During the dawn of chemistry
, when the temperature of the young Universe had fallen below some 4,000 kelvin, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse ...order of their ionization potential. With their higher ionization potentials, the helium ions He
and He
were the first to combine with free electrons, forming the first neutral atoms; the recombination of hydrogen followed. In this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH
through radiative association with protons. As recombination progressed, the destruction of HeH
created a path to the formation of molecular hydrogen. Despite its unquestioned importance in the evolution of the early Universe, the HeH
ion has so far eluded unequivocal detection in interstellar space. In the laboratory the ion was discovered
as long ago as 1925, but only in the late 1970s was the possibility that HeH
might exist in local astrophysical plasmas discussed
. In particular, the conditions in planetary nebulae were shown to be suitable for producing potentially detectable column densities of HeH
. Here we report observations, based on advances in terahertz spectroscopy
and a high-altitude observatory
, of the rotational ground-state transition of HeH
at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027. This confirmation of the existence of HeH
in nearby interstellar space constrains our understanding of the chemical networks that control the formation of this molecular ion, in particular the rates of radiative association and dissociative recombination.
The D/H ratio in cometary water has been shown to vary between 1 and 3 times the Earth’s oceans value, in both Oort cloud comets and Jupiter-family comets originating from the Kuiper belt. This has ...been taken as evidence that comets contributed a relatively small fraction of the terrestrial water. We present new sensitive spectroscopic observations of water isotopologues in the Jupiter-family comet 46P/Wirtanen carried out using the GREAT spectrometer aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The derived D/H ratio of (1.61 ± 0.65)×10−4 is the same as in the Earth’s oceans. Although the statistics are limited, we show that interesting trends are already becoming apparent in the existing data. A clear anti-correlation is seen between the D/H ratio and the active fraction, defined as the ratio of the active surface area to the total nucleus surface. Comets with an active fraction above 0.5 typically have D/H ratios in water consistent with the terrestrial value. These hyperactive comets, such as 46P/Wirtanen, require an additional source of water vapor in their coma, explained by the presence of subliming icy grains expelled from the nucleus. The observed correlation may suggest that hyperactive comets belong to a population of ice-rich objects that formed just outside the snow line, or in the outermost regions of the solar nebula, from water thermally reprocessed in the inner disk that was transported outward during the early disk evolution. The observed anti-correlation between the active fraction and the nucleus size seems to argue against the first interpretation, as planetesimals near the snow line are expected to undergo rapid growth. Alternatively, isotopic properties of water outgassed from the nucleus and icy grains may be different due to fractionation effects at sublimation. In this case, all comets may share the same Earth-like D/H ratio in water, with profound implications for the early solar system and the origin of Earth’s oceans.
In recent years, a plethora of observations with high spectral resolution of sub-millimetre and far-infrared transitions of methylidene (CH), conducted with Herschel and SOFIA, have demonstrated this ...radical to be a valuable proxy for molecular hydrogen that can be used for characterising molecular gas within the interstellar medium on a Galactic scale, including the CO-dark component. We report the discovery of the 13CH isotopologue in the interstellar medium using the upGREAT receiver on board SOFIA. We have detected the three hyperfine structure components of the ≈2 THz frequency transition from its X2Π1∕2 ground-state towards the high-mass star-forming regions Sgr B2(M), G34.26+0.15, W49(N), and W51E and determined 13CH column densities. The ubiquity of molecules containing carbon in the interstellar medium has turned the determination of the ratio between the abundances of the two stable isotopes of carbon, 12C/13C, into a cornerstone for Galactic chemical evolution studies. Whilst displaying a rising gradient with galactocentric distance, this ratio, when measured using observations of different molecules (CO, H2CO, and others), shows systematic variations depending on the tracer used. These observed inconsistencies may arise from optical depth effects, chemical fractionation, or isotope-selective photo-dissociation. Formed from C+ either through UV-driven or turbulence-driven chemistry, CH reflects the fractionation of C+, and does not show any significant fractionation effects, unlike other molecules that were previously used to determine the 12C/13C isotopic ratio. This makes it an ideal tracer for the 12C/13C ratio throughout the Galaxy. By comparing the derived column densities of 13CH with previously obtained SOFIA data of the corresponding transitions of the main isotopologue 12CH, we therefore derive 12C/13C isotopic ratios toward Sgr B2(M), G34.26+0.15, W49(N) and W51E. Adding our values derived from 12∕13CH to previous calculations of the Galactic isotopic gradient, we derive a revised value of 12C/13C = 5.87(0.45)RGC + 13.25(2.94).
Abstract
Atomic oxygen is a key species in the mesosphere and thermosphere of Venus. It peaks in the transition region between the two dominant atmospheric circulation patterns, the retrograde ...super-rotating zonal flow below 70 km and the subsolar to antisolar flow above 120 km altitude. However, past and current detection methods are indirect and based on measurements of other molecules in combination with photochemical models. Here, we show direct detection of atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.74 THz (63.2 µm). The atomic oxygen is concentrated at altitudes around 100 km with a maximum column density on the dayside where it is generated by photolysis of carbon dioxide and carbon monoxide. This method enables detailed investigations of the Venusian atmosphere in the region between the two atmospheric circulation patterns in support of future space missions to Venus.
Abstract
Atomic oxygen is a main component of the mesosphere and lower thermosphere of the Earth, where it governs photochemistry and energy balance and is a tracer for dynamical motions. However, ...its concentration is extremely difficult to measure with remote sensing techniques since atomic oxygen has few optically active transitions. Current indirect methods involve photochemical models and the results are not always in agreement, particularly when obtained with different instruments. Here we present direct measurements—independent of photochemical models—of the ground state
3
P
1
→
3
P
2
fine-structure transition of atomic oxygen at 4.7448 THz using the German Receiver for Astronomy at Terahertz Frequencies (GREAT) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). We find that our measurements of the concentration of atomic oxygen agree well with atmospheric models informed by satellite observations. We suggest that this direct observation method may be more accurate than existing indirect methods that rely on photochemical models.
Abstract
We report NASA-DLR SOFIA upGREAT circumstellar O
i
63.2
μ
m and C
ii
157.7
μ
m emission profiles and FIFI-LS O
i
63.2
μ
m, O
i
145.5
μ
m, and C
ii
157.7
μ
m fluxes obtained shortly ...after Betelgeuse’s 2019/2020 Great Dimming event. Haas et al. noted a potential correlation between the O
i
63.2
μ
m flux and
V
magnitude based on three Kuiper Airborne Observatory observations made with the CGS and FIFI instruments. The FIFI observation was obtained when V ≃ 0.88 and revealed a 3
σ
non-detection at a quarter of the previous CGS flux measurement made when
V
≃ 0.35. A potential explanation could be a change in dust-gas drag heating by circumstellar silicates caused by variations in the photospheric radiation field. SOFIA observations provide a unique test of this correlation because the
V
-band brightness went to its lowest value on record,
V
≃ 1.61, with the SOFIA observations being made when
V
FIFI−LS
≃ 1.51 and
V
upGREAT
≃ 1.36. The upGREAT spectra show a O
i
63.2
μ
m flux larger than previous space observatory measurements obtained when
V
≃ 0.58. The profile is consistent with formation in the slower, more turbulent inner S1 outflow, while the C
ii
157.7
μ
m profile is consistent with formation farther out in the faster S2 outflow. Modeling of dust-gas drag heating, combined with 25 yr of Wing three-filter and
V
photometry, reveals that it is unlikely that the S1 circumstellar envelope and O
i
63.2
μ
m fluxes are dominated by the dust-gas drag heating and that another heating source is also active. The O
i
63.2
μ
m profile is hard to reconcile with existing outflow velocity models.
Context. The C ii 158 μm fine structure line is one of the dominant cooling lines in star-forming active regions. Together with models of photon-dominated regions, the data is used to constrain the ...physical properties of the emitting regions, such as the density and the radiation field strength. According to the modeling, the C ii 158 μm line integrated intensity compared to the CO emission is expected to be stronger in lower metallicity environments owing to lower dust shielding of the UV radiation, a trend that is also shown by spectral-unresolved observations. In the commonly assumed clumpy UV-penetrated cloud scenario, the models predict a C ii line profile similar to that of CO. However, recent spectral-resolved observations by Herschel/HIFI and SOFIA/GREAT (as well as the observations presented here) show that the velocity resolved line profile of the C ii emission is often very different from that of CO lines, indicating a more complex origin of the line emission including the dynamics of the source region. Aims. The Large Magellanic Cloud (LMC) provides an excellent opportunity to study in great detail the physics of the interstellar medium (ISM) in a low-metallicity environment by spatially resolving individual star-forming regions. The aim of our study is to investigate the physical properties of the star-forming ISM in the LMC by separating the origin of the emission lines spatially and spectrally. In this paper, we focus on the spectral characteristics and the origin of the emission lines, and the phases of carbon-bearing species in the N159 star-forming region in the LMC. Methods. We mapped a 4′ × (3′–4′) region in N159 in C ii 158 μm and N ii 205 μm with the GREAT instrument on board SOFIA. We also observed CO(3–2), (4–3), (6–5), 13CO(3–2), and C i 3P1–3P0 and 3P2–3P1 with APEX. All spectra are velocity resolved. Results. The emission of all transitions observed shows a large variation in the line profiles across the map and in particular between the different species. At most positions the C ii emission line profile is substantially wider than that of CO and C i. We estimated the fraction of the C ii integrated line emission that cannot be fitted by the CO line profile to be 20% around the CO cores, and up to 50% at the area between the cores, indicating a gas component that has a much larger velocity dispersion than the ones probed by the CO and C i emission. We derived the relative contribution from C+, C, and CO to the column density in each velocity bin. The result clearly shows that the contribution from C+ dominates the velocity range far from the velocities traced by the dense molecular gas. Spatially, the region located between the CO cores of N159 W and E has a higher fraction of C+ over the whole velocity range. We estimate the contribution of the ionized gas to the C ii emission using the ratio to the N ii emission, and find that the ionized gas contributes ≤19% to the C ii emission at its peak position, and ≤15% over the whole observed region. Using the integrated line intensities, we present the spatial distribution of ICII/IFIR. Conclusions. This study demonstrates that the C ii emission in the LMC N159 region shows significantly different velocity profiles from that of CO and C i emissions, emphasizing the importance of velocity resolved observations in order to distinguish different cloud components.
In star-forming environments, shock-compressed magnetic fields occur in cloud-cloud collisions, in molecular clouds exposed to supernova remnants (SNRs), and in photo-dissociation regions (PDRs). ...Besides their dynamical role, they increase the cosmic ray flux above the Galactic average, and the trapped particles contribute to the heating of the shocked gas. The associated dust emission is polarized perpendicularly to the sky plane projection of the field, Bsky. In edge-on viewed shock planes, highly ordered polarization patterns are expected. In search of such a signature, the dust emission from the Orion bar (a prototypical PDR) and from a molecular cloud/SNR interface (IC443-G) was studied with a λ870μm polarimeter at the APEX (Wiesemeyer etal 2014 and references therein). While our polarization map of OMC1 confirms the hourglass shape of Bsky (e.g., Schleuning 1998, Houde etal 2004), a deep integration towards the Orion bar reveals an alignment of Bsky with the shock forming in response to the wind and to the ionizing radiation from the Trapezium cluster (Fig. 1). This structure suggests a compressed magnetic field accelerating cosmic-ray particles, a scenario proposed by Pellegrini et al. (2009) to explain the high excitation temperature of rotationally warm H2 and CO (Shaw et al. 2009, Peng et al. 2012, respectively).
ABSTRACT
We have mapped the NGC 2023 reflection nebula in the 63 and 145 $\mu$m transitions of O i and the 158 $\mu$m C ii spectral lines using the heterodyne receiver upGREAT on SOFIA. The ...observations were used to identify the diffuse and dense components of the photon-dominated region (PDR) traced by the C ii and O i emission, respectively. The velocity-resolved observations reveal the presence of a significant column of low-excitation atomic oxygen, seen in absorption in the O i 63 $\mu$m spectra, amounting to about 20–60 per cent of the oxygen column seen in emission in the O i 145 $\mu$m spectra. Some self-absorption is also seen in C ii, but for the most part it is hardly noticeable. The C ii and O i 63 $\mu$m spectra show strong red- and blue-shifted wings due to photoevaporation flows especially in the south-eastern and southern part of the reflection nebula, where comparison with the mid- and high-J CO emission indicates that the C+ region is expanding into a dense molecular cloud. Using a two-slab toy model the large-scale self-absorption seen in O i 63 $\mu$m is readily explained as originating in foreground low-excitation gas associated with the source. Similar columns have also been observed recently in other Galactic PDRs. These results have two implications: for the velocity-unresolved extragalactic observations this could impact the use of O i 63 $\mu$m as a tracer of massive star formation and secondly, the widespread self-absorption in O i 63 $\mu$m leads to underestimate of the column density of atomic oxygen derived from this tracer and necessitates the use of alternative indirect methods.
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