R. Canal

The binary progenitor of Tycho Brahe's 1572 supernova

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. Tychos supernova is one of 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.

Galactic chemical abundance evolution in the solar neighborhood up to the iron peak

We have developed a detailed standard chemi- cal evolution model to study the evolution of all the chem- ical 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 ∼ 1 Gyr, the halo and the thick disk were assembled out of primordial gas, while the thin disk formed in a second episode of in- fall of slightly enriched extragalactic gas, with much longer timescale. The model nicely reproduces the main observa- tional constraints of the solar neighborhood, and the cal- culated 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 iso- topes 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.

Type Ia Supernova Counts at High z: Signatures of Cosmological Models and Progenitors

Determination of the rates at which Type Ia supernovae (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 mR 23-26 are useful tests of the SNe Ia progenitor systems and cosmological tracers of a possible nonzero value of the cosmological constant, Λ. The SNe Ia counts at high redshifts compare differently with those at lower redshifts depending on the cosmological model. A flat, ΩΛ-dominated universe would show a more significant increase of the SNe Ia counts at z~1 than a flat, ΩM = 1 universe. Here we consider three sorts of universes: a flat universe with H0 = 65 km s-1 Mpc-1, ΩM = 1.0, ΩΛ = 0.0; an open universe with H0 = 65 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 0.0; and a flat, Λ-dominated universe with H0 = 65 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 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 = 0-1 range, contrary to what should be seen for other binary progenitors. A measurement of the SNe Ia counts up to z ~ 1 is within reach of ongoing SNe Ia searches at high redshifts.

The early spectral evolution of SN 2004dt

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 spectr oscopy 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 −1 , 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 featur es, whereas enhancing the density of outer layers significan tly 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.

Reclassification of gamma‐ray bursts

ABSTRACT We have applied two different automatic classifier algorithms to the BATSE CurrentGRB Catalog data and we obtain three different classes of GRBs. Our results confirmthe existence of a third, intermediate class of GRBs, with mean duration ∼ 25-50 s, asdeduced from a cluster analysis and from a neural network algorithm. Our analysesim-ply longer durations than those found by Mukherjee et al. (1998) and Horva´th (1998),whose intermediate class had durations ∼ 2-10 s. From the neural network analysis nodifference in hardness between the two longest classes is found, and from both meth-ods we find that the intermediate-duration class constitutes the most homogeneoussample of GRBs in its space distribution while the longest-duration class constitutesthe most inhomogeneous one with ∼ 0.1, being thus the deepest popula-tion of GRBs with z max ∼ 10. The trend previously found in long bursts, of spatialinhomogeneity increasingwith hardness, only holds for this new longest-durationclass.Key words: gamma-rays: bursts - methods: statistical - data analysis

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This data set provides unique spectral time series down to 2000 A. Significant diversity is seen in the near-maximum-light spectra (~2000-3500 A) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in the uvw1/F250W filter are found to correlate with the B-band light-curve shape parameter Δm 15(B), but with much larger scatter relative to the correlation in the broadband B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 mag 3σ), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.

Galactic Cosmic Rays from Superbubbles and the Abundances of Lithium, Beryllium, and Boron

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 (GCR) nucleosynthesis by particles accelerated in superbubbles. The chemical composition of the superbubble consists of varying proportions of interstellar medium (ISM) and freshly supernova-synthesized material. The observational trends of 6LiBeB evolution are nicely reproduced by models in which GCRs come from a mixture of 25% supernova material with 75% ISM, except for 6Li, for which perhaps an extra source is required at low metallicities. To account for 7Li evolution, several additional sources have been considered (neutrino-induced nucleosynthesis, nova outbursts, and C stars). The model fulfills the energetic requirements for GCR acceleration.

Type Ia Supernova Scenarios and the Hubble Sequence

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 SN 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 SN 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 SN Ia events should be brighter on average and should show larger mean velocities of the ejecta. The observational correlations suggest strongly that the characteristics of the SN 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.

The final stages of evolution of cold, mass-accreting white dwarfs

The evolution of solid C + O white dwarf models upon mass accretion is calculated up to the point of either explosive thermonuclear ignition or gravitational collapse. It is shown that both explosions and quiet collapses to a neutron star are possible for each of two different phase diagrams for high-density C + O mixtures. The ranges of initial masses and temperatures and of accretion rates leading to the different outcomes are determined. Problems concerning the chemical composition of the accreted matter and the effects of tidal dissipation are discussed. 68 references.

Identification of the Companion Stars of Type Ia supernovae

The nature of the binary systems giving rise to Type Ia supernovae (SNe Ia) remains an unsolved problem. In this 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, and red giant) following the explosion of the white dwarf that gives rise to the SNe Ia. 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; these velocities place proper-motion measurements within reach of the Hubble Space Telescope instruments. Detections would solve the long-standing problem of which kinds of binaries produce SNe Ia and would clear the way for an accurate physical modeling of the explosions.