244,891 research outputs found
Millisecond Oscillations During Thermonuclear X-ray Bursts
I review the basic phenomenology and theory of the millisecond brightness
oscillations observed during thermonuclear X-ray bursts from 13 of
approximately 70 accreting neutron stars in low-mass X-ray binaries. Compelling
observations indicate that the oscillations are produced by surface brightness
patterns on the rapidly rotating neutron stars. However, it remains to be
understood (1) why the brightness patterns producing them persist for up to 15
s during an X-ray burst, whereas the burning should cover the entire surface in
less than 1 s, and (2) why the frequencies drift upward by about 5 Hz during
the course of the burst. These peculiarities can probably be explained by
taking into account the expansion of the surface layers caused by the burning,
zonal flows that form due to pressure gradients between the equator and poles,
and Rossby-Alfven modes that are excited in the surface ocean. Further progress
toward understanding how burning progresses on the surface of the neutron star
can be made with a next-generation X-ray timing mission, which would provide a
larger sample of sources with oscillations, detect sideband signals produced by
the spectrum of modes that should be excited in the neutron star ocean, and
measure harmonic structure in the profiles of the oscillations. These
observations would be crucial for measuring the distribution of the rotation
rates of neutron stars, the progression of unstable nuclear burning in the
accreted ocean, and the curvature of the space-time around the neutron star.Comment: Review article for "X-Ray Timing 2003: Rossi and Beyond", ed. P.
Kaaret, F. K. Lamb, & J. H. Swank (Melville, NY: American Institute of
Physics). 6 pages, including 5 figure
Uncovering The Chemical Signature Of The First Stars In The Universe
The chemical abundance patterns observed in metal-poor Galactic halo stars contain the signature of the first supernovae, and thus allow us to probe the first stars that formed in the universe. We construct a theoretical model for the early chemical enrichment history of the Milky Way, aiming in particular at the contribution from pair-instability supernovae (PISNe). These are a natural consequence of current theoretical models for primordial star formation at the highest masses. However, no metal-poor star displaying the distinct PISN signature has yet been observed. We here argue that this apparent absence of any PISN signature is due to an observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, we predict that early PISN enrichment tends to "overshoot,'' reaching enrichment levels of [Ca/H] similar or equal to -2.5 that would be missed by current searches. We utilize existing observational data to place constraints on the primordial initial mass function (IMF). The number fraction of PISNe in the primordial stellar population is estimated to be 90%) contribution from PISNe is merely similar to 10(-4) to 5 x 10(-4). The corresponding fraction of stars formed from gas exclusively enriched by PISNe is a factor of similar to 4 smaller. With the advent of next-generation telescopes and new, deeper surveys, we should be able to test these predictions.NSF AST 07-08795Astronom
The Initial mass function of the first stars inferred from extremely metal-poor stars
This is an author-created, un-copyedited version of an article published in The Astrophysical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-4357/aab3de.We compare the elemental abundance patterns of ~200 extremely metal-poor (EMP; [Fe/H] < β3) stars to the supernova yields of metal-free stars, in order to obtain insights into the characteristic masses of the first (Population III or Pop III) stars in the universe. The supernova yields are prepared with nucleosynthesis calculations of metal-free stars with various initial masses (M = 13, 15, 25, 40 and 100 M β) and explosion energies (E 51 = E/1051[erg] = 0.5β60), to include low-energy, normal-energy, and high-energy explosions. We adopt the mixing-fallback model, to take into account possible asymmetry in the supernova explosions, and the yields that best fit the observed abundance patterns of the EMP stars are searched by varying the model parameters. We find that the abundance patterns of the EMP stars are predominantly best-fitted by the supernova yields with initial masses M < 40 M β, and that more than than half of the stars are best-fitted by the M = 25 M β hypernova (E 51 = 10) models. The results also indicate that the majority of the primordial supernovae have ejected 10β2β10β1 M β of 56Ni, leaving behind a compact remnant (either a neutron star or a black hole), with a mass in the range of ~1.5β5 M β. These results suggest that the masses of the first stars responsible for the first metal enrichment are predominantly <40 M β. This implies that the higher-mass first stars were either less abundant, directly collapsed into a black hole without ejecting heavy elements, or a supernova explosion of a higher-mass first star inhibits the formation of the next generation of low-mass stars at [Fe/H] < β3.Peer reviewedFinal Accepted Versio
Supernova Nucleosynthesis and Extremely Metal-Poor Stars
We investigate hydrodynamical and nucleosynthetic properties of the
jet-induced explosion of a population III star and compare the
abundance patterns of the yields with those of the metal-poor stars. We
conclude that (1) the ejection of Fe-peak products and the fallback of
unprocessed materials can account for the abundance patterns of the extremely
metal-poor (EMP) stars and that (2) the jet-induced explosion with different
energy deposition rates can explain the diversity of the abundance patterns of
the metal-poor stars. Furthermore, the abundance distribution after the
explosion and the angular dependence of the yield are shown for the models with
high and low energy deposition rates and . We also find that the
peculiar abundance pattern of a Si-deficient metal-poor star HE 1424--0241 can
be reproduced by the angle-delimited yield for of
the model with .Comment: 6 pages, 3 figures. To appear in "ORIGIN OF MATTER AND EVOLUTION OF
GALAXIES: From the Dawn of Universe to the Formation of Solar System", AIP
Conf. Proc. 1016 (December 2007, Sapporo), eds. T. Suda, T. Nozawa, et al.
(Melville: AIP
Cascading Behavior in Large Blog Graphs
How do blogs cite and influence each other? How do such links evolve? Does
the popularity of old blog posts drop exponentially with time? These are some
of the questions that we address in this work. Our goal is to build a model
that generates realistic cascades, so that it can help us with link prediction
and outlier detection.
Blogs (weblogs) have become an important medium of information because of
their timely publication, ease of use, and wide availability. In fact, they
often make headlines, by discussing and discovering evidence about political
events and facts. Often blogs link to one another, creating a publicly
available record of how information and influence spreads through an underlying
social network. Aggregating links from several blog posts creates a directed
graph which we analyze to discover the patterns of information propagation in
blogspace, and thereby understand the underlying social network. Not only are
blogs interesting on their own merit, but our analysis also sheds light on how
rumors, viruses, and ideas propagate over social and computer networks.
Here we report some surprising findings of the blog linking and information
propagation structure, after we analyzed one of the largest available datasets,
with 45,000 blogs and ~ 2.2 million blog-postings. Our analysis also sheds
light on how rumors, viruses, and ideas propagate over social and computer
networks. We also present a simple model that mimics the spread of information
on the blogosphere, and produces information cascades very similar to those
found in real life
Fossil Imprints of the First Generation Supernova Ejecta in Extremely Metal-Deficient Stars
Using results of nucleosynthesis calculations for theoretical core-collapse
supernova models with various progenitor's masses, it is shown that abundance
patterns of C, Mg, Si, Ca, and H seen in extremely metal-deficient stars with
[Fe/H] < -2.5 follow those seen in the individual first generation supernova
remnants (SNRs). This suggests that most of the stars with [Fe/H] < -2.5 were
made from individual supernova (SN) events. To obtain the ratio of heavy
elements to hydrogen, a formula is derived to estimate the mass of hydrogen
swept up by a SNR when it occurs in the interstellar matter with the primordial
abundances. We use [Mg/H] to indicate the metallicities instead of [Fe/H]. The
metallicities [Mg/H] predicted from these SNRs range from ~-4 to ~-1.5 and the
mass of Mg in a SN is well correlated with its progenitor's mass. Thus the
observed [Mg/H] in an extremely metal deficient star has a correspondence to
the progenitor's mass. A larger [Mg/H] corresponds to a larger progenitor's
mass. Therefore, so called `age-metallicity relation' does not hold for stars
with [Fe/H] < -2.5. In contrast, the [Mg/Fe] ratios in the theoretical SNRs
have a different trend from those in extremely metal-deficient stars. It is
also shown that the observed trend of [Mg/Fe] can predict the Fe yield of each
SN given the correspondence of [Mg/H] to the progenitor's mass. The Fe yields
thus obtained are consistent with those derived from SN light curve analyses.
This indicates that there is still a problem in modelling a core-collapse
supernova at its beginning of explosion or mass cut.Comment: 6 pages, 4 figures, 1 table; Accepted for publication in the
Astrophysical Journal Letter
Supernova Nucleosynthesis in the Early Universe
The first metal enrichment in the universe was made by supernova (SN)
explosions of population (Pop) III stars. The trace remains in abundance
patterns of extremely metal-poor (EMP) stars. We investigate the properties of
nucleosynthesis in Pop III SNe by means of comparing their yields with the
abundance patterns of the EMP stars. We focus on (1) jet-induced SNe with
various energy deposition rates [], and (2) SNe of stars with various main-sequence masses () and explosion energies [ergs]. The
varieties of Pop III SNe can explain varieties of the EMP stars: (1) higher
[C/Fe] for lower [Fe/H] and (2) trends of abundance ratios [X/Fe] against
[Fe/H].Comment: 5 pages, 4 figures. To appear in "Low-Metallicity Star Formation:
From the First Stars to Dwarf Galaxies", Proceedings of IAU Symposium 255
(June 2008, Rapallo), eds. L.K. Hunt, S. Madden, & R. Schneider (Cambridge
Univ. Press
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