253 research outputs found
Bounding inconsistency using a novel threshold metric for dead reckoning update packet generation
Human-to-human interaction across distributed applications requires that sufficient consistency be maintained among participants in the face of network characteristics such as latency and limited bandwidth. The level of inconsistency arising from the network is proportional to the network delay, and thus a function of bandwidth consumption. Distributed simulation has often used a bandwidth reduction technique known as dead reckoning that combines approximation and estimation in the communication of entity movement to reduce network traffic, and thus improve consistency. However, unless carefully tuned to application and network characteristics, such an approach can introduce more inconsistency than it avoids. The key tuning metric is the distance threshold. This paper questions the suitability of the standard distance threshold as a metric for use in the dead reckoning scheme. Using a model relating entity path curvature and inconsistency, a major performance related limitation of the distance threshold technique is highlighted. We then propose an alternative time—space threshold criterion. The time—space threshold is demonstrated, through simulation, to perform better for low curvature movement. However, it too has a limitation. Based on this, we further propose a novel hybrid scheme. Through simulation and live trials, this scheme is shown to perform well across a range of curvature values, and places bounds on both the spatial and absolute inconsistency arising from dead reckoning
Exploring the use of local consistency measures as thresholds for dead reckoning update packet generation
Human-to-human interaction across distributed applications requires that sufficient consistency be maintained among participants in the face of network characteristics such as latency and limited bandwidth. Techniques and approaches for reducing bandwidth usage can minimize network delays by reducing the network traffic and therefore better exploiting available bandwidth. However, these approaches induce inconsistencies within the level of human perception. Dead reckoning is a well-known technique for reducing the number of update packets transmitted between participating nodes. It employs a distance threshold for deciding when to generate update packets. This paper questions the use of such a distance threshold in the context of absolute consistency and it highlights a major drawback with such a technique. An alternative threshold criterion based on time and distance is examined and it is compared to the distance only threshold. A drawback with this proposed technique is also identified and a hybrid threshold criterion is then proposed. However, the trade-off between spatial and temporal inconsistency remains
The Ionizing Stars of the Galactic Ultra-Compact HII Region G45.45+0.06
Using the NIFS near-infrared integral-field spectrograph behind the facility
adaptive optics module, ALTAIR, on Gemini North, we have identified several
massive O-type stars that are responsible for the ionization of the Galactic
Ultra-Compact HII region G45.45+0.06. The sources ``m'' and ``n'' from the
imaging study of Feldt et a. 1998 are classified as hot, massive O-type stars
based on their K-band spectra. Other bright point sources show red and/or
nebular spectra and one appears to have cool star features that we suggest are
due to a young, low-mass pre-main sequence component. Still two other embedded
sources (``k'' and ``o'' from Feldt et al.) exhibit CO bandhead emission that
may arise in circumstellar disks which are possibly still accreting. Finally,
nebular lines previously identified only in higher excitation planetary nebulae
and associated with KrIII and SeIV ions are detected in G45.45+0.06.Comment: Latex, 28 pages, 10 figure
A molecular line study of NGC 1333/IRAS 4
Molecular line surveys and fully sampled spectral line maps at 1.3 and 0.87 mm are used to examine the
physical and chemical characteristics of the extreme Class I sources IRAS 4A and 4B in the L1450/NGC 1333
molecular cloud complex. A very well collimated, jetlike molecular outflow emanates from IRAS 4A, with a
dynamical age of a few thousand years. Symmetric, clumpy structure along the outflow lobes suggests that
there is considerable variability in the mass-loss rate or wind velocity even at this young age. Molecular emission
lines toward IRAS 4A and 4B are observed to be weak in the velocity range corresponding to quiescent
material surrounding the young stellar objects (YSOs). Depletion factors of 10-20 are observed for αll molecules,
including CO, even for very conservative mass estimates from the measured millimeter and submillimeter
dust continuum. However, abundances scaled with respect to CO are similar to other dark
molecular cloud cores. Such depletions could be mimicked by high dust optical depths or increased grain
emissivities at the observing frequencies of 230 and 345 GHz, but the millimeter and submillimeter spectral
energy distributions suggest that this is unlikely over the single-dish size scales of 5000-10,000 AU.
Dense, outflowing gas is found to be kinematically, but not spatially, distinct from the quiescent material on
these size scales. If CO is used as a chemical standard for the high-velocity gas, we find substantial enhancements
in the abundances of several molecules in outflowing material, most notably CS, SiO, and CH_30H. The
SiO emission is kinematically well displaced from the bulk cloud velocity and likely arises from directly
shocked material. As is the case for CO, however, the outflow features from more volatile species are centered
near the cloud velocity and are often characterized by quite low rotational temperatures. We suggest that
grain-grain collisions induced by velocity shear zones surrounding the outflow axes transiently desorb the
grain mantles, resulting in large abundance enhancements of selected species. Similar results have recently
been obtained in several other low-mass YSOs, where the outflowing gas is often both kinematically and spatially
distinct, and are illustrative of the ability of accretion and outflow processes to simultaneously modify
the composition of the gas and dust surrounding young stars
On the nature of the EXor accretion events: an unfrequent manifestation of a common phenomenology ?
We present the results of a comparison between classical and newly identified
EXor based on literature data and aimed at recognizing possible differences or
similarities of both categories. Optical and near-IR two-color diagrams,
modalities of fluctuations, and derived values of the mass accretion rates are
indicative of strong similarities between the two samples. We demonstrate how
the difference between the outburst and the quiescence spectral energy
distribution of all the EXor can be well fitted with a single blackbody, as if
an additional thermal component appears during the outbursting phase.
Temperatures of this additional component span between 1000 and 4500 K, while
the radii of the emitting regions (assumed to be a uniform disk) span between
0.01 and 0.1 AU, sizes typical of the inner portions of the circumstellar disk.
Spots persisting up to 50% of the outburst duration, not exceeding the 10% of
the stellar surface, and with temperatures compatible with the EXor mass
accretion rates, are able to account for both the appearance of the additional
thermal component and the dust sublimation in the inner structures of the disk.
We also compare the EXor events with the most significant color and magnitude
fluctuations of active T Tauri stars finding that (i} burst accretion phenomena
should also be important for this latter class; (ii} EXor events could be more
frequent then those accidentally discovered. Remarkable is the case of the
source V2493 Cyg, a T Tauri star recently identified as a strong outbursting
object: new optical and near-IR photometric and spectroscopic data are
presented trying to clarify its EXor or FUor nature.Comment: Accepted for publication in Ap
A Catalog of Young Stellar Groups and Clusters Within 1kpc of the Sun
We present a catalog of near-infrared surveys of young (<= a few 10^6 yr)
stellar groups and clusters within 1 kpc from the Sun, based on an extensive
search of the literature from the past ten years. We find 143 surveys from 69
published articles, covering 73 different regions. The number distribution of
stars in a region has a median of 28 and a mean of 100. About 80% of the stars
are in clusters with at least 100 members. By a rough classification of the
groups and clusters based on the number of their associated stars, we show that
most of the stars form in large clusters. The spatial distribution of cataloged
regions in the Galactic plane shows a relative lack of observed stellar groups
and clusters in the range 270 deg < l < 60 deg of Galactic longitude,
reflecting our location between the Local and Sagittarius arms. This
compilation is intended as a useful resource for future studies of nearby young
regions of multiple star formation.Comment: 16 pages, 2 tables, 2 figures. To appear in Astronomical Journa
Circumstellar Disks in the IC 348 Cluster
We report the results of the first sensitive L-band (3.4 micron) imaging
survey of the young IC 348 cluster in Perseus. In conjunction with previously
acquired JHK (1.25, 1.65, 2.2 micron) observations, we use L-band data to
obtain a census of the circumstellar disk population to m_K=m_L<=12.0 in the
central 110 square arcmin region of the cluster. An analysis of the JHKL colors
of 107 sources indicates that 65% +/- 8% of the cluster membership possesses
(inner) disks. This fraction is lower than those (86% +/- 8% and 80% +/- 7%)
obtained from similar JHKL surveys of the younger NGC 2024 and Trapezium
clusters, suggesting that the disk fraction in clusters decreases with cluster
age. Sources with circumstellar disks in IC 348 have a median age of 0.9 Myr,
while the diskless sources have a median age of 1.4 Myr, for a cluster distance
of 320 pc. Although the difference in the median ages between the two
populations is only marginally significant, our results suggest that over a
timescale of 2 - 3 Myr, more than a third of the disks in the IC 348 cluster
disappear. Moreover, we find that at a very high confidence level, the disk
fraction is a function of spectral type. All stars earlier than G appear
diskless, while stars with spectral types G and later have a disk fraction
ranging between 50% - 67%, with the latest type stars having the higher disk
fraction. This suggests that the disks around stars with spectral types G and
earlier have evolved more rapidly than those with later spectral types. The
L-band disk fraction for sources with similar ages in both IC 348 and Taurus is
the same, within the errors, suggesting that, at least in clusters with no O
stars, the disk lifetime is independent of environment.Comment: 25 pages, 4 figures. Paper to appear in April A
First occurrence of Xanthomonas campestris pv. raphanion wallflower (Erysimum cheiri) in the United Kingdom
In a study of diseases caused by Xanthomonas campestris on UK nursery-grown wallflowers (Erysimum cheiri), one isolate (P764) obtained in 1990 from Sussex differed from the others. The affected plants (cv. Bedder Mixed), showed distorted growth with chlorotic and necrotic spots, but no sectored V-shaped wilting, the most common symptom associated with xanthomonads in wallflowers. Other X. campestris isolates (including P763), were obtained from wallflowers with typical wilting. Infected plant material was comminuted in sterile 0.1% peptone solution and loopfuls streaked onto yeast dextrose chalk agar and nutrient dextrose agar (Lelliott & Stead, 1987). Plates were incubated for up to 72 hours at 28°C. Xanthomonas-like colonies were purified by re-streaking and isolates were maintained at -80°C (Protect System, UK)
V2494 cyg: A unique FU ori type object in the cygnus OB7 complex
A photometric and spectral study of the variable star V2494 Cyg in the L 1003 dark cloud is presented. The brightness of the star, formerly known as HH 381 IRS, increased by 2.5 mag in R (probably in the 1980s) and since then has remained nearly constant. Since the brightness increase, V2494 Cyg has illuminated a bipolar cometary nebula. The stellar spectrum has several features typical of the FU Ori (FUor) type, plus it exhibits very strong Ha and forbidden emissionlines with high-velocity components. These emission lines originate in the Herbig-Haro (HH) jet near the star. The kinematic age of the jet is consistent with it forming at the time of the outburst leading to the luminosity increase. V2494 Cyg also produces a rather extended outflow; it is the first known FUor with both an observed outburst and a parsec-sized HH flow. The nebula, illuminated by V2494 Cyg, possesses similar morphological and spectral characteristics to Hubble's variable nebula (R Monocerotis/NGC 2261). © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Laser Guide Star Adaptive Optics Integral Field Spectroscopy of a Tightly Collimated Bipolar Jet from the Herbig Ae star LkHa 233
We have used the integral field spectrograph OSIRIS and laser guide star
adaptive optics at Keck Observatory to obtain high angular resolution (0.06"),
moderate spectral resolution (R ~ 3800) images of the bipolar jet from the
Herbig Ae star LkHa 233, seen in near-IR [Fe II] emission at 1.600 & 1.644
microns. This jet is narrow and tightly collimated, with an opening angle of
only 9 degrees, and has an average radial velocity of ~ 100 km/s. The jet and
counterjet are asymmetric, with the red-shifted jet much clumpier than its
counterpart at the angular resolution of our observations. The observed
properties are in general similar to jets seen around T Tauri stars, though it
has a relatively large mass flux of (1.2e-7 +- 0.3e-7) M_sun/year, near the
high end of the observed mass flux range around T Tauri stars. We also
spatially resolve an inclined circumstellar disk around LkHa 233, which
obscures the star from direct view. By comparison with numerical radiative
transfer disk models, we estimate the disk midplane to be inclined i = 65 +- 5
degrees relative to the plane of the sky. Since the star is seen only in
scattered light at near-infrared wavelengths, we detect only a small fraction
of its intrinsic flux. Because previous estimates of its stellar properties did
not account for this, either LkHa 233 must be located closer than the
previously believed, or its true luminosity must be greater than previously
supposed, consistent with its being a ~4 M_sun star near the stellar birthline.Comment: Accepted for publication in the Ap
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