Synthetic Stellar Spectra Research Articles

The Wanderer: Charting WASP-77A b’s Formation and Migration Using a System-wide Inventory of Carbon and Oxygen Abundances

The elemental and isotopic abundances of volatiles like carbon, oxygen, and nitrogen may trace a planet’s formation location relative to H2O, CO2, CO, NH3, and N2 “snowlines,” or the distance from the star at which these volatile elements sublimate. By comparing the C/O and 12C/13C ratios measured in giant exoplanet atmospheres to complementary measurements of their host stars, we can determine whether the planet inherited stellar abundances from formation inside the volatile snowlines, or nonstellar C/O and 13C enrichment characteristic of formation beyond the snowlines. To date, there are still only a handful of exoplanet systems where we can make a direct comparison of elemental and isotopic CNO abundances between an exoplanet and its host star. Here, we present a 12C/13C abundance analysis for host star WASP-77A (whose hot Jupiter’s 12C/13C abundance was recently measured). We use MARCS stellar atmosphere models and the radiative transfer code TurboSpectrum to generate synthetic stellar spectra for isotopic abundance calculations. We find a 12C/13C ratio of 51 ± 6 for WASP-77A, which is subsolar (∼91) but may still indicate 13C enrichment in its companion planet WASP-77A b (12C/13C = 26 ± 16, previously reported). Together with the inventory of carbon and oxygen abundances in both the host and companion planet, these chemical constraints point to WASP-77A b’s formation beyond the H2O and CO2 snowlines and provide chemical evidence for the planet’s migration to its current location ∼0.024 au from its host star.

Three-Dimensional Nonlocal Thermodynamic Equilibrium Abundance Analyses of Late-Type Stars

The chemical compositions of stars encode the history of the universe and are thus fundamental for advancing our knowledge of astrophysics and cosmology. However, measurements of elemental abundance ratios, and our interpretations of them, strongly depend on the physical assumptions that dictate the generation of synthetic stellar spectra. Three-dimensional radiation-hydrodynamic (3D RHD) box-in-a-star simulations of stellar atmospheres offer a more realistic representation of surface convection occurring in late-type stars than do traditional one-dimensional (1D) hydrostatic models. As evident from a multitude of observational tests, the coupling of 3D RHD models with line formation in nonlocal thermodynamic equilibrium (non-LTE) today provides a solid foundation for abundance analysis for many elements. This review describes the ongoing and transformational work to advance the state of the art and replace 1D LTE spectrum synthesis with its 3D non-LTE counterpart. In summary: ▪3D and non-LTE effects are intricately coupled, and consistent modeling thereof is necessary for high-precision abundances; such modeling is currently feasible for individual elements in large surveys. Mean 3D (〈3D〉) models are not adequate as substitutes.▪The solar abundance debate is presently dominated by choices and systematic uncertainties that are not specific to 3D non-LTE modeling.▪3D non-LTE abundance corrections have a profound impact on our understanding of FGK-type stars, exoplanets, and the nucleosynthetic origins of the elements.

Synthetic stellar spectra for studying multiple populations in globular clusters

Synthetic stellar spectra for studying multiple populations in globular clusters

Titanium abundances in late-type stars

Context. Rapidly growing datasets from stellar spectroscopic surveys are providing unprecedented opportunities to analyse the chemical evolution history of our Galaxy. However, spectral analysis requires accurate modelling of synthetic stellar spectra for late-type stars, for which the assumption of local thermodynamic equilibrium (LTE) has been shown to be insufficient in many cases. Errors associated with LTE can be particularly large for Ti I, which is susceptible to over-ionisation, particularly in metal-poor stars. Aims. The aims of this work are to study and quantify the 1D non-LTE effects on titanium abundances across the Hertzsprung-Russell diagram for a large sample of stars. Methods. A large grid of departure coefficients, βν, were computed on standard MARCS model atmospheres. The grid extends from 3000 K to 8000 K in Teff, −0.5 dex to +5.5 dex in log g, and −5.0 to +1.0 in [Fe/H], with non-LTE effects in this grid reaching up to 0.4 dex. This was used to compute abundance corrections that were subsequently applied to the LTE abundances of over 70 000 stars selected from the GALAH survey in addition to a smaller sample of literature Keck data for metal-poor dwarfs. Results. The non-LTE effects grow towards lower [Fe/H], lower log g, and higher Teff, with a minimum and maximum ΔA(Ti)Ti I of 0.02 and 0.19 in the GALAH sample. For metal-poor giants, the non-LTE modelling reduces the average ionisation imbalance (ΔI−II) from −0.11 dex to −0.01 dex at [Fe/H] = −1.7, and the enhancement in titanium abundances from Ti I lines results in a [Ti/Fe] versus [Fe/H] trend that more closely resembles the behaviour of Ti II at low metallicities. At higher metallicities, the results are limited by the precision of the GALAH DR3 LTE abundances and the effects are within the errors. For the most metal-poor dwarfs from the Keck sample, the average ionisation imbalance increases from −0.1 dex to +0.2 dex, a shortcoming that is consistent with previous 1D non-LTE studies and which we speculate could be related to 3D effects. Conclusions. Non-LTE effects on titanium abundances are significant. Neglecting them may alter our understanding of Galactic chemical evolution. We have made our grid of departure coefficients publicly available, with the caveat that the Ti abundances of metal-poor dwarfs need further study in 3D non-LTE.

A Blueprint for the Milky Way’s Stellar Populations. IV. A String of Pearls—the Galactic Starburst Sequence

We continue our series of papers on phase-space distributions of stars in the Milky Way based on photometrically derived metallicities and Gaia astrometry, with a focus on the halo−disk interface in the local volume. To exploit various photometric databases, we develop a method of empirically calibrating synthetic stellar spectra based on a comparison with observations of stellar sequences and individual stars in the Sloan Digital Sky Survey, the SkyMapper Sky Survey, and the Pan-STARRS1 surveys, overcoming band-specific corrections employed in our previous work. In addition, photometric zero-point corrections are derived to provide an internally consistent photometric system with a spatially uniform metallicity zero-point. Using our phase-space diagrams, we find a remarkably narrow sequence in the rotational velocity (v ϕ ) versus metallicity ([Fe/H]) space for a sample of high proper-motion stars (>25 mas yr−1), which runs along Gaia Sausage/Enceladus (GSE) and the Splash substructures and is linked to the disk, spanning nearly 2 dex in [Fe/H]. Notably, a rapid increase of v ϕ from a nearly zero net rotation to ∼180 km s−1 in a narrow metallicity interval (−0.6 ≲ [Fe/H] ≲ −0.4) suggests that some of these stars emerged quickly on a short gas-depletion timescale. Through measurements of a scale height and length, we argue that these stars are distinct from those heated dynamically by mergers. This chain of high proper-motion stars provides additional support for recent discoveries suggesting that a starburst took place when the young Milky Way encountered the gas-rich GSE progenitor, which eventually led to the settling of metal-enriched gas onto the disk.

Automatic line selection for abundance determinations in large stellar spectroscopic surveys

Context. Over the past few years, new multiplex spectrographs have emerged to observe several millions of stars. The optimisation of these instruments (w.r.t. their resolution or wavelength range), their associated surveys (choice of instrumental set-up), and their parameterisation pipelines require methods that estimate which wavelengths (or pixels) contain useful information. Aims. We propose a method that establishes the usefulness of an atomic spectral line, whereby usefulness is defined by the purity of the line and its detectability. We demonstrate two applications of our code: a) optimising an instrument by comparing the number of detected useful lines at a given wavelength range and resolution; and b) optimising the line list for a given set-up, in the sense of creating a golden subsample of the least-blended lines that are detectable at a range of signal-to-noise ratio values. Methods. The method compares pre-computed normalised synthetic stellar spectra containing all of the elements and molecules with spectra solely containing the lines of specific elements. Then, the flux ratios between the full spectrum and the element spectrum are computed to estimate the line purities. The method automatically identifies: (i) the line’s central wavelength, (ii) its detectability based on its depth and a given signal-to-noise threshold, and (iii) its usefulness based on the purity ratio defined above. Results. We applied this method to compare the three WEAVE high-resolution set-ups (blue: 404–465 nm, green: 473–545 nm, red: 595–685 nm) and find that the green+red set-up both allows us to measure more elements and contains more numerous useful lines. However, there is a disparity in terms of which elements are detected over each of the set-ups that we have characterised. We also studied the performances of high-resolution (R ~ 20 000) and low-resolution (R ~ 6000) spectra covering the entire optical wavelength range. Assuming a purity threshold of 60%, we find that the high-resolution set-up contains a much wealthier selection of lines, for any of the considered elements; whereas the low-resolution set-up displays a ‘loss’ of 50% to 90% of the lines (depending on the nucleosynthetic channel considered), even when the signal-to-noise ratio is increased. Conclusions. The method presented here provides a vital diagnostic of where to focus to get the most out of a spectrograph. It is easy to implement for future instruments that have not yet determined their final configuration, as well as for pipelines that require line masks.

AlH lines in the blue spectrum of Proxima Centauri

ABSTRACT The recently computed ExoMol line lists for isotopologues of AlH are used to analyse the blue spectrum (4000–4500 Å) of Proxima Cen (M5.5 V). Comparison of the observed and computed spectra enables the identification of a large number of 27AlH lines of the A 1Π –X 1Σ+ band system: The spectral range covering 1-0, 0-0, and 1-1 bands are dominated by clearly resolved AlH lines. We reveal the diffuse nature of transitions close to the dissociation limit which appears in the form of increasingly wider (up to 5 Å) and shallower (up to the continuum confusion limit) AlH line profiles. The predicted wavelengths of AlH diffuse lines are systematically displaced. The effect of broadening by predissociation states on the line profiles is included by increasing the radiative damping rate by up to 5 orders of magnitude. We determine empirical values of damping rates for a number of the clean 0-0 Q-branch transitions by comparing the observed and synthetic stellar spectra. We find excellent agreement between our damping rates and lifetimes available in the literature. A comparison of 27Al1H ExoMol and REALH spectra shows that the observed spectrum is better described by the ExoMol line list. A search for 26Al1H lines in the Proxima Cen spectrum does not reveal any notable features; giving an upper limit of 27Al1H /26Al1H >100.

Accurate Modeling of Lyα Profiles and Their Impact on Photolysis of Terrestrial Planet Atmospheres

Accurately measuring and modeling the Lyα (Lyα; λ1215.67 Å) emission line from low-mass stars is vital for our ability to build predictive high energy stellar spectra, yet interstellar medium (ISM) absorption of this line typically prevents model-measurement comparisons. Lyα also controls the photodissociation of important molecules, like water and methane, in exoplanet atmospheres such that any photochemical models assessing potential biosignatures or atmospheric abundances require accurate Lyα host star flux estimates. Recent observations of three early M and K stars (K3, M0, M1) with exceptionally high radial velocities (>100 km s−1) reveal the intrinsic profiles of these types of stars as most of their Lyα flux is shifted away from the geocoronal line core and contamination from the ISM. These observations indicate that previous stellar spectra computed with the PHOENIX atmosphere code have underpredicted the core of Lyα in these types of stars. With these observations, we have been able to better understand the microphysics in the upper atmosphere and improve the predictive capabilities of the PHOENIX atmosphere code. Since these wavelengths drive the photolysis of key molecular species, we also present results analyzing the impact of the resulting changes to the synthetic stellar spectra on observable chemistry in terrestrial planet atmospheres.

GPS Measurements of Precipitable Water Vapor Can Improve Survey Calibration: A Demonstration from KPNO and the Mayall z-band Legacy Survey

Dual-band Global Positioning Satellite (GPS) measurements of precipitable water vapor (PWV) at the Kitt Peak National Observatory predict the overall per-image sensitivity of the Mayall z-band Legacy Survey (MzLS). The per-image variation in the brightness of individual stars is strongly correlated with the measured PWV and the color of the star. Synthetic stellar spectra through TAPAS transmission models successfully predict the observed PWV-induced photometric variation. We find that PWV absorption can be well approximated by a linear relationship with (airmass × PWV)0.6 and present an update on the traditional treatment in the literature. The MzLS zero-point sensitivity in electrons s−1 varies with a normalized-mean absolute deviation of 61 mmag. PWV variation accounts 23 mmag of this zero-point variation. The MzLS per-image absolute sensitivity decreases by 40 mmag per effective mm of PWV. The overall gray offset portion of this variation is corrected by the calibration to a reference catalog. But the relative calibration error between blue (r − z < 0.5 mag) versus red (1.2 mag < r − z) stars increases by 0.3–2 mmag per effective mm of PWV. We argue that GPS systems provide more precise PWV measurements than using differential measurements of stars of different colors and recommend that observatories install dual-band GPS as a low-maintenance, low-cost, auxiliary calibration system. We extend our results of the need for well-calibrated PWV measurements by presenting the calculations of the PWV photometric impact on three science cases of interest: stellar photometry, supernova cosmology, and quasar identification and variability.

Simulation of Stark-broadened Hydrogen Balmer-line Shapes for DA White Dwarf Synthetic Spectra

Abstract White dwarfs (WDs) are useful across a wide range of astrophysical contexts. The appropriate interpretation of their spectra relies on the accuracy of WD atmosphere models. One essential ingredient of atmosphere models is the theory used for the broadening of spectral lines. To date, the models have relied on Vidal et al., known as the unified theory of line broadening (VCS). There have since been advancements in the theory; however, the calculations used in model atmosphere codes have only received minor updates. Meanwhile, advances in instrumentation and data have uncovered indications of inaccuracies: spectroscopic temperatures are roughly 10% higher and spectroscopic masses are roughly 0.1 M ⊙ higher than their photometric counterparts. The evidence suggests that VCS-based treatments of line profiles may be at least partly responsible. Gomez et al. developed a simulation-based line-profile code Xenomorph using an improved theoretical treatment that can be used to inform questions around the discrepancy. However, the code required revisions to sufficiently decrease noise for use in model spectra and to make it computationally tractable and physically realistic. In particular, we investigate three additional physical effects that are not captured in the VCS calculations: ion dynamics, higher-order multipole expansion, and an expanded basis set. We also implement a simulation-based approach to occupation probability. The present study limits the scope to the first three hydrogen Balmer transitions (Hα, Hβ, and Hγ). We find that screening effects and occupation probability have the largest effects on the line shapes and will likely have important consequences in stellar synthetic spectra.

Disentangled Representation Learning for Astronomical Chemical Tagging

Abstract Modern astronomical surveys are observing spectral data for millions of stars. These spectra contain chemical information that can be used to trace the Galaxy’s formation and chemical enrichment history. However, extracting the information from spectra and making precise and accurate chemical abundance measurements is challenging. Here we present a data-driven method for isolating the chemical factors of variation in stellar spectra from those of other parameters (i.e., T eff, log g, [Fe/H]). This enables us to build a spectral projection for each star with these parameters removed. We do this with no ab initio knowledge of elemental abundances themselves and hence bypass the uncertainties and systematics associated with modeling that rely on synthetic stellar spectra. To remove known nonchemical factors of variation, we develop and implement a neural network architecture that learns a disentangled spectral representation. We simulate our recovery of chemically identical stars using the disentangled spectra in a synthetic APOGEE-like data set. We show that this recovery declines as a function of the signal-to-noise ratio but that our neural network architecture outperforms simpler modeling choices. Our work demonstrates the feasibility of data-driven abundance-free chemical tagging.

Fundamental Parameters of the Eclipsing Binary DD CMa and Evidence for Mass Exchange

Abstract We present a detailed photometric and spectroscopic analysis of DD CMa, based on published survey photometry and new spectroscopic data. We find an improved orbital period of P o = 2.0084530(6) days. Our spectra reveal Hβ and Hα absorptions with weak emission shoulders, and we also find a color excess in the Wide-field Infrared Survey Explorer multiband photometry, interpreted as signatures of circumstellar matter. We model the V-band orbital light curve derived from the ASAS and ASAS-SN surveys, assuming a semidetached configuration and using the mass ratio and temperature of the hotter star derived from our spectroscopic analysis. Our model indicates that the system consists of a B2.5 dwarf and a B9 giant of radii 3.2 and 3.7 R ⊙, respectively, orbiting in a circular orbit of radius 6.75 R ⊙. We also found M c = 1.7 ± 0.1 M ⊙, T c = 11,350 ± 100 K, and M h = 6.4 ± 0.1 M ⊙, T h = 20,000 ± 500 K, for the cooler and hotter star, respectively. We find broad single emission peaks in Hα and Hβ after subtracting the synthetic stellar spectra. Our results are consistent with mass exchange between the stars and suggest the existence of a stream of gas being accreted onto the early B-type star.

Atomic data for the Gaia-ESO Survey

Context. We describe the atomic and molecular data that were used for the abundance analyses of FGK-type stars carried out within the Gaia-ESO Public Spectroscopic Survey in the years 2012 to 2019. The Gaia-ESO Survey is one among several current and future stellar spectroscopic surveys producing abundances for Milky-Way stars on an industrial scale. Aims. We present an unprecedented effort to create a homogeneous common line list, which was used by several abundance analysis groups using different radiative transfer codes to calculate synthetic spectra and equivalent widths. The atomic data are accompanied by quality indicators and detailed references to the sources. The atomic and molecular data are made publicly available at the CDS. Methods. In general, experimental transition probabilities were preferred but theoretical values were also used. Astrophysical gf-values were avoided due to the model-dependence of such a procedure. For elements whose lines are significantly affected by a hyperfine structure or isotopic splitting, a concerted effort has been made to collate the necessary data for the individual line components. Synthetic stellar spectra calculated for the Sun and Arcturus were used to assess the blending properties of the lines. We also performed adetailed investigation of available data for line broadening due to collisions with neutral hydrogen atoms. Results. Among a subset of over 1300 lines of 35 elements in the wavelength ranges from 475 to 685 nm and from 850 to 895 nm, we identified about 200 lines of 24 species which have accurate gf-values and are free of blends in the spectra of the Sun and Arcturus. For the broadening due to collisions with neutral hydrogen, we recommend data based on Anstee-Barklem-O’Mara theory, where possible. We recommend avoiding lines of neutral species for which these are not available. Theoretical broadening data by R.L. Kurucz should be used for Sc II, Ti II, and Y II lines; additionally, for ionised rare-earth species, the Unsöld approximation with an enhancement factor of 1.5 for the line width can be used. Conclusions. The line list has proven to be a useful tool for abundance determinations based on the spectra obtained within the Gaia-ESO Survey, as well as other spectroscopic projects. Accuracies below 0.2 dex are regularly achieved, where part of the uncertainties are due to differences in the employed analysis methods. Desirable improvements in atomic data were identified for a number of species, most importantly Al I, S I, and Cr II, but also Na I, Si I, Ca II, and Ni I.

Assessing the performance of LTE and NLTE synthetic stellar spectra in a machine learning framework

ABSTRACT In the current era of stellar spectroscopic surveys, synthetic spectral libraries are the basis for the derivation of stellar parameters and chemical abundances. In this paper, we compare the stellar parameters determined using five popular synthetic spectral grids (INTRIGOSS, FERRE, AMBRE, PHOENIX, and MPIA/1DNLTE) with our convolutional neural network (CNN, StarNet). The stellar parameters are determined for six physical properties (effective temperature, surface gravity, metallicity, [α/Fe], radial velocity, and rotational velocity) given the spectral resolution, signal-to-noise ratio, and wavelength range of optical FLAMES-UVES spectra from the Gaia-ESO Survey. Both CNN modelling and epistemic uncertainties are incorporated through training an ensemble of networks. StarNet training was also adapted to mitigate differences between the synthetic grids and observed spectra by augmenting with realistic observational signatures (i.e. resolution matching, wavelength sampling, Gaussian noise, zeroing flux values, rotational and radial velocities, continuum removal, and masking telluric regions). Using the FLAMES-UVES spectra for FGK-type dwarfs and giants as a test set, we quantify the accuracy and precision of the stellar label predictions from StarNet. We find excellent results over a wide range of parameters when StarNet is trained on the MPIA/1DNLTE synthetic grid, and acceptable results over smaller parameter ranges when trained on the 1DLTE grids. These tests also show that our CNN pipeline is highly adaptable to multiple simulation grids.

Asymmetric spatial distribution of subsolar metallicity stars in the Milky Way nuclear star cluster

ABSTRACT We present stellar metallicity measurements of more than 600 late-type stars in the central 10 pc of the Galactic Centre. Together with our previously published KMOS data, this data set allows us to investigate, for the first time, spatial variations of the nuclear star cluster’s metallicity distribution. Using the integral-field spectrograph KMOS (VLT), we observed almost half of the area enclosed by the nuclear star cluster’s effective radius. We extract spectra at medium spectral resolution and apply full spectral fitting utilizing the PHOENIX library of synthetic stellar spectra. The stellar metallicities range from [M/H] = −1.25 dex to [M/H] &amp;gt; +0.3 dex, with most of the stars having supersolar metallicity. We are able to measure an anisotropy of the stellar metallicity distribution. In the Galactic north, the portion of subsolar metallicity stars with [M/H] &amp;lt; 0.0 dex is more than twice as high as in the Galactic south. One possible explanation for different fractions of subsolar metallicity stars in different parts of the cluster is a recent merger event. We propose to test this hypothesis with high-resolution spectroscopy and by combining the metallicity information with kinematic data.

To use or not to use synthetic stellar spectra in population synthesis models?

ABSTRACT Stellar population synthesis (SPS) models are invaluable to study star clusters and galaxies. They provide means to extract stellar masses, stellar ages, star formation histories, chemical enrichment, and dust content of galaxies from their integrated spectral energy distributions, colours, or spectra. As most models, they contain uncertainties that can hamper our ability to model and interpret observed spectra. This work aims at studying a specific source of model uncertainty: the choice of an empirical versus a synthetic stellar spectral library. Empirical libraries suffer from limited coverage of parameter space, while synthetic libraries suffer from modelling inaccuracies. Given our current inability to have both ideal stellar-parameter coverage with ideal stellar spectra, what should one favour: better coverage of the parameters (synthetic library) or better spectra on a star-by-star basis (empirical library)? To study this question, we build a synthetic stellar library mimicking the coverage of an empirical library, and SPS models with different choices of stellar library tailored to these investigations. Through the comparison of model predictions and the spectral fitting of a sample of nearby galaxies, we learned that predicted colours are more affected by the coverage effect than the choice of a synthetic versus empirical library; the effects on predicted spectral indices are multiple and defy simple conclusions; derived galaxy ages are virtually unaffected by the choice of the library, but are underestimated when SPS models with limited parameter coverage are used; metallicities are robust against limited HRD coverage, but are underestimated when using synthetic libraries.

Measuring the Magnetic Field of Young Stars Using iSHELL Observations: BP Tau and V347 Aur

Abstract While it has been suggested that there is a connection between the magnetic properties and the internal structure of young stars, there have not been enough magnetic measurements to firmly establish such a correlation at the earliest ages. Here, we contribute to this endeavor by presenting stellar parameters and magnetic field strength measurements of BP Tau and V347 Aur, both stars observed with the near-infrared spectrograph iSHELL. We first test the accuracy of our method by fitting synthetic stellar spectra to a sample of nine main and post-main-sequence stars. We report uncertainties of = 91 K in temperature and = 0.14 in gravity. We then apply the modeling technique to BP Tau and measure a surface magnetic field strength of = kG, confirming literature results. For this star, however, we obtain a much lower temperature value than previous optical studies ( K) and interpret this significant temperature difference as due to the relatively higher impact of starspots at near-infrared wavelengths than at optical wavelengths. We further apply this technique to the class I protostellar source V347 Aur and measure for the first time its magnetic field strength kG and its surface gravity log g = 3.25 . Lastly, we combine our measurements with pre-main-sequence stellar evolutionary models and illustrate the effects produced by starspots on the retrieved masses and ages of young stars.

Tracing Galactic Arm with 6614 Å Diffuse Interstellar Band and Gaia DR2 Distances

Enigmatic problem in astronomical spectroscopy is diffuse interstellar bands (DIBs). DIBs are ∼ 500 weak and broad absorption features seen in the spectra of stars or other astronomical objects located behind interstellar materials. Despite the unknown ions or molecules that caused the absorptions, except for two near infrared DIBs, DIBs can be used as a potential interstellar medium (ISM) tracer to map Galactic structure. Gaia is a mission of the European Space Agency (ESA) which was launched on 2013. It provides precise distances which greatly improve our knowledge of the structure and history of our Galaxy. In this work, we used previously determined DIB equivalent width (EW) measurements by means of DIB fitting that is a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profile to the Gaia–ESO Large Public Spectroscopic Survey data. From DIB measurements from stellar spectra of distributed target stars and together with Gaia DR2 distances, we investigated the ISM distribution along particular line of sight, i.e., (l,b)∼(213°, -2°) which probes Local and Perseus Arm.

Interstellar Lines and Diffuse Bands in the Spectrum of the Supergiant κ Cas

We have studied the interstellar lines and diffuse bands in the spectrum of the supergiant κ Cas using the CCD spectra obtained with the Echelle spectrograph of the 2 meter telescope at the peak Terskol observatory. A synthetic stellar spectrum was used to correctly reveal the interstellar features, computed for a model atmosphere with Teff = 21 500 K, log g = 2.60, v sin i = 83 kms-1, [M/H] = 0.0, ξ = 15 kms-1 and with partial selection of main chemical element abundances. Eleven interstellar bands and 91 diffuse interstellar bands were identified. A further 14 absorption features were revealed, possibly also diffuse bands. The spectrophotometric diffuse band parameters and their accuracies were tabulated. We present maps of observed and synthetic spectral regions containing interstellar diffuse bands.

Extracting diffuse interstellar bands from cool star spectra: tracing Galactic ISM structures, kinematics, and properties

Diffuse interstellar bands (DIBs) are broad and weak absorption features that appear in spectra recorded towards stars that lie behind interstellar (IS) clouds. They are known as a long-standing challenge in astronomical spectroscopy because of their unknown carriers. However, they are also a potential IS tracers, in particular to probe distant clouds because they are not easily saturated. We devised automated DIB-fitting methods appropriate for cool star spectra and multiple IS components. The data were fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. The initial number of DIB components and their radial velocity were guided by HI 21 cm emission spectrum, or, when available in the spectral range, IS neutral sodium (NaI) absorption lines. For NaI, radial velocities of IS NaI lines and DIBs were maintained linked during a global simultaneous fit. Using the tools, we have analyzed Gaia-ESO Survey (GES) spectra of stars that probe between 2-10 kpc long line-of-sight (LOS) in few different regions of the Milky Way (fields). For all fields, the DIB strength and target extinction are well correlated. We show that, toward distance-distributed target stars, DIBs can be used to locate dense clouds in the Galaxy (i.e., Galactic arm) and probe a LOS kinematical structure. In addition, we have devised the DIB-DIB global fit to study the relation between DIBs and can be a potential tool to trace interstellar environmental conditions.