5 An example

To illustrate and analyze the organisation of the Kohonen map applied to the articles from A&A between 1994 and 1996 (Fig. 4), we will study in detail a small area of that map concerning novae.

Looking at the map around the node labelled Novae, we find the general keywords: Nova, neutron stars, close binary stars, ISM. By definition, a nova is a cataclysmic variable, therefore a star that suddenly increases in brightness. Observations have demonstrated that novae are close binary stars of which one component is a white dwarf. When the companion star evolves and expands to fill it, material streams towards the white dwarfs, forming an accretion disc around it.

The neutron stars are formed in supernova explosions and are observed as pulsars. A stellar remnant of three solar masses or more will collapse into a black hole rather than a neutron star.

A quick comparison between the nova area with its specifications, and the general definition of a nova, are in agreement. The list of keywords attached to the Nova node, listed in Table 1, describes correctly a nova.

  

Table 1: Index term lists for Novae units

Let us look now at the articles attached to the Novae unit. We retrieve 77 documents from A&A (Fig. 5). This set of documents is too large for reading all the abstracts. This is why a more detailed map is proposed. The secondary map (Fig. 6) shows in fact a distinction between the documents concerning white dwarfs, novae and binary stars. At the new Nova node on the secondary map, the keywords are more limited but more precise. They are listed in Table 1. At this unit, we retrieve 38 documents; the first 7 documents are listed in Fig. 7.

  

Figure 5: Obtained by clicking in the Novae node on the opening page

All the documents deal with cataclysmic variables, nova-like stars, X-ray stars. Therefore they are clearly right within the subject.

Now, have a look at the nodes surrounding the Nova one on the main map. Above, we find documents concerning accretion disks, close binary stars and white dwarfs, to the left, pulsars and neutron stars, to the right, supernovae and molecular data and below, comets and meteors. There are logical links between these subjects.

Starting from the close binary stars node, we move into a wider stellar area described by different peaks defined by these main keywords: stars:fundamental parameters, stars:oscillations and stars:variables, stars:activity, X-rays:stars, and so on. Furthermore, we can move into the solar area by means of a link between the stellar oscillations and the solar oscillations.

In conclusion, the relationships presented in the map have been validated in most cases. We have also studied the evolution of annual maps, over the period of three years considered in this work. Little change was noted and we do not show these annual maps. Of course some well-known events relating to fashion or appearance of some astronomical phenomenon may well be reflected in our bibliographic data.
An alternative access to bibliography maps is implemented, allowing the user to select one or several keywords and to vizualize their positions on the maps. It is then possible to retrieve and select the relevant papers at these locations.

  

Figure 6: The local map corresponding to the Novae node

  

Figure 7: List of articles associated with the secondary map. This is a CDS page proposing, inter alia, access to abstracts of articles