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Aims. We study the optical spectroscopic properties of Type Ia Supernova (SN Ia) 2004dt, focusing our attention on the early epochs. Methods. Observation triggered soon after the SN 2004dt discovery allowed us to obtain a spectrophotometric coverage from day -10 to almost one year (~353 days) after the B band maximum. Observations carried out on an almost daily basis allowed us a good sampling of the fast spectroscopic evolution of SN 2004dt in the early stages. To obtain this result, low-resolution, long-slit spectroscopy was obtained using a number of facilities. Results. This supernova, which in some absorption lines of its early spectra showed the highest degree of polarization ever measured in any SN Ia, has a complex velocity structure in the outer layers of its ejecta. Unburnt oxygen is present, moving at velocities as high as ~16,700 km/s, with some intermediate-mass elements (Mg, Si, Ca) moving equally fast. Modeling of the spectra based on standard density profiles of the ejecta fails to reproduce the observed features, whereas enhancing the density of outer layers significantly improves the fit. Our analysis indicates the presence of clumps of high-velocity, intermediate-mass elements in the outermost layers, which is also suggested by the spectropolarimetric data., Comment: 13 pages, 15 figures, accepted for pubblication in Astronomy and Astrophysics

The variational method with constraints recently developed by Verkley and Gerkema to describe maximum-entropy atmospheric profiles is generalized to ideal gases but with temperature-dependent specific heats. In so doing, an extended and non standard potential temperature is introduced that is well suited for tackling the problem under consideration. This new formalism is successfully applied to the atmosphere of Venus. Three well defined regions emerge in this atmosphere up to a height of $100 km$ from the surface: the lowest one up to about $35 km$ is adiabatic, a transition layer located at the height of the cloud deck and finally a third region which is practically isothermal., Comment: 6 pages, 3 figures

We emphasize the inelasticity distribution of events detected at the IceCube neutrino telescope as an important tool for revealing new physics. This is possible because the unique energy resolution at this facility allows to separately assign the energy fractions for emergent muons and taus in neutrino interactions. As a particular example, we explore the possibility of probing second and third generation leptoquark parameter space (coupling and mass). We show that production of leptoquarks with masses \agt 250 GeV and diagonal generation couplings of O(1) can be directly tested if the cosmic neutrino flux is at the Waxman-Bahcall level., Comment: Matching version to be published in Phys. Rev. D

The final stages of the evolution of electron--degenerate ONe cores, resulting from carbon burning in ``heavy weight'' intermediate--mass stars ($8 M_{\sun}\la M \la 11 M_{\sun}$) and growing in mass, either from carbon burning in a shell or from accretion of matter in a close binary system, are examined in the light of their detailed chemical composition. In particular, we have modelled the evolution taking into account the abundances of the following minor nuclear species, which result from the previous evolutionary history: $^{12}$C, $^{23}$Na, $^{24}$Mg, and $^{25}$Mg. Both $^{23}$Na and $^{25}$Mg give rise to Urca processes, which are found to be unimportant for the final outcome of the evolution. $^{24}$Mg was formerly considered a major component of ONe cores (hence called ONeMg cores), but updated evolutionary calculations in this mass range have severely reduced its abundance. Nevertheless, we have parameterized it and we have found that the minimum amount of $^{24}$Mg required to produce NeO burning at moderate densities is $\sim 23%$, a value exceedingly high in the light of recent evolutionary models. Finally, we have determined that models with relatively small abundances of unburnt carbon ($X(^{12}$C)$\sim 0.015$) could be a channel to explosion at low to moderate density ($\sim 1\times 10^9$ g cm$^{-3}$). This is clearly below the current estimate for the explosion/collapse threshold and would have interesting consequences., Comment: 8 pages, 6 figures, to appear in Astronomy and Astrophysics

The brightness of type Ia supernovae, and their homogeneity as a class, makes them powerful tools in cosmology, yet little is known about the progenitor systems of these explosions. They are thought to arise when a white dwarf accretes matter from a companion star, is compressed and undergoes a thermonuclear explosion Unless the companion star is another white dwarf (in which case it should be destroyed by the mass-transfer process itself), it should survive and show distinguishing properties. Tycho's supernova is one of the only two type Ia supernovae observed in our Galaxy, and so provides an opportunity to address observationally the identification of the surviving companion. Here we report a survey of the central region of its remnant, around the position of the explosion, which excludes red giants as the mass donor of the exploding white dwarf. We found a type G0--G2 star, similar to our Sun in surface temperature and luminosity (but lower surface gravity), moving at more than three times the mean velocity of the stars at that distance, which appears to be the surviving companion of the supernova., Comment: accepted by Natute (subject to embargo policy), 31 pages, 4 figures

Monthly averages of the sunspot number visible on the sun, observed from 1749, Zurich Observatory, and from 1848 other observatories, have been analyzed. This time signal presents a frequency power spectra with a clear $1/f^\alpha$ behavior with $\alpha\simeq 0.8 \pm 0.2$. The well known cycle of approximately 11 years, clearly present in the spectrum, does not produce a sensible distortion of that behavior. The eventual characterization of the sunspot time series as a fractal is analyzed by means of the detrended fluctuation analysis. The jump size distribution of the signal is also studied., Comment: To appear in "Fractals"

The central regions of the remnants of Galactic SNe Ia have been examined for the presence of companion stars of the exploded supernovae. We present the results of this survey for the historical SN 1572 and SN 1006. The spectra of the stars are modeled to obtain Teff, log g and the metallicity. Radial velocities are obtained with an accuracy of 5--10 km s$^{-1}$. Implications for the nature of the companion star in SNeIa follow., Comment: 8 pages, 2 Postscript figures. Appeared in "From Twilight to Highlight: the Physics of Supernovae", ed. W. Hillebrandt & B. Leibundgut (Springer), pp. 140-147

In this article we study the galactic evolution of the LiBeB elements within the framework of a detailed model of the chemical evolution of the Galaxy that includes galactic cosmic ray nucleosynthesis by particles accelerated in superbubbles. The chemical composition of the superbubble consists of varying proportions of ISM and freshly supernova synthesized material. The observational trends of 6 LiBeB evolution are nicely reproduced by models in which GCR come from a mixture of 25% of supernova material with 75% of ISM, except for 6 Li, for which maybe an extra source is required at low metallicities. To account for 7 Li evolution several additional sources have been considered (neutrino-induced nucleosynthesis, nova outbursts, C-stars). The model fulfills the energetic requirements for GCR acceleration., Comment: 25 pages, 9 figures. Accepted for publication in the Astrophysical Journal

We have applied two different automatic classifier algorithms to the BATSE Current GRB Catalog data and we obtain three different classes of GRBs. Our results confirm the existence of a third, intermediate class of GRBs, with mean duration \sim 25-50 s, as deduced from a cluster analysis and from a neural network algorithm. Our analyses imply longer durations than those found by Mukherjee et al. (1998) and Horvath (1998), whose intermediate class had durations \sim 2-10 s. From the neural network analysis no difference in hardness between the two longest classes is found, and from both methods we find that the intermediate-duration class constitutes the most homogeneous sample of GRBs in its space distribution while the longest-duration class constitutes the most inhomogeneous one with \sim 0.1, being thus the deepest population of GRBs with z_max \sim 10. The trend previously found in long bursts, of spatial inhomogeneity increasing with hardness, only holds for this new longest-duration class., Comment: 8 pages, 11 figures, to appear in MNRAS

We extend our model of chemical evolution that successfully account for the main observables in the solar neighborhood (Alibes, Labay & Canal 2001) to the whole Milky Way halo and disk. We assume an inside-out scenario for the assembling of the Galaxy through the infall of external material. Two different compositions of the infalling material have been considered: primordial and slightly metal-enriched (0.1 Z_{\sun}). Our calculations follow the evolution of 76 isotopes up to the Iron peak, but we only present here results for the elements having well-measured gradients: C, N, O, Ne, Mg, Al, Si, S, Ar and Fe. The calculated current radial distributions of these elements are compared with large samples of data from HII regions, B stars, planetary nebulae and open clusters. We also discuss the time evolution of the abundance gradients. Good fits are achieved with both infall compositions, closer to the observational data in the case of primordial infall. We show that contributions from intermediate-mass stars allow to reproduce the measured abundances of C and N, but the profiles of their ratios to oxygen leave still open the nucleosynthetic origin of these elements. In agreement with several previous works, our models predict a flattening with time of the abundance gradients, most of which taking place during the early galactic evolution., Comment: 16 pages, 11 figures, submitted to A&A

A LiBeB evolution model including Galactic Cosmic Ray nucleosynthesis, the $\nu$-process, novae, AGB and C-stars is presented. We have included Galactic Cosmic Ray Nucleosynthesis (GCRN) in a complete Chemical Evolution Model that takes into account 76 stable isotopes from hydrogen to zinc. Any successful LiBeB evolution model should also be compatible with other observational constraints like the age-metallicity relation, the G-dwarf distribution or the evolution of other elements. At the same time, we have checked how different would be a model that took into account the last observations by Wakker et al. (1999) of metal-enriched clouds falling onto the disk, from a primordial infall model., Comment: 2 pages, 2 figures. To appear in `Cosmic Evolution' Conference at IAp, Paris 13-17 Nov 2000

We have developed a detailed standard chemical evolution model to study the evolution of all the chemical elements up to the iron peak in the solar vicinity. We consider that the Galaxy was formed through two episodes of exponentially decreasing infall, out of extragalactic gas. In a first infall episode, with a duration of $\sim$ 1 Gyr, the halo and the thick disk were assembled out of primordial gas, while the thin disk formed in a second episode of infall of slightly enriched extragalactic gas, with much longer timescale. The model nicely reproduces the main observational constraints of the solar neighborhood, and the calculated elemental abundances at the time of the solar birth are in excellent agreement with the solar abundances. By the inclusion of metallicity dependent yields for the whole range of stellar masses we follow the evolution of 76 isotopes of all the chemical elements between hydrogen and zinc. Those results are confronted with a large and recent body of observational data, and we discuss in detail the implications for stellar nucleosynthesis., Comment: 19 pages, 14 figures, submitted to A&A

The nature of the binary systems giving rise to Type Ia supernovae (SNeIa) remains an unsolved problem. In this {\it Letter} we calculate, from the statistics of initial conditions (masses and binary separations), the mass, luminosity, and velocity distributions of the possible binary companions (main-sequence star, subgiant, red giant) following the explosion of the white dwarf which gives rise to the SNeIa. Those companions could be detected from either their proper or their radial motions, by means of high-precision astrometric and radial-velocity measurements in young, nearby supernova remnants. Peculiar velocities typically ranging from 100 to 450 km s$^{-1}$ should be expected, which places proper-motion measurements within reach of HST instruments and makes radial-velocity ones feasible with 2.5-4m class telescopes from the ground. Detections would solve the long-standing problem of which kind of binaries do produce SNeIa and clear up the way to accurate physical modeling of the explosions., Comment: 17 pages, incl. 2 figures. Submittet to ApJ (Letters)

The present cosmic rate of Type Ia supernovae (SNeIa) suggests that about 6% of all stars in binary systems with primaries in the initial mass range $3-9\ M\sun$ end up as SNeIa. If that is confirmed, the unavoidable conclusion is that SNeIa can only be explained by the single degenerate scenario. At most 1% of stars in binary systems in the above range end up as CO + CO WD pairs, with total mass equal to or larger than the Chandrasekhar mass. Given that the number of mergers from pairs of CO + He WDs that reach the Chandrasekhar mass is even lower, the conclusion strongly favors binaries containing just one CO WD as the progenitors of SNeIa, since the SNeIa production efficiency (relative to the instantaneous star formation rate) predicted for double degenerate (DD) pairs lies more than $3\sigma$ below the observational data, and the DD scenario can be rejected at more than 99% confidence level. Only if the SFR measurements from $z\sim 0.1$ to $z\sim 0.5$ are being underestimated by a factor of 6 while SNeIa rates are not, can we escape the above conclusion. We evaluate the numbers and characteristics of double WD systems with different chemical compositions (CO and He WDs) that should form and compare them with the observations, in order to check our predictions. Our conclusions appear robust after that test., Comment: 17 pages, incl. 2 figures. Submitted to ApJ (Letters)

Using a time-dependent Galactic Cosmic Ray flux proportional to the halo Star Formation Rate and including astration and neutrino-induced nucleosynthesis, we have studied the evolution of lithium, beryllium and boron in the halo. Our results set limits to the production of LiBeB by the neutrino-induced nucleosynthesis in massive stars, in order to reproduce the observed constancy of Be/Fe and B/Fe ratios, the lithium plateau and the isotopic ratios evolution., Comment: latex 6 pages, 7 figures (Modified text content and figures). To appear in the Proceedings of the "Workshop on LiBeB, Cosmic Rays and Gamma-Ray Line Astronomy" (PASP)

An analysis of the influence of the Landau-Migdal-Pomeranchuk (LPM) effect on the development of air showers initiated by astroparticles is presented. The theory of Migdal is studied and compared with other theoretical methods, particularly the Blankenbecler and Drell approach. By means of realistic computer simulations and using algorithms that emulate Migdal's theory, including also the so-called dielectric suppression, we study the behavior of the relevant observables in the case of ultra-high energy primaries. We find that the LPM effect can significantly modify the development of high energy electromagnetic showers in certain cases., Comment: 18 pages, 13 figures, 1 table. To appear in Phys. Rev. D

An analysis of the influence of the Landau-Migdal-Pomeranchuck (LPM) effect on the development of air showers initiated by astroparticles, is presented. By means of computer simulations using algorithms that emulate Migdal's theory, including also the so-called dielectric suppression, we study the behaviour of the relevant observables in the case of ultra high energy primaries. We find that the LPM effect can significantly modify the development of high energy electromagnetic showers., Comment: 9 pages, 5 figures

We present a model for big bang nucleosynthesis which combines baryon inhomogeneities with the effects of the decays of massive particles (masses higher than a few GeV). Those particles, with half-lives longer than the standard nucleosynthesis epoch, give rise to both electromagnetic and hadron cascades which modify the abundances of the light nuclides resulting from the previous, inhomogeneous thermonuclear nucleosynthesis stage. We find a region in the parameter space of this composite model where there is agreement with currently inferred primordial abundances for values of $\Omega_{b}h^{2}_{50}$ as high as 0.35 ($\Omega_{b}$ being the baryon fraction of the closure density and $h_{50}$ the Hubble constant in units of 50 km/s/Mpc). Possible improvements of the model are pointed out., Comment: 4 pages, latex, including 1 figure. Summary of a talk given at the Ninth Nuclear Astrophysics Workshop (Ringberg Castle, March 23-27, 1998). To appear as MPA publication. Corrected some typos

We investigate the possibility of accounting for the currently inferred primordial abundances of D, 3He, 4He, and 7Li by big bang nucleosynthesis in the presence of baryon density inhomogeneities plus the effects of late-decaying massive particles (X), and we explore the allowed range of baryonic fraction of the closure density Omega_b in such context. We find that, depending on the parameters of this composite model (characteristic size and density contrast of the inhomogeneities; mass-density, lifetime, and effective baryon number in the decay of the X-particles), values as high as \Omega_{b}h_{50}^{2}\simeq 0.25-0.35 could be compatible with the primordial abundances of the light nuclides. We include diffusion of neutrons and protons at all stages, and we consider the contribution of the X particles to the energy density, the entropy production by their decay, the possibility that the X-products could photodissociate the light nuclei produced during the previous stages of nucleosynthesis, and also the possibility that the decay products of the X-particles would include a substantial fraction of hadrons. Specific predictions for the primordial abundance of Be are made., Comment: 17 pages, incl. 2 figs. Submitted to ApJ (Letters)

Determination of the rates at which supernovae of Type Ia (SNe Ia) occur in the early Universe can give signatures of the time spent by the binary progenitor systems to reach explosion and of the geometry of the Universe. Observations made within the Supernova Cosmology Project are already providing the first numbers. Here it is shown that, for any assumed SNe Ia progenitor, SNe Ia counts up to $m_{R}\simeq 23-26$ are useful tests of the SNe Ia progenitor systems and cosmological tracers of a possible non-zero value of the cosmological constant, $\Lambda$. The SNe Ia counts at high redshifts compare differently with those at lower redshifts depending on the cosmological model. Flat $\Omega_{\Lambda}$--dominated universes would show a more significant increase of the SNe Ia counts at $z \sim 1$ than a flat, $\Omega_{M} = 1$ universe. Here we consider three sorts of universes: a flat universe with $H_{0} = 65 km s^{-1} Mpc^{-1}$, $\Omega_{M} = 1.0$, $\Omega_{\Lambda} = 0.0$; an open universe with $H_{0} = 65 km s^{-1} Mpc^{-1}$, $\Omega_{M} = 0.3$, $\Omega_{\Lambda} = 0.0$; and a flat, $\Lambda$--dominated universe with $H_{0} = 65 km s^{-1} Mpc^{-1}$, $\Omega_{M} = 0.3$, $\Omega_{\Lambda} = 0.7$). On the other hand, the SNe Ia counts from one class of binary progenitors (double degenerate systems) should not increase steeply in the $z= 0$ to $z= 1$ range, contrary to what should be seen for other binary progenitors. A measurement of the SNe Ia counts up to $z \sim 1$ is within reach of ongoing SNe Ia searches at high redshifts., Comment: 16 pages, incl. 2 figures. To appear in ApJ (Letters)

The suggestion that roughly half the mass of the galactic halo might be in the form of white dwarfs, together with the limits on the infrared background light and on the initial metallicity of the galactic disk, would set strong constraints on the initial mass function (IMF) of the halo. Particular IMFs have been proposed to cut off both the numbers of low-mass stars contributing to the infrared background and of high-mass stars which contribute to the growth of metallicity when they promptly explode as gravitational-collapse (Type II and Type Ib/c) supernovae. Here we examine the further contribution to metallicity from the Type Ia (thermonuclear) supernovae which would later be produced from the halo white dwarf population. We find that, for most of the evolutionary scenarios for the Type Ia supernova progenitor systems proposed so far, the constraints on the white dwarf mass fraction in the halo from the predicted production of iron would be extremely severe. When the predicted iron excess is not so large, then the exceedingly high Type Ia supernova rate predicted for the present time would also exclude a major contribution of white dwarfs to the halo mass. The white dwarf contribution, in all cases, should be below 5-10%. Besides, for the IMFs considered, the duration of the halo burst should be shorter than 1 Gyr in order to avoid too large a spread in the iron abundances of Population II halo dwarfs, and the predicted halo $[O/Fe]$ ratio would be at odds with observations., Comment: 16 pages, including 2 figures. AAS macros; to appear in ApJ (Letters), Oct. 1997

The current status of the problem of whether neutron stars can form, in close binary systems, by accretion-induced collapse (AIC) of white dwarfs is examined. We find that, in principle, both initially cold C+O white dwarfs in the high-mass tail of their mass distribution in binaries and O+Ne+Mg white dwarfs can produce neutron stars. Which fractions of neutron stars in different types of binaries (or descendants from binaries) might originate from this process remains uncertain., Comment: 6 pages. To appear in "White Dwarfs", ed. J. Isern, M. Hernanz, and E. Garcia-Berro (Dordrecht: Kluwer)

A review of type Ia supernovae (SNe Ia) statistics as expected from the different scenarios and explosion models so far suggested is presented., Comment: ps file, 26 pages, incl. 6 figs. and 2 tables. To appear in "Thermonuclear Supernovae", ed. P. Ruiz-Lapuente, R. Canal, & J. Isern (Kluwer Acad. Publ.)

The occurrence and properties of Type Ia supernovae (SNe Ia) in single-degenerate binary systems (white dwarf [WD] + nondegenerate companion) is examined for galaxies of different types, and as a function of redshift. The rates and characteristics (peak luminosities, expansion velocities of the ejecta) expected from the explosion of mass-accreting WDs in symbiotic systems and "helium star cataclysmics" are found to be different from those arising in another class of candidate systems: cataclysmic-like (contact) systems (CLS), where a CO WD accretes hydrogen on a thermal time scale from a Roche-lobe filling main-sequence or subgiant companion. We derive the evolution of the SNe Ia rate and properties resulting from the thermonuclear explosion of sub-Chandrasekhar mass WDs in such systems when they detonate a helium layer accumulated from steady burning of hydrogen at the surface. A fraction of CLS are believed to form a subset of the observed luminous supersoft X-ray sources (SSS). Sub-Chandrasekhar explosions from CLS are disfavored in all types of galaxies at redshifts z > 1. On the other hand, CLS where the WD succeeds to grow to the Chandrasekhar mass are more likely found in spiral galaxies, and absent from early-type galaxies. SNe Ia statistics could (if the uncertainties still involved are reduced) help to discriminate among proposed SNe Ia scenarios. The range of variation of the characteristics of SNe Ia in the CLS scenario should be narrower than in symbiotics. The predicted correlation between peak luminosity and expansion velociy of the ejecta in SNe Ia coming from these systems is weak. For CLS, the distinction between the characteristics of SNe Ia respectively arising from sub-Chandrasekhar and from Chandrasekhar-mass explosions should be sharp, since all sub-Chandrasekhar, Comment: uuencoded gz-compressed .tar PostScript file containing 16 pages and 2 figures, to appear in ApJ (Letters), Jan. 10, 1996

The dependence of the Type Ia supernova (SN Ia) rate on galaxy type is examined for three currently proposed scenarios: merging of a Chandrasekhar--mass CO white dwarf (WD) with a CO WD companion, explosion of a sub--Chandrasekhar mass CO WD induced by accretion of material from a He star companion, and explosion of a sub--Chandrasekhar CO WD in a symbiotic system. The variation of the SNe Ia rate and explosion characteristics with time is derived, and its correlation with parent population age and galaxy redshift is discussed. Among current scenarios, CO + He star systems should be absent from E galaxies. Explosion of CO WDs in symbiotic systems could account for the SNe Ia rate in these galaxies. The same might be true for the CO + CO WD scenario, depending on the value of the common envelope parameter. A testable prediction of the sub--Chandrasekhar WD model is that the average brightness and kinetic energy of the SN Ia events should increase with redshift for a given Hubble type. Also for this scenario, going along the Hubble sequence from E to Sc galaxies SNe Ia events should be brighter on average and should show larger mean velocities of the ejecta. The observational correlations strongly suggest that the characteristics of the SNe Ia explosion are linked to parent population age. The scenario in which WDs with masses below the Chandrasekhar mass explode appears the most promising one to explain the observed variation of the SN Ia rate with galaxy type together with the luminosity--expansion velocity trend., Comment: 16 pages uuencoded compressed Postscript, 2 figures included. ApJ Letters, in press