453 research outputs found
On the Applicability of Low-Dimensional Models for Convective Flow Reversals at Extreme Prandtl Numbers
Constructing simpler models, either stochastic or deterministic, for
exploring the phenomenon of flow reversals in fluid systems is in vogue across
disciplines. Using direct numerical simulations and nonlinear time series
analysis, we illustrate that the basic nature of flow reversals in convecting
fluids can depend on the dimensionless parameters describing the system.
Specifically, we find evidence of low-dimensional determinism in flow reversals
occurring at zero Prandtl number, whereas we fail to find such signatures for
reversals at infinite Prandtl number. Thus, even in a single system, as one
varies the system parameters, one can encounter reversals that are
fundamentally different in nature. Consequently, we conclude that a single
general low-dimensional deterministic model cannot faithfully characterize flow
reversals for every set of parameter values.Comment: 9 pages, 4 figure
Optical Observations and Multiband Modelling of the Afterglow of GRB 041006: Evidence of A Hard Electron Energy Spectrum
We present the CCD Cousins R band photometric observations of the afterglow
of GRB 041006. The multiband afterglow evolution is modelled using an
underlying `hard' electron energy spectrum with a . The burst
appears to be of very low energy ( ergs) confined to a narrow
cone of opening angle . The associated supernova is
compared with SN1998bw and is found to be brighter.Comment: Accepted for publication in Bull. Astr. Soc. India (BASI
GRB000301C with peculiar afterglow emission
The CCD magnitudes in Johnson V and Cousins R and I photometric passbands are
determined for GRB 000301C afterglow starting ~ 1.5 day after the gamma-ray
burst. In fact we provide the earliest optical observations for this burst.
Light curves of the afterglow emissions in U, B, V, R, I, J and K' passbands
are obtained by combining the present measurements with the published data.
Flux decay shows a very uncommon variation relative to other well observed
GRBs. Overall, there is a steepening of the optical and near-infrared flux
decay caused by a geometric and sideways expanding jet. This is superimposed by
a short term variability especially during early time (Delta t < 8 days). The
cause of variability is not well understood, though it has occurred
simultaneously with similar amplitude in all the filters. We derive the early
and late time flux decay constants using jet model. The late time flux decay is
the steepest amongst the GRB OTs observed so far with alpha ~ 3. Steepening in
the flux decay seems to have started simultaneously around Delta t ~ 7.6 day in
all passbands. The value of spectral index in the optical-near IR region is ~
-1.0. Redshift determination with z=2.0335 indicates cosmological origin of the
GRB having a luminosity distance of 16.6 Gpc. Thus it becomes the second
farthest amongst the GRBs with known distances. An indirect estimate of the
fluence > 20 keV indicates, if isotropic,> =10^53 ergs of release of energy.
The enormous amount of released energy will be reduced, if the radiation is
beamed which is the case for this event. Using a jet break time of 7.6 days, we
infer a jet opening angle of ~ 0.15 radian. This means the energy released is
reduced by a factor of ~ 90 relative to the isotropic value.Comment: LaTeX file, 11 pages including 4 figures, uses psfig.sty, Bull.
Astron. Society of India(accepted, Sept, 2000 issue
X-ray emission from O-type stars : DH Cep and HD 97434
We present X-ray emission characteristics of the massive O-type stars DH Cep
and HD 97434 using archival XMM-Newton observations. There is no convincing
evidence for short term variability in the X-ray intensity during the
observations. However, the analysis of their spectra reveals X-ray structure
being consistent with two-temperature plasma model. The hydrogen column
densities derived from X-ray spectra of DH Cep and HD 97434 are in agreement
with the reddening measurements for their corresponding host clusters NGC 7380
and Trumpler 18, indicating that the absorption by stellar wind is negligible.
The X-ray emission from these hot stars is interpreted in terms of the standard
instability-driven wind shock model.Comment: 13 pages ; 2 figures; 2 tables (Accepted for publication in New
Astronomy
Star formation in young star cluster NGC 1893
We present a comprehensive multi-wavelength study of the star-forming region
NGC 1893 to explore the effects of massive stars on low-mass star formation.
Using near-infrared colours, slitless spectroscopy and narrow-band
photometry in the cluster region we have identified candidate young stellar
objects (YSOs) distributed in a pattern from the cluster to one of the nearby
nebulae Sim 129. The colour-magnitude diagram of the YSOs indicates
that majority of these objects have ages between 1 to 5 Myr. The spread in the
ages of the YSOs may indicate a non-coeval star formation in the cluster. The
slope of the KLF for the cluster is estimated to be , which agrees
well with the average value () reported for young clusters. For the
entire observed mass range the value of the slope of
the initial mass function, ', comes out to be , which is
in agreement with the Salpeter value of -1.35 in the solar neighborhood.
However, the value of ' for PMS phase stars (mass range ) is found to be which is shallower than the
value () obtained for MS stars having mass range indicating a break in the slope of the mass function at . Estimated ' values indicate an effect of mass segregation
for main-sequence stars, in the sense that massive stars are preferentially
located towards the cluster center. The estimated dynamical evolution time is
found to be greater than the age of the cluster, therefore the observed mass
segregation in the cluster may be the imprint of the star formation process.
There is evidence for triggered star formation in the region, which seems to
govern initial morphology of the cluster.Comment: Accepted for the publication in MNRAS, 21 pages, 26 figures, 10
table
BVRI CCD photometric standards in the field of GRB 990123
The CCD magnitudes in Johnson and Cousins photometric passbands are
determined for 18 stars in the field of GRB 990123. These measurements can be
used in carrying out precise CCD photometry of the optical transient of GRB
990123 using differential photometric techniques during non--photometric sky
conditions. A comparison with previous photometry indicates that the present
photmetry is more precise.Comment: Tex file, 5 pages with 1 figure. Bull. Astron. Society India, Vol. 27
(accepted
Stellar contents and Star formation in the young cluster Stock 18
We have carried out deep (V21 mag) \ubvri photometric study of the star
cluster Stock 18. These along with archival Infrared data have been used to
derive the basic cluster parameters and also to study the star formation
processes in and around the cluster region. The distance to the cluster is
derived as 2.80.2 kpc while its age is estimated as Myr.
Present study indicates that interstellar reddening is normal in the direction
of the cluster. The mass function slope is found to be -1.370.27 for the
mass range 111.9. There is no evidence found for the effect of
mass segregation in main-sequence stars of the cluster. A young stellar
population with age between 1-2 Myr have been found in and around the cluster
region. The presence of IRAS and AKARI sources with MSX intensity map also show
the youth of the Sh2-170 region.Comment: 25 pages, 11 figures, 3 tables (Accepted for publication in New
Astronomy
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