Daniel J. Stevens

A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host

The amount of ultraviolet irradiation and ablation experienced by a planet depends strongly on the temperature of its host star. Of the thousands of extrasolar planets now known, only six have been found that transit hot, A-type stars (with temperatures of 7,300–10,000 kelvin), and no planets are known to transit the even hotter B-type stars. For example, WASP-33 is an A-type star with a temperature of about 7,430 kelvin, which hosts the hottest known transiting planet, WASP-33b (ref. 1); the planet is itself as hot as a red dwarf star of type M (ref. 2). WASP-33b displays a large heat differential between its dayside and nightside, and is highly inflated–traits that have been linked to high insolation. However, even at the temperature of its dayside, its atmosphere probably resembles the molecule-dominated atmospheres of other planets and, given the level of ultraviolet irradiation it experiences, its atmosphere is unlikely to be substantially ablated over the lifetime of its star. Here we report observations of the bright star HD 195689 (also known as KELT-9), which reveal a close-in (orbital period of about 1.48 days) transiting giant planet, KELT-9b. At approximately 10,170 kelvin, the host star is at the dividing line between stars of type A and B, and we measure the dayside temperature of KELT-9b to be about 4,600 kelvin. This is as hot as stars of stellar type K4 (ref. 5). The molecules in K stars are entirely dissociated, and so the primary sources of opacity in the dayside atmosphere of KELT-9b are probably atomic metals. Furthermore, KELT-9b receives 700 times more extreme-ultraviolet radiation (that is, with wavelengths shorter than 91.2 nanometres) than WASP-33b, leading to a predicted range of mass-loss rates that could leave the planet largely stripped of its envelope during the main-sequence lifetime of the host star.

KELT-10b: the first transiting exoplanet from the KELT-South survey - a hot sub-Jupiter transiting a V = 10.7 early G-star

We report the discovery of KELT-10b, the first transiting exoplanet discovered using the KELT-South telescope. KELT-10b is a highly inflated sub-Jupiter mass planet transiting a relatively bright V = 10.7 star (TYC 8378-64-1), with T_(eff) = 5948 ± 74 K, log g = 4.319^(+0.020)_(−0.030) and [Fe/H] = 0.09^(+0.11)_(−0.10), an inferred mass M^* = 1.112^(+0.055)_(−0.061) M_⊙ and radius R^* = 1.209^(+0.047)_(−0.035) R_⊙. The planet has a radius Rp = 1.399^(+0.069)_(−0.049) RJ and mass Mp = 0.679^(+0.039)_(−0.038) MJ. The planet has an eccentricity consistent with zero and a semimajor axis a = 0.05250^(+0.00086)_(−0.00097) au. The best-fitting linear ephemeris is T_0 = 2457 066.720 45 ± 0.000 27 BJD_(TDB) and P = 4.166 2739 ± 0.000 0063 d. This planet joins a group of highly inflated transiting exoplanets with a larger radius and smaller mass than that of Jupiter. The planet, which boasts deep transits of 1.4 per cent, has a relatively high equilibrium temperature of T_(eq) = 1377^(+28)_(−23) K, assuming zero albedo and perfect heat redistribution. KELT-10b receives an estimated insolation of 0.817^(+0.068)_(−0.054) × 10^9 erg s^(−1) cm^(−2), which places it far above the insolation threshold above which hot Jupiters exhibit increasing amounts of radius inflation. Evolutionary analysis of the host star suggests that KELT-10b may not survive beyond the current subgiant phase, depending on the rate of in-spiral of the planet over the next few Gyr. The planet transits a relatively bright star and exhibits the third largest transit depth of all transiting exoplanets with V < 11 in the Southern hemisphere, making it a promising candidate for future atmospheric characterization studies.

KELT-8b: A HIGHLY INFLATED TRANSITING HOT JUPITER AND A NEW TECHNIQUE FOR EXTRACTING HIGH-PRECISION RADIAL VELOCITIES FROM NOISY SPECTRA

We announce the discovery of a highly inflated transiting hot Jupiter by the KELT-North survey. A global analysis including constraints from isochrones indicates that the V = 10.8 host star (HD 343246) is a mildly evolved, G dwarf with T_(eff)= 5754^(+54)_(-55)K, log g = 4.078^(0.049)_(0.054), [Fe/H] = 0.272 ± 0.038, an inferred mass M_* = 1.211_(0.066)^(+0.078)M_☉, and radius R_*=1.67 _(-0.12)^(+0.14) R_☉. The planetary companion has a mass Mp = 0.867 _(-0.061)^+(0.065) MJ, radius R_p 1.86_(-0.16)^(+0.18) R_J, surface gravity log g_p 2.793_(-0.075)^(+0.072), and density 0.167_(-0.038)^(+0.047) g cm^(−3). The planet is on a roughly circular orbit with semimajor axis ɑ 0.04571_(0.00084)^(+0.00096) AU and eccentricity e 0.035_(-0.025)^(+0.050). The best-fit linear ephemeris is T_0 = 2456883.4803 ± 0.0007 BJD_(TDB) and P = 3.24406 ± 0.00016 days. This planet is one of the most inflated of all known transiting exoplanets, making it one of the few members of a class of extremely low density, highly irradiated gas giants. The low stellar log g and large implied radius are supported by stellar density constraints from follow-up light curves, as well as an evolutionary and space motion analysis. We also develop a new technique to extract high-precision radial velocities from noisy spectra that reduces the observing time needed to confirm transiting planet candidates. This planet boasts deep transits of a bright star, a large inferred atmospheric scale height, and a high equilibrium temperature of T_(eq) 1675_(-55)^(+61)K, assuming zero albedo and perfect heat redistribution, making it one of the best targets for future atmospheric characterization studies.

KELT-17B: A HOT-JUPITER TRANSITING AN A-STAR IN A MISALIGNED ORBIT DETECTED WITH DOPPLER TOMOGRAPHY

We present the discovery of a hot-Jupiter transiting the V=9.23 mag main-sequence A-star KELT-17 (BD+14 1881). KELT-17b is a 1.31 -0.29/+0.28 Mj, 1.525 -0.060/+0.065 Rj hot-Jupiter in a 3.08 day period orbit misaligned at -115.9 +/- 4.1 deg to the rotation axis of the star. The planet is confirmed via both the detection of the radial velocity orbit, and the Doppler tomographic detection of the shadow of the planet over two transits. The nature of the spin-orbit misaligned transit geometry allows us to place a constraint on the level of differential rotation in the host star; we find that KELT-17 is consistent with both rigid-body rotation and solar differential rotation rates (alpha < 0.30 at 2 sigma significance). KELT-17 is only the fourth A-star with a confirmed transiting planet, and with a mass of 1.635 -0.061/+0.066 Msun, effective temperature of 7454 +/- 49 K, and projected rotational velocity v sin I_* = 44.2 -1.3/+1.5 km/s; it is amongst the most massive, hottest, and most rapidly rotating of known planet hosts.

KELT-11b: a highly inflated sub-Saturn exoplanet transiting the V=8 subgiant HD 93396

We report the discovery of a transiting exoplanet, KELT-11b, orbiting the bright (V = 8.0) subgiant HD 93396. A global analysis of the system shows that the host star is an evolved subgiant star with T_(eff) = 5370±51 K, M∗ = 1.438^(+0.061)_(−0.052) M⊙, R∗ = 2.72^(+0.21)_(−0.17) R⊙, log g∗= 3.727^(+0.040)_(−0.046), and [Fe/H]= 0.180 ± 0.075. The planet is a low-mass gas giant in a P = 4.736529 ± 0.00006 day orbit, with M_P = 0.195 ± 0.018 M_J, R_P = 1.37^(+0.15)_(−0.12) R_J, ρ_P = 0.093^(+0.028)_(−0.024) g cm^(−3) , surface gravity log g_P = 2.407^(+0.080)_(−0.086), and equilibrium temperature T_(eq) = 1712^(+51)_(−46) K. KELT-11 is the brightest known transiting exoplanet host in the southern hemisphere by more than a magnitude, and is the 6th brightest transit host to date. The planet is one of the most inflated planets known, with an exceptionally large atmospheric scale height (2763 km), and an associated size of the expected atmospheric transmission signal of 5.6%. These attributes make the KELT-11 system a valuable target for follow-up and atmospheric characterization, and it promises to become one of the benchmark systems for the study of inflated exoplanets.

The Mysterious Dimmings of the T Tauri Star V1334 Tau

We present the discovery of two extended ˜0.12 mag dimming events of the weak-lined T Tauri star V1334. The start of the first event was missed but came to an end in late 2003, and the second began in 2009 February, and continues as of 2016 November. Since the egress of the current event has not yet been observed, it suggests a period of >13 years if this event is periodic. Spectroscopic observations suggest the presence of a small inner disk, although the spectral energy distribution shows no infrared excess. We explore the possibility that the dimming events are caused by an orbiting body (e.g., a disk warp or dust trap), enhanced disk winds, hydrodynamical fluctuations of the inner disk, or a significant increase in the magnetic field flux at the surface of the star. We also find a ˜0.32 day periodic photometric signal that persists throughout the 2009 dimming which appears to not be due to ellipsoidal variations from a close stellar companion. High-precision photometric observations of V1334 Tau during K2 campaign 13, combined with simultaneous photometric and spectroscopic observations from the ground, will provide crucial information about the photometric variability and its origin.

A Posteriori Transit Probabilities

Given the radial velocity (RV) detection of an unseen companion, it is often of interest to estimate the probability that the companion also transits the primary star. Typically, one assumes a uniform distribution for the cosine of the inclination angle i of the companions orbit. This yields the familiar estimate for the prior transit probability of ~R/a, given the primary radius R and orbital semimajor axis a, and assuming small companions and a circular orbit. However, the posterior transit probability depends not only on the prior probability distribution of i but also on the prior probability distribution of the companion mass Mc, given a measurement of the product of the two (the minimum mass Mc sin i) from an RV signal. In general, the posterior can be larger or smaller than the prior transit probability. We derive analytic expressions for the posterior transit probability assuming a power-law form for the distribution of true masses, dΓ/dMc∝Mcα, for integer values -3 ≤ α ≤ 3. We show that for low transit probabilities, these probabilities reduce to a constant multiplicative factor fα of the corresponding prior transit probability, where fα in general depends on α and an assumed upper limit on the true mass. The prior and posterior probabilities are equal for α = -1. The posterior transit probability is ~1.5 times larger than the prior for α = -3 and is ~4/π times larger for α = -2, but is less than the prior for α≥0, and can be arbitrarily small for α > 1. We also calculate the posterior transit probability in different mass regimes for two physically-motivated mass distributions of companions around Sun-like stars. We find that for Jupiter-mass planets, the posterior transit probability is roughly equal to the prior probability, whereas the posterior is likely higher for Super-Earths and Neptunes (10 M⊕ - 30 M⊕) and Super-Jupiters (3 MJup - 10 MJup), owing to the predicted steep rise in the mass function toward smaller masses in these regimes. We therefore suggest that companions with minimum masses in these regimes might be better-than-expected targets for transit follow-up, and we identify promising targets from RV-detected planets in the literature. Finally, we consider the uncertainty in the transit probability arising from uncertainties in the input parameters, and the effect of ignoring the dependence of the transit probability on the true semimajor axis on i.

Recurring Occultations of RW Aurigae by Coagulated Dust in the Tidally Disrupted Circumstellar Disk

We present photometric observations of RW Aurigae, a Classical T Tauri system, that reveal two remarkable dimming events. These events are similar to that which we observed in 2010-2011, which was the first such deep dimming observed in RW Aur in a centurys worth of photometric monitoring. We suggested the 2010-2011 dimming was the result of an occultation of the star by its tidally disrupted circumstellar disk. In 2012-2013, the RW Aur system dimmed by ~0.7 mag for ~40 days and in 2014/2015 the system dimmed by ~2 mag for >250 days. The ingress/egress duration measurements of the more recent events agree well with those from the 2010-2011 event, providing strong evidence that the new dimmings are kinematically associated with the same occulting source. Therefore, we suggest that both the 2012-2013 and 2014-2015 dimming events, measured using data from the Kilodegree Extremely Little Telescope and the Kutztown University Observatory, are also occultations of RW Aur A by tidally disrupted circumstellar material. Recent hydrodynamical simulations of the eccentric fly-by of RW Aur B suggest the occulting body to be a bridge of material connecting RW Aur A and B. These simulations suggest the possibility of additional occultations, supported by the observations presented in this work. The color evolution of the dimmings suggest that the tidally stripped disk material includes dust grains ranging in size from small grains at the leading edge, typical of star forming regions, to large grains, ices or pebbles producing grey or nearly grey extinction deeper within the occulting material. It is not known whether this material represents arrested planet building prior to the tidal disruption event, or perhaps accelerated planet building as a result of the disruption event, but in any case the evidence suggests the presence of advanced planet building material in the space between RW Aur A and B.

A Bright Short Period M-M Eclipsing Binary from the KELT Survey: Magnetic Activity and the Mass–Radius Relationship for M Dwarfs

J.L. acknowledges support from an NSF REU site grant to Vanderbilt University (PHY-1263045). J.E.R. and K.G.S. acknowledge partial support from NSF PAARE grant AST1358862. Work performed by J.E.R. was supported by the Harvard Future Faculty Leaders Postdoctoral fellowship. Work by B.S.G. and D.J.S. was partially supported by NSF CAREER Grant AST-1056524.

KELT-20b: A Giant Planet with a Period of P ∼ 3.5 days Transiting the V ∼ 7.6 Early A Star HD 185603

We report the discovery of KELT-20b, a hot Jupiter transiting a V ~ 7.6 early A star, HD 185603, with an orbital period of P ≃ 3.47 days. Archival and follow-up photometry, Gaia parallax, radial velocities, Doppler tomography, and AO imaging were used to confirm the planetary nature of KELT-20b and characterize the system. From global modeling we infer that KELT-20 is a rapidly rotating (ν sin I* ≃ 120 km s^(-1)) A2V star with an effective temperature of T_(eff) = 8730^(+250)_(-260) K, mass of, M* = ^(+0.14)_(-0.20) M⊙ radius of, R* = 1.561^(+0.058)_(-0.064) R⊙ surface gravity of, log g* = 4.292^(+0.017)_(-0.020), and age of ≾600 Myr. The planetary companion has a radius of R_P = 1.735^(+0.070)_(-0.075) R_J, a semimajor axis of a = 0.0542^(+0.0014)_(-0.0021) au, and a linear ephemeris of BJD_(TDB) = 2457503.120049 ± 0.000190 + E(3.4741070 ± 0.0000019). We place a 3σ upper limit of ~3.5 M_J on the mass of the planet. Doppler tomographic measurements indicate that the planetary orbit normal is well aligned with the projected spin axis of the star (λ = 3.°4± 2.°1). The inclination of the star is constrained to 24.°4 < I* < 155.°6, implying a three-dimensional spin–orbit alignment of 1.°3 < ψ < 69.°8. KELT-20b receives an insolation flux of ~8 x 10^9 erg s^(-1) cm^(-2), implying an equilibrium temperature of of ~2250 K, assuming zero albedo and complete heat redistribution. Due to the high stellar T_(eff), KELT-20b also receives an ultraviolet (wavelength d ⩽ 91.2 nm) insolation flux of ~9.1 x 10^4 erg s^(-1) cm^(-2), possibly indicating significant atmospheric ablation. Together with WASP-33, Kepler-13 A, HAT-P-57, KELT-17, and KELT-9, KELT-20 is the sixth A star host of a transiting giant planet, and the third-brightest host (in V) of a transiting planet.