AAstronomy & Astrophysics manuscript no. rl_H2O2 c (cid:13)
ESO 2018October 14, 2018
Search for HOOH in Orion (cid:63) (Research Note)
R. Liseau and B. Larsson Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala,Sweden, e-mail: [email protected] AlbaNova University Centre, Stockholm University, Department of Astronomy, SE-106 91 Stockholm, SwedenReceived ... / Accepted ...
ABSTRACT
Context.
The abundance of key molecules determines the level of cooling that is necessary for the formation of stars and planetarysystems. In this context, one needs to understand the details of the time dependent oxygen chemistry, leading to the formation of O and H O. Aims.
We aim to determine the degree of correlation between the occurrence of O and HOOH (hydrogen peroxide) in star-formingmolecular clouds. We first detected O and HOOH in ρ Oph A, we now search for HOOH in Orion OMC A, where O has also beendetected. Methods.
We mapped a 3 (cid:48) × (cid:48) region around Orion H -Peak 1 with the Atacama Pathfinder Experiment (APEX). In addition to severalmaps in two transitions of HOOH, viz. 219.17 GHz and 251.91 GHz, we obtained single-point spectra for another three transitionstowards the position of maximum emission. Results.
Line emission at the appropriate LSR-velocity (Local Standard of Rest) and at the level of ≥ σ was found for two transitions,with lower S / N (2 . − . σ ) for another two transitions, whereas for the remaining transition, only an upper limit was obtained. Theemitting region, o ff set 18 (cid:48)(cid:48) south of H -Peak 1, appeared point-like in our observations with APEX. Conclusions.
The extremely high spectral line density in Orion makes the identification of HOOH much more di ffi cult than in ρ Oph A. As a result of having to consider the possible contamination by other molecules, we left the current detection status unde-cided.
Key words.
Astrochemistry – interstellar medium (ISM): general – ISM: individual objects: Orion H2-Peak 1 – ISM: molecules –ISM: abundances – Stars: formation
1. Introduction
Searches for the presumed key molecule O (Goldsmith &Langer 1978) in numerous star-forming regions have beenhighly unawarding (e.g. Goldsmith et al. 2000; Pagani et al.2003), with the definite detection of the molecule in merely twosources, viz. ρ Oph A (Larsson et al. 2007; Liseau et al. 2012)and Orion A (Goldsmith et al. 2011; Chen et al. 2014). Somecases have been either resolved or remained undecided (e.g.Goldsmith et al. 2002; Yıldız et al. 2013).The observed scarcity of O in the Interstellar Medium (ISM)called for the abandonment of pure gas-phase chemistry modelsand the invocation of grain-surface processes (Hollenbach et al.2009). Specific models addressed the conditions of the OrionBar PDR (photodissociation region), where searches had how-ever been unsuccessful in detecting the molecule (Melnick et al.2012). Surprisingly, perhaps, O was detected towards the hotcore, albeit at an LSR (Local Standard of Rest)-velocity of 10-12 km s − , i.e., significantly di ff erent from that of typical hotcore molecules ( ∼ − ; Goddi et al. 2011, and referencestherein). These authors also found a small region of emission in (cid:63) Based on observations with APEX, which is a 12 m diameter sub-millimetre telescope at 5100 m altitude on Llano Chajnantor in Chile.The telescope is operated by Onsala Space Observatory, Max-Planck-Institut für Radioastronomie (MPIfR), and European Southern Obser-vatory (ESO). NH inversion lines with velocities of about 11 kms. Overall, linewidths decrease with excitation from ∼ − to ∼ − .Chen et al. (2014) were able to pinpoint the location of the 9 (cid:48)(cid:48) O source, near the position identified as H -Peak 1 and some-what o ff set from the hot core centre. The non-detection of theO line at 1121 GHz led the authors to conclude that gas temper-atures do not exceed 50 K, with best-fit model values more like30 K. The excitation conditions thus resemble those in ρ Oph A(Liseau et al. 2012).Du et al. (2012) developed models for grain surface chem-istry, and as an example, they considered the particular case of ρ Oph A. According to these models, the existence of O in thegas phase is a transient phenomenon, lasting for some 10 years,and which may explain the extremely few detections. Thesemodels also predict the accompanying occurrence of hydrogenperoxide (HOOH or H O ) and hydroperoxyl (HO ), and waterof course, via the following major reactions on grain surfaces(Tielens & Hagen 1982; Parise et al. 2014):O + H → HO HO + H → HOOHHOOH + H → H O + OH,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 withO , the observation of ten other targets in lines of HOOH gave Article number, page 1 of 4 a r X i v : . [ a s t r o - ph . GA ] O c t & A proofs: manuscript no. rl_H2O2 null results (Parise et al. 2014), supporting the O -HOOH as-sociation. This included low- and high-mass star formation re-gions, where in particular the high-mass star formation regionshad strong UV fields, shocks and maser emissions. It was nat-ural, therefore, to search for the hydrogen peroxide moleculein 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 asfollows: in Sect. 2, the observations and data reduction are re-ported, with the results provided in Sect. 3. A brief discussion,together with our conclusions, follows in Sect. 4.
2. Observations and data reduction
The region around the position “Orion H -Peak 1" (Chen et al.2014) was observed with the Atacama Pathfinder Experiment(APEX; ) in 2014 during thetime August to December (Table 1). APEX is a 12 m singledish telescope at 5100 m altitude in northern Chile. We usedtwo receivers from the SHeFI suite, i.e., APEX-1 for (3 − ) 219 GHz and (6 − ) 252 GHz and APEX-2 for (4 − ) 269 GHz and (5 − ), (5 − ) 319 GHz, respectively .At these frequencies, the HPBW of APEX is 20 (cid:48)(cid:48) to 28 (cid:48)(cid:48) . Therms value of the telescope pointing accuracy is 2 (cid:48)(cid:48) .As seen in Table 1, maps were obtained on-the-fly in the219 GHz and 252 GHz lines, with a sampling rate of 9 (cid:48)(cid:48) / pxl,oversampling the 3 (cid:48) × (cid:48) region in these lines. The central J2000-coordinates are R.A. = h m s ·
70, Dec. = − ◦ (cid:48) (cid:48)(cid:48)·
0. To-wards the o ff set position (0 (cid:48)(cid:48) , − (cid:48)(cid:48) ), single position spectra wereobtained at 269 GHz and 319 GHz.For the instantaneous bandwidth of 2.5 GHz, we usedas backend the Fast Fourier Transform Spectrometer(FFTS) with 32768 spectral channels. We selected a spec-tral resolution of 76.3 kHz per channel, correspondingto a velocity resolution of ∼ − . The data werereduced with the software packages GILDAS / CLASS( ∼ brand/gag.html ) and xs ( ftp://yggdrasil.oso.chalmers.se/pub/xs/ ).
3. Results
An overview of the mapped region is shown in the left panel ofFig. 1, revealing that the core region near the centre is very com-pact. A blow-up, 36 (cid:48)(cid:48) in size, is shown in the right panel, wherea weak emission feature is shown on the wing of a stronger line.That feature corresponds to the (3 − ) line of HOOH at theLSR-velocity of 10.0 km s − , i.e., consistent with that of the O lines (Chen et al. 2014). It can also be seen that this feature is notmerely due to noise, but is repeatedly seen in di ff erent positions,albeit at lower intensity. The fact that HOOH is not detected out-side this limited region implies that the emission in the 219 GHzline is point-like to the 28 (cid:48)(cid:48) telescope beam.From the data in Table 2, it appears that only two out of fivelines were clearly detected ( ≥ σ ), and two were possibly de-tected at low S / N (2 . − . σ ). The quoted line widths (FWHMs)are only lower limits because of the di ffi culty in accurately de-termining the local continuum on sloping backgrounds. TheseHOOH widths are smaller than those for O reported by Chenet al. (2014). The 252 GHz line was not detected. However, thenoise level of that spectrum is very much higher than for theother observations (Table 2). Swedish Heterodyne Facility Instrument Energy level diagrams are found in Bergman et al. (2011).
Table 1.
Log of observations
HOOH-transition Frequency Date t int Sp.( J K a K c ) (cid:48) − ( J K a K c ) (cid:48)(cid:48) (GHz) yy–mm–dd (min)3 − − − − − α = h m s · δ = − ◦ (cid:48) (cid:48)(cid:48)· ff set (0 (cid:48)(cid:48) , − (cid:48)(cid:48) ). Table 2.
Measurements of HOOH features
HOOH E up / k υ LSR
FWHM T rms (cid:82) T A d υ S / NLine (K) (km s − ) (km s − ) (mK) (K km s − )3 −
31 10.0 1.4 26.1 0.172 4.66 −
66 11.6 0.8 257.0 0.242 1.24 −
41 10.3 1.0 41.9 0.118 2.85 −
53 11.8 0.9 28.8 0.102 3.55 −
67 12.3 1.5 31.5 0.174 4.0
4. Discussion and conclusions
The LSR-velocities of the HOOH features are clearly outside thehot core window, but seem consistent with those obtained for O .This could also indicate that in Orion HOOH can be tied to O .A major shortcoming, though, is the extremely high line densitytowards the hot core region, which makes proper line identifi-cation di ffi cult. In fact, several molecules in the 219 GHz spec-trum display similar hump features on their red wings (Fig. 2).This is not evidenced by the other transitions, but in view of therelatively lower S / N makes the HOOH identification apparentlynon-unique.
Acknowledgements.
The contributions by P. Bergman, including the interestingdiscussions, are highly appreciated. We also thank the Swedish APEX team andthe APEX sta ff on site for their help with the observations. As part of our Odinand Herschel work, this research has been supported by the Swedish NationalSpace Board (SNSB).
References
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Article number, page 2 of 4. Liseau and B. Larsson: Search for HOOH in Orion (RN)
R.A. = 05h 35m 13.70s Dec. = -‐05 o O (3 – 2 ) 219.167 GHz υ LSR$$ (km$s )1 )$ T A $$$ ( K ) $ RA offset [´´]
Fig. 1. Left: the 3 (cid:48) × (cid:48) mapped area, sampled at 9 (cid:48)(cid:48) with the origin at the Orion H -Peak 1 position, i.e., α = h m s · δ = − ◦ (cid:48) (cid:48)(cid:48)· Right:
Centred on (0 (cid:48)(cid:48) , − (cid:48)(cid:48) ), this partial map demonstrates that the 219.17 GHzfeature is a point source to the 28 (cid:48)(cid:48) beam. This weak spectral feature is identified inside the red markers. It is sitting on top of the red wing of amuch stronger line (HC N ( ν = -‐3000 -‐2000 -‐1000 0 1000 2000 3000 -‐40 -‐20 0 20 40 LSR – Velocity (km s -‐1 ) C O C H C N H C N H C N C O C H CN HC N HC N Orion H -‐Peak (0´´, -‐18´´) 217 – 221 GHz T A (K) Fig. 2. Left:
The 4 GHz wide spectrum, centred on 219 GHz, towards the o ff set position (0 (cid:48)(cid:48) , − (cid:48)(cid:48) ) relative to Orion H -Peak 1. Line identificationsfor 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. Article number, page 3 of 4 & A proofs: manuscript no. rl_H2O2
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