[Paper Review] Mean angular diameters, distances and pulsation modes of the classical Cepheids FF Aql and T Vul - CHARA/FLUOR near-infrared interferometric observations

A. Gallenne, P. Kervella+9 more|arXiv (Cornell University)|Mar 29, 2012

Stellar, planetary, and galactic studies28 references14 citations

TL;DR

This study presents the first near-infrared interferometric measurements of the angular diameters of classical Cepheids FF Aql and T Vul using the CHARA/FLUOR array. By combining these angular diameters with Hubble Space Telescope-FGS trigonometric parallaxes, the authors derive linear radii and conclude that both stars pulsate in the fundamental mode, resolving ambiguity in their pulsation classification using Period–Radius relations.

ABSTRACT

We report the first angular diameter measurements of two classical Cepheids, FF Aql and T Vul, that we have obtained with the FLUOR instrument installed at the CHARA interferometric array. We obtain average limb-darkened angular diameters of θ_LD = 0.878 +/- 0.013 mas and θ_LD = 0.629 +/- 0.013 mas, respectively for FF Aql and T Vul. Combining these angular diameters with the HST-FGS trigonometric parallaxes leads to linear radii R = 33.6 +/- 2.2 Rsol and R = 35.6 +/- 4.4 Rsol, respectively. The comparison with empirical and theoretical Period-Radius relations leads to the conclusion that these Cepheids are pulsating in their fundamental mode. The knowledge of the pulsation mode is of prime importance to calibrate the Period-Luminosity relation with a uniform sample of fundamental mode Cepheids.

Motivation & Objective

• To determine accurate angular diameters and linear radii of classical Cepheids FF Aql and T Vul using interferometry.
• To resolve the pulsation mode ambiguity for these stars, particularly FF Aql, which is often classified as a first-overtone pulsator due to its light curve characteristics.
• To improve the calibration of the Period–Luminosity relation by ensuring a homogeneous sample of fundamental-mode Cepheids.
• To test the consistency of empirical and theoretical Period–Radius relations with direct interferometric measurements.

Proposed method

• Conducted near-infrared (K′-band) interferometric observations with the FLUOR instrument on the CHARA Array over multiple pulsation cycles.
• Measured squared visibilities and calibrated them using K-type giant stars as secondary calibrators with known uniform-disk angular diameters.
• Fitted limb-darkened angular diameters using a model of the star's intensity profile and pulsation phase.
• Combined interferometric angular diameters with HST-FGS trigonometric parallaxes to derive linear radii.
• Applied the interferometric Baade-Wesselink method (IBWM) to estimate distances independently, while accounting for the p-factor uncertainty.
• Compared derived linear radii with empirical and theoretical Period–Radius relations to infer pulsation mode.

Experimental results

Research questions

• RQ1 Are FF Aql and T Vul pulsating in the fundamental mode or first overtone, given their light curve morphology?
• RQ2 How do direct interferometric measurements of angular diameter and radius compare with indirect estimates from P–R relations?
• RQ3 To what extent does the p-factor uncertainty affect distance estimates derived from the Baade-Wesselink method?
• RQ4 Can the combination of interferometric angular diameters and trigonometric parallaxes resolve pulsation mode ambiguity more reliably than light curve analysis alone?

Key findings

• The mean limb-darkened angular diameter of FF Aql is 0.878 ± 0.013 mas, and of T Vul is 0.629 ± 0.013 mas.
• The derived linear radius for FF Aql is 33.6 ± 2.2 R☉, and for T Vul is 35.6 ± 4.4 R☉, based on interferometric angular diameters and HST-FGS parallaxes.
• Both stars are consistent with the fundamental-mode Period–Radius relation within intrinsic dispersion, rejecting the first-overtone pulsation hypothesis.
• The radial radius of FF Aql is inconsistent with overtone pulsation models, suggesting it is a fundamental-mode pulsator despite its sinusoidal light curve.
• The distance estimate for FF Aql agrees with the HST-FGS parallax at 0.3σ, while T Vul's distance is 27% smaller but still within 1.1σ, indicating potential p-factor bias.
• The study confirms that T Vul is a fundamental-mode pulsator, and provides strong evidence that FF Aql is also fundamental-mode, challenging its traditional classification as an s-Cepheid in the first overtone.