Top PDF Stellar cycles from photometric data: CoRoT stars

Stellar cycles from photometric data: CoRoT stars

Stellar cycles from photometric data: CoRoT stars

Dynamo action occurs in the inner stellar layers, but its ef- fects are clearly observed in and above the photosphere, mostly through the magnetic activity cycles. Since the pioneering works by Wilson ( 1978 ) and Baliunas & Vaughan ( 1985 ), which pro- duced the first large sample of magnetic cycles and rotation in- formation validating stellar dynamo models, an increasing num- ber of studies have been dedicated to determining the empirical relations between the rotation period P rot , spectral type, cycle lengths, and other stellar parameters in an attempt to understand the mechanisms of stellar dynamos (e.g., Noyes et al. 1984 ; Saar & Baliunas 1992 ; Baliunas & Soon 1995 ; Brandenburg et al. 1998 ; Saar & Brandenburg 1999 ; Lorente & Montesinos 2005 ; Böhm-Vitense 2007 ). From these studies we know now that late- type stars exhibit several chromospheric variability behaviors, ranging from regular variations with multiyear periods, similar to the solar cycles, to irregular cycles with no clear pattern and to stars with no long-term variability ( Baliunas & Soon 1995 ). In addition, other studies found shown that short stellar mag- netic activity cycles, ranging from one to three years ( Fares et al. 2009 ; Metcalfe et al. 2010 ; Sanz-Forcada et al. 2013 ). Many au- thors have focused on the magnetic cycle properties of the Sun; based on sunspot behavior (see Hathaway 2010 , for a compre- hensive review). Generally, the temporal behavior of sunspots is  The CoRoT space mission was developed and is operated by the
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The variability behaviour of CoRoT M-giant stars

The variability behaviour of CoRoT M-giant stars

A total of 434 stars were found with previously determined periods ( Lebzelter 2011 ; De Medeiros et al. 2013 ). For approx- imately 96% of the targets, the differences between our periods and those periods of the other authors are less than 10%. We at- tribute the differences to four main reasons: first, the pipeline improvements made by the CoRoT team now result in better data for analysis; second, the combination of all observations provides a wider time window and thus more precise periods; third, detrending of long-term variations may distort a small por- tion of the LCs; and fourth, the first harmonic may be falsely identified as the main frequency when there are too few cycles, as explained in De Medeiros et al. ( 2013 ). The precision of the analysis of the nature of the variability, period, and amplitude in- creases with the number of cycles (the ratio of total time span to the variability period), as discussed in De Medeiros et al. ( 2013 ). According to those authors, the variability periods with more than three cycles for the CoRoT LCs, have a confidence level greater than 80%. The photometric instrumental jumps in the CoRoT data also hinder any analysis of potential long-period variability.
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New suns in the cosmos. IV. The multifractal nature of stellar magnetic activity in kepler cool stars

New suns in the cosmos. IV. The multifractal nature of stellar magnetic activity in kepler cool stars

average standard deviation of the subseries de fined by k- rotation periods (hereafter S á ph, k ñ), where k=5 is used. The values of S á ph, k ñ extracted from Mathur et al. ( 2014b ) are reported in Table 1 . This procedure divides a time series into macroscopic scales of order or higher than the rotational period. According to the authors, the standard deviation is a good indicator of the global magnetic activity based on photometric modulation and is consequently a classi fier of the stellar activity cycle. Speci fically, the standard deviation as a function of factor k is only a useful measure to quantify the amount of variation or dispersion in a statistical data set (Feigelson & Jogesh Babu 2012 ). However, the long-cadence data used here
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Wavelets: a powerful tool for studying rotation, activity, and pulsation in Kepler and CoRoT stellar light curves

Wavelets: a powerful tool for studying rotation, activity, and pulsation in Kepler and CoRoT stellar light curves

The CoRoT ( Baglin et al. 2009 ) and Kepler ( Borucki et al. 2010 ) space missions produced unique sets of light curves for about 300 000 stars, with excellent time sampling and unprece- dented photometric precision. These data, in addition to the ma- jor scientific goals of the missions (asteroseismology and the search for exoplanets) open new perspectives for studying differ- ent stellar properties, including rotation, magnetic activity, and binarity. For extracting information from raw signals, several mathematical transformations can be applied, such as Laplace transform ( Widder 1945 ), Z-transform ( Jury 1964 ), Wigner dis- tributions ( Boashash 1988 ), and Fourier transform ( Bochner & Chandrasekharan 1949 ), the last being the most widely used. The wavelet transform ( Torrence & Compo 1998 ) is a more re- cent tool applied for treating a large number of phenomena in different areas, including geophysics, atmospheric turbulence, health (cardiology), and astrophysics. This transformation has a major advantage, since it allows analysis of frequency varia- tions in time of a given signal. Analogous to sunspots and solar photospheric faculae, whose visibility is modulated by stellar ro- tation, stellar active regions consist of cool spots and bright fac- ulae caused by the magnetic field of the star. Such starspots are well established as tracers of rotation, but their dynamic behav- ior may also be used to analyze other relevant phenomena, such as magnetic activity and cycles (e.g., Mathur et al. 2014 ; García et al. 2009 ; Willson & Mordvinov 1999 ).
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Interplay of tidal evolution and stellar wind braking in the rotation of stars hosting massive close-in planets

Interplay of tidal evolution and stellar wind braking in the rotation of stars hosting massive close-in planets

The main studied cases were some systems with a central G star and age larger than some Gyr. In these cases we could reconstruct the possible past evolution of the orbital elements and the rotation of the star. We have shown that, independently of the initial rotation of the star, these solutions converge to a situation of equilibrium between the opposite tendencies imposed on the star ’s rotation by the magnetic wind braking and by the tidal torques due to the companion. The sample also included some systems with a central F star and ages larger than 1 Gyr. It is symptomatic that, in these cases, the rotation period does not differ signi ficantly from the orbital period of the companion. This has been interpreted as resulting from the absence of braking. In the absence of braking, the classical tide theories predict that the rotation period of the star evolves toward a stationary rotation slightly faster than the orbital motion. Finally, we have considered some younger systems and discussed the possibility of determining their ages from the simulations of the past evolution. In two cases, CoRoT-2 and CoRoT-18, the evolutionary paths through the current data converge to a fast rotation at some time in the past, thus giving an indication of the age of the star.
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Distribution of refractory and volatile elements in CoRoT exoplanet host stars

Distribution of refractory and volatile elements in CoRoT exoplanet host stars

The relative distribution of abundances of refractory, intermediate, and volatile elements in stars with planets can be an important tool for investigating the internal migration of a giant planet. This migration can lead to the accretion of planetesimals and the selective enrichment of the star with these elements. We report on a spectroscopic determination of the atmospheric parameters and chemical abundances of the parent stars in transiting planets CoRoT-2b and CoRoT-4b. Adding data for CoRoT-3 and CoRoT-5 from the literature, we find a flat distribution of the relative abundances as a function of their condensation temperatures. For CoRoT-2, the relatively high lithium abundance and intensity of its Li  resonance line permit us to propose an age of 120 Myr, making this star one of the youngest stars with planets to date. We introduce a new methodology to investigate a relation between the abundances of these stars and the internal migration of their planets. By simulating the internal migration of a planet in a disk formed only by planetesimals, we are able, for the first time, to separate the stellar fractions of refractory, intermediate, and volatile rich planetesimals accreting onto the central star. Intermediate and volatile element fractions enriching the star are similar and much larger than those of pure refractory ones. This result is opposite to what has been considered in the literature for the accreting self-enrichment processes of stars with planets. We also show that these results are highly dependent on the model adopted for the disk distribution regions in terms of refractory, intermediate, and also volatile elements and other parameters considered. We note however, that this self-enrichment mechanism is only efficient during the first 20–30 Myr or later in the lifetime of the disk when the surface convection layers of the central star for the first time attain its minimum size configuration.
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Overview of semi-sinusoidal stellar variability with the CoRoT satellite

Overview of semi-sinusoidal stellar variability with the CoRoT satellite

The color–period diagrams of the sample indicate several aspects compatible with rotational modulation. The increase in variability amplitude around (J − H)  0.55 corroborates stud- ies on rotating variable stars by Eker et al. ( 2008 ) and Gilliland et al. ( 2009 ). The overall behavior of the increasing period with rising color index is compatible with theoretical predictions of stellar rotation. Results from this investigation were compared with public data for field variables by Hartman et al. ( 2010 ). The distribution periods and variability amplitudes reported here are compatible with data in the corresponding color range. In addi- tion to our overall results, we identified a subset of three Sun- like candidates in the context of photometric period and color, which may be of particular interest for future studies. Moreover, we analyzed a subsample of more than 400 M-type giant stars, whose behavior seems compatible with recent studies of rota- tional modulation. In addition, the distribution of variability pe- riods for the CoRoT targets tends to be different when compared with Galactic center and anti-center directions. Finally, the be- havior of the variability period distribution in the period–color diagram appears to substantially depend on reddening correc- tion, which may significantly affect age–period analyses such as that carried out in Affer et al. ( 2012 ).
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Optimized cutting off transit algorithm to study stellar rotation from PLATO mission light curves

Optimized cutting off transit algorithm to study stellar rotation from PLATO mission light curves

We presented an optimized cutting off transit algorithm to study stellar rotation from space missions (CoRoT, Kepler, TESS and PLATO). We also presented some exper- iments to show method performance. The DT algorithm was applied in well-know CoRoT, Kepler, Kepler K2, and TESS photometric observed light curves as well as in a synthetic generated light curves expected to PLATO mission [ 29 ]. We have shown that when we precisely identify the rotation modulation and eliminate it from the light-curve, we better constrain the primary and secondary transit depths and better visually analyze the aliases on the Lomb-Scargle periodogram (see the examples of CoRoT 10268127 and TIC 34948050). From our analysis, we find good agreement with published measurements for the orbital period of four selected systems. We report likely rotational periods modulation for four eclipsing binary systems, where only EPIC 201408204 has been already analyzed by [ 23 ]. Measure rotation period for eclipsing binary is a relevant task for stellar evolution studies. These stars are fundamental targets to be used as benchmark objects, and allow an unprecedented accuracy in masses and radii of about 2%. Our rotation period measurement for EPIC 201408204 agrees with [ 23 ] determination. We validated our algorithm by applying a blind test to controlled synthetic light curves with known parameters. For PLATO, we generated light curves with spots modulation physics as used in ellc and system- atic noise as described by [ 31 ]. Our code is easy to be installed and is ready to be used in any machine with Python3.x and with standard libraries. It is an accessible tool to remove binary and planetary transit signals from low cadence observations without detrending the entire light curve.
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Stellar activity in high-precision photometric and spectroscopic transit observations

Stellar activity in high-precision photometric and spectroscopic transit observations

We defined the minimum size of a stellar spot as the limit for which the corresponding anomaly in the transit light-curve can- not be identified visually and will only be detectable by its e ffect on transit-timing measurements. We considered a system with a transiting planet with a size of R p /R ∗ = 0.05, 0.1, and 0.15 in a three-day orbit around a late-type spotted star. We assumed that the orbit of the planet is edge-on and circular. We assigned values of u1 = 0.29 and u2 = 0.34 to the star quadratic limb- darkening coefficients, and considered its rotational period to be nine days. We placed a dark stellar spot with a zero-brightness and filling factor of 1% on the longitude, which corresponds to 0.7 stellar radius in the time of overlap of planet and spot. To make the simulation closer to the real observation, we added a random Gaussian noise to each data point of the simulated light curve. The standard deviation of the Gaussian was chosen as the best standard deviation of a Kepler short-cadence obser- vation for one of its brightest target stars ( σ = 0.00017). We allowed the transit timing to vary as a free parameter and fit to this system a synthetic transit light-curve that was obtained without considering the effect of the overlap between the planet and the spot. The TTV values were obtained from the best-fit model. We note that we only used the TTVs generated by the spot anomaly in the transit light-curve as an indicator of the ex- istence of a spot. Since a random noise was added to the simula- tion, we repeated this process 100 times and obtained the mean value and standard deviations (shown as error bars in Fig. 8 ) of the TTVs. We then reduced the filling factor of the spot and determined the TTV value. Figure 8 shows the behavior of the highest TTV value as a function of spot filling factor.
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The connection between stellar activity cycles and magnetic field topology

The connection between stellar activity cycles and magnetic field topology

value with a star symbol in Fig. 2 (c) and see that it roughly co- incides with the inactive branch. It seems that the magnetic cycle period determined from ZDI for HD 78366 may be characteristic of the inactive branch while the chromospherically determined pe- riod is characteristic of the active branch. If this is true, then one would expect chromospheric observations with a time sampling of sufficient density to find an additional chromospheric cycle period of roughly 1.5 yr for HD 78366. The data for τ Boo, which has a similar spectral type to HD 78366, would also seem to favour such an interpretation. Just like HD 78366, τ Boo also has a long chromospheric cycle (11.6 yr; Baliunas et al. 1995 ) and a short magnetic cycle (2 yr or 8 months). However, in this case a shorter chromospheric cycle that is associated with the magnetic cycle has also been detected (116 d; Baliunas et al. 1997 ; Mengel et al. 2016 ). Similarly to HD 78366, we predict a cycle length for τ Boo from the two most likely time-scales (2 yr or 8 months) for the magnetic polarity flips and plot these with stars on Fig. 2 (c). If this scenario is true, HD 78366 finds itself in a curious position of having three cycle periods [two chromospherically determined cycles (Baliunas et al. 1995 ) and a short magnetic cycle (Morgenthaler et al. 2011 )] that cannot be explained by two dynamo modes as B¨ohm-Vitense ( 2007 ) suggests. We do note that the shorter chromospherically de- termined cycle period is only assigned a FAP of ‘fair’ by Baliunas et al. ( 1995 ). Additional, we also note that Baliunas et al. ( 1995 ) assigned a FAP of ‘poor’ to the 11.6 yr chromospheric cycle period that they determined for τ Boo. Correspondingly, the discussion in this section should be treated with caution.
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The variability behaviour of CoRoT M-giant stars

The variability behaviour of CoRoT M-giant stars

A total of 434 stars were found with previously determined periods ( Lebzelter 2011 ; De Medeiros et al. 2013 ). For approx- imately 96% of the targets, the differences between our periods and those periods of the other authors are less than 10%. We at- tribute the differences to four main reasons: first, the pipeline improvements made by the CoRoT team now result in better data for analysis; second, the combination of all observations provides a wider time window and thus more precise periods; third, detrending of long-term variations may distort a small por- tion of the LCs; and fourth, the first harmonic may be falsely identified as the main frequency when there are too few cycles, as explained in De Medeiros et al. ( 2013 ). The precision of the analysis of the nature of the variability, period, and amplitude in- creases with the number of cycles (the ratio of total time span to the variability period), as discussed in De Medeiros et al. ( 2013 ). According to those authors, the variability periods with more than three cycles for the CoRoT LCs, have a confidence level greater than 80%. The photometric instrumental jumps in the CoRoT data also hinder any analysis of potential long-period variability.
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First climatology of polar mesospheric clouds from GOMOS/ENVISAT stellar occultation instrument

First climatology of polar mesospheric clouds from GOMOS/ENVISAT stellar occultation instrument

This work yields a very rich PMC data set derived from the analysis of GOMOS pho- tometers global observations. The technique of stellar occultation allows a very accu- rate altitude retrieval and a very good geographical and temporal coverage. For the moment 8 PMC seasons have been studied, from 2002 to 2006 in both hemispheres. A total of approximately 10 000 mesospheric clouds were detected all over these four

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Reconstructing triangulated surfaces from unorganized points through local skeletal stars

Reconstructing triangulated surfaces from unorganized points through local skeletal stars

• Mesh partitioning into patches homeomorphic to disks. These patches are also known as charts. The surface mesh partitioning becomes mandatory when the surface is closed or has genus greater than zero. Roughly speaking, there are two ways of cutting surfaces into charts: segmentation techniques and seam generation techniques. For parametrisation purposes, segmentation techniques divide the surface into several charts in order to keep as short as possible the parametric distortion resulting from the cuts. Unlike segmentation, seam cutting techniques are capable of reducing the parametric distortion without cutting the surface into separate patches. For that purpose, they use seams (or partial cuts) to reduce a surface of genus greater than zero to a surface of genus zero. For more details about surface mesh partitioning, the reader is referred to Sheffer et al. [SPR06] and the references therein.
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Towards a Comprehensive Understanding of tiny stars in the near-infrared domain -  Determining stellar parameters of FGK and M dwarfs from their APOGEE spectra using the spectral synthesis method

Towards a Comprehensive Understanding of tiny stars in the near-infrared domain -  Determining stellar parameters of FGK and M dwarfs from their APOGEE spectra using the spectral synthesis method

Cuaresma (2019), where the codes mentioned above were compared for both the EW method and synthetic spectral fitting techniques. These discrepancies can be minimized by selecting the best spectral regions to analyze and having a uniform method for the analysis, which might not be possible for all scientific cases. Blanco-Cuaresma (2019) also concluded that iSpec parameters derived using these two methods have large differences that are a result of the intrinsic differences between the methods. Therefore, it is important to know and test the qual- ities and limitations of each available code before running a spectral synthesis analysis with iSpec. iSpec also includes multiple stellar models available for the user, such as the ATLAS and MARCS models described in section 3.2.2. Also, multiple solar abundances and line lists are pre-programmed and available for use. This allows for extensive customization of the syn- thesis and gives the user very high degrees of freedom to optimize their pipeline to obtain the best parameters from their spectra. However, and as shown in the referenced article, careful analysis is needed to confirm the best combination of code, stellar model and line list for each scientific case. These tests and comparisons for our sample stars and spectral region are shown in section 5.2.2.
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Provable data possession for securing the data from untrusted server

Provable data possession for securing the data from untrusted server

The model described for the use of Provable data Possession which allow the client to access the stored data at an Untrusted server that the server possesses the original data without retrieving it. This model executes the probabilistic proof of possession by random set of blocks which is derived from the server that dramatically reduces the cost of I/O. Sometimes the Client maintenance the constant amount of data which is used to verify the proof. The response protocol can transmit a small amount of data, which can minimize network communication. The two provably –Securer PDP Schemes presents more efficient schemes than previous solution .Even when compared with schemes that achieve weaker guarantees. It is the widely distributed storage systems. Using the experiment we can implement and verify the practicality of PDP and we can revel that the performance of the PDP that is bounded by disk I/O and that cannot be determined by computation.
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Stellar and planetary Cassini states

Stellar and planetary Cassini states

Colombo ( 1966 ) has shown that the second and third laws are independent of the first one, in the sense that even if the ro- tation rate is not synchronous, the second and third laws can still be satisfied since they correspond to the minimum dissipation of energy for the spin axis. For a non-synchronous Moon, only the angle between its equator and the ecliptic would change. Indeed, while the first law requires an triaxial ellipsoid to work, the two other laws only require an oblate spheroid. Moreover, Colombo ( 1966 ) generalised his theory to any satellite or planet whose nodal line on the invariant plane shifts because of perturbations, which can have a di fferent origin, such as the oblateness of the central body, perturbations from a third body, or both. Peale ( 1969 ) further generalised second and third laws to include the e ffects of an axial asymmetry and rotation rates commensurable with the orbital mean motion.
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Kepler rapidly rotating giant stars

Kepler rapidly rotating giant stars

By combining WISE and 2MASS IR information, we shown that a fraction of the reported Kepler rapidly rotating giants present a tendency toward a mid-IR excess. One interesting aspect in the present analysis emerges from the comparison between the corresponding IR behavior for these Kepler giants and main-sequence planet-hosting stars showing dust excess emission, based on the same IR excess diagnostic. The presence of two distinct IR excess levels is clearly observed for the rapidly rotating giants and stars with planets, with the mid-IR excess level for the stars with planets largely higher than that observed in a rapidly rotating giant. Nonetheless, the present study shows that mid-IR excess is not a general rule among rapidly rotating giants, with a fraction of the analyzed Kepler rapid rotators showing a standard behavior, without IR excess, in agreement with a simple blackbody emission. As it is now well established, a mid-IR excess, in general, points to the existence of warm circumstellar dust, an aspect that could also
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Development of automatic tools for measuring acoustic glitches in seismic data of solar-type stars

Development of automatic tools for measuring acoustic glitches in seismic data of solar-type stars

(4.3) where λ is a smoothing parameter that reduces the third derivative of the polyno- mial. Defining a value of λ = 0 results in the polynomial interpolating all the points and all the signal being explained by the smooth function and ignoring the existence of the signal from the glitches. However, setting the value too high can also cause a lot of the smooth signal from the frequencies to not be removed, contaminating the residuals and deteriorating the fit. Thus, it is important to try and define the lowest value that still doesn’t interfere with the residuals. According to Faria [ 2013 ] there is a range of values of λ where both the parameters of the fit and the their uncertainty remain approximately unchanged. In this implementation the value is chosen from within this range and takes into account the number of available points to use in the final fitting procedure.
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Retrievals from GOMOS stellar occultation measurements using characterization of modeling errors

Retrievals from GOMOS stellar occultation measurements using characterization of modeling errors

Our paper presents the algorithm for numerically efficient processing of GOMOS data, with modeling errors taken into account. It is based on using the Cholesky factor- ization and the subsequent change of variables. The “full covariance matrix” inversion was successfully implemented and tested on a large data set. Compared to the inver- sion without modeling errors, the following changes in inversion statistics and results

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Parametric stellar convection models

Parametric stellar convection models

The goal of this thesis is to adopt the state-of-the-art physics using a robust and already tested version of the code. The stellar models used in this work were obtained using the Aarhus STellar Evolution Code ( ASTEC, Christensen-Dalsgaard [2008]). In the interior model, we used OPAL opacities tables for high temperatures computed by [Iglesias and Rogers, 1996] and atmospheric opacities from Kurucz [1991], Alexander and Ferguson [1994] and low temperatures Ferguson et al. [2005] using the interpolation scheme developed by G. Houdek [Houdek and Rogl, 1996], the Livermore OPAL equation of state [Rogers et al., 1996], the NACRE nuclear reaction rates [Angulo et al., 1999] and the solar mixture from Grevesse and Noels [1993], that the ratio of the heavy elements to hydrogen Z/X = 0.0245(1 ± 0.1) at the solar surface and is in good agreement with the helioseismological properties. This abundances are at the moment, subject to debate. Asplund et al. [2004] have derived new solar abundances (AGS abundances) where solar metallicity is reduced by 30 %.However, including these new abundances in solar models has led to sound-speed profiles in disagreement with helioseismology.
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