CH3OH Abundance Research Articles

Radiolysis of Ices by Cosmic-Rays: CH4 and H2O Ices Mixtures Irradiated by 40 MeV 58Ni11+ Ions

Abstract Physico-chemical modifications induced by swift heavy ions on methane-water (CH4:H2O) ices at 15 K are analyzed. Ice films, at concentrations of (1:3) and (1:15), were irradiated by 40 MeV 58Ni11+ ions. Fourier transform transmission spectroscopy in the mid-range was used to monitor the evolution ices at 15 K as a function of projectile fluence. New IR bands appearing for the irradiated (CH4:H2O) (1:3) ice are attributed to the synthesized molecules: C3H8, HCO, H2CO, CO, CO2, H2O2, HCOOH, CH3OH, C2H5OH, and CH3CHO. For the irradiated (CH4:H2O) (1:15) ice, the abundances of the compounds containing two carbons atoms are lower than those for the (1:3) ice; in contrast, CH3OH and H2O2 abundances increase when compared to the values obtained with the (1:3) ice. After irradiation, the ices were warmed up until 110 K, when the IR spectra reveal features of complex organic molecules. The destruction and formation cross sections and the sputtering yields of the ice mixtures are estimated. These findings provide possible pathways for the occurrence of compounds rich in C, O, and H, which are indeed observed in the cold regions of the universe such as ices in grain mantles of the interstellar medium and circumstellar envelopes.

Seeds of Life in Space (SOLIS)

Context.It is nowadays clear that a rich organic chemistry takes place in protostellar regions. However, the processes responsible for it, that is, the dominant formation routes to interstellar complex organic molecules, are still a source of debate. Two paradigms have been evoked: the formation of these molecules on interstellar dust mantles and their formation in the gas phase from simpler species previously synthesised on the dust mantles.Aims.In the past, observations of protostellar shocks have been used to set constraints on the formation route of formamide (NH2CHO), exploiting its observed spatial distribution and comparison with astrochemical model predictions. In this work, we follow the same strategy to study the case of acetaldehyde (CH3CHO).Methods.To this end, we used the data obtained with the IRAM-NOEMA interferometer in the framework of the Large Program SOLIS to image the B0 and B1 shocks along the L1157 blueshifted outflow in methanol (CH3OH) and acetaldehyde line emission.Results.We imaged six CH3OH and eight CH3CHO lines which cover upper-level energies up to ~30 K. Both species trace the B0 molecular cavity as well as the northern B1 portion, that is, the regions where the youngest shocks (~1000 yr) occurred. The CH3OH and CH3CHO emission peaks towards the B1b clump, where we measured the following column densities and relative abundances: 1.3 × 1016cm−2and 6.5 × 10−6(methanol), and 7 × 1013cm−2and 3.5 × 10−8(acetaldehyde). We carried out a non-LTE (non-Local Thermodinamic Equilibrium) Large Velocity Gradient (LVG) analysis of the observed CH3OH line: the average kinetic temperature and density of the emitting gas areTkin~ 90 K andnH2~ 4 × 105cm−3, respectively. The CH3OH and CH3CHO abundance ratio towards B1b is 190, varying by less than a factor three throughout the whole B0–B1 structure.Conclusions.Comparison of astrochemical model predictions with the observed methanol and acetaldehyde spatial distribution does not allow us to distinguish whether acetaldehyde is formed on the grain mantles or in the gas phase, as its gas-phase formation, which is dominated by the reaction of ethyl radical (CH3CH2) with atomic oxygen, is very fast. Observations of acetaldehyde in younger shocks, for example those of ~102yr old, and/or of the ethyl radical, whose frequencies are not presently available, are necessary to settle the issue.

The chemical structure of the very young starless core L1521E

Context. L1521E is a dense starless core in Taurus that was found to have relatively low molecular depletion by earlier studies, thus suggesting a recent formation. Aims. We aim to characterize the chemical structure of L1521E and compare it to the more evolved L1544 pre-stellar core. Methods. We have obtained ~2.5 × 2.5 arcminute maps toward L1521E using the IRAM-30 m telescope in transitions of various species, including C17O, CH3OH, c-C3H2, CN, SO, H2CS, and CH3CCH. We derived abundances for the observed species and compared them to those obtained toward L1544. We estimated CO depletion factors using the C17O IRAM-30 m map, an N(H2) map derived from Herschel/SPIRE data and a 1.2 mm dust continuum emission map obtained with the IRAM-30 m telescope. Results. Similarly to L1544, c-C3H2 and CH3OH peak at different positions. Most species peak toward the c-C3H2 peak including C2S, C3S, HCS+, HC3N, H2CS, CH3CCH, and C34S. C17O and SO peak close to both the c-C3H2 and the CH3OH peaks. CN and N2H+ peak close to the Herschel dust peak. We found evidence of CO depletion toward L1521E. The lower limit of the CO depletion factor derived toward the Herschel dust peak is 4.3±1.6, which is about a factor of three lower than toward L1544. We derived abundances for several species toward the dust peaks of L1521E and L1544. The abundances of most sulfur-bearing molecules such as C2S, HCS+, C34S, C33S, and HCS+ are higher toward L1521E than toward L1544 by factors of ~2–20, compared to the abundance of A-CH3OH. The abundance of methanol is very similar toward the two cores. Conclusions. The fact that the abundances of sulfur-bearing species toward L1521E are higher than toward L1544 suggests that significant sulfur depletion takes place during the dynamical evolution of dense cores, from the starless to pre-stellar stage. The CO depletion factor measured toward L1521E suggests that CO is more depleted than previously found. Similar CH3OH abundances between L1521E and L1544 hint that methanol is forming at specific physical conditions in the Taurus Molecular Cloud Complex, characterized by densities of a few ×104 cm−3 and N(H2) ≳ 1022 cm−2, when CO starts to catastrophically freeze-out, while water can still be significantly photodissociated, so that the surfaces of dust grains become rich in solid CO and CH3OH, as already found toward L1544. Methanol can thus provide selective crucial information about the transition region between dense cores and the surrounding parent cloud.

Long-term monitoring of the outgassing and composition of comet 67P/Churyumov-Gerasimenko with the Rosetta/MIRO instrument

We present the analysis of ≈100 molecular maps of the coma of comet 67P/Churyumov-Gerasimenko that were obtained with the MIRO submillimeter radiotelescope on board the Rosetta spacecraft. From the spectral line mapping of H216O, H218O, H217O, CH3OH, NH3, and CO and some fixed nadir pointings, we retrieved the outgassing pattern and total production rates for these species. The analysis covers the period from July 2014, inbound to perihelion, to June 2016, outbound, and heliocentric distancesrh= 1.24–3.65 AU. A steep evolution of the outgassing rates with heliocentric distance is observed, typically inrh−16, with significant differences between molecules (e.g. steeper variation for H2O post-perihelion than for methanol). As a consequence, the abundances relative to water in the coma vary. The CH3OH and CO abundances increase after perihelion, while the NH3abundance peaks around perihelion and then decreases. Outgassing patterns have been modeled as 2D Gaussian jets. The width of these jets is maximum around the equinoxes when the bulk of the outgassing is located near the equator. From July 2014 to February 2015, the outgassing is mostly restricted to a narrower jet (full width at half-maximum ≈80°) originating from high northern latitudes, while around perihelion, most of the gaseous production comes from the southernmost regions ( − 80 ± 5° cometocentric latitude) and forms a 100°–130° (full width at half-maximum) wide fan. We find a peak production of water of 0.8 × 1028molec. s−1, 2.5 times lower than measured by the ROSINA experiment, and place an upper limit to a 50% additional production that could come from the sublimation of icy grains. We estimate the total loss of ices during this perihelion passage to be 4.18 ± 0.18 × 109kg. We derive a dust-to-gas ratio in the lost material of 0.7–2.3 (including all sources of errors) based on the nucleus mass loss of 10.5 ± 3.4 × 109kg estimated by the RSI experiment. We also obtain an estimate of the H218O/H217O ratio of 5.6 ± 0.8.

Preliminary results from prebiotic molecules with ALMA in the era of artificial intelligence

AbstractStudy of the composition from diverse sources of the Universe helps to us to understand their evolution. Molecular spectroscopy provides detailed information of the observed objects. We present a small study of the starburst NGC 253 with ALMA at 1mm. We detect the prebiotic molecules NH2CHO, and CNCHO. We obtain the integrated intensity maps and abundances of HNCO, CH3OH, H3O+ and CH3C2H. We propose the use of Artificial Intelligence for big data to find prebiotic molecules in galaxies.

Spatially Resolved Dense Molecular Gas Excitation in the Nearby LIRG VV 114

Abstract We present high-resolution observations (0.″2–1.″5) of multiple dense gas tracers, HCN and HCO+ (J = 1–0, 3–2, and 4–3), HNC (J = 1–0), and CS (J = 7–6) lines, toward the nearby luminous infrared galaxy VV 114 with the Atacama Large Millimeter/submillimeter Array. All lines are robustly detected at the central gaseous filamentary structure, including the eastern nucleus and the overlap region, the collision interface of the progenitors. We found that there is no correlation between star formation efficiency and dense gas fraction, indicating that the amount of dense gas does not simply control star formation in VV 114. We predict the presence of more turbulent and diffuse molecular gas clouds around the overlap region compared to those at the nuclear region, assuming a turbulence-regulated star formation model. The intracloud turbulence at the overlap region might be excited by galaxy-merger-induced shocks, which also explains the enhancement of gas-phase CH3OH abundance previously found there. We also present spatially resolved spectral line energy distributions of HCN and HCO+ for the first time, and derive excitation parameters by assuming optically thin and local thermodynamic equilibrium (LTE) conditions. The LTE model revealed that warmer, HCO+-poorer molecular gas medium is dominated around the eastern nucleus, harboring an active galactic nucleus (AGN). The HCN abundance is remarkably flat (∼3.5 × 10−9) independently of the various environments within the filament of VV 114 (i.e., AGN, star formation, and shock).

Modeling CO, CO2, and H2O Ice Abundances in the Envelopes of Young Stellar Objects in the Magellanic Clouds

Abstract Massive young stellar objects (MYSOs) in the Magellanic Clouds show infrared absorption features corresponding to significant abundances of CO, CO2, and H2O ice along the line of sight, with the relative abundances of these ices differing between the Magellanic Clouds and the Milky Way. CO ice is not detected toward sources in the Small Magellanic Cloud, and upper limits put its relative abundance well below sources in the Large Magellanic Cloud and the Milky Way. We use our gas-grain chemical code MAGICKAL, with multiple grain sizes and grain temperatures, and further expand it with a treatment for increased interstellar radiation field intensity to model the elevated dust temperatures observed in the MCs. We also adjust the elemental abundances used in the chemical models, guided by observations of H ii regions in these metal-poor satellite galaxies. With a grid of models, we are able to reproduce the relative ice fractions observed in MC MYSOs, indicating that metal depletion and elevated grain temperature are important drivers of the MYSO envelope ice composition. Magellanic Cloud elemental abundances have a subgalactic C/O ratio, increasing H2O ice abundances relative to the other ices; elevated grain temperatures favor CO2 production over H2O and CO. The observed shortfall in CO in the Small Magellanic Cloud can be explained by a combination of reduced carbon abundance and increased grain temperatures. The models indicate that a large variation in radiation field strength is required to match the range of observed LMC abundances. CH3OH abundance is found to be enhanced in low-metallicity models, providing seed material for complex organic molecule formation in the Magellanic Clouds.

Abundant Methanol Ice toward a Massive Young Stellar Object in the Central Molecular Zone*

Abstract Previous radio observations revealed widespread gas-phase methanol (CH3OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH3OH ice toward a star in the CMZ, based on a Subaru 3.4–4.0 μm spectrum, aided by NASA/IRTF L ′ imaging and 2–4 μm spectra. The star lies ∼8000 au away in projection from a massive young stellar object (MYSO). Its observed high CH3OH ice abundance ( 17 % ± 3 % relative to H2O ice) suggests that the 3.535 μm CH3OH ice absorption likely arises in the MYSO’s extended envelope. However, it is also possible that CH3OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH3OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH3OH abundance confirms the prominent 15.4 μm shoulder absorption observed toward GC MYSOs arises from CO2 ice mixed with CH3OH.

Binding energies: New values and impact on the efficiency of chemical desorption

Recent laboratory measurements have confirmed that chemical desorption (desorption of products due to exothermic surface reactions) can be an efficient process. The impact of including this process into gas-grain chemical models entirely depends on the formalism used and the associated parameters. Among these parameters, binding energies are probably the most uncertain ones for the moment. We propose a new model to compute binding energy of species to water ice surfaces. We have also compared the model results using either the new chemical desorption model proposed by Minissale et al. (2016) or the one of Garrod et al. (2007). The new binding energies have a strong impact on the formation of complex organic molecules. In addition, the new chemical desorption model from Minissale produces a much smaller desorption of these species and also of methanol. Combining the two effects, the abundances of CH3OH and COMs observed in cold cores cannot be reproduced by astrochemical models anymore.

MERGER-INDUCED SHOCKS IN THE NEARBY LIRG VV 114 THROUGH METHANOL OBSERVATIONS WITH ALMA

ABSTRACT We report the detection of two CH3OH lines (J K = 2 K –1 K and 3 K –2 K ) between the progenitor’s disks (“Overlap”) of the mid-stage merging galaxy VV 114 obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 and Band 4. The detected CH3OH emission shows an extended filamentary structure (∼3 kpc) across the progenitor’s disks with relatively large velocity width (FWZI ∼ 150 km s−1). The emission is only significant in the “overlap” and not detected in the two merging nuclei. Assuming optically thin emission and local thermodynamic equilibrium, we found the CH3OH column density relative to H2 ( ) peaks at the “Overlap” (∼8 × 10−9), which is almost an order of magnitude larger than that at the eastern nucleus. We suggest that kpc-scale shocks driven by galaxy–galaxy collision may play an important role to enhance the CH3OH abundance at the “Overlap.” This scenario is consistent with shock-induced large velocity dispersion components of ionized gas that have been detected in optical wavelength at the same region. Conversely, low at the nuclear regions might be attributed to the strong photodissociation by nuclear starbursts and/or a putative active galactic nucleus, or inefficient production of CH3OH on dust grains due to initial high-temperature conditions (i.e., desorption of the precursor molecule, CO, into gas phase before forming CH3OH on dust grains). These ALMA observations demonstrate that CH3OH is a unique tool to address kpc-scale shock-induced gas dynamics and star formation in merging galaxies.

THE DETECTION OF A HOT MOLECULAR CORE IN THE LARGE MAGELLANIC CLOUD WITH ALMA

ABSTRACT We report the first detection of a hot molecular core outside our Galaxy based on radio observations with ALMA toward a high-mass young stellar object (YSO) in a nearby low metallicity galaxy, the Large Magellanic Cloud (LMC). Molecular emission lines of CO, C17O, HCO+, H13CO+, H2CO, NO, SiO, H2CS, 33SO, 32SO2, 34SO2, and 33SO2 are detected from a compact region (∼0.1 pc) associated with a high-mass YSO, ST11. The temperature of molecular gas is estimated to be higher than 100 K based on rotation diagram analysis of SO2 and 34SO2 lines. The compact source size, warm gas temperature, high density, and rich molecular lines around a high-mass protostar suggest that ST11 is associated with a hot molecular core. We find that the molecular abundances of the LMC hot core are significantly different from those of Galactic hot cores. The abundances of CH3OH, H2CO, and HNCO are remarkably lower compared to Galactic hot cores by at least 1–3 orders of magnitude. We suggest that these abundances are characterized by the deficiency of molecules whose formation requires the hydrogenation of CO on grain surfaces. In contrast, NO shows a high abundance in ST11 despite the notably low abundance of nitrogen in the LMC. A multitude of SO2 and its isotopologue line detections in ST11 imply that SO2 can be a key molecular tracer of hot core chemistry in metal-poor environments. Furthermore, we find molecular outflows around the hot core, which is the second detection of an extragalactic protostellar outflow. In this paper, we discuss the physical and chemical characteristics of a hot molecular core in the low metallicity environment.

VLT/ISAAC infrared spectroscopy of embedded high-mass YSOs in the Large Magellanic Cloud: Methanol and the 3.47μm band

This study aims to elucidate a possible link between chemical properties of ices in star-forming regions and environmental characteristics of the host galaxy. We performed 3--4 micron spectroscopic observations toward nine embedded high-mass YSOs in the Large Magellanic Cloud (LMC) with the ISAAC at the VLT. Additionally, we analyzed archival ISAAC data of two LMC YSOs. As a result, we detected absorption bands due to solid H2O and CH3OH as well as the 3.47 micron absorption band. The 3.53 micron CH3OH ice absorption band for the LMC YSOs is found to be absent or very weak compared to that seen toward Galactic sources. The result suggests the low abundance of CH3OH ice in the LMC. The 3.47 micron absorption band is detected toward six out of eleven LMC YSOs. We found that the 3.47 micron band and the H2O ice band correlate similarly between the LMC and Galactic samples, but the LMC sources seem to require a slightly higher H2O ice threshold for the appearance of the 3.47 micron band. For the LMC sources with relatively large H2O ice optical depths, we found that the strength ratio of the 3.47 micron band relative to the water ice band is only marginally lower than those of the Galactic sources. We propose that grain surface reactions at a relatively high dust temperature (warm ice chemistry) are responsible for the observed characteristics of ice chemical compositions in the LMC. We suggest that this warm ice chemistry is one of the important characteristics of interstellar and circumstellar chemistry in low metallicity environments. The low abundance of CH3OH in the solid phase implies that formation of complex organic molecules from methanol-derived species is less efficient in the LMC. For the 3.47 micron band, the observed difference in the water ice threshold may suggest that a more shielded environment is necessary for the formation of the 3.47 micron band carrier in the LMC.

The compositional evolution of C/2012 S1 (ISON) from ground-based high-resolution infrared spectroscopy as part of a worldwide observing campaign

Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/Δλ∼2.5×104) infrared spectroscopy as part of a worldwide observing campaign. Spectra were obtained on UT 2013 October 26 and 28 with NIRSPEC at the W.M. Keck Observatory, and UT 2013 November 19 and 20 with CSHELL at the NASA IRTF. H2O was detected on all dates, with production rates increasing markedly from (8.7±1.5)×1027moleculess−1 on October 26 (Rh=1.12AU) to (3.7±0.4)×1029moleculess−1 on November 20 (Rh=0.43AU). Short-term variability of H2O production is also seen as observations on November 19 show an increase in H2O production rate of nearly a factor of two over a period of about 6h. C2H6, CH3OH and CH4 abundances in ISON are slightly depleted relative to H2O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C2H2, HCN and OCS abundances relative to H2O appear to be within the range of mean values, whereas H2CO and NH3 were significantly enhanced. There is evidence that the abundances with respect to H2O increased for some species but not others between October 28 (Rh=1.07AU) and November 19 (Rh=0.46AU). The high mixing ratios of H2CO/CH3OH and C2H2/C2H6 on November 19, and changes in the mixing ratios of some species with respect to H2O between October 28 to November 19, indicates compositional changes that may be the result of a transition from sampling radiation-processed outer layers in this dynamically new comet to sampling more pristine natal material as the outer processed layer was increasingly eroded and the thermal wave propagated into the nucleus as the comet approached perihelion for the first time. On November 19 and 20, the spatial distribution for dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributions for volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributions for H2O, HCN, C2H6, C2H2, and H2CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributions for NH3 and OCS may be consistent with extended sources for these species. Abundances of HCN and C2H2 on November 19 and 20 are insufficient to account for reported abundances of CN and C2 in ISON near this time. Differences in HCN and CN spatial distributions are also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distribution of CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. The spatial distributions for NH3 and NH2 are similar, suggesting that NH3 is the primary source of NH2 with no evidence of a significant dust source of NH2; however, the higher production rates derived for NH3 compared to NH2 on November 19 and 20 remain unexplained. This suggests a more complete analysis that treats NH2 as a distributed source and accounts for its emission mechanism is needed for future work.

HerschelHIFI observations of the Sgr A +50 km s-1Cloud

To date O2 has definitely been detected in only two sources, namely rho Oph A and Orion, reflecting the extremely low abundance of O2 in the interstellar medium. One of the sources in the HOP program is the +50 km/s Cloud in the Sgr A Complex in the centre of the Milky Way. The Herschel HIFI is used to search for the 487 and 774 GHz emission lines of O2. No O2 emission is detected towards the Sgr A +50 km/s Cloud, but a number of strong emission lines of methanol (CH3OH) and absorption lines of chloronium (H2Cl+) are observed. A 3 sigma upper limit for the fractional abundance ratio of (O2)/(H2) in the Sgr A +50 km/s Cloud is found to be X(O2) less than 5 x 10(-8). However, since we can find no other realistic molecular candidate than O2 itself, we very tentatively suggest that two weak absorption lines at 487.261 and 487.302 GHz may be caused by the 487 GHz line of O2 in two foreground spiral arm clouds. By considering that the absorption may only be apparent, the estimated upper limit to the O2 abundance of less than (10-20) x 10(-6) in these foreground clouds is very high. This abundance limit was determined also using Odin non-detection limits. If the absorption is due to a differential Herschel OFF-ON emission, the O2 fractional abundance may be of the order of (5-10) x 10(-6). With the assumption of pure absorption by foreground clouds, the unreasonably high abundance of (1.4-2.8) x 10(-4) was obtained. The rotation temperatures for CH3OH-A and CH3OH-E lines in the +50 km/s Cloud are found to be 64 and 79 K, respectively, and the fractional abundance of CH3OH is approximately 5 x 10(-7).

Spectroscopic constraints on CH3OH formation: CO mixed with CH3OH ices towards young stellar objects.

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 = 25000) spectra of the massive YSO AFGL 7009S and of the low-mass YSOL1489 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.

The gas phase origin of complex organic molecules precursors in prestellar cores

Complex organic molecules (COMs) have long been observed in the warm regions surrounding nascent protostars. The recent discovery of oxygen-bearing COMs like methyl formate or dimethyl ether in prestellar cores (Bacmann et al. [2]), where gas and dust temperatures rarely exceed 10–15 K, has challenged the previously accepted models according to which COM formation relied on the diffusion of heavy radicals on warm (∼30 K) grains. Following these detections, new questions have arisen: do non-thermal processes play a role in increasing radical mobility or should new gas-phase routes be explored? The radicals involved in the formation of the aforementioned COMs, HCO and CH3 O represent intermediate species in the grain-surface synthesis of methanol which proceeds via successive hydrogenations of CO molecules in the ice. We present here observations of methanol and its grain-surface precursors HCO, H2 CO, CH3 O in a sample of prestellar cores and derive their relative abundances. We find that the relative abundances HCO:H2 CO:CH3 O:CH3 OH are constant across the core sample, close to 10:100:1:100. Our results also show that the amounts of HCO and CH3 O are consistent with a gas-phase synthesis of these species from H2 CO and CH3 OH via radical-neutral or ion-molecule reactions followed by dissociative recombinations. Thus, while grain chemistry is necessary to explain the abundances of the parent volatile CH3 OH, and possibly H2 CO, the reactive species HCO and CH3 O might be daughter molecules directly produced in the gas-phase.

INFRARED SPECTROSCOPIC SURVEY OF THE QUIESCENT MEDIUM OF NEARBY CLOUDS. I. ICE FORMATION AND GRAIN GROWTH IN LUPUS

Infrared photometry and spectroscopy (1-25 um) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of the grains and the composition of the ices before they are incorporated into circumstellar envelopes and disks. H2O ices form at extinctions of Ak=0.25+/-0.07 mag (Av=2.1+/-0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H2O ice (2.3+/-0.1*10^-5 relative to Nh) is typical for quiescent regions, but lower by a factor of 3-4 compared to dense envelopes of YSOs. The low solid CH3OH abundance (<3-8% relative to H2O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared (>5 um) continuum extinction relative to Ak increases as a function of Ak. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to Rv~ 3.5 (Ak=0.71) and Rv~5.0 (Ak=1.47). For lines of sight with Ak>1.0 mag, the tau9.7/Ak ratio is a factor of 2 lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the tau9.7/Ak ratio. This process is likely related to grain growth by coagulation, as traced by the A7.4/Ak continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.

WIDESPREAD METHANOL EMISSION FROM THE GALACTIC CENTER: THE ROLE OF COSMIC RAYS

We report the discovery of a widespread population of collisionally excited methanol J = 4_{-1} to 3$_0 E sources at 36.2 GHz from the inner 66'x18' (160x43 pc) of the Galactic center. This spectral feature was imaged with a spectral resolution of ~16.6 km/s taken from 41 channels of a VLA continuum survey of the Galactic center region. The revelation of 356 methanol sources, most of which are maser candidates, suggests a large abundance of methanol in the gas phase in the Galactic center region. There is also spatial and kinematic correlation between SiO (2--1) and CH3OH emission from four Galactic center clouds: the +50 and +20 km/s clouds and G0.13-0.13 and G0.25+0.01. The enhanced abundance of methanol is accounted for in terms of induced photodesorption by cosmic rays as they travel through a molecular core, collide, dissociate, ionize, and excite Lyman Werner transitions of H2. A time-dependent chemical model in which cosmic rays drive the chemistry of the gas predicts CH3OH abundance of 10^{-8} to 10^{-7} on a chemical time scale of 5x10^4 to 5x10^5 years. The average methanol abundance produced by the release of methanol from grain surfaces is consistent with the available data.

A Sensitive Survey of Dense Parts of Outflows toward Massive Cores

A set of samples of 13 massive star-forming cores were observed in SiO (2-1), CH3OH (2-1) and C34S (2-1) thermal lines. Nine of these cores were detected in all three lines. Among the nine SiO detections, three were new detections, and relatively faint. Most of the lines have wide wings, which might be interpreted as the evidence of ongoing energetic outflows in the cores. The line widths of SiO are generally the broadest, which might further suggest that the SiO emissions are due to higher velocity outflow, and closer to the excited source. We derive the rotational temperatures, column densities and chemical relative abundances of the cores. There is a strong correlation between SiO and CH3OH abundances, with correlation coeffcient R = 0.77, but no correlation is observed between SiO and C34S.

AKARI observations of ice absorption bands towards edge-on young stellar objects

Context. Circumstellar disks and envelopes of low-mass young stellar objects (YSOs) contain significant amounts of ice. Such icy material will evolve to become volatile components of planetary systems, such as comets in our solar system. Aims. To investigate the composition and evolution of circumstellar ice around low-mass young stellar objects (YSOs), we observed ice absorption bands in the near infrared (NIR) towards eight YSOs ranging from class 0 to class II, among which seven are associated with edge-on disks. Methods. We performed slit-less spectroscopic observations using the grism mode of the InfraRed Camera (IRC) on board AKARI, which enables us to obtain full NIR spectra from 2.5 μ mt o 5μm, including the CO2 band and the blue wing of the H2O band, which are inaccessible from the ground. We developed procedures to carefully process the spectra of targets with nebulosity. The spectra were fitted with polynomial baselines to derive the absorption spectra. The molecular absorption bands were then fitted with the laboratory database of ice absorption bands, considering the instrumental line profile and the spectral resolution of the grism dispersion element. Results. Towards the class 0-I sources (L1527, IRC-L1041-2, and IRAS 04302), absorption bands of H2O, CO2 ,C O, and XCN are clearly detected. Column density ratios of CO2 ice and CO ice relative to H2O ice are 21−28% and 13−46%, respectively. If XCN is OCN − , its column density is as high as 2−6% relative to H2O ice. The HDO ice feature at 4.1 μm is tentatively detected towards the class 0-I sources and HV Tau. Non-detections of the CH-stretching mode features around 3.5 μm provide upper limits to the CH3OH abundance of 26% (L1527) and 42% (IRAS 04302) relative to H2O. We tentatively detect OCS ice absorption towards IRC-L1041-2. Towards class 0-I sources, the detected features should mostly originate in the cold envelope, while CO gas and OCN − could originate in the region close to the protostar, where there are warm temperatures and UV radiation. We detect H2O ice band towards ASR41 and 2MASSJ1628137-243139, which are edge-on class II disks. We also detect H2O ice and CO2 ice towards HV Tau, HK Tau, and UY Aur, and tentatively detect CO gas features towards HK Tau and UY Aur.