scholarly journals Direct evidence of a full dipole flip during the magnetic cycle of a sun-like star

2018 ◽  
Vol 620 ◽  
pp. L11 ◽  
Author(s):  
S. Boro Saikia ◽  
T. Lueftinger ◽  
S. V. Jeffers ◽  
C. P. Folsom ◽  
V. See ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Activity Cycle ◽  
Doppler Imaging ◽  
Dipolar Field ◽  
Magnetic Cycle ◽  
Activity Maximum ◽  
Cycle Minimum ◽  
Chromospheric Activity ◽  
Cycle Maximum

Context.The behaviour of the large-scale dipolar field, during a star’s magnetic cycle, can provide valuable insight into the stellar dynamo and associated magnetic field manifestations such as stellar winds.Aims.We investigate the temporal evolution of the dipolar field of the K dwarf 61 Cyg A using spectropolarimetric observations covering nearly one magnetic cycle equivalent to two chromospheric activity cycles.Methods.The large-scale magnetic field geometry is reconstructed using Zeeman Doppler imaging, a tomographic inversion technique. Additionally, the chromospheric activity is also monitored.Results.The observations provide an unprecedented sampling of the large-scale field over a single magnetic cycle of a star other than the Sun. Our results show that 61 Cyg A has a dominant dipolar geometry except at chromospheric activity maximum. The dipole axis migrates from the southern to the northern hemisphere during the magnetic cycle. It is located at higher latitudes at chromospheric activity cycle minimum and at middle latitudes during cycle maximum. The dipole is strongest at activity cycle minimum and much weaker at activity cycle maximum.Conclusions.The behaviour of the large-scale dipolar field during the magnetic cycle resembles the solar magnetic cycle. Our results are further confirmation that 61 Cyg A indeed has a large-scale magnetic geometry that is comparable to the Sun’s, despite being a slightly older and cooler K dwarf.

scholarly journals Magnetic field and chromospheric activity evolution of HD 75332: a rapid magnetic cycle in an F star without a hot Jupiter

2020 ◽  
Author(s):  
E L Brown ◽  
S C Marsden ◽  
M W Mengel ◽  
S V Jeffers ◽  
I Millburn ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Magnetic Activity ◽  
Doppler Imaging ◽  
Shallow Convection ◽  
Magnetic Cycle ◽  
Field Polarity ◽  
Chromospheric Activity ◽  
Magnetic Cycles ◽  
Hot Jupiter

Abstract Studying cool star magnetic activity gives an important insight into the stellar dynamo and its relationship with stellar properties, as well as allowing us to place the Sun’s magnetism in the context of other stars. Only 61 Cyg A (K5V) and τ Boo (F8V) are currently known to have magnetic cycles like the Sun’s, where the large-scale magnetic field polarity reverses in phase with the star’s chromospheric activity cycles. τ Boo has a rapid ∼240 d magnetic cycle, and it is not yet clear whether this is related to the star’s thin convection zone or if the dynamo is accelerated by interactions between τ Boo and its hot Jupiter. To shed light on this, we studied the magnetic activity of HD 75332 (F7V) which has similar physical properties to τ Boo and does not appear to host a hot Jupiter. We characterized its long term chromospheric activity variability over 53 yrs and used Zeeman Doppler Imaging to reconstruct the large-scale surface magnetic field for 12 epochs between 2007 and 2019. Although we observe only one reversal of the large-scale magnetic dipole, our results suggest that HD 75332 has a rapid ∼1.06 yr solar-like magnetic cycle where the magnetic field evolves in phase with its chromospheric activity. If a solar-like cycle is present, reversals of the large-scale radial field polarity are expected to occur at around activity cycle maxima. This would be similar to the rapid magnetic cycle observed for τ Boo, suggesting that rapid magnetic cycles may be intrinsic to late-F stars and related to their shallow convection zones.

scholarly journals The relation between stellar magnetic field geometry and chromospheric activity cycles – I. The highly variable field of ε Eridani at activity minimum

2017 ◽  
Vol 471 (1) ◽  
pp. L96-L100 ◽  
Author(s):  
S. V. Jeffers ◽  
S. Boro Saikia ◽  
J. R. Barnes ◽  
P. Petit ◽  
S. C. Marsden ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Activity Cycle ◽  
Doppler Imaging ◽  
The Sun ◽  
Chromospheric Activity ◽  
Large Scale Magnetic Field ◽  
Large Scale Field ◽  
Solar Type ◽  
Field Geometry

Abstract The young and magnetically active K dwarf ε Eridani exhibits a chromospheric activity cycle of about 3 yr. Previous reconstructions of its large-scale magnetic field show strong variations at yearly epochs. To understand how ε Eridani’s large-scale magnetic field geometry evolves over its activity cycle, we focus on high-cadence observations spanning 5 months at its activity minimum. Over this time-span, we reconstruct three maps of ε Eridani’s large-scale magnetic field using the tomographic technique of Zeeman–Doppler imaging. The results show that at the minimum of its cycle, ε Eridani’s large-scale field is more complex than the simple dipolar structure of the Sun and 61 Cyg A at minimum. Additionally, we observe a surprisingly rapid regeneration of a strong axisymmetric toroidal field as ε Eridani emerges from its S-index activity minimum. Our results show that all stars do not exhibit the same field geometry as the Sun, and this will be an important constraint for the dynamo models of active solar-type stars.

scholarly journals The large-scale magnetic field of Proxima Centauri near activity maximum

2020 ◽  
Vol 500 (2) ◽  
pp. 1844-1850
Author(s):  
Baptiste Klein ◽  
Jean-François Donati ◽  
Élodie M Hébrard ◽  
Bonnie Zaire ◽  
Colin P Folsom ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Circular Polarization ◽  
Large Scale ◽  
Doppler Imaging ◽  
Magnetic Star ◽  
Mass Star ◽  
Activity Maximum ◽  
Long Term Study ◽  
Large Scale Magnetic Field ◽  
Polarization Spectra

ABSTRACT We report the detection of a large-scale magnetic field at the surface of the slowly rotating fully convective (FC) M dwarf Proxima Centauri. 10 circular polarization spectra, collected from 2017 April to July with the HARPS-Pol spectropolarimeter, exhibit rotationally modulated Zeeman signatures suggesting a stellar rotation period of 89.8 ± 4.0 d. Using Zeeman–Doppler Imaging, we invert the circular polarization spectra into a surface distribution of the large-scale magnetic field. We find that Proxima Cen hosts a large-scale magnetic field of typical strength 200 G, whose topology is mainly poloidal, and moderately axisymmetric, featuring, in particular, a dipole component of 135 G tilted at 51° to the rotation axis. The large-scale magnetic flux is roughly 3× smaller than the flux measured from the Zeeman broadening of unpolarized lines, which suggests that the underlying dynamo is efficient at generating a magnetic field at the largest spatial scales. Our observations occur ∼1 yr after the maximum of the reported 7 yr-activity cycle of Proxima Cen, which opens the door for the first long-term study of how the large-scale field evolves with the magnetic cycle in an FC very low mass star. Finally, we find that Proxima Cen’s habitable zone planet, Proxima-b, is likely orbiting outside the Alfvèn surface, where no direct magnetic star–planet interactions occur.

Magnetic geometry and activity of cool stars

2018 ◽  
Vol 14 (S345) ◽  
pp. 341-342
Author(s):  
Sudeshna Boro Saikia ◽  
Theresa Lüftinger ◽  
Manuel Guedel
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Central Star ◽  
Doppler Imaging ◽  
Rotating Stars ◽  
Surface Magnetic Field ◽  
Chromospheric Activity ◽  
Field Geometry ◽  
Atmospheric Loss ◽  
Rapidly Rotating Stars

AbstractStellar magnetic field manifestations such as stellar winds and EUV radiation are the key drivers of planetary atmospheric loss and escape. To understand how the central star influences habitability, it is very important to perform detailed investigation of the star’s magnetic field. We investigate the surface magnetic field geometry and chromospheric activity of 51 sun-like stars. The magnetic geometry is reconstructed using Zeeman Doppler imaging. Chromospheric activity is measured using the Ca II H& K lines. We confirm that the Sun’s large-scale geometry is dominantly poloidal, which is also true for slowly rotating stars. Contrary to the Sun, rapidly rotating stars can have a strong toroidal field and a weak poloidal field. This separation in field geometry appears at Ro=1. Our results show that detailed investigation of stellar magnetic field is important to understand its influence on planetary habitability.

scholarly journals Circumstellar environment of 55 Cancri

2020 ◽  
Vol 633 ◽  
pp. A48 ◽  
Author(s):  
C. P. Folsom ◽  
D. Ó Fionnagáin ◽  
L. Fossati ◽  
A. A. Vidotto ◽  
C. Moutou ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Stellar Wind ◽  
Large Scale ◽  
Rotation Axis ◽  
Doppler Imaging ◽  
Magnetic Pole ◽  
Average Strength ◽  
Detailed Understanding ◽  
The Magnetic Field ◽  
Circumstellar Environment

Context. 55 Cancri hosts five known exoplanets, most notably the hot super-Earth 55 Cnc e, which is one of the hottest known transiting super-Earths. Aims. Because of the short orbital separation and host star brightness, 55 Cnc e provides one of the best opportunities for studying star-planet interactions (SPIs). We aim to understand possible SPIs in this system, which requires a detailed understanding of the stellar magnetic field and wind impinging on the planet. Methods. Using spectropolarimetric observations and Zeeman Doppler Imaging, we derived a map of the large-scale stellar magnetic field. We then simulated the stellar wind starting from the magnetic field map, using a 3D magneto-hydrodynamic model. Results. The map of the large-scale stellar magnetic field we derive has an average strength of 3.4 G. The field has a mostly dipolar geometry; the dipole is tilted by 90° with respect to the rotation axis and the dipolar strength is 5.8 G at the magnetic pole. The wind simulations based on this magnetic geometry lead us to conclude that 55 Cnc e orbits inside the Alfvén surface of the stellar wind, implying that effects from the planet on the wind can propagate back to the stellar surface and result in SPI.

scholarly journals The large scale magnetic field of the G0 dwarf HD 206860 (HN Peg)

2013 ◽  
Vol 9 (S302) ◽  
pp. 146-147
Author(s):  
Sudeshna Boro Saikia ◽  
Sandra V. Jeffers ◽  
Pascal Petit ◽  
Stephen Marsden ◽  
Julien Morin ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Spectral Type ◽  
Large Scale ◽  
Longitudinal Magnetic Field ◽  
Chromospheric Activity ◽  
Large Scale Magnetic Field ◽  
Debris Disk ◽  
Infrared Triplet

AbstractHD 206860 is a young planet (HN Peg b) hosting star of spectral type G0V and it has a potential debris disk around it. In this work we measure the longitudinal magnetic field of HD 206860 using spectropolarimetric data and we measure the chromospheric activity using Ca II H&K, H-alpha and Ca II infrared triplet lines.

scholarly journals A Model of Solar X-ray Bright Points and Ephemeral Active Regions

1979 ◽  
Vol 32 (6) ◽  
pp. 671 ◽  
Author(s):  
JH Piddington
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Large Scale ◽  
Flux Rope ◽  
Active Regions ◽  
Activity Cycle ◽  
Solar Magnetic Fields ◽  
Flux Tubes ◽  
X Ray ◽  
Field System

Solar ephemeral active regions may provide a larger amount of emerging magnetic flux than the active regions themselves, and the origin and disposal of this flux pose problems. The related X-ray bright points are a major feature of coronal dynamics, and the two phenomena may entail a revision of our ideas of the activity cycle. A new large-scale subsurface magnetic field system has been suggested, but it is shown that such a system is neither plausible nor necessary. The emerging magnetic bipoles merely represent loops in pre-existing vertical flux tubes which are parts of active regions or the remnants of active regions. These loops result from the kink (or helical) instability in a twisted flux tube. Their observed properties are explained in terms of the flux-rope theory of solar fields. The model is extended to some dynamical effects in emerging loops. Further observations of ephemeral active regions may provide important tests between the traditional and flux-rope theories of solar magnetic fields.

Formation of Coronal Holes and the Global Magnetic Field Distribution

1994 ◽  
Vol 144 ◽  
pp. 65-67 ◽  
Author(s):  
V. Bumba ◽  
M. Klvaňa ◽  
V. Rušin ◽  
M. Rybanský ◽  
G. T. Buyukliev
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Coronal Holes ◽  
Activity Cycle ◽  
Close Relation ◽  
Solar Activity Cycle ◽  
Cycle Maximum ◽  
Global Magnetic Field ◽  
Mutual Relations ◽  
The Individual

The photoelectric magnetograph of the Ondřejov Observatory was reconstructed in 1990 (Klvaňa and Bumba, 1994; Klvaňaet al, 1994). During 1991 and 1992, several hundred sets of measurements were obtained, mostly in line Fel 5253.47 Å. It has been found that some of the measurements are distributed very favorably around coronal holes, sometimes covering smaller parts and in a few cases even larger parts of their areas.Both 1991 and 1992 were exceptional as regards their relation to the phase of the ending solar activity cycle (No 22): while the period of the secondary cycle maximum (mainly the southern solar hemisphere) took place in 1991, the year 1992 coincided with the initial stage of its declining branch. Since the formation of coronal holes is in close relation to the dynamics of the global distribution of solar magnetic fields, we thought that before starting to investigate the detailed connections of the individual coronal holes with particular local magnetic fields, it might be interesting to study their mutual relations also on a large scale.

scholarly journals Spin evolution and saturation: new insights through 3D MHD simulations of young solar analogs

2019 ◽  
Vol 82 ◽  
pp. 233-240
Author(s):  
V. Réville ◽  
A.S. Brun
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Large Scale ◽  
Stellar Surface ◽  
Doppler Imaging ◽  
Momentum Loss ◽  
Mhd Simulations ◽  
Spin Evolution ◽  
Angular Momentum Loss ◽  
Solar Analogs

We examine how 3D MHD simulations can deliver clues on the mechanisms at the origin of angular momentum loss saturation of rapidly rotating solar-like stars. Based on a study of six targets, whose magnetic field has been observed by Zeeman Doppler Imaging (ZDI), we find that the saturation could be explained by a extremely strong coverage of the stellar surface of a large scale dipolar mode, in disagreement with recent works.

scholarly journals Zeeman-Doppler imaging of II Peg

2008 ◽  
Vol 4 (S259) ◽  
pp. 437-438 ◽  
Author(s):  
Thorsten A. Carroll ◽  
Markus Kopf ◽  
Klaus G. Strassmeier ◽  
Ilya Ilyin ◽  
Ilkka Tuominen
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Field Structure ◽  
Doppler Imaging ◽  
High Latitudes ◽  
Magnetic Vector ◽  
Magnetic Field Structure ◽  
Surface Magnetic Field ◽  
Large Scale Magnetic Field ◽  
The Magnetic Field

AbstractWe present Zeeman-Doppler images of the active K2 star II Peg for the years 2004 and 2007. The surface magnetic field was reconstructed with our new ZDI code iMap which provides a full polarized radiative transfer driven inversion to simultaneously reconstruct the surface temperature and magnetic vector field distribution. II Peg shows a remarkable large scale magnetic field structure for both years. The magnetic field is predominantly located at high latitudes and is arranged in active longitudes. A dramatic evolution in the magnetic field structure is visible for the two years, where a dominant and largely unipolar field in 2004 has changed into two distinct and large scale bipolar structures in 2007.

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