Observations of the Icy Universe Boogert, A.C. Adwin; Gerakines, Perry A; Whittet, Douglas C.B
Annual review of astronomy and astrophysics,
08/2015, Letnik:
53, Številka:
1
Journal Article
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Odprti dostop
Freeze-out of the gas-phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H
2
O, NH
3
, CO
2
, ...CO, CH
3
OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and Solar System environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze-out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state-of-the-art observations show promise for much progress in this area with planned infrared facilities.
Context. The formation of methanol (CH3OH) on icy grain mantles during the star formation cycle is mainly associated with the CO freeze-out stage. Yet there are reasons to believe that CH3OH also can ...form at an earlier period of interstellar ice evolution in CO-poor and H2O-rich ices. Aims. This work focuses on CH3OH formation in a H2O-rich interstellar ice environment following the OH-mediated H-abstraction in the reaction, CH4 + OH. Experimental conditions are systematically varied to constrain the CH3OH formation yield at astronomically relevant temperatures. Methods. CH4, O2, and hydrogen atoms are co–deposited in an ultrahigh vacuum chamber at 10–20 K. OH radicals are generated by the H + O2 surface reaction. Temperature programmed desorption – quadrupole mass spectrometry (TPD–QMS) is used to characterize CH3OH formation, and is complemented with reflection absorption infrared spectroscopy (RAIRS) for CH3OH characterization and quantitation. Results. CH3OH formation is shown to be possible by the sequential surface reaction chain, CH4 + OH → CH3 + H2O and CH3 + OH → CH3OH at 10–20 K. This reaction is enhanced by tunneling, as noted in a recent theoretical investigation Lamberts et al. (2017, A&A, 599, A132). The CH3OH formation yield via the CH4 + OH route versus the CO + H route is approximately 20 times smaller for the laboratory settings studied. The astronomical relevance of the new formation channel investigated here is discussed.
An important tracer of the origin and evolution of cometary ices is the comparison with ices found in dense clouds and toward young stellar objects (YSOs). We present a survey of ices in the 2–5 μm ...spectra of 23 massive YSOs, taken with the NASA InfraRed Telescope Facility SpeX spectrometer. The 4.90 μm absorption band of OCS ice is detected in 20 sight lines, more than 5 times the previously known detections. The absorption profile shows little variation and is consistent with OCS embedded in CH3OH-rich ices, and proton-irradiated H2S or SO2-containing ices. The OCS column densities correlate well with those of CH3OH and OCN−, but not with H2O and apolar CO ice. This association of OCS with CH3OH and OCN− firmly establishes their formation location deep inside dense clouds or protostellar envelopes. The median composition of this ice phase toward massive YSOs, as a percentage of H2O, is CO:CH3OH:OCN−:OCS = 24:20:1.53:0.15. CS, due to its low abundance, is likely not the main precursor to OCS. Sulfurization of CO is likely needed, although the source of this sulfur is not well constrained. Compared to massive YSOs, low-mass YSOs and dense clouds have similar or somewhat lower CO and CH3OH ice abundances, but less OCN− and more apolar CO, while OCS awaits detection. Comets tend to be underabundant in carbon-bearing species, but this does not appear to be the case for OCS, perhaps signalling OCS production in protoplanetary disks.
The relation between ices in the envelopes and disks surrounding young stellar objects (YSOs) and those in the quiescent interstellar medium (ISM) is investigated. For a sample of 31 stars behind ...isolated dense cores, ground-based and Spitzer spectra and photometry in the 1-25 Delta *mm wavelength range are combined. The baseline for the broad and overlapping ice features is modeled, using calculated spectra of giants, H2O ice and silicates. The adopted extinction curve is derived empirically. Its high resolution allows for the separation of continuum and feature extinction. The extinction between 13 and 25 Delta *mm is ~50% relative to that at 2.2 Delta *mm. The strengths of the 6.0 and 6.85 Delta *mm absorption bands are in line with those of YSOs. Thus, their carriers, which, besides H2O and CH3OH, may include NH+ 4, HCOOH, H2CO, and NH3, are readily formed in the dense core phase, before stars form. The 3.53 Delta *mm C-H stretching mode of solid CH3OH was discovered. The CH3OH/H2O abundance ratios of 5%-12% are larger than upper limits in the Taurus molecular cloud. The initial ice composition, before star formation occurs, therefore depends on the environment. Signs of thermal and energetic processing that were found toward some YSOs are absent in the ices toward background stars. Finally, the peak optical depth of the 9.7 Delta *mm band of silicates relative to the continuum extinction at 2.2 Delta *mm is significantly shallower than in the diffuse ISM. This extends the results of Chiar et al. to a larger sample and higher extinctions.
The prominent infrared absorption band of solid CO – commonly observed towards young stellar objects (YSOs) – consists of three empirically determined components. The broad ‘red component’ ...(2136 cm−1, 4.681 μm) is generally attributed to solid CO mixed in a hydrogen-bonded environment. Usually, CO embedded in the abundantly present water is considered. However, CO:H2O mixtures cannot reproduce the width and position of the observed red component without producing a shoulder at 2152 cm−1, which is not observed in astronomical spectra. Cuppen et al. showed that CO:CH3OH mixtures do not suffer from this problem. Here, this proposition is expanded by comparing literature laboratory spectra of different CO-containing ice mixtures to high-resolution (R = λ/Δλ = 25 000) spectra of the massive YSO AFGL 7009S and of the low-mass YSO L1489 IRS. The previously unpublished spectrum of AFGL 7009S shows a wide band of solid 13CO, the first detection of 13CO ice in the polar phase. In this source, both the 12CO and 13CO ice bands are well fitted with CO:CH3OH mixtures, while respecting the profiles and depths of the methanol bands at other wavelengths, whereas mixtures with H2O cannot. The presence of a gradient in the CO:CH3OH mixing ratio in the grain mantles is also suggested. Towards L1489 IRS, the profile of the 12CO band is also better fitted with CH3OH-containing ices, although the CH3OH abundance needed is a factor of 2.4 above previous measurements. Overall, however, the results are reasonably consistent with models and experiments about formation of CH3OH by the hydrogenation of CO ices.
We present an analysis of CO emission lines from a sample of T Tauri, Herbig Ae/Be, and transitional disks with known inclinations in order to study the structure of inner disk molecular gas. We ...calculate CO inner radii by fitting line profiles with a simple parameterized model. We find that, for optically thick disks, CO inner radii are strongly correlated with the total system luminosity (stellar plus accretion) and consistent with the dust sublimation radius. Transitional disk inner radii show the same trend with luminosity, but are systematically larger. Using rotation diagram fits, we derive, for classical T Tauri disks, emitting areas consistent with a ring of width ~0.15 AU located at the CO inner radius; emitting areas for transitional disks are systematically smaller. We also measure lower rotational temperatures for transitional disks, and disks around Herbig Ae/Be stars, than for those around T Tauri stars. Finally, we find that rotational temperatures are similar to, or slightly lower than, the expected temperature of blackbody grains located at the CO inner radius, in contrast to expectations of thermal decoupling between gas and dust.
Abstract
To study the demographics of interstellar ices in the Central Molecular Zone (CMZ) of the Milky Way, we obtain near-infrared spectra of 109 red point sources using NASA IRTF/SpeX at ...Maunakea. We select the sample from near- and mid-infrared photometry, including 12 objects in the previous paper of this series, to ensure that these sources trace a large amount of absorption through clouds in each line of sight. We find that most of the sample (100 objects) show CO band-head absorption at 2.3
μ
m, tagging them as red (super-) giants. Despite the photospheric signature, however, a fraction of the sample with
L
-band spectra (9/82 = 0.11) exhibit large H
2
O ice column densities (
N
> 2 × 10
18
cm
−2
), and six of them also reveal CH
3
OH ice absorption. As one of such objects is identified as a young stellar object (YSO) in our previous work, these ice-rich sight lines are likely associated with background stars in projection to an extended envelope of a YSO or a dense cloud core. The low frequency of such objects in the early stage of stellar evolution implies a low star-formation rate (≲0.02
M
⊙
yr
−1
), reinforcing the previous claim on the suppressed star-formation activity in the CMZ. Our data also indicate that the strong “shoulder” CO
2
ice absorption at 15.4
μ
m observed in YSO candidates in the previous paper arises from CH
3
OH-rich ice grains having a large CO
2
concentration
N
(CO
2
)/
N
(CH
3
OH) ≈ 1/3.
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