Exploring HNC and HCN line emission as probes of the protoplanetary disk temperature
Feng Long, Arthur D. Bosman, Paolo Cazzoletti, Ewine F. van Dishoeck, Karin I. Oberg, Stefano Facchini, Marco Tazzari, Viviana V. Guzman, Leonardo Testi
AAstronomy & Astrophysics manuscript no. aa © ESO 2021February 15, 2021
Exploring HNC and HCN line emission as probes of theprotoplanetary disk temperature
Feng Long , , Arthur D. Bosman , , Paolo Cazzoletti , Ewine F. van Dishoeck , , Karin I. Öberg , Stefano Facchini ,Marco Tazzari , Viviana V. Guzmán , and Leonardo Testi Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USAe-mail: [email protected] Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands Department of Astronomy, University of Michigan, 323 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748, Garching, Germany European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK Instituto de Astrofísica, Pontfíficia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, ChileReceived xx; accepted xx
ABSTRACT
Context.
The distributions and abundances of molecules in protoplanetary disks are powerful tracers of the physical and chemicaldisk structures. The abundance ratios of HCN and its isomer HNC are known to be sensitive to gas temperature. Their line ratiosmight therefore o ff er a unique opportunity to probe the properties of the emitting gas. Aims.
We investigate the HNC and HCN line emission in disks at (sub-)millimeter wavelengths and explore their potential utility forprobing disk temperature and other disk properties.
Methods.
Using the 2D thermochemical code DALI, we ran a set of disk models accounting for di ff erent stellar properties and radialand vertical disk structures, with an updated chemical network for the nitrogen chemistry. These modeling results were then comparedwith observations, including new observations obtained with the Atacama Large Millimeter / submillimeter Array (ALMA) of HNC J = − J = − Results.
Similar to CN, HCN and HNC have brighter line emission in models with larger disk flaring angles and higher UV fluxes.HNC and HCN are predicted to be abundant in the warm surface layer and outer midplane region, which results in ring-shapedemission patterns. However, the precise emitting regions and emission morphology depend on the probed transition, as well as onother parameters such as C and O abundances. The modeled HNC-to-HCN line intensity ratio increases from < . ffi cient HNC destruction at high temperatures. Disk-integratedHNC line fluxes from current scarce observations and its radial distribution in the TW Hya disk are broadly consistent with our modelpredictions. Conclusions.
The HNC-to-HCN flux ratio robustly increases with radius (decreasing temperature), but its use as a chemical ther-mometer in disks is a ff ected by other factors, including UV flux and C and O abundances. High-spatial resolution ALMA diskobservations of HNC and HCN that can locate the emitting layers would have the great potential to constrain both the disk thermaland UV radiation structures, and also to verify our understanding of the nitrogen chemistry. Key words. astrochemistry — protoplanetary disks — stars: pre-main sequence — planet formation
1. Introduction
Gas-rich protoplanetary disks around young stars provide rawmaterials for the assembly of planets. The final properties of theplanetary systems are therefore largely determined by the phys-ical and chemical conditions of disks. Emission lines of smallmolecules and radicals are excellent probes of disk ionization,density, and thermal structures (e.g., van Zadelho ff et al. 2001;Teague et al. 2016). In addition to the simple CO molecule, avariety of molecules with strong enough line emission (includ-ing CN, CS, HCN, HNC, C H, c-C H ,H CO, CH OH, andCH CN) at (sub-)millimeter wavelengths have been detected indisks, tracing the disk layers where they originate (e.g., Dutreyet al. 1997; Kastner et al. 1997, 2014, 2018; Thi et al. 2004;Öberg et al. 2010; Punzi et al. 2015; Walsh et al. 2016; Bergneret al. 2018, 2019; Pegues et al. 2020; Loomis et al. 2020). With the advent of high-resolution imaging, investigations ofthe detailed spatial distributions of disk materials become pos-sible. Strikingly, ring-like structures of dust grain distributionsemerge in most high-resolution continuum maps (e.g., Andrewset al. 2018; Long et al. 2018; van der Marel et al. 2019). Inthe so-called transition disks, where the inner regions are de-pleted of millimeter-sized grains, gas cavities are observed withCO isotopolog lines as well (van der Marel et al. 2016). Ring-shaped emission has also been found in other molecules, butthese molecular rings are not always associated with dust rings:N H + and DCO + emission rings trace regions near the CO snowline, where the disk temperature drops for CO gas to freeze outonto grains (Qi et al. 2013; Mathews et al. 2013; Öberg et al. Article number, page 1 of 16 a r X i v : . [ a s t r o - ph . E P ] F e b & A proofs: manuscript no. aa ; CN emission rings are strongly linked to the local UVfield (Cazzoletti et al. 2018); the formation of hydrocarbon rings(e.g., C H and C H ) requires not only a strong UV field, butalso an elevated C / O (Bergin et al. 2016; Miotello et al. 2019).These examples highlight the possibility of using chemical struc-tures as probes of disk conditions. Fully realizing the potentialutility of molecules as tracers of di ff erent disk properties wouldhave profound implications for studies of protoplanetary disksand planet formation.The simple organic molecule hydrogen cyanide (HCN) andits isomer HNC are of particular interests for this purpose. Iso-mers usually have connected formation and destruction path-ways regulated by physical characteristics, and their line ratiostherefore o ff er a unique opportunity to probe the properties ofthe emitting gas. HNC and HCN have been detected in a va-riety of astrophysical environments, including di ff erent phasesof star formation (Irvine & Schloerb 1984; Schilke et al. 1992;Hirota et al. 1998; Padovani et al. 2011), proto-brown dwarfs(Riaz et al. 2018), protoplanetary disks (Dutrey et al. 1997;Graninger et al. 2015), and also planetary nebulae (Schmidt &Ziurys 2017; Bublitz et al. 2019). These observations often re-veal a temperature-dependent HNC-to-HCN line ratio, for in-stance, with near-unity ratios found in cold dark clouds, and verylow ratios detected toward warm protostellar cores (Schilke et al.1992; Padovani et al. 2011) and hot ultracompact H II regionsrelated to massive star formation (Jin et al. 2015). A decreasein HNC-to-HCN line ratio with UV luminosity has been foundin planetary nebulae (Bublitz et al. 2019), but this could also be(partially) explained by the temperature e ff ect, with UV photonsheating the gas. Hacar et al. (2020) have recently establishedthe HNC-to-HCN line intensity ratio as a chemical thermometerfor the cold interstellar medium (ISM) based on observations to-wards the integral shape filament of Orion. Temperature is a fun-damental parameter in protoplanetary disks. It sets the locationsof molecular ice lines (Qi et al. 2019), regulates the physical andchemical evolution of the disk (e.g., Kenyon & Hartmann 1995;Dutrey et al. 2014), and is vital for determining the disk mass(e.g., Trapman et al. 2017). While the HCN lines are usuallybright and readily detected in disks (Öberg et al. 2011; Chapillonet al. 2012; Guzmán et al. 2015), the fainter HNC lines are rarelytargeted (Dutrey et al. 1997). Graninger et al. (2015) presentedthe first spatially resolved observations of HNC lines in diskswith a ring-shaped emission structure, which they interpretedas temperature-regulated HNC destruction. This result thereforedemonstrates that spatially resolved HNC and HCN observationsmay be employed to map the disk temperature structure.Because the excitation conditions for HNC and HCN aresimilar, the line intensity ratio would reflect their relative abun-dances and should be regulated by chemical reactions. The twoisomers are mainly produced by the dissociative recombinationof HCNH + ,HCNH + + e − → HCN + H (1)HCNH + + e − → HNC + H , (2)which can be generated from pathways involving NH , atomicN, and N (Loison et al. 2014). This reaction has an approx-imately equal branching ratio, therefore the abundance di ff er-ences between HCN and HNC are largely determined by the The emergence of an additional exterior DCO + emission ring couldbe explained by the nonthermal desorption of CO ice (Öberg et al.2015). main selective destruction pathways of HNC,HNC + C → HCN + C (3)HNC + H → HCN + H (4)HNC + O → CO + NH . (5)The rate coe ffi cient of reaction 3 is constant with temperature(Loison et al. 2014), while reactions 4 and 5 possess activationbarriers of 200 K (Graninger et al. 2014) and 20 K (Hacar et al.2020), respectively, to proceed. Reaction 4 is already active at ∼
20 K, but the reaction rate is orders of magnitude lower thanat 200 K. Because the majority of disk areas are warmer than20 K, the temperature dependence of the HNC-to-HCN line ra-tio is primarily controlled by reaction 4. Although the line ratioalso depends on elemental (e.g., carbon or oxygen) abundances,a high HNC-to-HCN line ratio would indicate a cold region ofthe disk, however.We present here the first exploration of HNC and HCN mod-eling in protoplanetary disks, employing the 2D thermochem-ical code called "dust and lines" (DALI, Bruderer et al. 2012;Bruderer 2013). Our goal is to investigate the HNC and HCNemission in protoplanetary disks based on our knowledge ofcyanide chemistry and to explore the potential usability of HNCand HCN as tracers of the disk physical and chemical condi-tions, especially as a disk thermometer. Section 2 describes thephysical framework and the chemical network. In Section 3 wepresent the results of molecular abundances and line emissionfrom our models, and we compare the results with currentlyavailable disk observations, including new ALMA observationsof HNC J = − J = −
2. Physical and chemical models
To model the HNC and HCN emission in protoplanetary disks,we employed the 2D thermochemical code DALI (Bruderer et al.2012; Bruderer 2013), which has been widely used to modelgas emission in disks (Miotello et al. 2014; van der Marel et al.2015; Kama et al. 2016; Facchini et al. 2017; Trapman et al.2017, 2019). This code includes calculations of radiative trans-fer, chemistry, thermal balance, and ray-tracing. Provided withthe dust and gas density structures, DALI first solves the dustcontinuum radiative transfer based on a Monte Carlo method toobtain the local UV flux and dust temperature T dust at each po-sition of the disk. T dust is used as an initial guess for the gastemperature T gas , and the abundances for chemical species arecalculated with a chemical network simulation and are then fedinto the non-LTE excitation calculation to solve T gas through theheating-cooling balance. Because both the abundance and exci-tation calculations depend on T gas , the final T gas is obtained by it-erating the above steps until a self-consistent solution is reached.Spectral line cubes are then created with a raytracer for a givensource distance and disk inclination. In this section, we describethe physical properties for the star and disk system, as well asthe chemical network used in our models. The main input for physical parameters is the disk density struc-ture. We adopt the gas surface density profile as for a vis-cous evolving accretion disk (viscosity ν ∝ R γ , Lynden-Bell & Article number, page 2 of 16eng Long et al.: HNC and HCN line emission in disks
Pringle 1974; Hartmann et al. 1998), Σ gas ( R ) = Σ c (cid:18) RR c (cid:19) − γ exp (cid:34) − (cid:18) RR c (cid:19) − γ (cid:35) , (6)where Σ c is the surface density at the characteristic radius R c andis set to yield the required total disk mass. The power-law index γ is taken as 1 (Hughes et al. 2008; Andrews et al. 2011) andassumed to not change with time. The gas density in the verti-cal direction follows a Gaussian distribution, with a scale-heightangle of h = h c ( R / R c ) ψ . The physical scale height is thus givenas H ∼ Rh . The dust surface density is scaled from the gas sur-face density assuming a typical gas-to-dust ratio of 100. Becauselarger grains are more settled toward the midplane, we considertwo populations of grains, following D’Alessio et al. (2006): asmall-grain population (0.005-1 µ m) with the same scale height h as the gas, and a large-grain population (1-1000 µ m) with a re-duced scale height of χ h , where χ = .
2. The fraction of dustsurface density distributed to the large grains is described by f large such that Σ dust = f large Σ large + (1 − f large ) Σ small . We havetested models in which we varied the f large parameter between0.9 and 0.99, but found no significant di ff erences in line emis-sion of cyanides (Cazzoletti et al. 2018). We took f large = .
99 inour models because large grains dominate the dust mass budget.High-energy radiation from the central star and interstellarmedium a ff ects the disk chemistry. The stellar spectrum is mod-eled as a blackbody for a given e ff ective temperature T e ff andstellar luminosity L ∗ . The FUV spectrum (6–13.6 eV) is particu-larly important for the chemistry of cyanides, therefore we con-sidered two types of stars: a T Tauri star with T e ff = L ∗ = L (cid:12) , and a Herbig Ae star with T e ff = L ∗ = L (cid:12) ,which we refer to as T Tauri and Herbig models throughout thepaper. Additional UV flux from accretion can also be includedfor T Tauri stars, modeled as a blackbody emitting at 10000 Kon to a 1 M (cid:12) and 1.5 R (cid:12) star, with the total accretion luminositycontrolled by the mass accretion rate ˙ M acc . We took the typi-cal accretion rate for young T Tauri stars of 10 − M (cid:12) yr − (Hart-mann et al. 2016; Manara et al. 2016) as the default in our modeland varied the value to explore the e ff ects of UV radiation oncyanide chemistry (see Figure 1 of Visser et al. 2018 for the UVflux change with varying accretion rates and Figure 11 of Caz-zoletti et al. 2018 for the consequences on CN emission). Weset an interstellar UV flux of G , as defined in Draine (1978),of ∼ . × − erg s − cm − between 6 and 13.6 eV range, anda cosmic-ray ionization rate of 5 × − s − per H . The X-rayspectrum was taken as thermal radiation at 7 × K between1 and 100 keV, with the X-ray luminosity of 10 erg s − . Thesame prescription was used for the T Tauri and Herbig models.While abundances of some molecules or molecular ions suchas HCO + may be sensitive to the choice of X-ray luminosity ortemperature, parameter studies have shown that most species, in-cluding HCN, HNC, and their ratio, have only small variationsover the wide range of X-ray values that is applicable to T Tauriand Herbig stars (Stäuber et al. 2005; Bruderer et al. 2009; Brud-erer 2013; Cleeves et al. 2017). A small grid of models was per-formed by varying disk mass, disk vertical height, and flaringfor the T Tauri and Herbig models to represent a wide range ofphysical and chemical conditions. The disk and stellar parame-ters used in the models are listed in Table 1. Our models used the new reduced network presented in Visseret al. (2018) for CNO chemistry, which is based on the network
Table 1.
Disk and stellar parameters for the models.
Parameter Range
Chemistry [C] / [H] 1 × − [O] / [H] 3.5 × − [N] / [H] 1.6 × − Physical structure γ ψ h c R c
60 au M gas − , − , − , − , − M (cid:12) f large χ Stellar spectrumT e ff + UV ( ˙ M = − M (cid:12) / yr),10000 K L bol
1, 10 L (cid:12) L X erg s − Dust properties
Dust 0.005-1 µ m (small)1-1000 µ m (large) Other parameters
Cosmic-ray ionizationrate per H × − s − External UV flux G Note: Chemistry – initial abundances for C, O, and N. Thee ff ect of di ff erent elemental abundances for the HNC and HCNabundances is also explored in Section 3.4. Physical structure –power-law index ( γ ) for surface density, power-law index ( ψ )for the scale height, scale height ( h c ) at critical radius ( R c ), diskmass, fraction of large-grain population, and its settlingparameter ( χ ).of Bruderer et al. (2012) and includes the reactions for cyanidesdiscussed in Loison et al. (2014). The entire chemical model isthe same as was used for the investigation of CN emission indisks (Cazzoletti et al. 2018), except for two changes made re-lating to the destruction of HNC: (1) we used an updated reac-tion rate coe ffi cient of HNC + H based on Graninger et al. (2014)with an energy barrier of 200 K, and (2) we included the reactionof HNC + O with an energy barrier of 20 K based on the recentstudy of the HNC-to-HCN ratio in molecular clouds by Hacaret al. (2020). The updated network has negligible e ff ects on theCN emission. Reactions involving isotopologs were not consid-ered. The same photodissociation rate was used for HCN andHNC; the actural HNC photodissociation rate may be a factor of2 higher than that of HCN (Aguado et al. 2017).The network contains gas-phase and grain-surface reactions,including neutral-neutral and ion-molecular chemistry, hydro-genation of simple species on ices, photodissociation and pho-toionization, freeze-out and desorption, X-ray and cosmic-rayinduced reactions, and reactions with vibrationally excited H .The details of these processes are elaborated in Bruderer et al.(2012) and Visser et al. (2018). We used binding energies tograins of 1600 K for CN and 2050 K for HNC and HCN, basedon the values from the Kinetic Database for Astrochemistry(KIDA; Wakelam et al. 2012). The chemistry was run in steady-state mode with initial ISM abundances for CNO as listed in Article number, page 3 of 16 & A proofs: manuscript no. aa R (AU)0255075100125150 z ( A U ) n gas (cm ) R (AU)0255075100125150 z ( A U ) F UV ( G ) R (AU)0255075100125150 z ( A U ) T dust (K) R (AU)0255075100125150 z ( A U ) T gas (K) Fig. 1.
Two-dimensional profiles of gas density, UV flux, dust temper-ature, and gas temperature for our fiducial model: a 10 − M (cid:12) disk with ψ = h c = G , Draine 1978). Temperatureonly shows a limited range to highlight the cold outer disk region. Table 1, thus with the ISM C / O. Given the uncertainties in indi-vidual rate coe ffi cients and physical structure, our model abun-dances and column densities have uncertainties of at least a fac-tor of a few. The trends in abundances and line fluxes with modelparameters are more robust.
3. Results
In this section, we present the abundance distribution and lineemission morphology for HNC and HCN in the fiducial model,which is a 10 − M (cid:12) disk with ψ = h c = Figure 2 shows the gas densities and abundances (with respectto the total gas density) for HNC and HCN in our fiducial TTauri disk model. In the hot inner disk ( < (Walsh et al. 2015), HCN is predicted to be orders ofmagnitude more abundant than the range covered in Figure 2. Wefocus on the outer disk where line emission at (sub-)millimeterwavelengths is generated mostly.Our model suggests similar spatial distributions for HNC andHCN, with regions of higher HNC and HCN abundance andnumber density located near the disk surface and extending inthe midplane from ∼
200 au outward (see also Figure 4 in Visseret al. 2018). Their distributions are expected to closely follow thejoint distribution of HCNH + and electrons (see Figure 2) becausethey are mainly produced through dissociative recombination ofHCNH + . At the surface of the disk, HCN and HNC are photodis-sociated into H and CN, explaining the upper boundaries on theHCN and HNC abundance distributions.Although the HCN and HNC dsitributions are largely sim-ilar in our model, there are also important di ff erences. Fig-ure 3 shows the 2D HNC-to-HCN abundance ratio distribu-tion for regions in which the HNC gas number density exceeds 5 × − cm − . Our model predicts overall low abundance ratios( < . − .
4) across the disk, with localized higher ratios seen inthe cold outer midplane and the warm intermediate layer around300 au. In the warm layer, the abundance ratio decreases inwardtoward the hotter region. This temperature-dependent abundanceratio is expected as the destruction of HNC through reaction 4 ismore e ffi cient in the warmer regions, leading to lower HNC-to-HCN ratios. The abundance di ff erence between HNC and HCNis also well observed in the column density profiles (see Fig-ure 4), in which the HCN profile is predicted to peak in the innerdisk, while the HNC profile shows a double-peak structure. For a given temperature structure and abundance profile, spec-tral line emission is obtained through ray-tracing. In this work,we focus on rotational transitions of J = − J = − (cid:48)(cid:48) . ◦ ) and placed at a distance of 150 pc.The simulated line emission from both molecules shows aring-like structure with an emission deficit in the inner disk inthe 1 − − − ∼
300 au for 1 − − − − − J emission in the outer disk islikely due to high critical densities of their corresponding tran-sitions (see also the discussion of CN emission in Cazzolettiet al. 2018). The critical density of the HNC 3 − ∼ × cm − at 20 K, typical gas temperature around 300 au)is a few times higher than the local gas density in the outer diskof our fiducial model (see Figure 1). We therefore expect the up-per level of this transition to be depopulated, resulting in weakline emission (see Figure A.1 for the line emission regions). Thelower J transitions with lower critical density ( ∼ × cm − )are still well populated in the low-density outer disk, however,leading to prominent emission there. In addition, the 3 − ff -center(see Figure 4, right panels). Ring-like emission for HNC andHCN is naturally produced in our full-disk models (with a cen-trally peaked surface density profile) because they cannot beabundantly formed in the dense inner midplane region wherethe electron density is low (see Figure 2). Molecular rings thusdo not necessarily correspond to dust rings that are frequentlyseen in continuum observations. The ring-like structures of HNCand HCN emission seen in the fiducial model are also presentin models with smaller characteristic radius ( R c =
15, 30 au),
Article number, page 4 of 16eng Long et al.: HNC and HCN line emission in disks R (AU)0255075100125150 z ( A U ) n ( HNC )/ n gas R (AU)0255075100125150 z ( A U ) n ( HCN )/ n gas R (AU)0255075100125150 z ( A U ) n ( HCNH + )/ n gas R (AU)0255075100125150 z ( A U ) n ( e )/ n gas R (AU)0255075100125150 z ( A U ) n ( HNC ) R (AU)0255075100125150 z ( A U ) n ( HCN ) R (AU)0255075100125150 z ( A U ) n ( HCNH + ) R (AU)0255075100125150 z ( A U ) n ( e ) Fig. 2. Upper:
Abundance maps of HNC, HCN, and HCNH + and electrons, the two main reactants for HNC and HCN formation. Abundancevalues are relative to n gas = n (H ) + n (H). Lower:
Gas number density maps of HNC, HCN, HCNH + and electrons. Data are shown for thefiducial model of a 10 − M (cid:12) disk with ψ = h c = R (AU) z ( A U ) n(HNC)/n(HCN) Fig. 3.
Two-dimensional profile of the HNC-to-HCN abundance ratio inthe fiducial model. Only regions with HNC gas number density above5 × − cm − are shown, highlighting the locations where most HNCand HCN emission is produced. but the ring peak is shifted inward (see Figure B.1 for intensityprofiles from models with di ff erent R c ). This can be understoodwhen we consider the disk environments that produce the HNCand HCN outer rings: low-density gas where both HCNH + andelectrons are present.Figure 5 shows the predicted radial profiles of HNC-to-HCNline ratio using the intensity profile cuts from the model images.The HNC-to-HCN line ratio generally increases outward, vary-ing from 0.1 to 0.6 in regions where most line emission orig-inates (50 to 350 au in the fiducial model). This increasing pat-tern is consistently seen in our models with di ff erent disk masses(Figure 5). The small bump around 200 au in the 0.1 M (cid:12) diskmodel might arise because disks with higher disk masses are ingeneral cooler, which slows the HNC destruction down.The radial profiles of the HNC-to-HCN line ratio from ourmodels are consistent with a scenario in which the line ratio isregulated by temperature in disks because the disk temperaturedecreases outward. To explore this scenario, we investigated therelation between disk temperature and the HNC-to-HCN ratio more directly. Figure 6 shows the extracted HNC-to-HCN abun-dance ratio and gas temperature at each position of the disk forall disk models with the mass of 0.01 M (cid:12) . We focus on diskradii outside of 10 au where most of HNC and HCN millime-ter emission originates. About half of the parameter space is notoccupied; high HNC-to-HCN abundance ratios are not expectedin the warmest disk regions. Furthermore, the highest achievedHNC-to-HCN abundance ratio in each temperature bin is pre-dicted to decrease with increasing gas temperature. There is noone-to-one relation between temperature and HNC-to-HCN ra-tio, however. Figure 3 shows that low ratios ( < .
2) are expectedto appear across the disk, corresponding to a wide range of gastemperatures, and high ratios ( > .
4) would emerge in the outerdisk from regions close to the cold midplane and from the warmlayer near the disk surface (note the z / r encoded by the colorscheme in Figure 6). We discuss the possible origin for this dis-tribution in Section 4. Integrated line fluxes are the most readily obtained observables.Thus it is instructive to explore the dependences of line fluxeson key disk properties. Figure 7 shows the predicted generalincreasing trend of the integrated HNC line fluxes with highertotal disk masses. Disks around Herbig stars, with higher UVradiation, are predicted to have brighter (high- J ) line emissionthan those around T Tauri stars. The reaction between atomic Nand vibrationally excited H (via FUV pumping), initializing onemajor pathway for the HCNH + formation (Visser et al. 2018),has a strong dependence on UV fluxes, which could then a ff ectthe production of HNC and HCN. Our results presented here fo-cus on HNC lines because similar patterns are also expected forHCN lines.A disk with a larger flaring angle will have more extendeddisk surface areas exposed to stellar radiation and therefore toUV radiation, which should boost the cyanide chemistry. Fig-ure 7 shows that increasing the disk flaring angle results in higherline fluxes of HNC 3 −
2. Our model also predicts that the e ff ectof disk flaring (UV radiation) on the emission of HNC 1 − − Article number, page 5 of 16 & A proofs: manuscript no. aa ( ) HNC J=1-0
HNC J=3-2
200 4000.00.51.0 N o r m a li z e d I n t e n s i t y J=3-2J=1-0
200 4000123 C o l u m n d e n s i t y ( x c m ) HNC )3210123 ( ) HCN J=1-0 ) HCN J=3-2
200 400R (AU)0.00.51.0 N o r m a li z e d I n t e n s i t y J=3-2J=1-0
200 400R (AU)0246810 C o l u m n d e n s i t y ( x c m ) HCN
Fig. 4.
From left to right: Ray-traced images, intensity profile cuts from images, and column density profile. The images are convolved with a 0 (cid:48)(cid:48) . − km s − . Intensity profiles are normalized to the peak of each profile. A distance of 150 pc has been adopted.
100 200 300
R (AU) I ( H N C ) / I ( H C N ) J=3-2J=1-0
100 200 300 400
R (AU) M disk = 0.01M
100 200 300 400 500
R (AU)
Fig. 5.
Radial profiles of HNC-to-HCN line intensity ratio for the two transitions for the R c =
60 au models. The dashed lines are shown for theHNC-to-H CN line ratio, reduced by a factor of 65 (the isotopic ratio), to illustrate the possible optical depth e ff ect. Models with disk masses of0.001, 0.01 (the fiducial model), and 0.1 M (cid:12) are shown from left to right. midplane in the outer disk, which is barely a ff ected by stellar UVradiation (see Figure A.1 in the Appendix).The UV fluxes in our models have contributions from thestellar blackbody spectrum and from an excess UV emissiondue to accretion. In addition to the default accretion rate of10 − M (cid:12) yr − , we ran a set of additional models with di ff erentaccretion rates (10 − , − , and 10 − M (cid:12) yr − , with order-of-magnitude di ff erences in FUV fluxes, see Visser et al. 2018) toexplore the e ff ect of UV fluxes on HNC lines. Similar to whatis seen for changing the disk flaring angles, the simulated linefluxes of the HNC 3 − − J = − N = − J HNC and HCN linetransitions.
Recent observations have suggested that substantial fractions ofthe volatile carbon and oxygen are missing in disks, as tracedby CO in the submillimeter (e.g., Favre et al. 2013; Miotelloet al. 2017; Long et al. 2017; Zhang et al. 2019) and H O va-por in the FIR (Hogerheijde et al. 2011; Du et al. 2017), mak-ing the gas-phase carbon and oxygen abundances largely uncer-tain. In addition, in contrast to an ISM-like C / O ( ∼ H emission indisks requires further oxygen depletion with a C / O > / O = Article number, page 6 of 16eng Long et al.: HNC and HCN line emission in disks
20 40 60 80 100 120 T gas (K) n ( H N C ) / n ( H C N ) z / r Fig. 6.
Extracted HNC and HCN abundance ratio and the correspond-ing gas temperature for all models with M gas = . (cid:12) , color-codedby scale height z / r . Only grids with an HNC gas number density higherthan 1 × − cm − are selected. The line ratio–gas temperature correla-tions from Hacar et al. (2020) are overplotted as gray curves. Figure 8 presents the column density profiles of HNC andHCN in the depletion models compared to our fiducial model.Our model indicates higher column densities with an increasinglevel of C and O depletion, especially in the inner disks, and thecorresponding line emission is expected to be brighter by a fac-tor of a few. We suspect that the removal of CO modifies theionization structure in the disk, which then favors the HCN andHNC formation route starting from N and He + (C.12 in Visseret al. 2018). Meanwhile, the destruction of CN with O wouldslow down with less initial oxygen, leading to a longer CN life-time and an increased production of HCN and HNC through CN + H reaction. The increase in HCN column densities with en-hanced C / O as predicted by our models is consistent with thechemical model results from Cleeves et al. (2018). With higherC / O, our model also predicts more centrally peaked column den-sity profile. The observed diverse morphologies of HCN emis-sion (ring-like or centrally peaked, Bergner et al. 2019) might beexplained by di ff erent C / O in individual disks.
The brighter HCN lines are commonly targeted and detected indisks (Öberg et al. 2010, 2011; Guzmán et al. 2015; Bergneret al. 2019). Low- to medium-resolution observations found thedisk-averaged column densities for HCN lines to be in the rangeof (1–10) × cm − for a sample of disks around T Tauri andHerbig Ae stars (Chapillon et al. 2012; Bergner et al. 2019).These values are broadly consistent with our models. However,the HCN column densities can reach 10 cm − in the inner100 au of disks, as revealed in the recent high-resolution imagesof the ALMA Large Program (MAPS, V. Guzman, J. Bergner,and G. Cataldi, private communication), which leads to high op-tical depths of >
10. High column densities like this could alsobe obtained from models with C and O depletion (Figure 8). Indisk regions in which the observed HCN column density is onthe order of 10 cm − , the line emission of J = − τ ∼ . − (cid:12) (Bergin et al. 2013) for TW Hya.For HD 163296, we take a range of gas disk masses (0.048–0.31 M (cid:12) ) from CO isotopolog and HD observations (Williams& Best 2014; Booth et al. 2019; Kama et al. 2020) with a dis-tance of 101 pc (Gaia Collaboration et al. 2018). Available SMAand IRAM 30m observations provide the disk-averaged HNC-to-HCN line ratio of 0.1–0.2 for the 3 − − − − The HNC J = − CASA version 5.1.1 and then continuum-subtracted with
CASA task uvcontsub . The HNC line data cube with channel widthsof 0.15 km s − was produced with tclean using Briggs weight-ing of robust = (cid:48)(cid:48) . × (cid:48)(cid:48) .
20 anda 1 σ noise level of 3.5 mJy beam − per velocity channel. Themoment-zero map of the HNC emission and the deprojected az-imuthally averaged radial profile, which are created includingonly pixels above 3 σ from the velocity range of 1.9–4 km s − ,are shown in Figure 9.The line emission in TW Hya exhibits a central component,a bright ring around 0 (cid:48)(cid:48) .
3, and a weaker bump around 1 (cid:48)(cid:48) .
0. Thisdouble-ring morphology with the brighter ring at closer radiusis consistent with our model predictions as described in Sec-tion 3.2. In Figure 9 we overplot the radial profile from a modelwith a smaller characteristic radius R c =
30 au (with an other-wise identical parameter setup as the fiducial model) to guidethe comparison. We note that the central emission component isnot predicted by our fiducial models. The data presented herelack the short-spacing baselines that might a ff ect the radial pro-file, thus future observations are needed to confirm the presenceof this central component. However, TW Hya is known to hostan inner disk cavity within 1 au (Andrews et al. 2016). Its brightC H emission also suggests a strong UV field and a high C / O inthe disk (Bergin et al. 2016). A detailed model optimized for TWHya is required to provide an improved match in emission com-ponents, ring locations, and structure amplitudes. This is beyondthe scope of this work, however. disks
The ALMA Lupus disk survey at Band 3 (PI: M. Tazzari,2016.1.00571.S) recorded the 3 mm continuum in frequency-
Article number, page 7 of 16 & A proofs: manuscript no. aa I n t e g r a t e d F l u x ( m J y k m s ) T Tauri
HNC J=1-0
Lupus stackingJ160830.7 10 Herbig
HNC J=1-0 Disk Mass (M )10 I n t e g r a t e d F l u x ( m J y k m s ) T Tauri
HNC J=3-2
TW Hya 10 Disk Mass (M )10 Herbig
HNC J=3-2
HD 163296=0.1=0.2=0.3
Fig. 7.
Disk-integrated line fluxes (calculated at a distance of 150 pc) of HNC as a function of disk mass (left for disks around T Tauri starsand right for Herbig stars). Colors represent di ff erent levels of disk flaring. Upper and lower panels are shown for di ff erent transitions. The onlydetection of HNC J = − C o l u m n D e n s i t y ( c m ) fiducial with ISM C/O0.1x[C] with ISM C/O0.01x[C] with ISM C/O0.01x[C] with C/O=1.5 HCN HNC n ( H N C ) / n ( H C N ) Fig. 8.
Column density profiles of HCN and HNC, as well as their ratio profiles, for disk models with di ff erent levels of C and O initial abundances:the fiducial model with C / O = / O, and the C abundance reduced by a factor of 100 from fiducial values with enhanced O depletion toreach a high C / O = division mode (FDM) with 3840 channels, designed forserendipitous line detections. The HNC 1 − tclean using natural weighting, resultingin a typical beam size of 0 (cid:48)(cid:48) . × (cid:48)(cid:48) .
40 with a velocity resolutionof ∼ . − . With 2–3 min on-source time, we reach a 1 σ noise level of 2–3 mJy beam − per 3 . − channel. To attempt to detect the HNC 1 − − channels andestimated the noise level from the signal-free zones. The tran-sition disk J160830.7-382827 (van der Marel et al. 2018) is thebrightest HNC emission source, with a peak S / N of 4.5 σ . This isincidentally also the brightest CN emission source in the sample(van Terwisga et al. 2019). Its HNC 1 − Article number, page 8 of 16eng Long et al.: HNC and HCN line emission in disks (") ( " ) TW HyaHNC 3-2
HNC (mJy/beam km/s) 0.0 0.5 1.0 1.5 2.0 radius (") N o r m a li z e d i n t e g r a t e d i n t e n s i t y model with Rc=30auTW Hya data (") ( " ) J160830.7-382827HNC 1-0 continuum contours
20 30 40 50 60 70 80I
HNC (mJy/beam km/s) 3210123 (") ( " ) StackingHNC 1-0
HNC (mJy/beam km/s)
Fig. 9. Upper panels:
HNC 3 − Lower panels:
Moment-zero maps for the HNC 1 − − take a source distance of 155 pc (Gaia Collaboration et al. 2018)and disk gas masses from CO observations (Ansdell et al. 2016;Miotello et al. 2017). In addition, HNC 1 − ff erent UV environment thanfull disks. Disk chemical models that include a central cavity areneeded to explore the origin of the observed morphology.We created the stacked image for the remaining 28 disksand obtained a disk-integrated 3 σ upper limit of ∼
10 mJy km s − ,which agrees with our models in the typical gas disk mass rangein the Lupus sample (Ansdell et al. 2016). The measured COgas disk sizes for a sample of Lupus disks span from 70 to ∼
500 au and are mostly smaller than 300 au (Ansdell et al. 2018).The CN observations also reveal that a fraction of Lupus disks are compact, consistent with models with a characteristic radius R c =
15 au (van Terwisga et al. 2019). The HNC 1 − ff erent radii in the sample.
4. Discussion
The HNC-to-HCN line ratio has been shown to correlate withtemperature in the ISM. The observed line ratio in cold darkclouds is much higher than what is found in the warmer gi-ant molecular clouds (Irvine & Schloerb 1984; Goldsmith et al.1986). A systematic study of the HNC and HCN distribution to-ward the OMC-1 region found that the HNC-to-HCN ratio is farbelow 0.1 around Orion-KL, but can reach unity in the coldercloud ridge (Schilke et al. 1992). Recently, Hacar et al. (2020)carried out a dedicated investigation of the HNC-to-HCN ratio inthe integral shape filament of Orion and established a strong cor-
Article number, page 9 of 16 & A proofs: manuscript no. aa A b un d a n c e HCNHNCHCNH+ e H OCO
R=300AU
Height from the midplane (AU) T g a s ( K ) fiducial+ISM C/O0.1x[C]+ISM C/O0.01x[C]+ISM C/O0.01x[C]+C/O=1.5 Fig. 10. Top : Abundance profiles along the vertical disk direction at theradius of 300 au for HNC and HCN in the fiducial model, as well asspecies relevant to their formation (HCNH + and e − ) and destruction (Hand O). The drop in CO abundance in the top layer reflects where COphotodissociation becomes important. Bottom : Gas temperature profilealong the vertical disk direction at 300 au for the fiducial model (solidline), as comparisons to carbon and oxygen depletion models (dashedline: a factor of 10; dash-dotted line: a factor of 100; and dotted line:C / O = relation between the HNC-to-HCN line ratio and the gas kinetictemperature (see the gray curves in Figure 6). The HNC-to-HCNratio has therefore been proposed to have a great potential as athermometer in interstellar and circumstellar environments. Thepredicted increasing trend of HNC-to-HCN line ratio with diskradius suggests that the HNC-to-HCN ratio can be used as probeof disk temperature. However, as discussed in Section 3.2 andFigure 6, a one-to-one correspondence between disk gas temper-ature and HNC-to-HCN ratio is more challenging to establish.The HNC-to-HCN abundance ratio map shown in Figure 3presents a vertical double-peak pattern that cannot simply beexplained by temperature-dependent HNC destruction becausethe gas temperature increases monotonically toward the surfacealong the vertical direction. To identify other possible explana-tions, we examined the abundance vertical cuts at 300 au forspecies involved in HNC / HCN formation and destruction (Fig-ure 10). The disk is assumed to be cold ( ∼ / HCN) could beexplained by the steep increase in O atom abundance around20 au, while we suspect the following increase in HNC abun- dace (and HNC / HCN) above a height of 25 au may relate tophotodissociation, which gradually takes over as the main de-struction pathway for HCN and HNC. Because similar photodis-sociation cross sections are assumed in the model for the twomolecules, we expect a HNC-to-HCN ratio that gradually ap-proaches unity in photodissociation-dominated regions. The COabundance profile, which starts to decrease above 50 au due tophotodissociation, provides the supporting evidence for this hy-pothesis because the photodissociation of HNC and HCN shouldoccur deeper in the disk than CO photodissociation.Because the destruction of HNC largely involves C and O,the HNC-to-HCN ratios would be a ff ected by the volatile C andO abundances. In the depletion models discussed in Section 3.4,the consumption of HNC through reactions with C and O is sup-pressed, and the HNC-to-HCN abundance ratio increases withhigher level of elemental depletion in regions close to disk mid-plane (see Figure D.1 in the Appendix, and also the right panelof Figure 8). Meanwhile, high HNC-to-HCN abundance ratiosin the intermediate disk layer gradually vanish because the gastemperature increases in the depletion models more rapidly (Fig-ure 10) and the activation of HNC reaction with H would begindeeper in the lower disk layers.In summary, although the HNC-to-HCN abundance ratio de-pends on gas temperature in disks, using this ratio as a ther-mometer in disks will be di ffi cult because the ratio also stronglydepends on other parameters, including UV penetration and ini-tial C and O abundances. However, observed abundance ratiosabove ∼ <
50 K)disk material. A promising way to isolate higher HNC-to-HCNratios due to low temperatures rather than photodissociation-dominated chemistry is to observe disks with moderate to highinclinations for which the midplane and elevated emitting regioncould be separated spatially and spectrally. With such observa-tions, the radially resolved HNC-to-HCN ratios in the disk mid-plane could be deployed as a thermometer, similar to its use inmolecular clouds.Currently available measurements in TW Hya and HD163296 return disk-averaged HNC-to-HCN J = − J = − The determination of physical properties of the emitting gas de-pends on the excitation condition of the lines employed. Whenthe emission lines are thermalized and kinetic temperature isknown, molecular column density can be derived from the op-tically thin lines. If the LTE condition is not satisfied, the infer-ence of physical quantities would not be so straightforward anddetailed radiative transfer calculation with model assumptionsare required (van Zadelho ff et al. 2001; Teague et al. 2018). Asdemonstrated in Section 3.2, some of the 3 − − − ∼
50% of the gas temperature. While the 1 − − Article number, page 10 of 16eng Long et al.: HNC and HCN line emission in disks even higher critical density. Analysis with HNC and HCN linesshould therefore take the non-LTE excitation e ff ects into accountfor more precise constraints of the disk physical conditions.
5. Summary
We presented chemical models of HNC and HCN in protoplan-etary disks. Using the 2D thermochemical code DALI, we ex-plored the dependence of HNC and HCN abundance distribu-tion and line emission at (sub-)millimeter wavelengths on vari-ous stellar and disk parameters and the potential usage of HNC-to-HCN line ratio as disk temperature probe. We also presentednew ALMA observational data of HNC J = − J = − − J = − J = − ff ected by UV radiation, however, because it primarily orig-inates from the cold midplane in the outer disk.3. In the models, the column densities of HNC and HCN in-crease with higher levels of C and O depletion, and theyare predicted to be very sensitive to C / O. With a superso-lar C / O = / O in disks.4. The line fluxes and line ratios from literature HNC obser-vations and the upper limit from new ALMA observationsfor Lupus disks are consistent with our model predictions.The transition disk J160830.7-382827 is the only detectionof HNC 1 − / O >
1. The preciserelation of the line ratio – gas temperature depends on otherparameters such as C and O abundances and UV penetration.
Acknowledgements.
We thank our referee, Joel Kastner, for his constructivecomments and suggestions. We thank Sean Andrews for providing the contin-uum image for J160830.7-382827 and Richard Teague and Romane Le Gal forhelpful discussions. We are grateful to Charlie Qi for fixing the doppler trackingissue in the SMA archival data. F.L. acknowledges support from the SmithsonianInstitution as the Submillimeter Array (SMA) Fellow. M.T. acknowledges sup-port from the UK Science and Technology research Council (STFC) consolidatedgrant ST / S000623 /
1, and by the European Union’s Horizon 2020 research andinnovation programme under the Marie Sklodowska-Curie grant agreement No.823823 (RISE DUSTBUSTERS project). This paper makes use of the followingALMA data: 2016.1.00571.S and 2017.1.01056.S. ALMA is a partnership ofESO (representing its member states), NSF (USA), and NINS (Japan), togetherwith NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Ko-rea), in cooperation with the Republic of Chile. The Joint ALMA Observatory isoperated by ESO, AUI / NRAO, and NAOJ.
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Appendix A: Line emission region
A two-dimensional view along the disk radius and the vertical di-rection of the line emission is shown in Figure A.1, in which thecontours indicate the locations from which most of the emissionoriginates. This comparison directly demonstrates the di ff erentemission structures of HNC lines seen above: emission of the J = − J = − Appendix B: Emission dependence on varying diskparameters
Observations reveal disks with a wide range of radial extensions.We investigated how the HNC line emission varies with di ff erentdisk sizes, characterized by R c in our models. Based on the fidu-cial model (e.g., ψ = h c = R c =
15 and 30 au, keeping the inner disk surface density profileidentical (see Figure B.1 in Cazzoletti et al. 2018). Figure B.1shows that the HNC line emission peak shifts inward with de-creasing R c . This is consistent with the fact that 1 − − − − ff ected by UVradiation. Appendix C: HNC-to-HCN line ratio compared toobservations
Disk-integrated HNC-to-HCN line ratios are rather constantin our models with varying stellar and disk conditions, with0.2 ∼ − ∼ − ff erences reflect their emitting regions. Ourmodel predictions are consistent with current available observa-tions of TW Hya and HD 163296. Appendix D: HNC-to-HCN abundance ratio with Cand O depletion
The volatile carbon and oxygen abundances a ff ect the HNC-to-HCN ratio as a disk thermometer because the reactions of HNCwith C and O are important pathways for HNC destruction. Withenhanced C and O depletion, the HNC-to-HCN abundance ratioincreases in the cold midplane regions. Appendix E: HNC line fluxes of the − and − transitions Figure E.1 summarizes the HNC J = − J = − J = − J = − J transitions are moresensitive to UV radiation. Article number, page 13 of 16 & A proofs: manuscript no. aa R (AU) z ( A U ) HNC J=1-0 . . . R (AU) z ( A U ) HNC J=3-2 . . . Fig. A.1.
Contribution function distribution on top of the disk gas temperature distribution. White contours indicate the locations from which 25%,50%, and 75% of the emission originates. HNC J = − J = −
100 200 300 400R (AU)0.00.10.20.30.40.5 I n t e n s i t y ( K k m s )
60 AU30 AU15 AU (a)
HNC J=1-0
100 200 300 400R (AU)0.00.10.20.30.40.5 I n t e n s i t y ( K k m s ) (b) HNC J=3-2
100 200 300 400
R (AU) I ( H N C ) / I ( H C N ) (c) J=3-2J=1-0
20 40 60 80 R c (AU) R i n g P e a k L o c a t i o n ( A U ) (d) Fig. B.1.
Line emission patterns in models with di ff erent R c values (disk sizes): (a) Radial intensity profiles for HNC 1 − − Accretion rate (
M yr ) I n t e g r a t e d F l u x ( m J y k m s ) (a) HNC 1-0HNC 3-2 10 Accretion rate (
M yr ) R i n g L o c a t i o n ( A U ) (b) HNC 3-2
Fig. B.2. (a) Disk-integrated line fluxes for models with di ff erent accretion rates, which are a measure of UV luminosity. (b) Emission peak locationchange for the HNC 3 − Disk Mass (M ) I ( H N C ) / I ( H C N ) J=1-0
HerbigT Tauri
HD 163296 10 Disk Mass (M ) I ( H N C ) / I ( H C N ) J=3-2
HerbigT Tauri
Fig. C.1.
Disk-averaged HNC and HCN line ratio for the T Tauri and Herbig star models and for both transitions. The uncertainties are given bythe changes in the vertical disk structure. Observations of TW Hya and HD 163296 are marked for comparison (Graninger et al. 2015). R (AU) z ( A U ) n(HNC)/n(HCN) R (AU) z ( A U ) n(HNC)/n(HCN) R (AU) z ( A U ) n(HNC)/n(HCN) Fig. D.1.
Similar plot as in Figure 3, but for models including initial elemental depletion. From left to right: (1) C and O depleted by a factor of10, (2) C and O depleted by a factor of 100, and (3) C depleted by a factor of 100, with O further depleted to reach C / O = & A proofs: manuscript no. aa I n t e g r a t e d F l u x ( m J y k m s ) T Tauri
HNC J=2-1 Herbig
HNC J=2-1 Disk Mass (M )10 I n t e g r a t e d F l u x ( m J y k m s ) T Tauri
HNC J=4-3 Disk Mass (M )10 Herbig
HNC J=4-3 =0.1=0.2=0.3
Fig. E.1.
Disk-integrated line fluxes (calculated at a distance of 150 pc) of HNC J = − J = − ff erent levels of disk flaring. The upper and lower panels show di ffff