Hiroyuki Mito

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.

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.

Stellar contents and star formation in the young open cluster Stock 8

We present UBVIc CCD photometry of the young open cluster Stock 8 with the aim of studying its basic properties such as the amount of interstellar extinction, distance, age, stellar contents and initial mass function (IMF). We also studied the star formation scenario in this region. From optical data, the radius of the cluster is found to be ∼6 arcmin (∼3.6 pc) and the reddening within the cluster region varies from E(B − V) = 0.40 to 0.60 mag. The cluster is located at a distance of 2.05 ± 0.10 kpc. Using Hα slitless spectroscopy and Two Micron All Sky Survey (2MASS) near-infrared (NIR) data we identified Hα emission and NIR-excess young stellar objects (YSOs), respectively. From their locations in the colour‐magnitude diagrams, majority of them seem to have ages between 1 and 5 Myr. The spread in their ages indicate a possible noncoeval star formation in the cluster. Massive stars in the cluster region reveal an average age of 2 Myr. In the cluster region (r 6 arcmin) the slope of the mass function (MF), � , in the mass range ∼1.0 M/M� < 13.4 can be represented by a power law having a slope of −1.38 ± 0.12, which agrees well with Salpeter value (−1.35). In the mass range 0.3 M/M� < 1.0, the MF is also found to follow a power law with a shallower slope of � =− 0.58 ± 0.23 indicating a break in the slope of the IMF at ∼ 1M � . The slope of the K-band luminosity function for the cluster (r 6 arcmin) is found to be 0.31 ± 0.02, which is smaller than the average value (∼0.4) obtained for embedded star clusters. A significant number of YSOs are distributed along a Nebulous Stream towards the east side of the cluster. A small cluster is embedded in the Nebulous Stream. The YSOs lying in the Nebulous Stream and in the embedded cluster are found to be younger than the stars in the cluster Stock 8. The radio continuum, MSX, IRAS mid- and far-infrared maps and the ratio of [S II]/Hα intensities indicate that the eastern region of Stock 8 is ionization bounded whereas the western region is density bounded. The morphology seems to indicate that the ionization/shock front caused by the ionizing sources located in the Stock 8 region and westwards of Stock 8 has not reached the Nebulous Stream. It appears that star formation activity in the Nebulous Stream and embedded cluster may be independent from that of Stock 8.

Kiso Supernova Survey (KISS) : Survey strategy

The Kiso Supernova Survey (KISS) is a high-cadence optical wide-field supernova (SN) survey. The primary goal of the survey is to catch the very early light of a SN, during the shock breakout phase. Detection of SN shock breakouts combined with multi-band photometry obtained with other facilities would provide detailed physical information on the progenitor stars of SNe. The survey is performed using a 2.2x2.2 deg field-of-view instrument on the 1.05-m Kiso Schmidt telescope, the Kiso Wide Field Camera (KWFC). We take a three-minute exposure in g-band once every hour in our survey, reaching magnitude g~20-21. About 100 nights of telescope time per year have been spent on the survey since April 2012. The number of the shock breakout detections is estimated to be of order of 1 during our 3-year project. This paper summarizes the KISS project including the KWFC observing setup, the survey strategy, the data reduction system, and CBET-reported SNe discovered so far by KISS.

Multicolor Shallow Decay and Chromatic Breaks in the GRB 050319 Optical Afterglow

Multiwavelength observations of the optical afterglow of GRB 050319 were performed from 1.31 to 9.92 hr after the burst. Our R-band light curves, combined with other published data, can be described by the smooth broken power-law function, with α1 = -0.84 ± 0.02 to α2 = -0.48 ± 0.03, 0.04 days after the gamma-ray burst. The optical light curves are characterized by shallow decays—as was also observed in the X-rays—which may have a similar origin, related to energy injection. However, our observations indicate that there is still a puzzle concerning the chromatic breaks in the R-band light curve (at 0.04 days) and the X-ray light curve (at 0.004 days) that remains to be solved.

KWFC: four square degrees camera for the Kiso Schmidt Telescope

The Kiso Wide Field Camera (KWFC) is a facility instrument for the 105-cm Schmidt telescope being operated by the Kiso Observatory of the University of Tokyo. This camera has been designed for wide-field observations by taking advantage of a large focal-plane area of the Schmidt telescope. Eight CCD chips with a total of 8k x 8k pixels cover a field-of-view of 2.2 degrees x 2.2 degrees on the sky. The dewar window works as a field flattener lens minimizing an image distortion across the field of view. Two shutter plates moving in parallel achieve uniform exposures on all the CCD pixels. The KWFC is equipped with a filter exchanger composed of an industrial robotic arm, a filter magazine capable of storing 12 filters, and a filter holder at the focal plane. Both the arm and the magazine are installed inside the tube framework of the telescope but without vignetting the beam. Wide-field survey programs searching for supernovae and late-type variable stars have begun in April 2012. The survey observations are performed with a management software system for facility instruments including the telescope and the KWFC. This system automatically carries out observations based on target lists registered in advance and makes appropriate decisions for implementation of observations by referring to weather conditions and status of the instruments. Image data obtained in the surveys are processed with pipeline software in real time to search for candidates of time-variable sources.

AKARI/FIS Mapping of the ISM-Wind Bow Shock around

We present 100 500scan maps around an M supergiant Ori at 65, 90, 140, and 160 m obtained with the AKARI Infrared Astronomy Satellite. Higher spatial resolution data with the exact analytic solution permit us to fit the de-projected shape of the stellar-wind bow shock around Ori to have a stand-off distance of 4:08, position angle of 55i and inclination angle of 56i. The shape of the bow shock suggests that the velocity of Ori with respect to the local medium is v = 40n 1=2 H , where nH is the hydrogen nucleus density at Ori. We found that the local medium is of nH = 1.5 to 1.9 cm 3 and the velocity of the local flow is at 11 km s 1 by using the most recent astrometric solutions for Ori under the assumption that the medium local to Ori is moving away from the Orion OB 1 association. AKARI images may also reveal a vortex ring due to instabilities on the surface of the bow shock, as demonstrated by numerical models. This research exemplifies the potential of AKARI All-Sky data as well as follow-up observations with Herschel Space Telescope and Stratospheric Observatory for Infrared Astronomy for this avenue of research in revealing the nature of the interaction between the stellar wind and the interstellar medium.

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Aims. The circumstellar dust shells of intermediate initial-mass (∼ 1t o 8M� ) evolved stars are generated by copious mass loss during the asymptotic giant branch phase. The density structure of their circumstellar shell is the direct evidence of mass loss processes, from which we can investigate the nature of mass loss. Methods. We used the AKARI infrared astronomy satellite and the Spitzer space telescope to obtain the surface brightness maps of an evolved star R Cas at far-infrared wavelengths, since the temperature of dust decreases as the distance from the star increases and one needs to probe dust at lower temperatures, i.e., at longer wavelengths. The observed shell structure and the star’s known proper motion suggest that the structure represents the interface regions between the dusty wind and the interstellar medium. The deconvolved structures are fitted with the analytic bow shock structure to determine the inclination angle of the bow shock cone. . . .

Orionis

Observations were made of the optical afterglow of GRB 020813 (Fox, Blake, & Price) with the Kiso observatory 1.05 m Schmidt telescope and the Bisei astronomical observatory 1.01 m telescope. Four-band (B, V, R, and I) photometric data points were obtained on 2002 August 13 (10:52-16:46 UT), or 0.346-0.516 days after the burst. In order to investigate the early-time (<1 day) evolution of the afterglow, four-band light curves were produced by analyzing the data taken at these two astronomical observatories as well as the publicly released data taken using the Magellan Baade telescope (Gladders & Hall). The light curves can be approximated by a broken power law, of which the indices are approximately 0.46 and 1.33 before and after a break at ~0.2 days, respectively. The optical spectral index stayed approximately constant at ~0.9 over 0.17-4.07 days after the burst. Since the temporal decay index after the break and the spectral index measured at that time are both consistent with those predicted by a spherical expansion model, the early break is unlikely to be a jet break but is likely to represent the end of an early bump in the light curve, as was observed in the optical afterglow of GRB 021004.

The interface between the stellar wind and interstellar medium around R Cassiopeiae revealed by far-infrared imaging

The unusual pre–main-sequence binary star KH 15D undergoes remarkably deep and long-lasting periodic eclipses. Some clues about the reason for these eclipses have come from the observed evolution of the systems light curve over the last century. Here we present UBVRI photometry of KH 15D from 1954 to 1997 based on photographic plates from various observatories. The system has been variable at the ≈1 mag level since at least 1965. There is no evidence of color variations, with a typical limit of Δ(B - V) < 0.2 mag. We confirm some previously published results that were based on a smaller sample of plates: from approximately 1965 to 1990, the total flux was modulated with the 48 day orbital period of the binary, but the maximum flux was larger, the fractional variations were smaller, and the phase of minimum flux was shifted by almost a half-cycle relative to the modern light curve. All these results are consistent with the recently proposed theory that KH 15D is being occulted by an inclined, precessing, circumbinary ring.