Search results for "Astrophysics - Solar and Stellar Astrophysics"
showing 10 items of 347 documents
Hydrodynamic Modeling of Accretion Impacts in Classical T Tauri Stars: Radiative Heating of the Pre-shock Plasma
2016
Context. It is generally accepted that, in classical T Tauri stars, the plasma from the circumstellar disc accretes onto the stellar surface with free-fall velocity and the impact generates a shock. The impact region is expected to contribute to emission in different spectral bands; many studies have confirmed that the X-rays arise from the post-shock plasma but, otherwise, there are no studies in the literature investigating the origin of the observed UV emission which is apparently correlated to accretion. Aims: We investigated the effect of radiative heating of the infalling material by the post-shock plasma at the base of the accretion stream, with the aim to identify in which region a …
Laboratory evidence for asymmetric accretion structure upon slanted matter impact in young stars
2020
Aims. Investigating the process of matter accretion onto forming stars through scaled experiments in the laboratory is important in order to better understand star and planetary system formation and evolution. Such experiments can indeed complement observations by providing access to the processes with spatial and temporal resolution. A previous investigation revealed the existence of a two-component stream: a hot shell surrounding a cooler inner stream. The shell was formed by matter laterally ejected upon impact and refocused by the local magnetic field. That laboratory investigation was limited to normal incidence impacts. However, in young stellar objects, the complex structure of magne…
Nanoflare Evidence from Analysis of the X-Ray Variability of an Active Region Observed with Hinode/XRT
2012
The heating of the solar corona is one of the big questions in astrophysics. Rapid pulses called nanoflares are among the best candidate mechanisms. The analysis of the time variability of coronal X-ray emission is potentially a very useful tool to detect impulsive events. We analyze the small-scale variability of a solar active region in a high cadence Hinode/XRT observation. The dataset allows us to detect very small deviations of emission fluctuations from the distribution expected for a constant rate. We discuss the deviations in the light of the pulsed-heating scenario.
Mean Interplanetary Magnetic Field Measurement Using the ARGO-YBJ Experiment
2011
The sun blocks cosmic ray particles from outside the solar system, forming a detectable shadow in the sky map of cosmic rays detected by the ARGO-YBJ experiment in Tibet. Because the cosmic ray particles are positive charged, the magnetic field between the sun and the earth deflects them from straight trajectories and results in a shift of the shadow from the true location of the sun. Here we show that the shift measures the intensity of the field which is transported by the solar wind from the sun to the earth.
A prospective new diagnostic technique for distinguishing eruptive and noneruptive active regions
2019
This research has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1 and the European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (grant agreement No. 647214). This work used the DiRAC@Durham facility managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BEIS capital funding via STFC capital grants ST/P002293/1, ST/R002371/1, and ST/S002502/1, Durham University and STFC operations grant ST/R000832/1. DiRAC is part of the National e-Infrastructure. S.L.Y. would like to acknowledge STFC for sup…
Sphinx measurements of the 2009 solar minimum x-ray emission
2012
The SphinX X-ray spectrophotometer on the CORONAS-PHOTON spacecraft measured soft X-ray emission in the 1-15 keV energy range during the deep solar minimum of 2009 with a sensitivity much greater than GOES. Several intervals are identified when the X-ray flux was exceptionally low, and the flux and solar X-ray luminosity are estimated. Spectral fits to the emission at these times give temperatures of 1.7-1.9 MK and emission measures between 4 x 10^47 cm^-3 and 1.1 x 10^48 cm^-3. Comparing SphinX emission with that from the Hinode X-ray Telescope, we deduce that most of the emission is from general coronal structures rather than confined features like bright points. For one of 27 intervals o…
The effect of ISM absorption on stellar activity measurements and its relevance for exoplanet studies
2017
Past UV and optical observations of stars hosting hot Jupiters have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening ISM. Inspired by this result, we study the effect of ISM absorption on activity measurements (S and logR'$_{\rm HK}$ indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several ins…
Correcting the effect of stellar spots on ARIEL transmission spectra – II. The limb-darkening effect
2021
This paper is part of an effort to correct the transmission spectra of a transiting planet orbiting an active star. In Paper I (Cracchiolo et al. 2020) we have demonstrated a methodology to minimize the potential bias induced by unocculted star spots on the transmission spectrum, assuming a spot model parameterized by filling factor and temperature. In this work we introduce the limb darkening effect, therefore the position of the spot in the stellar disk and the impact parameter of the transiting planet now play a key role. The method is tested on simulations of planetary transits of three representative kinds of planetary systems, at ARIEL resolution. We find that a realistic treatment of…
The GAPS programme at TNG
2020
Understanding stellar activity in M dwarfs is fundamental to improving our knowledge of the physics of stellar atmospheres and for planet search programmes. High levels of stellar activity (also with flare events) can cause additional variations in the stellar emission that contaminate the signal induced by a planet and that need to be corrected. The study of activity indicators in active stars can improve our capability of modelling this signal. Our aim is to understand the behaviour of stellar chromospheres of M stars, studying the more sensitive chromospheric activity indicators, characterising their variability and on finding the correlations among these indicators to obtain information…
Supersaturation and activity-rotation relation in PMS stars: the young Cluster h Per
2016
The magnetic activity of late-type MS stars is characterized by different regimes, and their activity levels are well described by Ro, the ratio between P_rot and the convective turnover time. Very young PMS stars show, similarly to MS stars, intense magnetic activity. However they do not show clear activity-rotation trends, and it still debated which stellar parameters determine their magnetic activity levels. To bridge the gap between MS and PMS stars, we studied the activity-rotation relation in the young cluster h Per, a ~13 Myr old cluster, that contains both fast and slow rotators, whose members have ended their accretion phase and have already developed a radiative core. It offers us…