| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- Bahar, Engin, et al.
(författare)
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First Chromospheric Activity and Doppler Imaging Study of PW And Using a New Doppler Imaging Code : SpotDIPy
- 2024
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 960:1
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Tidskriftsartikel (refereegranskat)abstract
- Measuring the coverage of dark spots on cool stars is important to understanding how stellar magnetic activity scales with the rotation rate and convection zone depth. In this respect, it is crucial to infer surface magnetic patterns on G and K stars, to reveal solar-like stellar dynamos in action. Molecular bands serve as invaluable indicators of cool spots on the surfaces of stars, as they play a crucial role in enabling accurate assessments of the extent of spot coverage across the stellar surface. Therefore, more reliable surface images can be obtained considering the inversion of atomic lines with molecular bands. In this context, we simultaneously carry out Doppler imaging (DI) using atomic lines as well as titanium oxide band profiles of PW And (K2 V) and also investigate chromospheric activity indicators for the first time in the literature, using high-resolution spectra. The surface spot distribution obtained from the inversion process represents both atomic line and TiO-band profiles quite accurately. The chromospheric emission is also correlated with photospheric spot coverage, except during a possible flare event during the observations. We detect frequent flare activity, using TESS photometry. We also introduce a new open-source, Python-based DI code SpotDIPy that allows performing surface reconstructions of single stars using the maximum entropy method. We test the code by comparing surface reconstruction simulations with the extensively used DoTS code. We show that the surface brightness distribution maps reconstructed via both codes using the same simulated data are consistent with each other.
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| 5. |
- Bellotti, S., et al.
(författare)
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Monitoring the large-scale magnetic field of AD Leo with SPIRou, ESPaDOnS, and Narval : Towards a magnetic polarity reversal?
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 676
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Tidskriftsartikel (refereegranskat)abstract
- Context: One clear manifestation of dynamo action on the Sun is the 22-yr magnetic cycle, exhibiting a polarity reversal and a periodic conversion between poloidal and toroidal fields. For M dwarfs, several authors claim evidence of activity cycles from photometry and analyses of spectroscopic indices, but no clear polarity reversal has been identified from spectropolarimetric observations. These stars are excellent laboratories to investigate dynamo-powered magnetic fields under different stellar interior conditions, that is partly or fully convective.Aims: Our aim is to monitor the evolution of the large-scale field of AD Leo, which has shown hints of a secular evolution from past dedicated spectropolarimetric campaigns. This is of central interest to inform distinct dynamo theories, contextualise the evolution of the solar magnetic field, and explain the variety of magnetic field geometries observed in the past.Methods: We analysed near-infrared spectropolarimetric observations of the active M dwarf AD Leo taken with SPIRou between 2019 and 2020 and archival optical data collected with ESPaDOnS and Narval between 2006 and 2019. We searched for long-term variability in the longitudinal field, the width of unpolarised Stokes profiles, the unsigned magnetic flux derived from Zeeman broadening, and the geometry of the large-scale magnetic field using both Zeeman-Doppler imaging and principal component analysis.Results: We found evidence of a long-term evolution of the magnetic field, featuring a decrease in axisymmetry (from 99% to 60%). This is accompanied by a weakening of the longitudinal field (-300 to -50 G) and a correlated increase in the unsigned magnetic flux (2.8-3.6 kG). Likewise, the width of the mean profile computed with selected near-infrared lines manifests a long-term evolution corresponding to field strength changes over the full time series, but does not exhibit modulation with the stellar rotation of AD Leo in individual epochs.Conclusions: The large-scale magnetic field of AD Leo manifested first hints of a polarity reversal in late 2020 in the form of a substantially increased dipole obliquity, while the topology remained predominantly poloidal and dipolar for 14 yr. This suggests that low-mass M dwarfs with a dipole-dominated magnetic field can undergo magnetic cycles.
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| 6. |
- Blazere, Aurore, et al.
(författare)
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Magnetic geometry and surface differential rotation of the bright Am star Alhena A
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 492:4, s. 5794-5810
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Tidskriftsartikel (refereegranskat)abstract
- Alhena A (y Gem A) is a bright Am star, with the strongest disc -integrated magnetic field strength reported so far for an Am star. Its spectrum exhibits standard circularly polarized Zeeman signatures, contrary to all previously studied Am stars that display abnormal signatures dominated by a single -signed lobe. We present here the result of follow-up observations of Athena, using very high signal-to-noise spectropolarimetric data obtained over 25 observing nights with NARVAL at Telescope Bernard Lyot, in the frame of the BRITE (BRIght Target Explorer) spectropolarimetric survey. We confirm that Alhena A is magnetic and we determine its surface magnetic properties using different methods. Inclined dipole models are used to reproduce the longitudinal field measurements, as well as the Stokes V line profiles themselves. In both cases, the model is consistent with a polar field strength of 30 G. This is confirmed by a Zeeman -Doppler Imaging (ZDI) model, which also unveils smaller scale magnetic structures. A rotational period of 8.975 d was identified using intensity line profile variations. The ZDI inversion suggests that the surface magnetic field is sheared by differential rotation, with a difference in rotation rate between high and low latitudes at about 15 per cent of the solar value. This result challenges theories of the development of surface differential rotation in intermediate mass main -sequence stars.
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| 7. |
- Boldt-Christmas, Linn, et al.
(författare)
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Optimising spectroscopic observations of transiting exoplanets
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 683
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Tidskriftsartikel (refereegranskat)abstract
- Context. When observing the atmospheres of transiting exoplanets using high-resolution spectroscopy, the aim is to detect well-resolved spectral features with high signal-to-noise ratios (S/Ns), as is possible today with modern spectrographs. However, obtaining such high-quality observations comes with a trade-off: a lower cadence of fewer, longer exposures across the transit collects more photons thanks to reduced overheads, enhancing the S/N of each observation, while a higher cadence of several shorter exposures minimises spectral feature smearing due to the continuously changing radial velocity of the planet.Aims. Considering that maximising S/N and minimising smearing are both beneficial to analysis, there is a need to identify the optimal compromise between the two for a given target. In this work, we aim to establish where this compromise lies for a typical exoplanet transit observation in order to benefit future data collection and subsequent interpretation.Methods. We modelled real transit events based on targets as they would be observed with VLT/CRIRES+ at Paranal Observatory, Chile. Creating four hypothetical scenarios, we simulated each set of transmission spectra across 100 realisations of the same transit event in order to vary the time resolution only. We removed telluric and stellar lines from these data sets using the SYSREM algorithm and analysed them through cross-correlation with model templates, measuring how successfully each time resolution and case detected the planetary signal and exploring how the results vary.Results. We demonstrate that there is a continuous change in the significance of the cross-correlation detection based on the trade-off between high and low time resolutions, and that, averaged over a large number of realisations, the function of this significance has clear maxima. The strength and location of these maxima vary depending on, for example, planet system parameters, instrumentation, and the number of removal iterations. We discuss why observers should therefore take several factors into account using a strategy akin to the 'exposure triangle' employed in traditional photography where a balance must be struck by considering the full context of the observation. Our method is robust and may be employed by observers to estimate the best observational strategies for other targets.
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| 8. |
- Creevey, O. L., et al.
(författare)
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Gaia Data Release 3 : Astrophysical parameters inference system (Apsis). I. Methods and content overview
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
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Tidskriftsartikel (refereegranskat)abstract
- Gaia Data Release 3 contains a wealth of new data products for the community. Astrophysical parameters are a major component of this release, and were produced by the Astrophysical parameters inference system (Apsis) within the Gaia Data Processing and Analysis Consortium (DPAC). The aim of this paper is to describe the overall content of the astrophysical parameters in Gaia DR3 and how they were produced. In Apsis, we use the mean BP /RP and mean RVS spectra along with astrometry and photometry, and we derive the following parameters: source classification and probabilities for 1.6 billion objects; interstellar medium characterisation and distances for up to 470 million sources, including a 2D total Galactic extinction map; 6 million redshifts of quasar candidates; 1.4 million redshifts of galaxy candidates; and an analysis of 50 million outlier sources through an unsupervised classification. The astrophysical parameters also include many stellar spectroscopic and evolutionary parameters for up to 470 million sources. These comprise T-eff, log g, and [M /H] (470 million using BP /RP, 6 million using RVS), radius (470 million), mass (140 million), age (120 million), chemical abundances (up to 5 million), diffuse interstellar band analysis (0.5 million), activity indices (2 million), H ff equivalent widths (200 million), and further classification of spectral types (220 million) and emission-line stars (50 000). This paper is the first in a series of three papers, and focusses on describing the global content of the parameters in Gaia DR3. The accompanying Papers II and III focus on the validation and use of the stellar and non-stellar products, respectively. This catalogue is the most extensive homogeneous database of astrophysical parameters to date, and is based uniquely on Gaia data. It will only be superseded by Gaia Data Release 4, and will therefore remain a key reference over the next four years, providing astrophysical parameters independent of other ground- and space-based data.
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| 9. |
- Das, Barnali, et al.
(författare)
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Discovery of Eight "Main-sequence Radio Pulse Emitters" Using the GMRT : Clues to the Onset of Coherent Radio Emission in Hot Magnetic Stars
- 2022
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Ingår i: Astrophysical Journal. - : IOP Publishing Ltd. - 0004-637X .- 1538-4357. ; 925:2
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Tidskriftsartikel (refereegranskat)abstract
- Main-sequence radio pulse emitters (MRPs) are magnetic early-type stars from which periodic radio pulses, produced via electron cyclotron maser emission (ECME), are observed. Despite the fact that these stars can naturally offer suitable conditions for triggering ECME, only seven such stars have been reported so far within a span of more than two decades. In this paper, we report the discovery of eight more MRPs, thus more than doubling the sample size of such objects. These discoveries are the result of our sub-GHz observation program using the Giant Metrewave Radio Telescope over the years 2015-2021. Adding these stars to the previously known MRPs, we infer that at least 32% of the magnetic hot stars exhibit this phenomenon, thus suggesting that observation of ECME is not a rare phenomenon. The significantly larger sample of MRPs allows us for the first time to perform a statistical analysis comparing their physical properties. We present an empirical relation that can be used to predict whether a magnetic hot star is likely to produce ECME. Our preliminary analysis suggests that the physical parameters that play the primary role in the efficiency of the phenomenon are the maximum surface magnetic field strength and the surface temperature. In addition, we present strong evidence of the influence of the plasma density distribution on ECME pulse profiles. Results of this kind further motivate the search for MRPs, as a robust characterization of the relation between observed ECME properties and stellar physical parameters can only be achieved with a large sample.
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| 10. |
- Dorn, R. J., et al.
(författare)
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CRIRES+ on sky at the ESO Very Large Telescope : Observing the Universe at infrared wavelengths and high spectral resolution
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 671
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Tidskriftsartikel (refereegranskat)abstract
- The CRyogenic InfraRed Echelle Spectrograph (CRIRES) Upgrade project CRIRES+ extended the capabilities of CRIRES. It transformed this VLT instrument into a cross-dispersed spectrograph to increase the wavelength range that is covered simultaneously by up to a factor of ten. In addition, a new detector focal plane array of three Hawaii 2RG detectors with a 5.3 mu m cutoff wavelength replaced the existing detectors. Amongst many other improvements, a new spectropolarimetric unit was added and the calibration system has been enhanced. The instrument was installed at the VLT on Unit Telescope 3 at the beginning of 2020 and successfully commissioned and verified for science operations during 2021, partly remotely from Europe due to the COVID-19 pandemic. The instrument was subsequently offered to the community from October 2021 onwards. This article describes the performance and capabilities of the upgraded instrument and presents on sky results.
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