K. Ogura

Wide-Field CCD Photometry around Nine Open Clusters

In this paper we study the evolution of the core and corona of nine open clusters using the projected radial density profiles derived from homogeneous CCD photometric data obtained with the 105 cm Kiso Schmidt telescope. The age and galactocentric distance of the target clusters vary from 16 to 2000 Myr and 9 to 10.8 kpc, respectively. Barring Be 62, which is a young open cluster, other clusters show a uniform reddening across the cluster region. The reddening in Be 62 varies from E(B - V)min = 0.70 mag to E(B - V)max = 1.00 mag. The coronae of six of the clusters in the present sample are found to be elongated; however, on the basis of the present sample it is not possible to establish any correlation between the age and shape of the core. The elongated core in the case of the young cluster Be 62 may reflect the initial conditions in the parental molecular cloud. The other results of the present study are as follows: (1) Core radius rc and corona size rcn/cluster radius rcl are linearly correlated. (2) The rc, rcn, and rcl are linearly correlated with the number of stars in that region. (3) In the age range 10-1000 Myr, the core and corona shrink with age. (4) We find that in the galactocentric distance range 9-10 kpc, the core and corona/cluster extent of the clusters increase with the galactocentric distance.

Deep Near-Infrared Observations of the W3 Main Star-forming Region

We present a deep JHKs-band imaging survey of the W3 Main star-forming region, using the near-infrared camera SIRIUS mounted on the University of Hawaii 2.2 m telescope. The near-infrared survey covers an area of ~24 arcmin2 with 10 σ limiting magnitudes of ~19.0, 18.1, and 17.3 in the J, H, and Ks bands, respectively. We construct JHK color-color and J versus J-H and K versus H-K color-magnitude diagrams to identify young stellar objects and estimate their masses. Based on these color-color and color-magnitude diagrams, a rich population of young stellar objects is identified that is associated with the W3 Main region. A large number of previously unreported red sources (H-K > 2) have also been detected around W3 Main. We argue that these red stars are most probably pre-main-sequence stars with intrinsic color excesses. We find that the slope of the Ks-band luminosity function (KLF) of W3 Main is lower than the typical values reported for young embedded clusters. The derived slope of the KLF is the same as that found in 1996 by Megeath and coworkers, from which analysis indicated that the W3 Main region has an age in the range of 0.3-1 Myr. Based on the comparison between models of pre-main-sequence stars and the observed color-magnitude diagram, we find that the stellar population in W3 Main is primarily composed of low-mass pre-main-sequence stars. We also report the detection of isolated young stars with large infrared excesses that are most probably in their earliest evolutionary phases.

A NEAR-INFRARED STUDY OF THE NGC 7538 STAR-FORMING REGION

We present subarcsecond (FWHM ~ 0farcs7), near-infrared (NIR) JHKs-band images and a high-sensitivity radio continuum image at 1280 MHz, using SIRIUS on the University of Hawaii 88 inch (2.2 m) telescope and the Giant Metrewave Radio Telescope (GMRT). The NIR survey covers an area of ~24 arcmin^2 with 10 σ limiting magnitudes of ~19.5, 18.4, and 17.3 in the J, H, and Ks bands, respectively. Our NIR images are deeper than any JHK surveys to date for the larger area of the NGC 7538 star-forming region. We construct JHK color-color and J - H/J and H - K/K color-magnitude diagrams to identify young stellar objects (YSOs) and to estimate their masses. Based on these color-color and color-magnitude diagrams, we identified a rich population of YSOs (Class I and Class II) associated with the NGC 7538 region. A large number of red sources (H - K > 2) have also been detected around NGC 7538. We argue that these red stars are most probably pre-main-sequence stars with intrinsic color excesses. Most of the YSOs in NGC 7538 are arranged from the northwest toward the southeast regions, forming a sequence in age: a diffuse H II region (northwest and oldest, where most of the Class II and Class I sources are detected), a compact IR core (center), and regions with an extensive IR reflection nebula and a cluster of red young stars (southeast and south). We find that the slope of the Ks-band luminosity function of NGC 7538 is lower than the typical values reported for young embedded clusters, although equally low values have also been reported in the W3 Main star-forming region. From the slope of the Ks-band luminosity function and the analysis by Megeath and coworkers, we infer that the embedded stellar population is composed of YSOs with an age of ~1 Myr. Based on the comparison of models of pre-main-sequence stars with the observed color-magnitude diagram, we find that the stellar population in NGC 7538 is primarily composed of low-mass pre-main-sequence stars similar to those observed in the W3 Main star-forming region. The radio continuum image from the GMRT observations at 1280 MHz shows an arc-shaped structure due to the interaction between the H II region and the adjacent molecular cloud. The ionization front at the interface between the H II region and the molecular cloud is clearly delineated by comparing the radio continuum, molecular line, and NIR images.

Triggered star formation and evolution of T-Tauri stars in and around bright-rimmed clouds

The aim of this paper is to quantitatively testify the ‘small-scale sequential star formation’ hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al., we have carried out BVI c photometry of four more BRC aggregates along with deeper re-observations of two previously observed BRCs. Again, quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer/Infrared Array Camera data also support this result. The global distribution of near-infrared excess stars in each H ii region studied here clearly shows evidence that a series of radiation-driven implosion processes proceeded in the past from near the central O star(s) towards the peripheries of the H ii region. We found that in general weak-line T-Tauri stars (WTTSs) are somewhat older than classical T-Tauri stars (CTTSs). Also the fraction of CTTSs among the T-Tauri stars (TTSs) associated with the BRCs is found to decrease with age. These facts are in accordance with the recent conclusion by Bertout, Siess & Cabrit that CTTSs evolve into WTTSs. It seems that in general the equivalent width of Hα emission in TTSs associated with the BRCs decreases with age. The mass function (MF) of the aggregates associated with the BRCs of the morphological type ‘A’ seems to follow that found in young open clusters, whereas ‘B/C’-type BRCs show significantly steeper MF.

Stellar contents and star formation in the young star cluster Be 59

We present UBV I c CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ∼10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between E(B - V) ≃ 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as 3.7 ± 0.3. The distance of the cluster is found to be 1.00 ± 0.05 kpc. Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between < 1 and ∼2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ∼2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, Γ, in the mass range 2.5 < M/M ⊙ ≤ 28 is found to be -1.01 ± 0.11 which is shallower than the Salpeter value (-1.35), whereas in the mass range 1.5 < M/M ⊙ ≤ 2.5 the slope is almost flat. The slope of the K-band luminosity function is estimated as 0.27 ± 0.02, which is smaller than the average value (∼0.4) reported for young embedded clusters. Approximately 32 per cent of Ha emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust.

2 Millimeter Observations of Bright-rimmed Clouds with IRAS Point Sources

We have made 2 mm continuum observations of 15 bright-rimmed clouds (BRCs) associated with IRAS point sources and S140 with the Nobeyama Bolometer Array (NOBA) mounted on the 45 m telescope of Nobeyama Radio Observatory. Continuum emission was detected in five BRCs and in S140. These five BRCs are known to be associated with near-infrared YSO clusters, mostly on the side facing toward the exciting star(s) and, therefore, are likely the sites of small-scale sequential star formation. The detected emission peaks of these five sources correspond to the IRAS positions within the positional errors, and the IRAS sources are considered to be protostars (Class 0/I-like sources) that were formed most recently in the BRCs. Four of them are clearly extended more than the telescope beam, suggesting the presence of circumstellar structures. The circumstellar masses derived from the 2 mm continuum are ~5–90 M⊙, which are more massive than those of the nearby Class 0/I objects (1 M⊙). This fact and the relatively large bolometric luminosities of these objects suggest that the mass of the cluster or star(s) most recently formed in these BRCs could be higher than those of the previously formed stars found in the near-infrared cluster. The comparisons with previous observations of Bok globules unassociated with bright rims and other objects are discussed. Most noteworthy is that the ratios of the bolometric luminosity to the circumstellar mass are significantly higher for these BRCs than for Bok globules.

Star formation in bright-rimmed clouds and clusters associated with the W5 E H ii region

The aim of this paper is to present the results of photometric investigations of the central cluster of the W5 E H II region as well as a follow-up study of the triggered star formation in and around bright-rimmed clouds (BRCs). We have carried out wide-field UBVI C and deep VI C photometry of the W5 E H II region. A distance of ∼2.1 kpc and a mean age of ∼1.3 Myr have been obtained for the central cluster. The young stellar objects (YSOs) associated with the region are identified on the basis of near-infrared and mid-infrared observations. We confirmed our earlier results that the average age of the YSOs lying on/inside the rim is younger than those lying outside the rim. The global distribution of the YSOs shows an aligned distribution from the ionizing source to the BRCs. These facts indicate that a series of radiation-driven implosion processes proceeded from near the central ionizing source towards the periphery of the W5 E H II region. We found that, in general, the age distributions of the Class II and Class III sources are the same. This result is apparently in contradiction to the conclusion by Bertout, Siess & Cabrit and Chauhan et al. that classical T Tauri stars evolve to weak-line T Tauri stars. The initial mass function of the central cluster region in the mass range 0.4 < M/M ⊙ ≤ 30 can be represented by Γ = ―1.29 ± 0.04. The cumulative mass functions indicate that in the mass range 0.2 ≤ M/M ⊙ ≤ 0.8, the cluster region and BRC NW have more low-mass YSOs compared to BRCs 13 and 14.

The population in the background of open clusters: tracer of the Norma‐Cygnus arm

We present colour-magnitude diagrams of open clusters, located in the range 112° < l < 252°, manifesting stellar populations in the background of clusters. Some of the populations are found to be located beyond the Perseus arm and may be the part of the Norma-Cygnus (outer) arm. The outer arm seems to be continued from l ∼ 120° to ∼235°. The background populations follow the downward warp of the Galactic plane around l ∼ 240°.

Young stellar population of bright-rimmed clouds BRC 5, BRC 7 and BRC 39

Bright-rimmed clouds (BRCs), illuminated and shaped by nearby OB stars, are potential sites of recent/ongoing star formation. Here we present an optical and infrared photometric study of three BRCs: BRC 5, BRC 7 and BRC 39 to obtain a census of the young stellar population, thereby inferring the star formation scenario, in these regions. In each BRC, the Class I sources are found to be located mostly near the bright rim or inside the cloud, whereas the Class II sources are preferentially outside, with younger sources closer to the rim. This provides strong support to sequential star formation triggered by radiation-driven implosion due to the ultraviolet radiation. Moreover, each BRC contains a small group of young stars being revealed at its head, as the next-generation stars. In particular, the young stars at the heads of BRC 5 and BRC 7 are found to be intermediate-/high-mass stars, which, under proper conditions, may themselves trigger further star birth, thereby propagating star formation out to long distances.

A multiwavelength census of stellar contents in the young cluster NGC 1624

We present a comprehensive multiwavelength analysis of the young cluster NGC 1624 associated with the H II region Sh2-212 using optical UBVRI photometry, optical spectroscopy and GMRT radio continuum mapping along with the near-infrared (NIR) JHK archival data. From optical observations of the massive stars, reddening E(B - V) and distance to the cluster are estimated to be 0.76-1.00 mag and 6.0 ± 0.8 kpc, respectively. The present analysis yields a spectral class of O6.5V for the main ionizing source of the region, and the maximum post-main-sequence age of the cluster is estimated as ∼4 Myr. Detailed physical properties of the young stellar objects (YSOs) in the region are analysed using a combination of optical/NIR colour-colour and colour-magnitude diagrams. The distribution of YSOs in the (J - H)/ (H - K) NIR colour-colour diagram shows that a majority of them have A V ≤ 4 mag. However, a few YSOs show A V values higher than 4 mag. Based on the NIR excess characteristics, we identified 120 probable candidate YSOs in this region, which yield a disc frequency of ∼20 per cent. However, this should be considered as a lower limit. These YSOs are found to have an age spread of ∼5 Myr with a median age of ∼2―3 Myr and a mass range of ∼0.1― 3.0 M ⊙ . A significant number of YSOs are located close to the cluster centre and we detect an enhanced density of reddened YSOs located/projected close to the molecular clumps detected by Deharveng et al. at the periphery of NGC 1624. This indicates that the YSOs located within the cluster core are relatively older in comparison to those located/projected near the clumps. From the radio continuum flux, the spectral class of the ionizing source of the ultracompact H II (UCH II) region at the periphery of Sh2-212 is estimated to be ∼B0.5V. From the optical data, the slope of the mass function (MF) Γ, in the mass range 1.2 ≤ M/M ⊙ < 27, can be represented by a single power law with a slope ―1.18 ± 0.10, whereas the NIR data in the mass range 0.65 ≤ M/M ⊙ < 27 yield Γ = ―1.31 ± 0.15. Thus the MF agrees fairly with the Salpeter value. The slope of the K-band luminosity function (KLF) for the cluster is found to be 0.30 ± 0.06, which is in agreement with the values obtained for other young clusters.