* Astronomy

Blobrana · L

Tycho's supernova

Blobrana · L

RE: Type Ia supernovae

Blobrana · L

A study of supernovae used as a 'standard candles' to measure the expansion of the universe has revealed how these exploding stars evolve. The results have major implications for studies of dark energy, researchers say.
The research, published last week in the Astrophysical Journal, looked at over 200 observations of 'Type Ia' supernovae over a period of five years.

Read more

Blobrana · L

It's an embarrassing gap in astronomers' knowledge. Despite relying on type Ia supernovae as tools to measure the dark energy speeding up the universe's expansion, they still don't know exactly what causes the blasts. Now the picture has got even fuzzier.
In the standard scenario, a white dwarf pulls matter from a companion star, and this extra mass triggers a supernova. Heavy white dwarfs were thought more likely to explode, since it takes less to push them over the edge.
Now Christopher Pritchet of the University of Victoria in British Columbia, Canada, and colleagues have observed galaxies dominated by lightweight white dwarfs producing type Ia supernovae just as efficiently as those dominated by heavier white dwarfs. These cases may be down to the collision of two white dwarfs, says co-author Andrew Howell of the University of Toronto. The study will appear in The Astrophysical Journal Letters.

Read more

Blobrana · L

Title: The Progenitors of Type Ia Supernovae
Authors: Christopher J. Pritchet, D. Andrew Howell, Mark Sullivan

Type Ia supernovae (SNe Ia) occur in both old, passive galaxies and active, star-forming galaxies. This fact, coupled with the strong dependence of SN Ia rate on star formation rate, suggests that SNe Ia form from stars with a wide range of ages. Here we show that the rate of SN Ia explosions is about 1% of the stellar death rate, independent of star formation history. The dependence of SN Ia rate on star formation rate implies a delay time distribution proportional to t^{-0.5±0.2}. The single degenerate channel for SNe Ia can be made to match the observed SN Ia rate -- SFR relation, but only if white dwarfs are converted to SNe Ia with uniform efficiency of ~1%, independent of mass. Since low-mass progenitors are expected to have lower conversion efficiencies than high mass progenitors, we conclude that some other progenitor scenario must be invoked to explain some, or perhaps all, SNe Ia.

Read more (66kb, PDF)

Blobrana · L

Title: Supernovae in Early-Type Galaxies: Directly Connecting Age and Metallicity with Type Ia Luminosity
Authors: J. S. Gallagher, P. M. Garnavich, N. Caldwell, R. P. Kirshner, S. W. Jha, W. Li, M. Ganeshalingam, A. V. Filippenko
(Version v3)

We have obtained optical spectra of 29 early-type (E/S0) galaxies that hosted type Ia supernovae (SNe Ia). We have measured absorption-line strengths and compared them to a grid of models to extract the relations between the supernova properties and the luminosity-weighted age/composition of the host galaxies. The same analysis was applied to a large number of early-type field galaxies selected from the SDSS spectroscopic survey. We find no difference in the age and abundance distributions between the field galaxies and the SN Ia host galaxies. We do find a strong correlation suggesting that SNe Ia in galaxies whose populations have a characteristic age greater than 5 Gyr are ~ 1 mag fainter at V(max) than those found in galaxies with younger populations. However, the data cannot discriminate between a smooth relation connecting age and supernova luminosity or two populations of SN Ia progenitors. We find that SN Ia distance residuals in the Hubble diagram are correlated with host-galaxy metal abundance, consistent with the predictions of Timmes, Brown & Truran (2003). The data show that high iron abundance galaxies host less-luminous supernovae. We thus conclude that the time since progenitor formation primarily determines the radioactive Ni production while progenitor metal abundance has a weaker influence on peak luminosity, but one not fully corrected by light-curve shape and colour fitters. Assuming no selection effects in discovering SNe Ia in local early-type galaxies, we find a higher specific SN Ia rate in E/S0 galaxies with ages below 3 Gyr than in older hosts. The higher rate and brighter luminosities seen in the youngest E/S0 hosts may be a result of recent star formation and represents a tail of the "prompt" SN Ia progenitors.

Read more (269kb, PDF)

Blobrana · L

Scientists have, for the first time, observed a flash of ultraviolet light from within a dying star giving vital evidence of how stars turn into supernovae.
An international team, including nine scientists from Oxford University, combined data from ground-bound telescopes observing visible light from supernovae with data from a space telescope looking for an earlier peak in ultraviolet light from an associated dying star. They were able to spot telltale signs of the shockwave that forms within a star before it explodes into a supernova.

Read more

Blobrana · L

Title: The Rate of Type Ia Supernovae at z~0.2 from SDSS-I Overlapping Fields
Authors: Assaf Horesh, Dovi Poznanski, Eran O. Ofek, Dan Maoz

In the course of the Sloan Digital Sky Survey (SDSS-I), a large fraction of the surveyed area was observed more than once due to field tiling overlap, usually at different epochs. We utilize some of these data to perform a supernova (SN) survey at a mean redshift of z=0.2. Our archival search, in ~ 5% of the SDSS-I overlap area, produces 29 SN candidates clearly associated with host galaxies. Using the Bayesian photometric classification algorithm of Poznanski et al., and correcting for classification bias, we find 17 of the 29 candidates are likely Type Ia SNe. Accounting for the detection efficiency of the survey and for host extinction, this implies a Type Ia SN rate of R=14.0+(2.5,1.4}-(2.5,1.1}+/-2.5 10^-14 h(70)^2 yr^-1 L_sun^-1, where the errors are Poisson error, systematic detection efficiency error, and systematic classification error, respectively. The volumetric rate is R=1.89+(0.42,0.18)-(0.34,0.15)+/-0.42 10^-5 yr^-1 h(70)^3 Mpc^-3. Our measurement is consistent with other rate measurements at low redshift. An order of magnitude increase in the number of SNe is possible by analysing the full SDSS-I database.

Read more (371kb, PDF)

Blobrana · L

Title: Time Dilation in Type Ia Supernova Spectra at High Redshift
Authors: S. Blondin, T. M. Davis, K. Krisciunas, B. P. Schmidt, J. Sollerman, W. M. Wood-Vasey, A. C. Becker, P. Challis, A. Clocchiatti, G. Damke, A. V. Filippenko, R. J. Foley, P. M. Garnavich, S. W. Jha, R. P. Kirshner, B. Leibundgut, W. Li, T. Matheson, G. Miknaitis, G. Narayan, G. Pignata, A. Rest, A. G. Riess, J. M. Silverman, R. C. Smith, J. Spyromilio, M. Stritzinger, C. W. Stubbs, N. B. Suntzeff, J. L. Tonry, B. E. Tucker, A. Zenteno

We present multiepoch spectra of 13 high-redshift Type Ia supernovae (SNe Ia) drawn from the literature, the ESSENCE and SNLS projects, and our own separate dedicated program on the ESO Very Large Telescope. We use the Supernova Identification (SNID) code of Blondin & Tonry to determine the spectral ages in the supernova rest frame. Comparison with the observed elapsed time yields an apparent aging rate consistent with the 1/(1+z) factor (where z is the redshift) expected in a homogeneous, isotropic, expanding universe. These measurements thus confirm the expansion hypothesis, while unambiguously excluding models that predict no time dilation, such as Zwicky's "tired light" hypothesis. We also test for power-law dependencies of the aging rate on redshift. The best-fit exponent for these models is consistent with the expected 1/(1+z) factor.

Read more (611kb, PDF)

Blobrana · L

Robert Fisher and Cal Jordan are among a team of scientists who will expend 22 million computational hours during the next year on one of the world's most powerful supercomputers, simulating an event that takes less than five seconds.
Fisher and Jordan require such resources in their field of extreme science. Their work at the University of Chicago's Centre for Astrophysical Thermonuclear Flashes explores how the laws of nature unfold in natural phenomena at unimaginably extreme temperatures and pressures. The Blue Gene/P supercomputer at Argonne National Laboratory will serve as one of their primary tools for studying exploding stars.

Read more

White dwarf deflagration by thermonuclear flame (Hi-res)

Click to Play | View Details