Using the Arecibo Observatory we have obtained neutral hydrogen (HI) absorption and emission spectral pairs in the direction of 26 background radio continuum sources in the vicinity of the Perseus molecular cloud. Strong absorption lines were detected in all cases allowing us to estimate spin temperature (T s ) and optical depth for 107 individual Gaussian components along these lines of sight. Basic properties of individual HI clouds (spin temperature, optical depth, and the column density of the cold and warm neutral medium, CNM and WNM) in and around Perseus are very similar to those found for random interstellar lines of sight sampled by the Millennium HI survey. This suggests that the neutral gas found in and around molecular clouds is not atypical. However, lines of sight in the vicinity of Perseus have on average a higher total HI column density and the CNM fraction, suggesting an enhanced amount of cold HI relative to an average interstellar field. Our estimated optical depth and spin temperature are in stark contrast with the recent attempt at using Planck data to estimate properties of the optically thick HI. Only ∼ 15% of lines of sight in our study have a column density weighted average spin temperature lower than 50 K, in comparison with ∼ > 85% of Planck's sky coverage. The observed CNM fraction is inversely proportional to the optical-depth weighted average spin temperature, in excellent agreement with the recent numerical simulations by Kim et al. While the CNM fraction is on average higher around Perseus relative to a random interstellar field, it is generally low, 10 − 50%. This suggests that extended WNM envelopes around molecular clouds, and/or significant mixing of CNM and WNM throughout molecular clouds, are present and should be considered in the models of molecule and star formation. Our detailed comparison of HI absorption with CO emission spectra shows that only 3/26 directions are clear candidates for probing the CO-dark gas as they have N (HI) > 10 21 cm −2 yet no detectable CO emission.

We investigate the impact of high optical depth on the HI saturation observed in the Perseus molecular cloud by using Arecibo HI emission and absorption measurements toward 26 radio continuum sources. The spin temperature and optical depth of individual HI components are derived along each line-of-sight, enabling us to estimate the correction for high optical depth. We examine two different methods for the correction, Gaussian decomposition and isothermal methods, and find that they are consistent (maximum correction factor ∼ 1.2) likely due to the relatively low optical depth and insignificant contribution from the diffuse radio continuum emission for Perseus. We apply the correction to the optically thin HI column density on a pixel-by-pixel basis, and find that the total HI mass increases by ∼10%. Using the corrected HI column density image and far-infrared data from the IRIS Survey, we then derive the H 2 column density on ∼0.4 pc scales. For five dark and star-forming sub-regions, the HI surface density is uniform with Σ HI ∼ 7-9 M ⊙ pc −2 , in agreement with the minimum HI surface density required for shielding H 2 against photodissociation. As a result, Σ H2 /Σ HI and Σ HI + Σ H2 show a tight relation. Our results are consistent with predictions for H 2 formation in steady state and chemical equilibrium, and suggest that H 2 formation is mainly responsible for the Σ HI saturation in Perseus. We also compare the optically thick HI with the observed "CO-dark" gas, and find that the optically thick HI only accounts for ∼20% of the "CO-dark" gas in Perseus.

The Late Devonian was a protracted period of low speciation resulting in biodiversity decline, culminating in extinction events near the Devonian–Carboniferous boundary. Recent evidence indicates that the final extinction event may have coincided with a dramatic drop in stratospheric ozone, possibly due to a global temperature rise. Here we study an alternative possible cause for the postulated ozone drop: a nearby supernova explosion that could inflict damage by accelerating cosmic rays that can deliver ionizing radiation for up to ∼100 ky. We therefore propose that the end-Devonian extinctions were triggered by supernova explosions at ∼20 pc, somewhat beyond the “kill distance” that would have precipitated a full mass extinction. Such nearby supernovae are likely due to core collapses of massive stars; these are concentrated in the thin Galactic disk where the Sun resides. Detecting either of the long-lived radioisotopes Sm146 or Pu244 in one or more end-Devonian extinction strata would confirm a supernova origin, point to the core-collapse explosion of a massive star, and probe supernova nucleosynthesis. Other possible tests of the supernova hypothesis are discussed.

Iron-TAML activators of peroxides are functional mimics of peroxidase and short-circuited cytochrome P450 enzymes that perform numerous transformations that appear by all measures to date to be fully life cycle compatible with the environment. Here we show how to design the same catalytic chemistry without the direct addition of H 2 O 2 , thereby removing the need to transport and store hydrogen peroxide. In this new approach, dioxygen is reduced in situ by D-glucose in the presence of glucose oxidase (GO) from Aspergillus niger. The resulting tandem TAML-GO system exhibits similar catalytic efficiency to the TAML/H 2 O 2 prototype. The tandem system is shown here to efficiently decolorize Orange II and oxidize NADH, both near neutral pH (7.5). Computational simulations of the kinetic data suggest that denaturing of GO by oxidized TAML intermediates does not occur throughout each process. This tandem system brings the advantage of in situ generation of low concentrations of H 2 O 2 during the catalytic cycles. This minimizes both unproductive H 2 O 2 consumption resulting from the catalase-like activity of iron TAMLs and suicidal inactivation. Kinetic data for oxidation of NADH by TAML/H 2 O 2 are also reported.