Using red clump stars to correct the Gaia DR1 parallaxes

3 MB·

by  Guy R. Davies

Astronomy&Astrophysicsmanuscriptno.Gaia_hack (cid:13)cESO2017
January11,2017
LettertotheEditor
Using red clump stars to correct the Gaia DR1 parallaxes
GuyR.Davies1,2(cid:63),MikkelN.Lund2,1,AndreaMiglio1,2,YvonneElsworth1,2,JamesS.Kuszlewicz1,2,
ThomasS.H.North1,2,BenRendle1,2,WilliamJ.Chaplin1,2,ThaíseS.Rodrigues3,4,TiagoL.Campante1,2,
Léo Girardi3,StevenJ.Hale1,2,OliverHall1,2,CaitlinD.Jones1,2,StevenD.Kawaler5,IanRoxburgh6,and
MathewSchofield1,2
1 SchoolofPhysicsandAstronomy,UniversityofBirmingham,Edgbaston,Birmingham,B152TT,UK
2 StellarAstrophysicsCentre,DepartmentofPhysicsandAstronomy,AarhusUniversity,NyMunkegade120,DK-8000AarhusC,
Denmark
3 OsservatorioAstronomicodiPadova–INAF,Vicolodell’Osservatorio5,I-35122Padova,Italy
7 4 DipartimentodiFisicaeAstronomia,UniversitàdiPadova,Vicolodell’Osservatorio2,I-35122Padova,Italy
1 5 DepartmentofPhysicsandAstronomy,IowaStateUniversity,Ames,IA50011,USA
0 6 AstronomyUnit,QueenMaryUniversityofLondon,MileEndRoad,LondonE14NS,UK
2
Received11.Nov.2016/Accepted10.Jan.2017
n
a
J ABSTRACT

0
1 RecentresultshavesuggestedthatthereistensionbetweentheGaiaDR1TGASdistancesandthedistancesobtainedusinglumi-
nositiesdeterminedbyeclipsingbinariesorasteroseismologyonredgiantstars.WeusetheKs-bandluminositiesofredclumpstars,
] identifiedandcharacterizedbyasteroseismology,tomakeindependentdistanceestimates.OurresultssuggestthatGaiaTGASdis-
R tancescontainasystematicerrorthatdecreaseswithincreasingdistance.Weproposeacorrectiontomitigatethisoffsetasafunction
S ofparallaxthatisvalidfortheKeplerfieldandvaluesofparallaxthatarelessthan∼1.6mas.Forparallaxesgreaterthanthiswefind
. agreementwithpreviouslypublishedvalues.WenotethattheTGASdistancestotheredclumpstarsoftheopenclusterM67showa
h highlevelofdisagreementthatisdifficulttocorrectfor.
p
- Keywords. Asteroseismology–Stars:fundamentalparameters–Stars:oscillations(includingpulsations)–AstronomicalDatabases
o –Parallaxes–Galaxy:structure
r
t
s
a
[1. Introduction parallaxes are compromised by incorrect interstellar extinction
correctionsand/orpoorlyknownbulkmetallicities,whichwould
1The Gaia mission promises trigonometric parallaxes for ∼109
introducesystematicsintheestimatedstellarluminosities.
vstarswithprecisionsoftensofmicroarcseconds.Thefirstdata
6release (DR1) – the Tycho-Gaia Astrometric Solutions sample Here, we have tested the TGAS and asteroseismic distance
0(Michaliketal.2015;GaiaCollaborationetal.2016)(hereafter scales independently by studying a subset of the stars from
5TGAS)–isbasedononly14monthsofdataandprovidesparal- DeR16, specifically those stars that have been robustly identi-
2laxestimatesforsome2millionstars.Initialcomparisonshave fied by asteroseismology as red clump (designated RC and de-
0
suggested that the TGAS sample contains a systematic offset finedtoincludecorehelium-burningstarsbutexcluderedgiant
.
1of around −0.25mas (Stassun & Torres 2016; Jao et al. 2016), branch (RGB), asymptotic giant branch (AGB), and secondary
0or that no correction is required (Lindegren et al. 2016; Sesar red clump (SC) stars). We also added a further six RC stars
7etal.2016).HerewetesttheTGASparallaxesusingasampleof fromtheopenclusterM67,whichhaveasteroseismicdetections
1starsobservedbytheNASAKepler missionthatprovideause- (Stello et al. 2016) in data collected by the re-purposed Kepler
v:fulprobeofthefarendofthedistancescaleintheKeplerfield Mission,K2.
Xiofview. WemakeuseofanimportantpropertyoftheRCstellarpop-
De Ridder et al. (2016) (hereafter DeR16) have recently ulation – specifically that RC stars have more or less the same
r
ashownthatthereisatensionbetweentheastrometricdistancesof luminosity (e.g., Cannon 1970; Paczyn´ski & Stanek 1998) – to
theTGASsampleandasteroseismicallydetermineddistancesof validatetheestimateddistances.AsshownbySalaris&Girardi
redgiantsfromRodriguesetal.(2014)(hereafterR14).Alinear (2002) and Girardi (2016), the K -band minimizes the intrinsic
s
fitoftheasteroseismicparallaxestotheTGASparallaxesreturns differences in the luminosites of RC stars due to differences in
aslopethatissignificantlydifferentfromunity,andanintercept their metallicities. Residual changes due to variations in stellar
that is significantly different from zero. We have replicated the masses,ages,andevolutionalongtheHe-burningphaseremain
DeR16resultinFigure1usinganorthogonaldistanceregression at the level of (cid:46)0.2 mag. The main limitation in the use of RC
(ODR,(Boggs&Rogers1990)),andfindcomparableresults.It stars as distance indicators stems from the difficulty in identi-
hasbeensuggestedbyDeR16thattheabovedeparturesindicate fying them among the wider red giant population (with the ex-
thateithertheTGASparallaxesarebiased;orthatasteroseismic ceptionofensemblesofstarslocatedatsimilardistances,e.g.in
clusters,theGalacticbulge,ornearbygalaxies).Thislimitation
(cid:63) e-mail:[email protected] hasnowbeenovercomethankstoasteroseismicconstraintsthat
Articlenumber,page1of5
A&Aproofs:manuscriptno.Gaia_hack
canunambiguouslydiscernRCfromSCandRGBorAGBstars relations(e.g.,Miglioetal.2013);theyaretabulatedasthe“dis-
(Beddingetal.2011).HenceRCstarscanbeusedtotestGaia tancesfromdirectmethod”intheonlinedataprovidedbyR14.
distances. This method has no knowledge of the evolutionary state of the
star and being model-independent does not force the estimated
luminosities to some assumed RC value. This ensures that we
3.0 RGB/AGB haveanindependentestimateofluminosityforeachstartouse
SC
asatestoftheassumptionthattheRChasalowscatterinlumi-
as) RC nosity.
m2.5
( WeselectedourRCsampleusinganasteroseismicclassifica-
ϖseis tion(Elsworthetal.2016)thatexploitsthediagnosticproperties
x,2.0 ofdipolemodesofmixedcharacter,allowingonetodiscriminate
a
all betweenhydrogen-shellandheliumcore-burningredgiantstars
par1.5 (Bedding et al. 2011). This classification has been shown to be
c
mi robust in the identification of RC stars and is capable of sepa-
oseis1.0 rating out secondary clump stars and with the addition of tem-
er perature from Pinsonneault et al. (2014), the horizontal branch
Ast stars.
0.5
0.0 3. Results
−0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
TGASparallax, ϖTGAS(mas) Figure2showstheresultsofusingasampleofRCstarsastests
of the asteroseismic and astrometric distance scales. We calcu-
Fig.1–ComparisonbetweenasteroseismicandTGASparallaxes.The late the theoretical dependence of the apparent K -band mag-
s
markers indicate the evolutionary state of the star, including the red nitudes of the RC stars on distance (and hence parallax, i.e.,
giant branch (RGB or AGB), the red clump (RC), and the secondary (cid:36) = 1000/d, with d in parsecs - in Figure 2 the dashed red
clump(SC).Theredlineshowsthe1:1relation.Theblacklineshows
line)using:
the linear relation obtained from an orthogonal distance regression
(ODR), which includes uncertainties on both parallax estimates. The mRC =µ +MRC +A , (3.1)
best-fittingrelationis(cid:36)seis = (1.21±0.07)×(cid:36)TGAS −(0.12±0.08). Ks 0 Ks Ks
Theshadedblueregionshowsthe1-σconfidenceintervalaroundthe where mRC are apparent K magnitudes obtained from the Two
best-fittingrelation. Ks s
Micron All Sky Survey (2MASS; Cutri et al. 2003; Skrutskie
etal.2006);µ =5log (d)−5isthedistancemodulus;A isthe
0 10 Ks
interstellarextinctionintheK band,determinedfromE(B−V)
s
reddening values from the 3D dust map by Green et al. (2015)
2. Theasteroseismicconstraints
(derivedfromstarsinthePan-STARRS1survey)attheastero-
Asteroseismology provides two key sets of constraints which seismic distance of a given star and we adopted an extinction-
maybeusedasindependentwaysofinferringdistances: to-reddeningratioofR = 0.355±0.1(Fitzpatrick1999).The
Ks
maximumvalueofA fromtheGreenetal.(2015)mapofour
◦ Radii of red giant stars can be estimated from the average Ks
RCsampleis0.08,withameanofA ≈0.02.MRC istheabso-
asteroseismicparametersthatcharacterizetheiracousticos- Ks Ks
luteK bandmagnitudefortheRCpopulation(seeGirardi2016,
cillation spectra: the so-called average large frequency sep- s
and references therein); we adopted a median of the literature
aration, and the frequency corresponding to the maximum
valuespresentedinGirardi(2016),i.e., MRC = −1.585±0.043,
observed oscillation power. Red giants, which show these Ks
wheretheuncertaintyincludestheRMSscatterbetweenthedif-
solar-likeoscillations,maythereforebeusedasaccuratedis-
ferentvalues.Forthe6RCstarsaddedfromtheM67clusterwe
tanceindicatorsjustasinthecaseofeclipsingbinaries:the
de-reddenedtheK magnitudesusingE(B−V)=0.041±0.004
distance to each red giant may be estimated from the abso- s
(Taylor2007),andfortheasteroseismicdistanceweadoptedthe
luteluminosity,whichisobtainedfromtheasteroseismically
valuegivenbyStelloetal.(2016)ofd =816±11pc.
determined radius (for example from asteroseismic scaling
The agreement in the left-hand panels of Figure 2 of the
relations)andTeff (e.g.,Miglioetal.2013).
asteroseismic parallax scale with the expected relation demon-
◦ Thankstothefrequenciesofdipolargravito-acousticmodes
stratesthatasampleofRCstarstogetherwithm canbeused
wecandiscernpristineRCstarsamongthezooofred-giant Ks
to test the distance scale. There is in contrast a clear system-
stars (Bedding et al. 2011). We note that these inferences
aticerrorintheTGASparallaxesversusdistance,evengiventhe
are independent of the constraints used to determine radii
largescatter.
(andhencedistances).OnceRCstarsareidentified,theirdis-
For the M67 RC stars we see noticeable differences in par-
tancesmaybedeterminedgiventheirintrinsicluminosity.
allaxes when compared with the expected values. We note that
RC stars in the solar neighborhood are expected to have simi- while it is possible to suggest a correction to the TGAS paral-
lar intrinsic K -band luminosities (to within ∼0.2 mag), so we laxes(seesection4),itisdifficulttoseehowacorrectionbased
s
anticipate a RC coherent, extended feature in a diagram show- solelyonparallaxorsomemeasureofcolorcouldsolvetheprob-
ing distance and apparent luminosity. This feature would show lempresentedbytheM67RCstars.
theexpecteddistancedependencesubjecttosomescatteroroff- Tocheckfortheeffectsofextinction,binarity,andRCpopu-
setfromtheeffectsofreddening(photometricbanddependent), lationheterogeneityontheresultswehavegeneratedasynthetic
stellarmultiplicity,andtheintrinsicscatterofRCluminosities. populationwiththepropertiesexpectedoftheKepler field(see
Weadoptasetofasteroseismicdistancescalculatedusinga Miglioetal.2014).Figure3plotsK -bandapparentmagnitudes
s
model-independentmethod,i.e.,usingtheasteroseismicscaling asafunctionofparallaxforaTRILEGALsimulatedpopulation
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GuyR.Daviesetal.:UsingredclumpstarstocorrecttheGaiaDR1parallaxes
10.5
RGB/AGB
10.0 SC
RC
9.5 M67 RC
g) 9.0
a
m
( 8.5
Ks
m
8.0
7.5
7.0
1.4
n 1.2 9.6
o
ti
a 1.0
el 9.2
R
/ 0.8
Ks
m 0.6 8.8
g)
a
0.4 8.4(m
as) 1.0 mKs
m 8.0
( 0.5
n
o
ti 0.0 7.6
a
Rel−0.5
− 7.2
−1.0
ϖ
0.5 1.0 1.5 2.0 2.5 3.0 −0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Asteroseismicparallax, ϖseis(mas) TGASparallax, ϖTGAS(mas)
Fig.2–Top:Relationbetweenasteroseismic(left)orTGAS(right)parallaxesandde-reddened2MASSK magnitudes(m )foroursample.The
s Ks
markersindicatetheevolutionarystateofthestar,includingtheredgiantbranch(RGBorAGB),theredclump(RC),andthesecondaryclump
(SC).Wenotethattheuncertaintiesonm aretypicallysmallerthanthemarkersize.Thedashedredlineinbothpanelsshowsthetheoretical
Ks
relationusedfortheresidualsinthebottompanels.TherelationisgivenbyEquation3.1usingthemediantheoreticalvalueof MRC fromthe
Ks
literature(seeGirardi2016,andreferencestherein);thegreyregionsaroundtherelationsindicatethespanoftheliteraturevalues.Thefilledred
markersdenotethesixasteroseismicRCstarsintheM67cluster(Stelloetal.2016)forwhichTGASparallaxeswereavailable.Middle:Fractional
differencesbetweentheobservedandpredictedm magnitudesfromEquation3.1attheasteroseismic(left)andTGAS(right)parallaxesofthe
Ks
RCstars.Bottom:residualsbetweenobservedandpredictedparallaxesfromEquation3.1attheasteroseismic(left)andTGAS(right)parallaxes
oftheRCstars.Thereddash-dotlinesandshadedconfidenceregionsfollowthepredictedoffsetof−0.39±0.08masoftheTGASparallaxes
byStassun&Torres(2016),adoptingtheeclipticlatitudeβ = 55◦±5oftheKeplerfield-of-view.Theinsertsinthemiddleandbottompanels
areviolinplotsofthedistributionsofthefractionaldifferencesandresiduals,withthehorizontalblacklinesgivingthemedianvalues;thecolors
indicatethem magnitudes.
Ks
(Girardi et al. 2005) of single RC stars. The expected depen- pected to achieve sufficiently high precision that the tests here
denceisclear,asistheintrinsicspreadintheRCK luminosity could be used to identify unresolved binaries and constrain in-
s
(seeEquation3.1).TheextinctionintheK band,A ,wasdeter- terstellarreddening.
minedfromtheA valuesinTRILEGALsandR =Ks0.355±0.1
(aspertherealdaVta)andR =3.1±0.1(CardelKlis etal.1989).
V
4. Suggestedcorrection
Asexpected,forthesmallvaluesofextinctionexpectedhere,
bandextinctionhaslittleeffectontheinferredasteroseismicdis-
Using the sample of RC stars we propose a correction to the
tances(whenusingthe Ks).Wehaveinvestigatedtheimpactof TGAS parallaxes. We find that the median offset between the
apoorlyestimatedextinctionbychangingthetrueextinctionby
TGASparallaxesandthevaluespredictedfromEquation3.1is
afactoroftwo.Theresultsofthistestshowthatevenwiththis
very close to −0.1mas (in the sense that TGAS overestimates
largesystematicerrorinextinction,theimpactontheestimated
the distance) which is less than either the −0.39mas correc-
distancesisoftheorderofonlyafewpercent. tion for the Kepler field ecliptic latitude or the bulk offset of
We have also used the synthetic population to estimate the −0.25mas given by Stassun & Torres (2016). However, we see
impact of unresolved binaries on the apparent K -band magni- the bias decrease at smaller parallaxes, and at parallaxes larger
s
tudes. We find that only for a few systems (< 1%), with lumi- than∼1.6masourcorrectioniscomparabletothatofStassun&
nosity ratios close to unity, does the change in apparent mag- Torres(2016).
nitude significantly impact the asteroseismic distance estimate. Figure 4 shows the correction we propose that should be
It is worth noting that later releases of Gaia parallaxes are ex- added to the TGAS parallaxes in the Kepler field as a function
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A&Aproofs:manuscriptno.Gaia_hack
10.5
2.4
9.6
10.0
2.0
9.5 as) 9.2
m 1.6
(mag) 89..50 (ϖTGAS 1.2 8.8ag)
mKs 8.0 allax, 0.8 8.4m(mKs
par 8.0
7.5 AS 0.4
G
T 7.6
7.0
0.0
7.2
n 1.1 −0.4
o 10.0
ati 0.4 0.8 1.2 1.6 2.0
Rel 1.0 9.5 TheoreticalRCparallax, ϖdist(mas)
/
Ks
m 0.9 9.0ag) Fofigt.he4R–CPasrtaalrlsaxaensdpthreodsiectoefdTfGroAmSE.qTuhaetifounll3r.e1daltintheeshmoKwssmthagen1i:t1udrees-
m
0.4 8.5m(Ks ltahteioans.soTchieatebdlac1k-σlinuenscegritvaeintthyegliivneenarbyfitthbeetbwleueenshthaedepdarraelglaioxness.wTihthe
on 0.2 8.0 greendashedlineshowsthepredictedoff-setof−0.39masfromStas-
ati sun & Torres (2016), adopting an ecliptic latitude of β = 55◦ of the
el 0.0
R 7.5 Kepler field-of-view. The marker colors indicate the m magnitudes
ϖ−−0.2 ofthestars.ThecorrectionofEquation4.1isgivenbyKtshedifference
7.0 betweenthe1:1relationandthelinearfit.
−0.4
0.5 1.0 1.5 2.0 2.5 3.0
TRILEGALparallax(mas)
WehavealsoconsideredsixRCstarswithasteroseismicdetec-
Fig.3–asFigure2butwithasimulatedRCpopulation. tionsintheopenclusterM67butfinddiscrepanciesintheTGAS
parallaxes that would be challenging to explain in terms of a
colororspatiallymotivatedsystemicerror.
of the theorectical RC parallax i.e., the parallax that would be
WehaveproposedacorrectiontotheTGASparallaxesasa
returnedbyanaccurateestimateofthedistance,(cid:36) :
dist function of distance (as described by parallax). This correction
∆(cid:36)=α+γ×(cid:36) , (4.1) isapplicabletotheKepler field-of-viewatdistances> 500pc,
dist butcouldbeusedatsimilareclipticlatitudes(β = 55◦±5)fol-
withparametersα=−0.15±0.06,γ=0.29±0.06,andaparam- lowingthefindingsofStassun&Torres(2016).Careshouldbe
eter correlation of rα,γ ≈ −0.95. Uncertainties were estimated exercisedwhenapplyingthiscorrectiontootherlatitudesshould
fromtheODR.Thecorrectionmayinpracticebeappliedinan the TGAS bias be position dependent. The correction we find
iterativemanner.Onebeginsbyadopting(cid:36)TGAS =(cid:36)disttocom- converges to the Stassun & Torres (2016) correction at paral-
puteacorrection∆(cid:36)usingEquation3.1,andhenceacorrected laxes larger that ∼1.6mas but is noticeably smaller at greater
parallax.ThiscorrectedparallaxisthenusedasaninputtoEqua- distances.
tion 3.1 to compute a new correction, and an iterated corrected
Ourresultsthenbridgethegapbetweenthepreviousworks
parallax.Theprocessisrepeateduntilgoodconvergenceisfound
thatfavoracorrectionbutaresensitivetonearbystars(Stassun&
(typicallythisrequiresonlyoneortwoiterations).
Torres2016;Jaoetal.2016),andworksthatfavornocorrection
This correction is calibrated with Kepler data and hence is
but are sensitive to more distant stars (Lindegren et al. 2016;
strictly applicable to stars in the Kepler field. If, as suggested
Sesar et al. 2016). We conclude that a correction to the TGAS
byStassun&Torres(2016),anycorrectionofTGASparallaxes
sampleisrequiredbutthatthiscorrectionbecomesnegligibleat
shouldbeafunctionofeclipticlatitude,thenthecorrectioncould
distancesgreaterthan∼1.2kpc.
beextendedtoallstarsatsimilarlatitudes.Careshouldbetaken
ifthiscorrectionistobeappliedatdifferentlatitudes. Acknowledgements. WethankJ.deRidder,D.Huber,J.Heifetzandtheanony-
mousrefereefortheirhelpandusefulcomments.Thispublicationisadirect
result of a hack session organised in the Solar and Stellar Physics group at
5. Conclusions theUniversityofBirmingham.ThisworkhasmadeuseofdatafromtheEu-
ropeanSpaceAgency(ESA)missionGaia(http://www.cosmos.esa.int/
Inthisletterwehaveusedredclump(RC)starsasdistanceesti- gaia),processedbytheGaiaDataProcessingandAnalysisConsortium(DPAC,
matorstotestthesystematicerrorintheGaiaDR1TGASsam- http://www.cosmos.esa.int/web/gaia/dpac/consortium).Fundingfor
theDPAChasbeenprovidedbynationalinstitutions,inparticulartheinstitu-
ple. We have reproduced the results of De Ridder et al. (2016)
tionsparticipatingintheGaiaMultilateralAgreement.Theauthorsacknowledge
showing there is a disagreement between the TGAS distances thesupportoftheUKScienceandTechnologyFacilitiesCouncil(STFC).MNL
and asteroseismic distances. By testing both results against the acknowledges the support of The Danish Council for Independent Research |
RC distance scale we have demonstrated that the asteroseismic NaturalScience(GrantDFF-4181-00415).FundingfortheStellarAstrophysics
Centre(SAC)isprovidedbyTheDanishNationalResearchFoundation(Grant
distances are in better agreement with the RC than the TGAS
DNRF106).LGandTSRacknowledgesupportfromPRININAF2014(CRA
distances.TheTGASsampleshowedamedianoffsetofaround 1.05.01.94.05)
−0.1mas (in the sense that TGAS overestimates the distance).
Articlenumber,page4of5
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