The Gemini Planet-Finding Campaign: The Frequency of Giant Planets Around Debris Disk Stars

Abstract

We have completed a high-contrast direct imaging survey for giant planets around 57 debris disk stars as part of the Gemini NICI Planet-Finding Campaign. We achieved median H-band contrasts of 12.4 mag at 0.5 ′′ and 14.1 mag at 1" separation. Follow-up observations of the 66 candidates with projected separation < 500 AU show that all of them are background objects. To establish statistical constraints on the underlying giant planet population based on our imaging data, we have developed a new Bayesian formalism that incorporates (1) non-detections, (2) single-epoch candidates, (3) astrometric and (4) photometric information, and (5) the possibility of multiple planets per star to constrain the planet population. Our formalism allows us to include in our analysis the previously known β Pictoris and the HR 8799 planets. Our results show at 95% confidence that <13% of debris disk stars have a ≥5M Jup planet beyond 80 AU, and <21% of debris disk stars have a ≥3M Jup planet outside of 40 AU, based on hot-start evolutionary models. We model the population of directly-imaged planets as d 2 N/dMda ∝ m α a β , where m is planet mass and a is orbital semimajor axis (with a maximum value of a max ). We find that β < −0.8 and/or α > 1.7. Likewise, we find that β < −0.8 and/or a max < 200 AU. For the case where the planet frequency rises sharply with mass (α > 1.7), this occurs because all the planets detected to date have masses above 5M Jup , but planets of lower mass could easily have been detected by our search. If we ignore the β Pic and HR 8799 planets (should they belong to a rare and distinct group), we find that <20% of debris disk stars have a ≥3M Jup planet beyond 10 AU, and β < −0.8 and/or α < −1.5. Likewise, β < −0.8 and/or a max < 125 AU. Our Bayesian constraints are not strong enough to reveal any dependence of the planet frequency on stellar host mass. Studies of transition disks have suggested that about 20% of stars are undergoing planet formation; our non-detections at large separations show that planets with orbital separation > 40 AU and planet masses > 3 M Jup do not carve the central holes in these disks.An interesting result of direct imaging searches so far is that many of the systems with exoplanet discoveries also have debris disks, i.e. β Pictoris, HR 8799 and Fomalhaut (Lagrange et al. 2009;Marois et al. 2008;Kalas et al. 2008). Even though the discovery rate for large-separation exoplanets in debris disk systems is low (<2%; this work), the discovery rate in non-debris disk systems is even lower. In addition, Wyatt et al. (2012) find that the RV-discovered systems with only Saturn-mass planets have a higher-than-expected debris disk fraction, 4 out of 6 (67% ) compared to 4 of 11 (36%) in the full sample of stars with RV planets (debris disk fraction in RV Jupiter-mass systems is constrained to <20%). The correlation between higher disk fraction and lower-mass planets suggests that the formation mechanism for Saturn-only systems results in large, stable debris disks which can produce dust for a l...

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