10,590 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
Molecular gas associated with IRAS 10361-5830
We analyze the distribution of the molecular gas and the dust in the
molecular clump linked to IRAS 10361-5830, located in the environs of the
bubble-shaped HII region Gum 31 in the Carina region, with the aim of
determining the main parameters of the associated material and investigating
the evolutionary state of the young stellar objects identified there.
Using the APEX telescope, we mapped the molecular emission in the J=3-2
transition of three CO isotopologues, 12CO, 13CO and C18O, over a 1.5' x 1.5'
region around the IRAS position. We also observed the high density tracers CS
and HCO+ toward the source. The cold dust distribution was analyzed using
submillimeter continuum data at 870 \mu\ obtained with the APEX telescope.
Complementary IR and radio data at different wavelengths were used to complete
the study of the ISM.
The molecular gas distribution reveals a cavity and a shell-like structure of
~ 0.32 pc in radius centered at the position of the IRAS source, with some
young stellar objects (YSOs) projected onto the cavity. The total molecular
mass in the shell and the mean H volume density are ~ 40 solar masses and
~(1-2) x 10 cm, respectively. The cold dust counterpart of the
molecular shell has been detected in the far-IR at 870 \mu\ and in Herschel
data at 350 \mu. Weak extended emission at 24 \mu\ from warm dust is projected
onto the cavity, as well as weak radio continuum emission.
A comparison of the distribution of cold and warm dust, and molecular and
ionized gas allows us to conclude that a compact HII region has developed in
the molecular clump, indicating that this is an area of recent massive star
formation. Probable exciting sources capable of creating the compact HII region
are investigated. The 2MASS source 10380461-5846233 (MSX G286.3773-00.2563)
seems to be responsible for the formation of the HII region.Comment: Accepted in A&A. 11 pages, 10 Postscript figure
Transient charge and energy flow in the wide-band limit
The wide-band limit is a commonly used approximation to analyze transport
through nanoscale devices. In this work we investigate its applicability to the
study of charge and heat transport through molecular break junctions exposed to
voltage biases and temperature gradients. We find that while this approximation
faithfully describes the long-time charge and heat transport, it fails to
characterize the short-time behavior of the junction. In particular, we find
that the charge current flowing through the device shows a discontinuity when a
temperature gradient is applied, while the energy flow is discontinuous when a
voltage bias is switched on and even diverges when the junction is exposed to
both a temperature gradient and a voltage bias. We provide an explanation for
this pathological behavior and propose two possible solutions to this problem.Comment: 11 pages, 9 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
Germanene: a novel two-dimensional Germanium allotrope akin to Graphene and Silicene
Using a gold (111) surface as a substrate we have grown in situ by molecular
beam epitaxy an atom-thin, ordered, two-dimensional multi-phase film. Its
growth bears strong similarity with the formation of silicene layers on silver
(111) templates. One of the phases, forming large domains, as observed in
Scanning Tunneling Microscopy, shows a clear, nearly flat, honeycomb structure.
Thanks to thorough synchrotron radiation core-level spectroscopy measurements
and advanced Density Functional Theory calculations we can identify it to a
xR(30{\deg}) germanene layer in coincidence with a
xR(19.1{\deg}) Au(111) supercell, thence, presenting the
first compelling evidence of the birth of a novel synthetic germanium-based
cousin of graphene.Comment: 16 pages, 4 figures, 1 tabl
Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
A significant fraction of the molecular gas in star-forming regions is
irradiated by stellar UV photons. In these environments, the electron density
(n_e) plays a critical role in the gas dynamics, chemistry, and collisional
excitation of certain molecules. We determine n_e in the prototypical strongly
irradiated photodissociation region (PDR), the Orion Bar, from the detection of
new millimeter-wave carbon recombination lines (mmCRLs) and existing far-IR
[13CII] hyperfine line observations. We detect 12 mmCRLs (including alpha,
beta, and gamma transitions) observed with the IRAM 30m telescope, at ~25''
angular resolution, toward the H/H2 dissociation front (DF) of the Bar. We also
present a mmCRL emission cut across the PDR. These lines trace the C+/C/CO gas
transition layer. As the much lower frequency carbon radio recombination lines,
mmCRLs arise from neutral PDR gas and not from ionized gas in the adjacent HII
region. This is readily seen from their narrow line profiles (dv=2.6+/-0.4
km/s) and line peak LSR velocities (v_LSR=+10.7+/-0.2 km/s). Optically thin
[13CII] hyperfine lines and molecular lines - emitted close to the DF by trace
species such as reactive ions CO+ and HOC+ - show the same line profiles. We
use non-LTE excitation models of [13CII] and mmCRLs and derive n_e = 60-100
cm^-3 and T_e = 500-600 K toward the DF. The inferred electron densities are
high, up to an order of magnitude higher than previously thought. They provide
a lower limit to the gas thermal pressure at the PDR edge without using
molecular tracers. We obtain P_th > (2-4)x10^8 cm^-3 K assuming that the
electron abundance is equal or lower than the gas-phase elemental abundance of
carbon. Such elevated thermal pressures leave little room for magnetic pressure
support and agree with a scenario in which the PDR photoevaporates.Comment: Accepted for publication in A&A Letters (includes language editor
corrections
Occurrence and persistence of magnetic elements in the quiet Sun
Turbulent convection efficiently transports energy up to the solar
photosphere, but its multi-scale nature and dynamic properties are still not
fully understood. Several works in the literature have investigated the
emergence of patterns of convective and magnetic nature in the quiet Sun at
spatial and temporal scales from granular to global. Aims. To shed light on the
scales of organisation at which turbulent convection operates, and its
relationship with the magnetic flux therein, we studied characteristic spatial
and temporal scales of magnetic features in the quiet Sun. Methods. Thanks to
an unprecedented data set entirely enclosing a supergranule, occurrence and
persistence analysis of magnetogram time series were used to detect spatial and
long-lived temporal correlations in the quiet Sun and to investigate their
nature. Results. A relation between occurrence and persistence representative
for the quiet Sun was found. In particular, highly recurrent and persistent
patterns were detected especially in the boundary of the supergranular cell.
These are due to moving magnetic elements undergoing motion that behaves like a
random walk together with longer decorrelations ( h) with respect to
regions inside the supergranule. In the vertices of the supegranular cell the
maximum observed occurrence is not associated with the maximum persistence,
suggesting that there are different dynamic regimes affecting the magnetic
elements
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