Fast-rotating Stars Research Articles

Stringent upper limit on Be star fractions produced by binary interaction

Context. Binary evolution can result in fast-rotating stars through accretion of angular momentum during mass-transfer phases. These fast-rotating stars are predicted to be observable as Be stars. Current models struggle to produce a satisfactory description of Be star populations, even though numerous pieces of observational evidence indicate that the accretion process might be the dominant Be formation channel. Aims. Given the distinct uncertainties in detailed binary evolution calculations, we investigate a rigorous and model-independent upper limit for the production of Be stars through binary interaction and aim to confront this limit with observations of Be stars in young star clusters. Methods. Using extreme assumptions, we calculate the number ratio of post-interaction to pre-interaction binary systems in a coeval population. This ratio describes an upper limit to Be star formation through mass transfer. A detailed comparison is made between our derived upper limit and relevant observations of Be stars, which allows us to probe several aspects of binary star physics. Results. We find that in coeval populations, binary interaction can at most account for one-third of all main-sequence stars being Be stars. Near the cluster turn-off region, this limit appears to be realised in the clusters studied. Away from the turn-off, a good fit to the observed Be fraction as a function of mass is obtained by applying simple assumptions about which systems undergo unstable mass-transfer produces. Conclusions. We find that assuming distinct physics, binary evolution alone can in principle match the high numbers of Be stars that are observed in open clusters. Whether the required binary physics is realised in nature remains to be investigated.

APOGEE-2S Discovery of Light- and Heavy-element Abundance Correlations in the Bulge Globular Cluster NGC 6380

Abstract We derive abundance ratios for nine stars in the relatively high-metallicity bulge globular cluster NGC 6380. We find a mean cluster metallicity between [Fe/H] = −0.80 and −0.73, with no clear evidence for a variation in iron abundances beyond the observational errors. Stars with strongly enhanced [N/Fe] abundance ratios populate the cluster and are anticorrelated with [C/Fe], trends that are considered a signal of the multiple-population phenomenon in this cluster. We detect an apparent intrinsic star-to-star spread (≳0.27 dex) in the slow neutron-capture process element (s-element) Ce ii. Moreover, the [Ce/Fe] abundance ratio exhibits a likely correlation with [N/Fe], and a somewhat weaker correlation with [Al/Fe]. If confirmed, NGC 6380 could be the first high-metallicity globular cluster where a N–Ce correlation is detected. Furthermore, this correlation suggests that Ce may also be an element involved in the multiple-population phenomenon. Currently, a consensus interpretation for the origin of the this apparent N–Ce correlation in high-metallicity clusters is lacking. We tentatively suggest that it could be reproduced by different channels—low-mass asymptotic giant branch stars in the high-metallicity regime or fast-rotating massive stars (“spinstars”), due to the rotational mixing. It may also be the cumulative effect of several pollution events including the occurrence of peculiar stars. Our findings should guide stellar nucleosynthesis models, in order to understand the reasons for its apparent exclusivity in relatively high-metallicity globular clusters.

12C/13C ratio and CNO abundances in the classical very old metal-poor dwarf HD 140283

Context. The isotope abundances provide powerful diagnostics of the chemical enrichment in our Galaxy. The star HD 140283 is one of the best-studied very metal-poor dwarf stars. It is very old, and the chemical abundance in this star is a good witness of the chemical composition of the matter in the early Galaxy. Aims. The aim of this work is to measure the precise abundances of carbon, nitrogen, oxygen, and mainly the 12C/13C isotopic ratio in this very old metal-poor star in order to have a good reference for the computations of the chemical evolution of the Galaxy. Methods. We used very high spectral resolution data, with extremely high signal-to-noise ratios obtained with the spectrographs ESPaDOnS at the CFHT, ESPRESSO at the VLT, and HARPS at the ESO 3.6 m telescope. Results. For the first time, we were able to measure the 12C/13C ratio in a very old metal-poor dwarf that was born at the very beginning of the Galaxy: 12C/13C = 33−6+12. We also obtained a precise determination of the abundance of the CNO elements in this star. These abundances give information about the early composition of the cloud from which HD 140283 was formed. They suggest that the effect of super-asymptotic giant branch stars or fast-rotating massive stars was significant in the early Galaxy.

TESS survey of rotational and pulsational variability of mercury–manganese stars

ABSTRACT Mercury–manganese (HgMn) stars are late-B upper main sequence chemically peculiar stars distinguished by large overabundances of heavy elements, slow rotation, and frequent membership in close binary systems. These stars lack strong magnetic fields typical of magnetic Bp stars but occasionally exhibit non-uniform surface distributions of chemical elements. The physical origin and the extent of this spot formation phenomenon remain unknown. Here, we use 2-min cadence light curves of 64 HgMn stars observed by the Transiting Exoplanet Survey Satellite (TESS) during the first 2 yr of its operation to investigate the incidence of rotational modulation and pulsations among HgMn stars. We found rotational variability with amplitudes of 0.1–3 mmag in 84 per cent of the targets, indicating ubiquitous presence of star-spots on HgMn-star surfaces. Rotational period measurements reveal six fast-rotating stars with periods below 1.2 d, including one ultra-fast rotator (HD 14228) with a 0.34-d period. We also identify several HgMn stars showing multiperiodic g-mode pulsations, tidally induced variation and eclipses in binary systems.

A three-dimensional hydrodynamics simulation of oxygen-shell burning in the final evolution of a fast-rotating massive star

ABSTRACT We perform for the first time a 3D hydrodynamics simulation of the evolution of the last minutes pre-collapse of the oxygen shell of a fast-rotating massive star. This star has an initial mass of 38 M⊙, a metallicity of ∼1/50 Z⊙, an initial rotational velocity of 600 km s−1, and experiences chemically homogeneous evolution. It has a silicon- and oxygen-rich (Si/O) convective layer at (4.7–17) × 108 cm, where oxygen-shell burning takes place. The power spectrum analysis of the turbulent velocity indicates the dominance of the large-scale mode (ℓ ∼ 3), which has also been seen in non-rotating stars that have a wide Si/O layer. Spiral arm structures of density and silicon-enriched material produced by oxygen-shell burning appear in the equatorial plane of the Si/O shell. Non-axisymmetric, large-scale (m ≤ 3) modes are dominant in these structures. The spiral arm structures have not been identified in previous non-rotating 3D pre-supernova models. Governed by such a convection pattern, the angle-averaged specific angular momentum becomes constant in the Si/O convective layer, which is not considered in spherically symmetrical stellar evolution models. Such spiral arms and constant specific angular momentum might affect the ensuing explosion or implosion of the star.

Maximum helium content of multiple populations in the globular cluster NGC 6752

Context. Multiple populations in globular clusters are usually explained by the formation of stars out of material with a chemical composition that is polluted to different degrees by the ejecta of short-lived, massive stars. But the nature of the “polluters” remains elusive. Different types of stars have been proposed to account for the observed chemical patterns of multiple populations. Among other things, these differ by the amount of helium they spread in the surrounding medium. Aims. In this study we investigate whether the present-day photometric method used to infer the helium content of multiple populations indeed gives the true value or underestimates it by missing very He-rich, but rare stars. This check is important to discriminate between the different polluter scenarios. We focus on the specific case of NGC 6752. Methods. We compute atmosphere models and synthetic spectra along isochrones produced for this cluster for a very broad range of He abundances covering the predictions of the different scenarios, including the extreme case of the fast-rotating massive star (FRMS) scenario. We use the same abundances in isochrones and atmosphere models to ensure consistency. We calculate synthetic photometry in HST filters best suited to study the helium content. We subsequently build synthetic clusters with various distributions of stars. We finally determine the maximum helium mass fraction of these synthetic clusters using a method similar to that applied to observational data. In particular, we select nonpolluted and very He-rich stars from the so-called chromosome map. Results. We re-determine the maximum helium mass fraction Y in NGC 6752, and find a value consistent with published results. We build toy models of clusters with various distributions of multiple populations and ensure that we are able to recover the input maximum Y. We then build synthetic clusters with the populations predicted by the FRMS scenario and find that while we slightly underestimate the maximum Y value, we are still able to detect stars much more He-rich than the current observed maximum Y. This result still holds even in synthetic clusters that contain less He-rich stars than predicted by the FRMS scenario. It is easier to determine the maximum Y on main sequence stars than on red giant branch stars, but qualitatively the results are unaffected by the sample choice. Conclusions. We show that in NGC 6752 it is unlikely that stars more He-rich than the current observational limit of about 0.3 (in mass fraction) are present.

Discovery of a young low-mass brown dwarf transiting a fast-rotating F-type star by the Galactic Plane eXoplanet (GPX) survey

ABSTRACT We announce the discovery of GPX-1 b, a transiting brown dwarf with a mass of 19.7 ± 1.6 MJup and a radius of 1.47 ± 0.10 RJup, the first substellar object discovered by the Galactic Plane eXoplanet (GPX) survey. The brown dwarf transits a moderately bright (V = 12.3 mag) fast-rotating F-type star with a projected rotational velocity $v\sin {\, i_*}=40\pm 10$ km s−1. We use the isochrone placement algorithm to characterize the host star, which has effective temperature 7000 ± 200 K, mass 1.68 ± 0.10 $\mathrm{\it M}_\odot$, radius 1.56 ± 0.10 $\mathrm{\it R}_\odot$, and approximate age $0.27_{-0.15}^{+0.09}$ Gyr. GPX-1 b has an orbital period of ∼1.75 d and a transit depth of 0.90 ± 0.03 per cent. We describe the GPX transit detection observations, subsequent photometric and speckle-interferometric follow-up observations, and SOPHIE spectroscopic measurements, which allowed us to establish the presence of a substellar object around the host star. GPX-1 was observed at 30-min integrations by TESS in Sector 18, but the data are affected by blending with a 3.4 mag brighter star 42 arcsec away. GPX-1 b is one of about two dozen transiting brown dwarfs known to date, with a mass close to the theoretical brown dwarf/gas giant planet mass transition boundary. Since GPX-1 is a moderately bright and fast-rotating star, it can be followed-up by the means of the Doppler tomography.

Maximum mass of compact stars from gravitational wave events with finite-temperature equations of state

We conjecture and verify a set of relations between global parameters of hot and fast-rotating compact stars which do not depend on the equation of state, including a relation connecting the masses of the mass-shedding (Kepler) and static configurations. We apply these relations to the GW170817 event by adopting the scenario in which a hypermassive compact star remnant formed in a merger evolves into a supramassive compact star that collapses into a black hole once the stability line for such stars is crossed. We deduce an upper limit on the maximum mass of static, cold neutron stars $2.{15}_{\ensuremath{-}0.17}^{+0.18}\ensuremath{\le}{M}_{\mathrm{TOV}}^{★}/{M}_{\ensuremath{\bigodot}}\ensuremath{\le}2.{24}_{\ensuremath{-}0.44}^{+0.45}$ for the typical range of entropy per baryon, $2\ensuremath{\le}S/A\ensuremath{\le}3$, and electron fraction ${Y}_{e}=0.1$ characterizing the hot hypermassive star. Our result implies that accounting for the finite temperature of the merger remnant relaxes previously derived constraints on the value of the maximum mass of a cold, static compact star.

The GAPS programme at TNG

Context. Transiting ultra-hot Jupiters are ideal candidates for studying the exoplanet atmospheres and their dynamics, particularly by means of high-resolution spectra with high signal-to-noise ratios. One such object is KELT-20b. It orbits the fast-rotating A2-type star KELT-20. Many atomic species have been found in its atmosphere, with blueshifted signals that indicate a day- to night-side wind. Aims. We observe the atmospheric Rossiter-McLaughlin effect in the ultra-hot Jupiter KELT-20b and study any variation of the atmospheric signal during the transit. For this purpose, we analysed five nights of HARPS-N spectra covering five transits of KELT-20b. Methods. We computed the mean line profiles of the spectra with a least-squares deconvolution using a stellar mask obtained from the Vienna Atomic Line Database (Teff = 10 000 K, log g = 4.3), and then we extracted the stellar radial velocities by fitting them with a rotational broadening profile in order to obtain the radial velocity time-series. We used the mean line profile residuals tomography to analyse the planetary atmospheric signal and its variations. We also used the cross-correlation method to study a previously reported double-peak feature in the FeI planetary signal. Results. We observed both the classical and the atmospheric Rossiter-McLaughlin effect in the radial velocity time-series. The latter gave us an estimate of the radius of the planetary atmosphere that correlates with the stellar mask used in our work (Rp+atmo∕Rp = 1.13 ± 0.02). We isolated the planetary atmospheric trace in the tomography, and we found radial velocity variations of the planetary atmospheric signal during transit with an overall blueshift of ≈10 km s−1, along with small variations in the signal depth, and less significant, in the full width at half maximum (FWHM). We also find a possible variation in the structure and position of the FeI signal in different transits. Conclusions. We confirm the previously detected blueshift of the atmospheric signal during the transit. The FWHM variations of the atmospheric signal, if confirmed, may be caused by more turbulent condition at the beginning of the transit, by a variable contribution of the elements present in the stellar mask to the overall planetary atmospheric signal, or by iron condensation. The FeI signal show indications of variability from one transit to the next.

Particle acceleration and radiation reaction in strong spherical electromagnetic waves

ABSTRACT Strongly magnetized and fast-rotating neutron stars are known to be efficient particle accelerators within their magnetosphere and wind. They are suspected to accelerate leptons, protons, and maybe ions to extreme relativistic regimes where the radiation reaction significantly feeds back to their motion. In the vicinity of neutron stars, magnetic field strengths are close to the critical value of Bc ∼ 4.4 · 109 T and particle Lorentz factors of the order γ ∼ 109 are expected. In this paper, we investigate the acceleration and radiation reaction feedback in the pulsar wind zone where a large-amplitude low-frequency electromagnetic wave is launched starting from the light cylinder. We design a semi-analytical code solving exactly the particle equation of motion including radiation reaction in the Landau–Lifshits approximation for a null-like electromagnetic wave of arbitrary strength parameter and elliptical polarization. Under conventional pulsar conditions, asymptotic Lorentz factor as high as 108−109 is reached at large distances from the neutron star. However, we demonstrate that in the wind zone, within the spherical wave approximation, radiation reaction feedback remains negligible.

Physics and evolution of the most massive stars in 30 Doradus

Context.The identification of stellar-mass black-hole mergers with up to 80 M⊙as powerful sources of gravitational wave radiation led to increased interest in the physics of the most massive stars. The largest sample of possible progenitors of such objects, very massive stars (VMS) with masses up to 300M⊙, have been identified in the 30 Dor star-forming region in the Large Magellanic Cloud (LMC). In this young starburst analogue, VMS were found to dominate stellar feedback. Despite their importance, the physics and evolution of VMS is highly uncertain, mainly due to their proximity to the Eddington limit.Aims.In this work, we investigate the two most important effects that are thought to occur near the Eddington limit: enhanced mass loss through optically thick winds and the formation of radially inflated stellar envelopes.Methods.We compute evolutionary models for VMS at LMC metallicity and perform a population synthesis of the young stellar population in 30 Dor. We adjust the input physics of our models to match the empirical properties of the single-star population in 30 Dor as derived in the framework of the VLT-Flames Tarantula Survey.Results.Enhanced mass loss and envelope inflation near the Eddington limit have a dominant effect on the evolution of the most massive stars. While the observed mass-loss properties and the associated surface He-enrichment are well described by our new models, the observed O-star mass-loss rates are found to cover a much larger range than theoretically predicted, with particularly low mass-loss rates for the youngest objects. Also, the (rotational) surface enrichment in the O-star regime appears to not be well understood. The positions of the most massive stars in the Hertzsprung-Russell diagram (HRD) are affected by mass loss and envelope inflation. For instance, the majority of luminous B supergiants in 30 Dor, and the lack thereof at the highest luminosities, can be explained through the combination of envelope inflation and mass loss. Finally, we find that the upper limit for the inferred initial stellar masses in the greater 30 Dor region is significantly lower than in its central cluster, R 136, implying a variable upper limit for the masses of stars.Conclusions.The implementation of mass-loss and envelope physics in stellar evolution models turns out to be essential for the modelling of the observable properties of young stellar populations. While the properties of the most massive stars (≳100 M⊙) are well described by our new models, the slightly less massive O stars investigated in this work show a much more diverse behaviour than previously thought, which has potential implications for rotational mixing and angular momentum transport. While the present models are a big step forward in the understanding of stellar evolution in the upper HRD, more work is needed to understand the mechanisms that regulate the mass-loss rates of OB stars and the physics of fast-rotating stars.

A global model of the magnetorotational instability in protoneutron stars

Context. Magnetars are isolated neutron stars characterized by their variable high-energy emission, which is powered by the dissipation of enormous internal magnetic fields. The measured spin-down of magnetars constrains the magnetic dipole to be in the range of 1014 − 1015 G. The magnetorotational instability (MRI) is considered to be a promising mechanism to amplify the magnetic field in fast-rotating protoneutron stars and form magnetars. This scenario is supported by many local studies that have shown that magnetic fields could be amplified by the MRI on small scales. However, the efficiency of the MRI at generating a dipole field is still unknown. Aims. To answer this question, we study the MRI dynamo in an idealized global model of a fast rotating protoneutron star with differential rotation. Methods. Using the pseudo-spectral code MagIC, we performed three-dimensional incompressible magnetohydrodynamics simulations in spherical geometry with explicit diffusivities where the differential rotation is forced at the outer boundary. We performed a parameter study in which we varied the initial magnetic field and investigated different magnetic boundary conditions. These simulations were compared to local shearing box simulations performed with the code Snoopy. Results. We obtain a self-sustained turbulent MRI-driven dynamo, whose saturated state is independent of the initial magnetic field. The MRI generates a strong turbulent magnetic field of B ≥ 2 × 1015 G and a nondominant magnetic dipole, which represents systematically about 5% of the averaged magnetic field strength. Interestingly, this dipole is tilted toward the equatorial plane. By comparing these results with shearing box simulations, we find that local models can reproduce fairly well several characteristics of global MRI turbulence such as the kinetic and magnetic spectra. The turbulence is nonetheless more vigorous in the local models than in the global ones. Moreover, overly large boxes allow for elongated structures to develop without any realistic curvature constraint, which may explain why these models tend to overestimate the field amplification. Conclusions. Overall, our results support the ability of the MRI to form magnetar-like large-scale magnetic fields. They furthermore predict the presence of a stronger small-scale magnetic field. The resulting magnetic field could be important to power outstanding stellar explosions, such as superluminous supernovae and gamma-ray bursts.

The evolution of the solar–stellar activity

We present a brief review of observational results contributing to modern ideas on the evolution of stellar activity. Basic laws, derived for both rotation–age and activity–rotation relationships, allow us to trace how the activity of low-mass stars changes with age during their stay on the main sequence. We focus on the evaluation of the activity properties of stars that could be analogs of the young Sun. Our study includes joint consideration of different tracers of activity, rotation and magnetic fields of Sun-like stars of various ages. We identify rotation periods, when the saturated regime of activity changes to the unsaturated mode, when the solar-type activity is formed: for G- and K-type stars, they are 1.1 and 3.3 days, respectively. This corresponds to an age interval of about 0.2–0.6 Gyr, when regular sunspot cycle began to be established on the early Sun. We discuss properties of the coronal and chromospheric activity in young Suns. Our evaluation of the EUV-fluxes in the spectral range of 1350−1750 Å shows that the far-UV radiation of the early Sun was a factor of 7 times more intense than that of the present-day Sun, and twice higher when the regular sunspot cycle was established. For the young Sun, we can estimate the possible mass loss rate associated with quasi-steady outflow as 10−12 M⊙/yr. The results of observations of the largest flares on solar-type stars are also discussed, leading to conclusion that the most powerful phenomena occur on the fast-rotating stars in the saturated activity regime. Our estimate of the stellar magnetic fields makes it possible to evaluate the maximal possible flare energy. This could help us better understand the origin of extreme events on the Sun in the past.

Publisher’s Note: Internal x-ray plateau in short GRBs: Signature of supramassive fast-rotating quark stars? [Phys. Rev. D 94 , 083010 (2016)

Publisher’s Note: Internal x-ray plateau in short GRBs: Signature of supramassive fast-rotating quark stars? [Phys. Rev. D <b>94</b> , 083010 (2016)

Fast-rotating giant stars behind the Coma Berenices star cluster

ABSTRACT In the frame of a study of the empirical isochrones of young stellar clusters, we have carried out BVIc Johnson–Cousins photometry of a sample of K and M stars of the Coma Berenices star cluster. All these stars have known rotational periods. Our main goal is to get a valuable reference on the colour–magnitude diagram, Mv versus B − V, for stars with ages within 400–800 Myr. For this purpose, we obtained BVIc photometry with an average upper limit for the precision of about 0.025 mag and used parallaxes from the Gaia Data Release 2. We found that one-third of our sample is located well above the cluster main sequence and these stars are confirmed as background giants by their radial velocities in the Gaia Data Release 2. This misclassification shows that giants with short-surface rotational periods can mimic main-sequence stars if they are located at the appropriate distance. We recommend caution when using rotational periods in order to determine cluster membership. Besides, the gyrochronology technique should be used only when the luminosity class of the stars is well known. Finally, our cleared sample supports an age of ∼600 Myr for Coma Berenices, rather than an age of ∼800 Myr.

Li-rich K giants, dust excess, and binarity

The origin of the Li-rich K giants is still highly debated. Here, we investigate the incidence of binarity among this family from a nine-year radial-velocity monitoring of a sample of 11 Li-rich K giants using the HERMES spectrograph attached to the 1.2 mMercatorTelescope. A sample of 13 non-Li-rich giants (8 of them being surrounded by dust according to IRAS, WISE, and ISO data) was monitored alongside. When compared to the binary frequency in a reference sample of 190 K giants (containing 17.4% of definite spectroscopic binaries – SB – and 6.3% of possible spectroscopic binaries – SB?), the binary frequency appears normal among the Li-rich giants (2/11 definite binaries plus 2 possible binaries, or 18.2% SB + 18.2% SB?), after taking account of the small sample size through the hypergeometric probability distribution. Therefore, there appears to be no causal relationship between Li enrichment and binarity. Moreover, there is no correlation between Li enrichment and the presence of circumstellar dust, and the only correlation that could be found between Li enrichment and rapid rotation is that the most Li-enriched K giants appear to be fast-rotating stars. However, among the dusty K giants, the binary frequency is much higher (4/8 definite binaries plus 1 possible binary). The remaining 3 dusty K giants suffer from a radial-velocity jitter, as is expected for the most luminous K giants, which these are.

Relation of X-ray activity and rotation in M dwarfs and predicted time-evolution of the X-ray luminosity

The relation of activity to rotation in M dwarfs is of high astrophysical interest because it provides observational evidence of the stellar dynamo, which is poorly understood for low-mass stars, especially in the fully convective regime. Previous studies have shown that the relation of X-ray activity to rotation consists of two different regimes: the saturated regime for fast-rotating stars and the unsaturated regime for slowly rotating stars. The transition between the two regimes lies at a rotation period of ∼10 d. We present here a sample of 14 M dwarf stars observed with XMM-Newton and Chandra, for which we also computed rotational periods from Kepler Two-Wheel (K2) Mission light curves. We compiled X-ray and rotation data from the literature and homogenized all data sets to provide the largest uniform sample of M dwarfs (302 stars) for X-ray activity and rotation studies to date. We then fit the relation between Lx − Prot using three different mass bins to separate partially and fully convective stars. We found a steeper slope in the unsaturated regime for fully convective stars and a nonconstant Lx level in the saturated regime for all masses. In the Lx/Lbol − RO space we discovered a remarkable double gap that might be related to a discontinuous period evolution. Then we combined the evolution of Prot predicted by angular momentum evolution models with our new results on the empirical Lx − Prot relation to provide an estimate for the age decay of X-ray luminosity. We compare predictions of this relationship with the actual X-ray luminosities of M stars with known ages from 100 Myr to a few billion years. We find remarkably good agreement between the predicted Lx and the observed values for partially convective stars. However, for fully convective stars at ages of a few billion years, the constructed Lx-age relation overpredicts the X-ray luminosity because the angular momentum evolution model underpredicts the rotation period of these stars. Finally, we examine the effect of different parameterizations for the Rossby number (RO) on the shape of the activity-rotation relation in Lx/Lbol − RO space, and we find that the slope in the unsaturated regime and the location of the break point of the dual power-law depend sensitively on the choice of RO.

Radial-velocity variations due to meridional flows in the Sun and solar-type stars: impact on exoplanet detectability

Context. Stellar variability due to magnetic activity and flows at different spatial scales strongly impacts radial velocities. This variability is seen as oscillations, granulation, supergranulation, and meridional flows. The effect of this latter process is currently poorly known but could affect exoplanet detectability. Aims. We aim to quantify the amplitude of the meridional flow integrated over the disc and its temporal variability, first for the Sun, as seen with different inclinations, and then for other solar-type stars. We then want to compare these amplitudes with low-mass exoplanetary amplitudes in radial velocity. Methods. We used long time series (covering two 11-yr cycles) of solar latitudinal meridional circulation to reconstruct its integrated contribution and study its properties. We then used scaling laws from hydrodynamical simulations relating the amplitude of the meridional flow variability with stellar mass and rotation rate to estimate the typical amplitude expected for other solar-type stars. Results. We find typical rms of the order of 0.5–0.7 m s−1 (edge-on) and 1.2–1.7 m s−1 (pole-on) for the Sun (peak-to-peak amplitudes are typically 1–1.4 m s−1 and 2.3–3.3 m s−1 resp.), with a minimal jitter for an inclination of 45–55°. This signal is significant compared to other stellar activity contributions and is much larger than the radial-velocity signal of the Earth. The variability is strongly related to the activity cycle, with maximum flows during the descending phase of the cycle, and possible variability on timescales lower than the cycle period. Extension to other solar-type stars shows that the variability due to meridional flows is dominated by the amplitude of the cycle of those stars (compared with mass and rotation rate), and that the peak-to-peak amplitudes can reach 4 m s−1 for the most variable stars when seen pole-on. The meridional flow contribution sometimes represents a high fraction of the convective blueshift inhibition signal, especially for quiet, low-mass stars. For fast-rotating stars, the presence of multi-cellular patterns should significantly decrease the meridional flow contribution to the radial-velocity signal. Conclusions. Our study shows that these meridional flows could be critical for exoplanet detection. Low inclinations are more impacted than edge-on configurations, but these latter still exhibit significant variability. Meridional flows also degrade the correlation between radial velocities due to convective blueshift inhibition and chromospheric activity indicators. This will make the correction from this signal challenging for stars with no multi-cellular patterns, such as the Sun for example, although there may be some configurations for which the line shape variations may be used if the precision is sufficient.

KIC 8975515: A fast-rotating (γ Dor – δ Sct) hybrid star with Rossby modes and a slower δ Sct companion in a long-period orbit

Aims. KIC 8975515 is a double-lined spectroscopic binary system with hybrid nature. Regular monitoring with the échelle spectrograph HERMES show that both components have matching atmospheric properties (Teff ∼ 7400 K), and that one of them is a fast rotator (v sin i = 162 versus 32 km s−1). Our aim is to study the Kepler light curve in order to determine the frequencies of the pulsations, to search for regular spacing patterns in the Fourier spectrum, if any, and to discuss their origin in the context of binarity and fast rotation. Methods. In this paper, we study the properties of the stellar pulsations based on a careful analysis of the Fourier spectrum. This was done by performing repeated frequency-search analyses with successive prewhitenings of all the significant frequencies detected in the spectrum. Moreover, we searched for regular period spacing among the g modes as well as frequency splitting among the g and p modes in frequency. Results. In the low-frequency regime, one series of prograde g modes and four series of retrograde r modes, were detected. The r modes are well-distributed with respect to the harmonics of the rotational frequency of the fast-rotating star frot = 1.647 d−1. The dominant g mode is f2 = 2.37 d−1. The strongest p mode is f1 = 13.97 d−1. We identified two multiplets of regularly split p modes with the mean values of 0.42 d−1 and 1.65 d−1. In addition, we also identified a few incidences of mode coupling (between f1 and a g or r mode). Conclusions. We detected five series of retrograde r and prograde g modes as well as two multiplets of p modes with rotational frequency splitttings related to both components of KIC 8975515. We identified the fast-rotating component as a hybrid pulsator with r modes, and the slowly-rotating component as a δ Sct pulsator.

LAMOST J040643.69+542347.8: The Fastest Rotator in the Galaxy

Abstract Rotation and binary interaction play important roles in understanding the nature of massive stars (≳8M ⊙). Some interesting transients, such as the long-duration gamma-ray bursts, are thought to be originated from fast-rotating massive stars. Because the strong stellar wind can effectively spin down a metal-rich massive star with fast rotation, it is very hard to find single massive stars rotating critically in the Galaxy. In the present work reported is the discovery of the fastest rotator in the Galaxy, LAMOST J040643.69+542347.8, with a projected rotational velocity km s−1, which is ∼100 km s−1 faster than that of the previous record holder HD 191423. The star has a spectral type of O6.5 Vnnn(f)p. Its He i λ4471 absorption line is blueshifted and asymmetric, while its He ii λ4686 and Hα have central absorption reversals in their emissions. It is also a runaway star, which implies an origin in a close binary interaction. Compared to VFTS 285 and VFTS 102 (their km s−1) in the Large Magellanic Cloud, LAMOST J040643.69+542347.8 has its own peculiar spectral characteristics and earlier spectral type. Moreover, LAMOST J040643.69+542347.8 is bright (B ∼ 13.9 mag) enough to allow future high-resolution spectroscopic follow-up.