Inclination Bias in Techniques Used to Identify Be Star Candidates

Several methods for identifying Be star candidates are reviewed for observational bias with respect to system inclination, that is the angle between the stellar/disk rotation axis and the observer’s line of sight, with focus on two photometric methods that leverage narrow-band filters centred on H$α$ and a spectroscopic method using a H$α$ peak-finding algorithm. Tests for bias were performed using a sample of 20,000 synthetic Be stars drawn from a Salpeter initial mass function and computed libraries of spectral energy distributions and H$α$ profiles. The spectroscopic method showed substantial bias against high inclinations ($i > 80^\circ$). Both photometric methods were biased against low inclinations, with one also biased against inclinations above $80^\circ$, resulting in a surplus in the Be star candidate detection rate for moderate inclinations ($ 50^\circ < i < 80^\circ$). Inclination probability distributions, including the random $\sin i$ factor, are given for the three methods that can be applied to observational samples.

💡 Research Summary

The paper “Inclination Bias in Techniques Used to Identify Be Star Candidates” investigates whether the most common observational methods for selecting classical Be (Be) star candidates introduce systematic biases with respect to the inclination angle (i), the angle between the stellar rotation axis (and its equatorial decretion disk) and the observer’s line of sight. The authors focus on two narrow‑band photometric techniques that employ Hα‑centered filters and a spectroscopic technique that uses an Hα peak‑finding algorithm. To quantify any inclination‑dependent selection effects, they generate a synthetic population of 20,000 Be stars drawn from a Salpeter initial mass function. For each synthetic star they compute a full spectral energy distribution (SED) and an Hα line profile using the Bedisk/Beray codes, sampling a wide range of stellar masses (3–20 M⊙), rotation rates (Ω/Ωcrit ≈ 0.8–0.99), disk density power‑law indices (n = 2.0–3.5), and disk outer radii (10–30 R⋆). The synthetic spectra are produced for inclinations from 0° (pole‑on) to 90° (edge‑on) in 1° steps, thereby providing a realistic testbed that captures the diversity of real Be stars.

Three candidate‑selection pipelines are then applied to the synthetic data:

1. Photometric Method A (Iqbal & Keller 2013) – uses the colour excess between an Hα narrow‑band filter and an adjacent continuum filter (r‑band). A star is flagged as a Be candidate if the colour exceeds a preset threshold.
2. Photometric Method B (Milone et al. 2018) – combines Hα with an