972 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
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
Star Formation in the Gulf of Mexico
We present an optical/infrared study of the dense molecular cloud, L935,
dubbed "The Gulf of Mexico", which separates the North America and the Pelican
nebulae, and we demonstrate that this area is a very active star forming
region. A wide-field imaging study with interference filters has revealed 35
new Herbig-Haro objects in the Gulf of Mexico. A grism survey has identified 41
Halpha emission-line stars, 30 of them new. A small cluster of partly embedded
pre-main sequence stars is located around the known LkHalpha 185-189 group of
stars, which includes the recently erupting FUor HBC 722.Comment: Submitted to A&A, 14 pages, 18 figure
Radio Spectra and NVSS Maps of Decametric Sources
We constructed radio spectra for ~1400 UTR-2 sources and find that 46% of
them have concave curvature. Inspection of NVSS maps of 700 UTR sources
suggests that half of all UTR sources are either blends of two or more sources
or have an ultra-steep spectrum (USS). The fraction of compact USS sources in
UTR may be near 10%. Using NVSS and the Digitized Sky Survey(s) we expect to
double the UTR optical identification rate from currently ~19%.Comment: 2 pages, no figures; to appear in Proc. "Observational Cosmology with
the New Radio Surveys", eds. M. Bremer, N. Jackson & I. Perez-Fournon, Kluwer
Acad. Pres
The Superluminal Character of the Compact Steep Spectrum Quasar 3C 216
We report the results of fourth epoch VLBI observations at 4990.99 MHz, with a resolution of ~1 mas, of the compact steep-spectrum quasar 3C216. Superluminal motion in this object is confirmed. Although a constant superluminal expansion at v_(app) = 3.9c ± 0.6 is not ruled out, our four epoch data are suggestive of component deceleration. In this paper we discuss the possibility of deceleration taking into account the compact steep spectrum nature of this quasar. We conclude that (a) compact steep spectrum sources may show the same beaming and orientation phenomena as extended sources and (b) the compact steep spectrum nature of the source could offer an explanation for the possible deceleration
IRAS 05436-0007 and the Emergence of McNeil's Nebula
We present a study of McNeil's Nebula, a newly appeared reflection nebula in
the L1630 cloud, together with photometry and spectroscopy of its source. New
IR photometry compared to earlier 2MASS data shows that the star has brightened
by about 3 magnitudes in the near-infrared, changing its location in a J-H/H-K
diagram precisely along a reddening vector. A Gemini NIRI K-band spectrum shows
strong CO-bandhead emission and Br-gamma is in emission, indicative of strong
accretion. A Gemini GMOS optical spectrum shows only a red, heavily veiled
continuum, with H-alpha strongly in emission and displaying a pronounced P
Cygni profile, with an absorption trough reaching velocities up to 600 km s-1.
This implies significant mass loss in a powerful wind. However, no evidence is
found for any shocks, as commonly seen in collimated outflows from young stars.
Apparently the eruption has dispersed a layer of extinction and this, together
with the intrinsic brightening of the IRAS source, has allowed an earlier
outflow cavity to be flooded with light, thus creating McNeil's Nebula.Comment: 9 pages, 5 figure
Observation of Feshbach-like resonances in collisions between ultracold molecules
We observe magnetically tuned collision resonances for ultracold Cs2
molecules stored in a CO2-laser trap. By magnetically levitating the molecules
against gravity, we precisely measure their magnetic moment. We find an avoided
level crossing which allows us to transfer the molecules into another state. In
the new state, two Feshbach-like collision resonances show up as strong
inelastic loss features. We interpret these resonances as being induced by Cs4
bound states near the molecular scattering continuum. The tunability of the
interactions between molecules opens up novel applications such as controlled
chemical reactions and synthesis of ultracold complex molecules
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
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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