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NASA NTRS1993

Abundance inhomogeneities and atmospheric structure in CN-bimodal globular cluster giants

It has been suggested by several authors that the sodium and aluminum abundance variations correlating with CN-band strength, frequently observed in CN-bimodal globular cluster giants, could be spurious manifestations of different temperature structures in the 'CN-strong' and 'CN-weak' stars, caused by different molecular line blanketing related to the C, N, and O trio. For stellar parameters generally appropriate to giants in the intermediate metallicity CN-bimodal cluster M4, we demonstrate through new model atmosphere calculations, employing opacity sampling and spherical geometry, that the observed abundance anomalies cannot be the result of atmospheric temperature structure. Our results using spherical geometry are compared to identical calculations performed with plane-parallel geometry: the effects of atmospheric extension on derived abundances for all lines considered amount to less than 0.1 dex.

Drake, Jeremy J.↗
NASA NTRS1993

The giant impact produced a precipitated Moon

The author's current simulations of Giant Impacts on the protoearth show the development of large hot rock vapor atmospheres. The Balbus-Hawley mechanism will pump mass and angular momentum outwards in the equatorial plane; upon cooling and expansion the rock vapor will condense refractory material beyond the Roche distance, where it is available for lunar formation. During the last seven years, the author together with several colleagues has carried out a series of numerical investigations of the Giant Impact theory for the origin of the Moon. These involved three-dimensional simulations of the impact and its aftermath using Smooth Particle Hydrodynamics (SPH), in which the matter in the system is divided into discrete particles whose motions and internal energies are determined as a result of the imposed initial conditions. Densities and pressures are determined from the combined overlaps of the particles, which have a bell-shaped density distribution characterized by a smoothing length. In the original series of runs all particle masses and smoothing lengths had the same values; the matter in the colliding bodies consisted of initial iron cores and rock (dunite) mantles. Each of 41 runs used 3,008 particles, took several weeks of continuous computation, and gave fairly good representations of the ultimate state of the post-collision body or bodies but at best crude and qualitative information about individual particles in orbit. During the last two years an improved SPH program was used in which the masses and smoothing lengths of the particles are variable, and the intent of the current series of computations is to investigate the behavior of the matter exterior to the main parts of the body or bodies subsequent to the collisions. These runs are taking times comparable to a year of continuous computation in each case; they use 10,000 particles with 5,000 particles in the target and 5,000 in the impactor, and the particles thus have variable masses and smoothing lengths (the latter are dynamically adjusted so that a particle typically overlaps a few tens of its neighbors). Since the matter in the impactor provides the majority of the mass left in orbit after the collision, and since the masses of the particles that originated in the impactor are smaller than those in the target, the mass resolution in the exterior parts of the problem is greatly improved and the exterior particles properly simulate atmospheres in hydrostatic equilibrium.

Cameron, A. G. W.↗
NASA NTRS1993

The formation process of the He I lambda 10830 line in cool giant stars

The Final Report on the formation process of the He I lambda 10830 line in cool giant stars is presented. The research involves observing a sample of cool giant stars with ROSAT. These stars were selected from the list of bright stars which display He I lambda 10830 in absorption or emission and lie on the cool side of the coronal dividing line. With measured x ray fluxes or upper limits measured by the Position Sensitive Proportional Counter (PSPC), the role x rays play in the formation of this important line was investigated using the non-LTE radiative transfer code PANDORA. Hydrodynamic calculations were performed to investigate the contributions of acoustic wave heating in the formation of this line as well.

Luttermoser, Donald G.↗
NASA NTRS1993

Core formation by giant impacts: Conditions for intact melt region formation

Among the many effects of high-speed, giant impacts is widescale melting that can potentially trigger catastrophic core formation. If the projectile is sufficiently large, the melt pools to form an intact melt region. The dense phase then segregates from the melt, forming a density anomoly at the melt region's base. If the anomoly produces a differential stress larger than a certain minimum, it overcomes the mantle's long-term elastic strength and rapidly forms a core. It was previously shown that giant impacts effectively trigger core formation in silicate bodies by the time they grow to the mass of Mercury and in icy bodies by the time they grow larger than Triton. In order for this process to be viable, an intact melt region must be formed. Conditions under which this occurs is examined in more detail than previously published.

Tonks, W. B.↗
NASA NTRS1994

Formation of dust grains with impurities in red giant winds

Among the several proposed carriers of diffuse interstellar bands (DIB's) are impurities in small dust grains, especially in iron oxide grains (Huffman 1977) and silicate grains (Huffman 1970). Most promising are single ion impurities since they can reproduce the observed band widths (Whittet 1992). These oxygen-rich grains are believed to originate mostly in the mass flows from red giants and in supernovae ejecta (e.g. Gehrz 1989). A question of considerable impact for the origin of DIB's is therefore, whether these grains are produced as mainly clean crystals or as some dirty materials. A formalism has been developed that allows tracking of the heterogeneous growth of a dust grain and its internal structure during the dust formation process. This formalism has been applied to the dust formation in the outflow from a red giant star.

Dominik, Carsten↗
NASA NTRS1994

The metallicity of M4: Accurate spectroscopic fundamental parameters for four giants

High-quality spectra, covering the wavelength range 5480 to 7080 A, have been obtained for four giant stars in the intermediate-metallicity CN-bimodal globular cluster M4 (NGC 6121). We have employed a model atmosphere analysis that is entirely independent from cluster parameters, such as distance, age, and reddening, in order to derive accurate values for the stellar parameters effective temperature, surface gravity, and microturbulence, and for the abundance of iron relative to the Sun, (Fe/H), and of calcium, Ca/H, for each of the four stars. Detailed radiative transfer and statistical equilibrium calculations carried out for iron and calcium suggest that departures from local thermodynamic equilibrium are not significant for the purposes of our analysis. The spectroscopically derived effective temperatures for our program stars are hotter by about 200 K than existing photometric calibrations suggest. We conclude that this is due partly to the uncertain reddening of M4 and to the existing photometric temperature calibration for red giants being too cool by about 100 K. Comparison of our spectroscopic and existing photometric temperatures supports the prognosis of a significant east-west gradient in the reddening across M4. Our derived iron abundances are slightly higher than previous high-resolution studies suggested; the differences are most probably due to the different temperature scale and choice of microturbulent velocities adopted by earlier workers. The resulting value for the metallicity of M4 is (Fe/H )(sub M4) = -1.05 + or - 0.15. Based on this result, we suggest that metallicities derived in previous high-dispersion globular cluster abundance analyses could be too low by 0.2 to 0.3 dex. Our calcium abundances suggest an enhancement of calcium, an alpha element, over iron, relative to the Sun, in M4 of (Ca/H) = 0.23.

Drake, J. J.↗
NASA NTRS1994

Soft gamma-ray repeaters: Black holes in giant molecular clouds?

The BATSE discovery of the near-simultaneous turnon of a soft gamma-ray repeater (SGR) source, located to apporximately 5 deg and possibly SGR 1900+14, and a new hard X-ray transient GRS 1915+105 (discovered by Granat), suggests they may be associated. Published positions for both the SGR and GRS sources do not preclude spatial coincidence for the two sources, or the GRS transient may be associated with another SGR source. We outline a model for SGR sources as due to thermal instabilities in spherical accretion onto black holes in giant molecular clouds. We show that the SIGMA position for the GRS 1915 source is consistent with it being in a giant molecular cloud and compare this possible identification with that suggested for the `1E' source in the Galactic center region. Our model would predict a strong concentration of SGR sources toward the Galactic plane (consistent with both SGR 1900+14 and SGR 1806-20) and so would require the only other known 'SGR,' the GRB 790305 superburst source apparently in the Large Magellanic Cloud, to be fundamentally different. We note that the recently discovered radio/infrared counterpart for the SGR 1806-20 may be consistent with this model, rather than the compact suprernova remnant/pulsar model suggested by Kulkarni and Frail.

Grindlay, Jonathan E.↗
NASA NTRS1994

A violent interaction between the dwarf galaxy UGC 7636 and the giant elliptical galaxy NGC 4472

We present new U, B, R, and H I imagery of the Virgo Cluster giant elliptical galaxy NGC 4472 and its interacting dwarf companion galaxy UGC 7636. Using a composite image reconstruction technique, we show that a trail of debris approx. 5 arcmin in length and approx. 1 arcmin in width (30x6 kpc for a Virgo cluster distance of 20 Mpc) is projected northward from the dwarf galaxy. A cloud of H I is projected along the northwest edge of the debris between the dwarf and gE. The dwarf's nuclear morphology is irregular and bow-shaped on what appears to be its leading edge. Apart from a number of isolated blue regions, most of of the trailing debris is similar in color to the dwarf's nucleus. Only a modest enhancement of star formation appears to have been induced by the interaction. Although separated by 15 kpc, the H I and stellar morphologies are remarkably similar. The stars and H I appear to have been tidally distorted in situ, prior to the cloud's removal by ram pressure. If the H I has maintained its shape by magnetic support, a magnetic field strength an order of magnitude larger than the galaxy's is required. Ram pressure deceleration due to the cloud's motion through NGC 4472's x-ray-emitting interstellar medium shold be sufficient for the cloud to become gravitationally bound to NGC 4472. The H I cloud is not self-gravitating and may fragment and be destroyed in the interaction. UGC 7636 will probably be disrupted by NGC 4472's strong tidal forces; the stellar debris will disperse into the Virgo cluster or become bound to NGC 4472's halo on eccentric orbits. The debris captured in the collision will have a negligible impact on NGC 4472's stellar and gaseous content. On the other hand, if similar interactions are common in giant elliptical galaxies, they could alter or deplete surrounding dwarf galaxy populations, fuel bursts of nuclear activity, and perhaps provide a source of magnetic energy to their interstellar media.

Mcnamara, Brian R.↗
NASA NTRS1994

ROSAT all-sky survey observations of X-ray variability in cool giant stars

We have identified 24 active late-type giant stars, including 11 RS CVn systems, with soft X-ray count rates high enough to allow the detection of statistically significant variability on a Roentgen Satellite (ROSAT) orbital timescale (96 minutes) as observed by the Position Sensitive Proportional Counter (PSPC) during the all-sky survey. Our sensitivity typically lies in the range of 10% - 25%, depending on the source count rate. Comparison is made to the daily, nonflare solar soft X-ray variability as observed by the Solrad satellites during solar minimum in 1969 and solar maximum in 1975. Seven of the 24 stars show significant variability; in two of these cases (HR 3922 and HR 8448) major flares were observed in which the peak count rate is enhanced by at least a factor of 3 above quiescent. While HR 3922 (G5 III) is not (yet) classified as an RS CVn star, its flare is more energetic (3 x 10(exp 31) ergs/s) than previously observed RS CVn flares. The apparently single giant HR 8167 (G8 III) also shows two flares. While one might expect to find an anticorrelation between saturated coronae and variability, we find no evidence of this: the two stars in our sample with the highest ratio of f(sub x)/f(sub v) both show variability. We also point out that Capella (G6 III + F9 III) is one of the stars manifesting variability.

Haisch, Bernhard↗
NASA NTRS1994

Spectroscopy of chromospheric lines of giants in the globular cluster

Spectroscopic observations of chromospheric transitions (Mg II, H-alpha, and Ca II K) from two red giants (A31 and A59) in the globular cluster NGC 6572 were made with the Goddard High Resolution Spectrograph on the Hubble Space Telescope and the coude spectrograph of the 1.9 m telescope at the Mount Stromlo Observatory. These measurements give evidence for chromospheric activity and outward motions within the atmospheres. The surface flux of the Mg II emission is comparable to that in disk population giants of similar (B-V) color. The Mg II profiles are asymmetric, which is most likely caused by absorption in an expanding stellar atmosphere and/or by possible interstellar features. Notches are found in the core of the H-alpha line of A59, which are similar to those found in Cepheids. This suggests that shocks are present in the atmosphere of A59 and indicates that hydrodynamic phenomena are influencing the levvel of chromospheric emission and producing upper atmospheric motions which may lead to mass loss.

Dupree, A. K.↗
NASA NTRS1994

Double-core evolution. 5: Three-dimensional effects in the merger of a red giant with a dwarf companion

The evolution of the common envelope phase of a binary system consisting of a 4.67 solar mass red giant and a 0.94 solar mass dwarf is studied using smoothed particle hydrodynamics. We demonstrate that the three-dimensional effects associated with the gravitational tidal torques lead to a rapid decay of the orbit on timescales approximately less than 1 yr. The relative orbit of the two cores in the common envelope is initally eccentric and tends to circularize as the orbital separation of the two cores decreases. The angular momentum lost from the orbital motion is distributed throughout the common envelope, and the double core does not evolve to a state of co-rotation for the evolutionary time followed. The energy dissipated from the relative orbit and deposited in the common envelope results in the ejection of approximately 13% of the mass of the envelope. The mass is ejected in all directions, but there is a preference for mass ejection in the orbital plane of the binary system. For example, approximately 80% of the ejected mass lies within 30 deg of the binary orbital plane. Because gravitational forces are long range, most of the energy and angular momentum is imparted to a small fraction of the common envelope resulting in an efficiency of the mass ejection process of approximately 15%. The core of the red giant executes significant displacement with respect to the center of mass of the system and contributes nearly equally to the total energy dissipation rate during the latter phases of the evolution. The degree of departure from synchronism of the initial binary system can be an important property of the system which can affect the outcome of the common envelope phase.

Terman, James L.↗
NASA NTRS1994

The chromospheric structure of the cool giant star g Herculis

Non-Local Thermodynamic Equilibrium (LTE) calculations of semiempirical chromospheric models are presented for 30 g Her (M6 III). This star is one of the coolest (T(sub eff) = 3250 K) SRb (semiregular) variable stars and has a mass perhaps as great as 4 solar mass. Chromospheric features we have observed in its spectrum include Mg II h and k; C II) UV0.01, which is sensitive to electron density; Mg I lambda 2852; Ca II H, K, and IRT; Ca I lambda 4227 and lambda 6573; Al II) UV 1; and H alpha. We pay special attention to fitting the C II intersystem lines and the Mg II resonance lines but use all the other features as constraints to some extent. The equations of radiative transfer and statistical equilibrium are solved self-consistently for H I, H(-), H2, He I, C I, C II, Na I, Mg I, Mg II, Al I, Al II, Ca I, and Ca II with the equivalent two-level technique. To simplify these calculations, a one-dimensional hydrostatic, plane-parallel atmosphere is assumed. We investigate 10 separate 'classical' chromospheric models, differing most importantly in total mass column density above the temperature minimum. Synthetic spectra from these models fit some but not all of the observations. These comparisons are discussed in detail. However, we find that no single-component classical model in hydrostatic equilibrium is able to reproduce both the Mg II line profiles and the relative strengths of the CII) lines. In all these models, chromospheric emission features are formed relatively close to the star (approximately less than 0.05 R(sub *). The circumstellar environment has a thick, cool component overlying the Mg II emission region, which is relatively static and very turbulent. Finally, we find that thermalization in the Mg II h and k lines in the coolest giant stars is controlled by continuum absorption from Ca I 4p 4p3 P0 bound-free opacity and not collisional de-excitation as is the case for warmer K giants.

Luttermoser, Donald G.↗
NASA NTRS1994

Molecular rotational line profiles from oxygen-rich red giant winds

We have developed a radiative transfer model of the dust and gas envelopes around late-type stars. The gas kinetic temperature for each star is calculated by solving equations of motion and the energy balance simultaneously. The main processes include viscous heating and adiabatic and radiative cooling. Heating is dominated by viscosity as the grains stream outward through the gas, with some contribution in oxygen-rich stars by near-IR pumping of H2O followed by collisional de-excitation in the inner envelope. For O-rich stars, rotational H2O cooling is a dominant mechanism in the middle part of the envelope, with CO cooling being less significant. We have applied our model to three well-studied oxygen-rich red giant stars. The three stars cover a wide range of mass-loss rates, and hence they have different temperature structures. The derived temperature structures are used in calculating CO line profiles for these objects. Comparison of the dust and gas mass-loss rates suggests that mass-loss rates are not constant during the asymptotic giant branch phase. In particular, the results show that the low CO 1-0 antenna temperatures of OH/IR stars reflect an earlier phase of much lower mass-loss rate.

Justtanont, K.↗
NASA NTRS1994

Chromospheric heating and metal deficiency in cool giants: Theoretical results versus observations

We compute acoustic shock wave-heated chromosphere models for moderately cool giant stars which differ greatly in metallicity. Subsequently, we simulate the emerging Mg II k lines assuming partial redistribution. The initial acoustic energy fluxes and the wave periods are taken from acoustic wave generation calculations based on traditional convection zone models. We find that the Mg II and Ca II core emissions are close to the observed basal flux limits which are common for giants and dwarfs. In addition, we find that the Mg II core emission is independent of the metallicity, in agreement with observations. We argue that these results should be considered as further evidence that the basal flux limits are indeed due to acoustic shock heating. The acoustic heating mechanism seems to be dominant in all nonmagnetic nonpulsating late-type stars.

Cuntz, M.↗
NASA NTRS1995

Tests of two convection theories for red giant and red supergiant envelopes

Two theories of stellar envelope convection are considered here in the context of red giants and red supergiants of intermediate to high mass: Boehm-Vitense's standard mixing-length theory (MLT) and Canuto & Mazzitelli's new theory incorporating the full spectrum of turbulence (FST). Both theories assume incompressible convection. Two formulations of the convective mixing length are also evaluated: l proportional to the local pressure scale height (H(sub P)) and l proportional to the distance from the upper boundary of the convection zone (z). Applications to test both theories are made by calculating stellar evolutionary sequences into the red zone (z). Applications to test both theories are made by calculating stellar evolutionary sequences into the red phase of core helium burning. Since the theoretically predicted effective temperatures for cool stars are known to be sensitive to the assigned value of the mixing length, this quantity has been individually calibrated for each evolutionary sequence. The calibration is done in a composite Hertzsprung-Russell diagram for the red giant and red supergiant members of well-observed Galactic open clusters. The MLT model requires the constant of proportionality for the convective mixing length to vary by a small but statistically significant amount with stellar mass, whereas the FST model succeeds in all cases with the mixing lenghth simply set equal to z. The structure of the deep stellar interior, however, remains very nearly unaffected by the choices of convection theory and mixing lenghth. Inside the convective envelope itself, a density inversion always occurs, but is somewhat smaller for the convectively more efficient MLT model. On physical grounds the FST model is preferable, and seems to alleviate the problem of finding the proper mixing length.

Stothers, Richard B.↗
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 NTRS1995

X-ray emission from giant H II regions in M101

We have examined the archival ROSAT Position Sensitive Proportional Counter (PSPC) and the High-Resolution Imager (HRI) images of the galaxy M101 to study the X-ray properties of its giant H II regions. All five giant H II regions, NGC 5447, NGC 5445, NGC 5461, NGC 5462, and NGC 5471, show X-ray emission. By observing their basic properties, such as size, hardness ratio, and estimated luminosity, and by comparisons with better known regions in the Large Magellanic Cloud (LMC), we are able to suggest some possible mechanisms for the sources of the X-ray emission.

Williams, Rosa Murphy↗
NASA NTRS1994

Calculations of the outflow velocity of envelopes of cool giants

We study the transfer of momentum from photons to dust grains to (molecular) gas in the outflow around cool giants (carbon-stars, Mira variables and OH/IR stars) beyond the radius where the dust grains condense. The problem is circular: radiation pressure determines the outflow velocity of the dust and thus also the dust density; on the other hand the dust density determines, via radiative transfer effects, the spectrum of the photons and thus the effective radiation pressure. This circular problem is solved by a rapidly converging iterative procedure. We compare our predictions with observed properties of a large sample of OH/IR stars and of Miras and find a good qualitative and quantitative agreement. We confirm a conclusion by Wood et al. (1993) that very luminous OH/IR stars in the Large Magellanic Cloud (LMC) owe their low outflow velocity to the low dust-to-gas ratio, a consequence of the low metallicity of the LMC. Similarly we consider a sample of about 100 OH/IR stars within 200 pc from the galactic center that has an average asymptotic giant branch (AGB) luminosity and an uncommonly high value of v(sub out); we conclude that these stars are probably very metal rich, perhaps even more than the stars in the Baade window studied by Rich (1990).

Habing, H. J.↗