Helium-rich Stars Research Articles

On the frequency of planetary nebula nuclei powered by helium burning and on the frequency of white dwarfs with hydrogen-deficient atmospheres

The evolutionary behavior of a model central star of a planetary nebula is examined as a function of the phase in its nuclear burning cycle when its progenitor leaves the asymptotic giant branch for the first time. Models may be assigned to one of six distinct groups, and estimates may be made of the probability that a real star will fall into a particular group. In particular, it is suggested that approximately 25% of all central stars of planetary nebulae will fall into groups whose members are expected to become hydrogen-deficient white dwarfs. This suggestion is not in disagreement with the observations. It is argued that winds of at least two types are responsible for this result: a low-velocity or slow wind (or ordinary or super strength) which operates when, and if, the central star returns to the asymptotic giant branch after experiencing a final helium shell flash; and a high-velocity or fast wind which operates when the central star is at high enough surface temperatures that emitted photons will excite nebular emission. It is also inferred that roughly half of all bright central stars of planetary nebulae are burning helium rather than hydrogen. Finally, an attempt is made tomore » assess the relevance of extant models for understanding exotic planetary nebulae such as A30 and A78, helium-rich stars of the R Coronae Borealis variety, and the chameleon star FG Sagittae.« less

A survey of spectral morphology and rotational velocities among the helium-rich stars

A spectroscopically homogeneous listing of all known members of the helium-rich B star category is presented together with uniform estimates of their rotational velocities. Equivalent widths of hydrogen, helium, and metal lines in the spectra of the helium-rich and comparison stars are given. The distribution of rotational velocities is found to be indistinguishable from that of normal early B dwarfs, except for a possible excess of rapid rotators. The existence of metal abundance anomalies in the helium-rich spectra is critically discussed, as is the identification of helium-rich stars as massive Population I objects with normal main sequence cores and surface helium enhancements, or as low mass highly evolved objects with processed cores.

Iron and magnesium in the white dwarf GD 40 - A test of diffusion theory

An outstanding problem in interpreting the properties of white-dwarf stars is related to the understanding of the chemical abundances in their atmospheres. The hydrogen-rich white-dwarf stars have monoelemental atmospheres, with small quantities of helium and no heavier elements observed in most, possibly all, stars with visible H lines. The helium-rich stars are more complex, and the existence of metallic lines in many of their spectra is now well confirmed. The DB star GD 40 (Gr 384) is the hottest He-rich white-dwarf star to show metal lines, apart from the extremely hot objects, such as HZ 21 and HD 149499B. The present investigation is concerned with IUE spectra which have been obtained of the GD 40. It is found that the near-UV is marked by strong Mg II lines along with broad blends of Fe II lines. An analysis is conducted of the abundances of Fe and Mg at the surface of GD 40 using a limited spectrum-synthesis technique with the measured equivalent widths of the best Fe II features.

PERIODIC SPECTRUM VARIATIONS IN HELIUM-RICH STARS.

ABSTRACT Spectroscopic observations of four helium-rich stars are presented. In HD 37776, antiphase variations of Si III and He I have been found, which represent another point of similarity to the Ap phenomenon. The remarkable H-alpha emission variations in Sigma Ori E are illustrated with uniform phase coverage, and strict periodicity over a five-year interval is shown. A radial-velocity study of HD 64740 shows constancy to within the accuracy of the observations. Finally, Delta Ori B is confirmed as a helium-rich star.

Open discussion

Conti: One of the main conclusions of the Wolf-Rayet symposium in Buenos Aires was that Wolf-Rayet stars are evolutionary products of massive objects. Some questions:–Do hot helium-rich stars, that are not Wolf-Rayet stars, exist?–What about the stability of helium rich stars of large mass? We know a helium rich star of ∼40 MO. Has the stability something to do with the wind?–Ring nebulae and bubbles : this seems to be a much more common phenomenon than we thought of some years age.–What is the origin of the subtypes? This is important to find a possible matching of scenarios to subtypes.

An Observing Campaign for Systematic Photoelectric Observations of Bright Be Stars

Most of our present-day knowledge of the photometric behaviour of Be stars still comes from various surveys only. Little attention was paid to systematic studies of possible photometric variability of particular objects on different time scales. Nevertheless, just this kind of information is necessary if one wants to study the physical relationship and/or differences between Be stars and other types of B stars like Beta Cepheids, supergiants, helium-rich stars etc., and to test various models of the Be phenomenon. Long systematic photoelectric observations do exist only for several Be stars. They invariably indicate that the long-term variability - if present - is always the most pronounced one. In some cases, this long-term variability masks the periodic light variations occuring on shorter time scales (see, e.g., the case of CX Dra, Koubský et al 1980).

Blue Stragglers as Long-Lived Stars

The existence of blue stragglers in old open clusters with apparent mass more than twice the mass of the turnoff argues against simple binary mass transfer as the mechanism of their origin. The excess of blue stragglers to the red of the termination of the core hydrogen burning main sequence suggests that blue stragglers are not evolving normally. Stellar evolution models invoking mixing in an extended core region can account for the distribution of blue stragglers in the H-R diagram. Such models live longer, brightening and evolving further to the red before core hydrogen exhaustion than do normal stars. The distribution of blue stragglers in NGC 7789 is consistent with a range of mixed core mass fraction ~30–90 per cent and a narrow range in mass ~1.7–2.1 M⊙. Such evolution will result in a class of helium rich stars which have lived longer than normal and whose total mass exceeds the Chandrasekhar limit.

The surface chemistry of stars. III - The electric field of a chemically inhomogeneous star

view Abstract Citations (15) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The surface chemistry of stars. III - The electric field of a chemically inhomogeneous star Alcock, C. Abstract The distribution of the electric field and chemical composition inside a star in diffusive equilibrium are derived. Hydrogen floats on the surface of a helium-rich star; a boundary layer of thickness about one pressure scale height produces a smooth transition in the electric field. Electrically driven mass loss does not occur unless the hydrogen content is extremely small. The electric field is dynamically unimportant outside the star, unless the star has a thermally driven wind. Arguments are given that suggest that white stars do not have such winds. It is also shown that if the star has a modest magnetic field, and if it rotates, the exterior electric field is dominated by the quadrupole contribution of the rotating magnetosphere. These results are important for the understanding of the spectra of white dwarf stars. Publication: The Astrophysical Journal Pub Date: December 1980 DOI: 10.1086/158506 Bibcode: 1980ApJ...242..710A Keywords: Chemical Composition; Electric Fields; Magnetohydrodynamics; White Dwarf Stars; Abundance; B Stars; Hydrogen; Ionized Gases; Stellar Rotation; Astrophysics full text sources ADS |

Masses and radii of white-dwarf stars. III - Results for 110 hydrogen-rich and 28 helium-rich stars

view Abstract Citations (201) References (72) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Masses and radii of white-dwarf stars. III. Results for 110 hydrogen-rich and 28 helium-rich stars. Shipman, H. L. Abstract The basic method for calculating stellar radii is briefly described. It is claimed that systematic errors in the method are approximately 5% for the hydrogen-rich stars, taking into account a discussion of systematic errors in support of that statement. Procedures strictly relevant to the calculations for the H-rich stars are given and procedures for the He-rich stars are discussed. Selection effects are also examined. It is found that the mean radius of a sample of 110 hydrogen-rich stars with parallaxes and good photometry is 0.0127 solar radius. The mean radius of a sample of 28 He-rich stars is 0.0111 solar radius, which, considering the uncertainties, is not significantly different from that of the DA stars. Publication: The Astrophysical Journal Pub Date: February 1979 DOI: 10.1086/156841 Bibcode: 1979ApJ...228..240S Keywords: Helium; Hydrogen; Radii; Stellar Mass; White Dwarf Stars; Astronomical Photometry; Atmospheric Models; Dimensional Measurement; Error Analysis; H Lines; Parallax; Stellar Evolution; Stellar Spectra; Stellar Temperature; Astrophysics; Diameters:White Dwarfs; Masses:White Dwarfs full text sources ADS | data products SIMBAD (134) Related Materials (2) Part 1: 1972ApJ...177..723S Part 3: 1980ApJ...235..177S

Spectral types for early-type stars observed by SKYLAB

MK spectral types are presented for 246 early-type stars observed with the S-019 ultraviolet stellar astronomy experiment on Skylab. K-line types are also given where applicable, and various peculiar stars are identified. The peculiar stars include five silicon stars, a shell star, a helium-rich star, a silicon-strontium star, a chromium-europium star, and two marginal metallic-line stars.

The Distribution of Masses and Radii of White-Dwarf Stars

The status of determinations of white dwarf radii by model atmosphere methods is reviewed in this paper. Details will appear elsewhere (Shipman 1978). In brief, the results are that (i) the mean radius of a sample of 95 hydrogen-rich stars with parallaxes is 0.0131 R⊙; (ii) the mean radius of a sample of 13 helium-rich stars is 0.011 R⊙, indistinguishably different from the radius of the hydrogen-rich stars; and (iii) that the most serious limitation on our knowledge of the mean radius of white dwarfs is the influence of selection effects. An estimate of the selection effects indicates that the true mean white dwarf radius is near 0.011 R⊙.

THE HELIUM-VARIABLE STAR HD 184927.

ABSTRACT The spectrum of HD 184927 is found to vary from that of a normal B2 star to that of a pronounced helium-rich star, with a period of 9.48 days. Low-amplitude light variation is also present, most noticeably in the ultraviolet below the Balmer discontinuity. The helium-variable stars are shown to be intermediate in surface temperature between the hotter helium-rich stars and the cooler helium-weak stars. Within the class of helium variables, the hottest stars are helium-rich throughout their cycles; and intermediate objects vary between helium-rich and normal or helium-weak; and the coolest stars are always helium-weak. Key words: helium variables - helium-rich stars - peculiar stars

On the nature of sigma ORI E.

A period of 1.19 has been found in observations of the H..cap alpha.. emission intensity, He absorption-line strengths, magnitudes, and colors of the helium-rich star sigma Ori E. The H..cap alpha.. emission variation is a double wave with two unequal maxima separated by about one-half the period. The primary H..cap alpha.. emission and He absorption maxima coincide. They are followed about one-quarter period later by a sharp continuum-flux minimum, the amplitude of which increases with decreasing wavelength. The radial velocities are uninformative. Some characteristics of the variations are reminiscent of the oblique rotator model, while others suggest the presence of a very low-mass companion.

Recent Results Pertaining to the Helium-Rich Stars

The term helium-rich stars here refers to those stars with apparent spectral types near B2 ana helium lines which are enhanced, but not stronger than the hydrogen lines. The previously known helium-rich star HD 184927 has been found to be a spectrum variable; the equivalent widths of the helium lines changed by an average of 46% between two observations. HD 186205 has been newly recognized as a member of the helium-rich class. Further work toward understanding the rapidly variable Ha emission in the prototype helium-rich star, Sigma Orionis E, is in progress in co-operation with Dr J. E. Hesser at CTIO.

NEW HELIUM SPECTRUM VARIABLE AND A NEW HELIUM-RICH STAR.

ABSTRACT HD 184927, known previously as a helium-rich star, has been found to have a variable helium spectrum; the equivalent widths of five He I lines are larger by an average of 46% on a 1974 spectrogram than on one obtained with the same equipment in 1970. HD 186205 has been found to be a new, pronounced helium-rich star. Key words: helium-rich stars - helium spectrum variables

The evolution of the helium star sigma Orionis E

view Abstract Citations (6) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The evolution of the helium star Sigma Orionis E. Odell, A. P. Abstract The helium-rich star sigma Ori E is a member of the quintuple star sigma Ori. The visual binary AB has recently been studied by Heintz, and a solution to its orbit and a distance was determined. It is shown that this new distance to the system resolves the problem of too large a mass for the helium star, and allows helium-rich models to approximate the present star. The upper age to the star is 10 m.y., and with a mass of less than 5 solar masses the star must have been formed with a large helium abundance. It is unlikely that component E is the result of binary mass transfer. It is difficult to understand the star in terms of mass loss from a very massive star. The idea that an original single star broke up into five stars is attractive to the time scale, but the angular momentum of the present system is very large. Alternatively, the system might have resulted from the capture of E, an old star which has been completely mixed through its evolution. Publication: The Astrophysical Journal Pub Date: December 1974 DOI: 10.1086/153285 Bibcode: 1974ApJ...194..645O Keywords: Abundance; Astronomical Models; Helium; Stellar Evolution; Angular Momentum; Mass Transfer; Orion Constellation; Stellar Mass Ejection; Stellar Rotation; Astrophysics full text sources ADS | data products SIMBAD (2)

A New Phenomenon in the Spectrum of Sigma Orionis E

view Abstract Citations (77) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A New Phenomenon in the Spectrum of Sigma Orionis E Walborn, Nolan R. Abstract A broad, variable emission has been found at Ha in the spectrum of the helium-rich star cr Ori E. The feature is a Woff-Rayet-like band of maximum width near 1800 km 1, which fills in the deep Ha absorption and reaches maximum intensities of nearly 30 percent above the continuum on either side. The emission intensity undergoes large variations with a time scale of a few hours. Radial-velocity measures do not establish binary motion, although a low-mass companion cannot be excluded. The nature of the energetic circumstellar activity reported, and its relationship to the helium anomaly, remain to be determined. SnUect headings: circumstellar shells - early-type stars - spectrum variables Publication: The Astrophysical Journal Pub Date: July 1974 DOI: 10.1086/181558 Bibcode: 1974ApJ...191L..95W full text sources ADS | data products SIMBAD (7)

ON THE CONTINUOUS SPECTRUM OF HD 184927

ABSTRACT New spectrophotometric scans from x3390 to x5840 are presented for the helium-rich star HD 184927. An apparent effective temperature is determined by comparing the unreddened continuum with continua predicted by model atrnospheres. Both the scans and the effective temperature are found to be in discordance with previously published results. Key words: helium-rich stars - spectrophotometry - effective temperature

CONTINUUM OBSERVATIONS OF SOME HOT HELIUM-RICH STARS

ABSTRACT Measurements are presented of the continuous energy distributions of six hot, helium-rich stars. The data have been used to determine the size of the Balmer discontinuities in all pro. gram stars and a search has been made for evidence for the presence of the helium discontinuity at A3422. Key words: helium-rich stars - spectrophotometry - Balmer and helium discontinuities

THE HE-RICH STAR HD 184927.

ABSTRACT A preliminary analysis of RD 184927 yields a helium-to-hydrogen ratio of 2.0, Teff = 22,000' K, and log g = 3.38. A comparison is made with similar type stars. Key words: helium abundance - helium-rich star