177 research outputs found
An ALMA study of the Orion Integral Filament : I. Evidence for narrow fibers in a massive cloud
© 2018 ESO. Reproduced with permission from Astronomy & Astrophysics. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder.Aim. We have investigated the gas organization within the paradigmatic Integral Shape Filament (ISF) in Orion in order to decipher whether or not all filaments are bundles of fibers. Methods. We combined two new ALMA Cycle 3 mosaics with previous IRAM 30m observations to produce a high-dynamic range N 2H + (1-0) emission map of the ISF tracing its high-density material and velocity structure down to scales of 0.009 pc (or ~2000 AU). Results. From the analysis of the gas kinematics, we identify a total of 55 dense fibers in the central region of the ISF. Independently of their location in the cloud, these fibers are characterized by transonic internal motions, lengths of ~0.15 pc, and masses per unit length close to those expected in hydrostatic equilibrium. The ISF fibers are spatially organized forming a dense bundle with multiple hub-like associations likely shaped by the local gravitational potential. Within this complex network, the ISF fibers show a compact radial emission profile with a median FWHM of 0.035 pc systematically narrower than the previously proposed universal 0.1 pc filament width. Conclusions. Our ALMA observations reveal complex bundles of fibers in the ISF, suggesting strong similarities between the internal substructure of this massive filament and previously studied lower-mass objects. The fibers show identical dynamic properties in both low- and high-mass regions, and their widespread detection in nearby clouds suggests a preferred organizational mechanism of gas in which the physical fiber dimensions (width and length) are self-regulated depending on their intrinsic gas density. Combining these results with previous works in Musca, Taurus, and Perseus, we identify a systematic increase of the surface density of fibers as a function of the total mass per-unit-length in filamentary clouds. Based on this empirical correlation, we propose a unified star-formation scenario where the observed differences between low- and high-mass clouds, and the origin of clusters, emerge naturally from the initial concentration of fibers.Peer reviewedFinal Published versio
Origin of the tetragonal-to-orthorhombic (nematic) phase transition in FeSe: a combined thermodynamic and NMR study
The nature of the tetragonal-to-orthorhombic structural transition at
K in single crystalline FeSe is studied using shear-modulus,
heat-capacity, magnetization and NMR measurements. The transition is shown to
be accompanied by a large shear-modulus softening, which is practically
identical to that of underdoped Ba(Fe,Co)As, suggesting very similar
strength of the electron-lattice coupling. On the other hand, a
spin-fluctuation contribution to the spin-lattice relaxation rate is only
observed below . This indicates that the structural, or "nematic", phase
transition in FeSe is not driven by magnetic fluctuations
Ferromagnetism and Lattice Distortions in the Perovskite YTiO
The thermodynamic properties of the ferromagnetic perovskite YTiO are
investigated by thermal expansion, magnetostriction, specific heat, and
magnetization measurements. The low-temperature spin-wave contribution to the
specific heat, as well as an Arrott plot of the magnetization in the vicinity
of the Curie temperature K, are consistent with a
three-dimensional Heisenberg model of ferromagnetism. However, a magnetic
contribution to the thermal expansion persists well above , which
contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a
comparison with the corresponding model system EuS. The pressure dependences of
and of the spontaneous moment are extracted using thermodynamic
relationships. They indicate that ferromagnetism is strengthened by uniaxial
pressures and is weakened by uniaxial
pressures and hydrostatic pressure.
Our results show that the distortion along the - and -axes is further
increased by the magnetic transition, confirming that ferromagnetism is favored
by a large GdFeO-type distortion. The c-axis results however do not fit
into this simple picture, which may be explained by an additional
magnetoelastic effect, possibly related to a Jahn-Teller distortion.Comment: 12 pages, 13 figure
Calorimetric Evidence of Multiband Superconductivity in Ba(Fe0.925Co0.075)2As2
We report on the determination of the electronic heat capacity of a slightly
overdoped (x = 0.075) Ba(Fe1-xCox)2As2 single crystal with a Tc of 21.4 K. Our
analysis of the temperature dependence of the superconducting-state specific
heat provides strong evidence for a two-band s-wave order parameter with gap
amplitudes 2D1(0)/kBTc=1.9 and 2D2(0)/kBTc=4.4. Our result is consistent with
the recently predicted s+- order parameter [I. I. Mazin et al., Phys. Rev.
Lett. 101, 057003 (2008)].Comment: 4 pages, 3 figure
- âŠ