arXiv subjects

Armando Domiciano de Souza

Publications and source records attributed to Armando Domiciano de Souza.

13 recordsLinked to original sources

arXiv2026

PYRA and MYTHRA: new statistical analysis for image reconstruction

Image reconstruction in optical interferometry remains a challenging inverse problem due to sparse Fourier sampling and the strong dependence of the reconstructed images on user-defined parameters. We present PYRA (Python for MiRA) and MYTHRA (Mean Astrophysical Images with PYRA), two tools designed to improve the robustness and reproducibility of interferometric imaging. PYRA automatically generates large ensembles of MiRA reconstructions by exploring a wide range of reconstruction parameters, while MYTHRA statistically analyzes the resulting images to identify the most consistent solutions and derive a final averaged image with associated uncertainty estimates. The methodology is validated using the blind datasets of the 2024 Interferometric Imaging Contest, where our reconstruction strategy achieved the best overall score. We further apply the method to VLTI/MATISSE observations of the triple AGB system π1 Gru and the A[e] supergiant 3 Pup. These results demonstrate that statistical analysis of large reconstruction ensembles provides a powerful framework for assessing image reliability and reducing reconstruction artifacts in optical interferometry.

astro-ph.IM↗

arXiv2026

Interferometric Survey of Stellar Parameters: Towards homogeneous FGK stars parameters and surface-brightness color relation in the context of PLATO space mission

The estimation of stellar angular diameters can be performed from the surface brightness - color relation (SBCR) and photometry. The SBCR have been considered by the PLATO space mission as an independent empirical alternative for estimating stellar radii of FGK stars. In this context, we have implemented an homogeneous approach not only for calibrating the SBCR for FGK-IV/V stars but, also for determining their fundamental parameters in order to place the stars reliably on the HR diagram and to study their impact on the SBCR. We have performed interferometric observations of 18 quiescent FGK-IV/V stars in the Gaia color range of $3.088 \leq G \leq 5.498$. For the first time, we used 3 different interferometers operating in the $R$, $H$ , and $K$ bands to measure polychromatic limb-darkened angular diameters (LDAD). In parallel, by using public domain spectra we have estimated the stellar parameters ($T_{eff}$, $log g$ and Z) by using the open python tool iSpec. We achieved an average accuracy of 2.3% for the LDAD based on polychromatic observations. However, we have observed that our SBCR does not follow the calibration of the relation between surface brightness and color in Gaia found in the literature. Furthermore, we found that $log g$ and $Z$ have no impact on the SBC relation; given the characteristics of our sample of quiescent stars, it constitutes an ideal set of targets for conducting a new SBCR calibration within the framework of the PLATO space mission. In this context, we reported a SBCR calibration with $σ_{RMS} = $ 0.012, 0.009, 0.009 in the $G, G_{BP}$ and $G_{RP}$ Gaia bands, respectively. This article is part of a series of papers reporting the first results obtained using a polychromatic approach to measure LDAD, employing a fully homogeneous methodology for both determining the fundamental parameters of stars and measuring the LDAD, $θ_{LD}$.

astro-ph.SR↗

arXiv2023

The discovery space of ELT-ANDES. Stars and stellar populations

The ArmazoNes high Dispersion Echelle Spectrograph (ANDES) is the optical and near-infrared high-resolution echelle spectrograph envisioned for the European Extremely Large Telescope (ELT). We present a selection of science cases, supported by new calculations and simulations, where ANDES could enable major advances in the fields of stars and stellar populations. We focus on three key areas, including the physics of stellar atmospheres, structure, and evolution; stars of the Milky Way, Local Group, and beyond; and the star-planet connection. The key features of ANDES are its wide wavelength coverage at high spectral resolution and its access to the large collecting area of the ELT. These features position ANDES to address the most compelling and potentially transformative science questions in stellar astrophysics of the decades ahead, including questions which cannot be anticipated today.

astro-ph.IM↗

arXiv2022

CHARA/SPICA: a 6-telescope visible instrument for the CHARA Array

With a possible angular resolution down to 0.1-0.2 millisecond of arc using the 330 m baselines and the access to the 600-900 nm spectral domain, the CHARA Array is ideally configured for focusing on precise and accurate fundamental parameters of stars. CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) aims at performing a large survey of stars all over the Hertzsprung-Russell diagram. This survey will also study the effects of the different kinds of variability and surface structure on the reliability of the extracted fundamental parameters. New surface-brightness-colour relations will be extracted from this survey, for general purposes on distance determination and the characterization of faint stars. SPICA is made of a visible 6T fibered instrument and of a near-infrared fringe sensor. In this paper, we detail the science program and the main characteristics of SPICA-VIS. We present finally the initial performance obtained during the commissioning.

astro-ph.IM↗

arXiv2022

Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel

In this paper we report on spatial intensity interferometry measurements within the H$α$ line on two stars: the Luminous Blue Variable supergiant \PCygni\,and the late-type B supergiant Rigel. The experimental setup was upgraded to allow simultaneous measurement of two polarization channels, instead of one in our previous setup, and the zero baseline correlation function on-sky to validate independent estimates obtained from the stellar spectrum and the instrumental spectral throughput. Combined with simultaneous spectra measurements and based on radiative transfer models calculated with the code CMFGEN, we were able to fit our measured visibility curves to extract the stellar distances. Our distance determinations for both \PCygni\ (1.61 $\pm$ 0.18 kpc) and Rigel (0.26 $\pm$ 0.02 kpc) agree very well with the values provided by astrometry with the Gaia and Hipparcos missions, respectively. This result for Rigel was obtained by adopting a stellar luminosity of $L_{\star}$ = 123000 $L_{\odot}$, which is reported in the literature as being consistent with the Hipparcos distance to Rigel. However, due to the lack of consensus on Rigel's luminosity, we also explore how the adoption of the stellar luminosity in our models affects our distance determination for Rigel. In conclusion, we support, in an independent way, the distance to Rigel as the one provided by the Hipparcos mission, when taking the luminosity of 123000 $L_{\odot}$ at face value. This study is the first successful step towards extending the application of the Wind Momentum Luminosity Relation method for distance calibration from an LBV supergiant to a more normal late-type B supergiant.

astro-ph.SR↗

arXiv2019

Classical and general relativistic post-Keplerian effects in binary pulsars hosting fast rotating main sequence stars

We consider a binary system composed of a pulsar and a massive, fast rotating, highly distorted main sequence star as a potential scenario to dynamically put to the test certain post-Keplerian effects of both Newtonian and post-Newtonian nature. We numerically produce time series of the perturbations $Δ\left(δτ\right)$ of the Rømer-like, orbital component of the pulsar's time delay $δτ$ induced over 10 years by the pN gravitoelectric mass monopole, quadrupole, gravitomagnetic spin dipole and octupole accelerations along with the Newtonian quadrupolar one. We do not deal with the various propagation time delays due to the travelling electromagnetic waves. It turns out that, for a Be-type star with $M = 15\ \textrm{M}_\odot$, $R_\textrm{e} = 5.96\ \textrm{R}_\odot$, $ν= 0.203$, $S = 3.41\times 10^{45}\ \textrm{J}\ \textrm{s}$, $J_2 = 1.92\times 10^{-3}$ orbited by a pulsar with an orbital period $P_\textrm{b}\simeq 40-70\ \textrm{d}$, the classical oblateness-driven effects are at the $\lesssim 4-150\ \textrm{s}$ level, while the pN shifts are of the order of $\lesssim 1.5-20\ \textrm{s}\ \left(GMc^{-2}\right)$, $\lesssim 10-40\ \textrm{ms}\ \left(GMR^2_\textrm{e} J_2 c^{-2}\right)$, $\lesssim 0.5 - 6\ \textrm{ms}\ \left(GSc^{-2}\right)$, $\lesssim 5 - 20\ μ\textrm{s}\ \left(GSR^2_\textrm{e} \varepsilon^2 c^{-2}\right)$, depending on their orbital configuration. The root-mean-square (rms) timing residuals $σ_τ$ of almost all the existing non-recycled, non-millisecond pulsars orbiting massive, fast rotating main sequence stars are $\lesssim\textrm{ms}$. Thus, such kind of binaries have the potential to become interesting laboratories to measure, or, at least, constrain, some Newtonian and post-Newtonian key features of the distorted gravitational fields of the fast rotating stars hosted by them [Abridged].

gr-qc↗

arXiv2018

The evolved fast rotator Sargas. Stellar parameters and evolutionary status from VLTI/PIONIER and VLT/UVES

Gravity darkening (GD) and flattening are important consequences of stellar rotation. The precise characterization of these effects across the HRD is crucial to a deeper understanding of stellar structure and evolution. We seek to characterize such important effects on Sargas, an evolved, fast-rotating, intermediate-mass star, located in a region of the HRD where they have never been directly measured as far as we know. We use our numerical model CHARRON to analyze interferometric (VLTI/PIONIER) and spectroscopic (VLT/UVES) observations through a MCMC model-fitting procedure. The visibilities and closure phases from the PIONIER data are particularly sensitive to rotational flattening and GD. Adopting the Roche approximation, we investigate two GD models: (1) the beta-model (classical von Zeipel's law), and (2) the omega-model. Using this approach we measure several physical parameters of Sargas, namely, equatorial radius, mass, equatorial rotation velocity, mean Teff, inclination and position angle of the rotation axis, and beta. In particular, we show that the measured beta leads to a surface flux distribution equivalent to the one given by the omega-model. Thanks to our results, we also show that Sargas is most probably located in a rare and interesting region of the H-R diagram: within the Hertzsprung gap and over the hot edge of the instability strip. These results show once more the power of optical/IR long-baseline interferometry, combined with high-resolution spectroscopy, to directly measure fast-rotation effects and stellar parameters, in particular GD. As was the case for a few fast rotators previously studied by interferometry, the omega-model provides a physically more profound description of Sargas' GD, without the need of a beta exponent.

astro-ph.SR↗

arXiv2017

Science cases for a visible interferometer

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41.

astro-ph.SR↗

arXiv2015

Capabilities of future intensity interferometers for observing fast-rotating stars: imaging with two- and three-telescope correlations

Future large arrays of telescopes, used as intensity interferometers, can be used to image the surfaces of stars with unprecedented angular resolution. Fast-rotating, hot stars are particularly attractive targets for intensity interferometry since shorter (blue) wavelength observations do not pose additional challenges. Starting from realistic surface brightness simulations of fast-rotating stars, we discuss the capabilities of future intensity interferometers for imaging effects such as gravity darkening and rotational deformation. We find that two-telescope intensity correlation data allow reasonably good imaging of these phenomena, but can be improved with additional higher order (e.g. three-telescope) correlation data, which contain some Fourier phase information.

astro-ph.IM↗

arXiv2012

Stars with the B[e] phenomenon seen by long baseline interferometry

Thanks to the high spatial resolution provided by long baseline interferometry, it is possible to understand the complex circumstellar geometry around stars with the B[e] phenomenon. These stars are composed by objects in different evolutionary stages, like high- and low-mass evolved stars, intermediate-mass pre-main sequence stars and symbiotic objects. However, up to now more than 50% of the confirmed B[e] stars are not well classified, being called unclassified B[e] stars. From instruments like VLTI/AMBER and VLTI/MIDI, we have identified the presence of gaseous and dusty circumstellar disks, which have provided us with some hints related to the nature of these objects. Here, we show our results for two galactic stars with the B[e] phenomenon, HD 50138 and CPD-529243, based on interferometric measurements.

astro-ph.SR↗

arXiv2011

The galactic unclassified B[e] star HD 50138. II. Interferometric constraints on the close circumstellar environment

HD 50138 is a southern star that presents the B[e] phenomenon, but its evolutionary stage is still not well known. This object presents spectral variability, which can be explained by outbursts or shell phases and spectropolarimetric observations have shown the presence of a non-spherically symmetric circumstellar environment that is responsible for the B[e] phenomenon. Based on recent optical long baseline interferometric observations from the VLTI/MIDI and VLTI/AMBER, and also from the Keck segment-tilting experiment, we study the structure of the circumstellar environment of HD 50138, through a geometrical analytical modeling, also using the recent LITpro software and considering a large space of parameters. We resolve and describe its circumstellar geometry for the first time in detail. The presence of a dusty circumstellar disk with an orientation onto the sky-plane of 71+-7 degrees, which is perpendicular to the polarimetric measurements from the literature, was derived. We also derived that HD 50138 is seen under an intermediate angle related to the line of sight, 56+-4 degrees. In addition, the structure of the disk and the flux contributions of the gas and dust components is discussed.

astro-ph.SR↗

arXiv2009

The galactic unclassified B[e] star HD 50138. I. A possible new shell phase

The observed spectral variation of HD 50138 has led different authors to classify it in a very wide range of spectral types and luminosity classes (from B5 to A0 and III to Ia) and at different evolutionary stages as either HAeBe star or classical Be. Aims: Based on new high-resolution optical spectroscopic data from 1999 and 2007 associated to a photometric analysis, the aim of this work is to provide a deep spectroscopic description and a new set of parameters for this unclassified southern B[e] star and its interstellar extinction. Methods: From our high-resolution optical spectroscopic data separated by 8 years, we perform a detailed spectral description, presenting the variations seen and discussing their possible origin. We derive the interstellar extinction to HD 50138 by taking the influences of the circumstellar matter in the form of dust and an ionized disk into account. Based on photometric data from the literature and the new Hipparcos distance, we obtain a revised set of parameters for HD 50138. Results: Because of the spectral changes, we tentatively suggest that a new shell phase could have taken place prior to our observations in 2007. We find a color excess value of E(B-V) = 0.08 mag, and from the photometric analysis, we suggest that HD 50138 is a B6-7 III-V star. A discussion of the different evolutionary scenarios is also provided.

astro-ph.SR↗

arXiv2003

Hot Stars Mass-loss studied with Spectro-Polarimetric INterferometry (SPIN)

We present a prospective work undertaken on Spectro-Polarimetric INterferometry (SPIN). Our theoretical studies suggest that SPIN is a powerful tool for studying the mass loss from early type stars. Based on Monte Carlo simulations, we computed the expected SPIN signal for numerous hot star spectral types covering a broad range of geometries and optical depths. The SPIN technique is based on the detection and comparison of the fringe characteristics (complex visibility) between two perpendicular directions of polarization. In particular, we demonstrate that the SPIN technique is very sensitive to the beta parameter from the so-called 'beta velocity law' for optically thin winds. Moreover, the location where the bulk of polarization is generated can be defined accurately. The required sensitivity for studying main sequence OB star winds is still very demanding (inferior to 0.5%), but the signal expected from denser winds or extended atmospheres is well within the capabilities of existing interferometers. The visibility curves obtained in two perpendicular polarizations for LBVs or WR stars can differ by more than 15%, and their corresponding limb-darkened radii obtained by the fit of these curves by more than 35%. The signal expected from the extended circumstellar environment of Be stars and B[e] appears also to be easy to detect, relaxing the required instrumental accuracy to 1%. For these spectral types, the SPIN technique provide a good tool to extract the highly polarized and spatially confined envelope contribution from the bright star emission.

astro-ph↗