The Astronomical Journal, 132:2653Y2664, 2006 December A # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A. NEAR-INFRARED OBSERVATIONS OF N11 IN THE LARGE MAGELLANIC CLOUD: TRIGGERED STAR FORMATION AROUND THE PERIPHERY OF LH 9 Hirofumi Hatano,1 Ryota Kadowaki,1 Yasushi Nakajima,2 Motohide Tamura,2 Tetsuya Nagata,3 Koji Sugitani,4 Toshihiko Tanabe´,5 Daisuke Kato,1 Mikio Kurita,1 Shogo Nishiyama,2 Daisuke Baba,1 Akika Ishihara,2 and Shuji Sato1 Received 2006 May 18; accepted 2006 August 11 ABSTRACT Near-infrared observations have been carried out to survey young stellar objects in the second-largest H ii region 2 in the Large Magellanic Cloud, N11. A total area of about 700 arcmin is covered in the J, H, and KS bands. We selected a total of 559 OB and 127 Herbig Ae/Be star candidates out of the detected sources based on their near- infrared colors and magnitudes. The existence of these young stellar objects indicates that star formation activity is underway in the whole N11 region. Many Herbig Ae/Be star candidates are distributed around the periphery of the OB association LH 9. Spatial correlations of the OB and Herbig Ae/Be star candidates with the objects observed at other wavelengths (optical, radio continuum, H , CO, and X-ray) suggest that the birth of the young stellar popula- tions in peripheral molecular clouds was triggered originally by LH 9. It is likely that the trigger for this star formation was an expanding supershell blown by the OB association. In N11 a new generation of stars would have been formed in the clouds developed from swept-up interstellar medium. Key words: infrared: stars — Magellanic Clouds — stars: formation — stars: preYmain-sequence Online material: machine-readable tables 1. INTRODUCTION located at the center of N11. The rich OB association LH 10 in N11B is situated to the north of LH 9, and LH 13 in N11C is to Massive stars play important roles in the large-scale evolution the east of LH 9. LH 14 in N11E lies to the northeast of LH 10. of their environment. Their violent actions, ultraviolet (UV) light, Parker et al. (1992) found that the slope of the initial mass func- stellar winds, and supernova (SN) explosions not only destroy tion (IMF) for LH 10 is significantly flatter than the slope for their parental clouds but also create various structures, for exam- LH 9 and that LH 10 is the younger of the two associations, and ple, giant H ii regions and superbubbles (e.g., Weaver et al. 1977), they proposed that the star formation in LH 10 possibly was trig- and sometimes trigger formation of the next generation of stars at gered by LH 9. Walborn & Parker (1992) proposed the two-stage their periphery (e.g., Elmegreen & Lada 1977; McCray & Kafatos starburst hypothesis: an initial, central starburst of LH 9 triggered 1987). a secondary burst around its periphery including LH 10 about Indeed, OB associations in the Large Magellanic Cloud (LMC) ; 6 show evidence for triggering the next generation of star formation 2 10 yr later. Rosado et al. (1996) also proposed that sequential star formation could have been triggered by LH 9 on a timescale around their peripheries. From an observational view, the LMC of at most a few times 106 yr. is located outside of our Galaxy, but its proximity of 50 kpc In this paper we present the results of our near-infrared (NIR) (e.g., Persson et al. 2004) enables us to resolve individual stars. observations toward N11. We can detect Herbig Ae/Be (HAEBE) In addition, the nearly face-on view of the LMC (e.g., Kim et al. stars down to 3 M in the LMC with a limiting magnitude of 1998) makes it easier to examine spatial correlations among global K 17 mag. They are intermediate-mass pre-main-sequence structures than in our Galaxy. (PMS) stars, whose lifetimes are 1Y3 Myr (Hillenbrand et al. The N11 complex is one of the most interesting star-forming 1992; Haisch et al. 2001; Fuente et al. 2002), yielding evidence regions in the LMC. It is the second-largest H ii region after the of ongoing star formation. Moreover, we show several pieces of 30 Dor nebula (Kennicutt & Hodge 1986) and is located in the northwestern part of the LMC. This H ii region was first cata- evidence for triggered star formation around the periphery of LH 9, based on comparison of our observations with those at other wave- loged by Henize (1956) in a survey of emission-line nebulae in the lengths (optical, radio continuum, H , CO, and X-ray). This study LMC. It consists of ionized nebulae (designated N11A to N11L) and filaments surrounding a central cavity. This cavity has been follows methodologically the study of Nakajima et al. (2005). evacuated by the OB association LH 9 (Lucke & Hodge 1970) 2. OBSERVATIONS AND DATA REDUCTION 1 Imaging observations of N11 were conducted on 2002 No- Department of Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464- vember 2, 6, and 7 and 2005 February 4 with the NIR camera 8602, Japan; [email protected]. 2 National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8858, SIRIUS (Simultaneous three-color InfraRed Imager for Unbiased Japan. Survey) installed on the IRSF (InfraRed Survey Facility) 1.4 m 3 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, telescope at the South African Astronomical Observatory in Japan. Sutherland. These observations are part of a comprehensive IRSF 4 Institute of Natural Sciences, Nagoya City University, Mizuho-ku, Nagoya 467-8501, Japan. Magellanic Clouds survey (D. Kato et al. 2006, in preparation). 5 Institute of Astronomy, School of Science, University of Tokyo, Mitaka, TheSIRIUScameraisequippedwiththree1024pixel; 1024 pixel Tokyo 181-0015, Japan. HAWAII arrays. The camera enables simultaneous observations 2653 2654 HATANO ET AL. Vol. 132 Fig. 1.—JHKS composite image of N11 using false colors (J,blue;H, green; KS , red). North is up, and east is to the left. in the J (1.25 m), H (1.63 m), and KS (2.14 m) bands by split- carried out only on photometric nights, and the typical seeing was ting the beam into the three colors with two dichroic mirrors 1B0 (FWHM) in the KS band. To make median sky frames for (Nagashima et al. 1999; Nagayama et al. 2003). The image scale the 14 fields we observed additional regions that are well off the of the array is 0B45 pixelÀ1, yielding a field of view of 7A7 ; 7A7. crowded stellar fields of the LMC. Twilight flat frames were ob- We observed 14 fields toward N11, and the total area covered tained before and after the observations. Dark frames were ob- is about 700 arcmin2 (see Fig. 1). We obtained 10 or 15 dithered tained at the end of the nights. We observed the standard stars frames with a 30 or 20 s exposure, respectively, resulting in a to- 9109 and 9119 in the faint NIR standard stars catalog of Persson tal exposure time of 300 s for each field. Our observations were et al. (1998) for photometric calibration. No. 6, 2006 NIR OBSERVATIONS OF N11 IN LMC 2655 image. The H ii regions that comprise the N11 complex can be seen as nebulosities. In particular, the largest one, N11B, lying on the north of LH 9, and N11C, on the southeast of N11B, are con- spicuous. N11B harbors the rich OB association LH 10. N11E appears in the northeast like a patch. The narrow filament in the south is N11F. In xx 3.1 and 3.2 we present the results of the photometry, selection of young stellar populations, namely, OB and HAEBE stars, and spatial distribution of the young stellar populations in N11. 3.1. Photometry and Source Selection Figures 2 and 3 show a J À H versus H À K color-color di- agram and a J À K versus K color-magnitude diagram for the point sources with photometric errors of 0.15 mag. The loci of dwarfs and giants (Allen 2000, p. 388; Bessel & Brett 1988; Whittet & van Breda 1980) are plotted in these diagrams. The locus of supergiants (Schmidt-Kaler 1982) is also plotted in the color-magnitude diagram. We adopt a distance modulus of 18.5 for the LMC (see, e.g., Persson et al. 2004). Reddening vectors are also plotted in these diagrams following the extinction law of Rieke & Lebofsky (1985): E(J À H )/E(H À K ) ¼ 1:7. Dash- dotted lines parallel to the reddening vector are shown in Fig- ure 2; the upper one is tangent to the upper shoulder of the locus of giants (position of K5 III), and the bottom one is tangent to Fig. 2.— J À H vs. H À K color-color diagram for the point sources that are the lower shoulder of the locus of dwarfs (B3 V). Hereafter we detected in all the bands and with photometric errors of 0.15 mag. We used the call this lower dashed line the ‘‘reddening line.’’ Stellar popula- values of J À H and H À K in the CTIO/CIT system after color conversion from tions in the color-color diagram are discussed in Appendix B of the IRSF system. The solid and dashed curves are the loci of dwarfs and giants, Nakajima et al. (2005). respectively. The data for O6YB8 dwarfs are from Whittet & van Breda (1980), and those for A0YM6 dwarfs and G0YM7 giants are from Bessel & Brett (1988). The majority of the sources are concentrated toward the lo- The dash-dotted lines show reddening vectors whose slope is 1.7.