Kepler photometry has revealed two unusual transiting companions orbiting an
early A-star and a late B-star. In both cases the occultation of the companion
is deeper than the transit. The occultation and transit with follow-up optical
spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a
companion in a 5.2 day orbit with a radius of 0.08 Rsun and a 10000 K late
B-star KOI-81 (KIC 8823868) that has a companion in a 24 day orbit with a
radius of 0.2 Rsun. We infer a temperature of 12250 K for KOI-74b and 13500 K
for KOI-81b.
  We present 43 days of high duty cycle, 30 minute cadence photometry, with
models demonstrating the intriguing properties of these object, and speculate
on their nature.Comment: 12 pages, 3 figures, submitted to ApJL (updated to correct KOI74
  lightcurve

Basri, Gibor

Batalha, Natalie

Borucki, William J.

Brown, Timothy M.

Caldwell, Douglas

Cochran, William D.

Dunham, Edward

Dupree, Andrea K.

Fortney, Jonathan J.

Gautier III, Thomas N.

Gilliland, Ronald L.

Howell, Steve B.

Jenkins, Jon

Koch, David

Latham, David W.

Lissauer, Jack . J.

Marcy, Geoff

Monet, David G.

Rowe, Jason F.

Sasselov, Dimitar

Welsh, William F.

English

arXiv

Borucki

Edmonds

Koch

Liebert

Lopez-Morales

Mandel

Pfahl

Provencal

Shkolnik

van Kerkwijk

Crossref

KEPLER

Kepler photometry has revealed two unusual transiting companions: one orbiting an early A-star and the other orbiting a late B-star. In both cases, the occultation of the companion is deeper than the transit. The occultation and transit with follow-up optical spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a companion in a 5.2 day orbit with a radius of 0.08R(circle dot) and a 10,000 K late B-star KOI-81 (KIC 8823868) that has a companion in a 24 day orbit with a radius of 0.2R(circle dot). We infer a temperature of 12,250 K for KOI-74b and 13,500 K for KOI-81b. We present 43 days of high duty cycle, 30 minute cadence photometry, with models demonstrating the intriguing properties of these objects, and speculate on their nature.NASA's Science Mission DirectorateMcDonald Observator

Gautier, Thomas N., III

Lissauer, Jack J.

Texas ScholarWorks

The Astrophysical Journal Letters, 713:L150–L154, 2010 April 20 doi:10.1088/2041-8205/713/2/L150C© 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.KEPLER OBSERVATIONS OF TRANSITING HOT COMPACT OBJECTSJason F. Rowe1,15, William J. Borucki1, David Koch1, Steve B. Howell2, Gibor Basri3, Natalie Batalha4,Timothy M. Brown5, Douglas Caldwell6, William D. Cochran7, Edward Dunham8, Andrea K. Dupree9,Jonathan J. Fortney10, Thomas N. Gautier III11, Ronald L. Gilliland12, Jon Jenkins6, David W. Latham9,Jack J. Lissauer1, Geoff Marcy3, David G. Monet13, Dimitar Sasselov9, and William F. Welsh141 NASA Ames Research Center, Moffett Field, CA 94035, USA2 National Optical Astronomy Observatory, Tucson, AZ 85719, USA3 University of California-Berkeley, Berkeley, CA 94720, USA4 San Jose State University, San Jose, CA 95192, USA5 Las Cumbres Observatory Global Telescope, Goleta, CA 93117, USA6 SETI Institute, Mountain View, CA 94043, USA7 University of Texas, Austin, TX 78712, USA8 Lowell Observatory, Flagstaff, AZ 86001, USA9 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA10 University of California, Santa Cruz, CA 95064, USA11 Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA 91109, USA12 Space Telescope Science Institute, Baltimore, MD 21218, USA13 US Naval Observatory, Flagstaff Station, Flagstaff, AZ 86001, USA14 San Diego State University, San Diego, CA 92182, USAReceived 2009 November 15; accepted 2010 March 8; published 2010 March 31ABSTRACTKepler photometry has revealed two unusual transiting companions: one orbiting an early A-star and the otherorbiting a late B-star. In both cases, the occultation of the companion is deeper than the transit. The occultation andtransit with follow-up optical spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a companionin a 5.2 day orbit with a radius of 0.08 R and a 10,000 K late B-star KOI-81 (KIC 8823868) that has a companionin a 24 day orbit with a radius of 0.2 R. We infer a temperature of 12,250 K for KOI-74b and 13,500 K forKOI-81b. We present 43 days of high duty cycle, 30 minute cadence photometry, with models demonstrating theintriguing properties of these objects, and speculate on their nature.Key words: stars: individual (KOI-74, KIC 6889235, KOI-81, KIC 8823868)Online-only material: color figures1. INTRODUCTIONThe Kepler Mission was designed to find transiting Earth-like planets in the habitable zones of other stars (Boruckiet al. 2010a). To accomplish this feat, Kepler was launchedinto a heliocentric, Earth-trailing orbit allowing stars to becontinuously monitored for the lifetime of the mission. A naturalproduct of the mission is that other objects that are Earth-sized or larger that transit the host star can be detected. Thisincludes stellar companions such as M-dwarfs and non-stellarcompanions such as brown dwarfs (BDs) and white dwarfs(WDs).Observations of a stellar system with a smaller companionthat orbits along our line of sight will be seen to move in front(transit) and behind (occultation) the host star. The transit lightcurve will have a curved “U” shape due to limb darkening of thestellar surface. The occultation will have a flat bottom and sharpegress and ingress as the flux contribution from the companionis completely obscured. From the shape of the stellar transit, theratio of the companion and stellar radii and the scaled semimajoraxis (a/R) can be determined. The term a/R is related to themean stellar density. The depth of the occultation gives theratio of the surface brightness of the companion and stellar hostintegrated over the observed bandpass.We present 43 days of high duty cycle, ultra-precise photome-try of two transiting objects that have radii similar to Jupiter and15 NASA Postdoctoral Program Fellow.effective temperatures >10,000 K. KOI-74b may have proper-ties similar to a low-mass WD, in that it is compact and hot andits radius combined with a rough estimate of its mass is consis-tent with an internal structure dominated by electron degeneracy.KOI-81b has a radius consistent with either a late M-dwarf, BD,or Jupiter-sized planet, but this picture is inconsistent with itsobserved temperature and is also likely a WD.2. KEPLER PHOTOMETRY AND TRANSIT FITSThe top panels of Figures 1 and 2 show 43 days of photometryfrom the Kepler instrument with 1σ error bars. The observationshave a cadence of 30 minutes. The gap seen near HJD 2454965is due to the transmission of data to the Earth and marks the gapbetween Q0 and Q1 observations (Koch et al. 2010a). The lightcurve for KOI-81 shows periodicity at 0.72 d. This signal wasremoved prior to the transit fits by masking off the transit andeclipsing and fitting a sinusoidal curve to the light curve.The bottom panels of Figures 1 and 2 show the observationsphased with the orbital period. The bottom curve and data arecentered on the transit, whereas the upper curve and data arecentered on the occultation. The points near the transit are shownwith stars and pluses where the different symbols indicate oddand even transits, respectively. Measurements taken during theoccultation are shown with open circles.The data were filtered with a running median boxcar witha width of 10 days. Transit models are computed using theanalytic formulas of Mandel & Agol (2002) with nonlinear limbL150No. 2, 2010 KEPLER OBSERVATIONS OF TRANSITING HOT COMPACT OBJECTS L151Figure 1. Phased light curve of KOI-74 containing eight transits and nine occultations observed by the Kepler photometry between 2009 May 2 and 2009 June 15.The upper panel shows the full 43-day time series after detrending. The bottom panel shows the light curve folded with the orbital period. The lower curve shows theoccultation, with the fitted transit model overplotted in red and corresponding scale to the left. The upper curve covers the expected time of occultation, with the fittedmodel overplotted in green with corresponding scale found to the right. Odd and even transits are marked with stars and pluses, respectively, and measurements nearthe occultation are shown with circles.(A color version of this figure is available in the online journal.)darkening. The data are fit for the center of transit time, period,impact parameter (b), the scaled planetary radius (R/R), andζ /R. The last term, ζ /R, is related to the transit duration(Td = 2(ζ/R)−1) and the mean stellar density (Pál et al.2009). The transit models are shown in Figures 1 and 2 bythe red (lower) line. The occultation is modeled by assuming nolimb darkening for the occulted object and shown by the green(upper) line. The transit model assumes that the companiondoes not emit. While not true, this is a good approximation asthe companions KOI-74b and KOI-81b are 800 and 200 timesless luminous than the host stars; thus, the dilution only alters thecompanion radius at the 1% level. We also assumed a circularorbit in our model fits.Optical spectroscopy obtained at the Kitt Peak 2.1 m wasused to estimate KOI-74 as spectral type A1V with Teff =9400 ± 150 K and KOI-81 as B9-A0V with Teff = 10000 ±150 K. With the stellar density from the transit light curveand Teff from spectroscopy, the stellar mass and radius wereestimated using the ρ method as described in Borucki et al.(2010b). This allows us to model the companion radii, orbitalperiod, inclination angle, and occultation depth, which we listin Table 1 as described in Koch et al. (2010b).2.1. Temperatures of the CompanionsThe occultation is deeper than the transit for both KOI-74and KOI-81, indicating the companions have a larger surfacebrightness as compared to the stellar host. The spectroscopicallydetermined effective temperature and the transit-determinedstellar density with the ρ method provide luminosity estimatesof the stars. If we assume that the occultation depths arebolometric, then the depth of the occultation gives the luminosityratio of the star and companion. The estimated luminosities ofthe stars and companions are listed in Table 1. Assuming thecompanions act as blackbodies and using the radii determinedfrom the transit fit, we find temperatures of 12289 ± 340 K and13430 ± 490 K for KOI-74b and KOI-81b, respectively.2.2. Mass EstimationKOI-74 and KOI-81 exhibit variations with a periodicityequal to one-half the orbital period and presenting amplitudesof ∼193 ppm and ∼50 ppm, respectively. The KOI-74 lightcurve also exhibits variability with an amplitude of ∼116 ppmwith a period equal to the orbital period. There are various waysto interpret these variations. We could be seeing spot activityinduced on the stellar surface from a magnetic interactionbetween the star and companion. This would be similar toactivity seen in the planet hosting system HD179949 (Shkolniket al. 2004) or τ Bootis (Walker et al. 2008). The variationscould also be phase locked due to tidal distortions of the hoststar. In the later scenario, we can attempt to estimate the massof the companion.L152 ROWE ET AL. Vol. 713Figure 2. Phased light curve of KOI-81 containing a single transit and occultation observed by the Kepler photometry between 2009 May 2 and 2009 June 15. Theupper panel shows the full 43 day time series after detrending. The bottom panel shows the transit and occultation after removal of periodicity seen 0.72 c day−1. SeeFigure 1 for further details.(A color version of this figure is available in the online journal.)Table 1Modeled Parameters for KOI-74 and KOI-81KOI-74 Star CompanionRadius 1.899+0.043−0.051 R 0.0393+0.0009−0.0013 RMass 2.22+0.10−0.14 M 0.02–0.11 MLuminosity 25.6 ± 2.4 L 0.0317 ± 0.0060 LTeff 9400 ± 150 K 12289 ± 340 KEpoch 2454958.87995 ± 0.00036 HJDPeriod 5.188754 ± 0.000083 daysi 88.8 ± 0.5 degOccultation depth 1238.2 ± 6.6 ppmKOI-81 Star CompanionRadius 2.93 ± 0.14 R 0.115 ± 0.006 RMass 2.71+0.19−0.11 M 0.02–0.2 MLuminosity 77.3 ± 9.6 L 0.387 ± 0.096 LTeff 10000 ± 150 K 13430 ± 490 KEpoch 2454976.06981 ± 0.00095 HJDPeriod 23.8776 ± 0.0020 daysi 88.2 ± 0.3 degOccultation depth 4977 ± 16 ppmNotes. The stellar radius, mass, and luminosity are determined from the ρmethod. The stellar temperatures are spectroscopically determined. The radiusof the companion, period, i, epoch, and occultation depth are based on fits tothe transit and occultation photometry. The mass of the planet is estimated fromthe amplitude of variations phase locked to the orbital period. The companionluminosities and temperatures are estimated assuming a bolometric occultationdepth and blackbody behavior.Ellipsoidal variations show two maxima and minima perorbital revolution. This variability is due the changing observedstellar cross section, reflection effects due to emission andabsorption from each component, and gravity darkening. Morris(1985) found from a cosine expansion of the double wavecharacteristic of the variations that the amplitude of the lightvariations is given by ΔM = f (τ, U1)qR3 sin3(i)/a3, whereq is the mass ratio of the companion and star, and τ and U1express the gravity and limb-darkening terms that describe thesurface profile of the distorted star with a radius of R witha companion in an orbit with inclination i with a semimajoraxis a. Adopting gravity and limb-darkening coefficients fromTable II of Beech (1989), we estimate the mass of KOI-74b ∼0.08 M and KOI-81b ∼ 0.19 M. These relations are validwhen the tidally perturbed stellar surface is in hydrostaticequilibrium. An important issue is addressed below.The ratio of the tidal acceleration to the star’s surface gravityis given byε ≡ MpM(Ra)3, (1)where M and Mp are the mass of the star and companion,respectively, R is the stellar radius, and a is the semimajor axis.This gives a rough measure of the amplitude of the ellipsoidalvariability. Assuming that the system is in tidal equilibrium,Equation (16) of Pfahl et al. (2008) can also be used to estimatethe mass of the system. Adding this prescription to our transitmodel allows us to fit for the companion mass. We find thatNo. 2, 2010 KEPLER OBSERVATIONS OF TRANSITING HOT COMPACT OBJECTS L153KOI-74b has a mass of ∼0.11 M and KOI-81b has a massof ∼0.21 M.The assumption of tidal equilibrium works well for shortperiod binaries where both members have similar masses. Tidaldissipation circularizes the orbit and synchronizes the stellarrotation period to the orbital period. When the masses are notsimilar there is no reason to expect the rotation and orbitalperiods to synchronize, thus the assumption of tidal equilibriummay be invalid.As shown by Pfahl et al. (2008), the tidal-equilibrium modelworks well for stars with deep convective atmospheres whereflux perturbations arise from changes in the local effectivegravity. This is not the case for stars such as KOI-74 andKOI-81 with radiative atmospheres. Tidal forcing of radiativeregions could produce significant deviations from hydrostaticbalance as gravity waves can penetrate the interiors. Simulationsindicate that the observed oscillations can be greater than 10εfor stars with M > 1.4 M.If the companion is the remnant of a much larger star andhas experienced an episode of mass loss, then the rotationaland orbital periods may have already been synchronized. In thiscase, the companions likely have significant mass. We plan toobtain follow-up spectroscopic observations to search for radialvelocity changes. Such observations will place firm limits onthe mass of the companions. Until then, we allow ε to be up to10 times larger in our estimate for the masses of the companionslisted in Table 1 to account for the possibility that the fluxvariations are produced by g-modes propagating to the surfaceof the star induced by a less massive companion.3. DISCUSSIONThe temperatures of the companions are high. If these objectsare planets then we can investigate their thermal properties whenthe only energy source is irradiation from the star. Assuminga Bond Albedo of zero and complete redistribution of heatfor reradiation, we can calculate the equilibrium temperature(Teq) assuming the companions are blackbodies. The light curvedoes not show any indication of variability at the orbital periodthat might indicate a day–night temperature gradient for atidally locked companion. The calculation of Teq estimates thetemperature of the planet assuming the only energy input isflux from the host star. For KOI-74b this gives an equilibriumtemperature of 2250 ± 50 K, and 1715 ± 50 K for KOI-81b.Comparing these values to the measured effective temperaturesallows us to conclude that the objects require an additionalenergy source. This could indicate prior evolution of thecompanions and we are watching them slowly cool.Figure 3 shows the positions of the host stars and companionsrelative to stars, planets, and WDs. The green and cyan points arepreviously known transiting extrasolar planets16 and their hoststars. The dark blue points show the first five Kepler extrasolarplanets (Borucki et al. 2010a). The red diamonds indicate theEarth, Uranus, Neptune, Saturn, and Jupiter. The magenta pointsmark low-mass stars from Lopez-Morales (2007). The blackcircles clustered near 1 M are WDs observed by Hipparcos(Provencal et al. 1998) and those near 0.2 M are a sample ofextremely low-mass WDs (Edmonds et al. 2001; Bassa et al.2006; Liebert et al. 2004; Kawka & Vennes 2009; van Kerkwijket al. 1996) that have degenerate companions. The orange lines,from top to bottom, show zero-temperature WD models ofZapolsky & Salpeter (1969) with compositions of H, He, Mg, C,16 The Extrasolar Planets Encyclopedia: http://www.exoplanet.eu.Figure 3. Location of KOI-74b and KOI-81b on the mass–radius diagram: starswith transiting exoplanets are shown in cyan and transiting exoplanets with well-defined masses and radii are shown in green. Kepler transiting planets are shownin blue. The positions of the Earth, Uranus, Neptune, Saturn, Jupiter, and the Sunare also indicated by red diamonds. Low-mass stars from Lopez-Morales (2007)are shown in magenta. WDs as observed by Hipparcos (Provencal et al. 1998)and a sample of extremely light WDs are shown with black o’s. The open circlenear KOI-81b is the millisecond pulsar companion discovered by Edmonds et al.(2001). The orange lines, from top to bottom, show zero-temperature models ofZapolsky & Salpeter (1969) with compositions of H, He, Mg, C, and Fe. Thered vertical line is the cooling curve for a 0.2026 M He-WD from Panei et al.(2007). KOI-74 and KOI-81 are shown as stars in the upper right portion of thediagram and the positions of KOI-74b and KOI-81b are labeled.(A color version of this figure is available in the online journal.)and Fe. The red vertical line is the cooling curve for a 0.2026 MHe-WD from Panei et al. (2007). The red vertical line shows thecooling model for a He-WD with a mass of 0.2026 M formedthrough mass transfer (Panei et al. 2007). The stars KOI-74 andKOI-81 are denoted in the upper right portion of the diagramby black stars and the positions of the companion KOI-74b andKOI-81b are labeled.The position of KOI-81b in the mass–radius diagram issimilar to low-mass stars at the hydrogen burning limit butits inferred temperature is much higher. It has similar physicalproperties to the cooling He-WD discovered in the globularcluster 47 Tuc by Edmonds et al. (2001). Stellar modelingfrom the ρ (Borucki et al. 2010b) method puts the age ofthe host star at 340 ± 67 Myr. The observed mass and radiusof the companion estimate ∼155 Myr since its creation fromthe cooling curves of Panei et al. (2007). KOI-74b and KOI-81borbit at distances of 16 R and 49 R, respectively. When thehot stars begin shell burning of hydrogen in their interiors, theirradii will dramatically increase and will likely result in masstransfer to the companions. That these objects are both foundaround hot stars is intriguing. The companions are inundatedwith UV radiation from the host star and are likely sufferingfrom substantial evaporation and ionization.The light curve for KOI-81 shows significant power near0.72 c day−1. There may be stellar pulsations induced by tidalinteractions with the companion similar to the activity has beenseen in the F-type binary star system HD 209295 (Handler et al.L154 ROWE ET AL. Vol. 7132002). Observations of stellar oscillations offer the opportunityto refine the stellar parameters through asteroseismology.KOI-74 and KOI-81 are interesting astrophysical systems.Confirmation of the companion masses with spectroscopic ra-dial velocities will help confirm the physical properties of theseobjects. Follow-up observations are planned as well as contin-ued with the Kepler instrument to help unravel their nature.Funding for this Discovery mission is provided by NASA’sScience Mission Directorate. We are indebted to the entireKepler team for all the hard work and dedication that madesuch discoveries possible.Facilities: KeplerREFERENCESBassa, C. G., van Kerkwijk, M. H., Koester, D., & Verbunt, F. 2006, A&A, 456,295Beech, M. 1989, Ap&SS, 152, 329Borucki, B., et al. 2010a, Science, 327, 997Borucki, B., et al. 2010b, ApJ, 713, L126Edmonds, P. D., et al. 2001, ApJ, 557, L57Handler, G., et al. 2002, MNRAS, 333, 262Kawka, A., & Vennes, S. 2009, A&A, 506, L25Koch, D. G., et al. 2010a, ApJ, 713, L79Koch, D. G., et al. 2010b, ApJ, 713, L131Liebert, J., et al. 2004, ApJ, 606, L147Lopez-Morales, M. 2007, ApJ, 660, 732Mandel, K., & Agol, E. 2002, ApJ, 580, L171Morris, S. 1985, ApJ, 295, 143Pál, A., et al. 2009, MNRAS, 401, 2665Panei, J. A., et al. 2007, MNRAS, 382, 779Pfahl, E., Arras, P., & Paxton, B. 2008, ApJ, 679, 783Provencal, J. L., et al. 1998, ApJ, 494, 759Shkolnik, E., Walker, G. A. H., & Bohlender, D. A. 2004, ApJ, 609,1197van Kerkwijk, M. H., Bergeron, P., & Kulkarni, S. R. 1996, ApJ, 467,L89Walker, G. A. H., et al. 2008, A&A, 482, 691Zapolsky, H. S., & Salpeter, E. E. 1969, ApJ, 158, 809

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Kepler Observations of Transiting Hot Compact Objects

Kepler Observations of Transiting Hot Compact Objects

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