Μm Feature Research Articles

Detection of PAH Absorption and Determination of the Mid-infrared Diffuse Interstellar Extinction Curve from the Sight Line toward Cyg OB2-12

Abstract The sight line toward the luminous blue hypergiant Cyg OB2-12 is widely used to study interstellar dust on account of its large extinction (A V ≃ 10 mag) and the fact that this extinction appears to be dominated by dust typical of the diffuse interstellar medium. We present a new analysis of archival Infrared Space Observatory Short Wavelength Spectrometer and Spitzer IRS observations of Cyg OB2-12 using a model of the emission from the star and its stellar wind to determine the total extinction A λ from 2.4 to 37 μm. In addition to the prominent 9.7 and 18 μm silicate features, we robustly detect absorption features associated with polycyclic aromatic hydrocarbons, including the first identification of the 7.7 μm feature in absorption. The 3.3 μm aromatic feature is found to be much broader in absorption than is typically seen in emission. The 3.4 and 6.85 μm aliphatic hydrocarbon features are observed with relative strengths that are consistent with observations of these features on sight lines toward the Galactic center. We identify and characterize more than 60 spectral lines in this wavelength range, which may be useful in constraining models of the star and its stellar wind. Based on this analysis, we present an extinction curve that extrapolates smoothly to determinations of the mean Galactic extinction curve at shorter wavelengths and to dust opacities inferred from emission at longer wavelengths, providing a new constraint on models of interstellar dust in the mid-infrared.

Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared

Abstract Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) may be present in H-rich and ultraviolet-poor benign regions. The addition of excess H atoms to PAHs converts the aromatic bonds into aliphatic bonds, the strongest of which falls near 3.4 μm. Therefore, superhydrogenated PAHs are often hypothesized to be a carrier of the 3.4 μm emission feature that typically accompanies the stronger 3.3 μm aromatic C–H stretching feature. To assess this hypothesis, we use density function theory to compute the infrared (IR) vibrational spectra of superhydrogenated PAHs and their ions of various sizes (ranging from benzene and naphthalene to perylene and coronene) and of various degrees of hydrogenation. For each molecule, we derive the intrinsic oscillator strengths of the 3.3 μm aromatic C–H stretch ( ) and the 3.4 μm aliphatic C–H stretch ( ). By comparing the computationally derived mean ratio of with the mean ratio of the observed intensities , we find that the degree of superhydrogenation—the fraction of carbon atoms attached with extra hydrogen atoms—is only ∼2.2% for neutral PAHs, which predominantly emit the 3.3 and 3.4 μm features. We also determine for each molecule the intrinsic band strengths of the 6.2 μm aromatic C–C stretch ( ) and the 6.85 μm aliphatic C–H deformation ( ). We derive the degree of superhydrogenation from the mean ratio of the observed intensities and for neutrals and for cations to be ≲3.1% for neutrals and ≲8.6% for cations. We conclude that astrophysical PAHs are primarily aromatic and are only marginally superhydrogenated.

Huge field enhancement and high transmittance enabled by terahertz bow-tie aperture arrays: a simulation study.

Sub-wavelength aperture arrays featuring small gaps have an extraordinary significance in enhancing the interactions of terahertz (THz) waves with matters. But it is difficult to obtain large light-substance interaction enhancement and high optical response signal detection capabilities at the same time. Here, we propose a simple terahertz bow-tie aperture arrays structure with a large electric field enhancement factor and high transmittance at the same time. The field enhancement factor can reach a high value of 1.9×104 and the transmission coefficient of around 0.8 (the corresponding normalized-to-area transmittance is about 14.3) at 0.04 µm feature gap simultaneously. The systematic simulation results show that the designed structure can enhance the intensity of electromagnetic hotspot by continuously reducing the feature gap size without affecting the intensity of the transmittance. We also visually displayed the significant advantages of extremely strong electromagnetic hot spots in local terahertz refractive index detection, which provides a potential platform and simple strategy for enhanced THz spectral detection.

Low-Power, Multimodal Laser Micromachining of Materials for Applications in sub-5 µm Shadow Masks and sub-10 µm Interdigitated Electrodes (IDEs) Fabrication.

Laser micromachining is a direct write microfabrication technology that has several advantages over traditional micro/nanofabrication techniques. In this paper, we present a comprehensive characterization of a QuikLaze 50ST2 multimodal laser micromachining tool by determining the ablation characteristics of six (6) different materials and demonstrating two applications. Both the thermodynamic theoretical and experimental ablation characteristics of stainless steel (SS) and aluminum are examined at 1064 nm, silicon and polydimethylsiloxane (PDMS) at 532 nm, and Kapton® and polyethylene terephthalate at 355 nm. We found that the experimental data aligned well with the theoretical analysis. Additionally, two applications of this multimodal laser micromachining technology are demonstrated: shadow masking down to approximately 1.5 µm feature sizes and interdigitated electrode (IDE) fabrication down to 7 µm electrode gap width.

Are fulleranes responsible for the 21 micron feature?

Recent detections of C60, C70, and C60+ in space induced extensive studies of fullerene derivatives in circumstellar environments. As the promising fullerene sources, protoplanetary nebulae (PPNe) shows a number of unidentified bands in their infrared spectra, among which a small sample exhibits an enigmatic feature at ∼21 µm. Hydrogenation converts fullerenes into fulleranes, which breaks the symmetry of fullerene molecules and produces new infrared bands. In this work, we investigate the possibility of fulleranes (C60Hm) as the carrier of the 21 µm feature in terms of theoretical vibrational spectra of fulleranes. The evidences favoring and disfavoring the fullerane hypothesis are presented. We made an initial guess for the hydrogen coverage of C60Hm that may contribute to the 21 µm feature.

Detecting and Characterizing Water Vapor in the Atmospheres of Earth Analogs through Observation of the 0.94 μm Feature in Reflected Light

Abstract The characterization of rocky, Earth-like planets is an important goal for future large ground- and space-based telescopes. In support of developing an efficient observational strategy, we have applied Bayesian statistical inference to interpret the albedo spectrum of cloudy true-Earth analogs that include a diverse spread in their atmospheric water vapor mixing ratios. We focus on detecting water-bearing worlds by characterizing their atmospheric water vapor content via the strong 0.94 μm H2O absorption feature, with several observational configurations. Water vapor is an essential signpost when assessing planetary habitability, and determining its presence is important in vetting whether planets are suitable for hosting life. We find that R = 140 spectroscopy of the absorption feature combined with a same-phase green-optical photometric point at 0.525–0.575 μm is capable of distinguishing worlds with less than 0.1× Earth-like water vapor levels from worlds with 1× Earth-like levels or greater at a signal-to-noise ratio of 5 or better with 2σ confidence. This configuration can differentiate between 0.01× and 0.1× Earth-like levels when the signal-to-noise ratio is 10 or better at the same confidence. However, strong constraints on the water vapor mixing ratio remained elusive with this configuration even at a signal-to-noise of 15. We find that adding the same-phase optical photometric point does not significantly help characterize the H2O mixing ratio, but does enable an upper limit on atmospheric ozone levels. Finally, we find that a 0.94 μm photometric point, instead of spectroscopy, combined with the green-optical point, fails to produce meaningful information about atmospheric water content.

Molecular gas inflows and outflows in ultraluminous infrared galaxies at z ∼ 0.2 and one QSO at z = 6.1

Aims. Our aim is to search for and characterize inflows and outflows of molecular gas in four ultraluminous infrared galaxies (ULIRGs; LIR > 1012L⊙) at z ∼ 0.2−0.3 and one distant quasi-stellar object (QSO) at z = 6.13. Methods. We used Herschel/PACS and ALMA Band 7 observations of the hydroxyl molecule (OH) line at rest-frame wavelength 119 μm, which in absorption can provide unambiguous evidence of inflows or outflows of molecular gas in nuclear regions of galaxies. Our study contributes to doubling the number of OH 119 μm observations of luminous systems at z ∼ 0.2−0.3, and pushes the search for molecular outflows based on the OH 119 μm transition to z ∼ 6. Results. We detect OH 119 μm high-velocity absorption wings in three of the four ULIRGs. In two cases, IRAS F20036−1547 and IRAS F13352+6402, the blueshifted absorption profiles indicate the presence of powerful and fast (∼200−500 km s−1) molecular gas outflows. Consistent with an inside-out quenching scenario, these outflows are depleting the central reservoir of star-forming molecular gas at a rate similar to that of intense star formation activity. For the starburst-dominated system IRAS 10091+4704, we detect an inverted P Cygni profile that is unique among ULIRGs and indicates the presence of a fast (∼400 km s−1) inflow of molecular gas at a rate of ∼100 M⊙ yr−1 towards the central region. Finally, we tentatively detect (∼3σ) the OH 119 μm doublet in absorption in the z = 6.13 QSO ULAS J131911+095051. The OH 119 μm feature is blueshifted with a median velocity that suggests the presence of a molecular outflow, although characterized by a modest molecular mass loss rate of ∼200 M⊙ yr−1. This value is comparable to the small mass outflow rates found in the stacking of the [C II] spectra of other z ∼ 6 QSOs and suggests that ejective feedback in this phase of the evolution of ULAS J131911+095051 has subsided.

The infrared bands of polycyclic aromatic hydrocarbons in the 1.6–1.7 μm wavelength region

Context. The 3.3 μm aromatic C–H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at ∼3.4–3.6 μm. One of these sources, IRAS 21282+5050, a planetary nebula, also exhibits a weak band at ∼1.68 μm. While the satellite features at ∼3.4–3.6 μm are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 μm feature. Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6–1.7 μm wavelength region. Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang–Landau random walk technique was employed. Results. All the dominant bands in the 1.6–1.7 μm wavelength range and in the 3.1–3.5 μm C–H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6–1.7 μm region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1–3.5 μm C–H stretching features to the bands in the 1.6–1.7 μm region, I3.1 − 3.5/I1.6 − 1.7, for both ground and vibrationally excited states. On average, we obtain ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈12.6 and ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈17.6 for PAHs at ground states and at vibrationally excited states, respectively.

Electrochemical synthesis of thin-layered graphene oxide-poly(CTAB) composite for detection of morphine

In this work, poly(cetyltrimethylammoniumbromide)/graphene oxide (poly(CTAB)/GO) composite was synthesized via in situ electropolymerisation and used as novel sensor for morphine (MO) detection. The CTAB functionalized graphene sheet offers large surface area which remarkably enhances the electrochemical response of MO compared to that of bare glassy carbon electrode (GCE). The surfactant CTAB with its hydrophilic head and hydrophobic tail plays a significant role in enhancing the catalytic property of the electrode. The electrostatic interaction between cationic group of poly (CTAB) and lone pair on nitrogen atom of tertiary amine group of MO favours the electro oxidation of tertiary amine group rather than phenolic group. A detection limit of 0.36 μM and a wide linear range of 50–60 μM feature the merits of the proposed composite in the analysis of MO. This sensor shows good repeatability, reproducibility, stability and anti interference properties. It is also tested in real samples. Schematic representation of the electrochemical behaviour of the modified electrode poly (CTAB)/GO/GCE towards morphine.

Robust Tracking of a Cost-Effective Micro-Stereolithography System Based on a Compliant Nanomanipulator.

Micro-stereolithography (MSL) has emerged as a promising and challenging technique in micro-/nano-scale additive manufacturing. Besides the requirement of the light source, the motion system requires ultra-high-precision tracking capability to reach the right location for every solidification event. To achieve single-digit micron feature size of the fabrication, we propose a robust control strategy to support a self-developed cost-effective MSL prototype based on a compliant nanomanipulator and a blue light-emitting diode (LED) module. In particular, the nonlinearity and parameter-variation of the compliant manipulator are dealt with by a robust radial basis function (RBF)-based neural network, and the repetitive control (RC) is innovatively integrated with RBF to improve the tracking performance of a closed pattern. Various simulations and real-time experiments are conducted to validate the proposed control strategy. The fabrication of a closed pattern will not begin by turning on the laser source until the tracking error reaches submicrons, and the fabrication results demonstrate that the cost-effective MSL system is capable of fabricating 2.5 µm feature size in a 0.5 mm working range.

The last pieces of the primitive inner belt puzzle: Klio, Chaldaea, Chimaera, and Svea

Aims.Several primitive families in the inner region of the main asteroid belt were identified as potential sources for two near-Earth asteroids (NEAs), (101955) Bennu and (162173) Ryugu, targets of the sample-return missions OSIRIS-REx and Hayabusa2, respectively. Four of the families, located at high proper inclinations (i> 10°), have not yet been compositionally studied: Klio, Chaldaea, Chimaera, and Svea. We want to characterize and analyze these families within the context of our PRIMitive Asteroid Spectroscopic Survey (PRIMASS), in order to complete the puzzle of the origins of the two NEAs.Methods.We obtained visible spectra (0.5–0.9μm) of a total of 73 asteroids within the Klio, Chaldaea, Chimaera, and Svea collisional families, using the instrument OSIRIS at the 10.4 m Gran Telescopio Canarias. We performed a taxonomical classification of these objects, and an analysis of the possible presence of absorption bands related to aqueous alterations, comparing the results with already studied primitive families in the inner main belt.Results.We present here reflectance spectra for 30 asteroids in the Klio family, 15 in Chaldaea, 20 in Chimaera, and 8 in Svea. We show that Klio, Chaldaea, and Chimaera members have moderately red spectral slopes, with aqueous alteration absorption bands centered around 0.7μm, characteristic of the group of primitive families known as Erigone-like. In contrast, Svea shows no 0.7μm features, and neutral and blue spectral slopes, and thus is a Polana-like family. While all four families might be related to (162173) Ryugu, the only family studied in this work that might be related to (101955) Bennu is Svea.

Study of CS, SiO, and SiS abundances in carbon star envelopes: Assessing their role as gas-phase precursors of dust.

We aim to determine the abundances of CS, SiO, and SiS in a large sample of carbon star envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in the surroundings of the central AGB star. We surveyed a sample of 25 carbon-rich AGB stars in the λ 2 mm band, more concretely in the J = 3 - 2 line of CS and SiO, and in the J = 7 - 6 and J = 8 - 7 lines of SiS, using the IRAM 30 m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient (LVG) method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. We also assessed the effect of infrared pumping in the excitation of the molecules. We detected CS in all 25 targeted envelopes, SiO in 24 of them, and SiS in 17 sources. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10-6 M⊙ yr-1 while it is detected in all envelopes with mass loss rates above that threshold. We found that CS and SiS have similar abundances in carbon star envelopes, while SiO is present with a lower abundance. We also found a strong correlation in which the denser the envelope, the less abundant are CS and SiO. The trend is however only tentatively seen for SiS in the range of high mass loss rates. Furthermore, we found a relation in which the integrated flux of the MgS dust feature at 30 μm increases as the fractional abundance of CS decreases. The decline in the fractional abundance of CS with increasing density could be due to gas-phase chemistry in the inner envelope or to adsorption onto dust grains. The latter possibility is favored by a correlation between the CS fractional abundance and the 30 μm feature, which suggests that CS is efficiently incorporated onto MgS dust around C-rich AGB stars. In the case of SiO, the observed abundance depletion with increasing density is most likely caused by an efficient incorporation onto dust grains. We conclude that CS, SiO (very likely), and SiS (tentatively) are good candidates to act as gas-phase precursors of dust in C-rich AGB envelopes.

Do evolved stars in the LMC show dual dust chemistry?

ABSTRACTWe study a group of evolved M-stars in the Large Magellanic Cloud, characterized by a peculiar spectral energy distribution. While the 9.7 $\mu$m feature arises from silicate particles, the whole infrared data seem to suggest the presence of an additional featureless dust species. We propose that the circumstellar envelopes of these sources are characterized by a dual dust chemistry, with an internal region, harbouring carbonaceous particles, and an external zone, populated by silicate, iron, and alumina dust grains. Based on the comparison with results from stellar modelling that describe the dust formation process, we deduce that these stars descend from low-mass (M < 2 M⊙) objects, formed 1–4 Gyr ago, currently evolving either in the post-AGB phase or through an after-pulse phase, when the shell CNO nuclear activity is temporarily extinguished. Possible observations able to confirm or disregard the present hypothesis are discussed.

Near-Earth asteroids spectroscopic survey atIsaac NewtonTelescope

Context.The population of near-Earth asteroids (NEAs) shows a large variety of objects in terms of physical and dynamical properties. They are subject to planetary encounters and to strong solar wind and radiation effects. Their study is also motivated by practical reasons regarding space exploration and long-term probability of impact with the Earth.Aims.We aim to spectrally characterize a significant sample of NEAs with sizes in the range of ~0.25–5.5 km (categorized as large), and search for connections between their spectral types and the orbital parameters.Methods.Optical spectra of NEAs were obtained using theIsaac NewtonTelescope (INT) equipped with the IDS spectrograph. These observations are analyzed using taxonomic classification and by comparison with laboratory spectra of meteorites.Results.A total number of 76 NEAs were observed. We spectrally classified 44 of them as Q/S-complex, 16 as B/C-complex, eight as V-types, and another eight belong to the remaining taxonomic classes. Our sample contains 27 asteroids categorized as potentially hazardous and 31 possible targets for space missions including (459872) 2014 EK24, (436724) 2011 UW158, and (67367) 2000 LY27. The spectral data corresponding to (276049) 2002 CE26 and (385186) 1994 AW1 shows the 0.7μm feature which indicates the presence of hydrated minerals on their surface. We report that Q-types have the lowest perihelia (a median value and absolute deviation of 0.797 ± 0.244 AU) and are systematically larger than the S-type asteroids observed in our sample. We explain these observational evidences by thermal fatigue fragmentation as the main process for the rejuvenation of NEA surfaces.Conclusions.In general terms, the taxonomic distribution of our sample is similar to the previous studies and matches the broad groups of the inner main belt asteroids. Nevertheless, we found a wide diversity of spectra compared to the standard taxonomic types.

30-micron sources in galaxies with different metallicities

Aims. We present an analysis and comparison of the 30 μm dust features seen in the Spitzer Space Telescope spectra of 207 carbon-rich asymptotic giant branch (AGB) stars, post-AGB objects, and planetary nebulae (PNe) 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 μm 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 these galaxies. Methods. Our analysis uses the “Manchester method” as a basis for estimating the temperature of dust for the carbon-rich AGB stars and the PNe in our sample. For post-AGB objects we changed the wavelength ranges used for temperature estimation, because of the presence of the 21 μm feature on the short wavelength edge of the 30 μm feature. We used a black-body function with a single temperature deduced from the Manchester method or its modification to approximate the continuum under the 30 μm feature. Results. We find that the strength of the 30 μm feature increases until dust temperature drops below 400 K. Below this temperature, the large loss of mass and probably the self-absorption effect reduces the strength of the feature. During the post-AGB phase, when the intense mass-loss has terminated, the optical depth of the circumstellar envelope is smaller, and the 30 μm feature becomes visible again, showing variety of values for post-AGB objects and PNe, and being comparable with the strengths of AGB stars. In addition, the AGB stars and post-AGB objects show similar values of central wavelengths – usually between 28.5 and 29.5 μm. However, in case of PNe the shift of the central wavelength towards longer wavelengths is visible. The normalised median profiles for AGB stars look uniformly for various ranges of dust temperature, and different galaxies. We analysed the profiles of post-AGB objects and PNe only within one dust temperature range (below 200 K), and they were also similar in different galaxies. In the spectra of 17 PNe and five post-AGB objects we found the broad 16–24 μm feature. Two objects among the PNe group are the new detections: SMP LMC 51, and SMP LMC 79, whereas in the case of post-AGBs the new detections are: IRAS 05370-7019, IRAS 05537-7015, and IRAS 21546+4721. In addition, in the spectra of nine PNe we found the new detections of 16–18 μm feature. We also find that the Galactic post-AGB object IRAS 11339-6004 has a 21 μm emission. Finally, we have produced online catalogues of photometric data and Spitzer IRS spectra for all objects that show the 30 μm feature. These resources are available online for use by the community. Conclusions. The most important conclusion of our work is the fact that the formation of the 30 μm feature is affected by metallicity. Specifically that, as opposed to more metal-poor samples of AGB stars in the MCs, the feature is seen at lower mass-loss rates, higher temperatures, and has seen to be more prominent in Galactic carbon stars. The averaged feature (profile) in the AGB, post-AGB objects, and PNe seems unaffected by metallicity at least between a fifth and solar metallicity, but in the case of PNe it is shifted to significantly longer wavelengths.

Modification of mechanical properties and resolution of printed stereolithographic objects through RAFT agent incorporation

Stereolithography (SL) is an additive manufacturing technique that uses light to cure liquid resins into thin layers and fabricate 3-dimensional objects layer by layer. SL is of high interest for small-volume manufacturing and rapid prototyping because of its ability to relatively quickly create objects with intricate 100 μm or smaller features. However, widespread adoption of SL faces a number of obstacles including unsuitable thermomechanical properties, anisotropic properties, and limited resolution and fidelity. In this work, we incorporate a reversible addition-fragmentation chain transfer (RAFT) agent into a glassy acrylate formulation to modify mechanical properties and improve resolution of objects printed using digital light processing (DLP) SL. Incorporating a small amount of a trithiocarbonate RAFT agent into the formulation leads to increased elongation and toughness accompanied by a small decrease in tensile modulus. To determine anisotropic properties of DLP SL, samples were printed in “horizontal” or “vertical” directions, where the long axis of the sample was printed in the x-axis or z-axis, respectively. RAFT samples printed in a vertical orientation exhibit a higher modulus than non-RAFT controls prior to post-cure in addition to a similar modulus with increased toughness upon UV post-cure due to the living/controlled nature of RAFT polymerization. Furthermore, incorporating a RAFT agent into the formulation allows significantly higher fidelity printing of a broad range of positive and negative features as small as 100 μm. The RAFT formulation allows objects to be printed with significantly better fidelity than non-RAFT formulations, even when a radical scavenger is incorporated to mimic reaction rates observed from the RAFT formulation. Additionally, the RAFT agent significantly increases the critical energy parameter determined from the SL working curve, indicating an increase in gel point conversion. This work demonstrates the benefits of using controlled/living polymerization in a highly cross-linked acrylate system to improve toughness, modify anisotropic properties, and print high-fidelity features with enhanced properties in 3D printed materials.

High resolution integral imaging display by using a microstructure array

A novel integral imaging method that can be used to display three-dimensional (3D) images with high resolution is proposed. The integral imaging system consists of a delicate microstructure array and microlens array. The microstructure array displays the information of the microimage array. Due to the application of the delicate microstructure array, a large number of efficiency pixels in the microimage array are regularly imaged through the microlens array, which will result in the 3D images with high resolution. In this paper, the relationships among the resolution of the 3D images, the focal length of the microlens, microstructure array/microlens array distance, and the feature size of the microstructure array are analyzed theoretically based on optical design theories. Related experiments are performed. The microstructure array with 1 μm feature size is fabricated by the lithographic method and applied on the integral imaging system, which forms the 3D images with the resolution of 10 lines/mm. Compared with a traditional integral imaging system, the resolution of the 3D images generated by the method is effectively enhanced. It is inferred that the method has great potential for static 3D displays such as 3D photographs, 3D advertisements, and so on.

Interplay between pulsation, mass loss, and third dredge-up: More about Miras with and without technetium

Context. We follow-up on a previous finding that AGB Mira variables containing the third dredge-up indicator technetium (Tc) in their atmosphere form a different sequence of K − [22] colour as a function of pulsation period than Miras without Tc. A near- to mid-infrared colour such as K − [22] is a good probe for the dust mass-loss rate of the stars. Contrary to what might be expected, Tc-poor Miras show redder K − [22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at a given period. Aims. Here, the previous sample is extended and the analysis is expanded towards other colours and dust spectra. The most important aim is to investigate if the same two sequences can be revealed in the gas mass-loss rate. Methods. We analysed new optical spectra and expanded the sample by including more stars from the literature. Near- and mid-IR photometry and ISO dust spectra of our stars were investigated where available. Literature data of gas mass-loss rates of Miras and semi-regular variables were collected and analysed. Results. Our results show that Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR, suggesting that the previous finding based on the K − [22] colour is not due to a specific dust feature in the 22 μm band. We establish a linear relation between K − [22] and the gas mass-loss rate. We also find that the 13 μm feature disappears above K − [22]≃2.17 mag, corresponding to Ṁg ∼ 2.6 × 10−7 M⊙ yr−1. No similar sequences of Tc-poor and Tc-rich Miras in the gas mass-loss rate vs. period diagram are found, most probably owing to limitations in the available data. Conclusions. Different hypotheses to explain the observation of two sequences in the P vs. K − [22] diagram are discussed and tested, but so far, none of them convincingly explains the observations. Nevertheless, we might have found an hitherto unknown but potentially important process influencing mass loss on the TP-AGB.

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Abstract We present insights into the behavior of the astronomical 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission complex as gleaned from analyzing synthesized spectra, utilizing the data and tools from the NASA Ames PAH IR Spectroscopic Database. We specifically study the influence of PAH size, charge, aliphatic content, and nitrogen substitution on the profile and peak position of the 7.7 μm feature (λ 7.7). The 7.7 μm band is known to vary significantly from object to object in astronomical observations, but the origin of these variations remains highly speculative. Our results indicate that PAH size can accommodate the largest shift in λ 7.7 (≃0.4 μm), where relatively small PAHs are consistent with class A spectra ( ≤ 60), while large PAHs are consistent with red/very red class B spectra. Aliphatic PAHs, of which our sample only contains a few, can produce redshifts typically around 0.15 μm; changes in ionization fraction, depending on the species, produce shifts of up to 0.1 μm; and nitrogen substitution has no effect on λ 7.7. Within the limits of our study, the class B → A transition is best explained with a changing PAH size distribution, with a relatively minor role assigned to aliphatic content and varying charge states. The resulting astronomical picture is that the photochemical evolution of PAHs moving from shielded class C/B environments into exposed ISM-like class A environments may be intrinsically different from the reverse class A → B transition of interstellar PAHs being incorporated into newly forming star systems.

Multi-scale 3D modelling of a DEMO prototype cable from strand to full-size conductor based on X-ray tomography and image analysis

The design of a superconducting magnet system of a fusion reactor candidate is usually based on the Cable in Conduit Conductor (CICC) concept. CICC consist of complex structures with several hundreds of highly packed, multistage twisted superconductor and copper strands, cooling structures – all wrapped in thin steel foils and jacketed in relatively thick stainless-steel pipes. As recently demonstrated, such complex morphology can be captured by fully 3D X-ray tomography. One of the three alternative winding pack (WP) options for the toroidal field coils of the European DEMO superconducting magnet system as proposed by the Italian ENEA, features a layer-wound WP design adopting a wind-and-react conductor with high aspect ratio rectangular cross section. A multi-physics modelling requiring high accuracy from the strand to the full-size conductor scale was tested on two X-ray tomography systems. For the microstructural integrity analysis of the internal tin one mm diameter Nb3Sn strand, manufactured by Western Superconducting Technologies, a microtomograph with ˜1 μm feature recognition was used. For the full-size conductor (66 x 25 mm2 internal dimensions) a high energy (> 300 kVp) microtomograph with voxel resolution of ˜20 μm was employed. Voids and/or geometrical imperfections were detected, accurately pin-pointed and locally investigated at high resolution before and after heat treatment. Dimensional measurements on the 3D internal structure of the strand showed the small dimensional changes of the strand after the heat treatment. A significant result was the determination of the twist-pitch factor in a purely non-invasive way. The 3D reconstructions of the CICC cables were used to verify the cabling and compaction processes as well as the structural integrity of the strands, wrapping foils and spiral cooling structures. Also, 3D reconstruction of strand trajectories provided a strong input for a multiphysical modelling/interpretation of mechanical–electrical properties of CICC in cryogenic–electromagnetic environments.