A new library of synthetic spectraand photometry for evolved C stars

K. Eriksson 1, W. Nowotny 2, S. Höfner 1, B. Aringer 2

1 Department of Physics and Astronomy, Uppsala University, Sweden2 Department of Astrophysics, University of Vienna, Austria

Acknowledgements: This research was funded by the Austrian Science Fund (FWF): P21988-N16, P23006, P23586. This work was supported by theSwedish Research Council. BA acknowledges funding by the contracts ASI-INAF I/016/07/0 and I/009/10/0. The computations wereperformed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at UPPMAX.

4. Contents of the library

● 229 models with wind / 311 models without● time-series of snapshots of the atmospheric structures:

- typically 2-3 separated sequences covering a few pulsation periods each - well-sampled light cycles

The user will get:- tabulated model properties (input parameters, wind characteristics)- tabulated photometric properties for rough characterisation

(mean magnitudes and colours, photometric amplitudes)- “fact sheets” = graphical illustration of each individual model- time-series synthetic spectra → 0.35–25µm, R=200- synthetic lightcurves in standard broadband filters BVRIJHKLL‘M

of the Johnson-Cousins-Glass system

1. Introduction

The evolution and spectral properties of stars on the AGB are significantly affectedby mass loss through dusty stellar winds. Dynamic atmosphere and wind modelsare an essential tool for studying these evolved stars, both individually and asmembers of stellar populations (to understand their contribution to the integratedlight and the chemical evolution of galaxies).

We have computed low-resolution spectra and photometry (in the wavelengthrange of 0.35–25µm) for a grid of 540 dynamic models with stellar parameterstypical of solar-metallicity C-type LPVs and with a range of pulsation amplitudes; itis a sub-sample of the grid by Mattsson et al. (2010). The models cover thepulsating atmosphere and the dusty outflow (if present), assuming sphericalsymmetry, and taking into account the opacities of gas-phase species and dustgrains (in the small-particle limit, SPL). Gas dynamics, including shocks, and non-equilibrium grain growth are described by detailed time-dependent equations. Thestellar pulsations are simulated by time-dependent sinusoidal inner boundaryconditions (piston) with period and amplitude as parameters.

Comparisons with various observed data in general show a quite satisfactoryagreement. The models are made available to the community as an electronicdatabase for use in various theoretical and observational studies.

2. Grid parameters and dynamic properties

Table 1: Combinations of fundamental stellar inputparameters covered by the model grid of the library.

Notes. The period is not an independent parameterbut coupled to the stellar luminosity via the period-luminosity relation from Feast et al. (1989). Foreach set of parameters listed, we varied the innerboundary velocity and luminosity amplitude (3 and 2values, respectively).

Fig. 1:Dynamic behaviour of the 1M models. For eachcombination of Teff and L, the effects of varying thecarbon excess (C–O) and the piston velocityamplitude Δup are shown.The colours denote:

blue = model without a wind,red = model with a steady wind,green = model with an episodic wind.

3. Comparing mass-loss results with previous studies

Fig. 2:Comparison of the mass-loss rates of our modelgrid with corresponding values computed from themass-loss formula given by Wachter et al. (2002)using the same stellar parameters. The filledsymbols denote a sub-sample of models havingparameters within the range for which the formulawas originally derived. The systematic differencesare due to the fact that the Wachter et al. modelsuse grey radiative transfer with a constant (low)value for the gas opacity.

A consequence is that using the Wachter-formula instellar evolution models probably leads to anoverestimation of the mass-loss rates.

6. Dynamic and photometric results vs. observations

Fig. 4:Mass-loss rates as a function of (J–K) colours forthe models of our grid which develop a wind(triangles) compared with the corresponding datafor observed galactic C-type Miras (Whitelock et al.2006). The red dashed line is a fit for LMC carbonstars given by Gullieuszik et al. (2012).

5. How do magnitudes and colours of the model grid compare with observed values?

Fig. 5:Colour-magnitude diagram showing mean K-magsvs. mean (J–K) colours for all the grid modelstogether with the observed sample of field CarbonMiras of Whitelock et al. (2006) and 2MASS data forBaade‘s window towards the Galactic Bulge (for adetailed description see Nowotny et al. 2013).

Fig. 6:Mean (J–H) vs. mean (H–K) colours for the wholemodel grid (triangles) and some observed carbonstar samples:

blue = galactic C-type Mirasmagenta = galactic C-type SRVsgreen = C stars with moderate variations (if at all)

The orange crosses denote the correspondingcolours of hydrostatic model atmospheres(COMARCS) adopted from Aringer et al. (2009).The dashed line is a fit for the Carbon Miras givenby Whitelock et al. (2006, their Eq. 7).

For more details, see our recent paper presenting the library:→ Eriksson et al. 2014, A&A, 566, A95

Synthetic spectra and photometry, as well as fact sheets summarising modelresults for each individual model can be downloaded from the CDS or from:

→ http://www.astro.uu.se/AGBmodels

Future plans:● replace SPL with size-dependent dust grain opacities● extend the parameter range to include smaller carbon excesses (C-O)

Fig. 3:Illustrative examples of the spectra as provided bythe library, based on a characteristic dynamicalmodel atmosphere resembling a typical C-rich Mirawith an intermediate mass loss of ≈ 4·10-6 M/yr.Adopted from Nowotny et al. (2011).

References: Aringer et al. 2009, A&A, 503, 913Bergeat et al. 2001, A&A, 369, 178Feast et al. 1989, MNRAS, 241, 375Gullieuszik et al. 2012, A&A, 537, A105Mattsson et al. 2010, A&A, 509, A14Nowotny et al. 2011, A&A, 529, A129Nowotny et al. 2013, A&A, 552, A20Wachter et al. 2002, A&A, 384, 452Whitelock et al. 2006, MNRAS, 369, 751

Presented at „Why Galaxies Care About AGB Stars III.“, July 28 – August 1, 2014, Vienna, Austria