4,137 research outputs found
Two populations of progenitors for type Ia SNe?
We use recent observations of type Ia Supernova (SN Ia) rates to derive, on
robust empirical grounds, the distribution of the delay time (DTD) between the
formation of the progenitor star and its explosion as a SN. Our analysis finds:
i) delay times as long as 3-4 Gyr, derived from observations of SNe Ia at high
redshift, cannot reproduce the dependence of the SN Ia rate on the colors and
on the radio-luminosity of the parent galaxies, as observed in the local
Universe; ii) the comparison between observed SN rates and a grid of
theoretical "single-population" DTDs shows that only a few of them are possibly
consistent with observations. The most successful models are all predicting a
peak of SN explosions soon after star formation and an extended tail in the
DTD, and can reproduce the data but only at a modest statistical confidence
level; iii) present data are best matched by a bimodal DTD, in which about 50%
of type Ia SNe (dubbed "prompt" SN Ia) explode soon after their stellar birth,
in a time of the order of 10^8 years, while the remaining 50% ("tardy" SN Ia)
have a much wider distribution, well described by an exponential function with
a decay time of about 3 Gyr. This fact, coupled with the well established
bimodal distribution of the decay rate, suggests the existence of two classes
of progenitors. We discuss the cosmological implications of this result and
make simple predictions. [Abridged]Comment: 11 pages, MNRAS, in press, modified after referee's comment
On the Evolution of the Cosmic Supernova Rates
Ongoing searches for supernovae (SNe) at cosmological distances have recently
started to provide a link between SN Ia statistics and galaxy evolution. We use
recent estimates of the global history of star formation to compute the
theoretical Type Ia and Type II SN rates as a function of cosmic time from the
present epoch to high redshifts. We show that accurate measurements of the
frequency of SN events in the range 0<z<1 will be valuable probes of the nature
of Type Ia progenitors and the evolution of the stellar birthrate in the
universe. The Next Generation Space Telescope should detect of order 20 Type II
SNe per 4'x 4' field per year in the interval 1<z<4.Comment: LaTeX, 19 pages, 3 figures, to be published in the MNRA
Traitement de texte et stratégies rédactionnelles
The objective of this experimental observation is to show how the use of a standard word processor changes the writing strategies devised by advanced users during the production of short texts. Empirical research has indicated that word processors, in fact, have a negative impact on writing strategies. Analysis of the conditions under which "man-machine" dialogue takes place, has shown that screen size and linear management both have an effect on writing. Before determining the ways in which a word processor can disrupt common writing practices, we must gain a better understanding of how a text is actually composed in real time, with or without a computer. Although the various writing processes have been clearly identified, the functional scenario describing the succession of writing phases and accompagnying activities is still poorly defined. The marks produced by writers on paper, whether linguistic (words, sentence fragments, sentences) or non-linguistic (arrows, underlining, indexation, diagrams, etc.), reflect the planning, translating, and revising processes being carried out by the writer. Sharples and Pemberton (1990) describe the exact functions of these marks in the elaboration of the ideas to be translated into text form (levels of organization). However, more knowledge about their frequency of use at the different stages of text composition is required. This is one of the goals of the present experimental observation. For the most part, such marks cannot be displayed and manipulated on the screen of a standard word processor as they can on paper. It is therefore crucial that we observe the means employed by writers to adapt their use of these necessary devices to word processing.
The main results indicate that writers who use a word processor still resort to "pencil and paper" for the initial planning. The small amount of text preparation done by computer users (manifested by chronological and hierarchical organization marks) compared to writers who produce without a word processor is compensated by extensive revision on the screen. However, while writing strategies are highly dependent on production conditions, the quality of the texts produced does not vary significantly. The possibility of eliminating one of the important drawbacks of computer-assisted writing i. e. the fact that the information must be displayed linearly on the screen, is currently being studied by designers of planning aids that accompagny word processors. Before such aids can actually be developed, however, more knowledge is needed of the phases of writing and the marks used by writers throughout the production process
Radial distributions of Gamma Ray Bursts and Supernovæ: Clues to their progenitors
In this paper wecomparetheobse rved radial distributions of Gamma Ray Bursts and of Supernovæ with respect to the host galaxy center. We investigate the possibility that the observed Gamma Ray Burst offset distribution (in kpc) is in
fact the distribution of I-b/c supernovæ modified by the kick received by the binary system when the first supernova explosion occurs. Our analysis lends support to the scenario in which all long-duration GRBs are produced by type Ib/c SNe. We ruled out that a significant fraction of long-duration GRBs could be due to merging of compact remnants of stellar evaluation
How many supernovae are we missing at high redshift?
Near-infrared and radio searches for core-collapse supernovae (CC SNe) in the
local universe have shown that the vast majority of the events occurring in
massive starburst are missed by the current optical searches as they explode in
very dusty environments. Recent infrared observations have shown that the
fraction of star-formation activity that takes place in very luminous dusty
starbursts sharply increases with redshift and becomes the dominant star
formation component at z>0.5. As a consequence, an increasing fraction of SNe
are expected to be missed by high-redshift optical searches. We estimate that
5-10% of the local CC SNe are out of reach of the optical searches. The
fraction of missing events rises sharply toward z=1, when about 30% of the CC
SNe will be undetected. At z=2 the missing fraction will be about 60%.
Correspondingly, for type Ia SNe, our computations provide missing fractions of
15% at z=1 and 35% at z=2. Such large corrections are crucially important to
compare the observed SN rate with the expectations from the evolution of the
cosmic star formation history, and to design the future SN searches at high
redshifts.Comment: 9 pages, MNRAS, in press, a few typos correcte
The Rate and Spatial Distribution of Novae in M101 (NGC 5457)
A new multi-epoch H-alpha imaging study of M101 (NGC 5457) has been carried
out as part of a larger campaign to study the rate and stellar population of
extragalactic novae. The survey yielded a total of 13 nova detections from 10
epochs of M101 observations spanning a three year period. After correcting for
the temporal coverage and survey completeness, a global nova rate of
11.7^{+1.9}_{-1.5} per year is found. This value corresponds to a
luminosity-specific nova rate of 1.23\pm0.27 novae per year per 10^{10}
L_{\sun,K} when the K luminosity is derived from the B-K color, or 1.94\pm0.42
novae per year per 10^{10} L_{\sun,K} when the K magnitude from the Two Micron
All Sky Survey is used. These values are consistent with previous estimates by
Shafter et al. that were based on more limited data. The spatial distribution
of the combined nova sample from the present survey and from the earlier
Shafter et al. survey shows that the specific frequency of novae closely
follows the integrated background light of the galaxy.Comment: 22 pages; 4 figures; accepted for publication in the Astrophysical
Journa
Coherent tunneling by adiabatic passage in an optical waveguide system
We report on the first experimental demonstration of light transfer in an
engineered triple-well optical waveguide structure which provides a classic
analogue of Coherent Tunnelling by Adiabatic Passage (CTAP) recently proposed
for coherent transport in space of neutral atoms or electrons among
tunneling-coupled optical traps or quantum wells [A.D. Greentree et al., Phys.
Rev. B 70, 235317 (2004); K. Eckert et al., Phys. Rev. A 70, 023606 (2004)].
The direct visualization of CTAP wavepacket dynamics enabled by our simple
optical system clearly shows that in the counterintuitive passage scheme light
waves tunnel between the two outer wells without appreciable excitation of the
middle well.Comment: submitted for publicatio
- …