3,745 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
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
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
The candidate filament close to the 3C295 galaxy cluster: optical and X-ray spectroscopy
We present a detailed analysis of the overdensity of X-ray sources colse to
the 3C 295 galaxy cluster (z=0.46) to assess whether it is associated with a
filament of the large-scale structure of the Universe. We obtained optical
spectra of the optical counterparts of eleven sources associated with the
filament, finding that one is at z=0.474. This is a type 1 AGN at 1.5 arcmin
from the cluster center. We found three more sources with a redshift in the
range 0.37 - 0.53. We extracted the stacked X-ray spectrum of 47 X-ray sources
belonging to the putative filament. We found a significant narrow (at the
resolution of the Chandra ACIS-I) line at E ~ 4.4 keV, the energy of the iron
Kalpha line at the redshift of the cluster. The detection of this line is
confirmed at a confidence level of better than 3sigma and its energy is
constrained to be in the range 6.2--6.47 (at a 90% confidence level), excluding
an identification with the 6.7 helium-like iron line from the hot cluster ICM
at better than 4 sigma. We conclude that the detection of the redshifted line
is a strong indication that at least several of the excess sources lie at z ~
0.46 and that AGNs are efficient tracers of the ``filament'' connected with the
central cluster of galaxies.Comment: 7 Pages 9 .ps figures, A&A in pres
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