A peculiar Of star in the Local Group galaxy IC 1613

Abstract

Context.Results from the theory of radiatively driven winds are nowadays incorporated in stellar evolutionary and population synthesis models, and are used in our interpretation of the observations of the deep Universe. Yet, the theory has been confirmed only until Small Magellanic Cloud metallicities. Observations and analyses of O-stars at lower metallicities are difficult, but much needed to prove the theory. Aims. We have observed GHV-62024, an O6.5 IIIf star in the low-metallicity galaxy IC 1613 (Z ≈ 0.15 Z ) to study its evolution and wind. According to a previous preliminary analysis that was subject to significant restrictions this star could challenge the radiatively driven wind theory at low metallicities. Here we present a complete analysis of this star. Methods. Our observations were obtained with VIMOS at VLT, at R ≈ 2000 and covered approximately between 4000 and 7000 Å. The observations were analysed using the latest version of the model atmosphere code FASTWIND, which includes the possibility of calculating the N iii spectrum. Results. We obtain the stellar parameters and conclude that the star follows the average wind momentum-luminosity relationship (WLR) expected for its metallicity, but with a high value for the exponent of the wind velocity law, β. Comparing this with values of other stars in the literature, we suggest that this high value may be reached because GHV-62024 could be a fast rotator seen at a low inclination angle. We also suggest that this could favour the appearance of the spectral "f"-characterictics. While the derived β value does not change by adopting a lower wind terminal velocity, we show that a wrong V ∞ has a clear impact on the position of the star in the WLR diagram. The N and He abundances are very high, consistent with strong CNO mixing that could have been caused by the fast rotation, although we cannot discard a different origin with present data. Stellar evolutionary model predictions are consistent with the star being still a fast rotator. We find again the well-known mass-discrepancy for this star. Conclusions. We conclude that the star follows the WLR expected for its metallicity. The results are consistent with GHV-62024 being a fast rotator seen close to pole-on, strongly contaminated at the surface with CNO products and with a wind structure altered by the fast rotation but without modifying the global WLR. We suggest that this could be a general property of fast rotators.

The young stellar population of IC 1613

“…Tramper et al (2011) report six stars with stronger wind momentum than expected from the poor metallicity of their host galaxies (∼1/7 Z ) from X-Shooter spectroscopic analyses, although error bars are too large for results to be determinant. Our analysis of an Of star in IC 1613 (Herrero et al 2012) concluded that the star may have a strong wind or, alternatively, a slower than expected wind acceleration. Lucy (2012) argues that the neglect of wind clumping may explain these findings, but also suggests that the discrepancy may be caused by an additional wind-driving mechanism operating only at certain metallicities, temperatures and luminosities, or negligible in the Galactic counterparts.…”

“…7. It includes all O-type stars in IC 1613 whose effective temperature has been derived from quantitative spectroscopic analysis: the eight stars analysed in this paper, plus five stars from Tramper et al (2011) and Herrero et al (2012). For comparison, we have added temperature determinations of SMC OB-stars from Mokiem et al (2007) and Massey et al (2009), and the temperature scale for SMC B-supergiants derived by Trundle et al (2007).…”

“…Lucy (2012) argues that the neglect of wind clumping may explain these findings, but also suggests that the discrepancy may be caused by an additional wind-driving mechanism operating only at certain metallicities, temperatures and luminosities, or negligible in the Galactic counterparts. The Herrero et al (2012) and Tramper et al (2011) examples, if confirmed by the detailed study of a large sample of objects, pose a challenge to the standard theory of radiation line-driven winds, since there are few metals to drive the wind. Nevertheless, this might explain why long-GRBs (typically associated with type Ic SN, Woosley & Heger 2006) are mostly found in metal-poor environments (Modjaz et al 2008;Levesque et al 2010) but require a strong wind to remove the H and He envelope in the pre-SN stages.…”

“…Bresolin et al (2007) published a catalogue of about 40 OB-stars in this galaxy, found from VLT-FORS2 MOS observations, but it only lists six O-types (4 of them were observed and analysed by Tramper et al 2011). These, together with the Of star analysed in Herrero et al (2012), make a total of seven O-type stars known in IC 1613, insufficient for a statistical characterization of the WLR at very low metallicities.…”

“…O-and early-B-stars are concentrated in boxes 1 and 2. The pink star represents 62 024, an Of star analysed by Herrero et al (2012). Red stars represent the sample of Tramper et al (2011) in IC 1613.…”