EDP Sciences
Free Access
Issue
A&A
Volume 576, April 2015
Article Number A45
Number of page(s) 15
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201323114
Published online 25 March 2015

© ESO, 2015

1. Introduction

Organic molecules containing more than six atoms, the so-called complex organics (Herbst & van Dishoeck 2009), are commonly found in the warm and dense gas (T > 100 K, n > 106 cm-3) around young stellar objects (YSOs), so-called molecular hot cores (e.g., Blake et al. 1987; Cazaux et al. 2003; Fuente et al. 2005). Abundances and abundance ratios of complex organics are found to vary substantially between (Helmich & van Dishoeck 1997) and within YSOs (e.g., Wyrowski et al. 1999). This suggests that formation and destruction routes are highly environment specific and that there is a sensitive dependence of the complex organic chemistry on chemical and physical initial conditions. In addition, different filling factors of the warm gas should play a role if there the complex organic products of cold and hot chemistry differ there.

The potential environmental dependencies and chemical memories lead to complex organics having a great potential as probes of the current and past physical and chemical conditions where they are found (Nomura & Millar 2004). Their potential utility is further increased by the fact that most complex organic molecules present large numbers of lines, spanning most excitation conditions found in space. Complex molecules are also of high interest for origins of life theories since they are the precursors of even more complex prebiotic material (Ehrenfreund & Charnley 2000). Using molecules as probes of physical conditions and advancements in prebiotic evolution from organics both rely on a detailed understanding of complex organic chemistry. The formation and destruction mechanisms and rates of most complex organics are, however, poorly constrained.

The formation of organic molecules around massive YSOs (MYSOs) was first thought to proceed through gas phase reactions in dense hot cores, following evaporation of ice grain mantles (e.g., Charnley et al. 1992). Recent laboratory experiments and modeling efforts point now toward a more complicated sequential scenario that relies to a greater extent on surface formation routes on submicron-sized dust particles. Herbst & van Dishoeck (2009) classify complex organic molecules in terms of generations according to the following scenario. In interstellar clouds and in the deeply embedded early phases of star formation, atoms and molecules accrete or form on the surface of dust grains, building up an icy mantle of simple species like H2O, CH4, and NH3 (Tielens & Hagen 1982). This icy mantle is processed at low temperature by atoms, which can diffuse even at the low temperatures in cloud cores, creating the zeroth generation of organic molecules. A good example of these species is CH3OH, which is efficiently formed at low temperature by the hydrogenation of CO ice (Watanabe & Kouchi 2002; Watanabe et al. 2003, 2004; Fuchs et al. 2009; Cuppen et al. 2009). First-generation complex organics form when heating the cold envelope up by the increasing luminosity of a central YSO and is due to a combination of photoprocessing of the ice resulting in radical production and a warming up (20 to 100 K) of the grains, thereby enhancing the mobility of radicals and molecules (Garrod et al. 2008; Öberg et al. 2010). When the icy grains move inward and reach a region warmer than 100 K, the icy mantle evaporates, bringing the zeroth- and first-generation organics into the gas phase, where additional chemical reactions give rise to the formation of the second-generation complex organics (e.g., Charnley et al. 1992; Doty et al. 2002; Viti et al. 2004).

In the proposed scenario of complex molecule formation, the initial ice mantle plays a critical role. The exact composition of this ice may therefore have a strong effect both on the product composition of formed organics and on their overall formation efficiency. Garrod et al. (2008) and Öberg et al. (2009) find, for example, that CH3OH ice is a key starting point for most complex organic formation. Rodgers & Charnley (2001) used a hot core chemistry model to show that the relative amount of NH3 in the ice has a large impact on the CH3CN/CH3OH protostellar abundance ratio. Observationally testing these relationships would provide key constraints on the formation pathways of complex organic molecules.

Isolated MYSOs with warm inner envelopes are good laboratories for testing this hypothesis as these sources are bright enough to observe a wide variety of organics and some of them present ice features from the cold outer protostellar envelope (Gibb et al. 2004). Sources presenting both complex gas and ice features are, however, rare as the sources need to be evolved to possibly display a bright hot core chemistry accessible to current observational facilities and young enough such that the ice material has not been completely consumed by accretion, warm up, and envelope dispersal. In the massive YSO sample studied by Bisschop et al. (2007), only three hot cores present ice spectra (see Table 1). Such a small number prevents any analysis of the correlation between ice and gas content and justifies our search for other objects that display both ice features and gas phase organics.

To extend the sample of sources with both complex organics and ice observations, we look for gas phase organics species around non-hot core MYSOs (absence or low-level of hot CH3OH emission) that also have ice observations available from the literature. These sources are called from now on organic-poor MYSOs (poor in lines of organic molecules). Complex molecule observations in such objects may additionally shed light on the conditions under which different kinds of complex molecules can form, i.e. which molecules require the presence of a hot core to be abundant.

Massive objects NGC 7538 IRS9, W3 IRS5 , and AFGL490 have been observed in the mid-infrared by the Infrared Space Observatory (ISO) and analyzed systematically for ice abundances by Gibb et al. (2004) and references therein. NGC 7538 IRS9 is a 6 × 104L luminous object located in Perseus. It is close to hot core source NGC 7538 IRS1 and displays at least three bipolar outflows, evidence for accretion (Sandell et al. 2005), and a hot component close to the central object. W3 IRS5 is associated with five YSOs, two of which are massive (van der Tak et al. 2005; Megeath et al. 2005; Rodón et al. 2008; Chavarría et al. 2010). It has a luminosity of 17 × 104L and presents strong S-bearing molecular lines (Helmich et al. 1994). AFGL490 is a very young medium-mass YSO of 4.6 × 103L, in transition to a Herbig Be star, which drives a high-velocity outflow (Mitchell et al. 1995) and shows evidence of a rotating disk (Schreyer et al. 2006). Since these sources are considered to potentally be at an earlier evolutionary stage than typical hot-core sources, it is difficult to predict the chemical complexity and the spatial emission of the organics that could be observed in these sources.

Table 1

Source characteristics and ice abundances.

In this study we use a combination of single-dish IRAM 30 m data and spatially resolved observations from the Submillimeter Array1 (SMA) to search for organic molecules around these three MYSOs and report on their complex organic abundances in the cool protostellar envelope and in a warmer region closer to the star. A subset of these data was used in the Öberg et al. (2013) to study the detailed radial distribution of molecules in NGC 7538 IRS9, while the present study focuses on the overall detection rate of organics in these organic-poor sources, and on how they compare with ice abundances and traditional hot core chemistry. The paper is organized as follows. The observations are described in Sect. 2, and the results of the line analysis are shown in Sects. 3.13.3. The chemistry in our sample is compared to the chemistry in traditional hot-core sources in Sect. 3.4. Section 3.5 presents correlation studies between ice and gas column densities and abundances, testing the impact of initial ice compositions on the complex chemistry. A discussion of the use of these line-poor sources to underpin the origins of complex chemistry is presented in Sect. 4, which is followed by the conclusions of this study.

thumbnail Fig. 1

Image of the CH3CN emission using the 130 − 120 at 239.138 GHz line acquired by the SMA for the massive young stellar objects NGC 7538 IRS9, W3 IRS5, and AFGL490 targeted in this study. The black contour presents the 50% line intensity, and the synthesized beam is shown in white at the bottom left. A 2′′ radius mask used to extract the spectra is overplotted in dashed red line. Images for the hot core source NGC 7538 IRS1 is presented as well. The latter source has been through the same program as the three other sources.

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2. Observations and analysis

2.1. Observations

The MYSOs NGC 7538 IRS9, W3 IRS5 , and AFGL490 located in Perseus NGC 7538 at 2.7 kpc, in Perseus W3 at 2.0 kpc, and in Camelopardalis OB1 at 1.4 kpc respectively (see Table 1) were observed with the IRAM 30 m and the SMA. The three sources were observed with the IRAM 30 m telescope on February 1920, 2012 using the EMIR 230 GHz receiver and the new FTS backend. At these frequencies the IRAM 30 m beam is ~10. The two sidebands cover 223231 GHz and 239247 GHz at a spectral resolution of ~0.2 km s-1 and with a sideband rejection of 15 dB (Carter et al. 2012). We checked the pointing every one to two hours and found to be accurate within 2′′ to 3′′.

Focus was checked every four hours and generally remained stable through most of the observations; i.e., corrections in the range of 0.2–0.4 were common, but a correction of 0.7 was required once. We acquired spectra in both position-switching and wobbler-switching modes. The resulting spectra had similar relative line intensities, indicative of no emission in the wobbler-off position. The wobbler-switching mode was considerably more stable, and we used these data alone for the quantitative analysis. The weather during the observations was excellent and the τ225 GHz varied between 0.05 and 0.15. We converted the raw IRAM spectra to main beam temperatures and fluxes using forward and beam efficiencies and antenna temperature to flux conversion values2. The spectra were reduced using CLASS3. A linear baseline was fitted to each 4 GHz spectral chunk using four to seven windows. The individual scans were baseline-subtracted and averaged4. The absolute flux scale of the lines were then set using calibrated SMA data as outlined in detail by Öberg et al. (2013).

SMA observations were acquired in the compact and extended array configurations. The data in the compact configuration were taken on 15 October 2011 for all sources and with seven antennas, resulting in baselines between 16 m and 77 m. The data in the extended configuration were obtained using eight antennas, resulting in 44 m to 226 m baselines and were acquired on 29 July 2011 for W3 IRS5 and AFGL490 and on the 15th of August 2011 for NGC 7538 IRS9. We set-up the SMA correlator to obtain a spectral resolution of ~1 km s-1 using 128 channels for each of the 46 chunks covering 227231 GHz in the lower sideband and 239243 GHz in the upper sideband. The τ225 GHz was 0.09 on 29 July, 0.1 on 15 August, and 0.07 on 15 October 20115.

We used the MIR package6 to perform the first data reduction steps (flux calibration and continuum subtraction). Absolute flux calibration is done with Callisto. The bandpass calibrators 1924292 and 3c84 were used for the compact observations, and 3c454.3 and 3c279 were used to calibrate 29 July and 15 July observations, respectively. The quasars 0014+612 and 0102+584 were used as gain calibrators for NGC 7538 IRS9, and 0244+624, 0359+509, and we used 0102+584 for W3 IRS5 and AFGL490. The compact and extended data were combined for each source with MIRIAD6 using natural or robust weighting, depending on the data quality, which resulted in synthesized beam sizes of 2.0″ × 1.7″ for NGC 7538 IRS9, 2.2″ × 2.8″ for W3 IRS5, and 2.3″ × 2.9″ for AFGL490.

thumbnail Fig. 2

239243 GHz spectral window from the IRAM 30 m displaying emission lines for typical hot core source NGC 7538 IRS1 and weak line MYSOs NGC 7538 IRS9, W3IRS5, AFGL490. The star-marked lines are CO ghost lines consistent with the sideband rejection for each source.

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2.2. Spectral extraction and rms

Both the IRAM and SMA data were frequency-calibrated using the bright 54 CH3OH ladder around 241.7 GHz, correcting for the intrinsic velocity of the different sources. We extracted the SMA spectra using a 2-radius mask around the continuum phase center of each source. The mask dimension was chosen to encompass a majority of the CH3CN line emission at 239.318 GHz that can be associated with a core component, as shown in Fig. 1. We selected a 2 mask size based on a combination of theory and data inspection, i.e. the optimal mask size should include all the hot emission and exclude as much as possible of the cold envelope emission. In all sources, the selected mask size should be bigger than the 100 K radius and thus incorporate all emission associated with a potential hot core. Toward NGC 7538 IRS9 and AFGL 490, where the 100 K radius should be smaller than 2, smaller masks were also explored to more exclusively trace the T > 100 K region, but the resulting spectra had generally too low signal-to-noise ratio to be useful for a quantitative analysis. Some colder chemistry contribution to the SMA spectra in these sources cannot, thus, be excluded a priori, but the succeeding analysis (see below) demonstrated that the emission is indeed dominated by hot gas.

The rms for the IRAM and SMA observations of each source was derived in a line free region of several hundred channels: the 229.37229.445 GHz region for the lower side band and the 240.7240.75 GHz region for the upper side band. The rms derived for the IRAM observations is between 15 and 20 mK, which is lower than any previous millimeter observations for these sources. For the SMA data, the rms for the lower side band is ~70 mK, and~100 mK for the upper side band.

Table 2

CH3OH lines data from IRAM 30 m spectra.

Table 3

CH3OH lines extracted from SMA observations with a 2-radius mask.

Table 4

CH3CN lines data from IRAM 30 m spectra.

Table 5

CH3CN lines data from SMA spectra.

Table 6

CH3CCH lines data from IRAM 30 m spectra.

Table 7

CH3CCH lines data from SMA spectra.

Table 8

HNCO, CH3CHO, and CH3OCH3 lines data from the IRAM 30 m spectra and HNCO line data from the SMA 2” radius compact region.

3. Results

3.1. Line identification and characterization

Figure 2 shows the IRAM 30 m 239243 GHz spectra for the three targeted line-poor MYSOs and the hot-core source NGC 7538 IRS1. The organic-poor MYSOs have, as expected, a lower line density, but also many line coincidences with the hot core. Of the lines from complex organics listed by Bisschop et al. (2007) found in their sample hot-core sources, CH3OH, CH3CN, CH3CCH, HNCO, CH3OCH3, and CH3CHO lines were identified in at least one of the organic-poor MYSOs using the splatalogue catalog tool7 and the CDMS8 and the JPL9 spectral databases (Müller et al. 2001; Pickett et al. 1998). All available lines in the observed spectral range were used for the quantitative analysis except for CH3OH where we only used the lines from the 54 ladder to simplify the excitation analysis.

We fitted the identified lines with a Gaussian function in IDL using the routine “gaussfit” for isolated lines and “mpfitfun” when a multiple Gaussian fit was required because of overlapping lines. A local baseline component was added to the fits when needed, and the presented uncertainties were output by the fitting routines. We calculated 3σ upper limits using an average FWHM for the different sources. Unresolved multiplets were treated in one out three ways depending on the nature of the overlapping lines: 1) if one of the possible contributing lines had a very low Einstein coefficient or high upper energy level and/or is not likely to be detected based on non-detections of the same species in other frequency ranges, then it was assumed to not contribute significantly and was not included in the fit. 2) If the lines came from the same species and the upper energy level and Einstein coefficients were identical or close to identical, then the degeneracies were added and the feature was treated as a single line; 3) if none of the two previous conditions were met, we did not include the multiplet in the analysis.

Line upper energy levels, Einstein coefficients, degeneracies, and quantum numbers from the Splatalogue are listed with the derived line fluxes and FWHM in Table 2 for CH3OH from the single-dish observations, in Table 3 for CH3OH from the SMA spectra, in Table 4 for CH3CN from IRAM, in Table 5 for CH3CN from the SMA, in Table 6 for CH3CCH from IRAM, in Table 7 for CH3CCH from the SMA, and in Table 8 for HNCO, CH3OCH3, and CH3CHO. Only the lines with an Einstein coefficient logarithm higher than −4.5 and their upper level energy below 400 K are displayed in the tables. Due to the high line density for CH3OCH3 and CH3CHO, only the lines with an upper energy level below 200 K are shown for these species. No other complex molecules were detected toward any of the sources. For molecules with weak lines, we only used the IRAM data since the SMA observations have lower spectral resolution and signal-to-noise ratio.

thumbnail Fig. 3

Spectral window with several CH3CN and CH3CCH lines from the single-dish (black lines, 0.2 MHz spectral resolution) and the 2′′ interferometric data (red line, 0.8 MHz spectral resolution).

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thumbnail Fig. 4

Spectral window with several CH3OH lines from the single-dish (black lines, 0.2 MHz spectral resolution) and the 2” interferometric data (red line, 0.8 MHz spectral resolution). CH3OH lines with upper level energies higher than 70 K are marked with a star in the NGC 7538 IRS9 to emphasize the increase in SMA/IRAM overlapping for lines with higher upper energy levels.

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thumbnail Fig. 5

Spectra of two CH3CHO lines at 242.106 GHz and 242.118 GHz (left panel) and the CH3OCH3 line at 241.946 GHz (right panel) from the single-dish (black line, 0.2 MHz spectral resolution) and 2′′ interferometric data (red line, 0.8 MHz spectral resolution). The three line frequencies are marked by black dotted line.

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3.2. Spatial origin of the line emission

Figures 35 present the line fluxes of key molecules from both the single-dish and SMA observations toward the three MYSOs. The IRAM beam is 6.2 to 7.2 times larger than the SMA mask ((IRAM radius at 227243 GHz: 5–5.4)2/(SMA mask radius: 2)2). That most emission lines in these figures do not display a factor of six or seven difference between the IRAM 30 m and SMA spectra demonstrates a non-uniform emission across the object. Some emission line fluxes, most notably CH3CN, are similar (within a factor of two) between the IRAM 30 m and SMA spectra, indicating of a large contribution from unresolved emission at the source center. In contrast, little or no CH3CCH flux from the IRAM is recovered by the SMA, which indicates extended emission. The fact that some CH3CCH IRAM 30 m fluxes are more than 6.2–7.2× higher than the corresponding SMA fluxes is explained by spatial filtering of large-scale emission and/or off-centered emission. CH3OH lines display a mixed behavior: lines with higher upper energies show more overlap between the IRAM 30 m and SMA spectra than the colder lines. The IRAM 30 m and SMA line fluxes for the 110, 11100, 10 HNCO line at 241.774 GHz are close for NGC 7538 IRS9 and similar for W3 IRS5, but none of the IRAM 30 m flux is recovered by the AFGL490 SMA observations. Based on these lines, HNCO emission appears to be coming from both the core and the envelope of NGC 7538 IRS9, from the core of W3 IRS5 alone, and from the envelope of AFGL490. This source-to-source difference could partially come from different excitation conditions in the three sources, and the excitation-abundance structure degeneracy can only be strictly broken by observation of additional lines. The simplest scenario for explaining our detection is for HNCO to have both an extended and a compact origin, however, and this is also supported by the reported excitation characteristics and emission profile of HNCO in other sources (Bisschop et al. 2007).

In Fig. 5 the signal-to-noise ratio is lower, but it is still clear that CH3CHO toward NGC 7538 IRS9 only has extended emission since none of the IRAM 30 m line flux is recovered in the SMA spectra. No CH3CHO lines are detected in the other two MYSOs in the spectral range where IRAM 30 m and SMA observations overlap. CH3OCH3 is detected toward NGC 7538 IRS9 and AFGL490, and in both cases tentative SMA detections suggest that the emission originates in the source centers. Based on the different emission patterns, the molecules found in these spectra are classified as follows: CH3CCH and CH3CHO are envelope organics, CH3CN and CH3OCH3 are core organics, and CH3OH and HNCO are intermediate cases with significant core and envelope contributions.

Table 9

Rotational temperatures and column densities for CH3OH, CH3CN, and CH3CCH derived from the rotational diagrams presented in Fig. 6.

3.3. Rotational temperatures, and column densities

The core and envelope classifications based on spatial emission patterns should be reflected in the rotational temperatures of the different molecules. Figure 6 shows the rotational diagrams for molecules with enough line detections, i.e., CH3OH (extracted from IRAM 30 m and SMA spectra) and CH3CN, CH3CCH, following the method described in Goldsmith & Langer (1999). The line fluxes from the IRAM observations were converted into main beam temperature using the flux-to-antenna temperature conversion factor and the beam and forward efficiencies listed online for the EMIR receiver10 and linearly extrapolated for each line frequency (Tmb(K) ≃ 0.2 × Flux(Jy)). The line fluxes from the SMA observation were converted into temperature using the Rayleigh-Jeans approximation with a circular beam of 2 radius coming from the mask dimension (Tmb(K) ≃ 1.3 × Flux(Jy)). Optically thin emission was assumed based on the low line intensities and lack of asymmetry in the line profiles. This assumption was verified by the shape of the rotational diagram; i.e., flattening or large scatter was observed for lower energy transitions (cf., Bisschop et al. 2007). Considering the possibilities of subthermal excitation in the envelope, the rotational temperatures are not expected to be the gas kinetics temperatures outside of the core. A 10% uncertainty was added to the line-integrated area and is listed in the tables to account for the line shapes sometimes deviating from the Gaussian shape assumed for the fit. We used the “linfit” IDL routine to derive the rotational temperatures, as well as the column densities, and the routine returned the corresponding uncertainties.

Table 9 presents the column densities and rotational temperatures derived for these molecules using the rotational diagrams in Fig. 6. The beam-averaged CH3OH column densities and rotational temperatures derived from the IRAM 30 m spectra agree with those found by van der Tak et al. (2000), based on JCMT single-dish telescope at higher frequencies. The rotational temperature and column densities derived for CH3OH from the SMA data are always higher than those derived by the IRAM 30 m, which is consistent with the SMA observations probing material closer to the MYSO centers. The E-/A- CH3OH ratio is consistent with unity within the uncertainties, which agrees with Wirström et al. (2011). The derived column densities for CH3CN from the IRAM data assume that the emission is only coming from the 2 radii encompassed by the SMA beam: i.e., we apply a dilution factor of 0.16 to account for the SMA extraction mask area (2′′ radius) to IRAM beam (5′′ radius) ratio. This assumption is justified by the CH3CN hot-core like rotational temperatures of 80110 K toward the different MYSOs, which are also consistent with the CH3OH excitation temperature derived from the SMA spectra. It is also consistent with the observed overlap between the IRAM and SMA line fluxes (see Fig. 3). The rotational temperatures of ~50 K obtained for the CH3CCH 1413 ladder from the IRAM spectra are consistent with an envelope origin, but suggests that it is mainly present in the luke-warm envelope regions rather than in the outermost cold envelope.

The rotational diagram method assumes that all data can be described by a single excitation temperature. To test this assumption for our data, a two-temperature fit was explored for the case of methanol. Two-temperature fits of CH3OH lines was investigated by van der Tak et al. (2000), Leurini et al. (2007), and Isokoski et al. (2013), among others. The full results are presented in Appendix A, but briefly: both a cold and warm component are recovered from the IRAM 30 m data. The derived column densities of the cold components are consistent with the single-component fits (within uncertainties), while the warm component is consistent with the single-component fit to the SMA data. The fit to the SMA line data was not improved by adding a second component, verifying our hypothesis that the 2 mask emission is dominated by a hot component for all sources.

For HNCO, CH3CHO, and CH3OCH3, no rotational diagrams could be built owing to the very small upper-level energy range of the observed transitions, and column densities were calculated using the envelope temperature (the CH3OH IRAM 30 m rotational temperature) if the molecule was classified as an envelope molecule and the core temperature (the CH3CN rotational temperature) if the molecule was classified as a core molecule, and both rotational excitation temperatures if the molecule was classified as intermediate, i.e. HNCO. As seen in Table 10, the calculated HNCO abundance with respect to CH3OH is almost identical regardless of the assumed spatial origin of the line emission. For core molecules, the same dilution factor as for CH3CN was applied (see Table 10). For molecules with multiple line detections, the column densities were derived by averaging the individual column densities found for each detected line and taking the square root of the sum of the individual uncertainties squared as uncertainty. Only the IRAM data were used to calculate these column densities since these data present a higher signal-to-noise ratio.

thumbnail Fig. 6

Rotational diagrams of CH3OH from the single dish data (first column), for CH3OH from the SMA spectra extracted with a 2 mask (second column), and CH3CN from the single-dish data (third column), and CH3CCH (fourth column) and for the sources NGC 7538 IRS9 (first row), W3 IRS5 (second row), and AFGL490 (third row).

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Table 10

Column densities for HNCO, CH3CHO, CH3OCH3 using excitation temperatures from Table 7.

Table 11

CH3CN, CH3CCH, HNCO, CH3CHO, CH3OCH3 abundances with respect to CH3OH for their respective spatial origin.

thumbnail Fig. 7

Gas abundance correlation between organics including upper limits. The black crosses are the abundances derived for the MYSOs, the red squares are derived for hot-core sources by Bisschop et al. (2007), and the blue diamonds are results for hot-core sources from Isokoski et al. (2013). An arbitrary error of 20% has been taken when not reported in the two latest studies.

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3.4. Organics in hot cores vs. organic-poor MYSOs

By definition, the organic-poor MYSOs reported in this study have less intense emission of complex organic molecules than do line-rich hot cores. The question for this section is whether the chemical composition with respect to CH3OH is different between the two source families. The CH3CN, CH3CCH, HNCO, CH3CHO, and CH3OCH3 abundances with respect to CH3OH obtained here and presented in Fig. 11 for the three MYSOs are compared to the hot core abundances derived by Bisschop et al. (2007) in Fig. 8. Moreover, three high-mass protostellar objects from Isokoski et al. (2013) have been added. These three objects were inferred to have large equatorial structures, but Isokoski et al. (2013) found no strong chemical differences in their chemistry compared to the Bisschop et al. (2007) hot-core sources. For the three MYSOs sources, the molecular abundances with respect to CH3OH are calculated using the CH3OH column densities derived for the envelope if the molecule has been classified as “envelope” molecules and using the CH3OH column density derived for the core (SMA-based) in the case of a core molecule. For the hot core sources, Bisschop et al. (2007) applied a dilution factor corresponding to the region where T> 100 K for CH3OH, CH3CN, HNCO, and CH3OCH3, but not for CH3CCH and CH3CHO. To calculate CH3CCH and CH3CHO abundances with respect to CH3OH, we removed the dilution factor for CH3OH applied by Bisschop et al. (2007). All other abundances were taken directly from Bisschop et al. (2007). Isokoski et al. (2013) could identify a cold and hot methanol emission using single-dish data and the derived column densities for both components have been used in Fig. 7 in the same way as for the sources analyzed in the present study.

The histograms in Fig. 8 show that the CH3CN, CH3OCH3, and HNCO core abundances with respect to CH3OH are similar for the organic-poor MYSOs and the hot-core sources. In contrast, the organic-poor MYSOs show higher complex organic envelope abundances, i.e., CH3CHO and CH3CCH, with respect to CH3OH compared to the hot core sources. This difference is most likely due to to our not being able to separate CH3OH core and envelope emission in the study by Bisschop et al. (2007), resulting in artificially low envelope ratios with respect to CH3OH when all CH3OH is implicitly assumed to originate in the envelope; in reality, the high excitation temperature of CH3OH in the hot core sources suggests that most of it really comes from the core.

A similar apparent separation between hot core and organic-poor MYSOs are visible in log–log correlations of molecular abundances with respect to CH3OH shown in Fig. 7. Furthermore, there is a clear correlation between envelope molecules CH3CHO and CH3CCH, but this may simply be due to the different abundance derivations of cold molecules with respect to CH3OH for the hot cores and the weak-line MYSOs, rather than signifying a chemical relationship. More interestingly, these log–log abundance ratio plots show that there is no correlation between the two N-bearing organics CH3CN and HNCO over an order of magnitude range. There is also no correlation between the two O-bearing complex species CH3OCH3 and CH3CHO, which is consistent with their inferred different origins in the organic-poor MYSOs.

thumbnail Fig. 8

Number of sources versus the logarithm of their gas phase organic ratio over methanol with respect to the mean for O-bearing species. The solid filled histograms correspond to sources observed and analyzed here, the unfilled histograms correspond to sources from Bisschop et al. (2007) and Isokoski et al. (2013). The left panel presents the O-bearing species data, while the right panel focuses on the N-bearing molecules. HNCO abundances were derived assuming either hot compact emission or cold extended emission, since its origin does not seem to be consistent between sources.

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3.5. An ice-gas connection?

The CH3OH ice content may be an important factor in whether a hot core chemistry developed, so we compare the CH3OH core column density toward our organic-poor MYSOs and hot cores with their CH3OH ice abundance (with respect to H2O) (see Table 1, Gibb et al. 2004). In the hot cores, the majority of the CH3OH gas originates in the core and we use the derived column densities from Bisschop et al. (2007), where all CH3OH emission is assumed to originate in the central region where the temperature is higher than 100 K. To ensure a fair comparison we calculated the size of the “hot core region” toward our sample using the relation between luminosity and temperature , which was shown by Bisschop et al. (2007) to approximate the 100 K radius well toward their source sample. We then assumed that all SMA CH3OH line flux originate in these regions, based on the derived rotational temperatures, and used an appropriate dilution factor when the 100 K area is smaller than the 2′′ mask used for spectral extraction. Figure 9 presents the resulting column density of hot CH3OH gas versus the initial CH3OH abundance on the grains. It appears that it is primarily the column density of CH3OH that is different between the line-rich and line-poor sources. No strong correlation with the ice content is observed, but more sources would allow the sample to be divided into luminosity and total mass bins, removing scatter due to initial physical conditions and size of 100 K region. Still, this plot suggests that initial CH3OH ice content alone does not determine the richness of the MYSO chemistry when correcting for the source luminosity.

thumbnail Fig. 9

CH3OH column density in the inner core (calculated area where T> 100 K) versus CH3OH ice abundance over H2O ice in the envelope. The stars present the line-poor sources analyzed in this study, while the empty squares are data from Bisschop et al. (2007) for hot-core sources.

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The initial ice composition may also affect the complex organic composition in both hot cores and organic-poor MYSOs cores and envelopes. Figure 10 presents correlation plots between ratios of the N-bearing organics and CH3OH in the ice and gas phases. The two gas-phase N-bearing organics are HNCO and CH3CN, and the two ice species are OCN and NH3. The ice abundances are listed in Table 1 and have been obtained by Gibb et al. (2004). For sake of consistency, only the abundances obtained through the analysis technique described and performed by Gibb et al. (2004) were used here though detailed analysis of specific ice species, taking ice environment and using multiple vibrational bands into account, have been conduced, e.g., by Taban et al. (2003) in the case of NH3 in W 33A.

When combining our new observations with data from the literature, a sample of seven MYSOs have both ice and gas observations. As seen in Fig. 9, only a fraction of them can be used to correlate specific ice ratios, however, because of multiple ice abundance upper limits for many of the sources. For example, W3 IRS5 is not included in any of the plots, because of its CH3OH, OCN and NH3 ice upper limits. This means that the current data set can only be used to search for tentative correlations or to note gross deviations from expected correlations, and not for a proper statistical correlation analysis.

The top lefthand panel of Fig. 10 shows no conclusive correlation between OCN ice and HNCO in the gas phase with respect to CH3OH. Qualitatively, such a correlation is expected since HNCO and OCN are linked through efficient thermal acid-base chemistry within the ice (e.g., Demyk et al. 1998; van Broekhuizen et al. 2004; Theule et al. 2011). The lack of a correlation may therefore simply be due to the difficulty determining the OCN abundance in the ice, so we also explore the correlation between HNCO gas and the better constrained NH3 ice. NH3 is likely the major source of nitrogen in the ice and may therefore be a proxy for the abundance of N-bearing ices in general. It may also affect the HNCO/OCN chemistry directly since it is a strong base. The top righthand panel of Fig. 10 shows that there is indeed a tentative correlation between gas phase abundance of HNCO over CH3OH with respect to NH3.

The relation between OCN in the ice and CH3CN in the gas is explored in the bottom lefthand panel of Fig. 10. These molecules do not appear to be correlated for the four sources presented here, despite both containing a CN functional group. Finally, Rodgers & Charnley (2001) predict a correlation between CH3CN gas a NH3 ice, but in this limited sample we find no correlation between the CH3CN/CH3OH gas ratio versus the NH3/CH3OH ratio in the ice.

thumbnail Fig. 10

Ice versus gas abundance correlation for N-bearing species with respect to CH3OH. The crosses are abundances derived for our organic-poor MYSOs, and the red squares are the values derived by Bisschop et al. (2007). An arbitrary error of 20% has been assumed for the latter values. For the two top plots, the black crosses represent the HNCO over CH3OH abundance derived for the compact component, while the blue crosses correspond to the HNCO abundance calculation for an extended component.

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4. Discussion

4.1. Organic-poor MYSOs versus hot cores

Previous observations of complex molecules toward MYSOs have generally focused on sources with a bright hot core that is responsible for most of the molecular emission. In such cases, either interferometric or single-dish observations are sufficient to determine complex organic abundances as long as the radius of the evaporation front close to the central protostar is known. Single-dish observations combined with the rotational diagram technique can also be used to derive abundances of molecules that are predominantly present in the outer envelope, since beam-averaged abundances can then be assumed. The real difficulty arises for molecules that are distributed throughout the envelope and core. For such molecules, single-dish and interferometric observations need to be combined to deduce what fraction of the molecular emission originates in the envelope and in the core, and then use these fractions to calculate the chemical composition of the two physically and chemically different regions. Based on this study, this class of molecules seems to mainly encompass zeroth-generation ices, i.e. CH3OH and HNCO, but as our sensitivity increases, we expect, based on model results, that many classical hot core molecules will present a significant envelope emission profile as well (Öberg et al. 2013).

Using the IRAM 30 m and SMA spectra, we could classify several complex organic molecules as belonging to the core, envelope, and both. The two envelope molecules, CH3CHO and CH3CCH, were similarly classified by Bisschop et al. (2007) based on excitation temperatures alone, suggesting that the envelopes around line-poor MYSOs and hot cores are chemically similar. In contrast we find that in the line-poor MYSOs, CH3OH and sometimes HNCO have significant emission contributions from the envelope, while Bisschop et al. (2007) find that in hot core sources, they have excitation temperatures above 100 K and were thus classified as originating exclusively in the core region; these sources have probably a similar envelope line flux to the one observed for the line-poor MYSOs, but in single-dish studies, this emission contribution is drowned out by the hot cores.

Overall, the chemistry in the young MYSOs is remarkably similar to what is observed in the hot cores, which suggests that they may be hot core precursors. CH3CN, CH3CCH, CH3CHO, HNCO, and CH3OCH3 are observed in both kinds of sources at comparable abundances with respect to CH3OH. CH3CH2OH and HCOOCH3 – two typical hot-core molecules – are not seen in the organic-poor MYSOs, but typical abundance of these molecules with respect to CH3OH from Bisschop et al. (2007) are consistent with non-detections. The hot-core precursor interpretation is also consistent with the observed lack of correlation between CH3OH core column density or hot-core activity on the initial CH3OH ice abundance.

4.2. The ice-gas connection: observations vs. theory

thumbnail Fig. 11

Model ice versus gas abundance correlation from the MAGICKAL model (Garrod 2013). Five initial ice abundances (M1 to M5) are used to run the models and derived N-bearing abundances with respect to CH3OH. The black plus signs present the results at 20 K, the blue crosses are the model results at 30 K, the red squares represents 50 K, and the green diamonds are the results at 100 K.

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Regardless or whether the overall hot-core chemistry depends on the initial ice composition, we expect that the ice composition will have an effect on the chemical composition in both hot cores and line-poor MYSOs. This dependence may look very different for complex molecules that form in the gas phase from evaporated ices compared to products of complex ice chemistry. That we do not observe a clear trend between NH3 in the ice and CH3CN in the gas suggests that the model of Rodgers & Charnley (2001) is missing important complex molecule formation pathways. We have therefore used the state-of the-art chemical model MAGICKAL (Garrod 2013) to explore the connection between ice and gas phase species further. The model uses a rate-equation/modified rate-equation approach, treating the gas phase, ice surface, and bulk ice as coupled, but distinct, chemical phases.

Garrod (2013) produced generic, single-point hot-core models that treated first a cold collapse to 107 cm-3, followed by a warm-up from 8 to 400 K at fixed density, assuming a typical grain size of 0.1 μm and 106 surface binding sites. We reran the warm-up phase, adopting the medium warm-up timescale of 2 × 105 years to reach 200 K (whose results appear best to fit various other observational results), but altering the ice abundances prior to warm-up. The original H2O ice abundance is retained, while the CH3OH, CH4, HNCO, and NH3 values are varied to mimic the observed ranges in the combined line-poor MYSOs and hot cores sample (see M1 to M5 in Table 12). Even though the ice abundance ratios correspond to the sources in this study, it is not possible to directly compare observations and simulation since key physical parameters such as densities and warm up rates of each specific object is not taken into account. The simulations are instead used here to investigate chemical trends. The resulting gas-phase abundances of HNCO, CH3CN, and CH3OH are reported at different temperatures during warm-up in Table 13.

Table 12

Initial ice abundances with respect to water used for the five chemical model simulation M1–5.

Table 13

Gas abundances with respect to hydrogen at various temperatures derived by the Garrod (2013) model for the initial ice abundances presented in Table 12.

The relationships during protostellar warm-up between gas-phase HNCO and CH3CN with respect to CH3OH, and OCN and NH3 initial ice abundances with respect to CH3OH ice are shown in Fig. 11 for temperatures between 20 K and 100 K. Since the model does not treat ion chemistry in the ice, a full conversion rate of HNCO ice into OCN has been assumed based on the efficient HNCO to OCN conversion derived experimentally by Demyk et al. (1998), van Broekhuizen et al. (2004), Theule et al. (2011), among others. The resulting complex molecular gas abundances are regulated by a combination of temperature and initial ice composition. The sensitivity to ice composition varies significantly with temperature however, the gas-phase HNCO/CH3OH ratio, for example, barely changes with ice composition when the grains are sitting at 30 K, because of the very limited sublimation at this temperature. To predict the complex chemistry thus clearly requires knowing both the temperature structure and the initial ice composition of a source.

Based on the observational and theoretical results that most HNCO and CH3CN emission start at high temperatures, we focus on the predictions at 100 K. At this temperature, the HNCO gas vs. methanol content is, as expected, correlated to the initial amount of OCN over methanol in the ice. This result suggests that with more sources and/or better constraints on OCN ice abundances, a clearer correlation in the observed data should appear as long as the model captures the dominant HNCO formation/destruction pathways. The observed tentative correlation between gas-phase HNCO and NH3 ice is consistent with model predictions, except for the M3 run. In the M3 model, the high absolute abundance of HNCO ice results in a longer HNCO desorption time scale, which shifts the abundance peak of HNCO to higher temperatures and results in the high [HNCO]/[CH3OH] ratio at 100 K; at higher temperatures, M3 no longer deviates from the trend. To fully explore the effects of NH3 and OCN on the final complex organics abundances clearly requires a much larger grid of models that covers all possible combinations of ice abundances as well as investigating the temperature dependences of the complex chemistry.

In contrast to what has been proposed by Rodgers & Charnley (2001), the abundance of CH3CN with respect to CH3OH does not correlate with either the NH3 ice content in the MAGICKAL code output or with the cyanide ice-related species OCN. This agrees with the observational results. In Rodgers & Charnley (2001) and Garrod (2013), CH3CN forms mainly through radiative association reaction in the gas phase between CH and desorbing HCN giving CH3CNH+. The correlation between CH3CN and NH3 predicted by Rodgers & Charnley (2001) comes from a cycled production of HCN from NH3. The latter is, however, not observed in MAGICKAL, which explains the lack of correlation between CH3CN and NH3.

In summary, there is some encouraging tentative agreement between model predictions and observations. To directly compare models and observations requires, however, that the appropriate model results are mapped onto the temperature-density profiles of individual sources, since both ice composition and temperature are shown to strongly affect the complex chemistry (Öberg et al. 2013). Thus, to draw any general conclusions requires a large sample of spatially-resolved gas-phase observations, along with ice observations of the same object. As shown here, organic-poor high-mass protostars contain detectable amounts of complex organic material and present a similar chemistry to bright hot cores. Most massive YSOs with existing ice observations could therefore be used to expand the sample of sources.

5. Conclusions

We detected complex organic molecules CH3CN, CH3CCH, CH3CHO, and CH3OCH3 together with HNCO and CH3OH, toward three massive YSOs without any previous evidence of hot-core chemistry activity. Using a combination of single-dish and interferometry observations, we found that CH3CN and CH3OCH3 emission originates in the central core region, CH3CHO and CH3CCH in an extended envelope, and CH3OH and, sometimes, HNCO have both envelope and core emission components. The inferred molecular emission locations are consistent with rotational temperatures derived from the single-dish observations, except for CH3OH, where single-dish data are dominated by the envelope.

The high-temperature abundances of complex organics with respect to CH3OH are indistinguishable for the organic-poor MYSOs and the sample of hot core sources from Bisschop et al. (2007) and Isokoski et al. (2013). The envelope chemistry also seems similar for both kinds of sources, but this analysis is limited by a lack of CH3OH envelope data toward hot core sources. No strong correlation between initial CH3OH ice abundance and hot CH3OH gas column density close to the central object was observed.

The NH3 ice abundances seem to affect the HNCO/CH3OH gas-phase abundances. This relationship is reproduced by the MAGICKAL astrochemical code, assuming fiducial collapse and warm-up rates and initial ice compositions that span the observed range.

More sources with both ice and gas data are required to settle how ice abundances affect complex molecule distributions around MYSOs. This could be achieved by collecting mid-IR ice spectra (using SOFIA for example), and performing spatially resolved millimetric observations of sources with no detected hot-core molecules.


1

The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. It is funded by the Smithsonian Institute and the Academia Sinica.

4

Reduced data are available through the dataverse network at http://dx.doi.org/10.7910/DVN/26562

5

Observations are available on the SMA archive website http://www.cfa.harvard.edu/cgi-bin/sma/smaarch.pl

7

Splatalogue website: http://www.cv.nrao.edu/php/splat/

9

JPL database website: http://spec.jpl.nasa.gov/

Acknowledgments

The authors thank the anonymous referee and the editor Malcolm Walmsley for helpful comments and suggestions. E.C.F. is supported by a Rubicon fellowship (680-50-1302), awarded by the Netherlands Organisation for Scientific Research (NWO). R.T.G. is funded by the NASA Astrophysics Theory Program, grant number NNX11AC38G. Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA), by a Royal Netherlands Academy of Arts and Sciences (KNAW) professor prize, and by the European Union A-ERC grant 291141 CHEMPLAN.

References

Appendix A: Two-component fit of the CH3OH rotational diagrams

thumbnail Fig. A.1

Results of a two component fit of the CH3OH rotational diagrams for the three line-poor MYSOs using the IRAM 30 m spectra.

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The IRAM 30 m and SMA CH3OH line data have been analyzed further to explore whether the data is better fit by two distinct temperature distributions than by the one assumed in the rotational diagrams in the main body text. In fact, two Boltzman distributions would be expected for the IRAM 30 m data, since these spectra include both the envelope and the warm core seen in the SMA spectra. Figure A.1 presents the two-component rotational diagrams for the three MYSOs. The fits were performed using the IDL routine MPFIT with the initial temperature guesses of 20 K and 100 K and initial column densities guesses of 5 × 1014 cm2 and 1 × 1015 cm2.

For all three MYSOs the data is well fit by two components, but the uncertainties in the derived excitation temperatures and column densities are very large, demonstrating that the signal-to-noise ratio of this data set is not sufficient for a quantitative two-component analysis. Still, it is clear that in each case, the cold component has a slightly lower excitation temperature than the single-component fit. The derived column densities agrees (taking the large uncertainties into account) between the two fits, supporting our assumption that the IRAM 30 m CH3OH spectra are dominated by the envelope. Each fit also results in a warm component. While the excitation temperatures have large uncertainties, they are all consistent with those derived from the SMA data, further supporting the conclusions based on that data set. Owing to these large uncertainties for the column densities and the hot component temperature, single-temperature fitting was used for the quantitative analysis in the paper. Higher signal-to-noise ratio single-dish data could clearly be used, however, to simultaneously constrain temperature and column densities of the two components, limiting the need for high-spatial-resolution observations for some sources.

We also attempted to fit the CH3OH SMA spectra with two temperature components using the same fitting routine and initial guesses. In each case, the outcome was that a single-component fit the data as well as two components; i.e., it was not possible to distinguish multiple components with different temperatures within the SMA masks. This confirms that the SMA beam samples only the hot CH3OH component and filters out the extended cold emission.

All Tables

Table 1

Source characteristics and ice abundances.

Table 2

CH3OH lines data from IRAM 30 m spectra.

Table 3

CH3OH lines extracted from SMA observations with a 2-radius mask.

Table 4

CH3CN lines data from IRAM 30 m spectra.

Table 5

CH3CN lines data from SMA spectra.

Table 6

CH3CCH lines data from IRAM 30 m spectra.

Table 7

CH3CCH lines data from SMA spectra.

Table 8

HNCO, CH3CHO, and CH3OCH3 lines data from the IRAM 30 m spectra and HNCO line data from the SMA 2” radius compact region.

Table 9

Rotational temperatures and column densities for CH3OH, CH3CN, and CH3CCH derived from the rotational diagrams presented in Fig. 6.

Table 10

Column densities for HNCO, CH3CHO, CH3OCH3 using excitation temperatures from Table 7.

Table 11

CH3CN, CH3CCH, HNCO, CH3CHO, CH3OCH3 abundances with respect to CH3OH for their respective spatial origin.

Table 12

Initial ice abundances with respect to water used for the five chemical model simulation M1–5.

Table 13

Gas abundances with respect to hydrogen at various temperatures derived by the Garrod (2013) model for the initial ice abundances presented in Table 12.

All Figures

thumbnail Fig. 1

Image of the CH3CN emission using the 130 − 120 at 239.138 GHz line acquired by the SMA for the massive young stellar objects NGC 7538 IRS9, W3 IRS5, and AFGL490 targeted in this study. The black contour presents the 50% line intensity, and the synthesized beam is shown in white at the bottom left. A 2′′ radius mask used to extract the spectra is overplotted in dashed red line. Images for the hot core source NGC 7538 IRS1 is presented as well. The latter source has been through the same program as the three other sources.

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In the text
thumbnail Fig. 2

239243 GHz spectral window from the IRAM 30 m displaying emission lines for typical hot core source NGC 7538 IRS1 and weak line MYSOs NGC 7538 IRS9, W3IRS5, AFGL490. The star-marked lines are CO ghost lines consistent with the sideband rejection for each source.

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In the text
thumbnail Fig. 3

Spectral window with several CH3CN and CH3CCH lines from the single-dish (black lines, 0.2 MHz spectral resolution) and the 2′′ interferometric data (red line, 0.8 MHz spectral resolution).

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In the text
thumbnail Fig. 4

Spectral window with several CH3OH lines from the single-dish (black lines, 0.2 MHz spectral resolution) and the 2” interferometric data (red line, 0.8 MHz spectral resolution). CH3OH lines with upper level energies higher than 70 K are marked with a star in the NGC 7538 IRS9 to emphasize the increase in SMA/IRAM overlapping for lines with higher upper energy levels.

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In the text
thumbnail Fig. 5

Spectra of two CH3CHO lines at 242.106 GHz and 242.118 GHz (left panel) and the CH3OCH3 line at 241.946 GHz (right panel) from the single-dish (black line, 0.2 MHz spectral resolution) and 2′′ interferometric data (red line, 0.8 MHz spectral resolution). The three line frequencies are marked by black dotted line.

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In the text
thumbnail Fig. 6

Rotational diagrams of CH3OH from the single dish data (first column), for CH3OH from the SMA spectra extracted with a 2 mask (second column), and CH3CN from the single-dish data (third column), and CH3CCH (fourth column) and for the sources NGC 7538 IRS9 (first row), W3 IRS5 (second row), and AFGL490 (third row).

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In the text
thumbnail Fig. 7

Gas abundance correlation between organics including upper limits. The black crosses are the abundances derived for the MYSOs, the red squares are derived for hot-core sources by Bisschop et al. (2007), and the blue diamonds are results for hot-core sources from Isokoski et al. (2013). An arbitrary error of 20% has been taken when not reported in the two latest studies.

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In the text
thumbnail Fig. 8

Number of sources versus the logarithm of their gas phase organic ratio over methanol with respect to the mean for O-bearing species. The solid filled histograms correspond to sources observed and analyzed here, the unfilled histograms correspond to sources from Bisschop et al. (2007) and Isokoski et al. (2013). The left panel presents the O-bearing species data, while the right panel focuses on the N-bearing molecules. HNCO abundances were derived assuming either hot compact emission or cold extended emission, since its origin does not seem to be consistent between sources.

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In the text
thumbnail Fig. 9

CH3OH column density in the inner core (calculated area where T> 100 K) versus CH3OH ice abundance over H2O ice in the envelope. The stars present the line-poor sources analyzed in this study, while the empty squares are data from Bisschop et al. (2007) for hot-core sources.

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In the text
thumbnail Fig. 10

Ice versus gas abundance correlation for N-bearing species with respect to CH3OH. The crosses are abundances derived for our organic-poor MYSOs, and the red squares are the values derived by Bisschop et al. (2007). An arbitrary error of 20% has been assumed for the latter values. For the two top plots, the black crosses represent the HNCO over CH3OH abundance derived for the compact component, while the blue crosses correspond to the HNCO abundance calculation for an extended component.

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In the text
thumbnail Fig. 11

Model ice versus gas abundance correlation from the MAGICKAL model (Garrod 2013). Five initial ice abundances (M1 to M5) are used to run the models and derived N-bearing abundances with respect to CH3OH. The black plus signs present the results at 20 K, the blue crosses are the model results at 30 K, the red squares represents 50 K, and the green diamonds are the results at 100 K.

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In the text
thumbnail Fig. A.1

Results of a two component fit of the CH3OH rotational diagrams for the three line-poor MYSOs using the IRAM 30 m spectra.

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In the text

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