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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 NTRS1994

Physical Theories of Winds From Cool Stars

Cool stars in the late stages of their evolution generally lose mass at a prodigious rate. This includes low mass stars on the red giant branch, on the asymptotic giant branch, and those transiting from the asymptotic giant branch to the planetary nebula phase, as well as massive supergiants. All of these objects are surrounded by dense circumstellar gas and often dust envelopes. This mass loss is an important source of gas and dust for the interstellar medium. For some of these objects, the mass loss rate exceeds the nuclear burning rate and, hence, mass loss determines the subsequent evolution of the star. A variety processes have been invoked to explain the mass loss of these objects. A consensus has developed over the last decade: photospheric processes create an extended atmosphere which extends to several stellar radii. At this height above the photosphere, dust grains can form and radiation pressure drives the dust out. The gas is dragged along by friction. While the detailed processes involved, in particular those lifting the atmosphere, may differ from object to object, this paradigm seems applicable to all of these objects. The process of mass loss breaks up into three parts: 1) The formation of the extended atmosphere; 2) the nucleation and condensation of dust; and 3) The radiation pressure driven wind. Each of these processes will be discussed with an emphasis on those processes that play a role in the mass loss from asymptotic giant branch stars for which the most detailed theories have been developed.

Tielens, A. G. G. M.↗

NASA NTRS1993

Interstellar grains in primitive meteorites - Diamond, silicon carbide, and graphite

Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O greater than 1), and apparently are 'stardust' formed in stellar atmospheres. Microdiamonds, of median size about 10 A, are most abundant (about 400-1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes. Silicon carbide, of grain size 0.2-10 microns and abundance about 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1-3 solar masses. Some grains appear to be not less than 10 exp 9 a older than the solar system. Graphite spherules of grain size 0.8-7 microns and abundance less than 2 ppm contain highly anomalous C and noble gases, as well as large amounts of fossil Mg-26 from the decay of extinct Al-26. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars.

Anders, Edward↗

NASA NTRS1983

Stellar populations in local group dwarf elliptical galaxies. I - NGC 147

A color-magnitude diagram of NGC 147 to an I magnitude of 23 is presented. The stellar population in the outer parts of this elliptical galaxy resembles that of the globular clusters of the Milky Way. Quantitative comparison of the giant branch with those of globular clusters yields a mean metallicity of -1.2 + or - 0.2, making NGC 147 a part of the general correlation between mass and metallicity seen in ellipticals. The giant branch appears to be broad, which suggests a metallicity dispersion. The absence of asymptotic giant branch stars at luminosities above that of the red giant branch tip sets an upper limit of 10 percent for the fraction of stars in this NGC 147 field that have ages less than 12 Gyr. This result contrasts with the situation in some of the related, but less massive, dwarf spheroidal systems. If the choice is made to assume, rather than determine the stellar content of NGC 147, a distance of 630 + or - 50 kpc is derived, similar to that of M31.

Mould, J. R.↗

DOE OSTI2021

Elemental Abundances in M31: Gradients in the Giant Stellar Stream

We analyze existing measurements of [Fe/H] and [α/Fe] for individual red giant branch (RGB) stars in the Giant Stellar Stream (GSS) of M31 to determine whether spatial abundance gradients are present. These measurements were obtained from low- (R ~ 3000) and moderate- (R ~ 6000) resolution Keck/DEIMOS spectroscopy using spectral synthesis techniques as part of the Elemental Abundances in M31 survey. From a sample of 62 RGB stars spanning the GSS at 17, 22, and 33 projected kpc, we measure a [Fe/H] gradient of -0.018 ± 0.003 dex kpc{sup -1} and negligible [α/Fe] gradient with M31-centric radius. We investigate GSS abundance patterns in the outer halo using additional [Fe/H] and [α/Fe] measurements for six RGB stars located along the stream at 45 and 58 projected kpc. These abundances provide tentative evidence that the trends in [Fe/H] and [α/Fe] beyond 40 kpc in the GSS are consistent with those within 33 kpc. We also compare the GSS abundances to 65 RGB stars located along the possibly related Southeast (SE) shelf substructure at 12 and 18 projected kpc. The abundances of the GSS and SE shelf are consistent, supporting a common origin hypothesis, although this interpretation may be complicated by the presence of [Fe/H] gradients in the GSS. We discuss the abundance patterns in the context of photometric studies from the literature and explore implications for the properties of the GSS progenitor, suggesting that the high &lang;[α/Fe]&rang; of the GSS (+0.40 ± 0.05 dex) favors a major merger scenario for its formation.

47 OTHER INSTRUMENTATION↗

DOE OSTI2021

Eridanus IV: an Ultra-faint Dwarf Galaxy Candidate Discovered in the DECam Local Volume Exploration Survey

We present the discovery of a candidate ultra-faint Milky-Way satellite, Eridanus IV (DELVE J0505−0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). Eridanus IV is a faint (M V = − 4.7 ± 0.2), extended (), and elliptical (ϵ = 0.54 ± 0.1) system at a heliocentric distance of , with a stellar population that is well described by an old, metal-poor isochrone (age of τ ∼ 13.0 Gyr and metallicity of [Fe/H] ≲ − 2.1 dex). These properties are consistent with the known population of ultra-faint Milky-Way satellite galaxies. Eridanus IV is also prominently detected using proper-motion measurements from Gaia Early Data Release 3, with a systemic proper motion of mas yr−1 measured from its horizontal branch and red-giant-branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1983

Evolutionary effects of helium diffusion in population II stars

Evolutionary calculations continuing until well past turnoff are presented for models of low-mass Population II stars which take into account the effects of the diffusion of helium relative to hydrogen. Evolutionary tracks, cluster isochrones and hydrogen distributions were obtained for stellar masses in the range 0.75 to 1.01 solar masses, both in the presence and absence of diffusion. It is found that for a star of a given mass, diffusion speeds up the evolutionary process on the main sequence, although after turnoff evolution is slowed with respect to the case without diffusion. As the stars ascend the red giant branch, their outer regions are remixed so that evidence of helium diffusion is erased, and the evolutionary tracks of the models with and without diffusion converge. Thus, if the age of a globular cluster is determined from the absolute magnitude at turnoff or from fitting isochrones, diffusion results in a 25% reduction in the derived age at a turnoff magnitude of 4.23, and a 14% reduction at a turnoff magnitude of 3.45.

Stringfellow, G. S.↗

NASA NTRS1983

Trail of the Crab progenitor star

It is proposed that the jet which has been observed protruding from the northern rim of the Crab Nebula is a tubular trail of material lost by the presupernova star during its red giant phase. The movement of the red giant's stellar wind is analyzed to show that the mass profile downstream should be concentrated within a narrow cylinder. The parameters of the tube predicted by this model are presented and an observational test of the model based on the hydrogen-to-helium abundance ratio in the jet and in the Nebula as a whole is suggested; the model predicts that the ratio in the jet should be larger.

Blandford, R. D.↗

NASA NTRS1983

The unique cometary nebula Parsamian 13

Parsamian 13 is found to be a cometary nebula, with a deeply embedded star suffering strong absorption by silicate grains. A feature near 2.7 microns, attributed to absorption by terminal OH groups, may represent the first detection in an astrophysical environment of water ice grains diluted by another molecule, for example by CO. The bolometric luminosity suggests that this star is either an extremely young T Tauri star or an evolved low-mass star, now a red giant, high on its convective track. Most unusual is the presence of cold (50 K) TiO gas in sufficient abundance to show in absorption in the optical and very near-infrared regions; this material probably represents very recently expelled photospheric layers.

Cohen, M.↗

NASA NTRS1986

Chromospheric dust formation, stellar masers and mass loss

A multistep scenario which describes a plausible mass loss mechanism associated with red giant and related stars is outlined. The process involves triggering a condensation instability in an extended chromosphere, leading to the formation of cool, dense clouds which are conducive to the formation of molecules and dust grains. Once formed, the dust can be driven away from the star by radiation pressure. Consistency with various observed phenomena is discussed.

Stencel, R. E.↗

NASA NTRS1986

Chromospheric dust formation, stellar masers and mass loss

Stencel, R. E.↗

NASA NTRS1987

On the evolution of tidal capture X-ray binaries - 4U 2127+12 (M15) to 4U 1820-30 (NGC 6624)

A new evolutionary scenario for X-ray binaries in globular clusters, which begins with a tidal capture of a main-sequence star by a neutron star and ends with a white dwarf-neutron star system, is presented. For tidal captures of main-sequence stars into orbits too wide to begin mass transfer immediately, the subsequent evolution of the secondary can lead to a common envelope binary similar to what the 9 hr X-ray binary 4U 2127+12 in M15 is suspected to be. If the common envelope is thick enough, it may cause the neutron star and the white dwarf core of the secondary to spiral in, producing a detached white dwarf-neutron star system. Subsequently, gravitational radiation losses may evolve this into the configuration seen in the 11 minute X-ray binary 4U 1820-30 in NGC 6624. This model appears more likely on statistical grounds than formation by collision of a neutron star and a red giant. In some circumstances, the latter process may result in unstable mass transfer, which would result in coalescence rather than a binary system like 4U 1820-30.

Bailyn, Charles D.↗

NASA NTRS1989

On the nature of emission-line clouds of quasars and active galactic nuclei

It is proposed that the clouds responsible for the emission lines in AGN and QSOs are stratified and not uniform in density. Specifically, it is suggested that they are winds from evolved stars, in particular red giants. This assumption appears to solve, in straightforward fashion, several problems concerning the origin, dynamics, and response of these clouds to changes in the continuum luminosity, while preserving the most successful feature of the current two-phase equilibrium photoionization models.

Kazanas, Demosthenes↗

NASA NTRS1992

SO and SO2 in mass-loss envelopes of red giants - Probes of nonequilibrium circumstellar chemistry and mass-loss rates

SO emission was searched for in one or more of four transitions toward 23 oxygen-rich red giant or supergiant stars and one S star, selected primarily on the basis of their nonmaser SiO emission. SO was detected in a total of 14 circumstellar envelopes, 13 of which are new detections. The circumstellar abundance of SO (and SO2) is significantly enhanced over the equilibrium value achieved in the photospheres of these stars. In general, the SO abundances are significantly larger than predicted by nonequilibrium circumstellar chemistry models. Sulfur cannot be significantly depleted onto circumstellar grains, and probably exists as H2S (and/or SH) in the inner regions of the envelopes. The SO rotational-level population in most circumstellar envelopes observed is characterized by excitation temperatures less than or approximately equal to 50 K. The circumstellar abundance of SO2 is comparable to, or larger than, that of SO, ruling out the 'large' value adopted for the unshielded photodissociation rate for SO2 in recent models.

Sahai, Raghvendra↗

NASA NTRS1994

The Diffuse Interstellar Bands: Contributed papers

Drawing a coherent picture of the observational characteristics of the Diffuse Interstellar Bands (DIB's) and the physical and chemical properties of its proposed carriers was the focus of this NASA sponsored conference. Information relating to absoption spectra, diffuse radiation carriers, carbon compounds, stellar composition, and interstellar extinction involving T-Tauri stars, Reflection Nebulae, Red Giants, and accretion discs are discussed from those papers presented at the conference, which are included in this analytic.

Tielens, Alexander G. G. M.↗

NASA NTRS2011

Microstructures of Rare Silicate Stardust from Nova and Supernovae

Most silicate stardust analyzed in the laboratory and observed around stellar environments derives from O-rich red giant and AGB stars [1,2]. Supernova (SN) silicates and oxides are comparatively rare, and fewer than 10 grains from no-va or binary star systems have been identified to date. Very little is known about dust formation in these stellar environments. Mineralogical studies of only three O-rich SN [3-5] and no nova grains have been performed. Here we report the microstructure and chemical makeup of two SN silicates and one nova grain.

Nguyen, A. N.↗

DOE OSTI2022

High-velocity Stars in SDSS/APOGEE DR17

We report 23 stars having Galactocentric velocities larger than 450 km s -1 in the final data release of the APOGEE survey. This sample was generated using space velocities derived by complementing the high-quality radial velocities from the APOGEE project in Sloan Digital Sky Survey's Data Release 17 (DR17) with distances and proper motions from Gaia early Data Release 3 (eDR3). We analyze the observed kinematics and derived dynamics of these stars, considering different potential models for the Galaxy. We find that three stars could be unbound depending on the adopted potential, but in general all of the stars show typical kinematics of halo stars. The APOGEE DR17 spectroscopic results and Gaia eDR3 photometry are used to assess the stellar parameters and chemical properties of the stars. All of the stars belong to the red giant branch, and, in general, they follow the abundance pattern of typical halo stars. There are a few exceptions that would deserve further analysis through high-resolution spectroscopy. In particular, we identify a high-velocity Carbon-Enhanced Metal-Poor star, with a Galactocentric velocity of 482 km s -1 . We do not confirm any hypervelocity star in the sample, but this result is very sensitive to the adopted distances and less sensitive to the Galactic potential.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback

We report the discovery of three faint and ultra-faint dwarf galaxies -- Sculptor A, Sculptor B and Sculptor C -- in the direction of NGC 300 (D=2.0 Mpc), a Large Magellanic Cloud-mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or HI gas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C (D=2.04$^{+0.10}_{-0.13}$ Mpc; $M_V=-9.1\pm 0.1$ mag or $L_V=(3.7^{+0.4}_{-0.3})\times 10^5L_{\odot})$ is consistent with being a satellite of NGC 300. Sculptor A (D=1.35$^{+0.22}_{-0.08}$ Mpc; $M_V=-6.9\pm 0.3$ mag or $L_V=(5^{+1}_{-1})\times 10^4 L_{\odot})$ is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultra-faint Tucana B in terms of its physical properties and environment. Sculptor B (D=2.48$^{+0.21}_{-0.24}$ Mpc; $M_V=-8.1\pm 0.3$ mag or $L_V=(1.5^{+0.5}_{-0.4})\times 10^5L_{\odot})$ is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are $\gtrsim 2-4 r_{vir}$ from NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram pressure stripping, reionization and internal feedback in influencing the star formation history of the faintest stellar systems.

79 ASTRONOMY AND ASTROPHYSICS↗