View Project: Gravitational Microlensing

Research Interests

My research efforts have focused on three areas which are:

(i) search for extrasolar planets,

(ii) the nature of dark matter, and

(iii) the nature of gamma-ray bursts.

(i) Search for extra-solar planets

(a) Through transits:

More than 200 extrasolar planets have been discovered around relatively nearby stars, primarily through the Doppler line shifts owing to the reflex motions of their host stars, and more recently through transits of some planets across the face of the host stars. The detection of planets with the shortest known periods, 1.2 to 2.5 days, has mainly resulted from transit surveys which have generally targeted stars more massive than 0.75 M_sun. Furthermore, almost all the planet detections so far have been around disk stars. As a result, our knowledge on the formation and frequency of planets is mostly confined to this single population.

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) .

(b) Through microlensing:

Gravitational microlensing provides a novel technique to look for planets around lens stars, and this is the only technique currently capable of detecting Earth-mass planets. Penny Sackett and I founded the PLANET collaboration in 1995 to search for planets through continuous monitoring of microlensing events, using a network of telescopes situated at appropriately spaced longitudes around the globe. After 10 years of intense monitoring of microlensing events, we discovered a 5.5 Earth-mass planetary companion at a separation of 2.8 AU from 0.2 solar-mass M-dwarf lens star. This is the lowest mass ever reported for an extrasolar planet orbiting a main sequence star. This detection suggests that such cool, sub-Neptune mass planets may be common, as predicted by the core accretion theory.

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)

(ii) Gravitational microlensing and implications on dark matter

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)

(iii) Gamma-ray bursts

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).

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