102 research outputs found
Breaking Sticks and Ambiguities with Adaptive Skip-gram
Recently proposed Skip-gram model is a powerful method for learning
high-dimensional word representations that capture rich semantic relationships
between words. However, Skip-gram as well as most prior work on learning word
representations does not take into account word ambiguity and maintain only
single representation per word. Although a number of Skip-gram modifications
were proposed to overcome this limitation and learn multi-prototype word
representations, they either require a known number of word meanings or learn
them using greedy heuristic approaches. In this paper we propose the Adaptive
Skip-gram model which is a nonparametric Bayesian extension of Skip-gram
capable to automatically learn the required number of representations for all
words at desired semantic resolution. We derive efficient online variational
learning algorithm for the model and empirically demonstrate its efficiency on
word-sense induction task
SN1993J VLBI (I): The Center of the Explosion and a Limit on Anisotropic Expansion
Phase-referenced VLBI observations of supernova 1993J at 24 epochs, from 50
days after shock breakout to the present, allowed us to determine the
coordinates of the explosion center relative to the quasi-stationary core of
the host galaxy M81 with an accuracy of 45 micro-arcsec, and to determine the
nominal proper motion of the geometric center of the radio shell with an
accuracy of 9micro-arcsec/yr. The uncertainties correspond to 160 AU for the
position and 160 km/s for the proper motion at the distance of the source of
3.63 Mpc. After correcting for the expected galactic proper motion of the
supernova around the core of M81 using HI rotation curves, we obtain a peculiar
proper motion of the radio shell center of only 320 +/- 160 km/s to the south,
which limits any possible one-sided expansion of the shell. We also find that
the shell is highly circular, the outer contours in fact being circular to
within 3%. Combining our proper motion values with the degree of circular
symmetry, we find that the expansion of the shockfront from the explosion
center is isotropic to within 5.5% in the plane of the sky. This is a more
fundamental result on isotropic expansion than can be derived from the
circularity of the images alone. The brightness of the radio shell, however,
varies along the ridge and systematically changes with time. The degree of
isotropy in the expansion of the shockfront contrasts with the asymmetries and
polarization found in optical spectral lines. Asymmetric density distributions
in the ejecta or more likely in the circumstellar medium, are favored to
reconcile the radio and optical results. We see no sign of any disk-like
density distribution of the circumstellar material, with the average axis ratio
of the radio shell of SN1993J being less than 1.04.Comment: 21 pages, LaTex + 5 Figures (encapulsated PostScript), Accepted for
Publication in the Astrophysical Journa
The Distribution of High and Low Redshift Type Ia Supernovae
The distribution of high redshift Type Ia supernovae (SNe Ia) with respect to
projected distance from the center of the host galaxy is studied and compared
to the distribution of local SNe. The distribution of high-z SNe Ia is found to
be similar to the local sample of SNe Ia discovered with CCDs, but different
than the sample discovered photographically. This is shown to be due to the
Shaw effect. These results have implications for the use of SNe Ia to determine
cosmological parameters if the local sample of supernovae used to calibrate the
light curve decline relationships is drawn from a sample discovered
photographically. A K-S test shows that the probability that the high redshift
SNe of the Supernova Cosmology Project are drawn from the same distribution as
the low redshift calibrators of Riess et al. is 0.1%. This is a potential
problem because photographically discovered SNe are preferentially discovered
farther away from the galaxy nucleus, where SNe show a lower scatter in
absolute magnitude, and are on average 0.3 magnitudes fainter than SNe located
closer to the center of their host galaxy. This raises questions about whether
or not the calibration SNe sample the full range of parameters potentially
present in high redshift SNe Ia. The limited data available suggest that the
calibration process is adequate; however, it would be preferable if high
redshift SNe and the low redshift SNe used to calibrate them were drawn from
the same sample, as subtle differences may be important. Data are also
presented which suggest that the seeming anti-Malmquist trend noticed by
Tammann et al.(1996, 1998) for SNe Ia in galaxies with Cepheid distances may be
due to the location of the SNe in their host galaxies.Comment: 16 pages, 2 figures Accepted for publication in the Astrophysical
Journa
Supernovae in isolated galaxies, in pairs and in groups of galaxies
In order to investigate the influence of the environment on the supernova
(SN) production we have performed a statistical investigation of the SNe
discovered in isolated galaxies, in pairs and in groups of galaxies. 22 SNe in
18 isolated galaxies, 48 SNe in 40 galaxies members of 37 pairs and 211 SNe in
170 galaxies members of 116 groups have been selected and studied. We found
that the radial distributions of core-collapse SNe in galaxies located in
different environments are similar, and consistent with that reported by
Bartunov, Makarova & Tsvetkov (1992). SNe discovered in pairs do not privilege
a particular direction with respect to the companion galaxy. Also the azimuthal
distributions inside the hosts members of galaxy groups are consistent with
being isotropics. The fact that SNe are more frequent in the brighter
components of the pairs and groups is expected from the dependence of the SN
rates on the galaxy luminosity. There is an indication that the SN rate is
higher in galaxy pairs compared with that in groups. This can be related to the
enhanced star formation rate in strongly interacting systems. It is concluded
that, with the possible exception of strongly interacting system, the parent
galaxy environment has no direct influence on the SN productionComment: 10 pages, accepted for publication to MNRA
Type Ibc supernovae in disturbed galaxies: evidence for a top-heavy IMF
We compare the radial locations of 178 core-collapse supernovae to the R-band
and H alpha light distributions of their host galaxies. When the galaxies are
split into `disturbed' and `undisturbed' categories, a striking difference
emerges. The disturbed galaxies have a central excess of core-collapse
supernovae, and this excess is almost completely dominated by supernovae of
types Ib, Ic and Ib/c, whereas type II supernovae dominate in all other
environments. The difference cannot easily be explained by metallicity or
extinction effects, and thus we propose that this is direct evidence for a
stellar initial mass function that is strongly weighted towards high mass
stars, specifically in the central regions of disturbed galaxies.Comment: 22 pages, 5 figures, accepted for publication in Ap
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