Michael J. Harris

Oxygen isotopic abundances in evolved stars. III. 26 carbon stars

The present O-isotope ratio measurements conducted for 21 ordinary N-type C stars and five C-13-rich J-type stars show the former to closely resemble those of MS and S stars, which is far higher than expected, while the latter ratios, together with N abundances supplied by Lambert et al. (1986), preclude the previously proposed envelope-burning hypothesis. Evidence is noted for a positive correlation of the O-16/O-17 ratio with the neutron exposure parameter required for an explanation of abundances of the s-process elements in the ordinary N-type stars. 45 references.

Oxygen isotopic abundances in evolved stars. IV. Five K giants

Oxygen isotopic ratios have been measured in the atmospheres of Alpha UMa, Beta Gem, Beta UMi, Alpha Ari, and Alpha Ser. It is shown that some of the stars in this and previous samples have probably experienced the helium core flash, and that the O-16/O-17 ratios tend to confirm that extramixing of CNO-cycled material into the stars envelopes has occurred as a result of the flash. The oxygen isotopic ratios are much smaller than the anomalously high ratios seen in stars in the later thermal-pulsing state of evolution. Hence, the anomalous ratios cannot be explained by the helium core flash. It is proposed that they are due to the addition by the third dredge-up mechanism of helium-burned material which is devoid of both O-17 and O-18 and therefore dilutes the O-17 and O-18 abundances acquired during earlier evolutionary stages. This process may also explain the oxygen isotope ratios seen in barium stars, if these are formed by mass transfer from a thermally pulsing binary companion. 57 references.

Oxygen isotopic abundances in evolved stars. II. Eight MS and S stars

Oxygen isotope ratios have been measured in five MS stars, two S stars, and the slightly s-process--rich M giant 30 Her. Both the /sup 16/O//sup 17/O and /sup 16/O//sup 18/O ratios are found to be much larger than expected from stellar evolution theories. Both /sup 16/O//sup 17/O and /sup 16/O//sup 18/O are found to correlate well with the abundance of /sup 12/C and with the degree of neutron exposure necessary to explain the s-process element abundances. These correlations cannot be explained by current theoretical models of AGB stars, nor by simple modifications of these models. Explosive nucleosynthesis in the envelope at extremely high temperatures during the helium core flash is tentatively proposed to explain them.

A Search for Hard-Spectrum Gamma-Ray Bursts Using SMM

We have searched over 9 yr of data from the Solar Maximum Mission (SMM) Gamma Ray Spectrometer for gamma-ray bursts (GRBs) with abnormally hard spectra that may have been missed in previous searches in the 0.35-0.8 MeV energy band. For this purpose, we searched the higher 0.8-10 MeV band. We have found only two new GRBs in this search, compared to 177 GRBs found in the 0.35-0.8 MeV band. We have made a careful examination of the threshold conditions for identifying the 0.8-10 MeV events, which confirms that there is no undiscovered population of hard-spectrum GRBs that have escaped detection by SMM, BATSE, and other instruments triggered by low-energy γ-ray emission. This conclusion does not apply to GRBs with duration less than 2 s, nor to those that are much fainter than the general population. Our results constrain models of GRBs that require or allow the peak in energy emission to be above 1 MeV.

Oxygen isotopic abundances in evolved stars. I: Six barium stars

Mesures des rapports isotopiques 16 O/ 17 O 16 O/ 18 O pour quatre etoiles Ba II: HD 121447, HD 101013, HD 178717 et HR 774 et pour deux etoiles Ba peu rigoureuses: ο Vir et 16 Ser

SMM observations of gamma-ray transients. 3: A search for a broadened, redshifted positron annihilation line from the direction of the Galactic center

We have searched for 1980-1988 Solar Maximum Mission gamma-ray spectrometer data for transient emission on timescales from hours to approximately 12 days of broad gamma-ray lines at energies approximately 400 keV, which were reported by the High Energy Astronomy Observatory (HEAO) 1 and SIGMA experiments from two sources lying toward the Galactic center. The lines have been interpreted as the product of the annihilation of positrons in pair plasmas surrounding the black hole candidate 1E 1740.7-2942 and the X-ray binary 1H 1822-371. Our results from a combined exposure of approximately 1.5 x 10(exp 7)s provide no convincing evidence for transient emission of this line on any timescale between approximately 9 hr and approximately 1 yr. Our 3 sigma upper limits on the line flux during approximately 12 day intervals are characteristically 4.8 x 10(exp -3) photon/sq cm/s, while for approximately 1 day intervals our 3 sigma upper limits are characteristically 4.9 x 10(exp -3) photon/sq cm/s. These results imply a duty cycle of less than 1.3% for the transient line measured from 1H 1822-371 during a approximately 3 week interval in 1977 by HEAO 1, and a duty cycle of less than or = 0.8% for the transient line detected in 1990 and 1992 from 1E 1740.7-2942 on approximately 1 day timescales by SIGMA.

A search for the 2.223 MeV neutron capture gamma-ray line from the directions of Cygnus and the Galactic center

Free neutrons produced by nuclear reactions in high-temperature plasmas may undergo proton capture in ambient material, giving rise to gamma-ray emisison in a line at 2.223 MeV. Plausible settings for this process include accretion disks around black holes, material accreting onto neutron stars, and the binary companions of TeV gamma-ray sources. Data accumulated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) between 1980 and 1989 have been searched for evidence of this line, during periods when the black hole candidates Cygnus X-1 and the Galactic center were in transit across the GRS aperture. During these periods the neutron-star X-ray binaries Cygnus X-3 and Scorpius X-1 were also in the respective GRS fields of view. A 3-sigma upper limit of 0.0001/sq cm s has been placed on the steady emission in the 2.223 MeV line from the Galactic center and Scorpius X-1. Upper limits in the range 0.00012-0.00022 gamma/sq cm s have been set for Cygnus X-1 according to different models of the origin of the emission. The 3-sigma upper limit to the phase-averaged steady emission from Cygnus X-3 was found to be 0.00012 gamma/sq cm s.

Spatial and temporal variability of the gamma radiation from Earth's atmosphere during a solar cycle

[1] The Solar Maximum Mission satellite’s Gamma Ray Spectrometer spent much of its 1980–1989 mission pointed at Earth, accumulating spectra of atmospheric albedo g-rays. Its 28� orbit ensured that a range of geomagnetic latitudes was sampled. We measured the variation with time and cutoff rigidity of some key g-ray lines which are diagnostic of the intensity of the Galactic cosmic radiation penetrating the geomagnetic cutoff and of the secondary neutrons produced in the atmosphere. We found that the intensities of nuclear lines at 1.6 MeV, 2.3 MeVand 4.4 MeV varied inversely with solar activity in cycles 21–22 as expected from the theory of solar modulation of cosmic rays. They were found to be strongly anticorrelated with cutoff rigidity, as expected from the theory of the cutoff, falling by a factor � 3.6 between the lowest ( 13 GV) rigidities sampled. The solar cycle modulation was particularly marked at the lowest rigidities, reaching an amplitude of16%.Theratios oftheintensities ofthelinesproduced bynuclearde-excitation (1.6 MeV, 2.3 MeV) and those from nuclear spallation (4.4 MeV) did not vary with either solar activity or cutoff rigidity, indicating that the shape of the secondary neutron spectrum in the atmosphere above 5 MeV was approximately constant over the times and regions sampled. If it is approximated by a power law in energy, we derive constraints on the absolute value of the power law index �� 1.15–� 1.45 and better constraints on its variability, � 5%overasolarcycle,and � 6%overSMM’srangeofsampledcutoffrigidities. We also measured the intensity of the electron-positron annihilation line at 0.511 MeV. This line also varies with the solar cycle, but its variation with cutoff rigidity is weaker than that of the nuclear lines, falling by a factor 2 (rather than 3.6) over SMM’s range of sampled cutoff rigidities. This can be understood in terms of the energy dependences of the cross sections for positron production and for the hadronic interactions which produce secondary neutrons. INDEX TERMS: 7554 Solar Physics, Astrophysics, and Astronomy: X rays, gamma rays, and neutrinos; 2104 Interplanetary Physics: Cosmic rays; 7536 Solar Physics, Astrophysics, and Astronomy: Solar activity cycle (2162); 7837 Space Plasma Physics: Neutral particles; KEYWORDS: Earth atmosphere neutrons, X rays, gamma rays, solar activity cycle, energetic particles

ON THE EVOLUTIONARY STATUS OF MU LEONIS.

The paper presents the abundances of carbon, nitrogen, and oxygen and the carbon and oxygen isotopic ratios measured in the CN-strong giant star Mu Leo from CO and CN lines derived from a new 2-2.5 spectrum and published equivalent widths of C2 and forbidden O I lines. The relative abundances of C-12, N-14, and O-16 and the C-12/C-13 ratio suggest that Mu Leonis has undergone the first red-giant dredge-up. The CNO abundances suggest that Mu Leo is probably a metal-rich red giant that has experienced the first dredge-up.

Search for variable line emission from galactic and extragalactic sources in the 1981–1989 SMM data

Transient line features in the energy range from 400 keV to ≳1 MeV have been reported from several sources over the past two decades. These lines have been explained in terms of electron‐positron annihilation occurring close to black holes (Cygnus X‐1 and 1E 1740.7–2942) or neutron stars (the Crab). We searched in the data obtained by the SMM Gamma Ray Spectrometer during 1981–1989 for evidence of these lines, using the method of Harris et al. (1993). We have not found any convincing evidence of the occurrence of any transient line. We present upper limits on transient fluxes from these sources in the broad continuum 0.6–7 MeV energy band, and in the following previously‐reported lines; narrow lines from the Crab in the range 400–600 keV on time‐scales ∼1 d; a broad 1 MeV line from the Galactic Center region and Cyg X‐1, on time‐scales ∼12 d; and a broadened, redshifted 511 keV line from the Galactic Center region, on time‐scales ∼1 d and longer.