpertsystemtohuntforplanetsofEarthmassandbelow
Galactic disk and the Galactic bulge, rather than facing a event OGLE 2005-BLG-390 over about a day, which was
restriction to the Solar neighbourhood.This will allow an showntobeduetoaplanetofabout5Earthmasses,orbiting
estimate of the abundance of planets in the Universe and astarwith0.2M at3AUwithaperiodof10years,where
⊙
provideapowerfultestoftheoreticalmodelsofplanetfor- all these values are uncertain to a factor of two (Beaulieu
mationandorbitalmigration. et al. 2006).An Earth-massplanetin the same spot would
Given the properties of the two stellar populations, a stillhavecausedasignalamplitudeof3%anddurationof
typicalmicrolensingeventonanobservedbulgestaratDS ∼ ∼ 12h.Ifoneassumesphotometricuncertaintiesof∼ 1%,
8.5 kpc involves a lens star with M ∼ 0.3 M⊙ at DL ∼ thediscoveryofsuchaplanetwouldonlyhavebeenpossi-
6.5kpc.ThisimpliesthatθE ∼350µas,sothatmicrolens- ble if the standardfollow-upsamplingof 2 h had been re-
ing is most sensitive to the detection of planets at a sepa- placed by high-cadence (10–15 min) anomaly monitoring
rationof1to10AU.Sincetherespectiveorbitalperiodby triggereduponthe firstsuspicionofa deviation.Real-time
far exceeds the duration of the planetary signal, the latter photometryand a promptresponse fromthe telescopes al-
reflectsasnapshotoftheplanetatitscurrentposition. lowtheSIGNALMEN anomalydetectortoidentifyongo-
ing anomalies by successively requesting further observa-
tions until an anomaly can be confirmed or rejected with
2 Microlensing observationcampaigns
therequiredsignificance.Bytriggeringonresidualswhose
absolute value is among the largest 5% of all data for the
Withonlyaboutoneinamillionmonitoredstarsbeingsig- respectivesite(suchtriggerpointsbeingmarkedbyarrows
nificantly brightened by the gravitational field of a fore- in Fig. 1), and eliminating the effect of outliers by means
groundstar(Kiraga&Paczyn´ski1994),theOGLE(Optical of robust-fitting techniques, SIGNALMEN carefully ad-
Gravitational Lensing Experiment) and MOA (Microlens- dresses the fact that reported photometric error bars fre-
ingObservationsinAstrophysics)surveysmonitormorethan quentlydonotproperlyrepresentthetrueuncertaintiesand
100millionstarsonadailybasis,whichresultsin700-1000 ingeneraldonotfollowaGaussiandistribution.
microlensing events per year being alerted on-line while
Thegiantsourcestar(R ∼9.6R )thatwasobserved
theyareinprogress(Udalskietal.1992;Murakietal.1999; ⋆ ⊙
inOGLE2005-BLG-390yieldedalargerprobabilitytode-
Bondetal.2001;Udalski2003).Theirsamplingishowever
tect a planetary signal and increased its duration, but re-
insufficient for detecting planets with masses significantly
duced its amplitude as compared to a main-sequence star.
belowthatofJupiter(Snodgrass,Tsapras,&Horne2004).
However,the SIGNALMEN anomalydetectorwouldalso
Thefirstmicrolensingfollow-upnetworkcombininghourly
allowtorevealanEarth-massplanetfroma5%deviationif
sampling with a round-the-clockcoveragewas established theobservedsourceisamain-sequencestar(R ∼1.2R ),
⋆ ⊙
byPLANET(ProbingLensingAnomaliesNetwork)in1995
provided that exposure times are chosen long enough for
(Albrow et al. 1998;Dominiket al. 2002).While this net-
achievinga photometricaccuracyof1–2%. While the mi-
workof1m-classtelescopesreliesonhumanobserversand
crolensingsearchesfaceareliablechanceoffirstdetecting
dedicatedobservingtime,thedemandofnotonlyanimme-
an extra-solar planet of Earth mass, the detection of plan-
diateresponse,butalsoaflexibleschedulingmakesrobotic
etswithmassesassmallas0.1M ischallengingbothby
⊕
telescopesideallysuitedtocarryoutsuchanobservingpro-
meansof the shortsignal durationand the tiny probability
gramme.Since 2004– andsince 2005in cooperationwith
forsignalsofappropriateamplitudestooccur,butpossible
PLANET –, microlensing observations have been carried
inprinciple(Dominiketal.2007).
outwith the RoboNet-1.0networkof UK-built2mrobotic
telescopes(Burgdorfetal.2007),usingapriorityalgorithm
that selects those targets for which a detection of a planet
4 The ARTEMiS concept (2008+)
is most likely to occur. In contrast to PLANET/RoboNet,
the MicroFUN team concentrateon a few quite promising
events, with a network only being activated on target-of- The detection of a significant number of terrestrial extra-
opportunitybasis. solarplanetsinreasonabletimerequiresawell-coordinated
With the deployment of the SIGNALMEN anomaly effort involving a network of either 2m or 4 × 1m tele-
detector (Dominik et al. 2007), a combined effort of mi- scopes at its sites and further smaller telescopes, in order
crolensing campaigns can realize a three-step strategy of toensurethatsufficientdatacanbeacquiredonanongoing
survey,follow-up,andanomalymonitoringthatallowsfora planetary anomaly, which could require that observations
substantialdetectionefficiencyevenforEarth-massplanets. are being scheduled at a suitable site within 10 min of an
alertbytheSIGNALMENanomalydetector.Comprisedof
theLiverpoolTelescope(LT),theFaulkesTelescopeNorth
3 Discovering planets ofEarth massand
(FTN), and the Faulkes Telescope South (FTS), RoboNet-
below 1.0 constituted the prototype of such a network, with the
two Faulkes telescopes now having been acquired by Las
On2005August10,PLANET/RoboNet,OGLE,andMOA CumbresObservatory,andfurthersuchinstrumentscanbe
observeda15%deviationtothelightcurveofmicrolensing expectedtobedeployedoverthenextthreeyears.
(cid:13)c 2007WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2007) 3
Fig.1 ThepossibledetectionofplanetsofEarthmassorbelow(simulations)inthreedifferentconfigurations(whered=δ/θE)with
therobotic2m-telescopesthatconstitutetheRoboNet-1.0network,namelytheLiverpoolTelescope(LT),theFaulkesTelescopeNorth
(FTN),andtheFaulkesTelescopeSouth(FTS),augmentedbytwosimilarhypothetical telescopeslocatedinChileandSouthAfrica.
ArrowsindicatetheepochswheretheSIGNALMENanomalydetectorrequestedfurtherobservations.(top)1-M⊕planetinthesame
spotasOGLE-2005-BLG-390Lb,withtheoriginalmodellightcurvefortherespectiveeventalsoplotted;(middle)1-M⊕planetinthe
samespot,butwithamain-sequencesourcestar;(bottom)0.1-M⊕planetatacloserdistance(d=1.25insteadofd=1.61)tothelens
star,andamain-sequencesourcestar.Forthetwolattercases,theorientationangleofthesourcetrajectorywithregardtotheplanet-star
axishasbeenmodifiedfromtheOGLE-2005-BLG-390modelinordertoproduceadeviationofthedesiredamplitude.
www.an-journal.org (cid:13)c 2007WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
4 M.Dominiketal.:ARTEMiS–AnexpertsystemtohuntforplanetsofEarthmassandbelow
Fig.2 ARTEMiS(AutomatedRoboticTerrestrialExoplanetMicrolensingSearch)anditsinteractionswiththeoutsideworld.
Acting as an expert system, ARTEMiS will determine alizationandinterpretationnotonlyallowsto link upwith
the optimal target to be observed by any site at any given professionalandamateurastronomersaroundtheworld,but
time bymeansofreal-timemodellingof real-timedatare- evenoffersanopportunitytocommunicateforefrontresearch
leasedbythemicrolensingobservingcampaignsandsubse- in progress to the general public as “Science live to your
quentassessmentbyapriorityalgorithmandtheSIGNAL- home”.
MENanomalydetector(seeFig.2).Withaflexiblestrategy
cateringforparametersthatallowobservingsitestodefine
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