186 research outputs found
Parameter Learning of Logic Programs for Symbolic-Statistical Modeling
We propose a logical/mathematical framework for statistical parameter
learning of parameterized logic programs, i.e. definite clause programs
containing probabilistic facts with a parameterized distribution. It extends
the traditional least Herbrand model semantics in logic programming to
distribution semantics, possible world semantics with a probability
distribution which is unconditionally applicable to arbitrary logic programs
including ones for HMMs, PCFGs and Bayesian networks. We also propose a new EM
algorithm, the graphical EM algorithm, that runs for a class of parameterized
logic programs representing sequential decision processes where each decision
is exclusive and independent. It runs on a new data structure called support
graphs describing the logical relationship between observations and their
explanations, and learns parameters by computing inside and outside probability
generalized for logic programs. The complexity analysis shows that when
combined with OLDT search for all explanations for observations, the graphical
EM algorithm, despite its generality, has the same time complexity as existing
EM algorithms, i.e. the Baum-Welch algorithm for HMMs, the Inside-Outside
algorithm for PCFGs, and the one for singly connected Bayesian networks that
have been developed independently in each research field. Learning experiments
with PCFGs using two corpora of moderate size indicate that the graphical EM
algorithm can significantly outperform the Inside-Outside algorithm
SiO Maser Survey of IRAS Sources in the Inner Galactic Disk
We have surveyed 401 color selected IRAS sources in the Galactic disk in the
SiO J=1--0 v= 1 and 2 maser lines at 43 GHz, resulting in 254 (239 new)
detections. The observed sources lie mostly in a strip of the inner Galactic
disk with boundaries -10<l<40 deg and |b|<3 deg. This survey provides radial
velocities of inner-disk stars for which optical measurements cannot be made
due to interstellar extinction. The SiO -- diagram in the area lv$ map does, indicating a slight difference of stellar
type between SiO and OH emitting stars. After identifying all of the SiO
detected sources in the 2MASS near-infrared catalog, we computed their
luminosity distances based on the infrared fluxes. We then mapped these objects
onto the first quadrant of the Galactic plane. Combining the distances with the
SiO radial-velocities, we obtained a pattern speed for SiO maser sources,
Omega_P=21 (+- 13) km s^{-1} kpc^{-1}, between the distances 1 and 5.5 kpc,
without the use of any dynamical models. The increase of the pattern speed
toward the Galactic center (up to 60 km s^{-1} kpc^{-1} between the distances,
5.5 and 7 kpc) suggests the presence of two pattern speeds in the Galaxy.Comment: 38 page 9 figures, high res. eps files are available as NRO report
No. 608 (http://www.nro.nao.ac.jp/library/report/list.html). PASJ 56 No. 4 in
pres
SiO Maser Survey of the Large-Amplitude Variables in the Galactic Center
We have surveyed ~400 known large-amplitude variables within 15' of the
galactic center in the SiO J=1--0 v=1 and 2 maser lines at 43 GHz, resulting in
179 detections. SiO lines were also detected from 16 other resulting in 180
detections. SiO lines were also detected from 16 other sources, which are
located within 20" (the telescope half beamwidth) of the program objects. The
detection rate of 48 percent is comparable to that obtained in Bulge IRAS
source surveys. Among the SiO detections, five stars have radial velocities
greater than 200 km/s. The SiO detection rate increases steeply with the period
of light variation, particularly for stars with P>500 d, where it exceeds 80%.
We found that, at a given period, the SiO detection rate is approximately three
times that for OH. These facts suggest that the large-amplitude variables in
the Nuclear Disk region are AGB stars similar in their overall properties to
the inner and outer Bulge IRAS/SiO sources. From the set of radial velocity
data, the mass distribution within 30 pc of the galactic center is derived by a
new method which is based on the collisionless Boltzmann equation integrated
along the line of sight. The mass within 30 pc is about 6.4 [\pm 0.7] \times
10^7 M_{\odot} and the mass of the central black hole is 2.7 [\pm 1.3] \times
10^6 M_{\odot}. Consideration of the line-of-sight velocity of each star and
its potential energy leads to the conclusion that the five high-velocity stars
come from galactocentric distances as high as 300 pc. The high-velocity
subsample of stars with negative radial velocities exhibits a tendency to have
brighter K magnitudes than the subsample of stars with positive velocities. The
origin of these high-velocity stars is discussed.Comment: Hires. figures are available as No.604 of NRO report at
http://www.nro.nao.ac.jp/library/report/list.html . PASJ 56 (april 28 issue)
in pres
Detections of SiO Masers from the Large-Amplitude Variables in the Galactic Nuclear Disk
We have surveyed known large-amplitude variables within 15' of the Galactic
center in the SiO J=1-0 v=1 and 2 maser lines at 43 GHz, resulting in 79
detections and 58 non-detections. The detection rate of 58 percent is
comparable to that obtained in Bulge IRAS source surveys. SiO lines were also
detected from four other sources near the program objects. The SiO detection
rate increases steeply with the period, particularly for stars with P>500 d,
where it exceeds 80%. We found at a given period that the SiO detection rate is
approximately double that for OH. These facts suggest that the large-amplitude
variables in the Nuclear Disk region are AGB stars similar in their overall
properties to the inner and outer bulge IRAS/SiO sources.Comment: 5 pages, 2 figures, 1 Table. PASJ 54, No 2 April 25 issue in pres
Trigonometric Parallaxes of Massive Star Forming Regions: II. Cep A & NGC 7538
We report trigonometric parallaxes for the sources NGC 7538 and Cep A,
corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04]
kpc, respectively. The distance to NGC 7538 is considerably smaller than its
kinematic distance and places it in the Perseus spiral arm. The distance to Cep
A is also smaller than its kinematic distance and places it in the Local arm or
spur. Combining the distance and proper motions with observed radial velocities
gives the location and full space motion of the star forming regions. We find
significant deviations from circular Galactic orbits for these sources: both
sources show large peculiar motions (> 10 km/s) counter to Galactic rotation
and NGC 7538 has a comparable peculiar motion toward the Galactic center.Comment: 21 pages, 8 figures; to appear in the Astrophysical Journa
The 2006 Radio Outburst of a Microquasar Cyg X-3: Observation and Data
We present the results of the multi-frequency observations of radio outburst
of the microquasar Cyg X-3 in February and March 2006 with the Nobeyama 45-m
telescope, the Nobeyama Millimeter Array, and the Yamaguchi 32-m telescope.
Since the prediction of a flare by RATAN-600, the source has been monitored
from Jan 27 (UT) with these radio telescopes. At the eighteenth day after the
quench of the activity, successive flares exceeding 1 Jy were observed
successfully. The time scale of the variability in the active phase is
presumably shorter in higher frequency bands.
We also present the result of a follow-up VLBI observation at 8.4 GHz with
the Japanese VLBI Network (JVN) 2.6 days after the first rise. The VLBI image
exhibits a single core with a size of <8 mas (80 AU). The observed image was
almost stable, although the core showed rapid variation in flux density. No jet
structure was seen at a sensitivity of K.Comment: 17 pages,6 figures; accepted by PAS
Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry -- Signatures of flow precession
NGC 7538 IRS1 is a high-mass (approx. 30 M_sun) protostar with a CO outflow,
an associated UCHII region, and a linear methanol maser structure, which might
trace a Keplerian-rotating circumstellar disk. The directions of the various
associated axes are misaligned with each other. We investigate the
near-infrared morphology of the source to clarify the relations among the
various axes. K'-band bispectrum speckle interferometry was performed at two
6-meter-class telescopes -- the BTA 6m telescope and the 6.5m MMT.
Complementary IRAC images from the Spitzer Space Telescope Archive were used to
relate the structures detected with the outflow at larger scales. High-dynamic
range images show fan-shaped outflow structure in which we detect 18 stars and
several blobs of diffuse emission. We interpret the misalignment of various
outflow axes in the context of a disk precession model, including numerical
hydrodynamic simulations of the molecular emission. The precession period is
approx. 280 years and its half-opening angle is 40 degrees. A possible
triggering mechanism is non-coplanar tidal interaction of an (undiscovered)
close companion with the circumbinary protostellar disk. Our observations
resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with
separation of 195 mas. We find indications for shock interaction between the
outflow activities in IRS1 and IRS2. Indications of outflow precession have
been discovered to date in a number of massive protostars, all with large
precession angles 20--45 degrees. This might explain the difference between the
outflow widths in low- and high-mass stars and add support to a common
collimation mechanism.Comment: 20 pages; 8 figures; Accepted by A&A on April 10, 2006; Image quality
reduced due to astro-ph file size limitations; Please download a version with
high-quality images from
http://www.mpifr-bonn.mpg.de/staff/tpreibis/ngc7538.pd
High Resolution CO Observations of Massive Star Forming Regions
Context. To further understand the processes involved in the formation of
massive stars, we have undertaken a study of the gas dynamics surrounding three
massive star forming regions. By observing the large scale structures at high
resolution, we are able to determine properties such as driving source, and
spatially resolve the bulk dynamical properties of the gas such as infall and
outflow. Aims. With high resolution observations, we are able to determine
which of the cores in a cluster forming massive stars is responsible for the
large scale structures. Methods. We present CO observations of three massive
star forming regions with known HII regions and show how the CO traces both
infall and outflow. By combining data taken in two SMA configurations with JCMT
observations, we are able to see large scale structures at high resolution.
Results. We find large (0.26-0.40 pc), massive (2-3 M_sun) and energetic (13-17
\times 10^44 erg) outflows emanating from the edges of two HII regions
suggesting they are being powered by the protostar(s) within. We find infall
signatures in two of our sources with mass infall rates of order 10-4 M_sun/yr.
Conclusions. We suggest that star formation is ongoing in these sources despite
the presence of HII regions. We further conclude that the source(s) within a
single HII region are responsible for the observed large scale structures; that
these large structures are not the net effect of multiple outflows from
multiple HII regions and hot cores.Comment: 8 pages,2 figures, accepted for publication in A&
- âŠ