Brown dwarfs in the Pleiades cluster - III. A deep IZ survey

ASTRONOMY & ASTROPHYSICS FEBRUARY I 1999, PAGE 537 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 134, 537–543 (1999) Brown dwarfs in the Pleiades cluster III. A deep IZ survey?,?? M.R. Zapatero Osorio1 , R. Rebolo1 , E.L. Martı́n1,??? , S.T. Hodgkin2 , M.R. Cossburn2 , A. Magazzù3,???? , I.A. Steele4 , and R.F. Jameson2 1 Instituto de Astrofı́sica de Canarias, C/, Vı́a Láctea s/n, E-38200 La Laguna, Tenerife, Spain Astronomy Group, Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK 3 Centro Galileo Galilei, Apartado 565, E-38700 Santa Cruz de La Palma, Islas Canarias, Spain 4 Astrophysics Research Institute, Liverpool John Moores University, Liverpool L3 3AF, UK 2 Received August 5; accepted September 16, 1998 Abstract. We present the results of a deep CCD-based IZ photometric survey of a ∼ 1 deg2 area in the central region of the Pleiades Galactic open cluster. The magnitude coverage of our survey (from I ∼ 17.5 down to 22) allows us to detect substellar candidates with masses between 0.075 and 0.03 M . Details of the photometric reduction and selection criteria are given. Finder charts prepared from the I-band images are provided. Key words: stars: low-mass, brown dwarfs — Galaxy: open clusters and associations: Pleiades catalogs for a review). The recent spectroscopic confirmations of Pleiades objects at the stellar-substellar boundary and genuine substellar members (Basri et al. 1996; Rebolo et al. 1996; Martı́n et al. 1998; Stauffer et al. 1998) previously discovered as a result of optical photometric surveys in small areas suggest that a numerous population of very low-mass objects may be found in this cluster. This encourages future surveys to discover BDs cooler and less massive than those (0.075 − 0.05 M ) previously detected by these surveys. The Pleiades offers a unique opportunity to establish the observational properties of these rather elusive objects and to characterize the initial mass function in the substellar mass regime. 1. Introduction 1.1. The survey The Pleiades star cluster is an ideal hunting ground for substellar objects mainly due to its richness of members, young age, proximity and scarce interstellar absorption. Taking advantage of these properties, several photometric searches aimed at finding brown dwarfs (BDs) have been performed during the last decade (see Hambly 1998 Send offprint requests to: M.R. Zapatero Osorio ? Based on observations made with the Isaac Newton Telescope (INT) and the Jacobus Kaptein Telescope (JKT) operated on the island of La Palma by the Isaac Newton Group at the Observatorio del Roque de los Muchachos of the Instituto de Astrofı́sica de Canarias; and on observations made with the Nordic Optical Telescope (NOT) at the same observatory. ?? Figure 5 is only available at the http://www.edpsciences.com ??? Present address: Astronomy Department, University of California, Berkeley, CA 94720, U.S.A. ???? Also at: Obsservatorio Astrofisico di Catania, Città Universitaria, Viale Andrea Doria 6, I-95125 Catania, Italy. Correspondence to: As part of an on-going search for BDs in the Pleiades, we have conducted a deep CCD-based IZ survey using the 2.5 m Isaac Newton Telescope (INT) located on the Observatorio del Roque de los Muchachos (ORM, island of La Palma). The area covered was 1.05 deg2 within the central region of the cluster (a small fraction of the total area was also observed using the R filter). More than 40 faint (I ≥ 17.5), very red (I − Z ≥ 0.5) objects have been detected down to I ∼ 22. Their location in the colourmagnitude diagram suggests cluster membership. In this paper we report on the details of this survey along with the selection criteria. We provide IZ magnitudes, coordinates and finder charts for all candidates. Preliminary results of this survey were presented in Zapatero Osorio et al. (1997a, 1998a). An extensive discussion on the membership of the candidates and derivation of the initial mass function will be given in a forthcoming paper (Zapatero Osorio et al. 1998b). 538 M.R. Zapatero Osorio et al.: Brown dwarfs in the Pleiades cluster. III. Table 1. Field (100 arcmin2 ) center coordinates RA (J2000) DEC (h m s ) (◦ 0 00 ) 3 43 35 3 43 43 3 44 15 3 44 30 3 44 30 3 45 00 3 45 15 3 45 45 3 45 45 3 45 50 3 46 25 3 46 30 3 46 30 3 47 20 3 47 22 3 47 30 3 47 52 3 48 00 3 48 00 3 48 05 3 48 05 3 48 15 3 48 15 3 48 19 3 48 20 3 48 30 3 48 40 3 48 40 3 48 45 3 48 57 3 49 15 3 49 20 3 49 20 3 49 35 3 49 41 3 50 00 3 50 15 3 50 25 3 50 30 3 53 30 a 24 30 00 24 45 00 23 40 30 23 55 00 24 40 00 24 37 00 23 54 35 24 10 00 24 46 00 23 55 00 24 42 00 24 25 00 24 37 00 23 25 00 22 39 40 24 26 00 24 10 00 24 00 00 24 34 00 23 39 32 24 45 00 23 32 30 24 28 00 25 32 00 23 45 00 23 57 00 22 42 00 24 45 00 23 40 00 24 18 00 24 31 00 23 32 00 24 45 00 23 55 00 25 15 40 24 28 00 23 40 00 24 05 00 23 50 00 23 40 00 Date (1996) Sep.a (arcmin) Filters 21 Sep. 12 Feb. 21 Sep. 21 Sep. 20 Sep. 20 Sep. 20 Sep. 20 Sep. 20 Sep. 20 Sep. 20 Sep. 20 Sep. 21 Sep. 21 Sep. 13 Feb. 20 Sep. 21 Sep. 21 Sep. 9 Feb. 9 Feb. 20 Sep. 21 Sep. 21 Sep. 11 Feb. 20 Sep. 21 Sep. 13 Feb. 20 Sep., 13 Feb. 20–21 Sep. 21 Sep. 20 Sep. 21 Sep. 20 Sep. 21 Sep. 11 Feb. 20 Sep. 21 Sep. 21 Sep. 21 Sep. 11 Feb. 53.02 58.69 46.15 36.32 47.44 40.54 27.02 17.37 42.55 20.00 35.89 19.25 30.77 42.25 87.48 20.19 12.23 15.39 30.25 31.24 40.76 38.55 27.06 86.85 28.62 22.87 88.07 44.27 36.15 28.84 38.97 47.48 49.54 37.40 77.72 46.03 52.18 46.83 50.94 93.29 IZ RI IZ IZ IZ IZ IZ IZ IZ IZ IZ IZ IZ IZ RIZ IZ IZ IZ RIZ RIZ IZ IZ IZ RI IZ IZ RIZ IZ IZ IZ IZ IZ IZ IZ RI IZ IZ IZ IZ RI Separation from the cluster center (3h 47m , +24◦ 70 ). 2. Observations and data reduction All of our CCD survey was carried out during 1996 in two campaigns which took place on February 9–12 and on September 19–20. We used the TEK (1024×1024 pixel2 ) detector mounted on the prime focus of the telescope, with a field of view of 10×10 arcmin2 . A total of 40 fields, with center coordinates listed in Table 1, were observed with the Harris (R)I and RGO Z broad-band filters providing a total survey area of 1.05 deg2 (∼ 6.5% of the whole cluster). Most of these fields were selected to avoid very bright stars and were located within 1 deg of the innermost region of the Pleiades (see Table 1), where the population of M dwarf proper motion members is much larger than in outer areas (Hambly et al. 1993; Bouvier et al. 1998). If as expected less massive Pleiades members show a similar 11 00 00 11 11 00 00 11 00 11 00 11 11 00 00 11 00 11 Fig. 1. Location of our fields (squares) within 3.◦ 5 × 3.◦ 5 of the Pleiades area. Open squares stand for those fields observed with IZ filters while the shaded squares depict the four fields observed with RI filters. Central coordinates (indicated with a cross) are 3h 47m , +24◦ 70 (Eq. 2000). The five fields which may have some amount of extra reddening according to the CO contours shown by Breger (1 (...truncated)