Triggered Star Formation Around the Periphery of Lh 9

Home , Large Magellanic Cloud, N11 (emission nebula), NGC 1994

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 populations 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 pixel1, 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 H versus H K color-color diagram and a J 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 H )/E(H 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 H vs. H 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 H and H 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. cus of K giants in these diagrams. There is another concentration around (J H; H K ) (0; 0) mag in the color-color diagram. We interpret the latter concentration as being composed of We applied the standard procedures of NIR array image re- OB stars with or without reddening and HAEBE stars with duction, including dark-current subtraction, flat-fielding, and sky subtraction, using the IRAF (Image Reduction and Analysis Fa- cility)6 software package. After subtraction of the average dark frame, each image was divided by the normalized flat-field image. Then the thermal emission pattern, the fringe pattern due to OH emission, and the reset-anomaly slope pattern of the HAWAII arrays were subtracted from each frame with a median sky frame. Identification and photometry of point sources were performed using the DAOFIND and DAOPHOT packages in IRAF. The 7 limiting magnitudes for point sources were estimated to be J 19:0 mag, H 18:0 mag, and KS 16:9 mag. A total number of 8919 sources were detected in all the JHKS bands with photometric errors of 0.15 mag (7 ). We applied color conversion equations from the IRSF to the CTIO/CIT system. Details of the conversion equations are given in Table 2 of Nakajima et al. (2005). Hereafter we present all magnitudes and colors in the CTIO/CIT system. Celestial coordinates of the detected sources were system- atically calculated in the International Celestial Reference Sys- tem (ICRS) by referencing the 2MASS Point Source Catalog (Skrutskie et al. 2006). The positional accuracy of the derived coordinates is estimated to be 0B1. 3. RESULTS

In Figure 1 we show a JHKS composite image of the observed ﬁelds. The central OB association LH 9 is located in the center of

Fig. 3.— J K vs. K color-magnitude diagram for the point sources detected 6 IRAF is distributed by the National Optical Astronomy Observatory, which in all the bands and with photometric errors of 0.15 mag. The data for giants are is operated by the Association of Universities for Research in Astronomy, Inc., taken from Allen (2000) and Bessel & Brett (1988), and those of supergiants and under cooperative agreement with the National Science Foundation. OB stars are taken from Schmidt-Kaler (1982) and Whittet & van Breda (1980). 2656 HATANO ET AL. Vol. 132

evolutionary sequence (groups II ! I ! III ) ordered by progressive decrease of the amount of circumstellar material. The authors showed that group I and many group II HAEBE stars are distributed in the region of J H 0:3, H K 0:5 mag in a J H versus H K color-color diagram (see Fig. 15 of Hillenbrand et al. 1992). As mentioned above, many OB stars are located around (J H; H K ) (0; 0) mag in Figure 2, where group III HAEBE stars exist. Moreover, Figure 2 of Dougherty et al. (1994) indicates that classical Be stars are also located in almost the same region. Since we cannot discriminate group III HAEBE stars from the other two stellar populations, we select only group I/II HAEBE stars, that is, younger stars than group III HAEBE stars. We pick up 127 HAEBE star candidates with the following criteria: (1) located under the reddening line, (2) having J H 0:3andH K 0:5 mag, and (3) having K 12 mag for J H > 1 mag. The last criterion is to reject Mira variables, which have magnitudes of K < 12 in the LMC (see, e.g., Ita Fig. 4.—Histogram of J K for sources in the region between the two dash- et al. 2002) and appear in the region J H > 1 mag. Table 2 dotted lines parallelpffiffiffiffi to the reddening vector in the color-color diagram. The error bars indicate N errors, where N is the number of stars in each bin. shows data on the HAEBE star candidates. Note that background galaxies with redshifts of z > 0:1 could also contaminate the small infrared excesses (group III HAEBE stars; Hillenbrand et al. HAEBE star candidates because of their colors (see Appendix B 1992). Figure 4 shows a histogram of J K colors for sources in of Nakajima et al. 2005). A total of 42 HAEBE star candidates the region between the two dash-dotted lines parallel to the red- have colors similar to those of the galaxies. dening vector (Fig. 2). In Figure 4 OB stars peak around J K 3.2. Spatial Distribution of the Young Stellar Populations 0:1, and most of them are between 0.2 and 0.1 mag. We select 559 OB star candidates from this range with the following The spatial distributions of the OB and HAEBE star candidates criteria: (1) located above the reddening line and with J K are shown in Figures 5 and 6 together with contours of their sur- 0:1 mag and (2) having K 13 mag. Early-type stars in our Gal- face stellar number density. We calculate the surface stellar num- axy and supergiants in the LMC are brighter than O3 V stars in the ber density by making 1000 separation grids and using a kernel LMC (K 13:4 mag; Fig. 3). We therefore made the latter cri- method (Gomez et al. 1993) with a smoothing parameter of 4500. terion to exclude these contaminations from the OB star candi- The contour levels of surface stellar number density larger than dates. Table 1 shows data on the OB star candidates. 1.0 and 0.4 stars (10 pc)2 for the OB and HAEBE star candi- We select HAEBE star candidates by referring to the catalog dates, respectively, indicate statistically significant concentrations. of Hillenbrand et al. (1992). The authors analyzed the spectral These thresholds are determined in the same way as in Gomez energy distributions (SEDs) of 47 nearby (d 1300 pc) HAEBE et al. (1993); a number of artificial sources equal to the 559 OB stars and classified them into three groups: group I, with large and 127 HAEBE candidates are randomly distributed over an area infrared excesses due to an optically thick circumstellar accre- of the same size as our observed area, and the surface number 4=3 tion disk, which have spectral slopes kFk k at k k 2:2 m; density is calculated for each. We repeat this process 10 times. group II, with large infrared excesses due to a dust envelope, whose Then, the averages of the highest peak values in 10 trials are SEDs are flat or rising toward the far-infrared; and group III, adopted as the thresholds to define the concentrations. The thresh- with small infrared excesses arising from free-free emission in olds are about 3 and 5 times larger than the observed averages of a circumstellar gas disk or envelope, whose SEDs appear to be 0.35 and 0.08 stars (10 pc)2 for the OB and HAEBE star similar to those of classical Be stars. These groups represent an candidates, respectively.

TABLE 1 OB Star Candidates in N11

a b a b a b a a c No. (J2000.0) (J2000.0) J J H H K K J H H K Comments

1...... 4 54 57.52 66 31 29.8 16.93 0.03 16.91 0.06 17.08 0.15 0.02 0.17 2...... 4 55 00.70 66 38 43.8 15.14 0.01 15.22 0.02 15.28 0.04 0.08 0.06 3...... 4 55 01.49 66 36 12.4 15.12 0.01 15.23 0.02 15.28 0.04 0.11 0.05 4...... 4 55 01.68 66 36 41.9 16.52 0.02 16.58 0.04 16.61 0.12 0.06 0.03 5...... 4 55 03.17 66 35 11.9 14.67 0.01 14.72 0.02 14.78 0.03 0.05 0.06 6...... 4 55 03.22 66 40 28.1 14.96 0.01 14.92 0.01 14.88 0.03 0.04 0.04 7...... 4 55 04.87 66 31 37.5 15.98 0.01 15.99 0.03 15.99 0.05 0.01 0.00 8...... 4 55 05.05 66 32 26.8 14.79 0.01 14.88 0.02 14.90 0.03 0.09 0.02 9...... 4 55 05.74 66 31 56.8 13.57 0.01 13.65 0.02 13.68 0.02 0.08 0.03 10...... 4 55 05.97 66 34 28.6 15.94 0.01 16.00 0.03 16.05 0.06 0.06 0.05

Note.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Table 1 is published in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its form and content. a These values represent magnitudes and colors in the CTIO/CIT system. b These values represent photometric errors in the JHK bands. c Comments contain the identiﬁcation with the objects detected in the previous observations. Optical PGMW numbers (with their spectral types) are from Parker et al. (1992). NIR BRRG numbers are from Barba´ et al. (2003). No. 6, 2006 NIR OBSERVATIONS OF N11 IN LMC 2657

TABLE 2 HAEBE Star Candidates in N11

a b a b a b a a c No. (J2000.0) (J2000.0) J J H H K K J H H K Comments 1...... 4 55 09.13 66 38 27.7 18.02 0.11 17.25 0.13 16.47 0.11 0.77 0.78 2...... 4 55 12.24 66 35 42.6 19.23 0.15 18.04 0.14 16.94 0.13 1.19 1.10 3...... 4 55 19.77 66 22 02.8 18.56 0.07 17.87 0.08 17.28 0.11 0.69 0.59 4...... 4 55 20.15 66 19 22.1 18.90 0.13 18.09 0.13 17.35 0.13 0.81 0.74 5...... 4 55 21.61 66 26 30.6 18.56 0.07 18.17 0.15 17.55 0.15 0.39 0.62 6...... 4 55 23.21 66 42 21.1 18.24 0.10 17.82 0.13 16.70 0.12 0.42 1.12 7...... 4 55 24.52 66 29 11.4 18.50 0.07 17.86 0.13 17.31 0.11 0.64 0.55 8...... 4 55 29.14 66 28 32.8 18.78 0.09 17.78 0.12 16.80 0.12 1.00 0.98 9...... 4 55 29.20 66 20 17.0 18.74 0.11 18.02 0.09 17.33 0.13 0.72 0.69 10...... 4 55 32.52 66 21 44.3 18.70 0.08 17.91 0.08 17.27 0.13 0.79 0.64

Note.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Table 2 is published in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its form and content. a These values represent magnitudes and colors in the CTIO/CIT system. b These values represent photometric errors in the JHK bands. c Comments contain the identiﬁcation with the objects detected in the previous observations. Optical PGMW numbers are from Parker et al. (1992). NIR BRRG numbers are from Barba´ et al. (2003).

There are four concentrations of the OB star candidates in the star spends 10%Y20% of its main-sequence lifetime buried in its observed area, and also six concentrations of the HAEBE star parental cloud (Wood & Churchwell 1989). This means that the candidates. We call the concentrations of the OB and HAEBE timescale for embedded massive stars is several 105 yr, while the star candidates OC 1Y4 and HC 1Y6, respectively (Figs. 5 and lifetime of the circumstellar dust disk/envelope of a HAEBE star 6). Concentrations of both the OB and HAEBE star candidates is estimated to be 1Y3 Myr (Hillenbrand et al. 1992; Haisch are seen in N11B (OC 2 and HC 1) and N11F (OC 4 and HC 4), et al. 2001; Fuente et al. 2002) and the main-sequence timescale while HC 2, HC 3, HC 5, and HC 6 have no counterparts among for OYB2 stars is 3Y10 Myr. Hence, C/UC H ii regions are the concentrations of the OB star candidates. good indicators of the most recent star formation activity. Four UC H ii regions are identified in our observed area on the 4. DISCUSSION basis of radio continuum at 3 and 6 cm and their Infrared Astro- nomical Satellite (IRAS ) colors by Indebetouw et al. (2004). We 4.1. Comparison with the Observations at Other Wavelengths plot them in Figure 7 with orange crosses. One of them is accom- N11 has been observed at various wavelengths from X-ray to panied by a methanol (CH3OH) maser, represented by a black radio. We examine spatial distributions of the OB and HAEBE cross in the same figure (Ellingsen et al. 1994). This maser emis- star candidates, optical clusters and associations, ultracompact sion is an excellent tracer of UC H ii regions and sites of recent H ii regions, methanol masers, H emission, molecular clouds, star formation (Caswell et al. 1995). These sources associated and diffuse X-ray emission in the observed area. with the concentrations of the young stellar populations are sum- marized in Table 3. Being associated with the UC H ii regions 4.1.1. Optical Clusters and Associations and/or a methanol maser, N11B and the concentration HC 3 sug- There are 49 optical stellar clusters and associations cataloged gest ongoing active star formation. by Bica et al. (1999) in our observed area. We plot the clusters and associations with ellipses in Figure 7. The nomenclature, 4.1.3. H, CO, and X-Ray size, and position angle are taken from Bica et al. (1999). In our Several H emission nebulae and filaments form a shell observed area, 11 clusters or associations have ages listed by structure surrounding LH 9 (see Fig. 1b of Mac Low et al. 1998). Bica et al. (1996). The authors categorized clusters and associ- Among them the two H ii regions N11B and N11C stand out. ations in the LMC into nine age groups, 0Y10 Myr, 10Y30 Myr, Many OB star candidates are associated with the H shell, and and so on. Two out of these 11 are just projection components they may be exciting their surroundings (Fig. 8). The giant arc (NGC 1776 and SL 153) that have no possible physical con- in the north of LH 10 might have been formed by BSDL 258, nection. Seven out of the remaining nine belong to the 0Y10 Myr which is detected as the concentration OC 1 to the northwest of age group. We determined that three of them are associated with LH 10. The distribution of H emission correlates well with many the concentrations of the HAEBE star candidates (Table 3), that concentrations of the HAEBE star candidates (Fig. 9). is, with young populations less than 3 Myr old (Hillenbrand et al. Israel et al. (2003) surveyed N11 and its periphery extensively 1992; Haisch et al. 2001; Fuente et al. 2002). The remaining in the J ¼ 1Y0 transition of 12CO and identified a total of 29 associations, LH 14 and BCDSP 1, belong to the 10Y30 Myr age individual molecular clouds (Fig. 1 of Israel et al. 2003). Around group. While most of our concentrations of the young stellar the main body of N11, the clouds are distributed in a ringlike populations have corresponding objects in optical stellar clusters structure surrounding LH 9. The virial masses of the individual 4 4 and associations, the two concentrations HC 3 and HC 6 do not clouds range from 0:5 ; 10 to 7:5 ; 10 M, and the total H2 5 have any counterparts in the catalog of Bica et al. (1999). mass in the ring area is estimated to be about 3:2 ; 10 M.TheCO ii generally follows the H emission, although their peaks are not 4.1.2. Ultracompact H Regions and a Methanol Maser always coincident (Caldwell & Kutner 1996). Israel & de Graauw Massive stars still embedded in their birth clouds are observed (1991) suggested that all the CO clouds around LH 9 have ap- as compact or ultracompact (C/ UC) H ii regions. A typical O-type parently been consumed, destroyed, or driven out. The CO clouds 2658 HATANO ET AL. Vol. 132

Fig. 5.—Distribution of the OB star candidates. Dots of different sizes represent the candidates; larger ones have brighter K-band magnitude. The contours show the surface stellar number density. The lowest level and the interval are both 0.5 stars (10 pc)2. The solid lines show the observed area. are obviously accompanied by the concentrations of the HAEBE At the point where hot gas stops a CO cloud exists, and it harbors star candidates (Fig. 9). In contrast, the HAEBE star candi- some HAEBE star candidates, denoted by HC 6 (Fig. 9). dates are hardly associated with the two CO clouds to the west 4.2. Star Formation in Each Re ion of LH 9. g Extended diffuse soft X-ray emission has been detected in N11 In this section we compare the spatial distributions of our OB using the XMM-Newton observatory (Fig. 5 of Naze´ et al. 2004). and HAEBE star candidates with the components shown in x 4.1. The emission covers LH 9, LH 10, and a region to the north of This comparison is made for the central region of N11, including these OB associations. The X-ray emission inside the central cav- LH 9, N11B, N11C, N11F, and N11I, and for the southernmost ity around LH 9 probably originates from hot, shocked gas within part of N11. a superbubble blown by the OB association, hidden supernova 4.2.1. LH 9 remnants (SNRs), colliding-wind binaries, and low-mass (2 M) PMS stars. The diffuse emission is well conﬁned within the H LH 9 is situated at the center of the cavity. The massive com- ﬁlaments delineating the superbubble. The same authors also pact cluster HD 32228 dominates this association. It contains a suggested that hot gas may be leaking out of the superbubble to WC star (Wolf-Rayet star with dominant carbon emission) and has the south of N11, where soft X-ray emission is detected (Fig. 8). an age of about 3.5 Myr (Walborn et al. 1999). In this association, No. 6, 2006 NIR OBSERVATIONS OF N11 IN LMC 2659

Fig. 6.—Distribution of the HAEBE star candidates. Dots of different sizes represent the candidates; larger ones have brighter K-band magnitude. The contours show the surface stellar number density. The lowest level and the interval are both 0.2 stars (10 pc)2. The solid lines show the observed area. several tens of O-type stars, including an O6 star, have been iden- stellar contents, including several O3 and zero-age main-sequence tiﬁed by Parker et al. (1992, 1996). (ZAMS) O stars (Parker et al. 1992), photoevaporate and ionize Many OB star candidates are detected here, and the most strik- conterminous molecular clouds (Barba´ et al. 2003). The latter ing concentration (OC 3) among N11 is seen in accordance with authors also detected several HAEBE star candidates in part of the previous observations. In contrast, few HAEBE star candi- N11B by means of NIR observations, suggesting that this region dates are detected inside the cavity. Assuming LH 9 is coeval is currently forming a new generation of stars. In spite of the with HD 32228 most of the intermediate-mass HAEBE stars, youth of this region, the presence of wind-blown bubbles around formed with massive OB stars early in LH 9, would have already concentrations of massive stars and individual O stars is shown disappeared, losing their circumstellar dust disks/envelopes by Naze´ et al. (2001). The X-ray emission encompassing LH 10 (recall their lifetime of 1Y3 Myr). Consequently, only OB stars would be a result of the hot gas ﬁlling the wind-blown bubbles remain in LH 9 now. (Naze´ et al. 2004). The concentrations OC 2 and HC 1 are detected in N11B. The 4.2.2. N11B second most striking concentration of OB star candidates after The brightest nebula, N11B, lies to the north of LH 9. The LH 9 is seen, as has been indicated by previous observations. A 4 youngest OB association, LH 10I, with an age of about 1 Myr dense molecular cloud (with a virial mass of 5:4 ; 10 M) (Walborn et al. 1999), is still embedded in this nebula. Its rich exists at the center of this region, but the peaks of CO and the 2660 HATANO ET AL. Vol. 132

Fig. 7.—Comparison of Bica’s clusters and associations and our OB and HAEBE star candidates. Ellipses represent clusters and associations in Bica’s catalog (Bica et al. 1999). The diameters of the major and minor axes and the position angles are from the catalog. The position and contours of the surface number density above the thresholds of concentrations for the OB and HAEBE star candidates are drawn in blue and red, respectively. The concentrations of the OB and HAEBE star candidates are denoted by OC 1Y4 and HC 1Y6. Crosses denote UC H ii regions (orange) and a methanol maser (black). The solid lines show the observed area. concentration HC 1 are not coincident. This may be caused by from the cluster as a result of dynamical interaction. Based on the destruction of CO clouds as a result of interaction with violent ages of Sk 66 41 (3.5Y5Myr)andHNT(100 25 Myr) and UV photons from massive stars in N11B. With several HAEBE from their stellar contents and kinematical properties, HNT has star candidates, two UC H ii regions, and the methanol maser no direct connection to Sk 66 41 and could be a line-of-sight detected in this region, we also conﬁrmed that star formation in object (Heydari-Malayeri et al. 2000). N11B is still ongoing actively. Since the tight Sk 66 41 cluster cannot be resolved in our data, no concentrations of OB star candidates are seen. Its po- 4.2.3. N11C sition corresponds to that of the X-ray emission from LH 13, and N11C, located to the east of LH 9, is one of the bright nebulae the emission could be well explained by the stars of the cluster of N11. This region has been studied extensively by Heydari- alone (Naze´ et al. 2004). Two molecular clouds are identiﬁed in Malayeri et al. (1987, 2000). LH 13, embedded in this nebula, is LH 13, and they appear to be divided by the powerful Sk 66 41 mainly comprised of two compact star clusters, Sk 66 41 and cluster. The concentration HC 2 is at the center of N11C and sup- HNT. OB stars surrounding Sk 66 41 might have been ejected ports the idea that this region is young. The concentration HC 2 is No. 6, 2006 NIR OBSERVATIONS OF N11 IN LMC 2661

TABLE 3 Concentrations of OB and HAEBE Star Candidates

Name (J2000.0)a (J2000.0)a Peak Densityb Bica’s Clusters and Associations SWBc Associated Sources Type References

OC 1..... 4 56 05.6 66 20 21 1.83 BSDL 258, HS 68 -, - ...... BSDL 247 - ...... OC 2..... 4 56 45.8 66 24 51 3.49 LH 10 (N11B) 0 B04566629 (W1) O9.5 V, UC H ii 1 ... B04566629 (W2) O9 V, UC H ii 1 ... MC 18 CH3OH maser 2 OC 3..... 4 56 37.8 66 28 41 4.71 LH 9, NGC 1761 -, 0 ...... HD 32228, N11F 0, 0 ...... BCDSP 1, BSDL 270 I, - ...... OC 4..... 4 56 56.5 66 32 51 1.75 N11F 0 ...... HC 1..... 4 56 39.1 66 24 30 0.50 LH 10 (N11B) 0 ...... HC 2..... 4 57 42.7 66 27 10 0.46 LH 13 (N11C) 0 ...... HC 3..... 4 57 43.0 66 30 30 0.52 ...... B04576632 B0 V, UC H ii 1 HC 4..... 4 56 32.8 66 32 00 0.63 N11F 0 ...... HC 5..... 4 55 42.7 66 34 30 0.69 N11I - ...... HC 6..... 4 56 21.4 66 37 10 0.45 ......

a These coordinates represent positions of the peaks of the concentrations. Units of right ascension are hours, minutes, and seconds. Units of declination are degrees, arcminutes, and arcseconds. b Peak values of surface stellar number density. Unit is stars (10 pc)2. c SWB indicates age groups: 0 is 0Y10 Myr, and I is 10 Y30 Myr (see Table 3 of Bica et al. 1996). A hyphen denotes no classification. References.— (1) Indebetouw et al. 2004; (2) Ellingsen et al. 1994. associated with the northern one of the two molecular clouds. To OB star candidates are associated with the H shell, and some the south of N11C, the concentration HC 3 is also seen but is less of them are concentrated in LH 10. Most of the HAEBE star can- associated with the southern one of the two molecular clouds. A didates, which indicate ongoing star formation, are located around UC H ii region is identified and associated with the concentration LH 9, and some of them are concentrated. The concentrations HC 3 (Fig. 7). In N11C star formation activity is also underway, of the HAEBE star candidates are associated with the molecular although the activity is not as strong as that in N11B. clouds that are distributed in the ringlike structure surrounding LH 9. Similarly to LH 10 (1 Myr) in N11B, the regions around 4.2.4. N11F LH 9 are younger than LH 9 (3.5 Myr), regions in which the The nebula N11F, lying to the south of LH 9, defines the HAEBE star candidates and the molecular clouds are distributed. southern boundary of the central cavity. Its filaments are well as- In addition to this age difference, spatial correlations between the sociated with many OB star candidates. In particular, the concen- young stellar populations and gaseous components suggest that tration OC 4 is seen in the southeastern filament. In the northern the star formation around the periphery of LH 9 was triggered part of N11F is the concentration HC 4, and its peak almost originally by the OB association. coincides with that of the molecular cloud identified there. It is McCray & Kafatos (1987) proposed that stellar winds and SN likely that the natal cloud is being dissipated, and some HAEBE explosions from an OB association may create an expanding super- star candidates have been exposed by the OB star candidates at shell, which is likely to develop a molecular (H2 and CO) layer the northern tip of N11F. within 1 Myr, and star formation can be triggered in this layer as a result of gravitational instability of the supershell. An expan- 4.2.5. N11I sion of the H shell around LH 9 with a velocity of 25 km s1 The faint nebula N11I lies to the southwest of LH 9 and is was found by Meaburn et al. (1989). Rosado et al. (1996) also ob- apart from the OB association. It is also situated at the eastern tip tained an expansion velocity of 45 km s1. There is also a giant of the most massive molecular cloud of N11 (with a virial mass H i shell (GS 16), centered at (;) ¼ (4h56m24s; 662500600) 4 of 7:6 ; 10 M). The cloud nurses the concentration HC 5, and with a radius of 11A4, or 170 pc (see Fig. 4a of Kim et al. 1999). the peaks of the cloud and HC 5 are nearly coincident. The OB This giant H i shell, extending outside of the H shell, is expand- star candidates in N11I would have been ionizing the cloud from ing with a velocity of 19.0 km s1. Meaburn et al. (1989) and the eastern side of it. Rosado et al. (1996) suggested that the expansion of this supershell is due to stellar winds and/or SN explosions of the interior 4.2.6. Southernmost Part of N11 massive stars. The latter authors estimated the kinematic age of As mentioned in x 4.1.3, the superbubble is probably leaking 2.5 Myr for the shell from the expansion velocity of 45 km s1. some hot gas to the south of N11, and a CO cloud exists at the This kinematic age is smaller than the age of the central LH 9 point where hot gas stops; this cloud harbors the concentration (3.5 Myr). Mac Low et al. (1998) also estimated that a kine- HC 6. There are no OB star candidates and little H emission in matic age of the shell is under 1 Myr, significantly shorter than the cloud. It is not clear whether physical connections between the the age of LH 9. Taking account of the kinematic age of 2.5 Myr leaking hot gas and the molecular cloud exist or not or whether and the age of LH 9, it probably took a long time (around 1 Myr) star formation there has been affected by the hot gas or not. until the OB association broke its natal cloud and started to blow the supershell. As mentioned in x 4.2.2, the presence 4.3. Triggered Star Formation of wind-blown bubbles around the massive stars in LH 10 The OB star candidates are most strikingly concentrated in the (1Myr)isshownbyNaze´ et al. (2001), but the bubbles are central OB association LH 9. Around the OB association, many much smaller than the supershell. This supports the delay of the 2662 HATANO ET AL. Vol. 132

Fig. 8.—Comparison of H emission (inverted gray scale; Mac Low et al. 1998), X-ray emission ( purple contours; Naze´ et al. 2004), and the distribution of the OB star candidates (blue dots and contours). The contours above the threshold of concentrations are shown. The solid lines show our observed area. onset of expansion of the shell. The CO clouds should be a didates. Moreover, in the southernmost part of N11, (1) no OB molecular layer of the supershell. All the concentrations of the star candidates appear, and (2) the natal cloud that formed the HAEBE star candidates are associated with molecular clouds. HAEBE star candidates therein appears to be almost pristine. By This suggests that a trigger for the star formation around the such comparison, N11I would be younger than N11B, N11C, periphery of LH 9 may be the expanding supershell blown by and N11F. It is likely that the southernmost part of N11 is much the OB association. younger than the others. That is, star formation activities in each In N11B, N11C, and N11F, (1) many OB star candidates exist, region would not be coeval, and the activities in N11I and the (2) each nebula has been ionized on a large scale by the OB star southernmost part of N11 may be more recent than those in N11B, candidates, and (3) the natal clouds that formed the HAEBE stars N11C, and N11F. Two of the most luminous HAEBE star can- therein would have been destroyed by the OB star candidates. didates (K 14 mag) are situated in N11I and the southernmost Conversely, in N11I, (1) few OB star candidates exist, (2) the neb- part of N11. Most of the HAEBE star candidates detected in our ula has been ionized on a small scale by the OB star candidates, observations are less luminous (K k 15 mag). Assuming that stars and (3) the natal cloud that formed the HAEBE stars therein would were formed in various mass ranges in their natal clouds, since have been destroyed partly at its eastern tip by the OB star can- brighter (higher mass) stars evolve more rapidly, this reinforces No. 6, 2006 NIR OBSERVATIONS OF N11 IN LMC 2663

Fig. 9.—Comparison of H emission (inverted gray scale; Mac Low et al. 1998), 12CO (1Y0) integrated map (green contours; Israel et al. 2003), and the distribution of the HAEBE star candidates (red dots and contours). The contours above the threshold of concentrations are shown. The solid lines show our observed area. evidence for the youth of N11I and the southernmost part of 5. CONCLUSION N11. A possible scenario for the evolution of the N11 complex is as 1. We have observed N11 in the Large Magellanic Cloud follows: (1) the central OB association LH 9 was formed 3.5 Myr and selected 559 OB and 127 HAEBE star candidates out of the ago, (2) OB stars in LH 9 ionized their natal cloud by UV light, detected sources, based on their NIR colors and magnitudes. (3) the OB stars started to blow a supershell by stellar winds, (4) as 2. We have discovered that, not only in N11B but also in the the supershell expanded into its surroundings, molecular clouds entire region of N11, star formation is in progress around the were formed from swept-up interstellar medium, (5) a new gen- periphery of the central OB association LH 9. eration of stars was formed in the clouds, earlier in N11B (1Myr 3. The spatial correlations of the OB and HAEBE star ago), N11C, and N11F and later in N11I and the southernmost candidates with the objects observed at other wavelengths (op- part of N11, (6) 3 Myr after the birth of LH 9 the HAEBE stars tical, radio continuum, H, CO, and X-ray) indicate that this star that formed early in LH 9 disappeared, and (7) N11B and N11C formation was triggered originally by LH 9. are currently being ionized largely by OB stars in LH 10 and LH 4. A trigger for the star formation around the periphery of 13. The star formation activities around the periphery of LH 9 are LH 9 could have been an expanding supershell blown by the OB ongoing. association. It is likely that in N11 a new generation of stars has 2664 HATANO ET AL. been formed in peripheral molecular clouds developed from search, Japan Society for the Promotion of Science. This pub- swept-up interstellar medium. lication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Mas- sachusetts and the Infrared Processing and Analysis Center/ We would like to thank the staff at the South African Astro- California Institute of Technology, funded by the National nomical Observatory for their support during the observations. Aeronautics and Space Administration and the National Science This research is supported by a Grant-in-Aid for Scientiﬁc Re- Foundation.

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