scholarly journals The CARMENES search for exoplanets around M dwarfs

2020 ◽  
Vol 641 ◽  
pp. A69 ◽  
Author(s):  
D. Baroch ◽  
J. C. Morales ◽  
I. Ribas ◽  
E. Herrero ◽  
A. Rosich ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Convective Motion ◽  
Active Regions ◽  
Radial Velocities ◽  
M Dwarfs ◽  
Stellar Activity ◽  
Novel Approach ◽  
Convective Motions ◽  
Solar Type ◽  
The Impact

Context. Variability caused by stellar activity represents a challenge to the discovery and characterization of terrestrial exoplanets and complicates the interpretation of atmospheric planetary signals. Aims. We aim to use a detailed modeling tool to reproduce the effect of active regions on radial velocity measurements, which aids the identification of the key parameters that have an impact on the induced variability. Methods. We analyzed the effect of stellar activity on radial velocities as a function of wavelength by simulating the impact of the properties of spots, shifts induced by convective motions, and rotation. We focused our modeling effort on the active star YZ CMi (GJ 285), which was photometrically and spectroscopically monitored with CARMENES and the Telescopi Joan Oró. Results. We demonstrate that radial velocity curves at different wavelengths yield determinations of key properties of active regions, including spot-filling factor, temperature contrast, and location, thus solving the degeneracy between them. Most notably, our model is also sensitive to convective motions. Results indicate a reduced convective shift for M dwarfs when compared to solar-type stars (in agreement with theoretical extrapolations) and points to a small global convective redshift instead of blueshift. Conclusions. Using a novel approach based on simultaneous chromatic radial velocities and light curves, we can set strong constraints on stellar activity, including an elusive parameter such as the net convective motion effect.

scholarly journals The impact of unresolved magnetic spots on high-precision radial velocity measurements

2020 ◽  
Vol 497 (3) ◽  
pp. 4009-4021
Author(s):  
M Lisogorskyi ◽  
S Boro Saikia ◽  
S V Jeffers ◽  
H R A Jones ◽  
J Morin ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Radial Velocity ◽  
High Precision ◽  
Large Scale ◽  
Magnetic Activity ◽  
Velocity Measurements ◽  
Stellar Activity ◽  
Solar Type ◽  
Rotating Star ◽  
The Impact

ABSTRACT The Doppler method of exoplanet detection has been extremely successful, but suffers from contaminating noise from stellar activity. In this work, a model of a rotating star with a magnetic field based on the geometry of the K2 star ϵ Eridani is presented and used to estimate its effect on simulated radial velocity (RV) measurements. A number of different distributions of unresolved magnetic spots were simulated on top of the observed large-scale magnetic maps obtained from 8 yr of spectropolarimetric observations. The RV signals due to the magnetic spots have amplitudes of up to 10 m s−1, high enough to prevent the detection of planets under 20 Earth masses in temperate zones of solar-type stars. We show that the RV depends heavily on spot distribution. Our results emphasize that understanding stellar magnetic activity and spot distribution is crucial for the detection of Earth analogues.

scholarly journals Future Directions for Radial-Velocity Research

1999 ◽  
Vol 170 ◽  
pp. 401-409
Author(s):  
David W. Latham
Keyword(s):  
Radial Velocity ◽  
Extrasolar Planets ◽  
Space Missions ◽  
Galactic Structure ◽  
M Dwarfs ◽  
Future Directions ◽  
The Status ◽  
The Impact

AbstractI review the status of ground-based radial-velocity searches for extrasolar planets and speculate about the new results that can be expected in this field over the coming years. Then I review the plans for astrometric space missions and speculate about the impact that these missions will have on ground-based radial-velocity work, citing the specific examples of extra-solar planet research, the mass-luminosity relation for M dwarfs and metal-poor stars, and Galactic structure and evolution.

scholarly journals Dealing with activity in RV planet searches

2015 ◽  
Vol 11 (A29A) ◽  
pp. 193-195
Author(s):  
Isabelle Boisse
Keyword(s):  
Time Series ◽  
Radial Velocity ◽  
Radial Velocities ◽  
Velocity Measurements ◽  
Stellar Activity ◽  
The Future

AbstractPrecise radial velocity measurements of a star allow to search for planets. But this method has to face with irregularly time series. Stellar variabilities: pulsation, granulation, stellar activity on a short and long timescale, also modify the measure of the radial velocities. There is indeed a growing literature of controversies on how a signal is interpreted as a planet or due to stellar activity. I present how the star variations change the measured RVs, which techniques and indices are used by several teams to disentangle activity and planets, and the future options that are being studied.

scholarly journals Stellar activity with LAMOST. III. Temporal variability pattern in Pleiades, Praesepe, and Hyades

2020 ◽  
Vol 495 (3) ◽  
pp. 2949-2965
Author(s):  
Xiang-Song Fang ◽  
Christian Moni Bidin ◽  
Gang Zhao ◽  
Li-Yun Zhang ◽  
Yerra Bharat Kumar
Keyword(s):  
Active Regions ◽  
Temporal Variations ◽  
Open Clusters ◽  
M Dwarfs ◽  
Stellar Activity ◽  
Cool Star ◽  
Level Of Activity ◽  
Variation Patterns ◽  
Wide Range ◽  
Chromospheric Emission

ABSTRACT We present the results from a systematic study of temporal variation of stellar activity in young late-type stars. We used multi-epoch LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) low-resolution spectra of over 300 member candidates in three young open clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band strength near 7050 Å (TiO2) and equivalent width of H α line (EWH α) are used as the tracers of cool spot coverage and chromospheric emission strength, respectively. The analysis of time-variation patterns of these two tracers suggested that there exist detectable variabilities in TiO2 and EWH α, and their time-scales are in the wide range from days to years. Results showed that more active stars, younger and fast rotators, tend to have larger activity variations. There is a tendency of anticorrelation between temporal variations in TiO2 and EWH α. Also, appreciable anticorrelation in the rotational phase between H α emission and K2 brightness is detected in some M dwarfs, indicating spatial co-location of the plages with cool star-spots; however, cool stars do not always show such co-location features. Furthermore, spot coverage and H α emission were evident at all rotational phases of several M dwarfs, indicating a basal level of activity, perhaps due to many small and randomly located active regions in the atmosphere.

scholarly journals Measuring precise radial velocities on individual spectral lines

2020 ◽  
Vol 633 ◽  
pp. A76 ◽  
Author(s):  
M. Cretignier ◽  
X. Dumusque ◽  
R. Allart ◽  
F. Pepe ◽  
C. Lovis
Keyword(s):  
Pearson Correlation ◽  
Active Regions ◽  
Stellar Atmosphere ◽  
Spectral Lines ◽  
Stellar Activity ◽  
Solar Type ◽  
Different Depths ◽  
The Difference ◽  
New Generation ◽  
First Time

Context. Although the new generation of radial-velocity (RV) instruments such as ESPRESSO are expected to reach the long-term precision required to find other earths, the RV measurements are contaminated by some signal from stellar activity. This makes these detections hard. Aims. Based on real observations, we here demonstrate for the first time the effect of stellar activity on the RV of individual spectral lines. Recent studies have shown that this is probably the key for mitigating this perturbing signal. By measuring the line-by-line RV of each individual spectral line in the 2010 HARPS RV measurements of α Cen B, we study their sensitivity to telluric line contamination and line profile asymmetry. After selecting lines on which we are confident to measure a real Doppler-shift, we study the different effects of the RV signal that is induced by stellar activity on spectral lines based on their physical properties. Results. We estimate that at least 89% of the lines that appear in the spectrum of α Cen B for which we measure a reliable RV are correlated with the stellar activity signal (Pearson correlation coefficient R > 0.3 at 2σ). This can be interpreted as those lines being sensitive to the inhibition of the convective blueshift observed in active regions. Because the velocity of the convective blueshift increases with physical depth inside the stellar atmosphere, we find that the effect induced by stellar activity on the RV of individual spectral lines is inversely proportional to the line depth. The stellar activity signal can be mitigated down to ~0.8–0.9 m s−1 either by selecting lines that are less sensitive to activity or by using the difference between the RV of the spectral lines that are formed at different depths in the stellar atmosphere as an activity proxy. Conclusions. This paper shows for the first time that based on real observations of solar-type stars, it is possible to measure the RV effect of stellar activity on the RV of individual spectral lines. Our results are very promising and demonstrate that analysing the RV of individual spectral lines is probably one of the solutions to mitigate stellar activity signal in RV measurements down to a level enabling the detection of other earths.

scholarly journals RAVE-Gaia and the impact on Galactic archeology

2017 ◽  
Vol 12 (S330) ◽  
pp. 176-180
Author(s):  
Andrea Kunder
Keyword(s):  
Radial Velocity ◽  
Effective Temperature ◽  
Milky Way ◽  
Radial Velocities ◽  
Surface Gravity ◽  
Individual Stars ◽  
Dynamic Analyses ◽  
Data Release ◽  
The Impact ◽  
Temperature Surface

AbstractThe new data release (DR5) of the RAdial Velocity Experiment (RAVE) includes radial velocities of 520,781 spectra of 457,588 individual stars, of which 215,590 individual stars are released in the Tycho-Gaia astrometric solution (TGAS) in Gaia DR1. Therefore, RAVE contains the largest TGAS overlap of the recent and ongoing Milky Way spectroscopic surveys. Most of the RAVE stars also contain stellar parameters (effective temperature, surface gravity, overall metallicity), as well as individual abundances for Mg, Al, Si, Ca, Ti, Fe, and Ni. Combining RAVE with TGAS brings the uncertainties in space velocities down by a factor of 2 for stars in the RAVE volume – 10 km s−1 uncertainties in space velocities are now able to be derived for the majority (70%) of the RAVE-TGAS sample, providing a powerful platform for chemo-dynamic analyses of the Milky Way. Here we discuss the RAVE-TGAS impact on Galactic archaeology as well as how the Gaia parallaxes can be used to break degeneracies within the RAVE spectral regime for an even better return in the derivation of stellar parameters and abundances.

scholarly journals HD 38858: a solar-type star with an activity cycle of ∼10.8 yr

2018 ◽  
Vol 620 ◽  
pp. A34 ◽  
Author(s):  
M. Flores ◽  
J. F. González ◽  
M. Jaque Arancibia ◽  
C. Saffe ◽  
A. Buccino ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Activity Cycle ◽  
Earth Planet ◽  
Stellar Activity ◽  
Balmer Lines ◽  
Solar Type ◽  
Activity Cycles ◽  
Novel Strategy ◽  
Solar Type Star

Context. The detection of chromospheric activity cycles in solar-analogue and twin stars can be used to place the solar cycle in a wider context. However, relatively few of these stars with activity cycles have been detected. It is well known that the cores of the Ca II H&K lines are modulated by stellar activity. The behaviour of the Balmer and other optical lines with stellar activity is not yet completely understood. Aims. We search for variations in the Ca II H&K, Balmer, and Fe II lines modulated by stellar activity. In particular, we apply a novel strategy to detect possible shape variations in the Hα line. Methods. We analysed activity signatures in HD 38858 using HARPS and CASLEO spectra obtained between 2003 and 2017. We calculated the Mount Wilson index (SMW), log(R′HK), and the statistical moments of the Ca II H&K, Balmer, and other optical lines. We searched for periodicities using the generalized Lomb-Scargle periodogram. Results. We detect a long-term activity cycle of 10.8 yr in Ca II H&K and Hα in the solar-analogue star HD 38858. In contrast, this cycle is marginally detected in the Fe II lines. We also detect a noticeable variation in radial velocity that seems to be produced by stellar activity. Conclusions. HD 38858 is the second solar-analogue star where we find a clear activity cycle that is replicated in the Balmer lines. Spectral indexes based on the shape of Hα line seem to be more reliable than the fluxes in the same line for detecting activity variations. The cyclic modulation we detected gives place to a variation in radial velocity that previously has been associated with a super-Earth planet. Finally, due to the similarity of HD 38858 with the Sun, we recommend to continue monitoring this star.

scholarly journals Precision Radial Velocities in the Near Infrared with TEDI

2008 ◽  
Vol 4 (S253) ◽  
pp. 157-161 ◽  
Author(s):  
James P. Lloyd ◽  
Agnieszka Czeszumska ◽  
Jerry Edelstein ◽  
David Erskine ◽  
Michael Feuerstein ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Near Infrared ◽  
Radial Velocities ◽  
Velocity Measurements ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Habitable Zones ◽  
Low Mass ◽  
Transit Method

AbstractThe TEDI (TripleSpec - Exoplanet Discovery Instrument) is a dedicated instrument for the near-infrared radial velocity search for planetary companions to low-mass stars with the goal of achieving meters-per-second radial velocity precision. Heretofore, such planet searches have been limited almost entirely to the optical band and to stars that are bright in this band. Consequently, knowledge about planetary companions to the populous but visibly faint low-mass stars is limited. In addition to the opportunity afforded by precision radial velocity searches directly for planets around low mass stars, transits around the smallest M dwarfs offer a chance to detect the smallest possible planets in the habitable zones of the parent stars. As has been the the case with followup of planet candidates detected by the transit method requiring radial velocity confirmation, the capability to undertake efficient precision radial velocity measurements of mid-late M dwarfs will be required. TEDI has been commissioned on the Palomar 200” telescope in December 2007, and is currently in a science verification phase.

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

2020 ◽  
Vol 644 ◽  
pp. A77
Author(s):  
N. Meunier ◽  
A.-M. Lagrange ◽  
S. Borgniet
Keyword(s):  
Time Series ◽  
Radial Velocity ◽  
High Precision ◽  
False Positive ◽  
Main Sequence ◽  
Habitable Zone ◽  
Stellar Activity ◽  
Detection Rates ◽  
Solar Type ◽  
Synthetic Time Series

Context. Stellar activity strongly affects and may prevent the detection of Earth-mass planets in the habitable zone of solar-type stars with radial velocity technics. Astrometry is in principle less sensitive to stellar activity because the situation is more favourable: the stellar astrometric signal is expected to be fainter than the planetary astrometric signal compared to radial velocities. Aims. We quantify the effect of stellar activity on high-precision astrometry when Earth-mass planets are searched for in the habitable zone around old main-sequence solar-type stars. Methods. We used a very large set of magnetic activity synthetic time series to characterise the properties of the stellar astrometric signal. We then studied the detectability of exoplanets based on different approaches: first based on the theoretical level of false positives derived from the synthetic time series, and then with blind tests for old main-sequence F6-K4 stars. Results. The amplitude of the signal can be up to a few times the solar value depending on the assumptions made for activity level, spectral type, and spot contrast. The detection rates for 1 MEarth planets are very good, however, with extremely low false-positive rates in the habitable zone for stars in the F6-K4 range at 10 pc. The standard false-alarm probability using classical bootstrapping on the time series strongly overestimates the false-positive level. This affects the detection rates. Conclusions. We conclude that if technological challenges can be overcome and very high precision is reached, astrometry is much more suitable for detecting Earth-mass planets in the habitable zone around nearby solar-type stars than radial velocity, and detection rates are much higher for this range of planetary masses and periods when astrometric techniques are used than with radial velocity techniques.

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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