174 research outputs found
Modelling stellar coronal magnetic fields
Our understanding of the structure and dynamics of stellar coronae has
changed dramatically with the availability of surface maps of both star spots
and also magnetic field vectors. Magnetic field extrapolations from these
surface maps reveal surprising coronal structures for stars whose masses and
hence internal structures and dynamo modes may be very different from that of
the Sun. Crucial factors are the fraction of open magnetic flux (which
determines the spin-down rate for the star as it ages) and the location and
plasma density of closed-field regions, which determine the X-ray and radio
emission properties. There has been recent progress in modelling stellar
coronae, in particular the relative contributions of the field detected in the
bright surface regions and the field that may be hidden in the dark star spots.
For the Sun, the relationship between the field in the spots and the large
scale field is well studied over the solar cycle. It appears, however, that
other stars can show a very different relationship.Comment: 6pages, 4 figure
The magnetic properties of the planet host star Kepler-78
Kepler-78 is host to a transiting 8.5-hour orbit super-Earth. In this paper,
the rotation and magnetic properties of the planet host star are studied. We
first revisit the Kepler photometric data for a detailed description of the
rotation properties of Kepler-78, showing that the star seems to undergo a
cycle in the spot pattern of ~1,300 day duration. We then use
spectropolarimetric observations with CFHT/ESPaDOnS to measure the circular
polarization in the line profile of the star during its rotation cycle, as well
as spectroscopic proxies of the chromospheric activity. The average field has
an amplitude of 16 G. The magnetic topology is characterized by a poloidal and
a toroidal component, encompassing 60% and 40% of the magnetic energy,
respectively. Differential rotation is detected with an estimated rate of
0.105+-0.039 rad/d. Activity tracers vary with the rotation cycle of the star;
there is no hint that a residual activity level is related to the planetary
orbit at the precision of our data. The description of the star magnetic
field's characteristics then may serve as input for models of interactions
between the star and its close-by planet, e.g., Ohmic dissipation and unipolar
induction
Magnetic field evolution of the K2 dwarf V471 Tau
Observations of the eclipsing binary system V471 Tau show that the time of
the primary eclipses varies in an apparent periodic way. With growing evidence
that the magnetically active K2 dwarf component might be responsible for
driving the eclipse timing variations (ETVs), it is necessary to monitor the
star throughout the predicted ~ 35 yr activity cycle that putatively fuels the
observed ETVs. We contribute to this goal with this paper by analysing
spectropolarimetric data obtained with ESPaDOnS at the Canada-France-Hawaii
Telescope in December 2014 and January 2015. Using Zeeman-Doppler Imaging, we
reconstruct the distribution of brightness inhomogeneities and large-scale
magnetic field at the surface of the K2 dwarf. Compared to previous tomographic
reconstructions of the star carried out with the same code, we probe a new
phase of the ETVs cycle, offering new constraints for future works exploring
whether a magnetic mechanism operating in the K2 dwarf star is indeed able to
induce the observed ETVs of V471 Tau.Comment: 12 pages, 10 figures, submitted to MNRA
Starspots and relativity: Applied Doppler imaging for the Gravity Probe B mission
We present Doppler images and surface differential rotation measurements for the primary of the RS CVn binary IM Pegasi, the guide star for the Gravity Probe B experiment. The data used is a subset of that taken during optical support of the mission and was obtained almost nightly over a near three year period from the Automatic Spectroscopic Telescope operated by Tennessee State University. Using the technique of least-squares deconvolution to increase the signal-to-noise ratio of the data, we have reconstructed 31 maximum entropy Doppler images of the star. The images show that the spot features are relatively stable for over a year (and possibly longer) with both a polar spot and lower latitude features. The most intense features are located on the side facing the secondary. In addition, we have incorporated a solar-like differential rotation law into the imaging process to determine the level of surface differential rotation for IM Peg for 22 epochs. A weighted least-squares average of the measurements gives a surface shear of 0.0142 ± 0.0007 rad/d, meaning that the equator takes âŒ440 ± 20 days to lap the poles. Although the level of surface differential rotation was shown to vary over the period of the observations, this may indicate an underestimate in the errors of the method rather than any temporal evolution in the differential rotation
Stable and Unstable Regimes of Mass Accretion onto RW Aur A
We present monitoring observations of the active T Tauri star RW Aur, from
2010 October to 2015 January, using optical high-resolution (R>10000)
spectroscopy with CFHT-ESPaDOnS. Optical photometry in the literature shows
bright, stable fluxes over most of this period, with lower fluxes (by 2-3 mag.)
in 2010 and 2014. In the bright period our spectra show clear photospheric
absorption, complicated variation in the Ca II 8542 A emission}profile shapes,
and a large variation in redshifted absorption in the O I 7772 and 8446 A and
He I 5876 A lines, suggesting unstable mass accretion during this period. In
contrast, these line profiles are relatively uniform during the faint periods,
suggesting stable mass accretion. During the faint periods the photospheric
absorption lines are absent or marginal, and the averaged Li I profile shows
redshifted absorption due to an inflow. We discuss (1) occultation by
circumstellar material or a companion and (2) changes in the activity of mass
accretion to explain the above results, together with near-infrared and X-ray
observations from 2011-2015. Neither scenario can simply explain all the
observed trends, and more theoretical work is needed to further investigate
their feasibilities.Comment: 23 pages, 11 figures, 4 tables, accepted by Astrophysical Journal;
some typos corrected on 4/18/201
Estimating magnetic filling factors from ZeemanâDoppler magnetograms
V.S., S.P.M., and A.J.F.acknowledge funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (grant agreement No. 682393 AWESoMeStars). S.B.S. acknowledges funding via the Austrian Space Application Programme (ASAP) of the Austrian Research Promotion Agency (FFG) within ASAP11, the FWF NFN project S11601-N16 and the sub-project S11604-N16. A. A.V. acknowledges funding received from the Irish Research Council Laureate Awards 2017/2018.Low-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterize them. ZeemanâDoppler imaging (ZDI) can determine the geometry of the large-scale magnetic field while Zeeman broadening can assess the total unsigned flux including that associated with small-scale structures such as spots. In this work, we study a sample of stars that have been previously mapped with ZDI. We show that the average unsigned magnetic flux follows an activity-rotation relation separating into saturated and unsaturated regimes. We also compare the average photospheric magnetic flux recovered by ZDI, BV, with that recovered by Zeeman broadening studies, BI. In line with previous studies, BV ranges from a few % to ~20% of BI. We show that a power-law relationship between BV and BI exists and that ZDI recovers a larger fraction of the magnetic flux in more active stars. Using this relation, we improve on previous attempts to estimate filling factors, i.e., the fraction of the stellar surface covered with magnetic field, for stars mapped only with ZDI. Our estimated filling factors follow the well-known activity-rotation relation, which is in agreement with filling factors obtained directly from Zeeman broadening studies. We discuss the possible implications of these results for flux tube expansion above the stellar surface and stellar wind models.Publisher PDFPeer reviewe
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