Previous studies of the angular momentum of young stars (Attridge & Herbst 1992; Bouvier et al. 1993,1995; Grankin 1993, 1994; Prosser 1993, 1994, 1995; Adams 1995; Choi & Herbst 1996) cover a wide range of stellar ages, but with exception of the ONC, the nTTs have been underrepresented. The nTTs are of particular interest since they are the youngest stars which are not members of composite systems. In a cTTs, there are three major flux contributors: the star, the disk and the boundary layer between them. Variability in any of these three components can cause a variability in the optical signal. The nTTs have only one primary contributor to the optical signal, the photosphere. This makes causes of variability easier to interpret and lowers the number of sources of possibly conflicting signals.
This makes period detection easier, especially under non--ideal conditions. Because some nTTs may have very recently lost their disks, they could be stars undergoing rapid change in their rotational properties. This would make them the most likely stars to occupy the 4 to 6 day gap (see Adams et al. 1996).
In this dissertation, I will discuss the results of observations
of nTTs in Orion.
Observations were made using both the local facility known as ``Mount
Stony Brook'',
and limited time allotments at large observatories. The target stars are
primarily in the Orion OB1a and OB1b sub--associations.
Estimates of the ages of these subgroups vary, but they are
generally thought to be less than 12 Myrs old (Brown 1996).
In Orion OB1b, I will concentrate
specifically on stars within 11 arc minutes of 
Orionis.
This region was chosen for its
compactness, which allowed many targets to be observed in a single field.
Other nTTs in the Orion OB1a association provide a larger
cross--section of the stellar populations between 1 and 10 million years
old.
The goal of this thesis is to study the rotational modulations of about
100 X--ray sources and to attempt to understand the effect of rotation
on other observable phenomena. Before focusing my efforts on the
rotational modulation, I first examine the general optical properties
of the X--ray selected sample and introduce 150 new PMS (primarily nTT)
stars. About 50 of these 150 stars were not identified as pre--main
sequence by their X--ray properties, but instead by their spectra and
optical photometry.
The rotational data improve our understanding of
the evolution of angular momentum among young stars by greatly improving
the statistical data base of the youngest stars with unobscured
photospheres. In the next
section I show that the stars near
Orionis are truly coeval and the stars in Orion OB1a
create a second coeval set. Because of this,
interpretive problems
introduced by age uncertainties can be minimized.
In Chapter 3, I discuss the difficulties inherent in measuring the
correct rotational periods of the candidate stars. In Chapter 4,
the period distributions and the false alarms in the individual
findings are discussed. There are several observables on which
angular momentum may be dependent including mass, age and IR--excess.
I discuss these, as well as the biases in the data set, in Chapter 5.
