Synthetic photometry for carbon-rich giants

Abstract

Late-type giant stars in the evolutionary stage of the asymptotic giant branch increasingly lose mass via comparatively slow but dense stellar winds. Not only do these evolved red giants contribute in this way to the enrichment of the surrounding interstellar medium, but the outflows also have a substantial influence on the spectro-photometric appearance of such objects. In the case of carbon-rich atmospheric chemistries, the developing cool circumstellar envelopes contain dust grains mainly composed of amorphous carbon. With increasing mass-loss rates, this leads to more and more pronounced circumstellar reddening. With the help of model calculations we aim at reproducing the observational photometric findings for a large sample of well-characterised galactic C-type Mira variables losing mass at different rates. We used dynamic model atmospheres, describing the outer layers of C-rich Miras, which are severly affected by dynamic effects. Based on the resulting structures and under the assumptions of chemical equilibrium as well as LTE, we computed synthetic spectra and synthetic broad-band photometry (Johnson-Cousins-Glass BVRIJHKL M). A set of five representative models with different stellar parameters describes a sequence from less to more evolved objects with steadily increasing mass-loss rates. This allowed us to study the significant influence of circumstellar dust on the spectral energy distributions and the (amplitudes of) lightcurves in different filters. We tested the photometric properties (mean NIR magnitudes, colours, and amplitudes) and other characteristics of the models (mass-loss rates, periods, and bolometric corrections) by comparing these with the corresponding observational data adopted from the literature. Using different kinds of diagrams we illustrate where the models are located in a supposed evolutionary sequence defined by observed C-type Mira samples. Based on comparisons of galactic targets with empirical relations derived for C stars in the Large Magellanic Cloud we discuss the relevance of metallicity and excess carbon (C-O) for the development of dust-driven winds. Having investigated the dynamic model atmospheres from different (mainly photometric) perspectives, we conclude that our modelling approach (meaning the combination of numerical method and a suitable choice of model parameters) is able to describe C-rich long-period variables over a wide range of mass-loss rates, i.e., from moderately pulsating objects without any dusty wind to highly dust-enshrouded Carbon Miras. Thus, we can trace the observed sequence of C-type Miras, which is mainly determined by the mass loss.

Luminosities and mass-loss rates of Local Group AGB stars and red supergiants

“…Polynomials are fitted separately to the data on either side of For the C stars, the BC for (J − K) agrees well with the quadratic relation by Kerschbaum et al (2010) in the range in common (J − K < 4). Our values are on the low side compared to observations by Whitelock et al (2006) and models by Nowotny et al (2013) and Eriksson et al (2014). The fitting excludes 24 C stars with (J − K) > 10 mag as well as the outliers SAGEMCJ054546 (near J − K ∼ 1.75 mag) and IRAS 05278 (near J − K ∼ 4.05 mag).…”

“…See Section 5.2 for an explanation of sources not fitted or plotted. For C stars in the top right panel, red stars indicate models by Nowotny et al (2013), green dots indicate models by Eriksson (2014), the dotted red line indicates the fit by Kerschbaum et al (2010), and the dashed red line indicates the fit by Whitelock et al (2006). For M stars in the lower-right panel, the straight red lines indicate relations "A", "B", and "C" from Kerschbaum et al (2010), valid in the range 1.0 < J − K < 1.6, 1.1 < J − K < 1.95, and 1.75 < J − K < 2.9 mag, respectively.…”

Mass loss of stars on the asymptotic giant branch

“…Eriksson et al (2014) produced synthetic low-resolution spectra and photometry for a sub-sample of the Mattsson et al (2010) grid with stellar parameters representative of solar-metallicity C-rich AGB stars. Furthermore, the study of photometric variability was extended by adding new dynamical models to the grid, with a higher luminosity amplitude but otherwise similar parameters, following up on the exploratory results of Nowotny et al (2011Nowotny et al ( , 2013.…”

Abundance analysis for long-period variables

“…L4C048AP): luminosity (L4), C/O ratio (0.48), the combination of the piston-velocity amplitude Δu p and the luminosity-amplitude parameter f L (A), as well as a simplified mass-loss information (P = only pulsating, PM = pulsating and mass-losing). Notation adopted from previous works (Lebzelter et al 2010;Nowotny et al 2010Nowotny et al , 2013. First column: parameters of the hydrostatic initial model (luminosity L , mass M , effective temperature T , metallicity [Fe/H]) together with the pulsation period P of the piston at the inner boundary.…”