464 research outputs found
Chemical Self-Enrichment of HII Regions by the Wolf-Rayet Phase of an 85 Msun star
It is clear from stellar evolution and from observations of WR stars that
massive stars are releasing metal-enriched gas through their stellar winds in
the Wolf-Rayet phase. Although HII region spectra serve as diagnostics to
determine the present-day chemical composition of the interstellar medium, it
is far from being understood to what extent the HII gas is already contaminated
by chemically processed stellar wind. Therefore, we analyzed our models of
radiative and wind bubbles of an isolated 85 Msun star with solar metallicity
(Kr\"oger et al. 2006) with respect to the chemical enrichment of the
circumstellar HII region. Plausibly, the hot stellar wind bubble (SWB) is
enriched with 14N during the WN phase and even much higher with 12C and 16O
during the WC phase of the star. During the short period that the 85 Msun star
spends in the WC stage enriched SWB material mixes with warm HII gas of solar
abundances and thus enhances the metallicity in the HII region. However, at the
end of the stellar lifetime the mass ratios of the traced elements N and O in
the warm ionized gas are insignificantly higher than solar, whereas an
enrichment of 22 % above solar is found for C. Important issues from the
presented study comprise a steeper radial gradient of C than O and a decreasing
effect of self-enrichment for metal-poor galaxies.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Depolarisation cooling of an atomic cloud
We propose a cooling scheme based on depolarisation of a polarised cloud of
trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the
coupling between the internal spin reservoir of the cloud and the external
kinetic reservoir via dipolar relaxation to reduce the temperature of the
cloud. By optical pumping one can cool the spin reservoir and force the cooling
process. In case of a trapped gas of dipolar chromium atoms, we show that this
cooling technique can be performed continuously and used to approach the
critical phase space density for BECComment: 8 pages, 5 figure
Damagnetization cooling of a gas
We demonstrate demagnetization cooling of a gas of ultracold Cr atoms.
Demagnetization is driven by inelastic dipolar collisions which couple the
motional degrees of freedom to the spin degree. By that kinetic energy is
converted into magnetic work with a consequent temperature reduction of the
gas. Optical pumping is used to magnetize the system and drive continuous
demagnetization cooling. Applying this technique, we can increase the phase
space density of our sample by one order of magnitude, with nearly no atom
loss. This method can be in principle extended to every dipolar system and
could be used to achieve quantum degeneracy via optical means.Comment: 10 pages, 5 figure
Observation of Feshbach resonances in an ultracold gas of Cr
We have observed Feshbach resonances in elastic collisions between ultracold
Cr atoms. This is the first observation of collisional Feshbach
resonances in an atomic species with more than one valence electron. The zero
nuclear spin of Cr and thus the absence of a Fermi-contact interaction
leads to regularly-spaced resonance sequences. By comparing resonance positions
with multi-channel scattering calculations we determine the s-wave scattering
length of the lowest potentials to be
\unit[112(14)]{a_0}, \unit[58(6)]{a_0} and -\unit[7(20)]{a_0} for S=6, 4,
and 2, respectively, where a_{0}=\unit[0.0529]{nm}.Comment: 4 pages, 2 figures, 1 tabl
Production of a chromium Bose-Einstein condensate
The recent achievement of Bose-Einstein condensation of chromium atoms [1]
has opened longed-for experimental access to a degenerate quantum gas with
long-range and anisotropic interaction. Due to the large magnetic moment of
chromium atoms of 6 {}B, in contrast to other Bose- Einstein condensates
(BECs), magnetic dipole-dipole interaction plays an important role in a
chromium BEC. Many new physical properties of degenerate gases arising from
these magnetic forces have been predicted in the past and can now be studied
experimentally. Besides these phenomena, the large dipole moment leads to a
breakdown of standard methods for the creation of a chromium BEC. Cooling and
trapping methods had to be adapted to the special electronic structure of
chromium to reach the regime of quantum degeneracy. Some of them apply
generally to gases with large dipolar forces. We present here a detailed
discussion of the experimental techniques which are used to create a chromium
BEC and alow us to produce pure condensates with up to {} atoms in an
optical dipole trap. We also describe the methods used to determine the
trapping parameters.Comment: 17 pages, 9 figure
The sensitivity of harassment to orbit: Mass loss from early-type dwarfs in galaxy clusters
We conduct a comprehensive numerical study of the orbital dependence of harassment on early-type dwarfs consisting of 168 different orbits within a realistic, Virgo-like cluster, varying in eccentricity and pericentre distance. We find harassment is only effective at stripping stars or truncating their stellar discs for orbits that enter deep into the cluster core. Comparing to the orbital distribution in cosmological simulations, we find that the majority of the orbits (more than three quarters) result in no stellar mass loss. We also study the effects on the radial profiles of the globular cluster systems of early-type dwarfs. We find these are significantly altered only if harassment is very strong. This suggests that perhaps most early-type dwarfs in clusters such as Virgo have not suffered any tidal stripping of stars or globular clusters due to harassment, as these components are safely embedded deep within their dark matter halo. We demonstrate that this result is actually consistent with an earlier study of harassment of dwarf galaxies, despite the apparent contradiction. Those few dwarf models that do suffer stellar stripping are found out to the virial radius of the cluster at redshift = 0, which mixes them in with less strongly harassed galaxies. However when placed on phase-space diagrams, strongly harassed galaxies are found offset to lower velocities compared to weakly harassed galaxies. This remains true in a cosmological simulation, even when haloes have a wide range of masses and concentrations. Thus phase-space diagrams may be a useful tool for determining the relative likelihood that galaxies have been strongly or weakly harassed
Determination of the s-wave Scattering Length of Chromium
We have measured the deca-triplet s-wave scattering length of the bosonic
chromium isotopes Cr and Cr. From the time constants for
cross-dimensional thermalization in atomic samples we have determined the
magnitudes and ,
where . By measuring the rethermalization rate of Cr over a
wide temperature range and comparing the temperature dependence with the
effective-range theory and single-channel calculations, we have obtained strong
evidence that the sign of is positive. Rescaling our Cr
model potential to Cr strongly suggests that is positive,
too.Comment: v3: corrected typo in y-axis scaling of Figs. 3 and
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