[Paper Review] Short-term variability and mass loss in Be stars V. Space photometry and ground-based spectroscopy of $\gamma$ Cas

C. C. Borre, D. Baade+10 more|arXiv (Cornell University)|Feb 11, 2020

Stellar, planetary, and galactic studiesPhysics and Astronomy60 references6 citations

TL;DR

This study investigates optical variability and mass loss in the prototypical Be star γ Cas using space photometry (SMEI, BRITE) and archival Hα spectroscopy (2006–2017). It confirms the 203.5-day orbital period via Hα line structure and identifies three distinct high-frequency variations—0.82 d⁻¹, 1.25 d⁻¹, and 2.48 d⁻¹—attributed to nonradial pulsations (NRPs), with the 2.48 d⁻¹ frequency likely present since the early 2000s, ruling out rotational origin and supporting NRPs as the dominant mechanism for disk mass ejection.

ABSTRACT

Context. Be stars are physically complex systems that continue to challenge theory to understand their rapid rotation, complex variability and decretion disks. $\gamma$ Cassiopeiae ($\gamma$ Cas) is one such star but is even more curious because of its unexplained hard thermal X-ray emission. Aims. We aim to examine the optical variability of $\gamma$ Cas and thereby to shed more light on its puzzling behaviour. Methods. Three hundred twenty-one archival H$\alpha$ spectra from 2006 to 2017 are analysed to search for frequencies corresponding to the 203.5 day orbit of the companion. Space photometry from the SMEI satellite from 2003 to 2011 and the BRITE-Constellation of nano-satellites between 2015 and 2019 is investigated in the period range from a couple of hours to a few days. Results. The orbital period of the companion of 203.5 days is confirmed with independent measurements from the structure of the H$\alpha$ line emission. A strong blue/red asymmetry in the amplitude distribution across the H$\alpha$ emission line could hint at a spiral structure in the decretion disk. With the space photometry, the known frequency of 0.82 d$^{-1}$ is confirmed in data from the early 2000s. A higher frequency of 2.48 d$^{-1}$ is present in the data from 2015 to 2019 and possibly also in the early 2000s. A third frequency at 1.25 d$^{-1}$ is proposed to exist in both SMEI and BRITE data. The only explanation covering all three rapid variations seems to be nonradial pulsation. The two higher frequencies are incompatible with rotation.

Motivation & Objective

• To understand the optical variability and mass loss mechanisms in γ Cas, a prototypical Be star with unexplained X-ray emission.
• To determine whether the observed high-frequency variations are due to rotation, orbital modulation, or pulsations.
• To investigate the origin of the Hα line profile asymmetry and its link to disk structure.
• To assess the long-term evolution of photometric and spectroscopic variability using multi-epoch data.

Proposed method

• Analysis of 321 archival Hα spectra (2006–2017) to detect periodicities and study line profile asymmetries.
• Time-series analysis of SMEI photometry (2003–2011) and BRITE-Constellation nano-satellite data (2015–2019) in the 0.1–5 d⁻¹ frequency range.
• Fourier and frequency analysis to identify significant periodicities and assess their stability over time.
• Cross-correlation of Hα amplitude distribution with orbital phase to detect phase-locked spiral structure effects.
• Re-analysis of SMEI data to test for the presence of the 2.48 d⁻¹ frequency in the early 2000s.
• Model comparison to distinguish between rotational, orbital, and nonradial pulsation (NRP) origins of the observed frequencies.

Experimental results

Research questions

• RQ1 Is the 203.5-day orbital period of γ Cas’s companion confirmed via spectroscopic analysis of Hα line structure?
• RQ2 Do the observed high-frequency photometric variations (0.82 d⁻¹, 1.25 d⁻¹, 2.48 d⁻¹) originate from rotation, orbital modulation, or nonradial pulsations?
• RQ3 Does the blue/red amplitude asymmetry in the Hα line profile indicate a spiral structure in the decretion disk phase-locked to the companion?
• RQ4 Is the 2.48 d⁻¹ frequency detectable in both SMEI and BRITE data, and was it present in the early 2000s?
• RQ5 Can the three observed frequencies be explained by a single physical mechanism, such as nonradial pulsations?

Key findings

• The 203.5-day orbital period of γ Cas’s companion is confirmed through the phase-locked modulation of Hα emission line structure.
• A strong blue/red amplitude asymmetry in the Hα line profile suggests a spiral structure in the decretion disk, likely induced by the companion’s gravitational influence.
• The 0.82 d⁻¹ frequency is confirmed in SMEI data from 2003 to 2011, with decreasing amplitude, and is absent in BRITE data (detection limit ~1 mmag).
• The 2.48 d⁻¹ frequency is dominant in BRITE photometry (2015–2019) and may have been present in SMEI data from 2003–2011, indicating long-term persistence.
• The 1.25 d⁻¹ frequency is detected in both SMEI and BRITE data, supporting its existence across multiple datasets.
• The three high-frequency variations (0.82 d⁻¹, 1.25 d⁻¹, 2.48 d⁻¹) are incompatible with rotation and most plausibly explained by nonradial pulsations, which are the primary driver of mass ejection in Be stars.