arXiv subjects

M. Jardine

Publications and source records attributed to M. Jardine.

At least 19 recordsLinked to original sources

arXiv2025

A magnetic field study of two fast-rotating, radio bright M dwarfs. StKM 1-1262 and V374 Peg

Radio observations at low frequencies are sensitive to the magnetic activity of stars and the plasma environment surrounding them. The accurate interpretation of the processes underlying the radio signatures requires a detailed characterisation of the stellar magnetism. We study two M dwarfs, StKM 1-1262 (M0 type, P$_\mathrm{rot}=1.24$ d) and V374 Peg (M4 type, P$_\mathrm{rot}=0.4455$ d), which were detected with the LOw Frequency ARray (LOFAR). StKM 1-1262 exhibited a type-II radio burst, potentially resulting from a coronal mass ejection event. V374 Peg manifested low-frequency radio emission typical of an electron-cyclotron maser instability emission mechanism. We analysed spectropolarimetric observations of these M dwarfs collected with the SpectroPolarim\`etre InfraRouge (SPIRou). Firstly, we refined the stellar parameters such as effective temperature, surface gravity, and metallicity, and measured the average surface magnetic flux via modelling of Zeeman broadening in unpolarised spectra. We then applied Zeeman-Doppler imaging to least-squares deconvolution line profiles in circular polarisation to reconstruct their large-scale magnetic fields. StKM 1-1262 has a total, unsigned magnetic field of $3.53\pm0.06$ kG on average and the large-scale magnetic field topology is dipolar and moderately axisymmetric, with an average strength of 300 G. V374 Peg has an unsigned magnetic field of $5.46\pm0.09$ kG and the large-scale field is dipolar and axisymmetric, with an average strength of 800 G. For StKM 1-1262, we found a strong anti-correlation between the total magnetic field and the effective temperature which is reminiscent of the tight link between small-scale magnetic fields and surface inhomogeneities. For V374 Peg, we found a moderate anti-correlation, possibly due to a more even distribution of surface features. (Abridged)

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arXiv2022

MOVES V. Modelling star-planet magnetic interactions of HD 189733

Magnetic interactions between stars and close-in planets may lead to a detectable signal on the stellar disk. HD 189733 is one of the key exosystems thought to harbor magnetic interactions, which may have been detected in August 2013. We present a set of twelve wind models at that period, covering the possible coronal states and coronal topologies of HD 189733 at that time. We assess the power available for the magnetic interaction and predict its temporal modulation. By comparing the predicted signal with the observed signal, we find that some models could be compatible with an interpretation based on star-planet magnetic interactions. We also find that the observed signal can be explained only with a stretch-and-break interaction mechanism, while that the Alfvén wings scenario cannot deliver enough power. We finally demonstrate that the past observational cadence of HD 189733 leads to a detection rate of only between 12 to 23%, which could explain why star-planet interactions have been hard to detect in past campaigns. We conclude that the firm confirmation of their detection will require dedicated spectroscopic observations covering densely the orbital and rotation period, combined with scarcer spectropolarimetric observations to assess the concomitant large-scale magnetic topology of the star.

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arXiv2021

Field linkage and magnetic helicity density

The helicity of a magnetic field is a fundamental property that is conserved in ideal MHD. It can be explored in the stellar context by mapping large-scale magnetic fields across stellar surfaces using Zeeman-Doppler imaging. A recent study of 51 stars in the mass range 0.1-1.34 M$_\odot$ showed that the photospheric magnetic helicity density follows a single power law when plotted against the toroidal field energy, but splits into two branches when plotted against the poloidal field energy. These two branches divide stars above and below $\sim$ 0.5 M$_\odot$. We present here a novel method of visualising the helicity density in terms of the linkage of the toroidal and poloidal fields that are mapped across the stellar surface. This approach allows us to classify the field linkages that provide the helicity density for stars of different masses and rotation rates. We find that stars on the lower-mass branch tend to have toroidal fields that are non-axisymmetric and so link through regions of positive and negative poloidal field. A lower-mass star may have the same helicity density as a higher-mass star, despite having a stronger poloidal field. Lower-mass stars are therefore less efficient at generating large-scale helicity.

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arXiv2020

Magnetic field and chromospheric activity evolution of HD75332: a rapid magnetic cycle in an F star without a hot Jupiter

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 $\sim$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 HD75332 (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 HD75332 has a rapid $\sim$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.

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arXiv2020

Even More Rapidly Rotating Pre-Main Sequence M Dwarfs with Highly Structured Light Curves: An Initial Survey in the Lower Centaurus-Crux and Upper Centaurus-Lupus Associations

Using K2, we recently discovered a new type of periodic photometric variability while analysing the light curves of members of Upper Sco (Stauffer \etal\ 2017). The 23 exemplars of this new variability type are all mid-M dwarfs, with short rotation periods. Their phased light curves have one or more broad flux dips or multiple arcuate structures which are not explicable by photospheric spots or eclipses by solid bodies. Now, using TESS data, we have searched for this type of variability in the other major sections of Sco-Cen, Upper Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC). We identify 28 stars with the same light curve morphologies. We find no obvious difference between the Upper Sco and the UCL/LCC representatives of this class in terms of their light curve morphologies, periods or variability amplitudes. The physical mechanism behind this variability is unknown, but as a possible clue we show that the rapidly rotating mid-M dwarfs in UCL/LCC have slightly different colors from the slowly rotating M dwarfs - they either have a blue excess (hot spots?) or a red excess (warm dust?). One of the newly identified stars (TIC242407571) has a very striking light curve morphology. At about every 0.05 in phase are features that resemble icicles, The "icicles" arise because there is a second periodic system whose main feature is a broad flux dip. Using a toy model, we show that the observed light curve morphology results only if the ratio of the two periods and the flux dip width are carefully arranged.

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arXiv2020

Magnetic field and prominences of the young, solar-like, ultra-rapid rotator V530 Per

We investigate signatures of magnetic fields and activity at the surface and in the prominence system of the ultra-rapid rotator V530 Per, a G-type solar-like member of the young open cluster $α$~Persei. This object has a rotation period shorter than all stars with available magnetic maps. With a time-series of spectropolarimetric observations gathered with ESPaDOnS over 2 nights on the CFHT, we reconstruct the surface brightness and large-scale magnetic field of V530 Per using the Zeeman-Doppler imaging method, assuming an oblate stellar surface. We also estimate the short term evolution of the brightness distribution through latitudinal differential rotation. Using the same data set, we finally map the spatial distribution of prominences through tomography of the Hαemission. The brightness map is dominated by a large, dark spot near the pole, accompanied by a complex distribution of bright and dark features at lower latitudes. The magnetic field map is reconstructed as well, most of the large-scale magnetic field energy is stored in the toroidal field component. The main radial field structure is a positive region of about 500 G, at the location of the dark polar spot. The brightness map of V530 Per is sheared by solar-like differential rotation, with a roughly solar value for the difference in rotation rate between the pole and equator. \halpha~is observed in emission, and is mostly modulated by the stellar rotation period. The prominence system is organized in a ring at the approximate location of the co-rotation radius, with significant evolution between the two observing nights. V530 Per is the first example of a solar-type star to have its surface magnetic field and prominences mapped together, which will bring important observational constraints to better understand the role of slingshot prominences in the angular momentum evolution of the most active stars.

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arXiv2020

Measuring stellar magnetic helicity density

Helicity is a fundamental property of a magnetic field but to date it has only been possible to observe its evolution in one star - the Sun. In this paper we provide a simple technique for mapping the large-scale helicity density across the surface of any star using only observable quantities: the poloidal and toroidal magnetic field components (which can be determined from Zeeman-Doppler imaging) and the stellar radius. We use a sample of 51 stars across a mass range of 0.1-1.34 M$_\odot$ to show how the helicity density relates to stellar mass, Rossby number, magnetic energy and age. We find that the large-scale helicity density increases with decreasing Rossby number $R_o$, peaking at $R_o \simeq 0.1$, with a saturation or decrease below that. For both fully- and partially-convective stars we find that the mean absolute helicity density scales with the mean squared toroidal magnetic flux density according to the power law: $|\langle{h\,}\rangle|$ $\propto$ $\langle{\rm{B_{tor}}^2_{}\,\rangle}^{0.86\,\pm\,0.04}$. The scatter in this relation is consistent with the variation across a solar cycle, which we compute using simulations and observations across solar cycles 23 and 24 respectively. We find a significant decrease in helicity density with age.

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arXiv2019

Slingshot prominences: coronal structure, mass loss and spin down

The structure of a star's coronal magnetic field is a fundamental property that governs the high-energy emission from the hot coronal gas and the loss of mass and angular momentum in the stellar wind. It is, however, extremely difficult to measure. We report a new method to trace this structure in rapidly-rotating young convective stars, using the cool gas trapped on coronal field lines as markers. This gas forms "slingshot prominences" which appear as transient absorption features in H-$α$. By using different methods of extrapolating this field from the surface measurements, we determine locations for prominence support and produce synthetic H-$α$ stacked spectra. The absorption features produced with a potential field extrapolation match well this those observed, while those from a non-potential field do not. In systems where the rotation and magnetic axes are well aligned, up to $50\%$ of the prominence mass may transit the star and so produces a observable feature. This fraction may fall as low as $~2\%$ in very highly inclined systems. Ejected prominences carry away mass and angular momentum at rates that vary by two orders of magnitude, but which may approach those carried by the stellar wind.

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arXiv2019

Tuning in to the radio environment of HD189733b

The hot Jupiter HD189733b is expected to be a source of strong radio emission, due to its close proximity to its magnetically active host star. Here, we model the stellar wind of its host star, based on reconstructed surface stellar magnetic field maps. We use the local stellar wind properties at the planetary orbit obtained from our models to compute the expected radio emission from the planet. Our findings show that the planet emits with a peak flux density within the detection capabilities of LOFAR. However, due to absorption by the stellar wind itself, this emission may be attenuated significantly. We show that the best time to observe the system is when the planet is near primary transit of the host star, as the attenuation from the stellar wind is lowest in this region.

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arXiv2019

MOVES II. Tuning in to the radio environment of HD189733b

We present stellar wind modelling of the hot Jupiter host HD189733, and predict radio emission from the stellar wind and the planet, the latter arising from the interaction of the stellar wind with the planetary magnetosphere. Our stellar wind models incorporate surface stellar magnetic field maps at the epochs Jun/Jul 2013, Sep 2014, and Jul 2015 as boundary conditions. We find that the mass-loss rate, angular momentum-loss rate, and open magnetic flux of HD189733 vary by 9%, 40%, and 19% over these three epochs. Solving the equations of radiative transfer, we find that from 10 MHz-100 GHz the stellar wind emits fluxes in the range of $10^{-3}$-$5$ $μ$Jy, and becomes optically thin above 10 GHz. Our planetary radio emission model uses the radiometric Bode's law, and neglects the presence of a planetary atmosphere. For assumed planetary magnetic fields of 1-10 G, we estimate that the planet emits at frequencies of 2-25 MHz, with peak flux densities of $\sim10^2$ mJy. We find that the planet orbits through regions of the stellar wind that are optically thick to the emitted frequency from the planet. As a result, unattenuated planetary radio emission can only propagate out of the system and reach the observer for 67% of the orbit for a 10 G planetary field, corresponding to when the planet is approaching and leaving primary transit. We also find that the plasma frequency of the stellar wind is too high to allow propagation of the planetary radio emission below 21 MHz. This means a planetary field of at least 8 G is required to produce detectable radio emission.

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arXiv2018

The magnetic field vector of the Sun-as-a-star. II. Evolution of the large-scale vector field through activity cycle 24

In the present work, we investigate how the large-scale magnetic field of the Sun, in its three vector components, has evolved during most of cycle 24, from 2010 Jan to 2018 Apr. To filter out the small-scale field of the Sun, present in high-resolution synoptic maps, we use a spherical harmonic decomposition method, which decomposes the solar field in multipoles with different l degrees. By summing together the low-l multipoles, we reconstruct the large-scale field at a resolution similar to observed stellar magnetic fields, which allows the direct comparison between solar and stellar magnetic maps. During cycle 24, the `Sun-as-a-star' magnetic field shows a polarity reversal in the radial and meridional components, but not in the azimuthal component. The large-scale solar field remains mainly poloidal with > 70% of its energy contained in the poloidal component. During its evolution, the large-scale field is more axisymmetric and more poloidal when near minima in sunspot numbers, and with a larger intensity near maximum. There is a correlation between toroidal energy and sunspot number, which indicates that spot fields are major contributors to the toroidal large-scale energy of the Sun. The solar large-scale magnetic properties fit smoothly with observational trends of stellar magnetism reported in See et al. The toroidal (Etor) and poloidal (Epol) energies are related as Etor ~Epol^{1.38 \pm 0.04}. Similar to the stellar sample, the large-scale field of the Sun shows a lack of toroidal non-axisymmetric field.

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arXiv2017

The open flux evolution of a solar-mass star on the main sequence

Magnetic activity is known to be correlated to the rotation period for moderately active main sequence solar-like stars. In turn, the stellar rotation period evolves as a result of magnetised stellar winds that carry away angular momentum. Understanding the interplay between magnetic activity and stellar rotation is therefore a central task for stellar astrophysics. Angular momentum evolution models typically employ spin-down torques that are formulated in terms of the surface magnetic field strength. However, these formulations fail to account for the magnetic field geometry, unlike those that are expressed in terms of the open flux, i.e. the magnetic flux along which stellar winds flow. In this work, we model the angular momentum evolution of main sequence solar-mass stars using a torque law formulated in terms of the open flux. This is done using a potential field source surface model in conjunction with the Zeeman-Doppler magnetograms of a sample of roughly solar-mass stars. We explore how the open flux of these stars varies with stellar rotation and choice of source surface radii. We also explore the effect of field geometry by using two methods of determining the open flux. The first method only accounts for the dipole component while the second accounts for the full set of spherical harmonics available in the Zeeman-Doppler magnetogram. We find only a small difference between the two methods, demonstrating that the open flux, and indeed the spin-down, of main sequence solar-mass stars is likely dominated by the dipolar component of the magnetic field.

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arXiv2017

Studying stellar spin-down with Zeeman-Doppler magnetograms

Magnetic activity and rotation are known to be intimately linked for low-mass stars. Understanding rotation evolution over the stellar lifetime is therefore an important goal within stellar astrophysics. In recent years, there has been increased focus on how the complexity of the stellar magnetic field affects the rate of angular momentum-loss from a star. This is a topic that Zeeman-Doppler imaging (ZDI), a technique that is capable of reconstructing the large-scale magnetic field topology of a star, can uniquely address. Using a potential field source surface model, we estimate the open flux, mass loss-rate and angular momentum-loss rates for a sample of 66 stars that have been mapped with ZDI. We show that the open flux of a star is predominantly determined by the dipolar component of its magnetic field for our choice of source surface radius. We also show that, on the main sequence, the open flux, mass- and angular momentum-loss rates increase with decreasing Rossby number. The exception to this rule is stars less massive than $0.3M_{\odot}$. Previous work suggests that low mass M dwarfs may possess either strong, ordered and dipolar fields or weak and complex fields. This range of field strengths results in a large spread of angular momentum-loss rates for these stars and has important consequences for their spin down behaviour. Additionally, our models do not predict a transition in the mass-loss rates at the so called wind dividing line noted from Ly$α$ studies.

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arXiv2017

A hot Jupiter around the very active weak-line T Tauri star TAP 26

We report the results of an extended spectropolarimetric and photometric monitoring of the weak-line T Tauri star TAP 26, carried out within the MaTYSSE programme with the ESPaDOnS spectropolarimeter at the 3.6 m Canada-France-Hawaii Telescope. Applying Zeeman-Doppler Imaging to our observations, concentrating in 2015 November and 2016 January and spanning 72 d in total, 16 d in 2015 November and 13 d in 2016 January, we reconstruct surface brightness and magnetic field maps for both epochs and demonstrate that both distributions exhibit temporal evolution not explained by differential rotation alone. We report the detection of a hot Jupiter (hJ) around TAP 26 using three different methods, two using Zeeman-Doppler Imaging (ZDI) and one Gaussian-Process Regression (GPR), with a false-alarm probability smaller than 6.10^-4. However, as a result of the aliasing related to the observing window, the orbital period cannot be uniquely determined; the orbital period with highest likelihood is 10.79 +/- 0.14 d followed by 8.99 +/- 0.09 d. Assuming the most likely period, and that the planet orbits in the stellar equatorial plane, we obtain that the planet has a minimum mass M.sin(i) of 1.66 +/- 0.31 M_Jup and orbits at 0.0968 +/- 0.0032 au from its host star. This new detection suggests that disc type II migration is efficient at generating newborn hJs, and that hJs may be more frequent around young T Tauri stars than around mature stars (or that the MaTYSSE sample is biased towards hJ-hosting stars).

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arXiv2016

The connection between stellar activity cycles and magnetic field topology

Zeeman Doppler imaging has successfully mapped the large-scale magnetic fields of stars over a large range of spectral types, rotation periods and ages. When observed over multiple epochs, some stars show polarity reversals in their global magnetic fields. On the Sun, polarity reversals are a feature of its activity cycle. In this paper, we examine the magnetic properties of stars with existing chromospherically determined cycle periods. Previous authors have suggested that cycle periods lie on multiple branches, either in the cycle period-Rossby number plane or the cycle period-rotation period plane. We find some evidence that stars along the active branch show significant average toroidal fields that exhibit large temporal variations while stars exclusively on the inactive branch remain dominantly poloidal throughout their entire cycle. This lends credence to the idea that different shear layers are in operation along each branch. There is also evidence that the short magnetic polarity switches observed on some stars are characteristic of the inactive branch while the longer chromospherically determined periods are characteristic of the active branch. This may explain the discrepancy between the magnetic and chromospheric cycle periods found on some stars. These results represent a first attempt at linking global magnetic field properties obtained form ZDI and activity cycles.

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arXiv2015

Could a change in magnetic field geometry cause the break in the wind-activity relation?

Wood et al suggested that mass-loss rate is a function of X-ray flux ($\dot{M} \propto F_x^{1.34}$) for dwarf stars with $F_x \lesssim F_{x,6} \equiv 10^6$ erg cm$^{-2}$ s$^{-1}$. However, more active stars do not obey this relation. These authors suggested that the break at $F_{x,6}$ could be caused by significant changes in magnetic field topology that would inhibit stellar wind generation. Here, we investigate this hypothesis by analysing the stars in Wood et al's sample that had their surface magnetic fields reconstructed through Zeeman-Doppler Imaging (ZDI). Although the solar-like outliers in the $\dot{M}$ -- $F_x$ relation have higher fractional toroidal magnetic energy, we do not find evidence of a sharp transition in magnetic topology at $F_{x,6}$. To confirm this, further wind measurements and ZDI observations at both sides of the break are required. As active stars can jump between states with highly toroidal to highly poloidal fields, we expect significant scatter in magnetic field topology to exist for stars with $F_x \gtrsim F_{x,6}$. This strengthens the importance of multi-epoch ZDI observations. Finally, we show that there is a correlation between $F_x$ and magnetic energy, which implies that $\dot{M}$ -- magnetic energy relation has the same qualitative behaviour as the original $\dot{M}$ -- $F_x$ relation. No break is seen in any of the $F_x$ -- magnetic energy

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arXiv2015

The energy budget of stellar magnetic fields

Spectropolarimetric observations have been used to map stellar magnetic fields, many of which display strong bands of azimuthal fields that are toroidal. A number of explanations have been proposed to explain how such fields might be generated though none are definitive. In this paper, we examine the toroidal fields of a sample of 55 stars with magnetic maps, with masses in the range 0.1-1.5$\,{\rm M}_\odot$. We find that the energy contained in toroidal fields has a power law dependence on the energy contained in poloidal fields. However the power index is not constant across our sample, with stars less and more massive than 0.5$\,{\rm M}_\odot$ having power indices of 0.72$\pm$0.08 and 1.25$\pm$0.06 respectively. There is some evidence that these two power laws correspond to stars in the saturated and unsaturated regimes of the rotation-activity relation. Additionally, our sample shows that strong toroidal fields must be generated axisymmetrically. The latitudes at which these bands appear depend on the stellar rotation period with fast rotators displaying higher latitude bands than slow rotators. The results in this paper present new constraints for future dynamo studies.

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arXiv2015

Activity and Magnetic Field Structure of the Sun-Like Planet Hosting Star HD 1237

We analyse the magnetic activity characteristics of the planet hosting Sun-like star, HD 1237, using HARPS spectro-polarimetric time-series data. We find evidence of rotational modulation of the magnetic longitudinal field measurements consistent with our ZDI analysis, with a period of 7 days. We investigate the effect of customising the LSD mask to the line depths of the observed spectrum and find that it has a minimal effect on shape of the extracted Stokes V profile but does result in a small increase in the S/N ($\sim$ 7%). We find that using a Milne-Eddington solution to describe the local line profile provides a better fit to the LSD profiles in this slowly rotating star, which also impacts the recovered ZDI field distribution. We also introduce a fit-stopping criterion based on the information content (entropy) of the ZDI maps solution set. The recovered magnetic field maps show a strong (+90 G) ring-like azimuthal field distribution and a complex radial field dominating at mid latitudes ($\sim$45 degrees). Similar magnetic field maps are recovered from data acquired five months apart. Future work will investigate how this surface magnetic field distribution impacts the coronal magnetic field and extended environment around this planet-hosting star.

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