arXiv subjects

A. Evans

Publications and source records attributed to A. Evans.

At least 127 records · Page 7Linked to original sources

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

The SuperWASP wide-field exoplanetary transit survey: Candidates from Fields 23hr < RA < 03hr

Photometric transit surveys promise to complement the currently known sample of extra-solar planets by providing additional information on the planets and especially their radii. Here we present extra-solar planet (ESP) candidates from one such survey called, the Wide Angle Search for Planets (WASP) obtained with the SuperWASP wide-field imaging system. Observations were taken with SuperWASP-North located in La Palma during the April to October 2004 observing season. The data cover fields between 23hr and 03hr in RA at declinations above +12. This amounts to over $\approx$400,000 stars with V magnitudes 8 to 13.5. For the stars brighter than 12.5, we achieve better than 1 percent photometric precision. Here we present 41 sources with low amplitude variability between $\approx$ 1 and 10 mmag, from which we select 12 with periods between 1.2 and 4.4 days as the most promising extrasolar planet candidates. We discuss the properties of these ESP candidates, the expected fraction of transits recovered for our sample, and implications for the frequency and detection of hot-Jupiters.

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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

Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high velocity ejecta interact with a pre-existing red giant wind, setting up shock systems analogous to those seen in Supernova Remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert (BAT) and X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25 keV band from t=0 to $t\sim6$ days. XRT observationsfrom 0.3-10 keV, started at 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column, $[N_H]_W$, and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, $[N_H]_W$ is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard Phase I remnant evolution terminated by $t\sim10$ days and the remnant then rapidly evolved to display behaviour characteristic of Phase III. Around t=26 days however, a new, luminous and highly variable soft X-ray source began to appear whose origin will be explored in a subsequent paper.

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arXiv2006

Infrared observations of V838 Mon

We describe the results of fitting simple spherically symmetric models to the first overtone CO and AlO A-X (2-0) bands in V838 Mon. We find that the temperature and column of both CO and AlO systematically decline over the period 2002 October - 2005 February and that an additional, hotter and denser, component is present from 2005 January. We also describe the results of an observation of the star and its light echo at 850micron. We do not detect the `infrared' echo at these wavelength, and place an upper limit of 3x10^7 cm^-2 on the column of grains.

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arXiv2006

An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi, a red giant. Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf, models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia. As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations.

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arXiv2006

Stellar Populations and Mass-Loss in M15: A Spitzer Detection of Dust in the Intra-Cluster Medium

We present Spitzer Space Telescope IRAC and MIPS observations of the galactic globular cluster M15 (NGC 7078), one of the most metal-poor clusters with a [Fe/H] = -2.4. Our Spitzer images reveal a population of dusty red giants near the cluster center, a previously detected planetary nebula (PN) designated K648, and a possible detection of the intra-cluster medium (ICM) arising from mass loss episodes from the evolved stellar population. Our analysis suggests 9 (+/-2) x 10^-4 solar masses of dust is present in the core of M15, and this material has accumulated over a period of approximately 10^6 years, a timescale ten times shorter than the last galactic plane crossing event. We also present Spitzer IRS follow up observations of K648, including the detection of the [NeII] 12.81 micron line, and discuss abundances derived from infrared fine structure lines.

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arXiv2006

The Real-Time Evolution of Sakurai's Star (V4334 Sgr) and other (V)LTP Objects

We report on the progress of our on-going campaign to monitor the evolution of the VLTP objects V4334 Sgr and V605 Aql, as well as the suspected (V)LTP object CK Vul. V4334 Sgr does not show signs of increased ionization compared to our previous observations in 2004. We obtained the first radio detection of V605 Aql, indicating a strong increase in radio flux since 1987. We also present the first radio detection of CK Vul and discuss the expansion of the material ejected during the 1670 event.

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arXiv2006

Spitzer Imaging of nearby ULIRGs and their Progeny: Fine-Structure Elliptical Galaxies

We present results from two mid-infrared imaging programs of ultraluminous infrared galaxies (ULIRGs) and fine-structure elliptical galaxies. The former are known to nearly all be recent mergers between two gas-rich spiral galaxies, while the latter are also believed to be even more aged merger remnants. An examination of these two classes of objects, which may represent different stages of the same putative merger sequence, should reveal similarities in the distribution of their stellar populations and dust content consistent with that expected for time-evolution of the merger. The data reveal for the first time extended dust emission in the ULIRG galaxy bodies and along their tidal features. However, contrary to expectation, we find that the vast majority of the fine-structure elliptical galaxies lack such structured emission. This likely results from the optical selection of the elliptical sample, resulting in a population that reflects the IR-activity (or lack thereof) of optically selected interacting galaxies. Alternately, this may reflect an evolutionary process in the distribution of the dust content of the galaxy bodies.

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arXiv2006

The first 8-13 micron spectra of globular cluster red giants: circumstellar silicate dust grains in 47 Tucanae (NGC 104)

We present 8-13 micron spectra of eight red giants in the globular cluster 47 Tucanae (NGC 104), obtained at the European Southern Observatory 3.6m telescope. These are the first mid-infrared spectra of metal-poor, low-mass stars. The spectrum of at least one of these, namely the extremely red, large-amplitude variable V1, shows direct evidence of circumstellar grains made of amorphous silicate.

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arXiv2005

Stellar mass loss and the Intra-Cluster Medium in Galactic globular clusters: a deep radio survey for HI and OH

We present the results of a survey, the deepest to date, for HI emission at 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720 MHz) in the direction towards the Galactic globular clusters M 15, M 2, NGC 6934, NGC 7006 and Pal 13. The aim is to measure the amount of hydrogen in the intra-cluster medium (ICM), and to find OH masers in the circumstellar envelopes of globular cluster red giants. We present a tentative detection of 0.3 Msun of neutral hydrogen in M 15 and possible detections of neutral hydrogen in M 2 and Pal 13. We derive upper limits to the neutral hydrogen content of NGC 6934 and NGC 7006. No OH emission is detected. We also present deep HI data of the northern tip of the Magellanic Stream behind Pal 13.

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arXiv2005

Infrared spectroscopy of Nova Cassiopeiae 1993 (V705 Cas). IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared spectral energy distribution with the DUSTY code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of ~2:1 by number (1.26:1 by mass) and a relatively flat size distribution. The 9.7micron and 18micron silicate features are consistent with freshly-condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions \~0.06micron; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova UIR features.

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arXiv2005

Infall and SiO emission in V838 Mon

We present moderate and high resolution infrared spectroscopy of the peculiar eruptive variable V838 Mon, which underwent a series of remarkable outbursts in early 2002. During the period covered by our observations, 2002 December-2003 December, the near-infrared spectrum continued to show many of the characteristics of a very cool supergiant. However, throughout this period the spectrum also revealed strong and variable SiO first overtone emission, and Paschen Beta emission. The 2003 December spectrum contained a series of Ti I lines with inverse P Cygni profiles. This is clear evidence that some material is falling inward towards the star.

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arXiv2004

The sub-millimetre evolution of V4334 Sgr (Sakurai's Object)

We report the results of monitoring of V4334 Sgr (Sakurai's Object) at 450 microns and 850 microns with SCUBA on the James Clerk Maxwell Telescope. The flux density at both wavelengths has increased dramatically since 2001, and is consistent with continued cooling of the dust shell in which Sakurai's Object is still enshrouded, and which still dominates the near-infrared emission. Assuming that the dust shell is optically thin at sub-millimetre wavelengths and optically thick in the near-infrared, the sub-millimetre data imply a mass-loss rate during 2003 of ~3.4(+/0.2)E-5 for a gas-to-dust ratio of 75. This is consistent with the evidence from 1-5micron observations that the mass-loss is steadily increasing.

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arXiv2004

Discovery of multiple shells around V838 Monocerotis

We report the discovery of multiple shells around the eruptive variable star V838 Mon. Two dust shells are seen in IRAS and MSX images, which themselves are situated in a shell of CO. This securely establishes V838 Mon as an evolved object. We revisit the light echo, which arises from scattering off the innermost resolved dust shell, to infer a distance to V838 Mon of >~5.5 kpc. The dynamical timescales of the ejected shells, location in the Milky Way and inferred luminosity are consistent with V838 Mon being a low-mass AGB star experiencing thermal pulses of which the 2002 eruption might have sent the star into the post-AGB phase. This scenario, however, is inconsistent with the presence of a (young and massive) B3 V companion.

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arXiv2004

Warm high velocity CO in the wind of Sakurai's Object (= V4334 Sgr)

We present UKIRT UIST spectra of Sakurai's Object (=V4334 Sgr) showing CO fundamental band absorption features around 4.7 microns. The line-centres are at heliocentric radial velocity of -170+/-30 km/s. The number and relative strengths of the lines indicate a CO gas temperature of 400+/-100 K and CO column density of 7(+3/-2)x10^17 per square cm. The gas was moving away from the central star at an average speed of 290+/-30 km/s in 2003 September. The lines appeared sometime between mid 1999 (well after the opaque dust shell formed) and mid 2000 and may have been somewhat more blue--shifted initially than they are now. The observed CO velocity and temperature indicate the continued presence of a fast wind in the object, previously seen in the He I 1.083 micron line beginning just prior to massive dust formation, and more recently in atomic and ionized lines. The dust continuum is consistent with a temperature of 350+/-30 K, indicating continued cooling of the shell. The similar CO temperature suggests that the bulk of the CO absorption occurs just outside of the dust continuum surface.

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arXiv2004

CO bands in V4334 Sgr (Sakurai's Object): the 12C/13C ratio

We present the results of our analysis of a high resolution (R ~ 30000) infrared spectrum of V4334 Sgr (Sakurai's Object) around the first overtone CO bands, obtained in 1998 July. The 12CO and 13CO bands are well-resolved, and we compute synthetic hydrogen-deficient model atmosphere spectra to determine the 12C/13C ratio. We find 12C/13C ~ 4 +/-1, consistent with the interpretation of V4334 Sgr as an object that has undergone a very late thermal pulse.

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arXiv2003

The Early Ultraviolet Evolution of the ONeMg Nova V382 Velorum 1999

We present a multiwavelength study of the ONeMg Galactic nova V382 Velorum 1999 using HST/STIS and FUSE ultraviolet spectra and comparisons with published groundbased optical spectra. We find a close match to the basic phenomenology of another well-studied ONeMg nova, V1974 Cygni (Nova Cygni 1992), in particular to the spectral development through the start of the nebular phase. Following an ``iron curtain'' phase, the nova proceeded through a stage of P Cygni line profiles on all important resonance lines, as in many ONeMg novae and unlike the CO class. Emergent emission lines displayed considerable structure, as seen in V1974 Cyg, indicating fragmentation of the ejecta at the earliest stages of the outburst. Analysis and modeling of our ultraviolet spectra suggest that 4 - 5 $\times$ 10$^{-4}$M$_{\odot}$ of material was ejected and that the distance to the nova is $\simeq 2.5$ kpc. Relative to solar values, we find the following abundances: He = 1.0, C = 0.6$\pm$0.3, N = 17$\pm$4, O = 3.4$\pm$0.3, Ne = 17$\pm$3, Mg = 2.6$\pm$0.1, Al = 21$\pm$2, and Si = 0.5$\pm$0.3. Finally, we briefly draw comparisons with Nova LMC 2000, another ONeMg nova, for which similar data were obtained with HST and FUSE.

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