535 research outputs found
Optical spectropolarimetry of V4332 Sagittarii
The eruption of V4332 Sgr was observed in 1994. During the outburst, the
object became extremely red, so it has been considered as to belong to red
transients of the V838 Mon type. Optical spectroscopy obtained a few years
after the eruption showed a faint M-type stellar spectrum underlying numerous
molecular and atomic emission features. It has recently been suggested that the
central object in V4332 Sgr is now hidden in a dusty disc and that the
photospheric spectrum of this object observed in the optical results from
scattering of the radiation of the central star on dust grains in the
circumstellar matter. Recent polarimetric photometry has shown that the optical
radiation of the object is indeed strongly polarized. We present and analyse
our spectropolarimetric observations of V4332 Sgr obtained with the VLT in the
optical region. The optical continuum of V4332 Sgr is linearly polarized with a
typical degree of 16.6%. A clear depolarization is observed in the spectral
regions where emission features contribute significantly to the observed flux.
The only prominent exception is the CaI 6573 emission line, which is polarized
in 21%. The results of our spectropolarimetric observations are in accord with
the proposed geometry of the system and the advocated nature of the observed
optical spectrum of V4332 Sgr. The continuum is not seen directly but results
from scattering on dust within the disc and polar outflow, while most of the
gas emission comes from the polar outflow excited by the radiation field of the
central source. Additionally, the observed polarization patterns suggest a
stratification of the outflow.Comment: accepted for publication in Astronomy & Astrophysic
Evolution of the stellar-merger red nova V1309 Scorpii: SED analysis
One very important object for understanding the nature of red novae is V1309
Sco. Its pre-outburst observations showed that, before its red-nova eruption in
2008, it was a contact binary quickly evolving to the merger of the components.
It thus provided us with a direct evidence that the red novae result from
stellar mergers. We analyse the spectral energy distribution (SED) of the
object and its evolution with time. From various optical and infrared surveys
and observing programmes carried out with OGLE, HST, VVV, Gemini South, WISE,
Spitzer, and Herschel we constructed observed SED in 2010 and 2012. Some
limited data are also available for the red-nova progenitor in 2007. We analyse
the data with our model of a dusty envelope surrounding a central star. Dust
was present in the pre-outburst state of V1309 Sco. Its high temperature
(900-1000 K) suggests that this was a freshly formed dust in a presumable
mass-loss from the spiralling-in binary. Shortly after its 2008 eruption,
V1309~Sco became almost completely embedded in dust. The parameters
(temperature, dimensions) of the dusty envelope in 2010 and 2012 evidence that
we then observed matter lost by the object during the 2008 outburst. Its mass
is at least . The object remains quite luminous, although
since its maximum brightness in September 2008, it has faded in luminosity by a
factor of ~50 (in 2012). Far infrared data from Herschel reveal presence of a
cold (~30 K) dust at a distance of a few thousand AU from the object. This
conclusion could be verified by submillimeter interferometric observations.Comment: 12 pages, 7 figures, accepted in Astronomy & Astrophysisc
High-resolution optical spectroscopy of V838 Monocerotis in 2009
V838 Mon erupted at the beginning of 2002. In the course of the outburst the
object evolved to low effective temperatures and declined as a very late M-type
supergiant. Among various scenarios proposed to explain the nature of the
outburst, the most promising is a stellar merger event. We aim at studying the
structure and evolution of the object in the decline from the 2002 eruption. We
obtained spectroscopic observations of V838 Mon in January--March 2009 with
UVES/VLT. The results are analysed and compared with similar observations
obtained in October 2005 with HIRES/Keck. The most striking difference between
2009 and 2005 is a complete absence of the B3V component and of the [FeII]
emission lines in 2009. The present spectrum displays only the spectrum of the
2002 eruption remnant. It resembles that of an M6 giant, although the molecular
bands in V838 Mon are deeper than those in standard stellar spectra of a
similar spectral class. Several atomic lines, which displayed P-Cyg profiles in
2005, are now dominated by pure absorptions. Some of these lines, however, show
a narrow emission component, which, as we argue, measures the radial velocity
of V838 Mon. The resulting heliocentric velocity is 71 km/s, which very well
agrees with the velocity obtained from SiO maser observations. The atomic lines
and the molecular bands show very complex kinematics. In some atomic lines and
high-excitation molecular bands we observe matter infalling in the object
atmosphere. The infall components were already observed in 2005, but were less
pronounced and present in fewer lines than in 2009. We argue that the most
negative radial velocity components seen in the resonance atomic lines and in
the low-excitation molecular bands were formed in the ejecta of the 2002
eruption. The B3V companion most probably became engulfed in an opaque dusty
matter of the 2002 V838 Mon ejecta.Comment: A&A, in pres
Quantum constraints, Dirac observables and evolution: group averaging versus Schroedinger picture in LQC
A general quantum constraint of the form (realized in particular in Loop Quantum Cosmology models) is
studied. Group Averaging is applied to define the Hilbert space of solutions
and the relational Dirac observables. Two cases are considered. In the first
case, the spectrum of the operator is assumed to be
discrete. The quantum theory defined by the constraint takes the form of a
Schroedinger-like quantum mechanics with a generalized Hamiltonian
. In the second case, the spectrum is absolutely continuous
and some peculiar asymptotic properties of the eigenfunctions are assumed. The
resulting Hilbert space and the dynamics are characterized by a continuous
family of the Schroedinger-like quantum theories. However, the relational
observables mix different members of the family. Our assumptions are motivated
by new Loop Quantum Cosmology models of quantum FRW spacetime. The two cases
considered in the paper correspond to the negative and, respectively, positive
cosmological constant. Our results should be also applicable in many other
general relativistic contexts.Comment: RevTex4, 32 page
Widespread HCN maser emission in carbon-rich evolved stars
Context. HCN is a major constituent of the circumstellar envelopes of
carbon-rich evolved stars, and rotational lines from within its vibrationally
excited states probe parts of these regions closest to the stellar surface. A
number of such lines are known to show maser action. Historically, in one of
them, the 177 GHz line in the -doubled bending mode has
been found to show relatively strong maser action, with results only published
for a single object, the archetypical high-mass loss asymptotic giant branch
(AGB) star IRC+10216. Aims. To examine how common 177 GHz HCN maser emission
is, we conducted an exploratory survey for this line toward a select sample of
carbon-rich asymptotic giant branch stars that are observable from the southern
hemisphere. Methods. We used the Atacama Pathfinder Experiment 12 meter
submillimeter Telescope (APEX) equipped with a new receiver to simultaneously
observe three HCN rotational transitions, the and -doublet components, and the line from
the (0,0,0) ground state. Results. The maser line is
detected toward 11 of 13 observed sources, which all show emission in the
(0,0,0) transition. In most of the sources, the peak intensity of the
line rivals that of the (0,0,0) line; in two sources,
it is even stronger. Except for the object with the highest mass-loss rate,
IRC+10216, the line covers a smaller velocity range
than the (0,0,0) line. Conclusions. Maser emission in the 177 GHz
line of HCN appears to be common in
carbon-rich AGB stars. (Abbreviated)Comment: 12 pages (including appendix), 3 figures / Astronomy & Astrophysics
(in press
Towards uniqueness of degenerate axially symmetric Killing horizon
We examine the linearized equations around extremal Kerr horizon and give
some arguments towards stability of the horizon with respect to generic
(non-symmetric) linear perturbation of near horizon geometry.Comment: 17 page
Space-time directional Lyapunov exponents for cellular automata
Space-time directional Lyapunov exponents are introduced. They describe the
maximal velocity of propagation to the right or to the left of fronts of
perturbations in a frame moving with a given velocity. The continuity of these
exponents as function of the velocity and an inequality relating them to the
directional entropy is proved
Reconfiguration of Dominating Sets
We explore a reconfiguration version of the dominating set problem, where a
dominating set in a graph is a set of vertices such that each vertex is
either in or has a neighbour in . In a reconfiguration problem, the goal
is to determine whether there exists a sequence of feasible solutions
connecting given feasible solutions and such that each pair of
consecutive solutions is adjacent according to a specified adjacency relation.
Two dominating sets are adjacent if one can be formed from the other by the
addition or deletion of a single vertex.
For various values of , we consider properties of , the graph
consisting of a vertex for each dominating set of size at most and edges
specified by the adjacency relation. Addressing an open question posed by Haas
and Seyffarth, we demonstrate that is not necessarily
connected, for the maximum cardinality of a minimal dominating set
in . The result holds even when graphs are constrained to be planar, of
bounded tree-width, or -partite for . Moreover, we construct an
infinite family of graphs such that has exponential
diameter, for the minimum size of a dominating set. On the positive
side, we show that is connected and of linear diameter for any
graph on vertices having at least independent edges.Comment: 12 pages, 4 figure
Reconfiguring Independent Sets in Claw-Free Graphs
We present a polynomial-time algorithm that, given two independent sets in a
claw-free graph , decides whether one can be transformed into the other by a
sequence of elementary steps. Each elementary step is to remove a vertex
from the current independent set and to add a new vertex (not in )
such that the result is again an independent set. We also consider the more
restricted model where and have to be adjacent
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