Abstract
A tomographic method to probe velocity fields within stellar atmospheres is described. It relies on the design of spectral masks collecting lines forming at given, pre-specified ranges of optical depths. Different masks thus probe different line-formation depths in the stellar atmosphere. The masks are cross-correlated with the observed spectrum to yield cross-correlation functions (CCFs). The cross-correlation has two advantages: (i) to overcome line crowding, and (ii) to reveal minute line asymmetries by adding together many lines. In pulsating stars (long-period variables or Cepheids), the CCFs are double-peaked around maximum light, when the shock front associated with the stellar pulsation is located in the layer probed by the considered mask. Double-peaked CCFs originate in stellar layers where upward- and downward-moving matter co-exist, on each side of the shock front. The application of the tomographic method to long-period variables and supergiants is illustrated.
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Notes
- 1.
The contribution function gives the relative contribution of the different atmospheric layers to an observed quantity, which can be the emergent intensity or flux, or the line depression in the surface intensity or flux.
- 2.
The high-resolution spectrograph at the Observatoire de Haute Provence. See Sect. 8.3.1 for details.
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Acknowledgements
The authors thank B. Plez, A. Chiavassa, and H. Van Winckel for useful discussions. SvE is FNRS research associate. Based on observations obtained with the Mercator Telescope and the HERMES spectrograph, supported by the Fund for Scientific Research of Flanders (FWO), the Research Council of K.U.Leuven, the Fonds National de la Recherche Scientifique (F.R.S.-FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de Genève, Switzerland and the Thüringer Landessternwarte Tautenburg, Germany.
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Jorissen, A., Van Eck, S., Kravchenko, K. (2016). Velocity Fields in Stellar Atmospheres Probed by Tomography. In: Boffin, H., Hussain, G., Berger, JP., Schmidtobreick, L. (eds) Astronomy at High Angular Resolution. Astrophysics and Space Science Library, vol 439. Springer, Cham. https://doi.org/10.1007/978-3-319-39739-9_8
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