* Astronomy

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RE: Type Ia supernovae

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Title: Improved Distances to Type Ia Supernovae with Two Spectroscopic Subclasses
Authors: Xiaofeng Wang (1,2), A. V. Filippenko (1), M. Ganeshalingam (1), W. Li (1), J. M. Silverman (1), L. Wang (3), R.Chornock (1), R.J.Foley (1,4), E.L.Gates (5), B. Macomber (1), F.J.D. Serduke (1), T.N.Steele (1), D. S. Wong (1) ((1) UC Berkeley, (2) Tsinghua University, (3) Texas A&M University, (4) Harvard University, (5) Lick Observatory)

We study the observables of 158 relatively normal Type Ia supernovae (SNe Ia) by dividing them into two groups in terms of the expansion velocity inferred from the absorption minimum of the Si II 6355 line in their spectra near B-band maximum brightness. One group ("Normal") consists of normal SNe Ia populating a narrow strip in the Si II velocity distribution, with an average expansion velocity v=10,600±400 km/s near B maximum; the other group ("HV") consists of objects with higher velocities, v > 11,800 km/s. Compared with the Normal group, the HV one shows a narrower distribution in both the peak luminosity and the luminosity decline rate dm_{15}. In particular, their B-V colours at maximum brightness are found to be on average redder by ~0.1, suggesting that they either are associated with dusty environments or have intrinsically red B-V colours. The HV SNe Ia are also found to prefer a lower extinction ratio Rv~1.6 (versus ~2.4 for the Normal ones). Applying such an absorption-correction dichotomy to SNe Ia of these two groups remarkably reduces the dispersion in their peak luminosity from 0.178 mag to only 0.125 mag.

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A. Burrows @ NU: Core Collapse of a Supernova (excerpt)

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A new technique for measuring the distances to supernovae more accurately than ever before has been developed by a team of scientists from Yale University, Lawrence Berkeley National Laboratory and a consortium of French laboratories.
Type Ia supernovae are exploding stars that made the discovery of dark energy possible in 1998. Astronomers continue to use supernovae to understand the acceleration of the universes expansion. In recent years, astronomers have only been able to measure the distances to these standard candles to within about 10 percent accuracy, and as such, can only test current models of dark energy to within a certain limit.

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Title: Using Spectral Flux Ratios to Standardise SN Ia Luminosities
Authors: S. Bailey, G. Aldering, P. Antilogus, C. Aragon, C. Baltay, S. Bongard, C. Buton, M. Childress, N. Chotard, Y. Copin, E. Gangler, S. Loken, P. Nugent, R. Pain, E. Pecontal, R. Pereira, S. Perlmutter, D. Rabinowitz, G. Rigaudier, K. Runge, R. Scalzo, G. Smadja, H. Swift, C. Tao, R. C. Thomas, C. Wu (The Nearby Supernova Factory)
(Version v2)

We present a new method to standardise Type Ia supernova (SN Ia) luminosities to ~<0.13 magnitudes using flux ratios from a single flux-calibrated spectrum per SN. Using Nearby Supernova Factory spectrophotomery of 58 SNe Ia, we performed an unbiased search for flux ratios which correlate with SN Ia luminosity. After developing the method and selecting the best ratios from a training sample, we verified the results on a separate validation sample and with data from the literature. We identified multiple flux ratios whose correlations with luminosity are stronger than those of light curve shape and colour, previously identified spectral feature ratios, or equivalent width measurements. In particular, the flux ratio R(642/443) = F(642 nm) / F(443 nm) has a correlation of 0.95 with SN Ia absolute magnitudes. Using this single ratio as a correction factor produces a Hubble diagram with a residual scatter standard deviation of 0.125 ± 0.011 mag, compared with 0.161 ± 0.015 mag when fit with the SALT2 light curve shape and colour parameters x1 and c. The ratio R(642/443) is an effective correction factor for both extrinsic dust reddening and intrinsic variations such as those of SN 1991T-like and SN 1999aa-like SNe. When combined with broad-band colour measurements, spectral flux ratios can standardise SN Ia magnitudes to ~0.12 mag. These are the first spectral metrics that improve over the standard normalisation methods based upon light curve shape and colour and they provide among the lowest scatter Hubble diagrams ever published.

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Title: Rates and Delay Times of Type Ia Supernovae
Authors: Ashley J. Ruiter (NMSU/CfA), Krzysztof Belczynski (LANL), Chris L. Fryer (LANL)

We analyse the evolution of binary stars to calculate synthetic rates and delay times of the most promising Type Ia Supernovae progenitors. We present and discuss evolutionary scenarios in which a white dwarf reaches the Chandrasekhar-mass and potentially explodes in a Type Ia supernova. We consider: Double Degenerate (DDS), Single Degenerate (SDS), and AM Canum Venaticorum scenarios. The results are presented for two different star formation histories; burst (elliptical-like galaxies) and continuous (spiral-like galaxies). It is found that delay times for the DDS in our standard model (with common envelope efficiency alpha = 1) follow a power-law distribution. For the SDS we note a wide range of delay times, while AM CVn progenitors produce a short burst of SNe Ia at early times. We point out that only the rates for two merging carbon-oxygen white dwarfs, the only systems found in the DDS, are consistent with the observed rates for typical Milky Way-like spirals. We also note that DDS progenitors are the dominant population in elliptical galaxies. The fact that the delay time distribution for the DDS follows a power-law implies more Type Ia supernovae (per unit mass) in young rather than in aged populations. Our results do not exclude other scenarios, but strongly indicate that the DDS is the dominant channel generating SNe Ia in spiral galaxies, at least in the framework of our adopted evolutionary models. Since it is believed that white dwarf mergers cannot produce a thermonuclear explosion given the current understanding of accreting white dwarfs, either the evolutionary calculations along with accretion physics are incorrect, or the explosion calculations are inaccurate and need to be revisited.

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Researchers have come up with a theory for how stars can end in a spectacular so-called Type Ia supernova in less than 100 million years.
While such early-stage supernovae are well-known, theory has been unable to explain them.
The secret, the researchers say, is that white dwarf stars steal mass from nearby "helium stars" until they have enough mass to initiate a supernova.

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A team of astronomers, led by Dr. Bo Wang from the Yunnan Observatory of the Chinese Academy of Sciences, have developed a new model which explains the formation of the most youthful type Ia supernovae. In a paper published in Monthly Notices of the Royal Astronomical Society, Dr. Bo Wang and his team show how the transfer of material from a 'helium star' to a compact white dwarf companion causes these cataclysmic events to take place early on in the life of the galaxy they formed in.
Most type Ia supernovae are believed to occur when a white dwarf (the superdense remnant that is the end state of stars like the Sun) draws matter from a companion star orbiting close by. When the white dwarf mass exceeds the so-called Chandrasekhar limit of 1.4 times the mass of the Sun, it eventually collapses and within a few seconds undergoes a runaway nuclear fusion reaction, exploding and releasing a vast amount of energy as a type Ia supernova.

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Title: The helium star donor channel for the progenitors of Type Ia supernovae
Authors: B. Wang, X. Meng, X. Chen and Z. Han

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution.

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Title: Helium star donor channel for the progenitors of type Ia supernovae
Authors: B. Wang, X. Meng, X. Chen, Z. Han

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution. There are several progenitor models for SNe Ia proposed in the past years. In this paper, we have carried out a detailed study of the He star donor channel, in which a carbon-oxygen white dwarf (CO WD) accretes material from a He main sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. Employing Eggleton's stellar evolution code with an optically thick wind assumption, and adopting the prescription of Kato & Hachisu (2004) for the mass accumulation efficiency of the He-shell flashes onto the WDs, we performed binary evolution calculations for about 2600 close WD binary systems. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period--secondary mass (log P^{i}-M^{i}_2) plane for various WD masses from this channel. The study shows that the He star donor channel is noteworthy for producing SNe Ia (i.e. ~ 1.2 x 10^-3 yr^-1 in the Galaxy), and that the progenitors from this channel may appear as supersoft X-ray sources. Importantly, this channel can explain SNe Ia with short delay times (\la 10^8 yr), which is consistent with recent observational implications of young populations of SN Ia progenitors.

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