5,579 research outputs found
On the Formation of Massive Primordial Stars
We investigate the formation by accretion of massive primordial protostars in
the range 10 to 300 Msun. The high accretion rate used in the models (4.4 x
10^{-3} Msun/yr) causes the structure and evolution to differ significantly
from those of both present-day protostars and primordial zero-age main sequence
stars. After an initial expansion of the radius (for < 12 Msun), the protostar
undergoes an extended phase of contraction (up to 60 Msun). The stellar surface
is not visible throughout most of the main accretion phase, since a photosphere
is formed in the infalling envelope. Also, significant nuclear burning does not
take place until a protostellar mass of about 80 Msun. As the interior
luminosity approaches the Eddington luminosity, the protostellar radius rapidly
expands, reaching a maximum around 100 Msun. Changes in the ionization of the
surface layers induce a secondary phase of contraction, followed by a final
swelling due to radiation pressure when the stellar mass reaches about 300
Msun. This expansion is likely to signal the end of the main accretion phase,
thus setting an upper limit to the protostellar mass formed in these
conditions.Comment: 11 pages and 4 figures. ApJL accepte
SiGMa: Simple Greedy Matching for Aligning Large Knowledge Bases
The Internet has enabled the creation of a growing number of large-scale
knowledge bases in a variety of domains containing complementary information.
Tools for automatically aligning these knowledge bases would make it possible
to unify many sources of structured knowledge and answer complex queries.
However, the efficient alignment of large-scale knowledge bases still poses a
considerable challenge. Here, we present Simple Greedy Matching (SiGMa), a
simple algorithm for aligning knowledge bases with millions of entities and
facts. SiGMa is an iterative propagation algorithm which leverages both the
structural information from the relationship graph as well as flexible
similarity measures between entity properties in a greedy local search, thus
making it scalable. Despite its greedy nature, our experiments indicate that
SiGMa can efficiently match some of the world's largest knowledge bases with
high precision. We provide additional experiments on benchmark datasets which
demonstrate that SiGMa can outperform state-of-the-art approaches both in
accuracy and efficiency.Comment: 10 pages + 2 pages appendix; 5 figures -- initial preprin
Formation of the First Stars by Accretion
The process of star formation from metal-free gas is investigated by
following the evolution of accreting protostars with emphasis on the properties
of massive objects. The main aim is to establish the physical processes that
determine the upper mass limit of the first stars. Although the consensus is
that massive stars were commonly formed in the first cosmic structures, our
calculations show that their actual formation depends sensitively on the mass
accretion rate and its time variation. Even in the rather idealized case in
which star formation is mainly determined by dot{M}acc, the characteristic mass
scale of the first stars is rather uncertain. We find that there is a critical
mass accretion rate dot{M}crit = 4 10^{-3} Msun/yr that separates solutions
with dot{M}acc> 100 Msun can form,
provided there is sufficient matter in the parent clouds, from others
(dot{M}acc > dot{M}crit) where the maximum mass limit decreases as dot{M}acc
increases. In the latter case, the protostellar luminosity reaches the
Eddington limit before the onset of hydrogen burning at the center via the
CN-cycle. This phase is followed by a rapid and dramatic expansion of the
radius, possibly leading to reversal of the accretion flow when the stellar
mass is about 100Msun. (abridged)Comment: 34 pages, 12 figures. ApJ, in pres
A birth-death process for feature allocation
We propose a Bayesian nonparametric prior over feature allocations for sequential data, the birth-death feature allocation process (BDFP). The BDFP models the evolution of the feature allocation of a set of N objects across a covariate (e.g. time) by creating and deleting features. A BDFP is exchangeable, projective, stationary and reversible, and its equilibrium distribution is given by the Indian buffet process (IBP). We show that the Beta process on an extended space is the de Finetti mixing distribution underlying the BDFP. Finally, we present the finite approximation of the BDFP, the Beta Event Process (BEP), that permits simplified inference. The utility of the BDFP as a prior is demonstrated on real world dynamic genomics and social network dat
Models of Social Groups in Blogosphere Based on Information about Comment Addressees and Sentiments
This work concerns the analysis of number, sizes and other characteristics of
groups identified in the blogosphere using a set of models identifying social
relations. These models differ regarding identification of social relations,
influenced by methods of classifying the addressee of the comments (they are
either the post author or the author of a comment on which this comment is
directly addressing) and by a sentiment calculated for comments considering the
statistics of words present and connotation. The state of a selected blog
portal was analyzed in sequential, partly overlapping time intervals. Groups in
each interval were identified using a version of the CPM algorithm, on the
basis of them, stable groups, existing for at least a minimal assumed duration
of time, were identified.Comment: Gliwa B., Ko\'zlak J., Zygmunt A., Models of Social Groups in
Blogosphere Based on Information about Comment Addressees and Sentiments, in
the K. Aberer et al. (Eds.): SocInfo 2012, LNCS 7710, pp. 475-488, Best Paper
Awar
A multi-color optical survey of the orion nebula cluster. II. The H-R diagram
We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical
photometry and spectroscopy.We study the color–color diagrams in BVI, plus a narrowband filter centered at 6200 Å, finding evidence that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC in the M
spectral-type range, while a better agreement is found employing intrinsic colors derived from synthetic photometry, constraining the surface gravity value as predicted by a pre-main-sequence isochrone.We refine these model colors even further, empirically, by comparison with a selected sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and extend this sample with the addition of 65 newly classified stars from slit spectroscopy and 182 M-type from narrowband photometry; in this way, we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive the stellar reddening and the optical excess due to accretion from the location of each star in the BVI color–color diagram. This enables us to accurately determine the extinction of the ONC members, together with an estimate of their accretion luminosities. We adopt a lower distance for the Orion Nebula than previously assumed, based on recent parallax measurements. With a careful choice of also the spectral-type–temperature transformation, we produce the new Hertzsprung–Russell diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a slightly lower luminosity for early types. We determine the age distribution of the population, peaking from ~2 to ~3 Myr depending on the model. We study the distribution of the members in the mass–age plane and find that taking into account selection effects due to incompleteness,
removes an apparent correlation between mass and age.We derive the initial mass function for low- and intermediate mass members of the ONC, which turns out to be model dependent and shows a turnover at M ≲ 0.2 M_⊙
Community Structure in Time-Dependent, Multiscale, and Multiplex Networks
Network science is an interdisciplinary endeavor, with methods and
applications drawn from across the natural, social, and information sciences. A
prominent problem in network science is the algorithmic detection of
tightly-connected groups of nodes known as communities. We developed a
generalized framework of network quality functions that allowed us to study the
community structure of arbitrary multislice networks, which are combinations of
individual networks coupled through links that connect each node in one network
slice to itself in other slices. This framework allows one to study community
structure in a very general setting encompassing networks that evolve over
time, have multiple types of links (multiplexity), and have multiple scales.Comment: 31 pages, 3 figures, 1 table. Includes main text and supporting
material. This is the accepted version of the manuscript (the definitive
version appeared in Science), with typographical corrections included her
Are There Age Spreads in Star Forming Regions?
A luminosity spread at a given effective temperature is ubiquitously seen in
the Hertzsprung-Russell (HR) diagrams of young star forming regions and often
interpreted in terms of a prolonged period (>=10 Myr) of star formation. I
review the evidence that the observed luminosity spreads are genuine and not
caused by astrophysical sources of scatter. I then address whether the
luminosity spreads necessarily imply large age spreads, by comparing HR diagram
ages with ages from independent clocks such as stellar rotation rate, the
presence of circumstellar material and lithium depletion. I argue that whilst
there probably is a true luminosity dispersion, there is little evidence to
support age spreads larger than a few Myr. This paradox could be resolved by
brief periods of rapid accretion during the class I pre main-sequence phase.Comment: To appear in the proceedings of JENAM10: Star Clusters in the Era of
Large Surveys, 8 page
Casimir force between planes as a boundary finite size effect
The ground state energy of a boundary quantum field theory is derived in
planar geometry in D+1 dimensional spacetime. It provides a universal
expression for the Casimir energy which exhibits its dependence on the boundary
conditions via the reflection amplitudes of the low energy particle
excitations. We demonstrate the easy and straightforward applicability of the
general expression by analyzing the free scalar field with Robin boundary
condition and by rederiving the most important results available in the
literature for this geometry.Comment: 10 pages, 2 eps figures, LaTeX2e file. v2: A reference is added, some
minor modifications made to clarify the text. v3: 9 pages, 3 eps figures,
LaTeX2e file, revtex style. Paper throughly restructured and rewritten. Much
more details are given, but essential results and conclusions are unchanged.
Version accepted for publicatio
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