6,581 research outputs found
The Magellanic Bridge cluster NGC 796: Deep optical AO imaging reveals the stellar content and initial mass function of a massive open cluster
NGC 796 is a massive young cluster located 59 kpc from us in the diffuse
intergalactic medium of the 1/5-1/10 Magellanic Bridge, allowing to
probe variations in star formation and stellar evolution processes as a
function of metallicity in a resolved fashion, providing a link between
resolved studies of nearby solar-metallicity and unresolved distant metal-poor
clusters located in high-redshift galaxies. In this paper, we present adaptive
optics H imaging of NGC 796 (at 0.5", which is ~0.14 pc at the
cluster distance) along with optical spectroscopy of two bright members to
quantify the cluster properties. Our aim is to explore if star formation and
stellar evolution varies as a function of metallicity by comparing the
properties of NGC 796 to higher metallicity clusters. We find from isochronal
fitting of the cluster main sequence in the colour-magnitude diagram an age of
20 Myr. Based on the cluster luminosity function, we derive a
top-heavy stellar initial mass function (IMF) with a slope =
1.990.2, hinting at an metallicity and/or environmental dependence of the
IMF which may lead to a top-heavy IMF in the early Universe. Study of the
H emission line stars reveals that Classical Be stars constitute a
higher fraction of the total B-type stars when compared with similar clusters
at greater metallicity, providing some support to the chemically homogeneous
theory of stellar evolution. Overall, NGC 796 has a total estimated mass of
990 , and a core radius of 1.40.3 pc which classifies
it as a massive young open cluster, unique in the diffuse interstellar medium
of the Magellanic Bridge.Comment: Accepted for publication in the Astrophysical Journal. Contains 14
pages, 11 figures, and 3 table
A pilot search for mm-wavelength recombination lines from emerging ionized winds in pre-planetary nebulae candidates
We report the results from a pilot search for radio recombination line (RRL)
emission at millimeter wavelengths in a small sample of pre-planetary nebulae
(pPNe) and young PNe (yPNe) with emerging central ionized regions. Observations
of the H30\alpha, H31a, H39a, H41a, H48b, H49b, H51b, and H55g lines at 1 and
3mm have been performed with the IRAM 30 m radio telescope. These lines are
excellent probes of the dense inner (<~150 au) and heavily obscured regions of
these objects, where the yet unknown agents for PN-shaping originate. We
detected mm-RRLs in three objects: CRL 618, MWC 922, and M 2-9. For CRL 618,
the only pPN with previous published detections of H41a, H35a, and H30a
emission, we find significant changes in the line profiles indicating that
current observations are probing regions of the ionized wind with larger
expansion velocities and mass-loss rate than ~29 years ago. In the case of MWC
922, we observe a drastic transition from single-peaked profiles at 3mm to
double-peaked profiles at 1mm, which is consistent with maser amplification of
the highest frequency lines; the observed line profiles are compatible with
rotation and expansion of the ionized gas, probably arranged in a disk+wind
system around a ~5-10 Msun central mass. In M 2-9, the mm-RRL emission appears
to be tracing a recent mass outburst by one of the stars of the central binary
system. We present the results from non-LTE line and continuum radiative
transfer models, which enables us to constrain the structure, kinematics, and
physical conditions (electron temperature and density) of the ionized cores of
our sample. (abridged). We deduce mass-loss rates of ~1e-6-1e-7 Msun/yr, which
are significantly higher than the values adopted by stellar evolution models
currently in use and would result in a transition from the asymptotic giant
branch to the PN phase faster than hitherto assumed.Comment: Accepted by Astronomy and Astrophysics. 28 pages, including figure
Time-dependent density functional theory on a lattice
A time-dependent density functional theory (TDDFT) for a quantum many-body
system on a lattice is formulated rigorously. We prove the uniqueness of the
density-to-potential mapping and demonstrate that a given density is
-representable if the initial many-body state and the density satisfy
certain well defined conditions. In particular, we show that for a system
evolving from its ground state any density with a continuous second time
derivative is -representable and therefore the lattice TDDFT is guaranteed
to exist. The TDDFT existence and uniqueness theorem is valid for any connected
lattice, independently of its size, geometry and/or spatial dimensionality. The
general statements of the existence theorem are illustrated on a pedagogical
exactly solvable example which displays all details and subtleties of the proof
in a transparent form. In conclusion we briefly discuss remaining open problems
and directions for a future research.Comment: 12 pages, 1 figur
870 micron continuum observations of the bubble-shaped nebula Gum 31
We are presenting here a study of the cold dust in the infrared ring nebula
Gum 31. We aim at deriving the physical properties of the molecular gas and
dust associated with the nebula, and investigating its correlation with the
star formation in the region, that was probably triggered by the expansion of
the ionization front. We use 870 micron data obtained with LABOCA to map the
dust emission. The obtained LABOCA image was compared to archival IR,radio
continuum, and optical images. The 870 micron emission follows the 8 micron
(Spitzer), 250 micron, and 500 micron (Herschel) emission distributions showing
the classical morphology of a spherical shell. We use the 870 micron and 250
micron images to identify 60 dust clumps in the collected layers of molecular
gas using the Gaussclumps algorithm. The clumps have effective deconvolved
radii between 0.16 pc and 1.35 pc, masses between 70 Mo and 2800 Mo, and volume
densities between 1.1x10^3 cm^-3 and 2.04x10^5 cm^-3. The total mass of the
clumps is 37600 Mo. The dust temperature of the clumps is in the range from 21
K to 32 K, while inside the HII region reaches ~ 40 K. The clump mass
distribution is well-fitted by a power law dN/dlog(M/Mo) proportional to
M^(-alpha), with alpha=0.93+/-0.28. The slope differs from those obtained for
the stellar IMF in the solar neighborhood, suggesting that the clumps are not
direct progenitors of single stars/protostars. The mass-radius relationship for
the 41 clumps detected in the 870 microns emission shows that only 37% of them
lie in or above the high-mass star formation threshold, most of them having
candidate YSOs projected inside. A comparison of the dynamical age of the HII
region with the fragmentation time, allowed us to conclude that the collect and
collapse mechanism may be important for the star formation at the edge of Gum
31, although other processes may also be acting.Comment: 15 pages, 10 figures. Accepted for publication in A&
Molecular gas and star formation towards the IR dust bubble S24 and its environs
We present a multi-wavelength analysis of the infrared dust bubble S24, and
its environs, with the aim of investigating the characteristics of the
molecular gas and the interstellar dust linked to them, and analyzing the
evolutionary status of the young stellar objects (YSOs) identified there. Using
APEX data, we mapped the molecular emission in the CO(2-1), CO(2-1),
CO(2-1), and CO(3-2) lines in a region of about 5'x 5' in size
around the bubble. The cold dust distribution was analyzed using ATLASGAL and
Herschel images. Complementary IR and radio data were also used.The molecular
gas linked to the S24 bubble, G341.220-0.213, and G341.217-0.237 has velocities
between -48.0 km sec and -40.0 km sec. The gas distribution
reveals a shell-like molecular structure of 0.8 pc in radius bordering
the bubble. A cold dust counterpart of the shell is detected in the LABOCA and
Herschel images.The presence of extended emission at 24 m and radio
continuum emission inside the bubble indicates that the bubble is a compact HII
region. Part of the molecular gas bordering S24 coincides with the extended
infrared dust cloud SDC341.194-0.221. A cold molecular clump is present at the
interface between S24 and G341.217-0.237. As regards G341.220-0.213, the
presence of an arc-like molecular structure at the northern and eastern
sections of this IR source indicates that G341.220-0.213 is interacting with
the molecular gas. Several YSO candidates are found to be linked to the IR
extended sources, thus confirming their nature as active star-forming regions.
The total gas mass in the region and the H ambient density amount to 10300
M and 5900 cm, indicating that G341.220-0.213, G341.217-0.237,
and the S24 HII region are evolving in a high density medium. A triggering star
formation scenario is also investigated.Comment: 17 pages, 16 figures. Submitted to A&A. Revised according to the
referee repor
Floquet Prethermalization in a Bose-Hubbard System
Periodic driving has emerged as a powerful tool in the quest to engineer new
and exotic quantum phases. While driven many-body systems are generically
expected to absorb energy indefinitely and reach an infinite-temperature state,
the rate of heating can be exponentially suppressed when the drive frequency is
large compared to the local energy scales of the system -- leading to
long-lived 'prethermal' regimes. In this work, we experimentally study a
bosonic cloud of ultracold atoms in a driven optical lattice and identify such
a prethermal regime in the Bose-Hubbard model. By measuring the energy
absorption of the cloud as the driving frequency is increased, we observe an
exponential-in-frequency reduction of the heating rate persisting over more
than 2 orders of magnitude. The tunability of the lattice potentials allows us
to explore one- and two-dimensional systems in a range of different interacting
regimes. Alongside the exponential decrease, the dependence of the heating rate
on the frequency displays features characteristic of the phase diagram of the
Bose-Hubbard model, whose understanding is additionally supported by numerical
simulations in one dimension. Our results show experimental evidence of the
phenomenon of Floquet prethermalization, and provide insight into the
characterization of heating for driven bosonic systems
The formation and disintegration of magnetic bright points observed by Sunrise/IMaX
The evolution of the physical parameters of magnetic bright points (MBPs)
located in the quiet Sun (mainly in the interwork) during their lifetime is
studied. First we concentrate on the detailed description of the magnetic field
evolution of three MBPs. This reveals that individual features follow
different, generally complex, and rather dynamic scenarios of evolution. Next
we apply statistical methods on roughly 200 observed MBP evolutionary tracks.
MBPs are found to be formed by the strengthening of an equipartition field
patch, which initially exhibits a moderate downflow. During the evolution,
strong downdrafts with an average velocity of 2.4 km/s set in. These flows,
taken together with the concurrent strengthening of the field, suggest that we
are witnessing the occurrence of convective collapses in these features,
although only 30% of them reach kG field strengths. This fraction might turn
out to be larger when the new 4 m class solar telescopes are operational as
observations of MBPs with current state of the art instrumentation could still
be suffering from resolution limitations. Finally, when the bright point
disappears (although the magnetic field often continues to exist) the magnetic
field strength has dropped to the equipartition level and is generally somewhat
weaker than at the beginning of the MBP's evolution. Noteworthy is that in
about 10% of the cases we observe in the vicinity of the downflows small-scale
strong (exceeding 2 km/s) intergranular upflows related spatially and
temporally to these downflows.Comment: 19 pages, 13 figures; final version published in "The Astrophysical
Journal
Forum participation plugin for Moodle: development and discussion
At present, a large amount of software has been created to analyze social networks, such as libraries to access online social networking APIs, software to draw graphs and tools to use and analyze networks. In fact, and because of the use of Moodle as standard Learning Management System at the University of Las Palmas de Gran Canaria, in 2009 was born the idea of creating a plugin for Moodle capable of analyzing forums in which students participate and of identifying the major players within the student network. This work is about the present state of such plugin, which provides useful information to teachers so that, through the use of social network analysis, allows them to make decisions to improve and promote participatory education. Here, we show the application of the plugin to three case studies, in two different universities, which allowed to evaluate its usefulness and to compare the information according to the variables that influenced each case study
Nanosized superconducting constrictions
Nanowires of lead between macroscopic electrodes are produced by means of an
STM. Magnetic fields may destroy the superconductivity in the electrodes, while
the wire remains in the superconducting state. The properties of the resulting
microscopic Josephson junctions are investigated.Comment: 3 pages,3 eps figures include
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