Arnold I. Boothroyd

A refined H3 potential energy surface

In evaluating some low temperature (T<1000 K) thermal rate coefficients for inelastic rotational excitation of H2 by H atoms, Sun and Dalgarno have found a marked sensitivity to the potential energy surface adopted for the calculations. We have investigated the origin of the discrepancies between previous H3 potential energy surfaces and have developed a refined surface which addresses these concerns. New quasiclassical trajectory calculations of cross sections for low energy rotational excitation are reported. The refined surface is based on 8701 ab initio energies, most newly computed for this purpose. It has the same functional form as our earlier (BKMP) surface, but since the fit of the parameters is more fully constrained than for any previous surface it is a more accurate representation. The refined surface matches the ab initio energies with an overall rms error of 0.27 mEh (i.e., 0.17 kcal/mol) and a maximum absolute deviation of 6.2 mEh (for a very compact high energy equilateral triangle conform...

Our Sun. III: Present and future

Self-consistent evolutionary models were computed for our Sun, using Los Alamos interior opacities and Sharp molecular opacities, starting with contraction on the Hayashi track, and fitting the observed present solar L, R, and Z/X at the solar age. This resulted in presolar Y=0.274 and Z=0.01954, and in present solar 37 Cl and 71 Ga neutrino capture rates or 6.53 and 123 SNU, respectively. We explored the Suns future. While on the hydrogen-burning main sequence, the Suns luminosity grows from 0.7 L ○. , 4.5 Gyr ago, to 2.2 L ○. , 6.5 Gyr from now

The creation of superrich lithium giants

A time-dependent «convective diffusion» algorithm has been coupled for the first time with a self-consistent full evolutionary computation in order to investigate theoretically the creation of superrich lithium stars on the asymptotic giant branch. Stars of 3, 4, 5, 6, and 7 M ⊙ stars are considered, for Z = 0.02 and Z = 0.001. Superrich lithium stars were produced when convective envelope base temperatures exceeded 50 × 10 6 K. Abundances of log e( 7 Li) ∼4.5 were obtained for stars of mass 4-6 M ⊙ , lying in the luminosity range M bol ∼ -6 to -7, in excellent agreement with observations

OUR SUN. V. A BRIGHT YOUNG SUN CONSISTENT WITH HELIOSEISMOLOGY AND WARM TEMPERATURES ON ANCIENT EARTH AND MARS

The relatively warm temperatures required on early Earth and Mars have been difficult to account for via warming from greenhouse gases. We tested whether this problem can be resolved for both Earth and Mars by a young Sun that is brighter than predicted by the standard solar model (SSM). We computed high-precision solar evolutionary models with slightly increased initial masses of Mi = 1.01-1.07 M☉; for each mass, we considered three different mass-loss scenarios. We then tested whether these models were consistent with the current high-precision helioseismic observations. The relatively modest mass-loss rates in these models are consistent with observational limits from young stars and estimates of the past solar wind obtained from lunar rocks and do not significantly affect the solar lithium depletion. For appropriate initial masses, all three mass-loss scenarios are capable of yielding a solar flux 3.8 Gyr ago high enough to be consistent with water on ancient Mars. The higher flux at the planets is due partly to the fact that a more massive young Sun would be intrinsically more luminous and partly to the fact that the planets would be closer to the more massive young Sun. At birth on the main sequence, our preferred initial mass Mi = 1.07 M☉ would produce a solar flux at the planets 50% higher than that of the SSM, namely, a flux 5% higher than the present value (rather than 30% lower, which the SSM predicts). At first (for 1-2 Gyr), the solar flux would decrease; subsequently, it would behave more like the flux in the SSM, increasing until the present. We find that all of our mass-losing solar models are consistent with the helioseismic observations; in fact, our preferred mass-losing case with Mi = 1.07 M☉ is in marginally (although insignificantly) better agreement with the helioseismology than is the SSM. The early solar mass loss of a few percent does indeed leave a small fingerprint on the Suns internal structure. However, for helioseismology to significantly constrain early solar mass loss would require higher accuracy in the observed solar parameters and input physics, namely, by a factor of ~3 for the observed solar surface composition and a factor of ~2 for the solar interior opacities, the p-p nuclear reaction rate, and the diffusion constants for gravitational settling.

An improved H3 potential energy surface

We report ab initio calculations of the ground state energy for 404 new conformations of H3, supplementing the set of 368 conformations reported previously by others. The entire dataset has been used to constrain an analytical functional form for the potential energy surface, building on that of Truhlar and Horowitz. The new surface extends the Truhlar and Horowitz surface to higher energies and offers some modest improvement at lower energies. In addition, we have eliminated a problem with derivatives of the London equation that was pointed out by Johnson. The new surface matches the 772 ab initio energies with an overall root‐mean‐square (rms) error of 0.25 mhartree (i.e., 0.16 kcal/mol) and a maximum absolute deviation of 1.93 mhartree (1.21 kcal/mol); for ‘‘noncompact’’ conformations (no interatomic distance smaller than 1.15 bohr), the rms error is 0.17 mhartree (0.11 kcal/mol) and the maximum absolute deviation is 1.10 mhartree (0.69 kcal/mol). The classical barrier height for H+H2→H2+H is estimated...

An accurate analytic H4 potential energy surface

The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. The 6101 ab initio H4 energies reported in 1991 by Boothroyd et al. demonstrated large inaccuracies in analytic H4 surfaces available at that time. Some undesirable features remained in the more accurate H4 surfaces fitted to these energies by Keogh and by Aguado et al., due in part to the relatively sparse coverage of the six-dimensional H4 conformation space afforded by the 6101 ab initio energies. To improve the coverage, 42 079 new ab initio H4 energies were calculated, using Buenker’s multiple reference (single and) double excitation configuration interaction program. Here the lowest excited states were computed as well as the ground state, and ...

Our Sun. I, The standard model : successes and failures

The results of computing a number of standard solar models are reported. A presolar helium content of Y = 0.278 is obtained, and a Cl-37 capture rate of 7.7 SNUs, consistently several times the observed rate of 2.1 SNUs, is determined. Thus, the solar neutrino problem remains. The solar Z value is determined primarily by the observed Z/X ratio and is affected very little by differences in solar models. Even large changes in the low-temperature molecular opacities have no effect on Y, nor even on conditions at the base of the convective envelope. Large molecular opacities do cause a large increase in the mixing-length parameter alpha but do not cause the convective envelope to reach deeper. The temperature remains too low for lithium burning, and there is no surface lithium depletion; thus, the lithium problem of the standard solar model remains. 103 refs.

RADIO POLARIMETRY OF THE ELAIS N1 FIELD: POLARIZED COMPACT SOURCES

We present deep polarimetric observations at 1420 MHz of the European Large Area ISO Survey North 1 region (ELAIS N1) as part of the Dominion Radio Astrophysical Observatory Planck Deep Fields project. By combining closely spaced aperture synthesis fields, we image a region of 7.43 deg^2 to a maximum sensitivity in Stokes Q and U of 78 μJy beam^(-1), and detect 786 compact sources in Stokes I. Of these, 83 exhibit polarized emission. We find that the differential source counts (log N-log p) for polarized sources are nearly constant down to p > 500 μJy, and that these faint polarized radio sources are more highly polarized than the strong source population. The median fractional polarization is 4.8% ± 0.7% for polarized sources with Stokes I flux density between 10 and 30 mJy, approximately 3 times larger than sources with I > 100 mJy. The majority of the polarized sources have been identified with galaxies in the Spitzer Wide Area Infrared Extragalactic Survey (SWIRE) image of ELAIS N1. Most of the galaxies occupy regions in the IRAC 5.8 μm/3.6 μm versus 8.0 μm/4.5 μm color-color diagram associated with dusty AGNs, or with ellipticals with an aging stellar population. A few host galaxies have colors that suggests significant PAH emission in the near-infrared. A small fraction, 12%, of the polarized sources are not detected in the SWIRE data. None of the polarized sources in our sample appears to be associated with an actively star-forming galaxy.

Accurate analytic He–H2 potential energy surface from a greatly expanded set of ab initio energies

The interaction potential energy surface (PES) of He–H2 is of great importance for quantum chemistry, as the simplest test case for interactions between a molecule and a closed-shell atom. It is also required for a detailed understanding of certain astrophysical processes—namely, collisional excitation and dissociation of H2 in molecular clouds—at densities too low to be accessible experimentally. A new set of 23 703 ab initio energies was computed for He–H2 geometries where the interaction energy was expected to be non-negligible. These have an estimated rms “random” error of ∼0.2 mhartree and a systematic error of ∼0.6 mhartree (0.4 kcal/mol). A new analytic He–H2 PES, with 112 parameters, was fitted to 20 203 of these new ab initio energies (and to an additional 4862 points generated at large separations). This yielded an improvement by better than an order of magnitude in the fit to the interaction region, relative to the best previous surfaces (which were accurate only for near-equilibrium H2 molecul...

Accurate abinitio potential energy computations for the H4 system: Tests of some analytic potential energy surfaces

The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry, as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely, collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. Accurate ab initio energies were computed for 6046 conformations of H4, using a multiple reference (single and) double excitation configuration interaction (MRD‐CI) program. Both systematic and ‘‘random’’ errors were estimated to have an rms size of 0.6 mhartree, for a total rms error of about 0.9 mhartree (or 0.55 kcal/mol) in the final ab initio energy values. It proved possible to include in a self‐consistent way ab initio energies calculated by Schwenke, bringing the number of H4 conformations to 6101. Ab initio energies were also computed for 404 conformations of H3; adding ab initio energies calculated by other authors yielded a total of 772 ...