We present 2.5-30 mum spectra from the Short-Wavelength Spectrometer of the Infrared Space Observatory for a total of 23 sources. The sources include embedded young stellar objects spanning a wide range of mass and luminosity, together with field stars sampling quiescent dark clouds and the diffuse interstellar medium. Expanding on results of previous studies, we use these spectra to investigate ice composition as a function of environment. The spectra reveal an extremely rich set of absorption features attributed to simple molecules in the ices. We discuss the observed properties of these absorption features and review their assignments. Among the species securely identified are H2O, CO, CO2, CH3OH, and CH4. Likely identified species include OCS, H2CO, and HCOOH. There is also evidence for NH3 and OCN- ice features, but these identifications are more controversial. Features that continue to defy identification include the 3.3-3.7 mum "ice band wing'' and the bulk of the 6.8 mum feature. In addition, we find evidence for excess absorption at 6.0 mum that cannot be attributed to H2O ice. We examine the degree of intercorrelation of the 6.8 mum, 4.62 mum ("XCN'') and 6.0 mum (excess) features. Our results are consistent with the interpretation of the 6.8 and 4.62 mum features as due to NH4+ and OCN- ions, respectively, though alternative explanations cannot currently be ruled out. We find that the optical depth correlations are dependent on the profile of the 6.8 mum feature but not on the mass of the YSO nor the ice temperature along the line of sight. We discuss the implications for our current understanding of ice processing. We briefly discuss the composition, origin, and evolution of interstellar ices.

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Interstellar Ice: The Infrared Space Observatory Legacy

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 (H2O, NH3, CO2, CO, CH3OH, 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.

Observations of the Icy Universe

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 (H2O, NH3, CO2, CO, CH3OH, 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.

Observations of interstellar linear polarization in the spectral range 0.35-2.2??m are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining published photometry and spectral classifications with photometry from the Two Micron All Sky Survey. The polarization and extinction curves are characterized in terms of the parameters ?max (the wavelength of maximum polarization) and RV (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical properties of the dust change systematically as a function of environment, considering stars observed through progressively more opaque (and thus progressively denser) regions of the cloud. At low visual extinctions (0 3, real changes in grain properties occur, characterized by observed RV values in the range 3.5-4.0. A simple model for the development of RV with AV suggests that RV may approach values of 4.5 or more in the densest regions of the cloud. The transition between normal extinction and dense cloud extinction occurs at AV ~ 3.2, a value coincident with the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in RV are thus either a direct consequence of mantle growth or occur under closely similar physical conditions. Dust in Taurus appears to be in a different evolutionary state compared with other nearby dark clouds, such as ? Oph, in which coagulation is the dominant physical process.

/pdf/interstellar-extinction-and-polarization-in-the-taurus-dark-110sxbmyig.pdf

Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H atoms, and has been benchmarked versus recent laboratory experiments on H2 formation on silicate surfaces. The results show that H2 formation on grain surfaces is efficient in the interstellar medium up to some 300 K. At low temperatures (≤100 K), H2 formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H2 formation proceeds through a reaction between two chemisorbed H atoms. We present simple analytical expressions for H2 formation that can be adopted to a wide variety of surfaces once their surface characteristics have been determined experimentally.

https://iopscience.iop.org/article/10.1086/342607/pdf

Molecular hydrogen formation in the interstellar medium

We report here observations made in November 1981 using the United Kingdom Infrared Telescope (UKIRT) at Mauna Kea of the 3 µm ice absorption feature in the spectra of several obscured stars in the Taurus interstellar clouds. The feature correlated in strength with extinction at visual wavelengths (Av), and is present in stars with Av as low as 4–6 mag, a remarkable result when compared with other regions of the Galaxy. Ice may be widespread in the Taurus clouds, vindicating ideas on grain composition and growth first reported nearly 50 yr ago1.

Interstellar ice grains in the Taurus molecular clouds

NGC3256 is a spectacular peculiar galaxy, with a highly chaotic nuclear region consisting of several bright knots. Two diffuse tidal tails are visible evidence of violent past history. The galaxy has been classified on the basis of this tidal damage as the remnant of two colliding galaxies in the final throes of merging1. The galaxy is highly luminous, with MB = −22.6, as well as being a bright, narrow emission-line galaxy2. At radio wavelengths, NGC3256 is one of the brightest in Wright's survey3 of interacting galaxies. We have discovered bright and exceptionally extended 10-μm emission that is evidence for an extremely luminous starburst extending over several kiloparsecs, which will leave the system severely gas-depleted. This depletion, in combination with the violent stellar relaxation which accompanies galaxy mergers, suggests that NGC3256 will become an elliptical galaxy.

NGC3256: an emerging elliptical galaxy

Supernova (SN) 1982g in NGC1332 was discovered by Maza on 28 March 1982 at mpg=14.0 (ref. 1) some 30kpc (Ho = 50 km s−1 Mpc−1) from the nucleus of the parent galaxy (distance, 30Mpc; type, SO)2. No other optical data have been reported so the SN type is uncertain, although no Type II SN has ever been identified in a galaxy earlier than SA3. We have obtained IR light curves for SN 1982g covering 100–250 days after its discovery. We show here that the IR radiation is due to heated dust. No evidence has been found for grain condensation in the SN ejecta. In fact, the light curves reveal that the IR emission originates from a pre-existing circumstellar dust cloud and is produced by an ‘echo’ of the UV and visible pulse of the SN.

Discovery of an IR echo from a supernova dust cloud

/pdf/radio-survey-of-classical-novae-2671lb9apw.pdf

Radio survey of classical novae

The possibility that point X-ray sources possess extended haloes arising from scattering of X rays by interstellar grains has been discussed by several authors (see, for example, refs 1, 2) and recent observations using the Einstein X-ray satellite have suggested that observational evidence for such haloes is now becoming available. For example, Rolf3 has shown that the X-ray source GX 339–4 has a discernible halo out to ∼20 arc min, while Catura4 has argued that several low latitude X-ray sources may have haloes that extend to ≳10 arc min. Much of the theoretical work on such haloes to date has involved steady (time-independent) X-ray sources. While haloes around such sources can provide useful information about the scattering medium, variable X-ray sources are potentially much more powerful probes of the interstellar dust distribution5,6. We discuss here the scattering of soft X rays emitted by dwarf novae in outburst, during which these objects undergo short (∼1 day) eruption at soft X-ray wavelengths7,8. We also consider the nature of the apparently transient soft X-ray ‘ring’ around the dwarf nova SU UMa (ref. 9) and we show that the properties of such a ring are consistent with those expected from the effect under consideration.

M. F. Bode

Papers

Interstellar ice grains in the Taurus molecular clouds

NGC3256: an emerging elliptical galaxy

Discovery of an IR echo from a supernova dust cloud

Radio survey of classical novae

Transient X-ray rings around dwarf novae