Novae dwarf

The most metal-rich stars in dwarf spheroidals (dSph) have been shown to have significantly lower even-Z abundance ratios than stars of similar metallicity in the Milky Way (MW). In addition, the most metal-rich dSph stars are dominated by an s-process abundance pattern in comparison to stars of similar metallicity in the MW. This has been interpreted as excessive contamination by Type la super-novae (SN) and asymptotic giant branch (AGB) stars ( Bonifacio et al. 2000, Shetrone et al. 2001, Smecker-Hane McWilliam 2002). By comparing these results to MW chemical evolution, Lanfranchi Matteucci (2003) conclude that the dSph galaxies have had a slower star formation rate than the MW (Lanfranchi Matteucci 2003). This slow star formation, when combined with an efficient galactic wind, allows the contribution of Type la SN and AGB stars to be incorporated into the ISM before the Type II SN can bring the metallicity up to MW thick disk metallicities. 

Novae and symbiotic stars have shells which are excited by extremely hot stars (several x 105 K) like some PNs, but denser, and display overabundances of elements up to Ne or beyond due to thermonuclear processes affecting matter accreted from a companion by a white dwarf. 

Beneath their placid exterior, white dwarfs are very sensitive to the addition of matter from without. The slightest excess ends in a brilliant flash of light (a nova) or an explosion (a type la supernova). 

Fig. 7.7. Stellar duo. The presence of a companion star can considerably perturb a star s evolution. Hence, mass transfer by accretion transforms a rather dull white dwarf into an erupting nova or a type la supernova. As an example, let us follow the life of a star with mass between 4 and 9 Mq and its little sister star with mass between 0.9 and 3 M , separated by a distance of between 1500 and 30000 Rq (where Rq is the solar radius). In childhood, the system is calm. The big star evolves more quickly than the small one, however, a universal feature of stellar evolution. It soon becomes an asymptotic giant, sweeping the companion star with its winds, and then a white dwarf. The oxygen- and carbon-built white dwarf shares an envelope with its partner and together they evolve beneath this cloak as one and the same star. The result is a pair comprising a white dwarf with mass between 0.9 and 1.2 M and a normal star with mass between 0.9 and 3 M , still evolving on the main sequence. The two components are separated by a distance of some 40-400 Rq, corresponding to a period of revolution of 30-800 days. The second star swells up and becomes a red giant. This is a boon for the dwarf. It captures the matter so generously donated. However, it cannot absorb it A tremendous wind is generated and, in the end, a cataclysmic explosion ensues. (After Nomoto et al. 2001.)...

However, not all pairs of stars will meet such a cataclysmic end. The proof stems from the existence of novas. These also arise from the tempestuous love affairs between a white dwarf and a healthy star. The difference is that their love bums in a more reasonable manner, ejecting only a small portion of then-envelope at a time (roughly 10 " Mq). These ejecta are nevertheless loaded like galleons with radioactive isotopes such as beryllium-7 and sodium-22. One day it is hoped that their signature will be detected by the great INTEGRAL observatory. 

Classical nova (CN) and dwarf nova (DN) systems have the same binary components (a low-mass main sequence star and a white dwarf) and the same orbital periods. An important question that therefore arises is are these systems really different (and if so, what is the fundamental difference ) or, are these the same systems, metamorphosing from one class to the other ... 

The first thing to note in this respect is that the white dwarfs in DN systems are believed to accrete continuously (both at quiescence and during eruptions). At the same time, both analytic (e.g. Fujimoto 1982) and numerical calculations show, that when sufficient mass accumulates on the white dwarf, a thermonuclear runaway (TNR) is obtained and a nova outburst ensues (see e.g. reviews by Gallagher and Starrfield 1978, Truran 1982). It is thus only natural, to ask the question, is the fact that we have not seen a DN undergo a CN outburst (in about 50 years of almost complete coverage) consistent with observations of DN systems ... 

We used a mass-radius relation for white dwarfs. Based on (1) — (4) we found that the probability for a nova not to occur in 50 years in these systems is 0.78-0.84, with the range resulting from differences in mass radius relations. We therefore find, that the fact that a nova outburst has not been observed in these systems is not surprising and does not imply that DN and CN systems are different. Incidentally, the systems found most likely to undergo a CN outburst in the near future were ... 

THE CHEMICAL COMPOSITION OF THE WHITE DWARFS IN CATACLYSMIC VARIABLE SYSTEMS WHICH PRODUCE NOVAE... 

Abstract We describe a mechanism that promises to explain how classical nova outbursts take place on white dwarfs of 1 M0 or less and for accretion rates of 4 x 10 M0 yr- or greater. 

Observations suggest that the average mass of white dwarfs in classical nova systems... 

One of the present authors proposed a model for superoutbursts and superhumps of SU UMa stars [1], In this model, the normal outbursts of SU UMa stars are thought to be essentially the same as the ordinary outbursts of dwarf nova and they are supposed to be caused by the disk instability (see e.g., Smak [2]). Superoutbursts are explained in the following way. The heating of the secondary s atmosphere by strong far UV and soft X-ray radiation due to accretion and the resulting increase in mass-overflow rate from the secondary may lead to a positive feed back instability between accretion and mass overflow in a certain circumstance. This "irradiation-induced mass-overflow instability" is the suggested mechanism of "superoutburst" of SU UMa stars. 

The Chemical Composition of the White Dwarfs in Cataclysmic Variable Systems Which Produce Novae... 

On the other hand, most, if not all, ordinary novas occur in double-star systems. For example, consider a red giant in the vicinity of a white dwarf. The gravitational field of the white dwarf may pull hydrogen from its larger companion, thereby initiating fusion and causing a nuclear explosion, a nova, that launches a small amount of gas into space. The process may repeat many times. 

Some 26A1 may also be produced by the classical nova explosions. In these the entire star does not explode rather, about 0.0001 solar masses of skin on a white dwarf experiences a thermonuclear runaway, some fraction of which is ejected in the nova outburst. This ejecta should contain live26 Al, especially from a subset of Mg-rich novae. Magnesium is the parent from which26 Al is created during the explosion. Dust grains that condense Al in the ejecta from the novae will carry excess 2 Mg from this initial content of26 Al, as Fred Hoyle and I pointed out in 1975-76. 

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