382 research outputs found
Neural network based path collective variables for enhanced sampling of phase transformations
We propose a rigorous construction of a 1D path collective variable to sample
structural phase transformations in condensed matter. The path collective
variable is defined in a space spanned by global collective variables that
serve as classifiers derived from local structural units. A reliable
identification of local structural environments is achieved by employing a
neural network based classification. The 1D path collective variable is
subsequently used together with enhanced sampling techniques to explore the
complex migration of a phase boundary during a solid-solid phase transformation
in molybdenum
Complementarity of Bs->mu+mu- and B-> K l+l- in New Physics searches
This year, LHC provided a very stringent bound on Br(Bs->mu+mu-), bringing it
closer to the value predicted by the Standard Model (SM). Bs->mu+mu- was
believed to be the golden mode at LHCb to find SUSY because a large enhancement
was expected in the regime of moderate and large values of tanb. Other
scenarios are still possible and a correlation with other decay channels is
needed. We show that a complementary information on New Physics (NP) can be
obtained model-indepedently from the B-> K l+l- decay mode. We provide a
prediction Br(B-> K l+l-) based on the first lattice QCD results for all three
relevant form factors, f0(q^2), f+(q^2) and fT(q^2). We were then able to
provide the model-independent bounds on the complex couplings to scalar and
pseudoscalr operators in the b->s sector.Comment: Contributed to the 16th International Conference in Quantum
ChromoDynamics (QCD12), Montpellier, France, 2-6 Jul 201
Complementarity of the constraints on New Physics from B_s -> mu+ mu- and from B -> K l+l- decays
We discuss the advantages of combining the experimental bound on Br(B_s ->
mu+ mu-) and the measured Br(B -> K l+l-) to get the model independent
constraints on physics beyond the Standard Model. Since the two decays give
complementary information, one can study not only the absolute values of the
Wilson coefficients that are zero in the Standard Model, but also their phases.
To identify the sector in which the new physics might appear, information about
the shapes of the transverse asymmetries in B -> K* l+l- at low q^2's can be
particularly useful. We also emphasize the importance of measuring the
forward-backward asymmetry in B -> K l+l- decay at large q^2's.Comment: 28 pp, 12 figures, 2 tables; v3: version as publishe
The X-ray emission from Z CMa during an FUor-like outburst and the detection of its X-ray jet
Accretion shocks have been recognized as important X-ray emission mechanism
for pre-main sequence stars. Yet the X-ray properties of FUor outbursts, events
that are caused by violent accretion, have been given little attention. We have
observed the FUor object Z CMa during optical outburst and quiescence with
Chandra. No significant changes in X-ray brightness and spectral shape are
found, suggesting that the X-ray emission is of coronal nature. Due to the
binary nature of Z CMa the origin of the X-ray source is ambiguous. However,
the moderate hydrogen column density derived from our data makes it unlikely
that the embedded primary star is the X-ray source. The secondary star, which
is the FUor object, is thus responsible for both the X-ray emission and the
presently ongoing accretion outburst, which seem however to be unrelated
phenomena. The secondary is also known to drive a large outflow and jet, that
we detect here for the first time in X-rays. The distance of the X-ray emitting
outflow source to the central star is higher than in jets of low-mass stars.Comment: 5 pages, accepted for publication in Astronomy & Astrophysic
Characterizing the structure of diffuse emission in Hi-GAL maps
We present a study of the structure of the Galactic interstellar medium
through the Delta-variance technique, related to the power spectrum and the
fractal properties of infrared/sub-mm maps. Through this method, it is possible
to provide quantitative parameters which are useful to characterize different
morphological and physical conditions, and to better constrain the theoretical
models. In this respect, the Herschel Infrared Galactic Plane Survey carried
out at five photometric bands from 70 to 500 \mu m constitutes an unique
database for applying statistical tools to a variety of regions across the
Milky Way. In this paper, we derive a robust estimate of the power-law portion
of the power spectrum of four contiguous 2{\deg}x2{\deg} Hi-GAL tiles located
in the third Galactic quadrant (217{\deg} < l < 225{\deg}, -2{\deg} < b <
0{\deg}). The low level of confusion along the line of sight testified by CO
observations makes this region an ideal case. We find very different values of
the power spectrum slope from tile to tile but also from wavelength to
wavelength (2 < \beta < 3), with similarities between fields attributable to
components located at the same distance. Thanks to the comparison with models
of turbulence, an explanation of the determined slopes in terms of the fractal
geometry is also provided, and possible relations with the underlying physics
are investigated. In particular, an anti-correlation between ISM fractal
dimension and star formation efficiency is found for the two main distance
components observed in these fields. A possible link between the fractal
properties of the diffuse emission and the resulting clump mass function is
discussed.Comment: Accepted by Ap
The spine of the swan: A Herschel study of the DR21 ridge and filaments in Cygnus X
In order to characterise the cloud structures responsible for the formation
of high-mass stars, we present Herschel observations of the DR21 environment.
Maps of the column density and dust temperature unveil the structure of the
DR21 ridge and several connected filaments. The ridge has column densities
larger than 1e23/cm^2 over a region of 2.3 pc^2. It shows substructured column
density profiles and branching into two major filaments in the north. The
masses in the studied filaments range between 130 and 1400 Msun whereas the
mass in the ridge is 15000 Msun. The accretion of these filaments onto the DR21
ridge, suggested by a previous molecular line study, could provide a continuous
mass inflow to the ridge. In contrast to the striations seen in e.g., the
Taurus region, these filaments are gravitationally unstable and form cores and
protostars. These cores formed in the filaments potentially fall into the
ridge. Both inflow and collisions of cores could be important to drive the
observed high-mass star formation. The evolutionary gradient of star formation
running from DR21 in the south to the northern branching is traced by
decreasing dust temperature. This evolution and the ridge structure can be
explained by two main filamentary components of the ridge that merged first in
the south.Comment: 8 pages, 5 figures, accepted for publication as a Letter in Astronomy
and Astrophysic
Cluster-formation in the Rosette molecular cloud at the junctions of filaments
For many years feedback processes generated by OB-stars in molecular clouds,
including expanding ionization fronts, stellar winds, or UV-radiation, have
been proposed to trigger subsequent star formation. However, hydrodynamic
models including radiation and gravity show that UV-illumination has little or
no impact on the global dynamical evolution of the cloud. The Rosette molecular
cloud, irradiated by the NGC2244 cluster, is a template region for triggered
star-formation, and we investigated its spatial and density structure by
applying a curvelet analysis, a filament-tracing algorithm (DisPerSE), and
probability density functions (PDFs) on Herschel column density maps, obtained
within the HOBYS key program. The analysis reveals not only the filamentary
structure of the cloud but also that all known infrared clusters except one lie
at junctions of filaments, as predicted by turbulence simulations. The PDFs of
sub-regions in the cloud show systematic differences. The two UV-exposed
regions have a double-peaked PDF we interprete as caused by shock compression.
The deviations of the PDF from the log-normal shape typically associated with
low- and high-mass star-forming regions at Av~3-4m and 8-10m, respectively, are
found here within the very same cloud. This shows that there is no fundamental
difference in the density structure of low- and high-mass star-forming regions.
We conclude that star-formation in Rosette - and probably in high-mass
star-forming clouds in general - is not globally triggered by the impact of
UV-radiation. Moreover, star formation takes place in filaments that arose from
the primordial turbulent structure built up during the formation of the cloud.
Clusters form at filament mergers, but star formation can be locally induced in
the direct interaction zone between an expanding HII--region and the molecular
cloud.Comment: A&A Letter, in pres
The <i>Herschel</i> view of the massive star-forming region NGC 6334
Aims: Fundamental to any theory of high-mass star formation are gravity and turbulence. Their relative importance, which probably changes during cloud evolution, is not known. By investigating the spatial and density structure of the high-mass star-forming complex NGC 6334 we aim to disentangle the contributions of turbulence and gravity.
Methods: We used Herschel PACS and SPIRE imaging observations from the HOBYS key programme at wavelengths of 160, 250, 350, and 500 μm to construct dust temperature and column density maps. Using probability distribution functions (PDFs) of the column density determined for the whole complex and for four distinct sub-regions (distinguished on the basis of differences in the column density, temperature, and radiation field), we characterize the density structure of the complex. We investigate the spatial structure using the Δ-variance, which probes the relative amount of structure on different size scales and traces possible energy injection mechanisms into the molecular cloud.
Results: The Δ-variance analysis suggests that the significant scales of a few parsec that were found are caused by energy injection due to expanding HII regions, which are numerous, and by the lengths of filaments seen everywhere in the complex. The column density PDFs have a lognormal shape at low densities and a clearly defined power law at high densities for all sub-regions whose slope is linked to the exponent α of an equivalent spherical density distribution. In particular with α = 2.37, the central sub-region is largly dominated by gravity, caused by individual collapsing dense cores and global collapse of a larger region. The collapse is faster than free-fall (which would lead only to α = 2) and thus requires a more dynamic scenario (external compression, flows). The column density PDFs suggest that the different sub-regions are at different evolutionary stages, especially the central sub-region, which seems to be in a more evolved stage
The Pipe Nebula as seen with Herschel: Formation of filamentary structures by large-scale compression ?
A growing body of evidence indicates that the formation of filaments in
interstellar clouds is a key component of the star formation process. In this
paper, we present new Herschel PACS and SPIRE observations of the B59 and Stem
regions in the Pipe Nebula complex, revealing a rich, organized network of
filaments. The asymmetric column density profiles observed for several
filaments, along with the bow-like edge of B59, indicates that the Pipe Nebula
is being compressed from its western side, most likely by the winds from the
nearby Sco OB2 association. We suggest that this compressive flow has
contributed to the formation of some of the observed filamentary structures. In
B59, the only region of the entire Pipe complex showing star formation
activity, the same compressive flow has likely enhanced the initial column
density of the clump, allowing it to become globally gravitationally unstable.
Although more speculative, we propose that gravity has also been responsible
for shaping the converging filamentary pattern observed in B59. While the
question of the relative impact of large-scale compression and gravity remains
open in B59, large-scale compression appears to be a plausible mechanism for
the initial formation of filamentary structures in the rest of the complexComment: 9 pages, 9 figures, accepted for publication in A&
A <i>Herschel</i> and BIMA study of the sequential star formation near the W 48A H II region
We present the results of Herschel HOBYS (Herschel imaging survey of OB Young Stellar objects) photometric mapping combined with Berkeley Illinois Maryland Association (BIMA) observations and additional archival data, and perform an in-depth study of the evolutionary phases of the star-forming clumps in W 48A and their surroundings. Age estimates for the compact sources were derived from bolometric luminosities and envelope masses, which were obtained from the dust continuum emission, and agree within an order of magnitude with age estimates from molecular line and radio data. The clumps in W 48A are linearly aligned by age (east-old to west-young): we find a ultra-compact (UC) H II region, a young stellar object (YSO) with class II methanol maser emission, a YSO with a massive outflow and finally the NH2D prestellar cores from Pillai et al. This remarkable positioning reflects the (star) formation history of the region. We find that it is unlikely that the star formation in the W 48A molecular cloud was triggered by the UC H II region and discuss the Aquila supershell expansion as a major influence on the evolution of W 48A. We conclude that the combination of Herschel continuum data with interferometric molecular line and radio continuum data is important to derive trustworthy age estimates and interpret the origin of large-scale structures through kinematic information
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