Y. Nakajima

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.

Deep Near-Infrared Imaging toward the Vela Molecular Ridge C. I. A Remarkable Embedded Cluster in RCW 36

We present deep near-infrared (J, H, and Ks) images toward an embedded cluster that lies in a C18O clump in cloud C of the Vela Molecular Ridge. This cluster has at least ~350 members and a radius of ~0.5 pc. The stellar surface number density is approximately 3000 stars pc-2 in the central 0.1 × 0.1 pc region of the cluster. This is much higher than most of the young clusters within 1 kpc of the Sun. From a comparison of the luminosity function and near-infrared excess fraction with those of other embedded clusters, we estimate that the age of this cluster is approximately 2-3 Myr. This cluster exhibits an excess of brighter stars in its central region, from which we conclude that the more massive stars are located near the cluster center.

Young Brown Dwarfs in the Core of the W3 Main Star-Forming Region

We present the results of deep and high-resolution (FWHM ~ 035) JHK near-infrared (NIR) observations with the Subaru telescope, to search for very low mass young stellar objects in the W3 Main star-forming region. The NIR survey covers an area of ~ 2.6 arcmin2 with 10σ limiting magnitude exceeding 20 mag in the JHK bands. The survey is sensitive enough to provide unprecedented details in W3 IRS 5 and IRS 3a regions and reveals a census of the stellar population down to objects below the hydrogen-burning limit. We construct JHK color-color and J – H/J and H – K/K color-magnitude diagrams to identify very low luminosity young stellar objects and to estimate their masses. Based on these color-color and color-magnitude diagrams, we identified a rich population of embedded YSO candidates with infrared excesses (Class I and Class II), associated with the W3 Main region. A large number of red sources (H – K > 2) have also been detected around W3 Main, which are arranged from the northwest toward the southeast regions. Most of these are concentrated around W3 IRS 5. We argue that these red stars are most probably pre-main-sequence (PMS) stars with intrinsic color excesses. We find that the slope of the K-band luminosity function of W3 Main is lower than the typical values reported for young embedded clusters. Based on the comparison between theoretical evolutionary models of very low mass PMS objects with the observed color-magnitude diagram, we find there exists a substantial substellar population in the observed region. The mass function does not show the presence of cutoff and sharp turnover around the substellar limit, at least at the hydrogen-burning limit. Furthermore, the mass function slope indicates that the number ratio of young brown dwarfs and hydrogen-burning stars in the W3 Main is probably higher than those in Trapezium and IC 348. The presence of mass segregation, in the sense that relatively massive YSOs lie near the cluster center, is seen. The estimated dynamical evolution time indicates that the observed mass segregation in the W3 Main may be the imprint of the star formation process.

One Month of Near-IR Imaging Photometry of Comet 9P/Tempel 1

We performed a month-long near-IR photometric and imaging observations of comet 9P/Tempel 1 in South Africa IRSF/SIRIUS. Our monitoring of comet brightness shows that a sustained comet activity and outbursts were not triggered by the impact. No color change was observed within 3

Polarimetry for the Disks around Young Brown Dwarfs

σ

Deep Near-Infrared Surveys and Young Brown Dwarf Populations in Star-Forming Regions

12 hours after the impact. This suggests the color change found by other IR observations by the impact may be a short-period phenomenum. Our observations of ejecta distribution detected large change in ejecta trajectory due to the solar radiation pressure.

NIR Survey of Variable Stars in the Large Magellanic Cloud

We report the near‐infrared (NIR) polarimetry of 34 young brown dwarfs (YBDs). This is the first NIR linear polarization survey for YBDs in the Taurus, rho Ophiuchi, and Chamaeleon dark clouds. The observations were conducted with SIRPOL on the IRSF 1.4‐m telescope in South Africa. SIRPOL is a polarimeter which has the capability to conduct the JHKs simultaneous imaging polarimetry. Our targets are inferred to have circumstellar disks based on optical and NIR spectroscopy and their spectral energy distribution (SED). The spectral types of these targets range from M6.25 to M9.5 and the masses range from about 80 to 10 MJupiter. We have detected significant linear polarization from 16 YBDs. In previous studies for YBD disks, the observed SEDs have been well fit by the flat or flared disk models, thus we consider significant polarization of three YBDs is due to photon scattering by the dust in circumstellar disks rather than in envelopes. Highly polarized sources serve as ‘direct‘ evidence for possible disk ...

Near-Infrared Study of M16: Star Formation in the Elephant Trunks

We are currently conducting three kinds of IR surveys of star forming regions (SFRs) in order to seek for very low-mass young stellar populations. First is a deep JHKs-bands (simultaneous) survey with the SIRIUS camera on the IRSF 1.4m or the UH 2.2m telescopes. Second is a very deep JHKs survey with the CISCO IR camera on the Subaru 8.2m telescope. Third is a high resolution companion search around nearby YSOs with the CIAO adaptive optics coronagraph IR camera on the Subaru. In this contribution, we describe our SIRIUS camera and present preliminary results of the ongoing surveys with this new instrument.

Simultaneous X-ray, radio, near-infrared and optical monitoring of young stellar objects in the Coronet cluster

The preliminary results of our ongoing NIR survey of the variable red giants in the Large Magellanic Cloud are presented. In the observed color-magnitude diagram (J — K s , K s ) and the stellar K s magnitude distribution, the tip of the first giant branch (TRGB, Ks 12.3 Nikolaev & Weinberg 2000) is clearly seen. Apart from the genuine AGB variables, we found many variable stars that populate just below the TRGB. From this result, we infer that most, if not all, of them are RGB variables