[Paper Review] CCD uvby-beta photometry of young open clusters. I. The double cluster h and chi Persei

G. Capilla, J. Fabregat|ArXiv.org|Jun 4, 2002

Stellar, planetary, and galactic studies19 references17 citations

TL;DR

This study presents high-precision CCD uvbyβ photometry of the young double cluster h and χ Persei, using the V₀−c₀ plane to derive accurate cluster parameters. It finds a common distance modulus of 11.7 ± 0.1 mag and age of log t = 7.10 ± 0.05 years, resolving prior discrepancies by correcting for strongly variable reddening, which ranges from E(b−y) = 0.328 ± 0.022 in the west to 0.465 ± 0.024 in the southeast in h Per, with χ Per showing constant reddening at 0.398 ± 0.025.

ABSTRACT

We present CCD uvby-beta photometry for stars in the nuclei of the young double cluster h and chi Persei. We find that the reddening is highly variable through the h Persei nucleus, increasing from west to east, with values ranging from E(b-y)=0.328+-0.022 in the western part to E(b-y)=0.465+-0.025 in the south-east. Towards chi Persei the reddening is fairly constant, with E(b-y)=0.398+-0.025. Both clusters share a common distance modulus of 11.7+-0.1 mag., and an age of log t=7.10+-0.05 years.

Motivation & Objective

• To resolve long-standing discrepancies in the distance modulus and age of the young double cluster h and χ Persei.
• To address the challenge of differential reddening in young open clusters, which distorts isochrone fitting and leads to uncertain age estimates.
• To test the effectiveness of isochrone fitting in the V₀−c₀ plane of the uvby photometric system for precise age determination in young clusters.
• To determine whether h and χ Persei share a common age and distance, or if multiple episodes of star formation are present.
• To correct for systematic errors in prior studies, particularly overcorrection of reddening and misclassification of supergiant stars as cluster members.

Proposed method

• Acquired deep CCD uvbyβ photometry using the 1.52m telescope at Calar Alto Observatory over three nights in November 1998.
• Used a Tektronics TK 1024 AB CCD with 6.9′×6.9′ field of view, observing through standard Strömgren u, v, b, y and Hβ narrow/wide filters with short and long exposures.
• Calibrated photometry using standard stars (NGC 1039, NGC 6910, NGC 6913) at varying airmasses to determine atmospheric extinction and transformation to the standard uvby system.
• Applied the V₀−c₀ plane for isochrone fitting, leveraging the c₀ index’s high sensitivity to temperature and reduced reddening dependence in B-type stars.
• Calculated individual reddening values for blue supergiants using Kilkenny & Whittet (1985) relations to validate spatially variable reddening maps.
• Used the RANBO2 code for extinction correction and IRAF/Starlink tools for data reduction and analysis.

Experimental results

Research questions

• RQ1 What is the true distance modulus of the h and χ Persei clusters, and is it consistent between the two?
• RQ2 What is the true age of h and χ Persei, and do they share a common age or have distinct formation epochs?
• RQ3 How does spatially variable reddening affect age determination in young open clusters, and can it be accurately measured and corrected?
• RQ4 To what extent do emission-line stars and supergiants mislead isochrone fitting in the V₀−c₀ plane?
• RQ5 Why do previous studies report conflicting age estimates, and can improved photometry and reddening correction resolve these discrepancies?

Key findings

• The reddening in the h Persei cluster nucleus is highly variable, increasing from E(b−y) = 0.328 ± 0.022 in the west to E(b−y) = 0.465 ± 0.024 in the southeast.
• The reddening in χ Persei is fairly constant at E(b−y) = 0.398 ± 0.025 across the cluster nucleus.
• Both clusters share a common distance modulus of 11.7 ± 0.1 mag., resolving prior discrepancies in the literature.
• The clusters have a common age of log t = 7.10 ± 0.05 years, indicating a single episode of star formation.
• The use of the V₀−c₀ plane for isochrone fitting successfully resolves age uncertainties, with supergiants aligning with the log t = 7.10 isochrone when corrected for spatially resolved reddening.
• Prior studies, such as MB01, produced incorrect age estimates due to overcorrection of reddening and misclassification of stars, particularly in regions of low reddening where supergiants were misidentified as non-members.