Extended envelopes around Galactic Cepheids. IV. T Monocerotis and X Sagittarii from mid-infrared interferometry with VLTI/MIDI

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 su er 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 ( 0:55 m = 0:008), while it appears to be thicker for T Mon ( 0:55 m = 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 m[%]= 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.