arXiv subjects

Yuri Aikawa

Publications and source records attributed to Yuri Aikawa.

At least 109 records · Page 6Linked to original sources

arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) XVI: Characterizing the impact of the molecular wind on the evolution of the HD 163296 system

During the main phase of evolution of a protoplanetary disk, accretion regulates the inner-disk properties, such as the temperature and mass distribution, and in turn, the physical conditions associated with planet formation. The driving mechanism behind accretion remains uncertain; however, one promising mechanism is the removal of a fraction of angular momentum via a magnetohydrodynamic (MHD) disk wind launched from the inner tens of astronomical units of the disk. This paper utilizes CO isotopologue emission to study the unique molecular outflow originating from the HD 163296 protoplanetary disk obtained with the Atacama Large Millimeter/submillimeter Array. HD~163296 is one of the most well-studied Class II disks and is proposed to host multiple gas-giant planets. We robustly detect the large-scale rotating outflow in the 12CO J=2-1 and the 13CO J=2-1 and J=1-0 transitions. We constrain the kinematics, the excitation temperature of the molecular gas, and the mass-loss rate. The high ratio of the rates of ejection to accretion (5 - 50), together with the rotation signatures of the flow, provides solid evidence for an MHD disk wind. We find that the angular momentum removal by the wind is sufficient to drive accretion through the inner region of the disk; therefore, accretion driven by turbulent viscosity is not required to explain HD~163296's accretion. The low temperature of the molecular wind and its overall kinematics suggest that the MHD disk wind could be perturbed and shocked by the previously observed high-velocity atomic jet. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales (MAPS) VI: Distribution of the small organics HCN, C2H, and H2CO

Small organic molecules, such as C2H, HCN, and H2CO, are tracers of the C, N, and O budget in protoplanetary disks. We present high angular resolution (10-50 au) observations of C2H, HCN, and H2CO lines in five protoplanetary disks from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program. We derive column density and excitation temperature profiles for HCN and C2H, and find that the HCN emission arises in a temperate (20-30 K) layer in the disk, while C2H is present in relatively warmer (20-60 K) layers. In the case of HD 163296, we find a decrease in column density for HCN and C2H inside one of the dust gaps near 83 au, where a planet has been proposed to be located. We derive H2CO column density profiles assuming temperatures between 20 and 50 K, and find slightly higher column densities in the colder disks around T Tauri stars than around Herbig Ae stars. The H2CO column densities rise near the location of the CO snowline and/or millimeter dust edge, suggesting an efficient release of H2CO ices in the outer disk. Finally, we find that the inner 50 au of these disks are rich in organic species, with abundances relative to water that are similar to cometary values. Comets could therefore deliver water and key organics to future planets in these disks, similar to what might have happened here on Earth. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

Molecules with ALMA at Planet-forming Scales. XX. The Massive Disk Around GM Aurigae

Gas mass remains one of the most difficult protoplanetary disk properties to constrain. With much of the protoplanetary disk too cold for the main gas constituent, H2, to emit, alternative tracers such as dust, CO, or the H2 isotopolog HD are used. However, relying on disk mass measurements from any single tracer requires assumptions about the tracer's abundance relative to \hh\ and the disk temperature structure. Using new Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program as well as archival ALMA observations, we construct a disk physical/chemical model of the protoplanetary disk GM Aur. Our model is in good agreement with the spatially resolved CO isotopolog emission from eleven rotational transitions with spatial resolution ranging from 0.15'' to 0.46'' (24-73 au at 159 pc) and the spatially unresolved HD J=1-0 detection from Herschel. Our best-fit model favors a cold protoplanetary disk with a total gas mass of approximately 0.2 solar masses, a factor of 10 reduction in CO gas inside roughly 100 au and a factor of 100 reduction outside of 100 au. Despite its large mass, the disk appears to be on the whole gravitationally stable based on the derived Toomre Q parameter. However, the region between 70 and 100 au, corresponding to one of the millimeter dust rings, is close to being unstable based on the calculated Toomre Q of <1.7. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

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arXiv2021

The JCMT Transient Survey: Four Year Summary of Monitoring the Submillimeter Variability of Protostars

We present the four-year survey results of monthly submillimeter monitoring of eight nearby ($< 500 $pc) star-forming regions by the JCMT Transient Survey. We apply the Lomb-Scargle Periodogram technique to search for and characterize variability on 295 submillimeter peaks brighter than 0.14 Jy beam$^{-1}$, including 22 disk sources (Class II), 83 protostars (Class 0/I), and 190 starless sources. We uncover 18 secular variables, all of them protostars. No single-epoch burst or drop events and no inherently stochastic sources are observed. We classify the secular variables by their timescales into three groups: Periodic, Curved, and Linear. For the Curved and Periodic cases, the detectable fractional amplitude, with respect to mean peak brightness, is $\sim4$ % for sources brighter than $\sim$ 0.5 Jy beam$^{-1}$. Limiting our sample to only these bright sources, the observed variable fraction is 37 % (16 out of 43). Considering source evolution, we find a similar fraction of bright variables for both Class 0 and Class I. Using an empirically motivated conversion from submillimeter variability to variation in mass accretion rate, six sources (7 % of our full sample) are predicted to have years-long accretion events during which the excess mass accreted reaches more than 40 % above the total quiescently accreted mass: two previously known eruptive Class I sources, V1647 Ori and EC 53 (V371 Ser), and four Class 0 sources, HOPS 356, HOPS 373, HOPS 383, and West 40. Considering the full protostellar ensemble, the importance of episodic accretion on few years timescale is negligible, only a few percent of the assembled mass. However, given that this accretion is dominated by events of order the observing time-window, it remains uncertain as to whether the importance of episodic events will continue to rise with decades-long monitoring.

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arXiv2021

Photoevaporation of Grain-Depleted Protoplanetary Disks around Intermediate-Mass Stars: Investigating Possibility of Gas-Rich Debris Disks as Protoplanetary Remnants

Debris disks are classically considered to be gas-less systems, but recent (sub)millimeter observations have detected tens of those with rich gas content. The origin of the gas component remains unclear; namely, it can be protoplanetary remnants and/or secondary products deriving from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for $\gtrsim10{\,\rm Myr}$. However, previous models predict $\lesssim 10{\,\rm Myr}$ lifetimes because of efficient photoevaporation at the late stage of disk evolution. In the present study, we investigate photoevaporation of gas-rich, optically-thin disks around intermediate-mass stars at a late stage of the disk evolution. The evolved system is modeled as those where radiation force is sufficiently strong to continuously blow out small grains ($\lesssim 4 {\,\rm μm}$), which are an essential component for driving photoevaporation via photoelectric heating induced by stellar far-ultraviolet (FUV). We find that the grain depletion reduces photoelectric heating, so that FUV photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of $2$--$5\times10^{-10}(Φ_{\rm EUV}/10^{41}{\,\rm s}^{-1})^{1/2}\,M_\odot\,{\rm yr}^{-1}$, where $Φ_{\rm EUV}$ is the EUV emission rate. The estimated lifetimes of the gas component are $\sim 50 (M_{\rm disk}/10^{-2}\,M_\odot)(Φ_{\rm EUV}/10^{41}\,{\rm s}^{-1})^{1/2}\,{\rm Myr}$ and depend on the ``initial'' disk mass at the point small grains have been depleted in the system. With an order estimation, we show that the gas component can survive for a much longer time around A-type stars than lower-mass stars. This trend is consistent with the higher frequency of gas-rich debris disks around A-type stars, implying the possibility of the gas component being protoplanetary remnants.

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arXiv2021

High Spatial Resolution Observations of Molecular Lines towards the Protoplanetary Disk around TW Hya with ALMA

We present molecular line observations of 13CO and C18O J=3-2, CN N = 3 - 2, and CS J=7-6 lines in the protoplanetary disk around TW Hya at a high spatial resolution of ~9 au (angular resolution of 0.15''), using the Atacama Large Millimeter/Submillimeter Array. A possible gas gap is found in the deprojected radial intensity profile of the integrated C18O line around a disk radius of ~58 au, slightly beyond the location of the au-scale dust clump at ~52 au, which resembles predictions from hydrodynamic simulations of planet-disk interaction. In addition, we construct models for the physical and chemical structure of the TW Hya disk, taking account of the dust surface density profile obtained from high spatial resolution dust continuum observations. As a result, the observed flat radial profile of the CN line intensities is reproduced due to a high dust-to-gas surface density ratio inside ~20 au. Meanwhile, the CO isotopologue line intensities trace high temperature gas and increase rapidly inside a disk radius of ~30 au. A model with either CO gas depletion or depletion of gas-phase oxygen elemental abundance is required to reproduce the relatively weak CO isotopologue line intensities observed in the outer disk, consistent with previous atomic and molecular line observations towards the TW Hya disk. {Further observations of line emission of carbon-bearing species, such as atomic carbon and HCN, with high spatial resolution would help to better constrain the distribution of elemental carbon abundance in the disk gas.

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arXiv2021

FAUST II. Discovery of a Secondary Outflow in IRAS 15398-3359: Variability in Outflow Direction during the Earliest Stage of Star Formation?

We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 au to 1800 au, as part of the ALMA Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and C18O line emission, which extends from the source along a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blue-shifted with respect to the systemic velocity. A velocity gradient of 1.2 km/s over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.

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arXiv2020

Chemical Variation among Protostellar Cores: Dependence on Prestellar Core Conditions

Hot corino chemistry and warm carbon chain chemistry (WCCC) are driven by gas-grain interactions in star-forming cores: radical-radical recombination reactions to form complex organic molecules (COMs) in the ice mantle, sublimation of CH$_4$ and COMs, and their subsequent gas-phase reactions. These chemical features are expected to depend on the composition of ice mantle which is set in the prestellar phase. We calculated the gas-grain chemical reaction network considering a layered ice-mantle structure in star-forming cores, to investigate how the hot corino chemistry and WCCC depend on the physical condition of the static phase before the onset of gravitational collapse. We found that WCCC becomes more active, if the temperature is lower, or the visual extinction is lower in the static phase, or the static phase is longer. Dependence of hot corino chemistry on the static-phase condition is more complex. While CH$_3$OH is less abundant in the models with warmer static phase, some COMs are formed efficiently in those warm models, since there are various formation paths of COMs. If the visual extinction is lower, photolysis makes COMs less abundant in the static phase. Once the collapse starts and visual extinction increases, however, COMs can be formed efficiently. Duration of the static phase does not largely affect COM abundances. Chemical diversity between prototypical hot corinos and hybrid sources, in which both COMs and carbon chains are reasonably abundant, can be explained by the variation of prestellar conditions. Deficiency of gaseous COMs in prototypical WCCC sources is, however, hard to reproduce within our models.

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arXiv2020

Disk structure around the Class I protostar L1489 IRS revealed by ALMA: a warped disk system

We have observed the Class I protostar L1489 IRS with the Atacama Millimeter/submillimeter Array (ALMA) in Band 6. The C$^{18}$O $J=$2-1 line emission shows flattened and non-axisymmetric structures in the same direction as its velocity gradient due to rotation. We discovered that the C$^{18}$O emission shows dips at a radius of ~200-300 au while the 1.3 mm continuum emission extends smoothly up to r~400 au. At the radius of the C$^{18}$O dips, the rotational axis of the outer portion appears to be tilted by ~15 degrees from that of the inner component. Both the inner and outer components with respect to the C$^{18}$O dips exhibit the $r^{-0.5}$ Keplerian rotation profiles until r~600 au. These results not only indicate that a Keplerian disk extends up to ~600 au but also that the disk is warped. We constructed a three dimensional warped disk model rotating at the Keplerian velocity, and demonstrated that the warped disk model reproduces main observed features in the velocity channel maps and the PV diagrams. Such a warped disk system can form by mass accretion from a misaligned envelope. We also discuss a possible disk evolution scenario based on comparisons of disk radii and masses between Class I and Class II sources.

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arXiv2020

Chemistry and physics of a low-metallicity hot core in the Large Magellanic Cloud

We present the results of 0.1-pc-scale observations in 250 GHz and 350GHz towards a newly-discovered hot molecular core in a nearby low-metallicity galaxy, the Large Magellanic Cloud (LMC), with the Atacama Large Millimeter/submillimeter Array. A variety of C/N/O/Si/S-bearing molecules are detected towards the high-mass young stellar object, ST16. A rotating protostellar envelope is for the first time detected outside our Galaxy by SO2 and 34SO lines. An outflow cavity is traced by CCH and CN. The isotope abundance of sulfur in the source is estimated to be 32S/34S = 17 and 32S/33S = 53 based on SO, SO2, and CS isotopologues, suggesting that both 34S and 33S are overabundant in the LMC. Rotation diagram analyses show that the source is associated with hot gas (>100K) traced by high-excitation lines of CH3OH and SO2, as well as warm gas (~50K) traced by CH3OH, SO2, 34SO, OCS, CH3CN lines. A comparison of molecular abundances between LMC and Galactic hot cores suggests that organic molecules (e.g., CH3OH, a classical hot core tracer) show a large abundance variation in low metallicity, where the present source is classified into an organic-poor hot core. Our astrochemical simulations suggest that different grain temperature during the initial ice-forming stage would contribute to the chemical differentiation. In contrast, SO2 shows similar abundances within all the known LMC hot cores and the typical abundance roughly scales with the LMC's metallicity. Nitrogen-bearing molecules are generally less abundant in LMC hot cores, except for NO. The present results suggest that chemical compositions of hot cores do not always simply scale with the metallicity.

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arXiv2019

The circumstellar environment around the embedded protostar EC 53

EC53 is an embedded protostar with quasi-periodic emission in the near-IR and sub-mm. We use ALMA high-resolution observations of continuum and molecular line emission to describe the circumstellar environment of EC 53. The continuum image reveals a disk with a flux that suggests a mass of 0.075 Msun, much less than the estimated mass in the envelope, and an in-band spectral index that indicates grain growth to centimeter sizes. Molecular lines trace the outflow cavity walls, infalling and rotating envelope, and/or the Keplerian disk. The rotation profile of the C17O 3-2 line emission cannot isolate the Keplerian motion clearly although the lower limit of the protostellar mass can be calculated as 0.3 +- 0.1 Msun if the Keplerian motion is adopted. The weak CH3OH emission, which is anti-correlated with the HCO+ 4-3 line emission, indicates that the water snow line is more extended than what expected from the current luminosity, attesting to bygone outburst events. The extended snow line may persist for longer at the disk surface because the lower density increases the freeze-out timescale of methanol and water.

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arXiv2019

Protostellar Evolution in Serpens Main: Possible Origin of Disk-Size Diversity

We have observed the submillimeter continuum condensations SMM2, SMM4, SMM9, and SMM11 in the star forming cluster Serpens Main using the Atacama Large Millimeter/submillimeter Array during Cycle 3 in the 1.3 mm continuum, 12CO J=2-1, SO J_N=6_5-5_4, and C18O J=2-1 lines at an angular resolution of ~0.55 (240 au). Sixteen sources have been detected in the 1.3 mm continuum, which can be classified into three groups. Group 1 consists of six sources showing extended continuum emission and bipolar/monopolar 12CO outflows. Although all the Group 1 members are classified as Class 0 protostars, our observations suggest evolutionary trends among them in terms of 12CO outflow dynamical time, SO emission distribution, C18O fractional abundance, and continuum morphology. Group 2 consists of four sources associated with a continuum filamentary structure and no 12CO outflows. Central densities estimated from the 1.3 mm continuum intensity suggest that they are prestellar sources in a marginally Jeans unstable state. Group 3 consists of six Spitzer sources showing point-like 1.3 mm continuum emission and clumpy 12CO outflows. These features of Group 3 suggest envelope dissipation, preventing disk growth from the present size, r <~ 60 au. The Group 3 members are protostars that may be precursors to the T Tauri stars associated with small disks at tens-au radii identified in recent surveys.

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arXiv2019

H$_2$ ortho-para spin conversion on inhomogeneous grain surfaces

We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) H$_2$ via the nuclear spin conversion on grain surfaces coated with water ice under physical conditions that are relevant to star- and planet-forming regions. We utilize the rate equation model that considers adsorption of gaseous H$_2$ on grain surfaces which have a variety of binding sites with a different potential energy depth, thermal hopping, desorption, and the nuclear spin conversion of adsorbed H$_2$. It is found that the spin conversion efficiency depends on the H$_2$ gas density and the surface temperature. As a general trend, enhanced H$_2$ gas density reduces the efficiency, while the temperature dependence is not monotonic; there is a critical surface temperature at which the efficiency is the maximum. At low temperatures, the exchange of gaseous and icy H$_2$ is inefficient (i.e., adsorbed H$_2$ does not desorb and hinders another gaseous H$_2$ to be adsorbed), while at warm temperatures, the residence time of H$_2$ on surfaces is too short for the spin conversion. Additionally, the spin conversion becomes more efficient with lowering the activation barriers for thermal hopping. We discuss whether the spin conversion on surfaces can dominate over that in the gas-phase in star- and planet-forming regions. Finally, we establish a simple but accurate way to implement the H$_2$ spin conversion on grain surfaces in existing astrochemical models.

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arXiv2019

Sulfur-Bearing Species Tracing the Disk/Envelope System in the Class I Protostellar Source Elias 29

We have observed the Class I protostellar source Elias 29 with Atacama Large Millimeter/submillimeter Array (ALMA). We have detected CS, SO, $^{34}$SO, SO$_2$, and SiO line emissions in a compact component concentrated near the protostar and a ridge component separated from the protostar by 4\arcsec\ ($\sim 500$ au). The former component is found to be abundant in SO and SO$_2$ but deficient in CS. The abundance ratio SO/CS is as high as $3^{+13}_{-2} \times 10^2$ at the protostar, which is even higher than that in the outflow-shocked region of L1157 B1. However, organic molecules (HCOOCH$_3$, CH$_3$OCH$_3$, CCH, and c-C$_3$H$_2$) are deficient in Elias 29. We attribute the deficiency in organic molecules and richness in SO and SO$_2$ to the evolved nature of the source or the relatively high dust temperature (\protect\raisebox{-0.7ex}{$\:\stackrel{\textstyle >}{\sim}\:$} 20 K) in the parent cloud of Elias 29. The SO and SO$_2$ emissions trace rotation around the protostar. Assuming a highly inclined configuration ($i \geq 65$\degr; 0\degr\ for a face-on configuration) and Keplerian motion for simplicity, the protostellar mass is estimated to be (0.8 -- 1.0) \Msun. The $^{34}$SO and SO$_2$ emissions are asymmetric in their spectra; the blue-shifted components are weaker than the red-shifted ones. Although this may be attributed to the asymmetric molecular distribution, other possibilities are also discussed.

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arXiv2019

Realizing the Unique Potential of ALMA to Probe the Gas Reservoir of Planet Formation

Understanding the origin of the astonishing diversity of exoplanets is a key question for the coming decades. ALMA has revolutionized our view of the dust emission from protoplanetary disks, demonstrating the prevalence of ring and spiral structures that are likely sculpted by young planets in formation. To detect kinematic signatures of these protoplanets and to probe the chemistry of their gas accretion reservoir will require the imaging of molecular spectral line emission at high angular and spectral resolution. However, the current sensitivity of ALMA limits these important spectral studies to only the nearest protoplanetary disks. Although some promising results are emerging, including the identification of the snowlines of a few key molecules and the first attempt at detecting a protoplanet's spiral wake, it is not yet possible to search for these important signatures in a population of disks in diverse environments and ages. Harnessing the tremendous power of (sub)mm observations to pinpoint and characterize the chemistry of planets in formation will require a major increase of ALMA's spectral sensitivity (5-10x), increase in instantaneous bandwidth (2x) at high spectral resolution, and improved angular resolution (2x) in the 2030 era.

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arXiv2019

The ice composition in the disk around V883 Ori revealed by its stellar outburst

Complex organic molecules (COMs), which are the seeds of prebiotic material and precursors of amino acids and sugars, form in the icy mantles of circumstellar dust grains but cannot be detected remotely unless they are heated and released to the gas phase. Around solar-mass stars, water and COMs only sublimate in the inner few au of the disk, making them extremely difficult to spatially resolve and study. Sudden increases in the luminosity of the central star will quickly expand the sublimation front (so-called snow line) to larger radii, as seen previously in the FU Ori outburst of the young star V883 Ori. In this paper, we take advantage of the rapid increase in disk temperature of V883 Ori to detect and analyze five different COMs, methanol, acetone, acetonitrile, acetaldehyde, and methyl formate, in spatially-resolved submillimeter observations. The COMs abundances in V883 Ori is in reasonable agreement with cometary values. This result suggests that outbursting young stars can provide a special opportunity to study the ice composition of material directly related to planet formation.

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arXiv2018

The Chemical Evolution from Prestellar to Protostellar Cores: A New Multiphase Model With Bulk Diffusion and Photon Penetration

We investigate the chemical evolution of a collapsing core that starts from a hydrostatic core and finally form a low-mass protostar. New multiphase gas-grain models that include bulk diffusion and photon penetration are simulated by the macroscopic Monte Carlo method in order to derive the chemical evolution. There are two types of species in the ice bulk in the new multiphase models. Interstitial species can diffuse and sublime at their own sublimation temperatures while normal species are locked in the ice bulk. Photodissociation rates of icy species are reduced by the exponential decay of UV flux within the ice mantle. Two-phase models and basic multiphase models without bulk diffusion and photon penetration are also simulated for comparison. Our physical model for the collapsing core is base on a one-dimensional radiation hydrodynamics model. Abundant icy radicals are produced at around 10 K in the new multiphase models. Interstitial radicals can diffuse inside ice mantles to form complex organic molecules (COMS) upon warming-up. Thus, COMs produced by radical recombination at higher temperatures in the new multiphase models are more than one order of magnitude higher than those in the two-phase and basic multiphase models. Moreover, COMs produced at around 10 K in the new multiphase models are about one order of magnitude higher than those in the two-phase model. Our model shows a reasonable agreement with observations toward low-mass protostars. Moreover, molecular oxygen abundances predicted by our new multiphase models agree reasonably well with that found in cometary materials.

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arXiv2018

Molecular-Cloud-Scale Chemical Composition III: Constraints of Average Physical Properties through Chemical Models

It is important to understand the origin of molecular line intensities and chemical composition in the molecular-cloud scale in the Galactic sources because it serves as a benchmark to compare with the chemical compositions of extragalactic sources. Recent observations of the 3-mm spectra averaged over the 10-pc scale show similar spectral pattern among sources for molecular lines HCN, HCO$^+$, CCH, HNC, HNCO, c-C$_3$H$_2$, CS, SO, N$_2$H$^+$, and CN. To constrain the average physical property emitting such spectral pattern, we model molecular spectra using a time-dependent gas-grain chemical model followed by a radiative transfer calculation. We use a grid of physical parameters such as the density $n=3 \times 10^2 - 3\times 10^4$ cm$^{-3}$, the temperature, $T=10-30$ K, the visual extinction $A_{\rm V} = 2,4,10$ mag, the cosmic-ray ionization rate $ζ= 10^{-17} - 10^{-16}$ s$^{-1}$, and the sulfur elemental abundance $S/H = 8\times 10^{-8} - 8\times 10^{-7}$. Comparison with the observed spectra indicates that spectra are well reproduced with the relatively low density of $n=(1-3) \times 10^3\,$cm$^{-3}$, $T=10\,$K, $ζ= 10^{-17}$ s$^{-1}$, and the short chemistry timescale of $10^5$ yrs. This short chemistry timescale may indicate that molecular clouds are constantly affected by the turbulence, and exposed to low-density, low $A_{\rm V}$ regions that "refreshes" the chemical clock by UV radiation. The relatively low density obtained is orders of magnitude lower than the commonly-quoted critical density in the optically thin case. Meanwhile, this range of density is consistent with results from recent observational analysis of molecular-cloud-scale mapping.

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