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| 2. |
- Airapetian, Vladimir S., et al.
(författare)
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One Year in the Life of Young Suns : Data-constrained Corona-wind Model of κ1 Ceti
- 2021
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 916:2
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Tidskriftsartikel (refereegranskat)abstract
- The young magnetically active solar-like stars are efficient generators of ionizing radiation in the form of X-ray and extreme-UV (EUV) flux, stellar wind, and eruptive events. These outputs are the critical factors affecting atmospheric escape and chemistry of (exo)planets around active stars. While X-ray fluxes and surface magnetic fields can be derived from observations, the EUV emission, and wind mass fluxes, coronal mass ejections and associated stellar energetic particle events cannot be directly observed. Here, we present the results of a three-dimensional magnetohydrodynamic (MHD) model with inputs constrained by spectropolarimetric data, Hubble Space Telescope/STIS far-UV and X-ray data, and stellar magnetic maps reconstructed at two epochs separated by 11 months. The simulations show that over the course of the year the global stellar corona had undergone a drastic transition from a simple dipole-like to a tilted dipole with multipole field components and thus provided favorable conditions for corotating interaction regions (CIRs) that drive strong shocks. The dynamic pressures exerted by CIRs are 1300 times larger than those observed from the Sun and can contribute to the atmospheric erosion of early Venus, Earth, Mars, and young Earth-like exoplanets. Our data-constrained MHD model provides the framework to model coronal environments of G-M planet-hosting dwarfs. The model outputs can serve as a realistic input for exoplanetary atmospheric models to evaluate the impact of stellar coronal emission, stellar winds, and CIRs on their atmospheric escape and chemistry that can be tested in the upcoming James Webb Space Telescope and ground-based observations.
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| 3. |
- Alecian, E., et al.
(författare)
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Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 567, s. A28-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The Magnetism in Massive Stars (MiMeS) project aims at understanding the origin of the magnetic fields in massive stars as well as their impact on stellar internal structure, evolution, and circumstellar environment. Aims. One of the objectives of the MiMeS project is to provide stringent observational constraints on the magnetic fields of massive stars; however, identification of magnetic massive stars is challenging, as only a few percent of high-mass stars host strong fields detectable with the current instrumentation. Hence, one of the first objectives of the MiMeS project was to search for magnetic objects among a large sample of massive stars, and to build a sub-sample for in-depth follow-up studies required to test the models and theories of fossil field origins, magnetic wind confinement and magnetospheric properties, and magnetic star evolution. Methods. We obtained high-resolution spectropolarimetric observations of a large number of OB stars thanks to three large programs (LP) of observations that have been allocated on the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric module HARPSpol of the HARPS spectrograph. We report here on the methods and first analysis of the HARPSpol magnetic detections. We identified the magnetic stars using a multi-line analysis technique. Then, when possible, we monitored the new discoveries to derive their rotation periods, which are critical for follow-up and magnetic mapping studies. We also performed a first-look analysis of their spectra and identified obvious spectral anomalies (e. g., surface abundance peculiarities, Ha emission), which are also of interest for future studies. Results. In this paper, we focus on eight of the 11 stars in which we discovered or confirmed a magnetic field from the HARPSpol LP sample (the remaining three were published in a previous paper). Seven of the fields were detected in early-type Bp stars, while the last field was detected in the Ap companion of a normal early B-type star. We report obvious spectral and multiplicity properties, as well as our measurements of their longitudinal field strengths, and their rotation periods when we are able to derive them. We also discuss the presence or absence of Ha emission with respect to the theory of centrifugally-supported magnetospheres.
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| 4. |
- Alecian, E., et al.
(författare)
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First HARPSpol discoveries of magnetic fields in massive stars
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 536, s. L6-
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Tidskriftsartikel (refereegranskat)abstract
- In the framework of the Magnetism in Massive Stars (MiMeS) project, a HARPSpol Large Program at the 3.6m-ESO telescope has recently started to collect high-resolution spectropolarimetric data of a large number of Southern massive OB stars in the field of the Galaxy and in many young clusters and associations. We report on the first discoveries of magnetic fields in two massive stars with HARPSpol - HD 130807 and HD 122451, and confirm the presence of a magnetic field at the surface of HD 105382 that was previously observed with a low spectral resolution device. The longitudinal magnetic field measurements strongly vary for HD 130807 from similar to-100 G to similar to 700 G. Those of HD 122451 and HD 105382 are less variable with values ranging from similar to-40 to -80 G, and from similar to-300 to -600 G, respectively. The discovery and confirmation of three new magnetic massive stars, including at least two He-weak stars, is an important contribution to one of MiMeS objectives: the understanding of the origin of magnetic fields in massive stars and their impact on stellar structure and evolution.
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| 5. |
- Alecian, E., et al.
(författare)
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The dramatic change of the fossil magnetic field of HD 190073 : Evidence of the birth of the convective core in a Herbig star?
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 549, s. L8-
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Tidskriftsartikel (refereegranskat)abstract
- In the context of the ESPaDOnS and Narval spectropolarimetric surveys of Herbig Ae/Be stars, we discovered and then monitored the magnetic field of HD 190073 over more than four years, from 2004 to 2009. Our observations all displayed similar Zeeman signatures in the Stokes V spectra, indicating that HD 190073 hosted an aligned dipole, stable over many years, consistent with a fossil origin. We obtained new observations of the star in 2011 and 2012 and detected clear variations of the Zeeman signature on timescales of days to weeks, indicating that the configuration of its field has changed between 2009 and 2011. Such a sudden change of external structure of a fossil field has never previously been observed in any intermediate or high-mass star. HD 190073 is an almost entirely radiative pre-main sequence star, probably hosting a growing convective core. We propose that this dramatic change is the result of the interaction between the fossil field and the ignition of a dynamo field generated in the newly-born convective core.
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| 6. |
- Alentiev, D., et al.
(författare)
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Discovery of the longest period rapidly oscillating Ap star HD177765
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925. ; 421:1, s. L82-L86
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Tidskriftsartikel (refereegranskat)abstract
- We present the discovery of a long-period, rapidly oscillating Ap (roAp) star, HD177765. Using high-resolution time-series observations obtained with the Ultraviolet and Visual Echelle Spectrograph at the European Southern Observatory Very Large Telescope, we found radial velocity variations with amplitudes 7-150 ms(-1) and a period of 23.6 min, exceeding that of any previously known roAp star. The largest pulsation amplitudes are observed for Eu III, Ce III and for the narrow core of H alpha. We derived the atmospheric parameters and chemical composition of HD177765, showing this star to be similar to other long-period roAp stars. Comparison with theoretical pulsational models indicates an advanced evolutionary state for HD177765. Abundance analyses of this and other roAp stars suggest a systematic variation with age of the rare-earth line anomalies seen in cool Ap stars.
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| 7. |
- Alvarado-Gomez, J. D., et al.
(författare)
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Activity and magnetic field structure of the Sun-like planet-hosting star HD 1237
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 582
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Tidskriftsartikel (refereegranskat)abstract
- We analyse the magnetic activity characteristics of the planet-hosting Sun-like star, HD 1237, using HARPS spectro-polarimetric time-series data. We find evidence of rotational modulation of the magnetic longitudinal field measurements that is consistent with our ZDI analysis with a period of 7 days. We investigate the effect of customising the LSD mask to the line depths of the observed spectrum and find that it has a minimal effect on the shape of the extracted Stokes V profile but does result in a small increase in the S/N (similar to 7%). We find that using a Milne-Eddington solution to describe the local line profile provides a better fit to the LSD profiles in this slowly rotating star, which also affects the recovered ZDI field distribution. We also introduce a fit-stopping criterion based on the information content (entropy) of the ZDI map solution set. The recovered magnetic field maps show a strong (+90 G) ring-like azimuthal field distribution and a complex radial field dominating at mid latitudes (similar to 45 degrees). Similar magnetic field maps are recovered from data acquired five months apart. Future work will investigate how this surface magnetic field distribution affeccts the coronal magnetic field and extended environment around this planet-hosting star.
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| 8. |
- Alvarado-Gomez, Julian D., et al.
(författare)
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Simulating the Space Weather in the AU Mic System : Stellar Winds and Extreme Coronal Mass Ejections
- 2022
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Ingår i: Astrophysical Journal. - : IOP Publishing Ltd. - 0004-637X .- 1538-4357. ; 928:2
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Tidskriftsartikel (refereegranskat)abstract
- Two close-in planets have been recently found around the M-dwarf flare star AU Microscopii (AU Mic). These Neptune-sized planets (AU Mic b and c) seem to be located very close to the so-called "evaporation valley" in the exoplanet population, making this system an important target for studying atmospheric loss on exoplanets. This process, while mainly driven by high-energy stellar radiation, will be strongly mediated by the space environment surrounding the planets. Here we present an investigation of this last area, performing 3D numerical modeling of the quiescent stellar wind from AU Mic, as well as time-dependent simulations describing the evolution of a highly energetic coronal mass ejection (CME) event in this system. Observational constraints on the stellar magnetic field and properties of the eruption are incorporated in our models. We carry out qualitative and quantitative characterizations of the stellar wind, the emerging CMEs, as well as the expected steady and transient conditions along the orbit of both exoplanets. Our results predict extreme space weather for AU Mic and its planets. This includes sub-Alfvenic regions for the large majority of the exoplanet orbits, very high dynamic and magnetic pressure values in quiescence (varying within 10(2)-10(5) times the dynamic pressure experienced by Earth), and an even harsher environment during the passage of any escaping CME associated with the frequent flaring observed in AU Mic. These space weather conditions alone pose an immense challenge for the survival of exoplanetary atmospheres (if any) in this system.
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| 9. |
- Asensio Ramos, A., et al.
(författare)
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Approximate Bayesian neural Doppler imaging
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The non-uniform surface temperature distribution of rotating active stars is routinely mapped with the Doppler imaging technique. Inhomogeneities in the surface produce features in high-resolution spectroscopic observations that shift in wavelength because of the Doppler effect, depending on their position on the visible hemisphere. The inversion problem has been systematically solved using maximum a posteriori regularized methods assuming smoothness or maximum entropy. Our aim in this work is to solve the full Bayesian inference problem by providing access to the posterior distribution of the surface temperature in the star compatible with the observations.Methods. We use amortized neural posterior estimation to produce a model that approximates the high-dimensional posterior distribution for spectroscopic observations of selected spectral ranges sampled at arbitrary rotation phases. The posterior distribution is approximated with conditional normalizing flows, which are flexible, tractable, and easy-to-sample approximations to arbitrary distributions. When conditioned on the spectroscopic observations, these normalizing flows provide a very efficient way of obtaining samples from the posterior distribution. The conditioning on observations is achieved through the use of Transformer encoders, which can deal with arbitrary wavelength sampling and rotation phases.Results. Our model can produce thousands of posterior samples per second, each one accompanied by an estimation of the log-probability. Our exhaustive validation of the model for very high-signal-to-noise observations shows that it correctly approximates the posterior, albeit with some overestimation of the broadening. We apply the model to the moderately fast rotator II Peg, producing the first Bayesian map of its temperature inhomogenities. We conclude that conditional normalizing flows are a very promising tool for carrying out approximate Bayesian inference in more complex problems in stellar physics, such as constraining the magnetic properties using polarimetry.
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| 10. |
- Bagnulo, S., et al.
(författare)
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Magnetic field measurements and their uncertainties : the FORS1 legacy
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 538, s. A129-
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Tidskriftsartikel (refereegranskat)abstract
- Context. During the last decade, the FORS1 instrument of the ESO Very Large Telescope has been extensively used to study stellar magnetism. A number of interesting discoveries of magnetic fields in several classes of stars have been announced, many of which obtained at a similar to 3 sigma level; some of the discoveries are confirmed by measurements obtained with other instruments, some are not.Aims. We investigate the reasons for the discrepancies between the results obtained with FORS1 and those obtained with other instruments.Methods. Using the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determination. We have applied this procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive (except for most of the observations obtained in multi-object spectropolarimetric mode). We have devised and applied a number of consistency checks to our field determinations, and we have compared our results to those previously published in the literature.Results. We find that for high signal-to-noise ratio measurements, photon noise does not account for the full error bars. We discuss how field measurements depend on the specific algorithm adopted for data reduction, and we show that very small instrument flexures, negligible in most of the instrument applications, may be responsible for some spurious field detections in the null profiles. Finally, we find that we are unable to reproduce some results previously published in the literature. Consequently, we do not confirm some important discoveries of magnetic fields obtained with FORS1 and reported in previous publications.Conclusions. Our revised field measurements show that there is no contradiction between the results obtained with the low-resolution spectropolarimeter FORS1 and those obtained with high-resolution spectropolarimeters. FORS1 is an instrument capable of performing reliable magnetic field measurements, provided that the various sources of uncertainties are properly taken into account.
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