1,354 research outputs found
A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function
NGC6231 is a massive young star cluster, near the center of the Sco OB1
association. While its OB members are well studied, its low-mass population has
received little attention. We present high-spatial resolution Chandra ACIS-I
X-ray data, where we detect 1613 point X-ray sources. Our main aim is to
clarify global properties of NGC6231 down to low masses through a detailed
membership assessment, and to study the cluster stars' spatial distribution,
the origin of their X-ray emission, the cluster age and formation history, and
initial mass function. We use X-ray data, complemented by optical/IR data, to
establish cluster membership. The spatial distribution of different stellar
subgroups also provides highly significant constraints on cluster membership,
as does the distribution of X-ray hardness. We perform spectral modeling of
group-stacked X-ray source spectra. We find a large cluster population down to
~0.3 Msun (complete to ~1 Msun), with minimal non-member contamination, with a
definite age spread (1-8 Myrs) for the low-mass PMS stars. We argue that
low-mass cluster stars also constitute the majority of the few hundreds
unidentified X-ray sources. We find mass segregation for the most massive
stars. The fraction of circumstellar-disk bearing members is found to be ~5%.
Photoevaporation of disks under the action of massive stars is suggested by the
spatial distribution of the IR-excess stars. We also find strong Halpha
emission in 9% of cluster PMS stars. The dependence of X-ray properties on
mass, stellar structure, and age agrees with extrapolations based on other
young clusters. The cluster initial mass function, computed over ~2 dex in
mass, has a slope Gamma~-1.14. The total mass of cluster members above 1 Msun
is 2280 Msun, and the inferred total mass is 4380 Msun. We also study the
peculiar, hard X-ray spectrum of the Wolf-Rayet star WR79.Comment: 25 pages, 36 figures, accepted for publication on Astronomy and
Astrophysic
Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks
We investigate the evolution of X-ray stellar activity from the age of the
youngest known star forming regions (SFR), < 1Myr, to about 100 Myr, i.e. the
zero age main sequence (ZAMS) for a ~ 1M_sun star. We consider five SFR of
varying age (Rho Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I,
and Eta Chamaeleontis) and two young clusters (the Pleiades and NGC 2516).
Optical and X-ray data for these regions are retrieved both from archival
observations and recent literature, and reanalyzed here in a consistent manner
so to minimize systematic differences in the results. We study trends of L_X
and L_X/L_bol as a function of stellar mass and association age. For low mass
stars (M < 1M_sun) we observe an increase in L_X/L_bol in the first 3-4 Myr and
a subsequent leveling off at the saturation level (L_X/L_bol ~ -3). Slowly
evolving very low mass stars then retain saturated levels down to the oldest
ages here considered, while for higher mass stars activity begins to decline at
some age after ~10^7 years. We find our data consistent with the following
tentative picture: low mass PMS stars with no circumstellar accretion disk have
saturated activity, consistently with the activity-Rossby number relation
derived for MS stars. Accretion and/or the presence of disks somehow lowers the
observed activity levels; disk dissipation and/or the decrease of mass
accretion rate in the first few Myrs of PMS evolution is therefore responsible
for the observed increase of L_X/L_bol with time.Comment: 17 pages, 8 figures, accepted for publication in A&
Dynamics of vitrimers: defects as a highway to stress relaxation
We propose a coarse-grained model to investigate stress relaxation in
star-polymer networks induced by dynamic bond exchange processes. We show how
the swapping mechanism, once activated, allows the network to reconfigure,
exploring distinct topological configurations, all of them characterised by
complete extent of reaction. Our results reveal the important role played by
topological defects in mediating the exchange reaction and speeding up stress
relaxation. The model provides a representation of the dynamics in vitrimers, a
new class of polymers characterized by bond swap mechanisms which preserve the
total number of bonds, as well as in other bond-exchange materials.Comment: 5 pages, 5 figures, with 6 pages SI appende
String attractors and combinatorics on words
The notion of string attractor has recently been introduced in [Prezza, 2017] and studied in [Kempa and Prezza, 2018] to provide a unifying framework for known dictionary-based compressors. A string attractor for a word w = w[1]w[2] · · · w[n] is a subset Γ of the positions 1, . . ., n, such that all distinct factors of w have an occurrence crossing at least one of the elements of Γ. While finding the smallest string attractor for a word is a NP-complete problem, it has been proved in [Kempa and Prezza, 2018] that dictionary compressors can be interpreted as algorithms approximating the smallest string attractor for a given word. In this paper we explore the notion of string attractor from a combinatorial point of view, by focusing on several families of finite words. The results presented in the paper suggest that the notion of string attractor can be used to define new tools to investigate combinatorial properties of the words
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
X-ray observations of IC348 in light of an updated cluster census
IC348 is an excellent laboratory for studies of low-mass star formation being
nearby, compact and rich. A Chandra observation was carried out early in the
satellite's lifetime. The extensive new data in optical and infrared
wavelengths accumulated in subsequent years have changed the cluster census
calling for a re-analysis of the X-ray data.Comment: poster paper to appear in Proc. of the 15th Workshop on Cool Stars,
Stellar Systems and the Su
Aging in short-ranged attractive colloids: A numerical study
We study the aging dynamics in a model for dense simple liquids, in which
particles interact through a hard-core repulsion complemented by a short-ranged
attractive potential, of the kind found in colloidal suspensions. In this
system, at large packing fractions, kinetically arrested disordered states can
be created both on cooling (attractive glass) and on heating (repulsive glass).
The possibility of having two distinct glasses, at the same packing fraction,
with two different dynamics offers the unique possibility of comparing --
within the same model -- the differences in aging dynamics. We find that, while
the aging dynamics of the repulsive glass is similar to the one observed in
atomic and molecular systems, the aging dynamics of the attractive glass shows
novel unexpected features.Comment: 8 pages, 11 figures, submited to Journal of Chemical Physic
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
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