Carbon stars with increased oxygen and nitrogen abundances: hydrostatic dust-free model atmospheres

Abstract

We have computed a grid of hydrostatic spherical COMARCS models for C stars covering metallicities from [Z/H] = 0 to −2 and values of the carbon excess [C−O] from 6.41 to 9.15, plus some temperature sequences, where the amount of oxygen and nitrogen is increased relative to a scaled solar element mixture. Such abundance variations may appear during the late stages of stellar evolution. Our study covers changes of [O/Z] and [N/Z] going up to +0.5. Based on the atmospheric structures we have calculated synthetic spectra and photometry for all of the models in a consistent way.The sequences with changed [O/Z] and [N/Z] can be used to derive correction terms, which are applied to the colours predicted for a certain combination of effective temperature, surface gravity, metallicity and carbon excess. If one neglects these shifts in case of a variable oxygen amount, taking [C−O] instead of C/O gives much better results, since the first quantity dominates the formation of many important molecular species. For the warmer C giants with weaker pulsation it is in principle possible to determine [C−O], [O/Z] or [N/Z] from high resolution spectra, when the opacities in the radiative transfer calculations for the models and observable properties are treated consistently. The corresponding changes due to the abundances often become significantly larger than the deviations caused by uncertainties of the stellar parameters or by a optically thin dust shell. Photometric data and low or medium resolution spectra are not sufficient to derive the mentioned quantities.

The VMC Survey

“…This is a limitation, and we try to estimate the possible effect now. Examination of the bolometric corrections computed by Aringer et al (2009Aringer et al ( , 2019, limited to the coolest of their spectra (T eff = 2600 K), reveals that the expected magnitude differences between a K passband (from Bessell & Brett 1989) and a 2MASS K s filter are typically smaller than 0.04 mag for O-rich stars, and smaller than 0.015 mag for C-rich stars. These differences are smaller for stars with hotter photospheres.…”

Extended view on the dust shells around two carbon stars

“…The ISO SWS spectrum shows a deep absorption feature around 5 µm that is ascribed to C 3 and is also not fully reflected in the COMARCS model atmospheres. It has been suggested that the feature depth must correlate with a particularly high C/O ratio (∼ 2, Gautschy-Loidl et al 2004;Aringer et al 2019). This is much higher than what is found by Lambert et al (1986) and used for the input radiation source in our model.…”

Extended view on the dust shells around two carbon stars

“…The ISO SWS spectrum shows a deep absorption feature around 5 µm that is ascribed to C 3 and is also not fully reflected in the COMARCS model atmospheres. It has been suggested that the feature depth must correlate with a particularly high C/O ratio (∼2, Gautschy-Loidl et al 2004;Aringer et al 2019). This is much higher than what is found by Lambert et al (1986) and used for the input radiation source in our model.…”

“…Concerning the selection of the central star, we follow the approach of our MoD fits, adopting a hydrostatic atmosphere model with fixed parameters. This was taken from the grid of Aringer et al (2016Aringer et al ( , 2019. Based on the data given in Table 1, the effective temperature was set to 3000 K, which is consistent with the choice for the MoD fits.…”

“…This difference between results based on some high-resolution measurements and the requirement to get a strong C 3 feature and a considerable amount of dust requires some further investigation. Such a study must include a consistent treatment of complete molecular opacities in the model construction and spectrum synthesis as well as possible dynamical and non-equilibrium effects (see Aringer et al 2019).…”