Analysis of the stellar population in the central area of the HII region Sh 2-284 ,

Abstract

Context. There is a lack of state-of-the-art information on very young open clusters with implications for determining the structure of the Galaxy. Aims. Our main objective is to study the timing and location of the star formation processes which yielded the generation of the giant HII region Sh 2-284. This includes the determination of different physical variables of the stars, such as distance, reddening, age and evolutionary stage, including pre-main-sequence (PMS) stars. Methods. The analysis is based on UBVR C I C CCD measurements of a field of 6.5 × 6.5 containing the cluster, and JHK s photometry in the 3.5 × 3.5 subfield, centered in the apparent higher condensation. The determination of cluster distance, reddening and age is carried out through a comparison with ZAMS, post-MS and PMS isochrones. The reference lines used are obtained from theoretical post-MS and PMS isochrones from the Geneva and Yale groups for a metallicity of Z = 0.004, which agrees with the spectroscopic metallicity determination published for several cluster members. Results. The results amount to E(B − V) = 0.78 ± 0.02, DM = 12.8 ± 0.2 (3.6 kpc), Log Age(yr) = 6.51 ± 0.07 (3.2 Myr). The distance result critically depends on the use of low metallicity ZAMS and isochrones. A PMS member sequence is proposed, with an age of Log Age(yr) = 6.7 ± 0.2 (4.7 Myr) which is therefore coeval within the errors with the post-MS cluster age. The mass function for this population in the mass range above 1.3−3.5 M is well fitted by a Salpeter mass function. The presence of a different star generation in the cluster with a distinctly older age of around 40 Myr is suggested. On the other hand, the NIR photometry results indicate a large number of sources with (H − K s ) excess, practically distinct from the optical PMS candidate members. Conclusions. The analysis of our deep UBVRIJHK photometry of Dolidze 25 therefore reveals a young cluster with coeval MS and PMS populations of age 3.2−5 Myr. In addition, a distinctly older cluster member population of the age of 40 Myr is suggested. The distance determined for the cluster from quantitative fits to ZAMS and isochrones is distinctly lower than previously published values. This result originates in the consistent use of low metallicity models for ZAMS fitting, applying published metallicity values for the cluster.

Molecular Environments of Three Large Supernova Remnants in the Third Galactic Quadrant: G205.5+0.5, G206.9+2.3, and G213.0–0.6

“…The molecular gas presents blueshifted wing profiles on the side of the cloud facing H ii region Sh 2-284 (e.g., see the red box and its spectra in Figure 20). Accounting for the low metallicity in the H ii region, the distance of Sh 2-284 is estimated to be about 4 kpc (Delgado et al 2010;Cusano et al 2011) or 4.5 kpc (Negueruela et al 2015), which is in good agreement with the kinematic distance from our CO observations (e.g., the MCs with V LSR ∼42-48 km s −1 physically associated with the H ii region Sh 2-284). However, we do not find any convincing evidence of SNR-MC interaction for such molecular components.…”

Pre-main-sequence stars in the star-forming complex Sh 2-284

“…On the basis of the above arguments, 23 bona fide PMS objects were identified. One of these, namely J06450075+0013356, was rejected as a PMS star by Delgado et al (2010), but our medium‐resolution spectrum ( R ∼ 2500) shows strong Hα emission and lithium absorption (see Table 4). In addition, its SED shows an IR‐excess (see and Fig.…”

“…However, none of these studies accounted for the low metallicity. More recently, Delgado et al (2010) estimated a distance of 3.6 kpc using a low‐metallicity isochronal fitting on UBV CM diagrams of Dolidze 25, but they claim that the distance could be as high as 4 kpc when considering systematic effects on metallicity determinations. In order to further investigate the distance of the Sh 2‐284 region, we performed a detailed analysis of three early‐type stars in the field of Dolidze 25.…”

“…The association of Dolidze 25 with Sh 2‐284 leads to the conclusion that the H ii region and the star‐forming sites in Sh 2‐284 are characterized by a low metallicity. Delgado, Djupvik & Alfaro (2010) proposed the presence of two populations of stars in Dolidze 25: one composed of young stars of 3–4 Myr and the other of older stars with an average age of 40 Myr. The average cluster reddening derived by Delgado et al (2010) was E ( B − V ) = 0.78, which is in good agreement with the value E ( B − V ) = 0.81 ± 0.11 estimated by Moffat & Vogt (1975).…”

“…They concluded that triggered star formation may be at work in the region. Both P09 and Delgado et al (2010) provide a photometric characterization of the sources, but no spectroscopic information on the low‐ and intermediate‐mass YSOs in Sh 2‐284 and members of Dolidze 25 is available.…”