Photometric and Spectroscopic Study of Stars in the Field of the Young Open Cluster Roslund 4

Abstract

We present photometric and spectroscopic observations performed in the field of the Galactic open cluster Roslund 4, which contains the two cataloged nebulae IC 4954 and IC 4955. UBVRI photometry was carried out in a field of 11' × 6' around the cluster center. Medium-resolution optical spectroscopy has been obtained for 41 stars, including previously selected main-sequence and pre–main-sequence candidate cluster members. Narrowband Hα, [S II], and continuum images have been secured. The observations allow us the measurement of different physical parameters for the cluster. Assuming an absorption coefficient AV/E(B - V) = 3.1 and a reddening slope E(U - B)/E(B - V) = 0.72, we obtain a color excess E(B - V) = 1.1 ± 0.2 and a distance modulus DM0 = 11.7 ± 0.5. Fitting of isochrones to the color-magnitude diagrams gives log age(yr) = 7.2 ± 0.2, and the spectroscopic measurements provide the value VR = -15.7 ± 5.2 km s-1 for the heliocentric radial velocity. Among the 41 stars with spectroscopic data, 11 stars are probable cluster members on the basis of their radial velocity, and another 10 are considered as possible members. Two probable member stars of spectral types A5 and G1 show hints of absorption in the Li I 6708 A line, with respective equivalent widths of 0.10 and 0.28 A, and 22 stars of spectral types from B2 to G0 show different degrees of emission in Hα, [N II], and [S II] lines. With the exception of three stars of spectral type earlier than A0, the emission of which is mainly photospheric, all other emissions seen in Hα and forbidden lines have a nebular origin. They arise in an ionized cloud that surrounds the cluster and is causing diffuse emission and, possibly, local variations in the extinction law from star to star. In addition to the stars, spectra of several nebular condensations with relatively higher excitation have been analyzed. Two of them have been suggested to be Herbig-Haro (HH) objects. Our spectra indicate that only one of these condensations could be considered as an HH object.