New Evidence for Mass Loss from delta Cephei from HI 21-cm Line Observations

TL;DR

This study provides direct observational evidence of mass loss from delta Cephei through HI 21-cm line data, revealing a nebula and quantifying the stellar wind properties, which helps address the mass discrepancy in Cepheid models.

Contribution

First direct detection and measurement of atomic hydrogen mass loss from delta Cephei using VLA HI observations, linking stellar wind properties to evolutionary models.

Findings

Detected a 1 pc HI nebula with a head-tail morphology.

Measured a hydrogen mass-loss rate of approximately 1x10^-6 solar masses per year.

Estimated a stellar wind velocity of about 36 km/s.

Abstract

Recently published Spitzer observations of the classical Cepheid archetype delta Cephei revealed an extended dusty nebula surrounding this star and its hot companion. The infrared emission resembles a bow shock aligned with the direction of space motion of the star, indicating that delta Cep is undergoing mass-loss through a stellar wind. Here we report HI 21-cm line observations with the VLA to search for neutral atomic hydrogen associated with this wind. Our VLA data reveal a spatially extended HI nebula (~13' or 1 pc across) surrounding the position of delta Cep. The nebula has a head-tail morphology, consistent with circumstellar ejecta shaped by the interaction between a stellar wind and the ISM. We directly measure a mass of circumstellar hydrogen M_HI\approx0.07M_odot, although the total HI mass may be larger. The HI data imply a stellar wind with an outflow velocity V_o=35.6\pm1.2 km/s and a mass-loss rate of M_dot=(1.0\pm0.8)x10**-6 M_dot/yr. We have computed theoretical evolutionary tracks that include mass loss across the instability strip and show that a mass-loss rate of this magnitude, sustained over the preceding Cepheid lifetime of delta Cep, could be sufficient to resolve a significant fraction of the discrepancy between the pulsation and evolutionary masses for this star. (abridged)