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N. W. Evans

Publications and source records attributed to N. W. Evans.

At least 181 records · Page 10Linked to original sources
arXiv2004

Simulations of the Population of Centaurs I: The Bulk Statistics

Large-scale simulations of the Centaur population are carried out. The evolution of 23328 particles based on the orbits of 32 well-known Centaurs is followed for up to 3 Myr in the forward and backward direction under the influence of the 4 massive planets. The objects exhibit a rich variety of dynamical behaviour with half-lives ranging from 540 kyr (1996 AR20) to 32 Myr (2000 FZ53). The mean half-life of the entire sample of Centaurs is 2.7 Myr. The data are analyzed using a classification scheme based on the controlling planets at perihelion and aphelion, previously given in Horner et al (2003). Transfer probabilities are computed and show the main dynamical pathways of the Centaur population. The total number of Centaurs with diameters larger than 1 km is estimated as roughly 44300, assuming an inward flux of one new short-period comet every 200 yrs. The flux into the Centaur region from the Edgeworth-Kuiper belt is estimated to be 1 new object every 125 yrs. Finally, the flux from the Centaur region to Earth-crossing orbits is 1 new Earth-crosser every 880 yrs

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arXiv2004

Mapping the Stellar Dynamics of M31

Using the Planetary Nebula Spectrograph, we have observed and measured the velocities for some 2764 PNe in the disk and halo of the Andromeda galaxy. Preliminary analysis using a basic ring model shows a rotation curve in good agreement with that obtained from HI data out to ~20kpc. Some substructure has also been detected within the velocity field, which can be modeled as the continuation of the tidal-remnant known as the Southern Stream, as it passes through Andromeda's disk.

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arXiv2004

Kinematically Cold Populations at Large Radii in the Draco and Ursa Minor Dwarf Spheroidals

We present projected velocity dispersion profiles for the Draco and Ursa Minor (UMi) dwarf spheroidal galaxies based on 207 and 162 discrete stellar velocities, respectively. Both profiles show a sharp decline in the velocity dispersion outside ~30 arcmin (Draco) and ~40 arcmin (UMi). New, deep photometry of Draco reveals a break in the light profile at ~25 arcmin. These data imply the existence of a kinematically cold population in the outer parts of both galaxies. Possible explanations of both the photometric and kinematic data in terms of both equilibrium and non-equilibrium models are discussed in detail. We conclude that these data challenge the picture of dSphs as simple, isolated stellar systems.

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arXiv2004

Classical novae from the POINT-AGAPE microlensing survey of M31 -- I. The nova catalogue

The POINT-AGAPE survey is an optical search for gravitational microlensing events towards the Andromeda Galaxy (M31). As well as microlensing, the survey is sensitive to many different classes of variable stars and transients. Here we describe the automated detection and selection pipeline used to identify M31 classical novae (CNe) and we present the resulting catalogue of 20 CN candidates observed over three seasons. CNe are observed both in the bulge region as well as over a wide area of the M31 disk. Nine of the CNe are caught during the final rise phase and all are well sampled in at least two colours. The excellent light-curve coverage has allowed us to detect and classify CNe over a wide range of speed class, from very fast to very slow. Among the light-curves is a moderately fast CN exhibiting entry into a deep transition minimum, followed by its final decline. We have also observed in detail a very slow CN which faded by only 0.01 mag day$^{-1}$ over a 150 day period. We detect other interesting variable objects, including one of the longest period and most luminous Mira variables. The CN catalogue constitutes a uniquely well-sampled and objectively-selected data set with which to study the statistical properties of classical novae in M31, such as the global nova rate, the reliability of novae as standard-candle distance indicators and the dependence of the nova population on stellar environment. The findings of this statistical study will be reported in a follow-up paper.

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arXiv2004

Distant field BHB stars and the mass of the Galaxy II: Photometry and spectroscopy of UKST candidates 16<B<19.5, 11<R<52 kpc

This is the second in a series of papers presenting a new calculation of the mass of the Galaxy based on radial velocities and distances for a sample of faint 16 < B < 21.3 field blue horizontal-branch (BHB) stars. We present accurate BV CCD photometry and spectra for 142 candidate A-type stars selected from ub_jr photometry of UK Schmidt telescope plates in six high-Galactic-latitude fields. Classification of these candidates produces a sample of 60 BHB stars at distances of 11-52 kpc from the Sun (mean 28 kpc), with heliocentric line-of-sight velocities accurate to 15 km/s, and distance errors < 10%. We provide a summary table listing coordinates and velocities of these stars. The measured dispersion of the radial component of the Galactocentric velocity for this sample is 108+-10 km/s, in agreement with a recent study of the distant halo by Sirko and coworkers. Measurements of the Ca II K line indicate that nearly all the stars are metal-poor with a mean [Fe/H] = -1.8 with dispersion 0.5. Subsequent papers will describe a second survey of BHBs to heliocentric distances 70 < R < 125 kpc and present a new estimate of the mass of the Galaxy.

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arXiv2004

Lightcurve Classification in Massive Variability Surveys II: Transients towards the Large Magellanic Cloud

Automatic classification of variability is now possible with tools like neural networks. Here, we present two neural networks for the identification of microlensing events -- the first discriminates against variable stars and the second against supernovae. The inputs to the networks include parameters describing the shape and the size of the lightcurve, together with colour of the event. The network computes the posterior probability of microlensing, together with an estimate of the likely error. An algorithm is devised for direct calculation of the microlensing rate from the output of the neural networks. We present a new analysis of the microlensing candidates towards the Large Magellanic Cloud (LMC). The neural networks confirm the microlensing nature of only 7 of the possible 17 events identified by the MACHO experiment. This suggests that earlier estimates of the microlensing optical depth towards the LMC may have been overestimated. A smaller number of events is consistent with the assumption that all the microlensing events are caused by the known stellar populations in the outer Galaxy/LMC.

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arXiv2004

A "Baedecker" for the Dark Matter Annihilation Signal

We provide a ``Baedecker'' or travel guide to the directions on the sky where the dark matter annihilation signal may be expected. We calculate the flux of high energy gamma-rays from annihilation of neutralino dark matter in the centre of the Milky Way and the three nearest dwarf spheroidals (Sagittarius, Draco and Canis Major), using realistic models of the dark matter distribution. Other investigators have used cusped dark halo profiles (such as the Navarro-Frenk-White) to claim a significant signal. This ignores the substantial astrophysical evidence that the Milky Way is not dark-matter dominated in the inner regions. We show that the annihilation signal from the Galactic Centre falls by two orders of magnitude on substituting a cored dark matter density profile for a cusped one. The present and future generation of high energy gamma-ray detectors, whether atmospheric Cerenkov telescopes or space missions like GLAST, lack the sensitivity to detect any of the monochromatic gamma-ray annihilation lines. The continuum gamma-ray signal above 1 GeV and above 50 GeV may however be detectable either from the dwarf spheroidals or from the Milky Way itself. If the density profiles of the dwarf spheroidals are cusped, then the best prospects are for detecting Sagittarius and Canis Major. However, if the dwarf spheroidals have milder, cored profiles, then the annihilation signal is not detectable. For GLAST, an attractive strategy is to exploit the wide field of view and observe the Milky Way at medium latitudes, as suggested by Stoehr et al. This is reasonably robust against changes in the density profile.

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arXiv2004

The POINT-AGAPE Survey I: The Variable Stars in M31

The POINT-AGAPE collaboration has been monitoring M31 for three seasons with the Wide Field Camera on the Isaac Newton Telescope. In each season, data are taken for one hour per night for roughly sixty nights during the six months that M31 is visible. The two fields of view straddle the central bulge, northwards and southwards. We have calculated the locations, periods and amplitudes of 35414 variable stars in M31 as a by-product of our microlensing search. The variables are classified according to their period and amplitude of variation. They are classified into population I and II Cepheids, Miras and semi-regular long-period variables. The population I Cepheids are associated with the spiral arms, while the central concentration of the Miras and long-period variables varies noticeably, the stars with brighter (and shorter) variations being much more centrally concentrated. A crucial role in the microlensing experiment is played by the asymmetry signal. It was initially assumed that the variable stars would not be a serious problem as their distributions would be symmetric. We demonstrate that this assumption is not correct. We find that differential extinction associated with the dust lanes causes the variable star distributions to be asymmetric. The size and direction of the asymmetry of the variable stars is measured as a function of period and amplitude of variation. The implications of this discovery for the successful completion of the microlensing experiments towards M31 are discussed. (Abridged)

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arXiv2003

Tracing the Star Stream Through M31 Using Planetary Nebula Kinematics

We present a possible orbit for the Southern Stream of stars in M31, which connects it to the Northern Spur. Support for this model comes from the dynamics of planetary nebulae (PNe) in the disk of M31: analysis of a new sample of 2611 PNe obtained using the Planetary Nebula Spectrograph reveals ~20 objects whose kinematics are inconsistent with the normal components of the galaxy, but which lie at the right positions and velocities to connect the two photometric features via this orbit. The satellite galaxy M32 is coincident with the stream both in position and velocity, adding weight to the hypothesis that the stream comprises its tidal debris.

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arXiv2003

The Anomaly in the Candidate Microlensing Event PA-99-N2

The lightcurve of PA-99-N2, one of the recently announced microlensing candidates towards M31, shows small deviations from the standard Paczynski form. We explore a number of possible explanations, including correlations with the seeing, the parallax effect and a binary lens. We find that the observations are consistent with an unresolved RGB or AGB star in M31 being microlensed by a binary lens. We find that the best fit binary lens mass ratio is about one hundredth, which is one of most extreme values found for a binary lens so far. If both the source and lens lie in the M31 disk, then the standard M31 model predicts the probable mass range of the system to be 0.02-3.6 solar masses (95 % confidence limit). In this scenario, the mass of the secondary component is therefore likely to be below the hydrogen-burning limit. On the other hand, if a compact halo object in M31 is lensing a disk or spheroid source, then the total lens mass is likely to lie between 0.09-32 solar masses, which is consistent with the primary being a stellar remnant and the secondary a low mass star or brown dwarf. The optical depth (or alternatively the differential rate) along the line of sight toward the event indicates that a halo lens is more likely than a stellar lens provided that dark compact objects comprise no less than 15 per cent (or 5 per cent) of haloes.

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arXiv2003

Theory of pixel lensing towards M31 -- II. The velocity anisotropy and flattening of the MACHO distribution

The POINT-AGAPE collaboration is currently searching for massive compact halo objects (MACHOs) towards the Andromeda galaxy (M31). The survey aims to exploit the high inclination of the M31 disk, which causes an asymmetry in the spatial distribution of M31 MACHOs. Here, we investigate the effects of halo velocity anisotropy and flattening on the asymmetry signal using simple halo models. For a spherically symmetric and isotropic halo, we find that the underlying pixel-lensing rate in far-disk M31 MACHOs is more than 5 times the rate of near-disk events. We find that the asymmetry is increased further by about 30% if the MACHOs occupy radial orbits rather than tangential orbits, but is substantially reduced if the MACHOs lie in a flattened halo. However, even for haloes with a minor-to-major axis ratio q = 0.3, the numbers of M31 MACHOs in the far-side outnumber those in the near-side by a factor of ~2. We show that, if positional information is exploited in addition to number counts, then the number of candidate events required to confirm asymmetry for a range of flattened and anisotropic halo models is achievable, even with significant contamination by variable stars and foreground microlensing events. For pixel-lensing surveys which probe a representative portion of the M31 disk, a sample of around 50 candidates is likely to be sufficient to detect asymmetry within spherical haloes, even if half the sample is contaminated, or to detect asymmetry in haloes as flat as q = 0.3 provided less than a third of the sample comprises contaminants. We also argue that, provided its mass-to-light ratio is less than 100, the recently observed stellar stream around M31 is not problematic for the detection of asymmetry. (Abstract slightly abridged.)

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arXiv2003

Fitting Gravitational Lenses: Truth or Delusion

The observables in a strong gravitational lens are usually just the image positions and sometimes the flux ratios. We develop a new and simple algorithm which allows a set of models to be fitted exactly to the observations. Taking our cue from the strong body of evidence that early-type galaxies are close to isothermal, we assume that the lens is scale-free with a flat rotation curve. However, our algorithm allows full flexibility as regards the angular structure of the lensing potential. Importantly, all the free parameters enter linearly into the model and so the lens and flux ratio equations can always be solved by straightforward matrix inversion. We use this new algorithm to examine some of the claims made for substructure on the basis of "anomalous flux ratios". We demonstrate by explicit construction that some of the lenses for which substructure has been claimed can be well-fit by smooth lens models. This is especially the case when the systematic errors in the flux ratios (caused by microlensing or differential extinction or scatter-broadening) are taken into account. However, there is certainly one system (B1422+231) for which the existing smooth models are definitely inadequate and for which substructure may be implicated. Within a few tens of kpc of galaxy centres, dynamical friction is efficient at dissolving any substructure. So, little substructure lies projected within the Einstein radius of most strong lenses. The numbers of strong lenses for which substructure is currently being claimed may be so large that this contradicts rather than supports cold dark matter theories.

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arXiv2003

The First Heroic Decade of Microlensing

We review the fundamentals of vanilla and exotic microlensing. Deviations from the standard form of an achromatic, time-symmetric lightcurve can be caused by the parallax and xallarap effects, finite source size effects and binarity. The pioneering experiments of MACHO, EROS and OGLE reported first candidate events exactly ten years ago, so now is an opportune moment to summarise the achievements of the First Heroic Decade. Three applications are reviewed in detail -- namely (i) searches for compact dark objects in the Galactic halo, (ii) probes of the baryonic mass distribution in the inner Galaxy and the Andromeda Galaxy and (iii) studies of the limb darkening of source stars. Finally, we suggest four projects for the Second Heroic Decade -- (i) K band microlensing towards the Bulge, (ii) pixel lensing towards the low luminosity spiral galaxy M33, (iii) polarimetry of on-going microlensing events and (iv) astrometric microlensing with the GAIA satellite.

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arXiv2003

The Populations of Comet-Like Bodies in the Solar system

A new classification scheme is introduced for comet-like bodies in the Solar system. It covers the traditional comets as well as the Centaurs and Edgeworth-Kuiper belt objects. At low inclinations, close encounters with planets often result in near-constant perihelion or aphelion distances, or in perihelion-aphelion interchanges, so the minor bodies can be labelled according to the planets predominantly controlling them at perihelion and aphelion. For example, a JN object has a perihelion under the control of Jupiter and aphelion under the control of Neptune, and so on. This provides 20 dynamically distinct categories of outer Solar system objects in the Jovian and trans-Jovian regions. The Tisserand parameter with respect to the planet controlling perihelion is also often roughly constant under orbital evolution. So, each category can be further sub-divided according to the Tisserand parameter. The dynamical evolution of comets, however, is dominated not by the planets nearest at perihelion or aphelion, but by the more massive Jupiter. The comets are separated into four categories -- Encke-type, short-period, intermediate and long-period -- according to aphelion distance. The Tisserand parameter categories now roughly correspond to the well-known Jupiter-family comets, transition-types and Halley-types. In this way, the nomenclature for the Centaurs and Edgeworth-Kuiper belt objects is based on, and consistent with, that for comets.

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arXiv2003

The Point-Agape survey: 4 high signal-to-noise ratio microlensing candidates detected towards M31

We have carried out a survey of the Andromeda galaxy for unresolved microlensing (pixel lensing). We present a subset of four short timescale, high signal-to-noise microlensing candidates found by imposing severe selection criteria: the source flux variation exceeds the flux of an R=21 magnitude star and the full width at half maximum timescale is less than 25 days. Remarkably, in three out of four cases, we have been able to measure or strongly constrain the Einstein crossing time of the event. One event, which lies projected on the M31 bulge, is almost certainly due to a stellar lens in the bulge of M31. The other three candidates can be explained either by stars in M31 and M32 or by MACHOs.

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arXiv2003

Supernovae with "Super-Hipparcos"

GAIA is the ``super-Hipparcos'' satellite scheduled for launch in 2010 by the European Space Agency. It is a scanning satellite that carries out multi-colour, multi-epoch photometry on all objects brighter than 20th magnitude. We conduct detailed simulations of supernovae (SNe) detection by GAIA. Supernovae of each type are chosen according to the observed distributions of absolute magnitudes, and located in nearby galaxies according to the local large-scale structure. Using an extinction model of the Galaxy and the scanning law of the GAIA satellite, we calculate how many SNe are detectable as a function of the phase of the lightcurve. Our study shows that GAIA will report data on 21400 SNe during the five-year mission lifetime, of which 14300 are SNe Ia, 1400 are SNe Ib/c and 5700 are SNe II. Using the simulations, we estimate that the numbers caught before maximum are 6300 SNe Ia, 500 SNe Ib/c and 1700 SNe II. GAIA will issue about 5 SNe alerts a day. The most distant SNe accessible to GAIA are at a redshift of 0.14 and so GAIA will provide a huge sample of local SNe. There will be many examples of the rarer subluminous events, over-luminous events, SNe Ib/c and SNe II-L. SNe rates will be found as a function of galaxy type, as well as extinction and position in the host galaxy. Amongst other applications, there may be about 26 SNe each year for which detection of gravitational waves is possible and about 180 SNe each year for which detection of gamma-rays is possible.

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arXiv2002

Five Years at the Movies

GAIA is an astrometric satellite which has been approved by the European Space Agency for launch in about 2010. It will measure the angles between objects in fields that are separated on the sky by about a radian. Data will stream continuously at 1 Mbps from GAIA's three telescopes, providing information on the positions and velocities of the billion or more astrophysical objects brighter than 20th magnitude. The motion of objects across the sky caused by their space motion and their parallactic motion, and the variability of objects in 15 wavebands, will be measured because each object is observed at least 150 times during the 5 year mission lifetime. From the raw time series, a three-dimensional movie of the motions of stars in the Galaxy will be synthesized. The alert despatcher in the GAIA mission will provide forewarning of many kinds of bursting and variable phenomena. We discuss two applications of the GAIA movie show in detail here. First, the astrometric microlensing signal will allow us to take a complete inventory of all objects -- no matter how dark -- in the solar neighbourhood. Second, the catalogue of supernovae detections will be the largest ever taken. It will provide opportunities both for follow-ups in other wavebands and numerous examples of scarce phenomena (like subluminous SNe).

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