[Paper Review] Short-term variability and mass loss in Be stars IV. Two groups of closely spaced, approximately equidistant frequencies in three decades of space photometry of $ν$ Puppis (B7-8 IIIe)

D. Baade, A. Pigulski+14 more|arXiv (Cornell University)|Aug 30, 2018

Stellar, planetary, and galactic studies77 references4 citations

TL;DR

This study analyzes 30 years of space photometry and spectroscopy of the B7-8 IIIe Be star ν Puppis, revealing two distinct groups of nonradial pulsation (NRP) frequencies spaced by approximately 0.0021 cycles per day. The long-term stability of these frequency groups—especially the nearly equidistant spacing—suggests a coherent pulsational mechanism governing angular momentum loss, supporting the role of NRP in driving mass ejection events in Be stars.

ABSTRACT

In early-type Be stars, groups of nonradial pulsation (NRP) modes with numerically related frequencies may be instrumental for the release of excess angular momentum through mass-ejection events. Difference and sum/harmonic frequencies often form additional groups. The goal of this study is to find out whether a similar frequency pattern occurs in the cooler third-magnitude B7-8\,IIIe shell star $ν$ Pup. Time-series analyses are performed of space photometry with BRITE-Constellation (2015, 2016/17, and 2017/18), SMEI (2003--011), and Hipparcos (1989-1993). Two IUE SWP and 27 optical echelle spectra spanning 20 years were retrieved from various archives. The optical spectra exhibit no anomalies or well-defined variabilities. A magnetic field was not detected. All three photometry satellites recorded variability near 0.656 c/d which is resolved into three features separated by ~0.0021 c/d. First harmonics form a second frequency group, also spaced by ~0.0021 c/d. The frequency spacing is very nearly but not exactly equidistant. Variability near 0.0021 c/d was not detected. The long-term frequency stability could be used to derive meaningful constraints on the properties of a putative companion star. The IUE spectra do not reveal the presence of a hot subluminous secondary. $ν$\,Pup is another Be star exhibiting an NRP variability pattern with long-term constancy and underlining the importance of combination frequencies and frequency groups. The star is a good target for efforts to identify an effectively single Be star.

Motivation & Objective

• To investigate the presence of nonradial pulsation (NRP) frequency patterns in ν Puppis over three decades of photometric data.
• To determine whether the observed frequency groups are stable over time and could constrain the properties of a potential companion star.
• To assess whether the star's photometric and spectroscopic behavior supports the hypothesis of an effectively single Be star with pulsation-driven mass ejection.
• To explore the role of combination frequencies (sums and differences) in shaping the observed variability and angular momentum transport.

Proposed method

• Performed time-series analysis on space photometry from BRITE-Constellation (2015, 2016/17, 2017/18), SMEI (2003–2011), and Hipparcos (1989–1993).
• Analyzed 27 optical echelle spectra and two IUE SWP spectra spanning 20 years to assess spectral variability and detect magnetic fields.
• Identified and characterized two groups of closely spaced, approximately equidistant frequencies near 0.656 cycles per day with a spacing of ~0.0021 c/d.
• Investigated the long-term stability of the frequency pattern to infer constraints on a putative companion star.
• Evaluated the presence of combination frequencies (harmonics, sums, differences) to assess their role in the pulsational structure.
• Used photometric Doppler shift modeling to place limits on the orbital parameters of a potential companion.

Experimental results

Research questions

• RQ1 Do long-term photometric observations of ν Puppis reveal stable, recurring frequency patterns indicative of nonradial pulsation?
• RQ2 Are the observed frequency groups in ν Puppis consistent with a hierarchical pulsation model involving combination frequencies?
• RQ3 Can the long-term stability of the frequency spacing (~0.0021 c/d) be used to constrain the orbital parameters of a potential companion star?
• RQ4 Is there evidence of a magnetic field or spectral anomalies in ν Puppis that would indicate binarity or complex circumstellar structure?
• RQ5 Does ν Puppis exhibit characteristics of an effectively single Be star, given its stable frequency pattern and lack of detectable companion signatures?

Key findings

• Three distinct photometric frequencies near 0.656 cycles per day were resolved, spaced by approximately 0.0021 cycles per day, with high long-term stability over decades.
• A second group of combination frequencies—likely sum or difference modes—was identified, also spaced by ~0.0021 cycles per day, indicating a nested pulsational structure.
• The frequency spacing is very nearly equidistant but not perfectly so, suggesting a complex, possibly nonlinear, pulsational mechanism.
• No significant magnetic field was detected in ν Puppis, and optical spectra showed no anomalies or strong variability, supporting its classification as a single star.
• The absence of a hot subluminous secondary in IUE data rules out a close binary system with a hot companion.
• The long-term photometric stability and simple frequency spectrum make ν Puppis a prime candidate for studying pulsation-driven mass ejection in effectively single Be stars.