Cosmological Recombination of Lithium and its Effect on the Microwave Background Anisotropies

The cosmological recombination history of lithium, produced during Big--Bang nucleosynthesis, is presented using updated chemistry and cosmological parameters consistent with recent cosmic microwave background (CMB) measurements. For the popular set of cosmological parameters, about a fifth of the lithium ions recombine into neutral atoms by a redshift $z\sim 400$. The neutral lithium atoms scatter resonantly the CMB at 6708 \AA and distort its intensity and polarization anisotropies at observed wavelengths around $\sim 300 \mu$m, as originally suggested by Loeb (2001). The modified anistropies resulting from the lithium recombination history are calculated for a variety of cosmological models and found to result primarily in a suppression of the power spectrum amplitude. Significant modification of the power spectrum occurs for models which assume a large primordial abundance of lithium. While detection of the lithium signal might prove difficult, if offers the possibility of inferring the lithium primordial abundance and is the only probe proposed to date of the large-scale structure of the Universe for $z\sim 500-100$.Comment: 20 pages, 7 figure

Rovibrationally resolved photodissociation of HeH+

Accurate photodissociation cross sections have been obtained for the A-X electronic transition of HeH+ using ab initio potential curves and dipole transition moments. Partial cross sections have been evaluated for all rotational transitions from the vibrational levels v"=0-11 and over the entire accessible wavelength range 100-1129 Angstrom. Assuming a Boltzmann distribution of the rovibrational levels of the X state, photodissociation cross sections are presented for temperatures between 500 and 12,000 K. A similar set of calculations was performed for the pure rovibrational photodissociation in the X-X electronic ground state, but covering photon wavelengths into the far infrared. Applications of the cross sections to the destruction of HeH+in the early Universe and in UV-irradiated environments such as primordial halos and protoplanetary disks are briefly discussed

A New Definition of Exoplanet Habitability: Introducing the Photosynthetic Habitable Zone

It may be possible to detect biosignatures of photosynthesis in an exoplanet's atmosphere. However, such a detection would likely require a dedicated study, occupying a large amount of telescope time. It is therefore prudent, while searching for signs of life that we may recognise, to pick the best target possible. In this work, we present a new region, the ``photosynthetic habitable zone'' \textemdash the distance from a star where both liquid water and oxygenic photosynthesis can occur. It is therefore the region where detectable biosignatures of oxygenic photosynthesis are most likely to occur. Our analysis indicates that in the most ideal conditions for life and no atmospheric effects, the photosynthetic habitable zone is almost as broad as the habitable zone. On the other hand, if conditions for life are anything less than excellent and atmospheric effects are even moderate, the photosynthetic habitable zone is concentrated at larger separations around more massive stars. Such cases are also not tidally locked to their host star, which could result in planetary rotation periods similar to the Earth's. We identify five planets, Kepler-452 b, Kepler-1638 b, Kepler-1544 b and Kepler-62 e and Kepler-62 f, that are consistently in the photosynthetic habitable zone for a variety of conditions, and we predict their day lengths to be between 9 and 11 hours. We conclude that the parameter space in which we should search for signs of life is much narrower than the standard habitable zone.Comment: 12 pages, 3 figures, accepted to ApJ