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John Southworth

Publications and source records attributed to John Southworth.

At least 145 records · Page 8Linked to original sources

Homogeneous studies of transiting extrasolar planets. IV. Thirty systems with space-based light curves

I calculate the physical properties of 32 transiting extrasolar planet and brown dwarf systems from existing photometric observations and measured spectroscopic parameters. The systems studied include fifteen observed by CoRoT, ten by Kepler and five by the Deep Impact spacecraft. Inclusion of the objects studied in previous papers leads to a sample of 58 transiting systems with homogeneously measured properties. The Kepler data include observations from Quarter 2, and my analyses of several of the systems are the first to be based on short-cadence data from this satellite. The light curves are modelled using the JKTEBOP code, with attention paid to the treatment of limb darkening, contaminating light, eccentricity, correlated noise, and numerical integration over long exposure times. The physical properties are derived using constraints from five sets of theoretical stellar model predictions. An alternative approach using a calibration from eclipsing binary star systems is explored and found to give comparable numbers. My results are in good agreement with published properties for most of the transiting systems, but discrepancies are identified for CoRoT-5, CoRoT-8, CoRoT-13, Kepler-5 and Kepler-7. Many of the errorbars quoted in the literature are underestimated. Refined orbital ephemerides are given for CoRoT-8 and for the Kepler planets. Asteroseismic constraints on the density of the host stars are in good agreement with the photometric equivalents for HD 17156 and TrES-2, but not for HAT-P-7 and HAT-P-11. Complete error budgets are generated for each transiting system. Whilst most would benefit from further photometry and spectroscopy, HD 17156, HD 80606, HAT-P-7 and TrES-2 are now extremely well characterised. HAT-P-11 is an exceptional candidate for studying starspots. The orbital ephemerides of some transiting systems are becoming uncertain and they should be re-observed.

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Absolute dimensions of detached eclipsing binaries. II. The metallic-lined system XY Ceti

We present phase-resolved spectroscopy and extensive survey photometry of the detached eclipsing binary system XY Cet, which is composed of two metallic-lined stars. We measure their masses to be 1.773 +/- 0.016 and 1.615 +/- 0.014 Msun and their radii to be 1.873 +/- 0.035 and 1.773 +/- 0.029 Rsun, resulting in logarithmic surface gravities of 4.142 +/- 0.016 and 4.149 +/- 0.014 (cgs). We determine effective temperatures of 7870 +/- 115 and 7620 +/- 125 K. The projected rotational velocities are 34.4 +/- 0.4 and 34.1 +/- 0.4 km/s, which are close to synchronous. Theoretical models cannot match all of these properties, but come closest for a solar helium and metal abundance and an age in the region of 850 Myr. We obtain the individual spectra of the two stars by the spectral disentangling method, and compare them to synthetic spectra calculated for the measured effective temperatures and a solar chemical composition. Both stars show enhanced abundances of iron-group elements and clear deficiencies of Ca I and Sc II, confirming their classification as Am stars. We also find strong overabundances of Zr II and the rare-earth species La II, Ce II and Nd II, a hallmark of chemically peculiar A-stars. XY Cet is a prime candidate for detailed spectroscopic analyses of metallic-lined stars whose masses and radii are known to accuracies of 1-2%.

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Kepler photometry of KIC 10661783: a binary star with total eclipses and delta Scuti pulsations

We present Kepler satellite photometry of KIC 10661783, a short-period binary star system which shows total eclipses and multi-periodic delta Scuti pulsations. A frequency analysis of the eclipse-subtracted light curve reveals at least 68 frequencies of which 55 or more can be attributed to pulsation modes. The main limitation on this analysis is the frequency resolution within the 27-day short-cadence light curve. Most of the variability signal lies in the frequency range 18 to 31 c/d, with amplitudes between 0.1 and 4 mmag. One harmonic term (2.f) and a few combination frequencies (f_i+f_j) have been detected. From a plot of the residuals versus orbital phase we assign the pulsations to the primary star in the system. The pulsations were removed from the short-cadence data and the light curve was modelled using the Wilson-Devinney code. We are unable to get a perfect fit due to the residual effects of pulsations and also to the treatment of reflection and reprocessing in the light curve model. A model where the secondary star fills its Roche lobe is favoured, which means that KIC 10661783 can be classified as an oEA system. Further photometric and spectroscopic observations will allow the masses and radii of the two stars to be measured to high precision and hundreds of delta Scuti pulsation frequencies to be resolved. This could lead to unique constraints on theoretical models of delta Scuti stars, if the evolutionary history of KIC 10661783 can be accounted for.

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High-speed photometry of the eclipsing cataclysmic variable 1RXS J180834.7+101041

We present high-speed photometry covering one eclipse of the cataclysmic variable system 1RXS J180834.7+101041. The data are modelled as arising from a close binary system containing a low-mass stars filling its Roche lobe plus a white dwarf with an accretion disc. We find an orbital inclination of 77.07 +/- 0.47 degrees and a mass ratio of 0.168 +/- 0.016. The relatively low inclination means that only the accretion disc and not the white dwarf are eclipsed by the low-mass star in this system.

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A much lower density for the transiting extrasolar planet WASP-7

We present the first high-precision photometry of the transiting extrasolar planetary system WASP-7, obtained using telescope defocussing techniques and reaching a scatter of 0.68 mmag per point. We find that the transit depth is greater and that the host star is more evolved than previously thought. The planet has a significantly larger radius (1.330 +/- 0.093 Rjup versus 0.915 +0.046 -0.040 Rjup) and much lower density (0.41 +/- 0.10 rhojup versus 1.26 +0.25 -0.21 rhojup) and surface gravity (13.4 +/- 2.6 m/s2 versus 26.4 +4.4 -4.0 m/s2) than previous measurements showed. Based on the revised properties it is no longer an outlier in planetary mass--radius and period--gravity diagrams. We also obtain a more precise transit ephemeris for the WASP-7 system.

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Orbital periods of cataclysmic variables identified by the SDSS. VIII. A slingshot prominence in SDSS J003941.06+005427.5?

We present VLT spectroscopy and NTT photometry of the faint cataclysmic binary SDSS J003941.06+005427.5. This object shows triple-peaked H-alpha emission with all three peaks variable in both strength and velocity. We measure an orbital period of 91.395 +/- 0.093 min from the velocity variations of the wings of the H-alpha emission line. Using the GALEX and SDSS photometry of this object, we determine a white dwarf temperature of 15000 K and a very late (>=L2) spectral type for the companion star. These measurements, plus the relatively long orbital period, suggest that SDSS J003941.06+005427.5 may be a post-bounce cataclysmic variable. Doppler maps of the H-alpha and He I 6678A emission features show an accretion disc with a non-uniform brightness and departures from Keplerian flow. The third emission peak is detected only in H-alpha and at a relatively low velocity amplitude of 202 +/- 3 km/s. We are unable to explain this emission as arising from either the white dwarf, the secondary star, or the accretion disc. We tentatively attribute this mysterious central peak to a coronal loop anchored at the secondary star. If confirmed, this would be the first example of a slingshot prominence in a CV with a low mass-transfer rate and/or a fully convective secondary star.

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Homogeneous studies of transiting extrasolar planets. III. Additional planets and stellar models

I derive the physical properties of 30 transiting extrasolar planetary systems using a homogeneous analysis of published data. The light curves are modelled with the JKTEBOP code, with attention paid to limb darkening and eccentricity. The light from some systems is contaminated by faint nearby stars, which if ignored will systematically bias the results. I show that this must be accounted for using external measurements of the amount of contaminating light. A contamination of 5% is enough to make the measurement of a planetary radius 2% too low. The physical properties of the 30 transiting systems are obtained by interpolating in stellar model predictions to find the best match to their measured quantities. The error budgets are used to compile a list of systems which would benefit from additional observations. The systematic errors arising from the inclusion of stellar models are assessed by using five different theoretical models. This model dependence sets a lower limit on the accuracy of measurements of the system properties, and at worst is 1% for the stellar mass. The correlations of planetary surface gravity and mass with orbital period have significance levels of only 3.1 sigma and 2.3 sigma. The division of planets into two classes based on Safronov number is increasingly blurred. Most of the objects studied here would benefit from more photometry and/or spectroscopy, as well as a better understanding of low-mass stars.

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High-precision photometry by telescope defocussing. III. The transiting planetary system WASP-2

We present high-precision photometry of three transits of the extrasolar planetary system WASP-2, obtained by defocussing the telescope, and achieving point-to-point scatters of between 0.42 and 0.73 mmag. These data are modelled using the JKTEBOP code, and taking into account the light from the recently-discovered faint star close to the system. The physical properties of the WASP-2 system are derived using tabulated predictions from five different sets of stellar evolutionary models, allowing both statistical and systematic errorbars to be specified. We find the mass and radius of the planet to be M_b = 0.847 +/- 0.038 +/- 0.024 Mjup and R_b = 1.044 +/- 0.029 +/- 0.015 Rjup. It has a low equilibrium temperature of 1280 +/- 21 K, in agreement with a recent finding that it does not have an atmospheric temperature inversion. The first of our transit datasets has a scatter of only 0.42 mmag with respect to the best-fitting light curve model, which to our knowledge is a record for ground-based observations of a transiting extrasolar planet.

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Orbital periods of cataclysmic variables identified by the SDSS. VII. Four new eclipsing systems

We present photometry of nine cataclysmic variable stars identified by the Sloan Digital Sky Survey, aimed at measuring the orbital periods of these systems. Four of these objects show deep eclipses, from which we measure their orbital periods. The light curves of three of the eclipsing systems are also analysed using the LCURVE code, and their mass ratios and orbital inclinations determined. SDSS J075059.97+141150.1 has an orbital period of 134.1564 +/- 0.0008 min, making it a useful object with which to investigate the evolutionary processes of cataclysmic variables. SDSS J092444.48+080150.9 has a period of 131.2432 +/- 0.0014 min and is probably magnetic. The white dwarf ingress and egress phases are very deep and short, and there is no clear evidence that this object has an accretion disc. SDSS J115207.00+404947.8 and SDSS J152419.33+220920.1 are nearly identical twins, with periods of 97.5 +/- 0.4 and 93.6 +/- 0.5 min and mass ratios of 0.14 +/- 0.03 and 0.17 +/- 0.03, respectively. Their eclipses have well-defined white dwarf and bright spot ingress and egress features, making them excellent candidates for detailed study. All four of the orbital periods presented here are shorter than the 2-3 hour period gap observed in the known population of cataclysmic variables.

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Physical properties of the 0.94-day period transiting planetary system WASP-18

We present high-precision photometry of five consecutive transits of WASP-18, an extrasolar planetary system with one of the shortest orbital periods known. Through the use of telescope defocussing we achieve a photometric precision of 0.47 to 0.83 mmag per observation over complete transit events. The data are analysed using the JKTEBOP code and three different sets of stellar evolutionary models. We find the mass and radius of the planet to be M_b = 10.43 +/- 0.30 +/- 0.24 Mjup R_b = 1.165 +/- 0.055 +/- 0.014 Rjup (statistical and systematic errors) respectively. The systematic errors in the orbital separation and the stellar and planetary masses, arising from the use of theoretical predictions, are of a similar size to the statistical errors and set a limit on our understanding of the WASP-18 system. We point out that seven of the nine known massive transiting planets (M_b > 3 Mjup) have eccentric orbits, whereas significant orbital eccentricity has been detected for only four of the 46 less massive planets. This may indicate that there are two different populations of transiting planets, but could also be explained by observational biases. Further radial velocity observations of low-mass planets will make it possible to choose between these two scenarios.

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Data Mining for Dwarf Novae in SDSS, GALEX and Astrometric Catalogues

By cross matching blue objects from SDSS with GALEX and the astrometric catalogues USNO-B1.0, GSC2.3 and CMC14, 64 new dwarf nova candidates with one or more observed outbursts have been identified. 14 of these systems are confirmed as cataclysmic variables through existing and follow-up spectroscopy. A study of the amplitude distribution and an estimate of the outburst frequency of these new dwarf novae and those discovered by the Catalina Real-time Transient Survey (CRTS) indicates that besides systems that are faint because they are farther away, there also exists a population of intrinsically faint dwarf novae with rare outbursts.

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Orbital periods of cataclysmic variables identified by the SDSS. VI. The 4.5-hr period eclipsing system SDSS J100658.40+233724.4

We present time-resolved spectroscopy and photometry of SDSS J100658.40+233724.4, which we have discovered to be an eclipsing cataclysmic variable with an orbital period of 0.18591324 days (267.71507 min). The observed velocity amplitude of the secondary star is 276 +/- 7 km/s, which an irradiation correction reduces to 258 +/- 12 km/s. Doppler tomography of emission lines from the infrared calcium triplet supports this measurement. We have modelled the light curve using the LCURVE code and Markov Chain Monte Carlo simulations, finding a mass ratio of 0.51 +/- 0.08. From the velocity amplitude and the light curve analysis we find the mass of the white dwarf to be 0.78 +/- 0.12 Msun and the masses and radii of the secondary star to be 0.40 +/- 0.10 Msun and 0.466 +/- 0.036 Rsun, respectively. The secondary component is less dense than a normal main sequence star but its properties are in good agreement with the expected values for a CV of this orbital period. By modelling the spectral energy distribution of the system we find a distance of 676 +/- 40 pc and estimate a white dwarf effective temperature of 16500 +/- 2000 K.

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High-precision photometry by telescope defocussing. II. The transiting planetary system WASP-4

We present and analyse light curves of four transits of the Southern hemisphere extrasolar planetary system WASP-4, obtained with a telescope defocussed so the radius of each point spread function was 17 arcsec (44 pixels). This approach minimises both random and systematic errors, allowing us to achieve scatters of between 0.60 and 0.88 mmag per observation over complete transit events. The light curves are augmented by published observations and analysed using the JKTEBOP code. The results of this process are combined with theoretical stellar model predictions to derive the physical properties of the WASP-4 system. We find that the mass and radius of the planet are M_b = 1.289 {+0.090 -0.090} {+0.039 -0.000} MJup and R_b = 1.371 {+0.032 -0.035} {+0.021 -0.000} RJup, respectively (statistical and systematic uncertainties). These quantities give a surface gravity and density of g_b = 17.03 +0.97 -0.54 m/s2 and rho_b = 0.500 {+0.032 -0.021} {+0.000 -0.008} rhoJup, and fit the trends for short-period extrasolar planets to have relatively high masses and surface gravities. WASP-4 is now one of the best-quantified transiting extrasolar planetary systems, and significant further progress requires improvements to our understanding of the physical properties of low-mass stars.

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High-precision photometry by telescope defocussing. I. The transiting planetary system WASP-5

We present high-precision photometry of two transit events of the extrasolar planetary system WASP-5, obtained with the Danish 1.54m telescope at ESO La Silla. In order to minimise both random and flat-fielding errors, we defocussed the telescope so its point spread function approximated an annulus of diameter 40 pixels (16 arcsec). Data reduction was undertaken using standard aperture photometry plus an algorithm for optimally combining the ensemble of comparison stars. The resulting light curves have point-to-point scatters of 0.50 mmag for the first transit and 0.59 mmag for the second. We construct detailed signal to noise calculations for defocussed photometry, and apply them to our observations. We model the light curves with the JKTEBOP code and combine the results with tabulated predictions from theoretical stellar evolutionary models to derive the physical properties of the WASP-5 system. We find that the planet has a mass of M_b = 1.637 +/- 0.075 +/- 0.033 Mjup, a radius of R_b = 1.171 +/- 0.056 +/- 0.012 Rjup, a large surface gravity of g_b = 29.6 +/- 2.8 m/s2 and a density of rho_b = 1.02 +/- 0.14 +/- 0.01 rhojup (statistical and systematic uncertainties). The planet's high equilibrium temperature of T_eq = 1732 +/- 80 K makes it a good candidate for detecting secondary eclipses.

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Homogeneous studies of transiting extrasolar planets. II. Physical properties

I present an homogeneous determination of the physical properties of 14 transiting extrasolar planetary systems for which good data are available. The input quantities for each system are the results of the light curve analyses (Paper 1), and published measurements of the stellar velocity amplitude, Teff and [Fe/H]. The physical properties are determined by interpolating within tabulated predictions from stellar theory. Statistical uncertainties are found using a perturbation algorithm, which gives a detailed error budget for every output quantity. Systematic uncertainties are assessed for each quantity by comparing the values found using different stellar models. As a theory-free alternative, physical properties are also calculated using an empirical mass-radius relation constructed using low-mass eclipsing binary stars. The properties of the planets depend mostly on parameters measured from the light and velocity curves, and have only a minor sensitivity to theoretical predictions. In contrast, the orbital semimajor axes and stellar masses have a strong dependence on theoretical predictions, and their systematic uncertainties can be substantially larger than the statistical ones. Using the empirical mass--radius relation instead, their values are smaller by up to 15%. Thus our understanding of extrasolar planets is currently limited by our lack of understanding of low-mass stars. Using all known transiting planets, I find that correlations between their orbital periods, masses and surface gravities are significant at the 2-3 sigma level. However, the separation of the known planets into two classes according to their Safronov number is weaker than previously found, and may not be statistically significant (abridged).

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The fainter the better: cataclysmic variable stars from the SDSS

The Sloan Digital Sky Survey has identified a total of 212 cataclysmic variables, most of which are fainter than 18th magnitude. This is the deepest and most populous homogeneous sample of cataclysmic variables to date, and we are undertaking a project to characterise this population. We have found that the SDSS sample is dominated by a great ``silent majority'' of old and faint CVs. We detect, for the first time, a population spike at the minimum period of 80 minutes which has been predicted by theoretical studies for over a decade.

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Orbital periods of cataclysmic variables identified by the SDSS. V. VLT, NTT and Magellan observations of nine equatorial systems

We present VLT and Magellan spectroscopy and NTT photometry of nine faint cataclysmic variables (CVs) which were spectroscopically identified by the SDSS. We measure orbital periods for five of these from the velocity variations of the cores and wings of their Halpha emission lines. Four of the five have orbital periods shorter than the 2-3 hour period gap observed in the known population of CVs. SDSS J004335.14-003729.8 has an orbital period of Porb = 82.325 +/- 0.088 min; Doppler maps show emission from the accretion disc, bright spot and the irradiated inner face of the secondary star. In its light curve we find a periodicity which may be attributable to pulsations of the white dwarf. SDSS J163722.21-001957.1 has Porb = 99.75 +/- 0.86 min. By combining this new measurement with a published superhump period we estimate a mass ratio of 0.16 and infer the physical properties and orbital inclination of the system. For SDSS J164248.52+134751.4 we find Porb = 113.60 +/- 1.5 min. The Doppler map of this CV shows an unusual brightness distribution in the accretion disc which would benefit from further observations. SDSS J165837.70+184727.4 had spectroscopic characteristics which were very different between the SDSS spectrum and our own VLT observations, despite only a small change in brightness. We measure Porb = 98.012 +/- 0.065 min from its narrow Halpha emission line. Finally, SDSS J223843.84+010820.7 has a comparatively longer period of Porb = 194.30 +/- 0.16 min. It contains a magnetic white dwarf and, with g = 18.15, is brighter than the other objects studied here. These results continue the trend for the fainter CVs identified by the SDSS to be almost exclusively shorter-period objects with low mass transfer rates.

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On the evolutionary status of short period cataclysmic variables

(Abridged) We present high-speed, three-colour photometry of seven short period (Porb <= 95 mins) eclipsing CVs from the Sloan Digital Sky Survey. We determine the system parameters via a parametrized model of the eclipse fitted to the observed lightcurve by chi2 minimization. Three out of seven of the systems possess brown dwarf donor stars and are believed to have evolved past the orbital period minimum. This is in line with predictions that 40-70 per cent of CVs should have evolved past the orbital period minimum. Therefore, the main result of our study is that the missing population of post-period minimum CVs has finally been identified. The donor star masses and radii are, however, inconsistent with model predictions; the donor stars are approximately 10 per cent larger than expected across the mass range studied here. One explanation for the discrepancy is enhanced angular momentum loss (e.g. from circumbinary discs), however the mass-transfer rates, as deduced from white dwarf effective temperatures, are not consistent with enhanced angular momentum loss. We show it is possible to explain the large donor radii without invoking enhanced angular momentum loss by a combination of geometrical deformation and the effects of starspots due to strong rotation and expected magnetic activity. Choosing unambiguously between these different solutions will require independent estimates of the mass-transfer rates in short period CVs.

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