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

2019 ◽  
Vol 627 ◽  
pp. A56 ◽  
Author(s):  
N. Meunier ◽  
A.-M. Lagrange ◽  
T. Boulet ◽  
S. Borgniet
Keyword(s):  
Time Series ◽  
Radial Velocity ◽  
Current Knowledge ◽  
Active Regions ◽  
Large Set ◽  
Activity Levels ◽  
Target Values ◽  
Wide Range ◽  
Chromospheric Emission ◽  
Synthetic Time Series

Context. Solar simulations and observations show that the detection of long-period Earth-like planets is expected to be very difficult with radial velocity techniques in the solar case because of activity. The inhibition of the convective blueshift in active regions (which is then dominating the signal) is expected to decrease toward lower mass stars, which would provide more suitable conditions. Aims. In this paper we build synthetic time series to be able to precisely estimate the effects of activity on exoplanet detectability for stars with a wide range of spectral type (F6-K4) and activity levels (old main-sequence stars). Methods. We simulated a very large number of realistic time series of radial velocity, chromospheric emission, photometry, and astrometry. We built a coherent grid of stellar parameters that covers a wide range in the (B–V, Log R′HK) space based on our current knowledge of stellar activity, to be able to produce these time series. We describe the model and assumptions in detail. Results. We present first results on chromospheric emission. We find the average Log R′HK to correspond well to the target values that are expected from the model, and observe a strong effect of inclination on the average Log R′HK (over time) and its long-term amplitude. Conclusions. This very large set of synthetic time series offers many possibilities for future analysis, for example, for the parameter effect, correction method, and detection limits of exoplanets.

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

2019 ◽  
Vol 632 ◽  
pp. A81 ◽  
Author(s):  
N. Meunier ◽  
A.-M. Lagrange ◽  
S. Cuzacq
Keyword(s):  
Time Series ◽  
Temporal Distribution ◽  
Active Regions ◽  
Activity Level ◽  
Large Set ◽  
Detection Rates ◽  
Chromospheric Emission ◽  
Spatio Temporal ◽  
The Difference ◽  
Geometrical Effects

Context. Inhibition of the convective blueshift in active regions is a major contribution to the radial velocity (RV) variations, at least for solar-like stars. A common technique to correct for this component is to model the RV as a linear function of chromospheric emission, because both are strongly correlated with the coverage by plages. Aims. This correction, although efficient, is not perfect: the aim of the present study is to understand the limits of this correction and to improve it. Methods. We investigate these questions by analysing a large set of synthetic time series corresponding to old main sequence F6-K4 stars modelled using a consistent set of parameters. We focus here on the analysis of the correlation between time series, in particular between RV (variability due to different processes) and chromospheric emission on different timescales. We also study the temporal variation for each time series. Results. We find that inclination strongly impacts these correlations, as well as the presence of additional signals (in particular granulation and supergranulation). Although RV and log R′HK are often well correlated, a combination of geometrical effects (butterfly diagrams related to dynamo processes and inclination) and activity level variations over time create an hysteresis pattern during the cycle, which produces a departure from an excellent correlation: for a given activity level, the RV is higher or lower during the ascending phase compared to the descending phase of the cycle depending on inclination, with a reversal for inclinations about 60° from pole-on. We find that this hysteresis is also observed for the Sun, as well as for other stars. This property is due to the spatio-temporal distribution of the activity pattern (and therefore to the dynamo processes) and to the difference in projection effects of the RV and chromospheric emission. Conclusions. These results allow us to propose a new method which significantly improves the correction for long timescales (fraction of the cycle), and could be crucial to improving detection rates of planets in the habitable zone around F6-K4 stars.

scholarly journals How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations

2020 ◽  
Vol 639 ◽  
pp. A139 ◽  
Author(s):  
K. L. Yeo ◽  
S. K. Solanki ◽  
N. A. Krivova
Keyword(s):  
Magnetic Flux ◽  
Active Regions ◽  
Total Solar Irradiance ◽  
Area Ratio ◽  
Sunspot Area ◽  
Activity Levels ◽  
Network Coverage ◽  
Chromospheric Emission ◽  
Activity Dependence ◽  
The Relationship

Context. How global faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars. Aims. We aim to extend and improve on earlier studies that established that the facular-to-sunspot-area ratio diminishes with total sunspot coverage. Methods. Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as “facular indices”, are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model, taking into account how active regions evolve and the possible non-linear relationship between plage emission, facular magnetic flux, and sunspot area. This model was incorporated into a model of total solar irradiance (TSI) to elucidate the implications for solar and stellar brightness variations. Results. The reconstruction of the facular indices from the sunspot indices with the model presented here replicates most of the observed variability, and is better at doing so than earlier models. Contrary to recent studies, we found the relationship between the facular and sunspot indices to be stable over the past four decades. The model indicates that, like the facular-to-sunspot-area ratio, the ratio of the variation in chromospheric emission and total network and facular magnetic flux to sunspot area decreases with the latter. The TSI model indicates the ratio of the TSI excess from faculae and network to the deficit from sunspots also declines with sunspot area, with the consequence being that TSI rises with sunspot area more slowly than if the two quantities were linearly proportional to one another. This explains why even though solar cycle 23 is significantly weaker than cycle 22, TSI rose to comparable levels over both cycles. The extrapolation of the TSI model to higher activity levels indicates that in the activity range where Sun-like stars are observed to switch from growing brighter with increasing activity to becoming dimmer instead, the activity-dependence of TSI exhibits a similar transition. This happens as sunspot darkening starts to rise more rapidly with activity than facular and network brightening. This bolsters the interpretation of this behaviour of Sun-like stars as the transition from a faculae-dominated to a spot-dominated regime.

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

2019 ◽  
Vol 629 ◽  
pp. A42 ◽  
Author(s):  
N. Meunier ◽  
A.-M. Lagrange
Keyword(s):  
Time Series ◽  
Main Sequence ◽  
Stellar Disk ◽  
Physical Parameters ◽  
Activity Levels ◽  
Main Sequence Stars ◽  
Low Mass Stars ◽  
Photometric Variability ◽  
Chromospheric Emission ◽  
Brightness Variability

Context. A number of high-precision time series have recently become available for many stars as a result of data from CoRoT, Kepler, and TESS. These data have been widely used to study stellar activity. Photometry provides information that is integrated over the stellar disk. Therefore, there are many degeneracies between spots and plages or sizes and contrasts. In addition, it is important to relate activity indicators, derived from photometric light curves, to other indicators (Log R′HK and radial velocities). Aims. Our aim is to understand how to relate photometric variability to physical parameters in order to help the interpretation of these observations. Methods. We used a large number of synthetic time series of brightness variations for old main sequence stars within the F6-K4 range. Simultaneously, we computed using consistent modeling for radial velocity, astrometry, and chromospheric emission. We analyzed these time series to study the effect of the star spectral type on brightness variability, the relationship between brightness variability and chromospheric emission, and the interpretation of brightness variability as a function of spot and plage properties. We then studied spot-dominated or plage-dominated regimes. Results. We find that within our range of activity levels, the brightness variability increases toward low-mass stars, as suggested by Kepler results. However, many elements can create an interpretation bias. Brightness variability roughly correlates to Log R′HK level. There is, however, a large dispersion in this relationship, mostly caused by spot contrast and inclination. It is also directly related to the number of structures, and we show that it can not be interpreted solely in terms of spot sizes. Finally, a detailed analysis of its relation with Log R′HK shows that in the activity range of old main-sequence stars, we can obtain both spot or plage dominated regimes, as was shown by observations in previous works. The same star can also be observed in both regimes depending on inclination. Furthermore, only strong correlations between chromospheric emission and brightness variability are significant. Conclusions. Our realistic time series proves to be extremely useful when interpreting observations and understanding their limitations, most notably in terms of activity interpretation. Inclination is crucial and affects many properties, such as amplitudes and the respective role of spots and plages.

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 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.

Attitudes and the Environment as Determinants of Active Travel in Adults: What Do and Don’t We Know?

2010 ◽  
Vol 7 (4) ◽  
pp. 551-561 ◽  
Author(s):  
Jenna Rachel Panter ◽  
Andy Jones
Keyword(s):  
Travel Behavior ◽  
Population Studies ◽  
Current Knowledge ◽  
Adult Population ◽  
Active Travel ◽  
Activity Levels ◽  
Environmental Correlates ◽  
Quantitative Studies ◽  
Psychological Correlates ◽  
Wide Range

Background:Walking and cycling for transport, or ‘active travel,’ has the potential to contribute to overall physical activity levels. However, a wide range of factors are hypothesized to be associated with adult’s active travel behavior. This paper describes current knowledge of the psychological and environmental determinants of active travel in adults, and considers ways in which the 2 domains can be better integrated.Methods:Quantitative studies were reviewed which examined psychological and environmental influences on active travel in an adult population. Studies were classified according to whether they examined psychological, environmental or both types of factor.Results:Fourteen studies were identified which examined psychological correlates of active travel behavior in adults, and 36 which examined environmental correlates. Seven studies were identified which considered both domains, of which only 2 of explored the interactions between personal, social and environmental factors. The majority of the evidence is helpful in identifying correlates rather than determinants of active travel behavior.Conclusions:To further our understanding of the influences of active travel, there is a need for more research which integrates both individual and environmental domains and examines how they interact.

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

2019 ◽  
Vol 628 ◽  
pp. A125 ◽  
Author(s):  
N. Meunier ◽  
A.-M. Lagrange
Keyword(s):  
Time Series ◽  
Radial Velocity ◽  
Spectral Type ◽  
Rotation Period ◽  
Activity Level ◽  
Cycle Period ◽  
Large Set ◽  
Data Set ◽  
Stellar Activity ◽  
Analysis Techniques

Context. The effect of stellar activity on radial velocity (RV) measurements appears to be a limiting factor in detecting Earth-mass planets in the habitable zone of a star that is similar to the Sun in spectral type and activity level. It is crucial to estimate whether this conclusion remain true for other stars with current correction methods. Aims. We built realistic time series in radial velocity and chromospheric emission for old main-sequence F6-K4 stars. We studied the effect of the stellar parameters we investigate on exoplanet detectability. The stellar parameters are spectral type, activity level, rotation period, cycle period and amplitude, latitude coverage, and spot constrast, which we chose to be in ranges that are compatible with our current knowledge of stellar activity. Methods. This very large set of synthetic time series allowed us to study the effect of the parameters on the RV jitter and how the different contributions to the RV are affected in this first analysis of the data set. The RV jitter was used to provide a first-order detection limit for each time series and different temporal samplings. Results. We find that the coverage in latitude of the activity pattern and the cycle amplitudes have a strong effect on the RV jitter, as has stellar inclination. RV jitter trends with B–V and Log R′HK are similar to observations, but activity cannot be responsible for RV jitter larger than 2–3 m s−1 for very quiet stars: this observed jitter is therefore likely to be due to other causes (instrumental noise or stellar or planetary companions, e.g.). Finally, we show that based on the RV jitter that is associated with each time series and using a simple criterion, a planet with one Earth mass and a period of one to two years probably cannot be detected with current analysis techniques, except for the lower mass stars in our sample, but very many observations would be required. The effect of inclination is critical. Conclusions. The results are very important in the context of future RV follow-ups of transit detections of such planets. We conclude that a significant improvement of analysis techniques and/or observing strategies must be made to reach such low detection limits.

scholarly journals Spatiotemporal development of the 2018–2019 groundwater drought in the Netherlands: a data-based approach

2021 ◽  
Author(s):  
Esther Brakkee ◽  
Marjolein van Huijgevoort ◽  
Ruud P. Bartholomeus
Keyword(s):  
Time Series ◽  
Preparation Method ◽  
Monitoring Data ◽  
Temperate Climate ◽  
Large Set ◽  
Data Preparation ◽  
Time Series Modelling ◽  
Groundwater Drought ◽  
Wide Range ◽  
Regional Groundwater

Abstract. The 2018–2019 drought in northwestern Europe caused severe damage to a wide range of sectors, and has made clear that even in temperate-climate countries adaptations are needed to cope with increasing future drought frequencies. A crucial component of drought management strategies is to monitor the status of groundwater resources. However, providing up-to-date assessments of regional groundwater drought development remains challenging due to the limited quality of available data. This limits many studies to small selections of groundwater monitoring sites, giving an incomplete image of drought dynamics. In this study, a time series modelling-based method for data preparation was developed and applied to map the spatiotemporal development of the 2018–2019 groundwater drought in the southeastern Netherlands, based on a large set of monitoring data. The data preparation method was evaluated for its usefulness and reliability for groundwater drought quantification and prediction. The analysis showed that the 2018–2019 meteorological drought caused extreme groundwater drought throughout the southeastern Netherlands, breaking 30-year records almost everywhere. Drought onset and duration were strongly variable in space, with especially higher elevated areas remaining in severe drought well into 2020. Groundwater drought development appeared to be governed dominantly by the spatial distribution of rainfall and the geological-topographic setting. The time series modelling-based data preparation method was found a useful tool to enable a detailed, consistent record of regional groundwater drought development. Applying a validation step before analysis turned out to be important for good results. The time series simulations were generally found to be reliable; however, the use of time series simulations rather than direct measurement series can bias drought estimations especially at a local scale, and underestimate spatial variability. Finally, time series modelling showed to be a promising tool for regional-scale drought nowcasting and prediction. Further development of time-series based validation and simulation methods, combined with accessible and consistent monitoring data, will be valuable to enable better groundwater drought monitoring in the future.

scholarly journals Solar chromospheric emission and magnetic structures from plages to intranetwork: Contribution of the very quiet Sun

2018 ◽  
Vol 615 ◽  
pp. A87 ◽  
Author(s):  
N. Meunier
Keyword(s):  
Time Series ◽  
Solar Cycle ◽  
Radial Velocity ◽  
Activity Level ◽  
The Sun ◽  
Stellar Activity ◽  
Quiet Sun ◽  
Magnetic Structures ◽  
Chromospheric Emission ◽  
The Impact

Context. We need to establish a correspondence between the magnetic structures generated by models and usual stellar activity indexes to simulate radial velocity time series for stars less active than the Sun. This is necessary to compare the outputs of such models with observed radial velocity jitters and is critical to better understand the impact of stellar activity on exoplanet detectability. Aims. We propose a coherent picture to describe the relationship between magnetic activity, including the so-called quiet Sun regions, and the chromospheric emission using the Sun as a test-bench and a reference. Methods. We analyzed a long time series of Michelson Doppler imaging (MDI) magnetograms jointly with chromospheric emission time series obtained at Sacramento Peak and Kitt Peak observatories. This has allowed us to study the variability in the quiet Sun over the solar cycle, and then, based on available relations between magnetic fields in active structures and chromospheric emission, to propose an empirical reconstruction of the solar chromospheric emission based on all contributions. Results. We show that the magnetic flux covering the solar surface, including in the quieted regions, varies in phase with the solar cycle, suggesting a long-term relationship between the global dynamo and the contribution of all components of solar activity. We have been able to propose a reconstruction of the solar S-index, including a relationship between the weak field component and its chomospheric emission, which is in good agreement with the literature. This allows us to explain that stars with a low average chromospheric emission level exhibit a low variability. Conclusions. We conclude that weak flux regions significantly contribute to the chromospheric emission; these regions should be critical in explaining the lower variability associated with the lower average activity level in other stars as compared to the Sun and estimated from their chromospheric emission.

scholarly journals Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series

2021 ◽  
Vol 13 (12) ◽  
pp. 2250
Author(s):  
Fabrizio Niro
Keyword(s):  
Time Series ◽  
Vegetation Index ◽  
Normalized Difference Vegetation Index ◽  
Large Set ◽  
Temporal Consistency ◽  
Wide Range ◽  
Data Record ◽  
Multi Temporal ◽  
Orbital Drift ◽  
The Impact

Multi-temporal consistency of space-borne observations is an essential requirement for studying inter-annual changes and trends of satellite-derived biophysical products. The Proba-V mission, launched in 2013, was designed to ensure the continuity of the SPOT-VEGETATION long-term data record of global daily observations for land applications. The suitability of Proba-V to provide a temporally consistent data record is, however, potentially jeopardized by the orbital drift effect, which is known to induce spurious trends in time series. The aim of this paper is therefore to evaluate, for the first time, the orbital drift effect on Proba-V surface reflectance time series at 1 km resolution. In order to reliably identify such an effect, a two-fold approach is adopted. A simulation study is first defined to predict the temporal anomalies induced by the drifting illumination conditions. The numerical simulations are used as a benchmark to predict the impact of the drift for a range of sun-viewing angles. Real observations are then analyzed over a large set of land sites, globally spread and spanning a wide range of surface and environmental conditions. The surface anisotropy is characterized using the Ross-Thick Li-Sparse Reciprocal (RTLSR) Bidirectional Reflectance Distribution Function (BRDF) model. Both the simulation and the analysis of real observations consistently show that the orbital drift induces distinct and opposite trends in the two sides of the sensor across-track swath. Particularly, a positive drift is estimated in backward and a negative one in the forward scattering direction. When observations from all angular conditions are retained, these opposite trends largely compensate, with no remaining statistically significant drifts in time series of surface reflectances or Normalized Difference Vegetation Index (NDVI). As such, the Proba-V archive at 1 km resolution can be reliably used for inter-annual vegetation studies.

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