ASTRONOMY and ASTROPHYSICS Dissolving Star Cluster Candidates
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A&A manuscript no. ASTRONOMY (will be inserted by hand later) AND Your thesaurus codes are: ASTROPHYSICS 05:10.15.1 Dissolving star cluster candidates E. Bica1, B.X. Santiago1, C.M. Dutra1, H. Dottori1, M.R. de Oliveira1 and D. Pavani1 Universidade Federal do Rio Grande do Sul, IF, CP 15051, Porto Alegre 91501–970, RS, Brazil Received 13.09.2000; accepted 14.11.2000 Abstract. We present a list of 34 neglected entries from dynamical stages of low mass globular clusters are still as- star cluster catalogues located at relatively high galac- sociated to relatively well populated star clusters, but one tic latitudes (|b| > 15◦) which appear to be candidate may wonder what subsequent stages would look like, prob- late stages of star cluster dynamical evolution. Although ably an underpopulated fossil core containing some double underpopulated with respect to usual open clusters, they and multiple stars. still present a high number density contrast as compared Among open clusters low MS depletion has been found to the galactic field. This was verified by means of (i) e.g. in the intermediate age (≈ 3 Gyr) cluster NGC3680 predicted model counts from different galactic subsystems (Anthony-Twarog et al. 1991). The dissolution of open in the same direction, and (ii) Guide Star Catalog equal clusters has been studied by Wielen (1971). Updated data solid angle counts for the object and surrounding fields. suggest that most open clusters dissolve in about 100 Myr This suggests that the objects are physical systems, possi- and this will probably not change much as one includes bly star clusters in the process of disruption or their fossil fainter clusters (Ahumada et al. 2000). Intermediate age remains. The sample will be useful for followup studies in open clusters are certainly survivors of initially massive view of verifying their physical nature. clusters (Friel 1995), and their updated age histogram con- taining more than 100 entries (Dutra & Bica 2000) sug- Key words: Galaxy: open clusters and associations: gen- gests a dissolution timescale of about 1 Gyr. Also, N-body eral simulations of star clusters in an external potential have shown typical dissolution times in the range 500-2500 Myr (Terlevich 1987, McMillan & Hut 1994, de la Fuente Mar- 1. Introduction cos 1997, Portegies Zwart et al. 2000). Mass segregation is expected to occur in cluster cores during one relaxation Star Clusters are known to dynamically evolve and stellar time, according to N-body simulations (Terlevich 1987, arXiv:astro-ph/0011280v1 14 Nov 2000 depletion effects eventually lead to the cluster dissolution. Portegies Zwart et al. 2000). An important effect of mass Fundamental questions are: (i) where are the clusters in segregation is the depletion of low mass MS stars by means process of dissolution? (ii) if fossils are left, can any rem- of evaporation due to the tidal field of the Galaxy and en- nant be detected? The present study aims at showing that counters with binary stars. This would imply that clusters several candidates for these effects occur in star cluster which are close to disruption have a core rich in compact catalogues themselves. and giant stars (Takahashi & Portegies Zwart 2000). Several poorly populated objects at relatively high Recently, evidence of an open cluster remnant has been galactic latitudes (from the open cluster perspective |b| > discussed by Bassino et al. (2000). They studied the rela- 15◦) are included in open cluster catalogues (Alter et al. tively high latitude concentration of stars M73 (NGC6994) 1970, Lyng˚a1987). Other objects were reported as clus- and derived an age of 2-3 Gyr from CMDs. They found ters in early studies (e.g. New General Catalogue and In- a significant number density contrast with respect to the dex Catalogue), or in modern ones like the ESO (B) Atlas galactic field CMD predicted by count models in the area. Survey Catalogue (Lauberts 1982). Carraro (2000) does not favour the object as an open clus- Depletion of Main Sequence (MS) stars has been de- ter or as a remnant. At any rate, if M73 is a physical tected or evidence of it has been found in luminosity func- system the open cluster classification is certainly not ade- tions and Colour-Magnitude Diagrams (CMDs) of some quate. Let us then suggest the acronym POCR - Possible Palomar or Palomar-like globular clusters such as E3 (Mc- Open Cluster Remnant. As an ongoing study of this ne- Clure et al 1985), ESO452-SC11 (Bica et al. 1999) and glected class of interesting objects and in view of future NGC6717 (Palomar 9) (Ortolani et al. 1999). Such evolved CMDs to determine parameters such as reddening, age Send offprint requests to: C.M. Dutra - [email protected] and distance, we present a list of candidates. We discuss 2 E. Bica et al.: Dissolving star cluster candidates their possible physical nature by checking whether they In the following we comment on some objects. The des- present a significant number density contrast with respect ignation NGC 1963 is often applied to an edge-on galaxy to their fields. In Section 2 we present the sample. In Sec- ≈ 11′ east of the POCR. However the New General Cata- tion 3 we analyze the significance of the excesses of stars logue describes a star cluster, and the original position by means of (i) equal solid angle counts in the object and (available e.g. in Sulentic & Tifft 1973) coincides with field areas, and (ii) galactic model counts. In Section 4 we that of the POCR. Lauberts (1982) correctly identified the discuss the results. Finally, in Section 5 we present the present object (NGC 1963 or ESO363SC5) and the galaxy concluding remarks. (IC 2135 or ESO363G7). On the other hand the POCR NGC 2314A has a neighbouring bright galaxy (NGC 2314 itself) located ≈ 5′ north. The New General Catalogue de- 2. The sample scription clearly refers to the galaxy, but the designation NGC 2314 has been applied to the stellar concentration In trying to identify fossil cluster remains we browsed rel- (Alter et al. 1970), which is also known as OCl-374. We ◦ atively high galactic latitude (|b| > 15 ) zones by means suggest the extension ’A’ to the POCR designation. NGC of 1st and 2nd generation Digitized Sky Survey Images 2664, NGC 1557 and NGC 1641 are poorly populated con- and Guide Star Catalog maps, searching for poorly pop- centrations of stars reported as clusters in the NGC cata- ulated objects described as star clusters in open cluster logue, but not included in modern open cluster catalogues. catalogues (Alter at al. 1970, Lyng˚a1987, Lauberts 1982), NGC 2664 is projected just a few cluster diameters away and also neglected NGC or IC entries originally described from the populous intermediate age open cluster M67, an as clusters. Although cluster remnants must be common old disk template spatially located quite high in the plane ◦ at low galactic latitudes we chose |b| > 15 because of (z = 430 pc - see Mermilliod’s (1996) WEBDA Open Clus- lower field contamination, also avoiding young disk and ter database for essential cluster parameters in the Web dust distribution details which affect models of low lati- interface http://obswww.unige.ch/webda). NGC 1901 is an tude zones (Section 3). open cluster or stellar group (Sanduleak & Philip 1968) Table 1 lists the 34 POCRs and two comparison clus- that is projected to the north of the LMC bar, whereas ters, by columns: (1) object designation, (2) and (3) galac- NGC 1641 and NGC 1557 are projected onto the outer tic coordinates, (4) and (5) J2000 equatorial coordinates, parts of the LMC. If physical stellar systems, these objects (6) and (7) the major and minor diameters, and finally (8) are clearly related to the Milky Way due to the brightness E(B-V)FIR reddening values. We measured coordinates of of their stars. The close projection of some open clusters the POCR centers and diameters on Digitized Sky Survey and POCRs also leaves the possibility of future investiga- images. The list is not intended to be extensive or com- tions, such as common origin, early cluster pairs or multi- plete, reflecting only the most interesting objects found in plets, fossil associations, or perhaps debris of dwarf galaxy the present search. We obtained the E(B-V)FIR reddening captures. values from Schlegel et al. (1998) dust emission reddening We show in Figure 1 the angular distribution of the 34 maps. From the study of star cluster directions Dutra & sample objects and the two comparison clusters. POCRs Bica (2000) called attention to the fact that Schlegel et can be basically found in any direction along the plane. al.’s reddening values reflect foreground and background The number of POCRs is small but concentrations could dust contributions. Most objects in Table 1 have rela- be present in the third Quadrant, which would suggest tively high galactic latitudes and their reddening values that several objects therein had common origin. We em- are quite low, probably corresponding to foreground dust phasize that our survey of POCRs was based on previ- in the pathsight. The only exception is NGC1901, which ously catalogued objects, and it would be important to is projected onto the LMC disk and the high reddening carry out a systematic search for new objects with similar value in Table 1 arises mainly from LMC dust (Dutra & characteristics in the same part of sky, as well as for |b| < Bica 2000). 15◦. Some POCRs present differences between their ma- Figure 2 shows a 2nd generation Digitized Sky Survey jor and minor diameters (Table 1) indicating ellipticity. (XDSS) image of the comparison cluster NGC 3680 which Numerical simulations by Terlevich (1987) and Portegies has low MS depletion (Anthony-Twarog et al.