Yoshiyuki Watase

Bio: Yoshiyuki Watase is an academic researcher from KEK. The author has contributed to research in topics: Gravitational microlensing & Annihilation. The author has an hindex of 19, co-authored 113 publications receiving 1982 citations.

Real-time difference imaging analysis of moa galactic bulge observations during 2000

Abstract: We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg2 were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.

MACHO Alert 95-30: First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

Abstract: We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event toward the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model Alert 95-30 was observed in real time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event We interpret the light-curve fine structure as indicating transit of the lens across the extended face of the source star This signifies resolution of a star several kiloparsecs distant We find a lens angular impact parameter θmin/θsource = 0715 ± 0003 This information, along with the radius and distance of the source, provides an additional constraint on the lensing system Spectroscopic and photometric data indicate the source is a M4 III star of radius 61 ± 12 R☉, located on the far side of the bulge at ~9 kpc We derive a lens angular velocity, relative to the source, of 215 ± 29 km s-1 kpc-1, where the error is dominated by uncertainty in the angular size of the source star Likelihood analysis yields a median lens mass of 067 -->+ 253−046 M☉, located with 80% probability in the Galactic bulge at a distance of 693 -->+ 156−225 kpc If the lens is a main-sequence star, we can include constraints on the lens luminosity This modifies our estimates to Mlens=053 -->+ 052−035 M☉ and Dlens=657 -->+ 099−225 kpc Spectra taken during the event show that the absorption-line equivalent widths of Hα and the TiO bands near 6700 A vary, as predicted for microlensing of an extended source This is most likely due to center-to-limb variation in the stellar spectral lines The observed spectral changes further support our microlensing interpretation These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly

MACHO Alert 95-30 : First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

Abstract: We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event towards the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model. Alert 95-30 was observed in real-time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event. We interpret the light-curve ``fine structure'' as indicating transit of the lens across the extended face of the source star. This signifies resolution of a star several kpc distant. We find a lens angular impact parameter theta_{min}/theta_{source} = 0.715 +/- 0.003. This information, along with the radius and distance of the source, provides an additional constraint on the lensing system. Spectroscopic and photometric data indicate the source is an M4 III star of radius 61 +/- 12 Rsun, located on the far side of the bulge at 9 kpc. We derive a lens angular velocity, relative to the source, of 21.5 +/- 4.9 km/s/kpc, where the error is dominated by uncertainty in the source radius. Likelihood analysis yields a median lens mass of 0.67{+2.53}{-0.46} Msun, located with 80% probability in the Galactic bulge at a distance of 6.93{+1.56}{-2.25} kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to M_lens = 0.53{+0.52}{-0.35} Msun and D_lens = 6.57{+0.99}{-2.25} kpc. Spectra taken during the event show that the absorption line equivalent widths of H alpha and the TiO bands near 6700 A vary, as predicted for microlensing of an extended source. This is most likely due to center-to-limb variation in the stellar spectral lines. These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly.

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

Abstract: We present observations of the microlensing event MACHO 98-BLG-35, which reached a peak mag- ni—cation factor of almost 80. These observations by the Microlensing Planet Search (MPS) and 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 ) 10~5 " v " 2 ) 10~4. A giant planet with v \ 10~3 is excluded from 95% of the region between 0.4 and 2.5 from the lens star, where is R E R E the Einstein ring radius of the lens. This exclusion region is more extensive than the generic ii lensing zone,ˇˇ which is 0.6¨1.6 For smaller mass planets, we can exclude 57% of the ii lensing zone ˇˇ for R E. v \ 10~4 and 14% of the lensing zone for v \ 10~5. The mass fraction v \ 10~5 corresponds to an Earth-mass planet for a lensing star of mass D0.3 A number of similar events will provide sta- M _ . tistically signi—cant 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 —nd that such a system is ruled out at the 90% con—dence level. A copy of the solar system with Jupiter replaced by a second Saturn-mass planet can be ruled out at 70% con—dence. Our low-mass planetary signal (few Earth masses to Neptune mass) is sig- ni—cant at the 4.5 p con—dence level. If this planetary interpretation is correct, the MACHO 98-BLG-35 lens system constitutes the —rst detection of a low-mass planet orbiting an ordinary star without gas giant planets.20 Subject headings: gravitational lensingplanetary systemsstars: low-mass, brown dwarfs

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

Abstract: 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 $4\times 10^{-5} \leq \epsilon \leq 2\times 10^{-4}$. A giant planet with $\epsilon = 10^{-3}$ 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 $\epsilon = 10^{-4}$ and 14% of the lensing zone for $\epsilon = 10^{-5}$. The mass fraction $\epsilon = 10^{-5}$ 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 $4.5\sigma$ 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.

Zonal flows in plasma—a review

Abstract: A comprehensive review of zonal flow phenomena in plasmas is presented. While the emphasis is on zonal flows in laboratory plasmas, planetary zonal flows are discussed as well. The review presents the status of theory, numerical simulation and experiments relevant to zonal flows. The emphasis is on developing an integrated understanding of the dynamics of drift wave–zonal flow turbulence by combining detailed studies of the generation of zonal flows by drift waves, the back-interaction of zonal flows on the drift waves, and the various feedback loops by which the system regulates and organizes itself. The implications of zonal flow phenomena for confinement in, and the phenomena of fusion devices are discussed. Special attention is given to the comparison of experiment with theory and to identifying directions for progress in future research.

The New Galaxy: Signatures of Its Formation

Abstract: ▪ Abstract The formation and evolution of galaxies is one of the great outstanding problems of astrophysics. Within the broad context of hierachical structure formation, we have only a crude picture of how galaxies like our own came into existence. A detailed physical picture where individual stellar populations can be associated with (tagged to) elements of the protocloud is far beyond our current understanding. Important clues have begun to emerge from both the Galaxy (near-field cosmology) and the high redshift universe (far-field cosmology). Here we focus on the fossil evidence provided by the Galaxy. Detailed studies of the Galaxy lie at the core of understanding the complex processes involved in baryon dissipation. This is a necessary first step toward achieving a successful theory of galaxy formation.