426 research outputs found
A new approach to the solar oxygen abundance problem
In this work we present new data that sets strong constraints on the solar
oxygen abundance. Our approach, based on the analysis of spectro-polarimetric
observations, is almost model-independent and therefore extremely robust. The
asymmetry of the Stokes V profile of the 6300 A [OI] and NiI blend is used as
an indicator of the relative abundances of these two elements. The peculiar
shape of the profile requires a value of EO = 730+/-100 ppm (parts per
million), or logEO = 8.86+/-0.07 in the logarithmic scale commonly used in
Astrophysics. The uncertainty range includes the model dependence as well as
uncertainties in the oscillator strengths of the lines. We emphasize that the
very low degree of model dependence in our analysis makes it very reliable
compared to traditional determinations.Comment: Accepted for publication in The Astrophysical Journal Letters. 12
pages, 3 figures, referee format. This is the replacement of a previous
version of the paper. Our revised analysis takes into consideration the
formation of molecules, resulting in a substantially larger value for the
derived Oxygen abundanc
The Solar Oxygen Abundance from an Empirical Three-Dimensional Model
The Oxygen abundance in the solar photosphere, and consequently the solar
metallicity itself, is still a controversial issue with far-reaching
implications in many areas of Astrophysics. This paper presents a new
determination obtained by fitting the forbidden OI line at 6300 A with an
observational 3D model. The approach presented here is novel because previous
determinations were based either on 1D empirical stratifications or on 3D
theoretical models. The resulting best-fit abundances are lg E(O)=8.90 and lg
E(Ni)=6.15. Nevertheless, introducing minor tweaks in the model and the
procedure, it is possible to retrieve very different values, even down to lg
E(O)=8.70. This extreme sensitivity of the abundance to possible systematic
effects is not something specific to this particular work but probably reflects
the real uncertainty inherent to all abundance determinations based on a
prescribed model atmosphere.Comment: Submitted to A&
The Three-Dimensional Structure of a Sunspot Magnetic Field
Here we report on observations of the three-dimensional structure of a
sunspot magnetic field from the photosphere to the chromosphere, obtained with
the new visible/infrared spectro-polarimeter SPINOR. The observations,
interpreted with a non-LTE modeling technique, reveal a surprisingly complex
topology with areas of opposite-sign torsion, suggesting that flux-ropes of
opposite helicities may coexist together in the same spot.Comment: To appear in the Astrophysical Journal Letter
Polarimetric Calibration of Large-Aperture Telescopes II: The sub-aperture method
A new method for absolute polarimetric calibration of large telescopes is
presented. The proposed method is highly accurate and is based on the
calibration of a small sub-aperture, which is then extended to the full system
by means of actual observations of an astronomical source. The calibration
procedure is described in detail along with numerical simulations that explore
its robustness and accuracy. The advantages and disadvantages of this technique
with respect to other possible alternatives are discussed.Comment: Journal of the Optical Society of America-A, submitte
Polarimetric Calibration of Large-Aperture Telescopes I: The Beam-Expansion Method
This paper describes a concept for the high-accuracy absolute calibration of
the instrumental polarization introduced by the primary mirror of a
large-aperture telescope. This procedure requires a small aperture with
polarization calibration optics (e.g., mounted on the dome) followed by a lens
that opens the beam to illuminate the entire surface of the mirror. The Jones
matrix corresponding to this calibration setup (with a diverging incident beam)
is related to that of the normal observing setup (with a collimated incident
beam) by an approximate correction term. Numerical models of parabolic on-axis
and off-axis mirrors with surface imperfections are used to explore its
accuracy.Comment: Journal of the Optical Society of America-A, in pres
Can a negative-mass cosmology explain dark matter and dark energy?
A recent study by Farnes (2018) proposed an alternative cosmological model in
which both dark matter and dark energy are replaced with a single fluid of
negative mass. This paper presents a critical review of that model. A number of
problems and discrepancies with observations are identified. For instance, the
predicted shape and density of galactic dark matter halos are incorrect. Also,
halos would need to be less massive than the baryonic component otherwise they
would become gravitationally unstable. Perhaps the most challenging problem in
this theory is the presence of a large-scale version of the `runaway' effect,
which would result in all galaxies moving in random directions at nearly the
speed of light. Other more general issues regarding negative mass in general
relativity are discussed, such as the possibility of time-travel paradoxes.Comment: Accepted for publication in A&
Milne-Eddington inversions of the He I 10830 {\AA} Stokes profiles: Influence of the Paschen-Back effect
The Paschen-Back effect influences the Zeeman sublevels of the He I multiplet
at 10830 {\AA}, leading to changes in strength and in position of the Zeeman
components of these lines. We illustrate the relevance of this effect using
synthetic Stokes profiles of the He I 10830 {\AA} multiplet lines and
investigate its influence on the inversion of polarimetric data. We invert data
obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum
Tower Telescope (VTT). We compare the results of inversions based on synthetic
profiles calculated with and without the Paschen-Back effect being included. We
find that when taking into account the incomplete Paschen-Back effect, on
average 16% higher field strength values are obtained. We also show that this
effect is not the main cause for the area asymmetry exhibited by many He I
10830 Stokes V-profiles. This points to the importance of velocity and magnetic
field gradients over the formation height range of these lines.Comment: Accepted for publication in A&A on Jun 12th 200
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