The Young Open Cluster Collinder 359 ( = Melotte 186) Was Selected

A PHOTOMETRIC STUDY OF THE YOUNG CLUSTER COLLINDER 359

N. LODIEU (1), BOUVIER (2) & M. J. McCAUGHREAN (1) .J.

(1) Astmphysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany (2) Laboratoire d'Astrophysique de Grenoble, BP 53, 38041 Grenoble Cedex, France

We present the first deep optical wide-field imaging survey of the young open cluster Collinder 359, complemented by near-infrared fo llow-up observations. We have surveyed 1.6 square degree in the cluster in the I and z filters with the CFH12K on the Canada.-France Hawaii 3.G-m telescope down to completeness and detection limits in both filters of 22.0 and 24.0, respectively. Ba.sect on their location in the optical (J,J-z) colour-magnitude diagram, we have extracted new cluster member candidates in Collinder 359 spanning 1.3-0.04 M0, as suming an age of 80 Myr and a distance of 500 pc for the cluster. We have derived the cluster luminosity and mass functions using the latest evolutionary models from the Lyon group: the best fit to the slope of the cluster mass function, when expressed as the mass spectrum, is

a= 0.30 ± 0.20 over the 0.55-0.035 M0 mass range. The derived slope appeais flatter than the estimates in the Pleiades and a Per open clusters.

Keywords: Open clusters and associations: individual: Collinder 359 - Stars: low-mass, brown dwarfs - Stars: luminosity function, mass function -- Techniques: photometric

1 The young open cluster Collinder 359

The young open cluster Collinder 359 ( = Melotte 186) was selected as a pre-main-sequence open cluster within the framework of a CFHT Key Programme to study the sensitivity of the low-mass stellar and substellar IMF to time and environment. The cluster is located in the Ophiuchus constellation around the supergiant 67 Oph. Collinder 359 was relatively unstudied and very little literature is available about it: in particular, no modern optical survey exists of the cluster core. The cluster was first seen on the Franklin-Adams Charts Plates and described by Melotte in 1915 21 as a large scattered group of bright stars around 67 Oph, covering an area of about 6

57 square de?;re<>s. In his catalogue of open clusters, Collinder (l\J31) 9 described the cluster as a group of about 15 stars with no appreciable concentration on the sky and 110 well-defined outline. A list of probable candidates was given in Collinder's paper, along with coordinates, magnitudes, and spectral types. The membership of these objects was, however, not well established and more than half of them were later rej ected by Van't Veer (1980) 30 and Rucinski (1987) 25 . Based on isochrone fitting, the distance of the cluster was estimated between 200 and 300 parsecs while the parallax measurements from HIPPARCOS suggested a distance of approximately 270 pc 15. A recent analysis of the cluster by Kharchenko et al. (2004, personal communication) derived a much larger distance of 650 pc, based on a larger number of objects included in the HIPPARCOS and Tycho 2 catalogues. The age of the cluster has previously been estimated as 30 l'viyr31 , as confirmed by Kharchenko et al. (2004).

2 The optical wide-field survey

A Canada-France-Hawaii Te lescope Key Programme (30 nights over 2 years) centred on wide field optical imaging of young, intermediate-age, and older open clusters was carried out within the framework of the European Research Training Network "The Formation and Evolution of Young Stellar Clusters" , to examine the sensitivity of the low-mass stellar and substellar IMF to time and environment. The survey was conducted with a large-CCD mosaic camera (CFH12K) in the I and z filtersdown to detection and completeness limits of I = 24.0 and 22.0, respectively. Five CFHl 2K frames were obtained on 18 and 20 June 2002 in Collinder 359 in the I and z filters, covering a total area of 1.6 square degrees in the cluster. Fields A, B, C, and D were covered on 18 June 2002 under photometric conditions with seeing � 0.8 arcsec. The final field, fieldE, was observed on 20 June 2002 under non-photometric conditions. Three sets of exposures were taken for each field-of-view: short, medium, and long exposures with integration times of 2,

30, and about 900 seconds, respectively. The long exposures were exposed 3 x 300 and 3 x 360 seconds in the I and z filter, respectively. The initial data reduction was provided by the Elixir pipeline, including bias subtraction, flat-fielding, correction for scattered light in both filters, combining the dithered frames in case of long exposures, and astrometric calibration. Zero-points were also provided by the pipeline from a repeated monitoring of several standard stars. The photometry was extracted with the SExtractor package coupled with a PSF fittingroutine (kindly provided by E. Bertin 6).

3 Identifii:;ation of cluster member candidates

Photometry has been extracted in the I and z filters, allowing us to draw a colour-magnitude diagram (I, I-z) shown in Figure 1. To take into account the uncertainties in the age and the distance of the cluster, we have selected all objects located to the right of the combined

Next Gen+ Dusty isochrones from the Lyon group 4 8, assuming a distance of 650 pc and an age of 80 Myr for the cluster. This value of the age takes into account the possibility of older ages as 26 27 suggested by the lithium test applied to the Pleiades , a Per , and IC2391 2. vVe have examined each cluster member candidate by eye both in the I and z images to reject extended objects, blended sources, and detections affected by bad pixels or bad columns. vVe have extracted a total of 1033 candidates ranging from I= 12.0 to I= 22.5 over 1.6 square degree area surveyed in Collinder 359. The new candidates are displayed as filled circles in Figure 1 and span masses between 1.3 and 0.04 M0 according to the tracks. We have also cross-correlated our sample of optically-selected candidates with the 2TvIASS database. All objects brighter than I= 17 have a 2MASS counterpart, and, therefore, near infrared magnitudes. In addition, we have obtained K'-band photometry with the infrared

58 .� '--

.i.'

Fig re 1: Colour-magnitude diagram (I, for the intermediate-age open cluster Collinder 359 over the full 1.G squareu degree area surveyed by the CFH12K1-z) camera. The large filled dots am all optically-selected cluster member candidates spanning 1.3-0.04 Mo. Overplottcd arc NextGen (solid line; Baraffe et al. 1998) the (daolied Dusty line; Cha brier et 2000) and the Cond (dotted line; Chabrier et al . 2000) isochrcmcs for 80 Myr, asotaning a distance of 500 pc al.for the cluster. The dash eel line at I� 20 inclicatr·s stellar/substellar boundary 070 the al 0 \L., . The mass scale (in solar ma.'5es) is giwn on the right side of the graph. reddening vector of 1 is indicated for comparison purposes. The open triangles depicts candidates A proper motion consistentAv = me1nbcrship. with v.,:ith cluster

59 camera (CFHTIR) on the CFH 3.6-m telescope to probe the contamination at and below the substellar limit (I � 20). This procedure rejected some objects exhibiting bluer colours than cluster members in the optical-to-infrared colour-magnitude diagram. We have used our optical survey in Collinder 359 complemented by near-infrared photometry to address the issue regarding the uncertainties on the age and the distance of the cluster. I First, we have followed the approach applied to the Per cluster by Stauffer et al. (2003). By comparing the location of the supergiant 670ph in thea colour-magnitude diagram (Mv, B-V) with theoretical solar metallicity isochrones including moderate overshoot 11, we have inferred an age of 60 ± 20 Myr for Collinder 359. Second, we have cross-correlated our list of candidates with the latest version of the USNO CCD Astrograph Catalog32 which provides proper motion for a large number of candidates. We have retained probable member candidates with photometric and proper motion measurements consistent with cluster membership. The location of these candidates in the colour-magnitude diagram suggested a most likely age of SO Myr and a distance of 500 pc.

4 The cluster luminosity function vVe have derived the cluster luminosity function using the list of cancliclatesextracted from the colour-magnitude diagram, assuming an ageof SO Myr and a distance of 500 pc for Collincler 359 (Figure 2). The luminosity function is drawn from all probable candidates whose optical and near-infrared photometry are consistent with cluster membership.

M, 6.5 8.5 10.5 12.5 14.5

0 v;

n(i; E :J z

o non-smoothed

15 17 19 21 23

Figure 2: The cluster luminosity function drawn for thecandidates selected to the right of the isochrones, assuming an age of 80 Myr and a distance of 500 pc for Collinder 359. The open squares represent the number of stars per bin of 0.5 magnitude whereas the filled circles indicate the number of stars in a 1.0 magnitude bin with an interval of 0.5 magnitudes. Poisson errors are indicated by vertical lines. We have employed two approaches to derive the cluster luminosity function. The first approach consisted in counting the number of stars per bin of 0.5 mag (open squares in Figure 2). The second approach "smoothed" the luminosity function to better characterise the faint encl,

60 i.e. we have counted the number of stars per interval of 1.0 magnitude by step of 0.5 magnitude (filled circles in Figure 2). Both methods yielded similar cluster luminosity functions. The optical colour-magnitude diagram (Figure 1) is suggestive of a large contamination at the high-massend (M 0.6 M0) as the cluster sequence merges with fieldstars. As a consequence, 2: we are unable at present to derive reliable luminosity and mass functions for Collinder 359 for masses larger than 0.6 M0. Additional follow-up observations such as optical spectroscopy are mandatory to estimate the level of contamination in this mass range. Two features are seen in the cluster luminosity function (Figure 2). First, a peak at I= 17.0-

17.5 mag (M1 = 8.5-9.0 mag) corresponding to M � 0.30 M0. Comparable peaks are seen in 3 different clusters at different magnitudes, including NGC2516 (150 Myr; M1 =11, 1 ) , M35 (150- 3 200 Myr; M1 = 9,1 ), and a Per (90 Myr; M1 = 10, ) but its origin is currently unknown. Second, a

dip around I= 20.5 mag (M � 0.070 M0) is present in the cluster luminosity function and clearly detected in the colour-magnitude diagram (Figure 1) well above our completeness limit. This

feature is comparable to the gap seen in the a Per luminosity function at M1 = 12.5 3 . This dip is 2 detected both in the field 4 and in young clusters, including (} Orionis 5, the Trapezium Cluster 16, IC348 17, the Pleiades 12, and IC 2391 2. This feature might originate from the formation of large dust grains at low temperatures around spectral types M7-M8 12.

5 The cluster mass function \Ve have converted the luminosity function into a mass function using the evolutionary models from the Lyon group, assuming an age of 80 Myr and a distance of 500 pc for the cluster.

10000 Muench (2002)

80 Myr 10 500 pc

1.00 0.10 0.01 Moss (solar moss)

Figure 3: The cluster mass function assuming an age of 80 Myr and a distance of 500 pc for Collinder 359. The cluster mass function (filled circles) is compared with the mass functions derived for the Pleiades (0.5-1.0; Bouvier et al. 1998; Martin et al 1998; Tej et al. 2002; Dobbie et al. 2002; Moraux et al. 2003) and the Trapezium Cluster (0.27 in the 0.12-0.025 mass range; Muench et al. 2002).

The cluster mass function exhibit a slow steepening from 0.6 M0 down to our completeness limit at about 0.040 M0. The best linear fitto the mass function in Collinder 359 is obtained for

an index a= 0.30 ± 0.2 in the 0.60-0.04 M0 mass range, when expressed as the mass spectrum

61 dN / dM oc M-°' (solid line in Figure 3). This estimate appears flatter than the ones in the Pleiades ± ± (a= 0.6 0.1; 22 ) and in c; Per (a= 0.59 0.05; 3) over the same mass range. Furthermore, a dip occurs around 0.070 M0 in the mass function, likely due to the dearth of objects with spectral types of M7-M8 objects 12, at the age and distance of the cluster. This optical survey of Collinder 359 constitutes a first step in the study of the cluster. Near-infrared imaging and low-resolution optical spectroscopy of the selected cluster member candidates is underway to refine the determination of the cluster mass function.

Acknowledgements

This work was supported by the European Research Training Network entitled "The Formation and Evolution of Young Stellar Clusters" (HPRN-CT-2000-00155), through funding to NL and the "Young Local Universe" meeting.

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