| 1. |
- Garcia-Lopez, R., et al.
(författare)
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The GRAVITY young stellar object survey XII. The hot gas disk component in Herbig Ae/Be stars
- 2024
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 684
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Tidskriftsartikel (refereegranskat)abstract
- Context. The region of protoplanetary disks closest to a star (within 1–2 au) is shaped by a number of different processes, from accretion of the disk material onto the central star to ejection in the form of winds and jets. Optical and near-IR emission lines are potentially good tracers of inner disk processes if very high spatial and/or spectral resolution are achieved. Aims. In this paper, we exploit the capabilities of the VLTI-GRAVITY near-IR interferometer to determine the location and kinematics of the hydrogen emission line Brγ. Methods. We present VLTI-GRAVITY observations of the Brγ line for a sample of 26 stars of intermediate mass (HAEBE), the largest sample so far analysed with near-IR interferometry. Results. The Brγ line was detected in 17 objects. The emission is very compact (in most cases only marginally resolved), with a size of 10–30 R∗(1–5 mas). About half of the total flux comes from even smaller regions, which are unresolved in our data. For eight objects, it was possible to determine the position angle (PA) of the line-emitting region, which is generally in agreement with that of the inner-dusty disk emitting the K-band continuum. The position-velocity pattern of the Brγ line-emitting region of the sampled objects is roughly consistent with Keplerian rotation. The exception is HD 45677, which shows more extended emission and more complex kinematics. The most likely scenario for the Brγ origin is that the emission comes from an MHD wind launched very close to the central star, in a region well within the dust sublimation radius. An origin in the bound gas layer at the disk surface cannot be ruled out, while accreting matter provides only a minor fraction of the total flux. Conclusions. These results show the potential of near-IR spectro-interferometry to study line emission in young stellar objects.
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| 2. |
- 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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| 3. |
- Saikia, S. Boro, et al.
(författare)
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Time evolution of magnetic activity cycles in young suns : The curious case of kappa Ceti
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Tidskriftsartikel (refereegranskat)abstract
- Context. A detailed investigation of the magnetic properties of young Sun-like stars can provide valuable information on our Sun's magnetic past and its impact on the early Earth.Aims. We determine the properties of the moderately rotating young Sun-like star kappa Ceti's magnetic and activity cycles using 50 yr of chromospheric activity data and six epochs of spectropolarimetric observations.Methods. The chromospheric activity was determined by measuring the flux in the Call H and K lines. A generalised Lomb-Scargle periodogram and a wavelet decomposition were used on the chromospheric activity data to establish the associated periodicities. The vector magnetic field of the star was reconstructed using the technique of Zeeman Doppler imaging on the spectropolarimetric observations.Results. Our period analysis algorithms detect a 3.1 yr chromospheric cycle in addition to the star's well-known similar to 6 yr cycle period. Although the two cycle periods have an approximate 1:2 ratio, they exhibit an unusual temporal evolution. Additionally, the spectropolarimetric data analysis shows polarity reversals of the star's large-scale magnetic field, suggesting a similar to 10 yr magnetic or Hale cycle.Conclusions. The unusual evolution of the star's chromospheric cycles and their lack of a direct correlation with the magnetic cycle establishes kappa Ceti as a curious young Sun. Such complex evolution of magnetic activity could be synonymous with moderately active young Suns, which is an evolutionary path that our own Sun could have taken.
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| 4. |
- Shultz, M. E., et al.
(författare)
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The magnetic early B-type stars - IV. Breakout or leakage? : H α emission as a diagnostic of plasma transport in centrifugal magnetospheres
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 499:4, s. 5379-5395
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Tidskriftsartikel (refereegranskat)abstract
- Rapidly rotating early-type stars with strong magnetic fields frequently show H alpha emission originating in centrifugal magnetospheres (CMs), circumstellar structures in which centrifugal support due to magnetically enforced corotation of the magnetically confined plasma enables it to accumulate to high densities. It is not currently known whether the CM plasma escapes via centrifugal breakout (CB), or by an unidentified leakage mechanism. We have conducted the first comprehensive examination of the H alpha emission properties of all stars currently known to display CM-pattern emission. We find that the onset of emission is dependent primarily on the area of the CM, which can be predicted simply by the value B-K of the magnetic field at the Kepler corotation radius R-K. Emission strength is strongly sensitive to both CM area and B-K. Emission onset and strength are not dependent on effective temperature, luminosity, or mass-loss rate. These results all favour a CB scenario; however, the lack of intrinsic variability in any CM diagnostics indicates that CB must be an essentially continuous process, i.e. it effectively acts as a leakage mechanism. We also show that the emission profile shapes are approximately scale-invariant, i.e. they are broadly similar across a wide range of emission strengths and stellar parameters. While the radius of maximum emission correlates closely as expected to R-K, it is always larger, contradicting models that predict that emission should peak at R-K.
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| 5. |
- Nowacki, H., et al.
(författare)
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Star-disk interactions in the strongly accreting T Tauri star S CrA N
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 678
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Tidskriftsartikel (refereegranskat)abstract
- Context. Classical T Tauri Stars are thought to accrete material from their surrounding protoplanetary disks through funnel flows along their magnetic field lines. Among them, those with high accretion rates (similar to 10(-7)M(circle dot) yr(-1) ) are ideal targets to test this magnetospheric accretion scenario in a sustained regime.Aims. We aimed at constraining the accretion-ejection phenomena around the strongly-accreting Northern component of the S CrA young binary system (S CrA N) by deriving its magnetic field topology and its magnetospheric properties, and by detecting ejection signatures, if any.Methods. We led a two-week observing campaign on S CrA N with the ESPaDOnS optical spectropolarimeter at the Canada-FranceHawaii Telescope. We recorded 12 Stokes I and V spectra over 14 nights. We computed the corresponding Least-Square Deconvolution (LSD) profiles of the photospheric lines and performed Zeeman-Doppler Imaging (ZDI). We analysed the kinematics of noticeable emission lines, namely He I lambda 5876 and the four first lines of the Balmer series, known to trace the accretion process. Results. We found that S CrA N is a low-mass (0.8 M-circle dot), young (similar to 1 Myr), and fully convective object exhibiting a strong and variable veiling (with a mean value of 7 +/- 2), which suggests that the star is in a strong accretion regime. These findings could indicate a stellar evolutionary stage between Class I and Class II for S CrA N. We reconstructed an axisymmetric large-scale magnetic field (similar to 70% of the total energy), primarily located in the dipolar component but with significant higher poloidal orders. From the He I lambda 5876 narrow emission component radial velocity curve, we derived a stellar rotation period of P-& lowast; = 7.3 +/- 0.2 days. We found a magnetic truncation radius of similar to 2 R-& lowast; which is significantly closer to the star than the corotation radius of similar to 6 R-& lowast;, suggesting that S CrA N is in an unstable accretion regime. The truncation radius being quite smaller than the size of the Br gamma line emitting region, as measured with the GRAVITY interferometer (similar to 8 R-& lowast;), supports the presence of outflows, which is nicely corroborated by the line profiles presented in this work.Conclusions. The findings from spectropolarimetry are complementary to those provided by optical long-baseline interferometry, allowing us to construct a coherent view of the innermost regions of a young, strongly accreting star. Yet, the strong and complex magnetic field reconstructed for S CrA N is inconsistent with the observed magnetic signatures of the emission lines associated to the post-shock region. We recommend a multi-technique, synchronized campaign of several days to put more constrains on a system that varies on a similar to 1 day timescale.
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| 6. |
- Shultz, M. E., et al.
(författare)
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MOBSTER-V. Discovery of a magnetic companion star to the magnetic beta Cep pulsator HD 156424
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 504:4, s. 4850-4864
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Tidskriftsartikel (refereegranskat)abstract
- HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4 association, previously noted to display both high-frequency radial velocity (RV) variability and magnetospheric H alpha emission. We have analysed the Transiting Exoplanet Survery Satellite (TESS) light curve, and find that it is a beta Cep pulsator with 11 detectable frequencies, 4 of which are independent p-modes. The strongest frequency is also detectable in RVs from ground-based high-resolution spectroscopy. RVs also show a long-term variation, suggestive of orbital motion with a period of similar to years; significant differences in the frequencies determined from TESS and RV data sets are consistent with a light-time effect from orbital motion. Close examination of the star's spectrum reveals the presence of a spectroscopic companion, however, as its RV is not variable it cannot be responsible for the orbital motion and we therefore infer that the system is a hierarchical triple with a so-far undetected third star. Reanalysis of least-squares deconvolution profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising presence of a strong magnetic field in the companion star, with < B-z > about +1.5 kG as compared to < B-z > similar to -0.8 kG for the primary. HD 156424 is thus the second hot binary with two magnetic stars. We are unable to identify a rotational period for HD 156424A. The magnetospheric H alpha emission appears to originate around HD 156424B. Using H alpha, as well as other variable spectral lines, we determine a period of about 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hot stars.
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| 7. |
- Squicciarini, V., et al.
(författare)
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A scaled-up planetary system around a supernova progenitor
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 664
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Tidskriftsartikel (refereegranskat)abstract
- Context. Virtually all known exoplanets reside around stars with M < 2.3 M⊙ either due to the rapid evaporation of the protostellar disks or to selection effects impeding detections around more massive stellar hosts.Aims. To clarify if this dearth of planets is real or a selection effect, we launched the planet-hunting B-star Exoplanet Abundance STudy (BEAST) survey targeting B stars (M > 2.4 M⊙) in the young (5-20 Myr) Scorpius-Centaurus association by means of the high-contrast spectro-imager SPHERE at the Very Large Telescope.Methods. In this paper we present the analysis of high-contrast images of the massive (M - 9 M⊙) star μ2 Sco obtained within BEAST. We carefully examined the properties of this star, combining data from Gaia and from the literature, and used state-of-the-art algorithms for the reduction and analysis of our observations.Results. Based on kinematic information, we found that μ2 Sco is a member of a small group which we label Eastern Lower Scorpius within the Scorpius-Centaurus association. We were thus able to constrain its distance, refining in turn the precision on stellar parameters. Around this star we identify a robustly detected substellar companion (14.4 ± 0.8 MJ)at a projected separation of 290 ± 10 au, and a probable second similar object (18.5 ± 1.5 MJ) at 21 ± 1 au. The planet-to-star mass ratios of these objects are similar to that of Jupiter to the Sun, and the flux they receive from the star is similar to those of Jupiter and Mercury, respectively.Conclusions. The robust and the probable companions of μ2 Sco are naturally added to the giant 10.9 MJ planet recently discovered by BEAST around the binary b Cen system. While these objects are slightly more massive than the deuterium burning limit, their properties are similar to those of giant planets around less massive stars and they are better reproduced by assuming that they formed under a planet-like, rather than a star-like scenario. Irrespective of the (needed) confirmation of the inner companion, μ2 Sco is the first star that would end its life as a supernova that hosts such a system. The tentative high frequency of BEAST discoveries is unexpected, and it shows that systems with giant planets or small-mass brown dwarfs can form around B stars. When putting this finding in the context of core accretion and gravitational instability formation scenarios, we conclude that the current modeling of both mechanisms is not able to produce this kind of companion. The completion of BEAST will pave the way for the first time to an extension of these models to intermediate and massive stars.
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| 8. |
- 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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| 9. |
- Grunhut, J. H., et al.
(författare)
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The magnetic field and magnetosphere of Plaskett's star : a fundamental shift in our understanding of the system
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 512:2, s. 1944-1966
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Tidskriftsartikel (refereegranskat)abstract
- Plaskett's 'star' appears to be one of a small number of short-period binary systems known to contain a hot, massive, magnetic star. We combine an extensive spectropolarimetric (Stokes V) data set with archival photometry and spectropolarimetry to establish the essential characteristics of the magnetic field and magnetosphere of the rapidly rotating, broad-line component of the system. We apply least-squares deconvolution (LSD) to infer the longitudinal magnetic field from each Stokes V spectrum. Using the time series of longitudinal field measurements, in combination with CoRoT photometry and equivalent width measurements of magnetospheric spectral lines, we infer the rotation period of the magnetic star to be equal to 1.21551(-0.00034)(+0.00028) d. Modelling the Stokes V LSD profiles with Zeeman-Doppler Imaging, we produce the first reliable magnetic map of an O-type star. We find a magnetic field that is predominantly dipolar, with an obliquity near 90 degrees and a polar strength of about 850 G. We update the calculations of the theoretical magnetospheric parameters, and in agreement with their predictions we identify clear variability signatures of the H alpha, H beta, and He II lambda 4686 lines confirming the presence of a dense centrifugal magnetosphere surrounding the star. Finally, we report a lack of detection of radial velocity (RV) variations of the observed Stokes V profiles, suggesting that historical reports of the large RV variations of the broad-line star's spectral lines may be spurious. This discovery may motivate a fundamental revision of the historical model of the Plaskett's star as a near-equal mass O + O binary system.
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| 10. |
- Tsvetkova, S., et al.
(författare)
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The large-scale magnetic field of the M dwarf double-line spectroscopic binary FK Aqr*
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 682
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Tidskriftsartikel (refereegranskat)abstract
- Context. This work is part of the BinaMIcS project, the aim of which is to understand the interaction between binarity and magnetism in close binary systems. All the studied spectroscopic binaries targeted by the BinaMIcS project encompass hot massive and intermediate-mass stars on the main sequence, as well as cool stars over a wide range of evolutionary stages. Aims. The present paper focuses on the binary system FK Aqr, which is composed of two early M dwarfs. Both stars are already known to be magnetically active based on their light curves and detected flare activity. In addition, the two components have large convective envelopes with masses just above the fully convective limit, making the system an ideal target for studying effect of binarity on stellar dynamos. Methods. We use spectropolarimetric observations obtained with ESPaDOnS at CFHT in September 2014. Mean Stokes I and V line profiles are extracted using the least-squares deconvolution (LSD) method. The radial velocities of the two components are measured from the LSD Stokes I profiles and are combined with interferometric measurements in order to constrain the orbital parameters of the system. The longitudinal magnetic fields B-l and chromospheric activity indicators are measured from the LSD mean line profiles. The rotational modulation of the Stokes V profiles is used to reconstruct the surface magnetic field structures of both stars via the Zeeman Doppler imaging (ZDI) inversion technique. Results. Maps of the surface magnetic field structures of both components of FK Aqr are presented for the first time. Our study shows that both components host similar large-scale magnetic fields of moderate intensity (B-mean similar or equal to 0.25 kG); both are predominantly poloidal and feature a strong axisymmetric dipolar component. Conclusions. Both components of FK Aqr feature a rather strong large-scale magnetic field (compared to single early M dwarfs with similar masses) with a mainly dipolar axisymmetric structure. This type of magnetic field is not typical for single early M dwarfs, and is rather reminiscent of fully convective dwarfs with later spectral types. The primary FK Aqr A is currently the most massive recognised main sequence M dwarf known to host this type of strong dipolar field.
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