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Photodissociation by ultraviolet radiation is the key destruction pathway for CS in photon-dominated regions, such as diffuse clouds. However, the large uncertainties of photodissociation cross sections and rates of CS, resulting from a lack of both laboratory experiments and theoretical calculations, limit the accuracy of calculated abundances of S-bearing molecules by modern astrochemical models. Here we show a detailed ab initio study of CS photodissociation. Accurate potential energy curves of CS electronic states were obtained by choosing an active space CAS(8,10) in MRCI+Q/aug-cc-pV(5+d)Z calculation with additional diffuse functions, with a focus on the B and C 1Σ+ states. Cross sections for both direct photodissociation and predissociation from the vibronic ground state were calculated by applying the coupled-channel method. We found that the C - X (0 - 0) transition has extremely strong absorption due to a large transition dipole moment in the Franck-Condon region, and the upper state is resonant with several triplet states via spin-orbit couplings, resulting in predissociation to the main atomic products C (3 P) and S (1 D). Our new calculations show that the photodissociation rate under the standard interstellar radiation field is 2.9-10-9 s-1, with a 57% contribution from C - X (0 - 0) transition. This value is larger than that adopted by the Leiden photodissociation and photoionization database by a factor of 3.0. Our accurate ab initio calculations will allow more secure determination of S-bearing molecules in astrochemical models.

Despite more than 30 years of searching, the compact object in Supernova (SN) 1987A has not yet been detected. We present new limits on the compact object in SN 1987A using millimeter, near-infrared, optical, ultraviolet, and X-ray observations from ALMA, VLT, HST, and Chandra. The limits are approximately 0.1 mJy ( erg s-1 cm-2 Hz-1) at 213 GHz, 1 L o ( erg s-1 cm-2 Hz-1) in the optical if our line of sight is free of ejecta dust, and 1036 erg s-1 ( erg s-1 cm-2 Hz-1) in 2-10 keV X-rays. Our X-ray limits are an order of magnitude less constraining than previous limits because we use a more realistic ejecta absorption model based on three-dimensional neutrino-driven SN explosion models. The allowed bolometric luminosity of the compact object is 22 L o if our line of sight is free of ejecta dust, or 138 L o if dust-obscured. Depending on assumptions, these values limit the effective temperature of a neutron star (NS) to -8 MK and do not exclude models, which typically are in the range 3-4 MK. For the simplest accretion model, the accretion rate for an efficiency η is limited to M o yr-1, which excludes most predictions. For pulsar activity modeled by a rotating magnetic dipole in vacuum, the limit on the magnetic field strength (B) for a given spin period (P) is G s-2, which firmly excludes pulsars comparable to the Crab. By combining information about radiation reprocessing and geometry, we infer that the compact object is a dust-obscured thermally emitting NS, which may appear as a region of higher-temperature ejecta dust emission.

NASA's Genesis mission reveals that the rare isotope 15N is approximately seven times more enriched than the rare isotopes 17O and 18O in the terrestrial planets relative to the Sun. Here, we explain this peculiar observation under the framework of self-shielding and the difference in chemical reactivity between the excited O(1 D) [N(2 D)] and the ground O(3 P) [N(4 S)] states produced by VUV photodissociation of CO [N2]. After weighting the absorption cross-sections for individual photodissociation bands, and taking into account the mutual shielding by H2, the CO/N2 ratio, and the partition of O and N among gas:ice:dust phases in the solar nebula, we show that the trapping of N(2 D) via hydrogenation is favored over that of O(1 D). This provides a possible explanation of the Genesis results and supports the self-shielding model as the primary mechanism for generating isotopic anomalies of O and N in the early solar nebula.

The branching ratios for the N(4S) + N(2D), N(4S) + N(2P), and N(2D) + N(2D) channels are measured for the photodissociation of in the vacuum ultraviolet (VUV) region of 100,808-122,159 cm-1 using the VUV-VUV pump-probe approach combined with velocity-map-imaging-photoion detection. No evidence of forming the ground-state N(4S) + N(4S) products is found. No potential barrier is observed for the N(2D) + N(2D) channel, but the N(4S) + N(2P) channel has a small potential barrier of ≈740 cm-1. The branching ratios are found to depend on the symmetry of predissociative N2 states instead of the total VUV excitation energy, indicating that N2 photodissociation is nonstatistical. When the branching ratios for N(4S) + N(2D) and N(4S) + N(2P) products are plotted as a function of the VUV excitation energy for the valence N21Πu and states, oscillations in these ratios are observed demonstrating how these channels are competing with each other. These data can be used to select both the velocity and internal states of the atomic products by picking the quantum state that is excited. High-level ab initio potential energy curves of the excited N2 states are calculated to provide insight into the mechanisms for the observed branching ratios. The calculations predict that the formation of both N(4S) + N(2D) and N(4S) + N(2P) channels involves potential energy barriers, in agreement with experimental observations. A discussion of the application of the present results to astronomy, planetary sciences, and comets is given.

Thirty years after the discovery of HIV-1, the early transmission, dissemination, and establishment of the virus in human populations remain unclear. Using statistical approaches applied to HIV-1 sequence data from central Africa, we show that from the 1920s Kinshasa (in what is now the Democratic Republic of Congo) was the focus of early transmission and the source of pre-1960 pandemic viruses elsewhere. Location and dating estimates were validated using the earliest HIV-1 archival sample, also from Kinshasa. The epidemic histories of HIV-1 group M and nonpandemic group O were similar until ~1960, after which group M underwent an epidemiological transition and outpaced regional population growth. Our results reconstruct the early dynamics of HIV-1 and emphasize the role of social changes and transport networks in the establishment of this virus in human populations.

The branching ratios for the spin-forbidden photodissociation channels of (12)C(16)O in the vacuum ultraviolet (VUV) photon energy region from 102,500 (12.709 eV) to 106,300 cm(-1) (13.180 eV) have been investigated using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to three (1)Σ(+) and six (1)Π Rydberg-type states, including the progression of W(3sσ) (1)Π(v' = 0, 1, and 2) vibrational levels of CO, have been identified and investigated. The branching ratios for the product channels C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) of these predissociative states are found to depend on the electronic, vibrational, and rotational states of CO being excited. Rotation and e/f-symmetry dependences of the branching ratios into the spin-forbidden channels have been confirmed for several of the (1)Π states, which can be explained using the heterogeneous interaction with the repulsive D'(1)Σ(+) state. The percentage of the photodissociation into the spin-forbidden channels is found to increase with increasing the rotational quantum number for the K(4pσ) (1)Σ(+) (v' = 0) state. This has been rationalized using a (1)Σ(+) to (1)Π to (3)Π coupling scheme, where the final (3)Π state is a repulsive valence state correlating to the spin-forbidden channel.