a r X i v :a s t r o -p h /0610653v 2 22 N o v 2006
A STROPHYS .J.L ETTERS 652(2006)L93
Preprint typeset using L A T E X style emulateapj v.6/22/04
HST WFPC2OBSERV ATIONS OF THE PECULIAR MAIN SEQUENCE OF THE DOUBLE STAR CLUSTER NGC 2011
IN THE LARGE MAGELLANIC CLOUD
D.A.G OULIERMIS 1,S.L IANOU 2,M.K ONTIZAS 2,
E.K ONTIZAS 3,A.D APERGOLAS 3
Astrophys.J.Letters 652(2006)L93
ABSTRACT
We report the serendipitous discovery of a peculiar main sequence in archived Hubble Space Telescope WFPC2observations of the young star cluster NGC 2011in the Large Magellanic Cloud.Th
e bright part of this main sequence exhibits a prominent double,fork-like feature,as if it consists of twin main sequences,one of them being redder.The color-magnitude diagram,constructed from the stars found in the only available WFPC2field of the cluster,is used to distinguish the stars according to their membership to each of these sequences and to study their spatial distribution.We find that there are two well distinguished populations in the sense that the redder main sequence is dominated by stars that belong to the main body of the cluster,while the stars of the bluer main sequence belong to the surrounding region.Providing that NGC 2011is a verified binary cluster,with the second companion unfortunately not observed,and taking into account the general region where this cluster is located,we discuss the possible scenarios from both star formation,and early dynamical evolution point-of-view that might explain this unique discovery.
Subject headings:Hertzsprung-Russell diagram —Magellanic Clouds —stars:evolution —galaxies:star
clusters —clusters:individual (NGC 2011)
1.INTRODUCTION
The Large Magellanic Cloud (LMC)contains an extraordi-nary sample of star clusters with a large var
iety in spatial dis-tribution,age and luminosity (e.g.Kontizas et al.1990;Bica et al.1999).This intriguing population of clusters include pe-culiar types of systems,like binary and elliptical clusters,that we do not see in the Galaxy.In this letter we present our find-ings from Hubble Space Telescope (HST)observations of the LMC cluster NGC 2011with the Wide Field Planetary Cam-era 2(WFPC2).NGC 2011is a young star cluster with age 6(±1)Myr (Elson &Fall 1988)located in a young region (Figure 1left)at the southern edge of the super-giant shell LMC 4(Meaburn 1980).It is considered as one of the bright-est and most populous double clusters in the LMC (Kontizas et al.1989).It was originally identified as a probable double cluster by Bhatia &Hatzidimitriou (1988)who conclude that among the LMC cluster population binary clusters constitute a statistically significant sample.
The multiplicity of the cluster,in the sense that projection effects cannot account for this pair,was verified by Kontizas et al.(1993),who studied the stellar content in the outer re-gion of the cluster by means of spectra classification from UK Schmidt objective prism spectra,and in the core of the cluster with low resolution International Ultraviolet Explorer (IUE)spectra.From the spectral classification it was found that both members of the pair show identical stellar content,dominated by early type stars,quite different from the stellar component of the neighboring field,where stars with sp
石筱磊happy songectral type later than A dominate.The UV integrated spectra taken with IUE showed that the stellar content in the core of both the brighter (named NGC 2011a)and fainter (NGC 2011b)members of
1
Max-Planck-Institut für Astronomie,Königstuhl 17,69117Heidelberg,Germany,dgoulier@mpia.de
2National and Kapodistrian University of Athens,Faculty of Physics,Dpt of Astrophysics,Astronomy and Mechanics,Panepistimiopolis,Zografos,15784Athens,Greece,s_lianou@,mkontiza@
3National Observatory of Athens,Institute of Astronomy and Astro-physics,Lofos Nymfon,Thiseio,P.O.Box 20048,11810Athens,Greece,a.gr,a.gr
the pair are found to be very similar.Kontizas et al.(1993)note that the core of NGC 2011a is elongated,so that another UV feature,which was named NGC 2011c,at an off-center position,but still in the dense core to be observed.The IUE spectra of NGC 2011b and NGC 2011c are found to be very similar.
Unfortunately the second component,NGC 2011b,is not covered in the WFPC2field presented here,b
ut the elonga-tion of the core of NGC 2011a is very well shown in these observations,and a part (if not all)of the third possible com-ponent,NGC 2011c,can be seen at the southern corner of the PC frame of the camera (Figure 1Right).In this Letter we report the peculiar behavior of the main sequence of NGC 2011,and we discuss its relation to the multiple nature of the cluster.
2.OBSERV ATIONS AND PHOTOMETRY
The results presented here are based on a single WFPC2field taken with the PC frame centered on NGC 2011a.The data were taken in filters F555W (∼V )and F814W (∼I )as part of the program GO-8134,and we retrieved them from the HST data archive 4.The six datasets (archive names U5AY0801R -U5AY0806R)include three exposures (2×350sec and 1×10sec)for each filter.The data were reduced using the HSTphot photometry package (Dolphin 2000).A detailed account of the photometry process with HSTphot is given by Gouliermis et al.(2005).
We combined the two images of 350seconds in each filter using the subroutine coadd to produce deep exposures.HST-phot is suited for HST/WFPC2observations and it is espe-cially designed to perform simultaneous photometry on both short-and long-exposure images and for more than one fil-ters.We,thus,performed our photometry on both deep (700sec)and shallow (10sec)images observ
ed in both filters to produce the final photometric catalog of 6,760stars found in the observed field.The detection limit of the short exposures
4
available from STScI at archive.stsci.edu/hst/search.php and ESO at /wdb/wdb/hst/science/form
2Gouliermis et
al.
F IG.1.—Left:A25′×25′image of the general region,where NGC2011is located with the observed WFPC2field overplotted.This image is selected from a digitized wide-field UKST photographic plate in the B-band,to exhibit the nebulosity of the region.The observed WFPC2field-of-view(FoV)was such that the PC frame covers the primary cluster NGC2011a.The second companion is unfortunatel
y outside this FoV(located to the right of PC frame)as it is shown in the inset image,constructed from an I-band UKST photographic plate,which does not show any emission from the nebula.Right:Map of stars found with HSTphot photometry based on WFPC2imaging of NGC2011.An outline is drawn to indicate the position of the possible component NGC2011c.Component NGC2011a is almost entirely covered by the PC frame of WFPC2.The images in both panels have almost identical orientation for reasons of comparison.
is V≈21.5mag,and the brightest magnitude observed in the
long exposures is V≈18.5mag.The present study is part
of an ongoing investigation of the phenomenon of mass seg-
regation in peculiar LMC clusters,and more details on the
photometry will be given in a forthcoming paper(S.Lianou
et al.,in preparation).
3.STELLAR POPULATIONS IN THE OBSERVED FIELD
3.1.The Fork-like Main Sequence
The color-magnitude diagram(CMD)of the
stars found in
the observed area of NGC2011is shown in Figure2.In this
CMD the upper main sequence shows to have two distinct
branches,one redder than the other,that form a fork-like pat-
tern.This feature is observed in the stellar samples from both
short and long exposures.The obvious resemblance of these
two main sequences suggest that they are populated by similar
kinds of stars and that the shift of the one to the red is prob-
ably due to reddening.Indeed,as it will be shown later the
right sequence(from hereon we will refer to it as the“Red”
main sequence)seems tofit the same isochrone models as the
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left(“Blue”main sequence)but assuming higher extinction.
In order to distinguish the areas where the stars of each of
these sequences are located and to check for any difference
in their spatial distributions,we selected two regions in the
CMD,covering the bright stars that populate each of the se-
quences,and we constructed the corresponding stellar maps.
We selected stars down to V=19mag,which is the faintest
magnitude where the two sequences are discriminable.The
map of both stellar groups is shown in Figure3,where the
stars of the Blue main sequence(MS)are plotted with open
and of the Red MS withfilled circles.In this map the loci
of the bright stars representative of each sequence can be eas-
ily distinguished.Specifically,the Red MS is found to cor-
respond to the main body of NGC2011(within∼0.′4from
F IG.2.—V−I,V Color-Magnitude Diagram(CMD)of all stars detected
in the area of NGC2011with WFPC2imaging.The bright fork-like feature
of the main sequence,that seems to consist of two distinct sequences,is quite
prominent.One of the branches of this feature is located to the red side of the
other and both show to be populated by the same kind of stars.Are these two
bright main sequences indicative of different stellar systems,or are they part
of the same peculiar stellar concentration?
HST/WFPC2Photometry of NGC 2011
3
F I
G .3.—Map of the bright stars of the Blue (open circles)and the Red (filled circles)main sequence.This map,which codes stellar brightness by symbol size,exhibits the discrepancy of the loc
i of the stars according to their membership to one of the branches of the fork-like main sequence observed in the area of NGC 2011.Most of the Red MS stars are strictly confined
within the cluster itself (within ∼0.
′4from its geometrical center),while the stars of the Blue MS are covering the remaining surrounding area,and they avoid the cluster region.
its center),observed in the PC frame of the WFPC2(in addi-tion to a loose strip of stars outwards to the east/north-east),while the stars of the Blue MS are located in the surrounding region,and they “avoid”the main cluster.This observation indicates that there is preferably higher extinction in the area of the cluster than in the surrounding region.
Indeed,as shown in Figure 1,NGC 2011is located in a region of high nebulosity.This region belongs to a stellar aggregate,along with several stellar associations and other young clusters in the stellar super-complex SC2.This com-plex,which is classified as an “active star forming complex”(Livanou et al.2006)is located at the southern edge of the super-giant shell LMC 4,the borders of which are character-ized by recent star formation (e.g.Yamaguchi et al.2001).An examination of the region of NGC 2011in the Magellanic Cloud Emission Line Survey (MCELS;Smith et al.2005)showed no trace
of interstellar gas that has been heated and energized by stars.On the other hand,observations with Spitzer within the SAGE survey (Meixner et al.2005)show a prominent bright infrared source on the cluster itself,giv-ing clear indications of recent star formation in the region of NGC 2011.Could it be that NGC 2011is a cluster on the act of forming stars?The observations presented so far seem to support this idea.
3.2.Characterization of the Observed Stellar Populations
黄景瑜起反应了If we assume that each of the two branches of the bright fork-like MS (Blue and Red)represents a stellar sample with a fully populated mass function (down to the detection limit)it would be interesting to see the whole CMD of each of these populations.Therefore,considering that each one of the ar-eas,which covers stars of the Blue and Red main sequences is well defined,the faint part of the corresponding CMD can easily be found if we select stars confined only in each of these areas.The PC frame is found to cover almost all of the bright-
est stars of the Red MS,without any contamination by stars of the Blue MS and therefore we selected all stars found in the PC frame,as well as in the small strip outwards from the cluster to the east/north-east direction (Figure 3)to construct the corresponding CMD down to the detected faintest
magni-tudes.For the full magnitude range CMD,which corresponds to the Blue MS,we selected two boxed areas,to the north-east and south of the PC frame,which are found to cover most of the bright young stars of the Blue MS (Figure 3).
Considering that the WFPC2FoV is too small to also cover a useful part of the general field of the LMC,and that the ob-served region is not representative of this field (Figure 1),one cannot expect to have any good measurement of the contri-bution of the LMC field population to the observed CMDs.Nevertheless,we selected the two most distant corners of the observed FoV (one to the far east and one to the far north of the PC frame)for plotting an indicative CMD of the general LMC field.The CMDs of all three selected areas are shown in Figure 4.It can be seen that indeed the whole magnitude range of stars,which belong to the Red and Blue MSs can be distinguished.This was further verified by applying isochrone fitting to each of the CMDs shown in Figure 4.The evolution-ary models of the Padova group in the HST WFPC2VEGA magnitude system (Girardi et al.2002)were used.
We found that both Blue and Red MSs are young with an age not older than about 107yr in agreement with the result of Elson &Fall (1988).Still there is a strong difference in reddening between the Blue and Red MSs.Specifically,we found that the CMD of the Blue MS shows a modest
color excess of E (B −V )≃0.01mag,equal to the one found for the field,while the color excess found from model fitting on the Red MS CMD is almost an order of a magnitude higher,E (B −V )≃0.15mag,which corresponds to optical extinction A V ≃0.48,assuming the reddening law R V =3.2(Mihalas &Binney 1981).This result verifies the suggestion mentioned earlier,that although both MSs are populated by similar kind of stars,there is a clear discrepancy in the absorption in each of the corresponding area,indicating that the main body of NGC 2011is embedded,strongly suggesting a recently formed star cluster,which did not have the time to expel its gas.Still,it should be noted that the suggested reddening for NGC 2011is much higher than the one given by van den Bergh (1981)who presented integrated colors for 147LMC clusters.
4.FINAL REMARKS
We showed evidence of the coexistence of two stellar groups in the same region of the LMC cluster NGC 2011,as it is observed with WFPC2.This is exhibited by a double pattern in the bright end of the observed main sequence.We found that each branch of this fork-like feature represents a fully populated main sequence (down to the detection limit),showing indications of twin stellar populations,the one lo-cated in the cluster itself being still embedded in its star form-ing gas.If these two populations belong to the same system or not is open for discussions,and the results presented
so far rise interesting questions concerning the relation between the peculiar main sequence of NGC 2011and its binary nature.Considering that the data presented here are the best avail-able today of this cluster,with no kinematic information on individual stars in the region,we can only speculate on the scenarios that explain our observations.
The simplest explanation would be that both main se-quences belong to the cluster itself,but some of the stars are not obscured by nebulosity,which is concentrated in the
4Gouliermis et
al.
F IG.4.—The V−I,V CMDs of the stars from our catalog confined within(1)the PC frame of WFPC2and a strip east/north-east from it,which covers the Red MS in the main body of NGC2011(left panel),(2)two boxed areas,one to the north-east and one to the south of thefirst(Figure3),which cover the population of the Blue MS(center panel),and(3)to the most distant east and north corners of the FoV,which provide an indicative population of the LMC generalfield (right panel).Isochrone models are overlayed.The model for logτ=6.6is overplotted on the Red MS with reddening A V≃0.48mag applied(the corresponding vector is shown on the right of the MS),and on the Blue MS with a modest reddening of A V≃0.03mag.In the central and right panels isochrones representative of thefield population of the LMC for ages1Gyr and1and3Gyr respectively are also shown.These isochronefits show that stars of both Blue and Red MSs seem to have the same age with logτ≤7,but the rddening in the corresponding areas is quite different.
core of NGC2011.But,in order to explain the existence of
massive stars away from the core,since dynamical evolution would tend to segregate these stars to the center(Meylan&
Heggie1997),one should assume that the cluster is under dis-ruption.Indeed,the elongated core observed in our WFPC2field supports this idea.Furthermore,the surface density pro-files constructed for NGC2011within our study of mass seg-regation(S.Lianou et al.,in preparation)deviate from King (1962)profiles,indicative of tidal stripping,although King models may not be the most suitable for such clusters,which are not tidally truncated(Elson et al.1987).Still,NGC2011 is a binary cluster with indications of tidal interaction from the companion cluster NGC2011b(Kontizas et al.1993), and with a third component observed in our WFPC2data,as a stellar“bridge”between NGC2011a and NGC2011b(Fig-ure1).
Under these circumstances,and since the stellar popula-
tions of both members of the binary cluster are identical,one may ask if the stars in the Blue MS are not members of NGC 2011a,but of the second companion,NGC2011b,stripped away from their host through dynamical interaction with the primary.According to recent N-body simulations,clusters with an initial separation smaller than1′-1.′3tend to merge in<∼60Myr due to loss of angular momentum from escap-
ing stars(Portegies Zwart&Rusli2006).Considering that the actual center to center separation of the NGC2011pair is
0.′95(Kontizas et al.1993)this is a possible future for this system.Are the two members of NGC2011binary cluster very young clusters in the process of early dynamical merg-ing?It is probable,but more observations preferably with the wider FoV of WFC of the Advanced camera for Surveys on HST will definitely give us a more clear-cut answer.
D.Gouliermis acknowledges the support of the German Research Foundation(Deutsche Forschungsgemeinschaft-DFG)through the individual grant1659/1-1.M.Kontizas and
E.Kontizas wish to thank the General Secretariat for Research and Technology and PYTHAGORAS II,project funded by the European Social Fund and National Resources(EPEAEK II),forfinancial support.This paper is based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Science Insti-tute.STScI is operated by the Association of Universities for Research in Astronomy,Inc.under NASA contract NAS5-26555.
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