Extended envelopes around Galactic Cepheids

IV. T Monocerotis and X Sagittarii from mid-infrared interferometry with VLTI/MIDI^⋆,⋆⋆

¹ Universidad de Concepción, Departamento de Astronomía, 160 Casilla, Concepción, Chile
e-mail: agallenne@astro-udec.cl
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

Abstract

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 (τ_0.55  μm = 0.008), while it appears to be thicker for T Mon (τ_0.55  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, f_{8.6   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.