On the cosmic origin of Fluorine

The cosmic origin of fluorine, the ninth element of the Periodic Table, is still under debate. The reason for this fact is the large difficulties in observing stellar diagnostic lines, which can be used for the determination of the fluorine abundance in stars. Here we discuss some recent work on the chemical evolution of fluorine in the Milky Way and discuss the main contributors to the cosmic budget of fluorine.Comment: 4 pages, 1 figure. Accepted for publication in JA

CNO abundances in the Galactic bulge

The carbon, nitrogen, and oxygen abundances and trends in the bulge are discussed in the context of our recent analysis of these elements in an on-going project based on near-IR spectra (Ryde et al. 2009). We obtained these using the CRIRES spectrometer on the VLT. The formation and evolution of the Milky Way bulge can be constrained by studying elemental abundances of bulge stars. Due to the large and variable visual extinction in the line-of-sight towards the bulge, an analysis in the near-IR is preferred.Comment: Contributed talk at Chemical Abundances in the Universe, Connecting First Stars to Planets, Proceedings of the International Astronomical Union, IAU Symposium, Volume 265, K. Cunha, M. Spite and B. Barbuy, eds, Cambridge University Press, in pres

A new nonlocal thermodynamical equilibrium radiative transfer method for cool stars

Context: The solution of the nonlocal thermodynamical equilibrium (non-LTE) radiative transfer equation usually relies on stationary iterative methods, which may falsely converge in some cases. Furthermore, these methods are often unable to handle large-scale systems, such as molecular spectra emerging from, for example, cool stellar atmospheres. Aims: Our objective is to develop a new method, which aims to circumvent these problems, using nonstationary numerical techniques and taking advantage of parallel computers. Methods: The technique we develop may be seen as a generalization of the coupled escape probability method. It solves the statistical equilibrium equations in all layers of a discretized model simultaneously. The numerical scheme adopted is based on the generalized minimum residual method. Result:. The code has already been applied to the special case of the water spectrum in a red supergiant stellar atmosphere. This demonstrates the fast convergence of this method, and opens the way to a wide variety of astrophysical problems.Comment: 13 pages, 9 figure

NLTE water lines in Betelgeuse-like atmospheres

The interpretation of water lines in red supergiant stellar atmospheres has been much debated over the past decade. The introduction of the so-called MOLspheres to account for near-infrared "extra" absorption has been controversial. We propose that non-LTE effects should be taken into account before considering any extra-photospheric contribution. After a brief introduction on the radiative transfer treatment and the inadequacy of classical treatments in the case of large-scale systems such as molecules, we present a new code, based on preconditioned Krylov subspace methods. Preliminary results suggest that NLTE effects lead to deeper water bands, as well as extra cooling.Comment: 9 pages, 5 figures, Proceedings of the Betelgeuse Workshop, Paris, 201

The Photospheric Temperatures of Betelgeuse during the Great Dimming of 2019/2020: No New Dust Required

The processes that shape the extended atmospheres of red supergiants (RSGs), heat their chromospheres, create molecular reservoirs, drive mass loss, and create dust remain poorly understood. Betelgeuse's V-band "Great Dimming" event of 2019 September /2020 February and its subsequent rapid brightening provides a rare opportunity to study these phenomena. Two different explanations have emerged to explain the dimming; new dust appeared in our line of sight attenuating the photospheric light, or a large portion of the photosphere had cooled. Here we present five years of Wing three-filter (A, B, and C band) TiO and near-IR photometry obtained at the Wasatonic Observatory. These reveal that parts of the photosphere had a mean effective temperature $(T_{\rm eff}$) significantly lower than that found by (Levesque & Massey 2020). Synthetic photometry from MARCS -model photospheres and spectra reveal that the V band, TiO index, and C-band photometry, and previously reported 4000-6800 Angstrom spectra can be quantitatively reproduced if there are multiple photospheric components, as hinted at by VLT-SPHERE images (Montarges et al. 2020). If the cooler component has $\Delta T_{\rm eff} \ge 250$ K cooler than 3650 K, then no new dust is required to explain the available empirical constraints. A coincidence of the dominant short- ($\sim 430$ day) and long-period ($\sim 5.8$ yr) V-band variations occurred near the time of deep minimum (Guinan et al. 2019). This is in tandem with the strong correlation of V mag and photospheric radial velocities, recently reported by Dupree et al. (2020b). These suggest that the cooling of a large fraction of the visible star has a dynamic origin related to the photospheric motions, perhaps arising from pulsation or large-scale convective motions.Comment: Accepted ApJ - 19 pages, 5 figure