Abstract
This paper investigates the impact of dust size distribution on magnetic resistivity. In particular, we focus on its impact when the maximum dust size significantly increases from a ...submicron level. The first half of the paper describes our calculation method for magnetic resistivity based on the model of Draine & Sutin and shows that the method reproduces the results of more realistic chemical reaction network calculations reasonably well. Then, we describe the results of the resistivity calculations for dust distributions with large maximum dust grains. Our results show that resistivity tends to decrease with dust growth, which is particularly true when the dust size power exponent
q
is
q
= 2.5. On the other hand, the decrease is less pronounced when the dust size power exponent
q
is
q
= 3.5, i.e., when the small dust is also responsible for the dust cross section. Our results suggest that detailed dust coagulation and fragmentation processes play a vital role in the magnetic resistivities in protostar formation.
As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path ...from submicron sized icy dust monomers to Earth-sized bodies outside the "snow line," located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier" does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1-1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10-100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with 10 times the mass of the Earth. The growth timescale depends on the turbulent strength of the circumnuclear disk and the black hole mass MBH, and it is comparable to the AGN's lifetime (∼108 yr) for low mass (MBH ∼ 106M ) SMBHs.
Abstract
In Wada et al. (2019), we proposed for the first time that a new class of planets,
blanets
, can be formed around supermassive black holes in the galactic center. Here, we investigate the ...dust coagulation processes and physical conditions of the blanet formation outside the snowline (
r
snow
∼ several parsecs) in more detail, especially considering the effect of the radial drift of the dust aggregates. We found that a dimensionless parameter
, where
v
t
is the turbulent velocity and
c
s
is the sound velocity, describing the turbulent viscosity should be smaller than 0.04 in the circumnuclear disk to prevent the destruction of the aggregates due to collision. The formation timescale of blanets
τ
GI
at
r
snow
is,
τ
GI
≃ 70–80 Myr for
α
= 0.01 − 0.04 and
M
BH
= 10
6
M
⊙
. The mass of the blanets ranges from ∼20
M
E
to 3000
M
E
in
r
< 4 pc for
α
= 0.02 (
M
E
is the Earth mass), which is in contrast with 4
M
E
–6
M
E
for the case without the radial drift. Our results suggest that blanets could be formed around relatively low-luminosity active galactic nuclei (
L
bol
∼ 10
42
erg s
−1
) during their lifetime (≲10
8
yr).
COVID-19 has accelerated the spread of telecommuting in Japan. In past studies regarding telecommuting, it was proved to be the result of i-deals, so it was discussed in the context of location ...flexibility i-deals (LFi-deals). The spread of COVID-19, however, has given rise to semi-compulsory telecommuting. Therefore, this study takes three groups: Group A, which continues to work at the office as before; Group B, which has started to telecommute (inexperienced telecommuters) so that telecommuting is regarded as semi-compulsory; and Group C, which has experience with telecommuting (experienced telecommuters) so that telecommuting is the result of making LFi-deals; and investigates the relationship that telecommuting has with the degree of self-determination (DSD) and productivity. Our analysis found that between Group B and Group C, which were both telecommuting, both DSD and productivity were significantly higher for Group C which has LFi-deals compared with Group B, for which telecommuting is semi-compulsory. However, DSD and productivity were higher for Group B than for Group A, so it is possible that starting to telecommute leads to more LFi-deals, a greater DSD, and higher productivity.
The formation of circumstellar disks is investigated using three-dimensional resistive magnetohydrodynamic simulations in which the initial prestellar cloud has a misaligned rotation axis with ...respect to the magnetic field. We examine the effects of (i) the initial angle difference between the global magnetic field and the cloud rotation axis (θ0) and (ii) the ratio of the thermal to gravitational energy ( 0). We study 16 models in total and calculate the cloud evolution until ∼5000 yr after protostar formation. Our simulation results indicate that an initial nonzero θ0 (>0) promotes disk formation but tends to suppress outflow driving for models that are moderately gravitationally unstable, 0 1. In these models, a large-sized rotationally supported disk forms and a weak outflow appears, in contrast to a smaller disk and strong outflow in the aligned case (θ0 = 0). Furthermore, we find that when the initial cloud is highly unstable with small 0, the initial angle difference θ0 does not significantly affect the disk formation and outflow driving.
ABSTRACT We investigate the chemistry in a radiation-hydrodynamics model of a star-forming core that evolves from a cold (∼10 K) prestellar core to the main accretion phase in ∼105 years. A ...rotationally supported gravitationally unstable disk is formed around a protostar. We extract the temporal variation of physical parameters in ∼1.5 × 103 SPH particles that end up in the disk, and perform post-processing calculations of the gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH particles migrate both inward and outward. Since a significant fraction of volatiles such as CO can be trapped in the water-dominant ice in the three-phase model, the ice mantle composition depends not only on the current position in the disk, but also on whether the dust grain has ever experienced higher temperatures than the water sublimation temperature. Stable molecules such as H2O, CH4, NH3, and CH3OH are already abundant at the onset of gravitational collapse and are simply sublimated as the fluid parcels migrate inside the water snow line. On the other hand, various molecules such as carbon chains and complex organic molecules (COMs) are formed in the disk. The COMs abundance sensitively depends on the outcomes of photodissociation and diffusion rates of photofragments in bulk ice mantle. As for S-bearing species, H2S ice is abundant in the collapse phase. In the warm regions in the disk, H2S is sublimated to be destroyed, while SO, H2CS, OCS, and SO2 become abundant.
We present results from our Submillimeter Array (SMA) observations and data analyses of the SMA archival data of the Class I protostar IRAS 04169+2702. The high-resolution (∼0 5) 13CO (3-2) image ...cube shows a compact (r 100 au) structure with a northwest (blue) to southeast (red) velocity gradient, centered on the 0.9 mm dust continuum emission. The direction of the velocity gradient is orthogonal to the axis of the molecular outflow as seen in the SMA 12CO (2-1) data. A similar gas component is seen in the SO (65-54) line. On the other hand, the C18O (2-1) emission traces a more extended (r ∼ 400 au) component with the opposite, northwest (red) to southeast (blue) velocity gradient. Such opposite velocity gradients in the different molecular lines are also confirmed from direct fitting to the visibility data. We have constructed models of a forward-rotating and counterrotating Keplerian disk and a protostellar envelope, including the SMA imaging simulations. The counterrotating model could better reproduce the observed velocity channel maps, although we could not obtain statistically significant fitting results. The derived model parameters are as follows: Keplerian radius of 200 au, central stellar mass of 0.1 M , and envelope rotational and infalling velocities of 0.20 and 0.16 km s−1, respectively. One possible interpretation for these results is the effect of the magnetic field in the process of disk formation around protostars, i.e., the Hall effect.
Abstract
Dust growth and its associated dynamics play key roles in the first phase of planet formation in young stellar objects. Observations have detected signs of dust growth in very young ...protoplanetary disks. Furthermore, signs of planet formation, gaps in the disk at a distance of several tens of au from the central protostar, are also reported. From a theoretical point of view, however, planet formation in the outer regions is difficult due to the rapid inward drift of dust, called the radial drift barrier. Here, on the basis of three-dimensional magnetohydrodynamical simulations of disk evolution with dust growth, we propose a mechanism called the “ashfall” phenomenon, induced by a powerful molecular outflow driven by a magnetic field that may circumvent the radial drift barrier. We find that the large dust that grows to a size of about a centimeter in the inner region of a disk is entrained by an outflow from the disk. Then, large dust decoupled from gas is ejected from the outflow due to centrifugal force, enriching the grown dust in the envelope and eventually falls onto the outer edge of the disk. The overall process is similar to the behavior of ashfall from volcanic eruptions. In the ashfall phenomenon, the Stokes number of dust increases by reaccreting to the less dense disk outer edge. This may allow the dust grains to overcome the radial drift barrier. Consequently, the ashfall phenomenon can provide a crucial assist for making the formation of the planetesimals in outer regions of the disk possible, and hence the formation of wide-orbit planets and gaps.
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