Spectral Lines Research Articles

MK-like spectral classification for hot subdwarf stars with LAMOST spectra

Abstract An MK-like spectral classification has been conducted for 1224 hot subdwarf stars with LAMOST DR9 low-resolution spectra. The whole sample was divided into four categories according to the spectral line characteristics: He-normal, He-weak, He-strong C, and He-strong. Each selected spectrum was assigned a spectral class, a luminosity class, and a helium class by comparing the line depth and width with standard spectra selected in LAMOST. Relationships between atmospheric parameters and spectral classification are also presented.

A Big Red Dot: Scattered light, host galaxy signatures and multi-phase gas flows in a luminous, heavily reddened quasar at cosmic noon

Abstract We present a deep X-Shooter rest-frame UV to optical spectral analysis of the heavily reddened quasar, ULASJ2315+0143 at z = 2.566, known to reside in a major-merger host galaxy. The rest-frame optical is best-fit by a dust-reddened quasar (E(B-V)QSO = 1.55) with black-hole mass $\rm log_{10}(H\beta , M\small {BH} [{\rm M}_{\odot }]) = 10.26 \pm 0.05$, bolometric luminosity $\rm L_{Bol}$ = $\rm 10^{48.16}\, erg\,\,s^{-1}$ and Eddington-scaled accretion rate log$_{10}(\rm \lambda _{Edd}) = -0.19$. We find remarkable similarities between ULASJ2315+0143 and the high-redshift Little Red Dots (LRDs). The rest-frame UV cannot be explained by a dusty quasar component alone and requires an additional blue component consistent with either a star-forming host galaxy or scattered AGN light. We detect broad high-ionisation emission lines in the rest-UV, supporting the scattered light interpretation for the UV excess. The scattering fraction represents just 0.05% of the total luminosity of ULASJ2315+0143 . Analysis of the mid infra-red SED suggests an absence of hot dust on torus-scales similar to what is observed for LRDs. The obscuring medium is therefore likely on galaxy scales. We detect narrow, blueshifted associated absorption line systems in 1 iv, 1 v, 1 iv and 1 iii. There is evidence for significant high-velocity (>1000 $\rm km\, s^{-1}$) outflows in both the broad and narrow line regions as traced by 1 iv and [1 iii] emission. The kinetic power of the [1 iii] wind is $\dot{\epsilon }_{k}^{ion} = 10^{44.61} \rm erg\, s^{-1} \sim 0.001\, L_{Bol}$. ULASJ2315+0143 is likely in an important transition phase where star formation, black-hole accretion and multi-phase gas flows are simultaneously occurring.

Dual-species sensor using time-division multiplexed laser absorption spectroscopy for hydrogen sulfide and carbon monoxide measurement in SF6 insulated equipment fault detection

To address the issues of high detection limits, long response times, and susceptibility to cross-interference in the detection of characteristic gases CO and H2S produced by SF6 electrical equipment failures, a dual-species laser sensor prototype for SF6 decomposition products in electrical equipment was developed based on tunable diode laser absorption spectroscopy (TDLAS). The absorption lines of 2334 nm and 1576 nm were selected for CO and H2S, respectively. The concentration inversion was performed using wavelength modulation spectroscopy (WMS) with first harmonic normalization of the second harmonic. The experimental results demonstrate that a rapid response time of ∼30 s is achieved and the sensor has detection accuracies of 0.59 % for CO and 2.09 % for H2S, with minimum detection limits of 0.46 ppb and 14.00 ppb, respectively. The dual-species laser sensor allows for convenient, rapid, and precise detection of CO and H2S, showing promising application prospects in electrical environment monitoring.

Cascaded multi-phonon stimulated Raman scattering near second-harmonic generation in a thin-film lithium niobate microdisk.

High-quality microresonators can greatly enhance light-matter interactions and are excellent platforms for studying nonlinear optics. Wavelength conversion through nonlinear processes is the key to many applications of integrated optics. The stimulated Raman scattering (SRS) process can extend the emission wavelength of a laser source to a wider range. Lithium niobate (LN), as a Raman active crystalline material, has remarkable potential for wavelength conversion. Here, we demonstrate the generation of cascaded multi-phonon Raman signals near the second-harmonic generation (SHG) peak in an X-cut thin-film lithium niobate (TFLN) microdisk. Fine tuning of the specific cascaded Raman spectral lines has also been made by changing the pump wavelength. Raman lines can reach a wavelength up to about 80 nm away from the SHG signal. We realize the SFG process associated with Raman signals in the visible range as well. Our work extends the use of WGM microresonators as effective optical upconversion wavelength converters in nonlinear optical applications.

The ammonia absorption spectrum between 3900 and 6350 cm-1: 15NH3 contribution and a recommended list for natural ammonia

Following previous contributions dedicated to the main isotopologue, 14NH3, in the present work, we examine the 15NH3 absorption spectrum between 3950 and 6350 cm-1 in order to elaborate an absorption line list for ammonia in natural isotopic abundance. For this purpose, several room temperature Fourier Transform absorption spectra of ammonia highly enriched in 15NH3 measured in the 90′s and available from the Kitt Peak data centre have been analysed. In addition, a more recent high-resolution spectrum recorded in Brussels between 4150 and 4600 cm-1 is also considered. The spectra were divided in three separate intervals (3900–4706, 4706–5650 and 5650–6350 cm-1) for which experimental lists including 3506, 3788, and 2317 lines, respectively, were retrieved and treated independently. By comparison with an ab initio line list (the BYTe list) and systematically validating the assignments by using lower state combination difference relations (LSCD), 2013, 1771 and 570 transitions could be assigned in the three regions, respectively. They belong to 16, 22, and 10 bands and correspond to 98.5, 92.6 and 78.6 % of the experimental intensities in the region, respectively. For comparison, only four 15NH3 bands were previously assigned in the literature in the entire studied region.The obtained 15NH3 assigned line lists were gathered with the previously elaborated line list of the main isotopologue to produce a recommended line list for natural ammonia between 3900 and 6350 cm-1.

Rovibrational Line Lists of Triplet and Singlet Methylene.

The methylene molecule (CH2) is a short-lived radical with lacking data on its spectral line intensities. Although the lifetime of CH2 is extremely short under Earth's conditions, it exists in a free form in interstellar media. CH2 is an important intermediate species in chemical reactions associated with the formation and destruction of complex hydrocarbons. We present the first rovibrational line lists of CH2 in its ground triplet and first excited singlet electronic state. To this end, our previously developed accurate ab initio potential energy surface (PES) was used for the ground electronic triplet state [Egorov et al. J. Comp. Chem. 2024. V. 45. (2). P. 83] while a new PES for the singlet state was constructed in this work using the single-reference coupled cluster approach [CCSD(T)] combined with the extrapolation to the complete basis set (CBS) limit based on the correlation-consistent orbital basis sets with the core-valence electron correlation effects [aug-cc-pCVXZ, X = T, Q, 5, and 6]. In addition, the contributions to the correlation energy from highly excited Slater determinants [CC(n), n = 3-5] were included as well as the scalar relativistic effects and DBOC. The most accurate description of the infrared band origins of singlet CH2 was thus achieved for the energy range where the impact of the nonadiabatic coupling due to the Renner-Teller effect can be neglected. To obtain the probabilities of the rovibrational transitions, new ab initio DMSs were constructed both for the triplet and singlet CH2 using the CCSD(T)/aug-cc-pCVQZ approach. Finally, the absorption spectra of triplet and singlet methylene were predicted from the variationally computed line lists.

A new code for low-resolution spectral identification of white dwarf binary candidates

Close white dwarf binaries (CWDBs) are considered to be progenitors of several exotic astronomical phenomena (e.g., type Ia supernovae, cataclysmic variables). These violent events are broadly used in studies of general relativity and cosmology. However, obtaining precise stellar parameter measurements for both components of CWDBs is a challenging task given their low luminosities, swift time variation, and complex orbits. High-resolution spectra (R$> 20 000$) are preferred but expensive, resulting in a sample size that is insufficient for robust population study. Recently, studies have shown that the more accessible low-resolution (R$ 2000$) spectra (LRS) may also provide enough information for spectral decomposition. To release the full potential of the less expensive low-resolution spectroscopic surveys, and thus greatly expand the CWDB sample size, it is necessary to develop a robust pipeline for spectra decomposition and analysis. We aim to develop a spectroscopic fitting program for white dwarf binary systems based on photometry, LRS, and stellar evolutionary models. The outputs include stellar parameters of both companions in the binary including effective temperature, surface gravity, mass, radius, and metallicity in the case of MS stars. We used an artificial neural network (ANN) to build spectrum generators for DA/DB white dwarfs and main-sequence stars. Characteristic spectral lines were used to decompose the spectrum of each component. The best-fit stellar parameters were obtained by finding the least $ solution to these feature lines and the continuum simultaneously. Compared to previous studies, our code is innovative in the following aspects: (1) implementing a sophisticated binary decomposition technique in LRS for the first time; (2) using flux-calibrated spectra instead of photometry plus spectral lines, in which the latter requires multi-epoch observations; (3) applying an ANN in binary decomposition, which significantly improves the efficiency and accuracy of generated spectra. We demonstrate the reliability of our code with two well-studied CWDBs, WD 1534+503 and PG 1224+309. We also estimate the stellar parameters of 14 newly identified CWDB candidates, most of which are fitted with double component models for the first time. Our estimates agree with previous results for the common stars and follow the statistical distribution in the literature. We provide a robust program for fitting binary spectra of various resolutions. Its application to a large volume of white dwarf binary candidates will offer important statistic samples to stellar evolution studies and future gravitational wave monitoring.

Searching for magnetic fields in featureless white dwarfs with the DIPOL-UF polarimeter at the Nordic Optical Telescope

About 20<!PCT!> of the white dwarfs possess a magnetic field that may be detected by the splitting and/or polarization of their spectral lines. As they cool, the effective temperatures of the white dwarfs become so low that no spectral lines can be seen in the visible wavelength range. If their atmospheres are not polluted by the debris of a planetary system, these cool white dwarfs have featureless optical spectra. Until quite recently, very little was known about the incidence of magnetic fields in these objects. However, when observed with polarimetric techniques, a significant number of featureless white dwarfs reveal strong magnetic fields in their optical continuum spectra. Measuring the occurrence rate and strength of magnetic fields in old white dwarfs may help us to understand how these fields are generated and evolve. We report the results of an ongoing survey of cool white dwarfs with the high-precision broad-band polarimeter DIPOL-UF, which is deployed at the Nordic Optical Telescope on La Palma, Spain. This survey has led to the firm discovery of 13 new cool magnetic white dwarfs in the solar neighborhood so far, including six new detections that we report in this paper.

Terahertz Frequency-Domain Spectroscopy with a Method for Suppressing Water Vapor Absorption Peaks for Analysis of Pharmaceutical Hydrate Samples

Measurements of terahertz absorption spectra of pharmaceutical hydrate samples are achieved under a normal humidity condition by combining terahertz frequency-domain spectroscopy with a newly proposed method for suppressing absorption peaks caused by water vapor. In this method, only simple mathematical operations such as subtraction, thresholding, interpolation, and smoothing are applied to extinction (or absorbance) data obtained by locating samples under a normal humidity condition. By considering the difference in spectral line width between narrow absorption peaks caused by water vapor and the relatively wide absorption peaks caused by active pharmaceutical ingredients (APIs) in solid forms, the absorption peaks caused by water vapor can be effectively suppressed without affecting the absorption peaks of the APIs in the samples. In the present study, levofloxacin hydrates were used as samples to investigate the performance of the proposed method. Spectra were obtained under both dry and normal humidity conditions. The temperature of the samples was raised from 300 to 363 K to dehydrate them and brought back to 313 K to observe hydration under the normal humidity condition. Spectra obtained under the normal humidity condition were processed with the proposed method. The spectra of the hydrates obtained under the dry condition were slightly different from those obtained under the normal humidity condition and processed by our method. Dehydration during the measurements under the dry condition was suggested. Stable and reliable results are expected by measuring spectra under normal humidity conditions and applying the proposed method to suppress absorption peaks by water vapor.

Colorado Ultraviolet Transit Experiment Near-ultraviolet Transmission Spectroscopy of the Ultrahot Jupiter KELT-9b

We present new near-ultraviolet (NUV, λ = 2479–3306 Å) transmission spectroscopy of KELT-9b, the hottest known exoplanet, obtained with the Colorado Ultraviolet Transit Experiment CubeSat. Two transits were observed on 2022 September 28th and September 29th, referred to as Visits 1 and 2 respectively. Using a combined transit and systematics model for each visit, the best-fit broadband NUV light curves are R p/R ⋆ = 0.136 −0.0146+0.0125 for Visit 1 and R p/R ⋆ = 0.111 −0.0190+0.0162 for Visit 2, appearing an average of 1.54× larger in the NUV than at optical wavelengths. While the systematics between the two visits vary considerably, the two broadband NUV light curves are consistent with each other. A transmission spectrum with 25 Å bins suggests a general trend of excess absorption in the NUV, consistent with expectations for ultrahot Jupiters. Although we see an extended atmosphere in the NUV, the reduced data lack the sensitivity to probe individual spectral lines.

FNet II: Spectral classification of quasars, galaxies, stars, and broad absorption line (BAL) quasars

FNet II: Spectral classification of quasars, galaxies, stars, and broad absorption line (BAL) quasars

Sensitivity Enhancement of Ultra-Wideline NMR by Progressive Saturation of the Proton Reservoir Under Magic-Angle Spinning.

Solid-state NMR of low-γ nuclides is often characterized by low sensitivity and significant spectral broadenings induced by the quadrupolar and the chemical-shift anisotropy interactions. Herein, we introduce an indirect acquisition method, termed PROgressive Saturation of the Proton Reservoir Under Spinning (PROSPRUS), which could facilitate the acquisition of ultra-wideline NMR spectra under magic-angle spinning, in systems with a sufficiently long dipolar relaxation time, T1D. PROSPRUS NMR relies on the generation of so‑called second-order dipolar order among abundant protons undergoing MAS, and on the subsequent depletion of this dipolar order by a series of looped cross-polarization events, transferring the proton order into polarization of the low-γI-nuclei as a function of the latter's offsets. While the spin dynamics of the ensuing experiment is complex, particularly when dealing with narrow I spectral lines, it is shown that PROSPRUS can lead to faithful lineshapes for ultra-wideline spin-1/2 and spin-1 species, providing high sensitivity with extremely low RF power requirements. It is also shown that the ensuing 1H-detected PROSPRUS experiments can efficiently characterize I-spin lineshapes in excess of 1 MHz without having to retune electronics, while providing improvements in sensitivity per unit time over current broadband direct-detection methods by up to a factor of four.

Unraveling the Structure-Spectrum Relationship of Yeast Phenylalanine Transfer RNA: Insights from Theoretical Modeling of Infrared Spectroscopy.

Yeast phenylalanine tRNA (tRNAphe) is a paradigmatic model in structural biology. In this work, we combine molecular dynamics simulations and spectroscopy modeling to establish a direct link between its structure, conformational dynamics, and infrared (IR) spectra. Employing recently developed vibrational frequency maps and coupling models, we apply a mixed quantum/classical treatment of the line shape theory to simulate the IR spectra of tRNAphe in the 1600-1800 cm-1 region across its folded and unfolded conformations and under varying concentrations of Mg2+ ions. The predicted IR spectra of folded and unfolded tRNAphe are in good agreement with experimental measurements, validating our theoretical framework. We then elucidate how the characteristic L-shaped tertiary structure of the tRNA and its modulation in response to diverse chemical environments give rise to distinct IR absorption peaks and line shapes. These calculations effectively bridge IR spectroscopy experiments and atomistic molecular simulations, unraveling the molecular origins of the observed IR spectra of tRNAphe. This work presents a robust theoretical protocol for modeling the IR spectroscopy of nucleic acids, which will facilitate its application as a sensitive probe for detecting the fluctuating secondary and tertiary structures of these essential biological macromolecules.

A fast-filament eruption observed in the Halpha spectral line. I. Imaging spectroscopy diagnostic

A fast-filament eruption observed in the Halpha spectral line. I. Imaging spectroscopy diagnostic

Quantitative Analysis of Pb in Soil Using Laser-Induced Breakdown Spectroscopy Based on Signal Enhancement of Conductive Materials.

Studying efficient and accurate soil heavy-metal detection technology is of great significance to establishing a modern system for monitoring soil pollution, early warning and risk assessment, which contributes to the continuous improvement of soil quality and the assurance of food safety. Laser-induced breakdown spectroscopy (LIBS) is considered to be an emerging and effective tool for heavy-metal detection, compared with traditional detection technologies. Limited by the soil matrix effect, the LIBS signal of target elements for soil heavy-metal detection is prone to interference, thereby compromising the accuracy of quantitative detection. Thus, a series of signal-enhancement methods are investigated. This study aims to explore the effect of conductive materials of NaCl and graphite on the quantitative detection of lead (Pb) in soil using LIBS, seeking to find a reliable signal-enhancement method of LIBS for the determination of soil heavy-metal elements. The impact of the addition amount of NaCl and graphite on spectral intensity and parameters, including the signal-to-background ratio (SBR), signal-to-noise ratio (SNR), and relative standard deviation (RSD), were investigated, and the mechanism of signal enhancement by NaCl and graphite based on the analysis of the three-dimensional profile data of ablation craters and plasma parameters (plasmatemperature and electron density) were explored. Univariate and multivariate quantitative analysis models including partial least-squares regression (PLSR), least-squares support vector machine (LS-SVM), and extreme learning machine (ELM) were developed for the quantitative detection of Pb in soil with the optimal amount of NaCl and graphite, and the performance of the models was further compared. The PLSR model with the optimal amount of graphite obtained the best prediction performance, with an Rp that reached 0.994. In addition, among the three spectral lines of Pb, the univariate model of Pb I 405.78 nm showed the best prediction performance, with an Rp of 0.984 and the lowest LOD of 26.142 mg/kg. The overall results indicated that the LIBS signal-enhancement method based on conductive materials combined with appropriate chemometric methods could be a potential tool for the accurate quantitative detection of Pb in soil and could provide a reference for environmental monitoring.

Attenuation from the optical to the extreme ultraviolet by dust associated with broad absorption line quasars: The driving force for outflows

Abstract We derive a mean attenuation curve out to the rest-frame extreme ultraviolet (EUV) for ‘BAL dust’ - the dust causing the additional extinction of active galactic nuclei (AGNs) with broad absorption lines (BALQSOs). In contrast to the normal, relatively-flat, mean AGN attenuation curve, BAL dust is well fit by a steeply-rising, SMC-like curve. We confirm the shape of the theoretical Weingartner & Draine SMC curve out to 700 Angstroms but the drop in attenuation at still shorter wavelengths is less than predicted. The similar SMC-like attenuation curve for low-ionization BALQSOs (LoBALs) does not support the idea that they are an early phase in the life of an AGN when it is breaking out of a cocoon of star-forming dust. Although the attenuation is only E(B − V) ∼ 0.03–0.05 in the optical, it rises to one magnitude in the EUV, which is an optimum value for radiative acceleration of dusty gas. Because the spectral energy distribution of AGNs peaks in the EUV, the force on the dust dominates the acceleration of BAL gas. Although the shape of the attenuation curve for LoBALs is similar to the shape for HiBALs, the LoBALs on average show negative attenuation in the optical. This is naturally explained if there is more light scattered into our line of sight in LoBALs compared with non-BALQSOs. We suggest that this and partial covering are causes when attenuation curves appear to be steeper in the UV than an SMC curve.

Terahertz bandpass and bandstop filter based on the babinet complementary metamaterials

In this study, we experimentally investigate the terahertz (THz) bandpass and bandstop filters based on the elliptical Babinet complementary structure, including an elliptical hole structure (EHS) and an elliptical ring structure (ERS). The experimental results indicate that the monolayer structure exhibits an obvious polarization selectivity with consistent filtering curves under TE and TM mode excitation, but the spectral line has an apparent shift of 0.1 THz. Based on Babinet's principle, the filtering mode matching is experimentally verified between bandpass and bandstop under a 90° change in polarization. Furthermore, we propose a double-layer filter by integrating EHS and ERS on both sides of the silicon substrate, and realize the bandpass and bandstop filtering simultaneously. The simulation results show that the isolation degree of bandpass and bandstop filtering has a maximum of nearly 40 dB based on the angle-tunable filtering performance, and the Q-value of the filter is further improved than that of the single-layer structure. The filter designed based on the Babinet principle has the advantages of simple structure, easy processing, and sharper filtering effect, and shows great applications in THz communication, imaging, and sensing systems.

Robust optimized weights spectrum: Enhanced interpretable fault feature extraction method by solving frequency fluctuation problem

Machine condition monitoring (MCM) plays a pivotal role in ensuring the reliability, safety, and efficiency of a production and operation system. Fault feature extraction (FFE), as an important step within MCM, aims to filter out interference components (ICs) and extract fault components (FCs) from raw signals. Consequently, it facilitates incipient fault detection and performance degradation assessment. A recently proposed optimized weights spectrum (OWS) provided a data-driven FFE methodology with a strong theoretical foundation, but it was found that the OWS is highly sensitive to a frequency fluctuation problem caused by rotational speed fluctuations and sensor errors, which may result in fake fault signatures. Therefore, a robust optimized weights spectrum (ROWS) is proposed in this paper to solve the frequency fluctuation problem. The ROWS is inspired by an intuitive idea that employs functions with adaptive frequency bandwidths instead of isolated spectral lines to represent frequency components, thus mitigating the impact of the frequency fluctuation problem. To adaptively determine the optimal parameters for these functions and estimate the corresponding ROWS, an alternative optimization strategy is adopted. Then, the proposed ROWS can be obtained when convergence conditions of this strategy are satisfied. Finally, the effectiveness and superiority of the proposed ROWS are verified by three real-world cases, i.e., the proposed ROWS can solve the frequency fluctuation problem and provide robust results for interpretable FFE.

Where is the supervirial hot gas? I: a pilot study with sightlines to Galactic X-ray binaries

ABSTRACT Hot, $\log (T/\mathrm{K})$$\sim$ 7.5, gas was recently discovered in the Milky Way in extragalactic sightlines. In order to determine its location, here we present sightlines to Galactic X-ray binaries (XRBs) passing through the Interstellar Medium (ISM). In this pilot study, we investigate absorption features of S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ in the spectra of three XRBs, namely 4U 1735–44, 4U 1820–30, and Cyg X-2, using Chandra High Energy Transmission Grating archival observations. We do not detect any of these lines. We determine the 2$\sigma$ upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions. We note that the 2$\sigma$ upper limits for S$_{\mathrm{XVI}}$ K$\alpha$ and Si$_{\mathrm{XIV}}$ K$\alpha$ are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at $\log (T/\mathrm{K})$$\gt\ 7$ is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that S$_{\mathrm{XVI}}$, Si$_{\mathrm{XIV}}$, and Ne$_{\mathrm{X}}$ absorption detected in extragalactic sightlines is not from the ISM, but is likely from a hot gas phase in the extraplanar region beyond the ISM or in the extended Circumgalactic Medium.

Bayesian Multi-line Intensity Mapping

Line intensity mapping (LIM) has emerged as a promising tool for probing the 3D large-scale structure through the aggregate emission of spectral lines. The presence of interloper lines poses a crucial challenge in extracting the signal from the target line in LIM. In this work, we introduce a novel method for LIM analysis that simultaneously extracts line signals from multiple spectral lines, utilizing the covariance of native LIM data elements defined in the spectral–angular space. We leverage correlated information from different lines to perform joint inference on all lines simultaneously, employing a Bayesian analysis framework. We present the formalism, demonstrate our technique with a mock survey setup resembling the SPHEREx deep-field observation, and consider four spectral lines within the SPHEREx spectral coverage in the near-infrared: Hα, [O iii], Hβ, and [O ii]. We demonstrate that our method can extract the power spectrum of all four lines at the ≳10σ level at z < 2. For the brightest line, Hα, the 10σ sensitivity can be achieved out to z ∼ 3. Our technique offers a flexible framework for LIM analysis, enabling simultaneous inference of signals from multiple line emissions while accommodating diverse modeling constraints and parameterizations.