The objective of this project is to detect and study Pre-Main Sequence stars in the Orion OB1a association. OB associations, although named for the prominent, massive Main Sequence or Post-Main Sequence O and B stars that are situated in them, also allow for study of the entire star formation process. They are considered to be fossil star formation regions, containing the final products in an open and easily observable region. While nebulae, where star formation directly occurs, would be the ideal location to study star formation, gas and dust obscure and prevent clear study. The implications of the study of OB associations are particularly fascinating because recent evidence suggests that the Sun may have formed in one, 4.6 Gyr ago. The study of the association Ori OB1a could be particularly instructive, since the Ori OB1 association is one of the nearest OB associations, and OB1a is believed to be the richest of the constituent members of the larger Orion OB1 association, which consists of four associations designated a, b, c, and d. There is still much to be learned about the star formation process.
The procedure followed capitalizes on powerful, modern astronomical techniques and methods to analyze imaging data. It begins with a series of images taken of square fields in the direction of the OB1a association with a 0.9-meter telescope at the Cerro Tololo Inter-American Observatory in Chile. OB1a is located NW of Orion's Belt at the canonical distance of 330 parsecs (pc), or approximately 1000 lightyears. The images analyzed are from four wavelength
bands for each field: blue, green, red, and infrared.
The analysis process consists of several distinct stages. The first step is to prepare the images for analysis by extracting numerical data from the pictures using the Image
Reduction and Analysis Facility (IRAF). This is accomplished by first using a procedure to locate the stars in the region, which must be checked and verified before proceeding. After this, photometry yields the instrumental magnitudes of the stars, which must then be corrected for the aperture of the instrument and atmospheric conditions through a calibration process, giving actual magnitudes. Following this, one knows the magnitudes of the stars at various wavelengths and can plot them on a Color Magnitude Diagram (CMD); V vs. V-I will be considered in this case. The difference in the magnitudes at different wavelengths is the color of the star, with a spectral type A0 star corresponding to a color of 0, hotter stars having negative colors, and cooler stars possessing positive colors.
On the CMD, the majority of the stars comprise the field background, and only appear to be located in the same vicinity of the sky as the association, which consists of physically close stars. A critical step is fitting a locus to determine which stars are actually low-mass PMS association members. Once this is done, characteristics of the constituents can be assessed: temperature (based on the colors), and masses and ages (based on temperature and luminosity).
I used a series of procedures and programming involving the Interactive Data Language (IDL) in the processes of the analysis described above, and produced a catalog of coordinates, magnitudes, and colors of the stars examined. Ultimately, my goal is to identify and categorize the population of the association, to compare mass distribution to theoretical models and data of the OB1b association in Orion's Belt, and to predict the age of the association.
This research was supported by the Simons Foundation.