arXiv subjects

Margarita Hernanz

Publications and source records attributed to Margarita Hernanz.

47 records · Page 3Linked to original sources

arXiv2003

The Impact of the Chemical Stratification of White Dwarfs on the Classification of Classical Novae

We analyse the impact of the initial abundances of the underlying white dwarf core on the nucleosynthesis accompanying classical nova outbursts, in the framework of hydrodynamic models of the explosion. Specifically, we take into account the chemical stratification of the white dwarf. It turns out that the presence of a thick CO-buffer on top of the ONe-rich core, as results from detailed calculations of previous evolution of the progenitor star, may lead to significant amounts of both 7Li and 26Al, after an outburst that, due to the lack of neon isotopes in the ejecta, would be misclassified as a non-neon nova (i.e., CO nova).

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arXiv2002

He-detonation in sub-Chandrasekhar CO white dwarfs: a new insight into energetics and p-process nucleosynthesis

He-accreting white dwarfs with sub-Chandrasekhar mass are revisited. The impact of the use of an extended reaction network on the predicted energy production and characteristics of the detonating layers is studied. It is shown that the considered scenario can be the site of an $α$p-process combined with a p-process and with a variant of the rp-process we refer to as the pn-process. We define the conditions under which the derived distribution of the abundances of the p-nuclides in the ejecta, including the puzzling light Mo and Ru isotopes, mimics the solar-system one.

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arXiv2001

Synthesis of intermediate-mass elements in classical novae: from Si to Ca

Thermonuclear runaways driven by accretion into degenerate white dwarf cores are the source that power classical nova outbursts. In this paper, we identify the dominant nuclear paths involved in the synthesis of intermediate-mass elements, from Si to Ca, during such violent events. New evolutionary sequences of 1.35 Mo ONe novae have been computed, using updated nuclear reaction rates. The main nuclear activity in this region is powered by the leakage from the NeNa-MgAl region, where the activity is confined during the early stages of the explosion. We discuss the critical role played by 30P(p,gamma) in the synthesis of nuclear species beyond sulfur and point out the large uncertainty that affects its rate, which has dramatic consequences for studies of nova nucleosynthesis in the Si-Ca mass region.

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arXiv2000

Influence of new reaction rates on 18F production in novae

Gamma-ray emission from classical novae is dominated, during the first hours, by positron annihilation resulting from the beta decay of radioactive nuclei. The main contribution comes from the decay of 18F and hence is directly related to 18F formation during the outburst. A good knowledge of the nuclear reaction rates of production and destruction of 18F is required to study 18F synthesis in novae and the resulting gamma-ray emission. The rates relevant for the main mode of 18F destruction (i.e, through proton captures) have been the object of many recent experiments. However, subsequent analyses were focused on providing rates for X-ray burst nucleosynthesis not valid at nova temperatures (lower than 3.5 10^8 K). Accordingly, it is crucial to propose and discuss new reaction rates, incorporating all new experimental results, down to the domain of nova nucleosynthesis. We show that in this temperature regime, the 18F(p,gamma) and (p,alpha) reaction rates remain uncertain and deserve further experimental and theoretical efforts. Our hydrodynamic calculations including the new nuclear rates demonstrate that their impact on 18F synthesis in nova explosions is quite large and, consequently, the early gamma-ray emission from classical novae is also affected.

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arXiv2000

Thermonuclear Runaways on Accreting White Dwarfs: Models of Classical Novae Explosions

The mechanism of classical novae explosions is explained, together with some of their observational properties. The scarce but not null impact of novae in the chemical evolution of the Milky Way is analyzed, as well as their relevance for the radioactivity in the Galaxy. A special emphasis is given to the predicted gamma-ray emission from novae and its relationship with the thermonuclear model itself and its related nucleosynthesis.

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arXiv1999

Nuclear uncertainties in the NeNa-MgAl cycles and production of 22Na and 26Al during nova outbursts

Classical novae eject significant amounts of nuclear processed material into the interstellar medium. Among the isotopes synthesized during such explosions, two radioactive nuclei deserve a particular attention: 22Na and 26Al. In this paper, we investigate the nuclear paths leading to 22Na and 26Al production during nova outbursts by means of an implicit, hydrodynamic code that follows the course of the thermonuclear runaway from the onset of accretion up to the ejection stage. New evolutionary sequences of ONe novae have been computed, using updated nuclear reaction rates relevant to 22Na and 26Al production. Special attention is focused on the role played by nuclear uncertainties within the NeNa and MgAl cycles in the synthesis of such radioactive species. From the series of hydrodynamic models, which assume upper, recommended or lower estimates of the reaction rates, we derive limits on the production of both 22Na and 26Al. We outline a list of nuclear reactions which deserve new experimental investigations in order to reduce the wide dispersion introduced by nuclear uncertainties in the 22Na and 26Al yields.

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arXiv1998

Nuclear uncertainties in the NeNa-MgAl cycles and synthesis of 22Na and 26Al in classical novae

Classical novae eject significant amounts of matter into the interstellar medium, as a result of thermonuclear runaways. Nucleosynthesis associated with nova outbursts includes products from explosive H burning, such as 17O, 15N and 13C, and also radioactive species like 7Be, 22Na and 26Al. In this paper we report on new hydrodynamic calculations of nova outbursts, from the onset of accretion to mass ejection. We stress the role played by the nuclear uncertainties associated with key reactions of the NeNa-MgAl cycles on the synthesis of 22Na and 26Al.

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arXiv1998

Evolutionary calculations of carbon dredge-up in helium envelope white dwarfs

We investigate the evolution of cooling helium atmosphere white dwarfs using a full evolutionary code, specifically developed for following the effects of element diffusion and gravitational settling on white dwarf cooling. The major difference between this work and previous work is that we use more recent opacity data from the OPAL project. Since, in general, these opacities are higher than those available ten years ago, at a given effective temperature, convection zones go deeper than in models with older opacity data. Thus convective dredge-up of observationally detectable carbon in helium atmosphere white dwarfs can occur for thicker helium layers than found by Pelletier et al (1986). We find that the range of observed C to He ratios in different DQ white dwarfs of similar effective temperature is well explained by a range of initial helium layer mass between $10^{-3}$ and $10^{-2} M_{\odot}$, in good agreement with stellar evolution theory, assuming a typical white dwarf mass of $0.6 M_{\odot}$. We also predict that oxygen will be present in DQ white dwarf atmospheres in detectable amounts if the helium layer mass is near the lower limit compatible with stellar evolution theory. Determination of the oxygen abundance has the potential of providing information on the profile of oxygen in the core and hence on the important $^{12}$C$(α,γ)^{16}$O reaction rate.

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arXiv1997

Nucleosynthesis in Classical Novae: ONe vs. CO White Dwarfs

Detailed nucleosynthesis in the ejecta of classical novae has been determined for a grid of hydrodynamic nova models. The reported 14 evolutionary sequences, followed from the onset of accretion up to the explosion and ejection stages, span a range of CO and ONe white dwarf masses (0.8-1.35 $M_o$) and mixing levels between the accreted envelope and the underlying white dwarf core (25-75%). The synthesis of each isotope, from ${}^1$H to ${}^{40}$Ca, is discussed along with its sensitivity to model parameters. Special emphasis is given to isotopes such as ${}^{13}$C, ${}^{15}$N, and ${}^{17}$O, which may account for a significant fraction of their Galactic content. Production of the radioactive isotopes ${}^7$Be, ${}^{22}$Na, and ${}^{26}$Al is also analyzed, since they may provide a direct test of the thermonuclear runaway model through their $γ$-ray emission. The resulting elemental yields reproduce fairly well the spectroscopic abundance determinations of several well studied classical novae.

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arXiv1997

The Physics of crystallizing white dwarfs

White dwarfs can be used as galactic chronometers and, therefore, provide important information about galactic evolution if good theoretical models of their cooling are available. Consequently, it is natural to wonder if all the sources or sinks of energy are correctly taken into account. One of these sources is partial differentiation of the chemical components of the white dwarf upon crystallization. In this paper we use a new formalism to show that if there is a redistribution of the elements inside the star, there is a net release of energy that has to be radiated away and that slows down the cooling rate of the white dwarf.

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arXiv1997

New results on 26Al production in classical novae

The production of 26Al by explosions of classical novae has been computed by means of a hydrodynamic code that follows both the accretion and the explosion stages. A special emphasis has been put on the analysis of the influence of the initial abundances of the accreted envelope, as well as on the nuclear reaction rates involved. With the most recent values of chemical composition and reaction rates available, 26Al production is lowered with respect to previous computations. According to our results, the final contribution of novae to the galactic 26Al is at most 0.4 M_sun, which is a small part of the estimated 26Al in the Galaxy derived from COMPTEL observations of the 1809 keV emission.

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