Solar Systems Around Nearby Red Dwarf Stars
While the overall pace of extrasolar planet discoveries continues to accelerate, less progress has been made in discovering planets around nearby red dwarf stars, which account for 75% of stars in the solar neighborhood. Recent analysis of data from the Kepler mission has demonstrated that a high fraction of red dwarfs host terrestrial planets (Dressing and Charbonneau 2013), but less is known about the populations of Jupiter-mass planets and substellar brown dwarf companions around such stars. Nearly all currently confirmed extrasolar planets have been discovered using transit or Doppler effect techniques. The former is geometrically biased towards planets with small orbits, while the latter is biased towards massive planets with short periods that exert large gravitational accelerations on their host stars. Astrometric techniques, which measure the positions of stars on the plane of the sky, are limited by the minimum detectable perturbation of a star's position due to a planet, but allow for the determination of orbit inclination and an accurate planetary mass.
Here we present results from 14 years of astrometric observations by the REsearch Consortium on Nearby Stars (RECONS, recons.org) from a mountaintop observatory (CTIO) in the Chilean Andes. Astrometric solutions are given for seven nearby stars with known planets: six red dwarfs (GJ 317, GJ 667C, GJ 581, GJ 849, GJ 876, and GJ 1214) and one more massive K dwarf (BD -10 3166). Observations have baselines of two to twelve years, and were made using the 0.9 m SMARTS telescope. Our astrometric techniques are most sensitive to Jupiter-mass planets in Jupiter-like orbits, and we find that the six red dwarfs have no brown dwarf or super-Jupiter companions. We provide improved parallaxes, and have used Monte Carlo simulations to determine the minimum detectable companion mass for each system. In the broader context, these results are consistent with the paucity of super-Jupiter and brown dwarf companions we find among the over 150 red dwarfs searched in our astrometric program. In our sample we find that at least 14% of red dwarfs have an equal or lower mass stellar companion, while less than 2% have brown dwarf companions, and none so far have super- Jupiter companions. Clearly much work remains to discover and understand the companions of our stellar neighbors.
This effort has been supported by the National Science Foundation via grant AST 09-08402 and the long-term cooperative efforts of the National Optical Astronomy Observatories and the members of the SMARTS Consortium.
References: Dressing, C. D. & Charbonneau, D. 2013, Astrophysical Journal, 767, 95, The Occurrence Rate of Small Planets Around Small Stars