11,799 research outputs found
Chemical Abundances from the Continuum
The calculation of solar absolute fluxes in the near-UV is revisited,
discussing in some detail recent updates in theoretical calculations of
bound-free opacity from metals. Modest changes in the abundances of elements
such as Mg and the iron-peak elements have a significant impact on the
atmospheric structure, and therefore self-consistent calculations are
necessary. With small adjustments to the solar photospheric composition, we are
able to reproduce fairly well the observed solar fluxes between 200 and 270 nm,
and between 300 and 420 nm, but find too much absorption in the 270-290 nm
window. A comparison between our reference 1D model and a 3D time-dependent
hydrodynamical simulation indicates that the continuum flux is only weakly
sensitive to 3D effects, with corrections reaching <10% in the near-UV, and <2%
in the optical.Comment: 10 pages, 5 figures, to appear in the proceedings of the conference A
Stellar Journey, a symposium in celebration of Bengt Gustafsson's 65th
birthday, June 23-27, 2008, Uppsal
A Reappraisal of the Solar Photospheric C/O Ratio
Accurate determination of photospheric solar abundances requires detailed
modeling of the solar granulation and accounting for departures from local
thermodynamical equilibrium (LTE). We argue that the forbidden C I line at 8727
A is largely immune to departures from LTE, and can be realistically modeled
using LTE radiative transfer in a time-dependent three-dimensional simulation
of solar surface convection. We analyze the [C I] line in the solar flux
spectrum to derive the abundance log epsilon(C)= 8.39 +/- 0.04 dex. Combining
this result with our parallel analysis of the [O I] 6300 A line, we find
C/O=0.50 +/- 0.07, in agreement with the ratios measured in the solar corona
from gamma-ray spectroscopy and solar energetic particles.Comment: 5 pages, 2 figures, to appear in ApJL July 1
Oxygen Abundances in Nearby FGK Stars and the Galactic Chemical Evolution of the Local Disk and Halo
Atmospheric parameters and oxygen abundances of 825 nearby FGK stars are
derived using high-quality spectra and a non-LTE analysis of the 777 nm O I
triplet lines. We assign a kinematic probability for the stars to be thin-disk
(P1), thick-disk (P2), and halo (P3) members. We confirm previous findings of
enhanced [O/Fe] in thick-disk (P2>0.5) relative to thin-disk (P1>0.5) stars
with [Fe/H]<-0.2, as well as a "knee" that connects the mean [O/Fe]-[Fe/H]
trend of thick-disk stars with that of thin-disk members at [Fe/H]>-0.2.
Nevertheless, we find that the kinematic membership criterion fails at
separating perfectly the stars in the [O/Fe]-[Fe/H] plane, even when a very
restrictive kinematic separation is employed. Stars with "intermediate"
kinematics (P1<0.7, P2<0.7) do not all populate the region of the [O/Fe]-[Fe/H]
plane intermediate between the mean thin-disk and thick-disk trends, but their
distribution is not necessarily bimodal. Halo stars (P3>0.5) show a large
star-to-star scatter in [O/Fe]-[Fe/H], but most of it is due to stars with
Galactocentric rotational velocity V-200 km/s
follow an [O/Fe]-[Fe/H] relation with almost no star-to-star scatter. Early
mergers with satellite galaxies explain most of our observations, but the
significant fraction of disk stars with "ambiguous" kinematics and abundances
suggests that scattering by molecular clouds and radial migration have both
played an important role in determining the kinematic and chemical properties
of solar neighborhood stars.Comment: ApJ, in press. Complete tables 2-6 are available in the source
(Download: Other formats -> Source
Type Ia Supernovae with Bi-Modal Explosions Are Common -- Possible Smoking Gun for Direct Collisions of White-Dwarfs
We discover clear doubly-peaked line profiles in 3 out of ~20 type Ia
supernovae (SNe Ia) with high-quality nebular-phase spectra. The profiles are
consistently present in three well-separated Co/Fe emission features. The two
peaks are respectively blue-shifted and red-shifted relative to the host
galaxies and are separated by ~5000 km/s. The doubly-peaked profiles directly
reflect a bi-modal velocity distribution of the radioactive Ni56 in the ejecta
that powers the emission of these SNe. Due to their random orientations, only a
fraction of SNe with intrinsically bi-modal velocity distributions will appear
as doubly-peaked spectra. Therefore SNe with intrinsic bi-modality are likely
common, especially among the SNe in the low-luminosity part on the Philips
relation (\Delta m15(B) >~ 1.3; ~40% of all SNe Ia). Such bi-modality is
naturally expected from direct collisions of white dwarfs (WDs) due to the
detonation of both WDs and is demonstrated in a 3D 0.64 M_Sun-0.64 M_Sun WD
collision simulation. In the future, with a large sample of nebular spectra and
a comprehensive set of numerical simulations, the collision model can be
unambiguously tested as the primary channel for type Ia SNe, and the
distribution of nebular line profiles will either be a smoking gun or rule it
out.Comment: To be published by MNRAS Letters. Minor changes of the main text, an
Appendix adde
- …