Opacity distribution functions for stellar spectra synthesis

Abstract

Context. Stellar spectra synthesis is essential for the characterization of potential planetary hosts. In addition, comprehensive stellar variability calculations with fast radiative transfer are needed to disentangle planetary transits from stellar magnetically-driven variability. The planet-hunting space telescopes, such as CoRoT, Kepler, and TESS will bring vast quantities of data, rekindling the interest in fast calculations of the radiative transfer. Aims. We revisit the Opacity Distribution Functions (ODF) approach routinely applied to speedup stellar spectral synthesis. To achieve a considerable speedup relative to the current state-of-the-art, we further optimize the approach and search for the best ODF configuration. Furthermore, we generalize the ODF approach for fast calculations of flux in various filters often used in stellar observations. Methods. In a parameter-sweep-fashion, we generated ODF in the spectral range from UV to IR with different setups. The most accurate ODF configuration for each spectral interval was determined. For calculations of the radiative fluxes through filters we adapted the wavelength grid based on the transmission curve, whereafter the normal ODF procedure was performed. Results. Our optimum ODF configuration allows for a three fold speedup, compared to the previously used ODF configurations. The ODF generalization to calculate fluxes through filters results in a speedup of more than two orders of magnitude.

MPS-ATLAS: A fast all-in-one code for synthesising stellar spectra

“…In most cases such an averaging would lead to a gross overestimation of the line opacity effect, and would essentially lead to trapping photons that otherwise would escape (see Fig. 2 and its detailed discussion in Cernetic et al 2019 This can be circumvented by taking averages over wavelength points with similar opacity values, which can be achieved by grouping points in a certain opacity range together. A straightforward way is to sort the points in one bin in an ascending order by their opacity value.…”

“…The resulting step-function in the entire bin [λ i , λ i+1 ], where each step is the averaged opacity κ s,i of the s-th sub-bin, is called opacity distribution function. For a more detailed de-scription of ODF and the importance of different sub-bin configurations, that is the number of sub-bins and their widths, see Cernetic et al (2019) and references therein.…”

“…Hence, it is important to have an option for changing the ODF wavelength grid. Furthermore, Cernetic et al (2019) showed that an optimal choice of the sub-bin grid leads to significant improvements in both accuracy and speed of the calculations. In particular, they showed that for most of the wavelength range two to four sub-bins are sufficient to reach the same accuracy as the standard 12 sub-bins (which are hard coded into DFSYNTHE).…”

Power spectrum of turbulent convection in the solar photosphere

“…625-627). The ODF approach is much faster, but accurate enough for our purposes (see the detailed analysis in Cernetic et al 2019). We computed the ODF for solar metallicity with a microturbulence of 2 km s −1 using a modified version of the spectrum synthesis program (DFSYNTHE;Castelli 2005a;Kurucz 2005).…”

Radiative transfer with opacity distribution functions: Application to narrow band filters

“…In other words, the detailed frequency dependence of the opacity is replaced by a few points, which are representative of the entire opacity distribution. ODFs have been extensively used and tested for 1D models of the solar atmosphere (see e.g., Kurucz 1974Kurucz , 1979Cernetic et al 2019).…”