arXiv subjects

Aline A. Vidotto

Publications and source records attributed to Aline A. Vidotto.

54 records · Page 3Linked to original sources

arXiv2021

Planet-induced radio emission from the coronae of M dwarfs

There have recently been detections of radio emission from low-mass stars, some of which are indicative of star-planet interactions. Motivated by these exciting new results, here we present stellar wind models for the active planet-hosting M dwarf AU Mic. Our models incorporate the large-scale photospheric magnetic field map of the star, reconstructed using the Zeeman-Doppler Imaging method. We use our models to assess if planet-induced radio emission could be generated in the corona of AU Mic, through a mechanism analogous to the sub-Alfvénic Jupiter-Io interaction. In the case that AU Mic has a mass-loss rate of 27 times that of the Sun, we find that both planets b and c in the system can induce radio emission from 10 MHz to 3 GHz in the corona of the host star for the majority of their orbits, with peak flux densities of 10 mJy. Our predicted emission bears a striking similarity to that recently reported from GJ 1151 by Vedantham et al. (2020), which is indicative of being induced by a planet. Detection of such radio emission would allow us to place an upper limit on the mass-loss rate of the star.

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arXiv2021

Planet-induced radio emission from the coronae of M dwarfs: the case of Prox Cen and AU Mic

There have recently been detections of radio emission from low-mass stars, some of which are indicative of star-planet interactions. Motivated by these exciting new results, in this paper we present Alfvén wave-driven stellar wind models of the two active planet-hosting M dwarfs Prox Cen and AU Mic. Our models incorporate large-scale photospheric magnetic field maps reconstructed using the Zeeman-Doppler Imaging method. We obtain a mass-loss rate of $0.25~\dot{M}_{\odot}$ for the wind of Prox Cen. For the young dwarf AU Mic, we explore two cases: a low and high mass-loss rate. Depending on the properties of the Alfvén waves which heat the corona in our wind models, we obtain mass-loss rates of $27$ and $590~\dot{M}_{\odot}$ for AU Mic. We use our stellar wind models to assess the generation of electron cyclotron maser instability emission in both systems, through a mechanism analogous to the sub-Alfvénic Jupiter-Io interaction. For Prox Cen we do not find any feasible scenario where the planet can induce radio emission in the star's corona, as the planet orbits too far from the star in the super-Alfvénic regime. However, in the case that AU Mic has a stellar wind mass-loss rate of $27~\dot{M}_{\odot}$, we find that both planets b and c in the system can induce radio emission from $\sim10$ MHz to 3 GHz in the corona of the host star for the majority of their orbits, with peak flux densities of $\sim10$ mJy. Detection of such radio emission would allow us to place an upper limit on the mass-loss rate of the star.

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arXiv2021

How does the mass and activity history of the host star affect the population of low-mass planets?

The evolution of the atmospheres of low and intermediate-mass planets is strongly connected to the physical properties of their host stars. The types and the past activities of planet-hosting stars can, therefore, affect the overall planetary population. In this paper, we perform a comparative study of sub-Neptune-like planets orbiting stars of different masses and different evolutionary histories. We discuss the general patterns of the evolved population as a function of parameters and environments of planets. As a model of the atmospheric evolution, we employ the own framework combining planetary evolution in MESA with the realistic prescription of the escape of hydrogen-dominated atmospheres. {We find that the final populations look qualitatively similar in terms of the atmospheres survival around different stars, but qualitatively different, with this difference accentuated for planets orbiting more massive stars. We show that a planet has larger chances of keeping its primordial atmosphere in the habitable zone of a solar mass star compared to M or K dwarfs and if it starts the evolution having a relatively compact envelope.} We also address the problem of the uncertain initial temperatures (luminosities) of planets and show that this issue is only of particular importance for planets exposed to extreme atmospheric mass-losses.

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arXiv2020

Identifying solar-like magnetic cycles with Zeeman-Doppler-Imaging (ZDI)

We are reaching the point where spectropolarimetric surveys have run for long enough to reveal solar-like magnetic activity cycles. In this paper we investigate what would be the best strategy to identify solar-like magnetic cycles and ask which large-scale magnetic field parameters best follow a solar-type magnetic cycle and are observable with the Zeeman-Doppler-Imaging (ZDI) technique. We approach these questions using the 3D non-potential flux transport simulations of \cite{Yeates2012} modelling the solar vector magnetic field over 15 years (centred on solar cycle 23). The flux emergence profile was extracted from solar synoptic maps and used as input for a photospheric flux transport model in combination with a non-potential coronal evolution model. We synthesise spectropolarimetric data from the simulated maps and reconstruct them using ZDI. The ZDI observed solar cycle is set into the context of other cool star observations and we present observable trends of the magnetic field topology with time, sunspot number and S-index. We find that the axisymmetric energy fraction is the best parameter of the ZDI detectable large-scale field to trace solar-like cycles. Neither the surface averaged large-scale field or the total magnetic energy is appropriate. ZDI seems also to be able to recover the increase of the toroidal energy with S-index. We see further that ZDI might unveil hints of the dynamo modes that are operating and of the global properties of the small-scale flux emergence like active latitudes.

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arXiv2020

Coupling thermal evolution of planets and hydrodynamic atmospheric escape in MESA

The long-term evolution of hydrogen-dominated atmospheres of sub-Neptune-like planets is mostly controlled by two factors: a slow dissipation of the gravitational energy acquired at the formation (known as thermal evolution) and atmospheric mass loss. Here, we use MESA to self-consistently couple the thermal evolution model of lower atmospheres with a realistic hydrodynamical atmospheric evaporation prescription. To outline the main features of such coupling, we simulate planets with a range of core masses (5-20 Mearth) and initial atmospheric mass fractions (0.5-30%), orbiting a solar-like star at 0.1 au. In addition to our computed evolutionary tracks, we also study the stability of planetary atmospheres, showing that the atmospheres of light planets can be completely removed within 1 Gyr, and that compact atmospheres have a better survival rate. From a detailed comparison between our results and the output of the previous-generation models, we show that coupling between thermal evolution and atmospheric evaporation considerably affects the thermal state of atmospheres for low-mass planets and, consequently, changes the relationship between atmospheric mass fraction and planetary parameters. We, therefore, conclude that self-consistent consideration of the thermal evolution and atmospheric evaporation is of crucial importance for evolutionary modeling and a better characterization of planetary atmospheres. From our simulations, we derive an analytical expression between the planetary radius and atmospheric mass fraction at different ages. In particular, we find that, for a given observed planetary radius, the predicted atmospheric mass fraction changes as age^0.11.

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arXiv2020

The impact of unresolved magnetic spots on high precision radial velocity measurements

The Doppler method of exoplanet detection has been extremely successful, but suffers from contaminating noise from stellar activity. In this work a model of a rotating star with a magnetic field based on the geometry of the K2 star Epsilon Eridani is presented and used to estimate its effect on simulated radial velocity measurements. A number of different distributions of unresolved magnetic spots were simulated on top of the observed large-scale magnetic maps obtained from eight years of spectropolarimetric observations. The radial velocity signals due to the magnetic spots have amplitudes of up to 10 m s$^{-1}$, high enough to prevent the detection of planets under 20 Earth masses in temperate zones of solar type stars. We show that the radial velocity depends heavily on spot distribution. Our results emphasize that understanding stellar magnetic activity and spot distribution is crucial for detection of Earth analogues.

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arXiv2020

Influence of the Sun-like magnetic cycle on exoplanetary atmospheric escape

Stellar high-energy radiation (X-ray and extreme ultraviolet, XUV) drives atmospheric escape in close-in exoplanets. Given that stellar irradiation depends on the stellar magnetism and that stars have magnetic cycles, we investigate how cycles affect the evolution of exoplanetary atmospheric escape. Firstly, we consider a hypothetical HD209458b-like planet orbiting the Sun. For that, we implement the observed solar XUV radiation available over one and a half solar cycles in a 1D hydrodynamic escape model of HD209458b. We find that atmospheric escape rates show a cyclic variation (from 7.6 to 18.5 $\times$ 10$^{10}$ g s$^{-1}$), almost proportional to the incident stellar radiation. To compare this with observations, we compute spectroscopic transits in two hydrogen lines. We find non-detectable cyclic variations in Ly$α$ transits. Given the temperature sensitiveness of the H$α$ line, its equivalent width has an amplitude of 1.9 mA variation over the cycle, which could be detectable in exoplanets such as HD209458b. We demonstrate that the XUV flux is linearly proportional to the magnetic flux during the solar cycle. Secondly, we apply this relation to derive the cyclic evolution of the XUV flux of HD189733 using the available magnetic flux observations of the star from Zeeman Doppler Imaging over nearly a decade. The XUV fluxes are then used to model escape in HD189733b, which shows escape rate varying from 2.8 to 6.5 $\times$ 10$^{10}$ g s$^{-1}$. Like in the HD209458b case, this introduces variations in Ly$α$ and H$α$ transits, with H$α$ variations more likely to be observable. Finally, we show that a strong stellar flare would enhance significantly Ly$α$ and H$α$ transit depths.

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arXiv2020

Alfvén-wave driven magnetic rotator winds from low-mass stars I: rotation dependences of magnetic braking and mass-loss rate

Observations of stellar rotation show that low-mass stars lose angular momentum during the main sequence. We simulate the winds of Sun-like stars with a range of rotation rates, covering the fast and slow magneto-rotator regimes, including the transition between the two. We generalize an Alfvén-wave driven solar wind model that builds on previous works by including the magneto-centrifugal force explicitly. In this model, the surface-averaged open magnetic flux is assumed to scale as $B_\ast f^{\rm open}_\ast \propto {\rm Ro}^{-1.2}$, where $f^{\rm open}_\ast$ and ${\rm Ro}$ are the surface open-flux filling factor and Rossby number, respectively. We find that, 1. the angular momentum loss rate (torque) of the wind is described as $τ_w \approx 2.59 \times 10^{30} {\rm \ erg} \ \left( Ω_\ast / Ω_\odot \right)^{2.82}$, yielding a spin-down law $Ω_\ast \propto t^{-0.55}$. 2. the mass-loss rate saturates at $\dot{M}_w \sim 3.4 \times 10^{-14} M_\odot {\rm \ yr^{-1}}$, due to the strong reflection and dissipation of Alfvén waves in the chromosphere. This indicates that the chromosphere has a strong impact in connecting the stellar surface and stellar wind. Meanwhile, the wind ram pressure scales as $P_w \propto Ω_\ast^{0.57}$, which is able to explain the lower-envelope of the observed stellar winds by Wood et al. 3. the location of the Alfvén radius is shown to scale in a way that is consistent with 1D analytic theory. Additionally, the precise scaling of the Alfvén radius matches previous works which used thermally-driven winds. Our results suggest that the Alfvén-wave driven magnetic rotator wind plays a dominant role in the stellar spin-down during the main-sequence.

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arXiv2020

Planetary transits at radio wavelengths: secondary eclipses of hot Jupiter extended atmospheres

When a planet transits in front of its host star, a fraction of its light is blocked, decreasing the observed flux from the star. The same is expected to occur when observing the stellar radio flux. However, at radio wavelengths, the planet also radiates, depending on its temperature, and thus modifies the transit depths. We explore this scenario simulating the radio lightcurves of transits of hot-Jupiters, Kepler-17b and WASP-12b, around solar-like stars. We calculated the bremsstrahlung radio emission at 17, 100, and 400 GHz originated from the star, considering a solar atmospheric model. The planetary radio emission was calculated modelling the planets in two scenarios: as a blackbody or with a dense and hot extended atmosphere. In both cases the planet radiates and contributes to the total radio flux. For a blackbody planet, the transit depth is in the order of 2-4% and it is independent of the radio frequency. Hot-Jupiters planets with atmospheres appear bigger and brighter in radio, thus having a larger contribution to the total flux of the system. Therefore, the transit depths are larger than in the case of blackbody planets, reaching up to 8% at 17 GHz. Also the transit depth is frequency-dependent. Moreover, the transit caused by the planet passing behind the star is deeper than when the planet transits in front of the star, being as large as 18% at 400GHz. In all cases, the contribution of the planetary radio emission to the observed flux is evident when the planet transits behind the star.

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arXiv2019

Do non-dipolar magnetic fields contribute to spin-down torques?

Main sequence low-mass stars are known to spin-down as a consequence of their magnetised stellar winds. However, estimating the precise rate of this spin-down is an open problem. The mass-loss rate, angular momentum-loss rate and the magnetic field properties of low-mass stars are fundamentally linked making this a challenging task. Of particular interest is the stellar magnetic field geometry. In this work, we consider whether non-dipolar field modes contribute significantly to the spin-down of low-mass stars. We do this using a sample of stars that have all been previously mapped with Zeeman-Doppler imaging. For a given star, as long as its mass-loss rate is below some critical mass-loss rate, only the dipolar fields contribute to its spin-down torque. However, if it has a larger mass-loss rate, higher order modes need to be considered. For each star, we calculate this critical mass-loss rate, which is a simple function of the field geometry. Additionally, we use two methods of estimating mass-loss rates for our sample of stars. In the majority of cases, we find that the estimated mass-loss rates do not exceed the critical mass-loss rate and hence, the dipolar magnetic field alone is sufficient to determine the spin-down torque. However, we find some evidence that, at large Rossby numbers, non-dipolar modes may start to contribute.

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arXiv2019

Estimating magnetic filling factors from Zeeman-Doppler magnetograms

Low-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterise them. Zeeman-Doppler 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, $\langle B_V\rangle$, with that recovered by Zeeman broadening studies, $\langle B_I\rangle$. In line with previous studies, $\langle B_V\rangle$ ranges from a few % to $\sim$20% of $\langle B_I\rangle$. We show that a power law relationship between $\langle B_V\rangle$ and $\langle B_I\rangle$ 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.

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arXiv2019

The Rotationally Modulated Polarization of $ξ$ Boo A

We have observed the active star $ξ$ Boo A (HD 131156A) with high precision broadband linear polarimetry contemporaneously with circular spectropolarimetry. We find both signals are modulated by the 6.43 day rotation period of $ξ$ Boo A. The signals from the two techniques are 0.25 out of phase, consistent with the broadband linear polarization resulting from differential saturation of spectral lines in the global transverse magnetic field. The mean magnitude of the linear polarization signal is ~4 ppm/G but its structure is complex and the amplitude of the variations suppressed relative to the longitudinal magnetic field. The result has important implications for current attempts to detect polarized light from hot Jupiters orbiting active stars in the combined light of the star and planet. In such work stellar activity will manifest as noise, both on the time scale of stellar rotation, and on longer time scales - where changes in activity level will manifest as a baseline shift between observing runs.

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arXiv2018

The Colorado Ultraviolet Transit Experiment (CUTE): A dedicated cubesat mission to study exoplanetary mass loss and magnetic fields

The Colorado Ultraviolet Transit Experiment (CUTE) is a near-UV (2550 - 3300 Angstrom) 6U cubesat mission designed to monitor transiting hot Jupiters to quantify their atmospheric mass loss and magnetic fields. CUTE will probe both atomic (Mg and Fe) and molecular (OH) lines for evidence of enhanced transit absorption, and to search for evidence of early ingress due to bow shocks ahead of the planet's orbital motion. As a dedicated mission, CUTE will observe more than 100 spectroscopic transits of hot Jupiters over a nominal seven month mission. This represents the equivalent of more than 700 orbits of the only other instrument capable of these measurements, the Hubble Space Telescope. CUTE efficiently utilizes the available cubesat volume by means of an innovative optical design to achieve a projected effective area of 28 sq. cm, low instrumental background, and a spectral resolving power of 3000 over the primary science bandpass. These performance characteristics enable CUTE to discern transit depths between 0.1 - 1% in individual spectral absorption lines. We present the CUTE optical and mechanical design, a summary of the science motivation and expected results, and an overview of the projected fabrication, calibration and launch timeline.

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arXiv2016

High surface magnetic field in red giants as a new signature of planet engulfment?

Context. Red-giant stars may engulf planets. This may increase the rotation rate of their convective envelope, which could lead to strong dynamo-triggered magnetic fields. Aims. We explore the possibility of generating magnetic fields in red giants that have gone through the process of a planet engulfment. We compare them with similar models that evolve without any planets. We discuss the impact of stellar wind magnetic braking on the evolution of the surface velocity of the parent star. Methods. With rotating stellar models with and without planets and an empirical relation between the Rossby number and the surface magnetic field, we deduce the evolution of the surface magnetic field along the red-giant branch. The effects of wind magnetic braking is explored using a relation deduced from MHD simulations. Results. The stellar evolution model of a 1.7 M$_\odot$ without planet engulfment and that has a time-averaged rotation velocity during the Main-Sequence equal to 100 km s$^{-1}$, shows a surface magnetic field triggered by convection larger than 10 G only at the base of the red giant branch, that means for gravities log $g > 3$. When a planet engulfment occurs, such magnetic field can also appear at much lower gravities, i.e. at much higher luminosities along the red giant branch. Typically the engulfment of a 15 M$_J$ planet produces a dynamo triggered magnetic field larger than 10 G for gravities between 2.5 and 1.9. We show that for reasonable wind magnetic braking laws, the high surface velocity reached after a planet engulfment may be maintained sufficiently long for being observable. Conclusions. High surface magnetic fields for red giants in the upper part of the red giant branch is a strong indication of a planet engulfment or of an interaction with a companion. Our theory can be tested by observing fast rotating red giants and check whether they show magnetic fields.

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arXiv2016

Star-planet interactions: II. Is planet engulfment the origin of fast rotating red giants?

Context. Fast rotating red giants in the upper part of the red giant branch have surface velocities that cannot be explained by single star evolution. Aims. We check whether tides between a star and a planet followed by planet engulfment can indeed accelerate the surface rotation of red giants for a sufficient long time in order to produce these fast rotating red giants. Methods. Using rotating stellar models, accounting for the redistribution of the angular momentum inside the star by different transport mechanisms, for the exchanges of angular momentum between the planet orbit and the star before the engulfment and for the deposition of angular momentum inside the star at the engulfment, we study how the surface rotation velocity at the stellar surface evolves. Results. We show that the surface velocities reached at the end of the orbital decay due to tidal forces and planet engulfment can be similar to values observed for fast rotating red giants. This surface velocity then decreases when the star evolves along the red giant branch but at a sufficiently slow pace for allowing stars to be detected with such a high velocity. More quantitatively, star-planet interaction can produce a rapid acceleration of the surface of the star, above values equal to 8 km s$^{-1}$, for periods lasting up to more than 30% the red giant branch phase. The changes of the surface carbon isotopic ratios produced by the dilution of the planetary material into the convective envelope is quite modest. Much more important might be the increase of the lithium abundance due to this effect. However lithium may be affected by many different, still uncertain, processes. Conclusions. The acceleration of the stellar surface to rotation velocities above limits that depend on the surface gravity does appear at the moment as the clearest signature of a star-planet interaction.[abridged]

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arXiv2016

An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ436 b

The warm Neptune GJ436b was observed with HST/STIS at three different epochs in the stellar Ly-alpha line, showing deep, repeated transits caused by a giant exosphere of neutral hydrogen. The low radiation pressure from the M-dwarf host star was shown to play a major role in the dynamics of the escaping gas. Yet by itself it cannot explain the time-variable spectral features detected in each transit. Here we investigate the combined role of radiative braking and stellar wind interactions using numerical simulations with the EVaporating Exoplanet code (EVE) and we derive atmospheric and stellar properties through the direct comparison of simulated and observed spectra. Our simulations match the last two epochs well. The observed sharp early ingresses come from the abrasion of the planetary coma by the stellar wind. Spectra observed during the transit can be produced by a dual exosphere of planetary neutrals (escaped from the upper atmosphere of the planet) and neutralized protons (created by charge-exchange with the stellar wind). We find similar properties at both epochs for the planetary escape rate (2.5x10$^{8}$ g/s), the stellar photoionization rate (2x10$^{-5}$ /s), the stellar wind bulk velocity (85 km/s), and its kinetic dispersion velocity (10 km/s). We find high velocities for the escaping gas (50-60 km/s) that may indicate MHD waves that dissipate in the upper atmosphere and drive the planetary outflow. In the last epoch the high density of the stellar wind (3x10$^{3}$ /cm3) led to the formation of an exospheric tail mainly composed of neutralized protons. The observations of GJ436 b allow for the first time to clearly separate the contributions of radiation pressure and stellar wind and to probe the regions of the exosphere shaped by each mechanism.

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arXiv2016

Star-planet interactions: I. Stellar rotation and planetary orbits

Context. As a star evolves, the planet orbits change with time due to tidal interactions, stellar mass losses, friction and gravitational drag forces, mass accretion and evaporation on/by the planet. Stellar rotation modifies the structure of the star and therefore the way these different processes occur. Changes of the orbits, at their turn, have an impact on the rotation of the star. Aims. Models accounting in a consistent way for these interactions between the orbital evolution of the planet and the evolution of the rotation of the star are still missing. The present work is a first attempt to fill this gap. Methods. We compute the evolution of stellar models including a comprehensive treatment of rotational effects together with the evolution of planetary orbits, so that the exchanges of angular momentum between the star and the planetary orbit are treated in a self-consistent way. The evolution of the rotation of the star accounts for the angular momentum exchange with the planet and also follows the effects of the internal transport of angular momentum and chemicals. Results. We show that rotating stellar models without tidal interactions can well reproduce the surface rotations of the bulk of the red giants. However, models without any interactions cannot account for fast rotating red giants in the upper part of the red giant branch, where, such models, whatever the initial rotation considered on the ZAMS, always predict very low velocities. For those stars some interaction with a companion is highly probable and the present rotating stellar models with planets confirm that tidal interaction can reproduce their high surface velocities. We show also that the minimum distance between the planet and the star on the ZAMS that will allow the planet to avoid engulfment and survive is decreased around faster rotating stars. [abridged]

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arXiv2014

The effects of stellar winds on the magnetospheres and potential habitability of exoplanets

Context: The principle definition of habitability for exoplanets is whether they can sustain liquid water on their surfaces, i.e. that they orbit within the habitable zone. However, the planet's magnetosphere should also be considered, since without it, an exoplanet's atmosphere may be eroded away by stellar winds. Aims: The aim of this paper is to investigate magnetospheric protection of a planet from the effects of stellar winds from solar-mass stars. Methods: We study hypothetical Earth-like exoplanets orbiting in the host star's habitable zone for a sample of 124 solar-mass stars. These are targets that have been observed by the Bcool collaboration. Using two wind models, we calculate the magnetospheric extent of each exoplanet. These wind models are computationally inexpensive and allow the community to quickly estimate the magnetospheric size of magnetised Earth-analogues orbiting cool stars. Results: Most of the simulated planets in our sample can maintain a magnetosphere of ~5 Earth radii or larger. This suggests that magnetised Earth analogues in the habitable zones of solar analogues are able to protect their atmospheres and is in contrast to planets around young active M dwarfs. In general, we find that Earth-analogues around solar-type stars, of age 1.5 Gyr or older, can maintain at least a Paleoarchean Earth sized magnetosphere. Our results indicate that planets around 0.6 - 0.8 solar-mass stars on the low activity side of the Vaughan-Preston gap are the optimum observing targets for habitable Earth analogues.

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