Spatially extended emission around the Cepheid RS Puppis in near-infrared hydrogen lines. Adaptive optics imaging with VLT/NACO

TL;DR

This study uses adaptive optics imaging to detect and analyze the circumstellar hydrogen envelope around the Cepheid RS Pup, quantifying its flux contribution and symmetry at near-infrared wavelengths.

Contribution

First direct near-infrared imaging detection and flux estimation of RS Pup's circumstellar hydrogen envelope with analysis of its symmetry and physical properties.

Findings

Detected extended emission with 38% flux contribution in one filter.

Detected extended emission with 24% flux contribution in another filter.

Found no detectable asymmetry in the circumstellar envelope.

Abstract

It has been recently discovered that Cepheids harbor circumstellar envelopes (CSEs). RS Pup is the Cepheid that presents the most prominent circumstellar envelope known, the origin of which is not yet understood. Our purpose is to estimate the flux contribution of the CSE around RS Pup at the one arcsecond scale (~2000 AU) and to investigate its geometry, especially regarding asymmetries, to constrain its physical properties. We obtained near-infrared images in two narrow band filters centered on \lambda = 1.644 and 2.180 \mu m (NB 1.64 and IB 2.18, respectively) that comprise two recombination lines of hydrogen: the 12-4 and 7-4 (Brackett \gamma) transitions, respectively. We used NACO's cube mode observations in order to improve the angular resolution with the shift-and-add technique, and to qualitatively study the symmetry of the spatially extended emission from the CSE with a statistical study of the speckle noise. We probably detect at a 2\sigma level an extended emission with a relative flux (compared with the star in the same filter) of 38 $\pm$ 17% in the NB 1.64 filter and 24 $\pm$ 11% in the IB 2.18 filter. This emission is centered on RS Pup and does not present any detectable asymmetry. We attribute the detected emission to the likely presence of an hydrogen envelope surrounding the star.