EDP Sciences
Free Access
Issue
A&A
Volume 555, July 2013
Article Number A18
Number of page(s) 7
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201220943
Published online 20 June 2013

© ESO, 2013

1. Introduction

Interstellar isocyanic acid (HNCO) was first detected in Sgr B2 molecular cloud complex, where it was found to be spatially extended and relatively strong (Snyder & Buhl 1972; Churchwell et al. 1986; Lindqvist et al. 1995; Kuan & Snyder 1996; Dahmen et al. 1997). Since its discovery, HNCO has been detected in various molecular clouds, including the dark cloud TMC-1 (e.g. Brown 1981; Jackson et al. 1984), as well as in hot cores in massive star-forming regions (e.g. MacDonald et al. 1996; Helmich & van Dishoeck 1997). Jackson et al. (1984) proposed that HNCO was a dense gas tracer due to the coincidence of HNCO emission with regions of high density (n ≥ 106 cm-3). Zinchenko et al. (2000) report a detection rate of 70% in a survey of 81 molecular clouds. HNCO has also been detected in some extragalactic sources (Nguyen-Q-Rieu et al. 1991; Meier & Turner 2005; Martín et al. 2009).

Within the Galactic center region, obvious different distribution of HNCO and C18O, which is thought to be a tracer of the total H2 column density, suggests possible different chemical properties of the different molecular complexes in the center of our Galaxy (Dehmen et al. 1997; Lindqvist et al. 1995; Martín et al. 2008). Based on the morphology of the emission and the HNCO abundance with respect to H2, several authors hypothesized that HNCO could be a good tracer of interstellar shocks (e.g., Zinchenko et al. 2000; Meier & Turner 2005; Minh & Irvine 2006). Martín et al. (2008, 2009) conducted a multitransition study of 13 molecular clouds towards the Galactic center and concluded that the HNCO/CS abundance ratio might provide a useful tool for distinguishing between the influence of shocks and radiation activity in the nuclear regions of galaxies. Rodríguez-Fernández et al. (2010) test the hypothesis by observing a low-mass molecular outflow where the chemistry is dominated by shocks. Their results indicate that shocks can actually produce the HNCO abundance measured in galactic nuclei, providing a solid basis to previous suggestions that the extended HNCO in galactic nuclei could trace large-scale shocks.

Chemical models have also been developed to investigate how HNCO forms. Both gas-phase reaction (e.g., Turner et al. 1999) and formation routes on grain surfaces (e.g. Hasegawa & Herbst 1993; Garrod et al. 2008) have been used to model HNCO abundance. Tideswell et al. (2010) find that HNCO is inefficiently formed when only gas-phase formation pathways are considered in the chemical network, and surface routes are needed to account for its abundance. Quan et al. (2010) have reproduced the abundances of HNCO and its isomers in cold and warm sources using gas-grain simulation, which contains both gas-phase and grain-surface syntheses.

Zinchenko et al. (2000) mapped three molecular clouds in HNCO transitions and found that HNCO emission is compact and centrally peaked. Because they are limited by angular resolutions, the size of HNCO clouds, the relationships with infrared sources, as well as the dominant excitation mechanism of HNCO emission remain unknown. From this consideration it is clear that high-sensitivity observations are needed to better understand the physical condition and chemical properties of HNCO clouds.

In this paper we present large-scale mapping observations of HNCO and C18O toward strong sources detected in Zinchenko et al. (2000) with the Purple Mountain Observatory 13.7 m (PMODLH 13.7 m) telescope. We first introduce the observations and data reduction in Sect. 2. In Sect. 3, we present the observational results. In Sect. 4, we discuss implication of our observations on the excitation mechanism and chemistry of HNCO, followed by a summary in Sect. 5.

2. Observations and data reductions

We performed mapping observations of HNCO 505 − 404 (109.905 GHz) and C18O 1−0 (109.782 GHz) lines simultaneously with the PMODLH 13.7 m located in Delingha, China in January 2011. The main beam size is about 55′′, and the pointing accuracy is estimated to be better than 9′′. A new cryogenically cooled 9-beam SIS receiver (3 × 3 with a separation of 174′′ between the centers of adjacent beams) working in the 85−115 GHz band were employed. A fast-Fourier transform spectrometer (FFTS) of 16 384 channels with a bandwidth of 1 GHz was used for each beam, supplying a velocity resolution of about 0.21 km s-1. Typical system temperatures were around 150−300 K, depending on the weather conditions. Observations were made in on-the-fly mode with nine beams. The telescope drifted in azimuth (with a rate of 20′′ s-1) and stepped in elevation (with a scan step of 15′′). Several maps were made and later combined to lower the rms noise levels. The mapping size was 10′ × 10′ for most sources. Mapping centers of all our sources are listed in Table 1.

Table 1

Source list.

Most of the sources were selected from dense cores showing strong HNCO 505 − 404 emission in the surveys of Zinchenko et al. (2000). W44, S140, and DR21S are also included in our sample. W44 is a molecular cloud interacting with a supernovae remnant (SNR), which provides a promising environment for production of HNCO. Both S140 and DR21S are massive star-forming regions with strong HC3N emission (Li et al. 2012). Table 1 lists information on the sources that have been mapped in this study. The distances were determined from an extensive literature search. Trigonometric parallax distances were used, if available, or otherwise photometric distances or kinematic distances based on rotation curve of Fich, Blitz & Stark (1989) were used.

The data processing was conducted using Gildas package1. A least-square fit to baselines in the spectra was carried out with the first-order polynomial. The baseline slopes were removed for all the sources. The individual spectra were averaged, and the resulting spectra were Hanning-smoothed to improve the signal-to-noise ratio (S/N) of the data. The line parameters were obtained by Gaussian fitting. We express the results in the unit of main beam brightness temperature (Tmb) assuming the main beam efficiencies of 0.5 (Chen et al. 2010, 2012).

To search for the mid-infrared (MIR) emission of young stellar objects (YSOs), we used the Midcourse Space Experiment (MSX) Galactic plane survey between 6 and 25 μm at 18′′ spatial resolution (Price et al. 2001). For S140, where no MSX data are available, the AKARI/IRC source catalog (Murakami et al. 2007; Onaka et al. 2007) of the AKARI all-sky survey is used to search for MIR emission (9 and 18 μm) with a spatial resolution of about 6′′. To search for the far-infrared (FIR) emission of YSOs, we used the AKARI/FIS bright source catalog (Kawada et al. 2007) centered at 65, 90, 140 and 160 μm, with spatial resolutions ranging from 37′′ to 61′′.

3. Observing results

thumbnail Fig. 1

HNCO 505 − 404 and C18O 1−0 spectra of S140 and W44 at (0, 0). Identification of the transitions is given to the right of each lines.

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Figure 1 presents the HNCO 505−404 and C18O 1−0 spectra of S140 and W44. Both of them are newly detected in HNCO. The derived line parameters of HNCO 505−404 and C18O 1−0 at (0, 0) are presented in Tables 2 and 3, respectively, including main beam brightness temperature (Tmb), integrated line intensity (dν), LSR velocity, and full-width half-maximum (FWHM) linewidth of HNCO 505−404 emission. All the parameters are determined from Gaussian fitting. Our derived parameters are similar but not identical to results of Zinchenko et al. (2000) because their main beam size is only two-thirds of our observations.

Table 2

Observational results of HNCO 505 − 404 transition.

Table 3

Observational results of C18O 1−0 transition.

HNCO 505−404 maps were obtained for nine sources. In Fig. 2, we present contour maps of HNCO 505−404 and C18O overlayed on the MSX 21.3 μm image on a linear scale. Except for Orion KL, eight other sources were mapped with the same telescope and toward the same positions with HNCO in the HC3N 10−9 transition (90.979 GHz) (Li et al. 2012). The HC3N contour maps were also overlayed for comparison. The MIR and FIR sources from AKARI catalogs are marked, along with the positions of water masers. Compared with the widespread C18O distribution, HNCO emission shows a rather compact distribution, with emissions concentrating on water masers. The HNCO emission peaks are offset from the C18O in Orion A, W51M, and S158. The recently improved resolution of infrared observations allow us to better understand the relationship between HNCO clumps and infrared sources. Nearly all the HNCO clumps are spatially coincident with MIR or FIR sources, which usually trace the warm dust emission of the cocoons of the OB star central exciting source (Crowther & Conti 2003). The morphology and spatial distribution of HNCO 505−404 emission are similar to the HC3N 10−9 transition, which is a dense gas tracer (e.g. Li et al. 2012). These results imply that HNCO emission is emitted from a small volume of warmer and denser gas located nearer to the embedded objects than the C18O emission.

thumbnail Fig. 2

Contour maps of HNCO (green solid line), HC3N (blue dotted line), and C18O (red dashed line) superimposed on the MSX 21.3 μm image for observing sources. The contour levels are 30%, 50%, 70%, and 90% of the map peak, reported in Table 3 (see Col. 3 for HNCO, Col. 7 for C18O, see Col. 3 in Table 3 of Li et al. (2012) for HC3N). The heavy lines represent 50% of the map peak. “” is used to mark the position of the AKARI FIR source. “∗” is used to mark the position of the AKARI MIR source in S140. Red filled squares are used to mark the position of water masers. The FWHM beam size for molecular lines (the big, red circle) and mid-infrared (the small, black circle) observations are shown in the lower left of the maps. (A color version of this figure is available in the online journal.)

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The size of clouds is characterized by using the beam deconvolved angular diameter and linear radius of a circle with the same area as the half peak intensity: (1)(2)where A1/2 is the area within the contour of half peak intensity, θbeam the FWHM beam size, and D the distance of the source. The deconvolved linear FWHM sizes and angular diameter of HNCO and C18O clouds are presented in Tables 2 and 3, respectively. The deconvolved linear sizes of HNCO clouds range from 0.05 to 2.88 pc, where the lowest value comes from Orion KL, and the highest comes from W51M. The HNCO clouds are significantly smaller than those of C18O for nearly all the sources except W75OH, in which the size of HNCO cloud is comparable to that of C18O.

Below we give comments on individual sources.

G121.30+0.66. We detected HNCO 505−404 emission with Tmb of 0.24 K. The HNCO clump has a centrally condensed structure, and the emission peak coincides with the MIR emission.

Orion KL. Orion KL is the closest massive star-forming region (414 pc; Menten et al. 2007). Several molecular components, such as the hot core, compact and extended ridges, and several luminous IR sources or radio sources are associated with Orion KL. The nature of the sources responsible for the luminous IR emission is still poorly known and much debated. Other than being powered by an embedded central heating source (e.g., Kaufman et al. 1998), an interesting alternative explanation for this region’s energetics is a protostellar merger event that released a few times 1047 erg of energy about 500 years ago (Bally & Zinnecker 2005; Zapata et al. 2011; Bally et al. 2011). Our observations show that the HNCO peak was displaced from the C18O peak and is centered on MSX 21.3 μm emission peak. The HNCO/C18O intensity ratio at (0, 0) is close to unity, while there are fewer than 0.25 in other sources. As shock enhancement of HNCO has been detected in L1157 molecular outflow (Rodríguez-Fernández et al. 2010), the relatively high HNCO/C18O intensity ratio seems to favor the idea that the chemistry in Orion KL should be shock-driven with much higher temperature than the chemistry in a typical hot core (Favre et al. 2011), and should produce high abundances of HNCO (Zinchenko et al. 2000).

W44. W44 is an SNR with shell-like morphologies (Dubner et al. 2000; Jones et al. 1993). It is adjacent to a giant molecular cloud that it is suspected of interacting with (Wootten 1981; Denoyer 1983). It contains OH 1720 MHz masers that are attributed to dense, shocked gas (Lockett et al. 1999). The MSX image of W44 consists of an SNR and dust emission heated by embedded YSOs. If it is correct that HNCO is enhanced in the presence of shocks due to its injection into the gas phase from the grain mantles (Zincheko et al. 2000), HNCO would be expected in molecular clouds interacting with SNRs. Strong HNCO emission was detected with Tmb of 0.58 K. The HNCO emission concentrates mainly around the YSO. More sources should be observed to investigate whether HNCO is abundant in molecular clouds interacting with SNRs.

W51M. This is a strong HNCO 505−404 emission source with Tmb of 0.48 K. Two cores were seen in HNCO clouds. The western peak, W51m-west, is stronger than the eastern peak in HNCO emission, but weaker than the eastern peak in HC3N and C18O emission. W51m-west is centered on the AKARI FIR sources, while the eastern peak is coincident with water masers and near to MIR sources.

ON1. We detected HNCO 505−404 emission with Tmb of 0.22 K. Similar to G121.30+0.66, the HNCO emission morphology has a centrally condensed structure, with the emission peak coinciding with the MIR emission.

DR21S. DR21S is a strong HC3N emission source in Li et al. (2012). This is a massive clump in the Cygnus X region. HNCO was detected in DR21S with Tmb of 0.12 K. The HNCO emission peak is near to the MIR emission peak position.

W75OH. W75OH is also a massive clump in the Cygnus X region, which hosts three massive dense cores at an early stage in their evolution. We detected HNCO 505 − 404 emission with Tmb of 0.38 K. Two HNCO cores were seen in W75OH. The southern peak is brighter than the northern peak in HNCO emission. The southern peak is near the AKARI FIR source, while the northern clump, W75OH-north, is near the strong MIR emission. Csengeri et al. (2011) report detecting velocity shears and low-velocity shocks in W75OH. Since HNCO is thought to be enhanced by low-velocity shocks, high-resolution observations of this source are expected to test this hypothesis by comparing the spatial distribution of HNCO and N2H+ convergent flows.

S158. S158 is also referred to as NGC 7538 (Moreno & Chavarría-K 1986). We detected HNCO 505 − 404 emission with Tmb of 0.44 K. Two clumps were seen in HNCO and HC3N clouds, with the stronger one coincident with the AKARI MIR source.

S140. S140 shows strong HC3N emission in Li et al. (2012). HNCO 505 − 404 was marginally detected with Tmb of 0.16 K. Two HNCO cores were seen with low S/N. One of them is associated with the AKARI MIR sources, while another one is starless.

4. Discussions

Churchwell et al. (1985) mapped 14 transitions of HNCO toward Sgr B2. Analysis of population distribution indicates the FIR radiation from warm dust is likely to be responsible for the excitation of HNCO. They proposed that the most likely excitation mechanism of HNCO in Sgr B2 was radiative rather than collisional. In this case HNCO was a good probe of the FIR radiation field but not of gas properties such as density and kinetic temperature. The physical environment of hot cores differ from the Galactic center, in which the gas density (103 − 104 cm-3) is much lower than in hot cores, thus the dominant excitation mechanism might be different. Zinchenko et al. (2000) explain the K-1 > 0 ladders of Orion KL with radiative excitation. For the K-1 = 0 transitions with a larger source size, they propose that the radiative excitation would become inefficient, and the collisional excitation may dominate.

thumbnail Fig. 3

Upper left: integrated intensities of HNCO 505 − 404 vs. HC3N 10−9; upper right: linewidth of HNCO 505 − 404 vs. HC3N 10−9; lower left: size of HNCO 505 − 404 vs. HC3N 10−9. The dotted line has a slop of 1; lower right: LSR velocity difference between HNCO 505 − 404 and HC3N 10−9 vs. LSR velocity difference between HNCO 505 − 404 and C18O 1−0. The source names were also labeled in the figure.

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In Fig. 3, we present plots that compare integrated intensities, linewidths, FWHM size, and LSR velocities of HNCO 505 − 404 and HC3N 10−9 (Li et al. 2012)2. The source names are also labeled in the figure. Good correlation is found for integrated intensities (rcorr = 0.63) and FWHM sizes (rcorr = 0.98) of HNCO and HC3N clumps. The linewidths of HNCO clumps also agree well with HC3N clumps (rcorr = 0.92), with the only exception being S140, which is possibly caused by the marginal detection of HNCO (<3σ) in this source (see Fig. 1). The velocity difference between HNCO and HC3N are within 3σ for all sources except W51M, which is possibly related to the complex structure of this source. The obvious correlations between line parameters, as well as the similar morphology of HNCO and HC3N clumps, suggest that these two molecule lines trace a similar volume of gas. The critical density of HC3N 10−9 is 106 cm-3 (e.g., Chung et al. 1991), which is comparable to the critical density of HNCO 505−404. The dominant excitation mechanism for HC3N is collisional excitation, thus collisional excitation is likely to be the dominant excitation mechanism for HNCO K-1 = 0 emission in galactic massive star-forming regions.

In sources mapped by Zinchenko et al. (2000), HNCO emission peaks are significantly displaced from any known IR sources. However, observations at that time were severely limited by the low resolution of infrared observations. With the greatly improved spatial resolution and sensitivity of IR observations, we found that nearly all the HNCO clumps in our observations are associated with MIR or FIR emission. Thus HNCO should be produced in hot gas. This is consistent with Bisschop et al. (2007), in which HNCO was classified as “hot” molecules based on rotation diagram analysis. Sanhueza et al. (2012) observed a sample of infrared dark clouds (IRDCs) and find that HNCO profiles show no evidence of being a tracer of shocks in most of the sources, but only in two sources, which represent 10% of the sources with HNCO detection, the HNCO spectrum presents a blue wing that is also observed in SiO. For results present here, we found possible shock enhancement of HNCO in two sources (Orion KL and W75OH). We conclude that HNCO is produced in a warm environment, while shock could enhance the HNCO abundance.

5. Summary and prospects

In this paper we present HNCO 505−404 mapping observations of nine massive star-forming regions with the PMODLH 13.7 m telescope. The C18O 1−0 maps of these sources are obtained simultaneously. We used MSX and AKARI satellite data to search for infrared emission from YSOs and investigated the spatial relationship between HNCO clumps and infrared sources.

We found good correlations between line parameters of HNCO and HC3N emission, which imply similar excitation mechanisms for these two molecules. The HNCO clumps are much smaller than the C18O clumps and comparable to the HC3N clumps. Thus collisional excitation is likely to be the dominant excitation mechanism for HNCO 505 − 404 transition in galactic massive star-forming regions.

We found that HNCO emission is compact and centrally condensed. Nearly all the HNCO clumps show signs of embedded infrared emission, supporting the idea that HNCO is a “hot” molecule. Future high-resolution observations of W75OH are expected to test the hypothesis that HNCO is enhanced by low-velocity shock by comparing the spatial distribution of HNCO and N2H+ convergent flows.

PMO is carrying out a CO, 13CO, and C18O 1−0 survey toward the Galactic plane with the DLH 13.7 m telescope. HNCO 505 − 404 could be observed simultaneously within the 1 GHz band, which would enable us to obtain a spatial distribution of strong HNCO emission in the Galactic plane and to better study the chemical properties of HNCO.


2

We obtain Tmb of HC3N by assuming the main beam efficiencies of 0.5.

Acknowledgments

We thank the refree for the helpful comments and constructive suggestions. This work is partly supported by the China Ministry of Science and Technology under the State Key Development Program for Basic Research (2012CB821800), and partly supported by the Natural Science Foundation of China under grants of 11103006, 10833006 and 10878010. We would like to thank the Key Laboratory of Radio Astronomy, Chinese Academy of Sciences. We are very grateful to the staff of Qinghai Station of Purple Mountain Observatory for their assistance with the observations and data reductions. This research made use of data products from the Midcourse Space Experiment. This research is based on observations with AKARI, a JAXA project with the participation of ESA. This research made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

References

All Tables

Table 1

Source list.

Table 2

Observational results of HNCO 505 − 404 transition.

Table 3

Observational results of C18O 1−0 transition.

All Figures

thumbnail Fig. 1

HNCO 505 − 404 and C18O 1−0 spectra of S140 and W44 at (0, 0). Identification of the transitions is given to the right of each lines.

Open with DEXTER
In the text
thumbnail Fig. 2

Contour maps of HNCO (green solid line), HC3N (blue dotted line), and C18O (red dashed line) superimposed on the MSX 21.3 μm image for observing sources. The contour levels are 30%, 50%, 70%, and 90% of the map peak, reported in Table 3 (see Col. 3 for HNCO, Col. 7 for C18O, see Col. 3 in Table 3 of Li et al. (2012) for HC3N). The heavy lines represent 50% of the map peak. “” is used to mark the position of the AKARI FIR source. “∗” is used to mark the position of the AKARI MIR source in S140. Red filled squares are used to mark the position of water masers. The FWHM beam size for molecular lines (the big, red circle) and mid-infrared (the small, black circle) observations are shown in the lower left of the maps. (A color version of this figure is available in the online journal.)

Open with DEXTER
In the text
thumbnail Fig. 3

Upper left: integrated intensities of HNCO 505 − 404 vs. HC3N 10−9; upper right: linewidth of HNCO 505 − 404 vs. HC3N 10−9; lower left: size of HNCO 505 − 404 vs. HC3N 10−9. The dotted line has a slop of 1; lower right: LSR velocity difference between HNCO 505 − 404 and HC3N 10−9 vs. LSR velocity difference between HNCO 505 − 404 and C18O 1−0. The source names were also labeled in the figure.

Open with DEXTER
In the text

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