THESPITZERICE LEGACY: ICE EVOLUTION FROM CORES TO PROTOSTARS

Abstract

Ices regulate much of the chemistry during star formation and account for up to 80% of the available oxygen and carbon. In this paper, we use the Spitzer c2d ice survey, complimented with data sets on ices in cloud cores and high-mass protostars, to 1 Hubble Fellow 2 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands 3 Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr.1, 85748 Garching, Germany -2 -determine standard ice abundances and to present a coherent picture of the evolution of ices during low-and high-mass star formation. The median ice composition H 2 O:CO:CO 2 :CH 3 OH:NH 3 :CH 4 :XCN is 100:29:29:3:5:5:0.3 and 100:13:13:4:5:2:0.6 toward low-and high-mass protostars, respectively, and 100:31:38:4:-:-:-in cloud cores. In the low-mass sample, the ice abundances with respect to H 2 O of CH 4 , NH 3 , and the component of CO 2 mixed with H 2 O typically vary by <25%, indicative of co-formation with H 2 O. In contrast, some CO and CO 2 ice components, XCN and CH 3 OH vary by factors 2-10 between the lower and upper quartile. The XCN band correlates with CO, consistent with its OCN − identification. The origin(s) of the different levels of ice abundance variations are constrained by comparing ice inventories toward different types of protostars and background stars, through ice mapping, analysis of cloud-to-cloud variations, and ice (anti-)correlations. Based on the analysis, the first ice formation phase is driven by hydrogenation of atoms, which results in a H 2 O-dominated ice. At later prestellar times, CO freezes out and variations in CO freeze-out levels and the subsequent CO-based chemistry can explain most of the observed ice abundance variations. The last important ice evolution stage is thermal and UV processing around protostars, resulting in CO desorption, ice segregation and formation of complex organic molecules. The distribution of cometary ice abundances are consistent with with the idea that most cometary ices have a protostellar origin.

The chemistry of planet-forming regions is not interstellar

“…Such bodies might be preferentially water-rich, if they formed outside the water snow line, but inside the CO 2 explain the high water column densities suggested by some slab models. Alternately, we may have underestimated the gas-to-dust ratio.…”

“…The elements in the material are differentially partitioned into gas and solids, and apart from a small contribution from nuclear reactions with stellar and cosmic radiation, their total abundances remain largely unchanged. Conversely, their molecular carriers, including those carrying the bulk of some elements, may change dramatically along this path, and depending on their volatility the local elemental abundances my be altered by hydrodynamic transport processes 1,2 . This rich history of pre-planetary matter is validated by strong differences in chemical content of primitive chondrites from the 3 AU region of the solar nebula 3 , comets that formed beyond 10s of AU 4 and protostellar envelopes 2 .…”

En Route to Destruction: The Evolution in Composition of Ices in Comet D/2012 S1 (Ison) Between 1.2 and 0.34 Au From the Sun as Revealed at Infrared Wavelengths*

“…A reanalysis of observations from the Spitzer survey of about 50 low-mass and 9 high-mass sources resulted in mean abundance ratios relative to H 2 O (Gibb et al 2004;Öberg et al 2011); these are included in Table 3. These ice abundances are generally larger than the "normal" abundances found among Table 1 for details including on-source integration times): (a) October 22 KL1, order 27.…”

Astrochemistry of dust, ice and gas: introduction and overview

“…Because of their lower binding energies, the snow lines of CO and CO 2 are outside that of H 2 O. This selective freeze-out of major ice reservoirs can change the overall elemental [C]/[O] abundance ratio in the gas and thus the composition of the atmospheres of giant planets that are formed there 94 . The CO snowline has been imaged with ALMA through N 2 H + observations in the nearby TW Hya disk 233 .…”