arXiv subjects

S. Sato

Publications and source records attributed to S. Sato.

63 records · Page 4Linked to original sources

arXiv2004

Deep Near Infrared Imaging toward Vela Molecular Ridge C - 1: A Remarkable Embedded Cluster in RCW 36 -

We present deep near-infrared (J, H, Ks) images toward an embedded cluster which lies in a C18O clump in the 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 pc x 0.1 pc region of the cluster. This is much higher than most of the young clusters within 1 kpc of the Sun. From the 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.

astro-ph↗

arXiv2004

Ultra-stable performance of an underground-based laser interferometer observatory for gravitational waves

In order to detect the rare astrophysical events that generate gravitational wave (GW) radiation, sufficient stability is required for GW antennas to allow long-term observation. In practice, seismic excitation is one of the most common disturbances effecting stable operation of suspended-mirror laser interferometers. A straightforward means to allow more stable operation is therefore to locate the antenna, the ``observatory'', at a ``quiet'' site. A laser interferometer gravitational wave antenna with a baseline length of 20m (LISM) was developed at a site 1000m underground, near Kamioka, Japan. This project was a unique demonstration of a prototype laser interferometer for gravitational wave observation located underground. The extremely stable environment is the prime motivation for going underground. In this paper, the demonstrated ultra-stable operation of the interferometer and a well-maintained antenna sensitivity are reported.

gr-qc↗

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↗

arXiv2003

An Optical and Near Infrared Search for Brown Dwarfs in the Pleiades Cluster

We have carried out a brown dwarf search over an area of 14' x 23' near the central portion of the Pleiades open cluster in five optical and near infrared bands (i', Z, J, H, Ks) with 10 sigma detection limits of i' = 22.0, J = 20.0 and Ks = 18.5 mag. The surveyed area has large extinction in excess of Av = 3 in the Pleiades region. We detected four new brown dwarf candidates from the colour-colour (J-K, i'-J) and the colour-magnitude (J, i'-K) diagrams. We estimated their masses as 0.046 Msun down to 0.028 Msun. The least massive one is estimated to have a mass smaller than Roque 25 or int-pl-IZ-69, and maybe the lowest mass object found so far in the Pleiades cluster.

astro-ph↗

arXiv2003

A chemically driven fluctuating ratchet model for actomyosin interaction

With reference to the experimental observations by T. Yanagida and his co-workers on actomyosin interaction, a Brownian motor of fluctuating ratchet kind is designed with the aim to describe the interaction between a Myosin II head and a neighboring actin filament. Our motor combines the dynamics of the myosin head with a chemical external system related to the ATP cycle, whose role is to provide the energy supply necessary to bias the motion. Analytical expressions for the duration of the ATP cycle, for the Gibbs free energy and for the net displacement of the myosin head are obtained. Finally, by exploiting a method due to Sekimoto (1997, J. Phys. Soc. Jpn., 66, 1234), a formula is worked out for the amount of energy consumed during the ATP cycle.

cond-mat.stat-mech↗

arXiv2002

Evolution of Elliptical Galaxies at z >~ 1 Revealed from a Large, Multicolor Sample of Extremely Red Objects

We study the evolution of elliptical galaxies at z >~ 1 on the basis of a sample of 247 Extremely Red Objects (EROs) with R-Ks >= 3.35 (AB) and Ks <= 22.1 (AB) constructed from BVRi'z'JHKs multicolor data of a 114 arcmin2 area in the Subaru/XMM Deep Survey Field. By fitting template spectra of old ellipticals (OEs) and young, dusty starbursts (DSs) to the multicolor data, we classify EROs into these two classes and estimate their redshifts. We find that 58% of the EROs in our sample belong to the OE class and that 24% of the OEs are fit by a spectrum having a disk component with the B-band bulge-to-total luminosity ratio of <= 0.9. We compare the observed surface density of OEs with predictions from passive evolution models. We then derive rest-frame B-band luminosity functions (LFs) of OEs in our sample at z=1-1.5 and 1.5-2.5. We find that the LF at z=1-1.5 roughly agrees with the LF of local ellipticals if a dimming of 1.3 mag from z=1.25 to the present epoch is assumed. On the other hand, the amplitude of the LF at z=1.5-2.5 is found to be lower than that of the local LF by a factor of ~ 3 over the whole range of magnitude observed. Taking account of a strong decrease in the number density of morphologically classified early-type galaxies at z >~ 1.5 found by several authors, we conclude that the majority of ellipticals seen at present have not established either a red color or a smooth 1/4-law profile before z ~ 1.5$. (abridged)

astro-ph↗

arXiv2002

Behavior of Sulfur Abundances in Metal-Poor Giants and Dwarfs

LTE and NLTE abundances of sulfur in 6 metal-poor giants and 61 dwarfs (62 dwarfs, including the Sun) were explored in the range of -3 \lsim [Fe/H] \lsim $+0.5$ using high-resolution, high signal-to-noise ratio spectra of the \SI 8693.9 Åand 8694.6 Ålines observed by us and measured by \Francois (1987, 1988) and Clegg et al. (1981). NLTE effects in S abundances are found to be small and practically negligible. The behavior of [S/Fe] vs. [Fe/H] exhibits a linear increasing trend without plateau with decreasing [Fe/H]. Combining our results with those available in the literature, we find that the slope of the increasing trend is -0.25 in the NLTE behavior of [S/Fe], which is comparable to that observed in [O/Fe]. The observed behavior of S may require chemical evolution models of the Galaxy, in which scenarios of hypernovae nucleosynthesis and/or time-delayed deposition into interstellar medium are incorporated.

astro-ph↗

arXiv2000

Oxygen Abundance Determination in Very Metal-Deficient Giants: Permitted O I Lines Versus Forbidden [O I] Lines

The abundance of oxygen was determined for selected very metal-poor G-K stars (six giants and one turn-off star) based on the high S/N and high-resolution spectra observed with Keck HIRES in the red through near-IR region comprising the permitted O I lines (7771-5, 8446) along with the [O I] forbidden line at 6363 A. It turned out that both the abundances from the permitted line features, O I 7771-5 and O I 8446, agree quite well with each other, while the forbidden line yields somewhat discrepant and divergent abundances with a tendency of being underestimated on the average. The former (7773/8446) solution, which we believe to be more reliable, gives a fairly tight [O/Fe] vs. [Fe/H] relation such that increasing steadily from [O/Fe] = 0.6 (at [Fe/H] = -1.5) to [O/Fe] = 1.0 (at [Fe/H] = -3.0), in reasonable consistency with the trend recently reported based on the analysis of the UV OH lines. We would suspect that some kind of weakening mechanism may occasionally act on the formation of [O I] forbidden lines in metal-poor stars. Therefore, [O I] lines may not be so a reliable abundance indicator as has been generally believed.

astro-ph↗

arXiv2000

On Planetary Companions to the MACHO-98-BLG-35 Microlens Star

We present observations of microlensing event MACHO-98-BLG-35 which reached a peak magnification factor of almost 80. These observations by the Microlensing Planet Search (MPS) and the MOA Collaborations place strong constraints on the possible planetary system of the lens star and show intriguing evidence for a low mass planet with a mass fraction $4\times 10^{-5} \leq ε\leq 2\times 10^{-4}$. A giant planet with $ε= 10^{-3}$ is excluded from 95% of the region between 0.4 and 2.5 $R_E$ from the lens star, where $R_E$ is the Einstein ring radius of the lens. This exclusion region is more extensive than the generic "lensing zone" which is $0.6 - 1.6 R_E$. For smaller mass planets, we can exclude 57% of the "lensing zone" for $ε= 10^{-4}$ and 14% of the lensing zone for $ε= 10^{-5}$. The mass fraction $ε= 10^{-5}$ corresponds to an Earth mass planet for a lensing star of mass $\sim 0.3 \msun$. A number of similar events will provide statistically significant constraints on the prevalence of Earth mass planets. In order to put our limits in more familiar terms, we have compared our results to those expected for a Solar System clone averaging over possible lens system distances and orientations. We find that such a system is ruled out at the 90% confidence level. A copy of the Solar System with Jupiter replaced by a second Saturn mass planet can be ruled out at 70% confidence. Our low mass planetary signal (few Earth masses to Neptune mass) is significant at the $4.5σ$ confidence level. If this planetary interpretation is correct, the MACHO-98-BLG-35 lens system constitutes the first detection of a low mass planet orbiting an ordinary star without gas giant planets.

astro-ph↗