Kinematics of Nearby Subdwarf Stars

0.37 MB·

by  M. I. Arifyanto

Astronomy&Astrophysicsmanuscriptno.aa030829 February5,2008
(DOI:willbeinsertedbyhandlater)
Kinematics of Nearby Subdwarf Stars
M.I.Arifyanto, B.Fuchs,H.Jahreiß, andR.Wielen
AstronomischesRechen-Institut,Mo¨nchhofstraße12-14,D-69120Heidelberg,Germany
Received/Accepted
5
0 Abstract. We present an analysis of the space motions of 742 subdwarf stars based on the sample of Carney et al.(1994).
0 Hipparcosparallaxes,TYC2+HIPpropermotionsandTycho2propermotionswerecombinedwithradialvelocitiesandmetal-
2
licitiesfromCLLA.Thekinematicalbehaviorisdiscussedinparticularinrelationtotheirmetallicities.Themajorityofthese

n samplestarshavemetalabundancesof[Fe/H]>−1andrepresentthethickdiskpopulation.Thehalocomponent,with[Fe/H]
a <−1.6,ischaracterizedbyalowmeanrotationvelocityandaradiallyelongatedvelocityellipsoid.Intheintermediatemetal-
J licity range (−1.6 ≤ [Fe/H] ≤ −1), we find a significant number of subdwarfs with disklike kinematics. We interpret this

2 populationofstarsasametal-weakthickdisk(MWTD)population.
1

Keywords.stars:subdwarfs–Galaxy:kinematicsanddynamics–Galaxy:solarneighbourhood
1
v
2
1 1. Introduction etal. 1988). Thehalo populationin thesolar neighborhoodis
2 not purely a relic of a monolithic, “rapid” collapse (Carney
1 Studies of the kinematicsof variousstellar populationsin the et al. 1996). There have been several suggestions of a two-
0 Galaxy,inparticularthethickdiskandthehalo,havelongbeen
componenthalo,with a flattenedcomponentinthe innerhalo
5 limited bythe availabilityoflargesamplesofstars with mea-
andamoresphericalouterhalo(Sommer-Larsen&Zhen1990;
0
surements of proper motions, radial velocities, distances, and
/ Carneyetal.1996).
h metallicities.Suchdataarerequiredinordertoconstrainplau-
TheGalacticthickdiskisthekinematicallyhottestportion
p sible scenarios for the formation and evolution of the Milky
- of the disk of the Galaxy, with a scale height of 1.0 to 1.5
Way. Samples of nearby subdwarf stars with high space mo-
o kpcandrotateswithavelocityofabout170kms−1 (Gilmore,
r tionsprovideanobservationallyconvenientprobeofthestruc-
t Wyse, & Kuijken 1989). The thick disk is usually considered
s ture of the Galaxy. The large, proper-motion-selected, stellar
a samples of Carney, Latham, Laird, and Aguilar (1994, here- to be dominated by stars in the range [Fe/H] > −1 (Freeman
: 1987), peaking at about [Fe/H] = −0.5 (Carney et al. 1989).
v afterCLLA)haveprovedparticularlyvaluableforstudyingthe
i kinematicsandchemicalabundanceswithinafewkiloparsecs Manyworkershaveclaimedtheexistenceofametalweaktail
X ofthethickdiskcomponentin therange−1.6 ≤[Fe/H]≤ −1
oftheSun.
r (MFF; Beers & Sommer-Larsen 1995; Chiba & Beers 2000).
a The correlation between kinematics and metallicity gives
MFF found a fraction of 72% of the stars in this metallicity
useful information for formulating theories of galactic struc-
rangeina”metal-weakthickdisk”(MWTD),rotatingrapidly
ture. Differencesin chemistryand space velocitiesare crucial
at V ≈ 170 km s−1. Another large fraction of MWTD was
indefiningthedifferentpopulationswithintheGalaxyandin- rot
foundalsobyBeers&Sommer-Larsen(1995).TheirMWTD,
ferringtheirorigins.Relevantstudiesofthekinematicalbehav-
rotatingatV ≈ 195kms−1,accountsforabout60%ofthe
iorofstars,inparticularinrelationtotheirmetallicities,were rot
stars in the range −1.6 ≤ [Fe/H] ≤ −1 in the solar neighbor-
presentedbye.g.Morrison,Flynn,&Freeman(1990,hereafter
hood, and it possesses an extremely metal-weak tail down to
MFF)usingasampleofKgiantswhosemetallicitiesaremea-
[Fe/H] ≤ −2. Chiba & Beers (2000) estimated the fraction of
sured using the DDO photometricsystem, Nissen & Schuster
MWTDatabout30%ofthemetal-poorstarsintheabundance
(1991)usinglateFandGdwarfsandsubgiants,Chiba&Yoshii
range −1.7 ≤ [Fe/H] ≤ −1, which is smaller than the frac-
(1998)usingredgiantsandRRLyraestars,Martin&Morrison
tionderivedbyMFFandBeers&Sommer-Larsen(1995),but
(1998) with a sample of nearby RR Lyrae stars and Chiba &
largerby∼10%thantheresultofChiba&Yoshii(1998)using
Beers(2000)using1203metal-poorsolar-neighborhoodstars.
solarneighborhoodredgiantsandRRLyraestars.
The Galactic halo is characterized by a roughly spherical
The investigation of thick-disk and halo kinematics may
space distribution with close to zero net rotation. Its stars are
only be applicable to a specific place in the Galaxy and may
metal poor, with a peak metallicity at [Fe/H] = −1.6 (Laird
have fine structure of the velocity distribution smoothed out
Send offprint requests to: M. Ikbal Arifyanto, e-mail: by the velocity resolution of the study (Martin & Morrison
[email protected] 1998). Here, we study the kinematics of solar neighborhood
2 M.I.Arifyantoetal.:KinematicsofNearbySubdwarfStars
100 All Data 100 [Fe/H]<-1.6
2
50 50
0 0
0 50 100 0 50 100
4
100 [Fe/H]>-1.0 100 -1.6<[Fe/H]<-1.0
50 50
0 0
6 0 50 100 0 50 100
100
50
8
0
0.2 0.4 0.6 0.8 1 1.2 1.4 0 50 100
B-V
Fig.1.Color-MagnitudediagramforallidentifiedCLLAstars. Fig.2. Hipparcos trigonometric parallaxes versus the photo-
Hipparcosparallaxeswereusedtodetermine M anditsstan- metricparallaxesofCLLA,for539stars(top-left)andfordif-
V
darderror. The fulllines indicate the zero age main sequence ferentmetallicitycuts:[Fe/H]<−1.6(top-right),[Fe/H]>−1
(ZAMS) and main sequences of the old open clusters M 67 (center-left),−1.6≤[Fe/H]≤−1(center-right),andforstarsat
andNGC188.ThedashedlineistheZAMSshiftedupwardby largedistances,π < 25mas(bottom-left).Thefulllineisa
Hip
∆M = 0.8 mag., used to remove the contamination by sub- linearfittothedata.
V
giantsandgiants.
subdwarfstarsbasedonthesampleofhighpropermotionstars
byCLLA.CLLAhavemeasuredphotometricparallaxes,radial metricCLLAdistancesdeterminedusingstarswithHipparcos
velocities,andmetallicitiesofmainlyAtoearlyGstars,many parallaxes.
late G and some early K stars in the Lowell Proper Motion In Sect. 2 we describethe selection of the sample andthe
Catalogue. In total the CLLA sample contains 1464 stars. In photometric correction. In the next section we determine the
theirpapertherearelisted1269starswithkinematicalparam- kinematicalproperties,i.e.the3Dvelocitieswithrespecttothe
etersand1261starswithmetallicityparameters,andthereare Sun,anddiscussthekinematicalbehaviorofthesubdwarfs,in
1447starswithradialvelocitiesintheircatalog.Theradialve- particularinrelationtotheirmetallicities.Ourconclusionsare
locity precision of their sample lies in the range of 0.4 to 1.3 summarizedinSect.4.
kms−1.About15%oftheirsamplearebinariesormultiplesys-
tems.Thetypicalaccuracyofthemetallicitieswasestimatedto
be±0.13dex.
The photometric parallax of CLLA was replaced in our 2. Data
study by using the high precision parallax of Hipparcoscata-
logue.WeusedtheAstrometricCatalogTYC2+HIP(Wielenet TheCLLAdatasetof1447starshasbeencross-identifiedwith
al. 2001) for the proper motions of stars with Hipparcos par- theAstrometricCatalogueTYC2+HIP(Wielenetal.2001)and
allaxes. This catalogue is derived from a combination of the we found 545 stars in common. About 700 CLLA stars did
HipparcosCataloguewithpropermotionsgivenintheTycho-2 not appear in the TYC2+HIP catalogue, but were then cross-
cataloguewithdirectsolutions(Wielenetal.2001)andprevi- identifiedwiththeTycho-2Catalogue(Høgetal.2000),andwe
ousearthboundmeasurements.We stillusethehighprecision found259subdwarfswithTycho-2propermotions.Theproper
radial velocities and metallicities of the CLLA catalogue for motionaccuracyof Tycho 2, derivedfroma comparisonwith
ourstudy.PreviousworkusingHipparcossubdwarfswasdone theAstrographicCatalogueand143otherground-basedastro-
byReid(1998)andFuchs,JahreißandWielen(1998;hereafter metriccatalogues,isabout2.5masyr−1.CLLAusedLuyten’s
FJW). Inthispreviousworkwediscussedthekinematicalbe- NLTTpropermotionsforthecalculationofthespacevelocity
haviour of the 560 subdwarfs for which improved parallaxes components.Thesepropermotionshavetypicalerrorsof20to
andpropermotionswereobtainedbyHipparcos,inrelationto 25 mas yr−1. Therefore,TYC2+HIP and Tycho-2 propermo-
theirmetallicities.Inthepresentpaperweincreasethesizeof tionsprovideanenormousimprovementintheaccuracyofthe
thesampleconsiderablybyapplyingacorrectiontothephoto- tangentialvelocities.
M.I.Arifyantoetal.:KinematicsofNearbySubdwarfStars 3
2.1.ColorMagnitudeDiagram
200 (a) 200 (d)
Fig.1showsthecolormagnitudediagramforall545identified
0 0
CLLA stars. The absolute magnitudes and their standard er-
rorsarebasedontheHipparcosparallaxesandtheirerrors.The -200 -200
B−V colorsweretakenfromtheHipparcoscatalogue.Some
-200 0 200 -200 0 200
starsforwhichnodistancewasgivenwerealreadyrecognized X [pc] X [pc]
byCLLA as subgiants.However,we cansee clearly fromthe
CM-diagraminFig.1thatthereisstillcontaminationbyprevi- 200 (b) 200 (e)
ouslyundetectedsubgiantsandgiants.Toavoidthese,wehave
0 0
removedallstarslyingabovealineintheCM-diagramdefined
by the zero age main sequence of stars with solar metallicity -200 -200
shiftedupwardby∆M = 0.8mag.About8%contaminating
V -200 0 200 -200 0 200
starswereeliminatedinthisway. X [pc] X [pc]
200 (c) 200 (f)
2.2.TestofPhotometricDistances
0 0
We determined the overall correction of the photometric dis-
tance scale of CLLA by analyzing the parallax difference. -200 -200
There are 539 CLLA stars which have both photometric and
-200 0 200 -200 0 200
trigonometric parallaxes in our sample (CLLA-TYC2+HIP). Y [pc] Y [pc]
WecomparedtheHipparcosparallaxwiththephotometricpar-
Fig.3. Spatial distribution of the samples CLLA-TYC2+HIP
allax of CLLA (as shown in Fig. 2). The error bars represent
(atoc)andCLLA-Tycho-2(dtof),respectively.Xpointsto-
both Hipparcos and CLLA parallax errors. A typical error in
wardstheGalacticcenter,Yinthedirectionofgalacticrotation
the absolute magnitude of CLLA stars of σ = 0.3 mag is
MV andZtowardstheGalacticnorthpole.
assumed.Weusedaleastχ2 methodapplicablewhenthedata
haveerrorsinbothcoordinates.Theχ2-functionischosenac-
cordingtoPressetal.(1992) 400 (a) 400 (d)
200 200
N (y −bx)2
χ2 =X i i (1) 0 0
σ2 +b2σ2
i=1 yi yi -200 -200
The slope of the regression b derivedfrom our 539 subd-
-400 -400
warfs, is b = 1.116±0.008. For comparison, a sample with
400 (b) 400 (e)
π ≤25masleadstoalargercorrectionofb=1.169±0.028.
Hip
We also tried cuts in the metallicties, with the result that for 200 200
moremetal-poorstarsalargercorrectionwasneeded.Forstars 0 0
with [Fe/H] > −1, which are dominated by thick disk stars,
-200 -200
−1.6≤[Fe/H]≤−1,andextrememetal-poorstarswith[Fe/H]
-400 -400
<−1.6wefindslopesoftheregressionlineb=1.093±0.010,
b=1.324±0.042andb=1.394±0.043,respectively.Jahreiß, 400 (c) 400 (f)
FuchsandWielen(1997)andFJWhavefoundsimilarcorrec- 200 200
tionsusing a smaller sample of subdwarfs.The data pointsat 0 0
theupperrightcornerofthefirstplot(AllData)andthefourth
-200 -200
plot(−1.6≤[Fe/H]≤−1)representthestarHIP57939,which
istheneareststarinoursamplewithπ = 109mas.Nosig- -400 -400
Hip
nificantchangesin the slopes of both plots (less than 1σ) are -3 -2 -1 0 -3 -2 -1 0
[Fe/H] [Fe/H]
found,ifweomitHIP57939whencalculatingtheslopes.
All these corrections are used to calibrate CLLA photo- Fig.4. Space velocity components (U,V ,W) versus metal-
rot
metric parallaxesin our sample with no Hipparcosparallaxes licity [Fe/H] of the samples CLLA-TYC2+HIP (a to c) and
andabout35starswithlowaccuracyHipparcosparallaxesi.e. CLLA-Tycho-2(dtof),respectively.
π <5masandπ /σ <3mas.
Hip Hip πHip
ofgalacticrotation,andnorthgalacticpole,respectively,have
3. KinematicalProperties
beencalculatedwithrespecttotheSunandthenreducedtothe
Using the parallaxes supplied by the Hipparcos catalogue, LSR (local standard of rest). For the latter Delhaye’s (1965)
propermotionsbytheTYC2+HIPandTycho-2catalogue,and values +9,+12,+7km s−1 were adopted for U⊙,V⊙,W⊙, re-
radialvelocitiesgiveninCLLA,thespacevelocitycomponents spectively.Finally,thevelocitycomponentsweretransformed
U,V,andW,whicharedirectedtotheGalacticcenter,direction onto a frame rotating with circular velocity V = −220 km
circ
4 M.I.Arifyantoetal.:KinematicsofNearbySubdwarfStars
400 spectively,whichare in goodagreementwith RR Lyraekine-
CLLA-TYC2+HIP sample
maticsofMartin&Morrison(1998)andLaydenetal.(1996).
300 Chiba&Beers(2000)foundalowervelocitydispersioninthe
U-direction,(σ ,σ ,σ )=(141±11,106±9,94±8)kms−1
U V W
200 fromtheir1203non-kinematicallyselectedstars.
200
150 The velocity dispersion components of the sample in the
moremetal-richabundancerangesdecreaseasthecontribution
100
100
ofthethickdiskcomponentprogressivelyincreases.Inpartic-
50
ular,for[Fe/H]>−1.0thecontributionofthehalocomponent
0 isexpectedtobenegligible.OurCLLA+TYC2+HIPsamplein
CLLA-Tycho2 sample thismetallicityrangehasvelocitydispersions(σ ,σ ,σ ) =
U V W
300 (74 ± 2,50 ± 1,37 ± 1) with Vrot = 176 km s−1, which is
in agreement with thick disk samples of Martin & Morrison
200 (1998) of RR Lyrae stars and Chiba & Beers (2000) of solar-
200
150 neighborhoodstars.
Our CLLA-Tycho2 sample is more sparsely distributed
100
100 than the CLLA-TYC2+HIP sample in the metallicity range
50
[Fe/H] > −1. To understand this, we note that the CLLA-
0 Tycho2samplewasdrawnfromtheCLLAstarsthatarenotin
-200 -100 0 100 200 300 theHipparcoscatalogue.ThismightimplythatmainlyCLLA
starswithmagnitudesbrighterthan10.5magfallinourCLLA-
Fig.5.Toomrediagram:(U2+W2)1/2versusVrot distributions TYC2+HIPsampleandstarswithmagnitudesfainterthan10.5
of CLLA-TYC2+HIP and CLLA-Tycho2 samples. The solid magareintheCLLA-Tycho2sample.Starswithfainterappar-
linesrepresenttotalvelocitiesof50,100,150and200kms−1, entmagnitudesareatlargerdistancesandvelocitiescompared
respectively. totheHipparcosstars.Theminimumdistancesforeachsample
are17and40pcforCLLA-TYC2+HIPandCLLA-Tycho2,re-
spectively.Figure3, wherewe plotthe spatialdistributionsin
s−1 relative to the LSR, i.e. the expected rest frame of our X,Y andZshowsthisclearly.We canfindtheminimumtan-
Galaxy (e.g. Wielen 1986). The rotational velocity is defined gentialvelocityusingtheminimumdistancesandmeanproper
asVrot =V −Vcirc. motionsforbothsamples,using
Figure4showstheU,V ,andWvelocitiesofthesamples
rot
CLLA-TYC2+HIPandCLLA-Tycho2asscatterplots.TheU-
hµi
distributionindicatesthatthepresentsamplewaskinematically V = 4.74 d (2)
Tmin min
1000
selected.TheCLLA Catalogisbasedonapropermotioncat-
alog so that stars with small tangentialvelocitiesare missing. wherehµi,d ,andV denotemeanpropermotionsin
min Tmin
We can see clearly that for small U-values the diagrams are masyr−1,minimumdistancesinparsecsandminimumtangen-
sparsely populated. This is also seen in the V-velocities. The tial velocities in km s−1 for each subdwarf sample. We found
starswithmetallicities[Fe/H]>−1lagontheaveragebyabout fortheCLLA-Tycho2sampleV >50kms−1,whichmight
Tmin
40km s−1, i.e. are thickdisk stars. The oldthin disk starsare explainwhytherearecomparativelyfewthickdiskstarsinthis
missing(cf.alsoFig.5).IntheW-velocitiesnokinematicalbias sample(cf.Fig.5).
isvisible;itisapparentlylostinprojection.However,sincewe
are mainly interested in the kinematics of the halo stars, this
3.1.V distributionsofsubdwarfs
bias is of no consequencein the present context. It is evident rot
from this figure that metal-poor stars with [Fe/H] < −1 have The corresponding V -velocity distributions for the samples
rot
largerrandommotionscomparedwith metal-richones[Fe/H] CLLA-TYC2+HIPandCLLA-Tycho2areshowninFigs.6and
> −1. This shows that the kinematicpropertieschangerather 7, respectively. The first group, [Fe/H] > −1 dex, represents
abruptlyat[Fe/H]≈ −1to−2,whichisprobablethetransition whatareobviouslythethickdiskstars.Thethirdgroup,[Fe/H]
regionfromhalotodiskcomponent(Ryan&Norris,1991and <−1.6dex,consistsofextrememetal-poorstars,dominatedby
Chiba&Yoshii,1998). members of the halo population. The histograms of the sam-
ThemeanmotionwithrespecttotheLSRandvelocitydis- ples CLLA-TYC2+HIP and CLLA-Tycho2 can be fitted by
persionswerecalculatedfordifferentgroupsin[Fe/H].There- Gaussiandistributions.
sultsarepresentedinTable1.Themostmetal-deficientstarsin The second group,−1.6 ≤ [Fe/H] ≤ −1 dex, shows a pe-
the samples, more metal poor than [Fe/H] = −1.6, are dom- culiar kinematics. A Kolmogorov-Smirnovtest, which avoids
inated by members of the halo population. These stars ex- binningofthedata,showsthatthevelocitydistributionofthe
hibit a radially elongated velocity ellipsoid (σ ,σ ,σ ) = verymetal-poorstars,[Fe/H]≤ −1.6,inthecombinedsample
U V W
(189±13,97±7,98±7) and (157±12,87±7,77±6) km isstatisticallydifferentfromthevelocitydistributionofthein-
s−1 andshownonetrotation,hV i = 1±13and15±14km termediate population,−1.6 ≤ [Fe/H] ≤ −1.0. The maximum
rot
s−1 fortheCLLA-TYC2+HIPandCLLA-Tycho2samples,re- deviation of the normalized cumulative distribution between
M.I.Arifyantoetal.:KinematicsofNearbySubdwarfStars 5
Table1.MeanVelocitiesandVelocityDispersionoftheSampleStars
N hUi hVi hWi σ σ σ
U V W
dex kms−1
CLLA-TYC2+HIP
[Fe/H]>−1.0 381 −11±4 −44±3 −4±2 74±2 50±1 37±1
−1.6≤[Fe/H]≤−1.0 49 −35±23 −163±12 −1±11 158±11 85±6 74±5
[Fe/H]<−1.6 53 −5±26 −219±13 −1±13 189±13 97±7 98±7
CLLA-Tycho2
[Fe/H]>−1.0 169 −9±8 −91±6 −5±4 110±4 81±3 58±2
−1.6≤[Fe/H]≤−1.0 50 51±17 −180±10 4±9 121±9 68±5 61±4
[Fe/H]<−1.6 40 36±25 −205±14 −19±12 157±12 87±7 77±6
5 3
[Fe/H]<-1.6
4 [Fe/H]<-1.6
2
3
2
1
1
0 0
-200 0 200 -200 0 200
Vrot [km/s] Vrot [km/s]
4 6
-1.6<[Fe/H]<-1.0 -1.6<[Fe/H]<-1.0
3
4
2
2
1
0 0
-200 0 200 -200 0 200
Vrot [km/s] Vrot [km/s]
40 15
[Fe/H]>-1.0
[Fe/H]> -1.0
30 10
20
5
10
0 0
-200 0 200 -200 0 200
Vrot [km/s] Vrot [km/s]
Fig.6. Rotational velocity (V ) distributions of sample Fig.7.ThesameasFig.6,butforsampleCLLA-Tycho2.
rot
CLLA-TYC2+HIP grouped according to their metallicities.
Thevelocitiesarereducedtothelocalstandardofrest.
4. SummaryandDiscussion
We have analyzed the kinematics of 742 nearby metal-poor
thetwogroupsis D = 0.241andthussignificantlylargerthan subdwarfstarsfromtheCLLAcatalogue.Thesubdwarfswere
thecriticalvalueD0.05 =0.196(Sachs,1988),whichleadstoa cross-identified with the TYC2+HIP and Tycho 2 Catalogues
rejectionofthehypothesisofthestatisticalsimilarityoftheve- to find accurate trigonometric parallaxes and proper motions.
locitydistributionsofthetwogroups.Similarlywehaveshown The accurate Hipparcos parallaxes lead to an upward correc-
thatthevelocitydistributionofthemetal-poorstarsissymmet- tionfactorof11%ofthephotometricdistancescaleofCLLA,
ricwithrespecttoVrot =0kms−1 (D=0.149,D0.05 =0.282), anditwasusedtocorrectthephotometricdistancesofCLLA-
whereas the velocity distribution of the intermediate group is Tycho2stars.
asymmetric(D=0.375,D =0.300).Thustheintermediate
0.05 Thepresentanalysisindicatesthatthesolar neighborhood
groupseemstorepresentadifferentpopulationofhalostars. subdwarfstarswith[Fe/H]<-1.6showhalokinematicscharac-
On the other hand, the asymmetric drift ratio hVi/σ2 = terizedbyaradiallyelongatedvelocityellipsoidandnosignif-
U
−0.007(CLLA-TYC2+HIP)isverysimilartothatofthethick icantrotation.Atametallicityrangeof[Fe/H]> −1,oursam-
disk stars, hVi/σ2 = −0.008. We conclude tentatively that plesshowdisklikekinematics.Inthemetallicityrange−1.6 ≤
U
thestarsinthe−1.6 ≤[Fe/H]≤ −1.0metallicityrangerepre- [Fe/H]≤−1.0wefoundasignificantnumberofstarswithkine-
sentapopulationofthedynamicallyhotmetal-weakthickdisk matics not of halo stars but that of a dynamically not-metal-
(MWTD). weaktailofthethickdisk.
6 M.I.Arifyantoetal.:KinematicsofNearbySubdwarfStars
Chiba&Beers(2000)obtainedafractionof30%oflow- Chiba,M.,&Yoshii,Y.1998,AJ,115,168
metallicitystarsintheirnonkinematicallyselectedsolarneigh- Delhaye, J.1965, inGalacticStructure,StarsandStellarSystems5,
borhoodsamplewith−1.7<[Fe/H]≤-1.0,whichisconsistent 61
with our result of 18%. Chiba & Yoshii (1998) analyzed the European Space Agency (ESA) 1997, The Hipparcos and Tycho
Catalogues(ESASP-1200)(Noordwijk:ESA)
kinematicsofredgiantsandRRLyraestarsinthesolarneigh-
Freeman,K.C.1987,ARA&A,25,603
borhoodbased on Hipparcosdata. Theyfoundin bothredgi-
Fuchs,B.,Jahreiß,H.,&Wielen,R.1998,Ap&SS,265,175(FJW)
antandRRLyraesamplesintherange−1.6<[Fe/H]≤-1.0,a
Gilmore,G.,&Wyse,R.F.G.1985,AJ,90,2015
fractionof∼10%ofstarsinapopulationwithameanvelocity
Gilmore,G.,Wyse,R.F.G.,&Kuijken,K.1989,ARA&A,27,555
hV i =195kms−1.
φ disk Gilmore,G.,Wyse,R.F.G.,&Norris,J.N.2002,AJ,574,L39
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Morrison, H. L., Flynn, C., & Freeman, K. C. 1990, AJ, 100, 1191
tainlystillappearstobeaminorconstituentoftheentirethick
(MFF)
diskpopulation(Beersetal.2002).
Nissen,P.E.,&Schuster,W.J.1991,A&A,251,457
If there is indeed a significant fraction of thick disk stars
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MWTD component may be the merging of satellite galaxies Ryan,S.G.,&Norris,J.E.1991,AJ,101,1835
(Searle& Zinn,1978),whichare thenaccretedbya thin,fast Sachs,L.1988,StatistischeMethoden,SpringerVerlag,Berlin.
rotating,possiblymetal-poor,Galacticdisk(Quinnetal.1993; Searle,L.,&Zinn,R.1978,ApJ,225,357
Sommer-Larsen,J.,&Zhen,C.1990,MNRAS,242,10
Wyse2001).Thedynamicalheatingofthestellarcomponentof
Wielen,R.1986,TransactionsIntern.Astron.Union19B,93
thisdiskinconnectionwiththeaccretionprocessproducesthe
Wielen, R., Schwan, H., Dettbarn, C., et al. 2001, Astrometric
thickdisk.Thekinematicsofthehalodependsonthedynamics
Catalogue TYC2+HIP Derived from a Combination of the
of the merging satellites, whereas the kinematics of the thick
HIPPARCOS Catalogue with the Proper Motions Given in the
disk are determined by the heating of the rotating thin disk.
TYCHO-2 Catalogue, Vero¨ff. Astron. Rechen-Inst. Heidelberg
Basedonthismergingpictureofgalaxyformation,onemight No.39
arguethatthe“shreddedsatellite”starsretainakinematicsig- Wyse,R.F.G.2001,inGalaxyDisksandDiskGalaxies,ASPConf.
nature distinct from the thick disk part that results from the Series230,ed.J.G.Funes&E.M.Corsini,71
heatedthindisk.Thekinematictraceofthedestroyedsatellite,
whichisprobablytheoriginoftheMWTDstars,wouldbevis-
ibleinthemeanorbitalrotationalvelocityofstars.Basedona
spectroscopicsurveyof∼2000F/Gstars0.5−5kpcabovethe
Galactic plane Gilmore et al. (2002) determineda mean rota-
tional velocity lag of the shredded galaxiesof ∼ 100 km s−1.
The actual lag expected from the shredded satellite depends
predominantly on the initial orbit and the amount of angular
momentumtransportinthemergerprocessandisnotinitially
predictableinaspecificcase(Gilmoreetal.2002).
Acknowledgements. M.I.A acknowledges support for this work as
partofaPh.DthesisfromaDAADscholarship.
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