Abstract
Breakthrough direct detections of planetary companions orbiting A-type stars confirm the existence of massive planets at relatively large separations, but dedicated surveys are required to estimate the frequency of similar planetary systems. To measure the first estimation of the giant exoplanetary systems frequency at large orbital separation around A-stars, we have conducted a deep-imaging survey of young (8−400 Myr), nearby (19−84 pc) A- and F-stars to search for substellar companions in the ~10−300 AU range. The sample of 42 stars combines all A-stars observed in previous AO planet search surveys reported in the literature with new AO observations from VLT/NaCo and Gemini/NIRI. It represents an initial subset of the International Deep Planet Survey (IDPS) sample of stars covering M- to B-stars. The data were obtained with diffraction-limited observations in H- and Ks-band combined with angular differential imaging to suppress the speckle noise of the central stars, resulting in typical 5σ detection limits in magnitude difference of 12 mag at 1′′, 14 mag at 2′′ and 16 mag at 5′′ which is sufficient to detect massive planets. A detailed statistical analysis of the survey results is performed using Monte Carlo simulations. Considering the planet detections, we estimate the fraction of A-stars having at least one massive planet (3−14 MJup) in the range 5−320 AU to be inside 5.9−18.8% at 68% confidence, assuming a flat distribution for the mass of the planets. By comparison, the brown dwarf (15−75 MJup) frequency for the sample is 2.0−8.9% at 68% confidence in the range 5−320 AU. Assuming power law distributions for the mass and semimajor axis of the planet population, the AO data are consistent with a declining number of massive planets with increasing orbital radius which is distinct from the rising slope inferred from radial velocity (RV) surveys around evolved A-stars and suggests that the peak of the massive planet population around A-stars may occur atseparations between the ranges probed by existing RV and AO observations. Finally, we report the discovery of three new close M-star companions to HIP 104365 and HIP 42334.
Highlights
An extensive population of exoplanets has been discovered down to sub-Jovian masses and at orbital separations below 5 astronomical units (AU) based on large-scale radial velocity (RV) surveys (Mayor & Udry 2008; Marcy et al 2008) and on transit surveys, which are identifying hundreds of new candidatesBased on observations collected at the European Southern Observatory, Chile, ESO programs 081.C-0519, 083.C-0706, 084.C-0605, 087.C-0559, 088.C-0477, and at the Gemini North observatory, Gemini programs GN-2007B-Q-59, GN-2008A-Q-77, GN-2008B-Q-64, GN-2009A-Q-80, GN-2009B-Q-93.Table A.1 is available in electronic form at http://www.aanda.org
Relative astrometry for each candidate companions (CCs) was obtained using 2D Gaussian fitting, and the position was refined by maximizing the flux in a 1.5λ/D aperture moved on a 2×2 pixel grid by steps of 0.1 pixel around the position found with the Gaussian fitting
There are some uncertainties induced by the distortion in the NIRI field of view, the precision was sufficient to clearly identify background or comoving objects
Summary
An extensive population of exoplanets has been discovered down to sub-Jovian masses and at orbital separations below 5 astronomical units (AU) based on large-scale radial velocity (RV) surveys (Mayor & Udry 2008; Marcy et al 2008) and on transit surveys, which are identifying hundreds of new candidates. (Cabrera et al 2009; Borucki et al 2011) These indirectly detected planets provide invaluable information on the distribution of close orbit planets (Cumming et al 2008) and on their frequency around nearby stars covering a large range of masses (Marcy et al 2008; Johnson et al 2010a). We report new measurements for 39 stars, and we include 3 A-stars from the literature that have been observed in previous surveys This survey intends to start defining the population statistics of massive planets and brown dwarfs (BDs) at orbital separations in the tens of AUs from their parent A-stars.
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