65 research outputs found
HD 46375: seismic and spectropolarimetric analysis of a young Sun hosting a Saturn-like planet
HD 46375 is known to host a Saturn-like exoplanet orbiting at 0.04 AU from
its host star. Stellar light reflected by the planet was tentatively identified
in the 34-day CoRoT run acquired in October-November 2008. We constrain the
properties of the magnetic field of HD 46375 based on spectropolarimetric
observations with the NARVAL spectrograph at the Pic du Midi observatory. In
addition, we use a high-resolution NARVAL flux spectrum to contrain the
atmospheric parameters. With these constraints, we perform an asteroseismic
analysis and modelling of HD 46375 using the frequencies extracted from the
CoRoT light curve. We used Zeeman Doppler imaging to reconstruct the magnetic
map of the stellar surface. In the spectroscopic analysis we fitted isolated
lines using 1D LTE atmosphere models. This analysis was used to constrain the
effective temperature, surface gravity, and chemical composition of the star.
To extract information about the p-mode oscillations, we used a technique based
on the envelope autocorrelation function (EACF). From the Zeeman Doppler
imaging observations, we observe a magnetic field of ~5 gauss. From the
spectral analysis, HD 46375 is inferred to be an unevolved K0 type star with
high metallicity [Fe/H]=+0.39. Owing to the relative faintness of the star
(m_hip=8.05), the signal-to-noise ratio is too low to identify individual
modes. However, we measure the p-mode excess power and large separation Delta
nu_0=153.0 +/- 0.7 muHz. We are able do constrain the fundamental parameters of
the star thanks to spectrometric and seismic analyses. We conclude that HD
46375 is similar to a young version of Alpha-CenB. This work is of special
interest because of its combination of exoplanetary science and
asteroseismology, which are the subjects of the current Kepler mission and the
proposed PLATO mission.Comment: Accepted in Astronomy & Astrophysics. 8 pages, 9 figure
Spectroscopic monitoring of the Herbig Ae star HD 104237. II. Non-radial pulsations, mode analysis and fundamental stellar parameters
Herbig Ae/Be stars are intermediate-mass pre-main sequence (PMS) stars
showing signs of intense activity and strong stellar winds, whose origin is not
yet understood in the frame of current theoretical models of stellar evolution
for young stars. The evolutionary tracks of the earlier Herbig Ae stars cross a
recently discovered PMS instability strip. Many of these stars exhibit
pulsations of delta Scuti type. HD 104237 is a well-known pulsating Herbig Ae
star. In this article, we reinvestigated an extensive high-resolution
quasi-continuous spectroscopic data set in order to search for very faint
indications of non-radial pulsations in the line profile. To do this, we worked
on dynamical spectra of equivalent photospheric (LSD) profiles of HD 104237. A
2D Fourier analysis (F2D) was performed of the entire profile and the temporal
variation of the central depth of the line was studied with the time-series
analysis tools Period04 and SigSpec. We present a mode identification
corresponding to the detected dominant frequency. We perform a new accurate
determination of the fundamental stellar parameters in view of a forthcoming
asteroseismic modeling. Following the previous studies on this star, our
analysis of the dynamical spectrum of recentered LSD profiles corresponding to
the 22nd -25th of April 1999 nights spectra has confirmed the presence of
multiple oscillation modes of low-degree l in HD 104237 and led to the first
direct detection of a non-radial pulsation mode in this star: the dominant mode
F1 was identified by the Fourier 2D method having a degree l value comprised
between 1 and 2, the symmetry of the pattern variation indicating an azimuthal
order of +1 or -1. The detailed study of the fundamental stellar parameters has
provided a Teff, log g and iron abundance of 8550 +/- 150K, 3.9 +/- 0.3 and
-4.38 +/- 0.19 (i.e. [Fe/H]=+0.16 +/- 0.19), respectively
Testing the chemical tagging technique with open clusters
Context. Stars are born together from giant molecular clouds and, if we
assume that the priors were chemically homogeneous and well-mixed, we expect
them to share the same chemical composition. Most of the stellar aggregates are
disrupted while orbiting the Galaxy and most of the dynamic information is
lost, thus the only possibility of reconstructing the stellar formation history
is to analyze the chemical abundances that we observe today.
Aims. The chemical tagging technique aims to recover disrupted stellar
clusters based merely on their chemical composition. We evaluate the viability
of this technique to recover co-natal stars that are no longer gravitationally
bound.
Methods. Open clusters are co-natal aggregates that have managed to survive
together. We compiled stellar spectra from 31 old and intermediate-age open
clusters, homogeneously derived atmospheric parameters, and 17 abundance
species, and applied machine learning algorithms to group the stars based on
their chemical composition. This approach allows us to evaluate the viability
and efficiency of the chemical tagging technique.
Results. We found that stars at different evolutionary stages have distinct
chemical patterns that may be due to NLTE effects, atomic diffusion, mixing,
and biases. When separating stars into dwarfs and giants, we observed that a
few open clusters show distinct chemical signatures while the majority show a
high degree of overlap. This limits the recovery of co-natal aggregates by
applying the chemical tagging technique. Nevertheless, there is room for
improvement if more elements are included and models are improved.Comment: accepted for publication in Astronomy and Astrophysics. Corrected
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A mathematical theory of resolution limits for super-resolution of positive sources
The superresolving capacity for number and location recoveries in the
super-resolution of positive sources is analyzed in this work. Specifically, we
introduce the computational resolution limit for respectively the number
detection and location recovery in the one-dimensional super-resolution problem
and quantitatively characterize their dependency on the cutoff frequency,
signal-to-noise ratio, and the sparsity of the sources. As a direct
consequence, we show that targeting at the sparest positive solution in the
super-resolution already provides the optimal resolution order. These results
are generalized to multi-dimensional spaces. Our estimates indicate that there
exist phase transitions in the corresponding reconstructions, which are
confirmed by numerical experiments. Our theory fills in an important puzzle
towards fully understanding the super-resolution of positive sources
Study on a miniaturized satellite payload for atmospheric temperature measurements
The atmospheric temperature reflects the thermal balance of the atmosphere and is a
valuable indicator of climate change. It has been widely recognized that the atmospheric
gravity wave activity has a profound effect on the large-scale circulation, thermal and
constituent structures in the mesosphere and lower thermosphere (MLT). Temperature
distribution in this region is an essential component to identify and quantify gravity
waves. Observation from remote sensing instruments on satellite platforms is an effective
way to measure the temperature in the MLT region.
A miniaturized satellite payload is developed to measure the atmospheric temperature
in the MLT region via observing the O2A-band emission. Following a Boltzmann
distribution, the relative intensities of the emission lines can be used to derive the temperature
profile. Based on the spatial heterodyne spectroscopy, this instrument is capable
of resolving individual emission lines in the O2A-band for the spatial and spectral
information simultaneously. The monolithic and compact feature of this spectrometer
makes it suitable for operating on satellite platforms.
In this work, the characterization of the instrument is investigated for the purpose
of simultaneously measuring multiple emission lines of the O2A-band. The instrument
is explored through a series of experimental methods, providing characteristics of the
instrument and evaluation of its performance. In spatial and spectral domain, Level-
0 and Level-1 data processors are developed to convert the raw data to the calibrated
spectral radiance for further temperature and gravity wave characterization.
Within this framework, the performance of the utilized detector is evaluated along
with its radiation tolerance in space environment. In the processor, the detector artifacts
are corrected based on the measurements in laboratory or in space. The radiometric
response of the instrument is characterized on a pixel-by-pixel basis using a blackbody.
An interferogram distortion correction algorithm is developed to correct for the spatial
and phase distortion induced by the detector optics. Further, localized phase distortion
correction is implemented to correct for the remaining phase error. Unwanted ghost
emission lines are removed based on two dimensional Fourier transform. In the spectral
domain, the processing steps mainly consist of wavelength calibration and instrument
spectral response correction, including filter response correction and modulation efficiency
correction.
As an in-orbit verification, the AtmoSHINE instrument was successfully deployed
in space on 22th of December, 2018. In the first test phase, the functionality and the
performance of the instrument in space were verified. The detector dark current measurement
in orbit is consistent with the ground-based results. Based on the the calibration
procedures and the developed data processing algorithms, the O2A-band emission
lines can be successfully resolved. A cross-verification of the AtmoSHINE limb radiance
profile with other satellite payload measurements indicates that the radiometric
performance of the instrument is within the expectation. The retrieved temperature parameters
are studied with respect to different number of samples and different objective
functions in the optimization. This work verifies the ability of the instrument to derive
the atmospheric temperature in the MLT region and its potential application in gravity
wave detections
Sparse Modeling of Grouped Line Spectra
This licentiate thesis focuses on clustered parametric models for estimation of line spectra, when the spectral content of a signal source is assumed to exhibit some form of grouping. Different from previous parametric approaches, which generally require explicit knowledge of the model orders, this thesis exploits sparse modeling, where the orders are implicitly chosen. For line spectra, the non-linear parametric model is approximated by a linear system, containing an overcomplete basis of candidate frequencies, called a dictionary, and a large set of linear response variables that selects and weights the components in the dictionary. Frequency estimates are obtained by solving a convex optimization program, where the sum of squared residuals is minimized. To discourage overfitting and to infer certain structure in the solution, different convex penalty functions are introduced into the optimization. The cost trade-off between fit and penalty is set by some user parameters, as to approximate the true number of spectral lines in the signal, which implies that the response variable will be sparse, i.e., have few non-zero elements. Thus, instead of explicit model orders, the orders are implicitly set by this trade-off. For grouped variables, the dictionary is customized, and appropriate convex penalties selected, so that the solution becomes group sparse, i.e., has few groups with non-zero variables. In an array of sensors, the specific time-delays and attenuations will depend on the source and sensor positions. By modeling this, one may estimate the location of a source. In this thesis, a novel joint location and grouped frequency estimator is proposed, which exploits sparse modeling for both spectral and spatial estimates, showing robustness against sources with overlapping frequency content. For audio signals, this thesis uses two different features for clustering. Pitch is a perceptual property of sound that may be described by the harmonic model, i.e., by a group of spectral lines at integer multiples of a fundamental frequency, which we estimate by exploiting a novel adaptive total variation penalty. The other feature, chroma, is a concept in musical theory, collecting pitches at powers of 2 from each other into groups. Using a chroma dictionary, together with appropriate group sparse penalties, we propose an automatic transcription of the chroma content of a signal
A novel NMF-based DWI CAD framework for prostate cancer.
In this thesis, a computer aided diagnostic (CAD) framework for detecting prostate cancer in DWI data is proposed. The proposed CAD method consists of two frameworks that use nonnegative matrix factorization (NMF) to learn meaningful features from sets of high-dimensional data. The first technique, is a three dimensional (3D) level-set DWI prostate segmentation algorithm guided by a novel probabilistic speed function. This speed function is driven by the features learned by NMF from 3D appearance, shape, and spatial data. The second technique, is a probabilistic classifier that seeks to label a prostate segmented from DWI data as either alignat, contain cancer, or benign, containing no cancer. This approach uses a NMF-based feature fusion to create a feature space where data classes are clustered. In addition, using DWI data acquired at a wide range of b-values (i.e. magnetic field strengths) is investigated. Experimental analysis indicates that for both of these frameworks, using NMF producing more accurate segmentation and classification results, respectively, and that combining the information from DWI data at several b-values can assist in detecting prostate cancer
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