Context. Giant stars, and especially C-rich giants, contribute significantly to the chemical enrichment of galaxies. The determination of precise parameters for these stars is a necessary prerequisite for a proper implementation of this evolutionary phase in the models of galaxies. Infrared interferometry opened new horizons in the study of the stellar parameters of giant stars, and provided new important constraints for the atmospheric and evolutionary models. Aims. We aim to determine which stellar parameters can be constrained by using infrared interferometry and spectroscopy; for C-stars in particular we aim to determine the precision that can be achieved as well as its limitations. Methods. For this purpose we obtained new infrared spectra and combined them with unpublished interferometric measurements for five mildly variable carbon-rich asymptotic giant branch stars. The observations were compared with a large grid of hydrostatic model atmospheres and with new isochrones that include the predictions of the thermally pulsing phase. Results. For the very first time we are able to reproduce spectra in the range between 0.9 and 4 μm, and K broad band interferometry with hydrostatic model atmospheres. Temperature, mass, log(g), C/O and a reasonable range for the distance were derived for all objects of our study. All our targets have at least one combination of best-fitting parameters that lies in the region of the HR-diagram where C-stars are predicted. Conclusions. We confirm that low-resolution spectroscopy is not sensitive to the mass and log(g) determination. For hydrostatic objects the 3 μm feature is very sensitive to temperature variations, therefore it is a very powerful tool for accurate temperature determinations. Interferometry can constrain mass, radius, and log(g), but a distance has to be assumed. The large uncertainty in the distance measurements available for C-rich stars remains a major problem.

In a previous paper, we assembled a collection of medium-resolution spectra of 35 carbon stars, covering optical and near-infrared wavelengths from 400 to 2400 nm. The sample includes stars from the Milky Way and the Magellanic Clouds, with a variety of (J − K s ) colors and pulsation properties. In the present paper, we compare these observations to a new set of high-resolution synthetic spectra, based on hydrostatic model atmospheres. We find that the broad-band colors and the molecular-band strengths measured by spectrophotometric indices match those of the models when (J − K s ) is bluer than about 1.6, while the redder stars require either additional reddening or dust emission or both. Using a grid of models to fit the full observed spectra, we estimate the most likely atmospheric parameters T eff , log(g), [Fe/H] and C/O. These parameters derived independently in the optical and near-infrared are generally consistent when (J − K s ) < 1.6. The temperatures found based on either wavelength range are typically within ±100 K of each other, and log(g) and [Fe/H] are consistent with the values expected for this sample. The reddest stars ((J − K s ) > 1.6) are divided into two families, characterized by the presence or absence of an absorption feature at 1.53 µm, generally associated with HCN and C 2 H 2 . Stars from the first family begin to be more affected by circumstellar extinction. The parameters found using optical or near-infrared wavelengths are still compatible with each other, but the error bars become larger. In stars showing the 1.53 µm feature, which are all large-amplitude variables, the effects of pulsation are strong and the spectra are poorly matched with hydrostatic models. For these, atmospheric parameters could not be derived reliably, and dynamical models are needed for proper interpretation.

Context. Based on the expansion velocities of their circumstellar shells, Galactic centre (GC) OH/IR stars can be divided into two groups that are kinematically different and therefore believed to have evolved from different stellar populations. Aims. We studied the metallicity distribution of the OH/IR stars population in the GC on the basis of a theoretical relation between EW(Na), EW(Ca), and EW(CO) and the metallicity.Methods. For 70 OH/IR stars in the GC, we obtained near-IR spectra. The equivalent line-widths of Na i, Ca i, 12 CO(2, 0), and the curvature of the spectrum around 1.6 µm due to water absorption were determined. Results. The near-IR spectrum of OH/IR stars is influenced by several physical processes. OH/IR stars are variable stars suffering high mass-loss rates. The dust that is formed around the stars strongly influences the near-IR spectra and reduces the equivalent linewidths of Na i and Ca i. A similar effect is caused by the water content in the outer atmosphere of the OH/IR star. Because of these effects, it is not possible to determine the metallicities of these stars with our low-resolution near-infrared spectroscopy.