A Likely Detection of a Two-planet System in a Low-magnification Microlensing Event

作者：Suzuki D; Bennett D P; Udalski A; Bond I A; Sumi T; Han C; Kim Ho il; Abe F; Asakura Y; Barry R K; Bhattacharya A; Donachie M; Freeman M; Fukui A; Hirao Y; Itow Y; Koshimoto N; Li M C A; Ling C H; Masuda K; Matsubara Y; Muraki Y; Nagakane M; Onishi K; Oyokawa H; Ranc C; Rattenbury N J; Saito To; Sharan A; Sullivan D J; Tristram P J; Yonehara A; Poleski R; Mroz P; Skowron J; Szymanski M K; Soszynski I

来源：Astronomical Journal, 2018, 155(6): 263.

DOI：10.3847/1538-3881/aabd7a

<jats:title>Abstract</jats:title>
<jats:p>We report on the analysis of a microlensing event, OGLE-2014-BLG-1722, that showed two distinct short-term anomalies. The best-fit model to the observed light curves shows that the two anomalies are explained with two planetary mass ratio companions to the primary lens. Although a binary-source model is also able to explain the second anomaly, it is marginally ruled out by 3.1<jats:italic>σ</jats:italic>. The two-planet model indicates that the first anomaly was caused by planet “b” with a mass ratio of <jats:inline-formula>
<jats:tex-math>
<?CDATA $q=({4.5}_{-0.6}^{+0.7})\times {10}^{-4}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn1.gif" xlink:type="simple" />
</jats:inline-formula> and projected separation in units of the Einstein radius, <jats:italic>s</jats:italic> = 0.753 ± 0.004. The second anomaly reveals planet “c” with a mass ratio of <jats:inline-formula>
<jats:tex-math>
<?CDATA ${q}_{2}=({7.0}_{-1.7}^{+2.3})\times {10}^{-4}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn2.gif" xlink:type="simple" />
</jats:inline-formula> with Δ<jats:italic>χ</jats:italic>
<jats:sup>2</jats:sup> ∼ 170 compared to the single-planet model. Its separation has two degenerated solutions: the separation of planet c is <jats:italic>s</jats:italic>
<jats:sub>2</jats:sub> = 0.84 ± 0.03 and 1.37 ± 0.04 for the close and wide models, respectively. Unfortunately, this event does not show clear finite-source and microlensing parallax effects; thus, we estimated the physical parameters of the lens system from Bayesian analysis. This gives the masses of planets b and c as <jats:inline-formula>
<jats:tex-math>
<?CDATA ${m}_{{\rm{b}}}={56}_{-33}^{+51}\,$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn3.gif" xlink:type="simple" />
</jats:inline-formula> and <jats:inline-formula>
<jats:tex-math>
<?CDATA ${m}_{{\rm{c}}}={85}_{-51}^{+86}\,{M}_{\oplus }$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn4.gif" xlink:type="simple" />
</jats:inline-formula>, respectively, and they orbit a late-type star with a mass of <jats:inline-formula>
<jats:tex-math>
<?CDATA ${M}_{\mathrm{host}}\,={0.40}_{-0.24}^{+0.36}\,{M}_{\odot }$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn5.gif" xlink:type="simple" />
</jats:inline-formula> located at <jats:inline-formula>
<jats:tex-math>
<?CDATA ${D}_{{\rm{L}}}={6.4}_{-1.8}^{+1.3}\,\mathrm{kpc}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn6.gif" xlink:type="simple" />
</jats:inline-formula> from us. The projected distances between the host and planets are <jats:inline-formula>
<jats:tex-math>
<?CDATA ${r}_{\perp ,{\rm{b}}}=1.5\pm 0.6\,\mathrm{au}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn7.gif" xlink:type="simple" />
</jats:inline-formula> for planet b and <jats:inline-formula>
<jats:tex-math>
<?CDATA ${r}_{\perp ,{\rm{c}}}={1.7}_{-0.6}^{+0.7}\,\mathrm{au}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn8.gif" xlink:type="simple" />
</jats:inline-formula> and <jats:inline-formula>
<jats:tex-math>
<?CDATA ${r}_{\perp ,{\rm{c}}}={2.7}_{-1.0}^{+1.1}\,\mathrm{au}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaabd7aieqn9.gif" xlink:type="simple" />
</jats:inline-formula> for the close and wide models of planet c. If the two-planet model is true, then this is the third multiple-planet system detected using the microlensing method and the first multiple-planet system detected in low-magnification events, which are dominant in the microlensing survey data. The occurrence rate of multiple cold gas giant systems is estimated using the two such detections and a simple extrapolation of the survey sensitivity of the 6 yr MOA microlensing survey combined with the 4 yr <jats:italic>μ</jats:italic>FUN detection efficiency. It is estimated that 6% ± 2% of stars host two cold giant planets.</jats:p>

• 出版日期2018-6