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NASA NTRS1994

Abundances, planetary nebulae, and stellar evolution

Among Henry Norris Russell's many achievements were his contributions to solar and stellar spectroscopy, in particular, to an analysis of the chemical composition of the solar atmosphere. The question of composition differences between stars was hotly debated; some distinguished astronomers argued that all stars had the solar composition. Some early challenges to this doctrine are described. Determinations of chemical compositions of gaseous nebulae were much more difficult. If we observe the lines of a given chemical element in one ionization stage in a stellar spectrum, we can deduce readily the abundance of that element. No such luxury is available for a planetary or diffuse gaseous nebula. We must measure lines of as many ionization stages as we can. Furthermore, a nebula is an extended object. Often detailed spectroscopy is at hand only for narrow pencil columns taken through the image. Different observers use a variety of apertures. Fortunately it is possible to calculate theoretical spectra for any arbitrary cross section taken through a symmetrical model, so UV, optical, and IR observations all can be compared properly with a prediction. The value of high-resolution spectra obtained with instruments such as the Hamilton Echelle Spectrograph at Lick Observatory is emphasized. Improved fluxes for weak but important transitions are found. Close blends of lines of different ions can be resolved, and checks can be made on predictions of atomic parameters such as Einstein A-values and collision strengths. High spectral resolution data have been obtained and reduced for 22 planetary nebulae of varying size, structure, stellar population membership, dustiness, level of excitation, evolutionary status, and chemical compositions. The promise seems justified that with such extensive, high quality data, additional insights on nebular genesis and late states of stellar evolution can be found. The present survey is confined to nebulae of high surface brightness, but among these are found some engaging objects such as NGC 7027, IC 4997, and NGC 6572.

Aller, Lawrence H.↗
NASA NTRS1994

Abundance gradients in cooling flow clusters: Ginga Large Area Counters and Einstein Solid State Spectrometer spectra of A496, A1795, A2142, and A2199

We analyze the Ginga Large Area Counters (LAC) and Einstein Solid State Spectrometer (SSS) spectra of four cooling flow clusters, A496, A1795, A2142, and A2199, each of which shows firm evidence of a relatively cool component. The inclusion of such cool spectral components in joint fits of SSS and LAC data leads to somewhat higher global temperatures than are derived from the high-energy LAC data alone. We find little evidence of cool emission outside the SSS field of view. Metal abundances appear to be centrally enhanced in all four clusters, with varying degrees of model dependence and statistical significance: the evidence is statistically strongest for A496 and A2142, somewhat weaker for A2199 and weakest for A1795. We also explore the model dependence in the amount of cold, X-ray-absorbing matter discovered in these clusters by White et al.

White, Raymond E., III↗
NASA NTRS1994

Pre- and postperihelon abundances of gas and dust in comet Halley

Photometrically calibrated spectra of comet P/Halley (1986 III) were recorded between 1985 September 12-1986 June 10 using the Ohio State University Image Dissector Scanner on the Perkins 72 inch telescope at the Lowell Observatory. Column densities of CN, C3, CH, C2, and NH2 were calculated from measured fluxes in these spectra, and molecular scale lengths were deduced from the radial distribution of CN, C3, C2, and NH2. Production rates were computed using the new scale lengths and a Haser model analysis. Continuum emission at 4260 A was used to derive gas-to-dust ratios. The data indicate than comet Halley was approximately 2-5 times more abundant in gas and dust at postperihelion than preperihelion. On 1986 June 8 we observed the onset of a cometary ourburst which appeared very strong in dust production. The gas-to-dust ratios appeared to be subject to changes as a result of short-term outbursts but otherwise did not exhibit any systematic dependence on heliocentric distance. Reflectivity gradients of the continuum were also measured from the spectra. While most of the continua were red, blue continua were also observed which may be correlated with dust outbursts.

Womack, Maria↗
NASA NTRS1994

Measuring glass abundances in lunar agglutinates

The purpose of this study is to develop a standard method for measuring modal abundances of glass in single agglutinates in the lunar regolith. Not only does agglutinitic glass increase in single agglutinates as clasts of older agglutinates get incorporated into newer agglutinates with increasing maturity, but it is in this glassy phase that nanophase superparamagnetic Fe metal originates as a result of reduction reactions during the agglutination process. We report the results of two sets of independent measurements using two different methods to determine the proportion of glass in single agglutinates. We have used polished grain mounts (PGM) of five hand-picked single agglutinates from Apollo 16 soil 61181. The ISI Scanning Electron Microscope (SEM) fitted with a high-resolution Backscattered Electron (BSE) detector was used to collect high-contrast BSE images of the agglutinates. Several images were collected to represent each single agglutinate. The contrast, brightness, and focus were adjusted to optimize each image collected. Histograms of the grayscale range for all images produced four 'peaks' corresponding to epoxy, glass, crystalline phases, and metal grains. We analyzed every grid point for 12 elements with a less than 1 micron electron beam using a CAMECA SX-50 electron probe microanalyzer (EPM). If any analysis was not within about 10% of the stoichiometry of a known lunar mineral, we considered that point to be nonmineralic. Our results show that there is a remarkable correspondence in the glass percentages obtained by the two methods. The EPM method may overestimate glass because secondary fluorescence from dusty clasts in agglutinitic glass can give the appearance of nonmineralic targets. The BSE method may underestimate glass because the diversity of compositions of agglutinitic glass may not be contained under one grayscale 'peak' during image analysis.

Strait, M. M.↗
NASA NTRS1995

Hot bottom burning in asymptotic giant branch stars and its effect on oxygen isotopic abundances

A self-consistent calculation of asymptotic giant branch (AGB) evolution was carried out, including nucleosynthesis at the base of the convective envelope (hot bottom burning). Hot bottom burning was found to occur for stars between approximately 4.5 and approximately 7 solar mass, producing envelopes with O-18/O-16 less than or equal to 10(exp -6) and 10(exp -3) approximately less than or equal O-17/O-16 approximately less than or equal to 10(exp -1). The O-17 abundance depends sensitively on the nuclear O-17-destruction rate; this rate is only loosely constrained by the requirement that first and second dredge-up models match O-isotope observations of red giant branch (RGB) stars (Boothroyd, Sackmann, & Wasserburg 1994). In some cases, high mass-loss rates can terminate hot bottom burning before further O-17 enrichment takes place or even before all O-18 is destroyed. These predictions are in accord with the very limited stellar observations of J type carbon stars on the AGB and with some of the circumstellar Al2O3 grains from meteorites. In contrast, precise data from a number of grains and data from most low-mass S and C AGB stars (approximately less than 1.7 solar mass) lie in a region of the O-18/O-16 versus O-17/O-16 diagram that is not accessible by first and second dredge-up or by hot bottom burning. We conclude that for AGB stars, the standard models of stellar evolution are not in accord with these observations. We surmise that an additional mixing mechanism must exist that transports material from the cool bottom of the stellar convective envelope to a depth at which O-18 is destroyed. This 'cool bottom processing' mechanism on the AGB is similar to extra mixing mechanisms proposed to explain the excess C-13 (and depleted C-12) observed in the earlier RGB stage of evolution and the large Li-7 depletion observed in low-mass main-sequence stars.

Boothroyd, Arnold I.↗
NASA NTRS1993

Temperature and abundances in the Jovian auroral stratosphere. 1: Ethane as a probe of the millibar region

We report infrared heterodyne spectroscopy (lambda/delta lambda is approximately 10(exp 6)) of C2H6 emission at 11.9 microns from the northern Jovian auroral region, in observations conducted over December 2-7, 1989. Accurately measured line shapes provide information on C2H6 abundance as well as temperature and permit retrieval of the source pressure region. Enhanced emission was observed in the longitude range approximately 150-180 deg at approximately 60 deg north latitude, approximately corresponding to the CH4 7.8 micron hot spot and the region of brightest UV aurora. Significant brightness variations were observed in the hot spot emissions on a time scale of approximately 20 hours. Analysis of the brightest hot spot spectra indicates C2H6 mole fractions of approximately (6.3-6.8) x 10(exp -6) at temperatures of approximately 182-184 K at 1 mbar, compared to mole fractions of (3.8 +/- 1.4) x 10(exp -6) averaged over spectra outside the hot spot at a temperature of approximately 172 K at the same pressure. Fixing the mole fraction to the lower limit retrieved in the quiescent (non-hot spot) region allows the temperature at 1 mbar to be as high as approximately 200 K within the hot spot. These results provide upper limits to the temperature increase near the source of the C2H6 thermal infrared emission. Combined with results from similar measurements of ethylene emission probing the approximately 10-microbar region (Kostiuk et al., this issue), altitude information on the thermal structure of the Jovian auroral stratosphere can be obtained for the first time.

Livengood, Timothy A.↗
NASA NTRS1995

The abundances of methane and ortho/para hydrogen on Uranus and Neptune: Implications of New Laboratory 4-0 H2 quadrupole line parameters

The tropospheric methane molar fraction (f(sub CH4, t) and the ortho/para hydrogen ratio are derived for Uranus and Neptune based on new determinations of spectroscopic parameters for key hydrogen features as reported by D. W./ Ferguson et al. (1993). For each planet, the relatively weak laboratory linestrengths (approximately 30 and 15% less than the theoretical 4-0 S(0) and S(1) linestrengths, respectively) results, when compared to analyses adopting theroetical values, in a approximately 30% decrease in the tropospheric methane ratio and a comparable increase in the pressure level of the optically thick cloudtop marking the bottom of the visible atmosphere (P(sub c/d)). The increase in the ratio of S(1)/S(0) linestrengths from 4.4 (theoretical) to approximately 5.9 (measured) results in a decrease in the range of viable ortho/para ratios; an equilibrium hydrogen distribution is now the best fit for both planets. The methane mixing ratios reported here are in agreement with the value of 0.023 derived by the Voyager Radio Occultation Experiment (G. F. Lindal, 1992) for Neptune, but slightly lower than the Voyager Uranus measurement of 0.023 reported by G. F. LIndel et al. (1987). The relative carbon-to-hydrogen abundances for Uranus and Neptune support planetary formation mechanisms involving the dissolution of carbon-bearing planetesimals in the atmospheres of both planets during their early stages of formation (e.g., J. B. Pollack et al., 1986).

Baines, Kevin H.↗
NASA NTRS1994

Infrared spectroscopic measurements of the total column abundance of ethan (C2H6) above Lauder, New Zealand

Total vertical column aboundances of atmospheric ethane (C2H6) have been retrieved form more than 350 high-resolution infrared solar spectra recorded on 129 days between December 1992 and March 1994 at the Network for the Detection of Stratospheric Change (NDSC) station in Lauder, New Zealand (latitude 45.04 deg S, 169.68 deg E, 0.37 km altitude). The results are based on nonlinear least squares fits to narrow spectral intervals containing the unresolved C2H6 nu (sub 7) band (P)Q(sub 3) and (R)Q(sub 0) subbranches at 2976.8 and 2986.7/cm. The measured Lauder column abundances show a pronounced seasonal cycle with a maximum value of (8.6 +/- 0.5) x 10(exp 15) molecules/sq cm in September and a minimum value of (3.8 +/- 0.5) x 10 (exp 15) molecules/sq cm in February. Assuming a realistic vertical distribution for C2H6, these values correspond to surface level volume-mixing ratios of 0.52 and 0.23 ppbv (parts per billion by volume), respectively. The phase and relative amplitude of the Lauder C2H6 seasonal cycle are similar to corresponding values determined from infrared measurements at the Jungfraujoch (46.55 deg N, 7.98 deg E, 3.58 km altitude) and Mauna Loa (latitude 19.5 deg N, longitude 155.6 deg W, 3.5 km altitude) NDSC stations, but the inferred absolute magnitudes of the Lauder surface level mixing ratios during the same season are significantly lower. The Lauder C2H6 measurements are also compared with published southern hemisphere surface level sampling measurements and two-dimensional model calculations.

Rinsland, Curtis P.↗
NASA NTRS1995

Stellar coronal abundances. 3: The solar first ionization potential effect determined from full-disk observation

In this paper we reanalayze the full-disk quiet-sun spectrum of Mallinovsky & Heroux (1973) with modern atomic data. The purposes of this are to check our atomic data and methods in other investigations using data from nearby stars obtained with the NASA Extreme Ultraviolet Explorer (EUVE) satellite, and to confirm that the solar first ionization potential (FIP) effect investigated by previous authors studying discrete solar regions is the same as that found in full-disk spectra. We recover the usual solar FIP effect of a coronal abundance enhancement of elements with a low FIP of a factor approximately 3-4 for lines formed at temperatures greater than approximately 10(exp 6) K. For lower temperatures, the FIP effect seems to be substantially smaller, in qualitative agreement with other data. Comparing our full-disk result with those from discrete solar structures suggest that the FIP effect is a function of altitude, with the lower temperature full-disk emission being dominated by the super-granulation network. We also compare the recent ionization balance of Arnaud & Raymond (1992) with that of Arnaud & Rothenflug (1985).

Laming, J. Martin↗
NASA NTRS1995

The abundances of major elements in Cas A and Tycho supernova remnants

The objective of this program was to map the abundances of major elements such as O, Si, S, and Fe in the supernova remnants, Tycho and Cas A. The approach was based upon using archival cosmic X-ray data from several space missions, notably, the Einstein Observatory, EXOSAT, ROSAT, BBSRT, and ASCA. Two of the missions, Einstein and ROSAT, had high resolution telescopes that provided excellent images, but no spectral information. Two missions with much poorer resolution telescopes, BBXRT and ASCA, gave good spectral information through pulse height of signals in their cooled solid state detector, but rather crude spatial information. Our goal was to extract spectral information from the combined analysis of the Einstein and ROSAT images of Cas A and Tycho and to verify or refine the spectral map by checking its agreement with the BBSRT or ASCA spectra results for larger regions. In particular, we note that the Einstein and ROSAT telescopes have different spectral responses. The Einstein bandwidth includes the 2-4 keV region which is absent from ROSAT. Hence, by forming linear combinations of the Einstein and ROSAT images, we are able to resolve the contributions of the 0.5-2 keV band from the 2-4 keV band. The former contains lines of O and Fe while the latter is dominated by Si and S. We correct for the expansion that has taken place in the remnants during the ten-year interval between the Einstein and ROSAT measurements, but we must assume that no significant spectral changes have occurred during that time. The analysis of the Tycho SNR was completed and the results have been published. A copy of the paper is included. The analysis of Cas A has proved to be more complicated. It is continuing with support from another program. Part of the problem may be due to difficulties in the aspect information which is needed to precisely register the ROSAT and Einstein images.

Gorenstein, P.↗
NASA NTRS1996

High Resolution Spectroscopy of Vega-like Stars: Abundances and Circumstellar Gas

Vega-like stars are main-sequence stars exhibiting excess infrared emission. In an effort to improve the information available on this class of star, 13 stars have been analyzed which have been classed as Vega-like, or have an infra-red excess attributable to dust in their circumstellar environment. In a separate paper stellar properties such as effective temperature and log g have been derived and in this poster we highlight the results of the photospheric abundance analysis also carried out during this work. King recently drew attention to the possible link between Vega-like stars and the photospheric metal-depleted class of A-stars, the Lambda Bootis stars. Since Vega-like stars are thought to have disks of dust, it might be expected that accretion of depleted gas onto the surface of these stars may cause this same phenomenon. In the 6 stars studied for depletions, none showed the extreme underabundance patterns observed in Lambda Bootis stars. However, depletions of silicon and magnesium were found in two of the sample, suggesting that these elements are in silicate dust grains in the circumstellar environment of these stars. Absorption lines attributed to circumstellar gas have been positively identified in three stars in our sample. Individual cases show evidence either of high-velocity outflowing gas, variability in the circumstellar lines observed, or evidence of circumstellar gas in excited lines of Fe II. No previous identification of circumstellar material has been made for two of the stars in question.

Dunkin, S. K.↗
NASA NTRS1997

Lithium Abundances in the Young Open Cluster IC 2602

We have obtained high-resolution spectra for 28 candidate late-type stars in the 30 Myr old cluster IC 2602. NLTE Li abundances have been derived from measured equivalent widths. The log n(Li) - T(sub eff) and log n(Li) - mass distributions for our sample stars have been compared with those of the Pleiades and alpha Persei. Our data show that F stars in the three clusters have the same lithium content, which corresponds to the initial content for Pop. I stars. G and early-K IC 2602 stars are, on average, somewhat more Li-rich than their counterparts in the two slightly older clusters. Finally, the latest-type IC 2602 stars are heavily Li depleted, with their Li content being as low as the lowest measured among the Pleiades. As in the Pleiades and alpha Per, a star-to-star scatter in lithium is observed among 30 Myr old late-K/early-K dwarfs in IC 2602, indicating that this spread develops in the pre-main sequence phases.

Randich, S.↗
NASA NTRS1996

Stratospheric NO and NO2 Abundances from ATMOS Solar-Occultation Measurements

Using results from a time-dependent photochemical model to calculate the diurnal variation of NO and NO2, we have corrected Atmospheric Trace MOlecule Spectroscopy (ATMOS) solar-occultation retrievals of the NO and NO2 abundances at 90' solar zenith angle. Neglecting to adjust for the rapid variation of these gases across the terminator results in potential errors in retrieved profiles of approximately 20% for NO2 and greater than 100% for NC at altitudes below 25 km. Sensitivity analysis indicates that knowledge of the local 03 and temperature profiles, rather than zonal mean or climatological conditions of these quantities, is required to obtain reliable retrievals of NO and NO2 in the lower stratosphere. Extremely inaccurate 03 or temperature values at 20 km can result in 50% errors in retrieved NO or NO2. Mixing ratios of NO in the mid-latitude, lower stratosphere measured by ATMOS during the November 1994 ATLAS-3 mission compare favorably with in situ ER-2 observations, providing strong corroboration of the reliability of the adjusted space-borne measurements.

Newchurch, M. J.↗
NASA NTRS1996

ASCA observations of A1367: Search for abundance and temperature gradients

The preliminary results of Abel 1367 (A 1367) observations with the Advanced Satellite for Cosmology and Astrophysics (ASCA) are presented. The A 1367 is a nearby dynamic young cluster. Four different pointings were performed to map the whole cluster. No evidence for significant abundance variations was found. However, the temperature increases along the major axis of A 1367 suggesting that the cluster was observed during the merger of a subcluster with the main cluster.

Churazov, E.↗
NASA NTRS1998

Quiet-Time Spectra and Abundances of Energetic Particles During the 1996 Solar Minimum

This report concerns the energy spectra and abundances of ions with atomic number, Z, in the interval 2 greater than or equal to Z and Z less than or equal to 36 and energies approximately 3-20 MeV/amu for solar and interplanetary quiet periods between November 1994 and April 1998 as measured by the large-geometry LEMT telescope on the Wind spacecraft near Earth. The energy spectra show the presence of galactic (GCR) and 'anomalous' cosmic ray (ACR) components, depending on the element. ACR components are reported for Mg and Si for the first time at 1 AU and the previous observation of S and Ar is confirmed. However, only GCR components are clearly apparent for the elements Ca, Ti, Cr, Fe, as well as for C. New limits are placed on a possible ACR contribution for other elements, including Kr.

Reames, Donald V.↗
NASA NTRS1999

Quiet-Time Spectra and Abundances of Energetic Particles During the 1996 Solar Minimum

We report the energy spectra and abundances of ions with atomic number, Z, in the interval Z is greater than or equal to 2 and Z is less than or equal to 36 and energies approximately 3-20 MeV/amu for solar and interplanetary quiet periods between 1994 November and 1998 April as measured by the large-geometry Low Energy Matrix Telescope (LEMT) telescope on the Wind spacecraft near Earth. The energy spectra show the presence of galactic (GCR) and "anomalous" cosmic ray (ACR) components, depending on the element. ACR components are reported for Mg and Si for the first time at 1 AU and the previous observation of S and Ar is confirmed. However, only GCR components are clearly apparent for the elements Ca, Ti, Cr, Fe, as well as for C. New limits are placed on a possible ACR contribution for other elements, including Kr.

Reames, Donald V.↗
NASA NTRS1999

Capella: Structure and Abundances

This grant covers the analysis of ASCA spectra of the cool star binary system Capella. This project has also required the analysis of simultaneous EUVE data. The ASCA spectrum of Capella could not be fit with standard models; by imposing models based on strong lines observed with EUVE, a problem wavelength region was identified. Correcting the problem required calculations of atomic collision strengths of higher principal quantum number than had ever been calculated, resulting in a paper in process by Liedahl and Brickhouse. With these new models applied to the ASCA spectrum, better fits were obtained. While solar abundance ratios are generally consistent with the ASCA data, the ratio of Ne/Fe is three to four times lower than solar photospheric values. Whether there is a general First Ionization Potential (FIP) effect or a specific neon anomaly cannot be determined from these data. Detailed discussion has been provided to NASA in the most recent annual report (1997). Two poster presentations have been made regarding modeling requirements. A substantial paper is in the final revision form, following review by six co-authors. The results of this work have wide implications, since the newly calculated emission lines almost certainly contribute to other problems in fitting not only other stellar spectra, but also composite supernova remnants, galaxies, and cooling flow clusters of galaxies. Furthermore, Liedahl and Brickhouse have identified other species for which lines of a similar nature (high principal quantum number) will contribute significant flux. For moderate resolution X-ray spectra, lines left out of the models in relatively isolated bands, will be attributed to continuum flux by spectral fitting engines, causing errors in line-to-continuum ratios. Thus addressing the general theoretical problem is of crucial importance.

Brickhouse, Nancy S.↗
NASA NTRS1999

On the Abundance of Lithium in T Coronae Borealis

We have obtained high resolution echelle spectroscopy of the recurrent nova T CrB (T Coronae Borealis). We find that the surface lithium abundance in T CrB is significantly enhanced compared to field M giants, where it is not detectable. We offer possible explanations for this in terms of either a delay in the onset of convection in the giant star, enhanced coronal activity due to star-spots or the enhancement of Li resulting from the nova explosion(s).

Shahbaz, T.↗