OB associations in the inner two kiloparsecs of M33

Details OB associations in the inner two kiloparsecs of M33 OB associations in the inner two kiloparsecs of M33

May 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991aj....101.1663w&link_type=abstract

Astronomical Journal (ISSN 0004-6256), vol. 101, May 1991, p. 1663-1680.

Astronomy and Astrophysics

Astronomy

53

Astronomical Photometry, B Stars, O Stars, Spiral Galaxies, Charge Coupled Devices, Molecular Clouds, Stellar Evolution, Ubv Spectra

Scientific paper

UBV CCD photometry for the inner 7 x 10 arcmin of the Local Group spiral galaxy M33 is used to identify the OB associations and to study their properties. The association boundaries are identified using a 'friends of friends' grouping algorithm, in which the major remaining source of subjectivity is the search radius chosen. The 41 associations found in this region of M33 contain at least ten blue stars each. One-third of the Wolf-Rayet stars in this region of the galaxy lie more than 75 pc away from the nearest OB association. Thus, in this galaxy, the most massive stars do not form exclusively in large associations. A comparison of the masses of the associations with the masses of molecular clouds in M33 implies a star formation efficiency of about 10 percent. The OB associations in the southern spiral arm show a weak age gradient perpendicular to the arm, consistent with the presence of a spiral density wave. The associations in the northern arm lack any stars more massive than about 20 solar masses while the southern arm contains several stars with masses of 60 solar masses. Either a recent cessation of star formation in the inner part of the northern arm or a difference in the upper mass cutoff to the initial mass function between the arms could explain this difference.

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