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 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 (tau(0.55microns) = 0.008), while it appears to be thicker for T Mon (tau(0.55micron) = 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(8.6micron)[%]= 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.Comment: Accepted for publication in Astronomy and Astrophysic

The nearby eclipsing stellar system δ Velorum IV. Differential astrometry with VLT/NACO at the 100 microarcsecond level ⋆

Context. The stellar system δVel contains the brightest eclipsing binary in the southern sky (δVel A), and a nearby third star located ≈ 0.6′ ′ away (δVel B). The proximity of δVel B (usable as a reference) makes it a particularly well-suited target to detect the astrometric displacement of the center of light of the eclipsing pair. Aims. We obtained NACO astrometric observations with two goals: 1) to confirm the orientation of the orbital plane of the eclipsing pair on the sky determined by interferometry (Paper III), and 2) to demonstrate the capabilities of narrow-angle adaptive optics astrometry on a simple system with predictable astrometric properties. Methods. We measured the angular separation vector between the eclipsing binary δVel A and the visual companion δVel B from narrow-band images at 2.17 µm obtained with the VLT/NACO adaptive optics system. Based on these observations and our previous determination of the orbital parameters of the wide binary δVel A-B, we derived the apparent displacement of the center of light of the eclipsing pair at 11 epochs over its orbital cycle. Results. We detect the astrometric wobble of the center of light of the δVel A pair relative to B with a typical measurement precision of ≈ 50 µas per epoch, for a total amplitude of the measured displacement of ≈ 2 mas. Conclusions. The detected wobble is in relatively good agreement with the model we presented in Paper III, and confirms the orientation of the Aab orbital plane on the sky. The residual dispersion compared to our model is 110 µas rms, which we tentativel