The amount of mass lost by stars during the red-giant branch (RGB) phase is one of the main parameters needed to fully understand later stages of stellar evolution. In spite of its importance, a fully-comprehensive physical understanding of this phenomenon is still missing, and we, mostly, rely on empirical formulations. The Galactic Globular Clusters are ideal targets to derive such formulations, but, until recently, the presence of multiple populations has been a major challenge.

We will discuss the insights on RGB mass loss that can be obtained from the study of the horizontal branch stars in such stellar associations. The estimates obtained via the study of the photometric data will be compared with recent and newly obtained estimates derived for few high metallicity open clusters and a large sample of field stars with asteroseismic techniques.

We measure the evolution of the UV upturn color for galaxies on the red sequence in clusters at 0 < z < 0.7 and to luminosity levels L ∼ L*. We show that the UV upturn color does not change until at least z = 0.55 but becomes significantly redder at z = 0.7. This is the first detection of evolution in the UV upturn. Our observations are inconsistent with all models proposed for its origin except the presence of a population of helium enriched stars, with helium abundances above 42 % and formed at z > 4.

We have performed photometric analysis of four Galactic globular clusters (GGCs): NGC 4147, NGC 4590, NGC 5053 and NGC 7492 using far-UV and near-UV filters of the Ultraviolet Imaging Telescope (UVIT) on-board AstroSat. With the help of color-magnitude diagrams (CMDs), we have identified ~150 blue horizontal branch stars (BHBs), and ~40 blue straggler stars (BSS) in the four GGCs. We study the temperature and radial distribution of BHBs and BSS for the four GGCs.

Once the age and metallicity are fixed, the colour distribution of horizontal branch stars in a globular cluster depends on few parameters: the helium abundance of the population and the mass lost during the pre-HB stages. These parameters are usually derived from the HB itself, hence they are degenerate. Breaking this degeneracy and understanding their role is a tricky and challenging problem that no study has solved yet. Combining the information obtained from the chromosome maps and the analysis of multi-band photometry with state of the art stellar evolution models, we can obtain a solid estimate of Y for the various stellar populations in a GC. We will then have, for the first time, the possibility to break the parameters’ degeneracy on the HB, understand the role of the mass loss, and lay the foundation to build another piece of the multiple populations mosaic.

We present the results obtained from Near-UV observations of the cluster NGC 5466 taken with UVIT onboard AstroSat to study the radial distribution of Blue Straggler Stars (BSSs), covering the cluster region up to a radius of 14'. Our study confirms that the BSSs are more centrally concentrated than Horizontal Branch (HB) stars in the cluster. We do not find a statistically significant rising trend in the radial distribution of BSSs for the outer regions, as most of the previously catalogued BSSs that are located in the cluster outskirts were found to be non-members, quasars or galaxies. This study highlights the importance of UV imaging combined with membership information to probe the radial distribution of BSSs.

ω Centauri is by far the most massive globular star cluster in the Milky Way, and possibly the remnant of a dwarf galaxy. As such, it contains a large number of variable stars of different classes. Here we report on an extensive, wide-field time-series study of ω Cen in the J and KS bands, which has allowed us to study the near-IR period-luminosity relations for different variability classes, including the first such relations for the SX Phoenicis stars.

Standard models of low-mass core helium burning stars typically give an asymptotic l=1 g-mode period spacing well below that inferred from observed mixed modes. We find that most physical uncertainties, such as mixing beyond the fully convective core, are not significant enough to be responsible for such a discrepancy. The solution to the problem may lie in a deviation of structure away from its canonical form, such as a more massive H-exhausted core, which we briefly explore here.

Thanks to the space missions CoRoT and Kepler new oscillation frequencies have been discovered in the Fourier spectra of Blazhko RR Lyrae stars. The period doubling (PD) yields half-integer frequencies between the fundamental mode and its harmonics. In many cases the first and/or second radial overtone frequencies also appear temporally. Some stars show extra frequencies that were identified as potential non-radial modes. We show here that all these frequencies can be explained by pure radial pulsation as linear combinations of the frequencies of radial fundamental and overtone modes.

Classical Cepheids form one of the foundations of modern cosmology and the extragalactic distance scale; however, cosmic microwave background observations measure cosmological parameters and indirectly the Hubble Constant, H0, to unparalleled precision. The coming decade will provide opportunities to measure H0 to 2% uncertainty thanks to the Gaia satellite, JWST, ELTs and other telescopes using Cepheids and other standard candles. In this work, we discuss the upcoming role for variable stars and asteroseismology in calibrating the distance scale and measuring H0 and what problems exist in understanding these stars that will feed back on these measurements.

The emission nebula around the subdwarf B (sdB) star PHL 932 is currently classified as a planetary nebula (PN) in the literature. Based on a large body of multi-wavelength data, both new and previously published, we show here that this low-excitation nebula is in fact a small Strömgren sphere (Hii region) in the interstellar medium around this star. We summarise the properties of the nebula and its ionizing star, and discuss its evolutionary status. We find no compelling evidence for close binarity, arguing that PHL 932 is an ordinary sdB star. We also find that the emission nebulae around the hot DO stars PG 0108 + 101 and PG 0109 + 111 are also Strömgren spheres in the ISM, and along with PHL 932, are probably associated with the same extensive region of high-latitude molecular gas in Pisces–Pegasus.

We present a calibration of the metallicity and physical parameters (temperature, luminosity, gravity, mass, radius) for RR Lyrae stars using the ugrizsdss photometric system. Our work is based on calculations of synthetic horizontal branches (HBs), fully taking into account evolutionary effects for a wide range in metallicities and HB morphologies. We provide analytical fits that are able to provide all quantities mentioned with very high (internal) precision, based solely on mean sdss magnitudes and colors.

We selected red horizontal-branch stars along the streams of the Sagittarius dwarf galaxy (Sgr) from sdss DR7 spectroscopic data using a theoretical model. We investigate the metallicity distribution of stars in the Sgr streams. The metallicity gradient is (−1.8 ± 0.3) × 10⊃3 dex deg−1 in the first wrap of the trailing arm and (−1.5 ± 0.4) × 10−3 dex deg−1 in the first wrap of the leading arm.

The method of statistical-parallax determination is used to adjust the photometric distance-scale zero points and test the selection of several samples of old and intermediate-age tracer objects in the Milky Way, including RR Lyrae- and Mira-type variables, blue and red horizontal-branch stars, and red giants. We obtain reasonable and stable absolute-magnitude (and, hence, photometric distance-scale) corrections for all samples except for the red horizontal-branch stars, which appear to be significantly contaminated by main-sequence turn-off and subgiant stars.

We report the detection of a planetary companion around HIP 13044, a metal-poor red horizontal branch star belonging to a stellar halo stream that results from the disruption of an ancient Milky Way satellite galaxy. The detection is based on radial velocity observations with FEROS at the 2.2-m MPG/ESO telescope. The periodic radial velocity variation of P = 16.2 days can be distinguished from the periods of the stellar activity indicators. We computed a minimum planetary mass of 1.25 Mjup and an orbital semimajor axis of 0.116 AU for the planet. This discovery is unique in three aspects: First, it is the first planet detection around a star with a metallicity much lower than few percent of the solar value; second, the planet host star resides in a stellar evolutionary stage that is still unexplored in the exoplanet surveys; third, the planetary system HIP 13044 most likely has an extragalactic origin in a disrupted former satellite of the Milky Way.

A core helium flash after the departure from the red giant branch (i.e. "hot-flasher scenario") offers one of the most promising explanations for the origin of He-sdO stars. Recently, Miller Bertolami et al. (2008) have shown that many surface properties of H-deficient sdO stars (the He-sdO stars) could be explained through this scenario if chemical diffusion is taken into account. In this context the He-sdO stars formed during a hot-flasher event would transform into H-rich hot-sdB stars (33000-38000 K) as a consequence of diffusion of the remaining H towards the surface of the star. Thus, some hot sdBs might be the descendants of He-sdO stars that have previously burnt most of their H-content and, thus, a very thin H envelope should be expected (10−9 to 10−10 M⊙, see Miller Bertolami et al. 2008 for details). Interestingly enough, the location of these sdB stars in the logg - Teff diagram should overlap with the domain of the rapidly pulsating (p-mode) EC 14026 stars. This fact opens the interesting possibility of employing asteroseismology to investigate the existence of hot-sdB stars characterized by such very thin H envelopes. In this preliminary investigation, we explore the sensitivity of the acoustic pulsation spectrum of EC 14026 stars to the thickness of the H envelope.

The pulsation analysis presented in this work was performed with the help of the adiabatic radial and nonradial pulsation code employed by our group in numerous asteroseismological studies of white dwarfs and pre-white dwarfs (see Córsico et al. 2008 and references therein). The stellar models adopted in the present study where extracted from the 0.48150 M⊙ (Z= 0.001) sequence of Miller Bertolami et al. (2008). Given the exploratory nature of this work, in order to analyze the effects of different thicknesses of the H-rich envelope on the pulsation spectrum of sdB stars we have artificially added a H-rich envelope in the outermost layers of our initial model. We have considered four different thicknesses of the H envelope (log Menv/M⊙ ~ −4, −6, −8, −10) in addition to the self-consistent model (which lacks a H envelope). Next, we have pursued the evolution of the five sequences during the evolution on the HB. For each stellar model, we have computed the radial and nonradial p-modes with periods longer than 20 seconds, thus comfortably covering the observed period range of EC 14026 stars (80 - 400 sec).

Our results show that the cycle of trapping is markedly smaller for the case of thick H-envelope models (Menv > 10−7 M⊙) than for the thin H-envelope models (Menv < 10−7 M⊙). We find that sdB stars with very thin H envelopes (Menv < 10−7 M⊙) would not display almost any kind of trapping features in their frequency distribution at the observed range in EC 14026 stars (ν < 13 mHz). Consequently their frequency spectra should be significantly different from that of normal sdB stars. We plan to explore in future works to which extent the shape of the chemical transitions (here adopted as simple gaussian profiles) affects the mode-trapping features.

We show that the current data of the HB branch of 47 Tuc show a particular feature that cannot be explained if a single population with an unique mechanism of mass loss is considered. We find that a spread in helium abundance among the stars is necessary, of ~0.02. We indicate that the same variation in helium is present among the sub giant branch stars and suggest that is responsible of the spread in luminosity of the bright sub giant branch, while only a small part of the second generation is characterized by C+N+O increase and gives the faint sub giant branch.

Globular clusters (GCs) are spheroidal concentrations typically containing of the order of 105 to 106, predominantly old, stars. Historically, they have been considered as the closest counterparts of the idealized concept of “simple stellar populations.” However, some recent observations suggest than, at least in some GCs, some stars are present that have been formed with material processed by a previous generation of stars. In this sense, it has also been suggested that such material might be enriched in helium, and that blue horizontal branch stars in some GCs should accordingly be the natural progeny of such helium-enhanced stars. In this contribution we show that, at least in the case of M3 (NGC 5272), the suggested level of helium enrichment is not supported by the available, high-precision observations.

Hipparcos orbiting observatory has revealed a large number of helium-core- burning “clump” stars of the Galactic field. These low-mass stars exhibit signatures of extra-mixing processes that require modeling beyond the standard stellar theory. In this contribution we overview available results of 12C, 13C, N and O abundances obtained by high-resolution spectra for clump stars and discuss them in the light of current predictions of stellar evolution models.

Large abundance anomalies have previously been detected in horizontal-branch B-type stars. We present the first high-resolution study of isotopic anomalies and chemical abundances in six horizontal-branch B-type stars in the globular clusters NGC 6397 and NGC 6752, carried out with UVES on the VLT and compare them to those observed in chemically peculiar main-sequence stars.

The aim of this work is to investigate the possible existence of the UV-excess in the Milky Way bulge and its correlation with the presence of sdB stars in the bulge. The integrated spectrum of a bulge region from the UV to the optical was constructed. Results show that this bulge region has only a very weak UV excess