The Sun is less active than other solar-like stars

Magnetic activity of the Sun and other stars causes their brightness to vary. We investigate how typical the Sun's variability is compared to other solar-like stars, i.e. those with near-solar effective temperatures and rotation periods. By combining four years of photometric observations from the Kepler space telescope with astrometric data from the Gaia spacecraft, we measure photometric variabilities of 369 solar-like stars. Most of the solar-like stars with well-determined rotation periods show higher variability than the Sun and are therefore considerably more active. These stars appear nearly identical to the Sun, except for their higher variability. Their existence raises the question of whether the Sun can also experience epochs of such high variability.Comment: Accepted for publication in Science. 3 (main) + 10 (supplementary) figure

Far beyond the Sun: II. Probing the stellar magnetism of the young Sun {\iota} Horologii from the photosphere to its corona

A comprehensive multi-wavelength campaign has been carried out to probe stellar activity and variability in the young Sun-like star $\iota$-Horologii. We present the results from long-term spectropolarimetric monitoring of the system by using the ultra-stable spectropolarimeter/velocimeter HARPS at the ESO 3.6-m telescope. Additionally, we included high-precision photometry from the NASA Transiting Exoplanet Survey Satellite (TESS) and observations in the far- and near-ultraviolet spectral regions using the STIS instrument on the NASA/ESA Hubble Space Telescope (HST). The high-quality dataset allows a robust characterisation of the star's rotation period, as well as a probe of the variability using a range of spectroscopic and photometric activity proxies. By analyzing the gradient of the power spectra (GPS) of the TESS lightcurves we constrained the faculae-to-spot driver ratio ($\rm S_{fac}/S_{spot}$) to 0.510$\pm$0.023, which indicates that the stellar surface is spot dominated during the time of the observations. We compared the photospheric activity properties derived from the GPS method with a magnetic field map of the star derived using Zeeman-Doppler imaging (ZDI) from simultaneous spectropolarimetric data for the first time. Different stellar activity proxies enable a more complete interpretation of the observed variability. For example, we observed enhanced emission in the HST transition line diagnostics C IV and C III, suggesting a flaring event. From the analysis of TESS data acquired simultaneously with the HST data, we investigate the photometric variability at the precise moment that the emission increased and derive correlations between different observables, probing the star from its photosphere to its corona

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.Comment: 46 pages, 8 figures; submitted to Experimental Astronomy on behalf of the ANDES Science Tea

Auctions for Renewable Energy Support II - First insights and results of the Horizon2020 project AURES II

This is the final version. Available from Funcas via the link in this recordThe Horizon2020 project AURES II aims at ensuring the effective implementation of auctions for renewable energies in the EU Member States (MS). In recent years, auction schemes for the allocation of support for renewable electricity sources (RES) have been advancing rapidly across Europe. Auctions are considered to have brought down support levels and increased planning capability for RES deployment and state budgets. In some unfortunate cases, they have, however, also resulted in delayed or unrealised projects and increased uncertainty for project developers. A variety of auction designs are still being tested and introduced in EU MS, as well as foreseen by European legislation. Therefore, there is still a need for further assessment and improvement of national auction design and implementation to ensure the future success of RES auctions in Europe. Applying different qualitative and quantitative methods in the various work packages (WPs), the AURES II project partners have already drafted and published a large number of reports and studies. This article aims at comprehensively presenting these results and provide a first overview.European Union Horizon 202

Inflection point in the power spectrum of stellar brightness variations

Context. Considerable effort has gone into using light curves observed by such space telescopes as CoRoT, Kepler, and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the light curves of many older and less active stars, such as stars that are similar to the Sun, are quite irregular. This hampers the determination of their rotation periods. Aims. We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves. Methods. We extended the Spectral And Total Irradiance Reconstruction approach for modeling solar brightness variations to Sun-like stars. We calculated the power spectra of stellar brightness variations for various combinations of parameters that define the surface configuration and evolution of stellar magnetic features. Results. The short lifetime of spots in comparison to the stellar rotation period, as well as the interplay between spot and facular contributions to brightness variations of stars with near solar activity, cause irregularities in their light curves. The power spectra of such stars often lack a peak associated with the rotation period. Nevertheless, the rotation period can still be determined by measuring the period where the concavity of the power spectrum plotted in the log–log scale changes its sign, that is, by identifying the position of the inflection point. Conclusions. The inflection point of the (log–log) power spectrum is found to be a new diagnostic for stellar rotation periods which is shown to work even in cases where the power spectrum shows no peak at the rotation rate

Biology, Difference, and Gender Discrimination

Context. Stellar rotation periods can be determined by observing brightness variations caused by active magnetic regions transiting visible stellar disk as the star rotates. Successful stellar photometric surveys stemming from the Kepler and TESS observations have led to the determination of rotation periods in tens of thousands of young and active stars. However, there is still a lack of information on the rotation periods of older and less active stars like the Sun. The irregular temporal profiles of light curves caused by the decay times of active regions, which are comparable to, or even shorter than, stellar rotation periods, in combination with the random emergence of active regions make period determination for such stars very difficult. Aims. We tested the performance of a new method for the determination of stellar rotation periods against stars with previously determined rotation periods. The method is based on calculating the gradient of the power spectrum (GPS) and identifying the position of the inflection point (i.e. point with the highest gradient). The GPS method is specifically aimed at determining rotation periods of low-activity stars like the Sun. Methods. We applied the GPS method to 1047 Sun-like stars observed by the Kepler telescope. We considered two stellar samples individually: one with near-solar rotation periods (24–27.4 d) and a broad range of effective temperatures (5000–6000 K) and the other with near-solar effective temperatures (5700–5900 K) and a broad range of rotation periods (15–40 d). Results. We show that the GPS method returns precise values for stellar rotation periods. Furthermore, it allows us to constrain the ratio between facular and spot areas of active regions at the moment of their emergence. We also show that the relative facular area decreases with the stellar rotation rate. Conclusions. Our results suggest that the GPS method can be successfully applied to retrieve the periods of stars with both regular and non-regular light curves

Voltammetric study of aminopurines on pencil graphite electrode in the presence of copper ions

Electrochemical oxidations of aminopurines (adenine, 2-aminopurine, 2,6-diaminopurine) and their complexes with Cu(I) on a pencil graphite electrode were investigated by means of linear sweep voltammetry (LSV) and elimination voltammetry with linear scan (EVLS). The anodic process of the Cu(I)-aminopurine complex, corresponding to the oxidation of Cu(I) to Cu(II), takes place in the potential range between 0.4 and 0.5 V (vs. Ag/AgCl/3 M KCl). At more positive potentials the aminopurines provide voltammetric peaks resulting from the oxidation of the purine ring. The stability of the accumulated complex layer was investigated by the adsorptive transfer stripping technique. © 2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim