The Exoplanet Massratio Function from the MOAII Survey: Discovery of a Break and Likely Peak at a Neptune Mass
Abstract
We report the results of the statistical analysis of planetary signals discovered in MOAII microlensing survey alert system events from 2007 to 2012. We determine the survey sensitivity as a function of planetstar mass ratio, q, and projected planetstar separation, s, in Einstein radius units. We find that the massratio function is not a single power law, but has a change in slope at q ∼ 10^{4}, corresponding to ∼20 M _{⊕} for the median hoststar mass of ∼0.6 {M}_{⊙ }. We find significant planetary signals in 23 of the 1474 alert events that are wellcharacterized by the MOAII survey data alone. Data from other groups are used only to characterize planetary signals that have been identified in the MOA data alone. The distribution of mass ratios and separations of the planets found in our sample are well fit by a broken powerlaw model of the form {{dN}}_{{pl}}/{(d{log}qd{log}s)=A(q/{q}_{{br}})}^{n}{s}^{m} {{dex}}^{2} for q > q _{br} and {{dN}}_{{pl}}/{(d{log}qd{log}s)=A(q/{q}_{{br}})}^{p}{s}^{m} {{dex}}^{2} for q < q _{br}, where q _{br} is the mass ratio of the break. We also combine this analysis with the previous analyses of Gould et al. and Cassan et al., bringing the total sample to 30 planets. This combined analysis yields A={0.61}_{0.16}^{+0.21}, n = 0.93 ± 0.13, m={0.49}_{0.49}^{+0.47}, and p={0.6}_{0.4}^{+0.5} for q _{br} ≡ 1.7 × 10^{4}. The unbroken powerlaw model is disfavored with a pvalue of 0.0022, which corresponds to a Bayes factor of 27 favoring the broken powerlaw model. These results imply that cold Neptunes are likely to be the most common type of planets beyond the snow line.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2016
 DOI:
 10.3847/15384357/833/2/145
 arXiv:
 arXiv:1612.03939
 Bibcode:
 2016ApJ...833..145S
 Keywords:

 gravitational lensing: micro;
 planetary systems;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 67 pages, 18 figures