Issue |
A&A
Volume 517, July 2010
|
|
---|---|---|
Article Number | A17 | |
Number of page(s) | 6 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200913830 | |
Published online | 26 July 2010 |
TIMASSS: the IRAS16293-2422 millimeter and
submillimeter spectral survey: tentative detection of deuterated methyl
formate (DCOOCH3)
,![[*]](/icons/foot_motif.png)
K. Demyk1,2 - S. Bottinelli1,2 - E. Caux1,2 - C. Vastel1,2 - C. Ceccarelli3 - C. Kahane3 - A. Castets3
1 - CESR (Centre d'étude Spatiale des Rayonnements), Université de Toulouse [UPS], 31028 Toulouse Cedex 4, France
2 -
CNRS, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
3 -
Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
Received 9 December 2009 / Accepted 14 April 2010
Abstract
Context. High deuterium fractionation is observed in various
types of environments such as prestellar cores, hot cores, and hot
corinos. It has proven to be an efficient probe for studying the
physical and chemical conditions of these environments. The study of
the deuteration of different molecules helps us to understand their
formation. This is especially interesting for complex molecules like
methanol and bigger molecules for which it may allow differentiation of
gas-phase and solid-state formation pathways.
Aims. Methanol exhibits a high deuterium fractionation in hot corinos. Since CH3OH
is thought to be a precursor of methyl formate we expect that
deuterated methyl formate is produced in such environments. We searched
for the singly-deuterated isotopologue of methyl formate, DCOOCH3, in IRAS 16293-2422, a hot corino well-known for its high degree of methanol deuteration.
Methods. We used the IRAM/JCMT unbiased spectral survey of IRAS 16293-2422, which allowed us to search for the DCOOCH3
rotational transitions within the survey spectral range
(80-280 GHz, 328-366 GHz). The expected emission of
deuterated methyl formate is modelled at LTE and compared with the
observations.
Results. We have tentatively detected DCOOCH3 in the
protostar IRAS 16293-2422. We assign eight lines detected in the IRAM
survey to DCOOCH3. Three of these lines are affected by blending
problems, and one is affected by calibration uncertainties;
nevertheless, the LTE emission model is compatible with the
observations. A simple LTE modelling of the two cores in IRAS
16293-2422, based on a previous interferometric study of HCOOCH3, allows us to estimate the amount of DCOOCH3 in IRAS 16293-2422. Adopting an excitation temperature of 100 K and a source size of 2
and 1
5 for the A and B cores, respectively, we find that
cm-2. The derived deuterium fractionation is
15%,
consistent with values for other deuterated species in this source and
much greater than expected from the deuterium cosmic abundance.
Conclusions. If its tentative detection is confirmed, DCOOCH3
should now be considered in theoretical models that study complex
molecule formation and their deuteration mechanisms. Experimental work
is also needed to investigate the different chemical routes leading to
the formation of deuterated methyl formate.
Key words: line: identification - methods: observational - ISM: molecules - ISM: abundances - ISM: individual objects: IRAS 16293-2422 - radio lines: ISM
1 Introduction
IRAS 16293-2422 (hereafter IRAS 16293) is a complex source hosting two
hot corinos (called ``A'' and ``B'') in which many complex organic
molecules have been observed: acetonitrile (CH3CN), methyl formate (HCOOCH3), ketene (H2CCO), formic acid (HCOOH), ethanol (C2H5OH), ethyl cyanide (C2H5CN), etc. (Cazaux et al. 2003; Bottinelli et al. 2004; Bisschop et al. 2008).
This source is also characterised by a high degree of deuteration with
singly, doubly, or triply deuterated molecules. HDCO, CH3OD, and CH2DOH are all detected in IRAS 16293 with a deuterium fractionation of 15%, 1.8% and 37%, respectively, while doubly-deuterated formaldehyde (D2CO) and methanol (CHD2OH) are observed with a deuteration of 5 and 7.4% (Parise et al. 2006). Triply deuterated methanol was also observed in IRAS 16293 with a deuterium fractionation ratio of 1.4% (Parise et al. 2004).
These values are much greater than expected from the cosmic D/H ratio (10-5).
The process of deuteration is a longstanding and challenging issue.
High deuterium fractionation is observed in cold regions such as
pre-stellar cores (Pillai et al. 2007; Bacmann et al. 2003), in warmer regions such as hot cores (eg. Orion, Jacq et al. 1993) and hot corinos and in photo dissociation regions (PDRs) (Pety et al. 2007; Leurini et al. 2006).
The detection of deuterated molecules allows one to study the chemical
pathways leading to their formation and to trace the chemical and
physical conditions of the observed environments. Whereas the deuterium
molecular enrichment is explained by gas phase chemistry in cold
environments (Roberts et al. 2003), in hot regions it is certainly driven by chemistry on the icy surface of the dust (Tielens 1983; Ceccarelli et al. 2001).
This is illustrated well by the detection of multiply deuterated
methanol since methanol cannot be efficiently formed in the gas phase (Turner 1998). Since methanol is thought to be involved in methyl formate formation on grains (Bennett & Kaiser 2007), the high deuterium fractionation of CH3OH suggests that deuterated methyl formate should be produced along with methyl formate.
In this study we present the tentative detection of deuterated methyl formate, DCOOCH3, in IRAS 16293 based on observations from the millimeter/submillimeter spectral survey performed toward the low-mass Class 0 protostar IRAS16293 (Caux et al. 2010). The observations are described in Sect. 2. The tentative detection of DCOOCH3 is presented in Sect. 3 and its formation is discussed in Sect. 4.
2 Observations
The survey was carried out with the IRAM-30m (frequency range 80-280
GHz) and JCMT-15m (frequency range 328-366 GHz) telescopes during
the
period January 2004 to August 2006. The angular resolution (HPBW) of
the observations varied between 9
and 33
,
depending on the telescope and frequency used, and the spectral
resolution ranged between 0.3 and 1.2 MHz. All observations were
performed in double-beam-switch observing mode, with a 90
throw.
Pointing and focus were regularly checked, and the resulting pointing
accuracy depended on the weather and the observed frequency but was
always better than 5
.
The lines assigned to deuterated methyl formate presented in this paper
were observed in the 190-270 GHz domain at the IRAM 30m, with
a spectral resolution of 1-1.25 MHz and an rms of about
5-15 mK (antenna temperature). For a detailed description of this
survey, see the paper presenting the observations and calibrations of
the whole survey (Caux et al. 2010). Once this paper has been published, the data will be made publicly available on the TIMASSS web site (http://www-laog.obs.ujf-grenoble.fr/heberges/timasss/).
3 Results
To search for deuterated methyl formate, we used the predicted spectrum calculated from the recent study by Margulès et al. (2010). Deuterated methyl formate DCOOCH3 is an asymmetric molecule described by the quantum numbers J,
,
and
.
It has a dense rotational spectrum in which each transition is split
into A and E components of the same intensity because of the internal
rotation of the methyl group.
Figure 1 shows the emission spectrum of DCOOCH3 modelled with the CASSIS software
in LTE, assuming optically thin conditions, with an excitation
temperature of 100 K (typical of hot core values), a line width of
5 km s-1, and an extended source (100
). The aim of this figure is to investigate the intensity ratios of the DCOOCH3
transitions, and it can indeed be seen that some lines are strikingly
more intense than the others. Each of these strong lines consists of 8
transitions with
and
,
0-0, 1-0, and 1-1 (4 for the A-species and 4 for the E-species, see Table 1) which are very close in frequency and are blended in the modelled spectrum because of the line width.
Table 1: Emission lines assigned to DCOOCH3 in the IRAS 16293 IRAM spectral survey.
Considering that the line width of complex molecules in IRAS 16293 is a few km s-1 (2-6 km s-1 for HCOOCH3, Bottinelli et al. 2004), these very intense lines are particularly favourable to search for DCOOCH3.
Eleven of these lines are in the observed spectral range of the IRAS
16293 spectral survey, eight in the IRAM data, and three in the
JCMT data. In the JCMT spectral range, because the modelled line
intensity decreases (Fig. 1) and the rms of the observed spectrum is high (15-25 mK,
main beam temperature), the line intensities predicted by the LTE model
described below are within the noise. Consequently, we do not detect
these transitions. We assigned eight lines detected in the IRAM
spectral range to the other eight transitions of DCOOCH3 (Table 1). All other lines from DCOOCH3 are below the detection limit. Indeed, for the other transitions of DCOOCH3, i.e. transitions with
and
and transitions with
and for all value of
(i.e. frequency <190 GHz), the transition frequencies are more
spaced out. Consequently the lines are not blended anymore, and their
intensity is low (Fig. 1). At 2 and 3 mm, this is combined with a beam dilution effect since the emission region is supposed to be compact.
![]() |
Figure 1: LTE emission model of DCOOCH3 calculated in optically thin conditions for an extended source, an excitation temperature of 100 K, with a linewidth of 5 km s-1. The line intensity has been normalised to the strongest line at 239.655 GHz. |
Open with DEXTER |
![]() |
Figure 2:
Observed lines assigned to DCOOCH3 in IRAS 16293 compared with a two-components LTE emission model of DCOOCH3. Black line: observed IRAM spectral lines. Red line: sum of the emission from region A (light blue line) modelled with
|
Open with DEXTER |
To compare with the observations, we modelled the DCOOCH3 emission in LTE. Although the single-dish observations do not allow us to constrain the spatial distribution of DCOOCH3
it seems reasonable to assume that it follows that of the main
isotopologue. Thus, following the interferometric observations of HCOOCH3 from Bottinelli et al. (2004) we used a two-component emission model characterised as follows: an angular size,
,
of 2
,
km s-1, and a line width
of 6 km s-1 for source A and
,
km s-1, and
km s-1 for source B. The excitation temperature was set to 100 K for both emission regions, a value derived for HCOOCH3 in the spectral survey (see below). A comparison between the observations and the model is presented in Fig. 2.
The transition at 198.824 GHz is potentially blended with a bright SO
line in the image sideband ((5,4)-(4,3) at 206.175 GHz, observed
K,
the line shows up at 198.826 GHz in the signal band), however,
there is a problem with the calibration of the observations at this
frequency. Indeed, adopting a sideband rejection of 15 dB (default
value), we find that the SO contaminating line from the image band
would have
mK, contradicting the observed line at 198.826 GHz (
mK).
The assumed sideband rejection thus seems to be incorrect and without
knowing the true rejection of this observation we cannot conclude
whether the line is caused by SO alone or by DCOOCH3 and either SO or another unidentified species. This line should thus be considered with caution.
The line at 209.033 GHz is partially blended with a line from HCOOCH3 (A species, 1711,6-1611,5,
209.030 GHz). Nonetheless, the observations are reproduced well
with a model including both the normal and deuterated species
(Fig. 2). To model the HCOOCH3 emission, we used the same two-component LTE model as for the deuterated species. Tens of HCOOCH3 lines are present in the spectral survey. They were modelled with an excitation temperature of 100 K and HCOOCH3 abundances of
cm-2 and
cm-2. The detailed analysis of this species will be published elsewhere, however these values are consistent with those of Bottinelli et al. (2004) within the uncertainties. An excess of emission on the high-frequency side of the HCOOCH3 lines at 209.023, 209.033, and 209.044 GHz (although for the latter the line wing is within the noise; Fig. 2, panel 2) may be seen in the observed spectrum. Since this excess of emission is attributed to DCOOCH3 in the 209.033 GHz line it is important to assess that it is not related to a HCOOCH3 line wing. This was checked on the numerous spectral lines assigned to HCOOCH3
in the spectral survey. It appears that they do not show any systematic
emission excess, as can be seen in the panel (4) of Fig. 2, which shows two methyl formate lines at 229.405 and 229.420 GHz that do no present such a feature.
The line at 219.242 GHz is partially blended with a strong acetone (CH3COCH3) line at 219.242 GHz (211,19-202,19). Acetone contributes to about two thirds of the intensity of the observed line (
cm-2,
K,
,
km s-1, and
km s-1) while DCOOCH3 contributes to one third (Fig. 2,
panels 3a and 3b). It should be noted that the intensity of the two
acetone lines at 219.220 GHz and 219.265 GHz (Fig. 2,
panel 3b) are partially reproduced. We carefully checked that the
adopted acetone model does not underestimate the abundance of acetone.
It is impossible to account for the whole line-integrated intensities
of these three lines without strongly contradicting other strong
acetone lines in the survey. Typically, the intensity of the strongest
acetone lines is overestimated by a factor 2 or 3 (depending on the
parameters of the model) when the intensity of the 219.220, 219.242,
and 219.265 GHz lines are correctly reproduced. Furthermore, the
acetone lines are optically thin, independent of the model parameters.
The optical depth of the strongest acetone lines do not exceed
,
we thus do not expect any opacity problem that could affect the line
intensities. The detailed analysis of acetone in the survey is
postponed to a future paper, so we conclude that these two lines are
blended with transitions of other species, although we have yet not
identified which species, and the 219.242 GHz band cannot be due
to only acetone and is consistent with additional emission from DCOOCH3 .
The transitions at 229.446, 239.654, 249. 860, and 260.064 GHz do
not appear to be blended. The observed line at 249. 860 GHz is
narrower than the modelled line. However, we have checked that it is
not an instrumental artefact or a spike, and it is probably caused by
the low signal-to-noise ratio (
).
The spectral range around the line at 270.26 GHz is characterised
by strong spectral confusion, however, although it is contaminated by
nearby lines from other (unknown) molecules, the line at 270.267 GHz is
reproduced by the DCOOCH3 emission model reasonably well.
Thus, among the eight lines in the IRAM spectral range assigned to DCOOCH3, four lines are not blended with other species (229.448, 239.654, 249.860, 260.064 GHz), two lines blended with known species (HCOOCH3 at 209.031 GHz and CH3COCH3 at 219.242 GHz). They are reproduced well by the model, and DCOOCH3 contributes about 30-35% to their intensity, independent of the adopted model used for acetone and methyl formate. The line at 270.267 GHz is heavily blended with unknown species, but the emission model agrees with the observations. The emission model is compatible with the observations at 198.824 GHz, however because this observation suffers from calibration uncertainty, it is impossible to conclude about the detection of this line.
For DCOOCH3, a reasonable agreement with the observations is found for a column density of
cm-2. High angular resolution observations are needed not only to confirm the adopted two-component model for DCOOCH3, but also to clarify the situation regarding HCOOCH3. Indeed, in their interferometric study of this species, Bottinelli et al. (2004) measured an apparent difference between the
and line widths of the two hot corinos, namely
km s-1 and
km s-1 for source A, and
km s-1 and
km s-1 for source B. However, the authors were unable to conclude whether the values for the
and line width in source B were real or caused by the distortion of optically thick HCOOCH3
lines in source B, in which case the blue wing of the (strongly
self-absorbed) broad line normally centred at 3.9 km s-1 could be mistaken for a narrow line centered at 2.7 km s-1. The DCOOCH3
lines are optically thin. Therefore, if the two-component model used in
this study is confirmed by higher angular resolution observations, it
would imply that the value of
and of the line width are indeed different in the two hot corinos in IRAS 16293.
Using the column density previously derived for HCOOCH3 and DCOOCH3, the degree of deuteration of methyl formate in IRAS 16293 is estimated to 15%.
This degree of deuteration is similar in sources A and B but dedicated
high spatial resolution observations are needed to confirm this (HCOOCH3
could be optically thick in the core B). Although this is a rough
estimate, the deuterium enrichment of methyl formate, greater than the
one expected from deuterium cosmic abundance, is compatible with other
singly deuterated species in this source such as HDCO or CH2DOH that have a deuterium enrichment of
15% and 37%, respectively (Parise et al. 2006).
4 Discussion
It has been established that methyl formate cannot be formed exclusively by gas-phase reactions (Horn et al. 2004) and that solid state reactions necessarily play a role in its formation. Methyl formate may form in the icy mantle of dust grains via the reaction between the metoxy radical, CH3O, with the formyl radical, HCO. Both CH3O and HCO are synthesised in the grain mantles either by hydrogenation of H2CO and CO, respectively (Garrod & Herbst 2006; Watanabe & Kouchi 2008) or via UV photodissociation of methanol, which produces CH3O and an H atom that reacts with CO to produce HCO (Bennett & Kaiser 2007). As for methyl formate, it is likely that deuterated methyl formate formation also requires grain chemistry. We discuss below two formation pathways, implying solid state reactions, that may lead to DCOOCH3.
The first one is similar to the formation of HCOOCH3. In that scenario, DCOOCH3 is formed directly via the reaction: DCO + CH3O
DCOOCH3
(1). At low temperature and in the absence of energetic processes, DCO
could be produced by deuteration reaction of CO with gas phase
deuterium atom accreted onto the grain, while CH3O results from the hydrogenation of H2CO. However, experiments from Hidaka et al. (2007)
have shown that the deuteration of CO ice is about 10 times less than
the hydrogenation rate because there is no tunnelling effect for
deuteration. The authors thus conclude that the deuteration of CO may
not be the first step in producing deuterated formaldehyde or methanol
in molecular clouds. This may also be the case for the formation of
deuterated methyl formate.
Energetic processes in ices, such as UV photolysis or cosmic rays bombardment of ices, may be more efficient at producing DCOOCH3 via reaction (1). Following the formation of methyl formate reported by Bennett & Kaiser (2007),
DCO could be formed from the reaction of CO with a deuterium atom
produced by the UV photodissociation of deuterated methanol. The energy
given by the UV photons should help overcome the activation energy of
the reaction CO + D
DCO. Among the deuterated isotopologues of methanol, CH3OD seems to be the best candidate for the UV photodissociation, since if the UV photons break the O-D bond of CH3OD, CH3O
and a D atom are easily produced. This is not the case for any other
deuterated methanol species, for which the simultaneous production of CH3O and D would require intermediate reactions.
A second route to forming deuterated methyl formate is the H/D substitution in solid HCOOCH3 during the warm-up of the ice mantles. This scenario is proposed for the formation of deuterated formaldehyde and participates in the formation of deuterated methanol species (Hidaka et al. 2009; Ratajczak et al. 2009). It may also be efficient at producing deuterated methyl formate. Once again, the H/D substitution could also be driven by energetic processes such as UV photolysis or cosmic-ray irradiation of the ice mantles (Weber et al. 2009). In their study of methane/deuterated-water ice, Weber et al. (2009) conclude that almost all organic species should undergo H/D substitution with the matrix in water ices exposed to UV radiation.
Isotopic substitutions are routinely used in laboratory experiments to disentangle the different chemical pathways leading to the formation of molecules. From that point of view, the formation of deuterated methyl formate is an interesting issue that could improve our understanding of the chemistry of these species. To our knowledge, experimental and theoretical works on the deuteration of methyl formate and other complex organic molecules do not exist yet. They are clearly needed to distinguish between the above scenarii. The astronomical detection of other deuterated isotopologues of methyl formate should also put some constraints on the different formation pathways.
5 Conclusion
We have tentatively detected the singly-deuterated isotopologue of methyl formate, DCOOCH3, in the protostar IRAS16293. We assigned eight observed lines to DCOOCH3
transitions. Four lines are not blended with other species (229.448,
239.654, 249.860, 260.064 GHz), and three are (209.031, 219.242,
270.267 GHz). Among the last, the first two lines are blended with
HCOOCH3 and CH3COCH3, respectively.
They are reproduced well by an emission model at LTE. We were not able
to identify the species responsible for the blending of the
270.267 GHz line, however the LTE model is compatible with the
observations and does not contradict the tentative identification of
DCOOCH3 in IRAS 16293. The LTE emission model is compatible
with the last observed line at 198.824 GHz, but we cannot conclude
anything about its detection because of calibration uncertainty. From
basic modelling in LTE, we estimate the abundance of deuterated methyl
formate to be
cm-2. This leads to a DCOOCH3/HCOOCH3 ratio of
15%, consistent with the deuteration fractionation of other singly deuterated species in this source.
Additional observations with better spectral resolution and higher sensitivity are needed for several reasons. First, considering the low spectral resolution and the relatively low signal-to-noise ratio of the lines assigned to DCOOCH3, such new observations would strengthen this tentative detection. Second, it would allow detection of additional lines among the hundreds of weaker lines from DCOOCH3 that are present in the spectral survey. Last, it would be an opportunity to search for other deuterated isotopologues of methyl formate. High angular resolution observations would also be helpful for investigating the existence of two emission sources for DCOOCH3, as observed for HCOOCH3 and for concluding about the reality of the physical differences between the two cores A and B. Such observations are also needed to properly estimate the methyl formate deuteration and, together with new experimental and theoretical work, to understand the deuteration mechanisms of complex organic molecules.
AcknowledgementsThe authors would like to thank Laurent Margulès for providing us with the DCOOCH3 spectral data prior to publication. We thank the IRAM staff for help provided during the observations. Support by the French National Agency (ANR-02-BLAN-0225-01) is acknowledged.
References
- Bacmann, A., Lefloch, B., Ceccarelli, C., et al. 2003, ApJ, 585, L55 [NASA ADS] [CrossRef] [Google Scholar]
- Bennett, C. J., & Kaiser, R. I. 2007, ApJ, 661, 899 [NASA ADS] [CrossRef] [Google Scholar]
- Bisschop, S. E., Jørgensen, J. K., Bourke, T. L., Bottinelli, S., & van Dishoeck, E. F. 2008, A&A, 488, 959 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Bottinelli, S., Ceccarelli, C., Neri, R., et al. 2004, ApJ, 617, L69 [NASA ADS] [CrossRef] [Google Scholar]
- Caux, E., Kahane, C., Castets, A., et al. 2010, A&A, submitted [Google Scholar]
- Cazaux, S., Tielens, A. G. G. M., Ceccarelli, C., et al. 2003, ApJ, 593, L51 [NASA ADS] [CrossRef] [Google Scholar]
- Ceccarelli, C., Loinard, L., Castets, A., et al. 2001, A&A, 372, 998 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Garrod, R. T., & Herbst, E. 2006, A&A, 457, 927 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Hidaka, H., Kouchi, A., & Watanabe, N. 2007, J. Chem. Phys., 126, 204707 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
- Hidaka, H., Watanabe, M., Kouchi, A., & Watanabe, N. 2009, ApJ, 702, 291 [NASA ADS] [CrossRef] [Google Scholar]
- Horn, A., Møllendal, H., Sekiguchi, O., et al. 2004, ApJ, 611, 605 [NASA ADS] [CrossRef] [Google Scholar]
- Jacq, T., Walmsley, C. M., Mauersberger, R., et al. 1993, A&A, 271, 276 [NASA ADS] [Google Scholar]
- Leurini, S., Rolffs, R., Thorwirth, S., et al. 2006, A&A, 454, L47 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Margulès, L., Huet, T. R., Demaison, J., et al. 2010, ApJ, 714, 1120 [NASA ADS] [CrossRef] [Google Scholar]
- Parise, B., Castets, A., Herbst, E., et al. 2004, A&A, 416, 159 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Parise, B., Ceccarelli, C., Tielens, A. G. G. M., et al. 2006, A&A, 453, 949 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Pety, J., Goicoechea, J. R., Hily-Blant, P., Gerin, M., & Teyssier, D. 2007, A&A, 464, L41 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Pillai, T., Wyrowski, F., Hatchell, J., Gibb, A. G., & Thompson, M. A. 2007, A&A, 467, 207 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Ratajczak, A., Quirico, E., Faure, A., Schmitt, B., & Ceccarelli, C. 2009, A&A, 496, L21 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Roberts, H., Herbst, E., & Millar, T. J. 2003, ApJ, 591, L41 [NASA ADS] [CrossRef] [Google Scholar]
- Tielens, A. G. G. M. 1983, A&A, 119, 177 [NASA ADS] [Google Scholar]
- Turner, B. E. 1998, ApJ, 501, 731 [NASA ADS] [CrossRef] [Google Scholar]
- Watanabe, N., & Kouchi, A. 2008, Prog. Surf. Sci., 83, 439 [NASA ADS] [CrossRef] [Google Scholar]
- Weber, A. S., Hodyss, R., Johnson, P. V., Willacy, K., & Kanik, I. 2009, ApJ, 703, 1030 [NASA ADS] [CrossRef] [Google Scholar]
Footnotes
- ...)
- Based on observations carried out with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
- ...
- Based on analysis carried out with the CASSIS software and the JPL (http://spec.jpl.nasa.gov/) and CDMS (http://www.ph1.uni-koeln.de/cdms/) spectroscopic databases. CASSIS has been developed by CESR-UPS/CNRS (http://cassis.cesr.fr).
- ... software
- The CASSIS software is a free interactive spectrum analyser aiming to interpret astrophysical spectra, for more details see http://cassis.cesr.fr
All Tables
Table 1: Emission lines assigned to DCOOCH3 in the IRAS 16293 IRAM spectral survey.
All Figures
![]() |
Figure 1: LTE emission model of DCOOCH3 calculated in optically thin conditions for an extended source, an excitation temperature of 100 K, with a linewidth of 5 km s-1. The line intensity has been normalised to the strongest line at 239.655 GHz. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Observed lines assigned to DCOOCH3 in IRAS 16293 compared with a two-components LTE emission model of DCOOCH3. Black line: observed IRAM spectral lines. Red line: sum of the emission from region A (light blue line) modelled with
|
Open with DEXTER | |
In the text |
Copyright ESO 2010
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.