Interstellar solid CO - Polar and nonpolar interstellar ices

Abstract

We present moderate-resolution (lambda/delta lambda approximately 1200) observations of the solid CO band in a sample of protostars. The spectra reveal two independent solid CO components along most lines of sight. One produces a narrow (delta nu approximately 5 cm-1) band generally centered at about 2140 cm-1 and the other a broader (delta nu approximately 10 cm-1) one at about 2136 cm-1. Both the peak position and width of the narrow, and generally strongest, component vary from object to object. The relative strengths of the two components vary considerably in this sample. Laboratory studies of the shape and peak position of the solid CO banD in astrophysically relevant mixtures show that the narrow CO band occurs in mixtures dominated by non-polar molecules (e.g., CO itself, CO2, O2, N2), while the broad feature is due to more polar mixtures, such as H2O ice. Calculations show that for mixtures dominated by CO (CO concentration > 0.3), the peak position and shape of the CO fundamental are strongly influenced by "surface modes," while for lower concentrations the laboratory measured absorption spectra provide very accurate representations of the small particle extinction spectrum. The observed variations in peak position and width of the interstellar 2140 cm-1 component can be attributed to variations in composition and/or physical characteristics of the grains (i.e., shape). These observations show that many lines of sight contain (at least) two independent grain mantle components: a polar mixture (H2O-rich) responsible for the 3.08 and 6.0 micrometers ice bands and a nonpolar one dominating the solid CO spectrum. These two independent grain mantle components may reflect chemical variations during accretion. Around luminous protostars, differences in volatility of the nonpolar and H2O-rich ices also may play an important role in determining their relative abundances.

Molecular dynamics simulations of D2O ice photodesorption

“…Our total photodesorption probability for D 2 O compares better with the experimental photodesorption yield than that for H 2 O, and also better at low ice temperatures. Presumably at higher ice temperatures long time scale processes become increasingly important, such as diffusion and thermal desorption, which are not covered in our picosecond simulations.…”

“…5(c)] is relatively small, and there are no differences between the average D 2 O and H 2 O photodesorption probabilities: both rise with ice temperature by only ∼30% going from 10 to 90 K. However, the D 2 O average kick-out photodesorption probability increases strongly with ice temperature (by ∼180% from 10 to 90 K), whereas the H 2 O kick-out average photodesorption probability shows a much weaker increase with ice temperature (by ∼50% from 10 to 90 K). Thus, the stronger trend with ice temperature for D 2 O results from the increase of the D 2 O kick-out photodesorption probability [ Fig. 5(b)].…”

“…The icy mantles contain mainly H 2 O, and also traces of other molecules (e.g., CO, CO 2 , NH 3 , CH 4 , among others). 2,3 Observed infrared (IR) spectra reveal that H 2 O and CO are the most abundant molecules in the icy mantles in the ISM. [2][3][4][5][6][7][8][9][10] Recent ground and space based observations have also detected heavy water (D 2 O) in the ISM.…”

Astrochemistry of dust, ice and gas: introduction and overview

“…4). The observational evidence for different layers comes from the shapes of various ice bands, in particular the high-quality line profiles of solid CO 95,96 : a CO molecule surrounded by a H 2 O molecule has a slightly different vibrational constant than a CO molecule embedded in CO itself, and these differences can be readily distinguished. The water-rich layer is thought to form early in the evolution of a cloud by hydro-genation of atomic O, once the extinction is a few mag 97 .…”

From Interstellar Polycyclic Aromatic Hydrocarbons and Ice to Astrobiology

“…[174][175][176][177][178] Many, but not all, of the lines-of-sight that contain H 2 O ice also contain CO ice and the relative strengths of the H 2 O and CO bands indicate CO/H 2 O ratios ranging from 0.0 to as much as 0.3. [118,151,[179][180][181][182][183][184] Although the CO band in a few objects has a position and profile consistent with CO frozen in H 2 O-rich matrices, most lines-of-sight exhibit profiles indicative of CO frozen in non-polar matrices, i.e., ices thought to be dominated by molecules such as CO, CO 2 , O 2 , and N 2 rather than H 2 O. [174,175,181,184,185] These are precisely the two sorts of mantles predicted on the basis of the H/H 2 ratio discussed earlier and sketched in Figure 12.…”