arXiv subjects

Keith Horne

Publications and source records attributed to Keith Horne.

At least 199 records · Page 11Linked to original sources

arXiv2005

Upper limits on the hot Jupiter fraction in the field of NGC 7789

We describe a method of estimating the abundance of short-period extrasolar planets based on the results of a photometric survey for planetary transits. We apply the method to a 21-night survey with the 2.5m Isaac Newton Telescope of \~32000 stars in a ~0.5 deg by 0.5 deg square field including the open cluster NGC 7789. From the colour-magnitude diagram we estimate the mass and radius of each star by comparison with the cluster main sequence. We search for injected synthetic transits throughout the lightcurve of each star in order to determine their recovery rate, and thus calculate the expected number of transit detections and false alarms in the survey. We take proper account of the photometric accuracy, time sampling of the observations and criteria (signal-to-noise and number of transits) adopted for transit detection. Assuming that none of the transit candidates found in the survey will be confirmed as real planets, we place conservative upper limits on the abundance of planets as a function of planet radius, orbital period and spectral type.

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arXiv2005

Photoionized HBeta Emission in NGC 5548: It Breathes!

Emission-line regions in active galactic nuclei and other photoionized nebulae should become larger in size when the ionizing luminosity increases. This 'breathing' effect is observed for the Hbeta emission in NGC 5548 by using Hbeta and optical continuum lightcurves from the 13-year 1989-2001 AGN Watch monitoring campaign. To model the breathing, we use two methods to fit the observed lightcurves in detail: (i) parameterized models and, (ii) the MEMECHO reverberation mapping code. Our models assume that optical continuum variations track the ionizing radiation, and that the Hbeta variations respond with time delays due to light travel time. By fitting the data using a delay map that is allowed to change with continuum flux, we find that the strength of the Hbeta response decreases and the time delay increases with ionizing luminosity. The parameterized breathing models allow the time delay and the Hbeta flux to depend on the continuum flux so that, the time delay is proportional to the continuum flux to the power beta, and the Hbeta flux is proportional to the continuum flux to the power alpha. Our fits give 0.1 < beta < 0.46 and 0.57 < alpha < 0.66. alpha is consistent with previous work by Gilbert and Peterson (2003) and Goad, Korista and Knigge (2004). Although we find beta to be flatter than previously determined by Peterson et al. (2002) using cross-correlation methods, it is closer to the predicted values from recent theoretical work by Korista and Goad (2004).

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arXiv2005

Results from the Wide Angle Search for Planets Prototype (WASP0) III: Planet Hunting in the Draco Field

The Wide Angle Search for Planets prototype (WASP0) is a wide-field instrument used to search for extra-solar planets via the transit method. Here we present the results of a monitoring program which targeted a 9-degree field in Draco. WASP0 monitored 35000 field stars for two consecutive months. Analysis of the lightcurves resulted in the detection of 11 multi-transit candidates and 3 single-transit candidates, two of which we recommend for further follow-up. Monte-Carlo simulations matching the observing parameters estimate the expected number of transit candidates from this survey. A comparison of the expected number with the number of candidates detected is used to discuss limits on planetary companions to field stars.

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arXiv2005

Results from the Wide Angle Search for Planets Prototype (WASP0) II: Stellar Variability in the Pegasus Field

Recent wide-field photometric surveys which target a specific field for long durations are ideal for studying both long and short period stellar variability. Here we report on 75 variable stars detected during observations of a field in Pegasus using the WASP0 instrument, 73 of which are new discoveries. The variables detected include 16 delta Scuti stars, 34 eclipsing binaries, 3 BY Draconis stars, and 4 RR Lyraes. We estimate that the fraction of stars in the field brighter than V ~ 13.5 exhibiting variable behaviour with an amplitude greater than 0.6% rms is ~ 0.4%. These results are compared with other wide-field stellar variability surveys and implications for detecting transits due to extra-solar planets are discussed.

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arXiv2005

A dearth of planetary transits in the direction of NGC 6940

We present results of our survey for planetary transits in the field of NGC 6940. We think nearly all of our observed stars are field stars. We have obtained high precision (3-10 millimags at the bright end) photometric observations of 50,000 stars spanning 18 nights in an attempt to identify low amplitude and short period transit events. We have used a matched filter analysis to identify 14 stars that show multiple events, and four stars that show single transits. Of these 18 candidates, we have identified two that should be further researched. However, none of the candidates are convincing hot Jupiters.

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arXiv2005

A Survey for Planetary Transits in the Field of NGC 7789

We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ~900 epochs, we obtained lightcurves and Sloan r-i colours for ~33000 stars, with ~2400 stars with better than 1% precision. We expected to detect ~2 transiting hot Jupiter planets if 1% of stars host such a companion and that a typical hot Jupiter radius is ~1.2RJ. We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81+0.09-0.00RJ. Three candidates remain that require follow-up observations in order to determine their nature.

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arXiv2004

Fireballs, Flares and Flickering: A Semi-analytic, LTE, Explosive Model from Accretion Disks to Supernovae

We derive simple analytic expressions for the continuum lightcurves and spectra of flaring and flickering events that occur over a wide range of astrophysical systems. We compare these results to data taken from the cataclysmic variable SS Cygni and also with SN 1987A, deriving physical parameters for the material involved. Fits to the data indicate a nearly time-independent photospheric temperature arising from the strong temperature dependence of opacity when hydrogen is partially ionized.

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arXiv2004

A Prototype for the PASS Permanent All Sky Survey

A prototype system for the Permanent All Sky Survey (PASS) project is presented. PASS is a continuous photometric survey of the entire celestial sphere with a high temporal resolution. Its major objectives are the detection of all giant-planet transits (with periods up to some weeks) across stars up to mag 10.5, and to deliver continuously photometry that is useful for the study of any variable stars. The prototype is based on CCD cameras with short focal length optics on a fixed mount. A small dome to house it at Teide Observatory, Tenerife, is currently being constructed. A placement at the antarctic Dome C is also being considered. The prototype will be used for a feasibility study of PASS, to define the best observing strategies, and to perform a detailed characterization of the capabilities and scope of the survey. Afterwards, a first partial sky surveying will be started with it. That first survey may be able to detect transiting planets during its first few hundred hours of operation. It will also deliver a data set around which software modules dealing with the various scientific objectives of PASS will be developed. The PASS project is still in its early phase and teams interested in specific scientific objectives, in providing technical expertise, or in participating with own observations are invited to collaborate.

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arXiv2004

An All-Sky Survey for the Detection of Transiting Extrasolar Planets and for Permanent Variable Star Tracking

An overview is given of the Permanent All Sky Survey (PASS) project. The primary goal of PASS is the detection of all transiting giant planets in the entire sky, complete for stellar systems of magnitudes ~ 5.5-10.5. Since the sample stars are fairly bright and relatively close, planets detected by PASS would be ideally suited for any follow-up study with ground- or space-based instrumentation. The survey would also allow the pursuit of a variety of work on temporal astronomical phenomena of any kind, and is intended to lead to a permanent all-sky tracking of variable stars with high temporal resolution. The instrument consists of arrays of CCD cameras with wide-field optics that cover the entire sky visible from their observing locations. Calculations of the instrument's noise sources and subsequent simulations indicate that the proposed design is able to achieve the prime objective of a full-sky survey for transits. An equation for the signal-to-noise ratio from photometry of unguided stellar images is given in the appendix, together with equations for the detection probability of planetary transits based on the observational coverage and the instrument's duty cycle.

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arXiv2004

Results from the Wide Angle Search for Planets Prototype (WASP0) I: Analysis of the Pegasus Field

WASP0 is a prototype for what is intended to become a collection of wide-angle survey instruments whose primary aim is to detect extra-solar planets transiting across the face of their parent star. The WASP0 instrument is a wide-field (9-degree) 6.3cm aperture F/2.8 Apogee 10 CCD camera (2Kx2K chip, 16-arcsec pixels) mounted piggy-back on a commercial telescope. We present results from analysis of a field in Pegasus using the WASP0 camera, including observations of the known transiting planet around HD 209458. We also present details on solving problems which restrict the ability to achieve photon limited precision with a wide-field commercial CCD. The results presented herein demonstrate that millimag photometry can be obtained with this instrument and that it is sensitive enough to detect transit due to extra-solar planets.

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arXiv2004

The abundance of galactic planets from OGLE-III 2002 microlensing data

From the 389 2002 OGLE-III observations of Galactic Bulge microlensing events we select 321 that are well described by a point-source point-lens lightcurve model. From this sample we identify n=1 event, 2002-BLG-055, which we regard as a strong planetary lensing candidate, and another, 2002-BLG-140, which is a possible candidate. If each of the 321 lens stars has 1 planet with a mass ratio q=m/M=10^{-3} and orbit radius a=R_E, the Einstein ring radius, analysis of detection efficiencies indicates that 14 planets should have been detectable with Δχ^2 > 25. Assuming our candidate is due to planetary lensing, then the abundance of planets with q=10^{-3} and a=R_E is n_p \approx n/14 = 7%. Conversion to physical units (M_Jup, and AU) gives the abundance of `cool Jupiters' (m \approx M_Jup, a \approx 4 AU) per lens star as n_p \approx n/5.5 = 18%. The detection probability scales roughly with q and (Δχ^2)^{-1/2}, and drops off from a peak at a \approx 4 AU like a Gaussian with a dispersion of 0.4 dex.

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arXiv2004

High-speed Keck II and RXTE spectroscopy of Cygnus X-2: (I) Three X-ray components revealed by correlated variability

We have performed simultaneous X-ray and optical spectroscopic observations of the Low Mass X-ray Binary Cygnus X-2. We have used a new data system attached to the Low Resolution Imaging Spectrograph instrument on Keck II to obtain spectra with a mean time resolution of 72.075 milliseconds, simultaneous with pointed X-ray observations using the PCA onboard XTE. In this paper, we have analysed the variability in both wavebands on timescales of 16 seconds. During our observations Cygnus X-2 covered all three branches of the Z-curve, allowing us to study how the changes in X-ray spectral state affect the optical emission. As the optical flux rises the X-ray intensity first rises on the Horizontal Branch (0<S_z<1) but then falls on the Normal Branch (1<S_z<2) and Flaring Branch (2<S_z<3), where S_z is a rank number characterising the position on the Z-curve. This linear increase in the optical flux with S_z indicates the optical flux is a good predictor of the accretion rate (possibly normalized by its own long-term average) inferred from the Z-state S_z. We have used this correlation to decompose the total X-ray count-rate into three distinct spectral components.

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arXiv2003

The Millennium Galaxy Catalogue: Star counts and the Structure of the Galactic Stellar Halo

We derive a star catalogue generated from the images taken as part of the 37.5 sq. deg Millennium Galaxy Catalogue. These data, alone and together with colours gained from the Sloan Digital Sky Survey Early Data Release, allow the analysis of faint star counts (B(MGC) < 20) at high Galactic latitude (41 < b < 63), as a function of Galactic longitude (239 < l < 353). We focus here on the inner stellar halo, providing robust limits on the amplitude of substructure and on the large-scale flattening. In line with previous results, the thick disk, an old, intermediate-metallicity population, is clearly seen in the colour-magnitude diagram. We find that the Galactic stellar halo within ~10 kpc (the bulk of the stellar mass) is significantly flattened, with an axial ratio of (c/a) =0.56 +/- 0.01, again consistent with previous results. Our analysis using counts-in-cells, angular correlation functions and the Lee 2D statistic, confirms tidal debris from the Sagittarius dwarf but finds little evidence for other substructure in the inner halo, at heliocentric distances of < 5 kpc. This new quantification of the smoothness in coordinate space limits the contribution of recent accretion/disruption to the build-up of the bulk of the stellar halo.

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arXiv2003

A Concept for an STJ-based Spectrograph

We describe a multi-order spectrograph concept suitable for 8m-class telescopes, using the intrinsic spectral resolution of Superconducting Tunneling Junction detectors to sort the spectral orders. The spectrograph works at low orders, 1-5 or 1-6, and provides spectral coverage with a resolving power of R~8000 from the atmospheric cutoff at 320 nm to the long wavelength end of the infrared H or K band at 1800 nm or 2400 nm. We calculate that the spectrograph would provide substantial throughput and wavelength coverage, together with high time resolution and sufficient dynamic range. The concept uses currently available technology, or technologies with short development horizons, restricting the spatial sampling to two linear arrays; however an upgrade path to provide more spatial sampling is identified. All of the other challenging aspects of the concept - the cryogenics, thermal baffling and magnetic field biasing - are identified as being feasible.

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arXiv2003

Microlensing limits on numbers and orbits of extra-solar planets from the 1998-2000 OGLE events

We analyze three years (1998-2000) of OGLE observations of microlensing events to place limits on the abundance of planets with a planet-to-star mass ratio $q=10^{-3}$ at distances $\sim 1-4$AU from their host stars, i.e. `cool Jupiters'. We fit a total of 145 events using a maximum likelihood fit that adjusts 6 parameters. Each data point on the lightcurve allows us to exclude planets close to the two images of the source appearing on opposite sides of the Einstein ring of the lens star. We proceed to compute detection probability maps for each event, using $Δχ^2$ threshold values of 25, 60, 100 and combine the results from all events to place global constraints. Our selection criteria returned 5 candidate events for a planet with mass ratio $q=10^{-3}$. Only two of these remained as plausible candidates after three were rejected due to poor data quality at the time of the anomalies. Our results suggest that less than 21 ($n$)% of the lens stars have Jupiter-mass planets orbiting them at an orbital radius of $1 < a < 4$ AU. $n \le 2$ is the number of planet anomaly candidates that are actually due to planets. The datasets presented here were obtained from the DoPhot analysis of the events available at the OGLE website. The main conclusion of this work is that observing time is more efficiently allocated by observing many events with sampling intervals that produce non-overlapping detection zones than using intensive sampling on a small number of events.

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arXiv2003

Status and Prospects of Planetary Transit Searches: Hot Jupiters Galore

The first transiting extrasolar planet, orbiting HD209458, was a Doppler wobble planet before its transits were discovered with a 10 cm CCD camera. Wide-angle CCD cameras, by monitoring in parallel the light curves of tens of thousands of stars, should find hot Jupiter transits much faster than the Doppler wobble method. The discovery rate could easily rise by a factor 10. The sky holds perhaps 1000 hot Jupiters transiting stars brighter than V=13. These are bright enough for follow-up radial velocity studies to measure planet masses to go along with the radii from the transit light curves. I derive scaling laws for the discovery potential of ground-based transit searches, and use these to assess over two dozen planetary transit surveys currently underway. The main challenge lies in calibrating small systematic errors that limit the accuracy of CCD photometry at milli-magnitude levels. Promising transit candidates have been reported by several groups, and many more are sure to follow.

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arXiv2003

Fireballs, Flares and Flickering

We review our understanding of the prototype ``Propeller'' system AE Aqr and we examine its flaring behaviour in detail. The flares are thought to arise from collisions between high density regions in the material expelled from the system after interaction with the rapidly rotating magnetosphere of the white dwarf. We show calculations of the time-dependent emergent optical spectra from the resulting hot, expanding ball of gas and derive values for the mass, lengthscale and temperature of the material involved. We see that the fits suggest that the secondary star in this system has reduced metal abundances and that, counter-intuitively, the evolution of the fireballs is best modelled as isothermal.

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arXiv2003

Searching for Planetary Transits in the Field of Open Cluster NGC 6819 - I

We present results from our survey for planetary transits in the field of the intermediate age (~2.5 Gyr), metal-rich ([Fe/H]~+0.07) open cluster NGC 6819. We have obtained high-precision time-series photometry for over 38,000 stars in this field and have developed an effective matched-filter algorithm to search for photometric transits. This algorithm identified 8 candidate stars showing multiple transit-like events, plus 3 stars with single eclipses. On closer inspection, while most are shown to be low mass stellar binaries, some of these events could be due to brown dwarf companions. The data for one of the single-transit candidates indicates a minimum radius for the companion similar to that of HD 209458b.

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