Stephen J. Fossey

Line-profile tomography of exoplanet transits – II. A gas-giant planet transiting a rapidly rotating A5 star★

Most of our knowledge of extrasolar planets rests on precise radial-velocity measurements, either for direct detection or for confirmation of the planetary origin of photometric transit signals. This has limited our exploration of the parameter space of exoplanet hosts to solar- and later-type, sharp-lined stars. Here we extend the realm of stars with known planetary companions to include hot, fast-rotating stars. Planet-like transits have previously been reported in the light curve obtained by the SuperWASP survey of the A5 star HD 15082 (WASP–33; V= 8.3, v sin i= 86 km s−1). Here we report further photometry and time-series spectroscopy through three separate transits, which we use to confirm the existence of a gas-giant planet with an orbital period of 1.22 d in orbit around HD 15082. From the photometry and the properties of the planet signal travelling through the spectral line profiles during the transit, we directly derive the size of the planet, the inclination and obliquity of its orbital plane and its retrograde orbital motion relative to the spin of the star. This kind of analysis opens the way to studying the formation of planets around a whole new class of young, early-type stars, hence under different physical conditions and generally in an earlier stage of formation than in sharp-lined late-type stars. The reflex orbital motion of the star caused by the transiting planet is small, yielding an upper mass limit of 4.1 MJupiter on the planet. We also find evidence of a third body of substellar mass in the system, which may explain the unusual orbit of the transiting planet. In HD 15082, the stellar line profiles also show evidence of non-radial pulsations, clearly distinct from the planetary transit signal. This raises the intriguing possibility that tides raised by the close-in planet may excite or amplify the pulsations in such stars.

On the detectability of habitable exomoons with Kepler-class photometry

In this paper we investigate the detectability of a habitable-zone exomoon around various configurations of exoplanetary systems with the Kepler Mission or photometry of approximately equal quality. We calculate both the predicted transit timing signal amplitudes and the estimated uncertainty on such measurements in order to calculate the confidence in detecting such bodies across a broad spectrum of orbital arrangements. The effects of stellar variability, instrument noise and photon noise are all accounted for in the analysis. We validate our methodology by simulating synthetic lightcurves and performing a Monte Carlo analysis for several cases of interest.

Detection of a transit by the planetary companion of HD 80606

We report the detection of a transit egress by the ∼3.9-Jupiter-mass planet HD 80606b, an object in a highly eccentric orbit (e ∼ 0.93) about its parent star of approximately solar type. The astrophysical reality of the signal of variability in HD 80606 is confirmed by observation with two independent telescope systems and checks against several reference stars in the field. Differential photometry with respect to the nearby comparison star HD 80607 provides a precise light curve. Modelling of the light curve with a full eccentric-orbit model indicates a planet/star-radius ratio of 0.1057 ± 0.0018, corresponding to a planet radius of 1.029RJ for a solar-radius parent star; and a precise orbital inclination of 89. ◦ 285 ± 0. ◦ 023, giving a total transit duration of 12.1 ± 0.4 hours. The planet hence joins HD 17156b in a class of highly eccentric transiting planets, in which HD 80606b has both the longest period and most eccentric orbit. The recently reported discovery of a secondary eclipse of HD 80606b by the Spitzer Space Observatory permits a combined analysis with the mid-time of primary transit in which the orbital parameters of the system can be tightly constrained. We derive a transit ephemeris of T tr = HJD (245 4876.344 ± 0.011) + (111.4277 ± 0.0032) × E.

Water in the atmosphere of HD 209458b from 3.6–8 μm IRAC photometric observations in primary transit

The hot Jupiter HD 209458b was observed during primary transit at 3.6, 4.5, 5.8 and 8.0 microns using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We detail here the procedures we adopted to correct for the systematic trends present in the IRAC data. The light curves were fitted including limb darkening effects and fitted using Markov Chain Monte Carlo and prayer-bead Monte Carlo techniques, finding almost identical results. The final depth measurements obtained by a combined Markov Chain Monte Carlo fit are at 3.6 microns, 1.469 +- 0.013 % and 1.448 +- 0.013 %; at 4.5 microns, 1.478 +- 0.017 % ; at 5.8 microns, 1.549 +- 0.015 % and at 8.0 microns 1.535 +- 0.011 %. Our results clearly indicate the presence of water in the planetary atmosphere. Our broad band photometric measurements with IRAC prevent us from determining the additional presence of other other molecules such as CO, CO2 and methane for which spectroscopy is needed. While water vapour with a mixing ratio of 10^-4-10^-3 combined with thermal profiles retrieved from the day-side may provide a very good fit to our observations, this data set alone is unable to resolve completely the degeneracy between water abundance and atmospheric thermal profile.

Of-type stars HD 16691 and HD 190429 show WN-like spectra in infrared K band

We present 2 micrometer K-band spectra of two early-type Of stars that have infrared emission-line morphology similar to that of WN stars. Archival International Ultraviolet Explorer (IUE) spectra of these two stars indicate they appear to be Of type, rather than WN. Recently acquired optical spectra of these stars are quantitatively similar to that in the past, namely, Of attributes. We suggest that these two Of stars have stellar wind characteristics closer to WN type than other Of stars. We discuss the consequences for K-band classification of highly obscured hot stars that might not otherwise be visible in optical or UV wavelengths.

Detailed maps of interstellar clouds in front of ω Centauri: small-scale structures in the Galactic disc–halo interface

The multiphase interstellar medium (ISM) is highly structured, on scales from the size of the Solar System to that of a galaxy. In particular, small-scale structures are difficult to Study and hence are poorly understood. We used the multiplex capabilities of the AAOmega spectrograph at the Anglo-Australian Telescope to create a half-square-degree map of the neutral and low-ionized ;ISM in front of the nearby (similar to 5 kpc), most massive Galactic globular cluster, omega Centauri. Its redshifted, metal-poor and hot horizontal branch stars probe the medium-strong Ca II K and Na I D-2 line absorption, and weak absorption in the lambda 5780 and lambda 5797 diffuse interstellar bands (DIBs), on scales around a parsec. The kinematical and thermodynamical picture emerging from these data is that we predominantly probe the warm neutral medium and weakly ionized medium of the Galactic Disc-Halo interface, similar to 0.3-1 kpc above the mid-plane. A comparison with Spitzer Space Telescope 24 mu m and DIRBE/IRAS maps of the warm and cold dust emission confirms that both Na I and Ca II trace the overall column density of the warm neutral and weakly ionized medium. Clear signatures are seen of the depletion of calcium atoms from the gas phase into dust grains. Curiously, the coarse DIRBE/IRAS map is a more reliable representation of the relative reddening between sightlines than the Na I and Ca II absorption-line measurements, most likely because the latter are sensitive to fluctuations in the local ionization conditions. The behaviour of the DIBs is consistent with the;0780 band being stronger than the lambda 5797 band in regions where the ultraviolet radiation level is relatively high, as in the Disc-Halo interface. This region corresponds to a or-type cloud in which Ca I and small diatomic molecules Such as CH and CN are usually absent. In all, Our maps and simple analytical model calculations show in unprecedented detail that small-scale density and/or ionization structures exist in the extra-planar gas of a spiral galaxy.

Small-scale structure in the interstellar medium: time-varying interstellar absorption towards κ Velorum

Ultra-high spectral resolution observations of time-varying interstellar absorption towards κ Vel are reported, using the Ultra-High Resolution Facility on the Anglo-Australian Telescope. Detections of interstellar Ca i, Ca ii, K i, Na i and CH are obtained, whilst an upper limit on the column density is reported for C_2. The results show continued increases in column densities of K i and Ca i since observations ∼4 yr earlier, as the transverse motion of the star carried it ∼10 au perpendicular to the line of sight. Line profile models are fitted to the spectra and two main narrow components (A and B) are identified for all species except CH. The column density N(K i) is found to have increased by 82^(+10)_(−9) per cent between 1994 and 2006, whilst N(Ca i) is found to have increased by 32 ± 5 per cent over the shorter period of 2002–2006. The line widths are used to constrain the kinetic temperature to T_(k,A) < 671^(+18)_(−17) K and T_(k,B) < 114^(+15)_(−14) K. Electron densities are determined from the Ca i/Ca ii ratio, which in turn place lower limits on the total number density of n_A ≳ 7 × 10^3 cm^(−3) and n_B ≳ 2 × 10^4 cm^(−3). Calcium depletions are estimated from the Ca i/K i ratio. Comparison with the chemical models of Bell et al. confirms the high number density, with n = 5 × 10^4 cm^(−3) for the best-fitting model. The first measurements of diffuse interstellar bands (DIBs) towards this star are made at two epochs, but only an upper limit of ≲ 40 per cent is placed on their variation over ∼9 yr. The DIBs are unusually weak for the measured E(B − V) and appear to exhibit similar behaviour to that seen in Orion. The ratio of equivalent widths of the λ5780 to λ5797 DIBs is amongst the highest known, which may indicate that the carrier of λ5797 is more sensitive to ultraviolet radiation than to local density.