1,316 research outputs found
Proper Motions of H2O Masers in the Water Fountain Source IRAS 19190+1102
We report on the results of two epochs of Very Long Baseline Array (VLBA)
observations of the 22 GHz water masers toward IRAS 19190+1102. The water maser
emission from this object shows two main arc-shaped formations perpendicular to
their NE-SW separation axis. The arcs are separated by ~280 mas in position,
and are expanding outwards at an angular rate of 2.35 mas/yr. We detect maser
emission at velocities between -53.3 km/s to +78.0 km/s and there is a distinct
velocity pattern where the NE masers are blueshifted and the SW masers are
redshifted. The outflow has a three-dimensional outflow velocity of 99.8 km/s
and a dynamical age of about 59 yr. A group of blueshifted masers not located
along the arcs shows a change in velocity of more than 35 km/s between epochs,
and may be indicative of the formation of a new lobe. These observations show
that IRAS 19190+1102 is a member of the class of "water fountain"'
pre-planetary nebulae displaying bipolar structureComment: Accepted for publication in ApJ, corrected typo
A Study of H2 Emission in Three Bipolar Proto-Planetary Nebulae: IRAS 16594-4656, Hen 3-401, and Rob 22
We have carried out a spatial-kinematical study of three proto-planetary
nebulae, IRAS 16594-4656, Hen 3-401, and Rob 22. High-resolution H2 images were
obtained with NICMOS on the HST and high-resolution spectra were obtained with
the Phoenix spectrograph on Gemini-South. IRAS 16594-4656 shows a
"peanut-shaped" bipolar structure with H2 emission from the walls and from two
pairs of more distant, point-symmetric faint blobs. The velocity structure
shows the polar axis to be in the plane of the sky, contrary to the impression
given by the more complex visual image and the visibility of the central star,
with an ellipsoidal velocity structure. Hen 3-401 shows the H2 emission coming
from the walls of the very elongated, open-ended lobes seen in visible light,
along with a possible small disk around the star. The bipolar lobes appear to
be tilted 10-15 deg with respect to the plane of the sky and their kinematics
display a Hubble-like flow. In Rob 22, the H2 appears in the form of an "S"
shape, approximately tracing out the similar pattern seen in the visible. H2 is
especially seen at the ends of the lobes and at two opposite regions close to
the unseen central star. The axis of the lobes is nearly in the plane of the
sky. Expansion ages of the lobes are calculated to be approximately 1600 yr
(IRAS 16594-4656), 1100 yr (Hen 3-401), and 640 yr (Rob 22), based upon
approximate distances
High-Resolution Near-Infrared Imaging and Polarimetry of Four Proto-Planetary Nebulae
High-resolution near-infrared HST NICMOS (F160W, F222M) images and
polarization (2 um) observations were made of four bipolar proto-planetary
nebulae (PPNs): IRAS 17150-3224, IRAS 17441-2411, IRAS 17245-3951, and IRAS
16594-4656. The first three of these are viewed nearly edge-on, and for the
first time the central stars in them are seen. Color maps reveal a reddened
torus between the bipolar lobes in the edge-on cases, with bluer lobes. The
polarization values are high, with maximum values ranging from 40 to 80%. The
polarization patterns are basically centrosymmetric, with some deviations in
the low polarization equatorial regions. For IRAS 17150-3224, circumstellar
arcs are seen at 1.6 um, along with a newly-discovered loop in the equatorial
region. Bright caps are seen at the end of the lobes, indicating that they are
not open-ended. A distinct point-symmetric pattern is seen in the strengths of
the polarization vectors, especially in IRAS 17150-3224. HST NICMOS
observations provide a valuable complement to the WFPC2 visible images in
deriving the basic structure of bipolar PPNs.Comment: 20 pages, 12 figures, accepted for publication in Astronomical
Journal higher resolution figures are available on
http://katherine.as.arizona.edu/~ksu/cv/su_nicmos7840.ps.g
Equatorial ozone characteristics as measured at Natal (5.9 deg S, 35.2 deg W)
Ozone density profiles obtained through electrochemical concentration cell (ECC) sonde measurements at Natal were analyzed. Time variations, as expected, are small. Outstanding features of the data are tropospheric densities substantially higher than those measured at other stations, and also a total ozone content that is higher than the averages given by satellite measurements
Melt-particle mixing in gas-stirred ladles with throughflow
An experimental study is performed on a gas-particle stirred ladle system with throughflow, using a simplified water model. Narrow ladles are used to produce 2-D flows. Flow visualization by the direct photographic method is employed to investigate the effects of ladle geometry, throughflow rate, air flow rate and its injection location on the melt-particle mixing performance. Image processing is applied to aid in determining the mixing performance. It is disclosed that an efficient mixing may be achieved if the gas at a higher flow rate is injected with particles through a nozzle near the bottom corner of the ladle wall on the melt inlet side. The mixing performance is better in a rectangular ladle (aspect ratio of 2) than in a square ladle (aspect ratio of unity). The effect of throughflow rate on mixing is minor. The study has an important application in manufacturing processes, such as continuous casting process, and materials processing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47063/1/348_2004_Article_BF00208073.pd
A collimated, ionized bipolar structure and a high density torus in the young planetary nebula IRAS 17347-3139
We present observations of continuum (lambda = 0.7, 1.3, 3.6 and 18 cm) and
OH maser (lambda = 18 cm) emission toward the young planetary nebula IRAS
17347-3139, which is one of the three planetary nebulae that are known to
harbor water maser emission. From the continuum observations we show that the
ionized shell of IRAS 17347-3139 consists of two main structures: one extended
(size ~1". 5) with bipolar morphology along PA=-30 degrees, elongated in the
same direction as the lobes observed in the near-infrared images, and a central
compact structure (size ~0". 25) elongated in the direction perpendicular to
the bipolar axis, coinciding with the equatorial dark lane observed in the
near-infrared images. Our image at 1.3 cm suggests the presence of dense walls
in the ionized bipolar lobes. We estimate for the central compact structure a
value of the electron density at least ~5 times higher than in the lobes. A
high resolution image of this structure at 0.7 cm shows two peaks separated by
about 0". 13 (corresponding to 100-780 AU, using a distance range of 0.8-6
kpc). This emission is interpreted as originating in an ionized equatorial
torus-like structure, from whose edges the water maser emission might be
arising. We have detected weak OH 1612 MHz maser emission at VLSR ~ -70 km/s
associated with IRAS 17347-3139. We derive a 3 sigma upper limit of < 35% for
the percentage of circularly polarized emission. Within our primary beam, we
detected additional OH 1612 MHz maser emission in the LSR velocity ranges -5 to
-24 and -90 to -123 km/s, associated with the sources 2MASS J17380406-3138387
and OH 356.65-0.15, respectively.Comment: 26 pages, 8 figures. Accepted for publication in Ap
Cognitive Radio Networks: Realistic or Not?
A large volume of research has been conducted in the cognitive radio (CR)
area the last decade. However, the deployment of a commercial CR network is yet
to emerge. A large portion of the existing literature does not build on real
world scenarios, hence, neglecting various important interactions of the
research with commercial telecommunication networks. For instance, a lot of
attention has been paid to spectrum sensing as the front line functionality
that needs to be completed in an efficient and accurate manner to enable an
opportunistic CR network architecture. This is necessary to detect the
existence of spectrum holes without which no other procedure can be fulfilled.
However, simply sensing (cooperatively or not) the energy received from a
primary transmitter cannot enable correct dynamic spectrum access. For example,
the low strength of a primary transmitter's signal does not assure that there
will be no interference to a nearby primary receiver. In addition, the presence
of a primary transmitter's signal does not mean that CR network users cannot
access the spectrum since there might not be any primary receiver in the
vicinity. Despite the existing elegant and clever solutions to the DSA problem
no robust, implementable scheme has emerged. In this paper, we challenge the
basic premises of the proposed schemes. We further argue that addressing the
technical challenges we face in deploying robust CR networks can only be
achieved if we radically change the way we design their basic functionalities.
In support of our argument, we present a set of real-world scenarios, inspired
by realistic settings in commercial telecommunications networks, focusing on
spectrum sensing as a basic and critical functionality in the deployment of
CRs. We use these scenarios to show why existing DSA paradigms are not amenable
to realistic deployment in complex wireless environments.Comment: Work in progres
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