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2001AJ....122.3017S - Astron. J., 122, 3017-3045 (2001/December-0)
High-mass, OB star formation in M51: Hubble Space Telescope Hα and Paα imaging.
SCOVILLE N.Z., POLLETTA M., EWALD S., STOLOVY S.R., THOMPSON R. and RIEKE M.
Abstract (from CDS):
To analyze the variations of H II region properties vis-à-vis the galactic structure, the spiral arm areas were defined independently from millimeter-CO and optical continuum imaging. Although the arms constitute only 25% of the disk surface area, the arms contain 45% of the cataloged H II regions. The luminosity function is somewhat flatter in spiral arm regions than in the interarm areas (-0.72 to -0.95); however, this is very likely the result of increased blending of individual H II regions in the arms that have higher surface density. No significant difference is seen in the sizes and electron densities of the H II regions in spiral arm and interarm regions. For 209 regions that had ≥5 σ detections in both Paα and Hα, the observed line ratios indicate visual extinctions in the range A_V_=0-6 mag. The mean extinction was AV=3.1 mag (weighting each region equally), 2.4 mag (weighting each by the observed Hα luminosity), and 3.0 mag (weighting by the extinction-corrected luminosity). On average, the observed Hα luminosities should be increased by a factor of ∼10, implying comparable increases in global OB star cluster luminosities and star formation rates. The full range of extinction-corrected Hα luminosities is between 1037 and 2x1040 ergs.s–1.
The most luminous regions have sizes ≥100 pc, so it is very likely that they are blends of multiple regions. This is clear based on their sizes, which are much larger than the maximum diameter (≤50 pc) to which an H II region might conceivably expand within the ∼3x106 yr lifetime of the OB stars. It is also consistent with the observed correlation (L∝D2) between the measured luminosities and sizes of the H II regions. We therefore generated a subsample of 1101 regions with sizes ≤50 pc, which is made up of those regions that might conceivably be ionized by a single cluster. Their extinction-corrected luminosities range between 2x1037 and 1039 ergs.s–1, or between two-thirds of M42 (the Orion Nebula) and W49 (the most luminous Galactic radio H II region). The upper limit for individual clusters is therefore conservatively ≤1039 ergs.s–1, implying QLyc,up≃7x1050.s–1 (with no corrections for dust absorption of the Lyman continuum or UV that escapes to the diffuse medium). This corresponds to cluster masses ≤5000 M☉ (between 1 and 120 M☉).
The total star formation rate in M51 is estimated from the extinction-corrected Hα luminosities to be ∼4.2 M☉.yr–1 (assuming a Salpeter initial mass function between 1 and 120 M☉), and the cycling time from the neutral interstellar medium into these stars is 1.2x109 yr.
We develop a simple model for the UV output from OB star clusters as a function of the cluster mass and age in order to interpret constraints provided by the observed luminosity functions. The power-law index at the high-luminosity end of the luminosity function (α=-1.01) implies N(Mcl)/dMcl∝M–2.01cl. This implies that high-mass star formation, cloud disruption due to OB stars, and UV production are contributed to by a large range of cluster masses with equal effects per logarithmic interval of cluster mass. The high-mass clusters (∼1000 M☉) have a mass such that the initial mass function is well sampled up to ∼120 M☉, but this cluster mass is ≤1% of that available in a typical giant molecular cloud. We suggest that OB star formation in a cloud core region is terminated at the point that radiation pressure on the surrounding dust exceeds the self-gravity of the core star cluster and that this is what limits the maximum mass of standard OB star clusters. This occurs at a stellar luminosity-to-mass ratio of ∼500-1000 (L/M)☉, which happens for clusters ≥750 M☉. We have modeled the core collapse hydrodynamically and have found that a second wave of star formation may propagate outward in a radiatively compressed shell surrounding the core star cluster–this triggered, secondary star formation may be the mechanism for formation of the super-star clusters seen in starburst galaxies.
Abstract Copyright: ∼
Journal keyword(s): Galaxies: ISM - Galaxies: Spiral - ISM: H II Regions - Stars: Early-Type
Simbad objects: 21
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