Horizontal branch stars: the interplay between observations and theory, and insights into the formation of the Galaxy

Abstract

We review and discuss horizontal branch (HB) stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC-an argument which, due to its strong reliance on the ancient RR Lyrae stars, is essentially independent of the chemical evolution of these systems after the very earliest epochs in the Galaxy's history. Convenient analytical fits to isochrones in the HB type-[Fe/H] plane are also provided. In this sense, a rediscussion of the second-parameter problem is also presented, focusing on the cases of NGC 288/NGC 362, M13/M3, the extreme outer-halo globular clusters with predominantly red HBs, and the metal-rich globular clusters NGC 6388 and NGC 6441. The recently revived possibility that the helium abundance may play an important role as a second parameter is also addressed, and possible constraints on this scenario discussed. We critically discuss the possibility that the observed properties of HB stars in NGC 6388 and NGC 6441 might be accounted for if these clusters possess a relatively minor population of helium-enriched stars. A technique is M. Catelan is John Simon Guggenheim Memorial Foundation Fellow.M. Catelan ( ) proposed to estimate the HB types of extragalactic globular clusters on the basis of integrated far-UV photometry. The importance of bright type II Cepheids as tracers of faint blue HB stars in distant systems is also emphasized. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyr, is also revisited. Taking into due account the evolutionary status of RR Lyr, the derived relation implies a true distance modulus to the LMC of (m-M) 0 = 18.44 ± 0.11. Techniques providing discrepant slopes and zero points for the M V (RRL)-[Fe/H] relation are briefly discussed. We provide a convenient analytical fit to theoretical model predictions for the period change rates of RR Lyrae stars in globular clusters, and compare the model results with the available data. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are also investigated.

Horizontal Branch Stars and the Ultraviolet Universe

“…This is not to mean that such semi-empirical recipes are also lacking; quite the contrary, in fact: as discussed in [15,17], there exist at present several different such recipes which are equally satisfactory at describing the available mass loss rates for red giant stars. Some such recipes are given in Figure 3 (see the Appendix in [15] for more details).…”

“…What is especially important to note here is that these expressions, though all roughly equivalent in their capacity to describe the available mass loss data ( [15]), imply integrated mass loss values that differ from one case to the next, thus also implying different metallicity and age dependencies (see also [15,17]). This is more clearly shown in Figure 4: in the upper panel, one sees the different metallicity dependencies for a fixed age (9 Gyr), with the absolute integrated mass loss values having been normalized to 0.10 M ⊙ at Fig.…”

“…These plots show very clearly that most of the available recipes predict both a metallicity and an age dependence that are stronger than in the case of the commonly adopted Reimers "law" -the only exception being, in fact, the milder age dependence predicted by the Goldberg expression. Unfortunately, until we are in a position to reliably decide which of these several expressions (if any; see [15,17] for several lingering caveats) provides a better description of the available data, we will remain fundamentally unable to reliably predict the UV output from a given stellar population. A safer approach at present would likely be to analyze independently the impact of each of these recipes upon the model predictions -which should give us a better handle of the systematic errors that are brought about by our lack of a unique, reliable description of mass loss rates in red giant stars.…”

“…They appear to have been discovered by [84] in an analysis of data collected by [82] for the globular cluster M3 (NGC 5272), and were first correctly identified as stars that burn helium in their cores and hydrogen in a shell by [41]. Recent reviews dealing with the general properties and astrophysical importance of these stars have been provided in [58,17], whereas a recent overview of low-mass stellar evolution can be found in [16].…”

“…The former plays the well-known role of "first parameter" in the determination of HB morphology, whereas the latter, which was the first suggested "second parameter," has recently regained popularity as a second-parameter candidate (see [17] for a recent review). In short, HB type tends to become redder with increasing Z, whereas it tends to become bluer with increasing Y .…”