EDP Sciences
Free Access
Issue
A&A
Volume 582, October 2015
Article Number L1
Number of page(s) 43
Section Letters
DOI https://doi.org/10.1051/0004-6361/201526255
Published online 02 October 2015

© ESO, 2015

1. Introduction

The spectral millimeter-wave survey of Orion KL carried out with the IRAM 30 m radio telescope (Tercero et al. 2010; Tercero 2012) shows more than 15 400 spectral features of which about 11 000 have been identified and attributed to 50 molecules (199 different isotopologues and vibrational modes). To date, there have been several works based on these data. As the result of a fruitful collaboration with spectroscopy laboratories, 3000 previously unidentified lines have been assigned to new species in the interstellar medium (ISM). We have detected in space 16 new isotopologues and vibrationally excited states of abundant molecules in Orion for the first time (Demyk et al. 2007; Margulès et al. 2009, 2010; Carvajal et al. 2009; Tercero et al. 2012; Motiyenko et al. 2012; Daly et al. 2013; Coudert et al. 2013; Haykal et al. 2014; López et al. 2014) as well as four new molecules (Tercero et al. 2013; Cernicharo et al. 2013; Kolesniková et al. 2014). These identifications reduce the number of unidentified lines and mitigate line confusion in the spectra. Nevertheless, many features still remain unidentified and correspond to new species that we have to search and identify. Formates, ethers, acetates, alcohols, and cyanides are the best candidates for this purpose in Orion.

The recent search for trans ethyl methyl ether (tEME) in selected hot cores (Sgr B2(N-LMH) and W51 e1/e2) by Carroll et al. (2015) only provides upper limits to tEME. Hence, the results from that work do not confirm the previous tentative identification of this species by Fuchs et al. (2005) towards W51 e1/e2.

A systematic line survey with most weeds removed permits us to address the problem of the abundances of isomers and derivatives of key species, such as methyl formate (A. López et al., in prep.), through combined IRAM and ALMA studies.

In this Letter, we report on the tentative detection of tEME towards the compact ridge (CR) of Orion KL. We have detected emission of features arising from the five spin states at 3, 2, and 1 mm with the IRAM 30 m telescope and the ALMA interferometer. In addition, several unidentified lines of these data have been identified as belonging to the gauche-trans conformer of n-propanol (an isomer of tEME). ALMA maps of organic saturated O-bearing species containing methyl, ethyl, and propyl groups, abundance ratios of related species, and upper limits to the column densities of non-detected ethers are presented and discussed in Sect. 4.

2. Observations

IRAM 30 m: new data of the IRAM 30 m telescope, which complement and improve those of Tercero et al. (2010), were collected in August 2013 and March 2014 towards Orion KL (see Tercero et al. 2010 and López et al. 2014, for information about the previous data set). Frequencies in the ranges 80.7116, 122.7161.2, 199.7291.0, 291.4306.7 GHz, were observed with the EMIR receivers connected to the FFTS (200 kHz of spectral resolution) spectrometers. We pointed towards IRc2 source at α2000.0 = 5h35m145, δ2000.0 = −5°22300, corresponding to the survey position (see Sect. 4). We observed an additional position to target the CR: α2000.0 = 5h35m143, δ2000.0 = −5°22370 (see Sect. 4). The observations were performed using the wobbler switching mode with a beam throw in azimuth of ±120′′. The intensity scale was calibrated using the atmospheric transmission model (ATM, Cernicharo 1985; Pardo et al. 2001). Focus and pointing were checked every 12 h on planets or nearby quasars. System temperatures were in the range of 100800 K from the lowest to highest frequencies. Half power beam width (HPBW) ranged from 31′′ to 8′′ from 80 to 307 GHz (HPBW[arcsec] = 2460/Freq.[GHz]). The data were reduced using the GILDAS package1.

ALMA SV: the ALMA Science Verification (SV) data2 were taken in January 2012 towards the IRc2 region in Orion. The observations were carried out with 16 antennas of 12 m in Band 6 (213.715–246.627 GHz). The primary beam was 27′′. Spectral resolution was 0.488 MHz corresponding to a velocity resolution of 0.64 km s-1. The observations were centred on coordinates: αJ2000 = 05h35m1435, δJ2000 = −05°223500. The CASA software3 was used for initial processing and then the visibilities were exported to the GILDAS package. The line maps were cleaned using the HOGBOM algorithm (Högbom 1974). The synthesized beam ranged from with a PA of 176° at 214.0 GHz to with a PA of 164° at 246.4 GHz. The brightness temperature to flux density conversion factor is 9 K for 1 Jy per beam.

3. Results

3.1. Search for trans ethyl methyl ether

thumbnail Fig. 1

Selected lines of trans ethyl methyl ether, t-CH3CH2OCH3, towards Orion KL detected with the ALMA interferometer in Position A (see text). A vLSR of +7.5 km s-1 is assumed.

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ALMA SV data: frequency predictions from Fuchs et al. (2003) and dipole moments measured by Hayashi & Kuwada (1975) of tEME were implemented in MADEX (Cernicharo 2012) to model the emission of this species and search for it towards Orion KL. Using the ALMA SV data, we extracted the averaged spectrum over 5 × 5 pixels (1′′ × 1′′) around the CH3OCH3 emission peak of the CR component (Position A; see Sect. 4). The advantage of ALMA with respect to single dish telescope data (see below) is the drastic reduction of the confusion limit. The ALMA SV data show the presence of tEME as shown in Fig. 1 (selected lines) and Fig. A.1 (all lines favourable for detection (corresponding to b-type transitions with upper level energies up to 300 K and large line strenghts, Sij ≥ 1) present in the ALMA SV frequency range). The model that best fits the data is shown with the red line. The assumed parameters are a source size of 3′′, vLSR = + 7.5 km s-1, Δv = 2.0 km s-1, and TK = 100 ± 20 K. Using MADEX and assuming local thermodynamic equilibrium (LTE), we obtain Ng.s.(tEME) ≤ (4.0 ± 0.8) × 1015 cm-2. In our models, rotation temperature and column density values are given with their corresponding uncertainty and we obtained them by fitting all available lines by eye. We adopted the source size in agreement with the emission of the maps (see below). In addition, a considerable number of unblended features allows us to fix the radial velocities and line widths. According to our model, in the ALMA frequency range only 33% of the detectable lines of tEME (102 lines) are totally hidden by the emission of stronger lines of other species. At least 46 lines (45% of the detectable lines) shown in Fig. A.1 are free of blending, i.e. these lines are present at the expected radial velocity and there are no other species with significant intensity at the same observed frequency (±3 MHz). Another point to ensure this tentative detection is that the forest of lines emitted by tEME between 215.5 and 215.7 GHz is not covered by lines of abundant molecules in the source allowing the detection of several lines that follow a straightened pattern (see Fig. 1). Hence, there are several clues that could reveal the presence of this species in the CR of Orion KL, but further analysis exploring new available ALMA data and modelling all the molecular content of the CR is needed to give the definitive detection in space of tEME. Table A.1 gives line parameters and blends of all lines of favourable transitions in the ALMA SV data. The spatial distribution of tEME is shown in Fig. 2. Lines that we found to be unblended at the Position A appear blended with emission from other components in the averaged spectrum (see the case of the 30 m data). We selected a line at 245.274 GHz, which is mixed with some emission from extreme velocities of 34SO2 and SO2. Nevertheless, the emission of tEME at Position A in Fig. 2 is not blended (see Sect. 4).

IRAM 30 m data: to search for tEME in the IRAM data, a synthetic spectra of tEME (red curve in Fig. A.2) was obtained with MADEX assuming LTE and adopting the following physical parameters: source diameter 3′′, TK = 100 ± 30 K, vLSR = + 7.5 km s-1, Δv = 1.5 km s-1; and a column density of (9 ± 3) × 1015 cm-2 for the ground state (g.s.) of tEME. According to our model, all favourable lines for detection in the 30 m data were detected or were blended with features from more abundant species. Nevertheless, owing to the weakness of the features (TMB< 0.1 K at 3 mm, TMB< 0.2 K at 2 mm, and TMB< 1 K at 1.30.9 mm) and the high level of line confusion at ~1 mm, only a few lines were mostly free of blending with other species in this domain. Whereas the synthetic beam of the ALMA SV is , in the 30 m the beam diameter ranging from 30′′ to 8′′. Therefore, in the 30 m data, the spectrum is a mix of all molecules from all source components (average spectrum over the beam) given rise to a high level of line blending and line confusion. Table A.2 shows line parameters, intensity provided by the model, and blends of all lines of favourable transitions in the 30 m data.

thumbnail Fig. 2

ALMA maps of organic saturated O-bearing molecules in Orion KL which have been detected containing both the methyl and the ethyl group, as well as a map of Gt-n-propanol and a continuum map at the central frequencies of the ALMA SV band (~230 GHz). Emission that probably arises from blended species in these maps is confined inside red rectangles. The yellow ellipse at the top left corner of the maps represents the ALMA synthetic beam. Triangle symbol: IRAM 30 m “survey position” (see Sect. 2). Cross symbol: IRAM 30 m compact ridge position (see Sect. 2). Position A: compact ridge (coordinates α2000.0 = 5h35m141, δ2000.0 = −5°22379). Position B: south hot core (coordinates α2000.0 = 5h35m144, δ2000.0 = −5°22349).

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3.2. Search for gauche-trans-n-propanol

All lines of Gt-CH3CH2CH2OH, an isomer of C3H8O (as well as tEME), reported by Maeda et al. (2006) and the dipole moments from Abdurakhmanov et al. (1969) were used to derive its rotational constants and to implement this species in MADEX. We conducted the search for Gt-n-propanol in the ALMA SV data at two different positions: Position A and the position where the emission peak of ethanol is located (Position B; see Sect. 4). We assign several unidentified lines in the source at Position B to this species. According to our model (dsou = 3′′, vLSR = + 8.0 km s-1, Δv = 3.0 km s-1, TK = 100 ± 20 K, and Ng.s ≤ (1.0 ± 0.2) × 1015 cm-2), many of the lines are below the detection limit although the strongest features are detected. Unfortunately, several lines remain blended (see Fig. A.3). A few lines of this species are also detected in the IRAM 30 m data at the survey position (Fig. A.2 bottom panel; model parameters: dsou = 3′′, vLSR = + 8.0 km s-1, Δv = 1.5 km s-1, TK = 100 ± 20 K, and Ng.s ≤ (2.0 ± 0.4) × 1015 cm-2). Table A.3 shows line parameters for the detected lines. The derived upper limit to its column density (assuming the same physical parameters than those of the tEME ALMA model) at Position A is (3.0 ± 0.6) × 1014 cm-2. The spatial distribution of this species around Position B is shown in Fig. 2. To perform the ALMA map, we averaged the emission between vLSR 6 and 9 km s-1 of two lines (lines at 236.138 and 244.765 GHz). Emission around source I should be due to other less abundant species in Orion (we did not find Gt-n-propanol at these positions).

4. Discussion

Species containing the functional groups formate, alcohol, and ether have been detected in Orion with both the methyl and ethyl groups (methyl formate (MF), ethyl formate (EF), methanol, ethanol, dimethyl ether (DME), and tEME). ALMA maps for the spatial distribution of these species as well as Gt-n-propanol are shown in Fig. 2. To address the flux filtered out by ALMA and the accuracy of the maps in a larger energy range, the following discussion is also based on the maps shown in Fig. 5 of Feng et al. (2015; maps performed mixing SMA and IRAM 30 m data) with MF, DME, methanol, and ethanol. For MF, DME, and methanol the spatial distribution and the position of the emission peaks are in agreement with those of the maps presented in this work (note, however, that the ALMA maps provide a more detailed structure at small scales, i.e. 5′′). For ethanol, we note a more extended spatial distribution in the map of Feng et al. (2015) mostly due to the lower energy of the transition involved. Nevertheless, the emission peak of ethanol is located at the same position.

For the methyl species, we note: i) a rather similar spatial structure: the three species present the V shape distribution of several clumps (at least six) studied by Favre et al. (2011) for the distribution of MF, which was mapped using data from the Plateau de Bure Interferometer (PdBI); ii) that although Brouillet et al. (2013) probed a striking similarity between the spatial distributions of CH3OCH3 and CH3OCOH, we found some differences in the relative intensities of both species. These differences could be mostly due to different excitation temperatures of the involved transitions; and iii) although methanol also follows this V shape structure, a displacement of the intensity peaks is observed with respect to MF. This behaviour suggests methanol as a possible precursor of MF and DME (see also Neill et al. 2011).

Comparing the methyl and ethyl species, we note: i) a reduced spatial distribution of the three ethyl species with respect to their methyl counterpart; ii) the two emission peaks of EF are correlated with those found in MF; iii) the emission peak of tEME is at the same position as the DME peak at the CR (Position A); and iv) the emission peak of ethanol (Position B) is displaced 2′′ south-west from the methanol peak.

Concerning the ethyl and propyl species, we note: i) a close correlation between EF and tEME; and ii) ethanol also presents a “V” shape structure (see Fig. 5 of Feng et al. 2015) with the bulk of the emission located away from the CR and coinciding with that of Gt-n-propanol. The ethanol/propanol peak is displaced 15 south from the ethylene glycol (CH2OH)2 peak (Brouillet et al. 2015), which is a double alcohol and we could naively expect to have the same spatial distribution. Whereas the ethylene glycol peak corresponds to the 13CH3OH peak, the ethanol/propanol peak is the same as that of deuterated methanol (CH2DOH; see Peng et al. 2012).

Table 1

Column densities and ratios.

Table 1 shows derived column densities and ratios for related species. The derived ratios and the spatial distribution of these molecules suggest important gas phase processes after the evaporation of the mantles of dust grains in hot cores. Possible reactions of the methoxy radical (CH3O), detected recently in space (Cernicharo et al. 2012), with other species could lead to the increase of chemical complexity in hot cores and hot corinos (Balucani et al. 2015). The spatial stratification of the different

species also suggests the time dependent effects on the chemistry of the gas. The detection of the less stable isomers of some species (Tercero et al. 2013) also points in this direction.

To summarize, a combined IRAM 30 m and ALMA SV data study allows us to provide a solid starting point to assess the identification of tEME in the ISM. In addition, some unidentified lines in the source have been assigned to another C3H8O isomer, Gt-n-propanol. ALMA maps show different spatial distributions for these species. Whereas tEME seems to mainly arises from the CR component (as well as EF) (Position A), emission from Gt-n-propanol could be located at the south hot core (at the same position as the emission peak of ethanol) (Position B). The CR is no longer the main host of all organic saturated O-bearing species in Orion (see also Peng et al. 2013, for the spatial distribution of acetone and A. López et al., in prep. for the acetic acid emission).


Acknowledgments

We thank Marcelino Agúndez for carefully reading the paper and providing useful comments and suggestions. B.T., J.C., and A.L. thank the Spanish MINECO for funding support under grants CSD2009-00038, AYA2009-07304, and AYA2012-32032 and also the ERC for funding support under grant ERC-2013-Syg-610256-NANOCOSMOS.

References

Online material

Appendix A: Online figures and tables

thumbnail Fig. A.1

Lines of trans ethyl methyl ether, t-CH3CH2OCH3, towards Orion KL detected with the ALMA interferometer in Position A (see text). (**): Features blended with SO (see Table A.1; artifacts in the spectrum due to the cleaning process). A vLSR of +7.5 km s-1 is assumed.

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thumbnail Fig. A.2

Top panel: selected lines of trans ethyl methyl ether, t-CH3CH2OCH3, towards Orion KL detected with the IRAM 30 m telescope. Data in the frequency range 124151 GHz are those of the survey position. From 201 to 293.5 GHz the data are those of the CR (see Sect. 2), where the emission peak of organic saturated O-rich species such as dimethyl ether (CH3OCH3) and methyl formate (CH3OCOH) is located (Favre et al. 2011; Brouillet et al. 2013). A vLSR of +7.5 km s-1 is assumed. Bottom panel: selected lines of gauche-trans-n-Propanol, Gt-n-CH3CH2CH2OH, towards Orion KL detected with the IRAM 30 m telescope. A vLSR of +7.5 km s-1 is assumed.

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thumbnail Fig. A.3

Lines of gauche-trans-n-propanol, Gt-n-CH3CH2CH2OH, towards Orion KL detected with the ALMA interferometer in Position B (see text). A vLSR of +8 km s-1 is assumed.

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Table A.1

Lines of trans-CH3CH2OCH3 in ALMA SV data.

Table A.2

Lines of trans-CH3CH2OCH3 in 30 m data.

Table A.3

Detected lines of gauche-trans-n-CH3CH2CH2OH.

All Tables

Table 1

Column densities and ratios.

Table A.1

Lines of trans-CH3CH2OCH3 in ALMA SV data.

Table A.2

Lines of trans-CH3CH2OCH3 in 30 m data.

Table A.3

Detected lines of gauche-trans-n-CH3CH2CH2OH.

All Figures

thumbnail Fig. 1

Selected lines of trans ethyl methyl ether, t-CH3CH2OCH3, towards Orion KL detected with the ALMA interferometer in Position A (see text). A vLSR of +7.5 km s-1 is assumed.

Open with DEXTER
In the text
thumbnail Fig. 2

ALMA maps of organic saturated O-bearing molecules in Orion KL which have been detected containing both the methyl and the ethyl group, as well as a map of Gt-n-propanol and a continuum map at the central frequencies of the ALMA SV band (~230 GHz). Emission that probably arises from blended species in these maps is confined inside red rectangles. The yellow ellipse at the top left corner of the maps represents the ALMA synthetic beam. Triangle symbol: IRAM 30 m “survey position” (see Sect. 2). Cross symbol: IRAM 30 m compact ridge position (see Sect. 2). Position A: compact ridge (coordinates α2000.0 = 5h35m141, δ2000.0 = −5°22379). Position B: south hot core (coordinates α2000.0 = 5h35m144, δ2000.0 = −5°22349).

Open with DEXTER
In the text
thumbnail Fig. A.1

Lines of trans ethyl methyl ether, t-CH3CH2OCH3, towards Orion KL detected with the ALMA interferometer in Position A (see text). (**): Features blended with SO (see Table A.1; artifacts in the spectrum due to the cleaning process). A vLSR of +7.5 km s-1 is assumed.

Open with DEXTER
In the text
thumbnail Fig. A.2

Top panel: selected lines of trans ethyl methyl ether, t-CH3CH2OCH3, towards Orion KL detected with the IRAM 30 m telescope. Data in the frequency range 124151 GHz are those of the survey position. From 201 to 293.5 GHz the data are those of the CR (see Sect. 2), where the emission peak of organic saturated O-rich species such as dimethyl ether (CH3OCH3) and methyl formate (CH3OCOH) is located (Favre et al. 2011; Brouillet et al. 2013). A vLSR of +7.5 km s-1 is assumed. Bottom panel: selected lines of gauche-trans-n-Propanol, Gt-n-CH3CH2CH2OH, towards Orion KL detected with the IRAM 30 m telescope. A vLSR of +7.5 km s-1 is assumed.

Open with DEXTER
In the text
thumbnail Fig. A.3

Lines of gauche-trans-n-propanol, Gt-n-CH3CH2CH2OH, towards Orion KL detected with the ALMA interferometer in Position B (see text). A vLSR of +8 km s-1 is assumed.

Open with DEXTER
In the text

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