High Helium Abundance Research Articles

THE CHEMICAL COMPOSITION OF NGC 288.

ABSTRACT Analysis of high-resolution spectra of stars in NGC 288 confirm a high overall metal abundance for this cluster, [Fe/H] = -1.0 ± 0.2, as well as an excess of oxygen relative to iron, [O/Fe] = +0.4. A deficiency of ZCNO or oxygen cannot explain the anomalous blue horizontal branch of this cluster, nor can excessive mass loss on the upper giant branch. A high helium abundance or rotation-induced mixing remain as possible alternatives.

The ages and compositions of old clusters

From published color-magnitude diagrams of 23 globular and open clusters we have measured the mean color index or absolute magnitude at various points along the principal sequences. By relating these quantities to the analogous parameters derived from theoretical isochrones, it is possible to solve simultaneously for age, heavy-element abundance, and helium abundance. This analysis leads us to conclude that there is no correlation of globular cluster age with composition and that the oldest open clusters are about one-half the age of the globular clusters. Although the substantial observational and theoretical uncertainties prevent a conclusive statement, the evidence suggests that the open clusters have a higher helium abundance than the globular clusters, that the globulars have an age of about 16.6 +- 0.5 Gyr, and that the metal-rich clusters 47 Tuc and M71 have (Fe/H)approx.-0.9. Changes in the metallicity scale or mixing length could substantially alter these values. Nevertheless, globular cluster ages as young as 10 Gyr would require large corrections and would yield cluster properties inconsistent with observation. If the Hubble constant is as large as 100 km s/sup -1/ Mpc/sup -1/, then fundamental changes in stellar evolution theory are required.

THE EFFECTS OF HELIUM ENRICHMENT IN CEPHEID ATMOSPHERES.

ABSTRACT Model atmospheres of Cepheids have been computed to determine if high helium abundance models could be distinguished from models with normal composition. Significant effects were found in the equivalent widths of metal lines and on broadband colors. However, both the colors and metal-line strengths depend critically on the effective temperature: therefore, the effective temperature must be accurately known for Cepheids to determine the helium abundance from broadband colors and metal-line strengths.

Helium abundance in ejecta from CP Lacertae and V446 Herculis

view Abstract Citations (24) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Helium abundance in ejecta from CP Lacertae and V446 Herculis. Ferland, G. J. Abstract Published emission-line intensities are used in this paper to measure the helium abundances in ejecta from the novae CP Lacertae and V446 Herculis. Recombination lines of hydrogen and helium indicate the helium abundance directly if corrections for unseen stages of ionization and recombination-line self-absorption are unimportant. These corrections are minimized by considering only spectra obtained during the late nebular phase of the outburst, when models show that ionization corrections are small and when self-absorption affects only certain lines. The physical conditions in the gas need not be accurately measured since ratios of recombination lines are only slightly affected by the density-temperature structure of the ejecta. Ejecta from the fast nova CP Lac (1936) have a nearly cosmic helium abundance (He/H = 0.11 + 0.02), but ejecta from V446 Her (1960) are enhanced (He/H = 0.19 + 0.03). Previous studies have found high (He/H = 0.2) abundances of helium in ejecta from HR Del, V544 Her, RR Pic, and RR Tel. Three novae are now known to have nearly cosmic helium abundances (CP Lac, V1500 Cyg, and DQ Her). No evidence for extreme overabundances of helium (He/H > 0.3) is found. The range in helium abundances in nova ejecta is similar to the range in planetary nebulae, but the sample of novae seems to be more clustered toward higher helium abundances. Cosmic or nearly cosmic helium abundances are probably an indication that the majority of the matter ejected in the nova outburst originated in the cool companion rather than in the white dwarf. There is no tight correlation between helium abundances and other properties of the nova outburst, but high helium abundances tend to occur in slower novae. Subject headings: stars: abundances - stars: individual - stars: novae Publication: The Astrophysical Journal Pub Date: August 1979 DOI: 10.1086/157243 Bibcode: 1979ApJ...231..781F Keywords: Abundance; Helium; Novae; Stellar Mass Ejection; Balmer Series; Chemical Composition; Emission Spectra; Hydrogen Recombinations; Photoionization; Self Absorption; Astrophysics; Helium:Novae; Nova V446 Herculis; Nova Herculis 1960; Nova Lacertae 1936 full text sources ADS | data products SIMBAD (9)

ABUNDANCE ANOMALIES IN THE HELIX NEBULA.

ABSTRACT High-spatial-resolution observations of the Helix nebula (NGC 7293) are presented. The derived helium abundance is abnormally large, NHe/NH at least about 0.17, and the oxygen abundance may be low. The N/O ratio is of order unity and therefore consistent with ratios observed in other planetaries with high helium abundance. In addition, the intensity of the forbidden Ne III line at 3869 A is correlated with that of the forbidden O II line at 3227 A, an effect also observed in the Ring and Dumbbell nebulae.

A Discussion on astronomy in the ultraviolet - The continuum energy distribution of the Wolf-Rayet star in y 2 Velorum

Near-ultraviolet S59 spectra of Ɣ 2 Vel (WC8 + 0 9 I) and £ Ori (09.5 I) are studied. The luminosity ratio O/WC of the binary components of Ɣ 2 Vel equals 1 at 2100 and 2500 Å. In the visible this ratio has been found to be 3.6 (Conti & Smith 1972). The empirical flux of the two components, expressed in monochromatic magnitudes, is compared with model energy distributions. An explanation for the high nearultraviolet flux of the WC8 star is suggested in terms of a higher helium abundance.

High-Helium Abundances in Two Planetary Nebulae

High-Helium Abundances in Two Planetary Nebulae

Main-Sequence Pulsation in Open Clusters

view Abstract Citations (45) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Main-Sequence Pulsation in Open Clusters Breger, Michel Abstract Studies of short-period pulsation in open clusters show that pulsation occurs in clusters of all ages provided stars are present in the instability strip (A2 V-F0 V). This includes -sequence stars. The value of the pulsation constant Q is similar in the clusters studied, i.e., (Q) = 0.026 days. This points toward pulsation in the first overtone. About 30 percent of the main-sequence cluster stars inside the instability strip show detectable variability. Hyades-type stars may show a particularly high incidence of pulsation, possibly connected to a high helium abundance. Statistically extremely significant correlations between slow rotation, metallicity, and pulsational stability are established for different clusters. The observed relations can be explained by a diffusion hypothesis: (i) Slowly rotating A dwarfs usually possess extreme stability in some subsurface zones which causes element segregation. This leads to metallicism as well as increased pulsational stability because of the low helium content iii the critical He ii ionization zone. (ii) Low rotation, however, is not a sufficient condition to cause metallicism. Some examples are given. These slow rotators may, and often do, pulsate. (ili) Dwarfs with rotational velocities greater than 100 km 1 may pulsate and do not become Am stars. These stars, however, are so close to stability that less than 50 percent have detectable variability. Publication: The Astrophysical Journal Pub Date: September 1972 DOI: 10.1086/151642 Bibcode: 1972ApJ...176..373B full text sources ADS | data products SIMBAD (23)

The Colour-Magnitude Diagram of the Globular Cluster NGC 7099

UBV photometry of the globular cluster NGC 7099 is reported. The ultraviolet excess of δ(U–B) = 0·33 mag shows that NGC 7099 is an extremely metal-poor cluster. The colour–magnitude diagram shows a strong blue population of the horizontal branch, with very few RR Lyrae variables and stars on the red side of the instability region. The giant branch shows very little spread in colour at a given magnitude. The metallicity parameter ∆V = 3·0 mag is consistent with the low metal abundance. A reddening of E(B–V) = 0·06 mag is derived from the two-colour diagram for field stars. The relative paucity of red giants in NGC 7099 probably indicates a high helium abundance.

Hydrogen and helium velocities in the solar wind

Examination of separately determined helium and hydrogen bulk speeds in the solar wind show these to be equal, both on time scales of 30 min and 3 min. Observations of two interplanetary shocks and 12 discontinuities show the changes in bulk speed across them to take place simultaneously for the two species. Observations made at times of high helium abundance following an interplanetary shock, and at times of observation of colliding streams in the plasma, confirm the conclusion that, if bulk speed differences between species occur, they do so very rarely.

The Blue Horizontal-Branch Stars of M15, M92, M13, and M3

view Abstract Citations (94) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Blue Horizontal-Branch Stars of M15, M92, M13, and M3 Newell, E. B. Abstract The physical characteristics of blue horizontal-branch stars in the northern globular clusters MIS, M92, Ni 113, and M3 are investigated on the basis of Sandage's (1969) UBV observations. These data are interpreted by means of the empideal correlations between broad-band colors and atmospheuc parameters found for the blue horizontal-branch stars of NGC 6397 and w Cen by Newell, Rodgers, and Searle. Use is made also of Sargent's low-dispersion spectra of several stars in NIlS and M92. A discussion of the results for the four northern clusters, together with the results pi eviously obtained for NGC 6397 and w Cen, leads to four principal conclusions: i) The temperature-color relations defined by blue horizontal-branch stars with T < 10000 K differ from those of the corresponding Population I stars. This effect is caused by differences in metal abundance and surface gravity. ii) Blue horizontal-branch stars have mass-luminosity ra tios of log = -2.0 t 0.1. iii) The evolution of stars described by Iben and Faulkner's models of high helium abundance (F 0.35) and a double energy source cannot satisfactorily ezplain the blue horizontal branch in all six clusters. iv) Helium is underabundant by a factor of at least 10 on the surface of star S-iS in MiS. Publication: The Astrophysical Journal Pub Date: February 1970 DOI: 10.1086/150322 Bibcode: 1970ApJ...159..443N full text sources ADS | data products SIMBAD (19)