H2 Excitation Structure on the Sightlines to delta Scorpius and zeta Ophiucus - First Results from the Sub-orbital Local Interstellar Cloud Experiment

TL;DR

This paper reports initial results from SLICE, a sub-orbital experiment that provides high-resolution UV spectra of interstellar clouds, revealing detailed physical conditions and molecular excitation states along sightlines to delta Sco and zeta Oph.

Contribution

First high-quality UV spectroscopic measurements of interstellar clouds using SLICE, improving understanding of molecular excitation and physical conditions in local interstellar medium.

Findings

Lower N(H2) for delta Sco than previous reports

Detection of highly excited H2 towards zeta Oph

Inferred higher density in excited H2 component of zeta Oph

Abstract

We present the first science results from the Sub-orbital Local Interstellar Cloud Experiment (SLICE): moderate resolution 1020-1070A spectroscopy of four sightlines through the local interstellar medium. High signal-to-noise (S/N) spectra of eta Uma, alpha Vir, delta Sco, and zeta Oph were obtained during a 21 April 2013 rocket flight. The SLICE observations constrain the density, molecular photoexcitation rates, and physical conditions present in the interstellar material towards delta Sco and zeta Oph. Our spectra indicate a factor of two lower total N(H2) than previously reported for delta Sco, which we attribute to higher S/N and better scattered light control in the new SLICE observations. We find N(H2) = 1.5 x 10^{19} cm^{-2} on the delta Sco sightline, with kinetic and excitation temperatures of 67 and 529 K, respectively, and a cloud density of n_{H} = 56 cm^{-3}. Our observations of the bulk of the molecular sightline toward zeta Oph are consistent with previous measurements (N(H2) ~ 3 x 10^{20} cm^{-2} at T_{01} = 66 K and T_{exc} = 350 K). However, we detect significantly more rotationally excited H2 towards zeta Oph than previously observed. We infer a cloud density in the rotationally excited component of n_{H} ~ 7600 cm^{-3} and suggest that the increased column densities of excited H2 are a result of the ongoing interaction between zeta Oph and its environment; also manifest as the prominent mid-IR bowshock observed by WISE and the presence of vibrationally-excited H2 molecules observed by HST.