K2-231 b: A sub-Neptune exoplanet transiting a solar twin in Ruprecht 147

The K2 survey of Ruprecht 147 finds a transiting planet Ruprecht 147 is the oldest nearby star cluster

While searching Everest light curves for stellar rotation, we noticed six transits in the light curve for this solar twin.

While looking for rotation, we found a planet

Surveyed with K2 during Campaign 7 (GO 7035, Science PI Curtis), R147 is the oldest (2.7 Gyr) nearby (300 pc) star cluster, making it a critical benchmark for stellar astrophysics: see, Curtis et al. (2013), Curtis (2016), and Torres et al. (2018).

This figure shows clusters with rotation data; those shaded red were surveyed with Kepler/K2. 0 20 40 60 80

Day since start of Campaign 7

0.999

1.000

1.001

1.002

Nor

mal

ized

Flu

x

The chromospheric emission is consistent with its siblings and empirical expectations (Mamajek & Hillenbrand 2008), as is its chemical composition (cluster [Fe/H] = +0.10 ± 0.04 dex).

K2-231 is a typical solar twin

3931 3932 3933 3934 3935 3936Wavelength (Angstroms)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Rel

ativ

e Fl

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ISM

M★ = 1.01 M¤ R★ = 0.95 R¤ Teff = 5695 K [Fe/H] = +0.14 dex log R’HK = −4.8 dex

Jason L. Curtis,1 Andrew Vanderbug,2 Guillermo Torres,3 Adam L. Kraus,2 Daniel Huber,4 Andrew W. Mann,5 Aaron C. Rizzuto,2 Howard Isaacson,6 Andrew W. Howard,7 Christopher E. Henze,8 Benjamin J. Fulton,7 Jason T. Wright9

(1) Columbia University (2) University of Texas, Austin (3) Harvard–Smithsonian Center for Astrophysics (4) University of Hawaii (5) University of North Carolina, Chapel Hill (6) University of California, Berkeley (7) California Institute of Technology (8) NASA Ames Research Center (9) Pennsylvania State University

Published in the Astronomical Journal (2018, 155, 173. arXiv 1803.07430)

Most binary scenarios are ruled out by combining precise RVs from HARPS, with high-resolution imaging from Keck/NIRC2 (NRM, AO), and isochrone modeling of the broadband photometry. Very-low-mass stars (VLM) and sub-stellar objects would negligibly dilute the transits.

Validation and Properties K2-231 is likely single

15 10 5 0 -5 -10 -15∆ Right Ascension (arcseconds)

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0 200 400 600 800 1000Projected Separation, ρ (mas)

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Hydrogen Burning Limit

VLM stars

RV ?

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0 100 200 300Projected Separation (AU)

0.07

0.120.200.300.400.500.600.700.85

Seco

ndar

y M

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(sol

ar)NRM Photometric Modeling

AO

Mitigating background blends

UKIRT/WFCAM image of K2-231 (big red circle), and neighboring stars blended in the K2 aperture (black circle). Transits are visible with the same depths in a smaller aperture (dashed black line), leaving one blend that could host the transits. Keck/HIRES RVs show it is not an EB.

Transit analysis and statistical validation

We re-extracted the light curve with a circular moving aperture to reject most background stars, corrected for K2 systematics (Vanderburg & Johnson et al 2014), then fit the transits with EXOFAST (Eastman et al. 2013).

We applied the BLENDER statistical validation technique (Torres et al. 2004, 2011, 2015) to calculate the false positive likelihood caused by unseen stars, constrained by high-resolution spectroscopy, AO imaging, precision RVs, and color information, and found a 99.86% probability that K2-231 b is a planet.

J.L.C. is funded by the National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1602662 and the National Aeronautics and Space Administration under grant NNX16AE64G issued through the K2 Guest Observer Program.

K2-231 b is a typical exoplanet

We estimated the planet mass with the probabilistic mass–radius relation Forecaster (Chen & Kipping 2017), and found Mp = 7 +5 −3 ME At V = 12.7, this may be measurable with existing precision radial velocity facilities.

The mass of K2-231 b

Comparison with planets located in the Kepler field

K2-231 b is found near a relative maximum in the distribution of planet size and orbital period, after accounting for the shorter duration of the K2 campaign (i.e., sensitive to Porb < 40 d).

(Figure adapted from Fulton et al. 2017)

The 23 published planets in open clusters (so far)

Planet Name

Pleiades (125 Myr):...

Praesepe (670 Myr):Pr0201 bPr0211 bPr0211 cK2-95 bK2-100 bK2-101 b K2-102 b K2-103 b K2-104 b EPIC 211901114 b

Hyades (727 Myr):eps Tau b HD 285507 bK2-25 b K2-136-A bK2-136-A cK2-136-A dHD 283869 b

NGC 2423 (740 Myr):TYC 5409-2156-1 b

NGC 6811 (1 Gyr):Kepler-66 bKepler-67 b

Ruprecht 147 (2.7 Gyr):K2-231 b

M67 (4 Gyr):YBP 401 b YBP 1194 bYBP 1514 bSAND 364 bSAND 978 b

NGC 6791 (8 Gyr):Several candidates from Montet et al.

References: (1) Barros et al. (2016); (2) Brucalassi et al. (2014); (3) Brucalassi et al. (2017); (4) Ciardi et al. (2017); (5) David et al. (2016); (6) Gaidos et al. (2017); (7) Libralato et al. (2016); (8) Lovis & Mayor (2007); (9) Malavolta et al. (2016); (10) Mann et al. (2016); (11) Mann et al. (2017a); (12) Mann et al. (2017b); (13) Meibom et al. (2013); (14) Obermeier et al. (2016); (15) Pepper et al. (2017); (16) Pope et al. (2016);(17) Quinn et al. (2012); (18) Quinn et al. (2014); (19) Sato et al. (2007); (20) Vanderburg et al. (2018).

KIC/EPICID

C4

211998346211936827211936827211916756211990866211913977211970147211822797211969807211901114

210754593210495452210490365247589423247589423247589423248045685

...

98361499532052

219800881

...211411531211416296211403356

...

DiscoveryMethod

...

RVRVRVTrTrTrTrTrTrTr

RVRVTrTrTrTrTr

RV

TrTr

Tr

RVRVRVRVRV

Tr

V(mag)

...

10.5212.1512.1517.2710.3712.5512.7614.6615.7716.49

3.5310.4715.8811.2011.2011.2010.6

9.45

15.316.4

12.71

13.7014.6814.779.809.71

Period(days)

...

4.432.15

>350010.141.6714.689.9221.171.971.65

594.96.093.4857.9817.3125.58~106

714.3

17.8215.73

13.84

4.0876.9605.118121151

Radius /M sin i

...

0.54 MJ1.844 MJ7.9 MJ3.7 RE3.5 RE2.0 RE1.3 RE2.2 RE1.9 RE9.6 RE

7.6 MJ0.917 MJ3.43 RE0.99 RE2.91 RE1.45 RE1.96 RE

10.6 MJ

2.80 RE2.94 RE

2.5 RE

0.42 MJ0.33 MJ0.40 MJ1.57 MJ2.18 MJ

HostInfo.

...

Late-FLate-GLate-G

0.43 M¤

1.18 M¤

0.80 M¤

0.77 M¤

0.61 M¤

0.51 M¤

0.46 M¤

2.7 M¤ GiantK4.5M4.5K5.5K5.5K5.5K5

Giant

1.04 M¤

0.87 M¤

Solar twin

F9VG5VG5VK3IIIK4III

Notes

None found

HJ, “two b’s”HJ, “two b’s”

Eccentric, 1st multi………………

Candidate

1st everEccentric HJ

…Stellar binaryStellar binaryStellar binary

Candidate (1 transit)

…

……

…

HJHJHJ…

Candidate

Kepler superstamp

References

6

17179

7, 11, 14, 151, 7, 11, 161, 7, 11, 16

1111

7, 1111

1918

5, 104, 124, 124, 1220

8

1313

This work

2, 32, 32, 32, 32, 3

-6 -4 -2 0 2 4 6Hours since mid-transit

0.99920.99940.99960.99981.00001.00021.0004

Normalize

dFlux

We estimated the number of planets that are detectable in K2 for the 126 RV-vetted members. Using the Fressin et al. (2013) occurrence rates for stars with at least one planet <29 d, we expect to find ~1 exoplanet bigger than Earth We apparently found it.

Expected Yield for R147

solarspectra

Ca II K spectrum

“K2 Survey of R147” results at this meeting:T.Bea6y:over-luminousbrowndwarf(poster77)G.Torres:eclipsingbinary(poster95)J.CurHs:gyrochronology(talk,Thurs9am)

10 100 1000 10000Distance (pc)

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Pleiades M35M34

M37Hyades

Praesepe NGC 6811NGC 752

NGC 6819Ruprecht 147M67

DayofObservaHon

Normalize

dFlux Porb = 13.84 d

Rp = 2.5 RE