arXiv subjects

Joao Fernandes

Publications and source records attributed to Joao Fernandes.

4 recordsLinked to original sources

arXiv2021

Testing the accuracy of Coimbra Astronomical Observatory solar filament historical series (1929-1941)

The present work aims to validate the positions of solar filaments published in the Annals of Coimbra University Astronomical Observatory, currently the Geophysical and Astronomical Observatory of the University of Coimbra, corresponding to years 1929 to 1941. The published Stonyhurst positions were obtained by an original method devised in the early 20th century that used a spherical calculator instrument, a wood-made model of the sun. We used the digital images of the original spectroheliograms to measure the positions of the filaments, and heliographic coordinates were determinate with the routines implemented on python package Sunpy. The correlation coefficients between both sets of coordinates are positive and highly significant. The results validate the method used at the Coimbra observatory and the data published. We conclude that Coimbra solar filaments catalogues are reliable and can therefore be considered for future solar activity studies.

astro-ph.SR↗

arXiv2019

Neural Network Forecast of the Sunspot Butterfly Diagram

Using neural networks as a prediction method, we attempt to demonstrate that forecasting of the Sun's sunspot time series can be extended to the spatial-temporal case. We employ this machine learning methodology to forecast not only in time but also in space (in this case the latitude) on a spatial-temporal dataset representing the solar sunspot diagram extending to a total of 142 years. The analysis shows that this approach seems to be able to reconstruct the overall qualitative aspects of the spatial-temporal series, namely the overall shape and amplitude of the latitude and time pattern of sunspots. This is, as far as we are aware, the first time neural networks have been used to forecast the Sun's sunspot butterfly diagram, and although the results are limited in the quantitative prediction aspects, it points the way to use the full spatial-temporal series as opposed to just the time series for machine learning approaches to forecasting. Further to that, we use the method to predict that the upcoming Cycle 25 maximum sunspot number will be around $R_{25}=57 \pm 17$. This implies a very weak cycle and that it will be the weakest cycle on record.

astro-ph.SR↗

arXiv2007

The `666' collaboration on OGLE transits: I. Accurate radius of the planets OGLE-TR-10b and OGLE-TR-56b with VLT deconvolution photometry

Transiting planets are essential to study the structure and evolution of extra-solar planets. For that purpose, it is important to measure precisely the radius of these planets. Here we report new high-accuracy photometry of the transits of OGLE-TR-10 and OGLE-TR-56 with VLT/FORS1. One transit of each object was covered in Bessel V and R filters, and treated with the deconvolution-based photometry algorithm DECPHOT, to ensure accurate millimagnitude light curves. Together with earlier spectroscopic measurements, the data imply a radius of 1.22 +0.12-0.07 R_J for OGLE-TR-10b and 1.30 +- 0.05 R_J for OGLE-TR-56b. A re-analysis of the original OGLE photometry resolves an earlier discrepancy about the radius of OGLE-TR-10. The transit of OGLE-TR-56 is almost grazing, so that small systematics in the photometry can cause large changes in the derived radius. Our study confirms both planets as inflated hot Jupiters, with large radii comparable to that of HD 209458$b$ and at least two other recently discovered transiting gas giants.

astro-ph↗

arXiv2004

Detailed theoretical models for extra-solar planet-host stars: The "red stragglers" HD37124 and HD46375

In this paper we analyse and discuss the HR Diagram position of two extra-solar planet-host stars - HD37124 and HD46375 - by means of theoretical stellar evolution models. This work was triggered by the results obtained by Laws et al. (2003) who found that these stars were in contradiction to the expectation based on their high metallicity. Fixing the age of both stars with the value based on their chromospheric activity levels and computing our own evolutionary models using the CESAM code, we are able to reproduce the observed luminosity, effective temperature and metallicity of both stars for a set of stellar parameters that are astrophysically reliable even if it is non-trivial to interpret the absolute values for these parameters. Our results are discussed in the context of the stellar properties of low mass stars.

astro-ph↗