Astronomy & Astrophysics

Journal: Astronomy & Astrophysics

Abbreviation

Astron. Astrophys.

Publisher

EDP Sciences

ISSN

0004-6361
1432-0746

Show all metadata

Search Results

Publications 1 - 10 of 1030

The SPHERE view of the planet-forming disk around HD 100546
Garufi, A.; Quanz, Sascha Patrick; Schmid, H.M.; et al. (2016)
Astronomy & Astrophysics
Collapsed, uncollapsed, and hidden magnetic flux on the quiet Sun
Stenflo, Jan Olof (2011)
Astronomy & Astrophysics
The influence of NLTE effects in Fe I lines on an inverted atmosphere
Smitha, H. Narayanamurthy; Holzreuter, René; van Noort, Michiel; et al. (2020)
Astronomy & Astrophysics
Context. Ultraviolet overionisation of iron atoms in the solar atmosphere leads to deviations in their level populations based on Saha-Boltzmann statistics. This causes their line profiles to form in non-local thermodynamic equilibrium (NLTE) conditions. When inverting such profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE. Aims. We investigate how the routinely employed LTE inversion of iron lines formed in NLTE underestimates or overestimates atmospheric quantities, such as temperature (T), line-of-sight velocity (vLOS), magnetic field strength (B), and inclination (γ) while the earlier papers have focused mainly on T. Our findings has wide-ranging consequences since many results derived in solar physics are based on inversions of Fe I lines carried out in LTE. Methods. We synthesized the Stokes profiles of Fe I 6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of a 3D magnetohydrodynamic simulation. The profiles were then inverted in LTE. We considered the atmosphere inferred from the inversion of LTE profiles as the fiducial model and compared it to the atmosphere resulting from the inversion of NLTE profiles. The observed differences have been attributed to NLTE effects. Results. Neglecting the NLTE effects introduces errors in the inverted atmosphere. While the errors in T can go up to 13%, in vLOS and B, the errors can go as high as 50% or above. We find these errors to be present at all three inversion nodes. Importantly, they survive degradation from the spatial averaging of the profiles. Conclusions. We provide an overview of how neglecting NLTE effects influences the values of T, vLOS, B, and γ that are determined by inverting the Fe I 6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region susceptible to NLTE effects. For an accurate determination of the atmospheric quantities and their stratification, it is, therefore, important to take the NLTE effects into account.
The nearby population of M-dwarfs with WISE: A search for warm circumstellar dust
Avenhaus, H.; Schmid, H.M.; Meyer, Michael R. (2012)
Astronomy & Astrophysics
Testing the ubiquitous presence of very high energy emission in gamma-ray bursts with the MAGIC telescopes
Abe, Shotaro; Abhir, Jayant; Abhishek, Abhishek; et al. (2025)
Astronomy & Astrophysics
Gamma-ray bursts (GRBs) are the most powerful transient objects in the Universe, and they are a primary target for the MAGIC Collaboration. Recognizing the challenges of observing these elusive objects with Imaging Atmospheric Cherenkov Telescopes (IACTs), we implemented a dedicated observational strategy that included an automated procedure for rapid re-pointing to transient sources. Since 2013, this automated procedure has enabled MAGIC to observe GRBs at a rate of approximately ten per year, which led to the successful detection of two GRBs at very high energies (VHE; E > 100 GeV). We present a comprehensive analysis of 42 non-detected GRBs (4 short GRBs) observed by MAGIC from 2013 to 2019. We derived upper limits (ULs) on the observed energy flux as well as on the intrinsic energy flux corrected for absorption by the extragalactic background light (EBL) from the MAGIC observations in selected energy and time intervals. We conducted a comprehensive study of their properties to investigate the reasons for these non-detections, including the possible peculiar properties of TeV-detected GRBs. We find that strong EBL absorption significantly hinders TeV detection for the majority of GRBs in our sample. For a subset of 6 GRBs with redshift z < 2, we compared the UL on the intrinsic flux in the VHE domain with the simultaneous X-ray flux, which is observed to be at the same level in the current population of TeV-detected GRBs. Based on these inferred MAGIC ULs, we conclude that a VHE component with a luminosity comparable to the simultaneously observed X-ray luminosity cannot be ruled out for this sample.
Transit-timing variations in the AU Mic system observed with CHEOPS
Boldog, Ádám; Szabó, Gyula M.; Kriskovics, Levente; et al. (2025)
Astronomy & Astrophysics
Context: AU Mic is a very active M dwarf star with an edge-on debris disk and two known transiting sub-Neptunes with a possible third planetary companion. The two transiting planets exhibit significant transit-timing variations (TTVs) that are caused by the gravi tational interaction between the bodies in the system. Aims: Using photometrical observations taken with the CHaracterizing ExOPlanet Satellite (CHEOPS), we aim to constrain the plan etary radii, the orbital distances, and the periods of AU Mic b and c. Furthermore, our goal is to determine the superperiod of the TTVs for AU Mic b and to update the transit ephemeris for both planets. Additionally, based on the perceived TTVs, we study the possible presence of a third planet in the system. Methods: We conducted ultra-high precision photometric observations with CHEOPS in 2022 and 2023. We used Allesfitter to fit the planetary transits and to constrain the planetary and orbital parameters. We combined our new measurements with results from previous years to determine the periods and amplitudes of the TTVs. We applied dynamical modelling based on TTV measurements from the 2018-2023 period to reconstruct the perceived variations. Results: We found that the orbital distances and periods for AU Mic b and c agree with the results from previous works. However, the values for the planetary radii deviate slightly from previous values, which we attribute to the effect of spots on the stellar surface. AU Mic c showed very strong TTVs, with transits that occurred ∼80 minutes later in 2023 than in 2021. Through a dynamical analysis of the system, we found that the observed TTVs can be explained by a third planet with an orbital period of ∼12.6 days and a mass of 0.203$_{0.024}^{0.022}$ M$_{\bigoplus}$. We explored the orbital geometry of the system and found that AU Mic c has a misaligned retrograde orbit. The limited number of AU Mic observations prevented us from determining the exact dynamical configuration and planetary parameters. Further monitoring of the system with CHEOPS might help to improve these results.
The SPHERE view of the Chamaeleon I star-forming region
Ginski, C.; Garufi, Antonio; Benisty, M.; et al. (2024)
Astronomy & Astrophysics
Context: The past few years have seen a revolution in the study of circumstellar disks. New instrumentation in the near-infrared and (sub)millimeter regimes have allowed us to routinely spatially resolve disks around young stars of nearby star-forming regions. As a result, we have found that substructures with scales of ~10 au in disks are common. We have also revealed a zoo of different morphologies, sizes, and luminosities that is as complex as the diversity of architectures found in evolved exoplanet systems. Aims: We study disk evolutionary trends as they appear in scattered light observations. Scattered light traces the micron-sized particles at the disk surface that are well coupled to the gas. This means that scattered light observations can be used to trace the distribution of the disk gas and its interaction with embedded perturbers. Methods: We used VLT/SPHERE to observe 20 systems in the Cha I cloud in polarized scattered light in the near-infrared. We combined the scattered light observations with existing literature data on stellar properties and with archival ALMA continuum data to study trends with system age and dust mass. We also connected resolved near-infrared observations with the spectral energy distributions of the systems. Results: In 13 of the 20 systems included in this study we detected resolved scattered light signals from circumstellar dust. For the CR Cha, CT Cha, CV Cha, SY Cha, SZ Cha, and VZ Cha systems we present the first detailed descriptions of the disks in scattered light. The observations found typically smooth or faint disks, often with little substructure, with the notable exceptions of SZ Cha, which shows an extended multiple-ringed disk, and WW Cha, which shows interaction with the cloud environment. New high S/N K-band observations of the HD 97048 system in our survey reveal a significant brightness asymmetry that may point to disk misalignment and subsequent shadowing of outer disk regions, possibly related to the suggested planet candidate in the disk. We resolve for the first time the stellar binary in the CS Cha system. Multiple wavelength observations of the disk around CS Cha have revealed that the system contains small, compact dust grains that may be strongly settled, consistent with numerical studies of circumbinary disks. We find in our sample that there is a strong anti-correlation between the presence of a (close) stellar companion and the detection of circumstellar material with five of our seven nondetections located in binary systems. We also find a correlation between disk mass, as inferred from millimeter observations, and the detection of scattered light signal. Finally, we find a tentative correlation between relative disk-to-star brightness in scattered light and the presence of a dust cavity in the inner (unresolved) disk, as traced by the system spectral energy distribution. At the same time, faint disks in our sample are generally younger than 2 Myr.
Multiple injections of energetic electrons associated with the flare and CME event on 9 October 2021
Jebaraj, Immanuel Christopher; Kouloumvakos, Athanasios; Dresing, Nina; et al. (2023)
Astronomy & Astrophysics
Context. We study the solar energetic particle (SEP) event observed on 9 October 2021 by multiple spacecraft, including Solar Orbiter. The event was associated with an M1.6 flare, a coronal mass ejection, and a shock wave. During the event, high-energy protons and electrons were recorded by multiple instruments located within a narrow longitudinal cone. Aims. An interesting aspect of the event was the multi-stage particle energisation during the flare impulsive phase and also what appears to be a separate phase of electron acceleration detected at Solar Orbiter after the flare maximum. We aim to investigate and identify the multiple sources of energetic electron acceleration. Methods. We utilised SEP electron observations from the Energetic Particle Detector (EPD) and hard X-ray (HXR) observations from the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter, in combination with radio observations at a broad frequency range. We focused on establishing an association between the energetic electrons and the different HXR and radio emissions associated with the multiple acceleration episodes. Results. We find that the flare was able to accelerate electrons for at least 20 min during the non-thermal phase, observed in the form of five discrete HXR pulses. We also show evidence that the shock wave contributed to the electron acceleration during and after the impulsive flare phase. The detailed analysis of EPD electron data shows that there was a time difference in the release of low- and high-energy electrons, with the high-energy release delayed. Also, the observed electron anisotropy characteristics suggest a different connectivity during the two phases of acceleration.
Detailed cool star flare morphology with CHEOPS and TESS
Bruno, Giovanni; Pagano, Isabella; Scandariato, Gaetano; et al. (2024)
Astronomy & Astrophysics
Context. White-light stellar flares are proxies for some of the most energetic types of flares, but their triggering mechanism is still poorly understood. As they are associated with strong X and ultraviolet emission, their study is particularly relevant to estimate the amount of high-energy irradiation onto the atmospheres of exoplanets, especially those in their stars’ habitable zone. Aims. We used the high-cadence, high-photometric capabilities of the CHEOPS and TESS space telescopes to study the detailed morphology of white-light flares occurring in a sample of 130 late-K and M stars, and compared our findings with results obtained at a lower cadence. Methods. We employed dedicated software for the reduction of 3 s cadence CHEOPS data, and adopted the 20 s cadence TESS data reduced by their official processing pipeline. We developed an algorithm to separate multi-peak flare profiles into their components, in order to contrast them to those of single-peak, classical flares. We also exploited this tool to estimate amplitudes and periodicities in a small sample of quasi-periodic pulsation (QPP) candidates. Results. Complex flares represent a significant percentage (≳30%) of the detected outburst events. Our findings suggest that high-impulse flares are more frequent than suspected from lower-cadence data, so that the most impactful flux levels that hit close-in exoplanets might be more time-limited than expected. We found significant differences in the duration distributions of single and complex flare components, but not in their peak luminosity. A statistical analysis of the flare parameter distributions provides marginal support for their description with a log-normal instead of a power-law function, leaving the door open to several flare formation scenarios. We tentatively confirmed previous results about QPPs in high-cadence photometry, report the possible detection of a pre-flare dip, and did not find hints of photometric variability due to an undetected flare background. Conclusions. The high-cadence study of stellar hosts might be crucial to evaluate the impact of their flares on close-in exoplanets, as their impulsive phase emission might otherwise be incorrectly estimated. Future telescopes such as PLATO and Ariel, thanks to their high-cadence capability, will help in this respect. As the details of flare profiles and of the shape of their parameter distributions are made more accessible by continuing to increase the instrument precision and time resolution, the models used to interpret them and their role in star-planet interactions might need to be updated constantly.
Scattering polarization in strong chromospheric lines
Holzreuter, R.; Stenflo, Jan Olof (2007)
Astronomy & Astrophysics

Publications 1 - 10 of 1030

Journal: Astronomy & Astrophysics

Abbreviation

Publisher

Journal Volumes

ISSN

Description

Filters

Search Results