NGC 6334 and NGC 6357. Insights from spectroscopy of their OB star populations

32 Pages·2017·10.08 MB·English

A&A607,A86(2017) Astronomy
DOI:10.1051/0004-6361/201629870
&
(cid:13)c ESO2017 Astrophysics
NGC 6334 and NGC 6357
Insights from spectroscopy of their OB star populations(cid:63)
D.Russeil1,C.Adami1,J.C.Bouret1,A.Hervé2,Q.A.Parker3,4,A.Zavagno1,andF.Motte5
1 AixMarseilleUniv,CNRS,LAM,Laboratoired’AstrophysiquedeMarseille,13013Marseille,France
e-mail:[email protected]
2 AstronomicalInstituteASCR,Fricova298,25165Ondrejov,CzechRepublic
3 DepartmentofPhysics,ChongYuetMingPhysicsBuilding,TheUniversityofHongKong,HongKong
4 AustralianAstronomicalObservatory,POBox915,NorthRyde,NSW1670,Australia
5 InstitutdePlanétologieetd’AstrophysiquedeGrenoble(IPAG),Univ.GrenobleAlpes,CNRS/INSU,BP53,
38041GrenobleCedex9,France
Received10October2016/Accepted4August2017
ABSTRACT
Aims.Theformationofhigh-massstarsisstilldebated.Forthisreason,severalprojectssuchasHerschel-HOBYSarefocussedonthe
studyoftheearliestphasesofmassivestarformation.Asaresult,massivestar-formingcomplexessuchasNGC6334andNGC6357
have been observed in the far-infrared to study their massive dense cores where massive stars are expected to form. However, to
better characterise the environments of these cores we need to understand the previous massive star formation history. To better
characterisetheenvironmentofthesemassivedensecoreswestudytheprevioushigh-massstarformationandhowthesestarsacton
theirenvironments.
Methods.ThisstudyisbasedonthespectralclassificationoftheOBstarsidentifiedtowardsNGC6334andNGC6357withspectra
taken with the AAOmega spectrograph on the Anglo-Australian Telescope (AAT). From the subsequent spectral classification of
109starsacrosstheseregionswewereabletoevaluatethefollowing:distance,age,mass,globalstar-formingefficiency(SFE),and
starformationrate(SFR)oftheregions.Thephysicalconditionsoftheionisedgasforbothcomplexeswasalsoderived.
Results.WeconfirmthatNGC6334andNGC6357belongtotheSaggitarius-Carinaarmwhich,inthisdirection,extendsfrom1kpc
to2.2kpc.FromthelocationofthestarsinHertzprung-Russelldiagramweshowthatstarsolderthan∼10Myrarebroadlyspread
ii
acrossthesecomplexes,whileyoungerstarsaremainlylocatedintheH regionsandstellarclusters.Ourdataalsosuggeststhat
someoftheyoungstarscanbeconsideredrunawaystars.WeevaluateaSFEof0.019+0.008and0.021+0.004andaSFRof1.1×103±
−0.007 −0.003
300 M Myr−1 and1.7×103±400 M Myr−1 forNGC6334andNGC6357,respectively.Wenotethat29OBstarshaveX-ray
(cid:12) (cid:12)
counterparts,mostofthembelongingtoNGC6357.ThissuggeststhatmolecularcloudsinNGC6357aremoreimpactedbyX-ray
fluxandstellarwindsthaninNGC6334.Finally,fromtheanalysisofnebularlines(Hα,[NII],and[SII])fromspectrafromseveral
regionsofionisedgas,weconfirmthatthefilamentsinNGC6357areshockheated.
Keywords. stars:distances–HIIregions–ISM:individualobjects:NGC6334–ISM:individualobjects:NGC6357
1. Introduction structureinthisdirectioninordertounderstandtheirrelationto
ii thesurroundingmolecularmaterialandtoestimatethestarfor-
The H regions NGC 6334 and NGC 6357 are two very ac-
mationratesfromthecensusofOBstarsineachcomplex.Inthis
tivestar-formingcomplexesinourGalaxy.Theyareapparently
paperweattempttoprobetherecentmassivestarformation(as
connectedbyafilamentarystructuresuchthatbothregionsmay
tracedbyO-B3stars)forcomparisonwiththepresentandlikely
belongtoasingleoverallcomplex(e.g.Russeiletal.2010),even
futuremassivestarformationastracedbytheadjacentmassive
thoughtheyexhibitdifferentmorphologies.WhileNGC6334is
ii molecular cores and clumps (see Tigéetal. 2017; Russeiletal.
the grouping of several optical H regions located around a
2010;Munozetal.2007).
main molecular/dust ridge (Persi&Tapia 2008), the morphol-
ogyofNGC6357isdominatedbyalargecavityshapedbythe As part of this program (Russeiletal. 2012, hereafter
massiveopenclusterPismis24(e.g.Lortetetal.1984)andalso PaperI) observed 4 square degrees in and around NGC 6334,
byabrightHii region(G353.2+0.9)thattracesasharpbound- NGC6357,andGM1-24intheU,B,andVphotometricbands
aryfacingPismis24. withtheVIMOScameraontheESO-VLT.FromtheU-Bversus
B-Vcolour-colourdiagram1302candidateOtoB3starswithV
To study how massive stars form in NGC 6334 and
between7.5and21werefound.ThedistancesoftheseOBstars
NGC6357weneedtodeterminethedistanceofthetworegions
were then established assuming they are main-sequence stars.
andthenplacetheminthecurrentbestmodelofthespiralarm
This hypothesis combined with their spectral types determined
(cid:63) FullTables2andA.1andthenormalisedobservedspectra fromthephotometryresultedinarelativedistanceuncertaintyof
displayedinFigs.B.1areavailableattheCDSviaanonymousftpto ∼50%.Thislargestellarsampleofsuchrarestarsallowedusto
cdsarc.u-strasbg.fr(130.79.128.5)orvia carryoutavaluablestatisticalstudyofthelocalstructureofthe
http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/607/A86 MilkyWayinthisdirection.Thedistributionofthestarsshowed
ArticlepublishedbyEDPSciences A86,page1of32
A&A607,A86(2017)
Table1.SummarydetailsofourAAT3.9m–2dFAAOmegaspectroscopicobservations.
Fieldcentre Obs.date Arm Wavelengthrange Resolution Gratings Exp. No.of
RA/Dec(J2000) (ddmmyy) (Å) FWHM(Å) (s) targets
NGC6334
17h.19m.34.77s.–35◦49(cid:48)59.30(cid:48)(cid:48) 03072014 Blue/Red 3358–5483/4955–8159 3.5/5.3 580V/385R 900 229
17h.19m.34.77s.–35◦49(cid:48)59.30(cid:48)(cid:48) 03072014 Blue/Red 3358–5483/4955–8159 3.5/5.3 580V/385R 5400 311
NGC6357
17h.24m.43.52s.–34◦28(cid:48)05.00(cid:48)(cid:48) 03072014 Blue/Red 3358–5483/4955–8159 3.5/5.3 580V/385R 900 160
17h.24m.43.52s.–34◦28(cid:48)05.00(cid:48)(cid:48) 03072014 Blue/Red 3358–5483/4955–8159 3.5/5.3 580V/385R 5400 297
peaksat1.0,1.8,2.6,and4.6kpcwhichwereattributedtodif- Standard spectral reduction was followed using the auto-
ferent spiral arms. The average distance of the NGC 6334 and mated 2dfDR1 data reduction package with default values. The
NGC6357combinedcomplexwasfoundtobe1.75kpcwhich blue and red arm spectra were extracted, bias and flat-field
places it in the Sagittarius-Carina arm, although it appeared to corrected, wavelength calibrated, and sky subtracted. The blue
belocatedslightlyfartherawaythanthefirstmainstellarpeak. and red part of the spectra were then combined to produce
Distances to NGC 6334 and to NGC 6357 are still under full wavelength range spectra taking care with the splicing to
debate. Recently, Limaetal. (2014) identified five embedded adjust continuum levels. We note that no flux calibration was
clusters within NGC 6357 and derived a distance of 1.78 ± attempted.
0.1 kpc (based on colour-magnitude diagrams), while Wuetal. Weplacedatotalof997fibres,874ofwhichwereplacedto-
(2014)andChibuezeetal.(2014)measuredtheparallaxofH O wardsthetargetstarswhiletheotherswereplacedtowardsstars,
2 ii
maserfeaturesassociatedwithNGC6334I(N)(adensemolecu- strategically located with respect to the H regions and clus-
larclumpassociatedwithNGC6334)anddeterminedadistance ters. A first analysis of the 997 spectra allowed us to identify,
of1.35±0.15and1.25±0.14kpc,respectively,forNGC6334. andthenexclude,159coolstarsand625spectraexhibitinglow
signal-to-noiseratios(S/N)thatpreventedadecentspectralclas-
In this paper we present new spectroscopic observations
sification. In parallel, 104 fibres, placed in the direction of the
taken with AAOmega fibre-spectroscopy system (Sharpetal.
faint stars, appeared to be fully dominated by nebular emission
2006) on the AAT for a sample of OB star candidates selected
linesinboththeblueandredspectraandwerethenusedtoprobe
fromPaperI.Ourgoalistoestablishtheirspectraltypesandso
basicionisedgasproperties.Fulldetailsoftheobservationsand
determinenewspectrophotometricdistanceofthesestarstogain
instrumentalconfigurationaregiveninTable1.
precision withrespect tothe photometric-onlydistance method
previouslyused,whichwillthenbeusedtorefinetheanalysisof
theNGC6334andNGC6357systems.
3. Spectralclassification
To assess spectral type for each stellar spectrum we compared
2. Observationsanddatareduction
our data to the spectral atlas of MaízApellánizetal. (2016)
(O stars) and Walborn&Fitzpatrick (1990) (B stars), both of
We observed a sample of 874 stars extracted from the photo-
whichprovidemedium-resolutionspectra(R = 2500and2933,
metric catalogue of PaperI selected to have a V-band magni-
respectively)inthe4000to4900Å range.Afterdegradingthe
tude fainter than 14 mag. This sample was observed with the
template spectra to our spectral resolution (R = 1253) we per-
AAOmega spectrograph on the AAT in service mode on July
3, 2014. AAOmega allows the simultaneous observation of up formedthefittingoverthekey4000–4900Å rangebetweenthe
to 400 targets (including objects and sky positions) over a 2◦ normalisedobservedspectraandthetemplates(seeFigs.B.1).
fieldwith2arcsecondon-skydiameterfibres(Sharpetal.2006). For most observed stars with V > 17 the S/N in the blue
AAOmegaisadualbeamsystemallowingbothblueandredarm was often too low to perform a reliable classification. How-
simultaneousobservations;inthiscasethelow-dispersion580V ever, for 11 stars where the blue spectra were inadequate we
and385Rgratingswereused.Forarobuststellarspectralclas- were able to use the red part of the spectrum instead following
sificationweprincipallyneededtocoverthespectralrangefrom thequalitativeapproachofWalborn(1980).Withthisapproach
it is not possible to determine the luminosity class, so dwarf
3850 Å to 5000 Å because to classify O-B3 stars we need to
class was assumed. In the red spectra, O-type stars are identi-
identify some of the HeI and HeII lines at 4009, 4026, 4121,
fied from the ratio of He I λ5876 Å to He II λ5411 Å, which
4200,4387,4471,4541,and4686Å.Thegratingcentralwave-
passesthroughunityattypeO7,whileHeIλ5876Å isabsent
lengthsweresetto4425Å forthebluearmand6563Å (Hα)
at type O3. The O III λ5592 Å is visible up to O9.5 V spec-
forredarm.Thesegratingsandsettingsdelivercentralspectral
resolutionsofR = 1253andR = 1234,respectively.Given2dF trum,whileHeIIλ5411Å persistsuntilB0.2V.AttypeB0.5V,
pointings were carefully selected to cover areas of each region SiIIIλ5740Å becomesvisibleandremainssothroughtotype
centredonNGC6334andNGC6357thatcansamplethecandi- B1.5V. Finally, B2.5 V to B3 V spectra are identified from the
date O-B3 stars well. Target samples were further divided into SiII+MgIIλ6347Å visibility.
two magnitude groups of V = 14 to 17 and V = 18 to 20. Ultimately, 109 stars were classified as hot (O to B3) stars
Thiswasdoneforobservationalefficiencybyadaptingexposure (see Table 2). Their spectra are displayed in Figs. B.1 and
timestomatchthemagnituderangesof15minper2dFfieldfor
thebrightsampleand1hour30minforthefaintsample. 1 http://www.aao.gov.au/science/software/2dfdr
A86,page2of32
D.Russeiletal.:NGC6334andNGC6357
Table2. Thefirsttenentriesofthe109OtoB3starsample.
No.a RA Dec Spec. Vc Bc Uc AV d M log(age) 2MASS
(J2000) (J2000) Typeb mag mag mag mag kpc M(cid:12) yr designation
1 17:16:03.8 –36:18:51.8 B2V 15.682±0.009 17.596±0.019 18.212±0.062 8.40±0.30 0.90±0.30 8.6 7.3 17160382–3618517
2 17:16:04.3 –36:16:55.2 B2V 15.880±0.009 17.597±0.019 18.056±0.058 7.71±0.29 1.88±0.81 11.8 7.1 17160431–3616548
3 17:16:29.5 –36:20:56.2 B3 16.286±0.011 18.057±0.023 18.554±0.067 7.69±0.31 1.11±0.82 6.4 7.6 17162955–3620558
4 17:16:35.0 –36:27:36.7 B2V 15.168±0.006 16.530±0.010 16.804±0.029 6.25±0.21 1.91±0.64 8.6 7.3 17163510–3627367
5 17:16:53.5 –36:19:22.4 B2V 15.534±0.008 16.909±0.014 17.215±0.036 6.30±0.23 2.20±0.74 8.6 7.3 17165359–3619225
6 17:17:00.5 –36:13:36.0 B3Ia 14.887±0.006 17.285±0.015 18.304±0.052 9.82±0.31 2.64±1.12 24.5 6.8 17170051–3613362
7 17:17:01.4 –36:22:01.6 B2V 15.988±0.010 17.713±0.021 18.209±0.063 7.66±0.30 1.45±0.48 8.6 7.3 17170147–3622014
8 17:17:02.4 –36:20:05.0 B2V 15.657±0.009 17.044±0.015 17.432±0.044 6.35±0.24 2.28±0.76 8.6 7.3 17170247–3620049
9 17:17:11.4 –36:19:39.5 B2V 16.083±0.010 17.494±0.018 17.900±0.055 6.44±0.26 2.66±0.89 8.6 7.3 17171143–3619393
10 17:17:18.5 –36:21:38.6 B3V 14.846±0.006 16.084±0.010 16.417±0.028 5.61±0.19 1.49±0.60 6.4 7.6 17171858–3621384
etc...
Notes.FulltableisavailableattheCDS.(a)StarswithanX-Raycounterpartareindicatedwithanasterisk;(b)starswithnoluminosityclassare
classifiedfromtheredpartoftheirspectrum(weassumetheyaredwarfstars);(c)UBVphotometryisfromRusseiletal.(2012).
theiralternativenames(fromtheSIMBADdatabase)andcross- of about 41%. The O-B3 star distances distribution is shown
identification with 2MASS and Gaia-DR1 (Lindegrenetal. Fig.2-upperpanel.Wecomplementedoursamplewith25stars
2016) catalogues are presented Table A.1. A visual inspection belongingtoNGC6357(14stars)andNGC6334(11stars),for
of the spectra was performed to check and precise the classi- whichthespectraltype(deducedfromspectralanalysisorUBV
fication following the qualifiers listed in Table 3 of Sotaetal. Hβphotometry)andphotometrywereretrievedfromthelitera-
(2011). Because of our low resolution it is difficult to identify ture(seeTableA.2).
binarystars2ortospecifyanyspectralpeculiarity.Indeed,spec- The distance histogram clearly shows a peak at around
tral peculiarity or binarity can modify the luminosity and then 1.7 kpc (with a sub-peak at 1.1 kpc), and two other features
impactthedistanceestimate.Forexample,Walbornetal.(2014) at around 2.3 and 2.7 kpc. Similarly, at l = 314◦ Galactic line
note that the Vz characteristic corresponds to lower luminosity. of sight, Carraro (2011) reported the detection of two different
In this frame we checked our star sample and find that no star groupsofyoungfieldstarsat1.5and2.5kpc.Thisisinagree-
fulfils the Vz criterion (Ariasetal. 2016) or exhibits detectable mentwithourpreviousresult(cf.Fig.7inRusseiletal.2012).
nitrogen, silicon, or carbon emission features. Comparing the From the Hou&Han (2014) Milky Way spiral arm model,
spectroscopic classification of this hot star subsample with the the line of sight in the direction of the overall NGC 6334 –
photometricequivalentfromPaperI,wenotethat27%haveless NGC6357complexissupposedtocrosstheSagittarius-Carina,
thanonetypeofdifference,23%haveoneoroneandhalftypes Scutum-Crux,andNormaspiralarmsatdistances∼0.9,∼3,and
of difference, while the others have a greater number of differ- ∼4.5 kpc, respectively, while in the Russeiletal. (2007) model
ences(fourtypedifferencesonaverage).Inadditionwenotethat it is supposed to cross these arms at slightly different distances
93%ofthesampleareBstars(whichbecomes84%whenadding of ∼1.5, ∼3, and ∼4.3 kpc (mainly because of the departure of
thebrightOBstarsfromtheliterature). the Sagittarius-Carina arm with respect to a logarithmic spiral
model in this direction). Consequently, the 2.3 to 2.7 kpc peak
canbeassignedtotheScutum-Cruxarm,whilethefirstpeakis
4. Discussion
assignedtotheSagittarius-Carinaarm.
4.1. Stellardistance Torefinethespiralarmdistances,wecollectedfromthelit-
erature star clusters known in the direction of NGC 6334 and
For the newly classified O to B3 stars we determine their dis-
NGC 6357 (Fig. A.1) and for which the distances have been
tances and the extinction (see Table 2) by combining their
determined independently of any velocity or association (see
present spectral types with their B and V photometry estab-
Table3).Thedistanceandreddeningareeitherfromstellarspec-
lished in PaperI and using the extinction relation A = R ×
V V trophotometric information or from isochrone fitting to the ob-
E(B − V), where R and E(B − V) are band integrated quan-
V servedcolour-magnitudedistributions.Forabroadercontext,we
tities (Cardellietal. 1989). To this end, we applied the tradi-
also collected, from Kharchenkoetal. (2013), clusters within
tionalspectrophotometricmethodandusedtheabsolutemagni- a 3.7◦ radius centred at l,b = 351.33◦, 0.07◦, covering both
tude M andintrinsiccolourcalibrations(Schmidt-Kaler1983;
V NGC6334andNGC6357comprisingatotalof39clusters.The
Humphreys&McElroy 1984; Vaccaetal. 1996; Sungetal.
distance histogram for these clusters is shown in Fig. 2-lower
2013), while the adequat RV value is taken from Russeiletal. panel.Itshowsmainlyabroaddistributionpeakingat∼1.2kpc
(2012). The uncertainty on the distance takes into account the
withaproportionofyoungclustersincreasingwithdistance.The
uncertainties from the photometry, the R and the calibration ii
V clustersexcitingtheH regionsinNGC6334andNGC6357
(adopting one class, and one spectral sub-type uncertainty).
belong mainly to the youngest and farthest part of the complex
Comparingthephotometricandspectroscopicclassificationsof
between1.6and2.1kpc.Theclusteragesdistributionsuggestsa
thespectraltypesallowsustoshowthatthemoreaccuratespec-
possiblestellaragegradientacrosstheCarinaarm.Thisgradient
troscopicclassificationresultsinastatisticaldecreaseindistance
wasalreadyreportedbyMelniketal.(1998).
ii
At higher Galactic latitude we note the H region
2 To identify possible binarity we used the “duplicate” flag from the
Sh2-10 (l,b = 352.44◦, +2.26◦; α, δ = 17h17m11.43s,
Gaia-DR1 catalogue, the photometric comparison between the Gaia
G-bandandV-band(PaperI)magnitudes,andthe“prox”flagfromthe –34◦03(cid:48)07.6294(cid:48)(cid:48)).ItisexcitedbytheSCO-OB4associationat
2MASSpointsourcecatalogueandfindthatonlythestars30,32,49, adistanceof1.1kpc(Kharchenkoetal.2005)similartothedis-
72,and82couldbebinaryormultiplestellarsystems(seeAppendixA). tance of cluster Bochum 13 located to the north of NGC 6334
A86,page3of32
A&A607,A86(2017)
BB B
B
O B
g B
e B Pismis 24
d OSh2-10
4 B B
3 B O O
- B OBB OOBBB OOBOBOBBOBBOB B O HII 353.09+0.63
BBOB B
B G352.838+0.934
B
B
B
B O B B
B B BB G351.462+0.556
B
B
B B
HII 353.24+0.60 O
B
B
B G351.261+1.016
BB
Bochum 13
BB B
IRAS 17211-3458 BB BB
B
CS73
B B B B
G352.132+0.663 BBB BO BB
B B
B B B B
g B B OBBBBB B B G350.706+0.998
e B B BB
d 6 GUM64b BBBB OO BOOB B
3 B B B
- IRAS 17183-3606 B B BB B
B
GUM61 BBBBB B B
HII 351.2+0.5 B
GM1-24
17h26m 17h20m 17h14m
Fig.1.GlobalHαview(AAO/UKSTHαimage,Parkeretal.2005)ofNGC6334andNGC6357.Thepositionoftheobjectsdiscussedinthetext
areindicated.ThesampleofOtoB3starsareoverlaid(labelled“O”and“B”).RedsymbolsindicatestarswithX-raycounterparts.Thegreenand
boldredsymbolsarestarsfromtheliterature.
ii
(see distance listed in Table 3). Both H regions can be as- this cluster we note four OB stars giving a distance of 2.37 ±
signedtotheclosestidentifiedstellarpeak(1.1kpc). 0.81kpc.
G352.838+0.934 (α, δ = 17h23m40.11s, –34◦26(cid:48)58.59(cid:48)(cid:48)):
ii
listed as a 2.85 arcmin radius H region by Andersonetal.
4.2. Distancetoparticularregions (2014) and associated with a ∼2 arcmin radius infrared bubble
(MWP1G352835+009401) by Simpsonetal. (2012), it has
Several of our newly identified and sampled O-B3 stars are
a previously assigned distance determined by assuming a
found in the direction of particular regions (Fig. 1), which
spatialgroupingwithNGC6357.ItexhibitsclearWISE-22µm
thereforeallowustoconstraintheirdistances.
Pismis 24 (α, δ = 17h24m38.4s, –34◦12(cid:48)49(cid:48)(cid:48)): four stars are emissioninitscentralpartandexhibitsasemi-arcSpitzer-8µm
photodissociationregion(PDR)withpillarspointingtowardsan
located towards Pismis 24 from which we determine a mean
B0Iabstar,forwhichweestablishadistanceof4.59±2.52kpc.
distanceof2.46±1.44kpc.Thisdistanceisinagreementwith
However,thespectraltypeofthestar(star69)isquiteuncertain
thevalueestablishedbyMasseyetal.(2001).
HII region 353.24+0.60 (α, δ = 17h26m06.12s, duetothepoorS/N(Fig.B.1).
–34◦18(cid:48)16.74(cid:48)(cid:48)): toward this Hii region, located in the CS73 (α, δ = 17h24m24.62s, –35◦21(cid:48)25.45(cid:48)(cid:48)): in the direction
of the bubble CS73 (l,b = 352.174◦, +0.297◦), which has
NGC 6357 star-forming complex, a cluster of young stellar
a reported average bubble radius of ∼5.5 arcmin identified
objects was described by Fangetal. (2012) and named the
in the mid-infrared by Churchwelletal. (2007), we find two
“B cluster” more recently by Massietal. (2015). Towards
A86,page4of32
D.Russeiletal.:NGC6334andNGC6357
Table3.Clustersummary.
Cluster Coordinates Assoc. Distance log(age) E(B−V) Radius Reference
J2000 region kpc yr arcmin.
NGC6334
Bochum13 17h.17m.24.0s.–35◦33(cid:48)00(cid:48)(cid:48) 1.34 7.25 0.82 9.30 1
1.08 6.82 7 3
MWSC2558 17h.17m.14.6s.–36◦20(cid:48)07(cid:48)(cid:48) GM1-24 2.02 6.00 1.46 5.40 1
MWSC2577 17h.20m.48.6s.–36◦06(cid:48)57(cid:48)(cid:48) G351.2+0.5 1.13 8.32 1.03 14.7 1
MWSC2575 17h.20m.38.8s.–35◦52(cid:48)59(cid:48)(cid:48) GUM64b 0.89 8.45 0.75 4.20 1
NGC6357
Pismis24 17h.24m.38.4s.–34◦12(cid:48)49(cid:48)(cid:48) G353.2+0.9 1.77 6.00 2.19 6.6 1
2.0 6.69 1.75 1.5 2
1.7 6.0 1.87 4
2.5 6.24 1.73 2.7 5
VVVCL167 17h.25m.09.0s.–34◦11(cid:48)13(cid:48)(cid:48) 1.6 6.95 1.47 0.6 2
AH03J1725-34.4 17h.25m.32.0s.–34◦24(cid:48)20(cid:48)(cid:48) Hii353.09+0.63 1.48 6.0 2.63 5.4 1
1.9 6.69 1.99 1.7 2
BDS101 17h.25m.34.0s.–34◦23(cid:48)09(cid:48)(cid:48) Hii353.09+0.63 1.6 6.69 1.95 0.7 2
MWSC2598 17h.26m.49.9s.–34◦40(cid:48)59(cid:48)(cid:48) 0.82 7.95 0.75 1 1
References.1=Kharchenkoetal.(2013)andKharchenkoetal.(2016);2=Limaetal.(2014);3=Moralesetal.(2013);4=Fangetal.(2012);
5=Masseyetal.(2001).
B-type stars with distances of 1.56±0.98 and 2.23±0.92 kpc, suggestingthateithertheregionisnotlinkedtoNGC6334orit
respectively.Thelatter,ofB2.5Vtype,ismorecentrallylocated hassomeinternalmotion.IfthedistanceofthestarandtheHα
withinthebubbleandsosuggestsadistanceofabout2kpcfor velocitysuggesttheregionisnotlinkedtoNGC6334,theradio
thebubbleitself.Inaddition,faintbutclearHαemissionfillsthe velocity and the morphological correlation between the dust
ii
bubble suggesting it is a H region located in the Sagittarius emission(Spitzer-8µm)andtheoptical(Hα)extinctionfeature
spiralarmbutinalow-densityarea. suggest the opposite. For this region we expect that Gaia will
IRAS 17211-3458 (α, δ = 17h24m27.6s, –35◦01(cid:48)02(cid:48)(cid:48)): we allowustosolvetheproblem.
find an O9 V star (d = 1.37±0.32 kpc) centred on this IRAS IRAS 17183-3606 (α, δ = 17h21m43.02s, –36◦09(cid:48)33.68(cid:48)(cid:48)): we
source (l,b = 351.474◦, +00.549◦), which was classified as an find a B0 V star (d = 0.93±0.30 kpc) centred on this IRAS
ultra-compact Hii region by Bronfmanetal. (1996). No Hα source (l,b = 351.203◦, +00.293◦). It is clearly seen as an
emissionisobserved,whileatSpitzer-8µmonlyafaintdiffuse extended but patchy source in WISE-22 µm and Spitzer-24 µm
emission(∼1.7arcminacross)isnotedaroundthestar. imageswithasizeof∼2×4arcmin.Itappearsasanarrowand
G352.132+0.663 (α, δ = 17h22m48.84s, –35◦11(cid:48)06.53(cid:48)(cid:48)): this smallfilamentatSpitzer-8µm.Despiteitsdistance,noHαand
ii
was identified as a candidate H region by Andersonetal. radioemissionisobserved.
(2014).WeidentifyaB3starinitsdirection.However,wewere G350.706+0.998 (α, δ = 17h17m26.75s, –36◦09(cid:48)47.92(cid:48)(cid:48)): in
notabletoassignaluminosityclassforthisstar.Assumingitis the centre of this region (Andersonetal. 2014), also identified
a main-sequence star, this gives a distance of 0.89±0.66 kpc. as the mid-infrared bubble CS98 (Churchwelletal. 2007),
Thisdistanceincreasesto1.5±0.67kpcifitisaclassIIIstar. we find a B1V star with a distance of 1.10 ± 0.48 kpc. A
G351.462+0.556 (α, δ = 17h21m22.70s, –35◦47(cid:48)50.79(cid:48)(cid:48)): bow-shock feature is also clearly seen at WISE-22 µm and
this region is listed by Andersonetal. (2014) as a radio quiet Herschel-70 µm, underlining the asymmetry of the bubble.
ii
H region and it has a small Hα counterpart with a Vlsr Thisfeaturecanbeproducedbyarunawaystarmovingathigh
of –8 kms−1(Russeiletal. 2016). Based on its mid-infrared relative velocity (Gvaramadzeetal. 2011) or by a stellar wind
ii
morphology this region was classified as a possible bow-shock within the H region associated with such a rapidly moving
due to a runaway star (Russeiletal. 2016). Here we identify a star(Mackeyetal.2016).BecausetheWISE-22µmarciswell
B1IIIstarwithadistanceof2.46±1.06kpc. insidetheSpitzer-8µmbubblewefavourthesecondhypothesis.
G351.261+01.016 (α, δ = 17h18m57.122s, –35◦41(cid:48)57.54(cid:48)(cid:48)): However,thisregionisnotseeninHα,probablybecauseofthe
in the direction of this region (Andersonetal. 2014) we find a strongextinctioninfrontofit.
B2Vstar(d = 4.11±1.37kpc).Theradiocontinuumemission GM 1-24 (α, δ = 17h17m04.0s, –36◦21(cid:48)14(cid:48)(cid:48)): several stars are
(S = 310 mJy) estimated by Andersonetal. (2011) corre- observed in the direction of GM 1-24 and its adjacent dark
9GHz
spondstoaO9.5/B0Vstar(basedonPanagia1973)suggesting area. A mean distance of 1.58±0.62 kpc is found from stars
thatthestarweidentifyisnotaloneinexcitingtheregion.This towardsthedarkarea,whilestarsobservedtowardtheGM1-24
is in agreement with its belonging to the cluster [BDS2003]99 Hα emission span from 1.44 to 2.66 kpc. In the direction
(Moralesetal. 2013). The radio recombination Vlsr of the of the radio source G350.5+0.95 located at the centre of the
regionis–3.72kms−1(givenbyWengeretal.2013),buttheHα mid-infrared bubble CS103 (Churchwelletal. 2007), a B2 V
velocity is measured around –11 kms−1 (Russeiletal. 2016) starat1.45±0.48kpcisobserved.
A86,page5of32
A&A607,A86(2017)
Fig. 2. Upper panel: distribution of O-B3 stars distances. The black Fig.3.HRdiagramforNGC6357(upperpanel)andNGC6334(lower
histogramcorrespondstothepresentO-B3sample,whilethefullwhite panel).Filledsquares,circles,andtrianglesidentifystarsofluminosity
histogramincludestheO-B3starscollectedfromtheliterature.Lower classesIVandV,giants(classesIIandIII),andsupergiants(classI),
panel:distributionofstellarclusterscollectedfromtheliterature(white, respectively. Open symbols in the lower panel are for GM1-24. The
hashed, and black histograms correspond to all clusters, clusters with blue and red curves are Geneva isochrones and evolutionary tracks,
10<age<100Myr,andclusterswithage≤10Myr,respectively). respectively.
4.3. Hertzprung-Russelldiagram:starformationefficiency calibrations) for O- and B-type stars, respectively. Similarly,
andrate from the V-band photometry and distance uncertainties we es-
timated∼0.32dexand∼0.45dexinlogL/L (withtheadopted
Inordertoderivethestarformationrate(SFR)andstarforma- (cid:12)
tion efficiency (SFE) we need to determine stellar masses and BCcalibration)forO-andB-typestars.
ages.Todothis,weplacedthestarsontheHertzprung-Russell From the previous section we assumed that stars between
(HR) diagram and compared their positions with the Geneva 1.1 and 2.3 kpc belong to the NGC 6334 and NGC 6357 star-
stellarisochronesandevolutionarytracks3(Lejeune&Schaerer formingcomplexes.HRdiagramswereproducedseparatelyfor
2001) computed for solar metallicity and no rotation. The star starsinthedirectionofNGC6334(andGM1-24)andNGC6357
bolometric luminosities were calculated using the observed V (Fig.3).StarswithlowS/Nwerenotusedintheseplots.Forev-
magnitudes (Russeiletal. 2012) and the spectral types, A ex- erystar,weestimatedtheageandtheinitialmassbyminimising
tinctionsanddistancesdeterminedinthispaper.Forthat,foVllow- thedifferencebetweentheBolometricLuminosityandthetem-
ing Wrightetal. (2015), the bolometric corrections (BC) were peraturewiththevaluesfromtheGenevamodelledevolutionary
taken from Martins&Plez (2006) for O-type stars and from tracks(Lejeune&Schaerer 2001).Wethenwereabletolookat
Crowtheretal. (2006) and Humphreys&McElroy (1984) for theageandspatialdistributionofthestars(Fig.4).Thetypical
B-typestars.Theeffectivetemperatureswereassignedasafunc- uncertaintiesare18%onthemassand37%and94%ontheage
tion of the spectral type using the tabulations of Martinsetal. forO-andB-typestars,respectively.
(2005)forO-typestarsandTrundleetal.(2007)forB-typestars. From Fig. 3 we note a large age spread among the mem-
Giventhetypicaluncertaintiesofhalfatypeinthespectralclas- bers for both regions suggesting the star formation has been in
sification and one class in luminosity, we evaluated uncertain- progressforatleast5Myr,whileastellarpopulationolderthan
ties of ∼0.04 dex and ∼0.07 dex in logTeff (from the adopted 12.6Myr(log(age)=7.1)isalsonoted.Inversely,starsasyoung
as1.3MyrarealsoobservedinNGC6357.Theoldpopulationis
3 Downloadedfromtheinteractivetoolswebpage:http://obswww. spatiallyspread(Fig.4)throughoutthefield,includingthearea
unige.ch/Recherche/evol/ betweenthetwomainregions.InNGC6334starsyoungerthan
A86,page6of32
D.Russeiletal.:NGC6334andNGC6357
77..31 7.3
7.1
6.5 7.3
g 7.3
e 7.3
d **6.5**
4 7.6
3- 7.1**6.6*7*.71.5 666.777...79.13 7.1 776766.67766761.....6..1...517.47.5501167176...7.16.56 7.66.7 **6.5**
776.73..1.73 6.9
7.1
6.6
7.5
7.3
7.5**6.5** 7.3 7.3
7.3 7.1 7.1
7.1
7.1
7.3
7.3 7.6
6.7
7.6
7.3
7.6
77..63
77..56 7.6
7.37.3 7.63.7
7.3
7.6 7.3 7.1 7.5
7.177..01 7.36.577.3.3
7.6 7.0
7.1 7.1 **7.1** 6.6
ge 7.17.37.3 7.1 67..6176.1.867.78.0.70.3 6.9 7.3
d 7.17.1 6.7 7.3
63 7.67.3 6.5 766...6557.3
- **6.8** 7.1 7.1
7.5 7.3 67..716.8
7.1
77.6.773..733.37.67.3
7.3
17h26m 17h20m
17h14m
Fig.4.Age(log(age))ofstarsoverlaidontheHαview(AAO/UKSTHαimage,Parkeretal.2005)ofNGC6334andNGC6357.Valuesbetween
double-starmarksidentifypossiblerunawaystars(seetext).Valuesingreen,red,andbluemarkstarswithlog(age) < 7withdistances1 ≤ d ≤
2.3kpc,d>2.3kpc,d<1kpc,respectively.
ii
10 Myr are mainly located in the H regions and give an age suggesting that the recent star formation proceeded nearly si-
between 1.9 and 3.9 Myr. GM1-24 seems not to have formed multaneouslyacrosstheregion.ForNGC6334,theyfoundclus-
starsforthelast5Myr. ters with ages between 0.7 and 2.3 Myr and a mean age of the
In NGC 6357, stars younger than 5 Myr can be split into dispersed population of 1.9 Myr. Because we found OB stars
a ∼1.4 Myr (2 stars) and ∼4.6 Myr (13 stars) population. We withagesof∼4.6MyrinNGC6357wesuspectthatafirststar
note that the two youngest stars belong to Pismis 24. In ad- formation event could have shaped the central cavity (and then
dition, if several young stars in NGC 6357 trace Pismis 24 the larger scale filaments), while the main star formation burst
several stars of similar age are noted in the external parts of occurred ∼1.4 Myr ago. For NGC 6334, a star formation burst
ii
NGC 6357 (Fig. 4). Some of these stars are identified as run- occurred∼4MyragoformingtheopticallyvisibleH regions
awaystarsbyGvaramadzeetal.(2011)expelledfromthestellar and has subsequently continued to form the youngest clusters
clusterPismis24orAH03J1725-34.4.Thiswasalreadyreported listedbyGetmanetal.(2014).
by Walbornetal. (2014) who found, at the periphery of the 30 In parallel, we can estimate the “immediate past” SFE and
Doradus LMC (Large Magellanic Cloud) nebula, a sample of SFR. If we assume that our O-type star census is complete,
Ostarsinanearlyevolutionarystate,whichtheyassumedwere we can estimate the total stellar mass expected to have been
expelled(fromthe30Dorclusters)runawaystars. formedinbothregions.AdoptingtheSalpeterinitialmassfunc-
Kuhnetal. (2014) identified young stellar clusters in tion (dN/dM = A m−2.35) and a minimum cut-off mass of
NGC6357andNGC6334.ForNGC6357Getmanetal.(2014) 0.08M weestimate(followingNguyenetal.2015)atotalstel-
(cid:12)
show that the age of the low-mass pre-main-sequence (PMS) lar mass of 4491±898 M (upper stellar mass 41.6 M ) and
(cid:12) (cid:12)
stars belonging to different clusters, as well as the more uni- 8528±1968 M (upper stellar mass 77.8 M ) for NGC 6334
(cid:12) (cid:12)
formly distributed stars, have similar ages of 1.0 to 1.5 Myr andNGC6357,respectively.ForNGC6357thisisinagreement
A86,page7of32
A&A607,A86(2017)
with the stellar mass of 2–6 × 103 M found for Pismis 24
(cid:12)
by Massietal. (2015). Adopting as total gas mass M =
tot
2.3 × 105 M (Willisetal. 2013, Schneideretal. 2015) and
(cid:12)
4×105 M (Cappaetal.2011)forNGC6334andNGC6357,
(cid:12)
respectively, we determine SFEs of 0.019+0.008 and 0.021+0.004.
−0.007 −0.003
These SFEs are in the range 0.002–0.2 found for Galactic
molecular clouds (Murray 2011; Francoetal. 1994), in agree-
ment with the inner Galaxy molecular cloud median value of
0.02 (Myersetal. 1986) and consistent with typical SFEs esti-
matedformoderatestarformationactivitystar-formingregions
(Federrath&Klessen2013).
To evaluate the “immediate past” SFR we assume that the
star-forming timescale is the age of the oldest massive star
used for the SFE calculation (4 and 5 Myr for NGC6334 and
NGC6357respectively).WeobtainaSFR equalto1.1×
imm.past
103±300M Myr−1and1.7×103±400M Myr−1forNGC6334
(cid:12) (cid:12)
and NGC 6357, respectively, which translate into log(Σ ) =
SFR Fig. 5. Relation between X-ray and bolometric luminosity. The stars
0.07 (in 933 pc2) and 0.13 (1260 pc2) M(cid:12) yr−1kpc−2, re- belongingtoNGC6357andNGC6334areindicatedbysquaresandcir-
spectively. This places both regions into the mini-starburst cles,respectively.TheOandBstarsarecodedasredandblacksymbols.
regime of the Schmidt-Kennicutt relation (following Fig. 7 Symbolsmarkedbyatrianglearestarswithpoorspectra,whilecrosses
of Nguyenetal. 2015). For NGC6334, these values can be indicatestarswithdistancesgreaterthan3kpcorlessthan1.1kpc.The
compared with the “recent” SFR evaluated from young stel- X-rayfluxesareevaluatedinthe[0.5–8keV]energybandandcorrected
lar objects (YSOs). Indeed, for NGC 6334, Willisetal. (2013) forinterstellarextinction.Thelinecorrespondstothescalingrelation
log(L /L )=−7.07.
give a total mass for class II and I YSOs of 2000 M (in X bol
(cid:12)
600pc2).Assumingameanlifetimeof2MyrforclassIIYSOs
(Evansetal. 2009) this gives SFRrecent = 1 × 103 M(cid:12) Myr−1, andobtainthefollowingscalinglaw4:
log(Σ (recent)) = 0.22. From Tigéetal. (2017), we can also
SFR
evaluate for NGC6334 the “present” SFR. Tigéetal. (2017), log(L /L )=−7.07±0.76. (1)
X bol
based on Herschel observations, give a census of 32 high-mass
protostars (in a studied area of 1119 pc2) and a statistical life- This result is in agreement with the well-established relation
timeof3×105yr.Assumingthattheseprotostarshavemassesbe- of Chlebowskietal. (1989) and that found for other regions,
tween8and50M(cid:12)weestimateSFRpresent =1.6×104 M(cid:12)Myr−1 as given by Nazéetal. (2011) for O stars in the Carina com-
and log(ΣSFR(present)) = 1.15. We can then imagine that plex and Rauwetal. (2015) for OB stars in the CygOB2 asso-
NGC6334ispresentlyexperiencingastarformationburst. ciation. Previously, Wangetal. (2007) led an X-ray census of
youngstarsinNGC6357.Theyfoundasimilarscalingrelation.
In addition, we confirm spectroscopically the B-type status for
4.4. X-raycounterpart
fourOBstarscandidateswhichwehaveincommon(stars140,
120,225,and527listedintheirTable7).Wealsoconfirmspec-
The impact of X-ray emission on molecular clouds can regu-
troscopically eight of the OB candidates listed by Povichetal.
late star formation and associated timescales. In recent simula-
(2017):theirstars5and8ofNGC6334and2,3,17,18,19,and
tionsHocuk&Spaans(2010)notethatstarformationcanbein-
22ofNGC6357.
duced and delayed by X-rays and that massive stars can form
with high efficiency. If the X-ray impact of YSOs is negligible Just as reported by Antokhinetal. (2008) we also find an
X-raypowerdemarcationbetweenBandOstarscorresponding
itcouldbemoreimportantwhenmassivestarsarepresent(e.g.
toL ∼1.4×1038erg/s.
Feigelson1997).X-rayemissionfrommassivestarscanbegen- bol
eratedbydifferentkindsofstellarwindshocks(e.g.Rauwetal. Amid our 29 X-ray OB star sub-sample, 24 (∼54 stars/◦2)
and 5 (∼31 stars/◦2) are located toward NGC 6357 and
2015; Chlebowskietal. 1989; Ezoeetal. 2006). Hence, X-ray
NGC 6334, respectively. Despite the high level of incomplete-
emission can also be used to identify massive stars with strong
nessofoursample,wecanthusexpectthatmolecularcloudsin
winds(Povichetal.2013).
NGC 6357 are likely to be more affected by X-rays and wind
To investigate this we cross-correlate (cone search 2(cid:48)(cid:48)) our
than in NGC 6334. The spectral type analysis shows us that
O-B3 stellar sample with the MOXC sample of Chandra/ACIS
21 stars are main-sequence stars, while 6 are giants, and that
X-ray point sources from Townsleyetal. (2014) to identify
16 and 13 are B- and O-type stars, respectively. In NGC 6357,
X-ray emitting stars. The MOXC catalogue covers the main
ridge of NGC 6334 (the regions GUM 61 and Hii 351.2+0.5 12 of the X-ray stars are in the central cavity, mainly beiilong-
ing to Pismis 24, 6 are located at the border of the H re-
have not been covered) and most of NGC 6357 (except for re- gion G353.2+0.7, within class I and class II YSOs aggregates
gion G353.43+0.46) corresponding to about an area of 0.16◦2
as listed by Fangetal. (2012), also identified as cluster “F” by
and0.44◦2 forNGC6334andNGC6357,respectively.Wefind
Kuhnetal.(2014),while2areisolatedintheexternalregionof
29 O-B3 stars with an X-Ray counterpart (16 from the present
NGC6357.ThesetwostarsareassociatedwithcompactWISE-
sample and 9 added from the literature). From the catalogued ii
22µmemission,oneofthemcertainlypoweringtheH region
MOXCX-Raytotalenergyfluxwedeterminetheextinctioncor- G352.838+0.934.ThefiveX-rayOBstarsobservedinthedirec-
rected X-ray flux (following Wilmsetal. 2000) and the X-Ray
tionofNGC6334arelocatedinthedirectionofGUM64band
luminosity(L )usingthepreviouslydeterminedstellardistances
X
and extinction (Table 2). We then investigated the relation be- 4 Thescalingfactorisobtainedbyaveragingthelog(L /L )otherthe
X bol
tweenX-Rayandbolometricextinctioncorrectedfluxes(Fig.5) sample.ThelineplottedinFig.5isthenlog(L )=log(L )−7.07.
X bol
A86,page8of32
D.Russeiletal.:NGC6334andNGC6357
-0.1 -1.3
-2.5
0.8
g
ed -0.0-14.5
5 -6.6
.53 -2.9 -3.3
-
-4.4 8.5 --66..89-4.7 0.2
--180.--4.1740.99.0-.191.9
-6.9 -9.4
0.45-41.278-..6.33.91.1-5.7 -37..-299.2 -7.7 -4.6
-5.2
-12.3 -8.8
-7.8-9.2
ge -11.5 -3-.170.6 -1.9
d -18.9
63- 4.7-1-31.06.4-14.18.4 -0.2
-9.8
17h20m 17h16m
1.2
-10.1
-7.1
2.7 -13.6
--98.6.8
-12.6 -5.7-7.1
-0.1
-1.8 -12.6
2.4-2-1.2.0-5.3
-4.4 93.4.3
ge -2-21.81.4
d -7.8 5.4
4
3 -14.7
- -6.6
0.5 -4.6 -1.2
-2.2
-5.1
-1.0 -8.2
-0.8-7.3 -12-.46.1
-0.7
2.2 -3.3
-10-2.9.0-5.2 -9.4 -7.6
g -7.8
e -13.6
d
5
.4 -8-.53-.24.3
3
-
0.2
17h28m 17h24m
Fig.6.Velocity(Vlsrinkms−1)measurementsoverlaidontheHαimagesofNGC6334(upperpanel)andNGC6357(lowerpanel).Velocities
morenegativethan–10kms−1 areindicatedinred.Thediamondsymbolsarediscussedinthetext.
ii
GUM 64c, two evolved H regions located on either side of 4.5. Nebularinformation
the molecular ridge (Tigéetal. 2017). They are found in small
YSO groups of class I and II listed by Willisetal. (2013) and For the 104 2dF fibres placed on nebular components across
alsoidentifiedasclusters“K”and“G”byKuhnetal.(2014). the two main regions studied, the resultant spectra are,
A86,page9of32
A&A607,A86(2017)
9944
110088 76
5555 220011 114466
7766 5577
3322 113355
6633 111955077 336688 115577
1799999 112255
222244
114466
126
111166 114499
117766
3322
112299
113300
227777
113300 118855
370
225522
445511
15 arcmin 9999
441144 226
44229955
7744 61
20 arcmin
7722
4477 28
4466 99116600 55105
8888
1111779988 45
111144
110077
117733 773399 220055
222277
113322
113399 112244
222255 118888 114477115522
111100
119977 119911
220011008844229977 118800 224411
115577
110000
7722
115577110099
55
Fig. 7. Spatial locations of individual electron density measurements obtained from nebula spectra (in cm−3) overlaid on the Hα images of
NGC6334(upperpanel)andNGC6357(lowerpanel).
unsurprisingly,completelydominatedbynebularemissionlines extinction and use the well-known nebular lines of Hα, [NII],
thatallowustoprobetheionisedgaspropertiesatdifferentpo- and [SII] to derive physical conditions of the ionised gas in-
sitionsoverbothNGC6334andNGC6357(Figs.6and7).We cluding electron density. In addition, despite the low resolution
focus on the red part of the spectrum which is less affected by (R∼1200atHα)givinga∼24kms−1 uncertaintybasedonthe
A86,page10of32