Hot HB stars in globular clusters - Physical parameters and consequences for theory. VI. The second parameter pair M 3 and M 13

We have determined the projected rotational velocities of thirteen blue horizontal-branch (BHB) stars in the globular cluster M13 via rotational broadening of metal absorption lines. Our sample spans the photometric gap observed in the horizontal branch distribution at Teff ~= 11000 K, and reveals a pronounced difference in stellar rotation on either side of this feature---bluewards of the gap, all the stars show modest rotations, vsini < 10 km/s, while to the red side of the...

We analysed red giant branch stars in 16 Galactic globular clusters, computing their atmospheric parameters both from the photometry and from excitation and ionisation balances. The spectroscopic parameters are lower than the photometric ones and this discrepancy increases decreasing the metallicity, reaching, at [Fe/H]~-2.5 dex, differences of ~350 K in effective temperature and ~1 dex in surface gravity. We demonstrate that the spectroscopic parameters are inconsistent with...

The Horizontal Branch (HB) second parameter of Globular Clusters (GCs) is a major open issue in stellar evolution. Large photometric and spectroscopic databases allow a re-examination of this issue. We derive median and extreme (90% of the distribution) colours and magnitudes of stars along the HB for about a hundred GCs. We transform these into median and extreme masses of stars on the HB taking into account evolutionary effects, and compare these masses with those expected ...

Inspection of the CM diagrams of globular clusters having similar heavy element content shows that the luminosity of the red giant bump relative to the turnoff (Delta V_{TO}^{bump}) differs by more than 0.1 mag between clusters with different horizontal branch morphology. Unfortunately, careful consideration of the data leaves us with only one pair (M3 and M13) of clusters good for a quantitative discussion. For this pair we consider differences in age and helium content as p...

We present spectroscopic analyses of 17 faint blue stars in the globular cluster NGC 6752. Effective temperatures, surface gravities, and helium abundances are determined and compared to theoretical predictions. All stars are helium deficient by factors of 3 to more than 100, indicative of gravitational settling of helium. Stars with Teff > 20000 K (sdBs) fit well to the evolutionary tracks, whereas the cooler stars show lower surface gravities than theoretically expected. Th...

We present high-precision V, B-V color-magnitude diagrams (CMDs) for the classic second parameter globular clusters M3 and M13 from wide-field deep CCD photometry. The data for the two clusters were obtained during the same photometric nights with the same instrument, allowing us to determine accurate relative ages. Based on a differential comparison of the CMDs using the Delta (B-V) method, an age difference of 1.7 +/- 0.7 Gyr is obtained between these two clusters. We compa...

Globular clusters are ideal laboratories to study the evolution of low-mass stars. In this work we concentrate on three types of hot stars observed in globular clusters: horizontal branch stars, UV bright stars, and white dwarfs. After providing some historical background and information on gaps and blue tails we discuss extensively hot horizontal branch stars in metal-poor globular clusters, esp. their abundance anomalies and the consequences for the determination of their a...

High-resolution optical spectra of eighteen blue horizontal-branch (BHB) stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the v sin i rotations appear to have a bimodal distribution, with eight stars at v sin i < 15 km/s and two...

It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is the rule, rather than the exception, among globular star clusters. An important prediction of this helium enhancement scenario is that the helium-enhanced blue horizontal branch (HB) stars should be brighter than the red HB stars which are not helium-enhanced. In this Letter, we test this prediction in the case of the Galactic globular cluster M3 (NGC 5272...

Atmospheric parameters (effective temperature, surface gravity) and radial velocities are derived for 12 candidate blue horizontal branch (HB) stars in the globular clusters 47 Tuc and NGC 362, which so far have been known to contain primarily red HB stars. The spectroscopic targets were selected from the catalog of hot stars detected in these clusters at 1600 A using the Ultraviolet Imaging Telescope (UIT). Spectroscopic analyses of these stars reveal, however, that one of t...