42 research outputs found
Synchronization Based Approach for Estimating All Model Parameters of Chaotic Systems
The problem of dynamic estimation of all parameters of a model representing
chaotic and hyperchaotic systems using information from a scalar measured
output is solved. The variational calculus based method is robust in the
presence of noise, enables online estimation of the parameters and is also able
to rapidly track changes in operating parameters of the experimental system.
The method is demonstrated using the Lorenz, Rossler chaos and hyperchaos
models. Its possible application in decoding communications using chaos is
discussed.Comment: 13 pages, 4 figure
Dynamics of stick-slip in peeling of an adhesive tape
We investigate the dynamics of peeling of an adhesive tape subjected to a
constant pull speed. We derive the equations of motion for the angular speed of
the roller tape, the peel angle and the pull force used in earlier
investigations using a Lagrangian. Due to the constraint between the pull
force, peel angle and the peel force, it falls into the category of
differential-algebraic equations requiring an appropriate algorithm for its
numerical solution. Using such a scheme, we show that stick-slip jumps emerge
in a purely dynamical manner. Our detailed numerical study shows that these set
of equations exhibit rich dynamics hitherto not reported. In particular, our
analysis shows that inertia has considerable influence on the nature of the
dynamics. Following studies in the Portevin-Le Chatelier effect, we suggest a
phenomenological peel force function which includes the influence of the pull
speed. This reproduces the decreasing nature of the rupture force with the pull
speed observed in experiments. This rich dynamics is made transparent by using
a set of approximations valid in different regimes of the parameter space. The
approximate solutions capture major features of the exact numerical solutions
and also produce reasonably accurate values for the various quantities of
interest.Comment: 12 pages, 9 figures. Minor modifications as suggested by refere
Stellar Populations in Compact Galaxy Groups: a Multi-Wavelength Study of HCGs 16, 22, and 42, their Star Clusters and Dwarf Galaxies
We present a multi-wavelength analysis of three compact galaxy groups, HCGs
16, 22, and 42, which describe a sequence in terms of gas richness, from space-
(Swift, HST, Spitzer) and ground-based (LCO, CTIO) imaging and spectroscopy. We
study various signs of past interactions including a faint, dusty tidal feature
about HCG 16A, which we tentatively age-date at <1 Gyr. This represents the
possible detection of a tidal feature at the end of its phase of optical
observability. Our HST images also resolve what were thought to be double
nuclei in HCG 16C and D into multiple, distinct sources, likely to be star
clusters. Beyond our phenomenological treatment, we focus primarily on
contrasting the stellar populations across these three groups. The star
clusters show a remarkable intermediate-age population in HCG 22, and identify
the time at which star formation was quenched in HCG 42. We also search for
dwarf galaxies at accordant redshifts. The inclusion of 33 members and 27
'associates' (possible members) radically changes group dynamical masses, which
in turn may affect previous evolutionary classifications. The extended
membership paints a picture of relative isolation in HCGs 16 and 22, but shows
HCG 42 to be part of a larger structure, following a dichotomy expected from
recent studies. We conclude that (a) star cluster populations provide an
excellent metric of evolutionary state, as they can age-date the past epochs of
star formation; and (b) the extended dwarf galaxy population must be considered
in assessing the dynamical state of a compact group.Comment: 29 pages, 13 figures. Accepted for publication in The Astrophysical
Journa
Constraining the optical emission from the double pulsar system J0737-3039
We present the first optical observations of the unique system J0737-3039
(composed of two pulsars, hereafter PSR-A and PSR-B). Ultra-deep optical
observations, performed with the High Resolution Camera of the Advanced Camera
for Surveys on board the Hubble Space Telescope could not detect any optical
emission from the system down to m_F435W=27.0 and m_F606W=28.3. The estimated
optical flux limits are used to constrain the three-component (two thermal and
one non-thermal) model recently proposed to reproduce the XMM-Newton X-ray
spectrum. They suggest the presence of a break at low energies in the
non-thermal power law component of PSR-A and are compatible with the expected
black-body emission from the PSR-B surface. The corresponding efficiency of the
optical emission from PSR-A's magnetosphere would be comparable to that of
other Myr-old pulsars, thus suggesting that this parameter may not dramatically
evolve over a time-scale of a few Myr.Comment: 16 pages, 3 figures, ApJ accepte
Star Clusters in the Tidal Tails of Interacting Galaxies: Cluster Populations Across a Variety of Tail Environments
We have searched for compact stellar structures within 17 tidal tails in 13
different interacting galaxies using F606W- and F814W- band images from the
Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The
sample of tidal tails includes a diverse population of optical properties,
merging galaxy mass ratios, HI content, and ages. Combining our tail sample
with Knierman et al. (2003), we find evidence of star clusters formed in situ
with Mv < -8.5 and V-I < 2.0 in 10 of 23 tidal tails; we are able to identify
cluster candidates to Mv = -6.5 in the closest tails. Three tails offer clear
examples of "beads on a string" star formation morphology in V-I color maps.
Two tails present both tidal dwarf galaxy (TDG) candidates and cluster
candidates. Statistical diagnostics indicate that clusters in tidal tails may
be drawn from the same power-law luminosity functions (with logarithmic slopes
~ -2 - -2.5) found in quiescent spiral galaxies and the interiors of
interacting systems. We find that the tail regions with the largest number of
observable clusters are relatively young (< 250 Myr old) and bright (V < 24 mag
arcsec^(-2)), probably attributed to the strong bursts of star formation in
interacting systems soon after periapse. Otherwise, we find no statistical
difference between cluster-rich and cluster-poor tails in terms of many
observable characteristics, though this analysis suffers from complex,
unresolved gas dynamics and projection effects.Comment: Accepted for publication in the Astrophysical Journal. 27 pages, 8
figure
An HI Threshold for Star Cluster Formation in Tidal Debris
Super star clusters are young, compact star clusters found in the central
regions of interacting galaxies. Recently, they have also been reported to
preferentially form in certain tidal tails, but not in others. In this paper,
we have used 21 cm HI maps and the Hubble Space Telescope Wide Field Planetary
Camera 2 images of eight tidal tail regions of four merging galaxy pairs to
compare the kiloparsec scale HI distribution with the location of super star
clusters found from the optical images. For most of the tails, we find that
there is an increase in super star cluster density with increasing projected HI
column density, such that the star cluster density is highest when log N(HI) >=
20.6 cm^{-2}, but equal to the background count rate at lower HI column
density. However, for two tails (NGC 4038/39 Pos A and NGC 3921), there is no
significant star cluster population despite the presence of gas at high column
density. This implies that the N(HI) threshold is a necessary but not
sufficient condition for cluster formation. Gas volume density is likely to
provide a more direct criterion for cluster formation, and other factors such
as gas pressure or strength of encounter may also have an influence. Comparison
of HI thresholds needed for formation of different types of stellar structures
await higher resolution HI and optical observations of larger numbers of
interacting galaxies.Comment: 19 pages, 6 figures, 3 tables, accepted for publication in MNRA
Episodic Star Formation Coupled to Reignition of Radio Activity in 3C 236
We present Hubble Space Telescope UV and optical imaging of the radio galaxy
3C 236, whose relic 4 Mpc radio jet lobes and inner 2 kpc CSS radio source are
evidence of multiple epochs of AGN activity. Our data confirm the presence of
four bright knots of FUV emission in an arc along the edge of the inner
circumnuclear dust disk in the galaxy's nucleus, as well as FUV emission
cospatial with the nucleus itself. We interpret these to be sites of recent or
ongoing star formation. We present photometry of these knots, as well as an
estimate for the internal extinction in the source using the Balmer decrement
from SDSS spectroscopy. We estimate the ages of the knots by comparing our
extinction-corrected photometry with stellar population synthesis models. We
find the four knots cospatial with the dusty disk to be young, of order 10^7 yr
old. The FUV emission in the nucleus is likely due to an episode of star
formation triggered ~10^9 yr ago. We argue that the young 10^7 yr old knots
stem from an episode of star formation that was roughly coeval with the event
resulting in reignition of radio activity, creating the CSS source. The 10^9 yr
old stars in the nucleus may be associated with the previous epoch of activity
that generated the 4 Mpc relic source, before it was cut off by exhaustion or
interruption. The ages of the knots, considered in context with the disturbed
morphology of the nuclear dust and the double-double morphology of the "old"
and "young" radio sources, present evidence for an episodic AGN/starburst
connection. We suggest that the AGN fuel supply was interrupted for ~10^7 yr
due to a minor merger event and has now been restored, and the resultant
non-steady flow of gas toward the nucleus is likely responsible for both the
new episode of infall-induced star formation and also the multiple epochs of
radio activity.Comment: 15 pages, 7 figures. Accepted for publication in ApJ