AKARI NIR spectroscopy of interstellar ices

Astronomy and Astrophysics, 2010

Aims. The aim of this study is to understand the chemical conditions of ices around embedded young stellar objects (YSOs) in the metal-poor Large Magellanic Cloud (LMC). Methods. We performed near-infrared (2.5−5 μm) spectroscopic observations toward 12 massive embedded YSOs and their candidates in the LMC using the infrared camera (IRC) onboard AKARI. We estimated the column densities of the H 2 O, CO 2 , and CO ices based on their 3.05, 4.27, and 4.67 μm absorption features, and we investigated the correlation between ice abundances and physical properties of YSOs. Results. The ice absorption features of H 2 O, CO 2 , 13 CO 2 , CO, CH 3 OH, and possibly XCN are detected in the spectra. In addition, hydrogen recombination lines and PAH emission bands are detected toward the majority of the targets. The derived typical CO 2 /H 2 O ice ratio of our samples (∼0.36 ± 0.09) is greater than that of Galactic massive YSOs (∼0.17 ± 0.03), while the CO/H 2 O ice ratio is comparable. It is shown that the CO 2 ice abundance does not correlate with the observed characteristics of YSOs: the strength of hydrogen recombination line and the total luminosity. Likewise, clear correlation is not seen between the CO ice abundance and YSO characteristics, but it is suggested that the CO ice abundance of luminous samples is significantly lower than in other samples. Conclusions. The systematic difference in the CO2 ice abundance around the LMC's massive YSOs, which was suggested by previous studies, is confirmed with the new near-IR data. We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC are responsible for the observed high abundance of the CO 2 ice. It is suggested that the internal stellar radiation does not play an important role in the evolution of the CO 2 ice around a massive YSO, while more volatile molecules like CO are susceptible to the effect of the stellar radiation.

The Astrophysical Journal, 2008

We present the first results of AKARI Infrared Camera near-infrared spectroscopic survey of the Large Magellanic Cloud (LMC). We detected absorption features of the H 2 O ice 3.05µm and the CO 2 ice 4.27µm stretching mode toward seven massive young stellar objects (YSOs). These samples are for the first time spectroscopically confirmed to be YSOs. We used a curve-of-growth method to evaluate the column densities of the ices and derived the CO 2 /H 2 O ratio to be 0.45±0.17. This is clearly higher than that seen in Galactic massive YSOs (0.17±0.03). We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC may be responsible for the observed high CO 2 ice abundance.

Publications of The Korean Astronomical Society, 2012

The absorption features due to interstellar ices, especially H 2 O and CO 2 ices, provide us with crucial information on present and past interstellar environments, and thus the evolutionary histories of galaxies. Before AKARI, however, few detections of ices were reported for nearby galaxies. The AKARI's unique capability of near-infrared spectroscopy with high sensitivity enables us to systematically study ices in nearby galaxies. Thus we have explored many near-infrared spectra (2.5-5 µm) of the 211 pointed observations, searching for the absorption features of ices. As a result, out of 122 nearby galaxies, we have significantly detected H 2 O ice from 36 galaxies and CO 2 ice from 9 galaxies. It is notable that the ices are detected not only in late-type galaxies but also in early-type galaxies. We find that CO 2 ice is more compactly distributed near the galactic center than H 2 O ice. Finally, we suggest that the gas density of a molecular cloud and UV radiation may be important factors to determine the abundance of ices.

The Astrophysical Journal

We discover two infrared objects that show deep absorption features of H 2 O, CO 2 , and CO ices in the AKARI/Infrared Camera (IRC) slit-less spectroscopic survey of the Galactic plane in 2.5-13 µm. Both objects are located neither in known star-forming regions nor in known dense clouds. For one of the objects, Object 1, we successfully extract a spectrum from 2.5 to 13 µm, which also shows several absorption features in 5-13 µm, including deep silicate absorption at 10 µm. For the other object, Object 2, only a spectrum from 3.1 to 5 µm is reliably extracted due to the presence of nearby overlapping objects and faint nebulosity. Both objects show warm (> 100 K) CO gas absorption in addition to the ice absorption features, suggesting that they are embedded young stellar objects (YSOs). On the other hand, both objects have spectral energy distributions (SEDs) that peak at around 5 µm and decrease towards longer wavelengths. These characteristics of the SEDs and the presence of deep absorption features cannot easily be accounted for by standard YSO models. They may be explained as background stars behind dense clouds. We discuss possible nature of the objects and implications of the present discovery.

Advances in Space Research, 1998

The first spectroscopic results horn IS0 (Infrared Space Observatory, have revealed a wealth of interesting features and in particular absorption signatures of a wide variety of solid state molecular species. We present here some new IS0 data obtained with SWS (Short Wavelength Spectrometer) toward the young deeply embedded object associated with RAFGL 7009s. Signatures of H20, CO, CO2 ices can be readily identified but also some much less abundant species such as 13C02, HzCO and CH4, all occurring in the near and mid-infrared region between 3 and 16 pm. The detection of CO2 at 4.27 and 15.2 pm confirms its presence in the IRAS-LRS spectra of several heavily absorbed sources. The very high extinction toward RAFGL 7009s makes it an excellent case to study other weak solid state absorption features, commonly measured in laboratory experiments. 01998 COSPAR. Published by Elsevier Science Ltd.

2009

We present spectroscopic observations of a sample of 15 embedded young stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS spectral range: the bending mode of CO 2 ice at 15.2 µm and the ice band between 5 and 7 µm that includes contributions from the bending mode of water ice at 6 µm amongst other ice species. The 5−7 µm band is difficult to identify in our LMC sample due to the conspicuous presence of PAH emission superimposed onto the ice spectra. We identify water ice in the spectra of two sources; the spectrum of one of those sources also exhibits the 6.8-µm ice feature attributed in the literature to ammonium and methanol. We model the CO 2 band in detail, using the combination of laboratory ice profiles available in the literature. We find that a significant fraction (50%) of CO 2 ice is locked in a water-rich component, consistent with what is observed for Galactic sources. The majority of the sources in the LMC also require a pure-CO 2 contribution to the ice profile, evidence of thermal processing. There is a suggestion that CO 2 production might be enhanced in the LMC, but the size of the available sample precludes firmer conclusions. We place our results in the context of the star formation environment in the LMC.

Astrophysical Journal, 2008

With the goal to study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low luminosity YSOs using 3-38 um Spitzer and ground-based spectra. The long-known 6.0 and 6.85 um bands are detected toward all sources, with the Class 0-type YSOs showing the deepest bands ever observed. In almost all sources the 6.0 um band is deeper than expected from the bending mode of pure solid H2O. The depth and shape variations of the remaining 5-7 um absorption indicate that it consists of 5 independent components, which, by comparison to laboratory studies, must be from at least 8 different carriers. Simple species are responsible for much of the absorption in the 5-7 um region, at abundances of 1-30% for CH3OH, 3-8% for NH3, 1-5% for HCOOH, ~6% for H2CO, and ~0.3% for HCOO- with respect to solid H2O. The 6.85 um band likely consists of one or two carriers, of which one is less volatile than H2O because its abundance relative to H2O is enhanced at lower H2O/tau_9.7 ratios. It does not survive in the diffuse interstellar medium (ISM), however. The similarity of the 6.85 um bands for YSOs and background stars indicates that its carrier(s) must be formed early in the molecular cloud evolution. If an NH4+ salt is the carrier its abundance with respect to solid H2O is typically 7%, and low temperature acid-base chemistry or cosmic ray induced reactions must have been involved in its formation. Possible origins are discussed for the carrier of an enigmatic, very broad absorption between 5 and 8 um. Finally, all the phenomena observed for ices toward massive YSOs are also observed toward low mass YSOs, indicating that processing of the ices by internal ultraviolet radiation fields is a minor factor in the early chemical evolution of the ices. [abridged]

Icarus, 1997

direction of the protostellar object RAFGL 7009S. Due to its extreme extinction this source represents a unique target for the detection of interstellar ices. Identified molecules include 1. INTRODUCTION H 2 O, CO, and CO 2 together with 13 CO 2 , CH 4 , OCS, and H 2 CO.

Astrophysical Journal, 2005

A powerful way to observe directly the solid state inventory of dense molecular clouds is by infrared spectroscopy of background stars. We present Spitzer/IRS 5-20 micron spectra of ices toward stars behind the Serpens and Taurus molecular clouds, probing visual extinctions of 10-34 mag. These data provide the first complete inventory of solid-state material in dense clouds before star formation begins. The spectra show prominent 6.0 and 6.85 micron bands. In contrast to some young stellar objects (YSOs), most (~75%) of the 6.0 micron band is explained by the bending mode of pure water ice. In realistic mixtures this number increases to 85%, because the peak strength of the water bending mode is very sensitive to the molecular environment. The strength of the 6.85 micron band is comparable to what is observed toward YSOs. Thus, the production of the carrier of this band does not depend on the energetic input of a nearby source. The spectra show large abundances of carbon monoxide and carbon dioxide (20-40% with respect to water ice). Compared to YSOs, the band profile of the 15 micron carbon dioxide bending mode lacks the signatures of crystallization, confirming the cold, pristine nature of these lines of sight. After the dominant species are removed, there are residuals that suggest the presence of minor species such as formic acid and possibly ammonia. Clearly, models of star formation should begin with dust models already coated with a fairly complex mixture of ices.

AIP Conference Proceedings, 2006

Recent results from the Raymond and Beverly Sackler Laboratory for Astrophysics on spectroscopy and processing of interstellar ice analogues are summarized. This includes thermal desorption studies of pure, layered and mixed CO, N2 and O2 ices, and infrared spectroscopy and heating of CO-CO2, CO-H2O, CO-HCOOH, CO-CH4 and CO-CH3OH layered and mixed ices. Laboratory data of CO-surface adsorbates show good agreement with the unidentified 2175 cm-1 interstellar feature. Complementary ab initio quantum chemical calculations and molecular dynamics simulations have been performed to provide insight into the gas-grain interactions and interstellar ice processing. This includes the first molecular dynamics study of the photodissociation of water ice and the corresponding photodesorption efficiencies. The relevance of these data in the analysis of astronomical data is emphasized throughout.