arXiv subjects

P. F. L. Maxted

Publications and source records attributed to P. F. L. Maxted.

At least 271 records · Page 15Linked to original sources

arXiv2008

WASP-7: The brightest transiting-exoplanet system in the Southern hemisphere

We report that a Jupiter-mass planet, WASP-7b, transits the V = 9.5 star HD197286 every 4.95 d. This is the brightest discovery from the WASP-South transit survey and the brightest transiting-exoplanet system in the Southern hemisphere. WASP-7b is among the densest of the known Jupiter-mass planets, suggesting that it has a massive core. The planet mass is 0.96 M_Jup, the radius 0.915 R_Jup, and the density 1.26 rho_Jup.

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arXiv2008

WASP-5b: a dense, very-hot Jupiter transiting a 12th-mag Southern-hemisphere star

We report the discovery of WASP-5b, a Jupiter-mass planet orbiting a 12th-mag G-type star in the Southern hemisphere. The 1.6-d orbital period places WASP-5b in the class of very-hot Jupiters and leads to a predicted equilibrium temperature of 1750 K. WASP-5b is the densest of any known Jovian-mass planet, being a factor seven denser than TrES-4, which is subject to similar stellar insolation, and a factor three denser than WASP-4b, which has a similar orbital period. We present transit photometry and radial-velocity measurements of WASP-5 (= USNO-B1 0487-0799749), from which we derive the mass, radius and density of the planet: M_P = 1.58 +0.13 -0.08 M_J, R_P = 1.090 +0.094 -0.058 R_J and Rho_P = 1.22 +0.19 -0.24 Rho_J. The orbital period is P = 1.6284296 +0.0000048 -0.0000037 d and the mid-transit epoch is T_C (HJD) = 2454375.62466 +0.00026 -0.00025.

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arXiv2008

WASP-4b: a 12th-magnitude transiting hot-Jupiter in the Southern hemisphere

We report the discovery of WASP-4b, a large transiting gas-giant planet with an orbital period of 1.34 days. This is the first planet to be discovered by the SuperWASP-South observatory and CORALIE collaboration and the first planet orbiting a star brighter than 16th magnitude to be discovered in the Southern hemisphere. A simultaneous fit to high-quality lightcurves and precision radial-velocity measurements leads to a planetary mass of 1.22 +/- 0.1 MJup and a planetary radius of 1.42 +/- 0.08 RJup. The host star is USNO-B1.0 0479-0948995, a G7V star of visual magnitude 12.5. As a result of the short orbital period, the predicted surface temperature of the planet is 1776 K, making it an ideal candidate for detections of the secondary eclipse at infrared wavelengths.

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arXiv2008

A survey for low mass spectroscopic binary stars in the young clusters around sigma Orionis and lambda Orionis

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs in the young clusters around sigma Ori and lambda Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their NaI 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its line width and shape. The sample covers the magnitude range Ic=14-18.9 (mass =~ 0.55-0.03 Msun), but all of the binary stars are brighter than Ic=16.6 (mass =~ 0.12Msun) and 10 are brighter than Ic=15.5 (mass =~ 0.23Msun). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of Ic magnitude with more than 99% confidence. The spectroscopic binary fraction for stars more massive than about 0.1Msun (Ic < 16.9) is f_bright=0.095(+0.012)(-0.028). The 90% confidence upper limit to the spectroscopic binary fraction for very low mass (VLM) stars (mass < 0.1Msun) and brown dwarfs (BDs) is f_faint < 7.5%. The hypothesis that f_bright and f_faint are equal can be rejected with 90% confidence. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and brown dwarfs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semi-major axes, or both. (Abridged)

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arXiv2007

The mass and radius of the M-dwarf in the short period eclipsing binary RR Caeli

We present new photometry and spectroscopy of the eclipsing white dwarf - M-dwarf binary star RR Cae. We use timings of the primary eclipse from white-light photo-electric photometry to derive a new ephemeris for the eclipses. We find no evidence for any period change greater than Pdot/P ~ 5E-12 over a timescale of 10 years. We have measured the effective temperature of the white dwarf, T_WD, from an analysis of two high resolution spectra of RR Cae and find T_WD = (7540 +- 175)K. We estimate a spectral type of M4 for the companion from the same spectra. We have combined new spectroscopic orbits for the white dwarf and M-dwarf with an analysis of the primary eclipse and cooling models for helium white dwarfs to measure the mass and radius of the M-dwarf. The mass of the M-dwarf is (0.182 - 0.183) +- 0.013 Msun and the radius is (0.203 - 0.215) +- 0.013 Rsun, where the ranges quoted for these values reflect the range of white dwarf models used. In contrast to previous studies, which lacked a spectroscopic orbit for the white dwarf, we find that the mass and radius of the M-dwarf are normal for an M4 dwarf. The mass of the white dwarf is (0.440 +-0.022) Msun. With these revised masses and radii we find that RR Cae will become a cataclysmic variable star when the orbital period is reduced from its current value of 7.3 hours to 121 minutes by magnetic braking in 9-20 Gyr. We note that there is night-to-night variability of a few seconds in the timing of primary eclipse caused by changes to the shape of the primary eclipse. We speculate as to the possible causes of this phenomenon. (Abridged)

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arXiv2006

A near-infrared spectroscopic detection of the brown dwarf in the post common envelope binary WD0137-349

We present a near-infrared spectrum of the close, detached white dwarf + brown dwarf binary WD0137-349 (Maxted et al 2006), that directly reveals the substellar companion through an excess of flux longwards of 1.95 microns. We best match the data with a white dwarf + L8 composite model. For ages ~1 Gyr, the spectral type of the cool secondary is in agreement with the mass determined by Maxted et al. (2006) from radial velocity measurements (0.053 +/- 0.006Msun), and supports an evolutionary scenario in which the brown dwarf survived a previous phase of common envelope evolution which resulted in the formation of this close binary. The brown dwarf is the lowest mass companion to a white dwarf yet found, and the lowest mass object known to have survived a common envelope phase. At 1300 < T < 1400K WD0137-349B is also the coolest known companion to a white dwarf. At a separation a = 0.65Rsun the hemisphere of the brown dwarf facing the 16,500K white dwarf is being heated through irradiation. We discuss the possible effects of this additional heating, with particular relevance to those other close binaries with substellar companions, the hot Jupiters. We propose future observations to investigate the likely temperature differences between the ``day'' and ``night'' sides of the brown dwarf.

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arXiv2006

The WASP Project and the SuperWASP Cameras

The SuperWASP Cameras are wide-field imaging systems sited at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, and the Sutherland Station of the South African Astronomical Observatory. Each instrument has a field of view of some ~482 square degrees with an angular scale of 13.7 arcsec per pixel, and is capable of delivering photometry with accuracy better than 1% for objects having V ~ 7.0 - 11.5. Lower quality data for objects brighter than V ~15.0 are stored in the project archive. The systems, while designed to monitor fields with high cadence, are capable of surveying the entire visible sky every 40 minutes. Depending on the observational strategy, the data rate can be up to 100GB per night. We have produced a robust, largely automatic reduction pipeline and advanced archive which are used to serve the data products to the consortium members. The main science aim of these systems is to search for bright transiting exo-planets systems suitable for spectroscopic followup observations. The first 6 month season of SuperWASP-North observations produced lightcurves of ~6.7 million objects with 12.9 billion data points.

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arXiv2006

Survival of a brown dwarf after engulfment by a red giant star

(Summary of paper) High resolution optical spectroscopy of the white dwarf WD0137-349 reveals a weak Halpha line in emission, due to a low mass companion in a close orbit. Using this emission feature, and the narrow NLTE core of the white dwarf's Halpha absorption line, we measure the orbital period at 116 minutes, the separation at 0.65Rsun, and the mass ratio m2/m1 = K1/K2 = 0.134+/-0.006. From the mass of the white dwarf (0.39+/-0.035Msun) we derive a mass for the companion of 0.053+/-0.006Msun. This is well below the limit of about 0.075Msun commonly used to distinguish stars from brown dwarfs. The observed infrared flux distribution of WD0137-349 is also consistent with a model of an old brown dwarf spectral type mid T or slightly earlier. The system is detached, and the brown dwarf must have survived a previous phase of common envelope evolution, providing a key observational test for models of this interaction.

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arXiv2006

SuperWASP Observations of the Transiting Extrasolar planet XO-1b

We report on observations of 11 transit events of the transiting extrasolar planet XO-1b by the SuperWASP-North observatory. From our data, obtained during May-September 2004, we find that the XO-1b orbital period is 3.941634 +/- 0.000137 days, the planetary radius is 1.34 +/- 0.12 Rjup and the inclination is 88.92 +/- 1.04 degrees, in good agreement with previously published values. We tabulate the transit timings from 2004 SuperWASP and XO data, which are the earliest obtained for XO-1b, and which will therefore be useful for future investigations of timing variations caused by additional perturbing planets. We also present an ephemeris for the transits. See http://www.superwasp.org for general project details.

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arXiv2006

Photometry of the Magnetic White Dwarf SDSS 121209.31+013627.7

The results of 27 hours of time series photometry of SDSS 121209.31+013627.7 are presented. The binary period established from spectroscopy is confirmed and refined to 0.061412 d (88.43 minutes). The photometric variations are dominated by a brightening of about 16 mmag, lasting a little less than half a binary cycle. The amplitude is approximately the same in V, R and white light. A secondary small brightness increase during each cycle may also be present. We speculate that SDSS 121209.31+013627.7 may be a polar in a low state.

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arXiv2006

Kinematic structure in the young Sigma Orionis association

We have used precise radial velocity measurements for a large number of candidate low-mass stars and brown dwarfs, to show that the young Sigma Ori ``cluster'' consists of two spatially superimposed components which are kinematically separated by 7 km/s in radial velocity, and which have different mean ages. We examine the relationship of these two kinematic groups to other populations in the Orion OB1 association and briefly discuss the consequence of mixed age samples for ongoing investigations of the formation and evolution of low-mass objects in this much-observed region.

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arXiv2005

Close binary systems among very low mass stars and brown dwarfs

Using Monte Carlo simulations and published radial velocity surveys we have constrained the frequency and separation (a) distribution of very low mass star (VLM) and brown dwarf (BD) binary systems. We find that simple Gaussian extensions of the observed wide binary distribution, with a peak at 4 au and 0.6< sigma_{log(a/au)}<1.0, correctly reproduce the observed number of close binary systems, implying a close (a<2.6 au) binary frequency of 17-30 per cent and overall frequency of 32-45 per cent. N-body models of the dynamical decay of unstable protostellar multiple systems are excluded with high confidence because they do not produce enough close binary VLMs/BDs. The large number of close binaries and high overall binary frequency are also completely inconsistent with published smoothed particle hydrodynamical modelling and argue against a dynamical origin for VLMs/BDs.

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arXiv2005

Subdwarf B binaries in the Edinburgh-Cape Survey

We give an update of the results of a campaign to obtain orbital solutions of subdwarf B stars from the Edinburgh-Cape survey (Stobie et al. 1997). To date we have obtained blue spectra of 40 subdwarf B stars from the Edinburgh-Cape catalogue using the grating spectrograph at the 1.9m Radcliffe telescope at the South African Astronomical Observatory. We find that 17 out of these 40 are certain binaries with a few other objects showing radial velocity variations of small amplitude. The binary fraction found in our sample, after correcting for our binary detection efficiency, is 48%. We have secured the orbital parameters for 4 of the 17 systems and narrowed down the orbits of another 7 to a small range of periods.

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arXiv2005

On the frequency of close binary systems among very low-mass stars and brown dwarfs

We have used Monte Carlo simulation techniques and published radial velocity surveys to constrain the frequency of very low-mass star (VLMS) and brown dwarf (BD) binary systems and their separation (a) distribution. Gaussian models for the separation distribution with a peak at a = 4 au and 0.6 =< sigma(log(a/au)) =< 1.0 correctly predict the number of observed binaries, yielding a close (a<2.6 au) binary frequency of 17-30 per cent and an overall VLMS/BD binary frequency of 32-45 per cent. We find that the available N-body models of VLMS/BD formation from dynamically decaying protostellar multiple systems are excluded at >99 per cent confidence because they predict too few close binary VLMS/BDs. The large number of close binaries and high overall binary frequency are also very inconsistent with recent smoothed particle hydrodynamical modelling and argue against a dynamical origin for VLMS/BDs.

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arXiv2005

Six detached white-dwarf close binaries

We determine the orbits of four double degenerate systems (DDs), composed of two white dwarfs, and of two white dwarf -- M dwarf binaries. The four DDs, WD1022+050, WD1428+373, WD1824+040, and WD2032+188, show orbital periods of 1.157155(5) d, 1.15674(2) d, 6.26602(6) d and 5.0846(3) d respectively. These periods combined with estimates for the masses of the brighter component, based on their effective temperatures, allow us to constrain the masses of the unseen companions. We estimate that the upper limit for the contribution of the unseen companions to the total luminosity in the four DDs ranges between 10 and 20 per cent. In the case of the two white dwarf - M dwarf binaries, WD1042-690 and WD2009+622, we calculate the orbital parameters by fitting simultaneously the absorption line from the white dwarf and the emission core from the M-dwarf. Their orbital periods are 0.337083(1) d and 0.741226(2) d respectively. We find signatures of irradiation on the inner face of WD2009+622's companion. We calculate the masses of both components from the gravitational redshift and the mass-radius relationship for white dwarfs and find masses of 0.75 -- 0.78 Msun and 0.61 -- 0.64 Msun for WD1042-690 and WD2009+622 respectively. This indicates that the stars probably reached the asymptotic giant branch in their evolution before entering a common envelope phase. These two white dwarf - M dwarf binaries will become cataclysmic variables, although not within a Hubble time, with orbital periods below the period gap.

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arXiv2004

Rotational period of WD1953-011 - a magnetic white dwarf with a star spot

WD1953-011 is an isolated, cool (7920 +/- 200K, Bergeron, Legget & Ruiz, 2001) magnetic white dwarf (MWD) with a low average field strength (~70kG, Maxted et al. 2000) and a higher than average mass (~0.74 M_sun, Bergeron et al. 2001). Spectroscopic observations taken by Maxted et al. (2000) showed variations of equivalent width in the Balmer lines, unusual in a low field white dwarf. Here we present V band photometry of WD1953-011 taken at 7 epochs over a total of 22 months. All of the datasets show a sinusoidal variation of approximately 2% peak-to-peak amplitude. We propose that these variations are due to a star spot on the MWD, analogous to a sunspot, which is affecting the temperature at the surface, and therefore its photometric magnitude. The variations have a best-fit period over the entire 22 months of 1.4418 days, which we interpret as the rotational period of the WD.

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arXiv2004

Subdwarf B Binaries from the Edinburgh--Cape Survey

We present the first results of a campaign to obtain orbital solutions of subdwarf B (sdB) stars from the Edinburgh-Cape survey. We have obtained blue spectra of 35 sdBs, 20 of which have been observed in more than two epochs. 15 out of the 35 are certain binaries with a few other objects showing radial velocity variations with small amplitude, possibly long period sdB binaries. We have secured the orbital parameters for 2 of the 15 systems and narrowed down the orbits of another one to a small range of periods. These preliminary results only use data taken up to December 2003.

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arXiv2004

Membership, binarity and accretion among very low-mass stars and brown dwarfs of the Sigma Orionis cluster

Intermediate resolution (R=7000) spectroscopy is presented for 76 photometrically selected very low mass (0.04<M<0.3M_{sun}) candidate members of the Sigma Orionis cluster. More than two thirds appear to be genuine cluster members on the basis of Li I absorption, weak Na I features and radial velocities. Photometric selection alone therefore appears to be very effective in identifying cluster members in this mass range. Only 6 objects appear to be certain non-members, however a substantial subset of 13 have ambiguous or contradictory indications of membership and lack Li absorption. Four candidate binary cluster members are identified. Consideration of sampling and precision leads us to conclude that either the fraction of very low mass stars and brown dwarfs in small separation (a<1au) binary systems is larger than in field M-dwarfs, or the distribution of separations is much less skewed towards large separations. This conclusion hinges critically on the correct identification of the small number of binary candidates, but is significant even when only Li-rich candidate members are considered. Broadened H alpha emission, indicative of circum(sub)stellar accretion discs is found in 5 or 6 of the candidate cluster members, 3 of which probably have substellar masses. The fraction of accretors (10+/-5 per cent) is similar to that found in stars of higher mass in the cluster using H alpha emission as a diagnostic, but much lower than found for very low mass stars and brown dwarfs of younger clusters. The timescale for accretion rates to drop to less than 10^{-11} M_sun/yr is hence less than the age of the Sigma Ori cluster (3 to 7 Myr) for most low-mass objects (abridged).

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