2014 Annual Science Report
Rensselaer Polytechnic Institute Reporting | SEP 2013 – DEC 2014
Project 1: Interstellar Origins of Preplanetary Matter
Interstellar space is rich in the raw materials required to build planets and life, including essential chemical elements (H, C, N, O, Mg, Si, Fe, etc.) and compounds (water, organic molecules, planet-building minerals). This research project seeks to characterize the composition and structure of these materials and the chemical pathways by which they form and evolve. The long-term goal is to determine the inventories of proto-planetary disks around young sun-like stars, leading to a clear understanding of the processes that led to our own origins and insight into the probability of life-supporting environments emerging around other stars.
We have made excellent progress in our long-term goal to understand the astrochemical processes that generate raw materials for new planetary systems. In the reporting period we have focused on understanding the distribution of chemical element abundances in environments near the diffuse-dense interstellar cloud transition. A detailed spectral decomposition analysis of solid-state absorbers in the spectrum of the prototypical star Zeta Ophiuchi was performed using archival observations from the NASA Spitzer Space Telescope and the NASA-ESA Infrared Space Observatory. We found clear evidence for the presence of sub-micron sized amorphous silicate grains, principally comprised of olivine-like composition. Evidence for thick ice mantles on large (a = 2.8 μm) grains is also presented – an order of magnitude larger than “typical” interstellar grains. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from our analysis and compared to standard reference abundances. We find that nearly all of elemental Mg and Si along the line of sight are present in amorphous silicate grains, but a substantial fraction of elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, and we conclude that the missing reservoir of elemental O must reside on large preplanetary grains that are nearly opaque to infrared radiation. These findings are directly relevant to the nature of the input material from prestellar clouds to protoplanetary disks. Our results are reported in a paper accepted and in press for Astrophysical Journal (Poteet et al. 2015).
Other progress during the reporting period included the successful thesis defense and graduation of NAI-supported RPI doctoral student Emily Hardegree-Ullman (PhD awarded spring 2014), and Whittet’s invited co-authorship of a major review article “Observations of the Icy Universe”, accepted and in press for the 2015 issue of Annual Reviews of Astronomy and Astrophysics.
PROJECT MEMBERS:Douglas Whittet
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Sources of prebiotic materials and catalysts