Other
Scientific paper
Jun 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt.........2d&link_type=abstract
PhD Thesis: Liverpool John Moores University, UK
Other
2
Novae, Cataclysmic Variables, Galaxies: Individual: M31
Scientific paper
Classical novae (CNe) are interacting binary systems in which the white dwarf undergoes unpredictable explosive outbursts. The energy of a nova outburst is only surpassed by that of gamma-ray bursts, supernovae and a small number of luminous blue variables. However, the outbursts of CNe are far more common than any of these other stars. Due to their brightness and occurrence in both Population I and Population II systems, novae are potentially important as extragalactic distance indicators and tools in the exploration of binary star evolution in galaxies.
The POINT-AGAPE survey is an optical search for gravitational micro-lensing events towards the Andromeda galaxy (M31). As well as micro-lensing, the survey is sensitive to many different classes of variable stars and transients, including CNe. In this work we describe the automated detection and selection pipeline used to identify M31 CNe and we present the resulting catalogue of 20 strong CN candidates observed over three seasons.
The CNe we discover are observed both in the M31 bulge region as well as over a wide area of the M31 disc. Nine of the CNe are caught during the final rise phase (which is often missed in Galactic novae) 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 classes, from very fast to very slow. Among the light curves is, for example, 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. The CN catalogue constitutes a uniquely well-sampled and objectively-selected data set with which to study the statistical properties of CNe in M31. As a by-product, we have detected other interesting variable objects, including one of the longest period and most luminous Mira variables.
An analysis of the MMRD relationship in M31 was performed using the resulting POINT-AGAPE CN catalogue. Within the limits of the uncertainties of extinction internal to M31, good fits were produced to the MMRD in two filters. The MMRD calibration was the first to be performed for Sloan r' and i' filters. However, we were unable to validate the existence of a t_{15} (or similar) relationship for either filter.
The subsequent analysis of the automated pipeline has provided us with the most thorough knowledge of the completeness of a CN survey to-date. Using this analysis we were able to probe the CN distribution of M31 and evaluate the global nova rate. Using models of the galactic surface brightness of M31, we were able to show that the observed CN distribution consisted of a separate bulge and disk population. We were also able show that the M31 bulge CN eruption rate per unit r' flux was up to an order of magnitude greater than that of the disk. This adds weight to the findings of some other authors with respect to nova rate versus stellar population type in extragalactic systems.
Through a combination of the completeness, M31 surface brightness model and our M31 CN eruption model, we were able to deduce a global M31 CN rate of 59±13 year^{-1}, a value much higher than found by previous surveys. Given our understanding of the completeness and an analysis of other sources of error, we are confidently able to conclude that the true global nova rate of M31 is at least 50% higher than was previously thought.
Finally, we go on to introduce an extension of our survey to the extragalactic systems M81 and NGC 2403. This involved a pilot project, using the Isaac Newton and Jacobus Kapteyn telescopes on La Palma, which is now being executed as an approved programme on the 2m Liverpool Telescope (LT). Preliminary results of the LT programme are presented and this, and other future work is discussed.
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