4,253 research outputs found
Theoretical regime diagrams for thermally driven flows in a beta-plane channel in the presence of variable gravity
The effect of a power law gravity field on baroclinic instability is examined, with a focus on the case of inverse fifth power gravity, since this is the power law produced when terrestrial gravity is simulated in spherical geometry by a dielectric force. Growth rates are obtained of unstable normal modes as a function of parameters of the problem by solving a second order differential equation numerically. It is concluded that over the range of parameter space explored, there is no significant change in the character of theoretical regime diagrams if the vertically averaged gravity is used as parameter
Numerical study of baroclinic instability
The effect of a power law gravity field on baroclinic instability is examined with emphasis on the case of inverse fifth power gravity, since this is the power law produced when terrestrial gravity is simulated in spherical geometry by electrostatic means. Growth rates of unstable normal modes were obtained as a function of parameters of the problem by solving a second order differential equation numerically. Results are compared with those from an earlier study where gravity was a constant. The conclusion is that, over the range of parameter space explored here, there is no significant change in the character of theoretical regime diagrams if the vertically averaged gravity is used as a parameter
A sample of relatively unstudied star clusters in the Large Magellanic Cloud: fundamental parameters determined from Washington photometry
To enlarge our growing sample of well-studied star clusters in the Large
Magellanic Cloud (LMC), we present CCD Washington CT1 photometry to T1 ~ 23 in
the fields of twenty-three mostly unstudied clusters located in the inner disc
and outer regions of the LMC. We estimated cluster radii from star counts.
Using the cluster Washington (T1,C-T1) colour-magnitude diagrams, statistically
cleaned from field star contamination, we derived cluster ages and
metallicities from a comparison with theoretical isochrones of the Padova
group. Whenever possible, we also derived ages using delta_T1 - the magnitude
difference between the red giant clump and the main sequence turn off - and
estimated metallicities from the standard giant branch procedure. We enlarged
our sample by adding clusters with published ages and metallicities determined
on a similar scale by applying the same methods. We examined relationships
between their positions in the LMC, ages and metallicities. We find that the
two methods for age and metallicity determination agree well with each other.
Fourteen clusters are found to be intermediate-age clusters (1-2 Gyr), with
[Fe/H] values ranging from -0.4 to -0.7. The remaining nine clusters turn out
to be younger than 1 Gyr, with metallicities between 0.0 and -0.4. Our 23
clusters represent an increase of ~ 30% in the current total amount number of
well-studied LMC clusters using Washington photometry. In agreement with
previous studies, we find no evidence for a metallicity gradient. We also find
that the younger clusters were formed closer to the LMC centre than the older
ones.Comment: 11 pages, 10 figures. A&A, in pres
Age Determination of Fifteen Old to Intermediate-Age Small Magellanic Cloud Star Clusters
We present CMDs in the V and I bands for fifteen star clusters in the Small
Magellanic Cloud (SMC) based on data taken with the Very Large Telescope (VLT,
Chile). We selected these clusters from our previous work, wherein we derived
cluster radial velocities and metallicities from Calcium II infrared triplet
(CaT) spectra also taken with the VLT. We discovered that the ages of six of
our clusters have been appreciably underestimated by previous studies, which
used comparatively small telescopes, graphically illustrating the need for
large apertures to obtain reliable ages of old and intermediate-age SMC star
clusters. In particular, three of these clusters, L4, L6 and L110, turn out to
be amongst the oldest SMC clusters known, with ages of 7.9 +- 1.1, 8.7 +- 1.2
and 7.6 +- 1.0 Gyr, respectively, helping to fill a possible "SMC cluster age
gap" (Glatt et al. 2008). Using the present ages and metallicities from Parisi
et al. (2009), we analyze the age distribution, age gradient and age
metallicity relation (AMR) of a sample of SMC clusters measured homogeneously.
There is a suggestion of bimodality in the age distribution but it does not
show a constant slope for the first 4 Gyr (Piatti 2011), and we find no
evidence for an age gradient. Due to the improved ages of our cluster sample,
we find that our AMR is now better represented in the intermediate/old period
than that we derived in Parisi et al. (2009), where we simply took ages
available in the literature. Additionally, clusters younger than aprox. 4 Gyr
now show better agreement with the bursting model, but we confirm that this
model is not a good representation of the AMR during the intermediate-age/old
period. A more complicated model is needed to explain the SMC chemical
evolution in that period.Comment: 76 pages, 32 figures. Accepted for publication in A
Influence of firing mechanisms on gain modulation
We studied the impact of a dynamical threshold on the f-I curve-the
relationship between the input and the firing rate of a neuron-in the presence
of background synaptic inputs. First, we found that, while the leaky
integrate-and-fire model cannot reproduce the f-I curve of a cortical neuron,
the leaky integrate-and-fire model with dynamical threshold can reproduce it
very well. Second, we found that the dynamical threshold modulates the onset
and the asymptotic behavior of the f-I curve. These results suggest that a
cortical neuron has an adaptation mechanism and that the dynamical threshold
has some significance for the computational properties of a neuron.Comment: 7 pages, 4 figures, conference proceeding
A Search for Old Star Clusters in the Large Magellanic Cloud
We report the first results of a color-magnitude diagram survey of 25
candidate old LMC clusters. For almost all of the sample, it was possible to
reach the turnoff region, and in many clusters we have several magnitudes of
the main sequence. Age estimates based on the magnitude difference
between the giant branch clump and the turnoff revealed that no new old
clusters were found. The candidates turned out to be of intermediate age (1-3
Gyr) We show that the apparently old ages as inferred from integrated UBV
colors can be explained by a combination of stochastic effects produced by
bright stars and by photometric errors for faint clusters lying in crowded
fields. The relatively metal poor candidates from the CaII triplet spectroscopy
also turned out to be of intermediate age. This, combined with the fact that
they lie far out in the disk, yields interesting constraints regarding the
formation and evolution of the LMC disk. We also study the age distribution of
intermediate age and old clusters This homogeneous set of accurate relative
ages allows us to make an improved study of the history of cluster
formation/destruction for ages Gyr. We confirm previous indications that
there was apparently no cluster formation in the LMC during the period from 3-8
Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3
Gyrs ago that peaked at about 1.5 Gyrs ago. Our results suggest that there are
few, if any, genuine old clusters in the LMC left to be found.Comment: LaTeX, to be published in Nov. 1997 Astronomical Journa
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