Solar-type Stars Research Articles

Cold Jupiters and improved masses in 38 Kepler and K2 small planet systems from 3661 HARPS-N radial velocities

The exoplanet population characterized by relatively short orbital periods (P < 100 d) around solar-type stars is dominated by super-Earths and sub-Neptunes. However, these planets are missing in our Solar System and the reason behind this absence is still unknown. Two theoretical scenarios invoke the role of Jupiter as the possible culprit: Jupiter may have acted as a dynamical barrier to the inward migration of sub-Neptunes from beyond the water iceline; alternatively, Jupiter may have considerably reduced the inward flux of material (pebbles) required to form super-Earths inside that iceline. Both scenarios predict an anti-correlation between the presence of small planets and that of cold Jupiters in exoplanetary systems. To test that prediction, we homogeneously analyzed the radial-velocity measurements of 38 Kepler and K2 transiting small planet systems gathered over nearly ten years with the HARPS-N spectrograph, as well as publicly available radial velocities collected with other facilities. We used Bayesian differential evolution Markov chain Monte Carlo techniques, which in some cases were coupled with Gaussian process regression to model non-stationary variations due to stellar magnetic activity phenomena. We detected five cold Jupiters in three systems: two in Kepler-68, two in Kepler-454, and a very eccentric one in K2-312. We also found linear trends caused by bound companions in Kepler-93, Kepler-454, and K2-12, with slopes that are still compatible with a planetary mass for outer bodies in the Kepler-454 and K2-12 systems. By using binomial statistics and accounting for the survey completeness, we derived an occurrence rate of 9.3−2.9+7.7% for cold Jupiters with 0.3–13 MJup and 1–10 AU, which is lower but still compatible at 1.3σ with the value measured from radial-velocity surveys for solar-type stars, regardless of the presence or absence of small planets. The sample is not large enough to draw a firm conclusion about the predicted anti-correlation between small planets and cold Jupiters; nevertheless, we found no evidence of previous claims of an excess of cold Jupiters in small planet systems. As an important byproduct of our analyses, we homogeneously determined the masses of 64 Kepler and K2 small planets, reaching a precision better than 5, 7.5, and 10σ for 25, 13, and 8 planets, respectively. Finally, we release the 3661 HARPS-N radial velocities used in this work to the scientific community. These radial-velocity measurements mainly benefit from an improved data reduction software that corrects for subtle prior systematic effects.

Long-term Evolution of Warps in Debris Disks—Application to the Gyr-old System HD 202628

We present the results of N-body simulations meant to reproduce the long-term effects of mutually inclined exoplanets on debris disks, using the HD 202628 system as a proxy. HD 202628 is a Gyr-old solar-type star that possesses a directly observable, narrow debris ring with a clearly defined inner edge and nonzero eccentricity, hinting at the existence of a sculpting exoplanet. The eccentric nature of the disk leads us to examine the effect on it over Gyr timescales from an eccentric and inclined planet, placed on its orbit through scattering processes. We find that, in systems with dynamical timescales akin to that of HD 202628, a planetary companion is capable of completely tilting the debris disk. This tilt is preserved over the Gyr age of the system. Simulated observations of our models show that an exoplanet around HD 202628 with an inclination misalignment ≳10° would cause the disk to be observably diffuse and broad, which is inconsistent with Atacama Large Millimeter Array (ALMA) observations. With these observations, we conclude that, if there is an exoplanet shaping this disk, it likely had a mutual inclination of less than 5° with the primordial disk. The conclusions of this work can be applied either to debris disks appearing as narrow rings (e.g., Fomalhaut and HR 4796) or to disks that are vertically thick at ALMA wavelengths (e.g., HD 110058).

Stable Fiber-illumination for Extremely Precise Radial Velocities with NEID

NEID is a high-resolution red–optical precision radial velocity (RV) spectrograph recently commissioned at the WIYN 3.5 m telescope at Kitt Peak National Observatory, Arizona, USA. NEID has an extremely stable environmental control system, and spans a wavelength range of 380–930 nm with two observing modes: a High Resolution mode at R ∼ 112,000 for maximum RV precision, and a High Efficiency mode at R ∼ 72,000 for faint targets. In this paper we present a detailed description of the components of NEID’s optical fiber feed, which include the instrument, exposure meter, calibration system, and telescope fibers. Many parts of the optical fiber feed can lead to uncalibratable RV errors, which cannot be corrected for using a stable wavelength reference source. We show how these errors directly cascade down to performance requirements on the fiber feed and the scrambling system. We detail the design, assembly, and testing of each component. Designed and built from the bottom-up with a single-visit instrument precision requirement of 27 cm s−1, close attention is paid to the error contribution from each NEID subsystem. Finally, we include the lab and on-sky tests performed during instrument commissioning to test the illumination stability, and discuss the path to achieving the instrumental stability required to search for a true Earth twin around a solar-type star.

Deuterium Escape on Photoevaporating Sub-Neptunes

We investigate the evolution of the deuterium-to-hydrogen (D/H) mass ratio driven by EUV photoevaporation of hydrogen-rich atmospheres of close-in sub-Neptunes around solar-type stars. For the first time, the diffusion-limited approach in conjunction with energy-limited photoevaporation is considered in evaluating deuterium escape from evolving exoplanet H/He envelopes. We find that the planets with smaller initial gas envelopes and thus smaller sizes can lead to weaker atmospheric escape, which facilitates hydrogen–deuterium fractionation. Specifically, in our grid of simulations with a low envelope mass fraction of less than 0.005, a low-mass sub-Neptune (4–5 M ⊕) at about 0.25–0.4 au or a high-mass sub-Neptune (10–15 M ⊕) at about 0.1–0.25 au can increase the D/H values by greater than 20% over 7.5 Gyr. Akin to the helium-enhanced envelopes of sub-Neptunes due to photoevaporating escape, the planets along the upper boundary of the radius valley are the best targets to detect high D/H ratios. The ratio can rise by a factor of ≲1.65 within 7.5 Gyr in our grid of evolutionary calculations. The D/H ratio is expected to be higher in thinner envelopes as long as the planets do not become bare rocky cores.

Mass Accretion, Spectral, and Photometric Properties of T Tauri Stars in Taurus Based on TESS and LAMOST

We present the analysis of 16 classical T Tauri stars (CTTSs) using LAMOST and TESS data, investigating spectral properties, photometric variations, and mass accretion rates. All 16 stars exhibit emissions in Hα lines, from which the average mass accretion rate of 1.76 × 10−9 M ⊙ yr−1 is derived. Two of the stars, DL Tau and Haro 6-13, show mass accretion bursts simultaneously in TESS, ASAS-SN, and/or the ZTF survey. Based on these observations, we find that the mass accretion rates of DL Tau and Haro 6-13 reach their maxima of 2.5 × 10−8 M ⊙ yr−1 and 2 × 10−10 M ⊙ yr−1, respectively, during the TESS observation. We detect 13 flares among these stars. The flare frequency distribution shows that the CTTSs’ flare activity is not only dominated by strong flares with high energy but also much more active than those of solar-type and young low-mass stars. By comparing the variability classes reported in the literature, we find that the transition timescale between different classes of variability in CTTSs, such as from stochastic (S) to bursting (B) or from quasi-periodic symmetric to quasi-periodic dipping, may range from 1.6 to 4 yr. We observe no significant correlation between inclination and mass accretion rates derived from the emission indicators. This suggests that inner disk properties may be more important than those of outer disks. Finally, we find a relatively significant positive correlation between the asymmetric metric M and the cold disk inclination compared to the literature. A weak negative correlation between the periodicity metric Q value and inclination has also been found.

Activity time series of old stars from late F to early K

Context. Stellar variability impacts radial velocities (hereafter RVs) at various timescales and therefore the detectability of exoplanets and the mass determination based on this technique. Detecting and characterising Earth-like planets in the habitable zone of solar-type stars represents an important challenge in the coming years, however. It is therefore necessary to implement systematic studies of this issue, for example to delineate the current limitations of RV techniques. Aims. A first aim of this paper is to investigate whether the targeted 10% mass uncertainty from RV follow-up of transits detected by PLATO can be reached. A second aim of this paper is to analyse and quantify Earth-like planet detectability for various spectral types. Methods. For this purpose, we implemented blind tests based on a large data set (more than 20 000) of realistic synthetic time series reproducing different phenomena leading to stellar variability such as magnetic activity patterns similar to the solar configuration as well as flows (oscillations, granulation, and supergranulation), covering F6-K4 stars and a wide range of activity levels. Results. We find that the 10% mass uncertainty for a 1 MEarth in the habitable zone of a G2 star cannot be reached, even with an improved version of the usual correction of stellar activity (here based on a non-linear relation with log R′HK and cycle phase instead of a linear correlation) and even for long-duration (10 yr) well-sampled observations. This level can be reached, however, for masses above 3 MEarth or for K4 stars alone. We quantify the maximum dispersion of the RV residuals needed to reach this 10% level, assuming the activity correction method and models do not affect the planetary signal. Several other methods, also based on a correction using log R′HK in various ways (including several denoising techniques and Gaussian processes) or photometry, were tested and do not allow a significantly improvement of this limited performance. Similarly, such low-mass planets in the habitable zone cannot be detected with a similar correction: blind tests lead to very low detection rates for 1 MEarth and to a very high level of false positives. We also studied the residuals after correction of the stellar signal, and found significant power in the periodogram at short and long timescales, corresponding to masses higher than 1 MEarth in this period range. Conclusions. We conclude that very significant and new improvements with respect to methods based on activity indicators to correct for stellar activity must be devised at all timescales to reach the objective of 10% uncertainty on the mass or to detect such planets in RV. Methods based on the correlation with activity indicators are unlikely to be sufficient.

The Correlations of Stellar Tidal Disruption Rates with Properties of Massive Black Holes and Their Host Galaxies

Stars can be either disrupted as tidal disruption events (TDEs) or swallowed whole by massive black holes (MBHs) at galactic centers when they approach sufficiently close to these MBHs. In this work, we investigate the correlations of such stellar consumption rates with both the MBH mass M BH and the inner slope of the host-galaxy mass density distribution α. We introduce a simplified analytical power-law model with a power-law stellar-mass density distribution surrounding MBHs and separate the contributions of two-body relaxation and stellar orbital precession for the stellar orbital angular momentum evolution in nonspherical galaxy potentials. The stellar consumption rates derived from this simplified model can be well consistent with the numerical results obtained with a more realistic treatment of stellar distributions and dynamics around MBHs, providing an efficient way to estimate TDE rates. The origin of the correlations of stellar consumption rates with M BH and α is explained by the dependence of this analytical model on those MBH/host-galaxy properties and by the separation of the stellar angular momentum evolution mechanisms. We propose that the strong positive correlation between the rates of stellar consumption due to two-body relaxation and α provides one interpretation for the overrepresentation of TDEs found in some rare E+A/poststarburst galaxies. We find high TDE rates for giant stars, up to those for solar-type stars. Understanding the origin of the correlations of the stellar consumption rates will be necessary for obtaining the demographics of MBHs and their host galaxies via TDEs.

The Activity of a Solar-Type Star TOI-1422 and Mass Loss of the Atmosphere of the Planet TOI-1422b

The results of the analysis of the activity of a solar type star G2V TOI-1422 are presented and estimates of the loss of atmospheric matter of the planet TOI-1422b are made. The planet can be attributed to the type of hot Neptunes. Compared to other exoplanets of a similar mass range, it is expected that the planet has an extensive gas envelope. According to the long-term photometric survey of Kamogata Wide-field Survey (KWS) the TOI-1422 activity was analyzed and the existence of possible activity cycles of 1650–1680 days and 2450 days was suggested. The probable value of the rotation period P of the star lies in the range of 27 (+19, –8) days, observations of the KWS survey in the V filter indicate the most possible value for the period P = 32 days. Mass loss of the planet’s atmosphere is determined using an approximation formula corresponding to the model of atmospheric loss with an energy restriction. To estimate the flux of XUV photons analytical dependences were used linking the flux values and the parameter log R_{{{text{HK}}}}^{'}. Calculations have shown that the atmosphere matter loss of TOI-1422b is dot {M} = 9.4 times {{10}^{8}} g/s; considering the existing errors in determining the parameters of the atmosphere it lies in the range from 6.8 times {{10}^{8}} to 1.4 times {{10}^{9}} g/s, and considering the uncertainty in the value of the chromospheric activity index—from 8.3 times {{10}^{8}} to 1.1 times {{10}^{9}} g/s.

THE ACTIVITY OF A SOLAR-TYPE STAR TOI-1422 AND MASS LOSS OF THE ATMOSPHERE OF THE PLANET TOI-1422b

The results of the analysis of the activity of a solar type star G2V TOI-1422 are presented and estimates of the loss of atmospheric matter of the planet TOI-1422b are made. The planet can be attributed to the type of hot neptunes. Compared to other exoplanets of a similar mass range it is expected that the planet has an extensive gas envelope. According to the long-term photometric survey of Kamogata Wide-field Survey (KWS) the TOI-1422 activity was analyzed and the existence of possible activity cycles of 1650-1680 days and 2450 days was suggested. The probable value of the rotation period P of the star lies in the range of 27 (+19, –8) days, observations of the KWS survey in the V filter indicate the most possible value for P = 32 days. Mass loss of the planet’s atmosphere is determined using an approximation formula corresponding to the model of atmospheric loss with an energy restriction. To estimate the flux of XUV photons analytical dependences were used linking the flux values and the parameter \(\log R_{{{\text{HK}}}}^{'}\). Calculations have shown that the mass loss of TOI-1422b is \(\dot {M} = 9.4 \times \) 108 g/s and taking into account the existing errors in determining the parameters of the atmosphere it lies in the range from 6.8 × 108 g/s to 1.4 × 109 g/s.

Preparation for an unsupervised massive analysis of SPHERE high-contrast data with PACO

Context. Despite tremendous progress in the detection and characterization of extrasolar planetary systems in the last 25 yr, we have not pinpointed any Solar System analogues. In particular, Jupiter-like planets (either mature or old) are barely detectable beyond 5 au with indirect techniques and they are still out of the reach of direct imaging techniques. Aims. Our study is aimed at a search for exoplanets throughout the whole ESO/VLT-SPHERE archive with an improved and unsupervised data analysis algorithm that could allow us to detect massive giant planets at 5 au. To prepare, test, and optimize our approach, we gathered a sample of 24 solar-type stars observed with SPHERE using angular and spectral differential imaging modes. Methods. We used PACO, a recently developed new-generation algorithm that has been shown to outperform classical methods. We also improved the SPHERE pre-reduction pipeline and optimized the outputs of PACO to enhance the detection performance. We developed custom-built spectral prior libraries to optimize the detection capability of the ASDI mode for both IRDIS and IFS. Results. Compared to previous works conducted with more classical algorithms, the contrast limits we derived with PACO are more reliable and significantly improved, especially at short angular separations, where a gain by a factor ten has been obtained between 0.2 and 0.5 arcsec. Under good observing conditions, planets down to 5 MJup, orbiting at 5 au could be detected around stars within 60 parsec. We identified two exoplanet candidates that will require a follow-up to test for a common proper motion. Conclusions. In this work, we use a small sample to demonstrate the benefits of PACO in terms of achievable contrast and of control of the confidence levels. In addition, we have developed custom tools to take full advantage of this algorithm and to quantity the total error budget on the estimated astrometry and photometry. This work paves the way towards an end-to-end, homogeneous, and unsupervised massive re-reduction of archival direct imaging surveys in the quest for new exo-Jupiters.

Magnetic Activity–Rotation–Age–Mass Relations in Late-pre-main-sequence Stars

We study the four-dimensional relationships between magnetic activity, rotation, mass, and age for solar-type stars in the age range 5–25 Myr. This is the late-pre-main-sequence (l-PMS) evolutionary phase when rapid changes in a star's interior may lead to changes in the magnetic dynamo mechanisms. We carefully derive rotational periods and spot sizes for 471 members of several l-PMS open clusters using photometric light curves from the Zwicky Transient Facility. Magnetic activity was measured in our previous Chandra-based study, and additional rotational data were obtained from other work. Several results emerge. Mass-dependent evolution of rotation through the l-PMS phase agrees with astrophysical models of stellar angular momentum changes, although the data suggest a subpopulation of stars with slower initial rotations than commonly assumed. There is a hint of the onset of unsaturated tachoclinal dependency of X-ray activity on rotation, as reported by Argiroffi et al., but this result is not confidently confirmed. Both X-ray luminosity and starspot area decrease approximately as t −1 for solar-mass stars, suggesting that spot magnetic fields are roughly constant and l-PMS stars follow the universal solar-scaling law between the X-ray luminosity and surface magnetic flux. Assuming convective dynamos are dominant, theoretical magnetic fluxes fail to reveal the universal law for l-PMS stars that enter late Henyey tracks. Altogether we emerge with a few lines of evidence suggesting that the transition from the turbulent to solar-type dynamo occurs at the later stages of l-PMS evolution as stars approach the zero-age main sequence.

Tidal Evolution of Close-in Exoplanets and Host Stars

Abstract The evolution of exoplanetary systems with a close-in planet is ruled by the tides mutually raised on the two bodies and by the magnetic braking of the host star. This paper deals with consequences of this evolution and some features that can be observed in the distribution of the systems two main periods: the orbital periods and the stars rotational periods. The results of the simulations are compared to plots showing both periods as determined from the light curves of a large number of Kepler objects of interest. These plots show important irregularities as a dearth of systems in some regions and accumulations of hot Jupiters in others. It is shown that the accumulation of short-period hot Jupiters around stars with rotation periods close to 25 days results from the evolution of the systems under the joint action of tides and braking, and requires a relaxation factor for solar-type stars of around 10 s−1.

Influence of planets on debris discs in star clusters – I. The 50 au Jupiter

ABSTRACT Although debris discs may be common in exoplanet systems, only a few systems are known in which debris discs and planets coexist. Planets and the surrounding stellar population can have a significant impact on debris disc evolution. Here, we study the dynamical evolution of debris structures around stars embedded in star clusters, aiming to determine how the presence of a planet affects the evolution of such structures. We combine NBODY6++GPU and REBOUND to carry out N-body simulations of planetary systems in star clusters ($N=8\, 000$; Rh = 0.78 pc) for a period of 100 Myr, in which 100 solar-type stars are assigned 200 test particles. Simulations are carried out with and without a Jupiter-mass planet at 50 au. We find that the planet destabilizes test particles and speeds up their evolution. The planet expels most particles in nearby and resonant orbits. Remaining test particles tend to retain small inclinations when the planet is present, and fewer test particles obtain retrograde orbits. Most escaping test particles with speeds smaller than the star cluster’s escape speed originate from cold regions of the planetary system or from regions near the planet. We identify three regions within planetary systems in star clusters: (i) the private region of the planet, where few debris particles remain (40–60 au), (ii) the reach of the planet, in which particles are affected by the planet (0–400 au), and (iii) the territory of the planetary system, most particles outside which will eventually escape (0–700 au).

Radiative hydrodynamical simulations of super-Eddington accretion flow in tidal disruption event: the accretion flow and wind

ABSTRACT One key question in tidal disruption events theory is how much of the fallback debris can be accreted to the black hole. Based on radiative hydrodynamic simulations, we study this issue for efficiently ‘circularized’ debris accretion flow. We find that for a black hole disrupting a solar-type star, $15{{\, \rm per\, cent}}$ of the debris can be accreted for a 107 M⊙ black hole. While for a 106 M⊙ black hole, the value is $43{{\, \rm per\, cent}}$. We find that wind can be launched in the super-Eddington accretion phase regardless of the black hole mass. The maximum velocity of the wind can reach 0.7c (with c being the speed of light). The kinetic power of wind is well above 1044 erg s−1. The results can be used to study the interaction of wind and the circumnuclear medium around quiescent supermassive black holes.

The winds of young Solar-type stars in the Pleiades, AB Doradus, Columba, and β Pictoris

ABSTRACT Solar-type stars, which shed angular momentum via magnetized stellar winds, enter the main sequence with a wide range of rotational periods Prot. This initially wide range of rotational periods contracts and has mostly vanished by a stellar age $t\sim {0.6}\, {\rm Gyr}$, after which Solar-type stars spin according to the Skumanich relation $P_\text{rot}\propto \sqrt{t}$. Magnetohydrodynamic stellar wind models can improve our understanding of this convergence of rotation periods. We present wind models of 15 young Solar-type stars aged ∼24 Myr to ∼0.13 Gyr. With our previous wind models of stars aged ∼0.26 and ∼0.6 Gyr we obtain 30 consistent three-dimensional wind models of stars mapped with Zeeman–Doppler imaging – the largest such set to date. The models provide good cover of the pre-Skumanich phase of stellar spin-down in terms of rotation, magnetic field, and age. We find the mass-loss rate $\dot{M}\propto \Phi ^{{0.9\pm 0.1}}$ with a residual spread of ∼150 per cent and the wind angular momentum loss rate $\dot{J}\propto {}P_\text{rot}^{-1} \Phi ^{1.3\pm 0.2}$ with a residual spread of ∼500 per cent where Φ is the unsigned surface magnetic flux. When comparing different magnetic field scalings for each single star we find a gradual reduction in the power-law exponent with increasing magnetic field strength.

GaiaData Release 3

Context.TheGaiaDR3 catalogue contains, for the first time, about 800 000 solutions with either orbital elements or trend parameters for astrometric, spectroscopic, and eclipsing binaries, and combinations of these three.Aims.With this paper, we aim to illustrate the huge potential of this large non-single-star catalogue.Methods.Using the orbital solutions and models of the binaries, we have built a catalogue of tens of thousands of stellar masses or lower limits thereof, some with consistent flux ratios. Properties concerning the completeness of the binary catalogues are discussed, statistical features of the orbital elements are explained, and a comparison with other catalogues is performed.Results.Illustrative applications are proposed for binaries across the Hertzsprung-Russell Diagram (HRD). Binarity is studied in the giant branch and a search for genuine spectroscopic binaries among long-period variables is performed. The discovery of new EL CVn systems illustrates the potential of combining variability and binarity catalogues. Potential compact object companions are presented, mainly white dwarf companions or double degenerates, but one candidate neutron star is also found. Towards the bottom of the main sequence, the orbits of previously suspected binary ultracool dwarfs are determined and new candidate binaries are discovered. The long awaited contribution ofGaiato the analysis of the substellar regime shows the brown dwarf desert around solar-type stars using true rather than minimum masses, and provides new important constraints on the occurrence rates of substellar companions to M dwarfs. Several dozen new exoplanets are proposed, including two with validated orbital solutions and one super-Jupiter orbiting a white dwarf, all being candidates requiring confirmation. Besides binarity, higher order multiple systems are also found.Conclusions.By increasing the number of known binary orbits by more than one order of magnitude,GaiaDR3 will provide a rich reservoir of dynamical masses and an important contribution to the analysis of stellar multiplicity.

Apocenter pileup and arcs: A narrow dust ring around HD 129590

Context. Observations of debris disks have significantly improved over the past decades, both in terms of sensitivity and spatial resolution. At near-infrared wavelengths, new observing strategies and post-processing algorithms allow us to drastically improve the final images and reveal faint structures in the disks. These structures inform us about the properties and spatial distribution of the small dust particles. Aims. We present new H-band observations of the disk around the solar-type star HD 129590, which display an intriguing arc-like structure in total intensity observations but not in total polarimetry, and we propose an explanation for the origin of this arc. Methods. Assuming geometric parameters for the birth ring of planetesimals, our model provides the positions of millions of particles of different sizes to compute scattered light images. The code can either produce images over the full size distribution or over several smaller intervals of grain sizes. Results. We demonstrate that if the grain size distribution is truncated or strongly peaks at a size larger than the radiation pressure blow-out size, we are able to produce an arc quite similar to the one detected in the observations. If the birth ring is radially narrow, given that particles of a given size have similar eccentricities, they will have their apocenters at the same distance from the star. Since this is where the particles spend most of their time, an “apocenter pileup” occurs that can look like a ring. Due to more efficient forward scattering, this arc only appears in total intensity observations and remains undetected in polarimetric data, which is in good agreement with our observations. Conclusions. For the secondary ring to be detected, sharp variations either in the grain size distribution or for the scattering efficiencies Qsca (or a combination of both) are required. We show that a wavy size distribution and a size-dependent phase function can strengthen the apocenter pileup. Overall, such arcs are rarely detected in other systems, which can mainly be explained by the fact that most parent belts are usually broad.

Unsigned magnetic flux proxy from solar optical intensity spectra

ABSTRACT The photospheric unsigned magnetic flux has been shown to be highly correlated with radial velocity (RV) variations caused by solar surface activity. This activity indicator is therefore a prime candidate to unlock the potential of RV surveys to discover Earth twins orbiting Sun-like stars. We show for the first time how a precise proxy of the unsigned magnetic flux (ΔαB2) can be obtained from Sun-as-a-star intensity spectra by harnessing the magnetic information contained in over 4000 absorption lines in the wavelength range from 380 to 690 nm. This novel activity proxy can thus be obtained from the same spectra from which RVs are routinely extracted. We derived ΔαB2 from 500 randomly selected spectra from the HARPS-N public solar data set, which spans from 2015 to 2018. We compared our estimates with the unsigned magnetic flux values from the Solar Dynamics Observatory (SDO) finding excellent agreement (median absolute deviation: 4.9 per cent). The extracted indicator ΔαB2 correlates with SDO’s unsigned magnetic flux estimates on the solar rotational time-scale (Pearson correlation coefficient 0.67) and on the 3-yr time-scale of our data set (correlation coefficient 0.91). We find correlations of ΔαB2 with the HARPS-N solar RV variations of 0.49 on the rotational time-scale and 0.78 on the 3-yr time-scale. The Pearson correlation of ΔαB2 with the RVs is found to be greater than the correlation of the classical activity indicators with the RVs. For solar-type stars, ΔαB2 therefore represents the best simultaneous activity proxy known to date.

Trajectories of coronal mass ejection from solar-type stars

ABSTRACT The Sun and other solar-type stars have magnetic fields that permeate their interior and surface, extend through the interplanetary medium, and are the main drivers of stellar activity. Stellar magnetic activity affects the physical processes and conditions of the interplanetary medium and orbiting planets. Coronal mass ejections (CMEs) are the most impactful of these phenomena in near-Earth space weather and consist of plasma clouds with a magnetic field, ejected from the solar corona. Precisely predicting the trajectory of CMEs is crucial in determining whether a CME will hit a planet and impact its magnetosphere and atmosphere. Despite the rapid developments in the search for stellar CMEs, their detection is still very incipient. In this work, we aim to better understand the propagation of CMEs by analysing the influence of initial parameters on CME trajectories, such as position, velocities, and the stellar magnetic field’s configuration. We reconstruct magnetograms for Kepler-63 (KIC 11554435) and Kepler-411 (KIC 11551692) from spot transit mapping, and use a CME deflection model, ForeCAT, to simulate trajectories of hypothetical CMEs launched into the interplanetary medium from Kepler-63 and Kepler-411. We apply the same methodology to the Sun, for comparison. Our results show that in general deflections and rotations of CMEs decrease with their radial velocity and increase with ejection latitude. Moreover, magnetic fields stronger than the Sun’s, such as Kepler-63’s, tend to cause greater CME deflections.

Testing angular momentum transport processes with asteroseismology of solar-type main-sequence stars

Context. Thanks to the so-called photometry revolution with the space-based missions CoRoT, Kepler, and TESS, asteroseismology has become a powerful tool to study the internal rotation of stars. The rotation rate depends on the efficiency of the angular momentum (AM) transport inside the star, and its study allows to constrain the internal AM transport processes, as well as improve our understanding of their physical nature. Aims. We compared the ratio of the rotation rate predicted by asteroseismology and starspot measurements of solar-type stars considering different AM transport prescriptions and investigated whether some of these prescriptions can be ruled out observationally. Methods. We conducted a two-step modelling procedure of four main-sequence stars from the Kepler LEGACY sample, which consists of an asteroseismic characterisation that serves as a guide for a modelling with rotating models, including a detailed and coherent treatment of the AM transport. The rotation profiles derived with this procedure were used to estimate the ratio of the mean asteroseismic rotation rate with the surface rotation rate from starspot measurements for each AM transport prescriptions. Comparisons between the models were then conducted. Results. In the hotter part of the Hertzsprung-Russell (HR) diagram (masses typically above ∼1.2 M⊙ at solar metallicity), models with only hydrodynamic transport processes and models with additional transport by magnetic instabilities are found to be consistent with previous measurements that observed a low degree (below 30%) of radial differential rotation between the radiative and convective zones. For these stars, which constitute a significant fraction of the Kepler LEGACY sample, a combination of asteroseismic constraints from the splitting of pressure modes and of the surface rotation rate does not allow us to conclude that an efficient AM transport is required in addition to transport by meridional circulation and shear instability alone. Even a model assuming local AM conservation cannot be ruled out. In the colder part of the HR diagram, the situation is different because of the efficient braking of the stellar surface by magnetised winds. We find a clear disagreement between the rotational properties of models that only include hydrodynamic processes and asteroseismic constraints, while models with magnetic fields correctly reproduce the observations, similarly to the solar case. Conclusions. This shows the existence of a mass regime corresponding to main-sequence F-type stars for which it is difficult to constrain the AM transport processes, unlike for hotter, Gamma Dor stars or colder, less massive solar analogues. The comparison between asteroseismic measurements and surface rotation rates enables us to easily rule out models with an inefficient transport of AM in the colder part of the HR diagram.