Mathematics – Logic
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........21p&link_type=abstract
Thesis (PH.D.)--MCMASTER UNIVERSITY (CANADA), 1995.Source: Dissertation Abstracts International, Volume: 57-03, Section: B, page
Mathematics
Logic
1
Scientific paper
We have used Hα and photometric data for two morphologically distinct Local Group galaxies, the spiral M33 and dwarf irregular NGC 6822, to study the distribution of the luminous blue O and B (OB) stars and HII regions in the galaxies as well as to determine whether individual regions of the galaxies are separately and/or collectively in a state of ionization balance. In the case of M33, we have concentrated on the inner 1 kpc region of the galaxy. Using the H alpha data, three distinct ionized gas environments (bright, halo and field) defined by the surface brightness of the Hα emission have been identified. We find that ~50% of the OB stars are located in the field, so that 1/2 of the lifetime of OB stars must be spent outside recognizable HII regions. We show that a possible origin for the large field OB population is that they were born in and subsequently percolated out of the ~10 ^3 giant molecular clouds with masses _sp{~}>10 ^3Modot predicted to exist within the inner kpc of the galaxy. Using UBV photometry and stellar ionization models, we predict Hα fluxes in the bright, halo and field regions. Our findings suggest that star formation rates obtained from luminosities must underestimate the true star formation rate within the inner region of M33. In the case of NGC 6822, four distinct components of the Hα emission (bright, halo, diffuse and field) differentiated by their surface brightnesses have been identified. We find that only 1/4 of the OB stars are found in the combined bright halo regions, suggesting that OB stars spend roughly 3/4 of their lifetimes outside "classical" HII regions. Molecular cloud lifetimes after forming OB stars could be as low as ~1 -3 times 10^6 yrs or 1/4 the typical main sequence lifetimes of OB stars if stars escape from bright HII regions by destroying their parent clouds. Additionally, the field population of OB stars cannot have originated in and percolated out of existing HII regions. Comparing the observed Hα emission with that predicted from BV photometry and stellar ionizing flux models, we find that although the bright, and diffuse regions are probably in a state of ionization balance, the field region, which is producing at least 6 times as much ionizing flux as is observed, is clearly not. Finally, we have performed optical spectroscopy of luminous blue stars in both NGC 6822 and M33 using respectively, the 4-m CTIO telescope plus ARGUS spectrograph and the 4-m Mayall telescope at KPNO plus HYDRA spectrograph. Due to the limited S/N ratio of the data, we have been able to classify only ~50% of the stars observed in NGC 6822 and ~40% of the stars observed in M33. Out of the 37 NGC 6822 and 15 M33 stars with classifiable spectra respectively 28 and 11 potential OB stars have been identified. We have used the NGC 6822 spectra to gauge the reliability of the principle stellar ionization model used for the ionization balance calculations in both galaxies. (Abstract shortened by UMI.).
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