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A&A
Volume 583, November 2015
Article Number A53
Number of page(s) 4
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201526830
Published online 27 October 2015

© ESO, 2015

1. Introduction

Searches for the presumed key molecule O2 (Goldsmith & Langer 1978) in numerous star-forming regions have been highly unawarding (e.g. Goldsmith et al. 2000; Pagani et al. 2003), with the definite detection of the molecule in merely two sources, viz. ρ Oph A (Larsson et al. 2007; Liseau et al. 2012) and Orion A (Goldsmith et al. 2011; Chen et al. 2014). Some cases have been either resolved or remained undecided (e.g. Goldsmith et al. 2002; Yıldız et al. 2013).

The observed scarcity of O2 in the interstellar medium (ISM) called for the abandonment of pure gas-phase chemistry models and the invocation of grain-surface processes (Hollenbach et al. 2009). Specific models addressed the conditions of the Orion Bar PDR (photodissociation region), where searches had however been unsuccessful in detecting the molecule (Melnick et al. 2012). Surprisingly, perhaps, O2 was detected towards the hot core, albeit at an LSR (Local Standard of Rest)-velocity of 10–12 km s-1, i.e., significantly different from that of typical hot core molecules (~5 km s-1; Goddi et al. 2011, and references therein). These authors also found a small region of emission in NH3 inversion lines with velocities of about 11 kms. Overall, line widths decrease with excitation from ~5 km s-1 to ~2 km s-1.

Chen et al. (2014) were able to pinpoint the location of the 9′′ O2 source, near the position identified as H2-Peak 1 and somewhat offset from the hot core centre. The non-detection of the O2 line at 1121 GHz led the authors to conclude that gas temperatures do not exceed 50 K, with best-fit model values more like 30 K. The excitation conditions thus resemble those in ρ Oph A (Liseau et al. 2012).

Du et al. (2012) developed models for grain surface chemistry, and as an example, they considered the particular case of ρ Oph A. According to these models, the existence of O2 in the gas phase is a transient phenomenon, lasting for some 105 years, and which may explain the extremely few detections. These models also predict the accompanying occurrence of hydrogen peroxide (HOOH or H2O2) and hydroperoxyl (HO2), and water of course, via the following major reactions on grain surfaces (Tielens & Hagen 1982; Parise et al. 2014):

and these two species were then also firstly detected in ρ Oph A (Bergman et al. 2011; Parise et al. 2012). As was the case with O2, the observation of ten other targets in lines of HOOH gave null results (Parise et al. 2014), supporting the O2-HOOH association. This included low- and high-mass star formation regions, where in particular the high-mass star formation regions had strong UV fields, shocks and maser emissions. It was natural, therefore, to search for the hydrogen peroxide molecule in Orion A, a site that was not listed in Table 4 of Parise et al. (2014).

The organisation of this Research Note is briefly outlined as follows: in Sect. 2, the observations and data reduction are reported, with the results provided in Sect. 3. A brief discussion, together with our conclusions, follows in Sect. 4.

Table 1

Log of observations.

2. Observations and data reduction

The region around the position “Orion H2-Peak 1” (Chen et al. 2014) was observed with the Atacama Pathfinder Experiment (APEX1) in 2014 during the time August to December (Table 1). APEX is a 12 m single dish telescope at 5100 m altitude in northern Chile. We used two receivers from the SHeFI2 suite, i.e., APEX-1 for (303−211) 219 GHz and (615−505) 252 GHz and APEX-2 for (404−312) 269 GHz and (505−413), (514−606) 319 GHz, respectively3. At these frequencies, the HPBW of APEX is 20′′ to 28′′. The rms value of the telescope pointing accuracy is 2′′.

As seen in Table 1, maps were obtained on-the-fly in the 219 GHz and 252 GHz lines, with a sampling rate of 9′′/pxl, oversampling the 3′ × 3′ region in these lines. The central J2000-coordinates are RA = 05h35m1370, Dec = −05° 22′ 090. Towards the offset position (0′′, −18′′), single position spectra were obtained at 269 GHz and 319 GHz.

For the instantaneous bandwidth of 2.5 GHz, we used as backend the Fast Fourier Transform Spectrometer (FFTS) with 32768 spectral channels. We selected a spectral resolution of 76.3 kHz per channel, corresponding to a velocity resolution of ~0.1 km s-1. The data were reduced with the software packages GILDAS/CLASS4 and xs5.

thumbnail Fig. 1

Left: the 3′×3′ mapped area, sampled at 9′′ with the origin at the Orion H2-Peak 1 position, i.e., α2000 = 05h35m1370, δ2000  = −05° 22′ 090. A core of intense emission is clearly seen just below the centre. Right: centred on (0′′, −18′′), this partial map demonstrates that the 219.17 GHz feature is a point source to the 28′′ beam. This weak spectral feature is identified inside the red markers. It is sitting on top of the red wing of a much stronger line (HC3N (ν7 = 3), Sutton et al. 1985).

thumbnail Fig. 2

Left: the 4 GHz wide spectrum, centred on 219 GHz, towards the offset position (0′′, −18′′) relative to Orion H2-Peak 1. Line identifications for the entire spectral region can be found in the paper by Sutton et al. (1985). Blow-ups of the labelled lines are found in the right-hand panel, where the LSR-velocity range of the putative HOOH line is indicated with the dashed vertical lines.

3. Results

An overview of the mapped region is shown in the left panel of Fig. 1, revealing that the core region near the centre is very compact. A blow-up, 36′′ in size, is shown in the right panel, where a weak emission feature is shown on the wing of a stronger line. That feature corresponds to the (303−211) line of HOOH at the LSR-velocity of 10.0 km s-1, i.e., consistent with that of the O2 lines (Chen et al. 2014). It can also be seen that this feature is not merely due to noise, but is repeatedly seen in different positions, albeit at lower intensity. The fact that HOOH is not detected outside this limited region implies that the emission in the 219 GHz line is point-like to the 28′′ telescope beam.

From the data in Table 2, it appears that only two out of five lines were clearly detected (≥4σ), and two were possibly detected at low signal-to-noise ratio (2.8−3.5σ). The quoted line widths (FWHMs) are only lower limits because of the difficulty in accurately determining the local continuum on sloping backgrounds. These HOOH widths are smaller than those for O2 reported by Chen et al. (2014). The 252 GHz line was not detected. However, the noise level of that spectrum is very much higher than for the other observations (Table 2).

Table 2

Measurements of HOOH features.

4. Discussion and conclusions

The LSR-velocities of the HOOH features are clearly outside the hot core window, but seem consistent with those obtained for O2. This could also indicate that in Orion HOOH can be tied to O2. A major shortcoming, though, is the extremely high line density towards the hot core region, which makes proper line identification difficult. In fact, several molecules in the 219 GHz spectrum display similar hump features on their red wings (Fig. 2). This is not evidenced by the other transitions, but in view of the relatively lower signal-to-noise ratio makes the HOOH identification apparently non-unique.

2

Swedish Heterodyne Facility Instrument.

3

Energy level diagrams are found in Bergman et al. (2011).

Acknowledgments

The contributions by P. Bergman, including the interesting discussions, are highly appreciated. We also thank the Swedish APEX team and the APEX staff on site for their help with the observations. As part of our Odin and Herschel work, this research has been supported by the Swedish National Space Board (SNSB).

References

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All Tables

Table 1

Log of observations.

In the text
Table 2

Measurements of HOOH features.

In the text

All Figures

thumbnail Fig. 1

Left: the 3′×3′ mapped area, sampled at 9′′ with the origin at the Orion H2-Peak 1 position, i.e., α2000 = 05h35m1370, δ2000  = −05° 22′ 090. A core of intense emission is clearly seen just below the centre. Right: centred on (0′′, −18′′), this partial map demonstrates that the 219.17 GHz feature is a point source to the 28′′ beam. This weak spectral feature is identified inside the red markers. It is sitting on top of the red wing of a much stronger line (HC3N (ν7 = 3), Sutton et al. 1985).

In the text
thumbnail Fig. 2

Left: the 4 GHz wide spectrum, centred on 219 GHz, towards the offset position (0′′, −18′′) relative to Orion H2-Peak 1. Line identifications for the entire spectral region can be found in the paper by Sutton et al. (1985). Blow-ups of the labelled lines are found in the right-hand panel, where the LSR-velocity range of the putative HOOH line is indicated with the dashed vertical lines.

In the text

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