NGC 4993 is the shell galaxy host of the GRB170817A short gamma-ray burst and the GW170817 gravitational-wave event produced during a binary neutron star coalescence. The galaxy shows signs, including the stellar shells, that it recently accreted a smaller late-type galaxy. The accreted galaxy could have been the original host of the binary neutron star. We measure the positions of the stellar shells of NGC 4993 in an HST/ACS archival image and use them to constrain the time of the galactic merger. According to the analytical model of the evolution of the shell structure in the expected gravitational potential of NGC 4993, the galactic merger happened at least 200 Myr ago with a probable time roughly around 400 Myr and the estimates and higher than 600 Myr being improbable. Because the galactic merger has likely shut down the star formation in the accreted galaxy, this constitutes the lower limit on the age of the binary neutron star, if it originated in this galaxy.

Using a merged beams apparatus we have measured the associative detachment (AD) reaction of H − + H → H 2 + e − for relative collision energies up to E r ≤ 4.83 eV. These data extend above the 1 eV limit of our earlier results. We have also updated our previous theoretical work to account for AD via the repulsive 2 Σ + g H − 2 potential energy surface and for the effects at E r ≥ 0.76 eV on the experimental results due to the formation of H 2 resonances lying above the H + H separated atoms limit. Merging both experimental data sets, our results are in good agreement with our new theoretical calculations and confirm the prediction that this reaction essentially turns off for E r 2 eV. Similar behavior has been predicted for the formation of protonium from collisions of antiprotons and hydrogen atoms.PACS numbers: 34.50. Lf, 52.20.Hv, 95.30.Ft, 97.10.Bt

We report experimental and theoretical results for associative detachment (AD) of D − + D → D2 + e − . We compare these data to our previously published results for H − + H → H2 + e − . The measurements show no significant isotope effect in the total cross section. This is to be contrasted with previously published experimental and theoretical work which has found a significant isotope effect in diatomic systems for partial AD cross sections, i.e., as a function of the rotational and vibrational levels of the final molecule formed. Our work implies that though the ro-vibrational distribution of flux is different for AD of H − + H and D − + D, the total flux for these two systems is essentially the same when summed over all possible final channels.

We present a new method to measure the vertical aerosol optical depth (VAOD) during clear nights using a wide-field imager—a CCD camera with a photographic lens on an equatorial mount. A series of 30 s exposures taken at different altitudes above the horizon can be used to measure the VAOD with a precision better than 0.008 optical depths within a few minutes. Such a measurement does not produce any light and is thus suitable for use at sites where other astronomical instruments are located. The precision of the VAOD measurement depends on the laboratory calibration of spectral properties of the system and of the response of the camera electronics to varying illumination levels, as well as careful considerations of the details of stellar photometry and modeling of the dependence of the measured stellar flux, the star color, and the position within the field of view. The results obtained with robotic setups at the future sites of the Cerenkov Telescope Array show good internal consistency and agreement with simultaneous measurements from a Sun/Moon photometer located at the same sites.