Open cluster Dolidze 25: Stellar parameters and the metallicity in the Galactic anticentre^⋆

¹ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politécnica SuperiorUniversidad de Alicante, Carretera de San Vicente del Raspeig s/n, 03690 San Vicente del Raspeig, Alicante, Spain
e-mail: ignacio.negueruela@ua.es
² Instituto de Astrofísica de Canarias, vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, Facultad de Física y Matemáticas, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, 38205 La Laguna, Tenerife, Spain
⁴ Argelander Institut für Astronomie, Auf den Hügel 71, 53121 Bonn, Germany

Received: 19 June 2015
Accepted: 25 August 2015

Abstract

Context. The young open cluster Dolidze 25, in the direction of the Galactic anticentre, has been attributed a very low metallicity, with typical abundances between −0.5 and −0.7 dex below solar.

Aims. We intend to derive accurate cluster parameters and accurate stellar abundances for some of its members.

Methods. We have obtained a large sample of intermediate- and high-resolution spectra for stars in and around Dolidze 25. We used the fastwind code to generate stellar atmosphere models to fit the observed spectra. We derive stellar parameters for a large number of OB stars in the area, and abundances of oxygen and silicon for a number of stars with spectral types around B0.

Results. We measure low abundances in stars of Dolidze 25. For the three stars with spectral types around B0, we find 0.3 dex (Si) and 0.5 dex (O) below the values typical in the solar neighbourhood. These values, even though not as low as those given previously, confirm Dolidze 25 and the surrounding H ii region Sh2-284 as the most metal-poor star-forming environment known in the Milky Way. We derive a distance 4.5 ± 0.3 kpc to the cluster (r_G ≈ 12.3 kpc). The cluster cannot be older than ~3 Myr, and likely is not much younger. One star in its immediate vicinity, sharing the same distance, has Si and O abundances at most 0.15 dex below solar.

Conclusions. The low abundances measured in Dolidze 25 are compatible with currently accepted values for the slope of the Galactic metallicity gradient, if we take into account that variations of at least ±0.15 dex are observed at a given radius. The area traditionally identified as Dolidze 25 is only a small part of a much larger star-forming region that comprises the whole dust shell associated with Sh2-284 and very likely several other smaller H ii regions in its vicinity.