4 research outputs found
Precise and efficient modeling of stellar-activity-affected solar spectra using SOAP-GPU
No full textContext. One of the main obstacles in exoplanet detection when using the radial velocity (RV) technique is the presence of stellar activity signal induced by magnetic regions. As the most advanced techniques to mitigate this signal are reaching a level better than one meter per second, it is difficult to evaluate their performance: instrumental systematics start to be similar in magnitude, and therefore it is impossible to know the ground truth of the stellar activity signal. In this context, a realistic simulated dataset that can provide photometry and spectroscopic outputs is needed for method development.
Aims. The goal of this paper is to describe two realistic simulations of solar activity obtained from SOAP-GPU and to compare them with real data obtained from the HARPS-N solar telescope. For this purpose, both simulated spectral time series cover the time window of HARPS-N solar observation, but nothing prevents SOAP-GPU from modeling the data over different time spans.
Methods. We describe two different methods of modeling solar activity using SOAP-GPU. The first models the evolution of active regions based on the spot number as a function of time. Other physical parameters are either drawn from observed solar distributions or modeled with empirical relations. The second method relies on the extraction of active regions from the Solar Dynamics Observatory (SDO) data. The location of spots and faculae on the solar disk at each timestamp are derived from the magnetogram and intensity maps and are fed into SOAP-GPU to simulate the corresponding spectra.
Results. The simulated spectral time series generated with the first method shows a long-term RV behavior similar to that seen in the HARPS-N solar observations. The effect of stellar activity induced by stellar rotation is also well modeled with prominent periodicities at the stellar rotation period and its first harmonic. The comparison between the simulated spectral time series generated using SDO images and the HARPS-N solar spectra shows that SOAP-GPU can precisely model the RV time series of the Sun to a precision better than 0.9 m/s. By studying the width and depth variations of each spectral line in the HARPS-N solar and SOAP-GPU data, we find a strong correlation between the observation and the simulation for strong spectral lines, therefore supporting the modeling of the stellar activity effect at the spectral level. The correlations are weaker for shallow lines, although it is likely that their lower signal-to-noise ratio does not allow a meaningful comparison.
Conclusions. We introduce two methods for modeling solar activity using SOAP-GPU. With only sunspot numbers as input, we accurately capture the long-term magnetic cycle and rotational features. Additionally, we effectively model shift and depth variations at the spectral line level by using data from SDO. These simulated solar spectral time series serve as a useful test bed for evaluating spectral-level stellar activity mitigation techniques
High Precision Spectroscopy of the Solar Surface
No full textContext: High-precision spectroscopy of the resolved solar surface provides benchmark observations for a vast field of stellar research. The solar spectrum exhibits variations from the disc centre to the limb. Observing the solar disc for varying centre-to-limb positions includes observing changing layers of the solar atmosphere. Moreover, as the Sun is the only star spatially resolvable, it serves as an ideal testbed for investigating the behaviour of solar-like stars.
Aims: The primary goal of this thesis was to deliver a high-resolution spectroscopic centre-to-limb solar atlas with high-frequency precision and accuracy. Additionally, the varying convective Doppler velocities across the solar disc are investigated. The comprehensive objective was to enhance our understanding of the solar atmosphere and contribute to refining atmospheric models.
Methods: High-resolution observations of the resolved quiet Sun were examined with a Fourier transform spectrograph at the Institut für Astrophysik und Geophysik in Göttingen. The spectra contain a wavelength range from 4200Å to 8000Å, with a resolving power R of 700,000 at ~6000Å. The Doppler velocities of more than 1000 Fe I lines were determined across the solar disc with respect to line depth and the formation temperature.
Results & Conclusion: The resolved solar atlas contains 14 heliocentric positions from the centre to the limb. The achieved radial-velocity precision for the disc centre is about 0.6 m/s but varies with the centre-to-limb position. While the spectra of the IAG atlas align with earlier observations, they differ significantly from multiple simulations. In general, strong Fe I lines exhibit less blueshift than shallow lines; the strongest lines may even display a redshift. Additionally, the convective blueshift of Fe I lines decreases towards the limb. Lines with decreasing formation temperature show a corresponding decrease in blueshift. Doppler velocities, as a function of formation temperature, can be fitted by a sigmoid function, converging to stable temperatures. The wavelength dependency of Doppler velocities is more pronounced when considering line depth but is still present considering formation temperatures. This atlas offers numerous opportunities to enhance atmospheric models for the Sun or solar-like stars and can, therefore, even improve exoplanet detection.2024-06-2
Solar carbon abundance from 3D non-LTE modelling of the diagnostic lines of the CH molecule
No full textInternational audienceContext. The spectral lines of the CH molecule are a key carbon (C) abundance diagnostic in FGKM-type stars. These lines are detectable in metal-rich and, in contrast to atomic C lines, also in metal-poor late-type stars. However, only 3D LTE analyses of the CH lines have been performed so far. Aims. We test the formation of CH lines in the solar spectrum, using for the first time, 3D Non-LTE (NLTE) models. We also aim to derive the solar photospheric abundance of C, using the diagnostic transitions in the optical (4218 - 4356 Å) and infrared (33025 - 37944 Å). Methods. We use the updated NLTE model molecule from Popa et al. (2023) and different solar 3D radiation-hydrodynamics model atmospheres. The models are contrasted against new spatially-resolved optical solar spectra, and the center-to-limb variation (CLV) of CH lines is studied. Results. The 1D LTE and 1D NLTE models fail to describe the line CLV, and lead to underestimated solar C abundances. The 3D NLTE modelling of diagnostic lines in the optical and IR yields a carbon abundance of A(C)= dex. The estimate is in agreement with recent results based on neutrino fluxes measured by Borexino. Conclusions. 3D NLTE modelling and tests on spatially-resolved solar data are essential to derive robust solar abundances. The analysis presented here focuses on CH, but we expect that similar effects will be present for other molecules of astrophysical interest
The He I D3 Line as a Proxy for Magnetic Activity Using EXPRES Solar Observations
No full textStellar activity remains one of the primary challenges in the detection and characterization of low-mass exoplanets, as it can induce radial velocity (RV) variations that mask or mimic planetary signals. Identifying reliable activity proxies is essential in order to distinguish stellar variability from genuine planetary signatures. In this study, we examine the variability of the chromospheric He I D3 line in high-resolution solar spectra and assess its potential as an activity indicator. We find a strong correlation between the He I D3 line intensity variation and the Sun’s unsigned magnetic flux derived from Solar Dynamics Observatory/Helioseismic and Magnetic Imager data as well as with the solar RVs. Our results suggest that the He I D3 line offers a promising and straightforward proxy for magnetic activity, which may complement existing stellar activity indicators. Its inclusion could help disentangle stellar signals in RV measurements and ultimately improve the detection of Earth-like exoplanets
