21,716 research outputs found
Near-Infrared spectroscopy of the super star cluster in NGC1705
We study the near-infrared properties of the super star cluster NGC1750-1 in
order to constrain its spatial extent, its stellar population and its age. We
use adaptive optics assisted integral field spectroscopy with SINFONI on the
VLT. We estimate the spatial extent of the cluster and extract its K-band
spectrum from which we constrain the age of the dominant stellar population.
Our observations have an angular resolution of about 0.11", providing an upper
limit on the cluster radius of 2.85+/-0.50 pc depending on the assumed
distance. The K-band spectrum is dominated by strong CO absorption bandheads
typical of red supergiants. Its spectral type is equivalent to a K4-5I star.
Using evolutionary tracks from the Geneva and Utrecht groups, we determine an
age of 12+/-6 Myr. The large uncertainty is rooted in the large difference
between the Geneva and Utrecht tracks in the red supergiants regime. The
absence of ionized gas lines in the K-band spectrum is consistent with the
absence of O and/or Wolf-Rayet stars in the cluster, as expected for the
estimated age.Comment: 5 pages, 4 figures. Research Note accepted in Astronomy and
Astrophysic
Further constraints on the optical transmission spectrum of HAT-P-1b
We report on novel observations of HAT-P-1 aimed at constraining the optical
transmission spectrum of the atmosphere of its transiting Hot-Jupiter
exoplanet. Ground-based differential spectrophotometry was performed over two
transit windows using the DOLORES spectrograph at the Telescopio Nazionale
Galileo (TNG). Our measurements imply an average planet to star radius ratio
equal to =(0.11590.0005). This result is consistent
with the value obtained from recent near infrared measurements of this object
but differs from previously reported optical measurements being lower by around
4.4 exoplanet scale heights. Analyzing the data over 5 different spectral bins
600\AA wide we observed a single peaked spectrum (3.7 level)
with a blue cut-off corresponding to the blue edge of the broad absorption wing
of sodium and an increased absorption in the region in between 6180-7400\AA. We
also infer that the width of the broad absorption wings due to alkali metals is
likely narrower than the one implied by solar abundance clear atmospheric
models. We interpret the result as evidence that HAT-P-1b has a partially clear
atmosphere at optical wavelengths with a more modest contribution from an
optical absorber than previously reported.Comment: Accepted by Ap
Bayesian acoustic prediction assimilating oceanographic and acoustically inverted data
The prediction of the transmission loss evolution on a day to week frame, in a given
oceanic area, is an important issue in modeling the sonar performance. It relies
primarily on acoustic propagation models, which convert water column and geometric/
geoacoustic parameters to ‘instantaneous’ acoustic field estimates. In practice, to model the acoustic field, even the most accurate acoustic models have to be fed with simplified environmental descriptions, due to computational issues and to a limited knowledge of the environment. This is a limitation, for example, in acoustic inversion methods, in which, by maximizing the proximity between measured and modeled acoustic signals, the estimated environmental parameters are deviated from reality, forming what is normally called an ‘acoustically equivalent environment’. This problem arises also in standard acoustic prediction, in which, the oceanographic forecasts and bottom data (typically from archives) are fed directly
to an acoustic model. The claim in the present work is that, by converting the oceanographic prediction and the bottom properties to ‘acoustically equivalent’
counterparts, the acoustic prediction can be obtained in an optimal way, adapted to the environmental model at hand. Here, acoustic prediction is formulated as a Bayesian estimation problem, in which, the observables are oceanographic forecasts,
a set of known bottom parameters, a set of acoustic data, and a set of water column
data. The predictive posterior PDF of the future acoustic signal is written as a function of elementary PDF functions relating these observables and ‘acoustically
equivalent’ environmental parameters. The latter are obtained by inversion of acoustic data. The concept is tested on simulated data based on water column measurements and forecasts for the MREA’03 sea trial.We thank the partial funding of Funda¸c˜ao para a Ciˆencia e Tecnologia - FCT
under POSI, POCTI and POCI programs, and scholarship no. SFRH/BD/9032/2002.
Acknowledgements are addressed also to Emanuel Coelho, for conducing the
MREA’03 sea trial, and to Peter Gerstoft, for prompt help and improvements
of the SAGA inversion package
Blind channel estimation with data from the INTIMATE'96 sea trial
Blind multipath channel estimation is studied by time-frequency (TF) analysis. For a linear frequency modulated source, the technique is based on its instantaneous frequency estimation, followed by an approximate formulation of matched- ltering. Tests concern at-sea recorded data during
the INTIMATE '96 experiment.Thanks are due to the INTIMATE '96 team for
the real data acquisition, to FCT, for funding, under fellowship PRAXIS XXI (BM/19298/99), and to A. Quinquis and C. Gervaise for the valuable reception at ENSIETA
From oceanographic to acoustic forecasting: acoustic model calibration using in situ acoustic measures
Sonar performance prediction relies heavily on acoustic propagation models and environmental representations of the oceanic area in which the sonar is to operate. The
performance estimate is derived from a predicted acoustic eld, which is the output of a propagation model. Though well developed nowadays, acoustic propagation modeling is limited in practice by simpli cations in the numerical methods, in the environmental structure to consider (for computational reasons), and even in the knowledge of some environmental properties. This is complicated by the fact that, in sonar performance prediction, the environmental properties need to be predicted for a far future, in the
order of hours or days. These limitations imply that the acoustic eld at the output of
the acoustic predictor is biased, in current methods. In mathematical terms, the prediction of the acoustic eld can be seen as a model parametrization problem, in which
the model is a numerical propagation model, and the parameters are environmental
descriptors which, when fed to the propagation model, best model the future acoustic field. Since the 1980's, signi cant research has been done in the development of propagation model parametrization, using techniques of the so-called \acoustic inversion" family. These techniques, having as objective the estimation of environmental properties of an oceanic area, use observed acoustic elds at the area, to be matched with candidate elds corresponding to candidate environmental pictures. At the end, the best acoustic match gives the estimated environment, in other words, the best model parameters to closely reproduce the measured acoustic eld. In the current work, the technique of acoustic inversion is used in the design of an acoustic predictor, together with oceanographic forecasts and measures. Synthetic acoustic data generated with oceanographic measures taken in the MREA'03 sea trial, is used to illustrate the proposed method. The results show that a collection of environments estimated by past
acoustic inversions, can ameliorate the acoustic estimates for future time, as compared to a conventional method
Finite orbit decomposition of endomaps
In this work we present a vectorized Matlab algorithm for the decomposition of an endomap into its finite
orbits
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