Faint solar analogues at the limit of no reddening

Precise atmospheric parameters from moderate low-resolution spectra^★

¹ ESO – European Southern Observatory, Karl-Schwarzchild-Strasse 2, 85748 Garching bei München, Germany
e-mail: rianoesc@gmail.com
² Observatório do Valongo, Universidade Federal do Rio de Janeiro, Ladeira do Pedro Antônio 43, 20080-090 Rio de Janeiro, RJ, Brazil
³ Departamento de Astronomia do IAG/USP, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP, Brazil
⁴ UEFS, Departamento de Física, Av. Transnordestina, s/n, Novo Horizonte, Feira de Santana, CEP 44036-900, BA, Brazil

Received: 16 May 2019
Accepted: 6 July 2019

Abstract

Context. The flux distribution of solar analogues is required for calculating the spectral albedo of solar system bodies such as asteroids and trans-Neptunian objects. Ideally a solar analogue should be comparatively faint as the target of interest, but very few analogues fainter than V = 9 mag have been identified so far. Only atmospheric parameters equal to solar guarantee a flux distribution equal to solar as well, while only photometric colours equal to solar do not. Reddening is also a factor to consider when selecting faint analogue candidates.

Aims. Our aim is to implement the methodology for identifying faint analogues at the limit of precision allowed by the current spectroscopic surveys. We quantify the precision attainable for the atmospheric parameters of effective temperature (T_eff), metallicity ([Fe/H]), and surface gravity (log g) when derived from moderately low-resolution (R = 8000) spectra with S∕N ~ 100. We estimate the significance of reddening at 100–300 pc from the Sun.

Methods. We used the less precise photometry in the HIPPARCOS catalogue to select potential analogues with V ~ 10.5 mag (located at ~135 pc). We calibrated T_eff and [Fe/H] as functions of equivalent widths of spectral indices by means of the principal component analysis regression. We derived log g, mass, radius, and age from the atmospheric parameters, Gaia parallaxes, and evolutionary tracks. We evaluated the presence of reddening for the candidates by underestimations of photometric T_eff with respect to those derived by spectral indices. These determinations were validated with extinction maps.

Results. We obtained the atmospheric parameters T_eff, [Fe/H], and log g with precision of 97 K, 0.06 dex, 0.05 dex, respectively. From 21 candidates analysed, we identify five solar analogues: HIP 991, HIP 5811, and HIP 69477 have solar parameters within 1σ errors, and HIP 55619 and HIP 61835 within 2σ errors. Six other stars have T_eff close to solar, but slightly lower [Fe/H]. Our analogues show no evidence of reddening except for four stars, that present E(B−V) ≥ 0.06 mag, translating to at least a 200 K decrease in photometric T_eff.