Ph.D. Thesis

The Stellar Content of M101

by Cynthia Anne Blaha

Under the supervision of Professor Roberta M. Humphreys

ABSTRACT

Photographic plates of spiral galaxy M101 were obtained through U, B, V, and R filters and scanned with the Automated Plate Scanner. Scanner data taken with four threshold levels were reconstructed into images and match between plates to obtain a comprehensive set of brightness and colors for the M101 images. Photoelectric measurements and CCD observations were made to provide the photometric calibrations transforming instrumental image diameters into apparent brightnesses. Profile information was used to separate the images into sets of foreground stars and background galaxies in the outer regions of the plates and stellar and nonstellar objects within M101.
 
Data sets of the brightest stars in our own Galaxy and the Magellanic Clouds were analyzed to determine the U, B, and V luminosity distributions and initial mass functions of these populations and used for comparison with the M101 results. Slopes for the upper end of the luminosity functions of the galaxies were found to be 0.7±0.1, similar to the value of 0.67 found for other nearby galaxies. The initial mass function s were consisted with a power law distribution S(M) ~ M^-2.0 with little variation in the slope between the three samples.
 
The observed apparent magnitude and color ranges of the stellar images within M101 corresponded to an evolved population of B through G supergiants with and absolute magnitude range of -8 to -10 and progenitor masses of 30-60 solar masses. Many of the previously identified brightest blue supergiant candidates were classified as nonstellar and may be compact clusters or multiple systems, thus questioning their use as extragalactic distance indicators. A slight flattening in the slope of the luminosity function with increasing galactocentric distance was noted and overall luminosity function corrected for contamination effects had a slope of 0.9-1.0, implying a steeper luminosity function for M101 that found in other galaxies. Crowding and saturation of images in the spiral arms prevent a complete count of the number of supergiants in M101. However the set of nearly 2000 stellar images found in M101 represents the largest sample of brightest supergiants yet studied and provides a testing ground for theories of stellar and galactic evolution.

Master's Thesis

Luminous Blue Variables in M31 and M33

by Cynthia Anne Blaha

Under the supervision of Professor Roberta M. Humphreys

ABSTRACT

The energy distributions of the luminous blue variables, the Hubble-Sandage variables, in M31 and M33, are determined from ultraviolet observations with the IUE, in combination with ground-based visual and infrared photometry. The observed distributions are corrected for interstellar reddening, and luminosities and temperatures are estimated from simple blackbody fits. The resulting bolometric luminosities confirm that these variable are hot, highly luminous stars very similar to the peculiar stars Eta Car, P Cyg, and S. Dor in our galaxy and the LMC. Several of the variables display infrared excess radiation which is attributed to free-free emission from their extended atmospheres. Rough mass loss estimates from this infrared excess yield rates of 10 E -5 solar masses/year. Physical interpretations of the observed variability and instabilities are presented and the role of the Hubble-Sandage variables in massive star evolution is discussed.