A massive association around the obscured open cluster RSGC3

1.2 MB·English

by  Ignacio Negueruela

Astronomy&Astrophysicsmanuscriptno.16102 (cid:13)c ESO2013
January9,2013
,⋆
A massive association around the obscured open cluster RSGC3
I.Negueruela1,C.Gonza´lez-Ferna´ndez1,A.Marco1,andJ.S.Clark2
1 DepartamentodeF´ısica,Ingenier´ıadeSistemasyTeor´ıadelaSen˜al,UniversidaddeAlicante,Apdo.99,E03080Alicante,Spain
e-mail:[email protected]
2 DepartmentofPhysicsandAstronomy,TheOpenUniversity,WaltonHall,MiltonKeynes,MK76AA,UK
Preprintonlineversion:January9,2013
1 ABSTRACT
1
0 Context.FourclustersofredsupergiantshavebeendiscoveredinaregionoftheMilkyWayclosetobaseoftheScutum-CruxArm
2 andthetipoftheLongBar.Populationsynthesismodelsindicatethattheymustbeverymassivetoharboursomanysupergiants.If
theclustersarephysicallyconnected,thisScutumComplexwouldbethelargestandmostmassivestar-formingregioneveridentified
n intheMilkyWay.
a Aims.Thespatialextentofoneoftheseclusters,RSGC3,hasnotbeeninvestigated.Inthispaperweexplorethepossibilitythata
J
populationofredsupergiantscouldbelocatedinitsvicinity.

1 Methods.Weutilised2MASS JHK photometry toidentify candidate obscured luminous redstarsinthevicinityof RSGC3.We
S
3 observedasampleofcandidateswiththeTWINspectrographonthe3.5-mtelescopeatCalarAlto,obtainingintermediate-resolution
spectroscopyinthe8000–9000Årange.Were-evaluatedanumberofclassificationcriteriaproposedintheliteratureforthisspectral

] rangeandfoundthatwecoulduseourspectratoderivespectraltypesandluminosityclasses.
A
Results.We measured the radial velocity of five members of RSGC3, finding velocities similar to the average for members of
G Stephenson2.Amongthecandidatesobservedoutsidethecluster,ourspectrarevealedeightM-typesupergiantsatdistances< 18′
from the centre of RSGC3, distributed in two clumps. The southern clump is most likely another cluster of red supergiants, with
.
h reddening and age identical to RSGC3. From 2MASS photometry, we identified four likely supergiant members of the cluster in
p addition to the five spectroscopically observed. The northern clump may be a small cluster with similar parameters. Photometric
- analysisoftheareaaroundRSGC3suggeststhepresenceofalarge(>30)populationofredsupergiantswithsimilarcolours.
o
Conclusions.OurdatasuggestthatthemassiveclusterRSGC3issurroundedbyanextendedassociation,whichmaybeverymassive
tr (>∼105 M⊙).WealsoshowthatsupergiantsintheScutumComplexmaybecharacterisedviaacombinationof2MASSphotometry
s andintermediate-to-high-resolutionspectroscopyintheZband.
a
[ Keywords.stars:evolution–earlytype–supergiant–Galaxy:structure–openclustersandassociations:individual:Alicante7–

RSGC3–Stephenson2
1
v
8
2 1. Introduction plane, between l = 24◦ and l = 29◦ (e.g., Figeretal. 2006;
0 Daviesetal. 2007; Clarketal. 2009a). These clusters are very
Aftercompletionofhydrogencoreburning,massivestarsevolve
0 heavily obscured, meaning that their heretofore observed pop-
. quicklytowardsthecoolsideoftheHRdiagram,wheretheyap- ulation consists solely of RSGs. However, population synthe-
2
pearasredsupergiants(RSGs).Evolutionarymodelsshowthat
0 sis models suggest that the clusters must be very massive to
thistransitionhappensatapproximatelyconstantbolometriclu-
1 harbour these populations (e.g., Daviesetal. 2007; Clarketal.
minosity, M , resulting in extremely high infrared luminosi-
1 bol 2009b). The first cluster found, RSGC1 (Figeretal. 2006), is
ties (Meynet&Maeder 2000; Marigoetal. 2008). As a conse-
: themostheavilyobscured,andalsotheyoungestatanestimated
v quence,RSGs arepowerfulbeaconsthatcanbeidentifiedeven
Xi behindhugeamountsofobscuration.Unfortunately,thephysical τA=li1c2a±nt2eM8,yrloacnadtedMiinnitiailt=s3i±m1m×ed1ia0t4eMv⊙ici(nDitayv,iesseeemtasl.sl2i0g0h8tl)y.
propertiesof RSGs are still poorlyconstrained(Levesqueetal.
r older and rather smaller (Negueruelaetal. 2010a). RSGC2 =
a 2005), although they are known to span a wide range of lu- Stephenson2 (Ste 2)is the least obscuredand apparentlymost
minosities, depending on their initial masses and evolutionary
massive of all, with τ=17±3 Myr and M =4±1 × 104M
status, displaying log(L /L ) ∼ 4.0–5.8 (Meynet&Maeder initial ⊙
bol ⊙ (Daviesetal. 2007; henceforth D07). Finally, RSGC3 lies at
2000). Their infrared spectra, however, do not reflect this va-
some distance from all the others and has an estimated τ=16–
riety.AllsupergiantswithspectraltypesbetweenmidGandlate
M display very similar infrared spectra, with a strong degen- 20MyrandaninferredMinitial=2–4×104M⊙(Clarketal.2009a;
Alexanderetal.2009).Collectively,theseclustershostasignif-
eracy between temperature and luminosity. As a consequence,
icantfractionoftheknownRSGpopulationintheGalaxy.
absolute magnitudes cannot be inferred from infrared spectra,
ThelineofsighttotheRSGclusters(RSGCs)passesthrough
implyingthatit isnotpossibleto assign physicallymeaningful
several Galactic arms and reaches an area of very high ob-
parameterstoanyindividualobscuredRSG.
scuration at a distance comparable to those of the clusters
Inthepastfewyears,aflurryofdiscoverieshasrevealedsev-
(Negueruelaetal.2010a).Stellardensitiesareextremelyhighat
eral clusters of RSGs located in a small region of the Galactic
moderateKmagnitudes,meaningthattheclusterpopulationsare
⋆ Partially based on observations collected at the 3.5-m telescope veryhardtoidentify,exceptfortheRSGs.Becauseofthis,mem-
(CalarAlto,Spain) bershipofindividualstarsisdifficulttoassessandtheactualex-
1
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
tentoftheclustersisuncertain.MembershipinRSGC1andSte2 2. Targetselectionandobservations
hasbeendefinedintermsofacommonradialvelocity,v ,mea-
rad
RSGC3wasdiscoveredasaconcentrationofverybrightinfrared
sured from high-resolution K-band spectra (Daviesetal. 2007,
sources (Clarketal. 2009a; Alexanderetal. 2009). Clarketal.
2008). In the case of RSGC1, the obscurationis very highand
(2009a) explored a circle of radius r = 7′ around the nominal
theRSGmembersareintrinsicallyverybright,resultinginarel-
centre of RSGC3, finding a strong concentration of RSGs to-
ativelyeasyselectionofcandidatemembers.Ontheotherhand,
wards the centre, but also several photometriccandidate mem-
the line of sight to Ste 2 is very rich in bright red stars. D07
bers at the edges of their search area. Alexanderetal. (2009)
find a large number of foreground giants and also several pu-
analysedasmallerarea.
tative RSGs with v apparently excluding membership, even
rad
thoughtheiranalysiswasconfinedtoacirclewithradiusr = 7′ Inregionsofhighextinction,traditionalphotometriccriteria
cannotalways be employedto select stars of a particularspec-
aroundthenominalclustercentre.Fortheotherclusters,dynam-
tral type and luminosity class. While RSGs are very bright in
icalanalysishasnotyetbeencarriedoutandthedetermination
K,severalclassesofredgiantsarealsobrightinfraredsources,
ofmembershipislesssecure.
butmuchmorenumerous.Despitethis,ifwearetodetectRSGs
With the data currently available, the exact nature of this
inthe areasurroundingRSGC3, wemightexpectthemto have
ScutumComplexisunclear.Therehavebeensuggestionsthatit
similarphotometriccharacteristicstoknownclustermembers.
representsagiantstarformationregiontriggeredbythedynam-
icalexcitationoftheGalacticBar,whosetipisbelievedtointer-
sect the Scutum-CruxArm close to this region (see discussion 2.1.Photometriccriteria
in D07; Garzo´netal. 1997). If allthe RSG clusters are located
atacommonnominaldistanceof6.5kpc,theyspan∼500pcin Wehaveused2MASSJHKSphotometry(Skrutskieetal.2006)
toidentifypossibleRSGstars.AsdiscussedinNegueruelaetal.
projection.Alternatively,ifthereisagiantMolecularRingatthe
(2010a),thereddening-freeparameterQ=(J−H)−1.8×(H−K )
endoftheBar(e.g.,Rathborneetal.2009),oursightlinecould S
cutitscrosssectionoveradistance∼3kpc. allowstheseparationofearly-typestars(Q<∼ 0.0)andlate-type
main-sequencestarsandredclumpgiants(Q>∼0.4).RSGsshow
Heretofore,all studies of the RSGCs have been carried out
apeculiarbehaviourandpresentverydifferentQvalues.Mostof
solelywith infrareddata.Intermediate-resolution K-bandspec-
themdereddentothepositionofyellowstars,withQ≈0.2−0.3,
troscopy has been used to observe the CO bandhead absorp-
thoughafractionfallclosetothepositionofredstars(Q≈0.4).
tionfeaturesnear2.3µm,whichidentifycool,evolvedstars.The
Forthispilotprogramme,weselectedstarswithin20′ofthe
equivalent width (EW) of the first bandhead feature is a valid
nominalcentreofRSGC3,with(J −K ) > 1.3.Thecolourcut
estimatorofspectraltypeatintermediateresolution,thoughthe S
was chosen to be redder than the intrinsic colours of any nor-
relationship is bimodal(showing a similar slope for giants and
mal, unreddenedstar, as we expected our objects to suffer sig-
supergiants, but higherEWs for the latter), and scatter is large
nificantextinction.Ontheotherhand,wedidnotwanttoforce
(Daviesetal. 2007; Negueruelaetal. 2010a). Alexanderetal.
candidates to have the same colours as cluster members, since
(2009) explore a similar relation for low-resolution spectra. In
extinctionmightbe variableonsmallscales overthisfield. We
both cases, the accuracy in the spectral-type determination is
selected stars brighterthan K = 7.0, as RSGs are expectedto
estimated at ±2 subtypes. Until the advent of infrared spectro- S
bebright.TypicalmagnitudesforconfirmedmembersofRSGC3
graphswithmultiplexingcapabilities,extendingthismethodol-
lie in the K = 5−6 range, and so K = 7.0 allows formuch
ogy to large samples will consume large amounts of telescope S S
heavierextinction.Finally,weimposed0.1≤ Q≤0.4,therange
time.
wheremostRSGslie,inordertoexcludeforegroundred-clump
Optical spectrographs with high multiplexing factors do, giantsandmain-sequenceM-typestars.Wenotethatthecriteria
however, exist. Highly obscured RSGs are, unfortunately, too are unlikely to select all RSGs, as some of them have colours
faintto be detectedin the opticalrange.The farred optical re- Q ≈ 0.4, or may be so heavily reddenedas to have K > 7.0.
S
gion, between 8000Å and 10000Å offers a good compromise, ThecriteriawerechosentoselectmostRSGs,whileleavingout
asitcanbereachedwithopticalinstrumentationandhaslower manylikelyinterlopers.
extinction.By usingexistingcatalogues,namelyB/DENISand We then checked our selection against the B/DENIS and
USNO-B1.0(Monetetal.2003),wehaveverifiedthatmorethan USNO-B1.0catalogues.StarsclearlydetectedintheDSS2blue
50% of the confirmed RSG members of Ste 2 and RSGC3 are plateswereeliminatedfromthesample.Inaddition,wesetacut
brighter than i = 16 and can be observed at intermediate-high at i > 9.5, as we did not expectour targetsto be brighterthan
resolutionwithexistinginstrumentation. thislimit,giventhatextinctionismuchhigherinithaninK.The
In thispaper,we presentapilotstudyto addressseveralis- sizeofthetelescopeforcesustoalimiti<15.5.Thislimitwas
sues that will allow us to evaluate the possibility of detecting never reached during the observations, as the seeing was rela-
RSGs overa wide area by using optical spectrographs.Firstly, tivelypoorthroughouttherun.Onlystarsbrighterthan i=14.5
we explore the use of photometric criteria and catalogue cross wereobserved.
correlationtodetectcandidateluminousredstars.Secondly,we Thereare∼ 50starsfulfillingthesecriteriaandlyingatdis-
re-evaluateseveralcriteriapresentedintheliteraturetoestimate tances > 7′ and < 20′ from the cluster centre. They are not
spectraltypesandluminositieswithintermediate-resolutionfar evenly spread, and we can identify three areas of high density,
red optical spectra. Finally, we assess the existence of an whichweselectforsystematicobservations(seeFig.1).Field1
extended population of RSGs around RSGC3 that may jus- lies14′South-WestofRSGC3.Thereisacompactgroupofcan-
tify wide-field searches. The paper is organised as follows. In didates with several other objects around. Field 2 contains an
Section 2, we set photometric criteria for selection of candi- obviousconcentrationofverybright(in K)stars,mostofwhich
dates and describe the observations of a sample of candidates. passallthephotometriccriteria.Itislocated ∼ 12′ North-West
InSection3,wediscussseveralmethodsofclassificationforred of the centre of RSGC3. We also include a few fainter stars a
starsinthenear-IR.InSection4,wepresenttheanalysisofour fewarcmintotheNorth.Finally,Field3doesnotappeartocon-
observations.Finally,wediscusstheirimplicationsinSection5. tain any obvious aggregate,instead comprising an area of uni-
2
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
Fig.1. Colour-composite 2MASS JHK finder of the area surveyed. The image covers ∼ 55′ × 35′ (roughly corresponding to
S
100×60pcat6kpc).ThecircleshowingthepositionofRSGC3hasradius6′.Yellowcirclesshowstarsthatpassallthephotometric
criteriainSection2.1.Redcirclesindicatestarswhich,inaddition,aredetectedasDENISorUSNO.B-1sourceswithi>13.2(see
Section5.3forthisselectioncriterion).CrossesindicatethestarsspectroscopicallyidentifiedasreddenedRSGsinthispaper.North
isupandEastisleft.
formly distributed photometric candidates. It lies ∼ 12′ South- to the horizon. Typical exposure times ranged from 150 s for
Eastoftheclustercore.Allthephotometriccandidatesarelisted the brightestobjects (i <∼ 10)to 2000s forobjectsfainterthan
inTable1,wheretheyaregivenanidentifieroftheformFnSnn, i=14.0.Toreduceintegrationtimes,mosttargetswereobserved
where the first digit identifies the field and the last two digits in pairs, aligned along the slit. Because of this, parallactic an-
identifytheindividualstar. gleswerenotused.ThArarclampswereobservedimmediately
before or after any exposure, and each time that the telescope
was moved more than a few arcmin. All exposures have been
2.2.Spectroscopy calibrated with, at least, one arc taken less than 1800 s before
orafterthemidpointoftheobservationand,inmanycases,are
Observations were carried out with the Cassegrain Twin
sandwichedbetweentwoarcs.
Spectrograph (TWIN) of the 3.5 m telescope at Calar Alto
All the data have been reduced using the Starlink
(Almer´ıa, Spain), during a run on 2009 July 10–11. We used
software packages ccdpack (Draperetal. 2000) and figaro
the red arm equipped with the SITe#20b 12 CCD and the T06
(Shortridgeetal. 1997) and analysed using figaro and dipso
(1200lnmm−1)grating.Thegratinganglewaschosentocover
(Howarthetal.1998).Arcframeswereextractedattheposition
the8000–9000Årange.The1′.′2slitgivesaresolutionelement
ofeachtargetstar,andtherespectivesolutionschecked.Typical
of 1.2Å,measuredon the arc lamps. Thisis equivalentto a re-
RMSvaluesforthefittoathird-degreepolynomialare≈ 0.1Å.
solvingpowerR = 7000.Forthispilotrun,wechosealongslit
Solutionsatdifferentpositionswithinagivenframearesignifi-
spectrograph,as it allows a muchbetterselection of individual
cantlyshifted,withdifferencesuptotwopixels.
targetsandobservationofstarsinthecrowdedclustercores.
The average signal-to-noise ratio (SNR) of our targets was
In addition to the stars in the three fields listed above, we nothigh,typically50–70perpixel,thoughhigherforthebright-
observedsevenobjectsfromthelistofmembersofSte2inD07 est targets, and lower for some objects (a few spectra have
to be used as referencesfor radial velocity measurements, and SNR∼ 35). As an example, in Fig. 2, we show the spectrum
fivephotometricmembersofRSGC3. ofstarD18(i.e.,star#18fromD07)inSte2,whichhasaSNR
The seeing was variable, with typical values ≈ 1′.′5 (the ∼90perpixel(∼145perresolutionelement),withanumberof
monitor was not active at the time, so only rough estimates featuresidentified,afterdifferentreferencesintheliterature,no-
wereavailable).Transparencywashigh,exceptforthefirst two tablyKirkpatricketal.(1991),Mallik(1997),andCenarroetal.
hours of the first night, when some clouds were present close (2001).
3
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
Table1.Candidatesselectedaspossibleredluminousstars,withtheircharacteristics.
ID 2MASS J1 H1 K 1 Q i1 Spectral Spectral E(J−K ) v
S S LSR
Type(TiO) Type (kms−1)
Field1
F1S01 18445403−0329009 7.55±0.02 6.26±0.04 5.71±0.02 0.30 11.08±0.02 M7 M7Ib/II 0.55 +43
F1S02 18444686−0331074 8.78±0.03 6.71±0.02 5.76±0.02 0.38 14.44±0.04 M1 M1Ia∗ 1.96 +73
F1S03 18444439−0334197 8.21±0.02 6.86±0.04 6.32±0.02 0.39 11.30±0.02 − − − −
F1S04 18443941−0330003 8.66±0.03 6.76±0.04 5.82±0.02 0.19 13.99±0.03 M2 M2Iab∗ 1.74 +112
F1S05 18443100−0330499 9.87±0.02 7.93±0.05 6.97±0.03 0.21 15.44±0.06 − − − −
F1S06 18442945−0330024 8.45±0.03 6.39±0.03 5.41±0.02 0.30 14.13±0.04 M3 M2Ia∗ 1.94 +93
F1S07 18442796−0329425 8.06±0.02 5.91±0.04 4.85±0.02 0.25 14.46±0.04 M5 M4Iab∗ 2.04 +93
F1S08 18443037−0328470 7.34±0.02 5.20±0.04 4.10±0.30 0.16 13.70±0.03 M4 M4Ib∗ 2.07 +75
F1S09 18442053−0328446 9.28±0.02 7.24±0.07 6.21±0.03 0.21 15.32±0.06 − − − −
F1S10 18442322−0324578 8.08±0.03 6.64±0.05 5.94±0.02 0.18 11.91±0.02 M6 M5Ib/II 0.95 +59
F1S11 18441700−0325360 8.68±0.03 7.40±0.04 6.89±0.02 0.36 11.44±0.02 − − − −
F1S12 18444065−0335199 8.90±0.03 7.49±0.03 6.90±0.02 0.37 12.22±0.03 − − − −
Field2
F2S01 18444846−0313495 9.14±0.02 7.33±0.03 6.50±0.02 0.32 14.01±0.04 M1 M0Ia∗ 1.63 +72
F2S02 18444431−0313350 8.69±0.02 6.39±0.03 5.23±0.02 0.22 15.52±0.07 − − − −
F2S03 18444044−0314142 8.06±0.02 6.22±0.03 5.31±0.02 0.21 13.21±0.03 M1 M1Iab∗ 1.67 +86
F2S04 18444283−0315168 8.53±0.02 6.69±0.04 5.80±0.02 0.24 13.69±0.03 M2 M1.5Iab∗ 1.64 +87
F2S05 18444023−0315329 7.21±0.03 5.51±0.03 4.72±0.03 0.30 12.40±0.03 M8 M7II 1.19 +61
F2S06 18442823−0312558 7.50±0.03 6.38±0.03 5.94±0.02 0.34 9.70±0.03 − − − −
F2S07 18443833−0316596 7.32±0.02 6.05±0.04 5.49±0.02 0.26 10.68±0.03 M7 M6III 0.58 +85
F2S08 18443324−0306334 8.44±0.03 7.34±0.06 6.88±0.04 0.27 10.73±0.02 − − − −
F2S09 18443220−0304597 9.04±0.03 7.29±0.05 6.44±0.04 0.23 13.77±0.03 M5 M5.5III 1.36 +94
F2S10 18443172−0302217 8.40±0.02 6.85±0.03 6.19±0.03 0.35 12.15±0.03 − − − −
F2S11 18444658−0302416 8.65±0.02 6.69±0.03 5.79±0.02 0.33 14.18±0.04 − − − −
Field3
F3S01 18461105−0330091 7.08±0.02 5.79±0.03 5.26±0.02 0.33 10.04±0.04 M3 M4II-III 0.65 −22
F3S02 18460994−0334581 8.10±0.02 6.80±0.05 6.26±0.02 0.33 10.94±0.02 M2 M3.5III 0.70 +10
F3S03 18455541−0331254 8.37±0.02 6.82±0.05 6.17±0.02 0.38 13.04±0.02 M8 M7III 0.90 +66
F3S04 18455357−0332069 9.30±0.02 7.31±0.04 6.38±0.03 0.31 14.76±0.04 − − − −
F3S05 18454257−0332406 6.68±0.02 5.27±0.02 4.60±0.02 0.19 11.67±0.02 M9 M7III 0.78 −37
F3S06 18453198−0331536 7.99±0.02 6.31±0.03 5.51±0.03 0.24 11.5±0.3 − − − −
F3S07 18453212−0334044 8.40±0.03 6.45±0.03 5.51±0.03 0.25 13.4±0.3 − − − −
F3S08 18453872−0336069 7.55±0.03 6.23±0.033 5.66±0.02 0.29 10.96±0.02 M6 M5II-III 0.68 +59
F3S09 18453660−0339191 7.20±0.02 5.83±0.04 5.24±0.02 0.30 10.42±0.03 M3 M4II 0.79 +35
F3S10 18460233−0338568 8.54±0.03 6.10±0.04 4.94±0.02 0.36 15.31±0.05 − − − −
(1) JHK magnitudesfrom2MASS.imagnitudesfromDENIS,exceptforF3S06andF3S07,whicharephotographicIfromUSNO.
S
(∗)LikelymembersoftheRSGC3association.
3. Spectralclassification luminosity class, but the results depend to some degree on the
resolutionused andthe way in whichthe continuumis defined
Theclassificationofredluminousstarsinthefarredregion has
(cf.Mallik1997).
been explored by several authors. Different criteria have been
Whilemainsequencestarsareeasytotellapart,differences
proposed depending on the resolution available. For example,
between giants and supergiants are more subtle. Amongst sev-
Boschietal.(2003),usingechellespectra(Munari&Tomasella
eral line strengths and line ratios studied by Carquillatetal.
1999;Marreseetal.2003),proposeclassificationcriteria based
on individual line ratios. At lower resolution, Ginestetetal. (1997), the strength of the Tii and Fei blend at 8468Å seems
(1994) and Carquillatetal. (1997) investigatedthe variation of tobethebestdiscriminantatagivenspectraltype.However,all
differentlinestrengthswithtemperatureandluminositywiththe luminosity criteria show a certain degree of dispersion due to
intentionoffindingvalidcriteriaforthedeterminationof effec- differences in chemical composition. In addition, many metal-
tivetemperature(T )andluminosity. lic linesshowdependenceonbothtemperatureandluminosity.
eff
As a consequence,the boundarybetween luminosity classes II
In general, at a given metallicity, the strength of the metal-
andIbmaybeuncertain(Ginestetetal.1994;Zhuetal. 1999),
lic spectrum increases with luminosity (Ginestetetal. 1994;
inparticularforlateKtypes(Ginestetetal.1994).
Carquillatetal.1997).Animportantcaseinpointisthestrength
of the Caii triplet. These lines dominate the I-band spectra of The different luminosity calibrations in the literature have
stars with spectral types between early F and M3 and their been derived from spectra with different resolutions, using di-
strengthisstronglycorrelatedwithluminosity(D´ıazetal.1989; versemeasuringmethods.Moreover,alltheseworkshaveused
Zhou 1991), although they also demonstrate a dependence on spectra with high SNR. As our spectra have moderately high
metallicity and spectral type. Several authors have studied the resolution and only moderate SNR, we choose to measure the
dependence of the equivalent width (EW) of these lines with EWoflinesrelativetothelocalcontinuum,usingwindowsde-
4
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
Fig.2. Intermediate resolution spectrum of star D18 in Ste 2,
with featuresidentified, and the position of the TiO bandheads
used for spectral classification of later-type stars marked. This
object is M1.5Iab according to the features in this spectral re-
gion. Also shown (displaced upwards by 0.4 continuum units) Fig.3. Calibration of the depth of the TiO bandhead at 8660Å
istheskytransmissioninthisspectralregion,fromHinkleetal. againstTeff fora sample of dwarfs (red circles), giants and su-
(2003). pergiants(black circles) with MK spectral types present in the
library of Cenarroetal. (2001). Adding stars from the library
ofRayneretal.(2009),observedatlowerresolution(asterisks),
finedbytheapparentlinewidth(which,forexample,turnoutto doesnotsignificantlychangethetrend.
be ≈14Å for Caii 8498Å and ≈18Å for Caii 8542Å in all our
stars). As there is an amount of subjectivity in this definition,
we estimate the internal errors in the measurements by taking To calibrate T against this measurementof the TiO band
eff
a large number of measurements with small differences in the strength, we used the spectral library of Cenarroetal. (2001)
definitionofthecontinuumandintegrationlimits.Typicallythe as a source of primary calibrators. In orderto check if there is
errors are found to be ≈ 3%. To calibrate our measurements, a dependence on spectral resolution, we also employed the li-
weutilisedthespectrallibraryofCenarroetal.(2001)sincethe braryofRayneretal.(2009).Thiscataloguecoversourspectral
spectra in this library have similar resolution to ours1. In addi- range at R = 2000, and again contains mostly stars classified
tion, many of the stars included are defined as secondary MK in Keenan&McNeil (1989). Both catalogues contain a fairly
standardsinKeenan&McNeil(1989)orhaveaccuratespectral widesampleofspectraltypes,includinggiants,supergiantsand
typesinKeenan&Pitts(1980).Wefind38K-typestarsand14 dwarfs.Overthespectralrangeandattheresolutioninquestion,
M-typestars(earlierthanM6)classifiedasgiantsorsupergiants giants and supergiantshave very similar spectra. Therefore we
inthesereferences. exploredthepossibilitythatasinglecalibrationcanbeusedfor
luminousstarsofallluminosityclasses.AsCenarroetal.(2001)
provideT determinationsforalltheirstars,weusedthissubset
3.1.Temperaturedeterminationandspectraltypes eff
toobtainbotharelationbetweenTiObanddepthandT anda
eff
Forlate-typestarsthedominantfeaturesintheI-bandspectraare relationbetweenT andG,anindexthatlabelsthespectraltype
eff
themolecularTiOabsorptionbands,withVObandsalsobecom- of the star (G = 0 fora putativeO0 star, increasingby one for
ingprominentforspectraltypesM6andlater(Kirkpatricketal. eachtype,reachingG = 30foranF0starandsoon).Wefound
1991). These bands are much stronger in luminous stars (gi- thatfortypeslaterthanA0,
ants and supergiants) than in stars close to the main sequence.
The strength of the TiO bandshas been used in the past as the G
log(T )=(4.155±0.008)−(9.79±0.16)× . (2)
basis of several temperature calibrations, via the definition of eff 1000
different photometric or spectroscopic indices (see for exam-
Initially, we fit the relationship between the TiO index and
ple Alvarez&Mennessier 1997; Cenarroetal. 2009). Ramsey
T forthestarsfromCenarroetal.(2001).Theresultsareplot-
(1981) found that the depth of the TiO δ(0,0) R-branch band- eff
tedinFig.3.Asisseen,theindexcorrelateswellwithtempera-
head at 8660Å correlated strongly with T for a sample of K
eff tureforspectraltypeslaterthanM0;theresultantrelationbeing
and M giants observed at high resolution. Here we build our
own temperature scale based on the strength of this feature on
T =(3848±38)−(1803±154)×TiO. (3)
intermediate-resolutionspectra.WemeasuretheTiOabsorption eff
between8860Åand8872Åagainstacontinuumestimationbe- Giantsandsupergiantscannotbeseparated.
tween 8850Å and 8857Å. The magnitude of this jump is then For stars in the catalogue of Rayneretal. (2009), we only
calculatedas: have spectral types. Consequently, we first estimated their T
eff
I via Eq. 2. With this estimate, we then checked that they
TiO=1− TiO . (1) follow the same (TiO,T ) relation as stars in Cenarroetal.
I eff
cont (2001) (Fig. 3). Typical errors are ±116 K for the stars from
1 ThespectrainCenarroetal.(2001)wereobservedindifferentruns Cenarroetal. (2001) and ±143 K for stars in Rayneretal.
atseveraltelescopes.Aroundtwothirdsofthespectrahave resolution (2009).Bothdispersionsarefullycompatible,asthelattervalue
(FWHM) of 1.2Å, around one third have resolution of 1.5Å, while a includes the errors from both Eqs. 2 and 3. Giants and super-
fewhavelowerresolutions. giantsfollowthesame relation,whilethe differenceinspectral
5
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
resolutionbetweenthetwosamplesdoesnotseemtoaffectthe 12.5
indexsignificantly,2.
Main-sequencestars,ontheotherhand,haveratherweaker
TiOfeatures,andfollowaverydifferentrelation,
10
T =(4058±140)−(9315±2190)×TiO. (4)
eff Å)
aninBcerceaausesedodfistpheerswioenak,wneisthsotyfptihcealTeiOrrobrasn±d3,5th6eKrefolartisotanrsshfroowms aII) ( 7.5
C
Cenarroetal.(2001). W(
E
Class III - no anomalies
3.2.Luminositycalibration 5
Class III - CP
Class II
We then proceededto calibrate the main luminosityindicators, Class Ib
namely the strength of the Caii triplet and the Tii dominated Class Ia/Iab
blend at 8468Å. For this, we measured the EWs of these fea- 2.5
G9 K0 K1 K2 K3 K4 K5 M0 M1 M2 M3 M4 M5 M6
tures on standard stars in Cenarroetal. (2001), using the same Spectral type
measuring technique as for our targets. As the number of su-
Fig.4. Measurements of the EW of Caii for stars in the li-
pergiants is low in this sample, we complement the dataset
brary of Cenarroetal. (2001). Circles are luminosity class III
with a few more stars from Ginestetetal. (1994), at slightly
higher resolution. The results for the two strongest Caii lines giants (open circles are stars whose spectral classification in-
cludes some chemical anomaly, while filled circles have no
are displayed in Fig. 4. We find “normal” luminosity class
III stars to have EW(Caii 8542+Caii 8662) in the range 6– suchqualifiers).DiamondsareluminosityclassIIbrightgiants.
Triangles are supergiants (triangles pointing upwards have lu-
8Å, with a clear tendency to lower values at early K types.
minosity class Ib, while those pointing downwards have class
Stars whose classification includes metallicity anomalies devi-
Iab).Withonesingleexception,giantsandsupergiantsarewell
ateslightlyfromthegeneraltrend.Bastarsgenerallyhave high separateduptospectraltypeM2.ThestrongestCaiitripletcor-
values, but all giants have EW(Caii 8542+Caii 8662)<8.5Å.
respondstotheK1Ia–IabsupergiantHD63302.
The few luminosity class II bright giants in the sample have
EW(Caii 8542+Caii 8662)≈8.5Å, while the seven supergiants
2
earlier than M3 have EW(Caii 8542+Caii 8662)>9Å. These
results agree qualitatively with those of Ginestetetal. (1994),
thoughourdefinitionofthe continuumleadsto smallerEWs,3.
TheEWincreasesonaveragewithspectraltype,untilitreaches 1.5
amaximumatK4–M1,andthendecreases,duetothedepression
Å)
obfytDhe´ıaczoenttianlu.u(1m98le9v)etlobysetphaeraTtieOsufepaetrugrieasnTtshferocmritebrriiognhtdegfiiannetds 8) (
at solar composition, namely EW(Caii 8542+Caii 8662)>9Å, 864 1
W(
also appears valid for our resolution and continuumdefinition,
E
atleastforspectraltypesearlierthanM3. Class III - no anomalies
Figure 5 shows the measurements of the 8468Å blend. We 0.5 Class III CP
Class II
find that “normal” class III stars have EW(8468) in the 0.6– Class Ib
0.8Å range, again with the lower values corresponding to the Class Ia/Iab
earlier types. Metallicity anomalies cause importantdispersion
0
here. In particular, stars classified as CN-strong have values in G9 K0 K1 K2 K3 K4 K5 M0 M1 M2 M3 M4 M5 M6
Spectral type
the 1.0–1.2Å range. These objects are believed to be to some
degreemetal-rich(Keenan&Heck1994).Allbrightgiantshave Fig.5. Measurements of the EW of Tii/Fei blend for stars in
EW(8468)=0.9–1.1Å, with all supergiantsdisplaying values > the library of Cenarroetal. (2001). Symbols are as in Fig. 4.
1.2Å. Our values agree very well with those of Ginestetetal. ExceptforafewdeviantpointsatK3,supergiantsareverywell
(1994,theirFig.13),asthelocalcontinuuminthisrangecoin- separatedfromnormal-compositiongiants.Thestrongestfeature
cides with their selected high points. We can thus confirm that ismeasuredagaininHD66302.
theEWofthisblendisaverygoodluminosityindicator,allow-
ingperhapsabetterseparationofsupergiantsandgiantsof nor-
mal composition than the Caii lines. Interestingly, all the CN- class III,butalwayslowerthansupergiants4. Thereforea com-
strong giants have EW(Caii 8542+Caii 8662)≈ 8Å, high for binationofEW(Caii)andEW(8468)allowstheidentificationof
metal-richgiantsandtheirseparationfromsupergiants.
2 Inordertoexploretheeffectofspectralresolution, weartificially Forspectral types later than ∼M4, the measurementof this
degradedtheresolutionofthespectrainCenarroetal.(2001)tolower
feature becomes very difficult, as it is located inside the TiO
resolutions.WefindthattheTiOindexstartstoloosesensitivitytothe
earliest M spectral types at resolving power R <∼ 1500 and deviates λλ 8432-8442-8452feature. Indeed, Fig. 5 shows high disper-
stronglyfromtherelationshowninEq.3forR<∼1000. sion with luminosity class for stars later than M2, the spectral
3 Ginestetetal. (1994) use a few high points in the spectra to de- typeatwhichthisTiOfeaturestartstobecomestrong.Asacon-
finethecontinuum,whilemostotherreferences(e.g.,D´ıazetal.1989; sequence, the luminosity classification for stars later than M3
Mallik1997)usesmallfeaturelesssectionsattheedgesofthespectral
range. As a consequence, Ginestetetal. (1994) measure higher EWs 4 Thisisnotsurprising,asthehighEWsoftheCaiilinesinsuper-
thanotherauthorsdo. giantsareduetothedevelopmentofbroadwingsatlowgravity
6
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
canonlybeachievedbyconsideringtherelativestrengthof the weakerDIBat8763Å(e.g.,Gredeletal.2001)andabroadshal-
wholemetallicspectrum. low DIB around 8648Å (Munarietal. 2008; Negueruelaetal.
2010b).Finally,interstellarlinescorrespondingtothe(2,0)band
of the C Phillips system are seen in stars with high redden-
3.3.Radialvelocities 2
ing(e.g.,Gredeletal.2001).Thestrongestfeatures,suchasthe
Cross-correlation, either with a model spectrum or a star with Q(2)andQ(4)transitionsatλλ8761,8764Å,reachintensitiesof
known radial velocity v , is the most effective way to derive afewhundredthsofthecontinuuminthespectrumofCygOB2
rad
radialvelocitiesviatheDopplershiftintroducedinthespectrum #12 (A ≈ 10; Gredeletal. 2001), but are not seen in any of
V
by motion in the radial direction. If the template spectrum is our spectra. This lack of detectability suggests that the cross-
chosen to match closely the stellar spectrum, accuraciesin v correlationisnotaffectedbythepresenceofinterstellarfeatures.
rad
ashighas<∼ 10%ofthespectralresolutioncanbeobtainedfor We derived an initial estimate of the accuracy of the mea-
spectraincludingalargenumberoflines. surements by performing cross-correlations against different
Wetriedtwodifferentapproachestoderiveradialvelocities. templates and among our targets. From this we estimated the
Initially,weemployeddirectcross-correlationbetweenourspec- internalerroroftheproceduretobearound±5kms−1.
tra utilising the hcross task within figaro. This allows direct
comparisonbetweenourtargets.Radialvelocitiesmaybemea-
suredrelativetomembersofSte2,whosevelocitiesintheLocal
StandardofRest(LSR)havebeenaccuratelymeasuredbyD07. 4. Results
Unfortunately,in the spectral range that we have observed,
A quick look at the spectra obtained (Figs. 6 to 9) clearly
the spectrum of late giants and supergiants changesdrastically
showsthat allthe stars selected by ourphotometriccriteria are
with T dueto theappearanceof strongTiO bands.Asa con-
eff
luminous late-type objects. Accurate classification can be ac-
sequence,we need to use astemplatesa set of spectra thatcan
complished using the criteria explored in the previous section
matchourheterogeneousdataset.ThestarsinSte2donotcover
and direct comparison to the standard stars in the library of
thewholerangeofspectraltypesofourtargets.
Cenarroetal. (2001). To determineluminosity,we use the EW
Inviewofthis,wedecidedtouseagainthespectrallibraryof
Cenarroetal.(2001),takingadvantageofthefactthatthespec- oftheCaiitripletlinesandthe8468Åblend,aswellasthedirect
tra cover the whole parameter space in T and luminosity at comparisonofthefullmetallicspectrumagainststandardstarsin
eff
a resolution comparableto ours. In addition, the spectra in the Cenarroetal.(2001)andCarquillatetal.(1997).Forstars later
libraryhavebeenshiftedtoacommonvelocity. thanM3,thedeterminationofluminosityisdifficult,andrelies
solelyontheapparentstrengthofthemetallicspectrum.
The algorithm that obtains the Doppler shift is a two-step
process. It starts with a first estimation of T via the TiO cal- Inadditiontospectralclassification,two otherspectralfea-
eff
ibration presented in Section 3.1, which is then used to select tures are important for the analysis. Firstly, the radial velocity
atemplateforcomparisonfromthelibrary.Successivevelocity of the objects can provideinformationabouttheirdistance. As
shifts, in stepsof1kms−1, areappliedtothe template,andthe shownbyDaviesetal.(2008),theGalacticrotationcurveallows
correlation maximum gives us a first estimation of v . Using theseparationofstarslocatedindifferentGalacticarms.Inthis
rad
this value, we shift the observed spectrum to the rest velocity, direction,thelineofsightcutstheSagittariusArmat ∼ 1.5kpc
andthenasecondTeffisderived.Thissecondestimateisinmost fromtheSun(wherevLSRvaluesare<∼25kms−1)andcrossesthe
caseswithin±1spectraltypesfromthefirstone.Althoughanew Scutum-CruxArmatadistance∼3.5kpc(Turner1980),where
templatecouldbeselected,suchasmallchangeinspectraltype vLSR values should be ∼ 50kms−1. Then it reaches the struc-
has a negligible effect on the derivation of v . Observed v ture close to the edge of the Long Bar, perhaps touching it at
rad rad
weretransformedintotheLSRreferencesystemusingIRAF’s,5 ∼5kpcandcrossingitfor2or3kpc.Itdoesnotreachanyother
rvcorrectpackage. arm untilit cuts againthe SagittariusArm at >∼10 kpc.Stars in
The measured values of vrad depend on the spectral range RSGC1haveanaveragevLSR =120kms−1(Daviesetal.2008),
chosen. Because of the presence of strong telluric bands, wide whilethoseinSte2havevLSR = 110kms−1 (D07).Starsinthe
regionsofthespectrumcannotbeusedinthecross-correlation. Scutum Complex thus have much higher radial velocities than
Asthehumiditywashighduringtheobservations,werestricted foregroundorbackgroundobjects.
thespectralrangeusedforcross-correlationto8370–8900Å.In Secondly, the DIB centred at 8620Å has been found to be
Fig.2,wedisplaytheskytransmissionfromHinkleetal.(2003). an excellent tracer of reddening. For moderate reddenings, a
Thestrongestatmosphericfeaturesinthisrangehave5%ofthe verytightcorrelation E(B−V) = 2.72(±0.03)× EW(DIB)(Å)
intensity of the O2 band centred at 8231Å. By scaling against has been found (Munarietal. 2008). The DIB does not show
the8231Åband,weestimatethatthestrongestatmosphericfea- any correlation with reddening for stars in the heavily red-
tures reach a depth of <∼ 0.03 continuum units, while the typ- dened(AV = 10.8)openclusterWesterlund1(Negueruelaetal.
ical metallic lines in the spectrum are close to 0.1 continuum 2010b). OB supergiants in this cluster show EW(DIB)= 1.1±
units. Therefore no atmospheric features are noticeable in our 0.1Å, suggesting that the band saturates at this value. Such a
spectraandtheyshouldnotcontributesignificantlytothecross- strongfeatureiseasilyrecognisableinthespectraofheavilyred-
correlationfunction. denedstarsandservesasanadditionalindicatorofhighdistance.
Likewise,interstellarfeaturesinthisrangearenotnumerous. InRSGs,itappearsblendedwiththeFei8621Åline(seeFig.2).
The only strong interstellar feature is the 8620Å diffuse inter- Because of this, its EW cannot be measured accurately at our
stellarband(DIB),discussedinthenextsection.Thereisalsoa resolution.However,thepresenceofthisfeaturecanbededuced
bycomparisonto Fei8611Å.Thisline is alwaysstrongerthan
5 IRAF is distributed by the National Optical Astronomy the Fei 8621Å feature at all spectral types. Observing a ratio
Observatories, which are operated by the Association of Universities
forResearchinAstronomy,Inc.,undercooperativeagreementwiththe Fei 8621Å/Fei 8611Å>1impliesthe presenceof a moderately
NationalScienceFoundation strongDIBblendedwiththeFei8621Åfeature.
7
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
valuesof100,111&93kms−1,correspondingtoanaverageof
101±9kms−1.
Basedonthesevalues,weestimatethatthetotalerrorinone
singlemeasurementofv usingourtechniqueis±10kms−1or
rad
slightly higher,6. However, the technique appears robust when
applied to a sample of objectsin a cluster. We assume that the
differenceof +8kms−1 representsoursystematicshiftwithre-
spect to the LSR system adopted by D07, based on the radial
velocityofArcturus.Theaverageradialvelocitythatweobtain
forSte2 is v = 102±4kms−1. Thedifferencewith theav-
LSR
erage value of D07 is +7 kms−1, consistent with a systematic
shift.
The reason for this shift is unclear. It may be due to a
different definition of the LSR. rvcorrect transforms using the
StandardSolarMotion(+20kms−1 towardsℓ = 56◦,b = 23◦).
We have calculated mean velocity correctionsfor the stars ob-
servedinSte2∆V⊙ = −4.5kms−1 and∆VLSR = +16.0kms−1.
In any case, the shift is of the same order of magnitude as
the dispersion in our measurements for cluster members, and
so could be statistical, as the number of members observed is
low. In addition to the accuracy of individual measurements,
a real dispersion in v amongst cluster members could be
rad
present.ObservationsoffiveRSGsintheassociationsurround-
ing χ Persei show random fluctuations in radial velocity with
amplitudes>∼5kms−1(Mermilliodetal.2008).
4.2.RSGC3
We observed five stars in RSGC3, those numbered S1–5 in
Clarketal. (2009a). Their spectra are displayed in Fig. 7. The
fiveobjectsareM-typesupergiants.Theirmeasuredv arecon-
rad
sistentwithintheaccuracydiscussedabove(S5showsthevalue
Fig.6.SpectraofsevenredsupergiantsinSte2.Notethepromi-
nent DIB at 8620Å compared to the neighbouringFei 8611Å. deviatingmostclearly,butitalsohasthepoorestspectrumwith
SNR∼35perresolutionelement).Theaveragevelocityforthese
TheFei8621Ålinecanbeseenasabumpontheredsideofthis
fivemembersisv =+95±7kms−1,wheretheerrorrepresents
feature,becauseit isdisplacedto λ8624duetothe highv of LSR
rad thestandarddeviation.Interestingly,Alexanderetal.(2009)find
theseobjects,exceptinD3,whichistoolatetoshowthisfeature.
that the CO distribution in this direction shows a maximum at
v = +95kms−1.Materialwiththisv showsaspatialdis-
LSR LSR
tributionstronglycorrelatedwiththeGLIMPSE8µmemission,
4.1.Stephenson2 andpresentsaholearoundthecluster,suggestingacavityblown
bystellarwindsandsupernovaexplosions.Thev forRSGC3
We obtained spectra of seven stars listed as members of Ste 2 LSR
is smaller than that found for Ste 2 (v = 102± 4 kms−1),
byD07.TheirspectraaredisplayedinFig.6.Alltheobjectsare LSR
thoughconsistentwithintheerrors.
M-typeluminousstars.InTable2,welisttheirspectraltypesas
Withthesenewspectraltypes,wecalculateindividualcolour
estimatedfromthestrengthoftheCO2.3µmbandheadinD07,
excesses E(J − K ), using the calibrations of Levesqueetal.
as estimated using the strength of the TiO 8660Å band and as (2005)tocalculateStheintrinsic(J−K) foreachspectraltype,7.
derivedfromdirectcomparisontostandardstars. 0
Thevaluesderived,showninTable 3, areverysimilarto those
The agreement in general is very good, except for D18,
of Clarketal. (2009a). Given the relatively good agreement
whichwasclassifiedM4byD07andappearsratherearlierhere.
between our spectral types and those derived using the CO-
The observations of D07 were taken in 2006, three years be-
bandheadcalibration,changesintheindividualabsolutemagni-
foreourrun.SomeRSGsareknowntoshowspectralvariability,
though this is a rare occurrence, perhaps confined to the most 6 Thisvalueisslightlyhigherthanexpected.However,wenotethat
luminousobjects(Levesqueetal.2005;Clarketal.2010). itisnotduetouseofexternaltemplates,asdirectcrossesbetweenthe
D07 find an average radial velocity for the cluster v = threespectraofD3usinghcrossgivesimilardispersion.Wecanonly
LSR
109.3 ± 0.7 kms−1, with the uncertainty representing Poisson speculateaboutthereasonwhytheerrorislargerthantheexpected10–
statisticsfor26memberswithmeasuredvalues.Thesevenstars 15%ofthespectralresolution.Itmayberelatedtothefactthatwetried
thatwehaveobservedhaveaveragev =109±2kms−1(where toalignseveralstarsineachslit,meaningthatobjectsareoccasionally
LSR
notwellcentred,perhapsresultingindifferentlightpathsforstarlight
the erroris the standard deviationof the seven measurements),
andthearclamplight.
i.e., are representative of the cluster velocity. Their individual
valuesrangefrom105kms−1forD29to112kms−1forD15. 7 Notethatthe(J−K)0coloursobtainedbyapplyingthecalibration
ofT versusspectraltypeinLevesqueetal.(2005)totheir(J−K) /T
Whenwecompareourvalues,wefindthatthedifferencesin eff 0 eff
relationshiparealmostidenticaltotheempiricalcalibrationofintrinsic
vwraidth(Dan07avmerianguesdoiuffrevraelnucees)orfa+n8gekmfrosm−1+an1d3kamstas−n1datord−d2ekvmiatsi−o1n, n(Jo−toKn)l0yfgoirvreesdsguipapnotsrtotfoSbtorathizˇcyasli&braLtaioznaus,skbuaittie˙n(d2i0ca0t9e)s.tThhaitsthager(eJe−mKen)t
0
of 5kms−1. For D3, we have three measurements, which give coloursofredgiantsandsupergiantsareindistinguishable.
8
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
Table2.NewspectraltypesformembersofStephenson2.
ID Co-ordinates i J H K Spectral Spectral Spectral v
S LSR
RA Decln. Type(CO) Type(TiO) Type (kms−1)
D3 183924.6 −060213.8 12.85 7.27 5.46 4.50 M4I M6 M5Iab +101
D11 183918.3 −060214.3 14.35 8.35 6.21 5.26 M4I M3 M3.5Ib +106
D15 183922.4 −060150.1 13.07 8.13 6.35 5.51 M2I M2 M1.5Iab +99
D16 183924.0 −060307.3 13.20 8.24 6.44 5.60 M3I M1 M1Ia +97
D18 183922.5 −060008.4 12.88 8.18 6.45 5.63 M4I M1 M1.5Iab +101
D20 183924.1 −060022.8 13.18 8.43 6.66 5.81 M2I M1 M1.5Iab +103
D29 183922.2 −060214.7 13.17a 8.61 6.88 6.15 M0I M1 M0Iab +107
(a)TheDENISmagnitudeseemstocorrespondtoablendoftwostars,D29and2MASSJ18392193−0602165.
Table3.PropertiesofconfirmedmembersofRSGC3.
ID Co-ordinates Ia Ja Ha K a Spectral Spectral Spectral E(J−K ) v
S S LSR
RA Decln. Type(CO) Type(TiO) Type (kms−1)
S1 184523.60 −032413.9 13.5 8.55 6.54 5.58 RSG M3 M3.5Ib 1.80 +97
S2 184526.54 −032335.3 b 8.53 6.62 5.75 M3I M2 M2Ia 1.68 +104
S3 184524.34 −032242.1 13.7c 8.54 6.43 5.35 M4I M4 M4I 2.02 +99
S4 184525.31 −032301.1 13.7c 8.42 6.39 5.31 M3I M2 M3Ia 1.97 +93
S5 184523.26 −032344.1 13.8 8.51 6.52 5.52 M2I M4 M4I 1.82 +84
(a) JHK magnitudesfrom2MASS.ImagnitudesfromUSNO-B1.0.
S
(b)NoUSNO-B1.0source,butthestarissimilarlybrighttotheotherfourmembersobservedintheDSS2Iimage.
(c)ThemagnitudecorrespondstoablendoftwostarsintheDSS2Iimage.
4.3.Field1
InField1,weselected12candidates(listedinTable1)andman-
aged to observe seven of them. All the stars observed turn out
to be M-type luminous stars. Two objects, F1S01 and F1S10,
haverelativelylow E(J −K ) andshowno evidenceforheavy
S
extinctionin theirspectra,as the 8620ÅDIB is notprominent.
Both are quite late and their exact luminosity classes are dif-
ficult to assign, but both look rather luminous, close to being
low-luminositysupergiants.
Theotherfiveobjectshavesimilarvaluesof E(J −K ),all
S
withinthesamerangeasstarsinRSGC3.AsseeninFig.8,all
fiveshowveryprominent8620ÅDIB,withEW≈1.3Å.Thefea-
ture is so broad that it is almost blended with Fei 8611Å. All
fiveobjectshavehighobservedv ,butourmeasurementsshow
rad
higherdispersionthanthoseforstarsinSte2andRSGC3.The
average of their velocities is v = +89 ± 16 kms−1, where
LSR
theerrorrepresentsagainthestandarddeviation.Alltheindivid-
ualmeasurementsarewithinonestandarddeviation,butdeviate
slightlymorethantheestimatederrorof±10−12kms−1(allthe
spectraaremoderatelypoorwithtypicalSNR∼50).
Among the objects that we did not observe, F1S03 with
(J−K )=1.89andF1S11with(J−K )=1.79musthaverel-
S S
ativelylowcolourexcessesiftheyareK–Mstars. Ontheother
hand,F1S05andF1S09havecolourssimilartothegroupoffive
supergiants. Below we argue that all these high-reddening ob-
jectsrepresentanewclusterofredsupergiants.
4.4.Field2
Fig.7.SpectraoffiveredsupergiantsinRSGC3.Notethepromi-
ThereisanobviousconcentrationofbrightstarsintheK image
S
nentDIBat8620ÅcomparedtotheneighbouringFei8611Å. ofthearea.Only11ofthempassourphotometriccut,defining
anapparentcompactgrouparound(RA:18:44:45,δ:−03:14:30)
andasmallergroup∼6′totheNorth.Wefindsevencandidates
tudesaresmall.Errorsin M aredominatedbytheuncertainty inthemaingroupandfourinthenortherngroup.Theircoordi-
K
inthedistancetothecluster. natesandmagnitudesarelistedinTable1.
9
I.Negueruelaetal.:AmassiveassociationaroundtheobscuredopenclusterRSGC3
Fig.8. Spectra of red supergiants in Field 1. The five top ob- Fig.9.SpectraofstarsinField2.Thethreetopobjectshavesim-
jectshavesimilarcoloursandareaffected byheavyreddening. ilarcoloursandareaffectedbyheavyreddening.Theyarelikely
The bottom spectrum corresponds to S10, which is less lumi- membersoftheRSGC3association.F2S05isaluminousfore-
nous than the other objects. It has much lower reddening than groundgiant.F2S09morphologicallylookslikeagiant(though
theotherstars(asshownbytheweak8620ÅDIB)andverydif- thespectrumhaslowSNR),butitsreddeningandvradaresimilar
ferentradialvelocity. tothoseofsupergiantsintheRSGC3association.
The four stars that we did not observe have redder colours
We observed five candidates in the main group. Of these,
than these giants, with F3S04 and F3S07 showing 2MASS
three objectsF2S01, F2S03 and F2S04 are M-type supergiants
colourssimilartomembersofRSGC3.
withsimilarcolours.Theircolourexcessesarealmostidentical,
and similar to the lowest values displayed by stars in RSGC3.
Theirobservedv arealsosimilarlyhigh.However,thereisno
rad 5. Discussion
strong evidence for a large cluster here. F2S05 and F2S07 are
bothlate-Mgiants.Theircolourexcessesaremuchlower,even We have observed a sample of RSG members of the massive
iftheirvrad arehigh.Ofthestarsthatwedidnotobserve,F2S06 clusters Ste 2 and RSGC3 in the region of the Caii triplet. In
has(J−KS)=1.56,andcannotbehighlyreddenedifitisared addition, we have observed 19 objects within 20′ of RSGC3,
star.F2S03,ontheotherhand,has(J−KS)=3.46,anditcould selectedbecausetheir2MASScolourssuggestedthattheymight
bearedderormorehighlyreddenedsupergiant. bereddenedluminouslate-typestars.
In the northern group, F2S09 has a late spectral type. The
spectrum is quite noisy, and the luminosity class is difficult to
5.1.Validityofcriteriaandcalibrations
determine.Itlookslikeagiant(seeFig.9),butdisplayshighred-
deningandhighvrad,similarlytothesupergiants.F2S11,which We haveselectedtargetsbasedontheirinfraredbrightness and
we did not observe, has colours and magnitudes similar to the colours. The criterion 0.1 ≤ Q ≤ 0.4 was expected to reject
threesupergiantsinthemaingroup. early-type stars and red dwarfs. Indeed, all the 19 objects se-
lectedaccordingtothesecriteriaforwhichwehavespectraturn
outto be luminousM-typestars with differentamountsof red-
4.5.Field3
dening.Eightobjectsarehighlyreddenedsupergiants.The rest
We observed six of the ten candidates selected in this area are mid and late M giants or low-luminosity supergiants with
(Table 1). All of them are mid or late M giants, with moder- differentamountsofreddening,rangingfrom E(J−K ) = 0.55
S
atereddening.ThoughtheircolourexcessesE(J−K )arequite for F1S01 to 1.36 for F2S09. Therefore we may conclude that
S
uniform,theydisplayaverywiderangeofobservedv . thecriteriadefinedinSection2.1toidentifyreddenedluminous
rad
10