Lboo A Bootis stars

- The Lambda X Bootis stars were defined within An Atlas of Stellar Spectra published in 1943 by Morgan, Keenan and Kellman. Apparently, k Bod stars are a group of metal-poor, Population I, A-type stars. They occupy the same position on the HR diagram as'Am stars that show a strong overabundance of many metals and the normal A-type stars. Not much is directly mentioned regarding variability as a characteristic of k Bod stars.

When you think about these stars being A-type, surely a thought relating to the 8 Scuti variables must briefly come to mind. In fact, some stars suspected of being X Boo stars are listed as 8 Scuti stars.

It is common practice to clearly identify the X Boo stars using spectroscopic means. Specifically, this group of stars is identified by a deficiency in the iron-group elements (Sr, Fe, Ti and Sc), a deficiency of Mg and Ca compared to Fe, and they possess moderately large rotational velocities, among other characteristics. Subject to various interpretations, some of these methods of identification disqualify the X Boo stars from being considered S Scuti stars.

Certainly, X Boo stars challenge our understanding of the various processes related to stars and they are fascinating members of the classical instability strip. Attempts to derive group properties with statistical methods have not been entirely successful because of the small number of unambiguously identified X Boo stars. On the whole, classification appears to be obscured by incorrect group membership and it is therefore essential to provide a sufficiently large catalog of definitive group members before modeling the X Boo phenomenon.

In the year 2000, an attempt was made to make a statistically sound analysis of their properties. In total, 708 program stars in the Galactic field, six open clusters, and the Orion OBI association were observed. This resulted in 26 newly discovered X Bootes stars and the confirmation of 18 candidates. Astroseismology was applied to the known members, resulting in 18 newly detected pulsators as well as 29 probably constant stars. The pulsational behavior of these stars is very similar to that of classical 5 Scuti stars, so careful measures must be taken to detect the difference.

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