1,193 research outputs found
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
Nonlinear ac conductivity of one-dimensional Mott insulators
We discuss a semiclassical calculation of low energy charge transport in
one-dimensional (1d) insulators with a focus on Mott insulators, whose charge
degrees of freedom are gapped due to the combination of short range
interactions and a periodic lattice potential. Combining RG and instanton
methods, we calculate the nonlinear ac conductivity and interpret the result in
terms of multi-photon absorption. We compare the result of the semiclassical
calculation for interacting systems to a perturbative, fully quantum mechanical
calculation of multi-photon absorption in a 1d band insulator and find good
agreement when the number of simultaneously absorbed photons is large.Comment: Dedicated to Thomas Nattermann on the occasion of his 60th birthday.
To appear in JSTAT. 5 pages, 2 figure
The initial mass distribution of the M82 star cluster system
We explore whether we can constrain the shape of the INITIAL mass
distribution of the star cluster population in M82's ~1 Gyr-old post-starburst
region "B", in which the present-day cluster mass function (CMF) is closely
approximated by a log-normal distribution. We conclude that the M82 B initial
CMF must have had a mean mass very close to that of the "equilibrium" CMF of
Vesperini (1998). Consequently, if the presently observed M82 B CMF has
remained approximately constant since its formation, as predicted, then the
INITIAL CMF must have been characterized by a mean mass that was only slightly
larger than the present mean mass. From our detailed analysis of the expected
evolution of CMFs, we conclude that our observations of the M82 B CMF are
inconsistent with a scenario in which the 1 Gyr-old cluster population
originated from an initial power-law mass distribution. Our conclusion is
supported by arguments related to the initial density in M82 B, which would
have been unphysically high if the present cluster population were the remains
of an initial power-law distribution.Comment: 10 pages, 4 figures, accepted for publication in MNRA
2.5-D retrieval of atmospheric properties from exoplanet phase curves: Application to WASP-43b observations
We present a novel retrieval technique that attempts to model phase curve observations of exoplanets more realistically and reliably, which we call the 2.5-dimensional (2.5D) approach. In our 2.5D approach we retrieve the vertical temperature profile and mean gaseous abundance of a planet at all longitudes and latitudes simultaneously, assuming that the temperature or composition, x, at a particular longitude and latitude (Î, Ί) is given by x(Î,Ί)=xÂŻ+(x(Î,0)âxÂŻ)cosnΊâ , where xÂŻ is the mean of the morning and evening terminator values of x(Î, 0), and n is an assumed coefficient. We compare our new 2.5D scheme with the more traditional 1D approach, which assumes the same temperature profile and gaseous abundances at all points on the visible disc of a planet for each individual phase observation, using a set of synthetic phase curves generated from a GCM-based simulation. We find that our 2.5D model fits these data more realistically than the 1D approach, confining the hotter regions of the planet more closely to the dayside. We then apply both models to WASP-43b phase curve observations of HST/WFC3 and Spitzer/IRAC. We find that the dayside of WASP-43b is apparently much hotter than the nightside and show that this could be explained by the presence of a thick cloud on the nightside with a cloud top at pressure <0.2 bar. We further show that while the mole fraction of water vapour is reasonably well constrained to (1â10) Ă 10â4, the abundance of CO is very difficult to constrain with these data since it is degenerate with temperature and prone to possible systematic radiometric differences between the HST/WFC3 and Spitzer/IRAC observations. Hence, it is difficult to reliably constrain C/O
Switching between dynamic states in intermediate-length Josephson junctions
The appearance of zero-field steps (ZFSâs) in the current-voltage characteristics of intermediate-length overlap-geometry Josephson tunnel junctions described by a perturbed sine-Gordon equation (PSGE) is associated with the growth of parametrically excited instabilities of the McCumber background curve (MCB). A linear stability analysis of a McCumber solution of the PSGE in the asymptotic linear region of the MCB and in the absence of magnetic field yields a Hillâs equation which predicts how the number, locations, and widths of the instability regions depend on the junction parameters. A numerical integration of the PSGE in terms of truncated series of time-dependent Fourier spatial modes verifies that the parametrically excited instabilities of the MCB evolve into the fluxon oscillations characteristic of the ZFSâs. An approximate analysis of the Fourier mode equations in the presence of a small magnetic field yields a field-dependent Hillâs equation which predicts that the major effect of such a field is to reduce the widths of the instability regions. Experimental measurements on Nb-NbxOy-Pb junctions of intermediate length, performed at different operating temperatures in order to vary the junction parameters and for various magnetic field values, verify the physical existence of switching from the MCB to the ZFSâs. Good qualitative, and in many cases quantitative, agreement between analytic, numerical, and experimental results is obtained
Zeewoorden: een speurtocht naar de naamsverklaring van zandbanken, geulen en andere 'zee-begrippen'. Kabeljauw; Paardenmarkt
Zeewoorden: een speurtocht naar de naamsverklaring van zandbanken, geulen en andere 'zee-begrippen'. Thorntonbank; Dolfijn
Zeewoorden: een speurtocht naar de naamsverklaring van zandbanken, geulen en andere 'zee-begrippen'. Negenvaam; Loods
Zeewoorden: een speurtocht naar de naamsverklaring van zandbanken, geulen en andere 'zee-begrippen'. Trapegeer; Zandbank
Zeewoorden: een speurtocht naar de naamsverklaring van zandbanken, geulen en andere 'zee-begrippen'. Vlakte van de Raan; Strand
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