During the months when the galactic bulge is visible from the southern hemisphere, there are typically about 8 to 10 on-going microlensing events at any given time. If the lensing stars have planets around them, then the signature of the planets can be seen as sharp, extra peaks on the microlensing light curves. And if every lensing star has a Jupiter around it, then the probability of detecting an extra spike is of the order of 10%. Thus continuous and frequent monitoring of the on-going microlensing events, with a sampling interval of a few hours, provides a powerful new method to search for planets around lensing stars. Such monitoring programs are now being carried out using a network of 1-meter class telescopes situated at appropriately spaced longitudes in the southern hemisphere (for example, by PLANET collaboration). However, the galactic bulge is visible from the south pole throughout this period, and hence a single automated telescope at the south-pole can provide an efficient means of carrying out the monitoring program. Up to about 20 events can be monitored during a single 3-month season with a 1-meter telescope, potentially leading to the detection of two planetary signals. The telescope can also be used for several other research projects involving microlensing and variability of stars.

Kailash C. Sahu

We present the discovery observations of the optical counterpart of the gamma-ray burster GRB990712 taken 4.16 hours after the outburst and discuss its light curve observed in the V, R and I bands during the first ~35 days after the outburst. The observed light curves were fitted with a power-law decay for the optical transient (OT), plus an additional component which was treated in two different ways. First, the additional component was assumed to be an underlying galaxy of constant brightness. The resulting slope of the decay is 0.97+/-0.05 and the magnitudes of the underlying galaxy are: V = 22.3 +/- 0.05, R = 21.75 +/- 0.05 and I = 21.35 +/- 0.05. Second, the additional component was assumed to be a galaxy plus an underlying supernova with a time-variable brightness identical to that of GRB980425, appropriately scaled to the redshift of GRB990712. The resulting slope of the decay is similar, but the goodness-of-fit is worse which would imply that either this GRB is not associated with an underlying supernova or the underlying supernova is much fainter than the supernova associated with GRB980425. The galaxy in this case is fainter: V = 22.7 +/- 0.05, R = 22.25 +/- 0.05 and I = 22.15 +/- 0.05; and the OT plus the underlying supernova at a given time is brighter. Measurements of the brightnesses of the OT and the galaxy by late-time HST observation and ground-based observations can thus assess the presence of an underlying supernova.

Kailash C. Sahu

We present refined coordinates and proper motion data for the high proper motion (HPM) stars in the Luyten Half-Second (LHS) catalogue. The positional uncertainty in the original Luyten catalogue is typically >10" and is often >30". We have used the digital scans of the Palomar Observatory Sky Survey (POSS) I and POSS II plates to derive more accurate positions and proper motions of the objects. Out of the 4470 candidates in the LHS catalogue, 4323 objects were manually re-identified in the POSS I and POSS II scans. A small fraction of the stars were not found due to the lack of finder charts and digitized POSS II scans. The uncertainties in the revised positions are typically ~2", but can be as high as ~8" in a few cases; this is a large improvement over the original data. Cross-correlation with the Tycho-2 and Hipparcos catalogues yielded 819 candidates (with m_R < 12). For these brighter sources, the position and proper motion data have been replaced with the more accurate Tycho/Hipparcos data. In total, we have revised proper motion measurements and coordinates for 4040 stars and revised coordinates for 4330 stars, which are presented here.

Gaspar A. Bakos Kailash C. Sahu Peter Nemeth

In this paper we present the mass functions in the substellar regime of three young open clusters, IC 348, $\sigma$ Orionis and Pleiades, as derived using the data from the 2 Micron All Sky Survey (2MASS) catalogue which has a limiting magnitude of K_s ~ 15, and the latest version of the Guide Star Catalogue (GSC) which has a limiting magnitude of F ~ 21 where F refers to the POSS II IIIa-F passband. Based on recent evolutionary models for low mass stars, we have formulated the selection criteria for stars with masses below 0.5 solar mass. Using a statistical approach to correct for the background contamination, we derive the mass function of objects with masses ranging from 0.5 solar mass down to the substellar domain, well below the Hydrogen Burning Mass Limit. The lowest mass bins in our analysis are 0.025, 0.045 and 0.055 solar masses for IC 348, sigma Orionis and Pleiades, respectively. The resultant slopes of the mass functions are 0.8 +/- 0.2 for IC 348, 1.2 +/- 0.2 for sigma Orionis and 0.5 +/- 0.2 for Pleiades, which are consistent with the previous results. The contribution of objects below 0.5 solar mass to the total mass of the cluster is ~40%, and the contribution of objects below 0.08 solar mass to the total mass is ~4%.

Anandmayee Tej Kailash C. Sahu T. Chandrasekhar N. M. Ashok

We have carried out further analysis of the tentative, short-term brightenings reported by Sahu et al. (2001), which were suggested to be possible lensings of Galactic-bulge stars by free-floating planets in the globular cluster M22. Closer examination shows that--unlikely as it may seem--small, point-like cosmic rays had hit very close to the same star in both of a pair of cosmic-ray-split images, which cause the apparent brightenings of stars at the times and locations reported. We show that the observed number of double hits is consistent with the frequency of cosmic rays in WFPC2 images, given the number of stars and epochs observed. Finally, we point to ways in which cosmic rays can be more directly distinguished.

Kailash C. Sahu Jay Anderson Ivan R. King

More than a dozen microlensing events have been detected so far towards the LMC and 2 towards the SMC. If all the lenses are in the Galactic halo, both the LMC and the SMC events are expected to have similar time scales. However, the first event towards the SMC, MACHO 97-SMC-1, had a time scale of 123 days which is much larger than the typical time scale for the LMC events. From optical depth estimates, we first show that the stars within the SMC play a dominant role as gravitational lenses and can fully account for the observed microlensing events, mainly due to its large physical depth. We also show that if the lenses are within the Magellanic Clouds, then the SMC events should be longer in duration than the LMC events, a fact that is consistent with the observations. The time scale of the event implies that the mass of the lens is >2 solar masses if it is in the Milky Way disk or halo, in which case the lens is expected to be bright and should reveal itself in the spectrum. Here, we present an optical spectrum of MACHO 97-SMC-1 which shows that the lens is not in the Milky Way disk or halo, but is a low-mass star within the SMC. It is worth noting here that MACHO SMC-98-1 is the only OTHER observed event towards the SMC. This was a binary lens event where the caustic crossing time-scale as observed by PLANET, MACHO, EROS and OGLE collaborations, suggests that the lens is within the SMC. Furthermore, the only LMC event where the location of the lens is known is the binary lens event MACHO LMC-9, for which the lens is also within the LMC. Thus, ALL the 3 microlensing events towards the Magellanic Clouds for which we have independent knowledge of the location of the lenses are due to self-lensing within the Magellanic Clouds.

Kailash C. Sahu M. S. Sahu

The cluster RX J1347.5-1145, the most luminous cluster in the X-ray wavelengths, was imaged with the newly installed Space Telescope Imaging Spectrograph (STIS) on-board HST. Its relatively high redshift (0.451) and luminosity indicate that this is one of the most massive of all known clusters. The STIS images unambiguously show several arcs in the cluster. The largest two arcs (> 5 arcsec in length) are symmetrically situated on opposite sides of the cluster, at a distance of ~ 35 arcsec from the central galaxy. The STIS images also show approximately 100 faint galaxies within the radius of the arcs whose combined luminosity is ~ 4 x 10^11 Lsun. We also present ground-based spectroscopic observations of the northern arc which show one clear emission line at 6730 A, which is consistent with an identification as [OII] 3727 A, implying a redshift of 0.81 for this arc. The southern arc shows a faint continuum but no emission features. The surface mass within the radius of the arcs (240 kpc), as derived from the gravitational lensing, is 6.3 x 10^14 Msun. The resultant mass-to-light ratio of ~1200 is higher than what is seen in many clusters but smaller than the value recently derived for some `dark' X-ray clusters (Hattori et al. 1997). The total surface mass derived from the X-ray flux within the radius of the arcs is ~2.1 - 6.8 x 10^14 Msun, which implies that the ratio of the gravitational to the X-ray mass is ~1 to 3. The surface GAS mass within this radius is ~3.5 x 10^13 Msun, which implies that at least 6% of the total mass within this region is baryonic.

Kailash C. Sahu Richard A. Shaw Mary Elizabeth Kaiser Stefi A. Baum Henry C. Ferguson Jeffrey J. E. Hayes Theodore R. Gull Robert J. Hill John B. Hutchings Randy A. Kimble Philip Plait Bruce E. Woodgate

In this paper, we explore the astrophysical implications of near-field microlensing and its effects on stellar transit observations, with a special emphasis on the Kepler mission. Kepler is a NASA-approved mission whose goal is to detect a large number of extrasolar, earth-like planets by obtaining near-continuous photometry of > 100,000 F, G, and K dwarfs for four years. The expected photometric precision of Kepler is 90 micromag (achieved in 15 minute samples), at which the effect of microlensing by a transiting companion can be significant. For example, for a solar-type primary transited by a white-dwarf secondary, the maximum depth of the transit is 0.01%, which is almost entirely compensated by the microlensing amplification when the white dwarf is at ~0.05 AU. The combined effect of microlensing and transit increases to a net amplification of 150 micromag at an orbital separation of 0.1 AU, and 2.4 millimag at an orbital separation of 1 AU. Thus, the effect of microlensing can be used to break the degeneracy between a planetary-mass object for which the microlensing effect is negligible, and a more massive object of the same size. For brown dwarfs at orbital separations of a few AU, the effect of microlensing is several percent of the transit depth, and hence the microlensing effect must be taken into account in deriving the physical parameters of the brown dwarf. The microlensing signal caused by a neutron star or a black hole in a binary can be several millimag, far exceeding the transit depth, and potentially detectable even from ground-based observations. Kepler will be sensitive to white dwarfs, neutron stars, and black holes in binaries through their microlensing signatures. These observations can be used to derive the frequency of such compact objects in binaries, and to determine their masses.

Kailash C. Sahu Ronald L. Gilliland

(Abridged) A close scrutiny of the microlensing results towards the Magellanic clouds reveals that the stars within the Magellanic clouds are major contributors as lenses, and the contribution of MACHOs to dark matter is 0 to 5%. The principal results which lead to this conclusion are the following. (i) Out of the ~17 events detected so far towards the Magellanic Clouds, the lens locations have been determined for four events where the lenses are most likely within the Magellanic clouds. (ii) If caused by MACHOs, the event timescales would imply that the MACHOs in the direction of the LMC and the SMC have masses of the order of 0.5 and 2 solar masses, respectively. This is inconsistent with even the most flattened model of the Galaxy. If caused by stars within the Magellanic Clouds, the masses of the lenses are of the order of 0.2 solar mass for both the LMC as well as the SMC. (iii) The fact that the two observed binary events are caused by lenses within the Magellanic Clouds would suggest that there should be a total of ~20 events caused by lenses within the Magellanic Clouds. This implies that most of the microlensing events observed so far are probably caused by stars within the Magellanic Clouds. (iv) If the microlensing events are caused by MACHOs of 0.5 solar mass, ~15 events should have been detected by now towards the SMC, with timescales of ~40 days. The fact that both the events detected towards the SMC have been shown to be due to self-lensing places severe constraints on the MACHO contribution and suggests that the contribution of MACHOs to dark matter is consistent with zero, with an upper limit of 5%.

Kailash C. Sahu

The exoplanets discovered so far have been mostly around relatively nearby and bright stars. As a result, the host stars are mostly (i) in the Galactic disk, (ii) relatively massive, and (iii) relatively metal rich. The aim of the SWEEPS project is to extend our knowledge to stars which (i) are in a different part of the Galaxy, (ii) have lower masses, and (iii) have a large range of metallicities. To achieve this goal, we used the Hubble Space Telescope to search for transiting planets around F, G, K, and M dwarfs in the Galactic bulge. We photometrically monitored 180,000 stars in a dense bulge field continuously for 7 days. We discovered 16 candidate transiting extrasolar planets with periods of 0.6 to 4.2 days, including a new class of ultra-short period planets (USPPs) with P < 1.2 days. Radial-velocity observations of the two brightest candidates support their planetary nature. These results suggest that planets are as abundant in the Galactic bulge as they are in the solar neighborhood, and they are equally abundant around low-mass stars (within a factor 2). The planet frequency increases with metallicity even for the stars in the Galactic bulge. All the USPP hosts are low-mass stars, suggesting either that close-in planets around higher-mass stars are irradiatively evaporated, or that the planets can migrate to close-in orbits only around such old and low-mass stars.

Kailash C. Sahu Stefano Casertano Jeff Valenti Howard E. Bond Thomas M. Brown T. Ed Smith Will Clarkson Dante Minniti Manuela Zoccali Mario Livio Alvio Renzini R. M. Rich Nino Panagia Stephen Lubow Timothy Brown Nikolai Piskunov