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), for which the helium-enhancement scenario predicts helium enhancements of > 0.02 in virtually all blue HB stars. Using high-precision Stroemgren photometry and spectroscopic gravities for blue HB stars, we find that any helium enhancement among most of the cluster's blue HB stars is very likely less than 0.01, thus ruling out the much higher helium enhancements that have been proposed in the literature.
We explore the possible contribution of classical and recurrent novae from isolated white dwarfs accreting from the intracluster medium to the abundances of "second generation" globular cluster stellar populations. We show that under reasonable assumptions the helium abundances of clusters can be enhanced substantially by these novae and argue that novae should be considered as an important, and perhaps even dominant channel in the evolution of the intracluster medium. We also discuss a possible test for whether helium enhancement really is the cause of the multiple main sequences in globular …
The recent measurements of internal variations of helium in Galactic and extragalactic Globular Clusters (GCs) set binding constraints to the models of formation of Multiple Populations (MPs) in GCs, and gave rise, at the same time, to crucial questions related with the influence of the environment on MP formation as well as with the role played by GCs in the early galactic formation. We present the most recent estimates of helium enrichment in the main populations of a large sample of Galactic and extragalactic GCs.
Context. Globular clusters host multiple populations of long-lived low-mass stars whose origin remains an open question. Several scenarios have been proposed to explain the associated photometric and spectroscopic peculiarities. They differ, for instance, in the maximum helium enrichment they predict for stars of the second population, which these stars can inherit at birth as the result of the internal pollution of the cluster by different types of stars of the first population. Aims. We present the distribution of helium-rich stars in present-day globular clusters as it is expected in the or…
Globular cluster progenitors may have been detected by \textit{HST}, and are predicted to be observable with \textit{JWST} and ground-based extremely-large telescopes with adaptive optics. This has the potential to elucidate the issue of globular cluster formation and the origins of significantly helium-enriched subpopulations, a problem in Galactic astronomy with no satisfactory theoretical solution. Given this context, we use model stellar tracks and isochrones to investigate the predicted observational properties of helium-enriched stellar populations in globular cluster progenitors. We fin…
Context. Globular clusters host stars with chemical peculiarities. The associated helium enrichment is expected to affect the evolution of stars, in general, and of low-mass stars, and in particular the progenitors of white dwarfs (WDs). Aims. We investigate the effects of different initial helium contents on the properties of white dwarfs such as their masses, compositions, and the time since their formation. Results. In a globular cluster at a given age, the He enrichment implies lower initial masses for stars at a given phase. Thus it leads to a decrease of the masses of white dwarfs reachi…
The leading method for the determination of relevant stellar population parameters of unresolved extragalactic Globular Clusters is through the study of their integrated spectroscopy, where Balmer line-strength indices are considered to be age sensitive. Previously, a splitting in the highly optimised spectral line-strength index H$β_o$ was observed in a sample of Galactic globular clusters at all metallicities resulting in an apparent "upper branch" and "lower branch" of globular clusters in the H$β_o$ - [MgFe] diagram. This was suggested to be caused by the presence of hot Blue straggler sta…
Recent work, based on data from the Hubble Space Telescope (HST) UV Legacy Survey of Galactic Globular Clusters (GCs), has revealed that all the analyzed clusters host two groups of first- (1G) and second-generation (2G) stars. In most GCs, both 1G and 2G stars host sub-stellar populations with different chemical composition. We compare multi-wavelength HST photometry with synthetic spectra to determine for the first time the average helium difference between the 2G and 1G stars in a large sample of 57 GCs and the maximum helium variation within each of them. We find that in all clusters 2G st…
We use Hubble Space Telescope (HST) imaging from the ACS Treasury Survey to determine fits for single population isochrones of 69 Galactic globular clusters. Using robust Bayesian analysis techniques, we simultaneously determine ages, distances, absorptions, and helium values for each cluster under the scenario of a "single" stellar population on model grids with solar ratio heavy element abundances. The set of cluster parameters is determined in a consistent and reproducible manner for all clusters using the Bayesian analysis suite BASE-9. Our results are used to re-visit the age-metallicity …
The horizontal branch (HB) morphology of globular clusters (GCs) is mainly described by metallicity. The fact that some clusters with almost the same metallicity exhibit different HB demonstrates that other parameters are at work. We present results from the analysis of the CMD of 72 GCs obtained with the Advanced Camera for Survey (ACS) of the Hubble Space Telescope (HST). We find a significant correlation between the HB color extension and the mass of the hosting cluster, while the color distance between the HB and the red-giant branch (RGB) depends on metallicity and age. We suggest that ag…
In a Globular Cluster (GC), the mass loss during the red-giant branch (RGB) phase and the helium content are fundamental ingredients to constrain the horizontal branch (HB) morphology. While many papers have been dedicated to the helium abundance in the different stellar populations, small efforts have been done to disentangle the effects of mass loss and helium content. We exploit the nearby GC NGC6121 (M4), which hosts two well-studied main stellar populations, to infer both helium and RGB mass loss. We combine multi-band Hubble Space Telescope photometry of RGB and main sequence (MS) stars …
Recent pieces of evidence have revealed that most, and possibly all, globular star clusters are composed of groups of stars that formed in multiple episodes with different chemical compositions. In this sense, it has also been argued that variations in the initial helium abundance ($Y$) from one population to the next are also the rule, rather than the exception. In the case of the metal-intermediate globular cluster M4 (NGC 6121), recent high-resolution spectroscopic observations of blue horizontal branch (HB) stars (i.e., HB stars hotter than the RR Lyrae instability strip) suggest that a la…
A recent study of hot (20,000 to 30,000 K) extreme horizontal branch (EHB) stars in globular clusters (GCs) has led to the discovery of their variability. It is suggested that this variability is driven by the projected rotation of magnetic spots on the stellar surfaces and is expected to have higher amplitudes at shorter wavelengths. Here, we present the analysis of such hot stars in the massive GC NGC 2808 using the Ultraviolet Imaging Telescope (UVIT), aboard AstroSat. We use the UVIT data in combination with the Hubble Space Telescope UV Globular Cluster Survey (HUGS) data for the central …
Globular clusters (GCs) are spheroidal concentrations typically containing of the order of 10^5 to 10^6, 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 nat…
Theory and observations indicate that the distribution of stars along the horizontal branch of Galactic globular clusters mainly depends on the metal content. However, the existence of globular clusters with similar metal content and absolute age but different horizontal branch morphologies, suggests the presence of another parameter affecting the star distribution along the branch. To investigate the variation of the horizontal branch morphology in Galactic globular clusters, we define a new photometric horizontal branch morphology index, overcoming some of the limitations and degeneracies af…
Recent studies suggest that the helium content of multiple stellar populations in globular clusters (GCs) is not uniform. The range of helium varies from cluster to cluster with more massive GCs having, preferentially, large helium spread. GCs with large helium variations also show extended-blue horizontal branch (HB). I exploit Hubble Space Telescope photometry to investigate multiple stellar populations in NGC6266 and infer their relative helium abundance. This cluster is an ideal target to investigate the possible connection between helium, cluster mass, and HB morphology, as it exhibits an…
Multiple populations in globular clusters are usually explained by the formation of stars out of material with a chemical composition that is polluted to different degrees by the ejecta of short-lived, massive stars of various type. Among other things, these polluters differ by the amount of helium they spread in the surrounding medium. In this study we investigate whether the present-day photometric method used to infer the helium content of multiple populations indeed gives the true value or underestimates it by missing very He-rich, but rare stars. We focus on the specific case of NGC6752. …
We combine ground and space-based photometry of the Galactic globular cluster 47 Tuc to measure four independent lines of evidence for a helium gradient in the cluster, whereby stars in the cluster outskirts would have a lower initial helium abundance than stars in and near the cluster core. First and second, we show that the red giant branch bump (RGBB) stars exhibit gradients in their number counts and brightness. With increased separation from the cluster center, they become more numerous relative to the other red giant (RG) stars. They also become fainter. For our third and fourth lines of…
The location of Galactic Globular Clusters' (GC) stars on the horizontal branch (HB) should mainly depend on GC metallicity, the "first parameter", but it is actually the result of complex interactions between the red giant branch (RGB) mass loss, the coexistence of multiple stellar populations with different helium content, and the presence of a "second parameter" which produces dramatic differences in HB morphology of GCs of similar metallicity and ages (like the pair M3--M13). In this work, we combine the entire dataset from the Hubble Space Telescope Treasury survey and stellar evolutionar…
Metallicity is the first parameter to influence the horizontal branch (HB) morphology of globular clusters (GCs). It has been found, however, that some other parameters may also play an important role in affecting the morphology. While the nature of these important parameters remains unclear, they are believed to be likely correlated with wind mass-loss of red giants, since this mass loss determines their subsequent locations on the HB. Unfortunately, the mass loss during the red giant stages of the stellar evolution is poorly understood at present. The stellar winds of red giants may be tidal…
Ultraviolet observations are the best tool to study hot stellar populations which emit most of their light at short wavelengths. As part of a large project devoted to the characterization of the UV properties of Galactic globular clusters (GGCs), we collected mid/far UV and optical images with the WFPC2@HST for 31 GGCs. These clusters cover a wide range in metallicity and structural parameters, thus representing an ideal sample for comparison with theoretical models. Here we present the first results from an ongoing analysis aimed at deriving the temperature distribution of Horizontal Branch s…
We use deep, high quality colour magnitude diagrams obtained with the Hubble Space Telescope to compute a simplified version of the Mironov index [SMI; B/(B+R)] to parametrize the horizontal branch (HB) morphology for 23 globular clusters in the M31 galaxy (Sample-A), all located in the outer halo at projected distances between 10 kpc and 100 kpc. This allows us to compare them with their Galactic counterparts, for which we estimated the SMI exactly in the same way, in the SMI vs. [Fe/H] plane. We find that the majority of the considered M31 clusters lie in a significantly different locus, in …
We use Cycle 21 Hubble Space Telescope (HST) observations and HST archival ACS Treasury observations of 30 Galactic Globular Clusters to characterize two distinct stellar populations. A sophisticated Bayesian technique is employed to simultaneously sample the joint posterior distribution of age, distance, and extinction for each cluster, as well as unique helium values for two populations within each cluster and the relative proportion of those populations. We find the helium differences among the two populations in the clusters fall in the range of ~0.04 to 0.11. Because adequate models varyi…
Globular clusters have long been considered the closest approximation to a physicist's laboratory in astrophysics, and as such a near-ideal laboratory for (low-mass) stellar evolution. However, recent observations have cast a shadow on this long-standing paradigm, suggesting the presence of multiple populations with widely different abundance patterns, and -- crucially -- with widely different helium abundances as well. In this review we discuss which features of the Hertzsprung-Russel diagram may be used as helium abundance indicators, and present an overview of available constraints on the h…
The pulsation periods of long period variables (LPVs) depend on their mass and helium abundance as well as on their luminosity and metal abundance. Comparison of the observed periods of LPVs in globular clusters with models is capable of revealing the amount of mass lost on the giant branch and the helium abundance.} {We aim to determine the amount of mass loss that has occurred on the giant branches of the low metallicity globular clusters NGC 362 and NGC 2808. We also aim to see if the LPVs in NGC 2808 can tell us about helium abundance variations in this cluster.} We have used optical monit…
For nearby K dwarfs, the broadening of the observed Main Sequence at low metallicities is much narrower than expected from isochrones with the standard helium-to-metal enrichment ratio DY/DZ=2. Though the latter value fits well the Main Sequence around solar metallicity, and agrees with independent measurements from HII regions as well as with theoretical stellar yields and chemical evolution models, a much higher DY/DZ~10 is necessary to reproduce the broadening observed for nearby subdwarfs. This result resembles, on a milder scale, the very high DY/DZ estimated from the multiple Main Sequen…
The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters is providing a major breakthrough in our knowledge of Globular Clusters (GCs) and their stellar populations. Among the main results, we discovered that all the studied GCs host two main discrete groups consisting of first generation (1G) and second generation (2G) stars. We exploit the multiwavelength photometry from this project to investigate, for the first time, the Red Giant Branch Bump (RGBB) of the two generations in a large sample of GCs. We identified, with high statistical significance, the RGBB of 1G and 2G sta…
We present Hubble Space Telescope (HST) photometric results for NGC 6402, a highly reddened very luminous Galactic globular cluster (GC). Recent spectroscopic observations of its red giant stars have shown a quite peculiar behavior in the chemistry of its multiple populations. These results have prompted UV and optical HST observations aimed at obtaining the cluster's "Chromosome map" (ChM), an efficient tool to classify GCs and characterize their multiple populations. We find that the discontinuity in the abundance distributions of O, Mg, Al and Na inferred from spectroscopy is more nuanced i…
We introduce the first g-mode pulsator found to reside on the classical blue horizontal branch. One year of Kepler observations of KIC 1718290 reveals a rich spectrum of low-amplitude modes with periods between one and twelve hours, most of which follow a regular spacing of 276.3 s. This mode structure strongly resembles that of the V1093Her pulsators, with only a slight shift towards longer periods. Our spectroscopy, however, reveals KIC 1718290 to be quite distinct from the sdB stars that show V1093Her pulsations, which all have surface gravities higher than log g = 5.1 and helium abundances…
Helium is a pivotal element in understanding multiple main sequences and extended horizontal branches observed in some globular clusters. Here we present a spectroscopic study of helium in the nearby globular cluster M4. We have obtained spectra of the chromospheric He I 10830 A line in 16 red horizontal branch, red giant branch, and asymptotic giant branch stars. Clear He I absorption or emission is present in most of the stars. Effective temperature is the principal parameter that correlates with 10830 A line strength. Stars with T_eff < 4450 K do not exhibit the helium line. Red horizontal …
We present a new set of horizontal-branch (HB) models computed with the MESA stellar evolution code. The models adopt $α$-enhanced \cite{ags09} metals mixtures and include the gravitational settling of He. They are used in our HB population synthesis tool to generate theoretical distributions of HB stars in order to describe the multiple stellar populations in the globular clusters 47Tuc, M3, and M13. The observed HB in 47Tuc is reproduced very well by our simulations for [Fe/H] $= -0.70$ and [$α$/Fe] $= +0.4$ if the initial helium mass fraction varies by $ΔY_0 \sim 0.03$ and approximately 21%…
Employing tidally enhanced stellar wind, we studied in binaries the effects of metallicity, mass ratio of primary to secondary, tidal enhancement efficiency and helium abundance on the formation of blue hook (BHk) stars in globular clusters (GCs). A total of 28 sets of binary models combined with different input parameters are studied. For each set of binary model, we presented a range of initial orbital periods that is needed to produce BHk stars in binaries. All the binary models could produce BHk stars within different range of initial orbital periods. We also compared our results with the …
Thermally pulsing asymptotic giant branch models of globular cluster stars are calculated using a synthetic model with the goal of reproducing the chemical composition, core masses and other observational parameters of the four known globular cluster planetary nebulae as well as roughly matching the overall cluster properties. The evolution of stars with an enhanced helium abundance ($Y$) and blue stragglers are modeled. New pre-thermally pulsing asymptotic giant branch mass-losses for red giant branch and early asymptotic giant branch stars are calculated from the Padova stellar evolution mod…
In this paper we present a multi-wavelength analysis of the complex stellar populations within the massive globular cluster Mayall II (G1), a satellite of the nearby Andromeda galaxy projected at a distance of 40 kpc. We used images collected with the Hubble Space Telescope in UV, blue and optical filters to explore the multiple stellar populations hosted by G1. The $m_{\rm F438W}$ versus $m_{\rm F438W}- m_{\rm F606W}$ colour-magnitude diagram shows a significant spread of the red giant branch, that divides $\sim 1$ mag brighter than the red clump. A possible explanation is the presence of two…
We exploit multi-band Hubble Space Telescope photometry to investigate multiple populations (MPs) along the red horizontal branches (HBs) and red clumps of fourteen metal-rich Globular Clusters (GCs), including twelve Milky Way GCs and the Magellanic Cloud GCs NGC 1978 and NGC 416. Based on appropriate two-color diagrams we find that the fraction of 1G stars in Galactic GCs correlates with cluster mass, confirming previous results based on red-giant branch (RGB) stars. Magellanic-Cloud GCs show higher fractions of 1G stars than Galactic GCs with similar masses, thus suggesting that the environ…
Our understanding of the formation and early evolution of globular clusters (GCs) has been totally overthrown with the discovery of the peculiar chemical properties of their long-lived host stars. As a consequence, the interpretation of the observed color-magnitude diagrams and of the properties of the GC stellar populations requires the use of stellar models computed with relevant chemical compositions. We present a grid of 224 stellar evolution for low-mass stars with initial masses between 0.3 and 1.0 Msun and initial helium mass fraction between 0.248 and 0.8 computed for [Fe/H]=-1.75 with…
Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters omega Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly …
Bulge globular clusters (GCs) with metallicities [Fe/H] <~ -1.0 and blue horizontal branches are candidates to harbor the oldest populations in the Galaxy. Based on the analysis of HST proper-motion-cleaned color-magnitude diagrams in filters F435W and F625W, we determine physical parameters for the old bulge globular clusters NGC 6522 and NGC 6626 (M28), both with well-defined blue horizontal branches. We compare these results with similar data for the inner halo cluster NGC 6362. These clusters have similar metallicities (-1.3 <= [Fe/H] <= -1.0) obtained from high resolution spectroscopy. We…
We present a detailed spectroscopic analysis of horizontal branch stars in the globular cluster NGC 3201. We collected optical (4580-5330 A), high resolution (~34,000), high signal-to-noise ratio (~200) spectra for eleven RR Lyrae stars and one red horizontal branch star with the multifiber spectrograph M2FS at the 6.5m Magellan telescope at the Las Campanas Observatory. From measured equivalent widths we derived atmospheric parameters and abundance ratios for α (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn) and s-process (Y) elements. We found that NGC 3201 is a homogeneous, mono-metallic ([Fe/H]=-…
We present the helium abundance of the two metal-poor clusters M30 and NGC6397. Helium estimates have been obtained by using the high-resolution spectrograph FLAMES at the ESO Very Large Telescope and by measuring the HeI line at 4471 A in 24 and 35 horizontal branch stars in M30 and NGC6397, respectively. This sample represents the largest dataset of He abundances collected so far in metal-poor clusters. The He mass fraction turns out to be Y=0.252+-0.003 (sigma=0.021) for M30 and Y=0.241+-0.004 (sigma=0.023) NGC6397. These values are fully compatible with the cosmological abundance, thus sug…
Recent work revealed that both the helium variation within globular clusters (GCs) and the relative numbers of first and second-generation stars (1G, 2G) depend on the mass of the host cluster. Precise determination of the internal helium variations and of the fraction of 1G stars are crucial constraints to the formation scenarios of multiple populations (MPs). We exploit multi-band Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one of the most-massive and distant GCs of the Galaxy, almost isolated from its tidal influence. We find that the 1G hosts the ~37% of the …
Aims. We use the spectra of more than 30,000 red giant branch (RGB) stars in 25 globular clusters (GC), obtained within the MUSE survey of Galactic globular clusters, to calibrate the Ca II triplet (CaT) metallicity relation and derive metallicities for all individual stars. We investigate the overall metallicity distributions as well as those of the different populations within each cluster. Methods. The Ca II triplet in the near-infrared at 8498, 8542, and 8662 AA is visible in stars with spectral types between F and M and can be used to determine their metallicities. In this work, we calibr…
We leverage new high-quality data from Hubble Space Telescope program GO-14164 to explore the variation in horizontal branch morphology among globular clusters in the Large Magellanic Cloud (LMC). Our new observations lead to photometry with a precision commensurate with that available for the Galactic globular cluster population. Our analysis indicates that, once metallicity is accounted for, clusters in the LMC largely share similar horizontal branch morphologies regardless of their location within the system. Furthermore, the LMC clusters possess, on average, slightly redder morphologies th…
Context. Stellar collision rates in globular clusters (GCs) do not appear to correlate with horizontal branch (HB) morphology, sug- gesting that dynamics does not play a role in the second-parameter problem. However, core densities and collision rates derived from surface-brightness may be significantly underestimated as the surface-brightness profile of GCs is not necessarily a good indicator of the dynamical state of GC cores. Core-collapse may go unnoticed if high central densities of dark remnants are present. Aims. We test whether GC HB morphology data supports a dynamical contribution to…
We present the results obtained from the UV photometry of the globular cluster NGC 1261 using Far-UV (FUV) and Near-UV (NUV) images acquired with the Ultraviolet Imaging Telescope (UVIT) onboard the Astrosat satellite. We utilized the UVIT data combined with HST, GAIA, and ground-based optical photometric data to construct the different UV colour-magnitude diagrams (CMDs). We detected blue HB (BHB), and two extreme HB (EHB) stars in FUV, whereas full HB, i.e., red HB (RHB), BHB as well as EHB is detected in NUV CMDs. The 2 EHB stars, identified in both NUV and FUV, are confirmed members of the…
We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the Horizontal Branch (HB) population of the We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the horizontal branch (HB) population of the Galactic globular cluster (GC) NGC 2419. A detailed analysis of the composite HB highlights three populations:(1) the blue luminous HB, hosting standard helium stars (Y=0.25) with a very small spread of mass, (2) a small population of stars with intermediate helium content (0.26<Y<=0.29), and (3) the well-popul…
We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color-magnitude diagrams (CMDs) are constructed for member stars in combination with HST UV Globular Cluster Survey (HUGS) data for the central region and Gaia and ground-based photometric data for the outer region. Blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium a…
M22 is an anomalous globular cluster that hosts two groups of stars with different metallicity and s-element abundance. The star-to-star light-element variations in both groups, with the presence of individual Na-O and C-N anticorrelations, demonstrates that this Milky-Way satellite has experienced a complex star-formation history. We have analysed FLAMES/UVES spectra for seven stars covering a small color interval, on the reddest horizontal-branch (HB) portion of this cluster and investigated possible relations between the chemical composition of a star and its location along the HB. Our chem…
Photometric and spectroscopic analyses have shown that the Galactic bulge cluster Terzan 5 hosts several populations with different metallicities and ages that manifest as a double red horizontal branch (HB). A recent investigation of the massive bulge cluster NGC 6569 revealed a similar, though less extended, HB luminosity split, but little is known about the cluster's detailed chemical composition. Therefore, we have used high-resolution spectra from the Magellan-M2FS and VLT-FLAMES spectrographs to investigate the chemical compositions and radial velocity distributions of red giant branch a…
We investigated the possibility of producing helium enhanced stars in globular clusters by accreting polluted matter during the pre-main sequence phase. We followed the evolution of two different classes of pre-main sequence accreting models, one which neglects and the other that takes into account the protostellar evolution. We analysed the dependence of the final central helium abundance, of the tracks position in the HR diagram and of the surface lithium abundance evolution on the age at which the accretion of polluted material begins and on the main physical parameters that govern the prot…
Hot subdwarf B stars (sdBs) are evolved core helium-burning stars with very thin hydrogen envelopes. In order to form an sdB, the progenitor has to lose almost all of its hydrogen envelope right at the tip of the red giant branch. In close binary systems, mass transfer to the companion provides the extraordinary mass loss required for their formation. However, apparently single sdBs exist as well and their formation is unclear since decades. The merger of helium white dwarfs leading to an ignition of core helium-burning or the merger of a helium core and a low mass star during the common envel…
UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in omega Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than…
Recently, the study of globular cluster (GC) CMDs has shown that some of them harbor multiple populations with different chemical compositions and/or ages. In the first case, the most common candidate is a spread in the initial helium abundance, but this quantity is difficult to determine spectroscopically due to the fact that helium absorption lines are not present in cooler stars, whereas for hotter GC stars gravitational settling of helium becomes important. As a consequence, indirect methods to determine the initial Y among populations are necessary. For that reason, in this series of pape…
The UV-initiative Hubble Space Telescope Treasury survey of Galactic globular clusters provides a new window into the phenomena that shape the morphological features of the horizontal branch (HB). Using this large and homogeneous catalog of UV and blue photometry, we demonstrate that the HB exhibits discontinuities that are remarkably consistent in color (effective temperature). This consistency is apparent even among some of the most massive clusters hosting multiple distinct sub-populations (such as NGC 2808, omega Cen, and NGC 6715), demonstrating that these phenomena are primarily driven b…
Subluminous B stars come in a variety of flavours including single stars, close and wide binaries, and pulsating and non-pulsating variables. A majority have helium-poor surfaces (helium by number nHe<1%), whilst a minority have extremely helium-rich surfaces (nHe>90%). A small number have an intermediate surface helium abundance (~ 10 - 30%), accompanied by peculiar abundances of other elements. The questions posed are i) whether these abundance peculiarities are associated with radiatively-driven and time-dependent stratification of elements within the photosphere as the star evolves from an…
A Helium-shell Burning Blue Horizontal Branch Star Produced from Common Envelope Evolution
Yunnan Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Hebei Normal University（河北师范大学）
