Open clusters in Auriga OB2

Abstract

We study the area around the HII region Sh 2-234, including the young open cluster Stock 8, to investigate the extent and definition of the association Aur OB2 and the possible role of triggering in massive cluster formation. We obtained Str\"omgren and J,H,Ks photometry for Stock 8 and Str\"omgren photometry for two other cluster candidates in the area, which we confirm as young open clusters and name Alicante 11 and Alicante 12. We took spectroscopy of 33 early-type stars in the area, including the brightest cluster members. We calculate a common distance of $2.80^{+0.27}_{-0.24}$ kpc for the three open clusters and surrounding association. We derive an age 4-6 Ma for Stock 8, and do not find a significantly different age for the other clusters or the association. The star LSV +34 23, with spectral type O8 II(f), is likely the main source of ionization of Sh 2-234. We observe an important population of pre-main sequence stars, some of them with disks, associated with the B-type members lying on the main-sequence. We interpret the region as an area of recent star formation with some residual and very localized ongoing star formation. We do not find evidence for sequential star formation on a large scale. The classical definition of Aur OB2 has to be reconsidered, because its two main open clusters, Stock 8 and NGC 1893, are not at the same distance. Stock 8 is probably located in the Perseus arm, but other nearby HII regions whose distances also place them in this arm show quite different distances and radial velocities and, therefore, are not connected.