13,501 research outputs found
Ricci Collineations for Non-Degenerate, Diagonal and Spherically Symmetric Ricci Tensors
The expression of the vector field generator of a Ricci Collineation for
diagonal, spherically symmetric and non-degenerate Ricci tensors is obtained.
The resulting expressions show that the time and radial first derivatives of
the components of the Ricci tensor can be used to classify the collineation,
leading to 64 families.
Some examples illustrate how to obtain the collineation vector
Variable stars in the globular cluster M28 (NGC 6626)
We present a new search for variable stars in the Galactic globular cluster
M28 (NGC 6626). The search is based on a series of BVI images obtained with the
SMARTS Consortium's 1.3m telescope at Cerro Tololo Inter-American Observatory,
Chile. The search was carried out using the ISIS v2.2 image subtraction
package. We find a total of 25 variable stars in the field of the cluster, 9
being new discoveries. Of the newly found variables, 1 is an ab-type RR Lyrae
star, 6 are c-type RR Lyrae, and 2 are long-period/semi-regular variables. V22,
previously classified as a type II Cepheid, appears as a bona-fide RRc in our
data. In turn, V20, previously classified as an ab-type RR Lyrae, could not be
properly phased with any reasonable period. The properties of the ab-type RR
Lyrae stars in M28 appear most consistent with an Oosterhoff-intermediate
classification, which is unusual for bona-fide Galactic globulars clusters.
However, the cluster's c-type variables do not clearly support such an
Oosterhoff type, and a hybrid Oosterhoff I/II system is accordingly another
possibility, thus raising the intriguing possibility of multiple populations
being present in M28. Coordinates, periods, and light curves in differential
fluxes are provided for all the detected variables.Comment: A&A, in pres
Further ALMA observations and detailed modeling of the Red Rectangle
We present new high-quality ALMA observations of the Red Rectangle (a well
known post-AGB object) in C17O J=6-5 and H13CN J=4-3 line emission and results
from a new reduction of already published 13CO J=3-2 data. A detailed model
fitting of all the molecular line data, including previous maps and single-dish
spectra, was performed using a sophisticated code. These observations and the
corresponding modeling allowed us to deepen the analysis of the nebular
properties. We also stress the uncertainties in the model fitting.
We confirm the presence of a rotating equatorial disk and an outflow, which
is mainly formed of gas leaving the disk. The mass of the disk is ~ 0.01 Mo,
and that of the CO-rich outflow is ~ 10 times smaller. High temperatures of ~
100 K are derived for most components. From comparison of the mass values, we
roughly estimate the lifetime of the rotating disk, which is found to be of
about 10000 yr. Taking data of a few other post-AGB composite nebulae into
account, we find that the lifetimes of disks around post-AGB stars typically
range between 5000 and more than 20000 yr. The angular momentum of the disk is
found to be high, ~ 9 Mo AU km/s, which is comparable to that of the stellar
system at present. Our observations of H13CN show a particularly wide velocity
dispersion and indicate that this molecule is only abundant in the inner
Keplerian disk, at ~ 60 AU from the stellar system. We suggest that HCN is
formed in a dense photodissociation region (PDR) due to the UV excess known to
be produced by the stellar system, following chemical mechanisms that are well
established for interstellar medium PDRs and disks orbiting young stars. We
further suggest that this UV excess could lead to the efficient formation and
excitation of PAHs and other C-bearing macromolecules, whose emission is very
intense in the optical counterpart.Comment: Astronomy & Astrohysics, in press; 17 pages, 18 figures, 1 tabl
ALMA observations of the Red Rectangle, a preliminary analysis
We aim to study equatorial disks in rotation and axial outflows in post-AGB
objects, as to disclose the formation and shaping mechanisms in planetary
nebulae. So far, both disks and outflows had not been observed simultaneously.
We have obtained high-quality ALMA observations of 12CO and 13CO J=3-2 and
12CO J=6-5 line emission in the Red Rectangle, the only post-AGB/protoplanetary
object in which a disk in rotation has been mapped up to date.
These observations provide an unprecedented description of the complex
structure of this source. Together with an equatorial disk in rotation, we find
a low-velocity outflow that occupies more or less the region placed between the
disk and the optical X-shaped nebula. From our observations and preliminary
modeling of the data, we confirm the previously known properties of the disk
and obtain a first description of the structure, dynamics, and physical
conditions of the outflow.Comment: 5 pages, 5 figure
Electronic States in Diffused Quantum Wells
In the present study we calculate the energy values and the spatial
distributions of the bound electronic states in some diffused quantum wells.
The calculations are performed within the virtual crystal approximation, spin dependent empirical tight-binding model and the surface Green
function matching method. A good agreement is found between our results and
experimental data obtained for AlGaAs/GaAs quantum wells with thermally induced
changes in the profile at the interfaces. Our calculations show that for
diffusion lengths {\AA} the transition (C3-HH3) is not
sensitive to the diffusion length, but the transitions (C1-HH1), (C1-LH1),
(C2-HH2) and (C2-LH2) display large "blue shifts" as L_{D} increases. For
diffusion lengths {\AA} the transitions (C1-HH1) and (C1-LH1)
are less sensitive to the L_{D} changes than the (C3-HH3) transition. The
observed dependence is explained in terms of the bound states spatial
distributions.Comment: ReVTeX file, 7pp., no macros, 4 figures available on the reques
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