uvby–β photometry of solar twins

The solar colors, model atmospheres, and the T_eff and metallicity scales^⋆,⋆⋆

¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas 4150-762, Porto, Portugal
e-mail: jorge@astro.up.pt
² Departamento de Astronomia do IAG/USP, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brasil
e-mail: jorge@astro.iag.usp.br
³ Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, Apartado Postal 877, Ensenada, B.C., CP 22800, Mexico
e-mail: schuster@astrosen.unam.mx
⁴ Max Planck Institute for Astrophysics, Postfach 1317, 85741 Garching, Germany
⁵ Núcleo de Astrofísica Teórica, Universidade Cruzeiro do Sul, R. Galvão Bueno, 868, Liberdade, 01506-000 São Paulo, Brasil

Received: 7 April 2010
Accepted: 5 July 2010

Abstract

Aims. Solar colors have been determined on the uvby–β photometric system to test absolute solar fluxes, to examine colors predicted by model atmospheres as a function of stellar parameters (T_eff, log g, [Fe/H]), and to probe zero-points of T_eff and metallicity scales.

Methods. New uvby–β photometry is presented for 73 solar-twin candidates. Most stars of our sample have also been observed spectroscopically to obtain accurate stellar parameters. Using the stars that most closely resemble the Sun, and complementing our data with photometry available in the literature, the solar colors on the uvby–β system have been inferred. Our solar colors are compared with synthetic solar colors computed from absolute solar spectra and from the latest Kurucz (ATLAS9) and MARCS model atmospheres. The zero-points of different T_eff and metallicity scales are verified and corrections are proposed.

Results. Our solar colors are (b − y)_⊙ = 0.4105  ±  0.0015, m_1, ⊙  = 0.2122  ±  0.0018, c_1, ⊙  = 0.3319  ±  0.0054, and β_⊙ = 2.5915  ±  0.0024. The (b − y)_⊙ and m_1, ⊙  colors obtained from absolute spectrophotometry of the Sun agree within 3-σ with the solar colors derived here when the photometric zero-points are determined from either the STIS HST observations of Vega or an ATLAS9 Vega model, but the c_1, ⊙  and β_⊙ synthetic colors inferred from absolute solar spectra agree with our solar colors only when the zero-points based on the ATLAS9 model are adopted. The Kurucz solar model provides a better fit to our observations than the MARCS model. For photometric values computed from the Kurucz models, (b − y)_⊙ and m_1, ⊙  are in excellent agreement with our solar colors independently of the adopted zero-points, but for c_1, ⊙  and β_⊙ agreement is found only when adopting the ATLAS9 zero-points. The c_1, ⊙  color computed from both the Kurucz and MARCS models is the most discrepant, probably revealing problems either with the models or observations in the u band. The T_eff calibration of Alonso and collaborators has the poorest performance ( ~ 140 K off), while the relation of Casagrande and collaborators is the most accurate (within 10 K). We confirm that the Ramírez & Meléndez uvby metallicity calibration, recommended by Árnadóttir and collaborators to obtain [Fe/H] in F, G, and K dwarfs, needs a small ( ~ 10%) zero-point correction to place the stars and the Sun on the same metallicity scale. Finally, we confirm that the c₁ index in solar analogs has a strong metallicity sensitivity.