We apply the theory of radiative torque (RAT) alignment for studying protoplanetary disks around a T-Tauri star and perform 3D radiative transfer calculations to provide the expected maps of ...polarized radiation to be compared with observations, such as with ALMA. We revisit the issue of grain alignment for large grains expected in the protoplanetary disks and find that mm-sized grains at the midplane do not align with the magnetic field since the Larmor precession timescale for such large grains becomes longer than the gaseous damping timescale. Hence, for these grains the RAT theory predicts that the alignment axis is determined by the grain precession with respect to the radiative flux. As a result, we expect that the polarization will be in the azimuthal direction for a face-on disk. It is also shown that if dust grains have superparamagnetic inclusions, magnetic field alignment is possible for (sub-)micron grains at the surface layer of disks, and this can be tested by mid-infrared polarimetric observations.
Context. Near- to mid-infrared observations of molecular emission from protoplanetary disks show that the inner regions are rich in small organic volatiles (e.g., C2H2 and HCN). Trends in the data ...suggest that disks around cooler stars (Teff ≈ 3000 K) are potentially (i) more carbon-rich; and (ii) more molecule-rich than their hotter counterparts (Teff ≳ 4000 K). Aims. We explore the chemical composition of the planet-forming region (<10 AU) of protoplanetary disks around stars over a range of spectral types (from M dwarf to Herbig Ae) and compare with the observed trends. Methods. Self-consistent models of the physical structure of a protoplanetary disk around stars of different spectral types are coupled with a comprehensive gas-grain chemical network to map the molecular abundances in the planet-forming zone. The effects of (i) N2 self shielding; (ii) X-ray-induced chemistry; and (iii) initial abundances, are investigated. The chemical composition in the “observable” atmosphere is compared with that in the disk midplane where the bulk of the planet-building reservoir resides. Results. M dwarf disk atmospheres are relatively more molecule rich than those for T Tauri or Herbig Ae disks. The weak far-UV flux helps retain this complexity which is enhanced by X-ray-induced ion-molecule chemistry. N2 self shielding has only a small effect in the disk molecular layer and does not explain the higher C2H2/HCN ratios observed towards cooler stars. The models underproduce the OH/H2O column density ratios constrained in Herbig Ae disks, despite reproducing (within an order of magnitude) the absolute value for OH: the inclusion of self shielding for H2O photodissociation only increases this discrepancy. One possible explanation is the adopted disk structure. Alternatively, the “hot” H2O (T ≳ 300 K) chemistry may be more complex than assumed. The results for the atmosphere are independent of the assumed initial abundances; however, the composition of the disk midplane is sensitive to the initial main elemental reservoirs. The models show that the gas in the inner disk is generally more carbon rich than the midplane ices. This effect is most significant for disks around cooler stars. Furthermore, the atmospheric C/O ratio appears larger than it actually is when calculated using observable tracers only. This is because gas-phase O2 is predicted to be a significant reservoir of atmospheric oxygen. Conclusions. The models suggest that the gas in the inner regions of disks around cooler stars is more carbon rich; however, calculations of the molecular emission are necessary to definitively confirm whether the chemical trends reproduce the observed trends.
Protoplanetary disks are thought to have lifetimes of several million yr in the solar neighborhood, but recent observations suggest that the disk lifetimes are shorter in a low-metallicity ...environment. We perform a suite of radiation hydrodynamics simulations of photoevaporating protoplanetary disks to study their long-term evolution of ∼10,000 yr and the metallicity dependence of mass-loss rates. Our simulations follow hydrodynamics, extreme and far-ultraviolet (FUV) radiative transfer, and nonequilibrium chemistry in a self-consistent manner. Dust-grain temperatures are also calculated consistently by solving the radiative transfer of the stellar irradiation and grain (re-)emission. We vary the disk metallicity over a wide range of 10 − 4 Z ≤ Z ≤ 10 Z . The photoevaporation rate is lower with higher metallicity in the range of 10 − 1 Z Z 10 Z , because dust shielding effectively prevents FUV photons from penetrating and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in 10 − 2 Z Z 10 − 1 Z , because FUV photoelectric heating becomes less effective than dust-gas collisional cooling. The temperature in the neutral region decreases, and photoevaporative flows are excited only in an outer region of the disk. At 10 − 4 Z ≤ Z 10 − 2 Z , H i photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with a roughly constant rate. The typical disk lifetime is shorter at Z = 0.3 Z than at Z = Z , being consistent with recent observations of the extreme outer galaxy.
We perform a suite of radiation hydrodynamics simulations of photoevaporating disks, varying the metallicity in a wide range of . We follow the disk evolution for over ∼5000 years by solving ...hydrodynamics, radiative transfer, and nonequilibrium chemistry. Our chemistry model is updated from the first paper of this series by adding X-ray ionization and heating. We study the metallicity dependence of the disk photoevaporation rate and examine the importance of X-ray radiation. In the fiducial case with solar metallicity, including the X-ray effects does not significantly increase the photoevaporation rate when compared to the case with ultraviolet (UV) radiation only. At subsolar metallicities in the range of , the photoevaporation rate increases as metallicity decreases owing to the reduced opacity of the disk medium. The result is consistent with the observational trend that disk lifetimes are shorter in low metallicity environments. In contrast, the photoevaporation rate decreases at even lower metallicities of , because dust-gas collisional cooling remains efficient compared to far-UV photoelectric heating whose efficiency depends on metallicity. The net cooling in the interior of the disk suppresses the photoevaporation. However, adding X-ray radiation significantly increases the photoevaporation rate, especially at . Although the X-ray radiation itself does not drive strong photoevaporative flows, X-rays penetrate deep into the neutral region in the disk, increase the ionization degree there, and reduce positive charges of grains. Consequently, the effect of photoelectric heating by far-UV radiation is strengthened by the X-rays and enhances the disk photoevaporation.
Abstract Carbon isotope fractionation of CO has been reported in the disk around TW Hya, where elemental carbon is more abundant than elemental oxygen (C/O elem > 1). We investigated the effects of ...the C/O elem ratio on carbon fractionation using astrochemical models that incorporate isotope-selective photodissociation and isotope exchange reactions. The 12 CO/ 13 CO ratio could be lower than the elemental carbon isotope ratio due to isotope exchange reactions when the C/O elem ratio exceeds unity. The observed 12 CO/ 13 CO and H 12 CN/H 13 CN ratios around TW Hya could be reproduced when the C/O elem ratio is 2–5. In the vicinity of the lower boundary of the warm molecular layer, the formation of ices leads to the gas-phase C/O elem ratio approaching unity, irrespective of the total (gas + ice) C/O elem ratio. This phenomenon reduces the variation in the 12 CO/ 13 CO ratio across different C/O elem ratios.
Abstract Recent high-resolution and sensitivity Atacama Large Millimeter/submillimeter Array observations have unveiled the carbon isotope ratios ( 12 C/ 13 C) of complex organic molecules (COMs) in ...a low-mass protostellar source. To understand the 12 C/ 13 C ratios of COMs, we investigated the carbon isotope fractionation of COMs from prestellar cores to protostellar cores with a gas-grain chemical network model. We confirmed that the 12 C/ 13 C ratios of small molecules are bimodal in the prestellar phase: CO and species formed from CO (e.g., CH 3 OH) are slightly enriched in 13 C compared to the local interstellar medium (by ∼10%), while those from C and C + are depleted in 13 C owing to isotope exchange reactions. COMs are mainly formed on the grain surface and in the hot gas (> 100 K) in the protostellar phase. The 12 C/ 13 C ratios of COMs depend on which molecules the COMs are formed from. In our base model, some COMs in the hot gas are depleted in 13 C compared to the observations. Thus, we additionally incorporate reactions between gaseous atomic C and H 2 O ice or CO ice on the grain surface to form H 2 CO ice or C 2 O ice, as suggested by recent laboratory studies. The direct C-atom addition reactions open pathways to form 13 C-enriched COMs from atomic C and CO ice. We find that these direct C-atom addition reactions mitigate 13 C-depletion of COMs, and the model with the direct C-atom addition reactions better reproduces the observations than our base model. We also discuss the impact of the cosmic-ray ionization rate on the 12 C/ 13 C ratio of COMs.
Abstract
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, but it could be protoplanetary remnants and/or secondary products from large bodies. In order to be protoplanetary in origin, the gas component of the parental protoplanetary disk is required to survive for
≳
10
Myr
. However, previous models predict
≲
10
Myr
lifetimes because of efficient photoevaporation at the late stage of disk evolution. 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 like those devoid of small grains (
≲
4
μ
m
). We find that grain depletion reduces photoelectric heating so that far-ultraviolet photoevaporation is not excited. Extreme-ultraviolet (EUV) photoevaporation is dominant and yields a mass-loss rate of the order of
1
×
10
−
11
(
Φ
EUV
/
10
38
s
−
1
)
1
/
2
M
⊙
yr
−
1
, where
Φ
EUV
is the EUV emission rate of the host star. The estimated gas–disk lifetimes are
∼
100
(
M
disk
/
10
−
3
M
⊙
)
(
Φ
EUV
/
10
38
s
−
1
)
1
/
2
Myr
and depend on the “initial” disk mass at the point small grains have been depleted in the system. We show that the gas component can survive for a much longer time around A-type stars than lower-mass (F-, G-, K-type) stars owing to their atypical low EUV (and X-ray) luminosities. 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.
Abstract
Observations have revealed that the elemental abundances of carbon and oxygen in the warm molecular layers of some protoplanetary disks are depleted compared to those in the interstellar ...medium by a factor of ∼10–100. Meanwhile, little is known about nitrogen. To investigate the time evolution of nitrogen, carbon, and oxygen elemental abundances in disks, we develop a one-dimensional plane-parallel model that incorporates dust settling, turbulent diffusion of dust and ices, as well as gas-ice chemistry including the chemistry driven by stellar UV/X-rays and galactic cosmic rays. We find that gaseous CO in the warm molecular layer is converted to CO
2
ice and locked up near the midplane via the combination of turbulent mixing (i.e., the vertical cold finger effect) and ice chemistry driven by stellar UV photons. On the other hand, gaseous N
2
, the main nitrogen reservoir in the warm molecular layer, is less processed by ice chemistry and exists as it is. Then, nitrogen depletion occurs solely through the vertical cold finger effect of N
2
. As the binding energy of N
2
is lower than that of CO and CO
2
, the degree of nitrogen depletion is smaller than that of carbon and oxygen depletion, leading to higher elemental abundance of nitrogen than that of carbon and oxygen. This evolution occurs within 1 Myr and proceeds further, when the
α
parameter for the diffusion coefficient is ≳10
−3
. Consequently, the N
2
H
+
/CO column density ratio increases with time. How the vertical transport affects the midplane ice composition is briefly discussed.
ABSTRACT Protoplanetary disks with non-axisymmetric structures have been observed. The Rossby wave instability (RWI) is considered as one of the origins of the non-axisymmetric structures. We perform ...linear stability analyses of the RWI in barotropic flow using four representative types of the background flow on a wide parameter space. We find that the co-rotation radius is located at the background vortensity minimum with large concavity if the system is marginally stable to the RWI, and this allows us to easily check the stability against the RWI. We newly derive the necessary and sufficient condition for the onset of the RWI in semi-analytic form. We discuss the applicability of the new condition in realistic systems and the physical nature of the RWI.
ABSTRACT In protoplanetary disks, micron-sized dust grains coagulate to form highly porous dust aggregates. Because the optical properties of these aggregates are not completely understood, it is ...important to investigate how porous dust aggregates scatter light. In this study, the light scattering properties of porous dust aggregates were calculated using a rigorous method, the T-matrix method, and the results were then compared with those obtained using the Rayleigh-Gans-Debye (RGD) theory and Mie theory with the effective medium approximation (EMT). The RGD theory is applicable to moderately large aggregates made of nearly transparent monomers. This study considered two types of porous dust aggregates-ballistic cluster-cluster agglomerates (BCCAs) and ballistic particle-cluster agglomerates. First, the angular dependence of the scattered intensity was shown to reflect the hierarchical structure of dust aggregates; the large-scale structure of the aggregates is responsible for the intensity at small scattering angles, and their small-scale structure determines the intensity at large scattering angles. Second, it was determined that the EMT underestimates the backward scattering intensity by multiple orders of magnitude, especially in BCCAs, because the EMT averages the structure within the size of the aggregates. It was concluded that the RGD theory is a very useful method for calculating the optical properties of BCCAs.
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