Horizontal Branch Stars In Globular Clusters Research Articles

SHOTGLAS

Aims. We want to study the population of blue horizontal branch (HB) stars in the centres of globular clusters (GC) for the first time by exploiting the unique combination of MUSE spectroscopy and HST photometry. In this work, we characterize their properties in the GCs ω Cen and NGC 6752. Methods. We use dedicated model atmospheres and grids of synthetic spectra computed using a hybrid LTE/NLTE modeling approach to fit the MUSE spectra of HB stars hotter than 8000 K in both clusters. The spectral fits provide estimates of the effective temperature (Teff), surface gravity (log ɡ), and helium abundance of the stars. The model grids are further used to fit the HST magnitudes of the stars, that is, their spectral energy distributions (SEDs). From the SED fits, we derive the average reddening, radius, luminosity, and mass of the stars in our sample. Results. The atmospheric and stellar properties that we derive for the stars in our sample are in good agreement with theoretical expectations. In particular, the stars cooler than ~15 000 K closely follow the theoretical predictions on radius, log ɡ, and luminosity for helium-normal (Y = 0.25) models. In ω Cen, we show that the majority of these cooler HB stars cannot originate from a helium-enriched population with Y > 0.35. The properties of the hotter stars (radii and luminosities) are still in reasonable agreement with theoretical expectations, but the individual measurements show a large scatter. For these hot stars, we find a mismatch between the effective temperatures indicated from the MUSE spectral fits and the photometric fits, with the latter returning Teff lower by ~3000 K. We use three different diagnostics, namely the position of the G-jump and changes in metallicity and helium abundances, to place the onset of diffusion in the stellar atmospheres at Teff between 11 000 and 11 500 K. Our sample includes two stars known as photometric variables; we confirm one to be a bona fide extreme HB object but the other is a blue straggler star. Finally, unlike what has been reported in the literature, we do not find significant differences between the properties (e.g., log ɡ, radius, and luminosity) of the stars in the two clusters. Conclusions. We show that our analysis method – combining MUSE spectra and HST photometry of HB stars in GCs – is a powerful tool for characterising their stellar properties. With the availability of MUSE and HST observations of additional GCs, we have a unique opportunity to combine homogeneous spectroscopic and photometric data to study and compare the properties of blue HB stars in different GCs.

THE ULTRAVIOLET UPTURN IN ELLIPTICAL GALAXIES AND ENVIRONMENTAL EFFECTS

It is suspected that the ultraviolet (UV) upturn phenomenon in elliptical galaxies and extended horizontal-branch stars in globular clusters have a common origin. An extremely high abundance of helium (Y~0.4) allows for a working hypothesis, but its origin is unclear. Peng & Nagai (2009) proposed that primordial helium sedimentation in dark haloes over cosmic timescales may lead to extreme helium abundances in galaxy cluster centers. In this scenario UV upturn should be restricted to brightest cluster galaxies (BCGs) only. This is a clear and testable prediction. We present tests of this hypothesis using galaxy clusters from Yoon et al. (2008) that were detected by both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer Medium Imaging Survey. Using a new UV classification scheme based on far-UV, near-UV, and optical photometry we found only 5% of cluster elliptical galaxies show a UV upturn, while 27% and 68% are classified as "recent star-formation" and "UV-weak" ellipticals, respectively. The data reveal a modest positive dependence of the UV upturn fraction on galaxy velocity dispersion, which is in agreement with the earlier findings of Burstein et al. (1988) and possibly with the helium sedimentation theory. However, we do not see any dependency on rank or luminosity of galaxies. Besides, BCGs do not show any marked difference in UV upturn fraction or strength, which is inconsistent with the prediction. We conclude that the aforementioned helium sedimentation theory and its inferred environmental effects are not supported by the available data.

Stellar fusion doesn’t stop at helium

Nelemans replies: Thanks to Mason Osborne for the clarification. Indeed, stars that are initially lighter than 8 solar masses also fuse helium into carbon and oxygen and end up with a degenerate carbon–oxygen core. Stars with initial masses of less than about 2 solar masses first develop a degenerate helium core, which in the helium flash is turned into a helium-burning core. Apart from the RR Lyrae stars, several other types, such as subdwarf B stars and horizontal-branch stars in globular clusters, are in the helium-burning-core stage (references in the original article and in Osborne’s letter). The point of my simplification in the box text was to distinguish between stars that develop a degenerate core to withstand gravity and more massive stars that do not form such a core. The less massive stars form white dwarfs after losing their hydrogen mantle, and the heavier ones ultimately become neutron stars or black holes. © 2007 American Institute of Physics.

LUMINOSITIES OF SX PHOENICIS, LARGE-AMPLITUDE DELTA SCUTI, AND RR LYRAE STARS

ABSTRACT Strömgren uvbyβ photometry of SX Phoenicis and large-amplitude δ Scuti variables is analyzed to determine the mean temperatures, metal abundances, and surface gravities of the stars. The mean temperatures and fundamental periods are used in conjunction with the pulsation equation and stellar models to derive masses, radii, surface gravities, and Mbol of the variables. The stars exhibit a period-luminosity relation. With the aid of Hipparcos trigonometric parallaxes the zero-point of the Mv, p is set. We find Mv = -3.725 log P - 1.933. This P-L relation is utilized to find the M-v values of RR Lyrae stars or horizontal-branch stars in globular clusters and the Carina galaxy-objects that contain both SX Phe, and RR Lyrae variables, or horizontal-branch stars. The Mv values of the metal-poor RR Lyrae stars are found to be consistent with Mv [Fe/H] calibrations derived by Sandage (1993) and by McNamara (1997) from revised Baade- Wesselink Mv values. At [Fe/H] = -1.9 Mv is 0.42. Feast and Catchpole (1997) suggestion that Mv = 0.25 at [Fe/H] = -1.9 for RR Lyrae is discussed. This Mv value is too luminous because it makes the (Teff) of RR Lyraes too high-well off any (Teff) = f (color index) calibration.

Precision CCD Photometry of the Horizontal Branch

The Strömgren four-color system is well suited to the measure and analysis of horizontal-branch stars. The increased accuracy of the CCD photometric system and the ability to measure fainter stars in very crowded regions make the combination of the CCD system and four-color photometry an excellent one to study horizontal-branch stars in globular clusters.

The horizontal-branch stars in globular clusters. 2: The second parameter phenomenon

view Abstract Citations (441) References (151) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Horizontal-Branch Stars in Globular Clusters. II. The Second Parameter Phenomenon Lee, Young-Wook ; Demarque, Pierre ; Zinn, Robert Abstract Using synthetic horizontal-branch models, we have investigated the origin of the systematic variation in horizontal-branch (HB) morphology with galactocentric distance (RG) among globular clusters. The variations in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed properties of the RR Lyrae variables or the observed main-sequence turnoffs in the clusters. There is also no clear evidence that the trend with RG is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age required to explain this effect are not in conflict with any observations. A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar (Fe/H) reveals reasonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertainty is the need for ad hoc hypotheses in the modeling of the HB morphologies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as approximately 5 Gyr) among the halo globular clusters. Our results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and we estimate this difference to be approximately 2 Gyr. Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173803 Bibcode: 1994ApJ...423..248L Keywords: Globular Clusters; Horizontal Branch Stars; Star Formation; Stellar Models; Abundance; Stellar Composition; Stellar Luminosity; Astrophysics; GALAXY: FORMATION; GALAXY: GLOBULAR CLUSTERS: GENERAL; STARS: EVOLUTION; STARS: HORIZONTAL-BRANCH full text sources ADS | data products SIMBAD (83) Related Materials (1) Part 1: 1990ApJ...350..155L

The horizontal-branch stars in globular clusters. I - The period-shift effect, the luminosity of the horizontal branch, and the age-metallicity relation

Synthetic models of the horizontal branches in globular clusters are constructed from a grid of the standard horizontal branch evolutionary tracks. The models are used to study the period shifts at constant T(eff) between RR Lyrae variables in globular clusters of different metallicities and the variation in horizontal-branch luminosity with the Fe/H ratio. The results suggest that the observed differences in the mean periods of the ab variables and the fraction of c-type variables between the two Oosterhoff groups are caused by a difference in the mean luminosity of the ab variables of about 0.18 bolometric mass and by the uneven distribution of variables across the instability strip in the group II clusters.

Faint Secondary Standards for Spectrophotometry and the Energy Distributions of Horizontal-Branch A-Stars

The energy distributions of 16 horizontal-branch A-stars and 11 horizontal-branch stars in globular clusters have been measured using the Harvard Scanners at KPNO and CTIO and the Oke multichannel spectrophotometer on the 5-m telescope at Mt. Palomar (Philip and Hayes 1983, Hayes and Philip 1983). Wavelengths between 3400 and 6800 Å were measured and reduced to absolute energy distributions on the system of Hayes and Latham (1975). The internal measuring errors were ± 0.034 mag. per observation for the 15th mag. globular cluster stars and ±0.025 mag. per observation for the 7th to 11th mag. field stars, averaged over all wavelengths. Eleven of the field stars have been observed over nine times each and have low internal measurement errors; these stars plus four globular cluster stars with low internal measurement errors are recommended as secondary standard stars. (See Table I.)

Chemical separation in horizontal-branch stars

Abundance anomalies are predicted to be produced by diffusion processes in horizontal-branch (HB) and halo Population II stars. If the outer envelope is stable enough for the gravitational settling of He to be efficient, it is here shown that overabundances of heavy elements are simultaneously produced. Abundance anomalies as large as 10/sup 3/--10/sup 4/ are expected. Detailed calculations of selective radiative accelerations and of the time evolution of abundances are presented for C, N, O, Ca, Fe, and Sr in a series of models representative of HB stars. These abundance anomalies may be important for what is called the ''second parameter problem'' in globular clusters. By detailed time dependent calculations, helium underabundances and relative overabundances of /sup 3/He are shown to be produced well within the lifetime of stars on the horizontal branch. The implications of helium underabundances on the pulsational properties of RR Lyrae variables are discussed. The role of turbulent mixing in reducing the abundance anomalies is discussed. It is shown that even with enough turbulence to prevent He from settling, heavy element abundance anomalies are possible. Even though chemical separation is very likely to be important in the blue horizontal-branch stars (T>15,000 K), it may not bemore » in the cooler stars where mass loss could eliminate any effect of chemical separation. This could be verified by observations of horizontal-branch stars in globular clusters.« less

Mass loss in globular cluster red giants.

Echelle spectra of the brightest globular-cluster red giants, taken with the 4 m telescope at Kitt Peak National Observatory, reveal the presence of emission features at H..cap alpha... These are interpreted as evidence for a circumstellar envelope, which is produced by mass loss. The gas is being lost at a velocity such that it is possible that the gas is escaping from the globular cluster. An estimate of the rate of mass loss (2 x 10/sup -9/ M/sub sun/ yr/sup -1/) is obtained, which agrees well with that expected from revolutionary tracks for the difference in mass between red-giant stars and horizontal-branch stars in globular clusters. (AIP)

Semiconvection and the RR Lyrae Variables

Theoretical computations (Hoyle and Schwarzschild, 1955; Faulkner, 1966; Iben and Rood, 1970; Demarque and Mengel, 1971a, b) have identified the horizontal-branch stars in globular clusters with the evolution phase in which helium burns within a convective core and hydrogen burns in a shell outside the convective core. Most computations for such double-energy-source models have indicated that the evolution proceeds smoothly on a nuclear time scale during the horizontal-branch phase, leading to small predicted rates of change in the RR Lyrae pulsation period (Iben and Rood, 1970). Sweigart and Demarque (1972) have recently considered the effects of semiconvection on the horizontal-branch evolution of typical Population II stars and have suggested that changes in the composition distribution within the core may occur on a time scale considerably shorter than the nuclear time scale during the phase immediately preceding core-helium exhaustion. It has been found that the composition distribution generated by the growth of a semiconvective zone in the layers surrounding the convective core can become unstable when Yc, the helium abundance within the convective core, decreases below roughly 0.12. The changes in the internal structure caused by this instability result in relatively rapid movement of the models in the HR diagram and consequently produce large predicted rates of change in the RR Lyrae pulsation period. The possibility that RR Lyrae period changes may be associated with the behavior of the semiconvective zone has been previously suggested by Schwarzschild (1970). A similar instability may occur in the late core-hydrogen burning phase for stars around 10 M⊙. Percy (1970) has noted the coincidence of β Cephei stars with stellar models containing semiconvective zones. It is tempting to suggest that such an instability in the semiconvective zone could also be related to the β Cephei phenomenon.

Photometric Classification of Blue Horizontal-Branch Stars

The Strömgren four-color system has been used to classify blue horizontal-branch stars in globular clusters and in the general field. A relation between δ log g (log gmain sequence–log gstar) and δc1 (c1observed–c1main sequence) has been determined which gives log g to within ±0.2 for stars with log g's between 4.4 and 2.0. Newell (1970) has determined a relation between (B–V)0 and θe. Thus the gravities and effective temperatures of blue horizontal-branch stars can be determined from photometric measures alone. Five globular clusters have been studied at the present time. The BHB stars in the globular cluster M3 and M13 have lower surface gravities than BHB stars in the other clusters studied. Iben and Rood (1970) have recently suggested that the BHB stars of M3 may have lost more mass than BHB stars in more metal poor clusters.

Horizontal-Branch Stars and Interstellar Lines at High Latitude

view Abstract Citations (45) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Horizontal-Branch Stars and Interstellar Lines at High Latitude Greenstein, Jesse L. Abstract Spectra of blue, horizontal-branch stars in globular clusters with weak He i are examined. The K-line is found strong; it is in part interstellar, and sometimes in part caused by the unresolved cluster back- ground of cool stars. Measures of interstellar Ca ii in hot stars at high galactic latitude show great strength. K reaches an equivalent width of 0.4 A near the pole. Preliminary measures of Na i give a similar limit. Most lines show a negative velocity, similar to, but smaller than, that of the high-velocity 21-cm clouds. I. INTRODUCTIO Publication: The Astrophysical Journal Pub Date: May 1968 DOI: 10.1086/149559 Bibcode: 1968ApJ...152..431G full text sources ADS | data products SIMBAD (25)