Radial Velocity Variations Research Articles

Unsigned Magnetic Flux as a Proxy for Radial-velocity Variations in Sun-like Stars

A major obstacle to detecting and characterizing long-period, low-mass exoplanets is the intrinsic radial-velocity (RV) variability of host stars. To better understand RV variability, we estimate disk-averaged RV variations of the Sun over its magnetic cycle, from the Fe i line observed by SDO/HMI, using a physical model for rotationally modulated magnetic activity that was previously validated against HARPS-N solar observations. We estimate the unsigned magnetic flux and show that a linear fit to it reduces the rms of RV variations by 62%, i.e., a factor of 2.6. We additionally apply the FF′ method, which predicts RV variations based on a star’s photometric variations. At cycle maximum, we find that additional processes must be at play beyond suppression of convective blueshift and velocity imbalances resulting from brightness inhomogeneities, in agreement with recent studies of RV variations. By modeling RV variations over the magnetic cycle using a linear fit to the unsigned magnetic flux, we recover injected planets at a period of ≈300 days with RV semi-amplitudes down to 0.3 m s−1. To reach 0.1 m s−1, we will need to identify and model additional phenomena that are not well traced by or FF′. This study motivates ongoing and future efforts to develop observation and analysis techniques to measure the unsigned magnetic flux at high precision in slowly rotating, relatively inactive stars like the Sun. We conclude that the unsigned magnetic flux is an excellent proxy for rotationally modulated, activity-induced RV variations, and could become key to confirming and characterizing Earth analogs.

Accounting for stellar activity signals in radial-velocity data by using change point detection techniques

Context.Active regions on the photosphere of a star have been the major obstacle for detecting Earth-like exoplanets using the radial velocity (RV) method. A commonly employed solution for addressing stellar activity is to assume a linear relationship between the RV observations and the activity indicators along the entire time series, and then remove the estimated contribution of activity from the variation in RV data (overall correction method). However, since active regions evolve on the photosphere over time, correlations between the RV observations and the activity indicators will correspondingly be anisotropic.Aims.We present an approach that recognizes the RV locations where the correlations between the RV and the activity indicators significantly change in order to better account for variations in RV caused by stellar activity.Methods.The proposed approach uses a general family of statistical breakpoint methods, often referred to as change point detection (CPD) algorithms; several implementations of which are available in R and python. A thorough comparison is made between the breakpoint-based approach and the overall correction method. To ensure wide representativity, we use measurements from real stars that have different levels of stellar activity and whose spectra have different signal-to-noise ratios.Results.When the corrections for stellar activity are applied separately to each temporal segment identified by the breakpoint method, the corresponding residuals in the RV time series are typically much smaller than those obtained by the overall correction method. Consequently, the generalized Lomb–Scargle periodogram contains a smaller number of peaks caused by active regions. The CPD algorithm is particularly effective when focusing on active stars with long time series, such asαCen B. In that case, we demonstrate that the breakpoint method improves the detection limit of exoplanets by 74% on average with respect to the overall correction method.Conclusions.CPD algorithms provide a useful statistical framework for estimating the presence of change points in a time series. Since the process underlying the RV measurements generates anisotropic data by its intrinsic properties, it is natural to use CPD to obtain cleaner signals from RV data. We anticipate that the improved exoplanet detection limit may lead to a widespread adoption of such an approach. Our test on the HD 192310 planetary system is encouraging, as we confirm the presence of the two hosted exoplanets and we determine orbital parameters consistent with the literature, also providing much more precise estimates for HD 192310 c.

HADES RV Programme with HARPS-N at TNG

Aims. We present the complete Bayesian statistical analysis of the HArps-n red Dwarf Exoplanet Survey (HADES), which monitored the radial velocities of a large sample of M dwarfs with HARPS-N at TNG over the last 6 yr. Methods. The targets were selected in a narrow range of spectral types from M0 to M3, 0.3 M⊙ < M★ < 0.71 M⊙, in order to study the planetary population around a well-defined class of host stars. We take advantage of Bayesian statistics to derive an accurate estimate of the detectability function of the survey. Our analysis also includes the application of a Gaussian Process approach to take into account stellar-activity-induced radial velocity variations and improve the detection limits around the most-observed and most-active targets. The Markov chain Monte Carlo and Gaussian process technique we apply in this analysis has proven very effective in the study of M-dwarf planetary systems, helping the detection of most of the HADES planets. Results. From the detectability function we can calculate the occurrence rate of small-mass planets around early-M dwarfs, either taking into account only the 11 already published HADES planets or adding the five new planetary candidates discovered in this analysis, and compare them with the previous estimates of planet occurrence around M-dwarf or solar-type stars: considering only the confirmed planets, we find the highest frequency for low-mass planets (1 M⊕ < mp sin i < 10 M⊕) with periods 10 d < P < 100 d, ${f_{{\rm{occ}}}} = 0.85_{ - 0.21}^{ + 0.46}$, while for short-period planets (1 d < P < 10 d) we find a frequency of ${f_{{\rm{occ}}}} = 0.10_{ - 0.03}^{ + 0.10}$, significantly lower than for later-M dwarfs; if instead we also take into account the new candidates, we observe the same general behaviours, but with consistently higher frequencies of low-mass planets. We also present new estimates of the occurrence rates of long-period giant planets and temperate planets inside the habitable zone of early-M dwarfs: in particular we find that the frequency of habitable planets could be as low as η⊕ < 0.23. These results, and their comparison with other surveys focused on different stellar types, confirm the central role that stellar mass plays in the formation and evolution of planetary systems.

Quaternionic Transform: A new Light on the Solar Power Spectrum

Asteroseismology, that is, the use of the frequency content of a time series caused by variations in brightness or radial velocity of a stellar object, is based on the hypothesis that such a series is harmonic and therefore can be described by a sum of sines and cosines. If this were not the case (e.g., the oscillations of an ellipsoid of revolution) it cannot be guaranteed that the Discrete Fourier transform is the least squares approximation to the time series. This report studies the effect of extending the Fourier kernel to a particular quaternion and exploring the impact when it is applied to the best time series that we have (GOLF/SoHO) from the closest star, our Sun. The results are consistent with a notable improvement in the signal-to-noise ratio in the low frequency range. This opens the possibility of detecting the elusive g modes of the Sun in future works.

The CARMENES search for exoplanets around M dwarfs

Context. The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Échelle Spectrographs (CARMENES) instrument is searching for periodic radial-velocity (RV) variations of M dwarfs, which are induced by orbiting planets. However, there are other potential sources of such variations, including rotational modulation caused by stellar activity. Aims. We aim to investigate four M dwarfs (Ross 318, YZ CMi, TYC 3529-1437-1, and EV Lac) with different activity levels and spectral sub-types. Our goal is to compare the periodicities seen in 22 activity indicators and the stellar RVs, and to examine their stability over time. Methods. For each star, we calculated generalised Lomb-Scargle periodograms of pseudo-equivalent widths of chromospheric lines, indices of photospheric bands, the differential line width as a measure of the width of the average photospheric absorption line, the RV, the chromatic index that describes the wavelength dependence of the RV, and parameters of the cross-correlation function. We also calculated periodograms for subsets of the data and compared our results to TESS photometry. Results. We find the rotation periods of all four stars to manifest themselves in the RV and photospheric indicators, particularly the TiO 7050 index, whereas the chromospheric lines show clear signals only at lower activity levels. For EV Lac and TYC 3529-1437-1, we find episodes during which indicators vary with the rotation period, and episodes during which they vary with half the rotation period, similarly to photometric light curves. Conclusions. The changing periodicities reflect the evolution of stellar activity features on the stellar surface. We therefore conclude that our results not only emphasise the importance of carefully analysing indicators complementary to the RV in RV surveys, but they also suggest that it is also useful to search for signals in activity indicators in subsets of the dataset, because an activity signal that is present in the RV may not be visible in the activity indicators all the time, in particular for the most active stars.

Chemo-kinematics of the Milky Way spiral arms and bar resonances: Connection to ridges and moving groups in the solar vicinity

Making use of a new high-resolution spiral galaxy simulation as well as Gaia DR2 and EDR3 data complemented by chemical abundances from the Galah DR3, APOGEE DR16, and LAMOST DR5 surveys, we explore the possible link between the Milky Way (MW) spiral arms, (R, vϕ) ridges, and moving groups in local vR-vϕ space. We show that the tightly wound main spiral arms in the N-body simulation can be successfully identified using overdensities in angular momentum (AM) or guiding space and also in the distribution of dynamically cold stars close to their guiding centres. Stars in the AM overdensities that travel over many kiloparsec in radius trace extended density ridges in (R, vϕ) space and overdensities in the vR-vϕ plane of a solar neighbourhood (SNd)-like region, similar to those observed in the Gaia data. Similarly, the AM space of the MW contains several overdensities that correlate with a wave-like radial velocity pattern; this pattern is also reproduced by stars well beyond the SNd. We find that the fraction of Gaia stars located near their guiding centres shows three large-scale structures that approximately coincide with the MW spiral arms traced by distributions of maser sources in the Sagittarius, Local, and Perseus arms. This approach does not work for the Scutum arm near the end of the bar. Similar to the simulation, the stars in the AM overdensities follow the main (R, vϕ) density ridges with nearly constant angular momentum. When these ridges cross the SNd, they can be matched with the main vR-vϕ features. Thus we suggest that the Hat is the inner tail of the Perseus arm, one of the Hercules components is the Sagittarius arm, and the Arcturus stream is likely to be the outermost tail of the Scutum-Centaurus arm. Based on previous work, the bar corotation is suggested to coincide with the second, vϕ ≈ −55 km s−1 Hercules stream ridge, and the OLR with the Sirius stream. The latter is supported by a sharp decrease in mean metallicity beyond the Sirius stream, which is an expected behaviour of the OLR, limiting migration of the metal-rich stars from the inner MW. In various phase-space coordinates, the AM overdensity stars have a mean metallicity that is systematically higher by about 0.05 dex than the metallicity of the surrounding stars. This is a predicted behaviour of the spiral arms. We show that the wave-like metallicity pattern can be traced at least up to |z|≈1 kpc. It is linked to radial velocity variations seen even farther (|z|≈2 kpc) from the Galactic mid-plane.

Multiplicity Statistics of Stars in the Sagittarius Dwarf Spheroidal Galaxy: Comparison to the Milky Way

We use time-resolved spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to examine the distribution of radial velocity (RV) variations in 249 stars identified as members of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy by Hayes et al. We select Milky Way (MW) stars that have stellar parameters (, T eff, and [Fe/H] ) similar to those of the Sagittarius members by means of a k-d tree of dimension 3. We find that the shape of the distribution of RV shifts in Sgr dSph stars is similar to that measured in their MW analogs, but the total fraction of RV variable stars in the Sgr dSph is larger by a factor of ∼2. After ruling out other explanations for this difference, we conclude that the fraction of close binaries in the Sgr dSph is intrinsically higher than in the MW. We discuss the implications of this result for the physical processes leading to the formation of close binaries in dwarf spheroidal and spiral galaxies.

LAMOST MRS-N Observation of the W80 Region

The spectral observations and analysis for the W80 region are presented by using the data of Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N) with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). A total of 2982 high-quality nebular spectra have been obtained in the 20 square degree field of view which covers the W80 complex, and the largest sample of spectral data has been established for the first time. The relative intensities, radial velocities (RVs), and full widths at half maximum (FWHMs) are measured with the high spectral resolution of LAMOST MRS-N, for Hα λ6563 Å, [N ii] λλ6548 Å, 6584 Å, and [S ii] λλ6716 Å, 6731 Å emission lines. In the field of view of the whole W80 region, the strongest line emissions are found to be consistent with the bright nebulae NGC 7000, IC 5070, and LBN 391, and weak line emissions also exist in the Middle Region, where no bright nebulae are detected by the wide-band optical observations. The large-scale spectral observations of the W80 region reveal the systematic spatial variations of RVs and FWHMs, and several unique structural features. A “curved feature” to the east of NGC 7000, and a “jet feature” to the west of LBN 391 are detected to be showing larger RVs. A “wider FWHM region” is identified in the eastern part of NGC 7000. The variations of [S ii]/Hα ratios display a gradient from southwest to northeast in the NGC 7000 region, and manifest a ring shape around the “W80 bubble” ionized by an O-type star in L935. Further spectral and multi-band observations are guaranteed to investigate in detail the structural features.

Search for exoplanets around northern circumpolar stars

Aims. We report the detection of long-period radial velocity (RV) variations in three giant stars, HD 19615, HD 150010, and HD 174205, using precise RV measurements. Methods. These detections are part of the Search for Exoplanets around Northern Circumpolar Stars (SENS) survey being conducted at the Bohyunsan Optical Astronomy Observatory (BOAO). The nature of the RV variations was investigated by analyzing the photometric and line shape variations. We found no variability with the RV period in these quantities and conclude that the RV variations are most likely caused by planetary companions. Results. Orbital solutions for the three stars yield orbital periods of 402 d, 562 d, and 582 d and minimum masses of 8.5 MJ, 2.4 MJ, and 4.2 MJ, respectively. These masses and periods are typical for planets around intermediate-mass stars, although some unclear interpretations and recent studies may being calling some planet convictions into question. Nevertheless, the SENS program is contributing to our knowledge of giant planets around intermediate-mass stars.

The η Aquilae System: Radial Velocities and Astrometry in Search of η Aql B

The classical Cepheid η Aql was not included in past Leavitt Law work because of a presumed complicating orbit due to a known B9.8V companion. To determine the orbit of η Aql B, we analyze a significant number of radial velocity (RV) measurements from eight sources. With these we establish the RV variation due to Cepheid pulsation, using a model with 12 Fourier coefficients, while solving for velocity offsets required to bring the RV data sets into coincidence. RV residuals provide no evidence of orbital motion, suggesting either nearly face-on orientation or a very long period. Reanalysis of Hubble Space Telescope Fine Guidance Sensor astrometry now includes reference star parallax and proper motion priors from Gaia EDR3. As modeling confirmation, we reanalyze ζ Gem in parallel, deriving values for its parallax and proper motion consistent with Gaia EDR3, and consistent with the Benedict 2007 Leavitt Law. In an effort to further characterize η Aql B, we hypothesize that η Aql residuals larger than those of the associated reference stars or a parallax inconsistent with EDR3 and the Benedict 2007 Leavitt Law indicate unmodeled orbital motion. Using the astrometric noise or parallax mismatch with EDR3, we estimate possible periods and mass for η Aql B. Ascribing photocenter motion to the photometric variation of the Cepheid, η Aql A, yields a plausible separation that is consistent with a long period and explains the lack of RV variation. None of these approaches yields an unassailable characterization of the η Aql A–B system.

Leveraging Space-based Data from the Nearest Solar-type Star to Better Understand Stellar Activity Signatures in Radial Velocity Data

Stellar variability is a key obstacle in reaching the sensitivity required to recover Earth-like exoplanetary signals using the radial velocity (RV) detection method. To explore activity signatures in Sun-like stars, we present SolAster, a publicly distributed analysis pipeline 10 10 https://tamarervin.github.io/SolAster/ that allows for comparison of space-based measurements with ground-based disk-integrated RVs. Using high-spatial-resolution Dopplergrams, magnetograms, and continuum filtergrams from the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory (SDO), we estimate “Sun-as-a-star” disk-integrated RVs due to rotationally modulated flux imbalances and convective blueshift suppression, as well as other observables such as unsigned magnetic flux. Comparing these measurements with ground-based RVs from the NEID instrument, which observes the Sun daily using an automated solar telescope, we find a strong relationship between magnetic activity indicators and RV variation, supporting efforts to examine unsigned magnetic flux as a proxy for stellar activity in slowly rotating stars. Detrending against measured unsigned magnetic flux allows us to improve the NEID RV measurements by ∼20% (∼50 cm s−1 in a quadrature sum), yielding an rms scatter of ∼60 cm s−1 over five months. We also explore correlations between individual and averaged spectral line shapes in the NEID spectra and SDO-derived magnetic activity indicators, motivating future studies of these observables. Finally, applying SolAster to archival planetary transits of Venus and Mercury, we demonstrate the ability to recover small amplitude (<50 cm s−1) RV variations in the SDO data by directly measuring the Rossiter–McLaughlin signals.

Comparative hydrodynamics study of fluidized bed gasifier incorporating static and rotating air distributor plates: A CFD approach

This paper studies hydrodynamics of bubbling fluidized bed gasifier using static and rotating perforated air distributor plates. Pressure drop across the bed is calculated numerically using a static distributor plate to validate the model results with available experimental data. CFD results are good enough within the acceptable range of 10% difference. Dynamic mesh methodology coupled with the siding mesh technique of ANSYS FLUENT is incorporated to rotate distributor plates for specified rotational velocity using a user-defined function (UDF) hooked to the solver. Hydrodynamics study of fluidized bed with varying operational parameters using rotating plate distributor is carried out extensively. It is revealed that the impact of tangential and radial velocities on the fluidized bed due to rotation of plate is highest for lower superficial velocities, shallow initial depth, and the maximum rotational velocity of the distributor plate. The tangential and radial velocities variations with column height up to 3Umf showed a similar trend that changed significantly for higher superficial velocities (4Umf, 5Umf), resulting in more chaotic solids. The comparative evaluation of static and rotating distributor plates results shows an appreciable difference. The pressure drop across the bed is increased about 6–7% and minimum fluidization velocity is decreased upto 10% when compared to static plate distributor. Moreover, bed height rise is on a higher side for static plate distributors due to the dominant axial component of air velocity compared to the rotating plate distributor. It is also revealed that the pressure fluctuations within the fluidized bed are significantly reduced (88%) when using a rotating distributor plate compared with static plate distributors.

Radial velocity variability and the evolution of hot subdwarf stars

Hot subdwarf stars represent a late and peculiar stage in the evolution of low-mass stars, since they are likely formed by close binary interactions. In this work, we perform a radial velocity (RV) variability study of a sample of 646 hot subdwarfs with multi-epoch radial velocities based on spectra from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST). The atmospheric parameters and RVs were taken from the literature. For stars with archival spectra but without literature values, we determined the parameters by fitting model atmospheres. In addition, we redetermined the atmospheric parameters and RVs for all the He-enriched sdO/Bs. This broad sample allowed us to study RV-variability as a function of the location in the Teff − log g- and Teff − log n(He)/n(H) diagrams in a statistically significant way. We used the fraction of RV-variable stars and the distribution of the maximum RV variations ΔRVmax as diagnostics. Both indicators turned out to be quite inhomogeneous across the studied parameter ranges. A striking feature is the completely dissimilar behaviour of He-poor and He-rich hot subdwarfs. While the former have a high fraction of close binaries, almost no significant RV variations could be detected for the latter. This has led us to the conclusion that there is likely no evolutionary connection between these subtypes. On the other hand, intermediate He-rich- and extreme He-rich sdOB/Os are more likely to be related. Furthermore, we conclude that the vast majority of this population is formed via one or several binary merger channels. Hot subdwarfs with temperatures cooler than ∼24 000 K tend to show fewer and smaller RV-variations. These objects might constitute a new subpopulation of binaries with longer periods and late-type or compact companions. The RV-variability properties of the extreme horizontal branch (EHB) and corresponding post-EHB populations of the He-poor hot subdwarfs match and confirm the predicted evolutionary connection between them. Stars found below the canonical EHB at somewhat higher surface gravities show large RV variations and a high RV variability fraction. These properties are consistent with most of them being low-mass EHB stars or progenitors of low-mass helium white dwarfs in close binaries.

A Polarimetric Radar Operator and Application for Convective Storm Simulation

In this study, a polarimetric radar forward model operator was developed for the Weather Research and Forecasting (WRF) model that was based on a scattering algorithm using the T-matrix methodology. Three microphysics schemes—Thompson, Morrison 2-moment, and Milbrandt-Yau 2-moment—were supported in the operator. This radar forward operator used the microphysics, thermodynamic, and wind fields from WRF model forecasts to compute horizontal reflectivity, radial velocity, and polarimetric variables including differential reflectivity (ZDR) and specific differential phase (KDP) for S-band radar. A case study with severe convective storms was used to examine the accuracy of the radar operator. Output from the radar operator was compared to real radar observations from the Weather Surveillance Radar–1988 Doppler (WSR-88D) radar. The results showed that the radar forward operator generated realistic polarimetric signatures. The distribution of polarimetric variables agreed well with the hydrometer properties produced by different microphysics schemes. Similar to the observed polarimetric signatures, radar operator output showed ZDR and KDP columns from low-to-mid troposphere, reflecting the large amount of rain within strong updrafts. The Thompson scheme produced a better simulation for the hail storm with a ZDR hole to indicate the existence of graupel in the low troposphere.

Discovery and analysis of three magnetic hot subdwarf stars: evidence for merger-induced magnetic fields

ABSTRACT Magnetic fields can play an important role in stellar evolution. Among white dwarfs, the most common stellar remnant, the fraction of magnetic systems is more than 20 per cent. The origin of magnetic fields in white dwarfs, which show strengths ranging from 40 kG to hundreds of MG, is still a topic of debate. In contrast, only one magnetic hot subdwarf star has been identified out of thousands of known systems. Hot subdwarfs are formed from binary interaction, a process often associated with the generation of magnetic fields, and will evolve to become white dwarfs, which makes the lack of detected magnetic hot subdwarfs a puzzling phenomenon. Here we report the discovery of three new magnetic hot subdwarfs with field strengths in the range 300–500 kG. Like the only previously known system, they are all helium-rich O-type stars (He-sdOs). We analysed multiple archival spectra of the three systems and derived their stellar properties. We find that they all lack radial velocity variability, suggesting formation via a merger channel. However, we derive higher than typical hydrogen abundances for their spectral type, which are in disagreement with current model predictions. Our findings suggest a lower limit to the magnetic fraction of hot subdwarfs of $0.147^{+0.143}_{-0.047}$ per cent, and provide evidence for merger-induced magnetic fields which could explain white dwarfs with field strengths of 50–150 MG, assuming magnetic flux conservation.

Characteristics of small protoplanetary disc warps in kinematic observations

ABSTRACT Many circumstellar discs appear to have misaligned central regions that give rise to shadows seen in scattered light observations. Small warps (&amp;lt;20° misalignment) are probably more common but are also more difficult to detect than the large misalignments studied previously. We present the characteristics of CO emission that may be used to identify a small disc warp, found from synthetic 13CO maps of a model misaligned circumbinary disc. The spectra are not symmetrical, so fitting a Keplerian model is not appropriate and can hide a warp or lead to spurious features such as spirals appearing in the residuals. We quantify the observed warp structure by fitting sinusoids to concentric annuli of the disc. From this we can trace the radial variation of the peak velocity and of the azimuth of the peak velocity, i.e. the twist. At near face-on inclinations, these radial profiles reveal the warp structure. The twist remains detectable at moderate inclinations (iouter disc ≲ 35°) in the absence of radial flows but the measured inclination must be accurate to ≲ 5° to allow detection of the radial variation. The observed twist does not provide a direct measure of the warp structure because of its dependence on optical depth. The warp causes broad asymmetries in the channel maps that span several channels and that are distinct from localized features caused by embedded planets and gravitational instability. We suspect that kinematic evidence of warps may have been missed and we suggest a few examples where the data may be revisited.

Dippers from TESS Full-frame Images. II. Spectroscopic Characterization of Four Young Dippers

Abstract Photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) has discovered not only periodic signals by transiting exoplanets but also episodic or quasiperiodic dimming around young stellar objects. The dimming mechanisms of these objects, the so-called “dippers,” are thought to be related to either the accretion property or the structure of protoplanetary disks especially in regions close to the host star. Recently, we have created a catalog of dippers from one year of TESS full-frame image data. In this paper, we report on the spectral features of four newly found dippers in that catalog and show that they potentially shed light on the dimming mechanisms. We found that all of the targets exhibit the Hα emission line, which is an indicator of ongoing accretion. Based on their line profiles and/or their variability, we characterized the properties of the disks of each source, which can support dimming mechanisms via a dusty disk wind or an accretion-driven inner-disk warp. Also, we found an interesting dipper (TIC 317873721), a “close-in binary dipper,” showing a complex variability of the line profile and a large radial velocity variation. Because the dimming intervals are similar to the orbital period of the binary, we suggest that the dips are caused by dust in the accretion warp from a circumbinary disk onto stars. Such a close-in (&lt;0.1 au) binary dipper has been rarely reported thus far; further investigation will reveal new aspects of disk evolution and planetary formation.

One year of AU Mic with HARPS – I. Measuring the masses of the two transiting planets

ABSTRACT The system of two transiting Neptune-sized planets around the bright, young M-dwarf AU Mic provides a unique opportunity to test models of planet formation, early evolution, and star–planet interaction. However, the intense magnetic activity of the host star makes measuring the masses of the planets via the radial velocity (RV) method very challenging. We report on a 1-yr, intensive monitoring campaign of the system using 91 observations with the HARPS spectrograph, allowing for detailed modelling of the ∼600 ${\rm m\, s^{-1}}$ peak-to-peak activity-induced RV variations. We used a multidimensional Gaussian Process framework to model these and the planetary signals simultaneously. We detect the latter with semi-amplitudes of Kb = 5.8 ± 2.5 ${\rm m\, s^{-1}}$ and Kc = 8.5 ± 2.5 ${\rm m\, s^{-1}}$, respectively. The resulting mass estimates, Mb = 11.7 ± 5.0 M⊕ and Mc = 22.2 ± 6.7 M⊕, suggest that planet b might be less dense, and planet c considerably denser than previously thought. These results are in tension with the current standard models of core-accretion. They suggest that both planets accreted a H/He envelope that is smaller than expected, and the trend between the two planets’ envelope fractions is the opposite of what is predicted by theory.

Efficient modeling of correlated noise

The radial velocity method is a very productive technique used to detect and confirm extrasolar planets. The most recent spectrographs, such as ESPRESSO or EXPRES, have the potential to detect Earth-like planets around Sun-like stars. However, stellar activity can induce radial velocity variations that dilute or even mimic the signature of a planet. A widely recognized method for disentangling these signals is to model the radial velocity time series, jointly with stellar activity indicators, using Gaussian processes and their derivatives. However, such modeling is prohibitive in terms of computational resources for large data sets, as the cost typically scales as the total number of measurements cubed. Here, we present S+LEAF 2, a Gaussian process framework that can be used to jointly model several time series, with a computational cost that scales linearly with the data set size. This framework thus provides a state-of-the-art Gaussian process model, with tractable computations even for large data sets. We illustrate the power of this framework by reanalyzing the 246 HARPS radial velocity measurements of the nearby K2 dwarf HD 138038, together with two activity indicators. We reproduce the results of a previous analysis of these data, but with a strongly decreased computational cost (more than two order of magnitude). The gain would be even greater for larger data sets.

The puzzling story of flare inactive ultra fast rotating M dwarfs. II. Searching for radial velocity variations

ABSTRACT Observations made using TESS revealed a sample of low-mass stars which show a periodic modulation on a period &amp;lt;0.2 d. Surprisingly, many of these ultra fast rotating (UFR) stars showed no evidence of flare activity which would be expected from such rapidly rotating stars. We present results from a spectroscopic survey of UFRs using the Nordic Optical Telescope to search for radial velocity variations which could reveal evidence for binarity. Our sample of 29 sources have a photometric period between 0.1–0.2 d, cover spectral classes of M0-4V, and show no evidence for flares. We detect only one source with clear radial velocity shifts, with another two having Gaia RUWE values which suggests they are binaries. Further observations reveal the former star possibly contains a brown dwarf companion with a mass of M2 &amp;gt; 58 MJup and probability P(M2 &amp;lt; 90 MJup) = 50 per cent. There is no evidence for the companion in our spectra, strengthening the case for a brown dwarf companion. We also examine the folded TESS light curves of all our targets, finding at least two are eclipsing binaries and one which has been contaminated by a spatially nearby δ Sct star. We estimate that around 1/4 of our targets may have been contaminated by short period variable stars. However, the majority of our targets are consistent with being single, low mass stars whose variability is due to starspots. We outline the possible reasons why they are not flare active despite being such rapid rotators.