[Paper Review] Subdwarf B Binaries from the Edinburgh--Cape Survey

L. Morales‐Rueda, P. F. L. Maxted+3 more|arXiv (Cornell University)|Oct 11, 2004

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This study presents the first orbital solutions for subdwarf B (sdB) binaries from the Edinburgh-Cape Survey, using radial velocity measurements from blue spectroscopy to identify and characterize 15 binaries, including three with confirmed orbital parameters. The results reveal short-period systems consistent with formation via common envelope ejection, offering critical data for testing binary evolution models and CE ejection efficiency.

ABSTRACT

We present the first results of a campaign to obtain orbital solutions of subdwarf B (sdB) stars from the Edinburgh-Cape survey. We have obtained blue spectra of 35 sdBs, 20 of which have been observed in more than two epochs. 15 out of the 35 are certain binaries with a few other objects showing radial velocity variations with small amplitude, possibly long period sdB binaries. We have secured the orbital parameters for 2 of the 15 systems and narrowed down the orbits of another one to a small range of periods. These preliminary results only use data taken up to December 2003.

Motivation & Objective

• To obtain orbital solutions for subdwarf B (sdB) binaries from the Edinburgh-Cape (EC) survey to create an unbiased sample of sdB binaries.
• To overcome selection biases in previous surveys (e.g., PG and KPD catalogues) that excluded sdB binaries with main sequence companions due to Ca II H-line filtering.
• To provide observational constraints on binary evolution models, particularly the common envelope ejection channel and CE ejection efficiency.
• To update the orbital period distribution of known sdB binaries with newly discovered systems from the EC survey.
• To assess the reliability of orbital solutions through statistical analysis of period aliases and systematic uncertainties.

Proposed method

• Obtained blue spectra of 35 sdB stars using the 1.9 m Radcliffe telescope at SAAO with a grating spectrograph (1200 grooves/mm), covering Hβ and Hγ with 0.5 Å/pixel dispersion and <1 Å resolution.
• Reduced spectra using standard procedures and measured radial velocities by fitting Balmer line profiles with a model of three Gaussians.
• Fitted radial velocity data with a sinusoidal function plus a constant (four free parameters: semi-amplitude, period, zero point, and systemic velocity) to determine orbital solutions.
• Identified the true orbital period when the Δχ² between the first and second alias exceeded 20, with statistical confidence levels (1% and 10%) calculated for period reliability.
• Accounted for unmodeled errors (e.g., slit-filling, intrinsic variability) by adding a systematic uncertainty in quadrature to raw errors to achieve a reduced χ² consistent with a 2.5% probability threshold.
• Used periodogram analysis to assess multiple aliases and determine the most likely orbital period, especially for systems with sparse data.

Experimental results

Research questions

• RQ1 What is the orbital period distribution of subdwarf B binaries in the Edinburgh-Cape Survey, and how does it compare to theoretical predictions?
• RQ2 Can reliable orbital solutions be determined for sdB binaries from the EC survey despite potential observational challenges like low signal-to-noise or sparse sampling?
• RQ3 What is the nature of the companions in newly identified sdB binaries, and do their orbital parameters support formation via the common envelope ejection channel?
• RQ4 How do the observed orbital periods inform the efficiency of common envelope ejection in binary evolution models?
• RQ5 To what extent does the EC survey provide an unbiased sample of sdB binaries compared to previous surveys like PG and KPD?

Key findings

• Orbital solutions were secured for two systems: EC00404−4429 with a period of 0.12834(4) days and EC02200−2338 with a period of 0.8022(7) days, both showing high confidence (Δχ² > 60).
• For EC12327−1338, the orbital period lies in a narrow range of 0.3628–0.3674 days, though insufficient observations prevent a precise determination.
• The companions in the three systems have minimum masses of 0.32–0.39 M⊙, consistent with white dwarf companions, supporting formation via the second common envelope path.
• The orbital periods of the three systems place them in the common envelope ejection channel, aligning with theoretical predictions from Han et al. (2003).
• The periodogram analysis showed that the probability of the true period lying beyond 1% or 10% of the favored value is extremely low (log10 probabilities < -15.18), indicating high confidence in the solutions.
• Systematic uncertainties of 2 km s⁻¹ were added to account for unmodeled effects, ensuring the reduced χ² values were consistent with statistical expectations.