A B S T R A C TWe 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 deg 2 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.

We analyze the data of the gravitational microlensing survey carried out by the Microlensing Observations in Astrophysics (MOA) group during 2000 toward the Galactic bulge (GB). Our observations are designed to detect efficient high-magnification events with faint source stars and short-timescale events, by increasing the sampling rate up to $6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg 2 . We use Monte Carlo simulations to estimate our microlensing event detection efficiency, where we construct the I-band extinction map of our GB fields in order to find dereddened magnitudes. We find a systematic bias and large uncertainty in the measured value of the timescale t E,out in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth ¼ 2:59 þ0:84 À0:64 Â 10 À6 toward the GB for events with timescales 0:3 < t E < 200 days. We consider disk-disk lensing, and obtain an optical depth bulge ¼ 3:36 þ1:11 À0:81 Â 10 À6 ½0:77=ð1 À f disk Þ for the bulge component assuming a 23% stellar contribution from disk stars. These observed optical depths are consistent with previous measurements by the MACHO and OGLE groups, and still higher than those predicted by existing Galactic models. We present the timescale distribution of the observed events, and find there are no significant short events of a few days, in spite of our high detection efficiency for short-timescale events down to t E $ 0:3 days. We find that half of all our detected events have high magnification (>10). These events are useful for studies of extrasolar planets.

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 θ min /θ 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 ± 12R ⊙ , 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 −1 kpc −1 , 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 M ⊙ , 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 M ⊙ and D lens = 6.57 +0.99 −2.25 kpc. Spectra taken during the event show that the absorption line equivalent widths of Hα and the TiO bands near 6700Å 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.Subject headings: dark matter -gravitational lensing -stars: low-mass, brown dwarfsstars: late-type -stars: atmospheres Table 4. Photometry of the source star in MACHO 95-30Observed Extinction Dereddened Abs Mag, 8 kpc Abs Mag, 9 kpc V = 16.21 A V = 1.35 V 0 = 14.86 M V = +0.34 M V = +0.59 K = 9.98 A K = 0.15 K 0 = 9.83 M K = −4.69 M K = −4.45 V − R = 1.39 E(V − R) = 0.34 V − R 0 = 1.05 J − K = 1.12 E(J − K) = 0.23 J − K 0 = 0.89 H − K = 0.26 E(H − K) = 0.08 H − K 0 = 0.18 V − K = 6.23 E(V − K) = 1.21 V − K 0 = 5.03 Bolometric BC K = −2.7 ± 0.1 M bol = −2.0 M bol = −2.25

We present observations of the microlensing event MACHO 98-BLG-35, which reached a peak magniÐcation 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 ¹ v ¹ 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 "" lensing zone,ÏÏ which is 0.6È1.6For smaller mass planets, we can exclude 57% of the "" lensing zone ÏÏ 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 D0.3A number of similar events will provide sta-M _ . tistically signiÐcant 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 Ðnd that such a system is ruled out at the 90% conÐdence level. A copy of the solar system with Jupiter replaced by a second Saturn-mass planet can be ruled out at 70% conÐdence. Our low-mass planetary signal (few Earth masses to Neptune mass) is signiÐcant at the 4.5 p conÐdence level. If this planetary interpretation is correct, the MACHO 98-BLG-35 lens system constitutes the Ðrst detection of a low-mass planet orbiting an ordinary star without gas giant planets.20

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Large‐scale astrometric and photometric data bases have been used to search for and confirm stellar membership of the open cluster IC 2391. 125 stars were found that satisfied criteria for membership based on proper motion components and BRI photometry from the United States Naval Observatory B (USNO‐B) catalogue and JHK photometry from the Two Micron All Sky Survey (2MASS) catalogue. This listing was compared with others recently published. A distance to the cluster of 147.7 ± 5.5 pc was found with mean proper motion components, from the Tycho2 catalogue of (−25.04 ± 1.53 mas yr−1; +23.19±1.23 mas yr−1). A revised Trumpler classification of II3r is suggested. Luminosity and mass functions for the candidate stars were constructed and compared with those of field stars and other clusters.

Presently scientists are working at Mt. John University Observatory in New Zealand using a technique known as gravitational microlensing to search for dark matter in our galaxy. This paper describes the current situation of the international collaboration known as MOA (Microlensing Observations in Astrophysics) that was formed by these scientists, together with future plans to improve the facilities. at Univ of Iowa-Law Library on May 27, 2015 http://ptps.oxfordjournals.org/ Downloaded from * ) Incidentally, MOA is also the name of a very large native bird which became extinct in New Zealand around two hundred years ago. at Univ of Iowa-Law Library on May 27, 2015 http://ptps.oxfordjournals.org/ Downloaded from * ) A second enhancement of the light curve was observed around 1084 days. This must be a variable star.

Optical photometry and H I synthesis observations of the southern edge-on Sc/Sd galaxy IC 5249 are reported. The rotation curve rises linearly out to a radius of 7 kpc and then appears to Ñatten out at D100 km s~1. The H I mass out to 24.5 kpc is D6 ] 109 or 10% of the total mass out to this M _ , radius. The color, central surface brightness, scale height, and scale length of the disk of IC 5249 are R [ I B 0.4, mag arcsec~2, 600^40 pc, and 11^2 kpc, respectively. Additional light k \ 20.6^0.1 R C to that predicted by an exponential disk is present at distances greater than 3 kpc from the disk. At 5 kpc the surface brightness is mag arcsec~2. The measured distribution of surface brightness is 27È28 R C used to constrain the abundance of low-mass main-sequence stars in the halo of the galaxy. A halo made up entirely of main-sequence stars heavier than 0.13 is excluded. We also Ðnd that less than 20% of M _ the halo can be composed of main-sequence stars heavier than 0.30Further observations are M _ . required to determine the rotation curve of IC 5249 to large radii and to determine precisely the abundance of low-mass main-sequence stars in the halo of the galaxy.