Volume 558, October 2013
|Number of page(s)||9|
|Section||Interstellar and circumstellar matter|
|Published online||22 October 2013|
Extended envelopes around Galactic Cepheids
1 Universidad de Concepción, Departamento de Astronomía, 160 Casilla, Concepción, Chile
2 European Southern Observatory, Alonso de Córdova 3107, 19001 Casilla, Santiago 19, Chile
3 LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
4 Nice Sophia-Antipolis University, CNRS UMR 6525, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
Received: 10 July 2013
Accepted: 30 August 2013
Aims. We study the close environment of nearby Cepheids using high spatial resolution observations in the mid-infrared with the VLTI/MIDI instrument, a two-beam interferometric recombiner.
Methods. We obtained spectra and visibilities for the classical Cepheids X Sgr and T Mon. We fitted the MIDI measurements, supplemented by B,V,J,H,K literature photometry, with the numerical transfer code DUSTY to determine the dust shell parameters. We used a typical dust composition for circumstellar environments.
Results. We detect an extended dusty environment in the spectra and visibilities for both stars, although T Mon might suffer from thermal background contamination. We attribute this to the presence of a circumstellar envelope (CSE) surrounding the Cepheids. This is optically thin for X Sgr (τ0.55 μm = 0.008), while it appears to be thicker for T Mon (τ0.55 μm = 0.15). They are located at about 15−20 stellar radii. Following our previous work, we derived a likely period–excess relation in the VISIR PAH1 filter, f8.6 μm[%]= 0.81(±0.04)P[day]. We argue that the impact of CSEs on the mid-IR period–luminosity (P − L) relation cannot be negligible because they can bias the Cepheid brightness by up to about 30%. For the K-band P − L relation, the CSE contribution seems to be lower (<5%), but the sample needs to be enlarged to firmly conclude that the impact of the CSEs is negligible in this band.
Key words: techniques: high angular resolution / techniques: interferometric / circumstellar matter / stars: variables: Cepheids
Table 3 is only available in electronic form at http://www.aanda.org
© ESO, 2013
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