We had a large HST project (105 orbits) designed to look for planets around stars in the Galactic bulge. This program is called SWEEPS (Sagittarius Window Eclipsing Extrasolar Planet Search, PI: K.C. Sahu) which was conceived as a transit survey that would fully exploit the high spatial resolution and high photometric precision capabilities of the Hubble Space Telescope. We used HST's Advanced Camera for Surveys (ACS) and chose a rich field, lying in the Sagittarius window of the Galactic bulge, which we monitored for periodic small dimmings over a continuous 7-day period. We discovered 16 candidates with orbital periods between 0.4 and 4.2 days, five of which orbit stars of 0.44 to 0.75 M_sun. In two cases, radial-velocity measurements support the planetary nature of the companions. Five candidates have orbital periods below 1.0 day, constituting a new class of ultra-short-period planets (USPPs), which occur only around stars of less than 0.88 M_sun. This indicates that those orbiting very close to more luminous stars might be evaporatively destroyed, or that jovian planets around lower-mass stars might migrate to smaller radii.
These results are published in:
Sahu, K.C. et al., 2006, Nature, 443, 1038 (astro-ph/0610098) .
Some publications on this topic are:
Beaulieu, J.P. et al. 2006, "Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing", Nature, 439, 43 (also: astro-ph/0601563)
Albrow, M. et al (i.e. PLANET Collaboration in alphabetical order), "Limits on the Abundance of Galactic Planets From Five Years of Planet Observations", 2001, ApJ, 556, L113 (also: astro-ph/0008078)
Sahu, K.C., ``Detecting Planets Through Microlensing'', Proc. of the ASP Conf. 119 on ``Planets beyond the solar system and the next generation of space missions", Ed. D. Soderblom, p. 73, 1997. (also: astro-ph/9704168)
Through independent determinations of the lens locations and other statistical arguments, I have shown that the stars within the Magellanic Clouds play a dominant role as gravitational lenses and that the contribution of MACHOs to the dark matter is small.
Some of my publications on this topic are:
Sahu, K.C., "Stars within the LMC as potential lenses for observed microlensing events", 1994, Nature, 370, 275. (also: astro-ph/9407065 )
Sahu, K.C., "Microlensing events of the LMC are better explained by stars within the LMC than by MACHOs", 1994, Pub. Astron. Soc. Pacific, 106, 942. (also: astro-ph/9408047)
Sahu, K.C., Sahu, M., "Spectroscopy of MACHO 97-SMC-1: self-lensing within the Small Magellanic Cloud", 1998, ApJ, 508, L147. (also: astro-ph/9810053)
Sahu, K.C., "Microlensing towards the Magellanic Clouds: Nature of the Lenses and Implications for Dark Matter", 2001, to appear in the Proceedings of STScI April Symposium on "The Dark Universe: Matter, Energy and Gravitation". (also: astro-ph/0302325)
I led a program of HST observations which showed, for the first time, that a GRB (GRB970228) was associated with an external galaxy, and that the GRB phenomenon is unrelated to the nuclear activity of the host galaxy.
Some publications on this topic are:
Sahu, K.C., Livio, M., Petro, et al. ``The optical counterpart to gamma-ray burst GRB970228 observed using the Hubble Space Telescope'', 1997, Nature, 387, 476. (also: astro-ph/9705184)
Sahu, K.C., et al. ``Discovery of the optical counterpart and early optical observations of GRB 990712", 2000, ApJ, 540, 74. (also: astro-ph/0003378)
I am also working on several other projects, some which are briefly described below.
(iv) Microlensing by high-proper motion stars: accurate mass determination of brown/white dwarfs (e.g. Bakos, G., Sahu, K.C., Nemeth, P., ``Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog", 2002, ApJ Suppl. Ser., 141, 187).
(v) Astrometric Microlensing of Stars (e.g. Dominik, M., Sahu, K.C., ``Astrometric microlensing of stars", 2000, ApJ, 534, 213)
(vi) Near-field Microlensing and its effects on Stellar Transit Observations by Kepler (e.g. Sahu, K. C., Gilliland, R. L., ``Near-Field Microlensing and Its Effects on Stellar Transit Observations by the Kepler Mission", 2003, ApJ, 584, 1042).
(vii) Imaging and spectroscopy of giant arcs, to measure mass and mass-to-light ratio in massive clusters. (e.g. Sahu, K.C., et al. ``Imaging and spectroscopy of arcs around the most luminous X-ray cluster RXJ 1347.5-1145'', 1998, ApJ, 492, L125).
(viii) Study of Planetary Nebulae (PN) in general, and young and proto-PN in particular (e.g. Bobrowsky, M., Sahu, K.C., Parthasarathy, M., Garcia-Lario, P.G., ``Birth and Rapid Evolution of the Stingray Nebula", 1998, Nature, 392, 469).