Probing the age and structure of the nearby very young open clusters NGC 2244 and NGC 2239
aa r X i v : . [ a s t r o - ph . GA ] J a n Mon. Not. R. Astron. Soc. , 1–15 (0000) Printed 30 October 2018 (MN L A TEX style file v2.2)
Probing the age and structure of the nearby very youngopen clusters NGC 2244 and NGC 2239
C. Bonatto ⋆ and E. Bica † Departamento de Astronomia, Universidade Federal do Rio Grande do SulAv. Bento Gon¸calves 9500, Porto Alegre 91501-970, RS, Brazil
30 October 2018
ABSTRACT
The very young open cluster (OC) NGC 2244 in the Rosette Nebula was studied withfield-star-decontaminated 2MASS photometry, which shows the main-sequence (MS)stars and an abundant pre-MS (PMS) population. Fundamental and structural pa-rameters were derived with colour-magnitude diagrams (CMDs), stellar radial densityprofiles (RDPs) and mass functions (MFs). Most previous studies centred NGC 2244close to the bright K0V star 12 Monocerotis, which is not a cluster member. Instead,the near-IR RDP indicates a pronounced core near the O5 star HD 46150. We de-rive an age within 1—6 Myr, an absorption A V = 1 . ± .
2, a distance from the Sun d ⊙ = 1 . ± . ≈ . χ = 0 . ± . ∼ M ⊙ . Its RDP is characterised by thecore and cluster radii R c ≈ . ′ ( ≈ . R RDP ≈ ′ ( ≈ . m MS + P MS ≈ M ⊙ ), young (5 ± V = 3 . ± . d ⊙ = 3 . ± . R c ≈ . ′ ≈ . R RDP ≈ . ′ ≈ . χ = 1 . ± .
06, is essentially Salpeter’s IMF.NGC 2244 is probably doomed to dissolution in a few 10 yr. Wide-field extractionsand field-star decontamination increase the stellar statistics and enhance both CMDsand RDPs, which is essential for faint and bright star clusters. Key words: (Galaxy:) open clusters and associations: general; (Galaxy:) open clus-ters and associations: individual: NGC 2244 and NGC 2239.
Still in the process of emerging from the parent molecu-lar cloud, star clusters younger than about 5 Myr usuallypresent a developing main sequence (MS) and a significantpopulation of pre-MS (PMS) stars. However, depending onthe initial cluster mass, star-formation efficiency and massof the more massive stars, the rapid early gas removal (fromsupernovae and massive star winds) may impart importantchanges to the original gravitational potential. One conse-quence of the reduced potential is that stars, especially thelow mass ones, moving faster than the scaled-down escapevelocity may be driven into the field. Over a time-scale of10 −
40 Myr, this effect can dissolve most of the very youngstar clusters (e.g. Goodwin & Bastian 2006). Indeed, esti-mates (e.g. Lada & Lada 2003) predict that only about 5% ⋆ E-mail: [email protected] † E-mail: [email protected] of the embedded clusters are able to dynamically evolve intobound open clusters (OCs).On observational grounds, the dramatic changes in thepotential affecting the early cluster spatial structure shouldbe reflected on the stellar radial density profile (RDP).Bochum 1 (Bica, Bonatto & Dutra 2008), for instance, canbe an example of this scenario, in which the irregular RDPdoes not follow a cluster-like profile. This suggests signifi-cant profile erosion or dispersion of stars from a primordialcluster.In the present paper we address the case of NGC 2244 inthe Rosette Nebula, which is also related to the Monocero-tis OB2 association (e.g. Rom´an-Z´u˜niga & Lada 2008). His-torically, in colour-magnitude diagram (CMD) studies someauthors centred the large-scale structure on 12 Mon, whichis a bright foreground star of spectral type K0V. When onlywide-field CMDs are considered, the adoption of this centredoes not affect the results. However, as will be explored in c (cid:13) C. Bonatto and E. Bica this work in the context of investigating the cluster struc-ture, that region is definitely at the cluster periphery.Based on Shanghai Observatory plates with baselines of34 and 87 yr, Chen, de Grijs & Zhao (2007) derived propermotions (PMs) and membership probabilities for NGC 2244.They found mass segregation, but no velocity-mass depen-dence, indicating a primordial mass segregation related tothe star-formation process. With arguments based on pub-lished initial mass functions (IMFs) and the measured inter-nal velocity dispersion of ≈
35 km s − , they concluded thatNGC 2244 will be dissolved on a short time-scale.Additionally, in the area of the Rosette Nebula, the clus-ter candidate NGC 2239 has been frequently included in cat-alogues, but hardly studied. Both NGC 2244 and NGC 2239are optical clusters, while the area includes numerous in-frared embedded clusters in the Rosette Molecular Cloud(e.g. Phelps & Lada 1997).This work employs 2MASS near-IR J , H , and K s pho-tometry. The 2MASS spatial and photometric uniformity al-low extractions of wide surrounding fields that provide highstar-count statistics. This property makes 2MASS an excel-lent resource to extract photometry of a broad variety of starclusters, the wide field ones in particular. For this purposewe have been developing quantitative tools to statisticallydisentangle cluster evolutionary sequences from field starsin CMDs. Decontaminated CMDs have been used to inves-tigate the nature of cluster candidates and derive their astro-physical parameters (e.g. Bica, Bonatto & Camargo 2008).Basically, we apply (i) field-star decontamination to measurethe statistical significance of the CMD morphology, which isfundamental to derive reddening, age, and distance from theSun, and (ii) colour-magnitude filters, which are essential forintrinsic stellar RDPs, as well as luminosity and mass func-tions (MFs). In particular, the use of field-star decontami-nation in the construction of CMDs has proved to constrainage and distance more than working with raw (observed)photometry, especially for low-latitude OCs (Bonatto et al.2006a).2MASS can be deep for nearby young or oldOCs. For instance, our group has studied the youngOCs NGC 6611 (Bonatto, Santos Jr. & Bica 2006) andNGC 4755 (Bonatto et al. 2006b). Abundant pre-MS (PMS)stars were seen in the ≈ ≈
14 Myrold NGC 4755. As nearby older OCs we cite NGC 2477(Bonatto & Bica 2005) and M 67 (Bonatto & Bica 2003).In this paper we apply our set of analytical tools to the2MASS photometry of the stars in the area of NGC 2244 toderive its fundamental parameters, structure and fraction ofMS and PMS stars. We also investigate the neighbouringcluster NGC 2239.This paper is organised as follows. In Sect. 2 we re-call literature data on NGC 2244. In Sect. 3 we describethe 2MASS photometry and compare it with the availableoptical data; we also describe the field-star decontamina-tion algorithm and build CMDs. In Sect. 4 we derive clusterfundamental parameters. In Sect. 5 we derive structural pa- The Two Micron All Sky Survey, All Skydata release (Skrutskie et al. 1997), available at rameters by means of stellar RDPs. In Sect. 6 we provideestimates of cluster mass. In Sect. 7 we compare the struc-tural parameters and dynamical state of the present clusterswith those of a sample of nearby OCs. Concluding remarksare given in Sect. 8.
Several studies on NGC 2244, especially photometric andspectroscopic ones, are available in the literature.The WEBDA database locates the cluster centre at α (2000) = 06 h m s and δ (2000) = +04 ◦ ′ ′′ , and pro-vides a distance from the Sun d ⊙ = 1 .
45 kpc, reddening E ( B − V ) = 0 .
46, and an age of 7.9 Myr.With UBV photometry, Ogura & Ishida (1981) ob-tained E ( B − V ) = 0 .
47, a total to selective ex-tinction ratio R V = 3 . d ⊙ = 1 .
42 kpc, 4 Myr ofage, and a total mass of 5000 M ⊙ . With similar data,Hensberge, Pavlovski & Verschueren (2000) derived an ageof 2 Myr, R V = 3 . ± . E ( B − V ) = 0 .
44, and d ⊙ =1 . ± . ′ , anage within 1 . − .
63 Myr, d ⊙ = 1 .
67 kpc, E ( B − V ) = 0 . M ⊙ for NGC 2244.Park & Sung (2002) found an average E ( B − V ) =0 . ± . R V = 3 . ± .
2, and d ⊙ = 1 .
66 kpc. By com-paring their photometric results with theoretical evolutionmodels, they derived a main-sequence turnoff (MSTO) ageof 1.9 Myr and a PMS age spread of about 6 Myr. The IMFslope calculated for the mass range 3 . . m ( M ⊙ ) .
100 isflat ( χ = − . ± . ±
3% was estimated formembers with masses above 0 . M ⊙ .In a Chandra study of NGC 2244, Wang et al. (2008)detected over 900 X-ray sources, 77% of which having opti-cal or FLAMINGOS NIR counterparts. Their X-ray-selectedpopulation is estimated to be nearly complete between 0.5and 3 M ⊙ . The K-band LFs indicate a normal Salpeter(1955) IMF for NGC 2244, which differs from the top-heavyone reported in earlier optical studies that lacked a goodcensus of . M ⊙ stars. The X-ray LF indicates a popula-tion of ∼ ∼ . R c = 1 . c (cid:13) , 1–15 he very young OCs NGC 2244 and NGC 2239 Figure 1.
Top-left panel: 1 ◦ × ◦ DSS B image of NGC 2244 and the Rosette Nebula, centred on the coordinates of Table 1. Right:30 ′ × ′ I image of the same region; circles indicate the cluster radii (Sect. 5) of NGC 2244 (East) and NGC 2239 (West). Several centresadopted for these clusters are identified in the right panel. The K0V star 12 Mon is indicated by position ’B’. Other positions as indicatedby the keys in Table 1. Bottom: K s image of NGC 2244 taken from the 2MASS Image Service focusing on the compact core within 3 ′ × ′ .Same centre as in the B image. Orientation: North to the top and East to the left. segregation, indicates that NGC 2244 is not in dynamicalequilibrium. The fraction of X-ray-selected members with K-band excesses caused by inner protoplanetary discs is 6%,slightly lower than the 10% disc fraction estimated fromFLAMINGOS. The Rosette X-ray spectra of OB stars aresoft and consistent with the standard model of small-scaleshocks in the inner wind of a single massive star.Recently, Rom´an-Z´u˜niga & Lada (2008) reviewed theRosette Complex, in particular they refer to the position oftwo OCs, NGC 2244 at position ’C’ in the top-right panel ofFig. 1, and the other at postion ’A’, designated as NGC 2237.We call attention that their NGC 2244 actually is in the region of the original NGC 2239, while NGC 2237 refers toNGC 2244, near the position of HD 46150.Rom´an-Z´u˜niga & Lada (2008) build a scenario wherean expanding H II region generated by a large OB associa-tion interacts with a giant molecular cloud, which harboursa number of embedded and open clusters.The wealth of papers on NGC 2244 reflects the com-plex - and, at the same time beautiful - nature of the inter-play between bright massive stars, faint pre-MS stars anda thinning dust shroud, all embodied in a single and rela-tively nearby object. Fig. 1 illustrates this scenario. In the c (cid:13) , 1–15 C. Bonatto and E. Bica
Table 1.
Previously adopted centres of NGC 2244 and NGC 2239Cluster α (2000) δ (2000) ℓ b Key R Reference(hms) ( ◦ ′ ′′ ) ( ◦ ) ( ◦ ) ( ′ )(1) (2) (3) (4) (5) (6) (7) (8)NGC 2237 06:31:58.5 +04:54:35.7 206.34 − .
07 A — Rom´an-Z´u˜niga & Lada (2008)NGC 2244 06:32:18.0 +04:52:00.0 206.42 − .
02 B — Sulentic & Tifft (1973)NGC 2244 06:30:36.1 +04:58:50.6 206.12 − .
34 C — Rom´an-Z´u˜niga & Lada (2008)NGC 2244 06:31:55.0 +04:58:30.0 206.28 − .
06 D — WEBDANGC 2244/12 Mon † − .
02 E ∼
24 SIMBADNGC 2244 06:31:55.4 +04:56:35.3 206.30 − . ∼
10 This workNGC 2239 06:30:54.0 +04:57:00.0 206.18 − .
29 F ∼
18 Sulentic & Tifft (1973)NGC 2239 06:30:57.3 +04:58:09.0 206.17 − . ∼ † ): 12 Mon as the centre of NGC 2244. Col. 7: cluster radius. DSS B image (top-left panel) NGC 2244 emerges from thethin dust of the Rosette central part, which is also sur-rounded by strong gas emission. Indeed, gas emission anddust absorption are nearly absent in the XDSS I image (top-right), and especially in the 2MASS K s image (bottom).Different centres adopted for NGC 2244 in previous (mostlyoptical) studies are indicated in the top-right panel. How-ever, when seen in K s , the cluster is highly concentratedon HD 46150 (bottom panel), suggesting a compact core. Asimilar centre for NGC 2244 had already been suggested by,e.g. P´erez, Th´e & Westerlund (1987).The different centres are summarised in Table 1, whichshows some confusion in the identification of the actual cen-tre of NGC 2244. As will be discussed in Sect. 5, we takeas centre the coordinates that present the maximum stellardensity (Fig. 11) computed within circles of 0.25 ′ in radius,for MS and PMS stars taken isolately (Sect. 3.3). The re-sulting coordinates (Table 1) are similar to those given byWEBDA. The same procedure was applied to find the centreof NGC 2239 (Sect. 5). Since the Rosette Nebula reaches about 1 ◦ , it is interestingto compare large-scale properties of the optical data withthose in the near-IR.2MASS J , H , and K s photometry was extracted in awide circular field with VizieR . The basic condition is thatthe extraction radius R ext should be large enough to al-low determination of the background level (Sect. 5). Weused R ext = 80 ′ (NGC 2244) and R ext = 30 ′ (NGC 2239),which are considerably larger than the respective clusterradii (Sect. 5 and Table 2). In the absence of significantdifferential absorption (Bonatto & Bica 2007a), wide extrac-tion areas provide statistics for a consistent colour and mag-nitude characterisation of field stars. For decontaminationpurposes, comparison fields were extracted within wide rings Extracted from the Canadian Astronomy Data Centre(CADC), at http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=II/246 located beyond the cluster radii. As photometric qualityconstraint, the 2MASS extractions were restricted to stars (i) brighter than the 99.9% Point Source Catalogue com-pleteness limit in the cluster direction, and (ii) with er-rors in J , H , and K s lower than 0.1 mag. The 99.9% com-pleteness limits refer to field stars, and depend on Galac-tic coordinates. Figure 2 (panel a) shows the distributionof uncertainties as a function of magnitude for the starsin the direction of NGC 2244. The fraction of stars with J , H , and K s uncertainties lower than 0.05 mag is ≈ ≈
70% and ≈ A J /A V = 0 . A H /A V = 0 . A K S /A V = 0 . A J = 2 . × E ( J − H ) (Dutra, Santiago & Bica 2002),with R V = 3 .
1. They stem from the extinction curve ofCardelli, Clayton & Mathis (1989).The available B and V photometry for NGC 2244 wastaken from SIMBAD within the same extraction radius asthat used for 2MASS. As expected, the number of detectedstars at a given radius in the optical is significantly lowerthan in the near-IR (Fig. 2, panel b). Indeed, the ratio ofthe number of stars detected in the near-IR to the optical N NIR /N opt increases with distance to the cluster centre, be-ing N NIR /N opt ≈ R . ′ (approximately the clusterregion) and N NIR /N opt ≈
24 for R . ′ . Except for theinnermost region, the stellar spatial distribution detectedwith 2MASS follows a cluster-like profile (Sect. 5), whilethe optical distribution deviates by a large amount (panelb). One conclusion is that analysis based on star-counts ina dust-rich region is more realistic in the near-IR than inthe optical. Also, panel (b) shows that dust is thicker atlarge radii. This may have introduced biases in some of theprevious optical studies.The spatial dependence of the colours towardsNGC 2244 is examined in Fig. 2 for the 2MASS (panel c)and optical (e) bands. The fiducial lines have been built asrunning averages of the raw (observed) data, with 10 pointsfor R < ′ , 100 for 2 ′ < R < ′ , and 1000 for R > ′ .Colours in both domains present a similar pattern, charac-terised by a blue core ( R . ′ ) containing essentially the MS According to the 2MASS Level 1 Requirement, at http://simbad.u-starsbg.fr/simbadc (cid:13) , 1–15 he very young OCs NGC 2244 and NGC 2239 C o l ou r M agn i t ude C o l ou r M agn i t ude N ( de t e c t ed s t a r s w i t h i n R ) J,H,K S (2MASS)B,V (SIMBAD)King−like fit C u m u l . D i s t r i b . ( % ) JHK S B−V BV JK S (J−H)(J−K S )(a) (b)(c) (d)(e) (f) Figure 2.
Panel (a): cumulative J , H and K s photometric errordistribution. (b): number of stars detected by 2MASS (emptycircles) and SIMBAD (filled squares) within a given radius; exceptfor the innermost bin, the 2MASS distribution follows a cluster-like profile (Sect. 5). (c) and (d): spatial dependence of the near-IRcolours and magnitudes. (e) and (f): same for the B and V bands. stars. For larger radii, foreground stars dominate the opticalphotometry, while the near-IR probes deeper regions. A sim-ilar effect occurs in the average magnitudes (panels d andf). As suggested by the ∆ R = 6 ′ linear extractions alongthe N-S and E-W directions (Fig. 3), a reasonable spatialuniformity level occurs with the 2MASS near-IR photome-try. Besides NGC 2244 itself, the next conspicuous bump iscaused by NGC 2239 at ≈ ′ to the West. These profilesguided the comparison field selection.Finally, in Fig. 4 (top panels) we show the spatialdistribution of the stellar surface-density ( σ , in units ofstars arcmin − ) around NGC 2244, measured by 2MASSphotometry. The surface density is computed in a rectan-gular mesh with cells of dimensions 2 . ′ × . ′ , with meshesreaching total offsets of | ∆ α | = | ∆ δ | ≈ ′ with respectto the centre (Table 1), in right ascension and declination.The respective isopleth surfaces are shown in the bottompanels, in which NGC 2239 shows up as a lower concen-tration at ≈ ′ to the West of NGC 2244. Two cases areconsidered in Fig. 4, the observed (raw) photometry (leftpanels) and the MS + PMS stars taken separately (right)by means of a colour-magnitude filter (Sect. 3.3). Since animportant fraction of the contaminant stars are excluded bythe colour-magnitude filter, the surface density distributionof NGC 2244 (and NGC 2239) is better defined with respectto the surroundings. −80−60−40−20020406080 ∆ R (arcmin)01234567 σ ( s t a r s a r c m i n − ) σ ( s t a r s a r c m i n − ) East WestNorth South
NGC2239NGC2244NGC2244
Figure 3.
Linear extractions centred on NGC 2244. The extrac-tions are 6 ′ wide in both directions. The clusters are ≈ ′ apartalong the EW direction. NGC2244 - RAW-20-15-10-5 0 5 10 15 20 ∆α -20-15-10-5 0 5 10 15 20 ∆δ σ -20-15-10-5 0 5 10 15 20 ∆α -20-15-10-5 0 5 10 15 20 ∆δ ∆α -20-15-10-5 0 5 10 15 20 ∆δ σ -20-15-10-5 0 5 10 15 20 ∆α -20-15-10-5 0 5 10 15 20 ∆δ Figure 4.
Top panels: stellar surface-density σ (stars arcmin − )of NGC 2244, computed for a mesh size of 2 . ′ × . ′ , centred onthe coordinates in Table 1. Bottom: the corresponding isoplethsurfaces. Left: observed (raw) photometry. Right: MS and PMSstars selected by means of the colour-magnitude filter (Fig. 6).NGC 2239 shows up at ≈ ′ West of NGC 2244. ∆ α and ∆ δ inarcmin.c (cid:13) , 1–15 C. Bonatto and E. Bica
NGC2244 - Decont.-20-15-10-5 0 5 10 15 20 ∆α -20-15-10-5 0 5 10 15 20 ∆δ σ -20-15-10-5 0 5 10 15 20 ∆α -20-15-10-5 0 5 10 15 20 ∆δ ∆α -15-10-5 0 5 10 15 20 25 ∆δ σ -15-10-5 0 5 10 15 20 25 ∆α -15-10-5 0 5 10 15 20 25 ∆δ Figure 5.
Similar to Fig. 4 for the decontaminated photometriesof NGC 2244 (left panels) and NGC 2239 (right). NGC 2239 is theconcentration ≈ ′ to the West of the prominent NGC 2244. CMDs displaying the J × ( J − K s ) and K s × ( J − K s ) coloursbuilt with the raw photometry of NGC 2244 are shown inFig. 6 (panels a and b). The sampled region ( R < ′ ) cor-responds to about half the cluster radius (Sect. 5). Whenqualitatively compared with the CMDs extracted from theequal-area comparison field (panels c and d), features typ-ical of a very young OC emerge. A relatively vertical andpopulous MS (at 0 . . ( J − H ) , ( J − K s ) . .
3) truncatedfor stars fainter than ≈ . . . M ⊙ - Sect. 4) inboth J and K s , stand out over the field contamination. As expected of low-Galactic latitude clusters (Table 1),the stellar surface-density in the direction of NGC 2244(Fig. 4) confirms that the field-star contamination, includingMon OB2 and disc stars, should be taken into account. Fur-ther confirmation is provided by the qualitative comparisonbetween the CMDs extracted within the cluster and field(Fig. 6). Obviously, the field contribution should be quanti-fied for a better definition of the intrinsic CMD morphology.Although difficult, decontamination is a very impor-tant step in the identification and characterisation of starclusters. Most of the different approaches (e.g. Mercer et al.2005) are based essentially on two different premises. Thefirst relies on spatial variations of the star-count density, butdoes not take into account CMD evolutionary sequences.Alternatively, stars of an assumed cluster CMD are sub-tracted according to similarity of colour and magnitude withthe stars of an equal-area comparison field CMD. Togetherwith the present one, these methods are based on photo-metric properties only. Ideally, more robust results on mem-bership determination would be obtained if another inde- −0.3 0.1 0.5 0.9 1.3 1.7(J−K S ) J Decontaminated (R<5’)SIMBAD stars79111315 J Raw (R<5’) −0.3 0.1 0.5 0.9 1.3 1.7(J−K S ) K S K S J Comparison Field K S (a) (b)(c) (d)(e) (f) Figure 6.
R < ′ region. Middle: equal-areacomparison field CMDs, extracted within 29 . ′ < R < ′ , in-cluding the Mon Ob2 association and disc contamination. Bottompanels: decontaminated CMDs with the MS fitted by the 3 MyrSolar-metallicity Padova isochrone. PMS tracks of different agesare shown. The shaded polygons correspond to the MS (dark-gray) and PMS (light-gray) colour-magnitude filters (Sect. 3.3).The bright stars listed in SIMBAD are identified as circles; 4 ofthese are within 5 . R ( ′ ) .
10. Arrows in the bottom panelsshow the reddening vector computed for A V = 2. pendent parameter, such as the PM of member and com-parison field stars, is taken into account. However, for PMto be useful the cluster should be relatively nearby (e.g.Alessi, Moitinho & Dias 2003) and/or to have been observedin widely-apart epochs preferentially with high resolution, asfor the globular cluster (GC) NGC 6397 (Richer et al. 2008).Neither condition is fully satisfied for NGC 2244, which isrelatively distant (Sect. 4) and was observed by 2MASS in asingle epoch. As a consequence, only about 50% of the starswithin R = 10 ′ of NGC 2244 have optical PM measured(Sect. 3.4).Our decontamination algorithm is fully described inBonatto & Bica (2007a), Bica, Bonatto & Camargo (2008)and Bica & Bonatto (2008). For clarity, we provide hereonly a brief description. The algorithm measures the rel-ative number densities of probable field and cluster stars incubic CMD cells with axes along the J magnitude and the( J − H ) and ( J − K s ) colours. It (i) divides the full rangeof CMD magnitude and colours into a 3D grid, (ii) esti-mates the number density of field stars in each cell basedon the number of comparison field stars with similar mag- c (cid:13) , 1–15 he very young OCs NGC 2244 and NGC 2239 S )111213141516J SIMBAD stars S )111213141516J Raw (R<4’) Comparison(Decont., d O =3.9kpc) (Decont., d O =1.7kpc)(Decont., d O =3.9kpc) (Decont., d O =1.7kpc)(a) (b)(c) (d)(e) (f) Figure 7.
Similar to Fig. 6 for the region
R < ′ (panel a) ofNGC 2239, and the equal-area comparison field (b). Panels (c)-(f):decontaminated CMD with different age/distance from the Sunsolutions. Bright stars with SIMBAD optical data are indicatedas open circles. Arrows show the reddening vector computed forA V = 2. nitude and colours as those in the cell, and (iii) subtractsthe expected number of field stars from each cell. Input al-gorithm parameters are the cell dimensions ∆ J = 1 . J − H ) = ∆( J − K s ) = 0 .
2; the comparison fields are lo-cated within R = 30 ′ − ′ (NGC 2244) and R = 20 ′ − ′ (NGC 2239). The equal-area field extractions (Figs. 6 and7) should be considered only for qualitative comparisons.The decontamination itself uses the large surrounding areaas described above. Among other statistical tests, decon-taminated CMDs of star clusters have integrated N σ ≫ N σ = 13 . , .
5, respectively for NGC 2244 and NGC 2239.
We take the decontaminated surface-density distributions(Fig. 5) as an efficiency indicator. For the present clusters,the central excesses have been significantly enhanced withrespect to the raw photometry (Fig. 4), while the residualsurface-density around the centre has been reduced to a min-imum level. By design, the decontamination depends essen-tially on the colour-magnitude distribution of stars locatedin different spatial regions. The fact that the decontami-nated surface-density presents a conspicuous excess only at the assumed cluster position implies significant differencesamong this region and the comparison field, both in termsof colour-magnitude and number of stars within the corre-sponding colour-magnitude bins. This meets cluster expec-tations, which can be characterised by a single-stellar pop-ulation, projected against a Galactic stellar field.The decontaminated CMDs are shown in the bottompanels of Figs. 6 and 7. As expected, essentially all contami-nation is removed, leaving stellar sequences typical of mildlyreddened young OCs, with a well-developed MS and a sig-nificant population of PMS stars, especially in NGC 2244.Although in both cases the MS width is rather tight andappears to be dominated by photometric errors, we cannotexclude the possibility of differential reddening to accountfor part of the observed spread, especially towards faintstars. To examine this issue we show, in Figs. 6 and 7 (pan-els e and f), reddening vectors computed with the 2MASSratios (Sect. 3.1) for a visual absorption A V = 2, approxi-mately the absorption derived for NGC 2244 and NGC 2239(Sect. 4). Together with the decontaminated CMDs, this ex-periment shows that differential reddening in both clustersis not significant.We conclude that the qualitative and quantitative ex-pectations of the decontamination algorithm have been sat-isfied by the output. In both cases, the decontaminated pho-tometry presents a relevant excess, with respect to the sur-roundings, in the surface-density distribution (Fig. 5). In ad-dition, field-decontaminated CMDs extracted from the spa-tial regions where the excesses occur (Figs. 6 and 7), presentstatistically significant cluster CMDs. To minimise CMD noise, we apply colour-magnitude filtersto the raw photometry to exclude stars with colours unlikethose of the cluster sequence. The filters are wide enough toinclude cluster MS stars and the 1 σ photometric uncertain-ties . For very young OCs such as NGC 2244 and NGC 2239,we also include filters to account for the PMS population.The colour-magnitude filters for the present OCs are shownin Figs. 6 and 7. Another indication of the star cluster nature of NGC 2239is provided by NOMAD PM data taken for the stars ex-tracted within the same spatial region as the 2MASS data.However, the correspondence between NOMAD and 2MASSdetections is not complete, with ≈
50% of the stars detectedwith 2MASS included in NOMAD, for the colour-magnitudefiltered photometry of both NGC 2244 and NGC 2239.In Fig. 8 we show histograms of the right ascension Colour-magnitude filter widths should also account for forma-tion or dynamical evolution-related effects, such as enhanced frac-tions of binaries (and other multiple systems) towards the centralparts of clusters, since such systems tend to widen the MS (e.g.Hurley & Tout 1998; Kerber et al. 2002; Bonatto & Bica 2007a;Bonatto, Bica & Santos Jr. 2005). http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/297. NO-MAD is based on the International Celestial Reference System(ICRS) with origin at the solar system barycenter.c (cid:13) , 1–15
C. Bonatto and E. Bica −40−30−20−10 0 10 20 30 40 µ α cos( δ ) (mas yr −1 )051015202530 N ( s t a r s ) ComparisonR<4’ N ( s t a r s ) ComparisonR<5’Members −40−30−20−10 0 10 20 30 40 µ δ (mas yr −1 )NGC2244 NGC2244NGC2239NGC2239 Figure 8.
Comparative histograms of the member+field (white)and comparison field stars (shaded), which was scaled to matchthe projected areas. The intrinsic distributions are shown by thedashed lines for the extraction
R < ′ of NGC 2244 (top panels)and R < ′ of NGC 2239 (bottom). ( µ α cos( δ )) and declination ( µ δ ) PM components measuredfor the member+field and field stars. We use the same clus-ter and comparison field extractions as those defined for thedecontamination process (Sect. 3.2). The field histogramshave been normalised to match the cluster projected area.Finally, the intrinsic PM distributions are obtained by sub-tracting the normalised field histogram from that of thefield+members.Both NGC 2244 and NGC 2239 present conspicuous PMexcesses over the field. As expected of an OC, the intrinsicPM distribution of NGC 2244 (Fig. 8, top panels) is essen-tially Gaussian. Although somewhat less defined, a similarconclusion applies to NGC 2239 (bottom panels). Besides,NGC 2244 shares essentially the same motion as the disc-field/Mon OB2 association, which is consistent with a rel-atively nearby OC. NGC 2239, on the other hand, appearsto be located at a different distance, especially because ofthe significant shift in µ α cos( δ ) between member and fieldstars. The field-decontaminated CMD morphologies (Sect. 3.1) canbe used to compute cluster fundamental parameters. BothNGC 2244 (Fig. 6) and NGC 2239 (Fig. 7) present MS and PMS stars that can be used as constraints. We adopt solarmetallicity isochrones because the clusters are young andnot far from the Solar circle (see below), a region essentiallyoccupied by [
F e/H ] ≈ . J , H , and K s filters . The tracks of Siess, Dufour & Forestini (2000)are used to characterise the PMS distributions.We take R ⊙ = 7 . ± . .Historically, different approaches have been used to ex-tract astrophysical parameters from isochrone fits. The sim-plest ones are based on a direct comparison of a set ofisochrones with the CMD morphology, while the more so-phisticated ones include photometric uncertainties, bina-rism, and metallicity variations. Most of these methodsare summarised in Naylor & Jeffries (2006), in which amaximum-likelihood CMD fit method is described. We cau-tion that, because of the 2MASS photometric uncertaintiesfor the lower sequences, a more sophisticated approach forisochrone fitting might lead to an overinterpretation.For the above reasons, fits are made by eye , with theMS and PMS stellar distributions as constraint. We also re-quire that, because of the probable presence of binaries, theadopted (single-star) MS isochrone should be shifted some-what to the left of the MS fiducial line, i.e. a median linethat takes into account the MS spread, including the photo-metric uncertainties as well (e.g. Bonatto, Bica & Santos Jr.2005, and references therein). In the following we discuss thepresent clusters individually. The decontaminated CMD morphology of NGC 2244(Fig. 6) shows a nearly-vertical MS at J .
13 and 0 . . ( J − K s ) . .
4, and a population of low-mass PMS stars at J & . . . ( J − K s ) . .
5. Taken together, thesestellar sequences unambiguously characterise a very youngOC. Allowing for photometric uncertainties, acceptable fitsto the decontaminated MS morphology are obtained withany isochrone with age in the range 1—6 Myr. The PMSstars in Fig. 6 are basically contained within the 0.2 Myrand 6 Myr PMS isochrones (Siess, Dufour & Forestini 2000),thus implying a similar age range as the MS. Accordingly,we take the 3 Myr isochrone as representative solution.With the adopted solution, the fundamental parametersof NGC 2244 are a near-IR reddening E ( J − H ) = 0 . ± .
02 (or E ( B − V ) = 0 . ± .
06 and A V = 1 . ± . m − M ) J = 11 . ± . m − M ) O = 11 . ± .
21, respectively, and a distance http://stev.oapd.inaf.it/cgi-bin/cmd . These isochronesare very similar to the Johnson-Kron-Cousins ones (e.g.Bessel & Brett 1988), with differences of at most 0.01 in colour(Bonatto, Bica & Girardi 2004). Other recent studies gave similar results, e.g. R ⊙ =7 . ± . R ⊙ = 7 . ± .
32 kpc (Eisenhauer et al. 2005) and R ⊙ = 7 . ± .
10 kpc(Nishiyama et al. 2006), with different approaches.c (cid:13) , 1–15 he very young OCs NGC 2244 and NGC 2239 S )−0.10.40.91.41.92.4 ( J − K S ) SIESS − 5Myr −0.10.40.91.41.92.4 ( J − K S ) SIESS − 3Myr −0.1 0.2 0.5 0.8 1.1 1.4(J−K S )111213141516J 68101214J O O A V = 1.7A V = 3.4 NGC2244NGC2239NGC2239 (R=4’, Decont.)(NGC2244, R=5’, Decont.) 3M O O O O O O O O O Figure 9.
Colour-colour diagrams with the decontaminated pho-tometry of NGC 2244 (top-left panel) and NGC 2239 (bottom-left). The 3 Myr and 5 Myr Siess, Dufour & Forestini (2000)isochrones,set with the derived reddening values, are used to showthe PMS and MS sequences simultaneously. Reddening vectors forA V = 1 . , . from the Sun d ⊙ = 1 . ± . R ⊙ = 7 . R GC = 8 . ± . ≈ . Although somewhat less-populated than NGC 2244, the de-contaminated CMD of NGC 2239 also presents MS and PMSsequences (Fig. 7), which suggests a similar age as that ofNGC 2244. However, this less-constrained CMD admits al-ternative age/distance solutions.Thus, before the CMD fit, we compute the distanceof the central bright stars projected on NGC 2239. Wefound 2 bright stars in SIMBAD in common with the2MASS detections. These are the A 2 star GSC 00154-01659( B = 13 . V = 12 . J = 11 . B = 13 . V = 13 . J = 12 . d ⊙ = 1 . d ⊙ = 4 . d ⊙ ≈ . ± . d ⊙ ≈ . ± . d ⊙ ≈ . ± E ( J − H ) = 0 . ± . E ( B − V ) = 1 . ± . A V = 3 . ± .
2, ( m − M ) J =13 . ± .
2, ( m − M ) O = 12 . ± . d ⊙ = 3 . ± . R GC = 10 . ± . ≈ . ≈ . When transposed to the near-IR colour-colour diagrams( J − K s ) × ( H − K s ), the age and reddening solutions ofNGC 2244 and NGC 2239 derived above consistently matchthe field-star decontaminated photometry of these OCs(Fig. 9, left panels). Since they include PMS stars, we usetracks of Siess, Dufour & Forestini (2000) to characterisethe age of NGC 2244 ( ∼ ∼ Right after formation, most of a cluster’s mass is storedin the PMS stars that, eventually, shed the dust lay-ers and emerge into the MS (Sect. 1). Thus, the num-ber of MS and PMS stars evolve in opposite directionswith cluster age, till all stars are in the MS after about30 Myr (Bonatto et al. 2006b, and references therein). In-deed, NGC 2244 and NGC 2239 present different fractionsof MS and PMS stars (Figs. 6 and 7), which is consistentwith the different ages.To further explore this issue we compute the ratio ofthe number of PMS to MS stars f PMS/MS = n PMS /n MS .Then we examine the age dependence of f PMS/MS forthe very young OCs, located at a similar distance asNGC 2244, studied by our group with the same methodsas those employed in NGC 2244 and NGC 2239. These con-ditions are satisfied by NGC 6611 (1 . ± . d ⊙ ≈ For consistency with the remaining clusters, we counted thenumber of PMS stars brighter than J = 16 in NGC 6611, since inBonatto, Santos Jr. & Bica (2006) we restricted them to J (cid:13) , 1–15 C. Bonatto and E. Bica n P M S / n M S N6611 N2244 N4755Bochum1N2239
Figure 10.
The ratio of the number of PMS to MS stars fol-lows an exponential-decay function (dashed line) with cluster age: n PMS /n MS ∝ exp( − age/τ ), with τ = 5 . ± . σ ) are within the shaded region. . ± d ⊙ ≈ . ± d ⊙ ≈ . f PMS/MS ratiosshown in Fig. 10 appear to be consistent with the expectedtrend with cluster age. Indeed, we found that the ratios fol-low the exponential-decay function f PMS/MS ∝ e − ( age/τ ) ,with the time-scale τ = 5 . ± . ≈ We use the projected stellar RDPs, defined as the stellarnumber density around the cluster centre, to derive struc-tural parameters. To minimise noise, we work with colour-magnitude filtered photometry to isolate the MS and PMSstars, which enhances the RDP contrast relative to the back-ground, especially in crowded fields (e.g. Bonatto & Bica2007a). However, field stars with colours similar to thoseof the cluster are expected to remain inside the colour- magnitude filter, affecting the intrinsic RDP in a way thatdepends on the relative densities of field and cluster stars.The contribution of the residual contamination to the ob-served RDP is statistically evaluated by means of its exten-sion into the field.Rings of increasing width with distance from the clustercentre are built to avoid oversampling near the centre andundersampling at large radii. The set of ring widths used is∆ R = 0 . , . , . , . , and 5 ′ , respectively for 0 ′ R < . ′ , 0 . ′ R < ′ , 2 ′ R < ′ , 5 ′ R < ′ , and R > ′ . The residual background level of each RDP correspondsto the average number-density of filtered field stars. The R coordinate (and uncertainty) of each ring corresponds to theaverage position and standard deviation of the stars insidethe ring.The colour-magnitude filtered RDPs of the clusters areshown in Fig. 11. As expected, minimisation of the numberof non-cluster stars by the colour-magnitude filter resultedin RDPs with a high contrast relative to the background.For NGC 2244 we also show the RDPs built with the MSand PMS stars separately (left panels). Interestingly, whilethe MS RDP (panel b) has a conspicuous density excessfor R ≈ . ′ , the PMS stars (panel c) are found only for R & . ′ . The presence of NGC 2239 causes a bump in theMS RDP of NGC 2244 (panel b). Similarly, NGC 2244 showsup in the RDP of NGC 2239 (d). Fig. 11 also shows the RDPproduced with the star 12 Mon as centre (panel d). It is clearthat 12 Mon cannot be the centre of NGC 2244.Most star clusters have RDPs that follow a well-definedanalytical profile e.g., the empirical, single mass, modifiedisothermal spheres of King (1966) and Wilson (1975) , andthe power law with a core of Elson, Fall & Freeman (1987).Each function is characterised by a different set of parame-ters that are related to cluster structure. For simplicity andconsidering the error bars of the RDPs in Fig. 11, we adoptthe function σ ( R ) = σ bg + σ / (1 + ( R/R c ) ), where σ bg isthe residual background density, σ is the central density ofstars, and R c is the core radius. It is similar to the functionintroduced by King (1962) to describe the surface brightnessprofiles in the central parts of GCs. To minimise degrees offreedom, σ and R c are obtained from the fit, while σ bg ismeasured in the field. The RDP bins corresponding to theneighbouring clusters were ignored in the fit. The best-fitsolutions are shown in Fig. 11, and the parameters are givenin Table 2. For absolute comparison with other clusters, Ta-ble 2 gives parameters in absolute units.Within uncertainties, the adopted King-like func-tion describes well the colour-magnitude filtered RDP ofNGC 2239 (panel d) along the full radius range. The sameapplies only for R & ′ for the RDP of NGC 2244. Theinnermost bin in the MS (and to a lesser degree to theMS+PMS) RDP (panel b) presents a several σ excess overthe fit. This RDP cusp basically corresponds to the de-tached grouping of stars (with a diameter of ≈ . ′ ) aroundHD 46150, seen in Fig. 1 (bottom-right panel). Our innerRDP shape agrees with that derived by Wang et al. (2008)with FLAMINGOS. In old star clusters, such a centralRDP excess can be attributed to a post-core collapse, like It assumes a pre-defined stellar distribution function and pro-duces more extended envelopes than King (1966).c (cid:13) , 1–15 he very young OCs NGC 2244 and NGC 2239 Table 2.
Derived cluster structural parametersCluster σ bg σ R c R RDP δ c ′ σ bg σ R c R RDP ( ∗ ′− ) ( ∗ ′− ) ( ′ ) ( ′ ) (pc) ( ∗ pc − ) ( ∗ pc − ) (pc) (pc)(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)NGC 2244 † . ± .
02 3 . ± .
65 5 . ± . . ± . . ± . . ± . . ± . . ± . . ± . ‡ . ± .
01 0 . ± .
43 1 . ± . . ± . . ± . . ± . . ± . . ± . . ± . ∗ . ± .
09 3 . ± .
61 5 . ± . . ± . . ± . . ± . . ± . . ± . . ± . † . ± .
05 12 . ± .
77 0 . ± . . ± . . ± . . ± . . ± . . ± . . ± . † ), MS ( ‡ ) and PMS ( ∗ ). Core ( R c ) and cluster ( R RDP ) radii are given inangular and absolute units. Col. 6: cluster/background density contrast parameter ( δ c = 1 + σ /σ bg ), measured in the colour-magnitudefiltered RDPs. Col. 7: arcmin to parsec scale. σ ( s t a r s a r c m i n − )
110 1 10R (arcmin)NGC2244 − MS + PMSNGC2244 − MSNGC2244 − PMS NGC2239 − MS + PMS12 MON
NGC2239 NGC2244 (a)(b)(c) (d) (e)
Figure 11.
Stellar RDPs built with colour-magnitude filteredphotometry. Solid line: best-fit King-like profile. Horizontalshaded polygon: background. Shaded regions: 1 σ King fit uncer-tainty. Note the central density excess in the RDP of NGC 2244in panels (a) and (b). The RDP with 12 Mon as centre in shownin (d). those detected in some GCs (e.g. Trager, King & Djorgovski1995). It has been detected as well in Gyr-class OCs,such as, e.g. NGC 3960 (Bonatto & Bica 2006) and LK 10(Bonatto & Bica 2009). Another very young cluster har-bouring such a detached core producing an RDP centralcusp is NGC 6823 (Bica, Bonatto & Dutra 2008). Clustersare not expected to dynamically evolve into a post-core col-lapse on short time-scales, and the cusp must have beencaused by star-forming effects. The compact core withinthe eroded profile of Bochum 1 (Bica, Bonatto & Dutra 2008) can be a long-lived structure in young clusters. Con-sequently, this central cusp in such a young cluster asNGC 2244 suggests a significant deviation from dynamicalequilibrium (Sect. 7).We also estimate the cluster radius ( R RDP ) by visuallycomparing the cluster RDP and background levels, i.e. R RDP is the distance from the cluster centre where both are sta-tistically indistinguishable (e.g. Bonatto & Bica 2005, andreferences therein). Most of the cluster stars are containedwithin R RDP , which should not be mistaken for the tidalradius . The cluster radii of the present objects are givenin angular and absolute scales (Table 2).The density contrast parameter δ c = 1 + σ /σ bg , whichis relatively high (3 . < δ c < .
2) for the present RDPs,is also given in Table 2. Since δ c is measured in colour-magnitude-filtered (lower noise) RDPs, it is usually higherthan the visual contrast produced by images (e.g. Fig. 1).Taken at face value, the core radius of NGC 2244 (forthe MS+PMS stars) R c ∼ . R c tail. Besides, assumingthe relation tidal radius ∼ × R RDP (Bonatto & Bica 2005),both clusters fall around the median value of the tidal radiusdistribution.
Both clusters clearly present distinct populations of MS andPMS stars (Figs. 6 and 7). As the first step to estimatethe cluster masses we build the luminosity functions (LFs)in the K s band for the MS and PMS stars separately, bymeans of the respective colour-magnitude filters (Sect. 3.3).We show them in Fig. 12, where the similar age, differentdistances and number of members are reflected, especiallyon the different MS and PMS cutoffs. In both cases thePMS LFs present the expected steep increase towards faint Tidal radii are derived from, e.g. the 3-parameter King-profilefit to RDPs (Bonatto & Bica 2008), which requires large sur-rounding fields and adequate errors. For instance, in populousand relatively high Galactic latitude OCs such as M 67, NGC 188,and NGC 2477, the tidal radii are a factor ∼ R RDP (Bonatto & Bica 2005).c (cid:13) , 1–15 C. Bonatto and E. Bica magnitudes (low-mass stars), which confirms that PMS starsare an important fraction of the members.For a more objective investigation on the stellar massdistribution we build the MFs (cid:0) φ ( m ) = dNdm (cid:1) for the cur-rent MS stars that, in turn, can be used to compute themass stored in stars. Similarly to the RDPs (Sect. 5), wework with colour-magnitude filtered photometry to minimisenoise. First we build the LF independently for each 2MASSband, both for the cluster region ( R < R
RDP ) and compari-son field. The intrinsic LFs are obtained by subtracting therespective (equal-area) comparison field LF from that of thecluster. The intrinsic LFs are transformed into MFs with themass-luminosity relations obtained from the correspondingage and distance from the Sun solutions (Sect. 4). The finalMF is produced by combining the J , H and K s MFs intoa single MF. Further details on MF construction are givenin Bica, Bonatto & Blumberg (2006). The effective MS stel-lar mass ranges are (4 . ± . m ( M ⊙ ) .
60 (NGC 2244)and (3 . ± . m ( M ⊙ ) .
14 (NGC 2239). As Fig. 12(bottom panels) shows, the MS MFs are rather smooth andpresent different lower and upper masses, which reflects thelower distance and younger age of NGC 2244 with respectto NGC 2239.Since PMS stars are abundant in both clusters, it is im-portant to build their MF as well. In Fig. 9 (right panels)we show the evolutionary tracks (Siess, Dufour & Forestini2000) of PMS stars of different masses superimposed on thedecontaminated CMDs of NGC 2244 and NGC 2239. It isclear, especially for NGC 2244, that PMS stars less massivethan 1 M ⊙ are the most abundant component. Similarly tothe MS, the PMS MFs are built with the number of PMSstars among any two tracks in the cluster region and compar-ison field. Finally, we add the MS and PMS MFs to producethe total MF of each cluster (Fig. 12).The number of MS ( n MS ) and PMS ( n PMS ) mem-bers in NGC 2244 (for R R RDP ) are derived by countingthe stars in the background-subtracted colour-magnitude fil-tered photometry. We apply the same approach as aboveto compute the PMS mass. There are n MS = 26 ± n PMS = 301 ±
60 stars; the corresponding mass values are m MS = 389 ± M ⊙ and m PMS = 236 ± M ⊙ (com-puted assuming the average mass between any two evolu-tionary tracks in Fig. 9). Thus, the total stellar mass ofNGC 2244 is m MS + PMS ≈ M ⊙ , which agrees with the770 M ⊙ mass estimated by P´erez (1991). We note that thisvalue is about 10% of the mass estimated by Ogura & Ishida(1981) for NGC 2244. However, this difference may arisefrom the present detailed analysis - especially the decon-tamination and the separation of MS and PMS stars inthe construction of the cluster MF. The same analysis ap-plied to NGC 2239 yields n MS = 26 ± n PMS = 70 ± m MS = 141 ± M ⊙ , m PMS = 160 ± M ⊙ , and the totalstellar mass m MS + PMS ≈ M ⊙ , about half the mass ofNGC 2244.Considering the MS stars isolately, the MFs can bewell represented by the function φ ( m ) ∝ m − (1+ χ ) , with theslopes χ = 0 . ± .
09 and χ = 0 . ± .
08, respectively forNGC 2244 and NGC 2239. Both values are flatter than the χ = 1 .
35 of Salpeter (1955) initial mass function (IMF). Aflat MF slope was also found for NGC 2244 by Park & Sung(2002). However, when the MS and PMS stars are takentogether, the slopes become steeper, χ = 0 . ± .
13 and S )1 10m (M O ) MS χ = 0.76 ± χ = 1.24 ± O )10 −1 φ ( m ) = d N / d m ( s t a r s M O − ) MS χ = 0.24 ± χ = 0.91 ± S )10 −1 L F ( K S ) = d N / d K S MSPMS
NGC2244 NGC2239NGC2244 NGC2239
Figure 12.
Top panels: K s -luminosity functions of the MS (filledcircles) and PMS (empty circles) stars. Bottom: mass functions(for the J , H and K s bands combined) of the MS (filled cir-cles) and MS+PMS (empty squares). Fits of the function φ ( m ) ∝ m − (1+ χ ) are shown by the dashed (MS) and dotted (MS+PMS)lines. χ = 1 . ± .
06. While within uncertainties the total MFof NGC 2239 is comparable to the Salpeter (1955) IMF, theMF of NGC 2244 remains somewhat flatter, but still consis-tent with the conclusions of Wang et al. (2008).
Constrained by isochrone fits (Sect. 4), we could derivefundamental and structural parameters of the young OCsNGC 2244 and NGC 2239, part of them for the first time.We use them to compare some of their properties with thoseof well-studied OCs.
We further investigate the nature of NGC 2244 andNGC 2239 with diagrams that examine relations amongastrophysical parameters of OCs in different environ-ments. They were introduced by Bonatto & Bica (2005).As reference sample we use some bright nearby OCs(Bonatto & Bica 2005; Bonatto et al. 2006b), and agroup of OCs projected towards the central parts ofthe Galaxy (Bonatto & Bica 2007a). Also included arethe young OCs NGC 6611 with the age ∼ . ∼ ∼ c (cid:13) , 1–15 he very young OCs NGC 2244 and NGC 2239 Age (Myr) R c o r e ( p c ) Age (Myr) R RD P ( p c ) Reference OCsNGC2244NGC2239Bochum1NGC6823NGC6611 d GC (kpc) R RD P ( p c ) R core (pc) R RD P ( p c ) R core (pc) M c l u s t e r ( M O ) σ mO =600M O pc −2 σ mO =30M O pc −2 Age (Myr) −1012 χ M F (a) (b)(c) (d)(e) (f) Figure 13.
Diagrams dealing with astrophysical parameters ofOCs. Gray-shaded circles: reference OCs. The young star clustersNGC 6611, NGC 6823 and Bochum 1 are indicated for comparisonpurposes. Analytical relations in panels (c) and (e) are discussedin the text. Dotted line in panel (f) shows Salpeter (1955) IMFslope χ = 1 . NGC 6611 and NGC 6823 serve as comparison with gravita-tionally bound objects of similar age, while Bochum 1 is astar cluster fossil remain that might be dynamically evolvinginto an OB association. The full sample of comparison OCsis characterised by ages in the range ∼ . ∼ . . R GC (kpc) . . R RDP ) and core ( R c )radii on cluster age, respectively. Most of the small-radiusOCs (especially in R RDP ) occur at an age ∼ . − R c too large when compared to the reference OCs.Core and cluster radii of the reference OCs follow therelation R RDP = (8 . ± . × R (1 . ± . (panel c) , sug-gesting a similar scaling for both kinds of radii. While Similar relations were also found byNilakshi, Pandey & Mohan (2002), Sharma et al. (2006),and Maciejewski & Niedzielski (2007).
NGC 2239 fits tightly in that relation, NGC 2244 deviatesagain probably because of the exceeding cluster radius. Adependence of OC size on Galactocentric distance is sug-gested by panel (d), as discussed by Lyng˚a (1982) andTadross et al. (2002). While NGC 2244 follows the trend,NGC 2239 deviates somewhat. This relation may be partlyprimordial, in the sense that the high molecular gas densityin central Galactic regions may have produced small clusters(e.g. van den Bergh, Morbey & Pazder 1991). After forma-tion, mass loss due to stellar and dynamical evolution (e.g.mass segregation and evaporation), together with tidal in-teractions with the Galactic potential and giant molecularclouds, also contribute to the depletion of star clusters, es-pecially the low-mass and centrally located ones.When the mass-density radial distribution fol-lows a King-like profile (e.g. Bonatto & Bica 2007b;Bonatto & Bica 2008; Bonatto, Bica & Santos Jr. 2008),the cluster mass inside R RDP can be computed as a functionof the core radius ( R c ) and the central mass-surface density( σ M ), M clus = π R σ M0 ln (cid:2) R RDP /R c ) (cid:3) . With theabove relation (panel c) between R c and R RDP , this equa-tion becomes M clus ≈ . σ M0 R . The observed relationof core radius and cluster mass is examined in panel (e).The reference OCs, together with NGC 2239 are containedwithin King-like distributions with central mass densitieswithin 30 . σ M0 ( M ⊙ pc − ) . In the present paper we employ the wide-field and near-IRdepth provided by 2MASS to focus on the Rosette Nebulacluster NGC 2244 and the nearby projected OC NGC 2239.Our approach relies essentially on field-star decontaminated2MASS photometry, which enhances cluster CMD evolu-tionary sequences and stellar radial density profiles, produc-ing more constrained fundamental and structural parame-ters.Previous studies were mostly based on optical photom-etry and/or near-IR with small angular fields. However,2MASS can still provide additional insight (Sect. 3). Theset of tools developed by our group allowed to unambigu-ously isolate MS and PMS stars that, in turn, resulted inwell-defined CMDs, RDPs and mass functions. In addition,we explore proper motion properties to investigate the othercluster in the area, NGC 2239.Taken together, the (decontaminated) MS and PMS se-quences of NGC 2244 provided an age range 1—6 Myr, an c (cid:13) , 1–15 C. Bonatto and E. Bica absorption A V = 1 . ± .
2, and a distance from the Sun d ⊙ = 1 . ± . ≈ . χ = 0 . ± . m MS + PMS ∼ M ⊙ derived in the present work, NGC 2244 is not as massiveas previously estimated (Sect. 2). The King-like profile fitto the (MS+PMS) stellar RDP was obtained with a coreradius R c ≈ . ′ ≈ . R RDP ≈ ′ ≈ . m MS + PMS ≈ M ⊙ ), young(5 ± V = 3 . ± . d ⊙ = 3 . ± . R c ≈ . ′ ≈ . R RDP ≈ . ′ ≈ . χ = 1 . ± .
06, its composite MS+PMS MFslope is essentially Salpeter’s IMF. These parameters char-acterise an average young OC, as compared to the referencenearby OCs (Sect. 7.1).While NGC 2239 is a normal young OC with MSand PMS stars distributed according to a cluster RDP,NGC 2244 appears to be another example, like Bochum 1(Bica, Bonatto & Dutra 2008), of an open cluster doomedto dissolution in a few 10 yr. The present work shows theimportance of field-star decontamination and wide-field ex-tractions to get the best stellar statistics and to producehigh-quality CMDs and RDPs. ACKNOWLEDGEMENTS
We thank the referee, Dr. M. P´erez, for comments. We ac-knowledge support from the Brazilian Institution CNPq.This publication makes use of data products from the TwoMicron All Sky Survey, which is a joint project of the Uni-versity of Massachusetts and the Infrared Processing andAnalysis Centre/California Institute of Technology, fundedby the National Aeronautics and Space Administration andthe National Science Foundation. This research has madeuse of the WEBDA database, operated at the Institute forAstronomy of the University of Vienna.
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