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A Substellar Mass Function for Alpha Persei?

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A Substellar Mass Function for Alpha Persei? A&A 395, 813–821 (2002) Astronomy DOI: 10.1051/0004-6361:20021262 & c ESO 2002 Astrophysics A substellar mass function for Alpha Persei? D. Barrado y Navascu´es1, J. Bouvier2,J.R.Stauffer3, N. Lodieu4, and M. J. McCaughrean4 1 Laboratorio de Astrof´ısica Espacial y F´ısica Fundamental, INTA, PO Box 50727, 28080 Madrid, Spain 2 Laboratoire d’Astrophysique, Observatoire de Grenoble, Universit´e Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France e-mail: [email protected] 3 IPAC, California Institute of Technology, Pasadena, CA 91125, USA e-mail: [email protected] 4 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [email protected],[email protected] Received 18 March 2002 / Accepted 28 August 2002 Abstract. We present a deep, wide-field optical survey of the young stellar cluster Alpha Per, in which we have discovered a large population of candidate brown dwarfs. Subsequent infrared photometric follow-up shows that the majority of them are probable or possible members of the cluster, reaching to a minimum mass of 0.035 M . We have used this list of members to derive the luminosity and mass functions of the substellar population of the cluster (α = 0:59 0:05, when expressed in the α ± mass spectrum form φ M− ) and compared its slope to the value measure for the Pleiades. This comparison indicates that the two cluster mass functions∝ are, indeed, very similar. Key words. open clusters and associations: individual: Alpha Per – stars: low-mass, brown dwarfs – stars: luminosity, mass functions 1. Introduction During the last years, different clusters and star forming regions have been studied intensively and their BD popula- In an ongoing effort to discover low-mass stars and brown tion revealed. The Pleiades, Alpha Per cluster, IC 2391, M 35, dwarfs (BDs) belonging to young open clusters, we have stud- NGC 2516, Taurus, the Trapezium cluster, Sigma Orionis ied the association Alpha Per. This is a well-known nearby cluster, Cha I dark cloud, Upper-Sco OB association, and cluster with (m M)0 = 6:23 (176 pc). The interstellar redden- IC348 have been investigated (Rebolo et al. 1995; Festin 1997; − ing is also low, AV = 0:30 (Pinsonneault et al. 1998). The nor- Bouvier et al. 1998; Stauffer et al. 1998a,b, 1999; Brice˜no mally quoted age for the cluster, based on isochrone fitting of et al. 1998; Barrado y Navascu´es et al. 1999, 2001a,b, 2002; the upper main sequence (MS), is of order 50 Myr (cf. Meynet Neuh¨auser & Comeron 1999; Zapatero-Osorio et al. 1999, et al. 1993), though models with a larger amount of convec- 2000; Lucas & Roche 2000; Luhman 1999, 2000; Mart´ın et al. tive core overshoot can yield ages up to about 80 Myr (Ventura 1999, 2000, 2001; B´ejar et al. 2001; Pinfield et al. 2000; Ardila et al. 1998). Recently, using the data we published in a prelim- et al. 2000; Najita et al. 2000; Moraux et al. 2001; etc.). All inary study of the BD population of the cluster (Stauffer et al. these works show that BDs are quite numerous and that the 1999), we estimated the age as 90 Myr, based on the location mass function (MF) usually presents an increase for very low- of the lithium depletion boundary. The theoretical background mass objects. In any case, the total mass below the substellar to this method can be found in Kumar (1963) and D’Antona & limit only contributes a few percent to the total mass of the par- Mazzitelli (1994), and when applied to the Pleiades (Basri et al. ent cluster, at least in the case of the Pleiades ( 3–5%, Bouvier 1996; Rebolo et al. 1996; Stauffer et al. 1998a) and IC 2391 et al. 1998; Hodgkin & Jameson 2000). However,∼ it is not clear (Barrado y Navascu´es et al. 1999) has also yielded ages 50% whether this MF is universal and this result can be extrapolated ∼ older than previously assumed. A review of these results may to other young clusters. be found in Basri (2000). In this paper, we present a new deep, wide-field optical sur- vey of the Alpha Per cluster. We have followed-up the optical Send offprint requests to: D. Barrado y Navascu´es, candidates in the near-infrared using new infrared imaging data e-mail: [email protected] and the 2MASS catalogue (Skrutskie et al. 1997). Using this ? Based on observations collected with the Kitt Peak National wealth of data, we have been able to establish the presence of Observatory, USA, and the Hispano-German Observatory of Calar a substantial population of BDs in the cluster, and derived its Alto, Spain. substellar MF. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021262 814 D. Barrado y Navascu´es et al.: A substellar mass function for Alpha Persei Table 1. Field centers for KPNO 4 m CCD Mosaic Images. Table 2. Previusly known Alpha Per members from Stauffer et al. (1999). Data from 2MASS survey. Field RA DEC (2000.0) Name JHKs A 03:27:10.0 49:24:00.0 AP300 15.595 0.061 14.859 0.074 14.538 0.083 B 03:28:00.0 48:42:00.0 AP301 15.233 0.061 14.526 0.072 14.251 0.082 C 03:24:50.0 48:42:00.0 AP302 15.572 0.066 15.101 0.092 14.758 0.102 D 03:21:40.0 48:42:00.0 AP303 14.951 0.043 14.278 0.049 14.127 0.062 E 03:19:35.0 49:27:00.0 AP304 16.102 0.107 15.418 0.134 14.995 0.130 F 03:33:45.0 49:35:00.0 G 03:33:45.0 49:04:00.0 AP305 15.729 0.077 14.906 0.084 14.675 0.100 H 03:33:45.0 48:34:00.0 AP306 15.262 0.050 14.522 0.063 13.964 0.051 I 03:33:45.0 50:06:00.0 AP307 14.948 0.043 14.280 0.055 14.069 0.060 J 03:30:30.0 50:09:00.0 AP308 14.677 0.035 14.128 0.048 13.613 0.049 K 03:27:00.0 50:13:00.0 AP309 14.495 0.032 13.882 0.044 13.575 0.045 L 03:23:10.0 48:11:00.0 AP275 15.031 0.046 14.488 0.059 14.128 0.066 M 03:21:00.0 50:48:00.0 AP310 15.543 0.065 15.110 0.104 14.622 0.085 AP311 15.450 0.058 14.709 0.069 14.348 0.069 AP312 16.274 0.118 15.483 0.131 15.132 0.135 2. Observations AP313 15.528 0.066 15.035 0.103 14.425 0.081 AP314 16.196 0.107 15.433 0.128 15.133 0.151 2.1. Optical survey AP315 15.894 0.089 15.234 0.111 14.672 0.104 AP318 15.049 0.049 14.502 0.068 14.108 0.071 We obtained deep optical imaging of the Alpha Per open clus- AP319 14.916 0.053 14.378 0.063 14.018 0.069 ter using the KPNO 4-m CCD mosaic camera, MOSA, on AP326 16.192 0.108 15.556 0.125 15.125 0.149 November 19–22 1998. Field centres were selected in order to cover as much of the central part of the cluster as possible while avoiding the brightest stars in the field (Table 1). All of the im- ages were obtained with the facility R and I filters. Weather of 53 Alpha Per candidates in the magnitude range K = 13:0– during the run was variable: Nov. 19 was generally overcast 0 19.2 mag, equivalent to I = 15:7–21.7 mag. All were observed with periods of only moderate cirrus; Nov. 20 was clear but c in the K filter, while 24 were also measured at J, and a handful with extremely poor seeing; Nov. 21 was clear with good see- 0 also at H. ing; and Nov. 22 was again partly cloudy. In order to provide calibrated photometry for all stars, we overlapped the survey We also searched the 2MASS point source catalogue fields so as to provide photometric calibration even for fields (Skrutskie et al. 1997), in order to extend the infrared data cov- whose deep images were obtained through light cirrus. The av- erage. Unfortunately, the present version of the 2MASS cata- erage seeing for the deep images was of order 1.0 arcsec. For logue does not completely cover the Alpha Per cluster, due to most fields, we obtained two 900 s R images and two 420 s I gaps between the scans in that part of the sky. Moreover, the images; a few of the fields just designed to tie together the pho- 2MASS survey does not go deep enough to detect all faint can- tometry were only observed with 100 or 200 s exposures. In all, didates in our survey. In summary, 47 of our candidates were we obtained deep imaging of 13 fields, thus corresponding to found in the 2MASS catalogue; 25 additional candidates lie in a total area on the sky of order 3 square degrees. Photometric the non-covered areas; and 21 are too faint to have been de- calibration was derived from short exposures of three Landolt tected.
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