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A Search for O_2 in CO-depleted Molecular Cloud Cores with Herschel

The general lack of molecular oxygen in molecular clouds is an outstanding problem in astrochemistry. Extensive searches with SWAS, Odin and Herschel have only produced two detections; upper limits to the O_2 abundance in the remaining sources observed are about 1000 times lower than predicted by chemical models. Previous atomic oxygen observations and inferences from observations of other molecules indicated that high abundances of O atoms might be present in dense cores exhibiting large amounts of CO depletion. Theoretical arguments concerning the oxygen gas-grain interaction in cold dense cores suggested that, if O atoms could survive in the gas after most of the rest of the heavy molecular material has frozen out on to dust, then O_2 could be formed efficiently in the gas. Using Herschel HIFI we searched a small sample of four depletion cores - L1544, L694-2, L429, Oph D - for emission in the low excitation O_2 N_J=3_3-1_2 line at 487.249 GHz. Molecular oxygen was not detected and we derive upper limits to its abundance in the range N(O_2)/N(H_2) = (0.6 - 1.6)x10^{-7}. We discuss the absence of O_2 in the light of recent laboratory and observational studies.

Tentative detections by SWAS (Goldsmith et al. 2002), and from ground-based observations of the 16 O 18 O isotopologue (Pagani et al. 1993), both remain unconfirmed (Pagani et al. 2003;Fuente et al. 1993;Marechal et al. 1997). Upper limits on the abundance of solid O 2 tend to rule out the possibility that the missing O 2 is hidden in icy grain mantles (Vandenbussche et al. 1999).
The SWAS and Odin limits on O 2 abundance are several hundred times less than predicted in chemical models. Herschel observations have pushed these limits down even more, to lie in the range ≈(0.1-5)×10 −8 for many sources (Goldsmith et al. 2011b). Thus far, O 2 has only been securely detected in Orion (Goldsmith et al. 2011a) and ρ Oph A (Larsson et al. 2007;Liseau et al. 2012).
Several theoretical explanations have been advanced to explain the low O 2 abundances (as summarised by Roberts & Herbst 2002), but calculations show that, most probably, the simple process of oxygen atom accretion and ice formation on cold dust grains is responsible Charnley et al. 2001;Viti et al. 2001;Roberts & Herbst 2002). These models show that, for cloud ages > ∼ 10 5 years, once CO formation is complete, significant O atom depletion is necessary to prevent the O 2 abundance from violating the observed limits. However, it is generally unknown if the elemental abundance of oxygen in the observed molecular clouds is consistent with this explanation. Observations and theoretical models available at the time of the Herschel mission indicated that O atoms might be transiently much more abundant than CO (and C atoms) in some dense cores, and so produce large abundances of O 2 in them. In this paper we report the results of a Herschel HIFI search for O 2 in four cores that exhibit high degrees of CO depletion. Molecular oxygen was not detected and we provide upper limits.

OXYGEN IN DENSE MOLECULAR CLOUDS
Our search for molecular oxygen was based on the state of observational and laboratory knowledge at the time of the Herschel mission.

Atomic Oxygen
Molecular oxygen is easy to form in both warm and cold dense gas through the neutral-neutral process and is rapidly destroyed in the chemical reaction with atomic carbon Once the conversion of atomic C to CO is almost complete (∼10 5 years), the O 2 abundance rises rapidly on a short time-scale. Collisions of O atoms with cold dust, leading to the formation and retention of water ice, can occur on a comparable time-scale and so inhibit interstellar O 2 production. However, there is no direct evidence for low atomic O abundances in cold clouds. In fact, large O/CO ratios have been measured in several sources, e.g. O/CO>15 towards W49N (Vastel et al. 2000); where it has been attributed to selective CO depletion on grains; and O/CO∼50 towards L1689N (Caux et al. 1999). High fractional abundances of atomic O, ≈10 −4 , have also been measured in the collapsing envelope of the low-mass binary system IRAS 16293-2422 . L1544 belongs to a family of cold starless cores in which CO molecules have been depleted from the gas Bacmann et al. 2002;Crapsi et al. 2005;Bergin & Tafalla 2007). Caselli et al. (2002) modelled the ionization of L1544, as determined from observations of HCO + , N 2 H + and their deuterated isotopologues, and concluded that a large fractional abundance of atomic O of ≈10 −4 must be present to destroy H + 3 there, despite CO being significantly depleted on to dust. Gupta et al. (2009) detected C 6 H − and C 6 H in the CO-depleted region of L1544. Subsequent modeling by Cordiner & Charnley (2012) determined that a similarly high gas-phase O atom abundance was required to destroy C 6 H − and so account for the observed anion/neutral abundance ratio in this core, as well as in others that also show CO depletion (e.g. L1512). In L1544, and other dense cores, some carbon-bearing molecules (CN, HCN, HNC) are also present in the COdepleted gas (Hily-Blant et al. 2010;Padovani et al. 2011) but at abundances much less than found in normal dark cloud gas, supporting the idea that C atoms are not particularly abundant in the gas.
In summary, there is some observational evidence that suggest oxygen atoms may not freeze out efficiently in some dense cores, even when CO and some other heavy molecules (e.g. CS) do. This, and the fact that the abundance of atomic C must also be significantly reduced, raises the possibility that CO-depleted cores could contain large abundances of O 2 .

Gas-grain Chemistry of O 2
Could O 2 form in CO-depletion cores and be detectable? Here we demonstrate that this is theoretically possible. Models of O 2 suppression have simply assumed that gas phase O atoms are hydrogenated to form H 2 O ices (and so removed) at the rates at which they collide and stick to grains (e.g. Bergin et al. 2000;Viti et al. 2001;Charnley et al. 2001;Roberts & Herbst 2002); experiments have confirmed the viability of this ice formation pathway (Dulieu et al. 2010). However, to maintain a population of O atoms in gas where CO is freezing out requires that collisions of oxygen atoms with cold grains do not result in 100% retention on the surface. Maret et al. (2006) have shown that the CO-depletion core B68 contains a significant abundance of N atoms as well as N 2 molecules and so it is reasonable to inquire whether this could also occur for O atoms.
Assumptions have been made in previous chemical models that could allow the return of O atoms to the gas at non-negligible rates (e.g. Hasegawa & Herbst 1993;Willacy & Williams 1993;Charnley et al. 2001;Viti et al. 2001;Caselli et al. 2002;Charnley & Rodgers 2002;Flower et al. 2005;Crapsi et al. 2005) and include: (i) lower than expected binding energies on the relevant surface, E 0 = 600 K, (cf. 800 K, Tielens & Allamandola 1987), or (ii) a low sticking efficiency, or (iii) a variety of non-thermal desorption processes such as photodesorption or cosmic ray impacts (Leger et al. 1985), or (iv) desorption induced by H 2 formation (Willacy & Millar 1998). Thus, accepting that at least one of the above assumptions holds, it is feasible for an accreted O atom to desorb from a cold grain. However, since clearly some oxygen atoms must be retained on grains, the gas-grain kinetics of O atoms and CO molecules has to incorporate some degree of selective retention on the dust. Hollenbach et al. (2009) have presented a model for photodissociation regions (PDRs), in which the O atoms can be thermally desorbed, and which treats water formation in a more kinetically realistic manner. On the grain surface, H atom addition first converts atomic O to OH, and these hydroxyl molecules can then form hydrogen bonds with preexisting water molecules; if an O atom does not meet a surface H atom there is a finite probability that it will desorb and return to the gas. The strengths of hydrogen bonds are typically at least a factor of ten greater than those for van der Waals (physisorption) bonding, and it is this chemical fixing which controls the rate at which gaseous atomic O can become depleted on grains and suppress O 2 production. Thus, O atom accretion onto grains by fixing, as opposed to simply sticking at the gas-grain collision rate, depends on the gas phase atomic O/H ratio, and is at its most efficient when the accreting gas has O/H<1, and least so when oxygen atoms are more abundant than hydrogen atoms, i.e. when O/H>1. This naturally introduces a dependence on the density of the accreting gas. Because O atom removal explicitly depends on the H atom kinetics, water ice formation can occur easily at low densities (∼10 4 cm −3 or less) where O/H<1 and the H/H 2 ratio has not yet reached a steady-state (corresponding to n(H) ∼ 1 cm −3 , Goldsmith & Li 2005). However, higher densities, n(H 2 )∼ 10 6 cm −3 , lead to an increased O/H ratio (>1), as H atoms are more efficiently converted to H 2 on grains, and consequently to less efficient O atom removal. CO freeze-out in cold cores (∼5-10 K) becomes apparent at hydrogen densities in the range n(H 2 )∼1.5×10 4 -6×10 5 cm −3 (Jørgensen et al. 2005), and so is concomitant with O/H>1. Thus, counterintuitively, the central regions of dense cores exhibiting significant CO depletion may contain large abundances of O 2 (∼10 −6 -10 −5 ).
An important point is that, at about 10 K, this O 2 will not react with H + 3 or any of its deuterated isotopologues, and so would not affect the observed ionization or D/H enhancements found in these regions. The failure of previous searches for O 2 towards starless cores could then be attributed to not studying cores of sufficiently high CO depletion (L183 only has f D (CO)∼5, Pagani et al. 2005), misalignment of the telescope beam with the depletion peak (∼2 ′ N of the TMC-2 position targeted by Fuente et al. 1993), beam dilution (SWAS and Odin, Goldsmith et al. 2000;Pagani et al. 2003), or some combination of the above.
One would expect that much of the H 2 O formed from atomic oxygen will freeze out on to the dust, so will O 2 suffer the same fate? N 2 freeze-out appears to set in at densities of n(H 2 )∼10 6 cm −3 above that of CO (Belloche & André 2004;Pagani et al. 2005Pagani et al. , 2012. The cause of his selective depletion in cold cores is poorly understood (Bergin & Tafalla 2007) but is unlikely to be connected to the difference in physisorption binding energies; these are measured to be very close: 855±25 K for CO and 790±25 K for N 2 (Öberg et al. 2005). Similarly, the measured O 2 binding energy of 912±15 K (Acharyya et al. 2007) is sufficiently close to those of CO and N 2 that it is uncertain whether O 2 could exhibit selective depletion more like CO or N 2 at high densities. If the observed CO/N 2 depletion is connected to the fact that these homonuclear diatomic molecules are nonpolar (Masel 1996), then it might be expected that O 2 will persist after CO has become depleted, up to the density where N 2 is depleted, and perhaps beyond.

HERSCHEL OBSERVATIONS
In order to test whether O 2 abundances could be be significantly enhanced in high density depletion cores, we used the HIFI instrument (de Graauw et al. 2010) on-board the Herschel Space Observatory (Pilbratt et al. 2010) to observe the N J =3 3 -1 2 O 2 line at 487.249 GHz towards a sample of such sources, listed in Table 1. The sources were selected based on the known correlation between enhanced abundances of deuterated molecules and high CO depletion, and are the ones showing the highest CO depletion factors, f D (CO), in the surveys of H 2 D + , N 2 H + and N 2 D + in starless cores by Crapsi et al. (2005) and Caselli et al. (2008). Observations targeted the N 2 D + emission peaks in the maps by Crapsi et al. (2005), which can be significantly offset from continuum core positions, but better tracing the depletion cores.
Note that gas densities and temperatures in these cores are typically n(H 2 ) > 10 5 cm −3 and T K ∼10 K, which makes the 487.249 GHz line (E u =26 K) the most favourable one to observe in emission with HIFI, and collision rates fast enough for the fractional population of all energy levels to be well described by a Boltzmann distribution at the kinetic temperature.
The dual beam switching mode was employed with the LO frequency in HIFI band 1a set to 494.93 GHz in both L and R polarisation, placing the line in the lower side band. Both the Wide-Band Spectrometer (WBS) and the High Resolution Spectrometer (HRS) were used, and because emission lines from the region are known to be narrow (N 2 D + FWHM ∼0.2-0.4 km s −1 ) the HRS was configured to high resolution mode with a band width of 230 MHz, corresponding to a spectral resolution of 125 kHz, or ∼0.08 km s −1 at this frequency. Observations were performed under observing program OT2 ewirst01 1 and the data presented here is available from the Herschel Science Archive 2 (HSA) under observing IDs given in Table 1.
The beam FWHM is 44 ′′ at this frequency, the forward efficiency η l = 96%, and the main-beam efficiency η mb = 76%. Detailed information about the HIFI calibration including beam efficiency, mixer sideband ratio, pointing, etc., can be found on the Herschel web site 3 . The in-flight performance is described by Roelfsema et al. (2012).
Spectra of both polarisations were reduced separately using the Herschel Interactive Processing Environment (Ott 2010). Subsequently, data FITS files were exported to the spectral analysis software XS 4 for further reduction and analysis. After linear baseline subtraction and frequency alignment, the two polarisations for each observing ID and spectrometer were averaged together, weighted by rms noise. Pointing offsets between polarisations were within 7 ′′ , i.e., less than 20% of the beam size. Figure 1 shows the observed Herschel HIFI Wide Band Spectrometer (WBS) polarization-averaged spectra towards the four sources in this study. Linear baselines have been subtracted and the resulting noise rms varies between 3-7 mK.

RESULTS AND ANALYSIS
Molecular oxygen was not detected towards any of the cores. Before further analysis, spectra were multiplied by the total efficiency factor of η l /η mb =1.27 (HIFI Observers' Manual version 2.4) in order to get them on the T mb scale. For estimates of the O 2 abundance upper limits the non-LTE radiative transfer code RADEX was used to model the level populations of O 2 molecules and 3σ line intensity upper limits. The spectroscopic data were adapted from Drouin et al. (2010) and collision rates with H 2 scaled from He + O 2 collision rates of Lique (2010). Physical conditions of the cores, listed in Table 2, were adapted from Caselli et al. (2008), while line widths were taken to be the same as for N 2 D + . Table 2 presents the beam averaged upper limits on O 2 column densities based on these calculations, and upper abundance limits to O 2 /H 2 in the range ≈(0.6 -1.6)×10 −7 , assuming a constant H 2 column density over an area larger than the beam.
If the O 2 gas is only present in the inner, most CO-depleted parts of the cores, the emission would be significantly diluted in the large HIFI beam, and higher abundances could remain undetected. For example, assuming Gaussian O 2 source distributions with half intensities close to the 90% contours of the N 2 D + maps by Crapsi et al. (2005), the abundance limits are instead of the order of 10 −6 , see Table 2  ing beam dilution factors and abundances.

DISCUSSION
Recent experiments indicate a much higher physisorption binding energy for O atoms on amorphous water ice (1660±60 K; He et al. 2015). This is almost twice the value previously considered in chemical models (800 K, Tielens & Allamandola 1987) and was in fact suggested by Hollenbach et al. (2009). Model calculations confirm that, in the case of PDRs, higher O atom sticking efficiencies lead to a significant reduction in the gas phase O 2 abundance (He et al. 2015). Nevertheless, the low O 2 abundances inferred in CO depletion cores can only be explained by these models if the gas-phase atomic oxygen abundance is significantly lower than 10 −5 .  Caselli et al. (2008). b From N 2 D + lines (Crapsi et al. 2005). c Beam averaged, 3σ upper limit. d Upper limit assuming the corresponding beam dilution factor f dil , see Sect. 4 for more details.
The inferred low abundance of atomic oxygen in L1544 is consistent with the low gaseous H 2 O abundance measured by Caselli et al. (2012) since higher O atom abundances would also produce more water (e.g. Bergin et al. 2000). On the other hand, it is too low to be consistent with the O atom abundances of ≤10 −4 required to explain the anion/neutral ratios in the same source (Cordiner & Charnley 2012). Experiments show that O atoms could diffuse on grain surfaces faster than by pure thermal hopping (Minissale et al. 2014) and so, when O/H>1 in the accreting gas, reactions between O atoms (Tielens & Hagen 1982) could form solid O 2 efficiently (cf. Charnley 2005), although some may be hydrogenated to H 2 O (Ioppolo et al. 2008;Oba et al. 2009). Photodesorption of this O 2 would be dissociative (Fayolle et al. 2013) and would not increase the O 2 abundance in the gas of, say, L1544. Also, O 2 is predicted to have a higher physisorption binding energy to amorphous water ice when considering a low fractional coverage (1310 K; He et al. 2016) compared to previous experimental estimates considering monolayer coverage (Acharyya et al. 2007). This effect is even larger for CO and N 2 , resulting in an O 2 sticking efficiency significantly lower than that of CO, and following more closely that of N 2 (He et al. 2016).
However, competition for binding sites between these O 2 molecules and CO molecules arriving on the surface could lead to their ejection into the gas (Noble et al. 2015). Our O 2 non-detections could therefore place limits on these processes in CO-depleted cores.
We did not detect molecular oxygen at abundances as high as 10 −6 in the survey of depletion cores. The derived upper limits of ∼10 −7 or less are similar to those obtained elsewhere. Yıldız et al. (2013) determined O 2 /H 2 ≤ 6 × 10 −9 in NGC 1333 IRAS 4A although there is a tentative O 2 detection in the surrounding cloud material. A low value for O 2 /H 2 of 5 × 10 −8 has been detected in ρ Oph A by Liseau et al. (2012). The widespread distribution of D-enriched formaldehyde and the detection of H 2 O 2 in ρ Oph A (Bergman et al. 2011a,b) both support the idea that the O 2 could be present following the evaporation of ice mantles from dust grains. A search for H 2 O 2 in Orion was unsuccessful (Liseau & Larsson 2015) although the upper limits for hydrogen peroxide in interstellar ices, H 2 O 2 /H 2 O< 9%, do not rule out it being quite abundant on dust grains (Smith et al. 2011). In Orion, the O 2 emission is confined to the H 2 Peak 1 position and the originally reported abundance ratio was O 2 /H 2 ∼ 10 −6 (Goldsmith et al. 2011b). A combination of postshock chemistry in a UV-illuminated MHD shock wave with a fortuitous line-of-sight can account for the presence of O 2 in Orion, and suggest that the O 2 abundance could possibly be even higher ∼10 −5 -10 −4 (Chen et al. 2014;Melnick & Kaufman 2015). In Sgr A, Sandqvist et al. (2015) find O 2 /H 2 ≤ 5 × 10 −8 but also suggest that higher abundances, O 2 /H 2 ≤ 1 − 2 × 10 −5 , could exist in foreground clouds.
In conclusion, as with much of the dense interstellar medium, O 2 is underabundant in CO-depletion cores, most probably due to a large degree of O atom freezout to form water ice. The reasons why it remains so generally underabundant and yet is detectable in a few sources remains a mystery, and there is little prospect of detection in additional sources in the near future with Herschel concluded and no follow-up space-based spectrometer in the pipeline. Nevertheless, understanding the interstellar O 2 deficiency can shed light on many poorly understood issues in astrochemistry (Melnick et al. 2012). E.S.W. acknowledges generous financial support from the Swedish National Space Board. The work of S.B.C. and M.A.C. were supported by NASA's Origins of Solar Systems Program.