Catalogue of ISO LWS observations of asteroids

0.55 MB·

by  Felix Hormuth

Astronomy&Astrophysicsmanuscriptno.lwsaster (cid:13)c ESO2009
January28,2009
⋆
Catalogue of ISO LWS observations of asteroids
F.Hormuth1,2 andT.G.Mu¨ller3
1 CentroAstrono´micoHispanoAlema´n,C/Jesu´sDurba´nRemo´n2-2◦,04004Almer´ıa,Spain
2 Max-Planck-Institutfu¨rAstronomie(MPIA),Ko¨nigstuhl17,69117Heidelberg,Germany
9
e-mail:[email protected]
0
3 Max-Planck-Institutfu¨rextraterrestrischePhysik(MPE),Giessenbachstrasse,85748Garching,Germany
0
e-mail:[email protected]
2

n Received;accepted
a
J ABSTRACT

8
2 Context.TheLongWavelengthSpectrometer(LWS)onboardtheInfraredSpaceObservatory(ISO)observedthefourlargemain-belt
asteroids(1)Ceres,(2)Pallas,(4)Vesta,and(10)Hygieamultipletimes.Thephotometricandspectroscopicdatacoverthewavelength

rangebetween43and197µm,andareauniquedatasetforfutureinvestigationsanddetailedcharacterisationsofthesebodies.
]
P Aims.Thestandard ISO archiveproducts, produced through the lastpost-mission LWSpipeline, werestillaffectedby instrument
E artefacts.Ourgoalwastoprovidethebestpossibledataproductstoexploitthefullscientificpotentialoftheseobservations.
. Methods.Forallasteroidobservationsweanalysedindetailthedarkcurrent,thecalibrationreferenceflashes,thespaceenvironment
h effects(glitches), memory effects,tracking influences, and various other sources of uncertainty. Weperformed arefinedreduction
p ofallmeasurements,correctedforthevariouseffects,andre-calibratedthedata.Weoutlinethedatareductionprocessandgivean
- overview of theavailable dataand thequality of the observations. Weapply athermophysical model tothe fluxmeasurements to
o
derivefar-IRbaseddiameterandalbedovaluesoftheasteroids.Themeasuredthermalrotationallightcurveof(4)Vestaiscompared
r
t tomodelpredictions.
s Results.ThecatalogueofLWS(LongWavelengthSpectrometer)observationsofasteroidscontains57manuallyreduceddatasets,
a
includingsevennon-standardobservations,whichassuchdidnothavefinalpipelineproductsavailablebefore.Intotal,thearchive
[
nowcontains11spectralscansand46fixedgratingmeasurementswithasimultaneousobservationat10keywavelengthsdistributed

1 over the full LWS range. The new data products are now accessible via the ISO data archive as highly processed data products
v (HPDP)⋆⋆.
7 Conclusions.Thequalityofthedataproductswascheckedagainststate-of-the-artthermophysicalmodelpredictionsandanexcellent
5 agreementwasfound.Theabsolutephotometricaccuracyisbetterthan10%.Thecalibratedspectrawillserveassourceforfuture
5 mineralogicalstudiesofdwarfplanetsanddwarfplanetcandidates.
4
Keywords.Minorplanets,asteroids–Radiationmechanisms:Thermal–Infrared:Solarsystem
.
1
0
9
1. Introduction ISO Handbook, Volume III (Gryetal. 2003). In total, the ISO
0
Data Archive(IDA1) contains51 successful LWS observations
: The Infrared Space Observatory (ISO; Kessleretal. 1996)
v of four different asteroids covering almost 18hours of satellite
i observed between 1995 and 1998 more than 40 asteroids in time.However,thescientificreturnoftheseprogrammeswasso
X
greatdetail,includingsomecompletespectrafrom2to200µm
far verylittle. Mostof the scientific questionscouldnotbe an-
r and large samplesof photometricmeasurements(Mu¨ller 2003;
a sweredmainlyduetotheinstrumentalartefactswhichremained
Dottoetal. 2002). The main goals of the about 100hours of
inthearchiveddataproducts.
asteroid observing time were the identification of surface min-
The LWS data set of asteroid observations covers a wave-
erals, composition, connection to meteorites and comets, sur-
lengthrangewhichisnotaccessiblefromtheground.Inthenear
face alteration processes, and the interpretation of taxonomic
future the Herschel mission will be capable of obtaining aster-
classes through the identification of mid-infrared features of
oid spectra between 57 and 210µm with PACS (Photodetector
well-knownminerals and meteorites. Mu¨lleretal. (2005) sum-
Array Camera & Spectrometer) and between 200 and 670µm
marisedthemainISOresultsfromthevarioussolarsystempro-
with SPIRE (Spectral and Photometric Imaging Receiver), but
grammesandfromallfourISOinstruments.
asteroidsarecurrentlynotpartoftheacceptedkeyprogrammes.
TheISOLongWavelengthSpectrometer(LWS;Cleggetal.
Although the ISO targetlist is very limited, it includesobjects
1996) performedphotometricand spectroscopic measurements
ofhighscientificinterest:(1)Ceresand(4)Vestaarethetargets
inthewavelengthrange43to197µmandopened,togetherwith
oftheDawn2spacecraftmission,(1)Ceresismeanwhileconsid-
ISOPHOT,anewwindowforfar-infraredastronomy.Thedetails
eredasadwarf-planet,and(2)Pallas,(4)Vesta,and(10)Hygiea
on the instrument and the data processing can be found in the
aredwarf-planetcandidates3.Ingeneral,itisbelievedthatthese
Sendoffprintrequeststo:F.Hormuth
⋆ BasedonobservationswithISO,anESAprojectwithinstruments 1 http://www.iso.esac.esa.int/ida/index.html
funded by ESA Member States (especially the PI countries: France, 2 http://dawn.jpl.nasa.gov/
Germany,theNetherlandsandtheUnitedKingdom)andwiththepar- 3 Dissertatio Cvm Nvncio Sidereo III, Nr. 3, August 16, 2006,
ticipationofISASandNASA. http://astro.cas.cz/nuncius/
2 F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids
largestbodiesinthemain-beltareprotoplanetsremainingintact of this data still remainsdifficult, since the relative spectralre-
sincetheirformation. sponsefunction(RSRF) ofthedetectorsusedinthecalibration
The LWS observations comprise spectral energy distribu- processisbasedonobservationsperformedinthestandardL01
tionsoverthefullwavelengthrangebetween43and197µmwith mode.TheremainingL99datasetsdidnotincludesufficientcal-
aspectralresolvingpowerof∼200andcanbeutilisedtosearch ibrationdataforreliabledarkcurrentdetermination.
forspectralsignaturesrelatedtotheasteroids’mineralogiccom- AllstandardobservationsarelistedinTbl.1,whilethenon-
position.Anoverviewofthespectroscopicfeaturesfromtheory standardobservationsaresummarisedinTbl.2.Contamination
and lab measurementsin the infrared wavelength regime up to by158µmCIIbackgroundemissionisindicatedwhereapplica-
25µm can be foundin e.g. Salisbury (1993) and in the context ble.
ofISO inDottoetal. (2002). Farinfraredlaboratoryspectraof Acomprehensiveoverviewoftheinstrumentdesign,observ-
various crystalline and amorphous minerals are digitally avail- ing modes, and detector properties can be found in the LWS
ablee.g.intheHeidelberg–Jena–St.Petersburg–Databaseof handbook(Gryetal.2003).
OpticalConstants(HJPDOC)4 (Henningetal. 1999). Although
thesemeasurementsareinprinciplesufficienttoallowtheanal-
2.2.Observationelements
ysisofthedatapresentedinthispaper,theirapplicationtosolid
surfaceswithnon-uniformgrainsizesandsubsurfaceradiationis Eachobservationconsistsnotonlyoffluxmeasurementsofthe
notstraightforward,renderingamineralogicinterpretationdiffi- targetitself, butalsoincludescalibrationdata,typicallysplitin
cult. two blocks at begin and end. By placing the Fabry-Pero´tspec-
Apartfromthat,theLWSobservationscangivevaluablein- trometerintothebeamandintentionallymisaligningtheetalons,
puttomodelsdescribingthethermalemissionofminorbodiesin lightfromthesourcedidnotreachthedetectors,eventuallyre-
ourownorothersolarsystems,andarekeyingredientstoestab- sulting in a dark currentmeasurement. Long observations, and
lishasteroidsascalibrationobjectsinthemiddleandfarinfrared especially the engineeringmode L99 data, contain intermittent
(e.g.Mu¨ller&Lagerros1998,2002). additionaldarkcurrentmeasurements.
Aneffortwasmadetoproduceahomogeneouslyreducedset Since the responsivity of the detectors is not constant, but
oftheseobservations,eventuallyleadingtoacataloguewhichis usually drifting upwards over time due to impacts of charged
availablethroughtheIDA.Wedescribethecontentsofthecata- particles,theactualresponsivityofeachdetectorhadtobedeter-
logue(Section2),outlinethedatareductionprocess,andgivean minedforeachobservationseparately.Thiswas donebyflash-
overview of the instrumental artefacts (Section 3) encountered ingthedetectorswithinternalilluminatorswhilelightfromthe
duringcataloguecompilation.Section 4 gives a brief overview sourcewasagainblockedwiththeFabry-Pero´t.Thisallowedto
of the catalogue structure. In Section 5 we discuss the quality quantifyresponsivitychangesduringthecourseofa single ob-
ofthefinalproducts,presentdiameterandalbedovaluesderived servation, and to apply the so-called ’responsivity drift correc-
from thermal IR data in the range 43–197µm for the observed tion’.
asteroids,andanalysethethermallightcurveof(4)Vesta. Absoluteresponsivitycalibrationwasperformedbycompar-
ingtheresponsivitiesmeasuredatthetimeoftheobservationto
2. Observations the ones measured during observations of Uranus, the primary
LWScalibrator.
2.1.Overview Forthetargetsourcefluxmeasurements,theFabry-Pero´twas
removedfromthebeam.Dependingontheobservationtemplate
MostoftheLWSasteroidobservationswerecarriedoutinfixed
andrequestedwavelengthcoverage,thegratingremainedeither
gratingmode,usingtheastronomicalobservingtemplate(AOT)
atafixedangleorwasmovedtoscanthedesiredspectralrange.
L02. This produced ten photometric measurements – one in
Thiscouldbedoneinunidirectionalmode,wherethegratingan-
eachdetector–at46.2µm,56.2µm,66.1µm,75.7µm,84.8µm,
gleconstantlyin-ordecreasedandwasresettoitsstartposition
102.4µm,141.8µm,160.6µm,and178.0µm(correspondingto
beforeeachscanrepetition.Alternatively,theobservationcould
detectorsSW1toSW5andLW1toLW5).
be performedin bidirectional mode, with the grating angle in-
OnlyfourfullgratingscansusingthetemplateL01wereper-
creasingandthendecreasingwiththesamerateduringthemea-
formed, covering the whole spectral range from 43 to 197µm
surement.Thisis referredto asup-anddown-scan,where’up’
with typical SNRs of 20–30 for (10) Hygiea and 150–200 for
and’down’doesnotcorrespondtoanincreaseordecreaseofthe
(1)Ceres.Thespectrumiscomposedoftensub-spectrawiththe
observedwavelength,butofthegratingpositionencodervalue.
sub-spectrabeinggeneratedbythegratingscanningovertheten
In fact, during an upscan the observing wavelength decreased,
LWS detectorssimultaneously.Signal-to-noisewas built up by
andincreasedduringdownscans.
takingmorethanonescan.
Additionally there exist 25 observations carried out in the
non-standard engineering mode L99, most of them similar to
3. Datareduction
L01 grating scans (SNR ≈150 for (1) Ceres and ≈100 for (4)
Vesta).Forthecatalogue,onlyL99on-sourcegratingscanswere 3.1.Instrumentalartefacts
considered,andnobackgroundmeasurementsareincluded.The
The most prominentartefacts, commonto all ISO instruments,
non-standardobservationsarecharacterisedbyexperimentalset-
are ’glitches’, sudden increases of the measured flux due to
tings of the detector parameters (bias voltages and heater cur-
chargedparticleshittinga detectororpartsofthereadoutelec-
rents),anunusuallyhighspectralsampling(13samplesperres-
tronics.Theinitialsuddensignalincreaseissometimesfollowed
olution element instead of four, as it was the standard setting
bya‘glitchtail’,along-livedexponentialdecay.Thoughglitches
later),andlongerintegrationtimespergratingposition.
canberecognisedandfilteredoutautomaticallytosomeextent,
From the available L99 observations we selected seven
glitch remnants are present in all observations throughout the
datasets for inclusion in this catalogue. The spectral analysis
archive,regardlessofinstrumentorobservingmode.Glitchesdo
4 http://www.mpia-hd.mpg.de/HJPDOC/ notonlyoccurduringobjectfluxmeasurements,butalsoduring
F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids 3
thecalibration,i.e.darkcurrentandresponsivitymeasurements, artefactsinthecalibrationdatawillaffecttheaccuracyandrelia-
and adversely affect the accuracy and reliability of the actual bilityofthewholeobservation.TheLIAroutineSHORT AALwas
observation.Special attention was therefore put on the manual usedtocreateISAP-compatibleoutputspectraintheLWSAuto-
removalofglitchremnantsineachobservationcontainedinthe Analysisformat,andtoconverttoW/cm2/µm,thefluxunitused
presentedcatalogue. for all LWS observations. This routine automatically strips all
Otherfrequentlyobservedinstrumentalartefactsarefringes calibrationdataandleavesonlythebasicinformationneededfor
in the spectra in the case of extended sources or off-axis ob- further scientific analysis, e.g. wavelength, flux, detector num-
servations,spuriousspectralfeaturesduetouncertaintiesinthe ber,andscandirection.
spectral response function used in the calibration process, and In the case of photometric L02 observations, the data was
memoryeffects,i.e.changesofdetectorresponsedependingon nowaveragedperdetectortocreatethefinalFITSproduct.
theilluminationhistory. In the case of grating scans a first round of manual de-
Observations towards the end of the satellite mission were glitchingwasperformed,probablythemosttimeconsumingstep
affectedbyso-called’warm-upeffects’,visibleasbroadspectral in the reduction process. Relative responsivity correction was
featuresinsomedetectors.Thiswaspresumablycausedbyperi- achievedbycomparingthemeanfluxineachdetectorandscan
odicbreaksoftheliquidheliumfilminthevicinityoftheLWS to the mean flux level of all scans together. The derived gains
strap location as the satellite’s helium tank came close to ex- werethenusedto bringallscansto thesame fluxlevel–sepa-
haustion.Fromtheobservationscontainedinthiscatalogue,this ratelyforeachdetectorandscandirection.
affectsoneL01typeobservationof(1)CerestakeninDecember Afterfine-zappingofremainingglitches,thedatawasaver-
1997, several months before ISO’s liquid helium depletion in aged per detector with a bin width of typically 0.06µm, corre-
April1998. spondingtoanoversamplingoffourwithrespecttothespectral
A thorough description of caveats and unexpected effects resolution.Inthecaseofbidirectionalscansthedatawasnowav-
concerning LWS data can be found in Chapter 6 of the LWS eragedoverthescandirections.Especiallyatlongerwavelengths
handbook (Gryetal. 2003). All datasets included in this cata- somespectraclearlyshowedfringing,probablycausedbyback-
logue were checked for the presence of the artefacts described groundemission. The affected detectorswere defringedwithin
aboveand data quality was quantifiedvia a set of well-defined ISAP,usingonlytheaveragedspectrumforfringedetectionand
flags. The criteria for setting these flags and the results of this defringingtogetthebestpossiblesignal-to-noiseforthefringe
quality check can be found in the technical documentationac- fitting(inthecaseofbidirectionaldata).
companyingthecatalogue(Hormuth&Mu¨ller2006).
4. Dataproductsandaccesstothecatalogue
3.2.Datareductionsteps
Datareductionwasperformedwiththeavailablestandardreduc- TheISODataArchive(IDA)5 containsspectroscopic,imaging,
tionpackagesLIA(LWSInteractiveAnalysis)Version10.2and photometric, and polarimetric measurements of more than 40
ISAP (ISO SpectralAnalysis Package)Version2.2. Additional differentasteroidsatwavelengthsbetween2and240µm.Aver-
ownIDL-procedureswereusedformanualeditingofthedataat satile Java based web-interface allows to extract observational
theSPD(StandardProcessedData)level,andthegenerationof datainvariousformats,rangingfromunprocessedrawdata,over
thefinalFITSandASCIIfiles.TheSPDdataispreprocessedin standardpipelineprocessedmeasurementsuptomanuallyrepro-
the sense that raw detector readouts have been converted into cessedhighqualitydataproducts.
photocurrents, saturated readouts flagged, and strong glitches All data files associated to this catalogue can be retrieved
automatically identified and removed. Housekeeping data not by searching for the specific TDT6 number of a measurement
necessaryforfurtherdatareductionhasbeenremoved,butdark asgiveninTbl.1and2,bysearchingforallLWSobservations
currentandresponsivitymeasurementshavebeenpreserved.In of a given asteroid, or by searching for LWS asteroid observa-
thefollowingwebrieflyoutlinetheorderofreductionstepsand tionsin general.The highlyprocesseddata products,including
givethenamesofthecorrespondingLIAprocedureswhereap- overviewplots, qualityflags, and cataloguedocumentationcan
plicable. beobtainedviatheHPDPbuttonlefttoeachentryinthesearch
In a first step, the pointing coordinatesof each observation resultwindowofthearchiveinterface.
werecheckedagainsttheephemerisoftheobservedasteroidto Thecataloguedocumentation(Hormuth&Mu¨ller2006),giving
detectpointingproblems.Themaximumdifferencebetweenas- athoroughdescriptionofthedatareductionsteps,isalsosepa-
teroidpositionandtelescopepointingwasfoundtobe2′.′5,much ratelyavailablefromthelistofallHPDPdatasets7.
smaller than the LWS beam profile (see Lloyd 2003). The re- InFig. 1andFig.2 weshowlogarithmicexampleplotsfor
duction process started at SPD level with a visual inspection all observingmodes. Differentcolourshave been chosen to al-
of the data in the time domain. Photometric L02 observations low distinguishmentof overlappingmeasurements by different
weremanuallyde-glitchedatthisstage,capturingminorglitches detectors. The error bars in Fig. 1 include measurement and
not automatically flagged during generation of the SPD data. calibration errors, incuding a 5% uncertainty attributed to the
Theavailablepipelinedataofgratingscanswasadditionallyin- Uranusmodelsusedforabsolutefluxcalibration.Theslightmis-
spectedwithISAPtogetaquickoverviewoftheavailablescans matchesbetweenmeasurementsfromdifferentdetectorsatsame
andscandirections. wavelengthsinFig.2arecausedbymemoryeffectsofindividual
Next, dark currentdetermination and subtraction were per- detectors.
formed using the LIA routine IA DARK, followed by the abso- The data sets are available both as FITS tables, compati-
luteresponsivitycorrectionwithIA ABSCORR.Theseinteractive ble with the ISAP software package, and as plain ASCII ta-
routinesallowvisualinspectionandmanualeditingofthedark
currentmeasurementsandtheinternalcalibrationsourceobser- 5 Accessibleviahttp://iso.esac.esa.int
vations. Deglitching of this data and the removal of measure- 6 “TargetDedicatedTime”,auniqueidentifierforISOobservations
mentsobviouslyaffectedbymemoryeffectsiscritical,sinceany 7 http://iso.esac.esa.int/ida/hpdp.php
4 F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids
TDT 23000306 − 210002 PALLAS − AOT L02 TDT 76903203 − 210001 CERES − AOT L01
10−16
10−17
10−17
m m
µ µ
2m/ 2m/
c c
W/ W/
x / x / 10−18
Flu 10−18 Flu
10−19
40 60 80 100 120 140 160 180 50 100 150 200
Wavelength / µm Wavelength / µm
TDT 84801302 − 210010 HYGIEA − AOT L02 TDT 05500201 − 210004 VESTA − AOT L01
10−17
10−17
m m
µ µ
2m/ 2m/ 10−18
W/c 10−18 W/c
x / x /
u u
Fl Fl 10−19
10−19 10−20
40 60 80 100 120 140 160 180 50 100 150 200
Wavelength / µm Wavelength / µm
Fig.1. Examples for L02 observations. Left: fixed grating ob- Fig.2.Examplesforfullwavelengthrangescans.Left:standard
servationof (2)Pallas. Right:shortgratingscan observationof L01observationof(1)Ceres.Right:EngineeringmodeL99ob-
(10)Hygiea. servationof(4)Vesta.
Table3.RatiosbetweenLWS02observationsandTPMpredic-
bles.Pleaserefertothecataloguedocumentationforinformation
tions,weightedmeanvaluesandstandarddeviationsperdetector
aboutindividualFITSheaderkeywordsandtablecolumns.
(SW1,2, 3,4,5andLW1,2, 3)andperasteroid.Thephotom-
etry of the LW4 and LW5 detectorsis less reliable due to dark
currentproblems.
5. Discussion
5.1.Qualityofthecatalogueproducts
(1)Ceres (2)Pallas (4)Vesta (10)Hygiea
In order to study the reliability of the photometric L02 data in λ 11Obs. 4Obs. 14Obs. 2Obs.
thecatalogue,wecomparedthemeasuredfluxwithpredictions [µm] Obs/Mod Obs/Mod Obs/Mod Obs/Mod
calculated by means of the thermophysical model (TPM; e.g.
46.4 1.04±0.05 1.00±0.02 1.02±0.04 1.21±0.05
Lagerros1998,andreferencestherein).ForTbl.3wecalculated 56.4 1.01±0.03 1.06±0.02 1.01±0.04 1.25±0.10
model fluxes for the specified targets and epochs and analysed 66.3 1.04±0.02 1.10±0.04 1.02±0.04 1.23±0.15
the observation-to-modelratios. Measurements affected by de- 75.9 1.02±0.03 1.02±0.03 1.04±0.04 1.24±0.12
tector warm-up artefacts or abnormally high dark currentwere 85.0 1.01±0.04 1.02±0.11 1.00±0.07 1.39±0.10
notincludedinthisqualitycheck. 102.9 1.09±0.04 1.13±0.04 1.24±0.06 1.40±0.26
The general agreement between the photometric observa- 122.7 0.96±0.05 0.95±0.03 1.07±0.05 1.30±0.07
tions (L02) and models (Tbl. 3) is better than 5% for Ceres, 142.2 0.89±0.04 0.84±0.07 1.10±0.11 1.33±0.04
161.0 0.99±0.05 0.99±0.04 — —
Pallas, and Vesta and better than 30% for Hygiea. The higher
178.4 1.01±0.10 — — —
uncertaintiesare dueto darkcurrenteffectsatthe lowfluxlev-
els. The discrepancy for Hygiea is not surprising. Due to the Mean 1.01±0.05 1.03±0.08 1.05±0.08 1.27±0.14
lack of suitable good quality thermal data the thermophysical
modelparametersarealso verypoor,andHygieais considered
asalowqualityfar-IRcalibrator(Mu¨ller&Lagerros1998).As
a rule of thumb, the absolute flux error is better than 10% for The high quality of the spectral scans can be seen in the
targetsbrighterthanabout10Jy. smoothandwell-connectedcurvesofthearchivebrowseimages
F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids 5
(see Fig. 2). For mineralogic studies it might still be useful to cludedthedatawhichareflaggedasbeingaffectedbyhighdark
investigate the up- and down-scans separately to confirm that currents or high glitch rates. In the table we give the full stan-
certainlow-level,broad-bandstructuresinthescansarereliable darddeviationof allN radiometricsolutionsto accountforthe
ornot.Lowlevelfeaturesextendingovertwoormoredetectors fact that different observations see the asteroid under different
as well as structures close to pronounced RSRF changes have observingandilluminationgeometries.
to be analysed carefully:the detectorsare notalways behaving
in exactly the same way and LWS should be considered as 10
individualspectrometers,oneperdetector. Table5.Radiometricdiameterandalbedovalues,derivedfrom
theLWSL02measurements.
5.2.Diameterandalbedocalculations
Object D [km] p N
eff V
Directsizemeasurements,evenforthelargemain-beltasteroids,
(1)Ceres 959.6±16.9 0.096±0.003 55
are still difficult. Up to now, only a small sample has been re- (2)Pallas 534.4±14.7 0.142±0.009 20
solvedviaimagingtechniquesfromspace(HST,seeDottoetal. (4)Vesta 548.5±12.5 0.317±0.015 70
(2002) and references therein) and adaptive optics techniques (10)Hygiea 469.3±26.5 0.056±0.006 20
from large ground-based telescopes (e.g. Marchisetal. 2006).
Occultation observations performed by well-organised groups
of amateur astronomers also reveal direct sizes, expressed in These diameter and albedo values are the first ones which
chord maps which represent the cross section of the asteroid are derived purely from far-IR observations. Nevertheless, the
at the time of the occultation (e.g. Millis&Dunham 1989) resultsagreeverywellwiththesofarpublishedvalues.
orhttp://www.psi.edu/pds/resource/occ.htmlformorerecentre- hemostaccuratevaluesforthe diameterof(1)Cerescome
sults). But the largest number of asteroid sizes are originating from HST observations (Thomasetal. 2005) and adaptive op-
fromthermalinfraredobservations(e.g.Tedescoetal.2002a,b) tics (AO)observationsusing theKeck IItelescope (Carryetal.
viaradiometricmethodsi.e.,anindirectmethodwhichrelieson 2008). The HST observations led to an oblate spheroid with
modelassumptions. axesofa=b=487.3kmandc=454.7km,resultinginaneffective
We used a well-established thermophysical model (TPM; diameter of 2 × (a × b × c)1/3 =952.4km. The AO data con-
Lagerros1996,1997,1998)toderiveabsoluteeffectivesizesand firmed the oblate spheroid,but with slightly differentvalues (a
albedosforour4asteroidsfromthenewL02datasets. =b=479.7±2.3km,c=444.4±2.1km)andaneffectivediame-
This model allows to use state-of-the-art shape models to- terof935.3km.Ourresultisderivedfromverylimitedviewing
gether with corresponding spin axis solutions derived from geometries, but it is in excellent agreement with the HST re-
lightcurve inversion techniques (e.g. Kaasalainenetal. 2002). sults. In this context it is interesting to note that the diameter
As part of the energy equation for the asteroids the amount of derivedfrom IRAS observationsat 12, 25, 60 and 100µm was
reflected sunlight is described via the H−G magnitude system 848.40±19.7km (Tedescoetal. 2002b). Also our albedo value
(Bowelletal.1989).Therestofthesolarenergyisabsorbedand agreesverywell withthe 0.0936derivedviastellar occultation
re-emittedasthermalemission.Knowingthesolarinsolationto- techniques(Shevchenko&Tedesco2006).
getherwith thedisk-integratedthermalemissionallowsthento (2) Pallas: Tedescoetal. (2002b) obtained from IRAS
solve the energy equation for the asteroid’s effective size and data values of 498.07±18.8km and a geometric albedo of
albedo(e.g.Harris&Lagerros2002).Thetrueilluminationand 0.1587±0.013. A combination of speckle, occultation and
observinggeometriesaretakenintoaccountin theTPM calcu- lightcurveobservationsof(2)Pallasleadtoanellipsoidalshape
lations.The surfaceroughnessis describedbythe r.m.s.values with dimensions of 574 ± 10 × 526 ± 3 × 501 ± 2 km and
ofthesurfaceslopes(ρ)andthefractionofthesurfacecovered a mean diameter of 533 ± 6km (Dunhametal. 1990). A re-
bycraters(f).Forthesurfaceroughness,aswellasforthether- centhigh-angularresolutionadaptiveopticsstudybyCarryetal.
mal properties to calculate the heat conduction into lower lay- (2007)resultedinanabsoluteeffectivesizeof519.5km(semi-
ers of the surface, we used “default properties” (ρ=0.7, f=0.6, major ellipsoidal axes values of 276×256×248km, ±10km).
Γ=15Jm−2s−0.5K−1) as described by Mu¨lleretal. (1999). The Drummond&Christou (2008) found 494±57km based on an-
emissivity models were taken from Mu¨ller&Lagerros (1998): othersetofAOobservations.Thevariousresultsshowthatthere
a standard wavelength-dependent emissivity model for Ceres, are still significant uncertainties in the overall shape and size
Pallas,adHygieaandaspecialoneforVestawhereloweremis- values,butthederivedeffectivesizesagreewithinthegivener-
sivitiesinthesubmm/mmrangewerefound. rorbarswithourradiometricdiameter.Therefore,webelievethat
TheTPM withthe “defaultthermalproperties”hasbeentested ouralbedoof0.142isthemostreliablevaluesofarpublishedfor
and validated extensively in the context of ISO for large, Pallas.
regolith-coveredmain-belt-asteroids(Mu¨ller&Lagerros2002). (4)Vestahasaverycomplexsurfacewithlargealbedovari-
Itisworthtonoteherethatthesurfaceroughnessandheatcon- ations on the surface (Binzeletal. 1997). Our diameter value
ductionpropertiesarelessimportantforourfar-IRanalysissince is about 4% larger than the effective diameter from HST ob-
they mainly influence the mid-IR radiation at the Wien-partof servations (axes 289, 280, and 229 ± 5 km; D = 529.2km;
eff
thespectralenergydistribution(e.g.Mu¨ller2002,Figs.2&3). Thomasetal.(1997),noteaddedinproof)andabout7%larger
Asteroid-specificmodelinputparametersandthecorresponding than the mean triaxial ellipsoid solution with 563±5×534±
referencesarelistedinTbl.4. 5 × 442 ± 7km (2 · (a · b · c)1/3 = 510.3± 5.6km) given by
OurderiveddiameterandalbedovaluesareshowninTbl.5. Drummond&Christou (2008). This is outside the given rms-
The diametersare effective diametersof a sphere of equalvol- scatterfromour70measurementswhichmightbeanindication
ume.Itcanbeconsideredasa“scalingfactor”forshapemodels that the special emissivity modelfor Vesta (Mu¨ller&Lagerros
which lack the absolute size information. These values are the 1998) needs a small adjustment for this wavelength range.
weightedmeanvalues,basedonthe photometricallymorereli- It looks like the emissivity drop which has been seen by
able shortwavelength detectors(46.4 to 85.0µm). We also ex- Redmanetal. (1992, 1998) occurs already at shorter wave-
6 F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids
Table4.ObjectspecificTPMinputparameters.Theλ andβ valuesaretheeclipticcoordinatesofthespinvectordirection,with
ecl ecl
β countedfromtheequator.
ecl
Object [H,G] P [h] λ ,β sourceofshape&spinvector
sid ecl ecl
(1)Ceres [3.28,0.05]1 9.074184 331.51◦,+77.88◦ HSTobservations4
(2)Pallas [4.13,0.16]2 7.813225 34.80◦,-11.75◦ Lightcurveinversiontechniques5,6
(4)Vesta [3.20,0.34]3 5.342129 319.45◦,+59.23◦ HSTobservations7
(10)Hygiea [5.43,0.07]2 27.623265 117.01◦,-28.53◦ Lightcurveinversiontechniques8
1 Lagerkvistetal. (1992); 2 Lagerkvistetal. (2001); 3 Mu¨ller&Lagerros (1998); 4 Thomasetal. (2005); 5 M. Kaasalainen, priv. comm.
22/Sep/2005;6Torppaetal.(2003);7Thomasetal.(1997);8M.Kaasalainen,priv.comm.31/Jan/2006;
lengths below 50µm. Almost all LWS observations of Vesta tionsareconnectedtostructures(e.g.,theOlbersfeature)which
were taken on one day (revolution 805) under very similar as- arevisibleintheHSTimagesof(4)Vesta(Binzeletal.1997).
pectangles.ThismightalsoinfluencetheoutcomeoftheTPM Before we combined TPM lightcurve predictions with the
technique which works best when combining data from differ- observedfluxes,weexploredtheinfluenceofTPMinputparam-
entwavelengths,phaseangles,rotationalphasesandaspectan- etersonthethermallightcurveamplitudeandphaseforthegiven
gles (Mu¨ller&Lagerros 2002). Shevchenko&Tedesco (2006) wavelength range and aspect angles. It turned out that neither
derivedanalbedoofp =0.370fromoccultationmeasurements, the thermal inertia (the range between 0 and 50Jm−2s−0.5K−1
H
whileweobtainedaradiometricvalueofp =0.32±0.02,again wasconsidered)northebeamingparameter,describingvarious
V
thisdiscrepancymightbeexplainedbyemissivityeffectsorthe roughnessscenarios,influencedthepredictionsforourcasesig-
limited aspect angle range. The albedo value in Tbl. 5 agrees nificantly. The predicted lightcurve amplitudes varied between
verywellwithpreviousstudiesbyMu¨ller&Lagerros(1998)(p 5−6% (peak-to-peak) and the lightcurve phases only by a few
V
= 0.33)based on a set of thermalobservationsfrom mid-IRto minutes between the low and high thermal inertia predictions.
mm-wavelengths. Ourdatasetswerethereforenotsufficienttoconfineanyofthese
Our result for (10) Hygiea deviates significantly from parameters.
previous studies: Tedescoetal. (2002b) found values of The Vesta L02 observations from revolution 805 (28-Jan-
407.12±6.8km and pV=0.0717±0.002. Mu¨ller&Lagerros 1998) were taken over a period of about 6hours, covering
(1998) gave 429.9km and 0.066. But Hygiea’s shape model slightlymorethanonefullrotationperiodof5.34hours.Onthe
is not that well defined due to a poor lightcurve coverage basis of the shape and spin-vectorsolution given in Tbl. 4 and
(in rotational phases) and some very low quality lightcurves the diameter and albedo results from Tbl. 5 we predicted the
(Kaasalainen,priv.comm.).There mightalso be a secondpole thermallightcurvesattheL02key-wavelengthsfortheLWSob-
solution at around λecl = 300◦ (instead of the 117◦ used here). servations(notethatthereferencetimeframeisthatoftheISO
Butneitherusingthesecondpolesolutionsnorusingaspherical satelliteandnottheasteroid-intrinsicone).
shape model lower the predicted size values (or the standard Figure 3 shows the observations at 102.3µm together with
deviations) significantly. One possibility for the discrepancies themodelpredictiononanabsolutefluxandtimescale.Figure4
in the diameters could be emissivity issues. If Hygiea’s far-IR showsthecombineddatabelow105µm,firstnormalisedperde-
emissivityislowerthenthecorrespondingdiameterwouldalso tector over the full observing period and then averaged for a
be smaller (and the albedo a bit larger). But the data quality given rotational phase angle. The quality of the data from the
and the very limited aspect angle range (all LWS observations longer wavelength channels beyond 105µm was not sufficient
were taken within 20days) are not sufficient to draw firm con- forthiskindofanalysis.
clusions.Especiallywhenlookingatthebestqualitydirectsize Both figures show that the observations follow the model
measurementbyRagazzonietal.(2000):Theygaveaneffective predictions. The errorbars in Fig. 4 correspond to the error of
diameter of 444±35km (with an axis-ratio of a/b=1.11),based the mean values plus a contribution from the flux uncertainty
on speckle techniques. Our radiometric results are still well in these detectors. This figure combines in total 78 indepen-
withintheseerrorbars. dentmeasurements(6detectors×13measurementsatdifferent
rotational phases). The model predictions match the observed
lightcurve amplitude as well as the lightcurve phase (in the
5.3.Far-IRthermallightcurveof(4)Vestaintherange
observer’s time frame). We also indicated the rotational phase
45−105µm
wheretheOlbers-feature(Zellneretal.1997;Binzeletal.1997)
The visual lightcurve of (4) Vesta is dominated by the influ- on Vesta’s surface would be visible. We cannot see any influ-
ence of the albedo variations (Degewijetal. 1979). Standard enceofthissurfacefeaturewithloweralbedo.Mu¨ller&Barnes
lightcurve inversion techniques failed to produce a reliable (2007)presentedamoredetaileddiscussiononhowvariousther-
shape model (Kaasalainen, priv. comm.), but high resolution malpropertiesofthissurfacefeaturewouldinfluencethethermal
HST imaging allowed a solution for the shape and spin vector lightcurvevariationsatdifferentwavelengths.Theyfoundstrong
(Thomasetal.1997).Redmanetal.(1992)foundthatthe1mm emissivity variations which they attributed to the Olbers struc-
light-curveisapparentlydominatedbythetriaxialshape,with- ture and a neighbouring region. The deviation from the model
out any significant contributionsfrom the optical albedo spots. predictionsatrotationalphasesbetween240−280◦inFig.4are
Mu¨ller&Barnes(2007)showedthatatevenlongerwavelengths alsoclosetotheOlbersregionandmightthereforebethefar-IR
(around3.2mm)themm-lightcurvefollowsforalargefraction signature of the ejecta material which is deposited on one side
of the rotational period the shape-introduced variations. They ofthe Olbersimpactstructureasdiscussedin Mu¨ller&Barnes
also demonstrated that the rotational phases with clear devia- (2007).
F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids 7
nicelytheshape-introducedvariations.Albedoandgraineffects
areimportantinthevisualandmm-lightcurves,butseemtoplay
averysmallroleatfar-IRwavelengths.
The catalogue of LWS observations of asteroids contains
photometricandspectroscopicmeasurementsofthemainrepre-
sentativesofthemainasteroidbelt.Thedatawillremainuseful
forthermophysicalandmineralogicalstudiesofthesebodies,es-
peciallyinthecontextoftheDawnspacemissiontoCeresand
Vesta.
Acknowledgements. WewouldliketothankTimGrundyandhiscolleaguesat
RALforpreprocessingthenon-standarddata,andfortheexplanationsconcern-
ingthepeculiaritiesoftheseobservations.
LIA is a joint development of the ISO-LWS Instrument Team at Rutherford
AppletonLaboratories(RAL,UK-thePIInstitute)andtheInfraredProcessing
andAnalysisCentre(IPAC/Caltech,USA).
TheISOSpectralAnalysisPackage(ISAP)isajointdevelopmentbytheLWS
andSWSInstrumentTeamsandDataCenters.ContributinginstitutesareCESR,
IAS,IPAC,MPE,RALandSRON.
References
Fig.3.Theobserved(dotswitherrorbars)andthepredictedther-
Binzel,R.P.,Gaffey,M.J.,Thomas,P.C.,etal.1997,Icarus,128,95
mallightcurve(continuousline)of(4)Vestaat102.3µm(detec- Bowell,E.,Hapke,B.,Domingue,D.,etal.1989,inAsteroidsII(Universityof
torLW1). ArizonaPress,Tucson),524–556
Carry,B.,Dumas,C.,Fulchignoni,M.,etal.2008,A&A,478,235
Carry, B., Kaasalainen, M., Dumas, C., et al. 2007, in AAS / Division for
PlanetarySciencesMeetingAbstracts,Vol.39,#30.08
Clegg,P.E.,Ade,P.A.R.,Armand,C.,etal.1996,A&A,315,L38
Degewij,J.,Tedesco,E.F.,&Zellner,B.1979,Icarus,40,364
Dotto, E.,Barucci, M. A.,Mu¨ller, T.G., Storrs, A. D.,& Tanga, P.2002, in
AsteroidsIII,ed.W.F.BottkeJr.,A.Cellino, P.Paolicchi, &R.P.Binzel,
219–234
Drummond,J.&Christou,J.2008,Icarus,197,480
Dunham,D.W.,Dunham,J.B.,Binzel,R.P.,etal.1990,AJ,99,1636
Gry, C., Swinyard, B., Harwood, A., & et al. 2003, The ISO Handbook:
LWS–TheLongWavelengthSpectrometer,ESASP-1262(EuropeanSpace
Agency)
Harris,A.W.&Lagerros,J.S.V.2002,inAsteroidsIII,ed.W.F.BottkeJr.,
A.Cellino,P.Paolicchi,&R.P.Binzel(UniversityofArizonaPress,Tucson),
205–218
Henning,T.,Il’In,V.B.,Krivova,N.A.,Michel, B.,&Voshchinnikov, N.V.
1999,A&AS,136,405
Hormuth,F.&Mu¨ller,T.G.2006,CatalogueofLWSobservationsofasteroids,
publishedonlineat:
http://ida.esac.esa.int:8080/hpdp/technical reports/technote36.pdf
Kaasalainen,M.,Mottola,S.,&Fulchignoni,M.2002,inAsteroidsIII,ed.W.
F.BottkeJr.,A.Cellino,P.Paolicchi,&R.P.Binzel(UniversityofArizona
Press,Tucson),139–150
Kessler,M.F.,Steinz,J.A.,Anderegg,M.E.,etal.1996,A&A,315,L27
Lagerkvist,C.-I.,Magnusson,P.,Williams,I.P.,etal.1992,A&AS,94,43
Lagerkvist, C. I., Piironen, J., & Erikson, A. 2001, Asteroid Photometric
Fig.4. The observed (dots with error bars, normalised and av- Catalogue,FifthUpdate,UppsalaAstron.Obs.
eraged over five detectors (SW1, SW2, SW3, SW4, SW5 and Lagerros,J.S.V.1996,A&A,310,1011
LW1)andthepredictedthermallightcurve(continuousline)of Lagerros,J.S.V.1997,A&A,325,1226
Lagerros,J.S.V.1998,A&A,332,1123
(4)Vestaasafunctionoftherotationalphase.
Lloyd,C.2003,inTheCalibrationLegacyoftheISOMission,ed.L.Metcalfe,
A.Salama,S.B.Peschke,&M.F.Kessler,ESASP-481,399
Marchis,F.,Kaasalainen,M.,Hom,E.F.Y.,etal.2006,Icarus,185,39
Millis,R.L.&Dunham,D.W.1989,inAsteroidsII,ed.R.P.Binzel,T.Gehrels,
&M.S.Matthews,148–170
6. Conclusions
Mu¨ller,T.G.2002,Meteoritics&PlanetaryScience,37,1919
Mu¨ller,T.G.2003,inExploitingtheISODataArchive-InfraredAstronomy
Viaradiometrictechniqueswederivedthefirstfar-IR-baseddi-
intheInternetAge,ed.C.Gry,S.B.Peschke,J.Matagne,P.Garc´ıa-Lario,
ameter and albedo values for the four large asteroids Ceres, R.Lorente,A.Salama,&E.Verdugo,ESASP-511,47–54
Pallas, Vesta, and Hygiea. Based on photometry in the wave- Mu¨ller,T.G.,A´braha´m,P.,&Crovisier,J.2005,SpaceScienceReviews,119,
length range 46–85µm we calculated effective diameters D 141
eff Mu¨ller,T.G.&Barnes,P.J.2007,A&A,467,737
of 960±17km, 534±15km, 549±13km, and 469±27km for
Mu¨ller,T.G.&Lagerros,J.S.V.1998,A&A,338,340
these four objects. The corresponding albedo values pV are Mu¨ller,T.G.&Lagerros,J.S.V.2002,A&A,381,324
0.096±0.003,0.142±0.009,0.317±0.015,and0.056±0.006,re- Mu¨ller,T.G.,Lagerros,J.S.V.,Burgdorf,M.,etal.1999,inTheUniverseas
spectively.We foundexcellentagreementwith direct measure- SeenbyISO,ed.P.Cox&M.F.Kessler,Vol.ESA-SP427,141–144
ments fromHST (Ceres and Vesta) and speckle/occultationre- Ragazzoni,R.,Baruffolo,A.,Marchetti,E.,etal.2000,A&A,354,315
Redman,R.O.,Feldman,P.A.,&Matthews,H.E.1998,AJ,116,1478
sults (Pallas and Hygiea). Our data contain also the first far-
Redman,R.O.,Feldman,P.A.,Matthews,H.E.,Halliday,I.,&Creutzberg,F.
IRlightcurveof anasteroid:Vesta’sthermallightcurvefollows 1992,AJ,104,405
8 F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids
Salisbury, J. W. 1993, in Remote Geochemical Analysis: Elemental and
MineralogicalComposition,ed.C.M.Pieters&P.A.J.Englert(Cambridge
UniversityPress,NewYork),79–98
Shevchenko,V.G.&Tedesco,E.F.2006,Icarus,184,211
Tedesco,E.F.,Egan,M.P.,&Price,S.D.2002a,AJ,124,583
Tedesco,E.F.,Noah,P.V.,Noah,M.,&Price,S.D.2002b,AJ,123,1056
Thomas,P.C.,Binzel,R.P.,Gaffey,M.J.,etal.1997,Icarus,128,88
Thomas,P.C.,Parker,J.W.,McFadden,L.A.,etal.2005,Nature,437,224
Torppa,J.,Kaasalainen,M.,Michalowski,T.,etal.2003,Icarus,164,346
Zellner,B.H.,Albrecht,R.,Binzel,R.P.,etal.1997,Icarus,128,83
F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids 9
Table1. OverviewofstandardLWSasteroidobservationsinourcatalogue.Fortheon-sourcepointingsthedistancetotheasteroid∆,theSun-
asteroiddistancer,andthephaseangleαaregiven,basedonMPC(MinorPlanetCenter)ephemerides.TheTDTnumberisauniqueidentifer,
allowingtofindtheobservationinthearchive.
Object TDT Mid-observationUTC AOT ∆[AU] r[AU] α[◦] Remarks
(1) Ceres 09300401 1996-02-18 04:31:18 L02 2.673 2.695 21.2
09304102 1996-02-18 15:15:23 L02 (off-sourcereferencemeasurementforTDT09300401)
10500402 1996-03-01 05:22:22 L02 2.520 2.705 21.5
11900214 1996-03-15 02:42:57 L02 2.344 2.715 21.1
11905611 1996-03-15 16:54:46 L02 (off-sourcereferencemeasurementforTDT11900214)
12600114 1996-03-22 02:03:54 L02 2.258 2.721 20.5
25800302 1996-07-31 19:30:27 L02 2.258 2.823 19.2
25805903 1996-08-01 08:55:54 L02 (off-sourcereferencemeasurementforTDT25800302)
26500301 1996-08-07 19:10:27 L02 2.351 2.828 20.0
26505602 1996-08-08 09:32:38 L02 (off-sourcereferencemeasurementforTDT26500301)
32100204 1996-10-02 15:31:35 L02 3.126 2.868 18.6
32103506 1996-10-03 03:52:44 L02 (off-sourcereferencemeasurementforTDT32100204)
53802209 1997-05-07 12:16:09 L02 3.112 2.971 18.9
57902409 1997-06-17 09:56:26 L02 2.575 2.978 19.4
59401908 1997-07-02 06:42:05 L02 2.391 2.980 17.9
72001901 1997-11-05 01:10:04 L02 2.522 2.973 18.6
74803304 1997-12-03 03:04:58 L02 2.901 2.966 19.3
74803403 1997-12-03 08:11:59 L01 2.904 2.966 19.3 containswarmupartefacts
75502902 1997-12-10 00:30:08 L01 (off-sourcereferencemeasurementforTDTs74803...)
75503003 1997-12-10 01:38:00 L02 2.994 2.964 19.0
76200502 1997-12-16 13:29:15 L02 3.081 2.962 18.6
76903102 1997-12-24 01:08:50 L02 3.177 2.960 18.0
76903203 1997-12-24 02:17:12 L01 3.178 2.960 18.0
(2) Pallas 23000306 1996-07-03 21:24:01 L02 2.497 2.823 20.9
23002907 1996-07-04 12:01:22 L02 (off-sourcereferencemeasurementforTDT23000306)
25100202 1996-07-24 19:49:12 L02 2.780 2.871 20.6
25103603 1996-07-25 09:58:26 L02 (off-sourcereferencemeasurementforTDT25100202)
26500503 1996-08-07 19:43:17 L02 2.967 2.903 19.9
26505204 1996-08-08 07:27:57 L02 (off-sourcereferencemeasurementforTDT26500503)
27200203 1996-08-14 18:19:41 L02 3.057 2.919 19.3
27202004 1996-08-15 03:27:01 L02 (off-sourcereferencemeasurementforTDT27200203)
(4) Vesta 24402202 1996-07-17 23:20:20 L02 1.593 2.149 26.6
24404603 1996-07-18 06:29:49 L02 (off-sourcereferencemeasurementforTDT24402202)
80500101 1998-01-28 10:36:25 L02 2.566 2.549 22.2
80500104 1998-01-28 11:02:50 L02 2.566 2.549 22.2
80500107 1998-01-28 11:29:15 L02 2.567 2.549 22.2
80500110 1998-01-28 11:55:40 L02 2.567 2.549 22.2
80500113 1998-01-28 12:22:05 L02 2.567 2.549 22.2
80500116 1998-01-28 12:48:30 L02 2.567 2.549 22.2
80500119 1998-01-28 13:14:55 L02 2.568 2.549 22.2
80500122 1998-01-28 13:41:20 L02 2.568 2.549 22.2
80500125 1998-01-28 14:07:45 L02 2.568 2.549 22.2
80500128 1998-01-28 14:34:10 L02 2.568 2.549 22.2
80500131 1998-01-28 15:00:35 L02 2.569 2.549 22.2
80500134 1998-01-28 15:27:00 L02 2.569 2.549 22.2
80500137 1998-01-28 15:53:25 L02 2.569 2.549 22.2
(10) Hygiea 83201702 1998-02-24 22:08:03 L01 3.179 3.444 16.6 CIIbackgroundemission
83201803 1998-02-24 22:44:05 L02 3.179 3.444 16.6
84801302 1998-03-12 16:13:06 L02 3.402 3.434 16.7
85303402 1998-03-17 16:09:46 L02 3.472 3.430 16.6
10 F.HormuthandT.G.Mu¨ller:CatalogueofISOLWSobservationsofasteroids
Table2. OverviewofL99asteroidobservationsinourcatalogue.AllobservationsareL01-likescanscoveringthefullwavelengthrange.
Object TDT Mid-observationUTC ∆[AU] r[AU] α[◦] Remarks
(1) Ceres 07500601 1996-01-31 07:52:11 2.891 2.682 19.9 CIIbackgroundemission
07500701 1996-01-31 10:10:56 2.890 2.682 19.9 CIIbackgroundemission
07501301 1996-01-31 16:08:13 2.887 2.682 19.9 CIIbackgroundemission,
non-standardbiasvoltages
07501401 1996-01-31 18:32:03 2.886 2.682 20.0 CIIbackgroundemission,
non-standardbiasvoltages
(4) Vesta 05500201 1996-01-11 07:10:35 2.313 2.250 24.8 non-standardbiasvoltages
05500301 1996-01-11 08:54:29 2.312 2.250 24.9 non-standardbiasvoltages
05500401 1996-01-11 10:39:34 2.311 2.250 24.9 non-standardbiasvoltages