CH3OH Abundance Research Articles

H2CO and CH3OH abundances in the envelopes around low-mass protostars

This paper presents the third in a series of single-dish studies of molecular abundances in the envelopes around a large sample of 18 low-mass pre- and protostellar objects. It focuses on typical grain mantle products and organic molecules, including H2CO, CH3OH and CH3CN. With a few exceptions, all H2CO lines can be fit by constant abundances of throughout the envelopes if ortho- and para lines are considered independently. The current observational dataset does not require a large H2CO abundance enhancement in the inner warm regions, but this can also not be ruled out. Through comparison of the H2CO abundances of the entire sample, the H2CO ortho-para ratio is constrained to be consistent with thermalization on grains at temperatures of 10-15 K. The H2CO abundances can be related to the empirical chemical network established on the basis of our previously reported survey of other species and is found to be closely correlated with that of the nitrogen-bearing molecules. These correlations reflect the freeze-out of molecules at low temperatures and high densities, with the constant H2CO abundance being a measure of the size of the freeze-out zone. An improved fit to the data is obtained with a “drop” abundance structure in which the abundance is typically a few 10-10 when the temperature is lower than the evaporation temperature and the density high enough so that the timescale for depletion is less than the lifetime of the core. The location of the freeze-out zone is constrained from CO observations. Outside the freeze-out zone, the H2CO abundance is typically a few . The observations show that the CH3OH lines are significantly broader than the H2CO lines, indicating that they probe kinematically distinct regions. CH3OH is moreover only detected toward a handful of sources and CH3CN toward only one, NGC 1333-IRAS2. For NGC 1333-IRAS2, CH3OH and CH3CN abundance enhancements of two-three orders of magnitude at temperatures higher than 90 K are derived. In contrast, the NGC 1333-IRAS4A and IRAS4B CH3OH data are fitted with a constant abundance and an abundance enhancement at a lower temperature of 30 K, respectively. This is consistent with a scenario where CH3OH probes the action of compact outflows on the envelopes, which is further supported by comparison to high frequency, high excitation CS and HDO line profiles which uniquely probe warm, dense gas. The extent to which the outflow dominates the abundance enhancements compared with the passively heated inner envelope depends on the filling factors of the two components in the observing beam.

The total column densities and fractional abun-dances of CH3CN, CH3OH, HCOOCH3 and 13CS in W51 cores

We present interferometric observations of the W51 region at 2.7 mm and 3.3 mm with the Berkeley-Illinois-Maryland Association Array. The lines of CH 3 CN (J = 6 − 5, K = 0,1,2,3,4), CH3OH (15 −2 −15 1 E), HCOOCH 3 -A (7 1,6 − 6 1,5 ), HCOOCH3 -E (7 1,6 − 6 1,5 ) and 13 CS (2-1) as well as the 2.7 mm continuum toward W51 have been observed. The derived total beam-averaged column densities NT and fractional abundances of CH 3 CN, CH 3 OH, HCOOCH 3 and 13 CS in W51 cores have been obtained.

The origin and structure of clumps along molecular outflows: the test case of CB3

We investigate the origin of the small, chemically rich molecular clumps observed along the main axis of chemically rich outflows such as CB3 and L1157. We develop a chemical model where we explore the chemical evolution of these clumps, assuming they are partially pre-existing to the outflow, or alternatively newly formed by the impact of the outflow on the surrounding medium. The effects of the impact of the outflow are reproduced by density and temperature changes in the clump. We find that the observed abundances of CH3OH, SO and SO2 are best reproduced by assuming a scenario where the dense molecular gas observed is probably pre-existing in the interstellar medium before the formation of their exciting (proto)stars and that the clumpiness and the rich chemistry of the clumps are a consequence of a pre-existing density enhancement and of its interaction with the outflow.

The Dependence of H 2 CO and CH 3 OH Formation on the Temperature and Thickness of H 2 O-CO Ice during the Successive Hydrogenation of CO

The formation of formaldehyde (H2CO) and methanol (CH3OH) by the successive addition of hydrogen atoms to CO molecules in H2O-CO mixed ice was measured at 10, 15, and 20 K. The maximum yield of CH3OH was obtained at 15 K, with the next highest yield at 10 K. At 20 K, the reactions proceeded very slowly, and the resulting yield of CH3OH was significantly smaller than at 10 and 15 K and was considered to be due to a fall in the sticking probability of hydrogen at this temperature. The observed temperature dependence indicates that the abundances of H2CO and CH3OH in a molecular cloud are strongly dependent on the temperature of dust as well as the flux of atomic hydrogen. The diffusion of hydrogen atoms in ice is also discussed based on measurements of the dependence of CO hydrogenation on ice thickness in the range of less than 30 monolayers.

Interaction between the Outflow and the Core in IRAM 04191+1522

We have carried out mapping observations of the molecular core associated with the young Class 0 protostar, IRAM 04191+1522, in the CH3OH (JK=2K-1K) and C34S (J=2-1) lines using the 45 m telescope at Nobeyama Radio Observatory. As well as an elongated envelope associated with the protostellar formation (size \~0.07 pc x 0.04 pc, mass ~ 2.3 Mo), two compact (~ 0.03 pc) condensations were found in the CH3OH line at the southern edge of the elongated envelope, where the blueshifted CO outflow emerging from the protostar is located. In contrast to the elongated envelope, those compact CH3OH condensations show much larger line width (up to 2.0 km s-1) with centroid velocities blueshifted by ~ 0.8 km s-1. The compact condensations have momenta (~ 0.06 Mo km s-1) comparable to that of the blueshifted molecular outflow. These results suggest that the compact condensations are probably formed in the course of interaction between the outflow and the ambient gas surrounding the protostar, and that such interaction may cause dissipation of a part of the ambient gas. No drastic, localized enhancement of the CH3OH abundance is, however, observed toward the compact condensations, implying that there seems to be no significant shock heating at the compact condensations. This may be because the CO outflow velocity (< 10 km s-1) is too low to cause effective heating to release CH3OH on dust grains into gas phase.

Methanol and Silicon Monoxide Observations toward Bipolar Outflows Associated with Class 0 Objects

We report Swedish-ESO Submillimetre Telescope observations of seven bipolar outflows thought to be associated with Class 0 objects in the υ = 0J = 3 → 2 and J = 2 → 1 transitions of SiO and Jk = 3k → 2k and Jk = 2k → 1k transitions of CH3OH. Methanol and silicon monoxide emission from outflowing gas were detected toward the lobes of four objects (NGC 2264G, IRAS 16293-2422, Serpens S68N, and Serpens SMM 4). The SiO line profiles are characteristics of C-type bow shocks, showing a peak at a radial velocity close to but displaced from the ambient cloud velocity and a gradual decrease in intensity from the peak toward higher flow velocities. There is a significant correlation between the column density of SiO and the terminal SiO flow velocity, which suggests a velocity-selective enhancement in the production of SiO molecules. We find that the SiO abundance in the lobes is enhanced with respect to that of the ambient cloud by a factor of at least 330 in IRAS 16293-2422 and SMM 4, 170 in NGC 2264G, and 80 in S68N. The CH3OH abundance is enhanced by a factor of 500 in IRAS 16293-2422, 330 in SMM 4, 80 in S68N, and 23 in NGC 2264G. In addition, we find that the dependence of the SiO/CO and CH3OH/CO abundance ratios with radial flow velocity shows a steep increase in the range from ~0 to ~4-5 km s-1 and a gradual decline toward higher flow velocities. In the remaining three sources (CG 30, IRAS 13036-7644, and VLA 1623-243), emission in methanol was detected from a narrow line at the velocity of the ambient cloud, and no emission was detected in silicon monoxide. Weak methanol emission from a low-velocity outflow component was detected toward CG 30.

A Comparison of the Spatial Distribution of H13CO+, CH3OH, and C34S Emission and Its Implication in Heiles Cloud 2

We have mapped the Heiles cloud 2 region in the Taurus molecular cloud complex with H13CO+ (J = 1-0), CH3OH (JK = 20-10 A+), and C34S (J = 2-1) lines. Dense gas traced by the mapped lines with critical densities higher than 104 cm-3 is concentrated in four condensations, that is, the TMC 1 and TMC 1C filaments, L1527, and TMC 1A. We have found that the three emission lines have remarkably different spatial distributions. The H13CO+ emission traces well dense cores harboring protostars, while the CH3OH emission is weak toward the protostars and is rather enhanced toward cores without protostars. We found that there are two starless cores with enhanced CH3OH emission at the northwestern ends of the TMC 1 and TMC 1C filaments, toward which the H13CO+ emission is barely seen. On the basis of the analyses using the large velocity gradient (LVG) model, we show that the CH3OH abundance relative to H13CO+ is enhanced by up to 1 order of magnitude in the cores without protostars. The C34S abundance relative to H13CO+ also shows a similar trend to that of CH3OH. Such an abundance variation between H13CO+ and CH3OH and C34S can be explained in the scheme of time-dependent gas-phase chemical evolution, which predicts that CH3OH and C34S are abundant in the early stages of chemical evolution and become deficient in the later stages. A comparison of the spatial-velocity structures in TMC 1 observed with the three molecular lines suggests that this cloud consists of multiple components with different velocities and different chemical compositions along the line of sight.

Identification of New Methanol Lines toward Sagittarius B2

The first astronomical measurements of the Jk = 182-181, 192-191, and 202-201 E transitions of methanol (CH3OH) are presented. A rotational temperature Trot = 170(13) K was determined for the Sgr B2N-LMH core by combining the data from the Jk = 132-131 and 172-171 E transitions. The total column density of CH3OH is NT(CH3OH) = 4.2(6) × 1017 cm-2, and the fractional abundance of CH3OH is X(CH3OH) ≈ 4 × 10-8. The H59α recombination line and a new unidentified line were detected.

Infrared Observations of Interstellar Ices

In the recent years revolutionary results concerning the nature of icy dust particles have been obtained with the help of the Infrared Space Observatory (ISO) and ground based observations. To date interstellar ice features of H2O, CO, CO2, CH3OH, CH4, H2CO, OCS and HCOOH as well as other minor species are observed. Interstellar grains act as important catalysts in the interstellar medium. Processes such as UV irradiation, cosmic ray processing and temperature variations determine the grain mantle growth and chemical evolution. ISO has revealed that ice segregation is an important and ubiquitous process in the vicinity of massive protostars and reflects the extensive thermal processing of grains in such environments.In this paper a recent view on the inventory of interstellar ices is presented. Constraints on the reservoirs of oxygen in dense clouds are discussed, taking into account recent measurements of oxygen-bearing species. Large abundances of CO2 and CH3OH in dense molecular clouds provide challenging perspectives to investigate the differences of ice chemistry in the vicinity of high and low-mass protostars. Accurate abundances of ice species and knowledge on the ice distribution in the protostellar regions are an important tool to define the environmental conditions in molecular clouds. A global understanding of interstellar ice chemistry also allows monitoring the incorporation and evolution of volatiles in planetesimals and comets and to reveal processes predominant in the early Solar System.

Lyman-alpha observations of comet Hyakutake with SWAN on SOHO

The SWAN instrument on board SOHO is a Lyman-α (Lα) photometer able to map the sky intensity with a resolution of 1°, and a capability of microstepping (0.1°). SWAN is primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background, but was in addition extensively used to map the Lα emission of several comets since its launch in December 1995. Here we report observations of comet C/1996 B2 (Hyakutake). Its Lα emission cloud extended over more than 60° while approaching the Earth at 0.102 AU. A comparison with a simple model allowed hydrogen and H 2O production rates to be derived, while the comet approached closer to the Sun from 1.12 AU to 0.53 AU distance to the Sun, pre-perihelion. The derived H 2O production rate was found in fair agreement with other derivations (IUE and ground-based in the IR and UV), validating the Lα method. The H 2O production by SWAN was related to several other measurements of minor constituents in order to derive new values of abundance of CO, HCN, H 2CO, CH 3OH and CH 3CN. Most important, the D H ratio in comet Hyakutake is now found at 3 × 10 −4, as in comet Halley, while a previous estimate based on a wrong H 2O number had indicated a value twice lower, with important cosmogonic consequences. The time evolution showed a fast surge on 21 March, coinciding with the time of fragmentation of the nucleus as detected 3 days later at Pic du Midi. This surge is also confirmed by the detailed comparison of H column densities (observed vs model) as a function of the distance to the nucleus, showing a larger ratio in the inner region (younger atoms) than in the outer region (older atoms) on 21 March, and then a progressive filling-in of the H envelope. After the surge, there was a plateau for 16 days around 1.8 × 10 29 H 2O mol s −1, and then an increase following approximately a R −2 law. This behavior is interpreted as the surge and plateau corresponding to the fragmentation and total disruption/evaporation of a fragment of the nucleus, of approximately 200 m. Finally, it is argued that the first detection of ethane C 2H 6 in this comet (IR observations) might have been the result of the special circumstances (a large fragment disrupted very near the Earth) rather than revealing a new special class of ethane-rich comets as argued by other authors.

Ices in star forming regions

IR spectra of sources associated with molecular cloud material show a variety of absorption features attributed to simple molecules, such as H2O, CO, CH3OH, CO2, CH4, and OCS in icy grain mantles. These identifications are reviewed. These molecules are formed through accretion and reaction of gas phase species on grain surfaces. The high abundance of CH3OH and CO2point towards the importance of hydrogenation and oxidation reactions in this process. Observations also show that thermal outgassing is of great importance for the composition of interstellar ice mantles. Both these processes are discussed in some detail.

Systematic Observations of Methanol and Other Organics in Comet P/Swift-Tuttle: Discovery of New Spectral Structure at 3.42 μm

We present a detailed study of the 3.4-μm "cometary organics feature" in Comet P/Swift-Tuttle (1992 XXVIII). The study is based on spectra acquired from Kitt Peak and Mauna Kea Observatories on UT 1992 November 8, 9, 12, 13, 26, and 27, during which time the heliocentric distance decreased from 1.12 to 0.99 AU. Two distinct components are present, cometary methanol (CH 3OH), and another as yet unidentified organic volatile responsible for an emission feature centered at ∼3.424 μm. We obtain a CH 3OH abundance of 4.3 ± 0.5% relative to H 2O, revising our previously reported value of 7 ± 1%. The revisions result from improved data analysis, inclusion of additional P/Swift-Tuttle observations, and use of corrected CH 3OH g-factors. The unidentified feature consistently accounted for one-half of the flux contained in the 3.4-μm feature and exhibited a heliocentric dependence consistent with a volatile parent species, in agreement with conclusions based on observations of several recent comets. We present the first echelle spectrum of this feature (resolving power λ/Δλ ≈ 8000), which reveals spectral structure in the form of a pronounced peak centered at a rest frequency of(2920.24 ± 0.09) cm -1. We suspect this to be representative of asymmetric CH 2 (or CH-X) vibrational stretching in some as yet unidentified organic compound. Our results indicate the presence of an additional abundant form of volatile carbon in comets. If the unidentified band(s) possess g-factors similar to those of the adjacent methanol bands, the inferred abundance of the unidentified emitting species would be comparable to that of methanol.

Organic molecules in oxygen-rich circumstellar envelopes: methanol and hydrocarbons

The existence of anomalously high abundances of gaseous CH4 has been invoked to explain the unexpectedly high abundances of the carbon-bearing molecules HCN and H2CO in the outflows from O-rich red giants. We have modelled the chemistry that proceeds in the outer envelope when CH4 is injected from the inner envelope. We find that photolysis by the interstellar radiation field drives an ion-neutral chemistry which produces several organic molecules. The calculated abundances of CH3OH, C2H and C2 can be comparable to those calculated for H2CO and HCN. Species such as C2H4, C2H2 and CH3CN can also be abundant. A search for CH3OH and C2H in several O-rich outflows known to exhibit strong HCN emission is needed. As it derives entirely from the CH4 photochain, is insensitive to the envelope temperature distribution, and has accessible transitions at millimetre wavelengths, the detection of the C2H radical would provide further indirect support for the presence of the hypothesized methane.

The circumstellar environment of IRAS 05338-0624.

Millimeter continuum and spectral line observations with 10", 30", and 60" resolution are used to characterize the structure and chemistry of the gas around the young, embedded star, IRAS 05338-0624. On arcminute scales, emission from dense gas tracers outline an isolated condensation centered on the IRAS source position. The condensation is characterized by a size of approximately 60", a density of 2 x 10(5) cm-3, and a virial mass of 40 M solar. Interferometric CS J = 2-1 observations show two peaks, one toward the continuum peak and the other toward a position 14" west and 8" south. Single-dish maps of SO, CH3OH, and SiO show pronounced wing emission to the west of the IRAS source, which interferometer observations reveal to be a compact region of outflow activity. CS emission as redshifted and blueshifted velocities reveals a bipolar outflow oriented with a position angle of 45 degrees, while SiO emission appears to be tracing a fast shock interaction region at the CS red-lobe peak, 14" west and 8" south of the IRAS source. Finally, H13CO+ emission traces clumps of quiescent gas toward the IRAS source and adjacent to the blue lobe of the outflow. Column densities and molecular fractional abundances are derived to explore the interaction between the surrounding condensation and the young stellar object. We find evidence for gas phase depletions within the overall condensation in several gas tracers (CO, CS, HCN, SO) but not in the region immediately around the young stellar object. Enhanced abundances of SO, CH3OH, and SiO (by factors of 4, >100, >1000, respectively) are observed in the shocked gas; these enhancements may be explained in terms of a nondissociative shock liberating mantle materials that contain some amount of refractory materials, a moderate velocity dissociative shock in which only minor sputtering of Si occurs, or a shock that impacts surrounding material with a range of speeds.