1,001 research outputs found
Saddles and softness in simple model liquids
We report a numerical study of saddles properties of the potential energy
landscape for soft spheres with different softness, i.e. different power n of
the interparticle repulsive potential. We find that saddle-based quantities
rescale into master curves once energies and temperatures are scaled by
mode-coupling temperature T_MCT, confirming and generalizing previous findings
obtained for Lennard-Jones like models.Comment: 2 pages, 2 figure
Circumstellar Disks in the Outer Galaxy: the Star-Forming Region NGC 1893
It is still debated whether star formation process depends on environment. In
particular it is yet unclear whether star formation in the outer Galaxy, where
the environmental conditions are, theoretically, less conducive, occurs in the
same way as in the inner Galaxy. We investigate the population of NGC1893, a
young cluster ~3-4 Myr in the outer part of the Galaxy (galactic radius >11
Kpc), to explore the effects of environmental conditions on star forming
regions. We present infrared observations acquired using the IRAC camera
onboard the Spitzer Space Telescope and analyze the color-color diagrams to
establish the membership of stars with excesses. We also merge this information
with that obtained from Chandra ACIS-I observations, to identify the Class III
population. We find that the cluster is very rich, with 242 PMS Classical
T-Tauri stars and 7 Class 0/I stars. We identify 110 Class III candidate
cluster members in the ACIS-I field of view. We estimate a disk fraction for
NGC1893 of about 67%, similar to fractions calculated for nearby star forming
regions of the same age. Although environmental conditions are unfavorable,
star formation can clearly be very successful in the outer Galaxy, allowing
creation of a very rich cluster like NGC1893.Comment: 10 pages,7 figures,4 table
The stellar population of Sco OB2 revealed by Gaia DR2 data
Sco OB2 is the nearest OB association, extending over approximately 2000
sq.deg. on the sky. Only its brightest members are already known (from
Hipparcos) across its entire size, while studies of its lower-mass population
refer only to small portions of its extent. In this work we exploit the
capabilities of Gaia DR2 measurements to search for Sco OB2 members across its
entire size and down to the lowest stellar masses. We use both Gaia astrometric
and photometric data to select association members, using minimal assumptions
derived mostly from the Hipparcos studies. Gaia resolves small details in both
the kinematics of individual Sco OB2 subgroups and their distances from the
Sun. We develop methods to explore the 3D kinematics of stellar populations
covering large sky areas. We find ~11000 pre-main sequence (PMS) Sco OB2
members (with <3% contamination), plus ~3600 MS candidate members with a larger
(10-30%) field-star contamination. A higher-confidence subsample of ~9200 PMS
(and ~1340 MS) members is also selected (<1% contamination for the PMS),
affected however by larger (~15%) incompleteness. We classify separately stars
in compact and diffuse populations. Most members belong to a few kinematically
distinct diffuse populations, whose ensemble outlines the association shape.
Upper Sco is the densest part of Sco OB2, with a complex spatial and
kinematical structure, and no global pattern of motion. Other dense subclusters
are found in Upper Centaurus-Lupus and in Lower Centaurus-Crux. Most clustered
stars appear to be younger than the diffuse PMS population, suggesting star
formation in small groups which rapidly disperse and dilute, while keeping
memory of their original kinematics. We also find that the open cluster IC 2602
has a similar dynamics to Sco OB2, and its PMS members are evaporating and
forming a ~10 deg halo around its double-peaked core.Comment: 27 pages, 37 figures. Accepted for publication in Astronomy and
Astrophysic
Quasi-saddles as relevant points of the potential energy surface in the dynamics of supercooled liquids
The supercooled dynamics of a Lennard-Jones model liquid is numerically
investigated studying relevant points of the potential energy surface, i.e. the
minima of the square gradient of total potential energy . The main findings
are: ({\it i}) the number of negative curvatures of these sampled points
appears to extrapolate to zero at the mode coupling critical temperature ;
({\it ii}) the temperature behavior of has a close relationship with the
temperature behavior of the diffusivity; ({\it iii}) the potential energy
landscape shows an high regularity in the distances among the relevant points
and in their energy location. Finally we discuss a model of the landscape,
previously introduced by Madan and Keyes [J. Chem. Phys. {\bf 98}, 3342
(1993)], able to reproduce the previous findings.Comment: To be published in J. Chem. Phy
Low mass star formation and subclustering in the HII regions RCW 32, 33 and 27 of the Vela Molecular Ridge. A photometric diagnostics to identify M-type stars
Most stars born in clusters and recent results suggest that star formation
(SF) preferentially occurs in subclusters. Studying the morphology and SF
history of young clusters is crucial to understanding early SF. We identify the
embedded clusters of young stellar objects (YSOs) down to M stars, in the HII
regions RCW33, RCW32 and RCW27 of the Vela Molecular Ridge. Our aim is to
characterise their properties, such as morphology and extent of the clusters in
the three HII regions, derive stellar ages and the connection of the SF history
with the environment. Through public photometric surveys such as Gaia, VPHAS,
2MASS and Spitzer/GLIMPSE, we identify YSOs with IR, Halpha and UV excesses, as
signature of circumstellar disks and accretion. In addition, we implement a
method to distinguish M dwarfs and giants, by comparing the reddening derived
in several optical/IR color-color diagrams, assuming suitable theoretical
models. Since this diagnostic is sensitive to stellar gravity, the procedure
allows us to identify pre-main sequence stars. We find a large population of
YSOs showing signatures of circumstellar disks with or without accretion. In
addition, with the new technique of M-type star selection, we find a rich
population of young M stars with a spatial distribution strongly correlated to
the more massive population. We find evidence of three young clusters, with
different morphology. In addition, we identify field stars falling in the same
region, by securely classifying them as giants and foreground MS stars. We
identify the embedded population of YSOs, down to about 0.1 Msun, associated
with the HII regions RCW33, RCW32 and RCW27 and the clusters Vela T2, Cr197 and
Vela T1, respectively, showing very different morphologies. Our results suggest
a decreasing SF rate in Vela T2 and triggered SF in Cr197 and Vela T1.Comment: Accepted for publication in A&A; 20 pages, 22 figures, 6 table
Dynamics in a supercooled molecular liquid: Theory and Simulations
We report extensive simulations of liquid supercooled states for a simple
three-sites molecular model, introduced by Lewis and Wahnstr"om [L. J. Lewis
and G. Wahnstr"om, Phys. Rev. E 50, 3865 (1994)] to mimic the behavior of
ortho-terphenyl. The large system size and the long simulation length allow to
calculate very precisely --- in a large q-vector range --- self and collective
correlation functions, providing a clean and simple reference model for
theoretical descriptions of molecular liquids in supercooled states. The time
and wavevector dependence of the site-site correlation functions are compared
with detailed predictions based on ideal mode-coupling theory, neglecting the
molecular constraints. Except for the wavevector region where the dynamics is
controlled by the center of mass (around 9 nm-1), the theoretical predictions
compare very well with the simulation data.
Test of the semischematic model for a liquid of linear molecules
We apply to a liquid of linear molecules the semischematic mode-coupling
model, previously introduced to describe the center of mass (COM) slow dynamics
of a network-forming molecular liquid. We compare the theoretical predictions
and numerical results from a molecular dynamics simulation, both for the time
and the wave-vector dependence of the COM density-density correlation function.
We discuss the relationship between the presented analysis and the results from
an approximate solution of the equations from molecular mode-coupling theory
[R. Schilling and T. Scheidsteger, Phys. Rev. E 56 2932 (1997)].Comment: Revtex, 10 pages, 4 figure
General features of the energy landscape in Lennard-Jones like model liquids
Features of the energy landscape sampled by supercooled liquids are
numerically analyzed for several Lennard-Jones like model systems. The
properties of quasisaddles (minima of the square gradient of potential energy
W=|grad V|^2), are shown to have a direct relationship with the dynamical
behavior, confirming that the quasisaddle order extrapolates to zero at the
mode-coupling temperature T_MCT. The same result is obtained either analyzing
all the minima of W or the saddles (absolute minima of W), supporting the
conjectured similarity between quasisaddles and saddles, as far as the
temperature dependence of the properties influencing the slow dynamics is
concerned. We find evidence of universality in the shape of the landscape:
plots for different systems superimpose into master curves, once energies and
temperatures are scaled by T_MCT. This allows to establish a quantitative
relationship between T_MCT and potential energy barriers for LJ-like systems,
and suggests a possible generalization to different model liquids.Comment: 8 pages, 5 figure
Relation between the High Density Phase and the Very-High Density Phase of Amorphous Solid Water
It has been suggested that high-density amorphous (HDA) ice is a structurally
arrested form of high-density liquid (HDL) water, while low-density amorphous
(LDA) ice is a structurally arrested form of low-density liquid (LDL) water.
Recent experiments and simulations have been interpreted to support the
possibility of a second "distinct" high-density structural state, named very
high-density amorphous (VHDA) ice, questioning the LDL-HDL hypothesis. We test
this interpretation using extensive computer simulations, and find that VHDA is
a more stable form of HDA and that in fact VHDA should be considered as the
amorphous ice of the quenched HDL.Comment: 5 pages, 4 fig
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