1,682 research outputs found
Hot Electrons and Cold Photons: Galaxy Clusters and the Sunyaev-Zel'dovich Effect
The hot gas in clusters of galaxies emits thermal bremsstrahlung emission
that can be probed directly through measurements in the X-ray band with
satellites like ROSAT and ASCA. Another probe of this gas comes from its effect
on the cosmic microwave background radiation (CMBR): the hot cluster electrons
inverse Compton scatter the CMBR photons and thereby distort the background
radiation from its blackbody spectral form. In the last few years, the
development of sensitive new instruments for measuring this distortion, called
the Sunyaev-Zel'dovich (SZ) effect, has sparked a revolution in the field.
Current radio interferometric arrays can now detect and map the SZ effect in
even distant (z ~ 1) clusters. It is well known that one of the purposes of
conducting such measurements is to determine the Hubble constant. In this
review I report on the progress that has been made in this area, quote the
current best estimate of Ho from the SZ effect of 8 galaxy clusters (44 - 64
km/s/Mpc +/- 17%), discuss important systematic uncertainties, and highlight
what else has been learned about galaxy clusters from these investigations.Comment: 4 pages, including 2 postscript figs, LaTeX. To appear in the
proceedings of IAU Symposium 188 "The Hot Universe" (held August 26-30, 1997,
Kyoto, Japan
Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables,
during its quiescence and outburst phases. We analyse high-resolution optical
spectra taken before, during, and after its 2008 outburst. In quiescence and
outburst, the star presents many permitted emission lines, including typical
CTTS lines and numerous neutral and ionized metallic lines. During the
outburst, the number of emission lines increases to over a thousand, with
narrow plus broad component structure (NC+BC). The BC profile is highly
variable on short timescales (24-72h). An active chromosphere can explain the
metallic lines in quiescence and the outburst NC. The dynamics of the BC line
profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform
accretion column that suffers velocity variations along the line-of-sight on
timescales of days. Assuming Keplerian rotation, the emitting region would be
located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity
profiles of the lines reveal a combination of rotation and infall. Line ratios
of ions and neutrals can be reproduced with a temperature of T~6500 K for
electron densities of a few times 10cm in the line-emitting
region. The data confirm that the 2008 outburst was an episode of increased
accretion, albeit much stronger than previous EX Lupi and typical EXors
outbursts. The line profiles are consistent with the infall/rotation of a
non-axisymmetric structure that could be produced by clumpy accretion during
the outburst phase. A strong inner disk wind appears in the epochs of higher
accretion. The rapid recovery of the system after the outburst and the
similarity between the pre-outburst and post-outburst states suggest that the
accretion channels are similar during the whole period, and only the accretion
rate varies, providing a superb environment for studying the accretion
processes.Comment: 15 pages plus 26 pages online material, accepted by A&
Efficient creation of molecules from a cesium Bose-Einstein condensate
We report a new scheme to create weakly bound Cs molecules from an atomic
Bose-Einstein condensate. The method is based on switching the magnetic field
to a narrow Feshbach resonance and yields a high atom-molecule conversion
efficiency of more than 30%, a factor of three higher than obtained with
conventional magnetic-field ramps. The Cs molecules are created in a single
-wave rotational quantum state. The observed dependence of the conversion
efficiency on the magnetic field and atom density shows scattering processes
beyond two-body coupling to occur in the vicinity of the Feshbach resonance.Comment: 7 pages, 4 figures, submitted to Europhysics Letter
Ab initio explanation of disorder and off-stoichiometry in Fe-Mn-Al-C kappa carbides
Carbides play a central role for the strength and ductility in many
materials. Simulating the impact of these precipitates on the mechanical
performance requires the knowledge about their atomic configuration. In
particular, the C content is often observed to substantially deviate from the
ideal stoichiometric composition. In the present work, we focus on Fe-Mn-Al-C
steels, for which we determined the composition of the nano-sized kappa
carbides (Fe,Mn)3AlC by atom probe tomography (APT) in comparison to larger
precipitates located in grain boundaries. Combining density functional theory
with thermodynamic concepts, we first determine the critical temperatures for
the presence of chemical and magentic disorder in these carbides. Secondly, the
experimentally observed reduction of the C content is explained as a compromise
between the gain in chemical energy during partitioning and the elastic strains
emerging in coherent microstructures
EX Lupi in quiescence
EX Lup is the prototype of EXors, a subclass of low-mass pre-main sequence
stars whose episodic eruptions are attributed to temporarily increased
accretion. In quiescence the optical and near-infrared properties of EX Lup
cannot be distinguished from those of normal T Tau stars. Here we investigate
whether it is the circumstellar disk structure which makes EX Lup an atypical
Class II object. During outburst the disk might undergo structural changes. Our
characterization of the quiescent disk is intended to serve as a reference to
study the physical changes related to one of EX Lupi's strongest known
eruptions in 2008 Jan-Sep. We searched the literature for photometric and
spectroscopic observations including ground-based, IRAS, ISO and Spitzer data.
After constructing the optical-infrared spectral energy distribution (SED), we
compared it with the typical SEDs of other young stellar objects and modeled it
using the Monte Carlo radiative transfer code RADMC. A mineralogical
decomposition of the 10 micron silicate emission feature and also the
description of the optical and near-infrared spectra were performed. The SED is
in general similar to that of a typical T Tauri star, though above 7 micron EX
Lup emits higher flux. The quiescent phase data suggest low level variability
in the optical-mid-infrared domain. Integrating the optical and infrared fluxes
we derived a bolometric luminosity of 0.7 L_Sun. The 10 micron silicate profile
could be fitted by a mixture consisting of amorphous silicates, no crystalline
silicates were found. A modestly flaring disk model with a total mass of 0.025
M_Sun and an outer radius of 150 AU was able to reproduce the observed SED. The
derived inner radius of 0.2 AU is larger than the sublimation radius, and this
inner gap sets EX Lup aside from typical T Tauri stars.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 6
figure
Pre-Main Sequence variables in the VMR-D : identification of T Tauri-like accreting protostars through Spitzer-IRAC variability
We present a study of the infrared variability of young stellar objects by
means of two Spitzer-IRAC images of the Vela Molecular Cloud D (VMR-D) obtained
in observations separated in time by about six months. By using the same
space-born IR instrumentation, this study eliminates all the unwanted effects
usually unavoidable when comparing catalogs obtained from different
instruments. The VMR-D map covers about 1.5 square deg. of a site where star
formation is actively ongoing. We are interested in accreting pre-main sequence
variables whose luminosity variations are due to intermittent events of disk
accretion (i.e. active T Tauri stars and EXor type objects). The variable
objects have been selected from a catalog of more than 170,000 sources detected
at a S/N ratio > 5. We searched the sample of variables for ones whose
photometric properties are close to those of known EXor's. These latter are
monitored in a more systematic way than T Tauri stars and the mechanisms that
regulate the observed phenomenology are exactly the same. Hence the modalities
of the EXor behavior is adopted as driving criterium for selecting variables in
general. We selected 19 bona fide candidates that constitute a well-defined
sample of new variable targets for further investigation. Out of these, 10
sources present a Spitzer MIPS 24 micron counterpart, and have been classified
as 3 Class I, 5 flat spectrum and 2 Class II objects, while the other 9 sources
have spectral energy distribution compatible with phases older than Class I.
This is consistent with what is known about the small sample of known EXor's,
and suggests that the accretion flaring or EXor stage might come as a Class
I/II transition. We present also new prescriptions that can be useful in future
searches for accretion variables in large IR databases.Comment: 35 pages, 12 figures To appear in Ap
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