A New Channel for the Detection of Planetary Systems through Microlensing. I. Isolated Events due to Planet Lenses

We propose and evaluate the feasibility of a new strategy to search for planets via microlensing observations. This new strategy is designed to detect planets in wide orbits, i.e., with orbital separation, a, greater than ~1.5RE. Planets in wide orbits may provide the dominant channel for the discovery of planets via microlensing, particularly low-mass (e.g., Earth-mass) planets. This paper concentrates on events in which a single planet serves as a lens, leading to an isolated event of short duration. We point out that a distribution of events due to lensing by stars with wide-orbit planets is necessarily accompanied by a distribution of shorter duration events. The fraction of events in the latter distribution is proportional to the average value of q1/2, where q is the ratio between planet and stellar masses. The position of the peak or peaks also provides a measure of the mass ratios typical of planetary systems. We study detection strategies that can optimize our ability to discover isolated short-duration events due to lensing by planets and find that monitoring employing sensitive photometry is particularly useful. If planetary systems similar to our own are common, even modest changes in detection strategy should lead to the discovery a few isolated events of short duration every year. We therefore also address the issue of the contamination due to stellar populations of any microlensing signal due to low-mass MACHOs. We describe how, even for isolated events of short duration, it will be possible to test the hypothesis that the lens was a planet instead of a low-mass MACHO, if the central star of the planetary system contributes a measurable fraction of the baseline flux.

[1]  E. Peng.  The Feasibility of Obtaining Finite Source Sizes from MACHO-Type Microlensing Light Curves , 1996, astro-ph/9608096.

[2]  H. Witt.  The Effect of the Stellar Size on Microlensing at the Baade Window , 1995 .

[3]  B. Peterson,et al.  A binary lensing event toward the LMC: Observations and dark matter implications , 1996 .

[4]  A. Gould,et al.  Stokes's Theorem Applied to Microlensing of Finite Sources , 1997 .

[5]  Bohdan Paczynski,et al.  Gravitational Microlensing in the Local Group , 1996 .

[6]  M. Kamionkowski.  Microlensing by Stars , 1994, astro-ph/9410062.

[7]  P. Bender,et al.  Multiresolution‐element imaging of extrasolar Earthlike planets , 1996 .

[8]  D. Soderblom.  Planets Beyond the Solar System and the Next Generation of Space Missions , 1997 .

[9]  M. J. Lehner,et al.  The MACHO Project LMC Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo , 1996 .

[10]  David P. Bennett,et al.  Detecting Earth-Mass Planets with Gravitational Microlensing , 1996, astro-ph/9603158.

[11]  S. Mao,et al.  Can lensed stars be regarded as pointlike for microlensing by MACHOs , 1994 .

[12]  Andrew Gould,et al.  Discovering Planetary Systems through Gravitational Microlenses , 1992 .

[13]  Andrew Gould,et al.  Proper Motions of MACHOs , 1994 .

[14]  D. Sasselov,et al.  Removing Degeneracy of Microlensing Light Curves through Narrowband Photometry of Giants , 1995, astro-ph/9504001.

[15]  A. Loeb,et al.  Microlensing of an Elliptical Source by a Point Mass , 1997, astro-ph/9702097.

[16]  Andrew Gould,et al.  Planet Parameters in Microlensing Events , 1996, astro-ph/9610123.

[17]  A. Esin,et al.  Blending of Light in Gravitational Microlensing Events , 1995, astro-ph/9506092.

[18]  Real-Time Detection of Gravitational Microlensing , 1995, astro-ph/9508039.

[19]  E. Milone,et al.  The origins, evolution, and destinies of binary stars in clusters , 1996 .

[20]  Austin B. Tomaney,et al.  Results from a Survey of Gravitational Microlensing toward M31 , 1996, astro-ph/9610065.

[21]  Christopher W. Stubbs,et al.  The macho project first-year large magellanic cloud results: The microlensing rate and the nature of the galactic dark halo , 1996 .

[22]  Cheongho Han.  Pixel Lensing Experiment Toward the M31 Bulge , 1995, astro-ph/9510161.

[23]  David G. Sandler,et al.  Optimization and Performance of Adaptive Optics for Imaging Extrasolar Planets , 1995 .

[24]  Antoine Labeyrie,et al.  Resolved imaging of extra-solar planets with future 10 100 km optical interferometric arrays , 1996, astro-ph/9602093.

[25]  C. Alcock.  Gravitational lenses , 1982, Nature.

[26]  M. J. Lehner,et al.  The MACHO Project: Limits on Planetary Mass Dark Matter in the Galactic Halo from Gravitational Microlensing , 1996, astro-ph/9604176.

[27]  B. Bromley.  Finite-Size Gravitational Microlenses , 1996 .

[28]  Goldenberg R.L. Etal.  Bed Rest in Pregnancy , 1994 .

[29]  R. Perna,et al.  Identifying Microlensing by Binaries , 1997, astro-ph/9702088.

[30]  Rosanne Di Stefano.  Microlensing and the Search for Extraterrestrial Life , 1999 .

[31]  S. Mao,et al.  Do Microlensing Events Repeat , 1996 .

[32]  M. J. Lehner,et al.  The Macho Project: 45 Candidate Microlensing Events from the First Year Galactic Bulge Data , 1997 .

[33]  J. Angel,et al.  An Imaging Nulling Interferometer to Study Extrasolar Planets , 1997 .

[34]  Joachim Wambsganss.  Discovering Galactic planets by gravitational microlensing: magnification patterns and light curves , 1997 .

[35]  A. Tomaney,et al.  MACHO Alert 95-30: First Real-Time Observation of Extended Source Effects in Gravitational Microlensing , 1997 .

[36]  Bohdan Paczynski,et al.  Gravitational microlensing by double stars and planetary systems , 1991 .

[37]  Austin B. Tomaney,et al.  Expanding the Realm of Microlensing Surveys with Difference Image Photometry , 1996 .

[38]  C. Kochanek,et al.  Astrophysical Applications of Gravitational Lensing , 1996 .