The young stellar population of IC 1613

Abstract

Context. To understand the structure and evolution of massive stars, systematic surveys of the Local Group galaxies have been undertaken, to find these objects in environments of different chemical abundances. We focus on the metal-poor irregular galaxy IC 1613 to analyze the stellar and wind structure of its low-metallicity massive stars. We ultimately aim to study the metallicitydependent driving mechanism of the winds of blue massive stars and use metal-poor massive stars of the Local Volume as a proxy for the stars in the early Universe. Aims. In a previous paper we produced a list of OB associations in IC 1613. Their properties are not only a powerful aid towards finding the most interesting candidate massive stars, but also reveal the structure and recent star formation history of the galaxy. We characterize these OB associations and study their connection with the galactic global properties. Methods. The reddening-free Q parameter is a powerful tool in the photometric analysis of young populations of massive stars, since it exhibits a smaller degree of degeneracy with OB spectral types than the B − V color. The color-magnitude diagram (Q vs. V) of the OB associations in IC 1613 is studied to determine their age and mass, and confirm the population of young massive stars. Results. We identified more than 10 stars with M ≥ 50 M . Spectral classification available for some of them confirm their massive nature, yet we find the common discrepancy with the spectroscopically derived masses. There is a general increasing trend of the mass of the most massive member with the number of members of each association, but not with the stellar density. The average diameter of the associations of this catalog is 40 pc, half the historically considered typical size of OB associations. Size increases with the association population. The distribution of the groups strongly correlates with that of neutral and ionized hydrogen. We find the largest dispersion of association ages in the bubble region of the galaxy where hydrogen is abundant, implying that recent star formation has proceeded over a longer period of time than in the rest of the galaxy, and is still ongoing. Very young associations are found at the west of the galaxy far from the bubble region, traditionally considered the sole locus of star formation, but still rich in neutral hydrogen. Conclusions. The contrast in the stellar properties derived from photometry and spectroscopy (when the latter is available) shows that the Q pseudo-color is very useful for estimating the parameters of OB stars when only photometric observations exist. This work helped define an extensive pool of candidate OB stars for subsequent spectroscopic analyses designed to study the structure and winds of metal-poor massive stars.

Ongoing star formation at the outskirts of Sextans A: Spectroscopic detection of early-O type stars.

“…s2, s3 and s4 align around the log age =6.19-6.5 dex isochrones (1.5-3.2 Myr) and it would be tempting to consider them a coeval OB-association. However, the population would subtend ∼ 25 arcsec (162 pc), one order of magnitude too loose compared to, e.g., the 17.5 pc typical sizes of associations in IC 1613 with 3 OB members ( Fig.7 from Garcia et al 2010). In sight of Fig.…”

“…We are finding that the location of massive stars and H i is always related. In IC 1613, as in Sextans A, O-stars and OBassociations are either found overlapping the highest concentrations of H i, or on the ridges of H i clouds (Garcia et al 2010).…”

Pursuing Local Group blue massive stars with WSO-ISSIS

“…1 of Garcia et al 2011a). Hence a one-to-one relation with intrinsic colors can be established for these types ((B − V ) 0 = −0.005 + 0.317 × Q for 0.08 Z stars, Massey et al 2000), which enables a correction for extinction.…”

“…To effectively find blue massive stars using only a photometric catalog, we have developed AUTOPOP (Garcia et al 2011a). AUTOPOP is an IDL code with three in- Garcia et al 2009).…”

Metallicity Distribution Functions of Four Local Group Dwarf Galaxies

“…As one of the nearest gas rich dwarf irregular galaxies at a distance of 721 kpc there have been numerous studies of the gas and stars in IC 1613 (McConnachie 2012). IC 1613 is similarly distant from M31 (517 kpc) however it is not considered to be a satellite of either the MW or M31 (McConnachie 2012), but without data on the proper motion the membership of IC 1613 to either (Garcia et al 2010).…”