1,225 research outputs found
Properties, evolution and morpho-kinematical modelling of the very fast nova V2672 Oph (Nova Oph 2009), a clone of U Sco
V2672 Oph reached maximum brightness V=11.35 on 2009 August 16.5. With
observed t2(V)=2.3 and t3(V)=4.2 days decline times, it is one of the fastest
known novae, being rivalled only by V1500 Cyg (1975) and V838 Her (1991) among
classical novae, and U Sco among the recurrent ones. The line of sight to the
nova passes within a few degrees of the Galactic centre. The reddening of V2672
Oph is E(B-V)=1.6 +/-0.1, and its distance ~19 kpc places it on the other side
of the Galactic centre at a galacto-centric distance larger than the solar one.
The lack of an infrared counterpart for the progenitor excludes the donor star
from being a cool giant like in RS Oph or T CrB. With close similarity to U
Sco, V2672 Oph displayed a photometric plateau phase, a He/N spectrum
classification, extreme expansion velocities and triple peaked emission line
profiles during advanced decline. The full width at zero intensity of Halpha
was 12,000 km/s at maximum, and declined linearly in time with a slope very
similar to that observed in U Sco. We infer a WD mass close to the
Chandrasekhar limit and a possible final fate as a SNIa. Morpho-kinematical
modelling of the evolution of the Halpha profile suggests that the overall
structure of the ejecta is that of a prolate system with polar blobs and an
equatorial ring. The density in the prolate system appeared to decline faster
than that in the other components. V2672 Oph is seen pole-on, with an
inclination of 0+/-6 deg and an expansion velocity of the polar blobs of 4800
+900/-800 km/s. On the basis of its remarkable similarity to U Sco, we suspect
this nova may be a recurrent. Given the southern declination, the faintness at
maximum, the extremely rapid decline and its close proximity to the Ecliptic,
it is quite possible that previous outbursts of V2672 Oph have been missed.Comment: in press in MNRA
The 2010 nova outburst of the symbiotic Mira V407 Cyg
The nova outburst experienced in 2010 by the symbiotic binary Mira V407 Cyg
has been extensively studied at optical and infrared wavelengths with both
photometric and spectroscopic observations. This outburst, reminiscent of
similar events displayed by RS Oph, can be described as a very fast He/N nova
erupting while being deeply embedded in the dense wind of its cool giant
companion. The hard radiation from the initial thermonuclear flash ionizes and
excites the wind of the Mira over great distances (recombination is observed on
a time scale of 4 days). The nova ejecta is found to progressively decelerate
with time as it expands into the Mira wind. This is deduced from line widths
which change from a FWHM of 2760 km/s on day +2.3 to 200 km/s on day +196. The
wind of the Mira is massive and extended enough for an outer neutral and
unperturbed region to survive at all outburst phases.Comment: MNRAS Letter, in pres
Historical light curve and search for previous outbursts of Nova KT Eridani (2009)
Context. Nova Eridani (2009) caught the eye of the nova community due to its
fast decline from maximum, which was initially missed, and its subsequent
development in the radio and X-ray wavelengths. This system also exhibits
properties similar to those of the much smaller class of recurrent novae;
themselves potential progenitors of Type Ia Supernovae. Aims. We aim to
determine the nature and physical parameters of the KT Eri progenitor system.
Methods. We searched the Harvard College Observatory archive plates for the
progenitor of KT Eri to determine the nature of the system, particularly the
evolutionary stage of the secondary.We used the data obtained to search for any
periodic signal and the derived luminosity to estimate a recurrence timescale.
Furthermore, by comparing the colours of the quiescent system on a
colour-magnitude diagram we may infer the nature of the secondary star.
Results. We identified the progenitor system of KT Eri and measured a quiescent
magnitude of = 14.7 \pm 0.4. No previous outburst was found. However, we
suggest that if the nova is recurrent it should be on a timescale of centuries.
We find a periodicity at quiescence of 737 days which may arise from reflection
effects and/or eclipses in the central binary. The periodicity and the
quiescence magnitude of the system suggest that the secondary star is evolved
and likely in, or ascending, the Red Giant Branch. A second period is evident
at 376 days which has a sinusoidal like light curve. Furthermore, the outburst
amplitude of ~ 9 magnitudes is inconsistent with those expected for fast
classical novae (~ 17 magnitudes) which may lend further support for an evolved
secondary. (Abridged)Comment: 6 pages, 5 figures, accepted for publication in A&
A modelâbased design floatingâpoint accumulator. Case of study: FPGA implementation of a support vector machine kernel function
Recent research in wearable sensors have led to the development of an advanced platform capable of embedding complex algorithms such as machine learning algorithms, which are known to usually be resourceâdemanding. To address the need for high computational power, one solution is to design custom hardware platforms dedicated to the specific application by exploiting, for example, Field Programmable Gate Array (FPGA). Recently, modelâbased techniques and automatic code generation have been introduced in FPGA design. In this paper, a new modelâbased floatingâpoint accumulation circuit is presented. The architecture is based on the stateâofâtheâart delayed buffering algorithm. This circuit was conceived to be exploited in order to compute the kernel function of a support vector machine. The implementation of the proposed model was carried out in Simulink, and simulation results showed that it had better performance in terms of speed and occupied area when compared to other solutions. To better evaluate its figure, a practical case of a polynomial kernel function was considered. Simulink and VHDL postâimplementation timing simulations and measurements on FPGA confirmed the good results of the standâalone accumulator
Study of a synchronization system for distributed inverters conceived for FPGA devices
In a multiple parallel-connected inverters system, limiting the circulating current phenomenon is mandatory since it may influence efficiency and reliability. In this paper, a new control method aimed at this purpose and conceived to be implemented on a Field Programmable Gate Array (FPGA) device is presented. Each of the inverters, connected in parallel, is conceived to be equipped with an FPGA that controls the Pulse-Width Modulation (PWM) waveform without intercommunication with the others. The hardware implemented is the same for every inverter; therefore, the addition of a new module does not require redesign, enhancing system modularity. The system has been simulated in a Simulink environment. To study its behavior and to improve the control method, simulations with two parallel-connected inverters have been firstly conducted, then additional simulations have been performed with increasing complexity to demonstrate the quality of the algorithm. The results prove the ability of the method proposed to limit the circulating currents to negligible values
On the Progenitor System of Nova V2491 Cygni
Nova V2491 Cyg is one of just two detected pre-outburst in X-rays. The light
curve of this nova exhibited a rare "re-brightening" which has been attributed
by some as the system being a polar, whilst others claim that a magnetic WD is
unlikely. By virtue of the nature of X-ray and spectroscopic observations the
system has been proposed as a recurrent nova, however the adoption of a 0.1 day
orbital period is generally seen as incompatible with such a system. In this
research note we address the nature of the progenitor system and the source of
the 0.1 day periodicity. Through the combination of Liverpool Telescope
observations with published data and archival 2MASS data we show that V2491
Cyg, at a distance of 10.5 - 14 kpc, is likely to be a recurrent nova of the U
Sco-class; containing a sub-giant secondary and an accretion disk, rather than
accretion directly onto the poles. We show that there is little evidence, at
quiescence, supporting a ~ 0.1 day periodicity, the variation seen at this
stage is likely caused by flickering of a re-established accretion disk. We
propose that the periodicity seen shortly after outburst is more likely related
to the outburst rather than the - then obscured - binary system. Finally we
address the distance to the system, and show that a significantly lower
distance (~ 2 kpc) would result in a severely under-luminous outburst, and as
such favour the larger distance and the recurrent nova scenario.Comment: 5 pages, 3 images, accepted for publication in A&A as a research not
CAOS spectroscopy of Am stars Kepler targets
The {\it Kepler} space mission and its {\it K2} extension provide photometric
time series data with unprecedented accuracy. These data challenge our current
understanding of the metallic-lined A stars (Am stars) for what concerns the
onset of pulsations in their atmospheres. It turns out that the predictions of
current diffusion models do not agree with observations. To understand this
discrepancy, it is of crucial importance to obtain ground-based spectroscopic
observations of Am stars in the {\it Kepler} and {\it K2} fields in order to
determine the best estimates of the stellar parameters.
In this paper, we present a detailed analysis of high-resolution
spectroscopic data for seven stars previously classified as Am stars. We
determine the effective temperatures, surface gravities, projected rotational
velocities, microturbulent velocities and chemical abundances of these stars
using spectral synthesis. These spectra were obtained with {\it CAOS}, a new
instrument recently installed at the observing station of the Catania
Astrophysical Observatory on Mt. Etna. Three stars have already been observed
during quarters Q0-Q17, namely: HD\,180347, HD\,181206, and HD\,185658, while
HD\,43509 was already observed during {\it K2} C0 campaign.
We confirm that HD\,43509 and HD\,180347 are Am stars, while HD 52403,
HD\,50766, HD\,58246, HD\,181206 and HD\,185658 are marginal Am stars. By means
of non-LTE analysis, we derived oxygen abundances from O{\sc
I}7771--5{\AA} triplet and we also discussed the results obtained with
both non-LTE and LTE approaches.Comment: accepted in MNRAS main journal 13 pages, 11 figures, 3 tables. arXiv
admin note: text overlap with arXiv:1404.095
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