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.
Aims. The aim of our study is to investigate the physical properties of the star-forming interstellar medium (ISM) in the Large Magellanic Cloud (LMC) by separating the origin of the emission lines ...spatially and spectrally. The LMC provides a unique local template to bridge studies in the Galaxy and high redshift galaxies because of its low metallicity and proximity, enabling us to study the detailed physics of the ISM in spatially resolved individual star-forming regions. Following Okada et al. (Okada, Y., Requena-Torres, M. A., Güsten, R., et al. 2015, A&A, 580, A54), we investigate different phases of the ISM traced by carbon-bearing species in four star-forming regions in the LMC, and model the physical properties using the KOSMA-τ PDR model. Methods. We mapped 3–13 arcmin2 areas in 30 Dor, N158, N160, and N159 along the molecular ridge of the LMC in C II 158 μm with GREAT on board SOFIA. We also observed the same area with CO(2-1) to (6-5), 13CO(2-1) and (3-2), C I 3P1–3P0 and 3P2–3P1 with APEX. For selected positions in N159 and 30 Dor, we observed O I 145 μm and O I 63 μm with upGREAT. All spectra are velocity resolved. Results. In all four star-forming regions, the line profiles of CO, 13CO, and C I emission are similar, being reproduced by a combination of Gaussian profiles defined by CO(3-2), whereas C II typically shows wider line profiles or an additional velocity component. At several positions in N159 and 30 Dor, we observed the velocity-resolved O I 145 and 63 μm lines for the first time. At some positions, the O I line profiles match those of CO, at other positions they are more similar to the C II profiles. We interpret the different line profiles of CO, C II and O I as contributions from spatially separated clouds and/or clouds in different physical phases, which give different line ratios depending on their physical properties. We modeled the emission from the CO, C I, C II, and O I lines and the far-infrared continuum emission using the latest KOSMA-τ PDR model, which treats the dust-related physics consistently and computes the dust continuum SED together with the line emission of the chemical species. We find that the line and continuum emissions are not well-reproduced by a single clump ensemble. Toward the CO peak at N159 W, we propose a scenario that the CO, C II, and O I 63 μm emission are weaker than expected because of mutual shielding among clumps.
The general features of the internet browser-accessible Cologne Database for Molecular Spectroscopy (CDMS) and recent developments in the CDMS are described in the present article. The database ...consists of several parts; among them is a catalog of transition frequencies from the radio-frequency to the far-infrared region covering atomic and molecular species that (may) occur in the interstellar or circumstellar medium or in planetary atmospheres. As of December 2004, 280 species are present in this catalog. The transition frequencies were predicted from fits of experimental data to established Hamiltonian models. We present some examples to demonstrate how the combination of various input data or a compact representation of the Hamiltonian can be beneficial for the prediction of the line frequencies.
Context. 13C II observations in several Galactic sources show that the fine-structure 12C II emission is often optically thick (the optical depths around 1 to a few). Aims. Our goal was to test ...whether this also affects the 12C II emission from nearby galaxies like the Large Magellanic Cloud (LMC). Methods. We observed three star-forming regions in the LMC with upGREAT on board SOFIA at the frequency of the C II line. The 4 GHz bandwidth covers all three hyperfine lines of 13C II simultaneously. For the analysis, we combined the 13C II F = 1−0 and F = 1−1 hyperfine components as they do not overlap with the 12C II line in velocity. Results. Three positions in N159 and N160 show an enhancement of 13C II compared to the abundance-ratio-scaled 12C II profile. This is likely due to the 12C II line being optically thick, supported by the fact that the 13C II line profile is narrower than 12C II, the enhancement varies with velocity, and the peak velocity of 13C II matches the O I 63 μm self-absorption. The 12C II line profile is broader than expected from a simple optical depth broadening of the 13C II line, supporting the scenario of several PDR components in one beam having varying 12C II optical depths. The derived 12C II optical depth at three positions (beam size of 14″, corresponding to 3.4 pc) is 1−3, which is similar to values observed in several Galactic sources shown in previous studies. If this also applies to distant galaxies, the C II intensity will be underestimated by a factor of approximately 2.
The age of dense interstellar cloud cores, where stars and planets form, is a crucial parameter in star formation and difficult to measure. Some models predict rapid collapse, whereas others predict ...timescales of more than one million years (ref. 3). One possible approach to determining the age is through chemical changes as cloud contraction occurs, in particular through indirect measurements of the ratio of the two spin isomers (ortho/para) of molecular hydrogen, H2, which decreases monotonically with age. This has been done for the dense cloud core L183, for which the deuterium fractionation of diazenylium (N2H(+)) was used as a chemical clock to infer that the core has contracted rapidly (on a timescale of less than 700,000 years). Among astronomically observable molecules, the spin isomers of the deuterated trihydrogen cation, ortho-H2D(+) and para-H2D(+), have the most direct chemical connections to H2 (refs 8, 9, 10, 11, 12) and their abundance ratio provides a chemical clock that is sensitive to greater cloud core ages. So far this ratio has not been determined because para-H2D(+) is very difficult to observe. The detection of its rotational ground-state line has only now become possible thanks to accurate measurements of its transition frequency in the laboratory, and recent progress in instrumentation technology. Here we report observations of ortho- and para-H2D(+) emission and absorption, respectively, from the dense cloud core hosting IRAS 16293-2422 A/B, a group of nascent solar-type stars (with ages of less than 100,000 years). Using the ortho/para ratio in conjunction with chemical models, we find that the dense core has been chemically processed for at least one million years. The apparent discrepancy with the earlier N2H(+) work arises because that chemical clock turns off sooner than the H2D(+) clock, but both results imply that star-forming dense cores have ages of about one million years, rather than 100,000 years.
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
We present analysis of O
i
63
μ
m and CO
J
= 5 − 4 and 8 − 7 multiposition data in the W3A region and use it to develop a model for the extended low-density foreground gas that produces ...absorption features in the O
i
and
J
= 5 − 4 CO lines. We employ the extinction to the exciting stars of the background H
ii
region to constrain the total column density of the foreground gas. We have used the Meudon photodissociation region code to model the physical conditions and chemistry in the region employing a two-component model with a high-density layer near the H
ii
region responsible for the fine-structure line emission and an extended low-density foreground layer. The best-fitting total proton density, constrained largely by the CO lines, is
n
(H) = 250 cm
−3
in the foreground gas and 5 × 10
5
cm
−3
in the material near the H
ii
region. The absorption is distributed over the region mapped in W3A and is not restricted to the foreground of either the embedded exciting stars of the H
ii
region or the protostar W3 IRS5. The low-density material associated with regions of massive-star formation, based on an earlier study by Goldsmith et al., is quite common, and we now see that it is extended over a significant portion of W3A. It thus should be included in modeling of fine-structure line emission, including interpreting low-velocity-resolution observations made with incoherent spectrometer systems, in order to use these lines as accurate tracers of massive-star formation.
Abstract
We used high-resolution C
ii
158
μ
m mapping of two nebulae IC 59 and IC 63 from SOFIA/upGREAT in conjunction with ancillary data of the gas, dust, and polarization to probe the kinematics, ...structure, and magnetic properties of their photodissociation regions (PDRs). The nebulae are part of the Sh 2-185 H
ii
region that is illuminated by the B0 IVe star
γ
Cas. The velocity structure of each PDR changes with distance from
γ
Cas, which is consistent with driving by the radiation. Based on previous far-ultraviolet (FUV) flux measurements of, and the known distance to,
γ
Cas, along with the predictions of 3D distances to the clouds, we estimated the FUV radiation field strength (
G
0
) at the clouds. Assuming negligible extinction between the star and clouds, we find their 3D distances from
γ
Cas. For IC 63, our results are consistent with earlier estimates of distance from Andersson et al., locating the cloud at ∼2 pc from
γ
Cas at an angle of 58° to the plane of the sky behind the star. For IC 59, we derive a distance of 4.5 pc at an angle of 70° in front of the star. We do not detect any significant correlation between the orientation of the magnetic field and the velocity gradients of C
ii
gas, which indicates a moderate magnetic field strength. The kinetic energy in IC 63 is estimated to be an order of 10 higher than the magnetic energies. This suggests that kinetic pressure in this nebula is dominant.
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.
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