scholarly journals So close, so different: characterization of the K2-36 planetary system with HARPS-N

2019 ◽  
Vol 624 ◽  
pp. A38 ◽  
Author(s):  
M. Damasso ◽  
L. Zeng ◽  
L. Malavolta ◽  
A. Mayo ◽  
A. Sozzetti ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Gaussian Process Regression ◽  
Magnetic Activity ◽  
Good Precision ◽  
Stellar Activity ◽  
Transiting Planets ◽  
Atmospheric Escape ◽  
Low Mass ◽  
Time Variations ◽  
Velocity Scatter

Context. K2-36 is a K dwarf orbited by two small (Rb = 1.43 ± 0.08 R⊕ and Rc = 3.2 ± 0.3 R⊕), close-in (ab = 0.022 au and ac = 0.054 au) transiting planets discovered by the Kepler/K2 space observatory. They are representatives of two distinct families of small planets (Rp < 4 R⊕) recently emerged from the analysis of Kepler data, with likely a different structure, composition and evolutionary pathways. Aims. We revise the fundamental stellar parameters and the sizes of the planets, and provide the first measurement of their masses and bulk densities, which we use to infer their structure and composition. Methods. We observed K2-36 with the HARPS-N spectrograph over ~3.5 yr, collecting 81 useful radial velocity measurements. The star is active, with evidence for increasing levels of magnetic activity during the observing time span. The radial velocity scatter is ~17 m s−1 due to the stellar activity contribution, which is much larger that the semi-amplitudes of the planetary signals. We tested different methods for mitigating the stellar activity contribution to the radial velocity time variations and measuring the planet masses with good precision. Results. We find that K2-36 is likely a ~1 Gyr old system, and by treating the stellar activity through a Gaussian process regression, we measured the planet masses mb = 3.9 ± 1.1 M⊕ and mc = 7.8 ± 2.3 M⊕. The derived planet bulk densities ρb = 7.2−2.1+2.5 g cm−3 and ρc = 1.3−0.5+0.7 g cm−3 point out that K2-36 b has a rocky, Earth-like composition, and K2-36 c is a low-density sub-Neptune. Conclusions. Composed of two planets with similar orbital separations but different densities, K2-36 represents an optimal laboratory for testing the role of the atmospheric escape in driving the evolution of close-in, low-mass planets after ~1 Gyr from their formation. Due to their similarities, we performed a preliminary comparative analysis between the systems K2-36 and Kepler-36, which we deem worthy of a more detailed investigation.

scholarly journals Physics of the Applegate mechanism: Eclipsing time variations from magnetic activity

2018 ◽  
Vol 620 ◽  
pp. A42 ◽  
Author(s):  
M. Völschow ◽  
D. R. G. Schleicher ◽  
R. Banerjee ◽  
J. H. M. M. Schmitt
Keyword(s):  
Binary Systems ◽  
Theoretical Work ◽  
Magnetic Activity ◽  
Close Binary ◽  
Magnetic Fluctuations ◽  
Common Envelope ◽  
Stellar Parameter ◽  
Low Mass ◽  
Time Variations ◽  
Binary Separations

Since its proposal in 1992, the Applegate mechanism has been discussed as a potential intrinsical mechanism to explain transit-timing variations in various types of close binary systems. Most analytical arguments presented so far focused on the energetic feasibility of the mechanism while applying rather crude one- or two-zone prescriptions to describe the exchange of angular momentum within the star. In this paper, we present the most detailed approach to date to describe the physics giving rise to the modulation period from kinetic and magnetic fluctuations. Assuming moderate levels of stellar parameter fluctuations, we find that the resulting binary period variations are one or two orders of magnitude lower than the observed values in RS-CVn like systems, supporting the conclusion of existing theoretical work that the Applegate mechanism may not suffice to produce the observed variations in these systems. The most promising Applegate candidates are low-mass post-common-envelope binaries with binary separations ≲1 R⊙ and secondary masses in the range of 0.30 M⊙ and 0.36 M⊙.

scholarly journals Biases in retrieving planetary signals in the presence of quasi-periodic stellar activity

2019 ◽  
Vol 489 (2) ◽  
pp. 2555-2571 ◽  
Author(s):  
M Damasso ◽  
M Pinamonti ◽  
G Scandariato ◽  
A Sozzetti
Keyword(s):  
Time Series ◽  
Gaussian Process Regression ◽  
Data Sets ◽  
Correlated Noise ◽  
Stellar Activity ◽  
Activity Component ◽  
Low Mass ◽  
Host Stars ◽  
Correlated Signals

Abstract Gaussian process regression is a widespread tool used to mitigate stellar correlated noise in radial velocity (RV) time series. It is particularly useful to search for and determine the properties of signals induced by small-sized low-mass planets (Rp < 4 R⊕, mp < 10 M⊕). By using extensive simulations based on a quasi-periodic representation of the stellar activity component, we investigate the ability in retrieving the planetary parameters in 16 different realistic scenarios. We analyse systems composed by one planet and host stars having different levels of activity, focusing on the challenging case represented by low-mass planets, with Doppler semi-amplitudes in the range 1–3 $\rm{\,m\,s^{-1}}$. We consider many different configurations for the quasi-periodic stellar activity component, as well as different combinations of the observing epochs. We use commonly employed analysis tools to search for and characterize the planetary signals in the data sets. The goal of our injection-recovery statistical analysis is twofold. First, we focus on the problem of planet mass determination. Then, we analyse in a statistical way periodograms obtained with three different algorithms, in order to explore some of their general properties, as the completeness and reliability in retrieving the injected planetary and stellar activity signals with low false alarm probabilities. This work is intended to provide some understanding of the biases introduced in the planet parameters inferred from the analysis of RV time series that contain correlated signals due to stellar activity. It also aims to motivate the use and encourage the improvement of extensive simulations for planning spectroscopic follow-up observations.

scholarly journals The effect of stellar activity on the spectroscopic stellar parameters of the young solar twin HIP 36515

2019 ◽  
Vol 490 (1) ◽  
pp. L86-L90 ◽  
Author(s):  
Jhon Yana Galarza ◽  
Jorge Meléndez ◽  
Diego Lorenzo-Oliveira ◽  
Adriana Valio ◽  
Henrique Reggiani ◽  
...  
Keyword(s):  
Effective Temperature ◽  
Signal To Noise Ratio ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Resolving Power ◽  
Strongly Correlated ◽  
Chemical Abundances ◽  
Stellar Activity ◽  
Time Variations ◽  
First Time

ABSTRACT Spectroscopic equilibrium allows us to obtain precise stellar parameters in Sun-like stars. It relies on the assumption of the iron excitation and ionization equilibrium. However, several works suggest that magnetic activity may affect chemical abundances of young active stars, calling into question the validity of this widely used method. We have tested, for the first time, variations in stellar parameters and chemical abundances for the young solar twin HIP 36515 (∼0.4 Gyr), along its activity cycle. This star has stellar parameters very well established in the literature and we estimated its activity cycle in ∼6 yr. Using HARPS spectra with high resolving power (115 000) and signal-to-noise ratio (∼270), the stellar parameters of six different epochs in the cycle were estimated. We found that the stellar activity is strongly correlated with the effective temperature, metallicity, and microturbulence velocity. The possibility of changes in the Li i 6707.8 Å line due to flares and star-spots was also investigated. Although the core of the line profile shows some variations with the stellar cycle, it is compensated by changes in the effective temperature, resulting in a non-variation of the Li abundance.

scholarly journals How the planetary research helps to the stellar dynamo understanding

2012 ◽  
Vol 8 (S294) ◽  
pp. 471-475
Author(s):  
I. Boisse ◽  
M. Oshagh ◽  
C. Lovis ◽  
N. C. Santos ◽  
X. Dumusque ◽  
...  
Keyword(s):  
Planetary System ◽  
Magnetic Activity ◽  
Doppler Imaging ◽  
Habitable Zone ◽  
Stellar Activity ◽  
Term Variation ◽  
Solar Type ◽  
Low Mass ◽  
Stellar Dynamo

AbstractMost of the exoplanet science is dependent on the stellar knowledge. One of them that has to be understood is the magnetic activity when we search for planets with radial velocity or photometry measurements. The main shape of stellar activity and spots properties have to be understood, for example, to choose the best targets to search for low-mass planets in the habitable zone or to derive the accurate parameters of a planetary system. With that aim, we show in this presentation how these studies lead to give clues on spots latitudes and on the long term variation of stellar activity. The properties of magnetic activity on the low rotators solar-type stars are not easily reachable by other techniques (spectropolarimetry or Doppler imaging) and these studies should be used to constrain theories of stellar dynamo.

scholarly journals Disentangling planetary orbits from stellar activity in radial-velocity surveys

2014 ◽  
Vol 13 (2) ◽  
pp. 155-157 ◽  
Author(s):  
R. D. Haywood ◽  
A. Collier Cameron ◽  
D. Queloz ◽  
S.C.C. Barros ◽  
M. Deleuil ◽  
...  
Keyword(s):  
Radial Velocity ◽  
High Accuracy ◽  
Stellar Surface ◽  
Stellar Activity ◽  
The Earth ◽  
Planetary Orbits ◽  
Low Mass ◽  
Do So

AbstractThe majority of extra-solar planets have been discovered (or confirmed after follow-up) through radial-velocity (RV) surveys. Using ground-based spectrographs such as High Accuracy Radial Velocity Planetary Search (HARPS) and HARPS-North, it is now possible to detect planets that are only a few times the mass of the Earth. However, the presence of dark spots on the stellar surface produces RV signals that are very similar in amplitude to those caused by orbiting low-mass planets. Disentangling these signals has thus become the biggest challenge in the detection of Earth-mass planets using RV surveys. To do so, we use the star's lightcurve to model the RV variations produced by spots. Here we present this method and show the results of its application to CoRoT-7.

scholarly journals The GAPS programme at TNG

2020 ◽  
Vol 642 ◽  
pp. A53
Author(s):  
C. Di Maio ◽  
C. Argiroffi ◽  
G. Micela ◽  
S. Benatti ◽  
A. F. Lanza ◽  
...  
Keyword(s):  
Magnetic Activity ◽  
Stellar Atmospheres ◽  
Activity Level ◽  
Mass Motion ◽  
Time Variability ◽  
Line Profiles ◽  
Low Mass Stars ◽  
Stellar Activity ◽  
Low Mass ◽  
M Stars

Context. Understanding stellar activity in M dwarfs is fundamental to improving our knowledge of the physics of stellar atmospheres and for planet search programmes. High levels of stellar activity (also frequently associated with flare events) can cause additional variations in the stellar emission that contaminate the signal induced by a planet and that need to be corrected. The study of activity indicators in active stars can improve our capability of modelling the signal generated by magnetic activity. Aims. In this work we present measurements of activity indicators at visible wavelength for a star with a high activity level, AD Leonis, observed with HARPS in 2006, and HARPS-N in 2018. Our aim is to understand the behaviour of stellar chromospheres of M stars, studying the more sensitive chromospheric activity indicators. We also focus on characterising their variability and on finding the correlations among these indicators to obtain information on the origin of the magnetic activity in low-mass stars. Methods. We performed a study of the main optical activity indicators (Ca II H&K, Balmer lines, Na I D1,2 doublet, He I D3, and other helium lines) measured for AD Leonis using the data provided by the HARPS-N high-resolution spectrograph at the Telescopio Nazionale Galileo in 2018, and by the HARPS instrument at La Silla observatory in 2006. Spectra were flux-calibrated in units of flux at the stellar surface. We measured excess flux of the selected activity indicators. The correlations between the different activity indicators as well as the temporal evolution of fluxes were analysed. A stellar flare was identified during the 2018 observing run and the Hα, Hβ, He I 4471 Å, and He I 5876 Å lines were analysed in detail by fitting the line profiles with two Gaussian components. Results. We found that the Ca II H&K flux excesses are strongly correlated with each other, but the Ca II H&K doublet is generally less correlated with the other indicators. Moreover, Hα is correlated with Na I doublet and helium lines. Analysing the time variability of flux of the studied lines, we found a higher level of activity of the star during the observations in 2018 than in 2006, while Ca II H&K showed more intense emission on spectra obtained during the observations in 2006. Thanks to a detailed analysis of selected line profiles, we investigated the flare evaluating the mass motion during the event.

scholarly journals A HARPS RV search for planets around young nearby stars

2020 ◽  
Vol 633 ◽  
pp. A44 ◽  
Author(s):  
A. Grandjean ◽  
A.-M. Lagrange ◽  
M. Keppler ◽  
N. Meunier ◽  
L. Mignon ◽  
...  
Keyword(s):  
Time Series ◽  
Radial Velocity ◽  
Spectroscopic Data ◽  
Imaging Techniques ◽  
Giant Planet ◽  
Magnetic Activity ◽  
Young Stars ◽  
Upper Limits ◽  
Nearby Stars ◽  
Low Mass

Context. Young nearby stars are good candidates in the search for planets with both radial velocity (RV) and direct imaging techniques. This, in turn, allows for the computation of the giant planet occurrence rates at all separations. The RV search around young stars is a challenge as they are generally faster rotators than older stars of similar spectral types and they exhibit signatures of magnetic activity (spots) or pulsation in their RV time series. Specific analyses are necessary to characterize, and possibly correct for, this activity. Aims. Our aim is to search for planets around young nearby stars and to estimate the giant planet (GP) occurrence rates for periods up to 1000 days. Methods. We used the HARPS spectrograph on the 3.6 m telescope at La Silla Observatory to observe 89 A−M young (<600 Myr) stars. We used our SAFIR (Spectroscopic data via Analysis of the Fourier Interspectrum Radial velocities) software to compute the RV and other spectroscopic observables. Then, we computed the companion occurrence rates on this sample. Results. We confirm the binary nature of HD 177171, HD 181321 and HD 186704. We report the detection of a close low mass stellar companion for HIP 36985. No planetary companion was detected. We obtain upper limits on the GP (<13 MJup) and BD (∈ [13;80] MJup) occurrence rates based on 83 young stars for periods less than 1000 days, which are set, 2−2+3 and 1−1+3%.

scholarly journals What to Expect from Transiting Multiplanet Systems

2008 ◽  
Vol 4 (S253) ◽  
pp. 173-179 ◽  
Author(s):  
Daniel C. Fabrycky
Keyword(s):  
Radial Velocity ◽  
Transit Time ◽  
Dynamical Evolution ◽  
Velocity Measurements ◽  
Search Methods ◽  
Tidal Evolution ◽  
Transiting Planets ◽  
Mean Motion ◽  
Time Variations ◽  
Mean Motion Resonance

AbstractSo far radial velocity measurements have discovered ~25 stars to host multiple planets. The statistics imply that many of the known hosts of transiting planets should have additional planets, yet none have been solidly detected. They will be soon, via complementary search methods of RV, transit-time variations of the known planet, and transits of the additional planet. When they are found, what can transit measurements add to studies of multiplanet dynamical evolution? First, mutual inclinations become measurable, for comparison to the solar system's disk-like configuration. Such measurements will give important constraints to planet-planet scattering models, just as the radial velocity measurements of eccentricity have done. Second, the Rossiter-McLaughlin effect measures stellar obliquity, which can be modified by two-planet dynamics with a tidally evolving inner planet. Third, transit-time variations are exquisitely sensitive to planets in mean motion resonance. Two planets differentially migrating in the disk can establish such resonances, and tidal evolution of the planets can break them, so the configuration and frequency of these resonances as a function of planetary parameters will constrain these processes.

scholarly journals Simulating radial velocity observations of trappist-1 with SPIRou

2019 ◽  
Vol 488 (4) ◽  
pp. 5114-5126 ◽  
Author(s):  
Baptiste Klein ◽  
J-F Donati
Keyword(s):  
White Noise ◽  
Radial Velocity ◽  
Gaussian Process Regression ◽  
Data Set ◽  
Stellar Activity ◽  
Sampling Schemes ◽  
Activity Curve ◽  
To Come ◽  
The Masses

ABSTRACT We simulate a radial velocity (RV) follow-up of the TRAPPIST-1 system, a faithful representative of M dwarfs hosting transiting Earth-sized exoplanets to be observed with SPIRou in the months to come. We generate an RV curve containing the signature of the seven transiting TRAPPIST-1 planets and a realistic stellar activity curve statistically compatible with the light curve obtained with the K2 mission. We find a ±5 m s−1 stellar activity signal comparable in amplitude with the planet signal. Using various sampling schemes and white noise levels, we create time-series from which we estimate the masses of the seven planets. We find that the precision on the mass estimates is dominated by (i) the white noise level for planets c, f, and e and (ii) the stellar activity signal for planets b, d, and h. In particular, the activity signal completely outshines the RV signatures of planets d and h that remain undetected regardless of the RV curve sampling and level of white noise in the data set. We find that an RV follow-up of TRAPPIST-1 using SPIRou alone would likely result in an insufficient coverage of the rapidly evolving activity signal of the star, especially with bright-time observations only, making statistical methods such as Gaussian Process Regression hardly capable of firmly detecting planet f and accurately recovering the mass of planet g. In contrast, we show that using bi-site observations with good longitudinal complementary would allow for a more accurate filtering of the stellar activity RV signal.

From the Sun to solar-type stars: radial velocity, photometry, astrometry and logR′HK time series for late-F to early-K old stars

2019 ◽  
Vol 15 (S354) ◽  
pp. 286-294
Author(s):  
Nadège Meunier ◽  
Anne-Marie Lagrange
Keyword(s):  
Radial Velocity ◽  
Large Range ◽  
Magnetic Activity ◽  
Activity Levels ◽  
The Sun ◽  
Lower Mass ◽  
Solar Type ◽  
Low Mass ◽  
Average Activity ◽  
The Impact

AbstractSolar simulations and observations showed that the detection of Earth twins around Sun-like stars is difficult in radial velocities with current methods techniques. The Sun has proved to be very useful to test processes, models, and analysis methods. The convective blueshift effect, dominating for the Sun, decreases towards lower mass stars, providing more suitable conditions to detect low mass planets. We describe the basic processes at work and how we extended a realistic solar model of radial velocity, photometry, astrometry and LogR′HK variability, using a coherent grid of stellar parameters covering a large range in mass and average activity levels. We present selected results concerning the impact of magnetic activity on Earth-mass planet detectability as a function of stellar type. We show how such realistic simulations can help characterizing the effect of stellar activity on RV and astrometric exoplanet detection.

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