[Paper Review] The Young Stellar Population of IC1613. I. A New Catalogue of OB Associations

M. García, A. Herrero+6 more|arXiv (Cornell University)|Apr 28, 2009

Stellar, planetary, and galactic studies60 references17 citations

TL;DR

This study presents a new, high-precision astrometric and photometric catalogue of stars in the metal-poor irregular galaxy IC 1613, identifying 163 OB associations using a friends-of-friends algorithm applied to color-selected massive star candidates. The key result is a detailed census of young stellar populations, revealing patchy extinction and concentration of associations in H ii regions, especially near the central bar and H ii shells, with high-accuracy targets ready for spectroscopic follow-up with the GTC.

ABSTRACT

Context: Determining the parameters of massive stars is crucial to understand many processes in galaxies and the Universe, since these objects are important sources of ionization, chemical enrichment and momentum. 10m class telescopes enable us to perform detailed quantitative spectroscopic analyses of massive stars in other galaxies, sampling areas of different metallicity. Relating the stars to their environment is crucial to understand the physical processes ruling their formation and evolution. Aims: In preparation for the GTC, our goal is to build a catalogue of massive star candidates in the metal-poor irregular galaxy IC1613 with high astrometric accuracy, apt for the current generation of multi-object spectrographs. A census of OB associations in this galaxy is also needed, to provide important additional information about age and environment of the candidate OB stars. Methods: From INT-WFC observations, we have built an astrometric and photometric catalogue of stars in IC1613. Candidate blue massive stars are preselected from their colors. A friends-of-friends algorithm is developed to find their clustering in the galaxy. While a common physical origin for all the members of the associations cannot be ensured, this is a necessary first step to place candidate OB stars in a population context. Results: We have produced a deep catalogue of targets in IC1613 that covers a large field of view. To achieve high astrometric accuracy a new astrometric procedure is developed for the INT-WFC data. We have also built a catalogue of OB associations in IC1613. We have found that they concentrate in the central regions, specially in the HII bubbles. The study of extinction confirms that it is patchy, with local values of color-excess above the foreground value.

Motivation & Objective

• To compile a high-astrometric-accuracy catalogue of stars in IC 1613 for use with next-generation multi-object spectrographs like those on the Gran Telescopio Canarias (GTC).
• To identify and characterize OB associations in IC 1613, a metal-poor irregular galaxy, to contextualize massive star formation and evolution.
• To improve target selection for spectroscopic surveys by combining photometry, astrometry, and clustering analysis, rather than relying solely on magnitude or location.
• To produce a reliable extinction map of IC 1613, revealing non-uniform and localized reddening effects beyond the foreground value.
• To provide a foundation for future studies of initial mass function (IMF) and star formation processes in low-metallicity environments.

Proposed method

• A deep multi-band photometric catalogue was constructed from INT-WFC observations, with a new astrometric solution developed to achieve high positional accuracy for spectroscopic target selection.
• Candidate OB stars were preselected using the reddening-free Q parameter and U-B color, leveraging the known correlation between Q and early-type spectral types.
• A friends-of-friends clustering algorithm was applied to identify OB associations, using a search radius of 6″ and a minimum of 3 members per association.
• The algorithm was tuned to detect both compact and extended associations, with validation against existing catalogues and spectral type cross-matches.
• Extinction was mapped by comparing observed colors to intrinsic color expectations, revealing local excesses beyond the foreground E(B-V) = 0.02–0.03.
• Associations were cross-identified with known H ii regions and structural features, particularly the central H ii shells and the H i cavity.

Experimental results

Research questions

• RQ1 How can we identify and catalog massive star candidates in a metal-poor galaxy like IC 1613 with high astrometric precision suitable for multi-object spectroscopy?
• RQ2 Where are the OB associations concentrated in IC 1613, and how do their spatial distributions relate to H ii regions and H i structures?
• RQ3 To what extent is extinction in IC 1613 non-uniform and patchy, and how does this affect the observed colors of massive stars?
• RQ4 How do the properties of the newly identified OB associations compare to previous catalogues, and what explains discrepancies in association counts?
• RQ5 What is the reliability of using the Q parameter and U-B color for selecting OB stars in low-metallicity environments?

Key findings

• A total of 163 OB associations were identified in IC 1613, with 24 associations containing 10 or more OB candidates, indicating significant clustering in the central regions.
• The associations are strongly concentrated in the central H ii shells and along the NE and SW lobes of the H i cavity, particularly near the bar and in the H ii bubbles.
• Extinction in IC 1613 is patchy and locally enhanced, with color excess values exceeding the foreground E(B-V) = 0.02–0.03, especially near the edges of the H i cavity.
• The new astrometric solution for the INT-WFC data achieved high positional accuracy, enabling precise target selection for future GTC spectroscopy.
• The catalogue includes 31 associations not previously identified, particularly in the outer regions and overlapping areas, due to a larger field of view and lower magnitude limits compared to prior studies.
• Four stars were found at the same position in the U-B vs Q diagram as the known WO3 Wolf-Rayet star, validating the photometric selection criteria.