1,721,476 research outputs found
Simbol-X capability of detecting the non-thermal emission of stellar flares
Full text linkWe investigate the capability of detecting, with Simbol-X, non-thermal emission during stellar flares, and distinguishing it from hot thermal emission. We find that flare non-thermal emission is detectable when at least ̃20 cts are detected with the CZT detector in the 20-80 keV band. Therefore Simbol-X will detect the non-thermal emission from some of the X-ray brightest nearby stars, whether the thermal vs. non-thermal relation, derived for solar flares, holds
Correcting the effect of stellar spots on ARIEL transmission spectra
Full text linkThe goal of this study is to assess the impact of the stellar spots on the extraction of the planetary transmission spectra observed by ARIEL. We develop a method to model the stellar spectrum of a star in the presence of spots by using the out-of-transit observations. It is based on a chi squared minimization procedure of the out-of-transit spectrum on a grid of stellar spectra with different sizes and temperatures of the spots. The approach allows us also to study the temporal evolution of the spots when comparing stellar spectra observed at different epochs. We also present a method to correct the transit depth variations due to non-occulted stellar spots and estimate the error we introduce if we apply the same correction to crossings over the stellar spots. The method is tested on three types of stellar targets that ARIEL will observe in its 4-yr mission lifetime. In all the explored cases, the approach allows us to reliably recover the spot parameters (size and temperature) from out-of-transit observations and, for non-occulted spots, to confidently recover the planetary atmosphere transmission spectrum within the noise level (with average uncertainty of at most 3.3 per cent of the planetary signal). Conversely, we find systematic biases in the inferred planetary spectra due to the occulted spots, with measurable effects for the brightest targets especially for more contrasted spots
Recent X-ray studies of stellar cycles and long-term variability
No full textWe discuss recent X-ray studies of stellar cycles and long-term variability
Modeling the long duration rise phase of a flare detected on the M star TWA 11 B
No full textWe present preliminary results from the analysis of a long duration flare that was observed in a serendipitous XMM-Newton detection of the M star CD-39 7717 B (TWA 11 B), member of the young stellar association TW Hya. Only the rise phase (with a duration of ~35 ks) and possibly the flare peak are present in the light-curve. The decay phase was not monitored. The fluorescent iron emission line at 6.4 keV was observed during this event. As far as we are concerned, since TWA 11 B seems to have no disk, this is only the third detection of Fe photospheric fluorescence. During the flare, the X-ray flux increased a factor of ~4. Taking the light curve and the evolution of the hardness ratio into account, we interpret the rise phase as resulting from the ignition of a first group of loops (part 1) which triggered a subsequent two-ribbon flare (part 2). Part 1 is analysed using the Reale's (2007) model, since it has been assumed to be dominated by a single loop. For part 2, the diagnostic method for two-ribbon flares developed by Kopp & Poletto (1984) is applied. Loop semi-lengths of about 2.5-3.0 stellar radii are obtained. These large structures were previously detected only in very young objects, this is the first time that are detected in a more evolved (~8 Myr) star
In-flight calibration of the ROSAT HRI ultraviolet sensitivity
No full textComparing measured and estimated count rates of a few selected sample stars, we confirm the validity and provide the in-flight calibration of the ROSAT HRI UV/visible effective area model in Zombeck et al. The count rate estimates for Betelgeuse derived with this model are in agreement with the measured HRI upper limit. This result is also confirmed in an erratum by Berghöfer et al. aimed at revising their previous calculation, which was overestimated by more than 2 orders of magnitude. Adopting this ROSAT HRI UV/visible effective area model and measured UV/visible spectra of a set of sample stars covering the range of Teff 3000-40,000 K, we have built the calibration curves to estimate UV/visible contamination count rates for any star of known Teff, mv, and NH
Activity-rotation relation in the young cluster h Per
No full textThe activity-rotation relation in PMS stars is still an open issue. To bridge the gap between the well constrained case of MS stars and the puzzling case of very young PMS stars, we studied the activity-rotation relation in the young cluster h Persei. Because of its age ( ̃13 Myr) h Per contains both fast and slow rotators, allowing us therefore to test the different regimes of stellar dynamo. We analyzed a deep Chandra/ACIS-I observation to constrain the activity level of h Per members. Rotational periods were derived by tet{MorauxArtemenko2013}. From the comparison of the 1002 detected X-ray sources, and the 586 h Per members with measured rotational period, we obtained a final catalog of 169 h Per members with measured X-ray luminosity (ranging between 3.5×10^{29} and 1.1×10^{31} {erg s^{-1}}), rotational period (ranging between 0.22 and 15.9 d), and mass (ranging between 0.2 and 1.8 {M_{⊙}}). We find clear evidence for supersaturation in stars with mass between 1.0 and 1.6 M_{⊙}. This phenomenon is unobserved for lower mass stars
A Detailed Study of the Rise Phase of a Long Duration X-Ray Flare in the Young Star TWA 11B
No full textWe analyzed a long duration flare observed in a serendipitous XMM-Newton detection of the M star CD-39 7717B (TWA 11B), member of the young stellar association TW Hya (~8 Myr). Only the rise phase (with a duration of ~35 ks) and possibly the flare peak were observed. We took advantage of the high count rate of the X-ray source to carry out a detailed analysis of its spectrum during the whole exposure. After a careful analysis, we interpreted the rise phase as resulting from the ignition of a first group of loops (event A) which triggered a subsequent two-ribbon flare (event B). Event A was analyzed using a single-loop model, while a two-ribbon model was applied for event B. Loop semi-lengths of ~4 R * were obtained. Such large structures had been previously observed in very young stellar objects (~1-4 Myr). This is the first time that they have been inferred in a slightly more evolved star. The fluorescent iron emission line at 6.4 keV was detected during event B. Since TWA 11B seems to have no disk, the most plausible explanation found for its presence in the X-ray spectrum of this star is collisional- or photo-ionization. As far as we are concerned, this is only the third clear detection of Fe photospheric fluorescence in stars other than the Sun
Activity and Rotation in the young cluster h Per
Full text linkWe study the stellar rotation-activity relation in the crucial age at which stars reach the fastest rotation. To this aim we have analyzed data of the young cluster h Per, very rich and compact, located at 2300 pc, that at an age of 13 Myr should be mainly composed of stars that have ended their contraction phase and that have not lost significant angular momentum viamagnetic breaking. To constrain the activity level of h Per members we have analyzed a deep Chandra/ACIS-I observation. Rotational periods of h Per members have been derived by Moraux et al. (2013) in the framework of the MONITOR project (Aigrain et al. 2007; Irwin et al. 2007). In the Chandra observation we have detected 1010 X-ray sources located in the central field of h Persei. Assuming a distance of 2300 pc their X-ray luminosity ranges between 2x10^29 and 6x10^31 erg/s. Among the 1010 x-ray sources ~600 have as optical counterpart candidate members of the cluster with masses ranging down to 0.3 solar mass, and ̃150 have also measured rotational period. For this sample of ̃150 h Per members we have compared X-ray luminosity and rotational periods for different mass ranges. We have found that solar type stars (~1.3 solar mass) show evidence of supersaturation for short periods. This phenomenon is unobserved for lower mass stars
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