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K. Ogura

Publications and source records attributed to K. Ogura.

33 records · Page 2Linked to original sources
arXiv2012

Multiwavelength Study of NGC 281 Region

We present a multiwavelength study of the NGC 281 complex which contains the young cluster IC 1590 at the center, using deep wide-field optical UBVI_c photometry, slitless spectroscopy along with archival data sets in the near-infrared (NIR) and X-ray. The extent of IC 1590 is estimated to be ~6.5 pc. The cluster region shows a relatively small amount of differential reddening. The majority of the identified young stellar objects (YSOs) are low mass PMS stars having age <1-2 Myr and mass 0.5-3.5 M_\odot. The slope (Γ) of the mass function for IC 1590, in the mass range 2 < M/M_\odot \le 54, is found to be -1.11+-0.15. The slope of the K-band luminosity function (0.37+-0.07) is similar to the average value (~0.4) reported for young clusters. The distribution of gas and dust obtained from the IRAS, CO and radio maps indicates clumpy structures around the central cluster. The radial distribution of the young stellar objects, their ages, Δ(H-K) NIR-excess, and the fraction of classical T Tauri stars suggest triggered star formation at the periphery of the cluster region. However, deeper optical, NIR and MIR observations are needed to have a conclusive view of star formation scenario in the region. The properties of the Class 0/I and Class II sources detected by using the Spitzer mid-infrared observations indicate that a majority of the Class II sources are X-ray emitting stars, whereas X-ray emission is absent from the Class 0/I sources. The spatial distribution of Class 0/I and Class II sources reveals the presence of three sub-clusters in the NGC 281 West region.

astro-ph.GA↗
arXiv2012

Soft X-ray harmonic comb from relativistic electron spikes

We demonstrate a new high-order harmonic generation mechanism reaching the `water window' spectral region in experiments with multi-terawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving uJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.

physics.plasm-ph↗
arXiv2011

Star formation in bright-rimmed clouds and cluster associated with W5 E H{\sc ii} region

The aim of this paper is to present the results of photometric investigations of the central cluster of the W5 E 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{\sc ii} region. A distance of $\sim$2.1 kpc and a mean age of $\sim$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 are younger than those lying outside the rim. The global distribution of the YSOs shows an aligned distribution from the ionising source to the BRCs. These facts indicate that a series of radiation driven implosion processes proceeded from near the central ionising source towards the periphery of the W5 E H{\sc 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 with the conclusion by Bertout, Siess & Cabrit (2007) and Chauhan et al. (2009) 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 $0.4 \le M/M_\odot \le 30$ can be represented by $Γ= -1.29 \pm 0.03$. The cumulative mass functions indicate that in the mass range $0.2 \le M/M_\odot \le 0.8$, the cluster region and BRC NW have more low mass YSOs in comparison to BRCs 13 and 14.

astro-ph.SR↗
arXiv2010

A multi-wavelength census of stellar contents in the young cluster NGC 1624

We present a comprehensive multi-wavelength 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. 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. Present analysis yields a spectral class of O6.5V for the main ionizing source of the region. The distribution of YSOs in (J-H)/ (H-K) NIR colour-colour diagram shows that a majority of them have A_V $\le$ 4 mag. Based on the NIR excess characteristics, we identified 120 probable candidate YSOs in this region which yield a disk frequency of ~ 20%. 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 $M_\odot$. 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 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, spectral class of the ionizing source of the ultra-compact H II region at the periphery of Sh2-212 is estimated to be ~ B0.5V. From optical data, slope of the mass function (MF) $Γ$, in the mass range $1.2 \le M/M_{\odot}<27$ can be represented by a single power law with a slope -1.18 +/- 0.10, whereas the NIR data in the mass range $0.65 \le M/M_{\odot}<27$ yields $Γ$ = -1.31 +/- 0.15. 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.

astro-ph.SR↗
arXiv2009

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 "{\it small-scale sequential star formation}" hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al. (2007, Paper I), we have carried out $BVI_{c}$ photometry of four more BRC aggregates along with deeper re-observations of 2 previously observed BRCs. Again quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer/IRAC data also support this result. The global distribution of NIR excess stars in each HII region studied here clearly shows evidence that a series of radiation driven implosion (RDI) processes proceeded in the past from near the central O star(s) towards the peripheries of the HII 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 et al. (2007) that CTTSs evolve into WTTSs. It seems that in general the EW of H$α$ 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.

astro-ph.SR↗
arXiv2009

Novel path towards compact laser ion accelerators for hadron therapy: Tenfold energy increase in laser-driven multi-MeV ion generation using a gas target mixed with submicron clusters

We demonstrate generation of 10-20 MeV/u ions with a compact 4 TW laser using a gas target mixed with submicron clusters, corresponding to tenfold increase in the ion energies compared to previous experiments with solid targets. It is inferred that the high energy ions are generated due to formation of a strong dipole vortex structure. The demonstrated method has a potential to construct compact and high repetition rate ion sources for hadron therapy and other applications.

physics.plasm-ph↗
arXiv2008

Mass functions and photometric binaries in nine open clusters

Using homogeneous CCD photometric data from the 105-cm Kiso Schmidt telescope covering a 50' x 50' field, we study the mass functions (MFs) of nine open clusters. The ages and Galactocentric distances of the target clusters vary from 16 - 2000 Myr and 9-10.8 kpc, respectively. The values of MF slopes vary from -1.1 to -2.1. The classical value derived by Salpeter (1955) for the slope of the IMF is Γ= -1.35. The MFs in the outer regions of the clusters are found to be steeper than in the inner regions, indicating the presence of mass segregation in the clusters.The MF slopes (in the outer region as well as the whole cluster) undergo an exponential decay with the evolutionary parameter τ(= age/ relaxation time). It seems that the evaporation of low-mass members from outer regions of the clusters is not significant at larger Galactocentric distances. It is concluded that the initial mass function (IMF) in the anticentre direction of the Galaxy might have been steeper than the IMF in the opposite direction. A comparison of the observed CMDs of the clusters with synthetic CMDs gives a photometric binary content of ~40%.

astro-ph↗
arXiv2007

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

We present $UBVI_c$ CCD photometry of the young open cluster Stock 8 with the aim to study the basic properties and star formation scenario in this region. The radius of the cluster is found to be $\sim 6^{\prime}$ ($\sim 3.6$ 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 $2.05 \pm 0.10$ kpc. Using H$α$ slitless spectroscopy and 2MASS NIR data we identified H$α$ emission and NIR excess young stellar objects (YSOs), respectively. The colour-magnitude diagrams of these YSOs reveal that majority of these objects have ages between 1 to 5 Myr indicating a non-coeval star formation in the cluster. Massive stars in the cluster region reveal an average age of $\le$ 2 Myr. In the cluster region ($r \le 6^\prime$) the slope of the mass function (MF), $Γ$, in the mass range $\sim 1.0 \le M/M_\odot < 13.4$ can be represented by a power law having a slope of $-1.38\pm0.12$, which agrees well with Salpeter value (-1.35). In the mass range $0.3 \le M/M_\odot < 1.0$, the MF is also found to follow a power law with a shallower slope of $Γ= -0.58\pm 0.23$ indicating a break in the slope of the IMF at $\sim 1 M_\odot$. The slope of the $K$-band luminosity function for the cluster ($r \le 6^\prime$) is found to be $0.31\pm0.02$, which is smaller than the average value ($\sim$ 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. It appears that star formation activity in the Nebulous Stream and embedded cluster may be independent from that of Stock 8.

astro-ph↗
arXiv2007

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

We present $UBVI_C$ 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 $\sim$ 10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between $E(B-V) \simeq$ 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as $3.7\pm0.3$. The distance of the cluster is found to be $1.00\pm0.05$ kpc. Using near-infrared 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 $<1$ Myr to $\sim$ 2 Myr, whereas the mean age of the massive stars in the cluster region is found to be $\sim$ 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 $2.5 < M/M_\odot \le 28$ is found to be $-1.01\pm0.11$ which is shallower than the Salpeter value (-1.35), whereas in the mass range $1.5 < M/M_\odot \le 2.5$ the slope is almost flat. The slope of the K-band luminosity function is estimated as $0.27\pm0.02$, which is smaller than the average value ($\sim$0.4) reported for young embedded clusters. Approximately 32% of H$α$ emission stars of Be 59 exhibit NIR excess indicating that inner disks of the T-Tauri star (TTS) population have not dissipated. The 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.

astro-ph↗
arXiv2007

Relativistic Tennis with Photons: Demonstration of Frequency Upshifting by a Relativistic Flying Mirror through Two Colliding Laser Pulses

Since the advent of chirped pulse amplification1 the peak power of lasers has grown dramatically and opened the new branch of high field science, delivering the focused irradiance, electric fields of which drive electrons into the relativistic regime. In a plasma wake wave generated by such a laser, modulations of the electron density naturally and robustly take the shape of paraboloidal dense shells, separated by evacuated regions, moving almost at the speed of light. When we inject another counter-propagating laser pulse, it is partially reflected from the shells, acting as relativistic flying (semi-transparent) mirrors, producing an extremely time-compressed frequency-multiplied pulse which may be focused tightly to the diffraction limit. This is as if the counterstreaming laser pulse bounces off a relativistically swung tennis racket, turning the ball of the laser photons into another ball of coherent X-ray photons but with a form extremely relativistically compressed to attosecond and zeptosecond levels. Here we report the first demonstration of the frequency multiplication detected from the reflection of a weak laser pulse in the region of the wake wave generated by the driver pulse in helium plasma. This leads to the possibility of very strong pulse compression and extreme coherent light intensification. This Relativistic Tennis with photon beams is demonstrated leading to the possibility toward reaching enormous electromagnetic field intensification and finally approaching the Schwinger field, toward which the vacuum nonlinearly warps and eventually breaks, producing electron-positron pairs.

physics.plasm-ph↗
arXiv2006

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 $112^\circ < l < 252^\circ$, 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 Norma-Cygnus (outer) arm. The outer arm seems to be continued from $l\sim120^\circ$ to $l\sim235^\circ$. The background populations follow the downward warp of the Galactic plane around $l\sim240^\circ$.

astro-ph↗
arXiv2006

Wide Field CCD photometry around nine open clusters

In this paper we study the evolution of core and corona of nine open clusters using the projected radial density profiles derived from homogeneous CCD photometric data obtained through the 105-cm Kiso Schmidt telescope. The age and galactocentric distance of the target clusters varies from 16 Myr to 2000 Myr and 9 kpcto 10.8 kpc respectively. Barring Be 62, which is young open cluster, other clusters show a uniform reddening across the cluster region. The reddening in Be 62varies from $E(B-V)_{min}$= 0.70 mag to $E(B-V)_{max}$= 1.00 mag. The corona of six of the clusters in the present sample is 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 young cluster Be 62 may reflect the initial conditions in the parental molecular cloud. The other results of the present study are (i) Core radius `$r_c$' and corona size $`r_{cn}$'/cluster radius $`r_{cl}$' are linearly correlated. (ii) The $r_c/r_{cn}/r_{cl}$ are linearly correlated with the number of stars in that region. (iii) In the age range 10-1000 Myr, the core and corona shrink with age. (iv) 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.

astro-ph↗
arXiv2004

A near-infrared study of the NGC 7538 star forming region

We present sub-arcsecond (FWHM ~ 0".7), NIR JHKs-band images and a high sensitivity radio continuum image at 1280 MHz, using SIRIUS on UH 88-inch telescope and GMRT. The NIR survey covers an area of ~ 24 arcmin^2 with 10-sigma limiting mags of ~ 19.5, 18.4, and 17.3 in J, H, and Ks-band, respectively. Our NIR images are deeper than any JHK surveys to date for the larger area of NGC 7538 star forming region. We construct JHK CC and J-H/J and H-K/K CM diagrams to identify YSOs and to estimate their masses. Based on these CC and CM 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 PMS stars with intrinsic color excesses. Most of YSOs in NGC 7538 are arranged from the N-W toward S-E regions, forming a sequence in age: the diffuse H II region (N-W, oldest: where most of the Class II and Class I sources are detected); the compact IR core (center); and the regions with the extensive IR reflection nebula and a cluster of red young stars (S-E and S). We find that the slope of the KLF of NGC 7538 is lower than the typical values reported for the young embedded clusters, although equally low values have also been reported in the W3 Main star forming region. From the slope of the KLF and the analysis by Megeath et al. (1996), we infer that the embedded stellar population is comprised of YSOs with an age of ~ 1 Myr. Based on the comparison between models of PMS stars with the observed CM diagram we find that the stellar population in NGC 7538 is primarily composed of low mass PMS stars similar to those observed in the W3 Main star forming region.

astro-ph↗
arXiv2004

Deep near-infrared observations of 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 2.2m telescope. The near-infrared survey covers an area of ~ 24 sq. arcmin with 10 sigma limiting magnitudes of ~ 19.0, 18.1, and 17.3 in J, H, and Ks-band, respectively. We construct JHK color-color and J/J-H and K/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 YSOs is identified which 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 of W3 Main is lower than the typical values reported for the young embedded clusters. The derived slope of the KLF is the same as that found by Megeath et al. (1996), from which analysis by Megeath et al. indicates 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 with 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 which are most probably in their earliest evolutionary phases.

astro-ph↗