Margarita Hernanz

Nucleosynthesis in Classical Novae: CO versus ONe 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☉) and mixing levels between the accreted envelope and the underlying white dwarf core (25%-75%). The synthesis of each isotope from 1H to 40Ca is discussed, along with its sensitivity to model parameters. Special emphasis is placed on isotopes such as 13C,15N, and 17O, whose synthesis may account for a significant fraction of their Galactic content. Production of the radioactive isotopes 7Be,22Na, and 26Al 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 the spectroscopic abundance determinations of several well-studied classical novae fairly well.

Presolar Grains from Novae

We report the discovery of five SiC grains and one graphite grain isolated from the Murchison carbonaceous meteorite whose major-element isotopic compositions indicate an origin in nova explosions. The grains are characterized by low 12C/13C (4-9) and 14N/15N (5-20) ratios, large excesses in 30Si (30Si/28Si ratios range to 2.1 times solar), and high 26Al/27Al ratios. These isotopic signatures are theoretically predicted for the ejecta from ONe novae and cannot be matched by any other stellar sources. Previous studies of presolar grains from primitive meteorites have shown that the vast majority formed in red giant outflows and supernova ejecta. Although a classical nova origin was suggested for a few presolar graphite grains on the basis of 22Ne enrichments, this identification is somewhat ambiguous since it is based on only one trace element. Our present study presents the first evidence for nova grains on the basis of major element isotopic compositions of single grains. We also present the results of nucleosynthetic calculations of classical nova models and compare the predicted isotopic ratios with those of the grains. The comparison points toward massive ONe novae if the ejecta are mixed with material of close-to-solar composition.

Gamma-ray bursts as collimated jets from neutron star/black hole mergers

THE distribution of more than 150 γ-ray bursts detected by the BATSE experiment is isotropic on the sky but radially non-uniform1,2. This raises the possibility that bursts are cosmological (at z≲l) and therefore very energetic events, releasing ∼1050 erg sr−1 on a timescale of seconds. The coalescence of two neutron stars3–7 or the accretion-induced collapse of a white dwarf8 can release up to 1053 erg in the form of neutrino-antineutrino pairs, so that the conversion of <1% into γ-rays by annihilation9 could generate γ-ray bursts, but in all such models an optically thick wind tends to form10,11, preventing the γ-rays from escaping and converting their energy into kinetic energy of the ejected material. We present here a possible solution to this difficulty. When a stellar-mass neutron star is disrupted by a black hole, it forms a thick disk which emits νν̄ pairs. These neutrinos expel a wind from the disk, but angular momentum conservation means that a clear funnel forms along the rotation axis. Neutrino annihilation within the matter-free funnel can then create γ-rays which escape to the distant observer.

The Imprint of Nova Nucleosynthesis in Presolar Grains

Infrared and ultraviolet observations of nova light curves have confirmed grain formation in their expanding shells that are ejected into the interstellar medium by a thermonuclear runaway. In this paper we present isotopic ratios of intermediate-mass elements up to silicon for the ejecta of CO and ONe novae, based on 20 hydrodynamic models of nova explosions. These theoretical estimates will help to properly identify nova grains in primitive meteorites. In addition, equilibrium condensation calculations are used to predict the types of grains that can be expected in the nova ejecta, providing some hints on the puzzling formation of C-rich dust in O > C environments. These results show that SiC grains can condense in ONe novae, in concert with an inferred (ONe) nova origin for several presolar SiC grains. Subject headingg dust, extinction — novae, cataclysmic variables — nuclear reactions, nucleosynthesis, abundances

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 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 a 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 that deserve new experimental investigations in order to reduce the wide dispersion introduced by nuclear uncertainties in the 22Na and 26Al yields.

Bright radio emission from an ultraluminous stellar-mass microquasar in M 31

A subset of ultraluminous X-ray sources (those with luminosities of less than 1040 erg s−1; ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of ∼5–20, probably by means of an accretion disk. The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 1039 erg s−1. The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.

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 M☉ 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,γ) 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.

Gamma‐ray emission from individual classical novae

Classical novae are important producers of radioactive nuclei, such as 7 Be, 13 N, 18 F, 22 Na and 26 Al. The disintegration of these nuclei produces positrons (except for 7 Be) that through annihilation with electrons produce photons of energies 511 keV and below. Furthermore, 7 Be and 22 Na decay producing photons with energies of 478 keV and 1275 keV, respectively, well in the -ray domain. Therefore, novae are potential sources of -ray emission. We have developed two codes in order to analyze carefully the -ray emission of individual classical novae: a hydrodynamical one, which follows both the accretion and the explosion stages, and a Monte-Carlo one, able to treat both the production and the transfer of -ray photons. Both codes have been coupled in order to simulate realistic explosions. The properties of -ray spectra and -ray light curves (for the continuum and for the lines at 511, 478 and 1275 keV) have been analyzed, with a special emphasis on the difference between carbon-oxygen and oxygen-neon novae. Predictions of detectability of individual novae by the future SPI spectrometer on board the INTEGRAL satellite are made. Concerning 26 Al, its decay produces photons of 1809 keV but it occurs on a timescale much longer than the typical time interval between nova outbursts in the Galaxy, making it undetectable in individual novae. The accumulated emission of 26 Al from many Galactic novae has not been modeled in this paper.

21Na(p,gamma)22Mg reaction and oxygen-neon novae.

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in oxygen-neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E(c.m.)=205.7+/-0.5 keV with a resonance strength omegagamma=1.03+/-0.16(stat)+/-0.14(sys) meV.

On the Synthesis of 7Li and 7Be in Novae

The production of 7Li and 7Be during the explosive hydrogen burning that occurs in nova explosions is computed by means of a hydrodynamic code able to treat both the accretion and the explosion stages. Large overproduction factors with respect to solar abundances are obtained, the exact value depending mainly on the chemical composition of the envelope. Although the final ejected masses are small, these results indicate that novae can contribute to the 7Li enrichment of the interstellar medium. Furthermore, since 7Be decays by emitting a gamma ray (478 keV), with a half-life of 53.3 days, the synthesis of 7Li could be tested during the INTEGRAL mission.