7,704 research outputs found
Fractional Fokker-Planck Equation for Ultraslow Kinetics
Several classes of physical systems exhibit ultraslow diffusion for which the
mean squared displacement at long times grows as a power of the logarithm of
time ("strong anomaly") and share the interesting property that the probability
distribution of particle's position at long times is a double-sided
exponential. We show that such behaviors can be adequately described by a
distributed-order fractional Fokker-Planck equations with a power-law
weighting-function. We discuss the equations and the properties of their
solutions, and connect this description with a scheme based on continuous-time
random walks
Theoretical models for classical Cepheids. VIII. Effects of helium and heavy elements abundance on the Cepheid distance scale
Previous nonlinear fundamental pulsation models for classical Cepheids with
metal content Z <= 0.02 are implemented with new computations at super-solar
metallicity (Z=0.03, 0.04) and selected choices of the helium-to-metal
enrichment ratio DeltaY/Delta Z. On this basis, we show that the location into
the HR diagram of the Cepheid instability strip is dependent on both metal and
helium abundance, moving towards higher effective temperatures with decreasing
the metal content (at fixed Y) or with increasing the helium content (at fixed
Z). The contributions of helium and metals to the predicted Period-Luminosity
and Period-Luminosity-Color relations are discussed, as well as the
implications on the Cepheid distance scale. Based on these new results, we
finally show that the empirical metallicity correction suggested by Cepheid
observations in two fields of the galaxy M101 may be accounted for, provided
that the adopted helium-to-metal enrichment ratio is reasonably high (Delta
Y/Delta Z ~ 3.5).Comment: 23 pages, including 6 postscript figures, accepted for publication on
Ap
Triggering the Formation of Halo Globular Clusters with Galaxy Outflows
We investigate the interactions of high-redshift galaxy outflows with
low-mass virialized (Tvir < 10,000K) clouds of primordial composition. While
atomic cooling allows star formation in larger primordial objects, such
"minihalos" are generally unable to form stars by themselves. However, the
large population of high-redshift starburst galaxies may have induced
widespread star formation in these objects, via shocks that caused intense
cooling both through nonequilibrium H2 formation and metal-line emission. Using
a simple analytic model, we show that the resulting star clusters naturally
reproduce three key features of the observed population of halo globular
clusters (GCs). First, the 10,000 K maximum virial temperature corresponds to
the ~ 10^6 solar mass upper limit on the stellar mass of GCs. Secondly, the
momentum imparted in such interactions is sufficient to strip the gas from its
associated dark matter halo, explaining why GCs do not reside in dark matter
potential wells. Finally, the mixing of ejected metals into the primordial gas
is able to explain the ~ 0.1 dex homogeneity of stellar metallicities within a
given GC, while at the same time allowing for a large spread in metallicity
between different clusters. To study this possibility in detail, we use a
simple 1D numerical model of turbulence transport to simulate mixing in
cloud-outflow interactions. We find that as the shock shears across the side of
the cloud, Kelvin-Helmholtz instabilities arise, which cause mixing of enriched
material into > 20% of the cloud. Such estimates ignore the likely presence of
large-scale vortices, however, which would further enhance turbulence
generation. Thus quantitative mixing predictions must await more detailed
numerical studies.Comment: 21 pages, 11 figures, Apj in pres
Age, Metallicity, and the Distance to the Magellanic Clouds From Red Clump Stars
We show that the luminosity dependence of the red clump stars on age and
metallicity can cause a difference of up to < ~0.6 mag in the mean absolute I
magnitude of the red clump between different stellar populations. We show that
this effect may resolve the apparent ~0.4 mag discrepancy between red
clump-derived distance moduli to the Magellanic Clouds and those from, e.g.,
Cepheid variables. Taking into account the population effects on red clump
luminosity, we determine a distance modulus to the LMC of 18.36 +/- 0.17 mag,
and to the SMC of 18.82 +/- 0.20 mag. Our alternate red clump LMC distance is
consistent with the value (m-M){LMC} = 18.50 +/- 0.10 adopted by the HST
Cepheid Key Project. We briefly examine model predictions of red clump
luminosity, and find that variations in helium abundance and core mass could
bring the Clouds closer by some 0.10--0.15 mag, but not by the ~0.4 mag that
would result from setting the mean absolute I-magnitude of the Cloud red clumps
equal to the that of the Solar neighborhood red clump.Comment: Accepted for publication in The Astrophysical Journal Letters, AASTeX
4.0, 10 pages, 1 postscript figur
On the stellar populations in NGC 185 and NGC 205, and the nuclear star cluster in NGC 205 from Hubble Space Telescope observations
[Abridged] We present a first detailed analysis of resolved stellar
populations in the dwarf galaxies NGC 185 and NGC 205 based on archival V- and
I-band WFPC2 pointings. For NGC 185 we deduce that star formation was probably
still active about 4 x 10^8 yr ago. Key abundance-related results are: (1) We
identify ancient stars with [Fe/H] <~ -1.5 dex by a well-defined horizontal
branch (HB). (2) We find a prominent RGB/ faint-AGB clump/ bump- like feature
with the same mean V-band magnitude as the HB, within uncertainties; from a
comparison with theory, ancient stars have [Fe/H] ~ -1.5 dex, with a higher
abundance level for intermediate-age stars. (3) From colour information we
infer that the median [Fe/H] > -1.11 +/- 0.08 dex for ancient stars. For NGC
205, we record (m-M)o = 24.76 +/- 0.1 mag, based on the RGB I-band tip
magnitude method. We find that stars were probably still forming less than 3 x
10^8 yr ago, which is compatible with star formation triggered by an
interaction with M31. Key abundance-related results are: (1) The RGB/ faint-AGB
is significantly skewed to redder values than that of a control field in the
outskirts of M31; it probably results from a relatively narrow metallicity and
or age range for a significant fraction of the dwarf's stars. (2) For ancient
stars we infer from colour information that the median [Fe/H] > -1.06+/-0.04
dex. We briefly compare the stellar populations of NGC 205, NGC 185 and NGC
147.
Finally, we find an apparent blue excess in the outer region of the nuclear
star cluster in NGC 205. It is as compact as a typical galactic globular
cluster, but is quite bright (10^6 L_solar,R); and by matching its blue colour
with models, its stellar population is young, up to a few times 10^8 yr old.Comment: To appear in the May edition of the Astronomical Journal. Some
figures have been degraded in quality for the purpose of submissio
Improving the mass determination of Galactic Cepheids
We have selected a sample of Galactic Cepheids for which accurate estimates
of radii, distances, and photometric parameters are available. The comparison
between their pulsation masses, based on new Period-Mass-Radius (PMR)
relations, and their evolutionary masses, based on both optical and NIR
Color-Magnitude (CM) diagrams, suggests that pulsation masses are on average of
the order of 10% smaller than the evolutionary masses. Current pulsation masses
show, at fixed radius, a strongly reduced dispersion when compared with values
published in literature.The increased precision in the pulsation masses is due
to the fact that our predicted PMR relations based on nonlinear, convective
Cepheid models present smaller standard deviations than PMR relations based on
linear models. At the same time, the empirical radii of our Cepheid sample are
typically accurate at the 5% level. Our evolutionary mass determinations are
based on stellar models constructed by neglecting the effect of mass-loss
during the He burning phase. Therefore, the difference between pulsation and
evolutionary masses could be intrinsic and does not necessarily imply a problem
with either evolutionary and/or nonlinear pulsation models. The marginal
evidence of a trend in the difference between evolutionary and pulsation masses
when moving from short to long-period Cepheids is also briefly discussed. The
main finding of our investigation is that the long-standing Cepheid mass
discrepancy seems now resolved at the 10% level either if account for canonical
or mild convective core overshooting evolutionary models.Comment: 14 pages, 4 postscript figures, accepted for publication on ApJ
Letter
Nonholonomic Constraints with Fractional Derivatives
We consider the fractional generalization of nonholonomic constraints defined
by equations with fractional derivatives and provide some examples. The
corresponding equations of motion are derived using variational principle.Comment: 18 page
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