arXiv:1608.02945v2 [astro-ph.SR] 11 Aug 2016 1 ( n eas lnt cuywdrobt rudAstars A around ( wider mmag, occupy several planets to because The and pulsational amount the which of . variations, because luminosity of successful difficulty has types most method the these transit of to application methods the planet-hunting caused in effect, problems selection observational by can an stars as main-sequence explained these be around deficit planet ent oe rni rbblt.Terda eoiymethod, velocity radial The probability. transit lower where main- strip, ( of instability Scuti classical masses the delta in the stars with A sequence remarkably coincides This 2007 announcement the After 1284 additional elusive. an remain of stars A ing oal i h RtrdAsas rjc ( project stars’ A ‘Retired the via notably planets. confirmed aiu o tr f1 of stars for maximum [email protected] olsas( around stars found of been cool have thousands candidates) its planet of (and Most planets date. to mission planet-finding ono ta.2011 al. et Johnson 2016 al. et Morton LNTI N80DOBTAON A AROUND 840-D AN IN PLANET A ailvlct bevtoso olsbgat,most sub-giants, cool of observations velocity Radial L using 2018 typeset 10, Preprint November version Draft h elrSaeTlsoei o h otsuccessful most the now is Telescope Space Kepler The ,sgetta lntocrec ae ec a reach rates occurrence planet that suggest ), ehv eetda1 M 12 a detected have We h ustoa hs hfsidcdb ria oin h lnth planet The motion. orbital by induced shifts phase pulsational the trkont otapae ihn1 within t planet using a discovery astrometr host first of to the known absence ours The makes A main-sequenc hosts imaging. orbiting these direct around planets by planets known or All method transit 0.15. the of eccentricity an and rcino h aaee pc hr eaeal odtc them orbits. detect Keywords: wide to able in are planets we high-mass where harbor space parameter the of fraction tla srpyisCnr,Dprmn fPyisadAst and Physics of Department Centre, Astrophysics Stellar ullye l 2015 al. et Mullally δ c)plaosaecmo.Teappar- The common. are pulsators Sct) 1. ynyIsiuefrAtooy(IA,Sho fPyis T Physics, of School (SIfA), Astronomy for Institute Sydney ,silfwrta 0Asashave stars A 20 than fewer still ), INTRODUCTION tr:oclain tr:variables: stars: – oscillations stars: cf. , A T E Kepler tl ATX .1.0 v. AASTeX6 style X . 9 − +0 eateto srnm,TeUiest fTko oy 113 Tokyo Tokyo, of University The Astronomy, of Department ly 2011 Lloyd 0 . . 1 5 M ,wieeolnt orbit- while ), xpaesi 06May 2016 in exoplanets ⊙ Jup ( eete l 2015 al. et Reffert io .Murphy J. Simon lntobtn no ertehbtbezn fami-euneAsta A main-sequence a of zone habitable the near or in orbiting planet OUAINO T PULSATIONS ITS OF MODULATION ,rsligi a in resulting ), ono tal. et Johnson σ ioooShibahashi Hiromoto ftehbtbezn.W n vdnefrpaesi large a in planets for evidence find We zone. habitable the of KEPLER ABSTRACT 1 ). n ioh .Bedding R. Timothy and δ ct lnt n aelts detection satellites: and planets – Scuti ANSQEC TRFUDFO PHASE FROM FOUND STAR A MAIN-SEQUENCE ooy ahsUiest,80 ahsC Denmark C, Aarhus 8000 University, Aarhus ronomy, d:tast n ietiaig h omrcat- former The imaging. direct includes and egory transits ods: stars, pulsation-timing host or their of astrometric, RV, motion methods. the the via via namely, stars A sequence 1,99 15 11ad1517; and 1171 1115, 959, 516, speiecok o h eeto fobtlmotion used orbital 2016 be of detection themselves the can for stars ( clocks A precise 1– of as to surveys limited is transit art the of state standard s the km precise 2 and a not measure, are lines wave-of the spectral Therefore their of pulsation. lengths by distorted these be and can lines, absorption lines shallower fewer, to lead tures ve- ro- rotational fast equatorial s typically the km 100 are of exceeding distribution stars mean locity na- A the the with by spectra. tators, hindered A-type particularly of is ture hand, other the on 1995 r atclrywl utdt hsts ( task this to suited well particularly are upye l 2014 al. et Murphy icvre aebe aeuigtoohrmeth- other two using made been have Discoveries main- orbiting discovered been yet have planets No otntl,tesm ustosta ii Vand RV limit that pulsations same the Fortunately, ). ; − oe ta.2007 al. et Royer 1 eUiest fSde,Australia Sydney, of University he rcso ( precision 1.1. hsspot h data stars A that idea the supports This . eobtlmto.I sas h first the also is It motion. orbital he co ailvlct eetosof detections radial-velocity or ic sa ria eido 840 of period orbital an as nw lnt rudAstars A around planets Known Kepler tr aebe on via found been have stars A e 03,Japan -0033, ekre l 2015 al. et Becker .Teplain of pulsations The ). .Terhg ffcietempera- effective high Their ). lnthss(elr1,462, (Kepler-13, hosts planet otne l 2016 al. et Morton − ). 1 ( opo tal. et Compton b Morrell & Abt ± via r 0d 20 δ c stars Sct ), 2 plus WASP-33 (Collier Cameron et al. 2010), HAT-P-57 (Hartman et al. 2015) and KELT-17 (Zhou et al. 2016). The orbital periods of these systems are all under 30d, except for Kepler-462b whose period is 85d. These planets are strongly irradiated and would have high sur- face temperatures. The imaged planets in the NASA Archive have very wide orbits. These include three objects (HD100546b, HIP78530b and κ And b) with masses above the canonical 13-MJup bound- ary between planets and low-mass brown dwarfs, which orbit B-type stars. Exoplanets orbiting A stars include Fomalhaut b (Stapelfeldt et al. 2004; Chiang et al. 2009), β Pic b (Lagrange et al. 2010), HD95086b (Rameau et al. 2013), the multi-planet sys- Figure 1. Position of KIC 7917485 on a Teff –log g diagram, according to LAMOST spectroscopy (filled circle) and re- tem HR 8799 b,c,d,e (Marois et al. 2008), and the planet vised KIC photometry (open square). The solid and dashed in the triple system HD 131399 Ab (Wagner et al. 2016), blue lines delineate the δ Sct and γ Dor instability strips, re- all of which have orbits ranging from 9 to 177 au. spectively. Evolutionary tracks of solar metallicity and mix- ing length αMLT = 1.8 are also shown (Grigahc`ene et al. Finally, there are some planets orbiting evolved 2005), with corresponding masses in M⊙ written where the hot stars, such as those discovered from eclipse tracks meet the zero-age main-sequence (black line). The +0.1 timing variations of the white-dwarf binary NN Ser peak of the planet occurrence rate distribution, at 1.9−0.5 M⊙ (Brinkworth et al. 2006; Beuermann et al. 2010; (Reffert et al. 2015), crosses the center of the δ Sct instability Parsons et al. 2010), the companion to the sdB strip. star V0391 Peg from pulsation timing variations (Silvotti et al. 2007), and companions to other sdB stars KIC10001893 (Silvotti et al. 2014) and the now- Table 1. Global atmospheric parameters from LAMOST spectroscopy (Frasca et al. 2016) and revised KIC photom- doubtful case of KIC5807616 (Charpinet et al. 2011, cf. etry (Huber et al. 2014). We evaluated these quantities Krzesinski 2015) from orbital brightness modulations. against evolutionary tracks to calculate a mass and lumi- In this paper we present the first planet discovered nosity, which are given below the mid-table break. around a main-sequence A star by the motion of the host star, and the first to be in or near the habitable Parameter Units Spectroscopy Photometry zone. We describe the observations and our method in ± +236 Sect. 2. The planet’s habitability and the implications Teff K 7067 192 7390−318 for planet occurrence posed by this discovery are dis- ± +0.14 log g (cgs) 4.00 0.14 4.08−0.31 cussed in Sect. 3. − ± − +0.30 [Fe/H] 0.02 0.15 0.26−0.36 2. OBSERVATIONS AND ANALYSIS v sin i km s−1 <120 The host star, KIC7917485, was observed for the full +0.15 +0.20 four of the Kepler mission in long-cadence mode M1 M⊙ 1.63−0.12 1.67−0.12 (30-min sampling). Our analysis used the multi-scale +4.8 +13.3 L1 L⊙ 9.9−3.3 10.1− 3.1 MAP data reduction of this data set (Smith et al. 2012; Stumpe et al. 2012). The time series spans 1461 d with a duty cycle of 91percent. This V = 13.2mag star was observed spectroscopi- transform of the light curve (Fig. 2a) is dominated by cally with LAMOST (De Cat et al. 2015). The stellar two oscillation modes at frequencies f1 = 15.3830026(1) −1 atmospheric parameters were extracted by Frasca et al. and f2 = 20.2628968(3)d , where the uncertainty on (2016), showing KIC7917485 to be a main-sequence A the final digit has been given in parentheses. These two star, in agreement with the photometric characteriza- modes were used in the PM analysis. Other signifi- tion by Huber et al. (2014). Its position on a Teff –log g cant peaks have amplitudes that are at least an order diagram is shown in Fig. 1, and atmospheric parameters of magnitude lower. Their signal-to-noise ratios are too are given in Table 1. The star is located in the δ Sct in- low to add usefully to the PM analysis, but their pres- stability strip and has a mass of approximately 1.63 M⊙. ence adds unwanted variance to the data. We therefore We detected the planet by phase modulation (PM) of subtracted all peaks above 50 µmag from the data, ex- the stellar pulsations (Murphy et al. 2014). A Fourier cept for f1 and f2, by fitting their frequencies to the 3

25 20 f2 15 f1 10 av 5 0 -5 -10

Time delay (s) -15 -20 -25 54800 55000 55200 55400 55600 55800 56000 56200 56400 56600 Time (d)

Figure 3. Delays in the light arrival time for the indepen- dent oscillation frequencies f1 (red squares) and f2 (blue cir- cles). Error bars are the formal least-squares uncertainties. Weighted averages of the two measurements in each 10-d segment are indicated as black diamonds.

20 15 10 5 0 -5 Time delay (s) -10 -15 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Orbital phase

Figure 4. Comparison of the weighted average time delays with the best-fitting orbital parameters from the MCMC analysis (given in Table 2). Figure 2. The Fourier transform of the light curve of KIC 7917485 before processing (a), also shown with verti- cal zoom (b). Panel (c) shows the high-pass-filtered data, with low-amplitude oscillations removed down to a 50-µmag node, ̟.1 We fitted a linear term to the time delays level. The horizontal axis is the same in each panel. as an additional parameter to correct for slight inaccu- racies in the oscillation frequencies. Proposed steps in all six parameters were made simultaneously. A total of 200000 steps were made, with a final acceptance rate light curve with a nonlinear least-squares algorithm. We of 0.20. For further details on the methodology, includ- also high-pass-filtered the light curve to remove any re- ing its verification by radial velocities, see Murphy et al. maining instrumental signal and low-frequency oscilla- (2016). tions, preserving all content at frequencies above 5d−1. Table 2 gives the best-fitting orbital parameters, The Fourier transform after the additional processing is which are compared with the observations in Fig. 4. shown in Fig. 2c. We divided the light curve into 10-d segments to look 3. DISCUSSION for shifts in the phases of f1 and f2. These phases were 3.1. Habitable zone converted into delays in the light arrival time (‘time The minimum mass of the sub-stellar companion to delays’) following the method of Murphy et al. (2014). KIC7917485 (11.8MJup) is close to the planet–brown- Time delays of f1 and f2 show identical periodic vari- dwarf mass boundary. Although such a body would not ation (Fig. 3), which we attribute to a sub-stellar com- be expected to have a solid surface, it could presumably panion. Values for the orbital parameters were initially host smaller exomoons. It is therefore interesting to obtained using formulae from Murphy & Shibahashi determine whether the planet lies within the habitable (2015), and then refined with an MCMC algorithm. The zone. MCMC analysis used a Metropolis–Hastings algorithm The habitable zone is broader for A stars than for (Metropolis et al. 1953; Hastings 1970) with symmet- cooler stars because the inner edge extends to higher in- ric proposal distributions based on gaussian-distributed cident surface fluxes for hotter stars (Kopparapu et al. random numbers. Trial runs were made to determine 2014). Their calculations indicate that, at the temper- appropriate step sizes in each of the five orbital parame- ature of KIC7917485, the inner edge lies between ∼1.2 ters fitted: the , Porb, projected light travel time across the orbit, a1 sin i/c, eccentricity, e, the phase of periastron passage calculated relative to the first time 1 We use ̟, rather than ω, because of the common use of ω to delay observation, φp, and the angle of the ascending represent angular oscillation frequencies in asteroseismology. 4

Table 2. Orbital parameters for the KIC 7917485 system. 3.2. Implications for planet occurrence rates The value of φp is calculated with respect to the first time- delay measurement at t0(BJD) = 2 454 958.39166. M1 is in- KIC7917485b is the least massive companion that ferred non-dynamically, from the spectroscopic parameters, we have found in Kepler data with the PM method which allows M2 sin i and a2 sin i to be calculated. K1 sin i, (Murphy et al. 2016, in preparation). We have also the projected semi-amplitude, is calculated from the orbital parameters and provided for reference; it is found two other stars with time delay variations consis- not used to derive the orbital solution. tent with planetary companions having periods longer than the 4-yr data set (KIC9700322, KIC8453431), but Parameter Units Values the finite duration of Kepler time-series does not allow +22 Porb d 840−20 the orbits to be fully parametrized. These detections +0.13 allow us to comment on the planet occurrence around A e 0.15 . −0 10 stars. +0.10 φp [0–1] 0.89−0.12 The detectability of low-mass companions is very sen- +1.1 × −7 f(m1,m2, sin i) M⊙ 5.3−1.0 10 sitive to the noise in the Fourier transform of the time delays, which is determined by the pulsation properties Star (Murphy et al. 2016). We quantified this noise level for +0.15 2040 pulsating single A stars and found that only five of M1 M⊙ 1.63− . 0 12 them had lower noise levels than KIC 7917485. Against +0.5 a1 sin i/c s 7.1−0.4 the same sample, KIC9700322 and KIC8453431 ranked +0.6 th nd ̟ rad 3.0−0.7 as the 9 lowest and 2 lowest, respectively. In other −1 +17 words, we have been able to detect a planetary-mass K1 sin i m s 186 −13 companion in one of the nine stars with the lowest noise Planet levels, and two others show variations that are consistent with planetary-mass companions. This fact is in strong +0.0008 M2 sin i M⊙ 0.0113−0.0006 support of existing observations that intermediate-mass +0.8 MJup 11.8−0.6 stars (‘retired A stars’) tend to host high-mass planets +136 in wide orbits (Johnson et al. 2011). a2 sin i/c s 1017−110 +0.27 a2 sin i au 2.03−0.22 +0.6 ̟ rad 6.1−0.7 4. CONCLUSIONS We have found a planetary companion of and 1.4 times the surface flux at Earth, depending on M sin i = 11.8 MJup near the inner edge of the habitable the mass of the potentially habitable body. zone of KIC 7917485. This is the first planet orbiting a The mean projected separation between the compo- main-sequence A star to be discovered via the motion nents, a sin i = a1 sin i + a2 sin i, is 2.05 au. At this of its host star. Other planets orbiting A stars have separation, the luminous A star irradiates the compan- been discovered via the transit method and have short ion to a surface flux ratio, S/S0, of 2.36 times the flux periods, or been discovered by direct imaging and at Earth. Statistical correction for random inclination have very long periods. Our finding is particularly gives S/S0 =1.76 as the most probable value. significant because no other method is presently capable The luminosity of the A star is not well constrained, of detecting non-transiting planets around these stars and is a strong function of main-sequence age. At the 1σ with periods of a few years, i.e., near their habitable lower luminosity limit (see Table 1), those values of S/S0 zones. reduce to 1.72 and 1.27, respectively. Thus the position KIC 7917485 has particularly low noise levels in its of KIC 7917485b is consistent with the habitable zone at light arrival time delays. We analyzed other stars with the 1σ level. The companion would have been closer to similarly low noise and for two of them we also found the center of the habitable zone earlier in the star’s life- evidence for planetary-mass companions with periods time. The luminosity will be refined substantially when similar to the 4-yr time span of Kepler data. This fact the Gaia mission provides a distance measurement, and strongly supports the idea that intermediate-mass stars the question of habitability can be reassessed. tend to host high-mass planets in wide orbits.

REFERENCES

Abt H. A., Morrell N. I., 1995, ApJS, 99, 135 Becker J. C., Johnson J. A., Vanderburg A., & Morton T. D.,

2015, ApJS, 217, 29 5

Beuermann K., et al., 2010, A&A, 521, L60 Metropolis N., Rosenbluth A., Rosenbluth M., Teller A., Teller Brinkworth C. S., Marsh T. R., Dhillon V. S., Knigge C., 2006, E., 1953, J. Chem. Phys., 21, 1087 MNRAS, 365, 287 Morton T. D., Bryson S. T., Coughlin J. L., Rowe J. F., Charpinet S., et al., 2011, Nature, 480, 496 Ravichandran G., Petigura E. A., Haas M. R., Batalha N. M., Chiang E., Kite E., Kalas P., Graham J. R., Clampin M., 2009, 2016, preprint, (arXiv 1605.02825) ApJ, 693, 734 Mullally F., et al., 2015, ApJS, 217, 31 Collier Cameron A., et al., 2010, MNRAS, 407, 507 Murphy S. J., Bedding T. R., Shibahashi H., Kurtz D. W., Compton D. L., Bedding T. R., Murphy S. J., Stello D., 2016, Kjeldsen H., 2014, MNRAS, 441, 2515 MNRAS, 461, 1943 Murphy S. J., Shibahashi H., 2015, MNRAS, 450, 4475 Murphy S. J., Shibahashi H., Bedding T., 2016, MNRAS, 461, De Cat P., et al., 2015, ApJS, 220, 19 4215 Frasca A., Molenda-Zakowicz J., De Cat P., et al., 2016, Parsons S. G., Marsh T. R., Copperwheat C. M., Dhillon V. S., preprint, (arXiv 1607.07879) Littlefair S. P., G¨ansicke B. T., Hickman R., 2010, MNRAS, Grigahc`ene A., Dupret M.-A., Gabriel M., Garrido R., Scuflaire 402, 2591 R., 2005, A&A, 434, 1055 Rameau, J., et al., 2013, ApJL, 779, L26 Hartman J. D., et al., 2015, AJ, 150, 197 Reffert S., Bergmann C., Quirrenbach A., Trifonov T., K¨unstler Hastings W. K., 1970, Biometrika, 57, 97 A., 2015, A&A, 574, A116 Huber D., et al., 2014, ApJS, 211, 2 Royer F., Zorec J., G´omez A. E., 2007, A&A, 463, 671 Johnson J. A., et al., 2007, ApJ, 665, 785 Silvotti R., et al., 2007, Nature, 449, 189 Johnson J. A., et al., 2011, ApJS, 197, 26 Silvotti R., et al., 2014, A&A, 570, A130 Kopparapu R. K., Ramirez R. M., SchottelKotte J., Kasting Smith J. C., et al., 2012, PASP, 124, 1000 J. F., Domagal-Goldman S., Eymet V., 2014, ApJL, 787, L29 Stapelfeldt K. R., et al., 2004, ApJS, 154, 458 Krzesinski J., 2015, A&A, 581, A7 Stumpe M. C., et al., 2012, PASP, 124, 985 Lagrange A.-M., et al., 2010, Science, 329, 57 Wagner K., Apai D., Kasper M., Kratter K., McClure M., Lloyd J. P., 2011, ApJL, 739, L49 Robberto M., Beuzit J.-L., 2016, preprint, (arXiv 1607.02525) Marois C., Macintosh B., Barman T., Zuckerman B., Song I., Zhou G., et al., 2016, preprint, (arXiv 1607.03512) Patience J., Lafreni`ere D., Doyon R., 2008, Science, 322, 1348