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تسجيل مستخدم جديدNeon Lights Up a Controversy: the Solar Ne/O Abundance
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The standard solar model was so reliable that it could predict the existence of the massive neutrino. Helioseismology measurements were so precise that they could determine the depth of the convection zone. This agreement between theory and observation was the envy of all astrophysics -- until recently when sophisticated three-dimensional hydrodynamic calculations of the solar atmosphere reduced the metal content by a factor of almost two. Antia & Basu (2005) suggested that a higher value of the solar neon abundance, Ne/O = 0.52, would resolve this controversy. Drake & Testa (2005) presented strong evidence in favor of this idea from a sample of 21 Chandra stars with enhanced values of the neon abundance, Ne/O = 0.41. In this paper, we have analyzed solar active region spectra from the archive of the Flat Crystal Spectrometer on Solar Maximum Mission, a NASA mission from the 1980s, as well as full-Sun spectra from the pioneering days of X-ray astronomy in the 1960s. These data seem consistent with the standard neon-to-oxygen abundance value, Ne/O = 0.15 (Grevesse & Sauval 1998). If these results prove to be correct, than the enhanced-neon hypothesis will not resolve the current controversy.
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A thorough critical literature survey has been carried out for reliable measurements of oxygen and neon abundances of planetary nebulae (PNe) and HII regions. By contrasting the results of PNe and of HII regions, we aim to address the issues of the e
volution of oxygen and neon in the interstellar medium (ISM) and in the late evolutionary phases of low- and intermediate-mass stars (LIMS), as well as the currently hotly disputed solar Ne/O abundance ratio. Through the comparisons, we find that neon abundance and Ne/O ratio increase with increasing oxygen abundance in both types of nebulae, with positive correlation coefficients larger than 0.75. The correlations suggest different enrichment mechanisms for oxygen and neon in the ISM, in the sense that the growth of neon is delayed compared to oxygen. The differences of abundances between PNe and HII regions, are mainly attributed to the results of nucleosynthesis and dredge-up processes that occurred in the progenitor stars of PNe. We find that both these alpha-elements are significantly enriched at low metallicity (initial oxygen abundance <= 8.0) but not at metallicity higher than the SMC. The fact that Ne/O ratios measured in PNe are almost the same as those in HII regions, regardless of the metallicity, suggests a very similar production mechanism of neon and oxygen in intermediate mass stars (IMS) of low initial metallicities and in more massive stars, a conjecture that requires verification by further theoretical studies. This result also strongly suggests that both the solar neon abundance and the Ne/O ratio should be revised upwards by ~0.22 dex from the Asplund, Grevesse & Sauval values or by ~0.14 dex from the Grevesse & Sauval values.
Context: Recent works with improved model atmospheres, line formation, atomic and molecular data, and detailed treatment of blends, have resulted in a significant downward revision of the solar oxygen abundance. Aims: Considering the importance of
the Sun as an astrophysical standard and the current conflict of standard solar models using the new solar abundances with helioseismological observations we have performed a new study of the solar oxygen abundance based on the forbidden [OI] line at 5577.34 A, not previously considered. Methods: High-resolution (R > 500 000), high signal-to-noise (S/N > 1000) solar spectra of the [O I] 5577.34 A line have been analyzed employing both three-dimensional (3D) and a variety of 1D (spatially and temporally averaged 3D, Holweger & Muller, MARCS and Kurucz models with and without convective overshooting) model atmospheres. Results: The oxygen abundance obtained from the [OI] 5577.3 A forbidden line is almost insensitive to the input model atmosphere and has a mean value of A(O) = 8.71 +/- 0.02 (sigma from using the different model atmospheres). The total error (0.07 dex) is dominated by uncertainties in the log gf value (0.03 dex), apparent line variation (0.04 dex) and uncertainties in the continuum and line positions (0.05 dex). Conclusions: The here derived oxygen abundance is close to the 3D-based estimates from the two other [OI] lines at 6300 and 6363 A, the permitted OI lines and vibrational and rotational OH transitions in the infrared. Our study thus supports a low solar oxygen abundance (A(O) ~ 8.7), independent of the adopted model atmosphere.
We examine the constraints imposed by helioseismic data on the solar heavy element abundances. In prior work we argued that the measured depth of the surface convection zone R_CZ and the surface helium abundance Y_surf were good metallicity indicator
s which placed separable constraints on light metals (CNONe) and the heavier species with good relative meteoritic abundances. The resulting interiors-based abundance scale was higher than some published studies based on 3D model atmospheres at a highly significant level. In this paper we explore the usage of the solar sound speed in the radiative interior as an additional diagnostic, and find that it is sensitive to changes in the Ne/O ratio even for models constructed to have the same R_CZ and Y_surf. Three distinct helioseismic tests (opacity in the radiative core, ionization in the convection zone, and the core mean molecular weight) yield consistent results. Our preferred O, Ne and Fe abundances are 8.86 +/-0.04, 8.15 +/-0.17 and 7.50 +/-0.05 respectively. They are consistent with the midrange of recently published 3D atmospheric abundances measurements. The values for O, Ne and Fe which combine interiors and atmospheric inferences are 8.83 +/-0.04, 8.08 +/-0.09 and 7.49 +/-0.04 respectively.
We have studied the O and Ne absorption features in the X-ray spectrum of Cyg X-2 observed with the Chandra LETG. The O absorption edge is represented by the sum of three absorption-edge components within the limit of the energy resolution and the ph
oton counting statistics. Two of them are due to the atomic O; their energies correspond to two distinct spin states of photo-ionized O atoms. The remaining edge component is considered to represent compound forms of oxide dust grains. Since Cyg X-2 is about 1.4 kpc above the galactic disk, the H column densities can be determined by radio (21 cm and CO emission line) and H alpha observations with relatively small uncertainties. Thus the O abundance relative to H can be determined from the absorption edges. We found that the dust scattering can affect the apparent depth of the edge of the compound forms. We determined the amplitude of the effect, which we consider is the largest possible correction factor. The ratio of column densities of O in atomic to compound forms and the O total abundance were respectively determined to be in the range 1.7^{+3.0}_{-0.9} to 2.8^{+5.1}_{-1.5} (ratio), and 0.63 +/- 0.12 solar to 0.74 +/- 0.14 solar (total), taking into account the uncertainties in the dust-scattering correction and in the ionized H column density. We also determined the Ne abundance from the absorption edge to be 0.75 +/- 0.20 solar. These abundance values are smaller than the widely-used solar values but consistent with the latest estimates of solar abundance.
The solar photospheric oxygen abundance has been determined from [OI], OI, OH vibration-rotation and OH pure rotation lines by means of a realistic time-dependent, 3D, hydrodynamical model of the solar atmosphere. In the case of the OI lines, 3D non-
LTE calculations have been performed, revealing significant departures from LTE as a result of photon losses in the lines. We derive a solar oxygen abundance of log O = 8.66 +/- 0.05. All oxygen diagnostics yield highly consistent abundances, in sharp contrast with the results of classical 1D model atmospheres. This low value is in good agreement with measurements of the local interstellar medium and nearby B stars. This low abundance is also supported by the excellent correspondence between lines of very different line formation sensitivities, and between the observed and predicted line shapes and center-to-limb variations. Together with the corresponding down-ward revisions of the solar carbon, nitrogen and neon abundances, the resulting significant decrease in solar metal mass fraction to Z = 0.0126 can, however, potentially spoil the impressive agreement between predicted and observed sound speed in the solar interior determined from helioseismology.
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