3rd International Workshop on Spectral Stellar LibrariesASI Conference Series, 2017, Vol. 14, pp 21–24Editors: P. Coelho, L. Martins & E. Griffin

Carbon stars in the X-Shooter Spectral Library

A. Gonneau1∗, A. Lançon2, S. C. Trager1, R. F. Peletier1, M. Lyubenova1,A. Arentsen3,B. Aringer4,5, Y.-P. Chen6, O. S. Choudhury3,M. Dries1, J. Falcón-Barroso7,8, P. Prugniel9, S. Meneses-Goytia10,W. Nowotny4, P. Sánchez-Blázquez11, A. Vazdekis7,8 and M. Koleva1Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen,The Netherlands2Observatoire Astronomique de Strasbourg, CNRS, UMR 7550, F-67000 Strasbourg, France3Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany4University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria5Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova, I-35122 Padova, Italy6New York University Abu Dhabi, Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates7Instituto de Astrofísica de Canarias, Vía Láctea s/n, La Laguna, Tenerife, Spain8Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain9CRAL-Observatoire de Lyon, Université de Lyon, Lyon I, CNRS, UMR5574, France10Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 3FX, UK11Universidad Autónoma de Madrid, Departamento de Física Teórica, E-28049 Cantoblanco,Madrid, Spain

Received July 1st, 2017 ; accepted Oct 15th, 2017

Abstract. We present a new collection of spectra of 35 carbon stars, ob-tained as part of the X-Shooter Spectral Library project. The sample con-tains stars with a broad range of (J − K) colours and pulsation properties,located in the Milky Way and the Magellanic Clouds. Two families ofspectra emerge when (J − K) is larger than 1.6, characterised by the pres-ence or absence of the absorption feature at 1.53 µm, generally associatedwith HCN and C2H2. We compare the observations with a new set of high-resolution synthetic spectra based on hydrostatic model atmospheres. Thefull spectrum fit of the observations to the grid of models allows us to iden-tify the most likely atmospheric parameters. Those parameters, derivedindependently in the optical and near-infrared, are generally very consis-tent for (J − K) < 1.6. For the reddest stars showing the 1.53 µm feature,which are all large amplitude variables, the effects of pulsation are strong

∗email: gonneau@astro.rug.nl

22 A. Gonneau et al.

and the spectra are poorly matched with hydrostatic models. Dynamicalmodels are needed to interpret those observations.

Keywords : X-Shooter – carbon stars – atmospheres – spectrum fitting

1. Introduction

On the thermally pulsing asymptotic giant branch (TP-AGB), some (lucky) stars ex-perience a drastic change of their spectral appearance. Thanks to the third dredge-up,some additional carbon is brought up from the core to the surface, which changesthe chemical composition of the envelope. These oxygen-rich stars then change intocarbon-rich stars, with bands of carbon-bearing molecules, such as CN and C2, dom-inating the spectra. Carbon stars (C stars) contribute significantly to the near-infraredlight of galaxies, but only small collections of C-star spectra exist to represent thisemission. Thanks to the X-Shooter Spectral Library (hereafter XSL, Chen et al. 2014),we obtained 35 C-star spectra covering the wavelength range 400–2400 nm, at R ∼8 000. Figure 1 illustrates the high quality of our spectra.

Our sample presents quite a diversity of spectral shape and absorption-line char-acteristics. It exhibits a bimodal behaviour for the reddest carbon stars. Indeed, someof our carbon stars with (J − Ks) > 1.6 display an absorption band at 1.53 µm that isusually associated with HCN and C2H2 (Gautschy-Loidl et al. 2004) (see the examplein Figure 2). This feature is thought to be an overtone of a strong absorption featureat 3 µm. In our sample, this feature appears essentially only in Miras, though not allour Miras show it.

Figure 1. Some classical bands of carbon-bearing molecules for our sample (Gonneau et al.2016)

Carbon stars in XSL 23

Figure 2. A close-up of theHCN+C2H2 feature (Gonneau et al.2016).

Furthermore, the presence of the 1.53 µmfeature is also associated with a smoother ap-pearance of the near-infrared spectrum for ourdata. Our favoured interpretation is that red car-bon stars are still enshrouded in hot circumstellardust that they have produced, which causes veil-ing.

2. Comparison with models

By using a grid of synthetic spectra to fit the fullobserved spectra, we estimated the most likelyvalues for the stellar atmospheric parameters Teff , log(g), [Fe/H] and C/O. Thesestate-of-the-art models are C-rich COMARCS model atmospheres, hydrostatic anddust-free (Aringer et al. 2016). They are are known to reproduce well the colours ofthe warmer carbon stars ((J − K) < 1.5) that have weak pulsation.

We performed a χ2 minimisation (with a multiplicative polynomial) over twowavelength ranges: 1) VIS, from 0.4 to 1.0 µm, and; 2) NIR, from 1.0 to 2.4 µm. Thepurpose was to check the compatibility of the stellar parameters that we had estimatedfrom both regions. This analysis was done at an intermediate spectral resolution:R ∼ 2 000, to avoid any velocity discontinuities driven by the pulsation.

We divided our sample into four groups, based on the (J−Ks) colour of our targetsand the presence of the 1.53 µm feature (see Gonneau et al. 2017 for more details).The left panel of Figure 3 shows our near-infrared colour indices, overplotted with thesynthetic colours.

Figure 3. Left: Colours for our sample of stars (filled symbols) and for the grid of hydrostaticmodels (open diamonds). Right: Comparison of the weighted averaged values found for theeffective temperatures for both wavelength ranges. (Gonneau et al. 2017).

For stars from Groups A to C, the effective temperatures derived separately from

24 A. Gonneau et al.

the VIS and NIR wavelength ranges overlap in general (typically within ±100K ofeach other), but the VIS-based temperatures tend to be warmer (see right panel ofFigure 3). Furthermore, the favoured surface gravity is log(g) = 0 and the favouredmetallicity [Fe/H] = −0.5, values which are satisfactory for TP-AGB stars and fora sample consisting mostly of LMC, SMC and Milky Way halo stars. It becomesprogressively more difficult to obtain reasonable matches of the data for the redder ofour observed spectra, in particular for stars from Group D, as they are increasinglyaffected by pulsation. The use of hydrostatic models is then a clear limitation.

3. Conclusion

The next obvious step is to compare the observations with dynamical models, i.e.,models that take into account the pulsating stellar interior as well as the developmentof dust-driven winds (Eriksson et al. 2014). It will also be interesting to revise theline lists corresponding to the main carriers (see Figure 4: C2 on one hand, HCN andC2H2 on the other).

Figure 4. Next challenges: the C2 feature (left panel) and the 1.53 µm feature (right panel)(Gonneau et al. 2017).

Acknowledgements

A warm thank you to everyone who helped organise this conference in Brazil. A.Gonneau also acknowledges the LKBF and the MLF for partial travel support.

References

Aringer B., et al., 2016, MNRAS, 457, 3611Chen Y. P., et al., 2014, A&A, 565, A117Eriksson K., et al., 2014, A&A, 566, A95Gautschy-Loidl R., et al., 2004, A&A, 422, 289Gonneau A., et al., 2016, A&A, 589, A36Gonneau A., et al., 2017, A&A, 2017arXiv170200819G