在16个富含碳的原行星状星云（PPNe）中发现的神秘的21微米发射特征自1989年发现以来一直没有得到确认。十几种材料被建议作为载体候选者。在这项工作中，我们定量研究了8种无机和1种有机载体候选者的元素丰度限制，而以前的研究主要集中在他们的光谱分布（这可能在很大程度上受到晶粒尺寸，形状和聚类效应）。发现：（1）5名候选人（TiC纳米团簇、与Ti原子配位的富勒烯、SiS $_2$、掺杂SiC和SiO $_2$涂覆的SiC粉末）违反了丰度约束（即，它们需要太多的Ti、S或Si来解释21微米带的发射功率，（2）三个候选物（碳和硅的混合物，Fe $_2$ O $_3$和Fe $_3$ O $_4$），在满足丰度限制的同时，表现出在21微米源中未检测到的次要特征，（3）纳米FeO，既没有超过丰度预算，也没有产生未被检测到的次级特征，似乎是一个可行的候选者，支持Posch等人2004年的建议。
英文摘要：
The mysterious 21micron 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 21micron 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 21micron 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 et al. 2004.
A large number of carbon-rich evolved objects (asymptotic giant branch stars, protoplanetary nebulae, and planetary nebulae) in both the Milky Way galaxy and the Magellanic Clouds exhibit an enigmatic broad emission feature at 30 micron. This feature, extending from 24 micron to 45 micron, is very strong and accounts for up to 30% of the total infrared luminosity of the object. In literature it is tentatively attributed to magnesium sulfide (MgS) dust. Using the prototypical protoplanetary nebula around HD 56126 for illustrative purpose, however, in this work we show that in order for MgS to b…
Void induced coronene C23H12++ was suggested to be a possible carrier of the astronomically observed polycyclic aromatic hydrocarbon (PAH), which shows unique molecular structure with carbon two pentagons connected with five hexagons. Well observed astronomical infrared spectrum from 3-15 micron could be almost reproduced based on density functional theory. However, there remain several discrepancies with observed spectra, especially on 11-15 micron band weaker intensity. Observed 11.2 micron intensity is comparable to 7.6-7.8 micron one. Methyl-modified molecule C24H14++ revealed that calcula…
An absorption feature is occasionally reported around 11 microns in astronomical spectra, including those of forming stars. Candidate carriers include water ice, polycyclic aromatic hydrocarbons (PAHs), silicon carbide, crystalline silicates or even carbonates. All are known constituents of cosmic dust in one or more types of environments, though not necessarily together. In this paper we present new ground-based 8-13 micron spectra of one evolved star, several embedded young stellar objects (YSOs) and a background source lying behind a large column of the interstellar medium (ISM) toward the …
PAH Spectral Diversity in NGC 7027 and the Evolution of Aromatic Carriers
The University of Western Ontario（韦仕兰大学）；Leiden University（莱顿大学）；NASA Ames Research Center（美国国家航空航天局艾姆斯研究中心）；University of Maryland, College Park（马里兰大学帕克分校）；SETI Institute（搜寻地外文明研究所）
The molecular carriers of the ubiquitous absorption features called the diffuse interstellar bands (DIBs) have eluded identification for many decades, in part because of the enormous parameter space spanned by the candidates and the limited set of empirical constraints afforded by observations in the diffuse interstellar medium. Detection of these features in circumstellar regions, where the environmental properties are more easily measured, is thus a promising approach to understanding the chemical nature of the carriers themselves. Here, using high resolution spectra from the APOGEE survey, …
The carriers of the 21 and 30\,$μ$m emission features in infrared spectra of circumstellar envelopes are a long-standing enigma. In this paper, we present the results of molecular line observations toward two circumstellar envelopes exhibiting the 21 and/or 30\,$μ$m features, IRAS\,21318+5631 and 22272+5435, aiming at investigating whether they have unusual gas-phase chemistry and searching for possible gas-phase precursor of the carriers of the two dust features. The spectra cover several discrete frequency ranges of 130--164\,GHz and 216.5--273\,GHz, resulting in a detection of 13 molecular …
We analyze the 11.2 μm unidentified infrared band (UIR) spectrum from NGC 7027 and identify a small fullerene (C24) as a plausible carrier. The blurring effects of lifetime and vibrational anharmonicity broadening obscure the narrower, intrinsic spectral profiles of the UIR band carriers. We use a spectral deconvolution algorithm to remove the blurring, in order to retrieve the intrinsic profile of the UIR band. The shape of the intrinsic profile, a sharp blue peak and an extended red tail, suggests that the UIR band originates from a molecular vibration-rotation band with a blue band head. Th…
We present new room-temperature 1100 - 1800 cm^{-1} spectra of melilite silicates and 600 - 2000 cm^{-1} spectra of three randomly orientated fine-grained carbonates to determine the possible carrier(s) of a 6.9~micron absorption feature observed in a variety of dense astronomical environments including young stellar objects and molecular clouds. We focus on the low-mass post-AGB star Sakurai's Object which has been forming substantial quantities of carbonaceous dust since an eruptive event in the 1990s. Large melilite grains cannot be responsible for the 6.9-micron absorption feature because …
Recent detections of C$_{60}$, C$_{70}$, and C$_{60}^+$ in space induced extensive studies of fullerene derivatives in circumstellar environments. As the promising fullerene sources, protoplanetary nebulae (PPNe) show a number of unidentified bands in their infrared spectra, among which a small sample exhibits an enigmatic feature at $\sim21$\,$μ$m. Hydrogenation of fullerenes can produce fulleranes emitting new infrared bands. In this paper, we investigate the possibility of fulleranes (C$_{60}$H$_m$) as the carrier of the 21\,$μ$m feature in terms of theoretical vibrational spectra of fuller…
The JWST Proto-PAH project. Computational modeling of the emission carriers
NASA Ames Research Center（美国宇航局艾姆斯研究中心）；SETI Institute（SETI研究所）；Space Telescope Science Institute（太空望远镜科学研究所）；University of North Carolina, Chapel Hill（北卡罗来纳大学教堂山分校）；The University of Western Ontario（韦仕敦大学（西安大略大学））；Cardiff University（卡迪夫大学）；California Institute of Technology（加州理工学院）；ACRI-ST；Centre d’Etudes et de Recherche de Grasse (CERGA)（格拉斯研究与研究中心）
A prominent, mysterious emission feature peaking at ~20.1 micrometer --- historically known as the ``21 micrometer' feature --- is seen in over two dozen Galactic and Magellanic Cloud carbon-rich post-asymptotic giant branch (post-AGB) stars. The nature of its carrier remains unknown since the first detection of the 21 micrometer feature in 1989. Over a dozen materials have been suggested as possible carrier candidates. However, none of them has been accepted: they either require too much material (compared to what is available in the circumstellar shells around these post-AGB stars), or exhib…
We present an analysis and comparison of the 30 micron dust features seen in the Spitzer Space Telescope spectra of 207 carbon-rich asymptotic giant branch (AGB) stars, post-AGB objects, and planetary nebulae located in the Milky Way, the Magellanic Clouds (MCs), or the Sagittarius dwarf spheroidal galaxy (Sgr dSph), which are characterised by different average metallicities. We investigated whether the formation of the 30 micron feature carrier may be a function of the metallicity. Through this study we expect to better understand the late stages of stellar evolution of carbon-rich stars in t…
We investigate the occurrence of crystalline silicates in oxygen-rich evolved stars across a range of metallicities and mass-loss rates. It has been suggested that the crystalline silicate feature strength increases with increasing mass-loss rate, implying a correlation between lattice structure and wind density. To test this, we analyse Spitzer IRS and Infrared Space Observatory SWS spectra of 217 oxygen-rich asymptotic giant branch stars and 98 red supergiants in the Milky Way, the Large and Small Magellanic Clouds and Galactic globular clusters. These encompass a range of spectral morpholog…
We present a comprehensive survey of diffuse interstellar bands (DIBs) in $0.91-1.32μ$m with a newly-developed near-infrared (NIR) spectrograph, WINERED, mounted on the Araki 1.3 m Telescope in Japan. We obtained high-resolution ($R=28,300$) spectra of 25 early-type stars with color excesses of $0.07<E(B-V)<3.4$. In addition to the five DIBs previously detected in this wavelength range, we identified 15 new DIBs, 7 of which were reported as DIB "candidates" by Cox. We analyze the correlations among NIR DIBs, strong optical DIBs, and the reddening of the stars. Consequently, we found that all N…
在中红外波段，渐近巨星分支变星的周期-光度关系与K_S（2.14微米）相似，即使在24微米处，星周尘埃的辐射占主导地位。LMC和NGC 6522的M，logP图上的位点基本相同，尽管年龄和金属丰度不同。斜率随波长的变化没有系统的趋势。视星等与logP关系的偏移量意味着两个场之间的距离模数差为3.8。变量的颜色证实log P > 1.75的主周期是可检测质量损失的必要条件。在观测到的最长波长24微米处，许多半规则变星的尘埃壳层的光度与Miras周围的尘埃壳层相当。在涉及24微米星等的LMC颜色星等图中有一个明显的分歧。
The tip of the red giant branch (TRGB) is a well-established standard candle used to measure distances to nearby galaxies. The TRGB luminosity is typically measured in the I-band, where the luminosity has little dependency on stellar age or stellar metallicity. As the TRGB is brighter at wavelengths redder than the I-band, observational gains can be made if the TRGB luminosity can be robustly calibrated at longer wavelengths. This is of particular interest given the infrared capabilities that will be available with the James Webb Space Telescope and an important calibration consideration for u…
The 3.3 $μ$m unidentified infrared emission feature is commonly attributed to C-H stretching band of aromatic molecules. Astronomical observations have shown that this feature is composed of two separate bands at 3.28 and 3.30 $μ$m and the origin of these two bands is unclear. In this paper, we perform vibrational analyses based on quantum mechanical calculations of 153 organic molecules, including both pure aromatic molecules and molecules with mixed aromatic/olefinic/aliphatic hydridizations. We find that many of the C-H stretching vibrational modes in polycyclic aromatic hydrocarbon (PAH) m…
We report near-infrared (2.5--5 micron) long-slit (~ 30 arcsec) spectroscopy of a young stellar object in the direction toward the Galactic center with the Infrared Camera on board the AKARI satellite. The present target is suggested to be AFGL 2006 based on its very red color and close location. The spectra show strong absorption features of H$_2$O and CO$_2$ ices, and emission of HI Br alpha recombination line and the 3.3 micron band, the latter of which originates from polycyclic aromatic hydrocarbons (PAHs) or materials containing PAHs. The spectra show a broad, complex absorption feature …
We have traced the spatial distributions of intermediate-age and old stars in nine dwarf galaxies in the distant parts of the Local Group, using multi-epoch 3.6 and 4.5 micron data from the DUST in Nearby Galaxies with Spitzer (DUSTiNGS) survey. Using complementary optical imaging from the Hubble Space Telescope, we identify the tip of the red giant branch (TRGB) in the 3.6 micron photometry, separating thermally-pulsating asymptotic giant branch (TP-AGB) stars from the larger red giant branch (RGB) populations. Unlike the constant TRGB in the I-band, at 3.6 micron the TRGB magnitude varies by…
Based on the results of chemical analysis and simulation of kerogens and immature coals, a large number of chemical structures carrying the 3.4-mum feature were studied by means of computer simulation codes. Further selection criteria were the integrated strength of the absorption lines in the aliphatic stretchings wavelength band, weak IR activity in the aromatic stretching band and absence of notable activity outside the astronomical UIBs (Unidentified Infrared Bands). Most of the structures that were retained can be classed as branched, short and oxygen-bridged CH_{2} chains, and naphtenic …
Low- and intermediate-mass stars are one of the important dust sources in the interstellar medium (ISM) of galaxies. The compositions of dust ejected from these stars are likely to affect those in the ISM. We investigate dust in post-Asymptotic Giant Branch (AGB) stars, which are in a late evolutionary phase for low- and intermediate-mass stars, and which produce a wide variety of dust grains. We are particularly targeting post-AGB stars in the Large Magellanic Cloud (LMC), which has about half of the solar metallicity, to investigate the effects of sub-solar metallicity on dust compositions. …
The Hubble morphological sequence from early to late galaxies corresponds to an increasing rate of specific star formation. The Hubble sequence also follows a banana-shaped correlation between 24 and 70 micron luminosities, both normalized with the K-band luminosity. We show that this correlation is significantly tightened if galaxies with central AGN emission are removed, but the cosmic scatter of elliptical galaxies in both 24 and 70 micron luminosities remains significant along the correlation. We find that the 24 micron variation among ellipticals correlates with stellar metallicity, refle…
A broad continuum excess in the near-infrared, peaking in the rest-frame at 2-5 micron, is detected in a spectroscopic sample of 88 galaxies at 0.5<z<2.0 taken from the Gemini Deep Deep Survey. Line emission from polycyclic aromatic hydrocarbons (PAHs) at 3.3 micron alone cannot explain the excess, which can be fit by a spectral component consisting of a template of PAH emission lines superposed on a modified blackbody of temperature T~850 K. The luminosity of this near-infrared excess emission at 3 micron is found to be correlated with the star formation rate of the galaxy. The origin of the …
