ON IRON MONOXIDE NANOPARTICLES AS A CARRIER OF THE MYSTERIOUS 21 μm EMISSION FEATURE IN POST-ASYMPTOTIC GIANT BRANCH STARS (original) (raw)
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On the carriers of the 21 μm emission feature in post-asymptotic giant branch stars
Monthly Notices of the Royal Astronomical Society, 2009
The mysterious 21 μm emission feature seen in sixteen C-rich proto-planetary nebulae (PPNe) remains unidentified since its discovery in 1989. Over a dozen of materials are suggested as the carrier candidates. In this work, we quantitatively investigate eight inorganic and one organic carrier candidates in terms of elemental abundance constraints, while previous studies mostly focus on their spectral profiles (which could be largely affected by grain size, shape and clustering effects). It is found that: (1) five candidates (TiC nanoclusters, fullerenes coordinated with Ti atoms, SiS 2 , doped-SiC and SiO 2 -coated SiC dust) violate the abundance constraints (i.e. they require too much Ti, S or Si to account for the emission power of the 21 μm band, (2) three candidates (carbon and silicon mixtures, Fe 2 O 3 and Fe 3 O 4 ), while satisfying the abundance constraints, exhibit secondary features which are not detected in the 21 μm sources and (3) nano FeO, neither exceeding the abundance budget nor producing undetected secondary features, seems to be a viable candidate, supporting the suggestions of Posch, Mutschke & Andersen.
Reconsidering the Origin of the 21 Micron Feature: Oxides in Carbon‐rich Protoplanetary Nebulae?
The Astrophysical Journal, 2004
The origin of the so-called '21' µm feature which is especially prominent in the spectra of some carbon-rich protoplanetary nebulae (PPNe) is the matter of a lively debate. A large number of potential band carriers have been presented and discarded within the past decade. The present paper gives an overview of the problems related to the hitherto proposed feature identifications, including the recently suggested candidate carrier silicon carbide. We also discuss the case for spectroscopically promising oxides. SiC is shown to produce a strong resonance band at 20-21 µm if coated by a layer of silicon dioxide. At low temperatures, core-mantle particles composed of SiC and amorphous SiO 2 indeed have their strongest spectral signature at a position of 20.1 µm, which coincides with the position of the '21' µm emission band. The optical constants of another candidate carrier that has been relatively neglected so far-iron monoxide-are proven to permit a fairly accurate reproduction of the '21' µm feature profile as well, especially when low-temperature measurements of the infrared properties of FeO are taken into account. As candidate carrier of the '21' µm emission band, FeO has the advantage of being stable against further oxidation and reduction only in a narrow range of chemical and physical conditions, coinciding with the fact that the feature, too, is detected in a small group of objects only. However, it is unclear how FeO should form or survive particularly in carbon-rich PPNe.
Astronomy and Astrophysics, 2002
We present a fit to the spectral energy distribution of OH 127.8+0.0, a typical asymptotic giant branch star with an optically thick circumstellar dust shell. The fit to the dust spectrum is achieved using non-spherical grains consisting of metallic iron, amorphous and crystalline silicates and water ice. Previous similar attempts have not resulted in a satisfactory fit to the observed spectral energy distributions, mainly because of an apparent lack of opacity in the 3-8 µm region of the spectrum. Nonspherical metallic iron grains provide an identification for the missing source of opacity in the near-infrared. Using the derived dust composition, we have calculated spectra for a range of mass-loss rates in order to perform a consistency check by comparison with other evolved stars. The L − [12 µm] colours of these models correctly predict the mass-loss rate of a sample of AGB stars, strengthening our conclusion that the metallic iron grains dominate the near-infrared flux. We discuss a formation mechanism for non-spherical metallic iron grains.
Unidentified species in envelopes around carbon stars
Proceedings of the International Astronomical Union
The infrared (IR) spectra of many evolved carbon-rich stars exhibit two prominent dust emission features peaking around 21um and 30um, with the former exclusively seen in proto-planetary nebulae (PPNe), while the latter seen in a much wider range of objects, including AGB stars, PPNe and planetary nebulae (PNe). The 30um feature is seen in all the 21um sources, but no correlation is found between these two features. Over a dozen carrier candidates have been proposed for the 21um feature, but none of them has been widely accepted and the nature of the 21um feature remains a mystery. The carrier of the 30um feature also remains unidentied. MgS dust, once widely accepted as a valid carrier, was ruled out because of the sulfur budget problem. In this work we examine nano-sized FeO dust as a carrier for the 21um feature. We calculate the IR emission spectrum of FeO nanodust which undergoes single-photon heating in PPNe. It is found that the 21um feature emitted by FeO nanodust is too bro...
Discovery of Variable Iron Fluorescence from Reflection Nebulae in the Galactic Center
The Astrophysical Journal, 2007
Based on three years of deep observations of the Galactic center with the Chandra X-ray Observatory, we report the discovery of changes in the intensities and morphologies of two hard X-ray nebulosities. The nebulosities are dominated by fluorescent iron emission, and are coincident with molecular clouds. The morphological changes are manifest on parsec scales, which requires that these iron features are scattered X-rays from a 2 or 3-year-long outburst of a point source (either Sgr A * or an X-ray binary) with a luminosity of at least 10 37 erg s −1. The variability precludes the hypotheses that these nebulae either are produced by keV electrons bombarding molecular clouds, or are iron-rich ejecta from supernovae. Moreover, the morphologies of the reflection nebulae implies that the dense regions of the clouds are filamentary, with widths of ≈0.3 pc and lengths of ≈ 2 pc.
[O/Fe] ESTIMATES FOR CARBON-ENHANCED METAL-POOR STARS FROM NEAR-INFRARED SPECTROSCOPY
The Astronomical Journal, 2011
We report on oxygen abundances determined from medium-resolution near-IR spectroscopy for a sample of 57 carbon-enhanced metal-poor (CEMP) stars selected from the Hamburg/ESO survey. The majority of our program stars exhibit oxygen-to-iron ratios in the range +0.5 < [O/Fe]< +2.0. The [O/Fe] values for this sample are statistically compared to available high-resolution estimates for known CEMP stars, as well as to high-resolution estimates for a set of carbon-normal metal-poor stars. Carbon, nitrogen, and oxygen abundance patterns for a sub-sample of these stars are compared to yield predictions for very metal-poor asymptotic giant-branch abundances in the recent literature. We find that the majority of our sample exhibit patterns that are consistent with previously studied CEMP stars having s-process-element enhancements, and thus have very likely been polluted by carbon-and oxygen-enhanced material transferred from a metal-poor asymptotic giant-branch companion.
The Astrophysical Journal, 2016
Over two decades ago, a prominent, mysterious emission band peaking at ∼20.1 μm was serendipitously detected in four preplanetary nebulae (PPNe; also known as "protoplanetary nebulae"). To date, this spectral feature, designated as the "21 μm" feature, has been seen in 27 carbon-rich PPNe in the Milky Way and the Magellanic Clouds. The nature of its carriers remains unknown although many candidate materials have been proposed. The 21 μm sources also exhibit an equally mysterious, unidentified emission feature peaking at 30 μm. While the 21 μm feature is exclusively seen in PPNe, a short-lived evolutionary stage between the end of the asymptotic giant branch (AGB) and planetary nebula (PN) phases, the 30 μm feature is more commonly observed in all stages of stellar evolution from the AGB through PPN to PN phases. We derive the stellar mass-loss rates (Ṁ) of these sources from their infrared (IR) emission, using the "2-DUST" radiative transfer code for axisymmetric dusty systems which allows one to distinguish the mass-loss rates of the AGB phase (M AGḂ) from that of the superwind (M SẆ) phase. We examine the correlation between M AGḂ or M SẆ and the fluxes emitted from the 21 and 30 μm features. We find that both features tend to correlate with M AGḂ , suggesting that their carriers are probably formed in the AGB phase. The nondetection of the 21 μm feature in AGB stars suggests that, unlike the 30 μm feature, the excitation of the carriers of the 21 μm feature may require ultraviolet photons which are available in PPNe but not in AGB stars.
[O/Fe] Estimates for Carbon-Enhanced Metal-Poor Stars from Near-IR Spectroscopy
2011
We report on oxygen abundances determined from medium-resolution near-IR spectroscopy for a sample of 57 carbon-enhanced metal-poor (CEMP) stars selected from the Hamburg/ESO survey. The majority of our program stars exhibit oxygen-to-iron ratios in the range +0.5 < [O/Fe]< +2.0. The [O/Fe] values for this sample are statistically compared to available high-resolution estimates for known CEMP stars, as well as