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.
Context. Magnetic activity strongly impacts stellar radial velocities (RVs) and therefore the search for small planets. We showed previously that in the solar case it induces RV variations with an ...amplitude over the cycle on the order of 8 m/s, with signals on both short and long timescales. The major component is the inhibition of the convective blueshift due to plages. Aims. In this paper we explore a new approach used to correct for this major component of stellar radial velocities in the case of solar-type stars. Methods. The convective blueshift depends on line depths; we use this property to develop a method that will characterize the amplitude of this effect and to correct for this RV component. We build realistic RV time series corresponding to RVs computed using different sets of lines, including lines in different depth ranges. We characterize the performance of the method used to reconstruct the signal without the convective component and the detection limits derived from the residuals. Results. We identified a set of lines which, combined with a global set of lines, allows us to reconstruct the convective component with a good precision and to correct for it. For the full temporal sampling, the power in the range 100−500 d significantly decreased, by a factor of 100 for a RV noise below 30 cm/s. We also studied the impact of noise contributions other than the photon noise, which lead to uncertainties on the RV computation, as well as the impact of the temporal sampling. We found that these other sources of noise do not greatly alter the quality of the correction, although they need a better noise level to reach a similar performance level. Conclusions. A very good correction of the convective component can be achieved providing very good RV noise levels combined with a very good instrumental stability and realistic granulation noise. Under the conditions considered in this paper, detection limits at 480 d lower than 1 MEarth could be achieved for RV noise below 15 cm/s.
Over the past decade, direct imaging has confirmed the existence of substellar companions on wide orbits from their parent stars. To understand the formation and evolution mechanisms of these ...companions, their individual as well as the full population properties must be characterized. We aim at detecting giant planet and/or brown dwarf companions around young, nearby, and dusty stars. Our goal is also to provide statistics on the population of giant planets at wide-orbits and discuss planet formation models. The observations were conducted with the ground-based adaptive optics system VLT/NaCo at L'-band (3.8 mu m). We resolve and characterize new visual binaries and do not detect any new substellar companion. The survey's median detection performance reaches contrasts of 10 mag at 0.5" and 11.5 mag at 1.0" . We will have to wait for the next generation of planet finders to start exploring that population, and even for the extremely large telescopes for a more complete overlap with other planet-hunting techniques.
Aims. In 2015, we initiated a survey of Scorpius-Centaurus A-F stars that are predicted to host warm-inner and cold-outer belts of debris similar to the case of the system HR 8799. The survey aims to ...resolve the disks and detect planets responsible for the disk morphology. In this paper, we study the F-type star HIP 67497 and present a first-order modelization of the disk in order to derive its main properties. Methods. We used the near-infrared integral field spectrograph (IFS) and dual-band imager IRDIS of VLT/SPHERE to obtain angular-differential imaging observations of the circumstellar environnement of HIP 67497. We removed the stellar halo with PCA and TLOCI algorithms. The disk emission was modeled with the GRaTeR code. Results. We resolve a ring-like structure that extends up to ~450 mas (~50 au) from the star in the IRDIS and IFS data. It is best reproduced by models of a non-eccentric ring with an inclination of 80 ± 1°, a position angle of −93 ± 1°, and a semi-major axis of 59 ± 3 au. We also detect an additional, but fainter, arc-like structure with a larger extension (0.65 arcsec) South of the ring that we model as a second belt of debris at ~130 au. We detect ten candidate companions at separations ≥1′′. We estimate the mass of putative perturbers responsible for the disk morphology and compare this to our detection limits. Additional data are needed to find those perturbers, and to relate our images to large-scale structures seen with HST/STIS.
Context. Magnetic activity and surface flows at different scales pertub radial velocity measurements. This affects the detectability of low-mass exoplanets. Aims. In these flows, the effect of ...supergranulation is not as well characterized as the other flows, and we wish to estimate its effect on the detection of Earth-like planets in the habitable zone of Sun-like stars. Methods. We produced time series of radial velocities due to oscillations, granulation, and supergranulation, and estimated the detection limit for a G2 star and a period of 300 days. We also studied in detail the behavior of the power when the signal of a 1 MEarth planet was superposed on the signal from the stellar flows. Results. We find that the detection rate does not reach 100% except for the supergranulation level we assume, which is still optimistic, and for an excellent sampling. Conclusions. We conclude that with current knowledge, it is a very challenging task to find Earth twins around Sun-like stars with our current capabilities.
Abstract
We present observations of the Herbig Ae star HD 169142 with the VLT/SPHERE instruments InfraRed Dual-band Imager and Spectrograph (IRDIS) (K1K2 and H2H3 bands) and the Integral Field ...Spectrograph (IFS) (Y, J and H bands). We detect several bright blobs at ∼180 mas separation from the star, and a faint arc-like structure in the IFS data. Our reference differential imaging (RDI) data analysis also finds a bright ring at the same separation. We show, using a simulation based on polarized light data, that these blobs are actually part of the ring at 180 mas. These results demonstrate that the earlier detections of blobs in the H and KS bands at these separations in Biller et al. as potential planet/substellar companions are actually tracing a bright ring with a Keplerian motion. Moreover, we detect in the images an additional bright structure at ∼93 mas separation and position angle of 355°, at a location very close to previous detections. It appears point-like in the YJ and K bands but is more extended in the H band. We also marginally detect an inner ring in the RDI data at ∼100 mas. Follow-up observations are necessary to confirm the detection and the nature of this source and structure.
Radial velocity (RV) time series are strongly impacted by the presence of stellar activity. In a series of papers, we have reconstructed solar RV variations over a full solar cycle from observed ...solar structures and studied their impact on the detectability of an Earth-mass planet in the habitable zone of the Sun as seen edge-on from a neighbor star in several typical cases. We found that the convective contribution dominates the RV times series. The objective of this paper is twofold: to determine detection limits on a Sun-like star seen edge-on with different levels of convection and to estimate the performance of the activity correction using a Ca index. We apply two methods to compute the detection limits: a correlation-based method and a local power analysis method, which both take into account the temporal structure of the observations. For Sun-like conditions and under the simplifying assumptions considered, we first conclude that the detection limit of a few Msubearth planet can be reached providing good sampling and Ca noise.
Context. The inhibition of small-scale convection in the Sun dominates the long-term radial velocity (RV) variability: it therefore has a critical effect on light exoplanet detectability using RV ...techniques. Aims: We here extend our previous analysis of stellar convective blueshift and its dependence on magnetic activity to a larger sample of stars in order to extend the Teff range, to study the impact of other stellar properties, and finally to improve the comparison between observed RV jitter and expected RV variations. Methods: We estimate a differential velocity shift for Fe and Ti lines of different depths and derive an absolute convective blueshift using the Sun as a reference for a sample of 360 F7-K4 stars with different properties (age, Teff, metallicity). Results: We confirm the strong variation in convective blueshift with Teff and its dependence on (as shown in the line list in Paper I) activity level. Although we do not observe a significant effect of age or cyclic activity, stars with a higher metallicity tend to have a lower convective blueshift, with a larger effect than expected from numerical simulations. Finally, we estimate that for 71% of the stars in our sample the RV and Log R' _HK variations are compatible with the effect of activity on convection, as observed in the solar case, while for the other stars, other sources (such as binarity or companions) must be invoked to explain the large RV variations. We also confirm a relationship between Log R' _HK and metallicity, which may affect discussions of the possible relationship between metallicity and exoplanets, as RV surveys are biased toward low Log R' _HK and possibly toward high-metallicity stars. Conclusions: We conclude that activity and metallicity strongly affect the small-scale convection levels in stars in the F7-K4 range, with a lower amplitude for the lower mass stars and a larger amplitude for low-metallicity stars. Full Table A.1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/607/A124
Stellar variability, at a variety of timescales, can strongly affect the ability to detect exoplanets, in particular when using radial velocity (RV) techniques. Accurately characterized solar ...variations are precious in this context to study the impact of stellar variations on planet detectability. Here we focus on the impact of small timescale variability. The objective of this paper is to model realistic RV time series due to granulation and super-granulation and to study in greater detail the impact of granulation and super granulation on RV times series in the solar case. We have simulated a collection of granules and super-granules evolving in time to reproduce solar photometric and RV time series. Synthetic time series are built over the full hemisphere over one solar cycle. To minimize the effect of granulation, the best strategy is to split the observing time during the night into several periods instead of observing over a consecutive duration. The results do not take the presence of pulsations into account.
Context. The transition disk around the T Tauri star T Cha possesses a large gap, making it a prime target for high-resolution imaging in the context of planet formation. Aims. We aim to find signs ...of disk evolutionary processes by studying the disk geometry and the dust grain properties at its surface, and to search for companion candidates. Methods. We analyze a set of VLT/SPHERE data at near-infrared and optical wavelengths. We performed polarimetric imaging of T Cha with IRDIS (1.6 μm) and ZIMPOL (0.5–0.9 μm), and obtained intensity images from IRDIS dual-band imaging with simultaneous spectro-imaging with IFS (0.9–1.3 μm). Results. The disk around T Cha is detected in all observing modes and its outer disk is resolved in scattered light with unprecedented angular resolution and signal-to-noise. The images reveal a highly inclined disk with a noticeable east-west brightness asymmetry. The significant amount of non-azimuthal polarization signal in the Uφ images, with a Uφ/Qφ peak-to-peak value of 14%, is in accordance with theoretical studies on multiple scattering in an inclined disk. Our optimal axisymmetric radiative transfer model considers two coplanar inner and outer disks, separated by a gap of 0.̋28 (~30 au) in size, which is larger than previously thought. We derive a disk inclination of ~69 deg and PA of ~114 deg. In order to self-consistently reproduce the intensity and polarimetric images, the dust grains, responsible for the scattered light, need to be dominated by sizes of around ten microns. A point source is detected at an angular distance of 3.5′′ from the central star. It is, however, found not to be co-moving. Conclusions. We confirm that the dominant source of emission is forward scattered light from the near edge of the outer disk. Our point source analysis rules out the presence of a companion with mass larger than ~8.5 Mjup between 0.̋1 and 0.̋3. The detection limit decreases to ~2 Mjup for 0.̋3 to 4.0′′.
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