2,590 research outputs found
Formulating representative features with respect to document genre classification
Genre classification (e.g. whether a document
is a scientific article or magazine article) is closely
bound to the physical and conceptual structure of document
as well as the level of depth involved in the text.
Hence, it provides a means of ranking documents retrieved
by search tools according to metrics other than
topical similarity. Moreover, the structural information
derived from genre classification can be used to locate
target information within the text. In previous studies,
the detection of genre classes has been attempted
by using some normalised frequency of terms or combinations
of terms in the document (here, we are using
term as a reference to words, phrases, syntactic
units, sentences and paragraphs, as well as other patterns
derived from deeper linguistic or semantic analysis).
These approaches largely neglect how the term is
distributed throughout the document. Here, we report
the results of automated experiments based on distributive
statistics of words in order to present evidence that
term distribution pattern is a better indicator of genre
class than term frequency.
Which effective viscosity?
Magmas undergoing shear are prime examples of flows that involve the transport of solids and gases by a separate (silicate melt) carrier phase. Such flows are called multiphase, and have attracted much attention due to their important range of engineering applications. Where the volume fraction of the dispersed phase (crystals) is large, the influence of particles on the fluid motion becomes significant and must be taken into account in any explanation of the bulk behaviour of the mixture. For congested magma deforming well in excess of the dilute limit (particle concentrations >40% by volume), sudden changes in the effective or relative viscosity can be expected. The picture is complicated further by the fact that the melt phase is temperature- and shear-rate-dependent. In the absence of a constitutive law for the flow of congested magma under an applied force, it is far from clear which of the many hundreds of empirical formulae devised to predict the rheology of suspensions as the particle fraction increases with time are best suited. Some of the more commonly used expressions in geology and engineering are reviewed with an aim to home in on those variables key to an improved understanding of magma rheology. These include a temperature, compositional and shear-rate dependency of viscosity of the melt phase with the shear-rate dependency of the crystal (particle) packing arrangement. Building on previous formulations, a new expression for the effective (relative) viscosity of magma is proposed that gives users the option to define a packing fraction range as a function of shear stress. Comparison is drawn between processes (segregation, clustering, jamming), common in industrial slurries, and structures seen preserved in igneous rocks. An equivalence is made such that congested magma, viewed in purely mechanical terms as a high-temperature slurry, is an inherently non-equilibrium material where flow at large Péclet numbers may result in shear thinning and spontaneous development of layering
SN Ib 1990I: Clumping and Dust in the Ejecta?
Photometry and spectra of the type Ib SN 1990I are presented and analysed,
covering about 400 days of evolution. The presence of optical helium lines is
shown. SN 1990I seems to show higher velocities compared to a sample of type Ib
events. The nebular emission lines display a high degree of asymmetry and the
presence of fine structures. Using the [O I] 6300,64A flux, we estimate a lower
limit on the oxygen mass to fall in the range (0.7-1.35) M_solar. The oxygen
mass requires a filling factor as small as 10^{-2} on day 254, indicating a
highly clumpy distribution of the oxygen material. A blueshift of the order 600
km/s is reported in the [O I] 6300,64A after day 254. The [Ca II] 7307.5A
emission profile appears blueshifted as well at late epochs . We recover the
quasi-bolometric "BVRI" light curve of SN 1990I. The constructed bolometric
light curve shows a change of slope at late phases, with an e-folding time of
60 +/-2 d in the [50:200] d time interval, considerably faster than the one of
{56}Co decay (i.e. 111.3 d), suggesting the gamma-rays escape with lower
deposition, owing to the low mass nature of the ejecta. After day 200, an
-folding time ~47 +/-2.8 d is measured. A simplified gamma-ray deposition
model is applied after adding a contribution of about 35% to the computed
pseudo-bolometric light curves to account for near-IR luminosities to estimate
the ejecta and {56}Ni masses (M({56}Ni)=0.11 M_solar and M_{ej}=3.7 M_solar).
The deficit in luminosity is estimated to be about 50% around day 308. The
observed spectral blueshift combined with the dramatic and sudden drop in the
pseudo-bolometric light curve and (B-V) colour is interpreted to be a
consequence of dust condensation in the ejecta of SN 1990I around day 250.Comment: 17 pages, 11 gigures and 5 tables. To appear in A & A (accepted
Dense gas in nearby galaxies XVI. The nuclear starburst environment in NGC4945
A multi-line millimeter-wave study of the nearby starburst galaxy NGC 4945
has been carried out using the Swedish-ESO Submillimeter Telescope (SEST). The
study covers the frequency range from 82 GHz to 354 GHz and includes 80
transitions of 19 molecules. 1.3 mm continuum data of the nuclear source are
also presented. A large number of molecular species indicate the presence of a
prominent high density interstellar gas component characterized by cm. Abundances of molecular species are calculated and
compared with abundances observed toward the starburst galaxies NGC 253 and M
82 and galactic sources. Apparent is an `overabundance' of HNC in the nuclear
environment of NGC 4945. While the HNC/HCN =1--0 line intensity ratio is
0.5, the HNC/HCN abundance ratio is 1. While HCN is subthermally
excited (8 K), CN is even less excited (3--4
K), indicating that it arises from a less dense gas component and that its
=2--1 line can be optically thin even though its =1--0 emission is
moderately optically thick. Overall, fractional abundances of NGC 4945 suggest
that the starburst has reached a stage of evolution that is intermediate
between those observed in NGC 253 and M 82. Carbon, nitrogen, oxygen and sulfur
isotope ratios are also determined. Within the limits of uncertainty, carbon
and oxygen isotope ratios appear to be the same in the nuclear regions of NGC
4945 and NGC 253. High O/O, low O/O and
N/N and perhaps also low S/S ratios appear to be
characteristic properties of a starburst environment in which massive stars
have had sufficient time to affect the isotopic composition of the surrounding
interstellar medium.Comment: 26 pages, 16 figures, accepted bt A&
The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters
NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high
spatial resolution near-IR photometry. We derive high T* for the WN stars in
NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~
2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars
make a disproportionate contribution to the global budget of their host
clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region.
Comparisons with stellar models calculated for the main-sequence evolution of
85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3
systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar
masses are supported by dynamical mass determinations for the components of NGC
3603 A1. We consider the predicted L_X of the R136 stars if they were close,
colliding wind binaries. R136c is consistent with a colliding wind binary
system. However, short period, colliding wind systems are excluded for R136a WN
stars if mass ratios are of order unity. Widely separated systems would have
been expected to harden owing to early dynamical encounters with other massive
stars in such a dense environment. From simulated star clusters, whose
constituents are randomly sampled from the Kroupa IMF, both clusters are
consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster
is either too old, exhibits a deficiency of very massive stars, or more likely
stellar masses have been underestimated - M_init for the most luminous stars in
the Arches cluster approach 200 Msun according to contemporary stellar &
photometric results. The potential for stars greatly exceeding 150 Msun within
metal-poor galaxies suggests that such pair-instability SNe could occur within
the local universe, as has been claimed for SN 2007bi (abridged).Comment: 20 pages, 14 figures, accepted for MNRAS. Version with higher
resolution figures is available from
http://pacrowther.staff.shef.ac.uk/R136.pdf See also
http://www.eso.org/public/news/eso1030/ from Wed 21 from noon (CEST
The chemistry of compact planetary nebulae
We report high-sensitivity millimetre observations of several molecular
species (13CO, HCN, HNC, CN, HCO+ and N2H+) in a sample of compact planetary
nebulae. Some species such as HCO+ and CN are particularly abundant compared to
envelopes around AGB stars or even interstellar clouds. We have estimated the
following average values for the column densities ratios: CN/HCN~2.6,
HCO+/HCN~0.5, and HNC/HCN~0.4. Thus, the chemical composition of the molecular
envelopes in these compact PNe appears somewhat intermediate between the
composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe
(such as the Ring, M4--9, or the Helix). From observations of the CO
isotopomers, we have estimated that the 12C/13C ratio is in the range 10 ~<
12C/13C ~< 40. These values are below those expected from standard asymptotic
giant branch models and suggest non-standard mixing processes. The observed
molecular abundances are compared to very recent modelling work, and we
conclude that the observations are well explained, in general terms, by
time-dependent gas-phase chemical models in which the ionization rate is
enhanced by several orders of magnitude with respect to the average
interstellar value. Thus, our observations confirm that the chemistry in the
neutral shells of PNe is essentially governed by the high energy radiation from
the hot central stars. The complexity of the chemical processes is increased by
numerous factors linked to the properties of the central star and the geometry
and degree of clumpiness of the envelope. Several aspects of the PN chemistry
that remains to be understood are discussed within the frame of the available
chemical models.Comment: 9 pages, 3 figures. "In press" in Astronomy and Astrophysic
Nucleosynthesis from massive stars 50 years after B2FH
We review some important observed properties of massive stars. Then we
discuss how mass loss and rotation affect their evolution and help in giving
better fits to observational constraints. Consequences for nucleosynthesis at
different metallicities are discussed. Mass loss appear to be the key feature
at high metallicity, while rotation is likely dominant at low and very low
metallicities. We discuss various indications supporting the view that very
metal poor stars had their evolution strongly affected by rotational mixing.
Many features, like the origin of primary nitrogen at low metallicity, that of
the C-rich extremely metal poor halo stars, of He-rich stars in massive
globular clusters, of the O-Na anticorrelation in globular clusters may be
related to the existence of a population of very fast rotating metal poor stars
that we tentatively call the {\it spinstars}. A fraction of these {\it
spinstars} may also be the progenitors of GRB in very metal poor regions. They
may avoid pair instability explosion due to the heavy mass loss undergone
during their early evolutionary phases and be, dependent on their frequency,
important sources of ionising photons in the early Universe.Comment: 46 pages, 17 figures, in Stellar Nucleosynthesis: 50 years after
B2FH, C. Charbonnel and J.P. Zahn (Eds.), EAS publications Serie
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