T. Yanagisawa

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

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 = 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 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 21.5 ± 2.9 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 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 Mlens=0.53 -->+ 0.52−0.35 M☉ and Dlens=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 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.

Study by MOA of extrasolar planets in gravitational microlensing events of high magnification

A search for extrasolar planets was carried out in three gravitational microlensing events of high magnification, MACHO 98-BLG-35, MACHO 99-LMC-2 and OGLE 00-BUL-12. Photometry was derived from observational images by the MOA and OGLE groups using an image subtraction technique. For MACHO 98-BLG-35, additional photometry derived from the MPS and PLANET groups was included. Planetary modelling of the three events was carried out in a supercluster computing environment. The estimated probability for explaining the data on MACHO 98-BLG-35 without a planet is < 1 per cent. The best planetary model has a planet of mass ∼ (0.4-1.5) X M E a r t h at a projected radius of either ∼ 1.5 or ∼ 2.3 au. We show how multiplanet models can be applied to the data. We calculate exclusion regions for the three events and find that Jupiter-mass planets can be excluded with projected radii from as wide as about 30au to as close as around 0.5 au for MACHO 98-BLG-35 and OGLE 00-BUL-12. For MACHO 99-LMC-2, the exclusion region extends out to around 10 au and constitutes the first limit placed on a planetary companion to an extragalactic star. We derive a particularly high peak magnification of ∼160 for OGLE 00-BUL-12. We discuss the detectability of planets with masses as low as Mercury in this and similar events.

Observation of the Halo of the Edge-On Galaxy IC 5249

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 flatten out at ~100 km s-1. The H I mass out to 24.5 kpc is ~6 × 109 M⊙, or 10% of the total mass out to this radius. The color, central surface brightness, scale height, and scale length of the disk of IC 5249 are R - I ≈ 0.4, μ = 20.6 ± 0.1RC mag arcsec-2, 600 ± 40 pc, and 11 ± 2 kpc, respectively. Additional light 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 27–28RC mag arcsec-2. The measured distribution of surface brightness is 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 M⊙ is excluded. We also find that less than 20% of the halo can be composed of main-sequence stars heavier than 0.30 M⊙. Further observations are 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.

Improving the prospects for detecting extrasolar planets in gravitational microlensing events in 2002

Gravitational microlensing events of high magnification have been shown to be promising targets for detecting extrasolar planets. However, only a few events of high magnification have been found using conventional survey techniques. Here we demonstrate that high-magnification events can be readily found in microlensing surveys using a strategy that combines high-frequency sampling of target fields with on-line difference imaging analysis. We present 10 microlensing events with peak magnifications greater than 40 that were detected in real-time towards the Galactic bulge during 2001 by the Microlensing Observations in Astrophysics (MOA) project. We show that Earth-mass planets can be detected in future events such as these through intensive follow-up observations around the event peaks. We report this result with urgency as a similar number of such events are expected in 2002.

Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud

ABSTRA C T 146 long-period red variable stars in the Large Magellanic Cloud (LMC) from the three-year Microlensing Observations in Astrophysics (MOA) project data base were analysed. A careful periodic analysis was performed on these stars and a catalogue of their magnitudes, colours, periods and amplitudes is presented. We convert our blue and red magnitudes to K-band values using 19 oxygen-rich stars. A group of red short-period stars separated from the Mira sequence has been found on a (log P,K) diagram. They are located at the short period side of the Mira sequence consistent with the work of Wood & Sebo. There are two interpretations for such stars; a difference in pulsation mode or a difference in chemical composition. We investigated the properties of these stars together with their colour, amplitude and periodicity. We conclude that they have small amplitudes and less regular variability. They are likely to be higher-mode pulsators. A large scatter has also been found on the longperiod side of the (log P,K) diagram. This is possibly a systematic spread given that the blue band of our photometric system covers both standard B and V bands and affects carbon-rich stars.

Determination of stellar shape in microlensing event MOA 2002-BLG-33

We report a measurement of the shape of the source star in microlensing event MOA 2002-BLG-33. The lens for this event was a close binary whose centre-of-mass passed almost directly in front of the source star. At this time, the source star was closely bounded on all sides by a caustic of the lens. This allowed the oblateness of the source star to be constrained. We found that a/b = 1.02 +0.04 -0.02 where a and b are its semi-major and semi-minor axes respectively. The angular resolution of this measurement is approximately 0.04 μarcsec. We also report HST images of the event that confirm a previous identification of the source star as an F8-G2 turn-off main-sequence star.

Multiple Outbursts of a Cataclysmic Variable in the Globular Cluster M22

We present a 4 yr light curve of a cataclysmic variable in M22, based on an analysis of accumulated data from the Microlensing Observations in Astrophysics (MOA) microlensing survey. The position of the star coincides with that of a transient event observed by the Hubble Space Telescope (HST) in 1999, originally attributed to microlensing but later suspected to be a dwarf nova outburst. Two outburst episodes, one in 2002 and one in 2003, with ΔI ~ 3 are seen in the MOA data, thus confirming that the HST event was a dwarf nova outburst. The MOA and HST data show that this dwarf nova underwent at least three outburst episodes during 1999-2004. Further close monitoring of this event is encouraged, as future outburst episodes are expected.

Photometry of Pulsating Stars in the Magellanic Clouds as Observed in the MOA Project

A review of the MOA (Microlensing Observations in Astrophysics) project is presented. MOA is a collaboration of approximately 30 astronomers from New Zealand and Japan established with the aim of finding and detecting microlensing events towards the Magellanic Clouds and the Galactic bulge, which may be indicative of either dark matter or of planetary companions. The observing program commenced in 1995, using very wide band blue and red filters and a nine-chip mosaic CCD camera. As a by-product of these observations a large database of CCD photometry for 1.4 million stars towards both LMC and SMC has been established. In one preliminary analysis 576 bright variable stars were confirmed, nearly half of them being Cepheids. Another analysis has identified large numbers of blue variables, and 205 eclipsing binaries are included in this sample. In addition 351 red variables (AGB stars) have been found. Light curves have been obtained for all these stars. The observations are carried out on a 61-cm f/6.25 telescope at Mt John University Observatory where a new larger CCD camera was installed in 1998 July. From this latitude (44 S) the Magellanic Clouds can be monitored throughout the year.