8,890 research outputs found
Pressure-induced structural, electronic, and magnetic effects in BiFeO3
We present a first-principles study of multiferroic BiFeO3 at high pressures.
Our work reveals the main structural (change in Bi's coordination and loss of
ferroelectricity), electronic (spin crossover and metallization), and magnetic
(loss of order) effects favored by compression and how they are connected. Our
results are consistent with the striking manifold transition observed
experimentally by Gavriliuk et al. [Phys. Rev. B 77, 155112 (2008)] and provide
an explanation for it.Comment: 4 pages with 4 figures embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
The nearby Galaxy structure toward the Vela Gum nebula
We report on photometry and spectroscopy for MK classification
purposes carried out in the fields of five open clusters projected against the
Vela Gum in the Third Galactic Quadrant of the Galaxy. They are Ruprecht 20,
Ruprecht 47, Ruprecht 60, NGC 2660 and NGC 2910. We could improve/confirm the
parameters of these objects derived before. The spectroscopic parallax method
has been applied to several stars located in the fields of four out of the five
clusters to get their distances and reddenings. With this method we found two
blue stars in the field of NGC 2910 at distances that make them likely members
of Vela OB1 too. Also, projected against the fields of Ruprecht 20 and Ruprecht
47 we have detected other young stars favoring not only the existence of Puppis
OB1 and OB2 but conforming a young stellar group at kpc from the Sun
and extending for more than 6 kpc outward the Galaxy. If this is the case,
there is a thickening of the thin Galactic disk of more than 300 pc at just 2-3
kpc from the Sun. Ruprecht 60 and NGC 2660 are too old objects that have no
physical relation with the associations under discussion. An astonishing result
has been the detection in the background of Ruprecht 47 of a young star at the
impressive distance of 9.5 kpc from the Sun that could be a member of the
innermost part of the Outer Arm. Another far young star in the field of NGC
2660, at near 6.0 kpc, may become a probable member of the Perseus Arm or of
the inner part of the Local Arm. The distribution of young clusters and stars
onto the Third Galactic Quadrant agrees with recent findings concerning the
extension of the Local Arm as revealed by parallaxes of regions of star
formation. We show evidences too that added to previous ones found by our group
explain the thickening of the thin disk as a combination of flare and warp.Comment: Accepted for publication in New Astronom
Photometric study of the young open cluster NGC 3293
Deep and extensive CCD photometric observations were
carried out in the area of the open cluster NGC 3293. The new data set allows
to see the entire cluster sequence down to , revealing that
stars with are evolving off the main sequence; stars with are
placed above it. According to our analysis, the cluster distance is () and its nuclear age is . NGC 3293 contains an important fraction of pre--main sequence (PMS) stars
distributed along a parallel band to the ZAMS with masses from 1 to 2.5 \cal
M_{\sun} and a mean contraction age of . This last value does not
differ too much from the nuclear age estimate. Actually, if we take into
account the many factors that may affect the PMS star positions onto the
colour--magnitude diagram, both ages can be perfectly reconciled. The star
formation rate, on the other hand, suggests that NGC 3293 stars formed surely
in one single event, therefore favouring a coeval process of star formation.
Besides, using the data, we detected nineteen stars with signs of
having emission in the region of NGC 3293, giving another
indication that the star formation process is still active in the region. The
computed initial mass function for the cluster has a slope value , a bit flatter than the typical slope for field stars and similar to the
values found for other young open clusters.Comment: 17 pages, 13 eps figures, in press in Astronomy and Astrophysic
Photometry of a Galactic field at l = 232, b = -6. The old open cluster Auner 1, the Norma-Cygnus spiral arm and the signature of the warped Galactic Thick Disk
We perform a detailed photometric study of the stellar populations in a
Galactic Field at l = 232, b = -6 in the Canis Major (CMa) constellation. We
present the first U,B,V,I photometry of the old open cluster Auner1 and
determine it to be 3.25 Gyr old and to lie at 8.9 kpc from the Sun. In the
background of the cluster, at more than 9 kpc, we detect a young population
most probably associated to the Norma Cygnus spiral arm. Furthermore, we detect
the signature of an older population and identify its Turn Off and Red Giant
Branch. This population is found to have a mean age of 7 Gyrs and a mean
metallicity of Z = 0.006 . We reconstruct the geometry of the stellar
distribution and argue that this older population - often associated to the
Canis Major {\it galaxy}- belongs in fact to the warped old thin/thick disk
component along this line of sight.Comment: 19 pages, 7 eps figures (some degraded), accepted for publication in
the Astronomical Journa
A Photometric Study of the Outer Halo Globular Cluster NGC 5824
Multi-wavelength CCD photometry over 21 years has been used to produce deep
color-magnitude diagrams together with light curves for the variables in the
Galactic globular cluster NGC 5824. Twenty-one new cluster RR Lyrae stars are
identified, bringing the total to 47, of which 42 have reliable periods
determined for the first time. The color-magnitude diagram is matched using
BaSTI isochrones with age of ~Gyr. and reddening is found to be ; using the period-Wesenheit relation in two colors the distance
modulus is corresponding to a distance of 30.9 Kpc.
The observations show no signs of populations that are significantly younger
than the ~Gyr stars. The width of the red giant branch does not allow for a
spread in [Fe/H] greater than dex, and there is no photometric
evidence for widened or parallel sequences. The pseudo-color
magnitude diagram shows a bifurcation of the red giant branch that by analogy
with other clusters is interpreted as being due to differing spectral
signatures of the first (75\%) and second (25\%) generations of stars whose age
difference is close enough that main sequence turnoffs in the color-magnitude
diagram are unresolved. The cluster main sequence is visible against the
background out to a radial distance of arcmin. We conclude that NGC
5824 appears to be a classical Oosterhoff Type II globular cluster, without
overt signs of being a remnant of a now-disrupted dwarf galaxy.Comment: 26 pages, 15 figures, 4 tables, accepted for publication in
Astronomical Journa
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