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.
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.
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).
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.
Abstract Hub–filament systems (HFSs) being the potential sites of formation of star clusters and high-mass stars, provide a testbed for the current theories that attempt to explain star formation ...globally. It is thus important to study a large number of HFSs using both intensity and velocity information to constrain these objects better observationally. Here, we present a study of the HFS associated with G6.55-0.1 using newly obtained observations of the radio continuum and the J = 2–1 transition of CO, 13 CO, and C 18 O. The radio continuum maps show multiple peaks that coincide with far-infrared dust continuum peaks, indicating the presence of more than one young massive star in the hub of the HFS. We used the velocity information from the C 18 O(2–1) map to (a) show that the source G6.55-0.1 is not physically associated with the supernova remnant W28 and (b) disentangle and identify the velocity components genuinely associated with G6.55-0.1. Among the velocity-coherent structures identified in the region, we conclude that only the two filaments at 13.8 and 17.3 km s −1 contribute a total mass accretion rate of 3000 M ⊙ Myr −1 to the hub. Both the filaments also show a V-shaped structure, characteristic of gravitational collapse, in their velocity profile at the location of the hub. The estimated mass per unit length of the segments of the filaments is smaller than the critical line masses derived from virial equilibrium considerations. This suggests that the filaments are not gravitationally collapsing as a whole, although their inner parts clearly show evidence of collapse in the form of young star-forming cores. We further conclude that the observed velocity gradients are consistent with the gravitational collapse of the main source in the region as estimated from its mass and size.
Spectral lines of ammonia, NH3, are useful probes of the physical conditions in dense molecular cloud cores. In addition to advantages in spectroscopy, ammonia has also been suggested to be resistant ...to freezing onto grain surfaces, which should make it a superior tool for studying the interior parts of cold, dense cores. Here we present high-resolution NH3 observations with the Very Large Array and Green Bank Telescope toward a prestellar core. These observations show an outer region with a fractional NH3 abundance of X(NH3) = (1.975 ± 0.005) × 10−8 (±10% systematic), but it also reveals that, after all, the X(NH3) starts to decrease above a H2 column density of ≈2.6 × 1022 cm−2. We derive a density model for the core and find that the break point in the fractional abundance occurs at the density n(H2) ∼ 2 × 105 cm−3, and beyond this point the fractional abundance decreases with increasing density, following the power law n−1.1. This power-law behavior is well reproduced by chemical models where adsorption onto grains dominates the removal of ammonia and related species from the gas at high densities. We suggest that the break-point density changes from core to core depending on the temperature and the grain properties, but that the depletion power law is anyway likely to be close to n−1 owing to the dominance of accretion in the central parts of starless cores.
Abstract We present SOFIA-upGREAT observations of C ii emission of Infrared Dark Cloud (IRDC) G035.39-00.33, designed to trace its atomic gas envelope and thus test models of the origins of such ...clouds. Several velocity components of C ii emission are detected, tracing structures that are at a wide range of distances in the Galactic plane. We find a main component that is likely associated with the IRDC and its immediate surroundings. This strongest emission component has a velocity similar to that of the 13CO(2–1) emission of the IRDC, but offset by ∼3 km s−1 and with a larger velocity width of ∼9 km s−1. The spatial distribution of the C ii emission of this component is also offset predominantly to one side of the dense filamentary structure of the IRDC. The C ii column density is estimated to be of the order of ∼1017–1018 cm−2. We compare these results to the C ii emission from numerical simulations of magnetized, dense gas filaments formed from giant molecular cloud (GMC) collisions, finding similar spatial and kinematic offsets. These observations and modellingof C ii add further to the evidence that IRDC G035.39-00.33 has been formed by a process of GMC–GMC collision, which may thus be an important mechanism for initiating star cluster formation.
We present Submillimeter Array (SMA) observations in the CO J = 3-2, SiO J = 5-4 and 8-7, and SO 98-87 lines, as well as Atacama Pathfinder EXperiment observations in the CO J = 6-5 line, of an ...extremely high-velocity and jet-like outflow in high-mass star-forming region HH 80-81. The outflow is known to contain two prominent molecular bullets, namely B1 and B2, discovered from our previous SMA CO J = 2-1 observations. While B1 is detected in all the CO, SiO, and SO lines, B2 is only detected in CO lines. The CO 3-2/2-1 line ratio in B1 is clearly greater than that in B2. We perform a large velocity gradient analysis of the CO lines and derive a temperature of 70-210 K for B1 and 20-50 K for B2. Taking into account the differences in the velocity, distance from the central source, excitation conditions, and chemistry between the two bullets, we suggest that the bullets are better explained by direct ejections from the innermost vicinity of the central high-mass protostar, and that we are more likely observing the molecular component of a primary wind rather than entrained or swept-up material from the ambient cloud. These findings further support our previous suggestions that the molecular bullets indicate an episodic, disk-mediated accretion in the high-mass star formation process.
Abstract
The FU Orionis–type objects (FUors) are low-mass pre-main-sequence objects that go through a short-lived phase (∼100 yr) of increased mass accretion rate (from 10
−8
to 10
−4
M
⊙
yr
−1
). ...These eruptive young stars are in the early stages of stellar evolution and thus still deeply embedded in a massive envelope that feeds material to the circumstellar disk that is then accreted onto the star. Some FUors drive molecular outflows, i.e., low-velocity wide-angle magnetohydrodynamical winds, that inject energy and momentum back to the surrounding envelopes and help clear the material surrounding the young star. Here we present a
12
CO (3–2),
13
CO (3–2), and
12
CO (4–3) survey of 20 FUor-type eruptive young stars observed with APEX. We use our
13
CO (3–2) observations to measure the masses of the envelopes surrounding each FUor and find an agreement with the FUor evolutionary trend found from the 10
μ
m silicate feature. We find outflows in 11 FUors, calculate their masses and other kinematic properties, and compare these with those of outflows found around quiescent young stellar objects gathered from the literature. This comparison indicates that outflows in FUors are more massive than outflows in quiescent sources, and that FUor outflows have a higher-ratio outflow mass with respect to the envelope than the quiescent sample, indicating that the eruptive young stars have lower star-forming efficiencies. Finally, we find that the outflow forces in FUors are similar to those of quiescent young stellar objects, indicating that their accretion histories are similar or that the FUor outflows have lower velocities.
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.
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