Low-frequency photospheric and wind variability in the early-B supergiant HD 2905

TL;DR

This study analyzes long-term spectroscopic and photometric data of the early-B supergiant HD 2905, revealing multi-periodic variability, low-frequency amplitude excess, and potential links to stellar pulsations and wind modulation.

Contribution

It provides the first detailed analysis of variability in HD 2905 using extensive ground-based spectroscopy and Hipparcos data, identifying multiple frequencies and their possible physical origins.

Findings

HD 2905 exhibits multi-periodic variability with timescales of days to weeks.

A dominant frequency at ~0.37 d$^{-1}$ was identified in both spectroscopic and photometric data.

Variability is linked to stellar pulsations, gravity waves, and wind modulation.

Abstract

Despite the important advances in space asteroseismology during the last decade, the early phases of evolution of stars with masses above $\sim$15 M$_{\odot}$ have been only vaguely explored up to now. Our goal is to detect, analyze and interpret variability in the early-B type supergiant HD\,2905 using long-term, ground based, high resolution spectroscopy. We gather a total of 1141 high-resolution spectra covering some 2900 days. We complement these observations with the $Hipparcos$ light curve, which includes 160 data points obtained during a time span of $\sim$1200 days. We investigate spectroscopic variability of up to 12 diagnostic lines by using the zero and first moments of the line profiles. We perform a frequency analysis of both the spectroscopic and photometric dataset. HD\,2905 is a spectroscopic variable with peak-to-peak amplitudes in the zero and first moments of the photospheric lines of up to 15\% and 30 \kms, respectively. The amplitude of the line-profile variability is correlated with the line formation depth in the photosphere and wind. All investigated lines present complex temporal behavior indicative of multi-periodic variability with timescales of a few days to several weeks. The Scargle periodograms of the {\it Hipparcos\} light curve and the first moment of purely photospheric lines reveal a low-frequency amplitude excess and a clear dominant frequency at $\sim$0.37 d$^{-1}$. In the spectroscopy, several additional frequencies are present in the range 0.1 - 0.4 d$^{-1}$. These may be associated with heat-driven gravity modes, convectively-driven gravity waves, or sub-surface convective motions. Additional frequencies are detected below 0.1 d$^{-1}$. In the particular case of H$\alpha$, these are produced by rotational modulation of a non-spherically symmetric stellar wind.