Revisiting the correlation between stellar activity and planetary surface gravity

Aims: We re-evaluate the correlation between planetary surface gravity and stellar host activity as measured by the index log($R'_{HK}$). This correlation, previously identified by Hartman (2010), is now analyzed in light of an extended measurements dataset, roughly 3 times larger than the original one. Methods: We calculated the Spearman's rank correlation coefficient between the two quantities and its associated p-value. The correlation coefficient was calculated for both the full dataset and the star-planet pairs that follow the conditions proposed by Hartman (2010). In order to do so, we considered effective temperatures both as collected from the literature and from the SWEET-Cat catalog, which provides a more homogeneous and accurate effective temperature determination. Results: The analysis delivers significant correlation coefficients, but with a lower value than those obtained by Hartman (2010). Yet, the two datasets are compatible, and we show that a correlation coefficient as large as previously published can arise naturally from a small-number statistics analysis of the current dataset. The correlation is recovered for star-planet pairs selected using the different conditions proposed by Hartman (2010). Remarkably, the usage of SWEET-Cat temperatures leads to larger correlation coefficient values. We highlight and discuss the role of the correlation betwen different parameters such as effective temperature and activity index. Several additional effects on top of those discussed previously were considered, but none fully explains the detected correlation. In light of the complex issue discussed here, we encourage the different follow-up teams to publish their activity index values in the form of log($R'_{HK}$) index so that a comparison across stars and instruments can be pursued.Comment: 11 pages, 3 figures, accepted for publication in A&

Exoplanets: Gaia and the importance of ground based spectroscopy follow-up

The search for extrasolar planets has developed rapidly and, today, more than 1700 planets have been found orbiting stars. Thanks to Gaia, we will collect high-accuracy astrometric orbits of thousands of new low-mass celestial objects, such as extra-solar planets and brown dwarfs. These measurements in combination with spectroscopy and with present day and future extrasolar planet search programs (like HARPS, ESPRESSO) will have a crucial contribution to several aspects of planetary astrophysics (formation theories, dynamical evolution, etc.). Moreover, Gaia will have a strong contribution on the stellar chemical and kinematic characterisation studies. In this paper we present a short overview of the importance of Gaia in the context of exoplanet research. As preparatory work for Gaia, we will then present a study where we derived stellar parameters for a sample of field giant stars

A pragmatic Bayesian perspective on correlation analysis: The exoplanetary gravity - stellar activity case

We apply the Bayesian framework to assess the presence of a correlation between two quantities. To do so, we estimate the probability distribution of the parameter of interest, $\rho$, characterizing the strength of the correlation. We provide an implementation of these ideas and concepts using python programming language and the pyMC module in a very short ($\sim$130 lines of code, heavily commented) and user-friendly program. We used this tool to assess the presence and properties of the correlation between planetary surface gravity and stellar activity level as measured by the log($R'_{\mathrm{HK}}$) indicator. The results of the Bayesian analysis are qualitatively similar to those obtained via p-value analysis, and support the presence of a correlation in the data. The results are more robust in their derivation and more informative, revealing interesting features such as asymmetric posterior distributions or markedly different credible intervals, and allowing for a deeper exploration. We encourage the reader interested in this kind of problem to apply our code to his/her own scientific problems. The full understanding of what the Bayesian framework is can only be gained through the insight that comes by handling priors, assessing the convergence of Monte Carlo runs, and a multitude of other practical problems. We hope to contribute so that Bayesian analysis becomes a tool in the toolkit of researchers, and they understand by experience its advantages and limitations.Comment: 10 pages, 1 figure, accepted for publication in the peer-reviewed proceedings of the conference "Habitability in the Universe: From the Early Earth to Exoplanets

Searching for the signatures of terrestrial planets in F-, G-type main-sequence stars

We have studied the volatile-to-refractory abundance ratios to investigate their possible relation with the low-mass planetary formation. We present a fully differential chemical abundance analysis using high-quality HARPS and UVES spectra of 61 late F- and early G-type main-sequence stars, 29 are planet hosts and 32 are stars without detected planets. As the previous sample of solar analogs, these stars slightly hotter than the Sun also provide very accurate Galactic chemical abundance trends in the metallicity range $-0.3<{\rm [Fe/H]}<0.4$. Stars with and without planets show similar mean abundance ratios. Moreover, when removing the Galactic chemical evolution effects, these mean abundance ratios, $\Delta {\rm [X/Fe]_{SUN-STARS}}$, versus condensation temperature tend to exhibit less steep trends with nearly null or slightly negative slopes. We have also analyzed a sub-sample of 26 metal-rich stars, 13 with and 13 without known planets and find the similar, although not equal, abundance pattern with negative slopes for both samples of stars with and without planets. Using stars at S/N $\ge 550$ provides equally steep abundance trends with negative slopes for both stars with and without planets. We revisit the sample of solar analogs to study the abundance patterns of these stars, in particular, 8 stars hosting super-Earth-like planets. Among these stars having very low-mass planets, only four of them reveal clear increasing abundance trends versus condensation temperature. Finally, we have compared these observed slopes with those predicted using a simple model which enables us to compute the mass of rocks which have formed terrestrial planets in each planetary system. We do not find any evidence supporting the conclusion that the volatile-to-refractory abundance ratio is related to the presence of rocky planets.Comment: Accepted for publication in A&

CNO behaviour in planet-harbouring stars. II. Carbon abundances in stars with and without planets using the CH band

Context. Carbon, oxygen and nitrogen (CNO) are key elements in stellar formation and evolution, and their abundances should also have a significant impact on planetary formation and evolution. Aims. We present a detailed spectroscopic analysis of 1110 solar-type stars, 143 of which are known to have planetary companions. We have determined the carbon abundances of these stars and investigate a possible connection between C and the presence of planetary companions. Methods. We used the HARPS spectrograph to obtain high-resolution optical spectra of our targets. Spectral synthesis of the CH band at 4300\AA was performed with the spectral synthesis codes MOOG and FITTING. Results. We have studied carbon in several reliable spectral windows and have obtained abundances and distributions that show that planet host stars are carbon rich when compared to single stars, a signature caused by the known metal-rich nature of stars with planets. We find no different behaviour when separating the stars by the mass of the planetary companion. Conclusions. We conclude that reliable carbon abundances can be derived for solar-type stars from the CH band at 4300\AA. We confirm two different slope trends for [C/Fe] with [Fe/H] because the behaviour is opposite for stars above and below solar values. We observe a flat distribution of the [C/Fe] ratio for all planetary masses, a finding that apparently excludes any clear connection between the [C/Fe] abundance ratio and planetary mass.Comment: 10 pages, 10 figures. Accepted to A&

From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio

The main goal of this work is to study element ratios that are important for the formation of planets of different masses. We study potential correlations between the existence of planetary companions and the relative elemental abundances of their host stars. We use a large sample of FGK-type dwarf stars for which precise Mg, Si, and Fe abundances have been derived using HARPS high-resolution and high-quality data. A first analysis of the data suggests that low-mass planet host stars show higher [Mg/Si] ratios, while giant planet hosts present [Mg/Si] that is lower than field stars. However, we found that the [Mg/Si] ratio significantly depends on metallicity through Galactic chemical evolution. After removing the Galactic evolution trend only the difference in the [Mg/Si] elemental ratio between low-mass planet hosts and non-hosts was present in a significant way. These results suggests that low-mass planets are more prevalent around stars with high [Mg/Si]. Our results demonstrate the importance of Galactic chemical evolution and indicate that it may play an important role in the planetary internal structure and composition.Comment: Accepted by A&A (Letter to the Editor

ARES v2 - new features and improved performance

Aims: We present a new upgraded version of ARES. The new version includes a series of interesting new features such as automatic radial velocity correction, a fully automatic continuum determination, and an estimation of the errors for the equivalent widths. Methods: The automatic correction of the radial velocity is achieved with a simple cross-correlation function, and the automatic continuum determination, as well as the estimation of the errors, relies on a new approach to evaluating the spectral noise at the continuum level. Results: ARES v2 is totally compatible with its predecessor. We show that the fully automatic continuum determination is consistent with the previous methods applied for this task. It also presents a significant improvement on its performance thanks to the implementation of a parallel computation using the OpenMP library.Comment: 4 pages, 2 Figures; accepted in A&A; ARES Webpage: www.astro.up.pt/~sousasag/are

Supernovae and their host galaxies - V. The vertical distribution of supernovae in disc galaxies

We present an analysis of the height distributions of the different types of supernovae (SNe) from the plane of their host galaxies. We use a well-defined sample of 102 nearby SNe appeared inside high-inclined (i > 85 deg), morphologically non-disturbed S0-Sd host galaxies from the Sloan Digital Sky Survey. For the first time, we show that in all the subsamples of spirals, the vertical distribution of core-collapse (CC) SNe is about twice closer to the plane of host disc than the distribution of SNe Ia. In Sb-Sc hosts, the exponential scale height of CC SNe is consistent with those of the younger stellar population in the Milky Way (MW) thin disc, while the scale height of SNe Ia is consistent with those of the old population in the MW thick disc. We show that the ratio of scale lengths to scale heights of the distribution of CC SNe is consistent with those of the resolved young stars with ages from ~ 10 Myr up to ~ 100 Myr in nearby edge-on galaxies and the unresolved stellar population of extragalactic thin discs. The corresponding ratio for SNe Ia is consistent with the same ratios of the two populations of resolved stars with ages from a few 100 Myr up to a few Gyr and from a few Gyr up to ~ 10 Gyr, as well as with the unresolved population of the thick disc. These results can be explained considering the age-scale height relation of the distribution of stellar population and the mean age difference between Type Ia and CC SNe progenitors.Comment: 11 pages, 6 figures, 6 tables, accepted for publication in MNRA