Quantitative spectroscopy of B-type supergiants

Context. B-type supergiants are versatile tools to address various astrophysical topics, ranging from stellar atmospheres over stellar and galactic evolution to the cosmic distance scale. Aims. A hybrid non-LTE approach - line-blanketed model atmospheres computed under the assumption of local thermodynamic equilibrium (LTE) in combination with line formation calculations that account for deviations from LTE - is tested for quantitative analyses of B-type supergiants with masses $M<30 M_{\odot}$, characterising a sample of 14 Galactic objects. Methods. Hydrostatic plane-parallel atmospheric structures and synthetic spectra computed with Kurucz's Atlas12 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 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 EDR3 parallaxes. Interstellar reddening towards the target stars is 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 considered. Turbulent pressure can become relevant for microturbulent velocities larger than 10 km s$^{-1}$. High precision and accuracy is achieved for all derived parameters by bringing multiple indicators to agreement simultaneously. Abundances for chemical species (He, C, N, O, Ne, Mg, Al, Si, S, Ar, Fe) are derived with uncertainties of 0.05 to 0.10 dex. The derived ratios N/C vs. N/O tightly follow the predictions from Geneva stellar evolution models.Comment: 31 pages, 24 figures, Accepted for publication in Astronomy & Astrophysics, Data: https://doi.org/10.5281/zenodo.680256

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-LTE (local thermodynamic equilibrium) 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 $d_\mathrm{NGC 3603}$ = 6250$\pm$150 pc. A cluster membership is also excluded as the hourglass nebula is unaffected by the vigorous stellar winds of the cluster stars and from the different excitation signatures of the hourglass nebula and the nebula around NGC 3603. Sher 25 turns out to have a luminosity of log L/L$_\odot$ = 5.48$\pm$0.14, equivalent to that of a $\sim$27 $M_\odot$ 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-69{\deg}202, 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.Comment: 27 pages, 22 figures, Accepted for publication in Astronomy & Astrophysics, Data: https://doi.org/10.5281/zenodo.823015

CRIRES high-resolution near-infrared spectroscopy of diffuse interstellar band profiles.: Detection of 12 new DIBs in the YJ band and the introduction of a combined ISM sight line and stellar analysis approach

Galaxie

CRIRES high-resolution near-infrared spectroscopy of diffuse interstellar band profiles

Aims. A high spectral resolution investigation of diffuse interstellar bands (DIBs) in the near-infrared (YJ band) is conducted to test new methods, to confirm and improve existing parameters, and to search for new DIBs. Methods. The CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) on the European Southern Observatory's Very Large Telescope was employed to obtain spectra of four reddened background supergiant stars (HD 183143, HD 165784, HD 92207, HD 111613) and an unreddened comparison star (HD 87737) at the highest resolution of R ≈ 100 000 currently achievable at near-infrared wavelengths, more than twice as high as accomplished in previous near-infrared DIB studies. The correction for telluric absorption was performed by a modelling approach. Non-local thermodynamic equilibrium spectral modelling of available optical and the new near-infrared stellar spectra facilitated a comprehensive characterisation of the atmospheric properties of the background stars. As a consequence, a more precise and accurate determination of the reddening and the reddening law along the respective sight lines could be achieved than feasible before by comparison of the observed and model spectral energy distributions. For DIBs that overlap with stellar lines the DIB profile shapes could be recovered. Results. Seventeen known near-infrared DIBs were confirmed, and 12 previously unknown and generally weaker DIBs were identified in the YJ band. Three DIBs that show uniform profiles along all sight lines were identified, possibly connected to transitions from a common lower state of the same carrier. The divergent extinction curve towards the frequently discussed DIB standard star HD 183143 could be reproduced for the first time, requiring extra absorption by ~3.5 mag due to polycyclic aromatic hydrocarbons (PAHs) to match the ultraviolet extinction bump. This extra absorption probably stems from a circumstellar bubble lying in front of the star which is intersected tangentially by the line of sight, making this particular sight line more peculiar than standard