arXiv subjects

K. Furuya

Publications and source records attributed to K. Furuya.

At least 19 recordsLinked to original sources

arXiv2026

Molecular and Atomic Gas Associated with the Gamma Ray Supernova Remnant RCW 103

We present a study of the interstellar molecular and atomic gas associated with the gamma-ray supernova remnant (SNR) RCW 103 using the Mopra $^{12}$CO($J$ = 1-0), Atacama Large Millimeter/submillimeter Array $^{13}$CO($J$ = 1-0), and the Australia Telescope Compact Array & Parkes HI data. We find that CO clouds in the velocity range from $-58.7$ to $-43.5$ km s$^{-1}$ show a clear spatial correspondence with the X-ray shell of RCW 103, particularly extending from the northwestern to the southeastern regions. The position-velocity diagram of the CO emission reveals an expanding gas motion with an expansion velocity of $\Delta V \sim$$7.5$ km s$^{-1}$, which may have been produced by shock waves and/or stellar winds from the progenitor system. We estimate the total cosmic-ray proton energy to be $\sim$$3.0^{+1.1}_{-0.6} \times 10^{47}$ erg, adopting a total interstellar proton density of $\sim$$810$ cm$^{-3}$. This value is more than an order of magnitude lower than those inferred for other gamma-ray emitting SNRs of comparable age. Given that a substantial amount of dense gas remains within the interior of RCW 103, where thermal X-ray emission is dominant, our results possibly suggest that a significant fraction of the shock energy may have been converted into thermal energy via gas heating, thereby reducing the efficiency of cosmic-ray acceleration.

astro-ph.HE↗

arXiv2026

Fizzy water ice in space: CO$_2$ adsorption, binding energies and its fate in a protoplanetary disk

CO2 is the third most abundant ice component found on dust grains in star-forming regions and a common ingredient of exoplanet atmospheres. Characterization of its adsorption properties on ices through the binding energy (BE) is essential for accurate astrochemical modelling and understanding chemical inheritance in planet formation. We aim to derive an accurate BE distribution of CO2 on water ices. Our goal is to understand the impact of the BE distribution on the abundance of gaseous and frozen CO2 in a generic protoplanetary disk and the spectral absorption features of frozen CO2. The ACO-FROST procedure is used for computing the BE distribution, where CO2 molecules are adsorbed on several sites of an amorphous water ice grain model. The BEs are computed using an ONIOM scheme. The BEs of CO2 follow a bimodal Gaussian distribution characterised by the following parameters: {\mu}1 = 1648K, {\sigma}1=229K, {\mu}2=2339K, {\sigma}2=274K.For each BE bin, the pre-exponential factor was estimated using two models and the Polanyi-Wigner relationship. Comparison with previous studies, both experimental and computational, show good agreement on the range of the BEs. The impact of the adsorption on water ice on the spectral features of CO2 molecule is evaluated. The coverage simulation shows the non-wetting properties of CO2 on the water ice surface. We discuss the impact of using a BE distribution and different pre-exponential factors to calculate the partitioning between the ice and gas in a generic protoplanetary disk. We confirm that the use of BE distribution to model the gas and ice fractionation in a protoplanetary disk causes the gas fraction to be significantly more extended. Furthermore, we show that the prefactor has a significant impact on where the snowline forms and on the final extent of the gas fraction in the disk.

astro-ph.EP↗

arXiv2026

Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya

The spatial distribution of the chemical reservoirs in protoplanetary disks is key to elucidate the composition of planets, especially habitable ones. However, the partitioning of the main elements among the refractory and volatile phases is still elusive. Key parameters such as the carbon-to-oxygen C/O elemental ratio and the ionization fraction remain poorly constrained, with the latter potentially orders of magnitude lower than in the interstellar medium. Moreover, the thermal structure of the gas is also poorly known, despite its deep influence on gas-phase chemistry. In this context, ortho-to-para ratios could provide selective and sensitive probes. Recent ALMA observations have measured the spatially resolved column density of ortho-and para-H2CO in the transition disk orbiting TW Hya and derived the radial profile of the ortho-to-para ratio. Yet, current disk models do not include the nuclear-spin-resolved chemistry required to interpret these observations. The present work aims to fill this gap, by combining a parametric disk physical model of TW Hya with the UGAN network, updated to include a comprehensive description of the nuclear-spin-resolved chemistry of formaldehyde. This new model reproduces the observed column density of H2CO to within a factor of 2, as well as the measured ortho-to-para ratio which varies from 1.5 in the outer disk to 3 inside 90au. In particular the low value of this ratio beyond 90au is well explained by our model. However, the statistical value of 3 measured below 70au cannot be reproduced, suggesting that additional processes involving ices may be involved. Our parameter space exploration shows that the abundance of H2CO is highly sensitive to the C/O elemental ratio and to the cosmic-ray ionization rate. Future observations of ortho-and para-H2CO, based on well selected rotational transitions, in a large sample of disks, appear highly desirable.

astro-ph.EP↗

arXiv2025

Simulating the spatial distributions of gas- and ice-phase molecules in galaxies: a new method and preliminary results

Recent observations have revealed significant variations in the abundances of gas- and ice-phase molecules in galaxies with different luminosities and types. In order to discuss the physical origins of these variations, we incorporate gas- and dust-phase interstellar chemistry into galaxy-scale simulations with various baryonic physics including dust formation, evolution, and destruction, all of which are essential for the calculations of 400 interstellar molecule species. The new simulations can accordingly predict the abundances of gas- and ice-phase molecular species such as H_2O and CO_2 ice within individual molecular gas cloud of galaxies based on gas density and temperature, dust temperature (T_dust), elemental abundances (e.g., CHNOPS), UV radiation strength (F_UV), and cosmic ray ionisation rate (zeta_CR) within the clouds. Since this is the first of the series of papers, we describe the details of the new simulations and present the preliminary results focused on the spatial distributions of H_2O, CO, CO_2, and CH_3OH ice species in a disk galaxy similar to the Milky Way. We particularly discuss how T_dust and gas-phase elemental abundances can control the spatial distributions of the above molecules in galaxies. We briefly discuss the total amount of H_2O and CO_2 ices and radial distributions of PN and PO molecules in the Galaxy.

astro-ph.GA↗

arXiv2025

Edge-On Disk Study (EODS) III: Molecular Stratification in the Flying Saucer Disk

Context: Investigating the vertical distribution of molecular content in protoplanetary disks remains difficult in most disks mildly inclined along the line of sight. In contrast, edge-on disks provide a direct (tomographic) view of the 2D molecular brightness. Aims: We study the radial and vertical molecular distribution as well as the gas temperature and density by observing the Keplerian edge-on disk surrounding the Flying Saucer, a Class II object located in Ophiuchus. Methods: We use new and archival ALMA data to perform a tomography of $^{12}$CO, $^{13}$CO, C$^{18}$O, CN, HCN, CS, H$_2$CO, c-C$_3$H$_2$, N$_2$D$^+$, DCN and $^{13}$CS. We analyze molecular tomographies and model data using the radiative transfer code DiskFit. Results: We directly measure the altitude above the mid-plane for each observed species. For the first time, we unambiguously demonstrate the presence of a common molecular layer and measure its thickness: most molecules are located at the same altitude versus radius. Beyond CO, as predicted by chemical models, the CN emission traces the upper boundary of the molecular layer, whereas the deuterated species (DCN and N2D+) resides below one scale-height. Our best fits from DiskFit show that most observed transitions in the molecular layer are thermalized because their excitation temperature is the same, around 17-20 K. Conclusions: These long-integration observations clearly reveal a molecular layer predominantly located around 1-2 scale height, at a temperature above the CO freeze-out temperature. The deuterated molecules are closer to the mid-plane and N2D+ may be a good proxy for the CO snowline. Some molecules, such as CN and H2CO, are likely influenced by the disk environment, at least beyond the mm dust disk radius. The direct observation of the molecular stratification opens the door to detailed chemical modeling in this disk which appears representative of T Tauri disks.

astro-ph.EP↗

arXiv2025

A D/H Ratio Consistent with Earth's Water in Halley-type Comet 12P from ALMA HDO Mapping

Isotopic measurements of Solar System bodies provide a primary paradigm within which to understand the origins and histories of planetary materials. The D/H ratio in particular, helps reveal the relationship between (and heritage of) different H$_2$O reservoirs within the Solar System. Here we present interferometric maps of water (H$_2$O) and semiheavy water (HDO) in the gas-phase coma of a comet (Halley-type comet 12P/Pons-Brooks), obtained using the Atacama Large Millimeter/submillimeter Array (ALMA). The maps are consistent with outgassing of both H$_2$O and HDO directly from the nucleus, and imply a coma D/H ratio (for water) of $(1.71 \pm 0.44)\times10^{-4}$. This is at the lower end of the range of previously-observed values in comets, and is consistent with D/H in Earth's ocean water. Our results suggest a possible common heritage between a component of the Oort cloud's water ice reservoir, and the water that was delivered to the young Earth during the early history of the Solar System.

astro-ph.EP↗

arXiv2025

Edge-On Disk Study (EODS) II: Thermal Structure of the Flying Saucer Disk

Context. The dust and gas temperature in proto-planetary disks play critical roles in determining their chemical evolution and influencing planet formation processes. Aims. We attempted an accurate measurement of the dust and CO temperature profile in the edge-on disk of the Flying Saucer. Methods. We used the unique properties of the Flying Saucer, its edge-on geometry and its fortunate position in front of CO clouds with different brightness temperatures to provide independent constraints on the dust temperature. We compared it with the dust temperature derived using the radiative transfer code DiskFit and the CO gas temperature. Results. We find clear evidence for a substantial gas temperature vertical gradient, with a cold (10 K) disk mid-plane and a warmer CO layer where T(r) is 27 K at 100 au, dropping with exponent 0.3. Direct evidence for CO depletion in the mid-plane, below about 1 scale height, is also found. At this height, the gas temperature is 15-20 K, consistent with the expected CO freeze out temperature. The dust disk appears optically thin at 345 GHz, and exhibits moderate settling.

astro-ph.EP↗

arXiv2023

ALMA Observations of Supernova Remnant N49 in the Large Magellanic Cloud. II. Non-LTE Analysis of Shock-heated Molecular Clouds

We present the first compelling evidence of shock-heated molecular clouds associated with the supernova remnant (SNR) N49 in the Large Magellanic Cloud (LMC). Using $^{12}$CO($J$ = 2-1, 3-2) and $^{13}$CO($J$ = 2-1) line emission data taken with the Atacama Large Millimeter/Submillimeter Array, we derived the H$_2$ number density and kinetic temperature of eight $^{13}$CO-detected clouds using the large velocity gradient approximation at a resolution of 3.5$''$ (~0.8 pc at the LMC distance). The physical properties of the clouds are divided into two categories: three of them near the shock front show the highest temperatures of ~50 K with densities of ~500-700 cm$^{-3}$, while other clouds slightly distant from the SNR have moderate temperatures of ~20 K with densities of ~800-1300 cm$^{-3}$. The former clouds were heated by supernova shocks, but the latter were dominantly affected by the cosmic-ray heating. These findings are consistent with the efficient production of X-ray recombining plasma in N49 due to thermal conduction between the cold clouds and hot plasma. We also find that the gas pressure is roughly constant except for the three shock-engulfed clouds inside or on the SNR shell, suggesting that almost no clouds have evaporated within the short SNR age of ~4800 yr. This result is compatible with the shock-interaction model with dense and clumpy clouds inside a low-density wind bubble.

astro-ph.GA↗

arXiv2023

Methanol formation through reaction of low energy $CH_{3}^{+}$ ions with an amorphous solid water surface at low temperature

We have performed experimental investigations of methanol formation via the reactions of low energy $CH_{3}^{+}$ ions with an amorphous solid water (ASW) surface around 10 K. A newly developed experimental apparatus enabled irradiation of the ASW surface by several eV ions and detection of trace amounts of reaction products on the surface. It was found that methanol molecules were produced by low-energy $CH_{3}^{+}$ irradiation of the ASW surface and that hydroxy groups in produced methanol originated from water molecules in ASW, as predicted in a previous theoretical study. Little temperature dependence of observed methanol intensity is apparent in the temperature range 12 - 60 K. Ab-initio molecular dynamics simulations under constant temperature conditions of 10 K suggested that this reaction spontaneously produced a methanol molecule and an $H_{3}O^{+}$ ion, regardless of the contact point of $CH_{3}^{+}$ on the ASW surface. We have performed simulation with an astrochemical model under molecular-cloud conditions, where the reaction between $CH_{3}^{+}$ and $H_{2}O$ ice, leading to methanol formation, was included. We found that the impact of the reaction on methanol abundance was limited only at the edge of the molecular cloud (< 1 mag) because of the low abundance of $CH_{3}^{+}$ in the gas phase, whereas the reaction between the abundant molecular ion $HCO^{+}$ and $H_{2}O$ ice, which has not yet been confirmed experimentally, can considerably affect the abundance of a complex organic molecule. This work sheds light on a new type of reaction between molecular ions and ice surfaces that should be included in astrochemical models.

astro-ph.GA↗

arXiv2022

The Isotopic Links from Planet Forming Regions to the Solar System

Isotopic ratios provide a powerful tool for understanding the origins of materials, including the volatile and refractory matter within solar system bodies. Recent high sensitivity observations of molecular isotopologues, in particular with ALMA, have brought us new information on isotopic ratios of hydrogen, carbon, nitrogen and oxygen in star and planet forming regions as well as the solar system objects. Solar system exploration missions, such as Rosetta and Cassini, have given us further new insights. Meanwhile, the recent development of sophisticated models for isotope chemistry including detailed gas-phase and grain surface reaction network has made it possible to discuss how isotope fractionation in star and planet forming regions is imprinted into the icy mantles of dust grains, preserving a record of the initial isotopic state of solar system materials. This chapter reviews recent progress in observations of molecular isotopologues in extra-solar planet forming regions, prestellar/protostellar cores and protoplanetary disks, as well as objects in our solar system -- comets, meteorites, and planetary/satellite atmospheres -- and discusses their connection by means of isotope chemical models.

astro-ph.EP↗

arXiv2021

The formation of planetary systems with SPICA

In this era of spatially resolved observations of planet forming disks with ALMA and large ground-based telescopes such as the VLT, Keck and Subaru, we still lack statistically relevant information on the quantity and composition of the material that is building the planets, such as the total disk gas mass, the ice content of dust, and the state of water in planetesimals. SPICA is an infrared space mission concept developed jointly by JAXA and ESA to address these questions. The key unique capabilities of SPICA that enable this research are (1) the wide spectral coverage 10-220 micron, (2) the high line detection sensitivity of (1-2) 10-19 W m-2 with R~2000-5000 in the far-IR (SAFARI) and 10-20 W m-2 with R~29000 in the mid-IR (SMI, spectrally resolving line profiles), (3) the high far-IR continuum sensitivity of 0.45 mJy (SAFARI), and (4) the observing efficiency for point source surveys. This paper details how mid- to far-IR infrared spectra will be unique in measuring the gas masses and water/ice content of disks and how these quantities evolve during the planet forming period. These observations will clarify the crucial transition when disks exhaust their primordial gas and further planet formation requires secondary gas produced from planetesimals. The high spectral resolution mid-IR is also unique for determining the location of the snowline dividing the rocky and icy mass reservoirs within the disk and how the divide evolves during the build-up of planetary systems. Infrared spectroscopy (mid- to far-IR) of key solid state bands is crucial for assessing whether extensive radial mixing, which is part of our Solar System history, is a general process occurring in most planetary systems and whether extrasolar planetesimals are similar to our Solar System comets/asteroids. ... (abbreviated)

astro-ph.EP↗

arXiv2021

ALMA observations of doubly deuterated water: Inheritance of water from the prestellar environment

Establishing the origin of the water D/H ratio in the Solar System is central to our understanding of the chemical trail of water during the star and planet formation process. Recent modeling suggests that comparisons of the D$_2$O/HDO and HDO/H$_2$O ratios are a powerful way to trace the chemical evolution of water and, in particular, determine whether the D/H ratio is inherited from the molecular cloud or established locally. We seek to determine the D$_2$O column density and derive the D$_2$O/HDO ratios in the warm region toward the low-mass Class 0 sources B335 and L483. The results are compared with astrochemical models and previous observations to determine their implications for the chemical evolution of water. We present ALMA observations of the D$_2$O transition at 316.8 GHz toward B335 and L483 at $<$0.5" ($<$ 100 au) resolution, probing the inner warm envelope gas. The column densities of D$_2$O, HDO, and H$_{2}^{18}$O are determined by synthetic spectrum modeling and direct Gaussian fitting, under the assumption of a single excitation temperature and similar spatial extent for the three water isotopologs. D$_2$O is detected toward both sources in the inner warm envelope. The derived D$_2$O/HDO ratios is $(1.0\pm0.2)\times10^{-2}$ for L483 and $(1.4\pm0.1)\times10^{-2}$ for B335. The high D$_2$O/HDO ratios are a strong indication of chemical inheritance of water from the prestellar phase down to the inner warm envelope. This implies that the local cloud conditions in the prestellar phase, such as temperatures and timescales, determine the water chemistry at later stages and could provide a source of chemical differentiation in young systems. In addition, the observed D$_2$O/H$_2$O ratios support an observed dichotomy in the deuterium fractionation of water toward isolated and clustered protostars, namely, a higher D/H ratio toward isolated sources

astro-ph.GA↗

arXiv2021

Modeling chemistry during star formation: Water deuteration in dynamic star-forming regions

Recent observations of the HDO/H$_2$O ratio toward protostars in isolated and clustered environments show an apparent dichotomy, where isolated sources show higher D/H ratios than clustered counterparts. Establishing which physical and chemical processes create this differentiation can provide insights into the chemical evolution of water during star formation and the chemical diversity during the star formation process and in young planetary systems. Methods: The evolution of water is modeled using 3D physicochemical models of a dynamic star-forming environment. The physical evolution during the protostellar collapse is described by tracer particles from a 3D MHD simulation of a molecular cloud region. Each particle trajectory is post-processed using RADMC-3D to calculate the temperature and radiation field. The chemical evolution is simulated using a three-phase grain-surface chemistry model and the results are compared with interferometric observations of H$_2$O, HDO, and D$_2$O in hot corinos toward low-mass protostars. Results: The physicochemical model reproduces the observed HDO/H$_2$O and D$_2$O/HDO ratios in hot corinos, but shows no correlation with cloud environment for similar identical conditions. The observed dichotomy in water D/H ratios requires variation in the initial conditions (e.g., the duration and temperature of the prestellar phase). Reproducing the observed D/H ratios in hot corinos requires a prestellar phase duration $t\sim$1-3 Myr and temperatures in the range $T \sim$ 10-20 K prior to collapse. This work demonstrates that the observed differentiation between clustered and isolated protostars stems from differences in the molecular cloud or prestellar core conditions and does not arise during the protostellar collapse itself.

astro-ph.GA↗

arXiv2019

ALMA observations of water deuteration: A physical diagnostic of the formation of protostars

The deuterium fractionation of water can serve as a tracer for the chemical and physical evolution of water during star formation and can constrain the origin of water in Solar System bodies. We determine the HDO/H$_2$O ratio in the inner warm gas toward three low-mass Class 0 protostars selected to be in isolated cores, i.e., not associated with any cloud complexes. Previous sources for which the HDO/H$_2$O ratio have been established were all part of larger star-forming complexes. Targeting these isolated protostars allows comparison of the water chemistry in isolated and clustered regions to determine the influence of local cloud environment. We present ALMA observations of the HDO $3_{1,2}$-$2_{2,1}$ and $2_{1,1}$-$2_{1,2}$ transitions at 225.897 GHz and 241.562 GHz along with the H$_2^{18}$O $3_{1,3}$-$2_{2,0}$ transition at 203.407 GHz. The high angular resolution (0\farcs3-1\farcs3) allow the study of the inner warm envelope gas. Model-independent estimates for the HDO/H$_2$O ratios are obtained and compared with previous determinations in the warm gas toward low-mass protostars. We detect the targeted water transitions toward the three sources with S/N > 5. We determine the HDO/H$_2$O ratio toward L483, B335 and BHR71-IRS1 to be ($2.2\pm0.4$)$\times 10^{-3}$, ($1.7\pm0.3$)$\times 10^{-3}$, and ($1.8\pm0.4$)$\times 10^{-3}$, respectively, assuming $T_\mathrm{ex} = 124$ K. The degree of water deuteration of these isolated protostars are a factor of 2-4 higher relative to Class 0 protostars that are members of known nearby clustered star-forming regions. The results indicate that the water deuterium fractionation is influenced by the local cloud environment. This effect can be explained by variations in either collapse timescales or temperatures, which depends on local cloud dynamics and could provide a new method to decipher the history of young stars.

astro-ph.SR↗

arXiv2016

Water delivery from cores to disks: deuteration as a probe of the prestellar inheritance of H2O

We investigate the delivery of regular and deuterated forms of water from prestellar cores to circumstellar disks. We adopt a semi-analytical axisymmetric two-dimensional collapsing core model with post-processing gas-ice astrochemical simulations, in which a layered ice structure is considered. The physical and chemical evolutions are followed until the end of the main accretion phase. When mass averaged over the whole disk, a forming disk has a similar H2O abundance and HDO/H2O abundance ratio as their precollapse values (within a factor of 2), regardless of time in our models. Consistent with previous studies, our models suggest that interstellar water ice is delivered to forming disks without significant alteration. On the other hand, the local vertically averaged H2O ice abundance and HDO/H2O ice ratio can differ more, by up to a factor of several, depending on time and distance from a central star. Key parameters for the local variations are the fluence of stellar UV photons en route into the disk and the ice layered structure, the latter of which is mostly established in the prestellar stages. We also find that even if interstellar water ice is destroyed by stellar UV and (partly) reformed prior to disk entry, the HDO/H2O ratio in reformed water ice is similar to the original value. This finding indicates that some caution is needed in discussions on the prestellar inheritance of H2O based on comparisons between the observationally derived HDO/H2O ratio in clouds/cores and that in disks/comets. Alternatively, we propose that the ratio of D2O/HDO to HDO/H2O better probes the prestellar inheritance of H2O. It is also found that icy organics are more enriched in deuterium than water ice in forming disks. The differential deuterium fractionation in water and organics is inherited from the prestellar stages.

astro-ph.GA↗

arXiv2015

Reconstructing the history of water ice formation from HDO/H2O and D2O/HDO ratios in protostellar cores

Recent interferometer observations have found that the D2O/HDO abundance ratio is higher than that of HDO/H2O by about one order of magnitude in the vicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice has sublimated. Previous laboratory and theoretical studies show that the D2O/HDO ice ratio should be lower than the HDO/H2O ice ratio, if HDO and D2O ices are formed simultaneously with H2O ice. In this work, we propose that the observed feature, D2O/HDO > HDO/H2O, is a natural consequence of chemical evolution in the early cold stages of low-mass star formation: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient, and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced due to the drop of the ortho-para ratio of H2, the weaker UV radiation field, etc. Using a simple analytical model and gas-ice astrochemical simulations tracing the evolution from the formation of molecular clouds to protostellar cores, we show that the proposed scenario can quantitatively explain the observed HDO/H2O and D2O/HDO ratios. We also find that the majority of HDO and D2O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H2O ice is formed. This work demonstrates the power of the combination of the HDO/H2O and D2O/HDO ratios as a tool to reveal the past history of water ice formation in the early cold stages of star formation and when the enrichment of deuterium in the bulk of water occurred. Further observations are needed to explore if the relation, D2O/HDO > HDO/H2O, is common in low-mass protostellar sources.

astro-ph.GA↗

arXiv2015

Water deuteration and ortho-to-para nuclear spin ratio of H2 in molecular clouds formed via the accumulation of HI gas

We investigate the water deuteration ratio and ortho-to-para nuclear spin ratio of H2 (OPR(H2)) during the formation and early evolution of a molecular cloud, following the scenario that accretion flows sweep and accumulate HI gas to form molecular clouds. We follow the physical evolution of post-shock materials using a one-dimensional shock model, with post-processing gas-ice chemistry simulations. This approach allows us to study the evolution of the OPR(H2) and water deuteration ratio without an arbitrary assumption concerning the initial molecular abundances, including the initial OPR(H2). When the conversion of hydrogen into H2 is almost complete, the OPR(H2) is already much smaller than the statistical value of three due to the spin conversion in the gas phase. As the gas accumulates, the OPR(H2) decreases in a non-equilibrium manner. We find that water ice can be deuterium-poor at the end of its main formation stage in the cloud, compared to water vapor observed in the vicinity of low-mass protostars where water ice is likely sublimated. If this is the case, the enrichment of deuterium in water should mostly occur at somewhat later evolutionary stages of star formation, i.e., cold prestellar/protostellar cores. The main mechanism to suppress water ice deuteration in the cloud is the cycle of photodissociation and reformation of water ice, which efficiently removes deuterium from water ice chemistry. The removal efficiency depends on the main formation pathway of water ice. The OPR(H2) plays a minor role in water ice deuteration at the main formation stage of water ice.

astro-ph.GA↗

arXiv2014

DNC/HNC and N2D+/N2H+ ratios in high-mass star forming cores

Chemical models predict that the deuterated fraction (the column density ratio between a molecule containing D and its counterpart containing H) of N2H+, Dfrac(N2H+), is high in massive pre-protostellar cores and rapidly drops of an order of magnitude after the protostar birth, while that of HNC, Dfrac(HNC), remains constant for much longer. We tested these predictions by deriving Dfrac(HNC) in 22 high-mass star forming cores divided in three different evolutionary stages, from high-mass starless core candidates (HMSCs, 8) to high-mass protostellar objects (HMPOs, 7) to Ultracompact HII regions (UCHIIs, 7). For all of them, Dfrac (N2H+) was already determined through IRAM-30m Telescope observations, which confirmed the theoretical rapid decrease of Dfrac(N2H+) after protostar birth (Fontani et al. 2011). Therefore our comparative study is not affected by biases introduced by the source selection. We have found average Dfrac(HNC) of 0.012, 0.009 and 0.008 in HMSCs, HMPOs and UCHIIs, respectively, with no statistically significant differences among the three evolutionary groups. These findings confirm the predictions of the chemical models, and indicate that large values of Dfrac(N2H+) are more suitable than large values of Dfrac(HNC) to identify cores on the verge of forming high-mass stars, likewise what found in the low-mass regime.

astro-ph.GA↗