VEGA/CHARA interferometric observations of Cepheids

Abstract

Context. The B-W method is used to determine the distance of Cepheids and consists in combining the angular size variations of the star, as derived from infrared surface-brightness relations or interferometry, with its linear size variation, as deduced from visible spectroscopy using the projection factor. The underlying assumption is that the photospheres probed in the infrared and in the visible are located at the same layer in the star whatever the pulsation phase. While many Cepheids have been intensively observed by infrared beam combiners, only a few have been observed in the visible. Aims. This paper is part of a project to observe Cepheids in the visible with interferometry as a counterpart to infrared observations already in hand. Methods. Observations of δ Cep itself were secured with the VEGA/CHARA instrument over the full pulsation cycle of the star. Results. These visible interferometric data are consistent in first approximation with a quasi-hydrostatic model of pulsation surrounded by a static circumstellar environment (CSE) with a size of θCSE = 8.9 ± 3.0 mas and a relative flux contribution of fCSE = 0.07 ± 0.01. A model of visible nebula (a background source filling the field of view of the interferometer) with the same relative flux contribution is also consistent with our data at small spatial frequencies. However, in both cases, we find discrepancies in the squared visibilities at high spatial frequencies (maximum 2σ) with two different regimes over the pulsation cycle of the star, φ = 0.0 − 0.8 and φ = 0.8 − 1.0. We provide several hypotheses to explain these discrepancies, but more observations and theoretical investigations are necessary before a firm conclusion can be drawn. Conclusions. For the first time we have been able to detect in the visible domain a resolved structure around δ Cep. We have also shown that a simple model cannot explain the observations, and more work will be necessary in the future, both on observations and modelling.

HARPS-N high spectral resolution observations of Cepheids I. The Baade-Wesselink projection factor of δ Cep revisited

“…However, a disagreement is still found between the interferometric and the infrared surface-brightness approaches (and their respective projection factors) in the case of δ Cep , while an agreement is found for the long-period Cepheid ℓ Car (Kervella et al 2004b). This suggests that the p-factors adopted in these approaches might be affected by something not directly related to the projection factor itself but rather to other effects in the atmosphere or close to it, such as a static circumstellar environment (Nardetto et al 2016). However, it is worth mentioning that the two methods currently provide absolutely the same results in terms of distances.…”

A thin shell of ionized gas as the explanation for infrared excess among classical Cepheids

“…Envelopes around Cepheids have been discovered by longbaseline interferometry in the K-Band with VLTI and CHARA Mérand et al 2006), and four Cepheids CSEs have been observed in the N band with VISIR and MIDI (Kervella et al 2009;Gallenne et al 2013) and one with NACO in the near-IR (Gallenne et al 2011). The presence of a motionless Hα absorption component using high-resolution spectroscopy around l Car (Nardetto et al 2008) has also been attributed to a CSE, and, recently, a CSE was detected in the visible domain with the VEGA/CHARA facility around δ Cep (Nardetto et al 2016). These various studies determined a CSE radius of 2.…”

Perspectives of a visible instrument on the VLTI

“…Characterizing these CSEs, and in particular their radial brightness profile in several spectral bands will enable one to unbias the period-luminosity relation. Recently, δ Cep was observed with the VEGA/CHARA instrument and an unexpected visible CSE contributing to 7% to the total flux was discovered [20]. Many more visible CSEs can be discovered with a more sensitive visible instrument.…”