Long-term spectral variability of the O4If⁺ star HD 15570 in the open cluster IC 1805

Extract

We present preliminary results concerning the O4If⁺ star HD 15570. Of-type supergiants are believed to represent an evolved evolutionary stage of very high mass stars (M_init > 40 M_⊙). Their low numbers and extreme peculiarity make each of these objects worth of continuous monitoring. HD 15570 dominates the very young open cluster IC 1805 and is thought to have had an initial mass ≥ 100 M_⊙. Low-, intermediate- and high-resolution spectra were collected at the Loiano and San Pedro Mártir telescopes since 1992. The comparison of our high- and low-resolution spectra shows clear variability of a number of spectral features. It is worth noticing, that the variation of Hα seems to follow a repeated secular trend, increasing its equivalent width from ~ 3Å to more than 8 Å in a few years. At the same time, its profile is varying, from a shape quite similar to the theoretical one corresponding to the Klein & Castor (1978) model C, to a much more developed P-Cygni profile, with a deep blue absorption wing. The possibility of instrumental effects is ruled out by the remarkable constancy of the nearby diffuse interstellar band at 6613 Å and of the He ii 6683 Å absorption. The Hβ profile variability is evident from the comparison of the 1996 and 1998 high-resolution spectra. The emission component, which is clearly visible at all epochs, is absent in the 1996 spectrum when the line appears in pure absorption. No relevant line-profile variation seems to be present in the He ii and N iii lines contributing to the ‘feature f’, that looks remarkably constant, as well as Hγ, while their equivalent widths seem to show a modest amount of random variability.