Extended Horizontal Branch Research Articles

DEEP IMAGING OF ERIDANUS II AND ITS LONE STAR CLUSTER*

ABSTRACT We present deep imaging of the most distant dwarf discovered by the Dark Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry reaches ∼3 mag deeper than previous work and allows us to confirm the presence of a stellar cluster whose position is consistent with Eri II’s center. This makes Eri II, at M V = − 7.1 , the least luminous galaxy known to host a (possibly central) cluster. The cluster is partially resolved, and at M V = − 3.5 it accounts for ∼4% of Eri II’s luminosity. We derive updated structural parameters for Eri II, which has a half-light radius of ∼280 pc and is elongated (ϵ ∼ 0.48) at a measured distance of D ∼ 370 kpc. The color–magnitude diagram displays a blue, extended horizontal branch, as well as a less populated red horizontal branch. A central concentration of stars brighter than the old main-sequence turnoff hints at a possible intermediate-age (∼3 Gyr) population; alternatively, these sources could be blue straggler stars. A deep Green Bank Telescope observation of Eri II reveals no associated atomic gas.

THE CURIOUS RADIAL DISTRIBUTIONS OF HORIZONTAL BRANCH STARS IN NGC 6441

NGC 6441 is one of the most massive and most metal-rich globular clusters in the galaxy, and is noted for an unusual extended horizontal branch which reaches past the instability strip. We find evidence that there are two different populations of stars within the heavily-populated red clump. Once a differential reddening correction is applied, a large but compact group of stars is found at the faint red end of the clump in the color magnitude diagram. Brighter, bluer stars in the clump are found to be more centrally concentrated within the cluster at very high level of significance. Curiously, the blue horizontal branch stars show a more complex distribution, and are not more centrally concentrated than the brighter red clump stars. The spatial distributions of clump stars is in agreement with the idea that the the brighter bluer part of the clump is a helium-enriched second generation. The blue horizontal branch stars may be showing evidence that they are being dynamically evaporated.

Evolutionary scenarios and chemical inhomogeneities of extended horizontal branch stars

Extended Horizontal Branch (EHB) stars are observed in many globular clusters and as field stars in the Galactic halo. They belong to old stellar populations of the halo and the old disk. Their evolutionary status is unclear, and still a current subject of debate. Current interest in these stars arise from their association with the discoveries of helium abundance inhomogeneities in the globular clusters ω Cen and NGC 2808. The origin of the inhomogeneities is not yet understood, but there are many interpretations.In order to better understand EHB stars, we explore the evolution of standard blue Horizontal Branch (HB) models using up-to-date physics. We present several grids of post Zero Age Horizontal Branch (post-ZAHB) evolutionary models to include both canonical and non-canonical evolutionary scenarios, as well as to compare models that contain semi-convection to models without semi-convection. We follow the models to the termination of nuclear helium burning. The detailed properties of the models, including shell flashes and breathing pulses, are described.

An extremely peculiar hot subdwarf with a 10 000-fold excess of zirconium, yttrium and strontium

Helium-rich subdwarf B (He-sdB) stars represent a small group of low-mass hot stars with luminosities greater than those of conventional subdwarf B stars, and effective temperatures lower than those of subdwarf O stars. By measuring their surface chemistry, we aim to explore the connection between He-sdB stars, He-rich sdO stars and normal sdB stars. LS IV-14 116 is a relatively intermediate He-sdB star, also known to be a photometric variable. High-resolution blue-optical spectroscopy was obtained with the Anglo-Australian Telescope. Analysis of the spectrum shows LS IV-14 116 to have effective temperature Teff = 34 000 +/- 500 K, surface gravity log g = 5.6 +/- 0.2, and surface helium abundance nHe = 0.16 +/- 0.03 by number. This places the star slightly above the standard extended horizontal branch, as represented by normal sdB stars. The magnesium and silicon abundances indicate the star to be metal poor relative to the Sun. A number of significant but unfamiliar absorption lines were identified as being due to germanium, strontium, yttrium and zirconium. After calculating oscillator strengths (for Ge, Y and Zr), the photospheric abundances of these elements were established to range from 3 dex (Ge) to 4 dex (Sr, Y and Zr) above solar. The most likely explanation is that these overabundances are caused by radiatively-driven diffusion forming a chemical cloud layer in the photosphere. It is conjectured that this cloud formation could be mediated by a strong magnetic field.

Formation of the Galactic globular clusters with He-rich stars in low-mass haloes virialized at high redshift

Abstract Recent observations have reported that the Galactic globular clusters (GCs) with unusually extended horizontal-branch (EHB) morphologies show a significantly lower velocity dispersion compared with that of the entire Galactic GC system. We consider that the observed distinctive kinematics of GCs with EHB has valuable information on the formation epochs of GCs and accordingly discuss this observational result based on cosmological N-body simulations with a model of GC formation. We assume that GCs in galaxies were initially formed in low-mass haloes at high redshifts and we investigate final kinematics of GCs in their host haloes at z= 0. We find that GCs formed in haloes virialized at z > 10 show lower velocity dispersions on average than those formed at z > 6 for haloes with GCs at z= 0. We thus suggest that the origin of the observed lower velocity dispersion for the Galactic GCs with EHBs is closely associated with earlier formation epochs (z > 10) of haloes initially hosting the GCs in the course of the Galaxy formation. Considering that the origin of EHBs can be due to the presence of helium-enhanced second-generation stars in GCs, we discuss the long-standing second parameter problem of GCs in the context of different degrees of chemical pollution in GC-forming gas clouds within low-mass haloes virialized at different redshifts.

AL 3 (BH 261): A New Globular Cluster in the Galaxy

AL~3 (BH 261), previously classified as a faint open cluster candidate, is shown to be a new globular cluster in the Milky Way, by means of B, V and I Color-Magnitude Diagrams. The main feature of AL~3 is a prominent blue extended Horizontal Branch. Its Color-Magnitude Diagrams match those of the intermediate metallicity cluster M~5. The cluster is projected in a rich bulge field, also contaminated by the disk main sequence. The globular cluster is located in the Galactic bulge at a distance from the Sun d$_{\odot}$ = 6.0$\pm$0.5 kpc. The reddening is E(B-V)=0.36$\pm$0.03 and the metallicity is estimated to be [Fe/H] $\approx$ -1.3$\pm$0.25. AL~3 is probably one of the least massive globular clusters of the Galaxy.

Multicolour high-speed photometry of pulsating subdwarf B stars with ULTRACAM

High-speed multicolour photometry is presented for two pulsating subdwarf B stars, KPD 2109+4401 and HS 0039+4302. The observations were obtained using the high-speed multichannel photometer ULTRACAM on the 4.2-m William Herschel Telescope with a sampling interval of 1 and 4 s, respectively, for the two targets. Both targets show multiperiodic oscillations, generally considered to be due to p-mode pulsations. The frequency spectra are similar to those observed previously, although the amplitudes of some modes have altered. The observations are compared with theoretical multicolour light curves for non-radially oscillating extreme horizontal branch stars using the amplitude ratio method. The radial and spherical degrees n and l of all unambiguously identified frequencies have been determined. In general, n 3 and l 2, but both stars show one l = 4 mode. The spectra of frequency versus spherical degree are compared with models for evolved extended horizontal branch stars. These confirm that KPD 2109+4401 has a post-zero-age horizontal branch age of approximately 47 Myr and an envelope mass ∼0.0002 M� . HS 0039+4302 lies on the upper edge of the horizontal branch and hence its envelope mass and age are less well determined spectroscopically. The pulsation properties suggest a more massive envelope and evolved structure. In both stars, the frequencies of the radial (l = 0) modes, if correctly identified, do not match the selected models well, suggesting that the density structure or opacity in the stellar envelopes may be incorrect.

An Historical Overview

Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (Teff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research.

Rotation of Hot Horizontal-Branch Stars in the Globular Clusters NGC 1904, NGC 2808, NGC 6093, and NGC 7078

We present high-resolution Very Large Telescope/Ultraviolet Visual Echelle Spectrograph observations of 56 stars in the extended horizontal-branch (EHB) of the Galactic globular clusters NGC 1904, NGC 2808, NGC 6093, and NGC 7078. Our data reveal for the first time the presence in NGC 1904 of a sizable population of fast (v sin i ≥ 20 km s-1) horizontal-branch (HB) rotators, confined to the cool end of the EHB, similar to that found in M13. We also confirm the fast rotators already observed in NGC 7078. The cooler stars (Teff < 11,500 K) in these three clusters show a range of rotation rates, with a group of stars rotating at ~15 km s-1 or less and a fast rotating group at ~30 km s-1. Apparently, the fast rotators are relatively more abundant in NGC 1904 and M13 than in NGC 7078. No fast rotators have been identified in NGC 2808 and NGC 6093. All the stars hotter than Teff ~ 11,500 K have projected rotational velocities of v sin i < 12 km s-1, but less than 20% have v sin i < 2 km s-1. The connection between photometric gaps in the HB and the change in the projected rotational velocities is not confirmed by the new data. However, our data are consistent with a relation between this discontinuity and the HB jump. We discuss a number of possibilities for the origin of the stellar rotation distribution along the HB. We conclude that none of them can yet provide a satisfactory explanation of the observations.

Evolutionary Oddities in Old Disk Population Clusters

With a luminosity zero point fixed by the kinematics of old disk superclusters (HR 1614, t = 6 Gyr, [Fe/H] = +0.1 dex) and groups (Arcturus, t = 14 Gyr, [Fe/H] = -0.65 dex), the luminosities and colors of evolved old disk stars, especially red horizontal branch (RHB), early asymptotic branch [AGB(1)], thermally pulsing asymptotic giant branch [AGB(2)], and sdOB stars in old disk clusters (NGC 6791, 47 Tuc, M71, M67, Mel 66, NGC 2420, NGC 2204, and NGC 2443) are discussed. (1) The RHB stars in the old disk all have MV = +0.7 ± 0.1 (MK = -1.3 ± 0.1) mag. (2) Large-amplitude red variables (LARVs) with quasi-stable periods and light curves are old disk stars on AGB(2). (3) AGB(1) objects include CH stars and semiregular (SRa) variables. (4) The populous and overabundant cluster NGC 6791 may be the only disk cluster with sdOB stars, populating the lower portion of the bifurcated extended horizontal branch that is usual in most blue tailed and high-density halo clusters. (5) Post–red giant branch (RGB) stars in old disk clusters show a B - V (b - y) defect when compared with RGB stars, possibly because of a change in the character of the atmospheres. (6) If the bulk of the LARVs are pulsating in the fundamental mode, R Vir (P = 145 days) is possibly a first-overtone pulsator. (7) The overabundant old disk clusters are within the solar circle, with Liller 1 being at the Galactic center. (8) Several probable RHB stars at the south Galactic pole are identified. (9) The period-age relation, combined with the known spatial distribution of Galactic LARVs, leads to a relation between age and scale height of distribution that monotonically increases with age, leaving no obvious reason for a bifurcation of the population.

The Nature of Faint Blue Stars in the Halo. II

view Abstract Citations (375) References (108) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Nature of Faint Blue Stars in the Halo. II Greenstein, Jesse L. ; Sargent, Anneila I. Abstract Spectra and colors of 189 hot (FB) stars selected colorimetrically and mostly within 30 of the galactic poles are analyzed quantitatively for surface gravity and effective temperatures. Palomar spectra give line intensities and Balmer line profiles, found in the Appendix tables. Using a network of model atmospheres, the g and 0 so determined give directly the light-to-mass ratio, and eventually the luminosities. The high-latitude hot stars cover a range of 106 in luminosity, and are classifiable into various groups. Spectroscopically normal B stars make up 26 percent of the sample; they rotate and have nearly normal line spectra. Interpreted as Population I stars, on the mass-luminosity relation, they have relatively high luminosities and large i-coordinates. For some, the nuclear lifetimes at present luminosities are less than transit times from the plane. Their high velocities present a problem for galactic kinematics. A few are slightly helium rich, while others are highly evolved Population II stars, from details of the spectroscopy. Some of the Population I stars may be in not-understood, short- lived evolutionary phases of high luminosity and low mass. The (g, 0)-relation can be transformed into luminosity-temperature diagrams if masses are known. Many fall along a relation log g = 2.35, have common properties of halo stars, and from 9500 to 40,000 K. The light-to-mass ratio for these is 68 (in solar units); most HB and sdB stars have weak metal and weak helium lines, i.e., are halo stars. We call this the extended horizontal branch (EHB). Quantitative classification gives 17 percent of the FB stars as horizontal-branch B and early A (HB) stars, and 16 percent subdwarf B (sdB) stars. There are 16 percent subdwarf 0 (sdO) stars, including planetary nuclei; these have strong, or very strong, helium lines. Assuming a constant mass, 0.66 rn0, the observed L/m gives Mb = +0.5. The EHB for field stars fits the globular cluster HB, and extends it to high Teff. The bolometric correction produces the change of Mv with color. The hot white dwarfs are 11 percent of the FB stars and appear as a bridge between the sdO stars and the ordinary DA white dwarfs. Composite stars are unresolved, noninteracting binaries with strange UB V colors; they require a faint Mv for the primary (e.g., sdO or sdB), and an unevolved G companion. The helium deficiency and the evolutionary problems are sharpest for the EHB stars. In a few, slightly stronger helium lines are accompanied by lines of peculiar elements (S iii, P ii) seen at high dispersion. While the L/m ratio is closely the same for sdO as for sdB and HB, the He lines become strong. Evolutionary tracks avoid the region of the hot EHB stars; the hottest sdO stars approach the helium main sequence. The low surface helium is almost certainly not cosmological in origin. Gravitational diffusion in a nonrotating star in the absence of convection is plausible; in the sdO and the peculiar sdB stars convection and helium flashes may have occurred. Numerous radial velocities were measured. Where possible, proper motions, luminosities, and radial-velocity dispersions were used for space motions. The Population I normal stars have abnormally high space motions, not greatly different from Population II stars. The luminosities derived from spectroscopy are consistent with those from peculiar motions. Subject headings: faint blue stars - high-velocity stars - horizontal-branch stars - subdwarfs - stellar evolution Publication: The Astrophysical Journal Supplement Series Pub Date: November 1974 DOI: 10.1086/190315 Bibcode: 1974ApJS...28..157G full text sources ADS | data products SIMBAD (209)