Search results for "Stellar atmosphere"
showing 10 items of 18 documents
The HADES RV Programme with HARPS-N@TNG. III. Flux-flux and activity-rotation relationships of early-M dwarfs
2016
(Abridged) Understanding stellar activity in M dwarfs is crucial for the physics of stellar atmospheres as well as for ongoing radial velocity exoplanet programmes. Despite the increasing interest in M dwarfs, our knowledge of the chromospheres of these stars is far from being complete. We aim to test whether the relations between activity, rotation, and stellar parameters and flux-flux relationships also hold for early-M dwarfs on the main-sequence. We analyse in an homogeneous and coherent way a well defined sample of 71 late-K/early-M dwarfs that are currently being observed in the framework of the HArps-n red Dwarf Exoplanet Survey (HADES). Rotational velocities are derived using the cr…
The Corona of the Sun as a Star
2006
We study the physics of the solar corona as a whole, i.e. of the Sun as a Star, in order to understand its global features and to provide a template for stellar coronae. In this process we strive to understand the features of various structures which compose the solar corona. This process in not straightforward given the problems of observing the Sun as a whole: e.g., no recent X‐ray wide‐band, medium‐resolution, spectrum of the Sun is avaible, unlike stars and no X‐ray spectral monitoring of the Sun at various activity phases is available. The presentation will discuss our work in this field; we present the method we have devised, based on Yohkoh/SXT data, to derive the Differential Emissi…
The Sun as a benchmark of flaring activity in stellar coronae
2009
The solar corona is a template to study and understand stellar activity. However the solar corona differs from that of active stars: the Sun has lower X‐ray luminosity, and on average cooler plasma temperatures. Active stellar coronae have a hot peak in their emission measure distribution, EM (T), at 8–20 MK, while the non‐flaring solar corona has a peak at 1–2 MK. In the solar corona significant amounts of plasma at temperature ∼10 MK are observed only during flares.To investigate what is the time‐averaged effect of solar flares we measure the disk‐integrated time‐averaged emission measure, EMF (T), of an unbiased sample of solar flares. To this aim we analyze uninterrupted GOES/XRS light …
Validating post-AGB candidates in the LMC and SMC using SALT spectra
2020
We selected a sample of post-AGB candidates in the Magellanic Clouds on the basis of their near- and mid-infrared colour characteristics. Fifteen of the most optically bright post-AGB candidates were observed with the South African Large Telescope in order to determine their stellar parameters and thus to validate or discriminate their nature as post-AGB objects in the Magellanic Clouds. The spectral types of absorption-line objects were estimated according to the MK classification, and effective temperatures were obtained by means of stellar atmosphere modelling. Emission-line objects were classified on the basis of the fluxes of the emission lines and the presence of the continuum. Out of…
Comparison of Hinode/XRT and RHESSI detection of hot plasma in the non-flaring solar corona
2009
We compare observations of the non-flaring solar corona made simultaneously with Hinode/XRT and with RHESSI. The analyzed corona is dominated by a single active region on 12 November 2006. The comparison is made on emission measures. We derive emission measure distributions vs temperature of the entire active region from multifilter XRT data. We check the compatibility with the total emission measure values estimated from the flux measured with RHESSI if the emission come from isothermal plasma. We find that RHESSI and XRT data analyses consistently point to the presence of a minor emission measure component peaking at log T ~ 6.8-6.9. The discrepancy between XRT and RHESSI results is withi…
X-ray Flares of EV Lac: Statistics, Spectra, Diagnostics
2010
We study the spectral and temporal behavior of X-ray flares from the active M-dwarf EV Lac in 200 ks of exposure with the Chandra/HETGS. We derive flare parameters by fitting an empirical function which characterizes the amplitude, shape, and scale. The flares range from very short (<1 ks) to long (10 ks) duration events with a range of shapes and amplitudes for all durations. We extract spectra for composite flares to study their mean evolution and to compare flares of different lengths. Evolution of spectral features in the density-temperature plane shows probable sustained heating. The short flares are significantly hotter than the longer flares. We determined an upper limit to the Fe…
The Extremely Metal‐poor, Neutron Capture–rich Star CS 22892‐052: A Comprehensive Abundance Analysis
2003
High-resolution spectra obtained with three ground-based facilities and the Hubble Space Telescope (HST) have been combined to produce a new abundance analysis of CS 22892-052, an extremely metal-poor giant with large relative enhancements of neutron-capture elements. A revised model stellar atmosphere has been derived with the aid of a large number of Fe-peak transitions, including both neutral and ionized species of six elements.Several elements, including Mo, Lu, Au, Pt and Pb, have been detected for the first time in CS 22892-052, and significant upper limits have been placed on the abundances of Ga, Ge, Cd, Sn, and U in this star. In total, abundance measurements or upper limits have b…
Multi-wavelength diagnostics of accretion in an X-ray selected sample of CTTSs
2010
High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in Classical T-Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies. We aim to test the hypothesis that the high density soft X-ray emission is from accretion by analysing optical accretion tracers from an X-ray selected sample of CTTSs in a homogeneous manner. We analyse optical spectra of a sample of CTTSs and calculate the accretion rates based on measuring optical emission lines. These are then compared to the accretion rates derived from the X-ray…
Supersonic Magnetic Upflows in Granular Cells Observed with Sunrise/IMaX
2010
Using the IMaX instrument on-board the Sunrise stratospheric balloon-telescope we have detected extremely shifted polarization signals around the Fe I 5250.217 {\AA} spectral line within granules in the solar photosphere. We interpret the velocities associated with these events as corresponding to supersonic and magnetic upflows. In addition, they are also related to the appearance of opposite polarities and highly inclined magnetic fields. This suggests that they are produced by the reconnection of emerging magnetic loops through granular upflows. The events occupy an average area of 0.046 arcsec$^2$ and last for about 80 seconds, with larger events having longer lifetimes. These supersoni…
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…