765 research outputs found
Microwave Induced Instability Observed in BSCCO 2212 in a Static Magnetic Field
We have measured the microwave dissipation at 10 GHz through the imaginary
part of the susceptibility, , in a BSCCO 2212 single crystal in an
external static magnetic field parallel to the c-axis at various fixed
temperatures. The characteristics of exhibit a sharp step at a
field which strongly depends on the amplitude of the microwave
excitation . The characteristics of vs. ,
qualitatively reveal the behavior expected for the magnetic field dependence of
Josephson coupling.Comment: 4 pages, 3 Postscript figure
Gas dynamics in Massive Dense Cores in Cygnus-X
We study the kinematic properties of dense gas surrounding massive protostars
recognized by Bontemps et a. (2010) in a sample of five Massive Dense Cores in
Cygnus-X. We investigate whether turbulent support plays a major role in
stabilizing the core against fragmentation into Jeans-mass objects or
alternatively, the observed kinematics could indicate a high level of dynamics.
We present IRAM 30m single-dish (HCO+ and H13CO+) and IRAM PdBI high
angular-resolution observations of dense gas tracers (H13CO+ and H13CN) to
reveal the kinematics of molecular gas at scales from 0.03 to 0.1 pc. Radiative
transfer modeling shows that H13CO+ is depleted within the envelopes of massive
protostars and traces the bulk of material surrounding the protostars rather
than their inner envelopes. H13CN shows a better correspondence with the peak
of the continuum emission, possibly due to abundance anomalies and specific
chemistry in the close vicinity of massive protostars. Analyzing the
line-widths we show that the observed line-dispersion of H13CO+ at the scale of
MDCs is smaller than expected from the quasi-static, turbulent-core model. At
large-scales, global organized bulk motions are identified for 3 of the MDCs.
At small-scales, several spectral components are identified in all MDCs showing
filamentary structures and intrinsic velocity gradients towards the continuum
peaks. The dynamics of these flows show diversity among the sample and we link
this to the specific fragmentation properties of the MDCs. No clear evidence is
found for a turbulence regulated, equilibrium scenario within the sample of
MDCs. We propose a picture in which MDCs are not in equilibrium and their
dynamics is governed by small-scale converging flows, which may initiate
star-formation via their shears
Massive Infrared-Quiet Dense Cores: Unveiling the Initial Conditions of High-Mass Star Formation
As Pr. Th. Henning said at the conference, cold precursors of high-mass stars
are now "hot topics". We here propose some observational criteria to identify
massive infrared-quiet dense cores which can host the high-mass analogs of
Class 0 protostars and pre-stellar condensations. We also show how far-infrared
to millimeter imaging surveys of entire complexes forming OB stars are starting
to unveil the initial conditions of high-mass star formation
The Carbon content in the Galactic CygnusX/DR21 star forming region
Observations of Carbon bearing species are among the most important
diagnostic probes of ongoing star formation. CO is a surrogate for H and is
found in the vicinity of star formation sites. There, [CI] emission is thought
to outline the dense molecular cores and extend into the lower density regions,
where the impinging interstellar UV radiation field plays a critical role for
the dissociation and ionization processes. Emission of ionized carbon ([CII])
is found to be even more extended than [CI] and is linking up with the ionized
medium. These different tracers emphasize the importance of multi-wavelength
studies to draw a coherent picture of the processes driving and driven by high
mass star formation. Until now, large scale surveys were only done with low
resolution, such as the COBE full sky survey, or were biased to a few selected
bright sources (e.g. Yamamoto et al. 2001, Schneider et al. 2003). A broader
basis of unbiased, high-resolution observations of [CI], CO, and [CII] may play
a key role to probe the material processed by UV radiation.Comment: 4 pages, 4 figure, to appear in "Proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium", ed. S. Pfalzner, C. Kramer, C. Straubmeier,
and A. Heithausen (Springer Verlag
First detection of CF+ towards a high-mass protostar
We report the first detection of the J = 1 - 0 (102.6 GHz) rotational lines
of CF+ (fluoromethylidynium ion) towards CygX-N63, a young and massive
protostar of the Cygnus X region. This detection occurred as part of an
unbiased spectral survey of this object in the 0.8-3 mm range, performed with
the IRAM 30m telescope. The data were analyzed using a local thermodynamical
equilibrium model (LTE model) and a population diagram in order to derive the
column density. The line velocity (-4 km s-1) and line width (1.6 km s-1)
indicate an origin from the collapsing envelope of the protostar.
We obtain a CF+ column density of 4.10e11 cm-2. The CF+ ion is thought to be
a good tracer for C+ and assuming a ratio of 10e-6 for CF+/C+, we derive a
total number of C+ of 1.2x10e53 within the beam. There is no evidence of carbon
ionization caused by an exterior source of UV photons suggesting that the
protostar itself is the source of ionization. Ionization from the protostellar
photosphere is not efficient enough. In contrast, X-ray ionization from the
accretion shock(s) and UV ionization from outflow shocks could provide a large
enough ionizing power to explain our CF+ detection.
Surprisingly, CF+ has been detected towards a cold, massive protostar with no
sign of an external photon dissociation region (PDR), which means that the only
possibility is the existence of a significant inner source of C+. This is an
important result that opens interesting perspectives to study the early
development of ionized regions and to approach the issue of the evolution of
the inner regions of collapsing envelopes of massive protostars. The existence
of high energy radiations early in the evolution of massive protostars also has
important implications for chemical evolution of dense collapsing gas and could
trigger peculiar chemistry and early formation of a hot core.Comment: 6 page
Understanding star formation in molecular clouds I. Effects of line-of-sight contamination on the column density structure
Column-density maps of molecular clouds are one of the most important
observables in the context of molecular cloud- and star-formation (SF) studies.
With the Herschel satellite it is now possible to determine the column density
from dust emission. We use observations and simulations to demonstrate how LOS
contamination affects the column density probability distribution function
(PDF). We apply a first-order approximation (removing a constant level) to the
molecular clouds of Auriga, Maddalena, Carina and NGC3603. In perfect agreement
with the simulations, we find that the PDFs become broader, the peak shifts to
lower column densities, and the power-law tail of the PDF flattens after
correction. All PDFs have a lognormal part for low column densities with a peak
at Av~2, a deviation point (DP) from the lognormal at Av(DP)~4-5, and a
power-law tail for higher column densities. Assuming a density distribution
rho~r^-alpha, the slopes of the power-law tails correspond to alpha(PDF)=1.8,
1.75, and 2.5 for Auriga, Carina, and NGC3603 (alpha~1.5-2 is consistent
gravitational collapse). We find that low-mass and high-mass SF clouds display
differences in the overall column density structure. Massive clouds assemble
more gas in smaller cloud volumes than low-mass SF ones. However, for both
cloud types, the transition of the PDF from lognormal shape into power-law tail
is found at the same column density (at Av~4-5 mag). Low-mass and high-mass SF
clouds then have the same low column density distribution, most likely
dominated by supersonic turbulence. At higher column densities, collapse and
external pressure can form the power-law tail. The relative importance of the
two processes can vary between clouds and thus lead to the observed differences
in PDF and column density structure.Comment: A&A accepted, 15.12. 201
Infrared Signature of the Superconducting State in Pr(2-x)Ce(x)CuO(4)
We measured the far infrared reflectivity of two superconducting
Pr(2-x)Ce(x)CuO(4) films above and below Tc. The reflectivity in the
superconducting state increases and the optical conductivity drops at low
energies, in agreement with the opening of a (possibly) anisotropic
superconducting gap. The maximum energy of the gap scales roughly with Tc as 2
Delta_{max} / kB Tc ~ 4.7. We determined absolute values of the penetration
depth at 5 K as lambda_{ab} = (3300 +/- 700) A for x = 0.15 and lambda_{ab} =
(2000 +/- 300) A for x = 0.17. A spectral weight analysis shows that the
Ferrell-Glover-Tinkham sum rule is satisfied at conventional low energy scales
\~ 4 Delta_{max}.Comment: 4 pages, 4 figure
Pairing in cuprates from high energy electronic states
The in-plane optical conductivity of Bi2Sr2CaCu2O8+d thin films with small
carrier density (underdoped) up to large carrier density (overdoped) is
analyzed with unprecedented accuracy. Integrating the conductivity up to
increasingly higher energies points to the energy scale involved when the
superfluid condensate builds up. In the underdoped sample, states extending up
to 2 eV contribute to the superfluid. This anomalously large energy scale may
be assigned to a change of in-plane kinetic energy at the superconducting
transition, and is compatible with an electronic pairing mechanism.Comment: 11 pages, 3 figure
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