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Register a new userModelling the nova rate in galaxies
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We compute theoretical nova rates as well as type Ia SN rates in galaxies of different morphological type (Milky Way, ellipticals and irregulars) by means of detailed chemical evolution models, and compare them with the most recent observations. The main difference among the different galaxies is the assumed history of star formation. In particular, we predict that the nova rates in giant ellipticals such as M87 are 100-300 nova/yr, about a factor of ten larger than in our Galaxy (25 nova/yr), in agreement with very recent estimates from HST data. The best agreement with the observed rates is obtained if the recurrence time of novae in ellipticals is assumed to be longer than in the Milky Way. This result indicates that the star formation rate in ellipticals, and in particular in M87, must have been very efficient at early cosmic epochs. We predict a nova rate for the LMC of 1.7 nova/yr, again in agreement with observations. We compute also the K- and B-band luminosities for ellipticals of different luminous mass and conclude that there is not a clear trend for the luminosity specific nova rate with luminosity among these galaxies. However, firm conclusions about ellipticals cannot be drawn because of possible observational biases in observing these objects. The comparison between the specific nova rates in the Milky Way and the LMC indicates a trend of increasing nova rate passing from the Galaxy towards late-type spirals and Magellanic irregulars.
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Theoretical modelling of the evolution of classical and recurrent novae plays an important role in studies of binary evolution, nucleosynthesis and accretion physics. However, from a theoretical perspective the observed statistical properties of novae remain poorly understood. In this paper, we have produced model populations of novae using a hybrid binary population synthesis approach for differing star formation histories (SFHs): a starburst case (elliptical-like galaxies), a constant star formation rate case (spiral-like galaxies) and a composite case (in line with the inferred SFH for M31). We found that the nova rate at 10;Gyr in an elliptical-like galaxy is $sim 10-20$ times smaller than a spiral-like galaxy with the same mass. The majority of novae in elliptical-like galaxies at the present epoch are characterized by low mass white dwarfs (WDs), long decay times, relatively faint absolute magnitudes and long recurrence periods. In contrast, the majority of novae in spiral-like galaxies at 10;Gyr have massive WDs, short decay times, are relatively bright and have short recurrence periods. The mass loss time distribution for novae in our M31-like galaxy is in agreement with observational data for Andromeda. However, it is possible that we underestimate the number of bright novae in our model. This may arise in part due to the present uncertainties in the appropriate bolometric correction for novae.
Recently, Shara and collaborators searched for novae in M87 in a series of images originally acquired in HST program #10543 (PI: Baltz), finding a surprisingly high nova rate of $363_{-45}^{+33}$ per year. In an attempt to reconcile this rate with previous ground-based estimates, we have undertaken an independent analysis of the HST data. Our results are in broad agreement with those of Shara et al., although we argue that the global nova rate in M87 remains uncertain, both due to the difficulty in identifying bona fide novae from incomplete lightcurves, and in extrapolating observations near the center of M87 to the entire galaxy. We conclude that nova rates as low as ~200 per year remain plausible.
Despite its fundamental importance, a reliable estimate of the Galactic nova rate has remained elusive. Here, the overall Galactic nova rate is estimated by extrapolating the observed rate for novae reaching $mleq2$ to include the entire Galaxy using a two component disk plus bulge model for the distribution of stars in the Milky Way. The present analysis improves on previous work by considering important corrections for incompleteness in the observed rate of bright novae and by employing a Monte Carlo analysis to better estimate the uncertainty in the derived nova rates. Several models are considered to account for differences in the assumed properties of bulge and disk nova populations and in the absolute magnitude distribution. The simplest models, which assume uniform properties between bulge and disk novae, predict Galactic nova rates of $sim$50 to in excess of 100 per year, depending on the assumed incompleteness at bright magnitudes. Models where the disk novae are assumed to be more luminous than bulge novae are explored, and predict nova rates up to 30% lower, in the range of $sim$35 to $sim$75 per year. An average of the most plausible models yields a rate of $50_{-23}^{+31}$ yr$^{-1}$, which is arguably the best estimate currently available for the nova rate in the Galaxy. Virtually all models produce rates that represent significant increases over recent estimates, and bring the Galactic nova rate into better agreement with that expected based on comparison with the latest results from extragalactic surveys.
A multi-epoch H$alpha$ survey of the early-type spiral galaxy M94 (NGC 4736) has been completed as part of a program to establish the galaxys nova rate. A total of four nova candidates were discovered in seven epochs of observation during the period from 2005 to 2007. After making corrections for temporal coverage and spatial completeness, a global nova rate of 5.0$^{+1.8}_{-1.4}$ yr$^{-1}$ was determined. This rate corresponds to a specific-luminosity nova rate of 1.4 $pm$ 0.5 novae per year per 10$^{10} L_{odot,K}$ when the $K$ luminosity is determined from the $B-K$ color, or 1.5 $pm$ 0.4 novae per year per 10$^{10} L_{odot,K}$ when the $K$ luminosity is derived from the Two Micron All Sky Survey. These values are slightly lower than that of other galaxies with measured nova rates, which typically lie in the range of $2-3$ novae per year per 10$^{10} L_{odot}$ in the $K$ band.
VLT and SALT spectroscopy of U Sco were obtained $sim$18 and $sim$30 months after the 2010 outburst. From these spectra the accretion disc is shown to take at least 18 months to become fully reformed. The spectral class of the companion is constrained to be F8$^{+5}_{-6}$,IV-V at the 95% confidence level when the irradiated face of the companion is visible.
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