English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We present the results of spectroscopic analyses of hot horizontal branch(HB) stars in M 13 and M 3, which form a famous ``second parameter'' pair. Fromthe spectra and Stroemgren photometry we derived -- for the first time in M 13-- atmospheric parameters (effective temperature and surface gravity). Forstars with Stroemgren temperatures between 10,000 and 12,000 K we foundexcellent agreement between the atmospheric parameters derived from Stroemgrenphotometry and those derived from Balmer line profile fits. However, for coolerstars there is a disagreement in the parameters derived by the two methods, forwhich we have no satisfactory explanation. Stars hotter than 12,000 K showevidence for helium depletion and iron enrichment, both in M 3 and M 13.Accounting for the iron enrichment substantially improves the agreement withcanonical evolutionary models, although the derived gravities and masses arestill somewhat too low. This remaining discrepancy may be an indication thatscaled-solar metal-rich model atmospheres do not adequately represent thehighly non-solar abundance ratios found in blue HB stars affected by diffusion.We discuss the effects of an enhancement in the envelope helium abundance onthe atmospheric parameters of the blue HB stars, as might be caused by deepmixing on the red giant branch or primordial pollution from an earliergeneration of intermediate mass asymptotic giant branch stars.Comment: 15 pages, 9 figures, LaTex, uses aa.cls (included), accepted for  publication in A&

Hot HB stars in globular clusters – Physical parameters and consequences for theory