arXiv subjects

Th. Henning

Publications and source records attributed to Th. Henning.

At least 343 records · Page 19Linked to original sources

arXiv2016

Orbital alignment and starspot properties in the WASP-52 planetary system

We report 13 high-precision light curves of eight transits of the exoplanet WASP-52b, obtained by using four medium-class telescopes, through different filters, and adopting the defocussing technique. One transit was recorded simultaneously from two different observatories and another one from the same site but with two different instruments, including a multi-band camera. Anomalies were clearly detected in five light curves and modelled as starspots occulted by the planet during the transit events. We fitted the clean light curves with the jktebop code, and those with the anomalies with the prism+gemc codes in order to simultaneously model the photometric parameters of the transits and the position, size and contrast of each starspot. We used these new light curves and some from the literature to revise the physical properties of the WASP-52 system. Starspots with similar characteristics were detected in four transits over a period of 43 days. In the hypothesis that we are dealing with the same starspot, periodically occulted by the transiting planet, we estimated the projected orbital obliquity of WASP-52b to be lambda = 3.8 \pm 8.4 degree. We also determined the true orbital obliquity, psi = 20 \pm 50 degree, which is, although very uncertain, the first measurement of psi purely from starspot crossings. We finally assembled an optical transmission spectrum of the planet and searched for variations of its radius as a function of wavelength. Our analysis suggests a flat transmission spectrum within the experimental uncertainties.

astro-ph.EP↗

arXiv2016

A Steeper than Linear Disk Mass-Stellar Mass Scaling Relation

The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887micron survey of the disk population around objects from 2 to 0.03Msun in the nearby 2Myr-old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45AU in radius. Assuming isothermal and optically thin emission, we convert the 887micron flux densities into dust disk masses, hereafter Mdust. We find that the Mdust-Mstar relation is steeper than linear with power law indices 1.3-1.9, where the range reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By re-analyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr-old regions of Taurus, Lupus, and Chamaeleon I share the same Mdust-Mstar relation, while the 10Myr-old Upper Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.

astro-ph.EP↗

arXiv2016

Are Infrared Dark Clouds Really Quiescent?

The dense, cold regions where high-mass stars form are poorly characterised, yet they represent an ideal opportunity to learn more about the initial conditions of high-mass star formation (HMSF), since high-mass starless cores (HMSCs) lack the violent feedback seen at later evolutionary stages. We present continuum maps obtained from the Submillimeter Array (SMA) interferometry at 1.1 mm for four infrared dark clouds (IRDCs, G28.34S, IRDC 18530, IRDC 18306, and IRDC 18308). We also present 1 mm/3 mm line surveys using IRAM 30 m single-dish observations. Our results are: (1) At a spatial resolution of 10^4 AU, the 1.1 mm SMA observations resolve each source into several fragments. The mass of each fragment is on average >10 Msun, which exceeds the predicted thermal Jeans mass of the whole clump by a factor of up to 30, indicating that thermal pressure does not dominate the fragmentation process. Our measured velocity dispersions in the 30 m lines imply that non-thermal motions provides the extra support against gravity in the fragments. (2) Both non-detection of high-J transitions and the hyperfine multiplet fit of N2H+(1-0), C2H(1-0), HCN(1-0), and H13CN(1-0) indicate that our sources are cold and young. However, obvious detection of SiO and the asymmetric line profile of HCO+(1-0) in G28.34S indicate a potential protostellar object and probable infall motion. (3) With a large number of N-bearing species, the existence of carbon rings and molecular ions, and the anti-correlated spatial distributions between N2H+/NH2D and CO, our large-scale high-mass clumps exhibit similar chemical features as small-scale low-mass prestellar objects. This study of a small sample of IRDCs illustrates that thermal Jeans instability alone cannot explain the fragmentation of the clump into cold (~15 K), dense (>10^5 cm-3) cores and that these IRDCs are not completely quiescent.

astro-ph.GA↗

arXiv2016

Investigating dust trapping in transition disks with millimeter-wave polarization

Spatially resolved polarized (sub-)mm emission has been observed for example in the protoplanetary disk around HL Tau. Magnetically aligned grains are commonly interpreted as the source of polarization. However, self-scattering by large dust grains with a high enough albedo is another polarization mechanism, becoming a compelling method independent of the spectral index to constrain the dust grain size in protoplanetary disks. We study the dust polarization at mm wavelength in the dust trapping scenario proposed for transition disks, when a giant planet opens a gap in the disk. We investigate the characteristic polarization patterns and their dependence on disk inclination, dust size evolution, planet position, and observing wavelength. We combine two-dimensional hydrodynamical simulations of planet-disk interactions with self-consistent dust growth models. These size-dependent dust density distributions are used for follow-up three-dimensional radiative transfer calculations to predict the polarization degree at ALMA bands due to scattered thermal emission. We find that the polarization pattern of a disk with a planetary gap after 1 Myr of dust evolution shows a distinctive three ring structure. Two narrow inner rings are located at the planet gap edges. For increasing observing wavelengths all three rings slightly change their position, where the innermost and outermost rings move inward. This distance is detectable comparing the results at ALMA bands 3, 6 and 7. Within the highest polarized intensity regions the polarization vectors are oriented in the azimuthal direction. For an inclined disk there is an interplay between polarization originating from a flux gradient and inclination-induced quadrupole polarization. For intermediate inclined transition disks the polarization degree is as high as ~ 2% at band 3, which is well above the detection limit of future ALMA observations.

astro-ph.EP↗

arXiv2016

New debris disks in nearby young moving groups

A significant fraction of nearby young moving group members harbor circumstellar debris dust disks. Due to their proximity and youth, these disks are attractive targets for studying the early evolution of debris dust and planetesimal belts. Here we present 70 and 160$μ$m observations of 31 systems in the $β$ Pic moving group, and in the Tucana-Horologium, Columba, Carina and Argus associations, using the Herschel Space Observatory. None of these stars were observed at far-infrared wavelengths before. Our Herschel measurements were complemented by photometry from the WISE satellite for the whole sample, and by submillimeter/millimeter continuum data for one source, HD 48370. We identified six stars with infrared excess, four of them are new discoveries. By combining our new findings with results from the literature, we examined the incidence and general characteristics of debris disks around Sun-like members of the selected groups. With their dust temperatures of <45 K the newly identified disks around HD 38397, HD 48370, HD 160305, and BD-20 951 represent the coldest population within this sample. For HD 38397 and HD 48370, the emission is resolved in the 70$μ$m PACS images, the estimated radius of these disks is ~90 au. Together with the well-known disk around HD 61005, these three systems represent the highest mass end of the known debris disk population around young G-type members of the selected groups. In terms of dust content, they resemble the hypothesized debris disk of the ancient Solar System.

astro-ph.SR↗

arXiv2016

The abundance and thermal history of water ice in the disk surrounding HD142527 from the DIGIT Herschel Key Program

The presence or absence of ice in protoplanetary disks is of great importance for the formation of planets. By enhancing the solid surface density and increasing the sticking efficiency, ice catalyzes the rapid formation of planetesimals and decreases the time scale for giant planet core accretion. Aims: In this paper we analyse the composition of the outer disk around the Herbig star HD~142527. We focus on the composition of the water ice, but also analyse the abundances of previously proposed minerals. Methods: We present new Herschel far infrared spectra and a re-reduction of archival data from the Infrared Space Observatory (ISO). We model the disk using full 3D radiative transfer to obtain the disk structure. Also, we use an optically thin analysis of the outer disk spectrum to obtain firm constraints on the composition of the dust component. Results: The water ice in the disk around HD~142527 contains a large reservoir of crystalline water ice. We determine the local abundance of water ice in the outer disk (i.e. beyond 130\,AU). The re-reduced ISO spectrum differs significantly from that previously published, but matches the new Herschel spectrum at their common wavelength range. In particular, we do not detect any significant contribution from carbonates or hydrous silicates, in contrast to earlier claims. Conclusions: The amount of water ice detected in the outer disk requires $\sim80\,$\% of the oxygen atoms. This is comparable to the water ice abundance in the outer solar system, in comets and in dense interstellar clouds. The water ice is highly crystalline while the temperatures where we detect it are too low to crystallize the water on relevant time scales. We discuss the implications of this finding.

astro-ph.EP↗

arXiv2016

Characterizing HR3549B using SPHERE

Aims. In this work, we characterize the low mass companion of the A0 field star HR3549. Methods. We observed HR3549AB in imaging mode with the the NIR branch (IFS and IRDIS) of SPHERE@VLT, with IFS in YJ mode and IRDIS in the H band. We also acquired a medium resolution spectrum with the IRDIS long slit spectroscopy mode. The data were reduced using the dedicated SPHERE GTO pipeline, purposely designed for this instrument. We employed algorithms such as PCA and TLOCI to reduce the speckle noise. Results. The companion was clearly visible both with IRDIS and IFS.We obtained photometry in four different bands as well as the astrometric position for the companion. Based on our astrometry, we confirm that it is a bound object and put constraints on its orbit. Although several uncertainties are still present, we estimate an age of ~100-150 Myr for this system, yielding a most probable mass for the companion of 40-50MJup and T_eff ~300-2400 K. Comparing with template spectra points to a spectral type between M9 and L0 for the companion, commensurate with its position on the color-magnitude diagram.

astro-ph.EP↗

arXiv2016

Earliest phases of star formation (EPoS): Dust temperature distributions in isolated starless cores

Constraining the temperature and density structure of dense molecular cloud cores is fundamental for understanding the initial conditions of star formation. We use Herschel observations of the thermal FIR dust emission from nearby isolated molecular cloud cores and combine them with ground-based submillimeter continuum data to derive observational constraints on their temperature and density structure. The aim of this study is to verify the validity of a ray-tracing inversion technique developed to derive the dust temperature and density structure of isolated starless cores directly from the dust emission maps and to test if the resulting temperature and density profiles are consistent with physical models. Using this ray-tracing inversion technique, we derive the dust temperature and density structure of six isolated starless cloud cores. We employ self-consistent radiative transfer modeling to the derived density profiles, treating the ISRF as the only heating source. The best-fit values of local strength of the ISRF and the extinction by the outer envelope are derived by comparing the self-consistently calculated temperature profiles with those derived by the ray-tracing method. We find that all starless cores are significantly colder inside than outside, with the core temperatures showing a strong negative correlation with peak column density. This suggests that their thermal structure is dominated by external heating from the ISRF and shielding by dusty envelopes. The temperature profiles derived with the ray-tracing inversion method can be well-reproduced with self-consistent radiative transfer models.

astro-ph.GA↗

arXiv2016

An optical transmission spectrum of the transiting hot Jupiter in the metal-poor WASP-98 planetary system

The WASP-98 planetary system represents a rare case of a hot Jupiter hosted by a metal-poor main-sequence star. We present a follow-up study of this system based on multi-band photometry and high-resolution spectroscopy. Two new transit events of WASP-98b were simultaneously observed in four passbands (g,r,i,z), using the telescope-defocussing technique, yielding eight high-precision light curves with point-to-point scatters of less than 1 mmag. We also collected three spectra of the parent star with a high-resolution spectrograph, which we used to remeasure its spectral characteristics, in particular its metallicity. We found this to be very low, Fe/H]=-0.49, but larger than was previously reported, [Fe/H]=-0.60. We used these new photometric and spectroscopic data to refine the orbital and physical properties of this planetary system, finding that the stellar and planetary mass measurements are significantly larger than those in the discovery paper. In addition, the multi-band light curves were used to construct an optical transmission spectrum of WASP-98b and probe the characteristics of its atmosphere at the terminator. We measured a lower radius at z compared with the other three passbands. The maximum variation is between the r and z bands, has a confidence level of roughly 6 sigma and equates to 5.5 pressure scale heights. We compared this spectrum to theoretical models, investigating several possible types of atmospheres, including hazy, cloudy, cloud-free, and clear atmospheres with titanium and vanadium oxide opacities. We could not find a good fit to the observations, except in the extreme case of a clear atmosphere with TiO and VO opacities, in which the condensation of Ti and V was suppressed. As this case is unrealistic, our results suggest the presence of an additional optical-absorbing species in the atmosphere of WASP-98b, of unknown chemical nature.

astro-ph.EP↗

arXiv2016

Appearance of Dusty Filaments at Different Viewing Angles

Context: In the last years, there have been many studies on the omnipresence and structures of filaments in star-forming regions, as well as their role in the process of star formation. Those filaments are normally identified as elongated fibres across the plane of the sky. But how would we detect filaments that are inclined? Aims: We aim to learn more about whether, and how, total column density or dust temperature change with respect to the line of sight. Such variations would enable observers to use dust observations to identify and study filaments at any inclination and gain more insight on the distribution and orientations of filaments within the Galactic plane. Methods: As a first step, we perform numerical calculations on simple cylindrical models to evaluate the influence of filament geometry on the average flux density. After that, we apply our three-dimensional Monte Carlo dust radiative transfer code on two models of star-forming regions and derive maps of effective total column density and dust temperature at different viewing angles. Results: We see only slight changes of average flux density for all cylinders we study. For our more complex models, we find that the effective dust temperature is not sensitive to viewing angle, while the total column density is strongly influenced, with differences exceeding an order of magnitude. The variations are not injective with the viewing angle and depend on the structure of the object. Conclusions: We conclude that there is no single quantity in our analysis that can uniquely trace the inclination and three-dimensional structure of a filament based on dust observations alone. However, observing wide variations in total column density at a given effective dust temperature is indicative of inclined filaments.

astro-ph.GA↗

arXiv2016

Inferring the evolutionary stages of the internal structures of NGC 7538 S and IRS1 from chemistry

To unambiguously diagnose the detailed physical mechanisms and the evolutionary status of high-mass star-forming regions (HMSFRs), we have performed 0.4" ($\sim1000$ AU) resolution observations towards NGC7538S and IRS1, using the Plateau de Bure Interferometer (PdBI). This paper presents a joint analysis of the 1.37 mm continuum emission and the line intensity of 15 molecular species. Assuming local thermal equilibrium, we derived molecular column densities and molecular abundances for each internal gas substructure which is spatially resolved. These derived quantities are compared with a suite of 1-D gas-grain models. NGC7538S is resolved into at least three dense gas condensations. Despite the comparable continuum intensity of these condensations, their differing molecular line emission is suggestive of an overall chemical evolutionary trend from the northeast to the southeast. Since these condensations are embedded within the same parent gas core, their differing chemical properties are the most likely due to the different warm-up histories, rather than the different dynamic timescales. Despite remaining spatially unresolved, in IRS1 we detect abundant complex organic molecules, indicating that IRS1 is the most chemically evolved hot molecular core presented here. We observe a continuum that is dominated by absorption features with at least three strong emission lines, potentially from $\rm CH_3OH$. The $\rm CH_3OH$ lines which are purely in emission have higher excitation than the ones showing purely absorption. Potential reasons for that difference are discussed. This is the first comprehensive comparison of observations of the high-mass cores NGC7538S and IRS1 and a chemical model. We have found that different chemical evolutionary stages can coexist in the same natal gas core. Our achievement illustrates the strength of chemical analysis for understanding HMSFRs.

astro-ph.GA↗

arXiv2016

Azimuthal asymmetries in the debris disk around HD61005

Debris disks offer valuable insights into the latest stages of circumstellar disk evolution, and can possibly help us to trace the outcomes of planetary formation processes. In the age range 10 to 100\,Myr, most of the gas is expected to have been removed from the system, giant planets (if any) must have already been formed, and the formation of terrestrial planets may be on-going. Pluto-sized planetesimals, and their debris released in a collisional cascade, are under their mutual gravitational influence, which may result into non-axisymmetric structures in the debris disk. High angular resolution observations are required to investigate these effects and constrain the dynamical evolution of debris disks. Furthermore, multi-wavelength observations can provide information about the dust dynamics by probing different grain sizes. Here we present new VLT/SPHERE and ALMA observations of the debris disk around the 40\,Myr-old solar-type star HD\,61005. We resolve the disk at unprecedented resolution both in the near-infrared (in scattered and polarized light) and at millimeter wavelengths. Thanks to the new observations, we propose a solution for both the radial and azimuthal distribution of the dust grains in the debris disk. We find that the disk has a moderate eccentricity ($e \sim 0.1$) and that the dust density is two times larger at the pericenter compared to the apocenter. With no giant planets detected in our observations, we investigate alternative explanations besides planet-disk interactions to interpret the inferred disk morphology. We postulate that the morphology of the disk could be the consequence of a massive collision between $\sim$\,1000\,km-sized bodies at $\sim$\,61\,au. If this interpretation holds, it would put stringent constraints on the formation of massive planetesimals at large distances from the star.

astro-ph.SR↗

arXiv2016

Giant molecular filaments in the Milky Way II: The fourth Galactic quadrant

Filamentary structures are common morphological features of the cold, molecular interstellar medium (ISM). Recent studies have discovered massive, hundred-parsec-scale filaments that may be connected to the large-scale, Galactic spiral arm structure. Addressing the nature of these Giant Molecular Filaments (GMFs) requires a census of their occurrence and properties. We perform a systematic search of GMFs in the fourth Galactic quadrant and determine their basic physical properties. We identify GMFs based on their dust extinction signatures in near- and mid-infrared and velocity structure probed by ^{13}CO line emission. We use the ^{13}CO line emission and ATLASGAL dust emission data to estimate the total and dense gas masses of the GMFs. We combine our sample with an earlier sample from literature and study the Galactic environment of the GMFs. We identify nine GMFs in the fourth Galactic quadrant; six are located in the Centaurus spiral arm and three in inter-arm regions. Combining this sample with an earlier study using the same identification criteria in the first Galactic quadrant results in 16 GMFs, nine of which are located within spiral arms. The GMFs have sizes of 80-160 pc and ^{13}CO-derived masses between 5-90 x 10^{4} Msun. Their dense gas mass fractions are between 1.5-37%, being higher in the GMFs connected to spiral arms. We also compare the different GMF-identification methods and find that emission and extinction based techniques overlap only partially, highlighting the need to use both to achieve a complete census.

astro-ph.GA↗

arXiv2016

Kepler-539: a young extrasolar system with two giant planets on wide orbits and in gravitational interaction

We confirm the planetary nature of Kepler-539b (aka Kepler object of interest K00372.01), a giant transiting exoplanet orbiting a solar-analogue G2 V star. The mass of Kepler-539b was accurately derived thanks to a series of precise radial velocity measurements obtained with the CAFE spectrograph mounted on the CAHA 2.2m telescope. A simultaneous fit of the radial-velocity data and Kepler photometry revealed that Kepler-539b is a dense Jupiter-like planet with a mass of Mp = 0.97 Mjup and a radius of Rp = 0.747 Rjup, making a complete circular revolution around its parent star in 125.6 days. The semi-major axis of the orbit is roughly 0.5 au, implying that the planet is at roughly 0.45 au from the habitable zone. By analysing the mid-transit times of the 12 transit events of Kepler-539b recorded by the Kepler spacecraft, we found a clear modulated transit time variation (TTV), which is attributable to the presence of a planet c in a wider orbit. The few timings available do not allow us to precisely estimate the properties of Kepler-539c and our analysis suggests that it has a mass between 1.2 and 3.6 Mjup, revolving on a very eccentric orbit (0.4<e<0.6) with a period larger than 1000 days. The high eccentricity of planet c is the probable cause of the TTV modulation of planet b. The analysis of the CAFE spectra revealed a relatively high photospheric lithium content, A(Li)=2.48 dex, which, together with both a gyrochronological and isochronal analysis, suggests that the parent star is relatively young.

astro-ph.EP↗

arXiv2016

Chemistry in Disks X: The Molecular Content of Proto-planetary Disks in Taurus

(abridged) We used the IRAM 30-m to perform a sensitive wideband survey of 30 protoplanetary disks in the Taurus Auriga region. We simultaneously observed HCO$^+$(3-2), HCN(3-2), C$_2$H(3-2), CS(5-4), and two transitions of SO. We combine the results with a previous survey which observed $^{13}$CO (2-1), CN(2-1), two o-H$_2$CO lines and one of SO. We use available interferometric data to derive excitation temperatures of CN and C$_2$H in several sources. We determine characteristic sizes of the gas disks and column densities of all molecules using a parametric power-law disk model. Our study is mostly sensitive to molecules at 200-400 au from the stars. We compare the derived column densities to the predictions of an extensive gas-grain chemical disk model, under conditions representative of T Tauri disks. This survey provides 20 new detections of HCO$^+$ in disks, 18 in HCN, 11 in C$_2$H, 8 in CS and 4 in SO. HCO$^+$ is detected in almost all sources, and its J=3-2 line is essentially optically thick, providing good estimates of the disk radii. The other transitions are (at least partially) optically thin. Variations of the column density ratios do not correlate with any specific property of the star or disk. Disks around Herbig Ae stars appear less rich in molecules than those around T Tauri stars, although the sample remains small. SO is only found in the (presumably younger) embedded objects, perhaps reflecting an evolution of the S chemistry due to increasing depletion with time. Overall, the molecular column densities, and in particular the CN/HCN and CN/C$_2$H ratios, are well reproduced by gas-grain chemistry in cold disks. This study provides a census of simple molecules in disks of radii $> 200-300$ au. Extending that to smaller disks, or searching for less abundant or more complex molecules requires a much more sensitive facility, i.e. NOEMA and ALMA.

astro-ph.EP↗

arXiv2016

An optical transmission spectrum of the giant planet WASP-36 b

We present broad-band photometry of five transits in the planetary system WASP-36, totaling 17 high-precision light curves. Four of the transits were simultaneously observed in four passbands (g, r, i, z), using the telescope-defocussing technique, and achieving scatters of less than 1 mmag per observation. We used these data to improve the measured orbital and physical properties of the system, and obtain an optical transmission spectrum of the planet. We measured a decreasing radius from bluer to redder passbands with a confidence level of more than 5 sigma. The radius variation is roughly 11 pressure scale heights between the g and the z bands. This is too strong to be Rayleigh scattering in the planetary atmosphere, and implies the presence of a species which absorbs strongly at bluer wavelengths.

astro-ph.EP↗

arXiv2016

Gaps, rings, and non-axisymmetric structures in protoplanetary disks - Emission from large grains

Dust grains with sizes around (sub)mm are expected to couple only weakly to the gas motion in regions beyond 10 au of circumstellar disks. In this work, we investigate the influence of the spatial distribution of such grains on the (sub)mm appearance of magnetized protoplanetary disks. We perform non-ideal global 3D magneto-hydrodynamic stratified disk simulations including particles of different sizes (50 micron to 1 cm), using a Lagrangian particle solver. We calculate the spatial dust temperature distribution, including the dynamically coupled submicron-sized dust grains, and derive ideal continuum re-emission maps of the disk through radiative transfer simulations. Finally, we investigate the feasibility to observe specific structures in the thermal re-emission maps with the Atacama Large Millimeter/submillimeter Array (ALMA). The pressure bump close to the outer edge of the dead-zone leads to particle trapping in ring structures. More specifically, vortices in the disk concentrate the dust and create an inhomogeneous distribution of the solid material in the azimuthal direction. The large-scale disk perturbations are preserved in the (sub)mm re-emission maps. The observable structures are very similar to those expected to result from planet-disk interaction. The larger dust particles increase the brightness contrast between gap and ring structures. We find that rings, gaps and the dust accumulation in the vortex could be traced with ALMA down to a scale of a few astronomical units in circumstellar disks located in nearby star-forming regions. Finally, we present a brief comparison of these structures with those recently found with ALMA in the young circumstellar disks of HL Tau and Oph IRS 48.

astro-ph.EP↗

arXiv2016

Ultra-Low-Temperature Reactions of Carbon Atoms with Hydrogen Molecules

The reactions of carbon atoms with dihydrogen have been investigated in liquid helium droplets at $T$ = 0.37 K. A calorimetric technique was applied to monitor the energy released in the reaction. The barrierless reaction between a single carbon atom and a single dihydrogen molecule was detected. Reactions between dihydrogen clusters and carbon atoms have been studied by high-resolution mass spectrometry. The formation of hydrocarbon cations of the type C$_m$H$_n^+$, with $m$ = 1-4 and $n$ = 1-15 was observed. With enhanced concentration of dihydrogen, the mass spectra demonstrated the main "magic" peak assigned to the CH$_5^+$ cation. A simple formation pathway and the high stability of this cation suggest its high abundance in the interstellar medium.

astro-ph.GA↗