4,419 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
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
Exogenous Transcription Factors Alter the Patterns of Condensed Tannin Accumulation in \u3ci\u3eLotus corniculatus\u3c/i\u3e
Spermiogenesis in the vermetid gastropod Dendropoma petraeum (Gastropoda, Prosobranchia)
The structure and maturation of the male gonad of the Mediterranean vermetid gastropod Dendropoma petraeum are described. Histological sections of the gonads were made throughout development and gonad activity was monitored at regular monthly intervals. During tha autumn monts the gonad is very small and is surrounded by a large quantity of connective tissue; it becomes more voluminous from December to August, with the highest growth peak in springtime. The stages of spermatogenesis were also observed and described
Evolution of angular-momentum-losing exoplanetary systems : Revisiting Darwin stability
We assess the importance of tidal evolution and its interplay with magnetic
braking in the population of hot-Jupiter planetary systems. By minimizing the
total mechanical energy of a given system under the constraint of stellar
angular momentum loss, we rigorously find the conditions for the existence of
dynamical equilibrium states. We estimate their duration, in particular when
the wind torque spinning down the star is almost compensated by the tidal
torque spinning it up. We introduce dimensionless variables to characterize the
tidal evolution of observed hot Jupiter systems and discuss their spin and
orbital states using generalized Darwin diagrams based on our new approach. We
show that their orbital properties are related to the effective temperature of
their host stars. The long-term evolution of planets orbiting F- and G-type
stars is significantly different owing to the combined effect of magnetic
braking and tidal dissipation. The existence of a quasi-stationary state, in
the case of short-period planets, can significantly delay their tidal evolution
that would otherwise bring the planet to fall into its host star. Most of the
planets known to orbit F-type stars are presently found to be near this
stationary state, probably in a configuration not too far from that they had
when their host star settled on the zero-age main sequence. Considering the
importance of angular momentum loss in the early stages of stellar evolution,
our results indicate that it has to be taken into account also to properly test
the migration scenarios of planetary system formation.Comment: 22 pages, 11 figures, accepted for publication in A&
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