scholarly journals Model-free inverse method for transit imaging of stellar surfaces

2019 ◽  
Vol 630 ◽  
pp. A122 ◽  
Author(s):  
Erik Aronson ◽  
Nikolai Piskunov
Keyword(s):  
Light Curve ◽  
Synthetic Data ◽  
Brightness Distribution ◽  
Stellar Atmosphere ◽  
Light Curves ◽  
Stellar Surface ◽  
Model Free ◽  
Very Large Telescope ◽  
Limb Darkening ◽  
Low Dispersion

Context. We present a model-free method for mapping surface brightness variations. Aims. We aim to develop a method that is not dependent on either stellar atmosphere models or limb-darkening equation. This method is optimized for exoplanet transit surveys such that a large database of stellar spot coverage can be created. Methods. The method uses light curves from several transit events of the same system. These light curves are phase-folded and median-combined to for a high-quality light curve without temporal local brightness variations. Stellar specific intensities are extracted from this light curve using a model-free method. We search individual light curves for departures from the median-combined light curve. Such departures are interpreted as brightness variations on the stellar surface. A map of brightness variations on the stellar surface is produced by finding the brightness distribution that can produce a synthetic light curve that fits observations well. No assumptions about the size, shape, or contrast of brightness variations are made. Results. We successfully reproduce maps of stellar disks from both synthetic data and archive observations from FORS2, the visual and near UV FOcal Reducer and low dispersion Spectrograph for the Very Large Telescope (VLT).

scholarly journals Comparison of the power-2 limb-darkening law from the STAGGER-grid to Kepler light curves of transiting exoplanets

2018 ◽  
Vol 616 ◽  
pp. A39 ◽  
Author(s):  
P. F. L. Maxted
Keyword(s):  
Light Curve ◽  
Probability Distributions ◽  
Binary Star ◽  
Magnetic Activity ◽  
Light Curves ◽  
Practical Implementation ◽  
Light Curve Analysis ◽  
Stagger Grid ◽  
Least Squares Fit ◽  
Limb Darkening

Context. Inaccurate limb-darkening models can be a significant source of error in the analysis of the light curves for transiting exoplanet and eclipsing binary star systems, particularly for high-precision light curves at optical wavelengths. The power-2 limb-darkening law, Iλ(µ) = 1 − c(1−µα), has recently been proposed as a good compromise between complexity and precision in the treatment of limb-darkening. Aims. My aim is to develop a practical implementation of the power-2 limb-darkening law and to quantify the accuracy of this implementation. Methods. I have used synthetic spectra based on the 3D stellar atmosphere models from the STAGGER-grid to compute the limb-darkening for several passbands (UBVRI, CHEOPS, TESS, Kepler, etc.). The parameters of the power-2 limb-darkening laws are optimized using a least-squares fit to a simulated light curve computed directly from the tabulated Iλ(μ) values. I use the transformed parameters h1 = 1 − c(1 − 2−α) and h2 = c2−α to directly compare these optimized limb-darkening parameters to the limb darkening measured from Kepler light curves of 16 transiting exoplanet systems. Results. The posterior probability distributions (PPDs) of the transformed parameters h1 and h2 resulting from the light curve analysis are found to be much less strongly correlated than the PPDs for c and α. The agreement between the computed and observed values of (h1, h2) is generally very good but there are significant differences between the observed and computed values for Kepler-17, the only star in the sample that shows significant variability between the eclipses due to magnetic activity (star spots). Conclusions. The tabulation of h1 and h2 provided here can be used to accurately model the light curves of transiting exoplanets. I also provide estimates of the priors that should be applied to transformed parameters h1 and h2 based on my analysis of the Kepler light curves of 16 stars with transiting exoplanets.

scholarly journals Interferometric Fringe Visibility Null as a Function of Spatial Frequency: A Probe of Stellar Atmospheres

2019 ◽  
Vol 08 (04) ◽  
pp. 1950012
Author(s):  
J. T. Armstrong ◽  
A. M. Jorgensen ◽  
D. Mozurkewich ◽  
H. R. Neilson ◽  
E. K. Baines ◽  
...  
Keyword(s):  
Spatial Frequency ◽  
Angular Size ◽  
Brightness Distribution ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Stellar Disk ◽  
Baseline Length ◽  
Fringe Visibility ◽  
Multiple Wavelengths ◽  
Limb Darkening

We introduce an observational tool based on visibility nulls in optical spectro-interferometry fringe data to probe the structure of stellar atmospheres. In a preliminary demonstration, we use both Navy Precision Optical Interferometer (NPOI) data and stellar atmosphere models to show that this tool can be used, for example, to investigate limb darkening. Using bootstrapping with either multiple linked baselines or multiple wavelengths in optical and infrared spectro-interferometric observations of stars makes it possible to measure the spatial frequency [Formula: see text] at which the real part of the fringe visibility [Formula: see text] vanishes. That spatial frequency is determined by [Formula: see text], where [Formula: see text] is the projected baseline length, and [Formula: see text] is the wavelength at which the null is observed. Since [Formula: see text] changes with the Earth’s rotation, [Formula: see text] also changes. If [Formula: see text] is constant with wavelength, [Formula: see text] varies in direct proportion to [Formula: see text]. Any departure from that proportionality indicates that the brightness distribution across the stellar disk varies with wavelength via variations in limb darkening, in the angular size of the disk, or both. In this paper, we introduce the use of variations of [Formula: see text] with [Formula: see text] as a means of probing the structure of stellar atmospheres. Using the equivalent uniform disk diameter [Formula: see text], given by [Formula: see text], as a convenient and intuitive parameterization of [Formula: see text], we demonstrate this concept by using model atmospheres to calculate the brightness distribution for [Formula: see text] Ophiuchi and to predict [Formula: see text], and then comparing the predictions to coherently averaged data from observations taken with the NPOI.

scholarly journals High-precision calculation of the light curve and its interpretation

2020 ◽  
Vol 633 ◽  
pp. A96 ◽  
Author(s):  
M. K. Abubekerov ◽  
N. Yu. Gostev
Keyword(s):  
Light Curve ◽  
High Precision ◽  
Binary Star ◽  
Practical Importance ◽  
Light Curves ◽  
Elliptical Orbit ◽  
Nonlinear Fitting ◽  
Precise Calculation ◽  
Adopted Model ◽  
Limb Darkening

We present a highly precise calculation of the theoretical light curve and its derivatives for a binary star-planet system in an elliptical orbit. We also describe an analytical fitting by limb-darkening coefficients to reduce the number of parameters for nonlinear fitting. We demonstrate the practical importance of the precision computation of theoretical light curves through the example of the interpretation of the light curve of HD 209458 and the synthetic light curve. We also compare the results obtained using our algorithm to those provided by a lower-precision algorithm to demonstrate the benefits of computing with a higher precision. We discuss the capability of making more accurate conclusions concerning the agreement of the observed light curve with the adopted model.

scholarly journals Polarimetry of the superluminous transient ASASSN-15lh

2020 ◽  
Vol 498 (3) ◽  
pp. 3730-3735
Author(s):  
J R Maund ◽  
G Leloudas ◽  
D B Malesani ◽  
F Patat ◽  
J Sollerman ◽  
...  
Keyword(s):  
Light Curve ◽  
Rest Frame ◽  
Uv Light ◽  
True Nature ◽  
V Band ◽  
Very Large Telescope ◽  
Imaging Polarimetry ◽  
The Subject ◽  
Low Dispersion ◽  
Curve Maximum

ABSTRACT ASASSN-15lh is the intrinsically brightest transient observed to date. Despite being the subject of concerted photometric and spectroscopic observing campaigns, there is still significant debate about the true nature of this transient and the mechanism responsible for its great luminosity. Here we report five epochs of imaging polarimetry and two epochs of spectropolarimetry conducted with the ESO Very Large Telescope (VLT) FOcal Reducer and low dispersion Spectrograph (FORS) polarimeter, spanning +28–91 d (rest frame) with respect to the light-curve maximum. The overall level of polarization across this period is seen to be low ${\sim} 0.5\!-\!0.8 {{\ \rm per\ cent}}$, however at +51.6 d, approximately corresponding to a dip in the ultraviolet (UV) photometric light curve, the polarization is seen to briefly rise to $1.2{{\ \rm per\ cent}}$ in the observed V band. We discuss this behaviour in the context of previous polarimetric observations of superluminous supernovae (SLSNe) and tidal disruption events (TDEs). Although the level of polarization could be consistent with polarization observed for SLSNe, the behaviour around the UV light-curve dip could also be consistent with a TDE observed almost edge on.

scholarly journals Detectability of shape deformation in short-period exoplanets

2019 ◽  
Vol 621 ◽  
pp. A117 ◽  
Author(s):  
B. Akinsanmi ◽  
S. C. C. Barros ◽  
N. C. Santos ◽  
A. C. M. Correia ◽  
P. F. L. Maxted ◽  
...  
Keyword(s):  
Light Curve ◽  
Accurate Determination ◽  
Light Curves ◽  
Shape Deformation ◽  
Love Number ◽  
Tidal Forces ◽  
Tidal Deformation ◽  
Short Period ◽  
Limb Darkening ◽  
Instrumental Precision

Context. Short-period planets are influenced by the extreme tidal forces of their parent stars. These forces deform the planets causing them to attain nonspherical shapes. The nonspherical shapes, modeled here as triaxial ellipsoids, can have an impact on the observed transit light-curves and the parameters derived for these planets. Aims. We investigate the detectability of tidal deformation in short-period planets from their transit light curves and the instrumental precision needed. We also aim to show how detecting planet deformation allows us to obtain an observational estimate of the second fluid Love number from the light curve, which provides valuable information about the internal structure of the planet. Methods. We adopted a model to calculate the shape of a planet due to the external potentials acting on it and used this model to modify the ellc transit tool. We used the modified ellc to generate the transit light curve for a deformed planet. Our model is parameterized by the Love number; therefore, for a given light curve we can derive the value of the Love number that best matches the observations. Results. We simulated the known cases of WASP-103b and WASP-121b which are expected to be highly deformed. Our analyses show that instrumental precision ≤50 ppm min−1 is required to reliably estimate the Love number and detect tidal deformation. This precision can be achieved for WASP-103b in ∼40 transits using the Hubble Space Telescope and in ∼300 transits using the forthcoming CHEOPS instrument. However, fewer transits will be required for short-period planets that may be found around bright stars in the TESS and PLATO survey missions. The unprecedented precisions expected from PLATO and JWST will permit the detection of shape deformation with a single transit observation. However, the effects of instrumental and astrophysical noise must be considered as they can increase the number of transits required to reach the 50 ppm min−1 detection limit. We also show that improper modeling of limb darkening can act to bury signals related to the shape of the planet, thereby leading us to infer sphericity for a deformed planet. Accurate determination of the limb darkening coefficients is therefore required to confirm planet deformation.

scholarly journals Fast and precise light-curve model for transiting exoplanets with rings

2019 ◽  
Vol 490 (1) ◽  
pp. 1111-1119 ◽  
Author(s):  
Edan Rein ◽  
Aviv Ofir
Keyword(s):  
Solar System ◽  
Light Curve ◽  
Source Code ◽  
Light Curves ◽  
Small Range ◽  
Silicate Material ◽  
Computationally Efficient ◽  
Curve Model ◽  
In Transit ◽  
Limb Darkening

ABSTRACT The presence of silicate material in known rings in the Solar system raises the possibility of ring systems existing even within the snow line – where most transiting exoplanets are found. Previous studies have shown that the detection of exoplanetary rings in transit light curves is possible, albeit challenging. To aid such future detection of exoplanetary rings, we present the Polygon + Segments model for modelling the light curve of an exoplanet with rings. This high-precision model includes full ring geometry as well as possible ring transparency and the host star’s limb darkening. It is also computationally efficient, requiring just a 1D integration over a small range, making it faster than existing techniques. The algorithm at its core is further generalized to compute the light curve of any set of convex primitive shapes in transit (e.g. multiple planets, oblate planets, moons, rings, combination thereof, etc.) while accounting for their overlaps. The python source code is made available.

scholarly journals Analytic approximations for transit light curve observables and uncertainties

2008 ◽  
Vol 4 (S253) ◽  
pp. 392-393
Author(s):  
Joshua A. Carter ◽  
Jennifer C. Yee ◽  
Jason Eastman ◽  
B. Scott Gaudi ◽  
Joshua N. Winn
Keyword(s):  
Parameter Estimation ◽  
Light Curve ◽  
Brightness Distribution ◽  
Analytic Approximations ◽  
Computationally Intensive ◽  
Limb Darkening ◽  
Parameters Of Interest

AbstractThe light curve of an exoplanetary transit can be used to estimate the planetary radius and other parameters of interest. Because accurate parameter estimation is a non-analytic and computationally intensive problem, it is often useful to have analytic approximations for the parameters as well as their uncertainties and covariances. Here we give such formulas, for the case of an exoplanet transiting a star with a uniform brightness distribution. When limb darkening is significant, our parameter sets are still useful, although our analytic formulas underpredict the covariances and uncertainties.

scholarly journals Two-Spot Modeling of Synoptic Light Curves of II PEG

1983 ◽  
Vol 71 ◽  
pp. 185-188
Author(s):  
G. La Fauci ◽  
M. Rodonò
Keyword(s):  
Binary System ◽  
Light Curve ◽  
Computer Code ◽  
Light Curves ◽  
Stellar Surface ◽  
Single Line ◽  
Relative Location ◽  
Single Spot ◽  
Spectroscopic Binary ◽  
Circular Spot

The light curve (LC) of the single-line spectroscopic binary system II Peg (K2-3, IV-III) generally shows an asymmetrical and highly variable shape (Fig.l, left). Therefore, single circular spot models can be successfully applied only to symmetric LCs, as the 1977 one by Vogt (1981). When asymmetrical or almost flat LCs develop, as appears to be the rule for II Peg, at leasts two-spot modeling is required (Bopp and Noah 1980, Dorren and Guinan 1982).From classical single-spot models (Torres and Ferraz Mello 1973, Friedman and Gurtler 1975, Bopp and Evans 1973) we have developed a computer code including two separate circular spots of different size which are allowed to assume any relative location on the stellar surface. The model is symmetric with respect to the equator. The spots are assumed to radiate as back bodies with temperature ranging from 100 to 2000 degree lower than the temperature of the unspotted photosphere (4500°K).

scholarly journals Analysis Of Kepler-71 Activity Through Planetary Transit

2016 ◽  
Vol 12 (S328) ◽  
pp. 140-142
Author(s):  
Eber A. Gusmão ◽  
Caius L. Selhorst ◽  
Alexandre S. Oliveira
Keyword(s):  
Light Curve ◽  
Mean Value ◽  
Disk Center ◽  
Light Curves ◽  
Stellar Surface ◽  
Physical Parameters ◽  
Detectable Change ◽  
Active Star ◽  
Maximum Value ◽  
Parent Star

AbstractAn exoplanet transiting in front of the disk of its parent star may hide a dark starspot causing a detectable change in the light curve, that allows to infer physical characteristics of the spot such as size and intensity. We have analysed the Kepler Space Telescope observations of the star Kepler-71 in order to search for variabilities in 28 transit light curves. Kepler-71 is a star with 0.923 M⊙ and 0.816 R⊙ orbited by the hot Jupiter planet Kepler-71b with radius of 1.0452 RJ. The physical parameters of the starspots are determined by fitting the data with a model that simulates planetary transits and enables the inclusion of spots on the stellar surface with different sizes, intensities, and positions. The results show that Kepler-71 is a very active star, with several spot detections, with a mean value of 6 spots per transit with size 0.6 RP and 0.5 IC, as a function of stellar intensity at disk center (maximum value).

scholarly journals A New Method of Light Curve Analysis: Application to V444 Cygni

1980 ◽  
Vol 88 ◽  
pp. 73-78
Author(s):  
S.A.H. Smith ◽  
A. C. Theokas
Keyword(s):  
Light Curve ◽  
Narrow Band ◽  
Curve Analysis ◽  
Light Curves ◽  
New Method ◽  
Light Curve Analysis ◽  
Arbitrary Degree ◽  
Analysis Application ◽  
Limb Darkening

A new method of light curve analysis is introduced for systems involving an atmospheric eclipse. Occultation or partial eclipses may be treated, with or without the presence of transparency effects. Besides admitting an arbitrary degree of transparency to the eclipsing component, an arbitrary law of limb darkening may also be given the eclipsed star.The method has successfully been applied to the study of narrow band continuum light curves of V444 Cygni obtained by Cherepashchuk and Khaliullin. Primary and secondary minima are examined separately.

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