119089633
3He in Planetary Nebulae: A Challenge to Stellar Evolution Models
The discrepancy between the observed abundances of 3He in the interstellar medium and those predicted by stellar and Galactic chemical evolution remains largely unexplained. In this paper, we attempt to shed some light on this unsolved problem by presenting a quantitative comparison of the 3He abundances recently measured in six planetary nebulae (IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, NGC 7662) with the corresponding predictions of stellar evolution theory. The determination of the mass of the planetary nebulae progenitors allows us to dismiss, with a good degree of confidence, the hypothesis that the abundance of 3He in the envelopes of all low-mass stars (M 2.5 M?) is strongly reduced with respect to the standard theoretical values by some mixing mechanism acting in the latest phases of stellar evolution. The abundance versus mass correlation, with allowance made for the limitation of the sample, is in fact found to be fully consistent with the classical prediction of stellar evolution. We examine the implications of this result on the Galactic evolution of 3He with the help of a series of models with standard and nonstandard (i.e.,3He-depleted) nucleosynthesis prescriptions in varying percentages of low-mass stars. The results are found to be consistent with the abundances determined in the presolar material and in the local interstellar medium only if the vast majority of low-mass stars (more than 70%-80%) follow nonstandard prescriptions. This implies that either the sample of planetary nebulae under examination is highly biased and therefore not representative of the whole population of low-mass stars, or the solution to the 3He problem lies elsewhere.
18590
The Chemistry of the early universe
The process of molecule formation in the primor- dial gas is considered in the framework of Friedmann cosmo- logical models from redshift z =1 0 4 to z =0 . First, a com- prehensive analysis of 87 gas phase reaction rates (both col- lisional and radiative) relevant in the physical environment of the expanding universe is presented and critically discussed. On this basis, calculations are carried out of the abundance of 21 molecular species as function of redshift, for different values of the cosmological parameters 0, and H0, evaluating con- sistently the molecular heating and cooling due to H2 ,H D and LiH molecules. One of the major improvements of this work is the use of a better treatment of H recombination that leads to a reduction of a factor 2-3 in the abundance of electrons and H + at freeze-out, with respect to previous studies. The lower residual ionization has a negative effect on the chemistry of the primordial gas in which electrons and protons act as cat- alysts in the formation of the first molecules. We find that in the standard model (h =0 :67, 10 =4 :5, 0 =1 and (D/H) =4 :3 10 5 ), the residual fractional ionization at z =1 is (e= H) =3 :0210 4 , and the main molecular species fractional abundances (H2= H) =1 :1 10 6 , (HD=H 2 )=1 :1 10 3 , (HeH + = H) =6 :2 10 13 , (LiH + = H) =9 :4 10 18 and (LiH=LiH + )=7 :6 10 3 . We devise a reduced chemical network that reproduces with excellent accuracy the numerical results of the complete model and allows to follow the chemical compositions and the thermal properties of a primordial gas in the presence of an external radiation field. Finally, we provide accurate cooling functions of H2, HD and LiH in a wide range of density and temperature that can be conveniently used in a variety of cosmological applications.
15769380
Modeling the magnetic field in the protostellar source NGC 1333 IRAS 4A
Context. Magnetic fields are believed to play a crucial role in the process of star formation. Aims. We compare high-angular resolution observations of the submillimeter polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a low-mass protostar, with models of the collapse of magnetized molecular cloud cores. Methods. Assuming a uniform dust alignment efficiency, we computed the Stokes parameters and synthetic polarization maps from the model density and magnetic field distribution by integrations along the line-of-sight and convolution with the interferometric response. Results. The synthetic maps are in good agreement with the data. The best-fitting models were obtained for a protostellar mass of 0.8 M� ,o f age 9× 10 4 yr, formed in a cloud with an initial mass-to-flux ratio ∼2 times the critical value. Conclusions. The magnetic field morphology in NGC 1333 IRAS 4A is consistent with the standard theoretical scenario for the formation of solar-type stars, where well-ordered, large-scale, rather than turbulent, magnetic fields control the evolution and collapse of the molecular cloud cores from which stars form.
119561246
Formation of OB associations in galaxies
We consider the formation of OB associations from two perspectives: (1) the fractional gas consumption in star formation, , per dynamical timescale tdyn in a galaxy, and (2) the origin of the so-called Kennicutt-Schmidt law, that the rate of star formation per unit area is proportional to a power, α, of the surface density in H I and H2 gas when certain thresholds are crossed. The empirical findings that ≈ 10-2 and α ≈ 1.4 or 1.5 have simple explanations if the rate of star formation is magnetically regulated. An empirical test of the ideas resides in an analysis of why giant OB associations are "strung out like pearls along the arms" of spiral galaxies.
14748555
Possible signature of hypernova nucleosynthesis in a beryllium‐rich halo dwarf★
As part of a large survey of halo and thick disc stars, we found one halo star, HD 106038, exceptionally overabundant in beryllium. In spite of its low metallicity, [Fe/H]=−1.26, the star has log(Be/H) =−10.60, which is similar to the solar meteoritic abundance, log(Be/H) =−10.58. This abundance is more than 10 times higher the abundance of stars with similar metallicity and cannot be explained by models of chemical evolution of the Galaxy that include the standard theory of cosmic ray spallation. No other halo star exhibiting such a beryllium overabundance is known. In addition, overabundances of Li, Si, Ni, Y and Ba are also observed. We suggest that all these chemical peculiarities, excepting the Ba abundance, can be simultaneously explained if the star was formed in the vicinity of a hypernova.
53662238
MAGNETIC INTERACTIONS IN PRE-MAIN-SEQUENCE BINARIES
Young stars typically have strong magnetic fields, so that the magnetospheres of newly formed close binaries can interact, dissipate energy, and produce synchrotron radiation. The V773 Tau A binary system, a pair of T Tauri stars with a 51 day orbit, displays such a signature, with peak emission taking place near periastron. This paper proposes that the observed emission arises from the change in energy stored in the composite magnetic field of the system. We model the fields using the leading order (dipole) components and show that this picture is consistent with current observations. In this model, the observed radiation accounts for a fraction of the available energy of interaction between the magnetic fields from the two stars. Assuming antisymmetry, we compute the interaction energy E int as a function of the stellar radii, the stellar magnetic field strengths, the binary semimajor axis, and orbital eccentricity, all of which can be measured independently of the synchrotron radiation. The variability in time and energetics of the synchrotron radiation depend on the details of the annihilation of magnetic fields through reconnection events, which generate electric fields that accelerate charged particles, and how those charged particles, especially fast electrons, are removed from the interaction region. However, the major qualitative features are well described by the background changes in the global magnetic configuration driven by the orbital motion. The theory can be tested by observing a collection of pre-main-sequence binary systems.
121570479
Compatibility and possible implications of the experimental results on solar neutrinos
Abstract The effect of varying the assumed value of the cross-section for the proton-proton fusion weak reaction of the solar cycle on the solar neutrino problem is discussed. It is shown that an increase of about 20% of the relevant astrophysical factor fields compatibility (within the present experimental accuracy) of the available results with theory and among themselves.
118241507
Searching for micro black holes at LHC
An exciting feature of brane-world models with large extra dimensions [1] is that the fundamental scale of gravity M G could be as low as the electroweak scale (M G ≃ 1TeV). Since the mass of a black hole must be larger than M G, otherwise the classical concept of event horizon would be lost, micro black holes of a few TeV’s may therefore be produced at the LHC in this scenario (for some recent reviews, see [2]).
54034724
The HERA-B electromagnetic calorimeter pre-trigger system
SummaryIn this document the pretrigger system developed for the electromagnetic calorimeter of the HERA-B experiment is described. The system owns a lot of frontier features for calorimetry-based triggers and its design performances are such that some possible implementations of it can be foreseen for the future high-energy collider experiments.
118941207
Present knowledge of the Cabibbo-Kobayashi-Maskawa matrix
A complete review of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements and of the experimental methods for their determination is presented. A critical analysis of the relevant experimental results, and in particular of the most recent ones, allows to improve the accuracies of all the matrix elements. A chi-square minimization with the three-family unitarity constraint on the CKM matrix is performed to test the current interpretation of the CP violating phenomena inside the Standard Model. A complete and unambiguous solution satisfying all the imposed constraints is found. As a by-product of the fit, the precision on the values of the matrix elements is further increased and it is possible to obtain estimates for the important CP violation observables $sin 2\beta$, $sin 2\alpha$ and $\gamma$. Finally, an independent estimation of the CKM elements based on a Bayesian approach is performed. This complementary method constitutes a check of the results obtained, providing also the probability functions of the CKM elements and of the related quantities.
67702235
The Task Manager for the LHCb On-Line Farm