1/11	  Re-characterization of a gravitydarkened and precessing
planetary system PTFO 8-8695

Shoya Kamiaka*, Kento Masuda*, Yuxin Xue*, Yasushi Suto*,Tsubasa Nishioka**, Risa Murakami**, Koichiro Inayama**, Madoka
Saitoh**, Michisuke Tanaka**, and Atsunori Yonehara***University of Tokyo, **Kyoto Sangyo University

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PTFO 8-8695：unusual lightcurves	

20092010~0.1 days~0.1 daystime（sec）

Normalized ﬂux

Normalized ﬂux

Ø T-Tauri star + hot JupiterØ Transits were observed in 2009 and 2010.
(van Eyken et al. 2012)

time（sec）

What makes such a diﬀerence？ (Barnes et al. 2013)
Photometric data by courtesy of Dr. Julian van Eyken and Prof. Masahiro Ikoma

Rotationally-deformed star and close-inplanetØ Rapid stellar rotation (period < 0.671days) induces
gravity darkening.
Ø Planetary orbit is less than twice the stellar radius(orbital period = 0.448 days).

brighterdarker< 2 R★
c.f., Barnes et al. 2013	

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Spin-orbit nodal precession	
Stellar spin and planetary orbital axes precess with theperiod of ~ 1 yr.	  	

c.f., Barnes et al. 2013	

Gravity darkening + nodal precession→ unusual and time-variable lightcurves!	
2010
Normalized ﬂux

Normalized ﬂux

2009

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time（sec）

time（sec）←planethost star

PTFO 8-8695 is an ideal system to check the model ofgravity darkening and nodal precession.
c.f., Barnes et al. 2013	

Previous work：Barnes et al. (2013)6/11	

Ø Estimated system parametersthat can satisfactory reproduce
the observed feature(spin-orbit angle：φ=69 )Ø adopted spin-orbit synchronous condition：
Pspin (< 0.671 days) = Porb (0.448 days)

Standard tidal model does NOT favor such a
synchronized state with signiﬁcant misalignment.
→ need to extend the analysis to the case of Pspin

Porb

Three classes of solutions

B13	
Stellar spin period（days）

Precession period（days）

475 days

827 days

Reduced chi square

Reduced chi square

199 daysd.o.f = 565

Reduced chi square

Planet-to-star mass ratioØ Slightly better solutions are
obtained outside thesynchronous condition.
Ø They are devided into threegroups with diﬀerent precessionperiods.

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Precession period（days）

8/11	How to distinguish them?
Ø All the three solutions reproduce 09 and 10 data well.
Ø They are diﬀerent during unobserved epochs.
09
obs

10
obs

09
data

red, green, and blue：199, 475, and 827 days solutions

10
data

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Preliminary comparison with photometry at
Koyama Astronomical ObservatoryØ Predicted transit depth is ~1% for 199 days solution,
while over 10% for 475 and 827 solutions.
Ø No clear transit signal beyond the noise level of data
→ Data seems to prefer the 199 days solution.

red, green, and blue：199, 475, and 827 days solutionsSummary

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Ø PTFO 8-8695 is an ideal system to check the models ofgravity darkening and nodal precession, which accountfor the time-variable transit lightcurves.
Ø Analysis discarding synchronous condition reveals thatthree diﬀerent solutions reproduce the observed data,
whose precession periods are 199, 475, and 827 days.
Ø Diﬀerence in transit depth is particularly useful todistinguish them with the frequent monitoring of thesystem.

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DiscussionØ The stellar gravity darkening serves as the methodology
to characterize the exoplanetary systems which consistof rapid rotators (Barnes et al. 2011, Zhou and Huang
2013, Philippov and Raﬁkov 2013, Ahlers et al. 2014,
Masuda 2015).
Ø This method is expected to unveil the properties ofyounger or hotter stars, which are known to be rotating
more rapidly than older or cooler ones.