10,448 research outputs found
Scatter broadening of pulsars and implications on the interstellar medium turbulence
Observations reveal a uniform Kolmogorov turbulence throughout the diffuse
ionized interstellar medium (ISM) and supersonic turbulence preferentially
located in the Galactic plane. Correspondingly, we consider the Galactic
distribution of electron density fluctuations consisting of not only a
Kolmogorov density spectrum but also a short-wave-dominated density spectrum
with the density structure formed at small scales due to shocks. The resulting
dependence of the scatter broadening time on the dispersion measure (DM)
naturally interprets the existing observational data for both low and high-DM
pulsars. According to the criteria that we derive for a quantitative
determination of scattering regimes over wide ranges of DMs and frequencies
, we find that the pulsars with low DMs are primarily scattered by the
Kolmogorov turbulence, while those at low Galactic latitudes with high DMs
undergo more enhanced scattering dominated by the supersonic turbulence, where
the corresponding density spectrum has a spectral index . Besides,
by considering a volume filling factor of the density structures with the
dependence on as in the supersonic turbulence, our
model can also explain the observed shallower scaling of the scattering
time than the Kolmogorov scaling for the pulsars with relatively large DMs. The
comparison between our analytical results and the scattering measurements of
pulsars in turn makes a useful probe of the properties of the large-scale ISM
turbulence, e.g., an injection scale of pc, and also characteristics
of small-scale density structures.Comment: 11 pages, 3 figures, accepted for publication in Ap
Lifelong Learning CRF for Supervised Aspect Extraction
This paper makes a focused contribution to supervised aspect extraction. It
shows that if the system has performed aspect extraction from many past domains
and retained their results as knowledge, Conditional Random Fields (CRF) can
leverage this knowledge in a lifelong learning manner to extract in a new
domain markedly better than the traditional CRF without using this prior
knowledge. The key innovation is that even after CRF training, the model can
still improve its extraction with experiences in its applications.Comment: Accepted at ACL 2017. arXiv admin note: text overlap with
arXiv:1612.0794
Star formation associated with a large-scale infrared bubble
Using the data from the Galactic Ring Survey (GRS) and Galactic Legacy
Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), we performed a study for a
large-scale infrared bubble with a size of about 16 pc at a distance of 2.0
kpc. We present the 12CO J=1-0, 13CO J=1-0 and C18O J=1-0 observations of HII
region G53.54-0.01 (Sh2-82) obtained at the the Purple Mountain Observation
(PMO) 13.7 m radio telescope to investigate the detailed distribution of
associated molecular material. The large-scale infrared bubble shows a
half-shell morphology at 8 um. H II regions G53.54-0.01, G53.64+0.24, and
G54.09-0.06 are situated on the bubble. Comparing the radio recombination line
velocities and associated 13CO J=1-0 components of the three H II regions, we
found that the 8 um emission associated with H II region G53.54-0.01 should
belong to the foreground emission, and only overlap with the large-scale
infrared bubble in the line of sight. Three extended green objects (EGOs, the
candidate massive young stellar objects), as well as three H II regions and two
small-scale bubbles are found located in the G54.09-0.06 complex, indicating an
active massive star-forming region. C18O J=1-0 emission presents four cloud
clumps on the northeastern border of H II region G53.54-0.01. Via comparing the
spectral profiles of 12CO J=1-0, 13CO J=1-0, and C18O J=1-0 peak at each clump,
we found the collected gas in the three clumps, except for the clump coincided
with a massive YSO (IRAS 19282+1814). Using the evolutive model of H II region,
we derived that the age of H II region G53.54-0.01 is 1.5*10^6 yr. The
significant enhancement of several Class I and Class II YSOs around G53.54-0.01
indicates the presence of some recently formed stars, which may be triggered by
this H II region through the collect and collapse (CC) process.Comment: 9 pages, 6 figures, accepted for publication in A&
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