Michael M. Dworetsky

Neon abundances in normal late-B and mercury–manganese stars

ABSTRA C T We make new non-local thermodynamic equilibrium calculations to deduce the abundances of neon from visible-region echelle spectra of selected Ne i lines in seven normal stars and 20 HgMn stars. We find that the best strong blend-free Ne line that can be used at the lower end of the effective temperature Teff range is l6402, although several other potentially useful Ne i lines are found in the red region of the spectra of these stars. The mean neon abundance in the normal starsOlog Aa 8:10U is in excellent agreement with the standard abundance of neon (8.08). However, in HgMn stars neon is almost universally underabundant, ranging from marginal deficits of 0.1‐0.3 dex to underabundances of an order of magnitude or more. In many cases, the lines are so weak that only upper limits can be established. The most extreme example found is y Her with an underabundance of at least 1.5 dex. These underabundances are qualitatively expected from radiative acceleration calculations, which show that Ne has a very small radiative acceleration in the photosphere, and that it is expected to undergo gravitational settling if the mixing processes are sufficiently weak and there is no strong stellar wind. According to theoretical predictions, the low Ne abundances place an important constraint on the intensity of such stellar winds, which must be less than 10 214 M( yr 21 if they are non-turbulent.

Mars, Jupiter, Saturn, and Uranus: 3.3-mm brightness temperatures and a search for variations with time or phase angle

Abstract Extensive 3.3-mm observations yield disk-average brightness temperatures for Mars, Jupiter, Saturn, and Uranus of 180° ± 18°, 153° ± 1.25° ± 13°, and 105° ± 13° K (1 σ ), respectively. The values for the first three planets are corrected to heliocentric distances of 1.524, 5.203, and 9.540 a.u., respectively. Any variation of the temperature of Mars with phase angle is less than a few percent; this upper limit is consistent with an expected variation of ⪅3%. The measurements from mid-1965 to November 1969 contain no confirmed variations in the brightness temperatures of Jupiter and Saturn larger than ≈4 and ≈7%, respectively.

The Spectrum of Pt II in Chemically Peculiar Stars

We have obtained a reliable estimate of the platinum abundance in HgMn stars for the first time. It is as high as log A(Pt) = 6, on the scale log A(H) = 12; the Pt abundance often matches the remarkably high abundances of Hg already known in these stars. Our results are based on calculations of theoretical oscillator strengths which were applied to high resolution spectra taken with the International Ultraviolet Explorer satellite. We have used the method of spectrum synthesis to obtain the abundances. Astrophysical absolute oscillator strengths for the optical Pt II lines are derived based on the spectrum of the star χ Lupi.

Xenon in mercury–manganese stars

Previous studies of elemental abundances in mercury-manganese (HgMn) stars have occasionally reported the presence of lines of the ionized rare noble gas Xe II, especially in a few of the hottest stars with T eff ∼ 13 000-15 000 K. A new study of this element has been undertaken using observations from Lick Observatorys Hamilton Echelle Spectrograph. In this work, the spectrum synthesis program UCLSYN has been used to undertake abundance analysis assuming local thermodynamic equilibrium. We find that in the Smith & Dworetsky sample of HgMn stars, Xe is vastly overabundant in 21 of 22 HgMn stars studied, by factors of 3.1-4.8 dex. There does not appear to be a significant correlation of Xe abundance with T eff . A comparison sample of normal late B stars shows no sign of Xe II lines that could be detected, consistent with the expected weakness of lines at normal abundance. The main reason for the previous lack of widespread detection in HgMn stars is probably due to the strongest lines being at longer wavelengths than the photographic blue. The lines used in this work were λ4603.03, λ4844.33 and λ5292.22.

Isotopic Anomalies of Platinum in the Mercury-Manganese Star HR 7775

High-resolution spectra have been obtained for the regions of the five strongest optical lines of Pt II in the spectrum of the cool HgMn star HR 7775, which is one of the sharpest-lined HgMn stars known. Model lines have been constructed from the isotopic and hyperfine structure laboratory analysis of Engleman. Abundances of the individual isotopes have been determined from spectrum synthesis. The total abundance of Pt is 4.46 dex greater than the adopted solar abundance. The isotopic composition is clearly nonterrestrial, with a pronounced relative enhancement of the heaviest isotope,198Pt, and deficiencies of isotopes lighter than 196Pt. The pattern of isotopic composition does not follow the widely assumed fractionation formalism; the lighter isotopes are far more deficient than a single-parameter fractionation pattern would predict.

RADIO SOURCES: 3.3-mm FLUX AND VARIABILITY MEASUREMENTS.

Abstract : Graphical and tabular summaries of 3.3-mm (90-GHz) flux measurements of 35 discrete galactic and extragalactic sources are presented, including results of extensive monitoring of nine sources. Variability at 3.3-mm is certain for NGC 1068, NGC 1275, 3C120, 3C273, 3C279, VRO 42.22.01, and 3C454.3; repeated outbursts with time scales ranging from a week to several months have been observed for all of these except NGC 1068 and 3C454.3. (Author)

Radiative accelerations on Ne in the atmospheres of late B stars

Radiative accelerations on Ne are calculated for the atmospheres of main-sequence stars with 11 000 Teff 15 000 K. This range corresponds to that of the mercury-manganese (HgMn) stars. The calculations take into account neon fine structure as well as shadowing of neon lines using the entire Kurucz line list, bound‐bound, bound‐free and free‐free opacity of H, He and C as well as some non-LTE effects. Non-LTE effects are found to modify the radiative acceleration by a factor of the order of 10 2 in the outer atmosphere and are crucial for dm < 10 −3 gc m −2 . The dependence of the radiative accelerations on the Ne abundance, effective temperature and gravity is studied. Radiative accelerations are found to be well below the gravitational acceleration over the entire range of Teff and it is predicted that in stable atmospheres devoid of disturbing motions, Ne should sink and be observed as under-abundant. This agrees with recent observations of low Ne abundances in HgMn stars.

SAGITTARIUS A: OBSERVATIONS OF THE GALACTIC CENTER AT 3.3 mm.

Abstract : Observations at 3.3 mm of the radio source Sagittarius A have been used to obtain a flux estimate. The value is approximately half that predicted by extrapolation of results at longer wavelengths. The mean half-intensity diameter of Sagittarius A appears to be smaller supporting the suggestion of Maxwell and Taylor that the apparent size of Sagittarius A decreases with increasing radio frequency. (Author)

Helium abundances in HgMn and normal stars

The parameter-free model of diffusion in the atmospheres of HgMn stars (Michaud 1986; Michaud et al 1979) predicts that helium should sink below the He II ionization zone in order that diffusion of other elements may take place, and that all HgMn stars should have deficits of helium in their photospheres, with a minimum deficit of 0.3 dex. In this study, the Smith & Dworetsky (1993) sample of HgMn stars and normal comparison stars is examined, and the helium abundances determined by spectrum synthesis using echelle spectra taken at Lick Observatory and the AAT. The prediction is confirmed; all HgMn stars are deficient in He by as much as 1.5 dex. Also, two HgMn stars, HR7361 and HR7664, show clear evidence of helium stratification.

A search for ultraviolet objects

The observations of the sky-survey telescope in the TD-1A satellite are being searched for ultraviolet objects. The first results, covering about 10 % of the sky, are presented and rough limits on space density and luminosity are derived. It is concluded that the frequency population of hot objects lying a few magnitudes below the main sequence is probably much greater than had been thought hitherto. Some examples of the observed ultraviolet spectra are presented.