[Paper Review] FIRST, a fibered aperture masking instrument

Elsa Huby, Gaspard Duchêne+8 more|arXiv (Cornell University)|Dec 16, 2013

Calibration and Measurement Techniques31 references13 citations

TL;DR

This paper presents FIRST-18, a fibered pupil remapping interferometric instrument that achieves diffraction-limited spectroscopy at visible wavelengths, enabling direct measurement of the spectral flux ratio of the Capella binary system with R ~ 300. It resolves both components at all three epochs, achieves 1 mas astrometric precision, and reveals that molecular features—particularly CN bands—are deeper than predicted by current models, indicating incomplete line lists or underestimated oscillator strengths in stellar atmosphere models.

ABSTRACT

Aims. FIRST is a prototype instrument built to demonstrate the capabilities of the pupil remapping technique, using single-mode fibers and working at visible wavelengths. Our immediate objective is to demonstrate the high angular resolution capability of the instrument and to show that the spectral resolution of the instrument enables characterization of stellar companions. Methods. The FIRST-18 instrument is an improved version of FIRST-9 that simultaneously recombines two sets of nine fibers instead of one, thus greatly enhancing the (u, v) plane coverage. We report on observations of the binary system Capella at three epochs over a period of 14 months (≳4 orbital periods) with FIRST-18 mounted on the 3 m Shane telescope at Lick Observatory. The binary separation during our observations ranges from 0.8 to 1.2 times the diffraction limit of the telescope at the central wavelength of the spectral band. Results. We successfully resolved the Capella binary system at all epochs, with an astrometric precision as good as 1 mas under the best observing conditions. FIRST also gives access to the spectral flux ratio between the two components directly measured with an unprecedented spectral resolution of R ~ 300 over the 600−850 nm range. In particular, our data allow detection of the well-known overall slope of the flux ratio spectrum, leading to an estimation of the “pivot” wavelength of 0.64 ± 0.01 μm, at which the cooler component becomes the brightest. Spectral features arising from the difference in effective temperature of the two components (specifically the Hα line, TiO, and CN bands) have been used to constrain the stellar parameters. The effective temperatures we derive for both components are slightly lower (5−7%) than the well-established properties for this system. This difference mainly comes from deeper molecular features than those predicted by state-of-the-art stellar atmospheric models, suggesting that molecular line lists used in the photospheric models are incomplete and/or oscillator strengths are underestimated, most likely concerning the CN molecule. Conclusions. These results demonstrate the power of FIRST, which is a fibered pupil remapping-based instrument, in terms of high angular resolution and show that the direct measurement of the spectral flux ratio provides valuable information to characterize little known companions.

Motivation & Objective

• To demonstrate the high angular resolution and spectral flux ratio capability of a fibered pupil remapping instrument like FIRST at visible wavelengths.
• To test the instrument's ability to resolve and characterize close binary stars near the diffraction limit.
• To investigate discrepancies between observed spectral flux ratios and synthetic spectra from PHOENIX models in the context of the well-characterized Capella binary system.
• To assess the reliability of stellar atmospheric models by comparing observed flux ratio spectra with synthetic models at high spectral resolution.

Proposed method

• FIRST-18 uses 18 single-mode fibers to recombine light from non-redundant sub-pupils on the 3-m Shane telescope, enabling high-precision visibility and closure phase measurements.
• The instrument operates in a self-calibration mode that retrieves complex visibilities without requiring dedicated photometric channels.
• Spectral flux ratio measurements are obtained across 600–850 nm at R ~ 300, enabling direct comparison with synthetic spectra from PHOENIX models.
• A self-calibration algorithm mitigates flux fluctuations from fiber coupling inefficiencies, improving dynamic range and measurement accuracy.
• The data are analyzed using closure phases and flux ratio spectra to derive astrometric and spectroscopic parameters of the Capella binary.
• Stellar parameters are derived by fitting observed flux ratio spectra to synthetic spectra from the PHOENIX model grid, adjusting T_eff, log g, and metallicity.

Experimental results

Research questions

• RQ1 Can a fibered pupil remapping instrument achieve diffraction-limited spectral flux ratio measurements of a close binary system at visible wavelengths with R ~ 300?
• RQ2 To what extent do observed spectral flux ratio features in Capella (e.g., Hα, TiO, CN bands) match predictions from current PHOENIX stellar atmosphere models?
• RQ3 Why do the derived effective temperatures from FIRST data differ by 5–7% from well-established values for Capella?
• RQ4 What causes the observed flux ratio spectrum to deviate from model predictions, particularly in molecular band regions?
• RQ5 Is the discrepancy due to incomplete molecular line lists or underestimated oscillator strengths in the models, especially for CN molecules?

Key findings

• FIRST-18 successfully resolved the Capella binary system at all three observation epochs, achieving an astrometric precision of 1 mas under optimal conditions.
• The instrument measured the spectral flux ratio between the two components with a resolution of R ~ 300 across 600–850 nm, enabling direct spectroscopic characterization.
• The flux ratio spectrum revealed a pivot wavelength of 0.64 ± 0.01 µm, where the cooler component becomes brighter, consistent with known system properties.
• Effective temperatures derived from the flux ratio spectrum were 5–7% lower than the established values of 4920 K and 5680 K, indicating a model-data discrepancy.
• Deeper molecular bands—especially CN and TiO—were observed than predicted by PHOENIX models, suggesting incomplete line lists or underestimated oscillator strengths, with CN being the most likely culprit.
• The mismatch between observations and models is astrophysical rather than instrumental, as it persists across different spectral features and is independent of closure phase shifts.