arXiv subjects

John Southworth

Publications and source records attributed to John Southworth.

At least 127 records · Page 7Linked to original sources

arXiv2014

Variable Emission from a Gaseous Disc around a Metal-Polluted White Dwarf

We present the discovery of strongly variable emission lines from a gaseous disc around the DA white dwarf SDSS J1617+1620, a star previously found to have an infrared excess indicative of a dusty debris disc formed by the tidal disruption of a rocky planetary body. Time-series spectroscopy obtained during the period 2006-2014 has shown the appearance of strong double-peaked Ca II emission lines in 2008. The lines were weak, at best, during earlier observations, and monotonically faded through the remainder of our monitoring. Our observations represent unambiguous evidence for short-term variability in the debris environment of evolved planetary systems. Possible explanations for this extraordinary variability include the impact onto the dusty disc of either a single small rocky planetesimal, or of material from a highly eccentric debris tail. The increase in flux from the emission lines is sufficient that similar events could be detected in the broadband photometry of ongoing and future large-area time domain surveys.

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arXiv2014

High-precision photometry by telescope defocussing. VI. WASP-24, WASP-25 and WASP-26

We present time-series photometric observations of thirteen transits in the planetary systems WASP-24, WASP-25 and WASP-26. All three systems have orbital obliquity measurements, WASP-24 and WASP-26 have been observed with Spitzer, and WASP-25 was previously comparatively neglected. Our light curves were obtained using the telescope-defocussing method and have scatters of 0.5 to 1.2 mmag relative to their best-fitting geometric models. We used these data to measure the physical properties and orbital ephemerides of the systems to high precision, finding that our improved measurements are in good agreement with previous studies. High-resolution Lucky Imaging observations of all three targets show no evidence for faint stars close enough to contaminate our photometry. We confirm the eclipsing nature of the star closest to WASP-24 and present the detection of a detached eclipsing binary within 4.25 arcmin of WASP-26.

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arXiv2014

Warm Ice Giant GJ 3470b. II Revised Planetary and Stellar Parameters from Optical to Near-infrared Transit Photometry

It is important to explore the diversity of characteristics of low-mass, low-density planets to understand the nature and evolution of this class of planets. We present a homogeneous analysis of 12 new and 9 previously published broadband photometric observations of the Uranus-sized extrasolar planet GJ 3470b, which belongs to the growing sample of sub-Jovian bodies orbiting M dwarfs. The consistency of our analysis explains some of the discrepancies between previously published results and provides updated constraints on the planetary parameters. Our data are also consistent with previous transit observations of this system. We also provide new spectroscopic measurements of GJ 3470 from 0.33 to 2.42 $μ$$m$ to aid our analysis. We find $R_{\star}$ = 0.48$\pm$0.04 $R_{\odot}$, $M_{\star}$ = 0.51$\pm$0.06 $M_{\odot}$, and $T_{\rm eff}$ = 3652$\pm$50 K for GJ 3470, along with a rotation period of $20.70\pm{0.15}$ d and an R-band amplitude of 0.01 mag, which is small enough that current transit measurements should not be strongly affected by stellar variability. We also present the most precise orbital ephemeris for this system: T$_{o}$ = 2455983.70472$\pm$0.00021 BJD$_{TDB}$, P = 3.3366487$^{+0.0000043}_{-0.0000033}$ d, and we see no evidence for transit timing variations greater than 1 minute. Our reported planet to star radius ratio is 0.07642$\pm$0.00037. The physical parameters of this planet are $R_{p}$ = 3.88$\pm$0.32 $R_{\oplus}$, and $M_{p}$ = 13.73$\pm$1.61 $M_{\oplus}$. Because of our revised stellar parameters, the planetary radius we present is smaller than previously reported values. We also perform a second analysis of the transmission spectrum of the entire ensemble of transit observations to date, supporting the existence of a H$_{2}$ dominated atmosphere exhibiting a strong Rayleigh scattering slope.

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arXiv2014

KIC 7582608: A new Kepler roAp star with frequency variability

We analyse the fifth roAp star reported in the Kepler field, KIC 7582608, discovered with the SuperWASP project. The object shows a high frequency pulsation at 181.7324 d$^{-1}$ (P=7.9 min) with an amplitude of 1.45 mmag, and low frequency rotational modulation corresponding to a period of 20.4339 d with an amplitude of 7.64 mmag. Spectral analysis confirms the Ap nature of the target, with characteristic lines of Eu II, Nd III and Pr III present. The spectra are not greatly affected by broadening, which is consistent with the long rotational period found from photometry. From our spectral observations we derive a lower limit on the mean magnetic field modulus of = 3.05$\pm$0.23 kG. Long Cadence Kepler observations show a frequency quintuplet split by the rotational period of the star, typical for an oblique pulsator. We suggest the star is a quadrupole pulsator with a geometry such that $i\sim66^\circ$ and $β\sim33^\circ$. We detect frequency variations of the pulsation in both the WASP and Kepler data sets on many time scales. Linear, non-adiabatic stability modelling allows us to constrain a region on the HR diagram where the pulsations are unstable, an area consistent with observations.

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arXiv2014

HSTEP -- Homogeneous Studies of Transiting Extrasolar Planets

I give a summary of the HSTEP project: an effort to calculate the physical properties of the known transiting extrasolar planets using a homogeneous approach. I discuss the motivation for the project, list the 83 planets which have already been studied, run through some important aspects of the methodology, and finish with a synopsis of the results. The results have been compiled into an online catalogue: TEPCat.

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arXiv2013

CoRoT 223992193: A new, low-mass, pre-main sequence eclipsing binary with evidence of a circumbinary disk

We present the discovery of CoRoT 223992193, a double-lined, detached eclipsing binary, comprising two pre-main sequence M dwarfs, discovered by the CoRoT space mission during a 23-day observation of the 3 Myr old NGC 2264 star-forming region. Using multi-epoch optical and near-IR follow-up spectroscopy with FLAMES on the Very Large Telescope and ISIS on the William Herschel Telescope we obtain a full orbital solution and derive the fundamental parameters of both stars by modelling the light curve and radial velocity data. The orbit is circular and has a period of $3.8745745 \pm 0.0000014$ days. The masses and radii of the two stars are $0.67 \pm 0.01$ and $0.495 \pm 0.007$ $M_{\odot}$ and $1.30 \pm 0.04$ and $1.11 ~^{+0.04}_{-0.05}$ $R_{\odot}$, respectively. This system is a useful test of evolutionary models of young low-mass stars, as it lies in a region of parameter space where observational constraints are scarce; comparison with these models indicates an apparent age of $\sim$3.5-6 Myr. The systemic velocity is within $1σ$ of the cluster value which, along with the presence of lithium absorption, strongly indicates cluster membership. The CoRoT light curve also contains large-amplitude, rapidly evolving out-of-eclipse variations, which are difficult to explain using starspots alone. The system's spectral energy distribution reveals a mid-infrared excess, which we model as thermal emission from a small amount of dust located in the inner cavity of a circumbinary disk. In turn, this opens up the possibility that some of the out-of-eclipse variability could be due to occultations of the central stars by material located at the inner edge or in the central cavity of the circumbinary disk.

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arXiv2013

Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-Wavelength Variability Monitoring of WISE J104915.57-531906.1AB

We present two epochs of MPG/ESO 2.2m GROND simultaneous 6-band ($r'i'z'JHK$) photometric monitoring of the closest known L/T transition brown dwarf binary WISE J104915.57-531906.1AB. We report here the first resolved variability monitoring of both the T0.5 and L7.5 components. We obtained 4 hours of focused observations on the night of UT 2013-04-22, as well as 4 hours of defocused (unresolved) observations on the night of UT 2013-04-16. We note a number of robust trends in our light curves. The $r'$ and $i'$ light curves appear to be anticorrelated with $z'$ and $H$ for the T0.5 component and in the unresolved lightcurve. In the defocused dataset, $J$ appears correlated with $z'$ and $H$ and anticorrelated with $r'$ and $i'$, while in the focused dataset we measure no variability for $J$ at the level of our photometric precision, likely due to evolving weather phenomena. In our focused T0.5 component lightcurve, the $K$ band lightcurve displays a significant phase offset relative to both $H$ and $z'$. We argue that the measured phase offsets are correlated with atmospheric pressure probed at each band, as estimated from 1D atmospheric models. We also report low-amplitude variability in $i'$ and $z'$ intrinsic to the L7.5 component.

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arXiv2013

Stellar Magnetism and starspots: the implications for exoplanets

Stellar variability induce by starspots can hamper the detection of exoplanets and bias planet property estimations. These features can also be used to study star-planet interactions as well as inferring properties from the underlying stellar dynamo. However, typical techniques, such as ZDI, are not possible for most host-stars. We present a robust method based on spot modelling to map the surface of active star allowing us to statistically study the effects and interactions of stellar magnetism with transiting exoplanets. The method is applied to the active Kepler-9 star where we find small evidence for a possible interaction between planet and stellar magnetosphere which leads to a 2:1 resonance between the spot rotation and orbital period.

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arXiv2013

The solar-type eclipsing binary system LL Aquarii

The eclipsing binary LL Aqr consists of two late-type stars in an eccentric orbit with a period of 20.17 d. We use an extensive light curve from the SuperWASP survey augmented by published radial velocities and UBV light curves to measure the physical properties of the system. The primary star has a mass of 1.167 +/- 0.009 Msun and a radius of 1.305 +/- 0.007 Rsun. The secondary star is an analogue of the Sun, with a mass and radius of 1.014 +/- 0.006 Msun and 0.990 +/- 0.008 Rsun respectively. The system shows no signs of stellar activity: the upper limit on spot-induced rotational modulation is 3 mmag, it is slowly rotating, has not been detected at X-ray wavelengths, and the calcium H and K lines exhibit no emission. Theoretical stellar models provide a good match to its properties for a sub-solar metal abundance of Z = 0.008 and an age of 2.5 Gyr. Most low-mass eclipsing binary systems are found to have radii larger than expected from theoretical predictions, blamed on tidally-enhanced magnetic fields in these short-period systems. The properties of LL Aqr support this scenario: it exhibits negligible tidal effects, shows no signs of magnetic activity, and matches theoretical models well.

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arXiv2013

An extremely high photometric precision in ground-based observations of two transits in the WASP-50 planetary system

We present photometric observations of two transits in the WASP-50 planetary system, obtained using the ESO New Technology Telescope and the defocussed-photometry technique. The rms scatters for the two datasets are 258 and 211\,ppm with a cadence of 170 to 200\,s, setting a new record for ground-based photometric observations of a point source. The data were modelled and fitted using the \textsc{prism} and \textsc{gemc} codes, and the physical properties of the system calculated. We find the mass and radius of the hot star to be $0.861\pm 0.057\Msun$ and $0.855\pm0.019\Rsun$, respectively. For the planet we find a mass of $1.437\pm 0.068\Mjup$, a radius of $1.138\pm0.026\Rjup$ and a density of $0.911\pm0.033\pjup$. These values are consistent with but more precise than those found in the literature. We also obtain a new orbital ephemeris for the system: $ T_0 = {\rm BJD/TDB} \,\, 2\,455\,558.61237 (20) \, + \, 1.9550938 (13) \times E $.

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arXiv2012

Transits and starspots in the WASP-19 planetary system

We have developed a new model for analysing light curves of planetary transits when there are starspots on the stellar disc. Because the parameter space contains a profusion of local minima we developed a new optimisation algorithm which combines the global minimisation power of a genetic algorithm and the Bayesian statistical analysis of the Markov chain. With these tools we modelled three transit light curves of WASP-19. Two light curves were obtained on consecutive nights and contain anomalies which we confirm as being due to the same spot. Using these data we measure the star's rotation period and velocity to be $11.76 \pm 0.09$ d and $3.88 \pm 0.15$\kms, respectively, at a latitude of 65$^\circ$. We find that the sky-projected angle between the stellar spin axis and the planetary orbital axis is $λ= 1.0^{\circ} \pm 1.2^{\circ}$, indicating axial alignment. Our results are consistent with and more precise than published spectroscopic measurements of the Rossiter-McLaughlin effect.

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arXiv2012

Homogeneous studies of transiting extrasolar planets. V. New results for 38 planets

I measure the physical properties of 38 transiting extrasolar planetary systems, bringing the total number studied within the Homogeneous Studies project to 82. Transit light curves are modelled using JKTEBOP, with careful attention paid to limb darkening, eccentricity and third light. The physical properties of each system are obtained from the photometric parameters, published spectroscopic measurements and five sets of theoretical stellar model predictions. Statistical errors are assessed using Monte Carlo and residual-permutation algorithms and propagated via a perturbation algorithm. Systematic errors are estimated from the interagreement between results calculated using five theoretical stellar models. The headline result is a major upward revision of the radius of the planet in the OGLE-TR-56 system, from 1.23-1.38 Rjup to 1.734 +/- 0.051 +/- 0.029 Rjup (statistical and systematic errors, respectively). Its density is three times lower than previously thought. This change comes from the first complete analysis of published high-quality photometry. Significantly larger planetary radii are also found for Kepler-15, KOI-428, WASP-13, WASP-14 and WASP-21 compared to previous work. I present the first results based on Kepler short-cadence data for Kepler-14, Kepler-15 and KOI-135. More extensive long-cadence data s used to improve the measured properties of KOI-196, KOI-204, KOI-254, KOI-423 and KOI-428. Detailed analyses are given for HAT-P-3, HAT-P-6, HAT-P-9, HAT-P-14 and WASP-12, based on more extensive datasets than considered in previous studies. I revisit the correlations between orbital period and surface gravity, and orbital period and mass of the transiting planets, finding both to be significant at the 4 sigma level. I conclude by discussing the opportunities for follow-up observations, the sky positions and the discovery rate of the known transiting planets.

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arXiv2012

High-precision photometry by telescope defocussing. IV. Confirmation of the huge radius of WASP-17b

We present photometric observations of four transits in the WASP-17 planetary system, obtained using telescope defocussing techniques and with scatters reaching 0.5 mmag per point. Our revised orbital period is 4.0 +/- 0.6 s longer than previous measurements, a difference of 6.6 sigma, and does not support the published detections of orbital eccentricity in this system. We model the light curves using the JKTEBOP code and calculate the physical properties of the system by recourse to five sets of theoretical stellar model predictions. The resulting planetary radius, Rb = 1.932 +/- 0.052 +/- 0.010 Rjup (statistical and systematic errors respectively), provides confirmation that WASP-17b is the largest planet currently known. All fourteen planets with radii measured to be greater than 1.6 Rjup are found around comparatively hot (Teff > 5900 K) and massive (MA > 1.15 Msun) stars. Chromospheric activity indicators are available for eight of these stars, and all imply a low activity level. The planets have small or zero orbital eccentricities, so tidal effects struggle to explain their large radii. The observed dearth of large planets around small stars may be natural but could also be due to observational biases against deep transits, if these are mistakenly labelled as false positives and so not followed up.

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arXiv2012

Physical properties and radius variations in the HAT-P-5 planetary system from simultaneous four-colour photometry

The radii of giant planets, as measured from transit observations, may vary with wavelength due to Rayleigh scattering or variations in opacity. Such an effect is predicted to be large enough to detect using ground-based observations at multiple wavelengths. We present defocussed photometry of a transit in the HAT-P-5 system, obtained simultaneously through Stromgren u, Gunn g and r, and Johnson I filters. Two more transit events were observed through a Gunn r filter. We detect a substantially larger planetary radius in u, but the effect is greater than predicted using theoretical model atmospheres of gaseous planets. This phenomenon is most likely to be due to systematic errors present in the u-band photometry, stemming from variations in the transparency of Earth's atmosphere at these short wavelengths. We use our data to calculate an improved orbital ephemeris and to refine the measured physical properties of the system. The planet HAT-P-5b has a mass of 1.06 +/- 0.11 +/- 0.01 Mjup and a radius of 1.252 +/- 0.042 +/- 0.008 Rjup (statistical and systematic errors respectively), making it slightly larger than expected according to standard models of coreless gas-giant planets. Its equilibrium temperature of 1517 +/- 29 K is within 60K of that of the extensively-studied planet HD 209458b.

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arXiv2012

Eclipsing Binary Stars: the Royal Road to Stellar Astrophysics

Russell (1948) famously described eclipses as the "royal road" to stellar astrophysics. From photometric and spectroscopic observations it is possible to measure the masses and radii (to 1% or better!), and thus surface gravities and mean densities, of stars in eclipsing binary systems using nothing more than geometry. Adding an effective temperature subsequently yields luminosity and then distance (or vice versa) to high precision. This wealth of directly measurable quantities makes eclipsing binaries the primary source of empirical information on the properties of stars, and therefore a cornerstone of stellar astrophysics. In this review paper I summarise the current standing of eclipsing binary research, present an overview of useful analysis techniques, and conclude with a glance to the future.

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arXiv2011

Refined physical properties of the HAT-P-13 planetary system

We present photometry of four transits of the planetary system HAT-P-13, obtained using defocussed telescopes. We analyse these, plus nine datasets from the literature, in order to determine the physical properties of the system. The mass and radius of the star are M_A = 1.320 +/- 0.048 +/- 0.039 Msun and R_A = 1.756 +/- 0.043 +/- 0.017 Rsun (statistical and systematic errorbars). We find the equivalent quantities for the transiting planet to be M_b = 0.906 +/- 0.024 +/- 0.018 Mjup and R_b = 1.487 +/- 0.038 +/- 0.015 Rjup, with an equilibrium temperature of 1725 +/- 31 K. Compared to previous results, which were based on much sparser photometric data, we find the star to be more massive and evolved, and the planet to be larger, hotter and more rarefied. The properties of the planet are not matched by standard models of irradiated gas giants. Its large radius anomaly is in line with the observation that the hottest planets are the most inflated, but at odds with the suggestion of inverse proportionality to the [Fe/H] of the parent star. We assemble all available times of transit midpoint and determine a new linear ephemeris. Previous findings of transit timing variations in the HAT-P-13 system are shown to disagree with these measurements, and can be attributed to small-number statistics.

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arXiv2011

Discovery of a ZZ Ceti in the Kepler Mission Field

We report the discovery of the first identified pulsating DA white dwarf in the field of the Kepler mission, WD J1916+3938 (Kepler ID 4552982). This ZZ Ceti star was first identified through ground-based, time-series photometry, and follow-up spectroscopy confirm it is a hydrogen-atmosphere white dwarf with an effective temperature = 11,129 $\pm$ 115 K and log g = 8.34 $\pm$ 0.06, placing it within the empirical ZZ Ceti instability strip. The object shows up to 0.5 percent amplitude variability at several periods between 800 -- 1450 s. Extended Kepler observations of this object could yield the best lightcurve, to-date, of any pulsating white dwarf, allowing us to directly study the interior of an evolved object representative of the fate of the majority of stars in our Galaxy.

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arXiv2011

Homogeneous studies of transiting planets: an online catalogue

The derived physical properties of the known transiting extrasolar planetary systems come from a variety of sources, and are calculated using a range of different methods so are not always directly comparable. I present a catalogue of the physical properties of 58 transiting extrasolar planet and brown dwarf systems which have been measured using homogeneous methods, resulting in quantities which are internally consistent and well-suited to detailed statistical study. The main results for each object, plus a critical compilation of literature values for all known systems, have been placed in an online catalogue. TEPCat can be found at: http://www.astro.keele.ac.uk/~jkt/tepcat/

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