Influence of magnetic cycles on stellar prominences and their mass loss rates

Funding: The authors acknowledge support from STFC consolidated grant number ST/R000824/1.Observations of rapidly-rotating cool stars often show coronal “slingshot” prominences that remove mass and angular momentum when they are ejected. The derived masses of these prominences show a scatter of some two orders of magnitude. In order to investigate if this scatter could be intrinsic, we use a full magnetic cycle of solar magnetograms to model the coronal structure and prominence distribution in a young Sun, where we scale the field strength in the magnetograms with angular velocity according to B∝Ω−1.32. We reproduce both the observed prominence masses and their scatter. We show that both the field strength and the field geometry contribute to the prominence masses that can be supported and to the rate at which they are ejected. Predicted prominence masses follow the magnetic cycle, but with half the period, peaking both at cycle maximum and at cycle minimum. We show that mass loss rates in prominences are less than those predicted for the stellar wind. We also investigate the role of small-scale field that may be unresolved in typical stellar magnetograms. This provides only a small reduction in the predicted total prominence mass, principally by reducing the number of large magnetic loops that can support slingshot prominences. We conclude that the observed scatter in prominence masses can be explained by underlying magnetic cycles.PostprintPeer reviewe

Heating and cooling in stellar coronae: coronal rain on a young Sun

Recent observations of rapidly-rotating cool dwarfs have revealed H$\alpha$ line asymmetries indicative of clumps of cool, dense plasma in the stars' coronae. These clumps may be either long-lived (persisting for more than one stellar rotation) or dynamic. The fastest dynamic features show velocities greater than the escape speed, suggesting that they may be centrifugally ejected from the star, contributing to the stellar angular momentum loss. Many however show lower velocities, similar to coronal rain observed on the Sun. We present 2.5D magnetohydrodynamic simulations of the formation and dynamics of these condensations in a rapidly rotating ($P_{\rm rot}~=~ 1 \ \mathrm{day}$) young Sun. Formation is triggered by excess surface heating. This pushes the system out of thermal equilibrium and triggers a thermal instability. The resulting condensations fall back towards the surface. They exhibit quasi-periodic behaviour, with periods longer than typical periods for solar coronal rain. We find line-of-sight velocities for these clumps in the range $50 \ \mathrm{km} \ \mathrm{s}^{-1}$ (blue shifted) to $250 \ \mathrm{km} \ \mathrm{s}^{-1}$(red shifted). These are typical of those inferred from stellar H$\alpha$line asymmetries, but the inferred clump masses of$3.6\times 10^{14}\ \mathrm{g}$are significantly smaller. We find that a maximum of$\simeq~3\%$ of the coronal mass is cool clumps. We conclude that coronal rain may be common in solar like stars, but may appear on much larger scales in rapid rotators.Comment: 11 pages, 5 figure

Heating and cooling in stellar coronae: coronal rain on a young Sun

Funding: SD-Y and MJ acknowledge support from STFC consolidated grant number ST/R000824/1. This work was performed using the DiRAC Data Intensive service at Leicester, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BEIS capital funding via STFC capital grants ST/K000373/1 and ST/R002363/1 and STFC DiRAC Operations grant ST/R001014/1. DiRAC is part of the National e-Infrastructure. CDJ acknowledges support from the NASA GSFC Internal Scientist Funding Model (competitive work package) programme.Recent observations of rapidly rotating cool dwarfs have revealed H α line asymmetries indicative of clumps of cool, dense plasma in the stars’ coronae. These clumps may be either long-lived (persisting for more than one stellar rotation) or dynamic. The fastest dynamic features show velocities greater than the escape speed, suggesting that they may be centrifugally ejected from the star, contributing to the stellar angular momentum loss. Many, however, show lower velocities, similar to coronal rain observed on the Sun. We present 2.5D magnetohydrodynamic simulations of the formation and dynamics of these condensations in a rapidly rotating (Prot = 1 d) young Sun. Formation is triggered by excess surface heating. This pushes the system out of thermal equilibrium and triggers a thermal instability. The resulting condensations fall back towards the surface. They exhibit quasi-periodic behaviour, with periods longer than typical periods for solar coronal rain. We find line-of-sight velocities for these clumps in the range of 50 km s−1 (blueshifted) to 250 km s−1 (redshifted). These are typical of those inferred from stellar H α line asymmetries, but the inferred clump masses of 3.6 × 1014 g are significantly smaller. We find that a maximum of ${\simeq}3~{{ \rm per\ cent}}$ of the coronal mass is cool clumps. We conclude that coronal rain may be common in solar-like stars, but may appear on much larger scales in rapid rotators.Publisher PDFPeer reviewe

X-ray Emission From Nearby M-dwarfs: the Super-saturation Phenomenon

A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary M-dwarf systems. X-ray luminosities for the majority of both single & binary M-dwarf systems with periods below $\simeq 5-6$ days (equatorial velocities, V$_{eq}>$ 6 km~s$^{-1}$) are consistent with the current rotation-activity paradigm, and appear to saturate at about $10^{-3}$ of the stellar bolometric luminosity. The single M-dwarf data show tentative evidence for the super-saturation phenomenon observed in some ultra-fast rotating ($>$ 100 km~s$^{-1}$) G & K-dwarfs in the IC 2391, IC 2602 and Alpha Persei clusters. The IC 2391 M star VXR60b is the least X-ray active and most rapidly rotating of the short period (P$_{rot}<$ 2 days) stars considered herein, with a period of 0.212 days and an X-ray activity level about 1.5 sigma below the mean X-ray emission level for most of the single M-dwarf sample. For this star, and possibly one other, we cautiously believe that we have identified the first evidence of super-saturation in M-dwarfs. If we are wrong, we demonstrate that only M-dwarfs rotating close to their break up velocities are likely to exhibit the super-saturation effect at X-ray wavelengths.Comment: 12 pages, 4 figures, accepted by MNRA

Magnetic support of stellar slingshot prominences

The authors acknowledge support from STFC.We present models for the magnetic support of the ‘slingshot prominences’ observed in the coronae of rapidly rotating stars. We calculate mechanical equilibria of loops in a spherical geometry. Prominence-forming loops are found first for dipolar and quadrupolar stellar fields that are fully closed. Equilibria are then found within the stellar wind for a dipolar field that becomes open beyond a given radius. We identify two physical processes that may produce gaps in the distribution of prominence heights: the location of this opening radius, and the behaviour of the buoyancy force. The buoyancy may differ from one prominence-bearing loop to another if they are at different temperatures, thus potentially smearing out any gap in observed height distributions. We produce synthetic prominence distributions and compare to the observations of two well-observed stars: AB Doradus and Speedy Mic. The model recovers the more compact prominence distribution observed for Speedy Mic and reproduces better the overall shape of the height distributions for both stars when the opening radius is beyond the co-rotation radius.Publisher PDFPeer reviewe

Amplitude modulation of short-timescale hot spot variability

Funding Information: L.I.B., L.A.P., and J.L. acknowledge support from NASA through an Astrophysics Data Analysis Program grant to Lowell Observatory (grant 80NSSC20K1001). A.C.C. and M.M.J. acknowledge support from the Science and Technology Facilities Council (STFC) consolidated grant No. ST/R00824/1, and the support of the visiting scientist program at Lowell Observatory in 2019 January and 2020 January. Data were obtained using the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.Variability of Classical T Tauri stars (CTTS) occurs over a vast range of timescales. CTTS in particular are subject to variability caused by accretion shocks, which can occur stochastically, periodically, or quasi-periodically on timescales over a few days. The detectability of young planets within these systems is likely hampered by activity; therefore, it is essential that we understand the origin of young star variability over a range of timescales to help disentangle stellar activity from signatures of planetary origin. We present an analysis of the stochastic small-amplitude photometric variability in the K2 lightcurve of CI Tau occurring on timescales of ≲1 day. We find the amplitude of this variability exhibits the same periodic signatures as detected in the large-amplitude variability, indicating that the physical mechanism modulating these brightness features is the same. The periods detected are also in agreement with the rotation period of the star (∼6.6 days) and the orbital period of the planet (∼9.0 days) known to drive pulsed accretion onto the star.PostprintPeer reviewe

X-ray emission from nearby M-dwarfs: the super-saturation phenomenon

A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary M-dwarf systems. X-ray luminosities for the majority of both single and binary M-dwarf systems with periods below ≃5-6 d (equatorial velocities, Veq≳6 km s−1) are consistent with the current rotation-activity paradigm, and appear to saturate at about 10−3 of the stellar bolometric luminosity. The single M-dwarf data show tentative evidence for the super-saturation phenomenon observed in some ultra-fast rotating (≳100 km s−1) G- and K-dwarfs in the IC 2391, IC 2602 and Alpha Persei clusters. The IC 2391 M star VXR60b is the least X-ray active and most rapidly rotating of the short period (Prot≲2 d) stars considered herein, with a period of 0.212 d and an X-ray activity level of about 1.5 sigma below the mean X-ray emission level for most of the single M-dwarf sample. For this star, and possibly one other, we cautiously believe that we have identified the first evidence of super-saturation in M-dwarfs. If we are wrong, we demonstrate that only M-dwarfs rotating close to their break-up velocities are likely to exhibit the super-saturation effect at X-ray wavelengths. The M-dwarf X-ray data also show that there is no evidence for any difference in the X-ray behaviour between the single and binary systems, because for the single stars, the mean log LxLbol=−3.21±0.04 (0.2≲Prot≲10.1 d), whereas for the binary stars, the mean log LxLbol=−3.19±0.10 (0.8≲Prot≲10.4 d). Furthermore, we show that extremely X-ray active M-dwarfs exhibit a blue excess of about 0.1 magnitudes in U-B compared with less active field M-dwarfs. Such an excess level is comparable to that observed for extremely chromospherically active M-dwarfs. Moreover, as is the case for M-dwarf Ca ii H and K activity levels, there is an exclusion zone of X-ray activity between the extremely active M-dwarfs and the less active one

Radio masers on WX UMa : hints of a Neptune-sized planet, or magnetospheric reconnection?

RDK acknowledges funding received from the Irish Research Council (IRC) through the Government of Ireland Postgraduate Scholarship Programme. RDK and AAV acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 817540, ASTROFLOW). We acknowledge the provisions of the Space Weather Modelling Framework (SWMF) code from the Center for Space Environment Modeling (CSEM) at the University of Michigan, and the computational resources of the Irish Centre for High End Computing (ICHEC), both of which were utilised in this work.The nearby M dwarf WX UMa has recently been detected at radio wavelengths with LOFAR. The combination of its observed brightness temperature and circular polarisation fraction suggests that the emission is generated via the electron-cyclotron maser instability. Two distinct mechanisms have been proposed to power such emission from low-mass stars: either a sub-Alfvénic interaction between the stellar magnetic field and an orbiting planet, or reconnection at the edge of the stellar magnetosphere. In this paper, we investigate the feasibility of both mechanisms, utilising the information about the star’s surrounding plasma environment obtained from modelling its stellar wind. Using this information, we show that a Neptune-sized exoplanet with a magnetic field strength of 10 – 100 G orbiting at ∼0.034 au can accurately reproduce the observed radio emission from the star, with corresponding orbital periods of 7.4 days. Due to the stellar inclination, a planet in an equatorial orbit is unlikely to transit the star. While such a planet could induce radial velocity semi-amplitudes from 7 to 396 m s−1, it is unlikely that this signal could be detected with current techniques due to the activity of the host star. The application of our planet-induced radio emission model here illustrates its exciting potential as a new tool for identifying planet-hosting candidates from long-term radio monitoring. We also develop a model to investigate the reconnection-powered emission scenario. While this approach produces less favourable results than the planet-induced scenario, it nevertheless serves as a potential alternative emission mechanism which is worth exploring further.PostprintPeer reviewe

Rođenje perspektive iz duha muzike

Understanding the origins of stellar radio emission can provide invaluable insight into the strength and geometry of stellar magnetic fields and the resultant space weather environment experienced by exoplanets. Here, we present the first model capable of predicting radio emission through the electron cyclotron maser instability using observed stellar magnetic maps of low-mass stars. We determine the structure of the coronal magnetic field and plasma using spectropolarimetric observations of the surface magnetic fields and the X-ray emission measure. We then model the emission of photons from the locations within the corona that satisfy the conditions for electron cyclotron maser emission. Our model predicts the frequency, and intensity of radio photons from within the stellar corona. We have benchmarked our model against the low-mass star V374 Peg. This star has both radio observations from the Very Large Array and a nearly simultaneous magnetic map. Using our model we are able to fit the radio observations of V374 Peg, providing additional evidence that the radio emission observed from low-mass stars may originate from the electron cyclotron maser instability. Our model can now be extended to all stars with observed magnetic maps to predict the expected frequency and variability of stellar radio emission in an effort to understand and guide future radio observations of low-mass stars.Comment: Accepted for publication in Ap