58 research outputs found
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 )
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 K cooler than 3650 K, then no
new dust is required to explain the available empirical constraints. A
coincidence of the dominant short- ( day) and long-period (
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
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