by the distance to a $n$th. neighbor point (see @Casertano:1985uq for a description of this method in the particular case of stellar cluster identification; some examples of its use for embedded clusters can be found in @Roman-Zuniga:2008aa [@Gutermuth:2009aa] and @Roman:2015aa). The `gather` algorithm is adequate for identifying individual clusters in a relatively simple layout, optimizing the value of $n$ that defines the surface density measurement and the size of the smoothing kernel used to construct the surface density map.

We made individual `gather` maps for each of the candidate YSO class lists. These maps are shown in Figure \[fig:gather\]. The maps show the concentration of Class I sources at the north IRDC region. Class II sources are distributed over the entire region of study, but still concentrate in a few clearly defined groups. This led us to define some populations of stars, which we use as samples for our analysis. We named the two known clusters as DR15-C and DR15-N because their location center and north of our region of study. We identified and named as well three other groups: DR15-W, DR15-SW and DR15-SE. The purpose of this selection is to compare the age and age spread of these populations 