KOALA, a new ATLAS9 database -- I. Model atmospheres, opacities, fluxes, bolometric corrections, magnitudes and colours

TL;DR

The paper addresses the need for a comprehensive, regularly sampled grid of LTE ATLAS9 model atmospheres and fluxes with updated solar abundances to support accurate photometric transformations and chemical analyses. The authors present KOALA, a new grid featuring Opacity Distribution Functions and emergent fluxes across an expanded range of $[\mathrm{M/H}]$ from $-5.0$ to $+0.5$ and $[\alpha/\mathrm{Fe}]$ from $-0.4$ to $+0.4$, together with finer $T_{ m eff}$ sampling below $7000$ K. They compute 51663 model atmospheres, five microturbulent velocities, and associated photometric quantities in UBVRI, 2MASS, Hipparcos-Tycho, SDSS, Euclid, Gaia DR3, and Gaia G-band bolometric corrections, including extinction coefficients. The study quantifies how $[\mathrm{M/H}]$ and $[\alpha/\mathrm{Fe}]$ influence the thermal and pressure structure and the synthetic colours, demonstrating the importance of using chemical mixtures consistent with the intended spectral analysis; the dataset is publicly accessible for broad use in stellar and galactic studies.

Abstract

We present the KOALA database, a new set of LTE, line-blanketed model atmospheres calculated with the code ATLAS9, together with the corresponding Opacity Distribution Functions and emergent fluxes. The latter were used also to calculated G-band bolometric corrections and theoretical magnitudes and colours for several photometric systems, i.e. UBVRI, 2MASS, Hypparcos-Tycho, SDSS, Galex, Euclid and Gaia DR3. With respect to the previous grids of ATLAS9 model atmospheres, we adopted the solar mixture by Caffau/Lodders and we extend the sampling in metallicity (from -5.0 to -2.5 dex with step of 0.5 dex, and from -2.5 dex to +0.5 dex with step of 0.25 dex) and in [alfa/Fe] (from -0.4 to +0.4 dex with a step of 0.2 dex). Also, we provide a finer sampling in Teff for Teff lower than 7000 K. This finer grid allows for more accurate interpolation of colours and in many cases it makes not necessary to compute a new model atmosphere, since one of the grid can be used directly. A total of 51663 model atmospheres and emergent fluxes have been computed. Finally, we discuss the impact of [M/H] and [alfa/Fe] on the thermal and pressure structures of the model atmospheres and on theoretical colours.

Figures (7)

• Figure 2: Main panel: comparison among ATLAS9 fluxes for the Sun calculated with suitable ODFs adopting different solar chemical mixtures: Caffau-Lodders (this work, grey line), gs98, grevesse07, magg22. Left panels: percentage difference in temperature (upper panel) and logarithm of the gas pressure (lower panel) as a function of the Rosseland optical depth of the model atmospheres with respect to that computed with Caffau-Lodders chemical mixture (same colour-code of the main panel).
• Figure 3: Run of temperature (upper panels), logarithm of the gas pressure (middle panels) and logarithm of the electron number density (lower panels) as a function of $\log{\tau_{Ross}}$ for the model atmospheres of a dwarf (${\rm T_{\rm eff}}$=6500 K and $\log {\rm g}$=4.5, left panels) and a giant (${\rm T_{\rm eff}}$=4500 K and $\log {\rm g}$=1.5, right panels) star, colour-coded according to the metallicity [M/H]. We adopt [$\alpha$/Fe]=+0.0 dex for all the models.
• Figure 4: Run of fraction of electrons provided by H (upper panels), and Mg (lower panels) as a function of $\log{\tau_{Ross}}$ for the model atmospheres of a dwarf (${\rm T_{\rm eff}}$=6500 K and $\log {\rm g}$=4.5, left panels) and a giant (${\rm T_{\rm eff}}$=4500 K and $\log {\rm g}$=1.5, right panels) star, colour-coded according to the metallicity [M/H]. We adopt [$\alpha$/Fe]=+0.0 dex for all the models.
• Figure 5: Model atmospheres with ${\rm T_{\rm eff}}$=4500 K, $\log {\rm g}$=1.5, [M/H]=+0.0 dex and different [$\alpha$/Fe] . The panels show the behaviour of temperature, logarithm of the gas pressure, fractions of free electrons provided by Mg and Fe. The model with [$\alpha$/Fe]=--0.4 dex, showing a different behaviour with respect to the other ones, is plotted with a thicker curve.
• Figure 6: Synthetic spectra for the second line of the Ca II triplet calculated with model atmospheres with ${\rm T_{\rm eff}}$= 4500 K, $\log {\rm g}$= 1.5, [M/H]=+0.0 dex and three different values of [$\alpha$/Fe]. Despite the latter value, all the synthetic spectra were computed assuming [Ca/Fe]=+0.2 dex.