Spatially resolved spectroscopy across stellar surfaces

Dravins, Dainis; Ludwig, H.‐G.; Freytag, B.

doi:10.1051/0004-6361/202039995

Cited by 10 publications

(12 citation statements)

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“…Such information is very challenging to obtain, however, 3D MHD simulations (e.g. Johnson et al 2021b;Dravins et al 2021) and resolved spectroscopic observation of other stars due to planetary transits (e.g. Dravins et al 2017) could significantly help.…”

Section: Discussionmentioning

confidence: 99%

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SOAP-GPU: Efficient spectral modeling of stellar activity using graphical processing units

Zhao

Dumusque

2023

A&A

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Context. Stellar activity mitigation is one of the major challenges for the detection of earth-like exoplanets in radial velocity measurements. Several promising techniques are now investigating the use of spectral time-series, to differentiate between stellar and planetary perturbations. In this context, developing a software that can efficiently explore the parameter space of stellar activity at the spectral level is of great importance. Aims. The goal of this paper is to present a new version of the Spot Oscillation And Planet (SOAP) 2.0 code that can model stellar activity at the spectral level using graphical processing units (GPUs). Methods. We take advantage of the computational power of GPUs to optimise the computationally expensive algorithms behind the original SOAP 2.0 code. For that purpose, we developed GPU kernels that allow to model stellar activity on any given wavelength range. In addition to the treatment of stellar activity at the spectral level, SOAP-GPU also includes the change of spectral line bisectors from center to limb, and can take as input PHOENIX spectra to model the quiet photosphere, spots and faculae, which allow to simulate stellar activity for a wider space in stellar properties. Results. Benchmark calculations show that for the same accuracy, this new code improves the computational speed by a factor of 60 compared with a modified version of SOAP 2.0 that generates spectra, when modeling stellar activity on the full visible spectral range with a resolution of R=115'000. Although the code now includes the variation of spectral line bisector with center-to-limb angle, the effect on the derived RVs is small. We also show that it is not possible to fully separate the flux from the convective blueshift effect when modeling spots, due to their lower temperature and thus the appearance of molecular absorption in their spectra. Rather negligible for the Sun, this degeneracy between the flux and convective blueshift effect become more important when we move to cooler stars, however, this issue does not impact the estimation of the total effect (flux plus convection), and therefore users can trust this output. Conclusions. The publicly available SOAP-GPU code allows to efficiently model stellar activity at the spectral level, which is essential to test further stellar activity mitigation techniques working at the level of spectral timeseries not affected by other sources of noise. Besides a huge gain in performance, SOAP-GPU also includes more physics and is able to model different stars than the Sun, from F to K dwarfs, thanks to the use of the PHOENIX spectral library. We however note that due to the limited understanding of stellar convection and activity on other stars than the Sun, the more we go away from the solar case, the more the output of the code should be taken with care.

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Section: Discussionmentioning

confidence: 99%

“…Those could come from 3D MHD simulations (e.g. Johnson et al 2021b;Dravins et al 2021), or thanks to spatially resolved spectroscopic observation across stellar surfaces thanks to transiting Fig. 13.…”

Section: Limitation Of Soap-gpu When Moving Away From Solar Twinsmentioning

confidence: 99%

SOAP-GPU: Efficient spectral modeling of stellar activity using graphical processing units

Zhao

Dumusque

2023

A&A

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“…The radial-velocity wobble induced by a small exoplanet is much smaller than stellar microvariability, therefore effects of the latter must somehow be calibrated and corrected for. Numerous authors have studied empirical correlations between various stellar activity indices and corresponding excursions in apparent radial velocity (e.g., Cegla et al 2014Cegla et al , 2019Collier Cameron et al 2020;de Beurs et al 2020;Dumusque 2016;Feng et al 2016;Haywood et al 2016;Hojjatpanah et al 2020;Kosiarek & Crossfield 2020;Lanza et al 2019;Luhn et al 2020a;Marchwinski et al 2015;Meunier et al 2010Meunier et al , 2015Meunier et al , 2017bMiklos et al 2020;Oshagh et al 2017;Sulis et al 2020a;Tayar et al 2019;Thompson et al 2020).…”

Section: The Quest For Earth-like Exoplanetsmentioning

confidence: 99%

“…Tests of 3D hydrodynamic models of photospheric convection become possible through comparisons to synthetic spectral-line profiles at various center-to-limb positions, computed as temporal and spatial averages over their simulation sequences. In Paper IV (Dravins et al 2021), photospheric spectral-line shapes and shifts were examined in the 400-700 nm region from complete stellar spectra synthesized at hyper-high spectral resolution (λ/∆λ > 1 000 000), obtained from 3D hydrodynamic CO 5 BOLD simulations (Freytag et al 2012, Ludwig et al, in prep.). These spectral signatures encode the detailed properties of the dynamic stellar photosphere, and can serve as a diagnostic tool for testing and verifying hydrodynamic simulations for various classes of stars.…”

Section: Introductionmentioning

confidence: 99%

Spatially resolved spectroscopy across stellar surfaces

Dravins

Ludwig

Freytag

2021

A&A

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Context. High-precision stellar analyses require hydrodynamic 3D modeling. Testing such models is feasible by retrieving spectral line shapes across stellar disks, using differential spectroscopy during exoplanet transits. Observations were presented in Papers I, II, and III, while Paper IV explored synthetic data at hyper-high spectral resolution for different classes of stars, identifying characteristic patterns for Fe I and Fe II lines. Aims. Anticipating future observations, the observability of patterns among photospheric lines of different strength, excitation potential and ionization level are examined from synthetic spectra, as observed at ordinary spectral resolutions and at different levels of noise. Time variability in 3D atmospheres induces changes in spectral-line parameters, some of which are correlated. An adequate calibration could identify proxies for the jitter in apparent radial velocity to enable adjustments to actual stellar radial motion. Methods. We used spectral-line patterns identified in synthetic spectra at hyper-high resolution in Paper IV from 3D models spanning Teff = 3964–6726 K (spectral types ~K8 V–F3 V) to simulate practically observable signals at different stellar disk positions at various lower spectral resolutions, down to λ/Δλ = 75 000. We also examined the center-to-limb temporal variability. Results. Recovery of spatially resolved line profiles with fitted widths and depths is shown for various noise levels, with gradual degradation at successively lower spectral resolutions. Signals during exoplanet transit are simulated. In addition to Rossiter-McLaughlin type signatures in apparent radial velocity, analogous effects are shown for line depths and widths. In a solar model, temporal variability in line profiles and apparent radial velocity shows correlations between jittering in apparent radial velocity and fluctuations in line depth. Conclusions. Spatially resolved spectroscopy using exoplanet transits is feasible for main-sequence stars. Overall line parameters of width, depth and wavelength position can be retrieved already with moderate efforts, but a very good signal-to-noise ratio is required to reveal the more subtle signatures between subgroups of spectral lines, where finer details of atmospheric structure are encoded. Fluctuations in line depth correlate with those in wavelength, and because both can be measured from the ground, searches for low-mass exoplanets should explore these to adjust apparent radial velocities to actual stellar motion.

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“…To characterize stellar jitter, we use existing computational capabilities to perform 3D radiative models of stellar convection and use the resulting models of atmospheres to compute synthetic spectra. This approach has been used to investigate stellar surface effects by various authors (e.g., Cegla et al 2012Cegla et al , 2013Dravins et al 2017Dravins et al , 2021a. The present work presents initial results of modeling several target stars and computing spectral line syntheses over the stellar disks to investigate the jitter properties.…”

Section: Introductionmentioning

confidence: 99%

3D Realistic Modeling of Solar-Type Stars to Characterize the Stellar Jitter

2020

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Detection of Earth-mass planets with the radial velocity method requires a precision of about 10cm/s to identify a signal caused by such a planet. At the same time, noise originating in the photospheric and subphotospheric layers of the parent star is of the order of meters per second. Understanding the physical nature of the photospheric noise (so-called stellar jitter) and characterizing it are critical for developing techniques to filter out these unwanted signals. We take advantage of current computational and technological capabilities to create 3D realistic models of stellar subsurface convection and atmospheres to characterize the photospheric jitter. We present 3D radiative hydrodynamic models of several solar-type target stars of various masses and metallicities, discuss how the turbulent surface dynamics and spectral line characteristics depend on stellar properties, and provide stellar jitter estimates for these stars.

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Spatially resolved spectroscopy across stellar surfaces

Cited by 10 publications

References 97 publications

SOAP-GPU: Efficient spectral modeling of stellar activity using graphical processing units

SOAP-GPU: Efficient spectral modeling of stellar activity using graphical processing units

Spatially resolved spectroscopy across stellar surfaces

3D Realistic Modeling of Solar-Type Stars to Characterize the Stellar Jitter

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