We present a 0.8-5 μm spectral library of 210 cool stars observed at a resolving power of R ≡ λ/Δλ ~ 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars have well-established MK spectral classifications and are mostly restricted to near-solar metallicities. The sample not only contains the F, G, K, and M spectral types with luminosity classes between I and V, but also includes some AGB, carbon, and S stars. In contrast to some other spectral libraries, the continuum shape of the spectra is measured and preserved in the data reduction process. The spectra are absolutely flux calibrated using the Two Micron All Sky Survey photometry. Potential uses of the library include studying the physics of cool stars, classifying and studying embedded young clusters and optically obscured regions of the Galaxy, evolutionary population synthesis to study unresolved stellar populations in optically obscured regions of galaxies and synthetic photometry. The library is available in digital form from the IRTF Web site.

The infrared telescope facility (irtf) spectral library: cool stars

Freeze-out of the gas-phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H2O, NH3, CO2, CO, CH3OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and Solar System environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze-out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state-of-the-art observations show promise for much progress in this area with planned infrared facilities.

Observations of the Icy Universe

We present models for the evolution of the elemental abundances in the gas and dust phases of the interstellar medium (ISM) of our Galaxy by generalizing standard models for its dynamical and chemical evolution. In these models, the stellar birthrate history is determined by the infall rate of primordial gas and by its functional dependence on the mass surface density of the stars and gas. We adopt a two-component Galaxy consisting of a central bulge and an exponential disk with different infall rates and stellar birthrate histories. Condensation in stellar winds, Type Ia and Type II supernovae, and the accretion of refractory elements onto preexisting grains in dense molecular clouds are the dominant contributors to the abundance of elements locked up in the dust. Grain destruction by sputtering and evaporative grain-grain collisions in supernova remnants are the most important mechanisms that return these elements back to the gas phase. Guided by observations of dust formation in various stellar sources, and by the presence of isotopic anomalies in meteorites, we calculate the production yield of silicate and carbon dust as a function of stellar mass. We find that Type II supernovae are the main source of silicate dust in the Galaxy. Carbon dust is produced primarily by low-mass stars in the ~2-5 M☉ range. Type Ia SNe can be important sources of metallic iron dust in the ISM. We also analyze the origin of the elemental depletion pattern and find that the observed core + mantle depletion must reflect the efficiency of the accretion process in the ISM. We also find that grain destruction is very efficient, leaving only ~10% of the refractory elements in grain cores. Observed core depletions are significantly higher, requiring significant UV, cosmic ray, or shock processing of the accreted mantle into refractory core material. Adopting the current grain destruction lifetimes from Jones et al., we formulate a prescription for its evolution in time. We make a major assumption, that the accretion timescale evolves in a similar fashion, so that the current ratio between these quantities is preserved over time. We then calculate the evolution of the dust abundance and composition at each Galactocentric radius as a function of time. We find that the dust mass is linearly proportional to the ISM metallicity and is equal to about 40% of the total mass of heavy elements in the Galaxy, independent of Galactocentric radius. The derived relation of dust mass with metallicity is compared to the observed Galactic dust abundance gradient, and to the Mdust versus log (O/H) relation that is observed in external dwarf galaxies. The dependence of dust composition on the mass of the progenitor star and the delayed recycling of newly synthesized dust by low-mass stars back to the ISM give rise to variations in the dust composition as a function of time. We identify three distinct epochs in the evolution of the dust composition, characterized by different carbon-to-silicate mass ratios. Two such epochs are represented by the Galaxy and the SMC. The third is characterized by an excess of carbon dust (compared to the Milky Way Galaxy), and should be observed in galaxies or star-forming regions in which the most massive carbon stars are just evolving off the main sequence. Our models provide a framework for the self-consistent inclusion of dust in population synthesis models for various pre-galactic and galactic systems, allowing for the calculation of their UV to far-infrared spectral energy distribution at various stages of their evolution.

The Evolution of the Elemental Abundances in the Gas and Dust Phases of the Galaxy

It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.

https://science.sciencemag.org/content/sci/249/4967/366.full.pdf

Cometary delivery of organic molecules to the early Earth.

▪ Abstract In this contribution, a review is presented on the ample data obtained on post-AGB stars, both on the central stars and their circumstellar material. The fast evolutionary phase is characterized by a rapid change in the properties of the objects, but the variety is so large that there is yet no clear consensus on how the detailed studies of individual objects are linked together by evolutionary channels. The absence of strong molecular veiling in the photospheres of the central stars, together with a spread in intrinsic metallicity make post-AGB stars very useful in constraining AGB chemical evolutionary models. We discuss the surprisingly wide variety of chemical signatures observed. The onset in the creation process of the panoply of structures and shapes observed in planetary nebulae occurs during the short post-AGB evolution, but the physical nature of the processes involved is still badly understood. In the rapidly growing field of circumstellar mineralogy, post-AGB stars have their story ...

Post-AGB Stars

In this letter, far-infrared spectrophotometry (30-55 microns) and photometry (53-200 microns) are presented which define, for the first time, the long wavelength limit of the previously unidentified 30 micron emission feature found in certain extreme carbon star spectra. The spectral similarities are sufficiently similar to those of solid MgS that MgS is proposed to be the band carrier. This is interpreted as the first direct evidence that chemical surface reactions occur on dust grains in circumstellar environments. The presence of MgS indicates that either the oxygen abundance is relatively low and/or the sulfur abundance is high in extreme carbon stars.

MgS grain component in circumstellar shells

A study of all full-scan spectra of optically thin oxygen-rich circumstellar dust shells in the database produced by the Short Wavelength Spectrometer on ISO reveals that the strength of several infrared spectral features correlates with the strength of the 13 ?m dust feature. These correlated features include dust features at 19.8 and 28.1 ?m and the bands produced by warm carbon dioxide molecules (the strongest of which are at 13.9, 15.0, and 16.2 ?m). The database does not provide any evidence for a correlation of the 13 ?m feature with a dust feature at 32 ?m, and it is more likely that a weak emission feature at 16.8 ?m arises from carbon dioxide gas rather than dust. The correlated dust features at 13, 20, and 28 ?m tend to be stronger with respect to the total dust emission in semiregular and irregular variables associated with the asymptotic giant branch than in Mira variables or supergiants. This family of dust features also tends to be stronger in systems with lower infrared excesses and thus lower mass-loss rates. We hypothesize that the dust features arise from crystalline forms of alumina (13 ?m) and silicates (20 and 28 ?m).

Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features*

Laboratory spectra and mass absorption coefficients of MgS, CaS, FeS, SiS2, FeS2, Fe3C, and a commercial iron carbide are presented over the wavelength range 125-15 microns. These spectra confirm that MgS is the most likely source of the unidentified 30-micron emission in carbon-rich sources and that FeS, Fe3C, and 'iron carbide' cannot be responsible for this feature although they could contribute to the continuum in this region. CaS and FeS2 may contribute to the 30-micron feature; however, both higher resolution and higher precision astronomical observations are needed before their presence can be established. SiS2 has a peak near 22 microns and therefore cannot be a significant component of the dust in such regions.

Laboratory infrared spectra of predicted condensates in carbon-rich stars

Complete 0.75-13 micron spectrometry of a carbon-rich, Mira-class variable star is presented for the first time. It is noted that although the near-infrared is dominated by photospheric absorption bands of the CN red system, the infrared becomes progressively dominated by the bands of the polyatomic molecules HCN and C2H2. Since the band at 3.1 microns is known to be due to HCN and C2H2, it is possible to associate bands at 1.04, 1.53, 1.85, 2.5, 2.7, 3.56, 3.85, 4.8, and 7.1 microns with HCN and C2H2. The spectrum suggests that radiative transfer in the carbon Mira class cannot be discussed quantitatively without the inclusion of HCN and C2H2 opacity. On the basis of the carbon star models of Querci and Querci (1974), it is deduced that the abundance ratio of HCN to C2H2 can be used to indicate whether 3-alpha-processed or CNO-processed material is in the outer atmosphere. An 11.3 micron SiC dust-emission feature is present, although it differs significantly from the 11.7 micron SiC feature in Y CVn. A featureless emission is present from 4 to 13 microns and can be ascribed to optically thin graphite grains having a temperature of 450 K.

Identification of new infrared bands in a carbon-rich Mira variable

The paper deals with spectrophotometric observations covering the essentially complete wavelength interval between 1.2 and 30.0 microns. The observations confirm the identification of the C3 band at 5.2 microns. They show that if SiC2 is present, the SiC1 absorption band at 5.7 microns would be obscured by C3 at a 1% spectral resolution. Silicon carbide emission at 11.5 microns exists simultaneously with C3 absorption at 5.2 microns, requiring a contribution of both species to the violet opacity of Y CVn.

J. H. Goebel

Papers

MgS grain component in circumstellar shells

Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features*

Laboratory infrared spectra of predicted condensates in carbon-rich stars

Identification of new infrared bands in a carbon-rich Mira variable

The infrared spectrum of the carbon star Y Canum Venaticorum between 1.2 and 30 microns