Asymmetry Factor (Af)

The distribution of Asymmetry Factors, defined as the ratio of the equivalent width blueward of the rest velocity to the ratio of the equivalent width redward of the rest velocity, is displayed in Figure 5.6.

In general, the Asymmetry Factor is slightly larger than one. Most Pa Beta profiles have Af's between 1 and 2. Even though the same can be said about the Br Gamma distribution, it is also true that the latter has a significantly larger spread than that of the Pa Beta line profiles. About 21% of the Br Gamma lines have Af's between roughly 3 and 4.

Edwards et al. (1994) use the Asymmetry Factor as a diagnostic for determining whether hydrogen emission lines form in magnetospheric accretion zones. This diagnostic is based on the line profile modeling performed by Hartmann et al. (1994). While Hartmann and co-workers have not yet published results for Pa Beta line profiles in the magnetospheric accretion scenario, Br Gamma model line profiles have been published by Muzerolle, Calvet & Hartmann (1998). The Af's of the observations presented here are consistent with those that result from modeling the line profile with accretion flows.

The histograms discussed above result from considering all of the observed line profiles. The redshifted absorption features seen in a relatively high percentage of line profiles imply that a bias might be present in the distribution of the various parameters. For example, one expects the value of Af to be larger than 1 when a redshifted absorption is present and also the velocity corresponding to the line peak to be blueshifted. To avoid this potential problem one computes the distributions obtained for Type I and for Type IV R line profiles separately. Ideally, one would analyze Types II and III separately as well. However, there are too few members of these types in the sample presented here, to make statistical analysis appropriate.

The statistics of the line parameters by class of line profile is discussed in the sections that follow.