A 3-5 mu m VLT spectroscopic survey of embedded young low mass stars I - Structure of the CO ice
By K. Pontoppidan, Helen J. Fraser, E. Dartois, W. Thi, E. F. Dishoeck, A. Boogert, L. D'hendecourt, A. Tielens, S. Bisschop. 2003
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Medium resolution (λ/Δλ = 5000-10000) VLT-ISAAC M-band spectra are presented of 39 young stellar objects in nearby low-mass star forming clouds showing the 4.67 μm stretching vibration mode of solid CO. By taking advantage of the unprecedentedly large sample, high S/N ratio and high spectral resolution, similarities in the ice profiles from source to source are identified. It is found that excellent fits to all the spectra can be obtained using a phenomenological decomposition of the CO stretching vibration profile at 4.67 μm into 3 components, centered on 2143.7 cm^(-1),2139.9 cm^(-1), and 2136.5 cm^(-1) with fixed widths of 3.0, 3.5 and 10.6 cm ^(-1), respectively. All observed interstellar CO profiles can thus be uniquely described by a model depending on only 3 linear fit parameters, indicating that a maximum of 3 specific molecular environments of solid CO exist under astrophysical conditions. A simple physical model of the CO ice is presented, which shows that the 2139.9 cm^(-1) component is indistinguishable from pure CO ice. It is concluded, that in the majority of the observed lines of sight, 60-90% of the CO is in a nearly pure form. In the same model the 2143.7 cm^(-1) component can possibly be explained by the longitudinal optical (LO) component of the vibrational transition in pure crystalline CO ice which appears when the background source is linearly polarised. The model therefore predicts the polarisation fraction at 4.67 μm, which can be confirmed by imaging polarimetry. The 2152 cm^(-1) feature characteristic of CO on or in an unprocessed water matrix is not detected toward any source and stringent upper limits are given. When this is taken into account, the 2136.5 cm ^(-1) component is not consistent with the available water-rich laboratory mixtures and we suggest that the carrier is not yet fully understood. A shallow absorption band centered between 2165 cm^(-1) and 2180 cn^(1) is detected towards 30 sources. For low-mass stars, this band is correlated with the CO component at 2136.5 cm^(-1), suggesting the presence of a carrier different from XCN at 2175 cm^(-1). Furthermore the absorption band from solid ^(13)CO at 2092 cm^(-1) is detected towards IRS 51 in the ρ Ophiuchi cloud complex and an isotopic ratio of ^(12)CO/^(13)CO = 68 ± 10 is derived. It is shown that all the observed solid ^(12)CO profiles, along with the solid ^(13)CO profile, are consistent with grains with an irregularly shaped CO ice mantle simulated by a Continuous Distribution of Ellipsoids (CDE), but inconsistent with the commonly used models of spherical grains in the Rayleigh limit.
Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy
View Paper: https://www.semanticscholar.org/paper/61e6af9b2735c4db91dbd00a740521a413f66eef
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Citations in the corpus
- Testing grain-surface chemistry in massive hot-core regions (2007, background): “Abundances for some ice species such as CH3OH and OCN− are large, up to 30% with respect to H2O ice or 3 × 10−5 with respect to H2 (Brooke et al. 1999; Dartois et al. 1999; Pontoppidan et al. 2003; van Broekhuizen et al. 2005).”
- Formation of compact solid water after ion irradiation at 15 K (2006, background): “As discussed above comparisons of laboratory spectra with the observed solid CO band suggest the presence of CO embedded in compact solid water (Pontoppidan et al. 2003; Fraser et al. 2004).”
- Organic molecules in protoplanetary disks around T Tauri and Herbig Ae stars (2004, background): “Both CH3OH and H2CO have been detected in icy mantles, with the CH3OH abundance varying strongly from source to source (Dartois et al. 1999, Keane et al. 2001, Pontoppidan et al. 2003).”
- Observations of the Icy Universe (2015, background): “An alternative explanation by absorption of linearly polarized light in a crystalline CO ice phase (the longitudinal optical mode; Pontoppidan et al. 2003) appears less likely.”
- Grain Surface Models and Data for Astrochemistry (2017, methodology): “Using VLT data on the CO band of a large collection of YSOs (Pontoppidan et al. 2003), peak width, peak position and upper limits on the 2152 cm−1 band were extracted and compared to laboratory spectra.”
- Cosmic ray induced explosive chemical desorption in dense clouds (2004, background): “…n ra te c oe fic ie nt s (m ol ec ul es g ra in -1 s -1 ) AV Spot heating Whole grain heating Explosive + whole grain heating Observations indeed confirm that in dense clouds most solid CO is embedded in an ice mantle which is dominated by CO itself (Chiar et al. 1995; Pontoppidan et al. 2003).”
- Competition between CO and N2 Desorption from Interstellar Ices (2005, background): “It is known from observations that in many pre- and protostellar cores, up to 90% of the CO is frozen out in a distinct layer separated from the H2O ice (Tielens et al. 1991; Pontoppidan et al. 2003).”
- The c2d Spitzer Spectroscopic Survey of Ices around Low-Mass Young Stellar Objects. I. H2O and the 5-8 μm Bands (2007, background): “In agreement with previous studies (Dartois et al. 1999; Pontoppidan et al. 2003b), the CH3OH column varies considerably with respect to solid H2O: from low upper limits of a few percent (3σ) in several sources, to detections of up to 30% in a few others.”
- THE SPITZER ICE LEGACY: ICE EVOLUTION FROM CORES TO PROTOSTARS (2011, methodology): “In addition, the combined sample partly overlaps with a 3–5 µm VLT survey of CO and XCN ice toward low-mass protostars (Pontoppidan et al. 2003b; van Broekhuizen et al. 2005) and additional CO observations were obtained with Keck/NIRSPEC (Paper II).”