arXiv subjects

John Southworth

Publications and source records attributed to John Southworth.

At least 91 records · Page 5Linked to original sources

arXiv2020

Binary stars: a cheat sheet

I present a brief summary of three different types of binary star - astrometric, spectroscopic and eclipsing - and tabulate the properties of these systems that can be determined directly from observations. Eclipsing binary stars are the most valuable of these, as they are our main source of direct mass and radius measurements for normal stars. In good cases, masses and radii can be obtained to better than 1% precision and accuracy using only photometry, spectroscopy and geometry. These measurements constitute vital empirical data against which theoretical models of stars can be verified and improved. I give examples of the use of these systems for constraining stellar theory and the distance scale, and conclude with a presentation of preliminary results for the solar-type eclipsing binary 1SWASP J034114.25+201253.5.

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arXiv2019

OGLE-2015-BLG-1649Lb: A gas giant planet around a low-mass dwarf

We report the discovery of an exoplanet in microlensing event OGLE-2015-BLG-1649. The planet/host-star mass ratio is $q =7.2 \times 10^{-3}$ and the projected separation normalized by the Einstein radius is $s = 0.9$. The upper limit of the lens flux is obtained from adaptive optics observations by IRCS/Subaru, which excludes the probability of a G-dwarf or more massive host star and helps to put a tighter constraint on the lens mass as well as commenting on the formation scenarios of giant planets orbiting low-mass stars. We conduct a Bayesian analysis including constraints on the lens flux to derive the probability distribution of the physical parameters of the lens system. We thereby find that the masses of the host star and planet are $M_{L} = 0.34 \pm 0.19 M_{\odot}$ and $M_{p} = 2.5^{+1.5}_{-1.4} M_{Jup}$, respectively. The distance to the system is $D_{L} = 4.23^{+1.51}_{-1.64}$kpc. The projected star-planet separation is $a_{\perp} = 2.07^{+0.65}_{-0.77}$AU. The lens-source relative proper motion of the event is quite high, at $\sim 7.1 \, {\rm mas/yr}$. Therefore, we may be able to determine the lens physical parameters uniquely or place much stronger constraints on them by measuring the color-dependent centroid shift and/or the image elongation with additional high resolution imaging already a few years from now.

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arXiv2019

Transit timing variations in the WASP-4 planetary system

Transits in the planetary system WASP-4 were recently found to occur 80s earlier than expected in observations from the TESS satellite. We present 22 new times of mid-transit that confirm the existence of transit timing variations, and are well fitted by a quadratic ephemeris with period decay dP/dt = -9.2 +/- 1.1 ms/yr. We rule out instrumental issues, stellar activity and the Applegate mechanism as possible causes. The light-time effect is also not favoured due to the non-detection of changes in the systemic velocity. Orbital decay and apsidal precession are plausible but unproven. WASP-4b is only the third hot Jupiter known to show transit timing variations to high confidence. We discuss a variety of observations of this and other planetary systems that would be useful in improving our understanding of WASP-4 in particular and orbital decay in general.

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arXiv2019

OGLE-2017-BLG-1186: first application of asteroseismology and Gaussian processes to microlensing

We present the analysis of the event OGLE-2017-BLG-1186 from the 2017 Spitzer microlensing campaign. This is a remarkable microlensing event because its source is photometrically bright and variable, which makes it possible to perform an asteroseismic analysis using ground-based data. We find that the source star is an oscillating red giant with average timescale of $\sim 9$ d. The asteroseismic analysis also provides us source properties including the source angular size ($\sim 27~μ{\rm as}$) and distance ($\sim 11.5$ kpc), which are essential for inferring the properties of the lens. When fitting the light curve, we test the feasibility of Gaussian Processes (GPs) in handling the correlated noise caused by the variable source. We find that the parameters from the GP model are generally more loosely constrained than those from the traditional $χ^2$ minimization method. We note that this event is the first microlensing system for which asteroseismology and GPs have been used to account for the variable source. With both finite-source effect and microlens parallax measured, we find that the lens is likely a $\sim 0.045~M_{\odot}$ brown dwarf at distance $\sim 9.0$ kpc, or a $\sim 0.073~M_{\odot}$ ultracool dwarf at distance $\sim 9.8$ kpc. Combining the estimated lens properties with a Bayesian analysis using a Galactic model, we find a $\sim 35$ per cent probability for the lens to be a bulge object and $\sim 65$ per cent to be a background disc object.

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arXiv2019

Qatar Exoplanet Survey: Qatar-8b, 9b and 10b --- A Hot Saturn and Two Hot Jupiters

In this paper we present three new extrasolar planets from the Qatar Exoplanet Survey (QES). Qatar-8b is a hot Saturn, with Mpl = 0.37 Mjup and Rpl = 1.3 Rjup, orbiting a solar-like star every Porb = 3.7 days. Qatar-9b is a hot Jupiter with a mass of Mpl = 1.2 Mjup and a radius of Rpl = 1 Rjup, in a Porb = 1.5 days orbit around a low mass, Mstar = 0.7 Msun, mid-K main-sequence star. Finally, Qatar-10b is a hot, Teq ~ 2000 K, sub-Jupiter mass planet, Mpl = 0.7 Mjup, with a radius of Rpl = 1.54 Rjup and an orbital period of Porb = 1.6 days, placing it on the edge of the sub-Jupiter desert.

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arXiv2019

Spitzer Microlensing parallax reveals two isolated stars in the Galactic bulge

We report the mass and distance measurements of two single-lens events from the 2017 Spitzer microlensing campaign. The ground-based observations yield the detection of finite-source effects, and the microlens parallaxes are derived from the joint analysis of ground-based observations and Spitzer observations. We find that the lens of OGLE-2017-BLG-1254 is a $0.60 \pm 0.03 M_{\odot}$ star with $D_{\rm LS} = 0.53 \pm 0.11~\text{kpc}$, where $D_{\rm LS}$ is the distance between the lens and the source. The second event, OGLE-2017-BLG-1161, is subject to the known satellite parallax degeneracy, and thus is either a $0.51^{+0.12}_{-0.10} M_{\odot}$ star with $D_{\rm LS} = 0.40 \pm 0.12~\text{kpc}$ or a $0.38^{+0.13}_{-0.12} M_{\odot}$ star with $D_{\rm LS} = 0.53 \pm 0.19~\text{kpc}$. Both of the lenses are therefore isolated stars in the Galactic bulge. By comparing the mass and distance distributions of the eight published Spitzer finite-source events with the expectations from a Galactic model, we find that the Spitzer sample is in agreement with the probability of finite-source effects occurrence in single lens events.

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arXiv2019

The PDS 110 observing campaign - photometric and spectroscopic observations reveal eclipses are aperiodic

PDS 110 is a young disk-hosting star in the Orion OB1A association. Two dimming events of similar depth and duration were seen in 2008 (WASP) and 2011 (KELT), consistent with an object in a closed periodic orbit. In this paper we present data from a ground-based observing campaign designed to measure the star both photometrically and spectroscopically during the time of predicted eclipse in September 2017. Despite high-quality photometry, the predicted eclipse did not occur, although coherent structure is present suggesting variable amounts of stellar flux or dust obscuration. We also searched for RV oscillations caused by any hypothetical companion and can rule out close binaries to 0.1 $M_\odot$. A search of Sonneberg plate archive data also enabled us to extend the photometric baseline of this star back more than 50 years, and similarly does not re-detect any deep eclipses. Taken together, they suggest that the eclipses seen in WASP and KELT photometry were due to aperiodic events. It would seem that PDS 110 undergoes stochastic dimmings that are shallower and shorter-duration than those of UX Ori variables, but may have a similar mechanism.

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arXiv2019

$Spitzer$ Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-Rotating Low-Mass Brown Dwarf

The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens parallax and finite-source effects are needed in order to recover the full information. Here, we combine $Spitzer$ satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either $18\pm1M_J$ or $20\pm1M_J$. This is the lowest isolated-object mass measurement to date, only $\sim$45\% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either $3.9\pm0.1$ kpc or $4.1\pm0.1$ kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population.

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arXiv2018

Physical properties and optical-infrared transmission spectrum of the giant planet XO-1b

We present ten high-precision light curves of four transits in the XO-1 planetary system, obtained using $u$, $g$, $r$, redshifted H$α$, $I$ and $z$ filters. We use these to measure the physical properties, orbital ephemeris, and a transmission spectrum of the planet covering the full optical wavelength range. We augment this with published HST/WFC3 observations to construct a transmission spectrum of the planet covering 0.37 to 1.65 $μ$m. Our best-fitting model to this spectrum is for a H$_2$/He-rich atmosphere containing water (3.05$σ$ confidence), nitrogen-bearing molecules NH$_3$ and HCN (1.5$σ$) and patchy cloud (1.3$σ$). We find that adding the optical to the near-infrared data does not lead to more precise constraints on the planetary atmosphere in this case. We conduct a detailed investigation into the effect of stellar limb darkening on our results, concluding that the choice of limb darkening law and coefficients is unimportant; such conclusions may not hold for other systems so should be reassessed for all high-quality datasets. The planet radius we measure in the $g$-band is anomalously low and should be investigated with future observations at a higher spectral resolution. From the measured times of transit we determine an improved orbital ephemeris, calculate a lower limit on the modified stellar tidal quality factor of $Q_\star^{\,\prime} > 10^{5.6}$, and rule out a previously postulated sinusoidal variation in the transit times.

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arXiv2018

The Transiting Exoplanet Community Early Release Science Program for JWST

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, time-series observations required for such investigations have unique technical challenges, and prior experience with other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative datasets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the time-series modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, time-series instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.

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arXiv2017

Possible detection of a bimodal cloud distribution in the atmosphere of HAT-P-32\,A\,b from multi-band photometry

We present high-precision photometry of eight separate transit events in the HAT-P-32 planetary system. One transit event was observed simultaneously by two telescopes of which one obtained a simultaneous multi-band light curve in three optical bands, giving a total of 11 transit light curves. Due to the filter selection and in conjunction with using the defocussed photometry technique we were able to obtain an extremely high precision, ground-based transit in the \textit{u}-band (350\,nm), with an rms scatter of $\approx 1$\,mmag. All 11 transits were modelled using \textsc{prism} and \textsc{gemc}, and the physical properties of the system calculated. We find the mass and radius of the host star to be $1.182\pm 0.041\Msun$ and $1.225\pm0.015\Rsun$, respectively. For the planet we find a mass of $0.80\pm 0.14\Mjup$, a radius of $1.807\pm0.022\Rjup$ and a density of $0.126\pm0.023\pjup$. These values are consistent with those found in the literature. We also obtain a new orbital ephemeris for the system $ T_0 = {\rm BJD/TDB} \,\, 2\,454\,420.447187 (96) \, + \,2.15000800 (10) \times E $. We measured the transmission spectrum of HAT-P-32\,A\,b and compared it to theoretical transmission spectra. Our results indicate a bimodal cloud particle distribution consisting of Rayleigh--like haze and grey absorbing cloud particles within the atmosphere of HAT-P-32\,A\,b.

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arXiv2017

Qatar Exoplanet Survey : Qatar-3b, Qatar-4b and Qatar-5b

We report the discovery of Qatar-3b, Qatar-4b, and Qatar-5b, three new transiting planets identified by the Qatar Exoplanet Survey (QES). The three planets belong to the hot Jupiter family, with orbital periods of $P_{Q3b}$=2.50792 days, $P_{Q4b}$=1.80539 days, and $P_{Q5b}$=2.87923 days. Follow-up spectroscopic observations reveal the masses of the planets to be $M_{Q3b}$=4.31$\pm0.47$ $M_{\rm J}$, $M_{Q4b}$=6.10$ \pm0.54$ $M_{\rm J}$, and $M_{Q5b}$ = 4.32$ \pm0.18$ $M_{\rm J}$, while model fits to the transit light curves yield radii of $R_{Q3b}$ = 1.096$ \pm0.14$ $R_{\rm J}$, $R_{Q4b}$ = 1.135$ \pm0.11$ $R_{\rm J}$, and $R_{Q5b}$ = 1.107$ \pm0.064$ $R_{\rm J}$. The host stars are low-mass main sequence stars with masses and radii $M_{Q3}$ = 1.145$ \pm0.064$ $M_{\odot}$, $M_{Q4}$ = 0.896$ \pm0.048$ $M_{\odot}$, $M_{Q5}$ = 1.128$ \pm0.056$ $M_{\odot}$ and $R_{Q3}$ = 1.272$ \pm0.14$ $R_{\odot}$, $R_{Q4}$ = 0.849$\pm0.063$ $R_{\odot}$ and $R_{Q5}$ = 1.076$\pm0.051$ $R_{\odot}$ for Qatar-3, 4 and 5 respectively. The V magnitudes of the three host stars are $V_{Q3}$=12.88, $V_{Q4}$=13.60, and $V_{Q5}$=12.82. All three new planets can be classified as heavy hot Jupiters (M > 4 $M_{J}$).

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arXiv2017

Detection of the atmosphere of the 1.6 Earth mass exoplanet GJ 1132b

Detecting the atmospheres of low-mass low-temperature exoplanets is a high-priority goal on the path to ultimately detect biosignatures in the atmospheres of habitable exoplanets. High-precision HST observations of several super-Earths with equilibrium temperatures below 1000K have to date all resulted in featureless transmission spectra, which have been suggested to be due to high-altitude clouds. We report the detection of an atmospheric feature in the atmosphere of a 1.6 Mearth transiting exoplanet, GJ 1132b, with an equilibrium temperature of ~600K and orbiting a nearby M dwarf. We present observations of nine transits of the planet obtained simultaneously in the griz and JHK passbands. We find an average radius of 1.43 +/- 0.16 Rearth for the planet, averaged over all the passbands, and a radius of 0.255 +/- 0.023 Rsun for the star, both of which are significantly greater than previously found. The planet radius can be decomposed into a "surface radius" at ~1.375 Rearth overlaid by atmospheric features which increase the observed radius in the z and K bands. The z-band radius is 4sigma higher than the continuum, suggesting a strong detection of an atmosphere. We deploy a suite of tests to verify the reliability of the transmission spectrum, which are greatly helped by the existence of repeat observations. The large z-band transit depth indicates strong opacity from H2O and/or CH4 or a hitherto unconsidered opacity. A surface radius of 1.375 +/- 0.16 Rearth allows for a wide range of interior compositions ranging from a nearly Earth-like rocky interior, with ~70% silicate and ~30% Fe, to a substantially H2O-rich water world.

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arXiv2016

Peculiar architectures for the WASP-53 and WASP-81 planet-hosting systems

We report the detection of two new systems containing transiting planets. Both were identified by WASP as worthy transiting planet candidates. Radial-velocity observations quickly verified that the photometric signals were indeed produced by two transiting hot Jupiters. Our observations also show the presence of additional Doppler signals. In addition to short-period hot Jupiters, we find that the WASP-53 and WASP-81 systems also host brown dwarfs, on fairly eccentric orbits with semi-major axes of a few astronomical units. WASP-53c is over 16 $M_{\rm Jup} \sin i_{\rm c}$ and WASP-81c is 57 $M_{\rm Jup} \sin i_{\rm c}$. The presence of these tight, massive companions restricts theories of how the inner planets were assembled. We propose two alternative interpretations: a formation of the hot Jupiters within the snow line, or the late dynamical arrival of the brown dwarfs after disc-dispersal. We also attempted to measure the Rossiter--McLaughlin effect for both hot Jupiters. In the case of WASP-81b we fail to detect a signal. For WASP-53b we find that the planet is aligned with respect to the stellar spin axis. In addition we explore the prospect of transit timing variations, and of using Gaia's astrometry to measure the true masses of both brown dwarfs and also their relative inclination with respect to the inner transiting hot Jupiters.

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arXiv2016

WASP-20 is a close visual binary with a transiting hot Jupiter

We announce the discovery that WASP-20 is a binary stellar system, consisting of two components separated by $0.2578\pm0.0007^{\prime\prime}$ on the sky, with a flux ratio of $0.4639\pm 0.0015$ in the $K$-band. It has previously been assumed that the system consists of a single F9 V star, with photometric and radial velocity signals consistent with a low-density transiting giant planet. With a projected separation of approximately $60$ au between the two components, the detected planetary signals almost certainly originate from the brighter of the two stars. We reanalyse previous observations allowing for two scenarios, `planet transits A' and `planet transits B', finding that both cases remain consistent with a transiting gas giant. However, we rule out the `planet transits B' scenario because the observed transit duration requires star B to be significantly evolved, and therefore have an age much greater than star A. We outline further observations which can be used to confirm this finding. Our preferred `planet transits A' scenario results in the measured mass and radius of the planet increasing by 4$σ$ and 1$σ$, respectively.

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arXiv2016

Testing the asteroseismic scaling relations for Red Giants with Eclipsing Binaries Observed by Kepler

Given the potential of ensemble asteroseismology for understanding fundamental properties of large numbers of stars, it is critical to determine the accuracy of the scaling relations on which these measurements are based. From several powerful validation techniques, all indications so far show that stellar radius estimates from the asteroseismic scaling relations are accurate to within a few percent. Eclipsing binary systems hosting at least one star with detectable solar-like oscillations constitute the ideal test objects for validating asteroseismic radius and mass inferences. By combining radial-velocity measurements and photometric time series of eclipses, it is possible to determine the masses and radii of each component of a double-lined spectroscopic binary. We report the results of a four-year radial-velocity survey performed with the échelle spectrometer of the Astrophysical Research Consortium's 3.5-m telescope and the APOGEE spectrometer at Apache Point Observatory. We compare the masses and radii of 10 red giants obtained by combining radial velocities and eclipse photometry with the estimates from the asteroseismic scaling relations. We find that the asteroseismic scaling relations overestimate red-giant radii by about 5% on average and masses by about 15% for stars at various stages of red-giant evolution. Systematic overestimation of mass leads to underestimation of stellar age, which can have important implications for ensemble asteroseismology used for Galactic studies. As part of a second objective, where asteroseismology is used for understanding binary systems, we confirm that oscillations of red giants in close binaries can be suppressed enough to be undetectable, an hypothesis that was proposed in a previous work.

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arXiv2016

Orbital obliquities of transiting planets from starspot occultations

When a planet passes in front of a starspot during a transit of its host star, it causes a small upward blip in the light curve. Modelling the transit with the starspot allows the size, brightness and position of the spot to be measured. If the same spot can be observed in two different transits, it is possible to track the motion of the spot due to the rotation of the star. The rotation period and velocity of the star (Prot and Vsini) and the sky-projected orbital obliquity of the system (lambda) can then be determined. If one has three or more observations of the same spot, the true orbital obliquity (psi) can be measured. We are performing this analysis for a number of cool stars orbited by transiting planets. We present our results so far and compile a catalogue of lambda and psi measurements from spot crossing events. The method is particularly useful for cool stars, and is therefore complementary to studies of the Rossiter-McLaughlin effect, which perform better on hotter and faster-rotating stars.

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arXiv2016

Contamination from a nearby star cannot explain the anomalous transmission spectrum of the ultra-short period giant planet WASP-103b

The planet in the WASP-103 system is an excellent candidate for transmission spectroscopy because of its large radius and high temperature. Application of this technique found a variation of radius with wavelength which was far too strong to be explained by scattering processes in the planetary atmosphere. A faint nearby star was subsequently detected, whose contamination of the transit light curves might explain this anomaly. We present a reanalysis of published data in order to characterise the faint star and assess its effect on the measured transmission spectrum. The faint star has a mass of 0.72 +/- 0.08 Msun and is almost certainly gravitationally bound to the planetary system. We find that its effect on the measured physical properties of the planet and host star is small, amounting to a planetary radius larger by 0.6 sigma and planetary density smaller by 0.8 sigma. Its influence on the measured transmission spectrum is much greater: the spectrum now has a minimum around 760 nm and opacity rises to both bluer and redder wavelengths. It is a poor match to theoretical spectra and the spectral slope remains too strong for Rayleigh scattering. The existence of the faint nearby star cannot therefore explain the measured spectral properties of this hot and inflated planet. We advocate further observations of the system, both with high spatial resolution in order to improve the measured properties of the faint star, and with higher spectral resolution to confirm the anomalous transmission spectrum of the planet.

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