A&A 584, A77 (2015) Astronomy DOI: 10.1051/0004-6361/201526784 & c ESO 2015 Astrophysics Open cluster Dolidze 25: Stellar parameters and the metallicity in the Galactic anticentre I. Negueruela1, S. Simón-Díaz2,3,J.Lorenzo1,N.Castro4, and A. Herrero2,3 1 Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politécnica Superior, Universidad de Alicante, Carretera de San Vicente del Raspeig s/n, 03690 San Vicente del Raspeig, Alicante, Spain e-mail: [email protected] 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 −0.5and−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 previ- ously, 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 (rG ≈ 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. Key words. open clusters and associations: individual: Dolidze 25 – Galaxy: abundances – Galaxy: disk – stars: formation – stars: early-type 1. Introduction Later, Turbide & Moffat (1993) obtained CCD pho- tometry of the cluster core. From a fit to a sequence The young open cluster Dolidze 25 (Do 25 = C0642+0.03 = of 12 likely members, they estimated an average colour excess OCL-537) is located near the centre of the large H ii region E(B − V) = 0.83 ± 0.08 and an age of 6 Myr. For solar metal- ◦ ◦ Sh2-284 (Galactic coordinates: l = 211.9, b = −01.3), and licity, this would imply a distance of d = 5.5 ± 0.4 kpc, while is assumed to be its ionisation source. With distance esti- for the lower metallicity expected at this Galactocentric radius, mates to Do 25 ranging from ∼4to∼6 kpc, it is believed d = 5.0 ± 0.4 kpc. Making use of more extensive photome- to be one of the massive star formation regions at the largest try, Delgado et al. (2010) determined E(B − V) = 0.78 ± 0.02 galactocentric radii, and an excellent testbed for massive star and an age of 3.2 Myr. Isochrone fits, using the value of metal- formation in the Galactic anticentre. Using photoelectric pho- licity determined by Lennon et al. (1990), place the cluster at tometry of the brightest members, Moffat & Vogt (1975)de- d = 3.6 kpc, while the use of solar neighbourhood abundances termined that the cluster contained O-type stars and was lo- would place the cluster at a distance approaching 5 kpc, and thus cated at a distance d = 5.25 kpc. Lennon et al. (1990)used compatible with that of Turbide & Moffat (1993). Strömgren photometry to select spectroscopic targets and took intermediate-resolution spectra of two O-type and one B0 star Even though all photometric studies have been limited to this / in the cluster. From atmosphere model fitting, they determined central concentration, Spitzer IRAC observations (Puga et al. surprisingly low abundances for some elements, implying an 2009) revealed the complexity of the region, showing evidence overall metallicity around one-sixth of solar (Lennon et al. of widespread star formation over the whole extent of Sh2-284. 1990). They detected dust emission extending not only across the ob- served H ii region (which is almost circular with a diame- ter ∼20), but also to larger distances (over 30). The dust emis- Based on observations made with the Nordic Optical Telescope, the sion surrounds the ionised gas, even though it is less prominent Mercator Telescope, and the telescopes of the Isaac Newton Group. to the south-west, a fact that Puga et al. (2009) attribute to a Article published by EDP Sciences A77, page 1 of 15 A&A 584, A77 (2015) density gradient within the molecular cloud. In addition to this Throughout the paper, proposed members of Do 25 will be very large bubble, they found two much smaller, almost circular identified by their number in the WEBDA database, when avail- bubbles close to the edges of the H ii region. From an analysis able, marked with a prefix “S”. Other stars will be identified of point-like sources over the whole field and cometary globules with their standard names in SIMBAD. Spectroscopic distances detected at the rim of the main bubble, Puga et al. (2009) con- are calculated when precision optical photometry is available by clude that star formation is currently taking place in the area, using the intrinsic colour calibration of Fitzgerald (1970)and that the age spread is small, and that triggered star formation the absolute magnitude calibration of Turner (1980), and from is likely taking place at the bright rim of the H ii region. Later, 2MASS data by using the intrinsic colours for O-type stars from Cusano et al. (2011) took spectra of several low-mass pre-main- Martins & Plez (2006) or B-type stars from Winkler (1997), sequence (PMS) candidates in the region, confirming their na- combined with the Turner (1980) calibration. ture and concluding that triggered star formation was taking place. Recently, Kalari & Vink (2015) used spectra of low-mass PMS stars to conclude that accretion rates are not significantly 2. Observations different from those observed in similar stars of solar metallicity. With a physical size comparable to that of the Rosette Targets for spectroscopy were selected by making use of avail- able photometry in the area. The original list of possible mem- Nebula, but at a Galactocentric distance of at least 12 kpc, ff Sh2-284 is an excellent laboratory to study star formation in the bers of Mo at & Vogt (1975) was scanned by generating outer Galaxy. Our current understanding, however, still presents colour–magnitude diagrams (CMDs) with JHKS photometry many gaps. If the cluster contains only two O9 V stars, as as- from the 2MASS catalogue (Skrutskie et al. 2006). Stars with sumed by Lennon et al. (1990), why is the H ii region so large? In colours indicative of early type, and positions in the CMDs com- patible with the members confirmed in the works of Turbide & contrast, the ionising cluster ot the Rosette Nebula, NGC 2244, ff contains seven O-type stars, including two early ones (e.g. Mo at (1993)andDelgado et al. (2010) were selected for an ini- Martins et al. 2012). Moreover, why should a Milky Way cluster tial survey. In addition, we selected bright catalogued emission- have a metallicity comparable to that of the Small Magellanic line stars, as well as other stars with colours suggestive of early- Cloud? Even though the outer regions of the Milky Way are type that could be associated with bright dust emission observed known to have lower metallicity than the solar neighbourhood, in the Spitzer images. Afterwards, inspection of spectra for three values typically accepted for the slope of the Galactic metallicity stars in the immediate vicinity of Sh2-284 that has been observed gradient (e.g. Rolleston et al. 2000; Esteban et al. 2013) support as part of the IACOBsweG survey (Simón-Díaz et al. 2015) sug- 1 gested that some early-type stars in the surrounding area could abundances ∼0.3 dex below solar for distances rG = 12–13 kpc . The comparative analysis of a large sample of stars with spec- be at exactly the same distance as the cluster. This led to the tral types around B0 located towards the anticentre (Rolleston observation of some catalogued OB stars in the area surround- et al. 2000) only found one other cluster with similarly low ing Sh2-284 whose 2MASS magnitudes were compatible with abundances, the cluster associated with the H ii region Sh2-289, O-type stars at the distance of the cluster. which lies at a much higher distance (Moffat et al. 1979; An initial survey of the brightest catalogued cluster mem- Rolleston et al.