Interstellar Reddening Research Articles

Quantitative spectroscopy of multiple OB stars. I. The quadruple system HD 37061 at the centre of Messier 43

The majority of massive stars are located in binary or multiple star systems. This poses additional challenges to quantitative analyses that use model atmospheres because little information is currently available on the chemical composition of such systems. The members of the quadruple star system HD 37061, which excites the H ii region Messier 43 in Orion, are fully characterised. Accurate and precise abundances for all elements with lines traceable in the optical spectrum are derived for the first time. A hybrid non-local thermodynamic equilibrium (non-LTE) approach, using line-blanketed hydrostatic model atmospheres computed with the Atlas12 code in combination with non-LTE line-formation calculations with Detail and Surface was employed. A high-resolution composite spectrum was analysed for the atmospheric parameters and elemental abundances of the individual stars. Fundamental stellar parameters were derived based on stellar evolution tracks, and the interstellar reddening was characterised. We determined the fundamental parameters and chemical abundances for three stars in the HD\,37061 system. The fourth and faintest star in the system shows no distinct spectral features, as a result of its fast rotation. However, this star has noticeable effects on the continuum. The derived element abundances and determined ages of the individual stars are consistent with each other, and the abundances coincide with the cosmic abundance standard. We find an excellent agreement between our spectroscopic distance and the Gaia Data Release 3 parallax distance.

A network of cooler white dwarfs as infrared standards for flux calibration

ABSTRACT The accurate flux calibration of observational data is vital for astrophysics and cosmology because absolute flux uncertainties of stellar standards propagate into scientific results. With the ever higher precision achieved by telescopic missions (e.g. JWST) in the infrared (IR), suitable calibrators are required for this regime. The basis of the Hubble Space Telescope (HST) flux scale is defined by model fits of three hot (${T_\mathrm{eff}} > 30\, 000$ K) hydrogen-atmosphere (DA) white dwarfs, which achieve an accuracy better than 1 per cent at optical wavelengths but falls below this level in the IR range. We present a network of 17 cooler DA white dwarfs with ${T_\mathrm{eff}} < 20\, 000$ K as spectrophotometric flux standards that are equally, if not more, accurate at IR wavelengths. Cooler white dwarfs do not suffer from non-local thermal equilibrium effects in continuum flux or from ultraviolet metal line blanketing, have a larger sky density, are generally closer to Earth with little or negligible interstellar reddening, and have energy distributions peaking in the optical or near-IR. Using the latest grid of DA local thermal equilibrium atmosphere models with three-dimensional convection, the observed Space Telescope Imaging Spectrometer (STIS) and Wide Field Camera 3 (WFC3) fluxes of our network are accurate to 3 per cent over most of the range $1450\!\! - \!\!16\, 000$ Å, with a median standard deviation of 1.41 per cent. Fitting the HST STIS and WFC3 white dwarf spectral energy distributions (SEDs) and Balmer lines independently yields SEDs that agree within $3\sigma$, which demonstrates the precision of the models for our network.

Spin parity of spiral galaxies. IV. Differential reddening of globular cluster systems of nearby spiral galaxies

Abstract The north-west side of the disk of M 31 is known as the near side because of the differential reddening of globular clusters found from their photographic photometry. This paper reports a simple geometric model to evaluate the visibility of the effect and its application to published CCD photometry on globular cluster systems of three spiral galaxies, M 31, M 33, and NGC 253. The color difference of globular cluster systems due to differential reddening was confirmed for M 31 and NGC 253; however, the data for M 33 were insufficient. The analysis reaffirms the currently adopted interpretation that the side on the minor axis of the galactic disk, where more conspicuous dust features and interstellar reddening are visible, is the nearer side to us and provides an additional basis for using spiral galaxies with identified spiral windings, S-wise or Z-wise, to study the large-scale spin distribution of galaxies in the universe.

On the impact of «red leak» of filters on brightness estimates of late stars using the example of observations of rapid variability of symbiotic stars

The results of modeling the dependence of the «red leak» of photometric filters on various factors (color index 𝑉 −𝑅, luminosity class, interstellar reddening, air mass and PWV) during observations of stars are presented. The error arising from not taking into account the «red leak» in the case of filters used on the 0.6-m telescope of the CMO SAI can amount to 0.6 𝑚 − 0.8 𝑚 for late stars. Algorithms for reducing observational data are presented for filters 𝑈 and 𝐵. The results of observations of the rapid variability of two symbiotic stars CH Cyg and SU Lyn with cold components of very late spectral types are presented. For CH Cyg, rapid variability was detected on both observation dates. Taking into account the «red leak» effect, the amplitude in the 𝐵 band was 0.10𝑚 on November 6, 2019 and 0.19𝑚 on December 15, 2022, with a characteristic variability time of about 20 minutes. For SU Lyn, no rapid brightness variability was detected in the 𝐵 band on February 2, 2023 (with an accuracy of 0.003𝑚)

A catalogue of planetary nebulae chemical abundances in the Galactic bulge

ABSTRACT In this paper, the third of a series of papers, we present well-determined chemical abundances for 124 planetary nebulae (PNe) in the Galactic bulge from deep, long-slit FORS2 spectra from the 8.2 m ESO Very Large telescope (VLT). Prior to this work there were only ∼240 bulge PNe with chemical abundances previously determined over a ∼50 yr period and of highly variable quality. For 34 of these PNe we are presenting their abundances for the first time, which adds ∼14 per cent to the available sample of bulge PNe abundances. The interstellar reddening, physical conditions (electron densities, ne, temperatures, Te), and chemical compositions are derived as single values for each PN but also using different line diagnostics. Selected comparisons with the best literature fluxes for 75 PNe in common reveal that these significant new data are robust, reliable, and internally self-consistent forming the largest independent, high-quality, and well-understood derivation of PNe abundances currently available for study.

An HST COS ultraviolet spectroscopic survey of 311 DA white dwarfs – I. Fundamental parameters and comparative studies

ABSTRACT White dwarf studies carry significant implications across multiple fields of astrophysics, including exoplanets, supernova explosions, and cosmological investigations. Thus, accurate determinations of their fundamental parameters (Teff and log g) are of utmost importance. While optical surveys have provided measurements for many white dwarfs, there is a lack of studies utilizing ultraviolet (UV) data, particularly focusing on the warmer ones that predominantly emit in the UV range. Here, we present the medium-resolution far-UV spectroscopic survey of 311 DA white dwarfs obtained with Cosmic Origins Spectrograph (COS) onboard Hubble Space Telescope confirming 49 photometric Gaia candidates. We used 3D extinction maps, parallaxes, and hydrogen atmosphere models to fit the spectra of the stars that lie in the range $12\, 000 \lt \mbox{$T_{\mathrm{eff}}$}\lt 33\, 000$ K, and $7 \le \mbox{$\log g$}\lt 9.2$. To assess the impact of input physics, we employed two mass–radius relations in the fitting and compared the results with previous studies. The comparisons suggest the COS Teff are systematically lower by 3 per cent, on average, than Balmer line fits while they differ by only 1.5 per cent from optical photometric studies. The mass distributions indicate that the COS masses are smaller by ≈0.05 and 0.02 M⊙ than Balmer lines and photometric masses, respectively. Performing several tests, we find that the discrepancies are either arising due to issues with the COS calibration, broadening theories for hydrogen lines, or interstellar reddening which needs further examination. Based on comparative analysis, we identify 30 binary candidates drawing attention for follow-up studies to confirm their nature.

The blue supergiant Sher 25 revisited in the Gaia era

Aims. The evolutionary status of the blue supergiant Sher 25 and its membership to the massive cluster NGC 3603 are investigated. Methods. A hybrid non-local thermodynamic equilibrium (non-LTE) spectrum synthesis approach is employed to analyse a high-resolution optical spectrum of Sher 25 and five similar early B-type comparison stars in order to derive atmospheric parameters and elemental abundances. Fundamental stellar parameters are determined by considering stellar evolution tracks, Gaia Data Release 3 (DR3) data, and complementary distance information. Interstellar reddening and the reddening law along the sight line towards Sher 25 are constrained employing UV photometry for the first time in addition to optical and infrared data. The distance to NGC 3603 is reevaluated based on Gaia DR3 data of the innermost cluster O-stars. Results. The spectroscopic distance derived from the quantitative analysis implies that Sher 25 lies in the foreground of NGC 3603, which is found to have a distance of dNGC3603 = 6250 ± 150 pc. A cluster membership is also excluded as the hourglass nebula is unaffected by the vigorous stellar winds of the cluster stars and from the different excitation signatures of the hourglass nebula and the nebula around NGC 3603. Sher 25 turns out to have a luminosity of log L/L⊙ = 5.48 ± 0.14, equivalent to that of a ~27 M⊙ supergiant in a single-star scenario, which is about half of the mass assumed so far, bringing it much closer in its characteristics to Sk−69º202, the progenitor of SN 1987A. Sher 25 is significantly older than NGC 3603. Further arguments for a binary (merger) evolutionary scenario of Sher 25 are discussed.

Gaia Data Release 3

Context. Diffuse interstellar bands (DIBs) are common interstellar absorption features in spectroscopic observations but their origins remain unclear. DIBs play an important role in the life cycle of the interstellar medium (ISM) and can also be used to trace Galactic structure. Aims. Here, we demonstrate the capacity of the Gaia-Radial Velocity Spectrometer (RVS) in Gaia DR3 to reveal the spatial distribution of the unknown molecular species responsible for the most prominent DIB at 862 nm in the RVS passband, exploring the Galactic ISM within a few kiloparsecs from the Sun. Methods. The DIBs are measured within the GSP-Spec module using a Gaussian profile fit for cool stars and a Gaussian process for hot stars. In addition to the equivalent widths and their uncertainties, Gaia DR3 provides their characteristic central wavelength, width, and quality flags. Results. We present an extensive sample of 476 117 individual DIB measurements obtained in a homogeneous way covering the entire sky. We compare spatial distributions of the DIB carrier with interstellar reddening and find evidence that DIB carriers are present in a local bubble around the Sun which contains nearly no dust. We characterised the DIB equivalent width with a local density of 0.19 ± 0.04 Å kpc−1 and a scale height of 98.60−8.46+11.10 pc. The latter is smaller than the dust scale height, indicating that DIBs are more concentrated towards the Galactic plane. We determine the rest-frame wavelength with unprecedented precision (λ0 = 8620.86 ± 0.019 Å in air) and reveal a remarkable correspondence between the DIB velocities and the CO gas velocities, suggesting that the 862 nm DIB carrier is related to macro-molecules. Conclusions. We demonstrate the unique capacity of Gaia to trace the spatial structure of the Galactic ISM using the 862 nm DIB.

Mass–Metallicity Relationship of SDSS Star-forming Galaxies: Population Synthesis Analysis and Effects of Star Burst Length, Extinction Law, Initial Mass Function, and Star Formation Rate

We investigate the mass–metallicity relationship of star-forming galaxies by analyzing the absorption line spectra of ∼200,000 galaxies in the Sloan Digital Sky Survey. The galaxy spectra are stacked in bins of stellar mass, and a population synthesis technique is applied yielding the metallicities, ages, and star formation history of the young and old stellar population together with interstellar reddening and extinction. We adopt different lengths of the initial starbursts and different initial mass functions for the calculation of model spectra of the single stellar populations contributing to the total integrated spectrum. We also allow for deviations of the ratio of extinction to reddening R V from 3.1 and determine the value from the spectral fit. We find that burst length and R V have a significant influence on the determination of metallicities, whereas the effect of the initial mass function is small. The R V values are larger than 3.1. The metallicities of the young stellar population agree with extragalactic spectroscopic studies of individual massive supergiant stars and are significantly higher than those of the older stellar population. This confirms galaxy evolution models where metallicity depends on the ratio of gas to stellar mass and where this ratio decreases with time. Star formation history is found to depend on galaxy stellar mass. Massive galaxies are dominated by stars formed at early times.

TESS Observations of the Pleiades Cluster: A Nursery for δ Scuti Stars

We studied 89 A- and F-type members of the Pleiades open cluster, including five escaped members. We measured projected rotational velocities () for 49 stars and confirmed that stellar rotation causes a broadening of the main sequence in the color–magnitude diagram. Using time-series photometry from NASA’s TESS Mission (plus one star observed by Kepler/K2), we detected δ Scuti pulsations in 36 stars. The fraction of Pleiades stars in the middle of the instability strip that pulsate is unusually high (over 80%), and their range of effective temperatures agrees well with theoretical models. On the other hand, the characteristics of the pulsation spectra are varied and do not correlate with stellar temperature, calling into question the existence of a useful relation for δ Scutis, at least for young main-sequence stars. By including δ Scuti stars observed in the Kepler field, we show that the instability strip is shifted to the red with increasing distance by interstellar reddening. Overall, this work demonstrates the power of combining observations with Gaia and TESS for studying pulsating stars in open clusters.

Quantitative spectroscopy of late O-type main-sequence stars with a hybrid non-LTE method

Context. Late O-type stars at luminosities log L/L⊙ ≲ 5.2 show weak winds with mass-loss rates lower than 10−8 M⊙ yr−1. This implies that, unlike their more massive and more luminous siblings, their photospheric layers are not strongly affected by the stellar wind. Aims. A hybrid non-local thermodynamic equilibrium (non-LTE) approach – line-blanketed hydrostatic model atmospheres computed under the assumption of LTE in combination with non-LTE line-formation calculations – is tested for analyses of late O-type stars with masses up to ~25 M⊙. A sample of 20 mostly sharp-lined Galactic O stars of spectral types O8 to O9.7 and luminosity classes V and IV, previously studied in the literature using full non-LTE model atmospheres, is investigated. Methods. Hydrostatic and plane-parallel atmospheric structures and synthetic spectra computed with Kurucz’s ATLAS12 code together with the non-LTE line-formation codes DETAIL and SURFACE, which account for the effects of turbulent pressure on the atmosphere, were employed. High-resolution spectra were analysed for atmospheric parameters using hydrogen lines, multiple ionisation equilibria, and elemental abundances. Fundamental stellar parameters were derived by considering stellar evolution tracks and Gaia Early Data Release 3 (EDR3) parallaxes. Interstellar reddening was characterised by fitting spectral energy distributions from the UV to the mid-IR. Results. A high precision and accuracy is achieved for all derived parameters for 16 sample stars (4 objects show composite spectra). Turbulent pressure effects turn out to be significant for the quantitative analysis. Effective temperatures are determined to 1–3% uncertainty levels, surface gravities to 0.05 to 0.10 dex, masses to better than 8%, radii to better than 10%, and luminosities to better than 20% uncertainty typically. Abundances for C, N, O, Ne, Mg, Al, and Si are derived with uncertainties of 0.05–0.10 dex and for helium within 0.03–0.05 dex (1σ standard deviations) in general. Overall, results from previous studies using unified photosphere plus wind (full) non-LTE model atmospheres are reproduced, and with higher precision. The improvements are most pronounced for elemental abundances, and smaller microturbulent velocities are found. An overall good agreement is found between our spectroscopic distances and those from Gaia. Gaia EDR3-based distances to the Lac OB1b association and to the open clusters NGC 2244, IC 1805, NGC 457, and IC 1396 are determined as a byproduct. The derived N/C versus N/O abundance ratios tightly follow the predictions from stellar evolution models. Two ON stars show a very high degree of mixing of CNO-processed material and appear to stem from binary evolution.

Quantitative spectroscopy of B-type supergiants

Context. B-type supergiants are versatile tools to address a number of highly-relevant astrophysical topics, ranging from stellar atmospheres over stellar and galactic evolution to the characterisation of interstellar sightlines and to the cosmic distance scale. Aims. A hybrid non-local thermodynamic equilibrium (LTE) approach, involving line-blanketed model atmospheres computed under the assumption of LTE in combination with line formation calculations that account for deviations from LTE, is tested for quantitative analyses of B-type supergiants of mass up to about 30 M⊙, characterising a sample of 14 Galactic objects in a comprehensive way. Methods. Hydrostatic plane-parallel atmospheric structures and synthetic spectra computed with Kurucz’s ATLAS 12 code together with the non-LTE line-formation codes DETAIL/SURFACE are compared to results from full non-LTE calculations with TLUSTY, and the effects of turbulent pressure on the models are investigated. High-resolution spectra at signal-to-noise ratio >130 are analysed for atmospheric parameters, using Stark-broadened hydrogen lines and multiple metal ionisation equilibria, and for elemental abundances. Fundamental stellar parameters are derived by considering stellar evolution tracks and Gaia early data release 3 (EDR3) parallaxes. Interstellar reddening and the reddening law along the sight lines towards the target stars are determined by matching model spectral energy distributions to observed ones. Results. Our hybrid non-LTE approach turns out to be equivalent to hydrostatic full non-LTE modelling for the deeper photospheric layers of the B-type supergiants under consideration, where most lines of the optical spectrum are formed. Turbulent pressure can become relevant for microturbulent velocities larger than 10 km s−1. The changes in the atmospheric density structure affect many diagnostic lines, implying systematic changes in atmospheric parameters, for instance an increase in surface gravities by up to 0.05 dex. A high precision and accuracy is achieved for all derived parameters by bringing multiple indicators to agreement simultaneously. Effective temperatures are determined to 2–3% uncertainty, surface gravities to better than 0.07 dex, masses to about 5%, radii to about 10%, luminosities to better than 25%, and spectroscopic distances to 10% uncertainty typically. Abundances for chemical species that are accessible from the optical spectra (He, C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe) are derived with uncertainties of 0.05–0.10 dex (1σ standard deviations). The observed spectra are reproduced well by the model spectra. The derived N/C versus N/O ratios tightly follow the predictions from Geneva stellar evolution models that account for rotation, and spectroscopic and Gaia EDR3 distances are closely matched. Finally, the methodology is tested for analyses of intermediate-resolution spectra of extragalactic B-type supergiants.

A WISE view on extreme AGB stars

Context. Variability is a key property of stars on the asymptotic giant branch (AGB). Their pulsation period is related to the luminosity and mass-loss rate (MLR) of the star. Long-period variables (LPVs) and Mira variables are the most prominent of all types of variability of evolved stars. However, the reddest, most obscured AGB stars are too faint in the optical and have eluded large variability surveys. Aims. Our goal is to obtain a sample of LPVs with large MLRs by analysing WISE W1 and W2 light curves (LCs) for about 2000 sources, photometrically selected to include known C-stars with the 11.3 μm silicon carbide dust feature in absorption, and Galactic O-stars with periods longer than 1000 days. Methods. Epoch photometry was retrieved from the AllWISE and NEOWISE database and fitted with a sinus curve. Photometry from other variability surveys was also downloaded and fitted. For a subset of 316 of the reddest stars, spectral energy distributions (SEDs) were constructed, and, together with mid-infrared (MIR) spectra when available, fitted with a dust radiative transfer programme in order to derive MLRs. Results. WISE based LCs and fits to the data are presented for all stars. Periods from the literature and periods from refitting other literature data are presented. The results of the spatial correlation with several (IR) databases is presented. About one-third of the sources are found to be not real, but it appears that these cannot be easily filtered out by using WISE flags. Some are clones of extremely bright sources, and in some cases the LCs show the known pulsation period. Inspired by a recent paper, a number of non-variable OH/IRs are identified. Based on a selection on amplitude, a sample of about 750 (candidate) LPVs is selected of which 145 have periods > 1000 days, many of them being new. For the subset of the stars with the colours of C-rich extremely red objects (EROs) the fitting of the SEDs (and available MIR spectra) separates them into C- and O-rich objects. Interestingly, the fitting of MIR spectra of mass-losing C-stars is shown to be a powerful tracer of interstellar reddening when AV ≳ 2 mag. The number of Galactic EROs appears to be complete up to about 5 kpc and a total dust return rate in the solar neighbourhood for this class is determined. In the LMC 12 additional EROs are identified. Although this represents only about 0.15% of the total known LMC C-star population adding their MLRs increases the previously estimated dust return by 8%. Based on the EROs in the Magellanic Clouds, a bolometric period luminosity is derived. It is pointed out that due to their faintness, EROs and similar O-rich objects are ideal targets for a NIR version of Gaia to obtain distances, observing in the K-band or, even more efficiently, in the L-band.

MUSE crowded field 3D spectroscopy in NGC 300

Aims.A quantitative spectral analysis of BA-type supergiants and bright giants in an inner spiral arm region of the nearby spiral galaxy NGC 300 is presented, based on observations with the Multi Unit Spectroscopic Explorer (MUSE) on the European Southern Obsevatory, Very Large Telescope. The flux-weighted gravity–luminosity relationship (FGLR), a stellar spectroscopic distance determination method for galaxies, is extended towards stars at lower luminosities.Methods.Point spread function fitting 3D spectroscopy was performed with PampelMUSE on the datacube. The 16 stars with the highest signal-to-noise ratios are classified with regard to their spectral type and luminosity class using Galactic templates. They were analysed using hybrid non-local thermodynamic equilibrium model spectra to fit the strongest observed hydrogen, helium, and metal lines in the intermediate-resolution spectra. Supplemented by photometric data, this facilitates fundamental stellar parameters and interstellar reddening which have yet to be determined.Results.Effective temperatures, surface gravities, reddeningE(B−V), bolometric magnitudes and luminosities, as well as radii and masses are presented for the sample stars. The majority of the objects follow the FGLR as established from more luminous BA-type supergiants in NGC 300. An increase in the scatter in the flux-weighted gravity–luminosity plane is observed at these lower luminosities, which is in line with predictions from population synthesis models.

A survey for variable young stars with small telescopes – V. Analysis of TX Ori, V505 Ori, and V510 Ori, the HST ULLYSES targets in the σ Ori cluster

ABSTRACT Investigations of the formation of young stellar objects (YSOs) and planets require the detailed analysis of individual sources as well as statistical analysis of a larger number of objects. The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) project provides such a unique opportunity by establishing a UV spectroscopic library of young high- and low-mass stars in the local universe. Here, we analyse optical photometry of the three ULLYSES targets (TX Ori, V505 Ori, V510 Ori) and other YSOs in the σ Ori cluster taken at the time of the HST observations to provide a reference for those spectra. We identify three populations of YSOs along the line of sight to σ Ori, separated in parallax and proper motion space. The ULLYSES targets show typical YSO behaviour with pronounced variability and mass accretion rates of the order of 10−8 M⊙ yr−1. Optical colours do not agree with standard interstellar reddening and suggest a significant contribution of scattered light. They are also amongst the most variable and strongest accretors in the cluster. V505 Ori shows variability with a 7 d period, indicating an inner disc warp at the co-rotation radius. Uncovering the exact nature of the ULLYSES targets will require improved detailed modelling of the HST spectra in the context of the available photometry, including scattered light contributions as well as non-standard reddening.

Search for LBVs in the Local Volume galaxies: study of four stars in NGC 4449

ABSTRACT We continue to search for LBV stars in galaxies outside the Local Group. In this work, we have investigated four luminous stars in NGC 4449. Multiple spectral observations carried out for J122810.94+440540.6, J122811.70+440550.9, and J122809.72+440514.8 revealed the emission features in their spectra that are characteristic of LBVs. Photometry showed noticeable brightness changes of J122809.72+440514.8 (ΔI = 0.69 ± 0.13m) and J122817.83+440630.8 (ΔR = 2.15 ± 0.13m), while the variability of J122810.94+440540.6 and J122811.70+440550.9 does not exceed 0.3m regardless of the filter. We have obtained estimates of the interstellar reddening, photosphere temperatures, and bolometric luminosities log (LBol/L⊙) ≈ 5.24–6.42. Using the cmfgen code, we have modelled the spectrum of the cold state of J122809.72+440514.8 ($T_{\text{eff}}=9300\,$ K) and have obtained possible value of the mass-loss rate $\dot{M} = 5.2\times 10^{-3}\, \mathrm{M}_{\odot }\, \mathrm{yr}^{-1}$. Based on the observational properties, J122809.72+440514.8 and J122817.83+440630.8 were classified as LBVs, while the other two stars were classified as LBV candidates or B[e]-supergiants candidates.

The VMC Survey – XL. Three-dimensional structure of the Small Magellanic Cloud as derived from red clump stars

ABSTRACT Galaxy interactions distort the distribution of baryonic matter and can affect star formation. The nearby Magellanic Clouds are a prime example of an ongoing galaxy interaction process. Here, we use the intermediate-age (∼1–10 Gyr) red clump (RC) stars to map the 3D structure of the Small Magellanic Cloud (SMC) and interpret it within the context of its history of interaction with the Large Magellanic Cloud (LMC) and the Milky Way. RC stars are selected from near-infrared colour–magnitude diagrams based on data from the Visible and Infrared Survey Telescope for Astronomy survey of the Magellanic Clouds. Interstellar reddening is measured and removed, and the corrected brightness is converted to a distance, on a star-by-star basis. A flat plane fitted to the spatial distribution of RC stars has an inclination i = 35°–48° and position angle, PA=170°–186°. However, significant deviations from this plane are seen, especially in the periphery and on the eastern side of the SMC. In the latter part, two distinct populations are present, separated in distance by as much as 10 kpc. Distant RC stars are seen in the north of the SMC, and possibly also in the far west; these might be associated with the predicted ‘Counter-Bridge’. We also present a dust reddening map, which shows that dust generally traces stellar mass. The structure of the intermediate-age stellar component of the SMC bears the imprints of strong interaction with the LMC a few Gyr ago, which cannot be purely tidal but must have involved ram pressure stripping.

Infrared Spectroscopy of YZ Reticuli (Nova Reticuli 2020)

Abstract IR spectroscopy from three epochs is presented for the very bright, X-ray nova YZ Reticuli (Nova Reticuli 2020). The first spectrum, from 2.5 months after outburst, shows a nova well into the coronal phase, with strong lines of [Si vi] and [Si vii], and with little or no interstellar reddening, and no dust emission. The second spectrum, from one month later, shows a four-fold drop in brightness and a measurable increase in the coronal lines relative to the lower excitation features. The final spectrum, from 7 months post outburst, is a factor of six fainter and shows a decrease in the relative strength of the coronal lines from the previous epoch. Emission lines from O i and [N i] are present at all epochs, indicating the persistence of a region of largely neutral gas in this otherwise high excitation object.

Runaway blue main-sequence stars at high Galactic latitudes

Context. The ESA Gaia mission is a remarkable tool for stellar population analysis through its accurate Hertzsprung–Russell diagram. Its precise astrometry has propelled detailed kinematic studies of the Milky Way and the identification of high-velocity outliers. Aims. Motivated by the historical identification of runaway main-sequence (MS) stars of an early spectral type at high Galactic latitudes, we test the capability of Gaia at identifying new such stars. Methods. We selected ≈2300 sources with Gaia magnitudes of GBP − GRP ≤ 0.05, which are compatible with the colors of low-extinction MS stars that are earlier than a mid-A spectral type, and obtained low-resolution optical spectroscopy for 48 such stars. By performing detailed photometric and spectroscopic analyses, we derived their atmospheric and physical parameters (effective temperature, surface gravity, radial velocity, interstellar reddening, spectrophotometric distance, mass, radius, luminosity, and age). The comparison between spectrophotometric and parallax-based distances enabled us to disentangle the MS candidates from older blue horizontal branch (BHB) candidates. Results. We identify 12 runaway MS candidates, with masses between 2 and 6 M⊙. Their trajectories were traced back to the Galactic disk in order to identify their most recent Galactic plane crossings and the corresponding flight times. All 12 candidates are ejected from the Galactic disk within 2 to 16.5 kpc from the Galactic center and possess flight times that are shorter than their evolutionary ages, which is compatible with a runaway hypothesis. Three MS candidates have ejection velocities exceeding 450 km s−1, thus, they appear to challenge the canonical ejection scenarios for late B-type stars. The fastest star of our sample also has a non-negligible Galactic escape probability if its MS nature can be confirmed. We identify 27 BHB candidates, and the two hottest stars in our sample are rare low-mass stars of late O- and early B-types, evolving towards the white dwarf cooling sequence. Conclusions. The combination of Gaia parallaxes and proper motions can lead to the efficient selection of runaway blue MS candidates up to 10 kpc away from the Sun. High resolution spectra are needed to confirm the MS status, via precise measurements of projected rotational velocities and chemical compositions.

Calibrating the Galactic Cepheid Period–Luminosity Relation from the Maximum-likelihood Technique

Abstract We present a realization of the maximum-likelihood technique, which is one of the latest modifications of the Baade–Becker–Wesselink method. Our approach is based on nonlinear calibrations of the effective temperature and bolometric correction, which take into account metallicity and surface gravity. It allows one to estimate the key Cepheid parameters, the distance modulus, and the interstellar reddening, combining photometric and spectroscopic data (including the effective temperature data). This method is applied to a sample of 44 Galactic Cepheids for which multiphase temperature measurements are available. The additional data correction is performed to subtract the impact of the component in binary/multiple systems. We also study the effect of shock waves, whose presence in the stellar atmosphere distorts the observational data and leads to systematic errors in the obtained parameters. We determine the optimal restriction on the input data to eliminate this effect. This restriction provides accurate period–radius and period–luminosity relations that are consistent with the results in previous studies. We found the following relations: log R = (0.68 ± 0.03) · log P + (1.14 ± 0.03), M v = − (2.67 ± 0.16) · (log P − 1) − (4.14 ± 0.05).