R. Cowsik

CONSTRAINTS ON STELLAR GRAIN FORMATION FROM PRESOLAR GRAPHITE IN THE MURCHISON METEORITE

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3E9 km) extracted from the Murchison meteorite. The spherules have 12C/13C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission elec- tron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite struc- ture consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3E4 nm in diameter, with little graphitic layering order. These sheets prob- ably condensed as isolated particles that subsequently coalesced to form the cores, after which the sur- rounding graphitic mantles were added by vapor deposition. We also detected internal crystals of metal carbides in one-third of the spherules. These crystals (5E200 nm) have compositions ranging from nearly pure TiC to nearly pure Zr-Mo carbide. Some of these car- bides occur at the centers of the spherules and are surrounded by well-graphitized carbon, having evi- dently served as heterogeneous nucleation centers for condensation of carbon. Others were entrained by carbon as the spherules grew. The chemical and textural evidence indicates that these carbides formed prior to carbon condensation, which indicates that the C/O ratios in the stellar sources were very close to unity. Only one of the 67 spherules studied in the TEM contained SiC, from which we infer that carbon condensation nearly always preceded SiC formation. This observation places stringent limits on the possible delay of graphite formation and is consistent with the predictions of equilibrium thermody- namics in the inferred range of pressure and C/O ratios. We model the formation of the observed refractory carbides under equilibrium conditions, both with and without s-process enrichment of Zr and Mo, and show that the chemical variation among internal crystals is consistent with the predicted equilibrium condensation sequence. The compositions of most of the Zr-Mo-Ti carbides require an s-process enrichment of both Zr and Mo to at least 30 times their solar abundances relative to Ti. However, to account for crystals in which Mo is also enriched relative to Zr, it is necessary to suppose that Zr is removed by separation of the earliest formed ZrC crystals from their parent gas. We also explore the formation constraints imposed by kinetics, equilibrium thermodynamics, and the observation of clusters of carbide crystals in some spherules, and conclude that relatively high formation pressures dynes cm~2), and/or condensable carbon number densities cm~3) are required. (Z0.1 (Z108 The graphite spherules with 12C/13C ratios less than the solar value may have originated in AGB stellar winds. However, in the spherically symmetric AGB atmospheres customarily assumed in models of stellar grain formation, pressures are much too low (by factors of to produce carbide crystals or Z102) graphite spherules of the sizes observed within plausible timescales. If some of the graphite spherules formed in the winds from such stars, it thus appears necessary to assume that the regions of grain forma- tion are density concentrations with length scales less than a stellar radius. Some of the spherules with both 12C/13C ratios greater than the solar value and 28Si excesses probably grew in the ejecta of super- novae. The isotopic compositions and growth constraints imply that they must have formed at high den- sities (e.g., with g cm~3) from mixtures of inner-shell material with material from the C-rich

Superluminal neutrinos at OPERA confront pion decay kinematics.

Violation of Lorentz invariance (VLI) has been suggested as an explanation of the superluminal velocities of muon neutrinos reported by OPERA. In this Letter, we show that the amount of VLI required to explain this result poses severe difficulties with the kinematics of the pion decay, extending its lifetime and reducing the momentum carried away by the neutrinos. We show that the OPERA experiment limits α=(ν(ν)-c)/c<4×10(-6). We then take recourse to cosmic-ray data on the spectrum of muons and neutrinos generated in Earths atmosphere to provide a stronger bound on VLI: (ν-c)/c<10(-12).

CONSTRAINTS ON GRAIN FORMATION AROUND CARBON STARS FROM LABORATORY STUDIES OF PRESOLAR GRAPHITE

We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, with and without previously formed internal crystals of titanium carbide (TiC). A lower mass limit of 1.1 Mfor stars capable of contributing grains to the solar nebula isderived. Thismass limit, in conjunction with a mass-luminosity relation for carbon stars, identifies the region of the H-R diagram relevant to the production of presolar graphite. Detailed dynamical models of AGB outflows, along with constraints provided by kinetics and equilibrium ther- modynamics, indicate that grain formation occurs at radiifrom2.3 to 3.7 AU for AGB carbon stars in the 1.1-5M� range. This analysis also yields time intervals available for graphite growth that are on the order of a few years. By considering the luminosity variations of carbon stars, we show that grains formed during minima in the luminosity are likely to be evaporated subsequently, while those formed at luminosity maxima will survive. We calculate strict upper limits on grain sizes for graphite and TiC in spherically symmetric AGB outflows. Graphite grains can reach diameters in the observed micron size range (1-2 � m) only under ideal growth conditions (perfect sticking efficiency,noevaporation,nodepletionofgasspeciescontributingtograingrowth),andthenonlyinoutflowsfrom carbon stars with masses P2.5 M� . The same is true for TiC grains that are found within presolar graphite, which have mean diameters of 24 � 14 nm. In general, the mass-loss rates that would be required to produce the observed grain sizes in spherically symmetric outflows are at least an order of magnitude larger than the maximum observed AGB carbon star mass-loss rates. These results, as well as pressure constraints derived from equilibrium ther- modynamics,forceustoconcludethatpresolargraphiteandTiCmustforminregionsofenhanceddensity(clumps, jets)inAGBoutflowshavingsmallangularscales.AsshowninthecompanionpaperbyCroatetal.,theenrichment of 12 C in many AGB graphites, and the overabundances of the s-process elements Mo, Zr, and Ru in the carbides found within them, often greatly exceed the values observed astronomically in AGB outflows. These observations notonlylendfurthersupporttotheideathattheoutflowsareclumpy,butalsoimplythattheoutflowingmatterisnot well mixed in the circumstellar envelope out to the radii where grain condensation takes place. Subject headingg dust, extinction — stars: AGB and post-AGB — stars: carbon

The afterglow and the host galaxy of GRB 011211

We present optical, near-infrared, and X-ray observations of the optical afterglow (OA) of the X-ray rich, long- duration gamma-ray burst GRB 011211. Hubble Space Telescope (HST) data obtained 14, 26, 32, and 59 days after the burst, show the host galaxy to have a morphology that is fairly typical of blue galaxies at high redshift. We measure its magnitude to be R= 24:95 0:11. We detect a break in the OA R-band light curve which is naturally accounted for by a collimated outflow geometry. By fitting a broken power-law to the data we find a best fit with a break 1 :56 0:02 days after the burst, a pre-break slope of1 = 0:95 0:02, and a post-break slope of2 = 2:11 0:07. The UV-optical spectral energy distribution (SED) around 14 hours after the burst is best fit with a power-law with index = 0:56 0:19 reddened by an SMC-like extinction law with a modest AV= 0:08 0:08 mag. By comparison, from the XMM-Newton X-ray data at around the same time, we find a decay index ofX= 1:62 0:36 and a spectral index ofX= 1:21 +0:10 0:15 . Interpolating between the UV-optical and X-ray implies that the cooling frequency is located close to10 16 Hz in the observer frame at the time of the observations. We argue, using the various temporal and spectral indices above, that the most likely afterglow model is that of a jet expanding into an external environment that has a constant mean density rather than a wind-fed density structure. We estimate the electron energy index for this burst to be p 2:3.

Sources of cosmic ray electrons

The spectrum of cosmic ray electrons is discussed in terms of the contributions from discrete sources distributed over the Galaxy. The analysis shows that one needs sources situated within a few hundred parsecs of the solar system, in order that the radiative losses of energy do not induce a premature cutoff in the energy spectrum. If the mean spatial density of the sources is roughly uniform over the galactic plane, then there should be at least 3 x 10/sup 4/ active sources accelerating cosmic rays in the Galaxy. Therefore it is very unlikely that supernovae are the only sources of cosmic ray electrons in the energy range 1--1000 GeV.

The origin of the spectral intensities of cosmic-ray positrons

We present a straightforward model of cosmic-ray propagation in the Galaxy that can account for the observed cosmic-ray positrons entirely as secondary products of cosmic-ray interactions with the interstellar medium. In addition to accounting for the observed energy dependence of the ratio of positrons to total electrons, this model can accommodate both the observed energy dependence of secondary to primary nuclei, like boron/carbon, and the observed bounds on the anisotropy of cosmic rays. This model also predicts the energy dependence of the positron fraction at energies higher than those measured to date, with the ratio rising to ~0.7 at very high energies. The model presented in this paper arises as a natural extension of the widely used current models and allows one to include the spatial and temporal discreteness of the sources of cosmic rays.

Conditions in stellar outflows inferred from laboratory studies of presolar grains

Presolar grains from meteorites provide direct information on the characteristics of stellar condensates. We review the state of knowledge regarding the origins and properties of presolar grains that has been derived from microanalytical studies of individual particles in the laboratory. We show how these observations can be interpreted through kinetic and equilibrium condensation models to give insights into the physico-chemical conditions in mass outflows from stars.

Comments on stochastic acceleration of cosmic rays

The spectrum of particles subjected to second-order Fermi acceleration is studied, and the spectra of both the primary particles such as C and O and their nuclear secondaries such as Li, Be, and B are derived explicitly. The second-order term is seen to be of prime importance in the acceleration process. The secondary-to-primary ratio is shown to increase logarithmically with energy contrary to observation. These considerations are extended to arbitrary stochastic processes, and it is concluded that cosmic rays are not accelerated in the interstellar medium.

X-ray polarimetry with the Polarization Spectroscopic Telescope Array (PolSTAR)

This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASA’s 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3–50 keV (requirement; goal: 2.5–70 keV) X-rays probing the behavior of matter, radiation and the very fabric of spacetime under the extreme conditions close to the event horizons of black holes, as well as in and around magnetars and neutron stars. The PolSTAR design is based on the technology developed for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission launched in June 2012. In particular, it uses the same X-ray optics, extendable telescope boom, optical bench, and CdZnTe detectors as NuSTAR. The mission has the sensitivity to measure ∼1% linear polarization fractions for X-ray sources with fluxes down to ∼5 mCrab. This paper describes the PolSTAR design as well as the science drivers and the potential science return.

Early Observations of the Afterglow of GRB 000301c

We report multiband observations of the optical transient associated with GRB 000301c carried out 2000 March 2-4 using the 2.34 m Vainu Bappu Telescope at Kavalur, India. When combined with other reported data, the initial decline in the R-band magnitude with log(t - t0), the time since the burst is fitted with a slope of α1 = -0.70 ± 0.07, which steepens after about 6.0 days to a slope of α2 = -2.44 ± 0.29. This change in slope does not occur smoothly, but there is an indication for a bimodal distribution. The available measurements of the evolution of (B-R) color do not show any discernible evolution in the first 12 days.