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Issue
A&A
Volume 538, February 2012
Article Number A12
Number of page(s) 11
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201117608
Published online 26 January 2012

© ESO, 2012

1. Introduction

The Orion Molecular Cloud 1 (OMC-1) is a complex region of the interstellar medium (ISM) stretching over more than 2.4 pc (20′, roughly north-south) on the sky (Kutner et al. 1976). Its densest part, toward which the Great Orion Nebula (M42), a classical compact “blister” Hii region, and its associated Orion Nebular Cluster (ONC) appear in projection, is one of the best-studied regions in astronomy. It is a test bed for studies of (proto)stars and clusters and the formation of low-, intermediate-, and high-mass stars (for a review of this region, see O’Dell 2001; O’Dell et al. 2008). Much of this region’s prominence is due to its distance of just 414 ± 7 pc (Menten et al. 2007), which makes the ONC and OMC-1 the closest regions of recent (few million years old) and ongoing high-mass star formation. In the following, we shall use the term “OMC-1 core” or even just OMC-1 for the roughly 8′ × 8′ – or 1 pc2 – sized dense molecular cloud region, which is located closely (0.1−0.2 pc) behind M42 and most of the stars in the ONC (see Zuckerman 1973; Genzel & Stutzki 1989, the latter give a comprehensive overview of this region and its phenomena).

The OMC-1 core region may be divided into three main zones, all of which show bright (sub)millimeter wavelength emission from warm dust and molecular gas: the Becklin-Neugebauer/Kleinmann-Low (BN/KL) region, Orion South (OMC-1S or Orion-S) and the Orion Bar.

The Orion BN/KL and Orion South regions are considered to be “twin” high-mass star forming regions because they have similar masses (≲100 M⊙) and bolometric luminosities (104−105 L⊙, Mezger et al. 1990; Drapatz et al. 1983) and show comparable levels of star-forming activity (O’Dell et al. 2008). The BN/KL region harbors the eponymous “hot core” (Masson et al. 1984), which was (and commonly still is) taken to be the prototype of the hot dense regions observed around many newly formed stars (see, e.g., Kurtz et al. 2000). An interesting alternative explanation for this region’s energetics (other than being powered by an embedded central heating source) 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).

The Orion Bar is a well-described photon-dominated region (PDR) located at the side of the OMC-1 core, facing M42, which is heated and partially ionized by far-ultraviolet (FUV) photons from the young massive stars (most of them from θ1 C, a spectral type O5–O7 star) that form the “Trapezium” at the center of the ONC (see, e.g., Hollenbach & Tielens 1997; Walmsley et al. 2000). In addition, the Orion Bar appears to be located at the edge of the Hii blister tangential to the line of sight.

On giant molecular cloud (GMC) scales, low-rotational level (J) 12CO emission (commonly from the J = 1−0 line) of the ambient, low density gas is usually used to trace the mass of the molecular ISM under a range of assumptions (Bloemen et al. 1984; Dame & Thaddeus 1985). However, in massive star forming regions with much higher densities and temperatures, observations of the submillimeter and far-infrared (FIR) wavelength mid- or high-J transitions of 12CO and its 13C and 18O isotopologues are required for determinations of the gas temperature and density, usually in conjunction with Large Velocity Gradient (LVG) radiative transfer and PDR modelings.

Much of the 12CO emission from the OMC-1 core has been proposed to arise from the neutral and partially ionized back side of the Hii blister, i.e., the PDR (Genzel & Stutzki 1989). The pioneering submillimeter observations of the 12CO J = 7−6, 6−5 and 13CO J = 7−6 transitions by Schmid-Burgk et al. (1989) and Graf et al. (1990) revealed high density gas (n ≥  104 cm-3) and elevated temperatures (T ≥  50 K) in all parts of the OMC-1 core region. Observations of the even more highly excited 12CO J = 9−8 line by Marrone et al. (2004) with the Receiver Lab Telescope (84′′ resolution) showed that the hot and broad velocity emission in the line profiles arises mainly from the BN/KL region, while much of the narrower (~3−6 km s-1 wide) emission arises from the PDR. This was also observed in the spectra of the 12CO J = 9−8 transition taken by Kawamura et al. (2002), which indicated warm molecular gas from the extended “quiescent ridge” region, north of Orion BN/KL. Furthermore, the observations of 12CO J = 7−6 and J = 4−3 made by Wilson et al. (2001) together with LVG modeling suggest that the broad line widths of these lines from the BN/KL region are probably due to shock heating, while most of the narrow line extended emission can be explained by PDR models. Recently, Furuya & Shinnaga (2009) presented the BN/KL region images in the 12CO J = 7−6 line.

Clearly, multi-transition, high angular resolution imaging studies are needed to disentangle the relative contributions of shock and radiative excitation to the CO emission, and to identify possible energy sources. All of the spectral line observations described above report either single pointing toward selected positions in OMC-1 or limited mapping of special areas such as the Bar (Lis et al. 1998) or the BN/KL region (Furuya & Shinnaga 2009). The only exceptions are the 12CO J = 7−6 map by Schmid-Burgk et al. (1989) and the 12CO J = 4−3 and J = 7−6 maps by Wilson et al. (2001). However, the first of these, which covers the whole OMC-1 core, was taken with the poor resolution of the Kuiper Airborne Observatory.

Here we present the first high (better than 10″) resolution large scale maps of the OMC-1 core using the 2 × 7 pixel submillimeter CHAMP+ array receiver on the APEX telescope. Seven of the receiver units cover the 620 to 720 GHz frequency range and seven the 780 to 950 GHz range, allowing imaging of two lines simultaneously. As we shall see in Sect. 2, we imaged the OMC-1 core in 12CO J = 6−5 plus J = 7−6 in one run and 13CO J = 8−7 plus C18O J = 6−5 in another. The fast mapping afforded by the 14 unit instrument resulted in a superior image consistency. The mid-J CO data were complemented by the maps of the 13CO and C18O J = −2 lines.

2. Observations

Table 1

Observational parameters.

The observations were performed in 2007 November and 2008 July with the 12 m APEX telescope on Llano de Chajnantor in Chile (Güsten et al. 2006)2.

Large-scale maps of the 13CO and C18O J = 3−2 lines were obtained in the on-the-fly (OTF) observations with the APEX-2a facility receiver (Risacher et al. 2006). This double sideband (DSB) heterodyne receiver provided receiver DSB temperatures of 60 K and typical system noise from 100 to 200 K. Maps of size  ~300″ × 350″ were centered on the Orion BN source (, ). For all spectra, position switching to an emission-free reference position at offsets ( − 500′′, 0′′) was used. The Fast Fourier Transform Spectrometers (FFTS, Klein et al. 2006) were operated with a 1000 MHz bandwidth and a  ~0.1 km s-1 spectral resolution.

The submm observations of the high-J CO lines were performed with the dual-color heterodyne array CHAMP+ (Kasemann et al. 2006; Güsten et al. 2008), operating 2 × 7 elements simultaneously in the 450 μm and 350 μm atmospheric windows. We observed two setups: in 2007 November the 12CO J = 6−5 and J = 7−6 lines were mapped simultaneously, while in the second coverage in 2008 July, the 13CO J = 8−7 and C18O J = 6−5 transitions were observed in parallel. The MPIfR Array Correlator Spectrometer (MACS) was used as the backend during the former run, and the central pixels of each array were always connected to the FFT spectrometers mentioned above. Resolutions were  ~0.42 km s-1 for MACS and  ~0.05 km s-1 for the FFTS. For the 2008 July observations, MACS had been replaced with a new FFTS array backend, operating for each pixel with a 2 × 1.5 GHz bandwidth and 8192 channels per FFTS. We mapped the OMC-1 core region in a size of 6′ × 8′ (0.72 × 0.96 pc2) in OTF slews in the right ascension direction (each  ~ 3′ long), dumping data every 4″. Subsequent scans were also spaced by 4″ in declination so that the Nyquist sampled maps for each pixel were obtained. The angular resolution varies with frequency between (881 GHz) and (658 GHz). The same reference position at offsets ( − 500″,0″) was used. The on-source integration time per dump and per pixel was 1 second only. With the above over-sampling strategy, all pixels of CHAMP+ covered a given grid position at least once.

Data calibration was performed every  ~10 min with cold and ambient temperature loads. The data were processed with the APEX online calibrator, assuming an image sideband suppression of 10 dB. The CHAMP+ observing parameters, the telescope beam sizes, and typical values for the Precipitable Water Vapor (PWV) are summarized in Table 1. Typical single sideband (SSB) system temperatures were well below 2000 K and 4000 K for the low- and high-frequency array of CHAMP+, respectively.

Telescope pointing was established by 13CO and C18O J = 3−2 line pointings every hour on nearby R Lup for the APEX-2a observations, resulting in an accuracy of  ~2″. In the shorter-wavelength submm windows, pointing is more difficult because of the much reduced system sensitivities and the paucity of suitable pointing sources, strong enough for peak-up on their continuum and/or line emission. During the 2007 CHAMP+ observations, o Cet was used as a line pointing source in the 12CO J = 6−5 mapping, but with an on-sky distance of  ≈50° to the OMC-1 core, the absolute pointing on the latter was not accurately corrected. The wider IF processors and backends that became available in 2008 improved the situation significantly: simultaneous observations of the 12CO J = 6−5 line and the CH3CN Jk = 369 − 359 multiplet (near 660.6 GHz) became possible in the low-frequency array of CHAMP+. We assume that the highly rotationally excited (lower energy level E/k = 133 K above the ground state) methyl cyanide line emission does peak on the Orion Hot Core proper, as defined by the centroid of 92 GHz CH3CN Jk = 5k − 4k emission determined with the BIMA interferometer, i.e., , (Wilner et al. 1994). Thereby, we established a pointing reference local to the OMC-1 core complex. While those small pointing maps toward the Orion Hot Core were made, the 12CO J = 7−6 transition in the high-frequency array was observed in parallel, and provided the link to the 2007 observations of the main CO isotopologues. The central pixels between the two arrays are co-aligned (better than one arcsecond). In practice, we shifted the 2007 maps such that the 12CO J = 7−6 line wing emission from  − 50 to  − 30 km s-1 match the pointed 2008 observations. The overall accuracy in the higher-J CO observations with CHAMP+ is estimated to be  ≲ 4″.

All spectra were converted to the main beam brightness temperature unit, (ηMB = Beff/Feff), using a forward efficiency (Feff) of 0.95 and beam coupling efficiencies (Beff) as determined toward Jupiter (Table 1). The latter is motivated by the average size of the emission in velocity space (Fig. 4). All data were reduced using the standard procedures in the CLASS and GREG programs from the GILDAS package3.

thumbnail Fig. 1

The OMC-1 core 12CO J = 6−5 integrated intensity [−25,  + 30] km s-1 image overlaid with the J = 7−6 integrated intensity [− 25,  + 30] km s-1 contours running from 300 to 1000 K km s-1 in steps of 100 K km s-1, and the subsequent contours are plotted from 1200 to 7200 K km s-1 in steps of 600 K km s-1. The 12CO J = 6−5 (black) and 12CO J = 7−6 (red) spectra are shown for Orion BN, Orion South, Orion Bar, and Orion East. Both images have been smoothed using Gaussian profiles (a width of 91 for 12CO J = 6−5 and 78 for 12CO J = 7−6) from their original Nyquist-sampled images. The black stars mark the positions of the five Trapezium stars (θ1 Ori A, B, C, D, and E). The original beam sizes are shown at the bottom left of the image for 12CO J = 6−5 and J = 7−6 in red (96) and white (82), respectively.

Table 2

CO isotopologues measurements in the selected sources of the OMC-1 core.

thumbnail Fig. 2

The OMC-1 core 13CO J = 8−7 integrated intensity [+5,  +15] km s-1 image overlaid with the C18O J = 6−5 [+4,  +12] km s-1 contours running from 15 to 65 K km s-1 in steps of 10 K km s-1, and the subsequent contours are plotted from 80 to 200 K km s-1 in steps of 30 K km s-1. The 13CO J = 8−7 (black) and C18O J = 6−5 (red) spectra are shown for Orion BN, Orion South, Orion Bar, and Orion East at the same positions as in Fig. 1. Both images have been smoothed using Gaussian profiles (a width of 71 for 13CO J = 8−7 and 95 for C18O J = 6−5) from their original Nyquist-sampled images. The black stars mark the positions of the five Trapezium stars. The original beam sizes are shown at the bottom left of the image for 13CO J = 8−7 and C18O J = 6−5 in red (75) and white (101), respectively.

thumbnail Fig. 3

The OMC-1 core 13CO J = 3−2 integrated intensity [+5,  +15] km s-1 image overlaid with the C18O J = 3−2 [+4,  +12] km s-1 contours running from 24 to 80 K km s-1 in steps of 8 K km s-1. The 13CO J = −2 (black) and C18O J = 3−2 (red) spectra are shown for Orion BN, Orion South, Orion Bar, and Orion East at the same positions as in Fig. 1. The black stars mark the positions of the five Trapezium stars. The beam size of 211 is shown at the bottom left of the image.

thumbnail Fig. 4

Low-velocity channel maps of the 12CO J = 6−5 line with the J = 7−6 contours in the OMC-1 core. The contours are running from 50 to 200 K in steps of 25 K. Orion BN is located at the offset (0′′, 0′′), and white stars mark the positions of the five Trapezium stars.

thumbnail Fig. 5

Upper panel is the 12CO J = 6−5 to C18O J = 6−5 ratio channel maps (θHPBW = 10″) at different velocities from 6 to 11 km s-1 in the OMC-1 core. The edge cut and empty area are due to different image sizes and data blanking (2 σ level of the C18O J = 6−5 temperature). Lower panel is the corresponding C18O J = 6−5 to J = 3−2 ratio channel maps (θHPBW = 20″). The black arrow in the 10 km s-1 image indicates the ratio gradient across the Orion Bar (see text). The Orion Hot Core (HC) position is marked, and Orion BN is located at the offset (0′′, 0′′). The positions of the five Trapezium stars are marked in each map.

3. Results and discussion

3.1. Large scale maps – overall morphology

Figure 1 shows  ≈ 6′ × 8′-size maps of the OMC-1 core in the intensity of the 12CO J = 6−5 and J = 7−6 lines integrated over the local standard of rest (LSR) velocity interval from  −25 to  +30 km s-1. The emission distributions of both lines, clearly resolved in our observations, resemble each other closely and are consistent with the JCMT/SCUBA 850 μm image obtained by Johnstone & Bally (1999), where several regions are discernible. The most prominent of which are: (a) the emission maximum centered on the BN/KL region, (b) a secondary maximum coinciding with Orion South (OMC-1S), (c) a region  ≈ 3′ east of the Trapezium, which is known as Orion East (OMC-1 E; see below), and (d), the prominent straight Orion Bar PDR. The Bar actually appears connected to the Orion South region, forming a mirrored letter L shape. Furthermore, there are two minor maxima at N and NNE of the BN/KL peak position.

Apart from their spatial distributions, the regions above can be distinguished by their centroid LSR velocities, VLSR, and their line widths, ΔV. The Orion BN/KL region displays a spectacularly broad line up to 200 km s-1 wide, centered around 6 km s-1 with a narrow (ΔV ≈ 5 km s-1) feature at VLSR ≈ 8 km s-1 superposed. Orion South is characterized by  ~6 km s-1 wide lines, with higher values at positions with outflow activities. The 12CO J = 6−5 and J = 7−6 lines observed toward the Orion Bar have FWHM widths of  ~4 km s-1 and are centered around VLSR = 10 km s-1. The Orion East region has previously been mapped by Houde et al. (2004) and Herrmann et al. (1997), but otherwise has received relatively little attention. The observed line widths and LSR velocities of Orion East, as well the high intensity of its CO emission (TMB ≳ 160 K) clearly demonstrate its PDR nature. All our measurements are in good agreement with the results of Schmid-Burgk et al. (1989), Graf et al. (1990), Wilson et al. (2001), Kawamura et al. (2002), and Marrone et al. (2004). The measurements of the CO isotopologues in the OMC-1 core are summarized in Table 2.

Figure 2 shows the integrated intensity of 13CO J = 8−7 and C18O J = 6−5 maps in a similar area as 12CO but in a smaller range of radial velocities (from  +5 to  +15 km s-1 for 13CO and from  +4 to  +12 km s-1 for C18O). Both maps also show a good correspondence but with the integrated emission from C18O J = 6−5 fainter than 13CO J = 8−7. The two isotopologues have fainter and less broader lines compared with the 12CO line emission which is expected for optically thick lines (see the spectra in Figs. 1 and 2). The Orion Bar is well resolved in both lines and also shows a sharp edge. In the mid-J images (Figs. 1 and 2), the strong peak in the center of the Bar is also seen in the recent Herschel/SPIRE high-J12CO and 13CO data obtained by Habart et al. (2010). In addition, the line widths of 13CO J = 8−7 and C18O J = 6−5 toward all four main regions are narrow, even in the outflow zones, i.e., Orion BN/KL and South.

thumbnail Fig. 6

Velocity channel maps (5, 10, and 15 km s-1) of the 12CO J = 7−6 to J = 6−5 ratio (θHPBW = 10″) in the OMC-1 core. The 10 km s-1 image is overlaid with the 12CO J = 6−5 contours as shown in Fig. 1. The empty area are due to data blanking (3 σ level of the 12CO J = 7−6 temperature). The dashed-lines indicate that these ratio gradients are likely caused by outflows or filaments. Black triangles mark the Orion Hot Core position.

thumbnail Fig. 7

a) The OMC-1 core C18O J = 3−2 to 13CO J = 3−2 integrated intensity ratio map (θHPBW = 20″) overlaid with the 12CO J = 6−5 contours (grey) as shown in Fig. 1. The black contour represents the C18O/13CO abundance ratio of 0.122 (60/490). b) The OMC-1 core C18O J = 3−2 to J = 6−5 integrated intensity ratio map (θHPBW = 20″) overlaid with the 12CO J = 6−5 contours as shown in Fig. 1. Black triangles mark the Orion Hot Core position.

The 13CO J = 3−2 and C18O J = 3−2 integrated intensity maps are shown in Fig. 3. These images were made in a similar area as higher-J12CO and in a range of radial velocities equal to 13CO J = 8−7 and C18O J = 6−5. The C18O J = −2 emission is only present where the 13CO J = 3−2 emission is strong. The 13CO and C18O line profiles at this rotational transition toward the Orion BN/KL and South regions are very broad and show well-defined wings. The southwestern part of the Bar is especially strong in this lower-J transition for both isotopologues, which is also shown in the Herschel/SPIRE high-J12CO and 13CO images reported by Habart et al. (2010).

3.2. Velocity structure

The 12CO J = 6−5 and J = 7−6 velocity channel maps are shown in Fig. 4. These maps reveal a well-resolved and complex structure of the OMC-1 core at ambient velocities as well as higher radial velocities. The outflows that are located mainly at Orion BN/KL, Orion South, and toward the north of Orion BN/KL are clearly seen in these images (see Zapata et al. 2006, 2009), e.g., VLSR = 0−6 km s-1 for the BN/KL region and VLSR = 12−14 km s-1 for the Orion South region. Besides, no clear north-south velocity gradients across the OMC-1 core region are observed.

The compact and warm structures toward the BN/KL region traced by the 12CO J = 6−5 and J = 7−6 lines, which display a very broad range of velocities (up to about  ± 100 km s-1), are part of the enigmatic molecular outflow that seems to be produced by a violent explosion during the disruption of a massive young stellar system (Bally & Zinnecker 2005; Zapata et al. 2009; Bally et al. 2011). Our observations reveal that some faint filamentary structures are very likely associated with the high-velocity 12CO bullets (Peng et al., in prep.) reported by Zapata et al. (2009).

3.3. Line ratios

In Figs. 5–7, we show the line intensity ratio maps between the CO isotopologues and the different rotational transitions.

From the image of the 12CO to C18O J = 6−5 ratio (Fig. 5 upper panel), we can see the spatial distribution of high optical depth regions at different velocities. The north-south dense ridge from Orion BN/KL to Orion South is clearly seen at 9 km s-1. This high optical depth ridge is similar to the filamentary structures of the NH3 emission (Wiseman & Ho 1996, 1998), but has some morphological differences in the north of the Orion BN/KL region. Besides, the straight shape of the Orion Bar is pronounced at VLSR = 10−11 km s-1, where three high optical depth regions are seen at the two ends and center of the Bar at VLSR = 10 and 11 km s-1, respectively. In the lower panel of Fig. 5, the C18O J = 6−5 to J = −2 ratios around 10 − 11 km s-1 show a gradient in the Orion Bar which goes in the direction from the Trapezium stars. It indicates that C18O J = 6−5 is strongly excited at the edge of the Bar by UV photons from the Trapezium stars.

Figure 6 shows the ratio between 12CO J = 7−6 and J = 6−5 at three different radial velocities of 5, 10, and 15 km s-1. The three panels show clear variations across the integrated line emission. It is interesting to note that at the cloud velocity of 10 km s-1, there are high 7−6/6−5 ratios located very close to the position of the Trapezium stars toward Orion BN/KL. These gradients are likely produced by those massive stars that heat the molecular cloud, causing the stronger 12CO J = 7−6 line emission compared with the J = 6−5 line. A gradient is also seen in the Orion Bar which goes in the perpendicular direction to the Trapezium stars (Fig. 6 at VLSR = 10 km s-1), and some patches with higher 12CO J = 7−6 brightness temperatures are seen inside the Bar or behind the ionization front. Some horizontal and vertical strips, artifacts from the OTF mapping, are also seen in the Orion Bar and South regions. These artifacts can affect the intensity ratio by  ~ 14% given the calibration errors of 10% for both 12CO J = 6−5 and J = 7−6. However, some structures seen in the 12CO J = 7−6 to J = 6−5 ratio maps, e.g., the dashed-lines in Fig. 6, are unlikely due to the calibration error or scanning strips during the observation, and seem to be footprints of filaments or outflows. Some similar filament structures have been noticed in the dust continuum emission observed by Johnstone & Bally (1999).

thumbnail Fig. 8

a) The C18O column density image of the OMC-1 core overlaid with black contours of the JCMT 850 μm dust continuum image. The 850 μm image is taken from the JCMT data archive. The contours represent 0.2%, 0.5%, 1%, 2%, 4%, 7%, 20%, 40%, 60%, and 80% of the peak intensity (87 Jy beam-1). b) The excitation temperature image for 12CO and C18O overlaid with the 35 mm VLA-GBT continuum emission (Dicker et al. 2009) in black contours running from 15% to 95% in steps of 8% of the peak intensity (0.94 Jy beam-1). The stars mark the positions of the five Trapezium stars, and black triangles represent the position of the Orion Hot Core.

In the ratio map of C18O J = −2 to 13CO J = −2 shown in Fig. 7, a clear elongated and dense ridge is seen in the north-south direction, where the 13CO J = −2 intensity is weaker in the Orion BN/KL region than at Orion South and the north of BN/KL. Therefore, the lower CO column density derived from the lower C18O J = −2 to 13CO J = −2 ratio toward Orion BN/KL may be misleading. Even though the 13CO J = −2 line is optically thick and may be self-absorbed, this will result in a higher C18O J = −2 to 13CO J = −2 ratio instead of a lower ratio in Orion BN/KL. As Goldsmith et al. (1997) pointed out, the column density calculated using low-J C18O lines is accurate except for the temperature  ≥ 150 K, which is the case in the OMC-1 core. Hence, higher-J CO observations are critical to determine the CO column density in the OMC-1 core region. In addition, already shown in Fig. 6, the low C18O J = −2 to J = 6−5 ratios indicate that the C18O J = 6−5 intensity is strongly enhanced toward the Orion BN/KL, South, and Bar regions, and is likely due to their PDR nature. Nevertheless, shocks/outflows may also play an important role in the Orion BN/KL and South regions, leading to an extra heating in the cloud.

3.4. Excitation temperature and density estimates

The 12CO J = 6−5 and C18O J = 6−5 data are used here to derive the gas excitation temperature and density of the OMC-1 core. In the submillimeter regime where the Rayleigh-Jeans approximation is often not valid, the observed radiation temperature in local thermodynamic equilibrium (LTE) can be expressed as (1)where Tex is the excitation temperature and τν is the optical depth at a specific molecular line transition. The background brightness temperature Tbg includes the cosmic background radiation of 2.73 K and the radiation from warm dust, ranging from about 5 K for less dense gas to about 26 K for the Orion Hor Core (Goldsmith et al. 1997). The second term inside the brackets can be neglected since it only contributes  ≲ 2% of . By making the assumption of the same excitation temperature for 12CO and C18O J = 6−5, the optical depths of 12CO and C18O J = 6−5 can be determined from the relation (2)Since the optical depth of 12CO J = 6−5 ≫ 1, the optical depth of C18O J = 6−5 can be directly obtained (~0.08) by assuming that the optical depth ratio is approximated to the isotopologic abundance ratio, and here we adopted [12CO]/[C18O] of 490 (Boreiko & Betz 1996; Wilson & Matteucci 1992) and a beam filling factor of unity (). The excitation temperatures of 12CO and C18O J = 6−5 can be estimated via (3)Then the total C18O column density can be derived from where Eup is 110.6 K for the C18O J = 6−5 transition and Qrot is the rotational partition function. In the end, the H2 column density can be estimated by adopting a [12CO]/[C18O] abundance ration of 490 and a 12CO abundance of 8 × 10-5 (Wilson & Matteucci 1992).

The results of the H2 column density and excitation temperature distributions are shown in Fig. 8, where the north-south dense ridge near the Trapezium cluster and the Orion Bar in the southeast are clearly seen. The average τ(C18O) at the J = 6−5 transition over the whole OMC-1 core region is about 0.08 with an average excitation temperature of about 115 K, which are consistent with the 12CO J = 9−8 observations by Kawamura et al. (2002). Besides, the minimum excitation temperature in the OMC-1 core is  ~30 K, indicating a generally warm environment. The average C18O column density in the OMC-1 core is about 8.7 × 1015 cm-2, corresponding to an H2 column density of 5.5 × 1022 cm-2 assuming a 12CO abundance of 8 × 10-5 (Wilson & Matteucci 1992). Therefore, the dense gas mass is  ~140 M⊙ in an area of 0.13 pc2 in the OMC-1 core. The H2 column density agrees with the density of 5 × 1021 − 7 × 1022 cm-2 derived from the mid-J CO isotopologue observations by Wirström et al. (2006). Our results agree with the recent measurement by Wilson et al. (2011) using the 13CO J = 6−5 line with a lower [12CO]/[H2] ratio of 2 × 10-5. Additionally, Habart et al. (2010) derived an H2 column density of about 9 × 1022 cm-2 toward the Orion Bar from the Herschel/SPIRE high-J CO observations, assuming a lower excitation temperature of 85 K and a higher 12CO to C18O ratio of 560. Their result is consistent with ours, where we obtained an H2 column density of  ≈ 1023 cm-2 toward the Orion Bar with a higher excitation temperature ( ≳ 150 K).

thumbnail Fig. 9

The RADEX modeling results shown in the 12CO J = 6−5 peak temperatures and the 13CO/C18O ratios for three different 12CO column densities. The column densities of 13CO and C18O were fixed at the isotopologic abundance ratio of 1/60 and 1/490 of the 12CO column density. The line widths of 12CO, 13CO, and C18O were fixed at 5, 5, and 4 km s-1, respectively. The red and blue contours denote the temperature and H2 number density, respectively, and the color points represent the data in the different regions of the OMC-1 core. Each point represents a peak 12CO J = 6−5 temperature and a temperature ratio between 13CO J = 8 − 7 and C18O J = 6−5 from a single pixel with a resolution of 20″. a) The modeling input column density of 12CO is 1 × 1018 cm-2, and the data are plotted in a 12CO column density density of 7.5 × 1017 − 2.5 × 1018 cm-2. b) The modeling input column density of 12CO is 5 × 1018 cm-2, and the data are plotted in a density range of 2.5 × 1018 − 7.5 × 1018 cm-2. c) The modeling input column density of 12CO is 1 × 1019 cm-2, and the data are plotted in the density of  ≥7.5 × 1018 cm-2. Upper right panel shows the 12CO column density map toward Orion BN/KL in contours of 5, 10, 20, 30, and 40 × 1018 cm-2. The stars mark the positions of the five Trapezium stars, and the Orion Hot Core (HC) position is also marked.

The JCMT 870 μm dust continuum emission image (Johnstone & Bally 1999) shown in Fig. 8 agrees with our density map well, where the dust emission traces mostly the dense ridge (Orion BN/KL and South) and Bar regions. It is interesting to note that the dense ridge has an offset from the peak temperature positions, especially in the Orion South region (~30″) and the north of the Orion BN/KL region (~20″). This dense ridge seems to extend farther to the north of the OMC-1 core, and is probably related to the NH3 filamentary structure seen by Wiseman & Ho (1998, 1996). Additionally, as Fig. 8b demonstrates, the interface between the ionized and the molecular warm dense gas is very pronounced, where Orion BN/KL and Orion South are located at the north-south dense ridge, and the Orion Bar and Orion East are part of the high temperature enclosed structure well illustrated by the free-free continuum emission. Furthermore, our large-scale images are consistent with the [Cii] and [Oi] emission maps obtained by Herrmann et al. (1997), where the rather uniform [Cii] emission indicates that PDRs are present over the whole OMC-1 core region.

3.5. RADEX modeling

The non-LTE radiative transfer program RADEX (van der Tak et al. 2007) was used to investigate the accuracy of the temperature and density calculation in LTE shown above. The optical depth effects are taken into account by RADEX using the escape probability approximation, where a uniform sphere geometry was chosen. The different transitions of 12CO, 13CO, and C18O were used in the modeling, and their line widths were fixed at 5, 5, and 4 km s-1, respectively, which are the average values from our spectra. The molecular data using in the modeling are taken from the Leiden Atomic and Molecular Database4 (LAMDA; Schöier et al. 2005). In addition, only H2 was chosen as a collision partner, and 2.73 K was adopted as the background temperature. The input parameters are kinetic temperature and H2 number density. Each model was iterated with a kinetic temperature ranging from 50 to 450 K and an H2 number density ranging from 104 to 108 cm-3. The input column densities of these three molecules were chosen, so that their ratios were fixed at their isotopologic abundance ratios, i.e., 490 for [12CO]/[C18O] and 60 for [12CO]/[13CO]. Three models were iterated with three different 12CO column densities of 1 × 1018, 5 × 1018, and 1 × 1019 cm-2 which were obtained from the LTE calculation. Since these models did not take outflows into account, the output molecular line radiation temperatures (TR) were directly compared with the peak temperatures instead of the integrated intensities to avoid the strong line wing emission. The modeling results are shown in Fig. 9.

Figure 9 reveals that most regions in the OMC-1 core have an H2 number density of  ~ 104−106 cm-3. The Orion Bar and Orion East emission peaks (Fig. 9a), however, show higher densities. The distribution of the H2 number density in the Bar is consistent with 104−105 cm-3 for a homogeneous medium (Wyrowski et al. 1997). The modeling results in the Orion BN/KL, South, and Bar regions all show similar trends of a diverse kinetic temperature (about 100−200 K), which are consistent with the LTE calculation results. In addition, the non-LTE modeling results indicate that the central region of Orion BN/KL close to the Hot Core (in 20′′ size) has a number density of about 105 cm-3 and a temperature of about 200 K, which is also close to the excitation temperature calculated in the LTE case. Noteworthily, the Orion BN/KL region and part of the Orion South region (Fig. 9a) show very high kinetic temperatures ( > 350 K) which may indicate an extra heating mechanism, e.g., outflow/shock heating. The suggestion of outflow/shock heating is supported by the larger 12CO line widths (Fig. 1) and a great amount of known outflows in these regions (see Henney et al. 2007). However, the high kinetic temperatures (≥250 K) in the Orion Bar and Orion East regions (Fig. 9a) are hardly explained by the outflow/shock heating for the lack of outflow activities or broad 12CO line widths. Instead, as PDRs, the Orion Bar and Orion East are expected to be heated by FUV photons from the Trapezium OB stars. Moreover, a recent study of the Orion Bar (Pellegrini et al. 2009) suggests an extra heating by the excess density of cosmic rays, which are trapped in the compressed magnetic field.

3.6. Mass of the OMC-1 core

By adopting a radius of 40′′–60′′ and an H2 density of 5 × 104 − 2 × 105 cm-3 for Orion BN/KL, we can estimate a gas mass of 7 − 85 M⊙ with the assumption of a spherical volume at a distance of 414 pc (Menten et al. 2007). In a similar way, the gas mass in Orion South is 3 − 49 M⊙ with a 30′′–50′′ radius and an H2 density of 5 × 104−2 × 105 cm-3. For Orion East, the H2 density is  ~2 × 105 cm-3 with a 15′′–25′′ radius, and the gas mass is 2 − 7 M⊙. In contrast, it is very difficult to estimate the gas mass in the Orion Bar because of its geometry. We adopted a cylindrical geometry and an H2 density of 5 × 104−2 × 105 cm-3 as the lower limit since an edge-on plane may contain much more gas. With a length of 300′′ and a radius of 20′′–30′′, the gas mass of the Orion Bar is 9−79 M⊙. The estimated total dense H2 gas mass in the OMC-1 core is 21 − 220 M⊙ within a radius of 0.3 pc. This is also consistent with the mass estimated (~140 M⊙) from the LTE calculation above. If we include the warm diffuse gas (104 cm-3) in the same volume, the total warm gas mass is 86 − 285 M⊙ in the OMC-1 core. This mass estimate for the OMC-1 core is close to that derived from the Odin 12CO and 13CO J = 5 − 4 observations (Wirström et al. 2006), i.e., 320 M⊙ in the molecular ridge for a farther distance of 500 pc. The result of Wilson et al. (2001) also suggests a similar estimate of a warm gas mass of 310 − 430 M⊙ from the observations of 12CO J = 7−6 and J = 4 − 3, again for a distance of 500 pc. Scaling these two literature values to the distance of 414 pc, results in 213 − 295 M⊙ for the total mass of the OMC-1 core. This is comparable to the value derived by us. Hence, we conclude that the higher-J CO lines do trace most of the gas in the OMC-1 core which is heated mainly by the Trapezium stars.

4. Summary

In this paper, we present the first large-scale images of highly excited 12CO, 13CO, and C18O lines in the OMC-1 core. The high excitation temperatures (~150−200 K) reveal a hot enclosed structure which is mostly heated by the Trapezium cluster. The LTE approximation and non-LTE RADEX program were used to derive the excitation temperature and H2 density in the OMC-1 core. We found a typical density of 104−106 cm-3 in this region. The clear dense ridge in the north-south direction is seen and is offset from the high temperature enclosed structure. Orion BN/KL and Orion South are located at this dense ridge, and the Orion Bar and East are part of the high temperature enclosed structure. The estimated mass of total warm gas is 86−285 M⊙, assuming different geometries for different regions. In addition, the higher-J CO lines trace most of the molecular gas in the OMC-1 core. The detailed investigation of individual region of the Orion BN/KL, Orion South, Orion Bar, and Orion East will be presented in the following papers.

2

This publication is based on the data acquired with the Atacama Pathfinder Experiment (APEX) telescope. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.

Acknowledgments

We would like to thank the APEX staff for the support during the observations and Simon Dicker for providing the VLA-GBT map. This work was supported by the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne.

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All Tables

Table 1

Observational parameters.

In the text
Table 2

CO isotopologues measurements in the selected sources of the OMC-1 core.

In the text

All Figures

thumbnail Fig. 1

The OMC-1 core 12CO J = 6−5 integrated intensity [−25,  + 30] km s-1 image overlaid with the J = 7−6 integrated intensity [− 25,  + 30] km s-1 contours running from 300 to 1000 K km s-1 in steps of 100 K km s-1, and the subsequent contours are plotted from 1200 to 7200 K km s-1 in steps of 600 K km s-1. The 12CO J = 6−5 (black) and 12CO J = 7−6 (red) spectra are shown for Orion BN, Orion South, Orion Bar, and Orion East. Both images have been smoothed using Gaussian profiles (a width of 91 for 12CO J = 6−5 and 78 for 12CO J = 7−6) from their original Nyquist-sampled images. The black stars mark the positions of the five Trapezium stars (θ1 Ori A, B, C, D, and E). The original beam sizes are shown at the bottom left of the image for 12CO J = 6−5 and J = 7−6 in red (96) and white (82), respectively.
In the text
thumbnail Fig. 2

The OMC-1 core 13CO J = 8−7 integrated intensity [+5,  +15] km s-1 image overlaid with the C18O J = 6−5 [+4,  +12] km s-1 contours running from 15 to 65 K km s-1 in steps of 10 K km s-1, and the subsequent contours are plotted from 80 to 200 K km s-1 in steps of 30 K km s-1. The 13CO J = 8−7 (black) and C18O J = 6−5 (red) spectra are shown for Orion BN, Orion South, Orion Bar, and Orion East at the same positions as in Fig. 1. Both images have been smoothed using Gaussian profiles (a width of 71 for 13CO J = 8−7 and 95 for C18O J = 6−5) from their original Nyquist-sampled images. The black stars mark the positions of the five Trapezium stars. The original beam sizes are shown at the bottom left of the image for 13CO J = 8−7 and C18O J = 6−5 in red (75) and white (101), respectively.
In the text
thumbnail Fig. 3

The OMC-1 core 13CO J = 3−2 integrated intensity [+5,  +15] km s-1 image overlaid with the C18O J = 3−2 [+4,  +12] km s-1 contours running from 24 to 80 K km s-1 in steps of 8 K km s-1. The 13CO J = −2 (black) and C18O J = 3−2 (red) spectra are shown for Orion BN, Orion South, Orion Bar, and Orion East at the same positions as in Fig. 1. The black stars mark the positions of the five Trapezium stars. The beam size of 211 is shown at the bottom left of the image.
In the text
thumbnail Fig. 4

Low-velocity channel maps of the 12CO J = 6−5 line with the J = 7−6 contours in the OMC-1 core. The contours are running from 50 to 200 K in steps of 25 K. Orion BN is located at the offset (0′′, 0′′), and white stars mark the positions of the five Trapezium stars.

In the text
thumbnail Fig. 5

Upper panel is the 12CO J = 6−5 to C18O J = 6−5 ratio channel maps (θHPBW = 10″) at different velocities from 6 to 11 km s-1 in the OMC-1 core. The edge cut and empty area are due to different image sizes and data blanking (2 σ level of the C18O J = 6−5 temperature). Lower panel is the corresponding C18O J = 6−5 to J = 3−2 ratio channel maps (θHPBW = 20″). The black arrow in the 10 km s-1 image indicates the ratio gradient across the Orion Bar (see text). The Orion Hot Core (HC) position is marked, and Orion BN is located at the offset (0′′, 0′′). The positions of the five Trapezium stars are marked in each map.
In the text
thumbnail Fig. 6

Velocity channel maps (5, 10, and 15 km s-1) of the 12CO J = 7−6 to J = 6−5 ratio (θHPBW = 10″) in the OMC-1 core. The 10 km s-1 image is overlaid with the 12CO J = 6−5 contours as shown in Fig. 1. The empty area are due to data blanking (3 σ level of the 12CO J = 7−6 temperature). The dashed-lines indicate that these ratio gradients are likely caused by outflows or filaments. Black triangles mark the Orion Hot Core position.

In the text
thumbnail Fig. 7

a) The OMC-1 core C18O J = 3−2 to 13CO J = 3−2 integrated intensity ratio map (θHPBW = 20″) overlaid with the 12CO J = 6−5 contours (grey) as shown in Fig. 1. The black contour represents the C18O/13CO abundance ratio of 0.122 (60/490). b) The OMC-1 core C18O J = 3−2 to J = 6−5 integrated intensity ratio map (θHPBW = 20″) overlaid with the 12CO J = 6−5 contours as shown in Fig. 1. Black triangles mark the Orion Hot Core position.

In the text
thumbnail Fig. 8

a) The C18O column density image of the OMC-1 core overlaid with black contours of the JCMT 850 μm dust continuum image. The 850 μm image is taken from the JCMT data archive. The contours represent 0.2%, 0.5%, 1%, 2%, 4%, 7%, 20%, 40%, 60%, and 80% of the peak intensity (87 Jy beam-1). b) The excitation temperature image for 12CO and C18O overlaid with the 35 mm VLA-GBT continuum emission (Dicker et al. 2009) in black contours running from 15% to 95% in steps of 8% of the peak intensity (0.94 Jy beam-1). The stars mark the positions of the five Trapezium stars, and black triangles represent the position of the Orion Hot Core.
In the text
thumbnail Fig. 9

The RADEX modeling results shown in the 12CO J = 6−5 peak temperatures and the 13CO/C18O ratios for three different 12CO column densities. The column densities of 13CO and C18O were fixed at the isotopologic abundance ratio of 1/60 and 1/490 of the 12CO column density. The line widths of 12CO, 13CO, and C18O were fixed at 5, 5, and 4 km s-1, respectively. The red and blue contours denote the temperature and H2 number density, respectively, and the color points represent the data in the different regions of the OMC-1 core. Each point represents a peak 12CO J = 6−5 temperature and a temperature ratio between 13CO J = 8 − 7 and C18O J = 6−5 from a single pixel with a resolution of 20″. a) The modeling input column density of 12CO is 1 × 1018 cm-2, and the data are plotted in a 12CO column density density of 7.5 × 1017 − 2.5 × 1018 cm-2. b) The modeling input column density of 12CO is 5 × 1018 cm-2, and the data are plotted in a density range of 2.5 × 1018 − 7.5 × 1018 cm-2. c) The modeling input column density of 12CO is 1 × 1019 cm-2, and the data are plotted in the density of  ≥7.5 × 1018 cm-2. Upper right panel shows the 12CO column density map toward Orion BN/KL in contours of 5, 10, 20, 30, and 40 × 1018 cm-2. The stars mark the positions of the five Trapezium stars, and the Orion Hot Core (HC) position is also marked.

In the text

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