Abstract
Recent simulations find that hot gas accretion onto compact accretors is often highly turbulent and diskless, and shows a power-law density profile with slope
α
ρ
≈ −1. These results are ...consistent with observational constraints, but do not match existing self-similar solutions of radiatively inefficient accretion flows. We develop a theory for this new class of accretion flows, which we dub simple convective accretion flows (SCAFs). We use a set of hydrodynamic simulations to provide a minimalistic example of SCAFs, and develop an analytic theory to explain and predict key flow properties. We demonstrate that the turbulence in the flow is driven locally by convection, and argue that radial momentum balance, together with an approximate up–down symmetry of convective turbulence, yields
α
ρ
= −1 ± few 0.1. Empirically, for an adiabatic hydrodynamic flow with
γ
≈ 5/3, we get
α
ρ
≈ −0.8; the resulting accretion rate (relative to the Bondi accretion rate),
M
̇
/
M
̇
B
∼
(
r
acc
/
r
B
)
0.7
, agrees very well with the observed accretion rates in Sgr A*, M87*, and a number of wind-fed supergiant X-ray binaries. We also argue that the properties of SCAFs are relatively insensitive to additional physical ingredients such as cooling and magnetic field; this explains their common appearance across simulations of different astrophysical systems.
In coalescing neutron star (NS) binaries, tidal force can resonantly excite low-frequency (≲500 Hz) oscillation modes in the NS, transferring energy between the orbit and the NS. This resonant tide ...can induce phase shift in the gravitational waveforms, and potentially provide a new window of studying NS interior using gravitational waves. Previous works have considered tidal excitations of pure g-modes (due to stable stratification of the star) and pure inertial modes (due to Coriolis force), with the rotational effect treated in an approximate manner. However, for realistic NSs, the buoyancy and rotational effects can be comparable, giving rise to mixed inertial-gravity modes. We develop a nonperturbative numerical spectral code to compute the frequencies and tidal coupling coefficients of these modes. We then calculate the phase shift in the gravitational waveform due to each resonance during binary inspiral. Given the uncertainties in the NS equation of state and stratification property, we adopt polytropic NS models with a parametrized stratification. We derive relevant scaling relations and survey how the phase shift depends on various properties of the NS. We find that for canonical NSs (with mass M=1.4 M⊙ and radius R=10 km) and modest rotation rates (≲300 Hz), the gravitational wave phase shift due to a resonance is generally less than 0.01 radian. But the phase shift is a strong function of R and M, and can reach a radian or more for low-mass NSs with larger radii (R≳15 km). Significant phase shift can also be produced when the combination of stratification and rotation gives rise to a very low frequency (≲20 Hz in the inertial frame) modified g-mode. As a by-product of our precise calculation of oscillation modes in rotating NSs, we find that some inertial modes can be strongly affected by stratification; we also find that the m=1 r-mode, previously identified to have a small but finite inertial-frame frequency based on the Cowling approximation, in fact has essentially zero frequency, and therefore cannot be excited during the inspiral phase of NS binaries.
Abstract
In protoplanetary disks, sufficiently massive planets excite pressure bumps, which can then be preferred locations for forming new planet cores. We discuss how this loop may affect the ...architecture of multiplanet systems and compare our predictions with observations. Our main prediction is that low-mass planets and giant planets can each be divided into two subpopulations with different levels of mass uniformity. Low-mass planets that can and cannot reach the pebble isolation mass (the minimum mass required to produce a pressure bump) develop into intra-system similarity “super-Earths” and more diverse “Earths,” respectively. Gas giants that do and do not accrete envelopes quickly develop into similar “Jupiters” and more diverse “Saturns,” respectively. Super-Earths prefer to form long chains via repeated pressure-bump planet formation, while Jupiter formation is usually terminated at pairs or triplets due to dynamical instability. These predictions are broadly consistent with observations. In particular, we discover a previously overlooked mass uniformity dichotomy among the observed populations of both low-mass planets (Earths versus super-Earths) and gas giants (Saturns versus Jupiters). For low-mass planets, planets well below the pebble isolation mass (≲3
M
⊕
or ≲1.5
R
⊕
for Sun-like stars) show significantly higher intra-system pairwise mass differences than planets around the pebble isolation mass. For gas giants, the period ratios of intra-system pairs show a bimodal distribution, which can be interpreted as two subpopulations with different levels of mass uniformity. These findings suggest that pressure-bump planet formation could be an important ingredient in shaping planetary architectures.
Abstract
We formulate a parameterized model of embedded protostellar disks and test its ability to estimate disk properties by fitting dust-continuum observations. The main physical assumptions of ...our model are motivated by a recent theoretical study of protostellar disk formation; these assumptions include that the disk should be marginally gravitationally unstable, and that the dominant dust heating mechanism is internal accretion heating instead of external protostellar irradiation. These assumptions allow our model to estimate reliably the disk mass even when the observed emission is optically thick and to determine self-consistent disk (dust) temperatures. Using our model to fit multiwavelength observations of 163 disks in the VANDAM Orion survey, we find that the majority (57%) of this sample can be fit well by our model. Using our model, we produce new estimates of Orion protostellar disk properties. We find that these disks are generally warm and massive, with a typical star-to-disk mass ratio
M
d
/
M
⋆
=
(
1
)
in Class 0/I. We also discuss why our estimates differ from those in previous studies and the implications of our results on disk evolution and fragmentation.
Abstract
Recent observations suggest that the first stages of planet formation likely take place in the Class 0/I phase of young stellar object evolution, when the star and the disk are still ...embedded in an infalling envelope. In this study we perform grain coagulation calculations to investigate the very first stage of planet formation, the collisional growth of dust grains, in Class 0/I disks. We find that the slow increase in grain mass by high-velocity collision with much smaller grains (“sweep-up”) allows ∼50
M
⊕
of grains to grow well beyond the fragmentation barrier into ∼kilogram pebbles by the end of Class 0/I (0.1 Myr). We analyze the linear growth and saturation of sweep-up to understand our results quantitatively, and test whether the sweep-up outcome is sensitive to disk parameters and details of the grain coagulation model. The sweep-up pebble population could be important for planet formation, because they are less well-coupled to the gas (compared to the main population below the fragmentation barrier) and therefore more favorable to known mechanisms of dust clump formation (which initiate planetesimal formation). It also contains enough mass to form all planet cores, based on observational estimates of the planet mass budget. Our findings motivate future studies of grain growth and planetesimal formation in Class 0/I disks, including the subsequent evolution of this sweep-up population.
Abstract
Embedded class 0/I protostellar disks represent the initial condition for planet formation. This calls for a better understanding of their bulk properties and the dust grains within them. We ...model multiwavelength dust continuum observations of the disk surrounding the class I protostar TMC1A to provide insight on these properties. The observations can be well fit by a gravitationally self-regulated (i.e., marginally gravitationally unstable and internally heated) disk model with surface density Σ ∼ 1720(
R
/10 au)
−1.96
g cm
−2
and midplane temperature
T
mid
∼ 185(
R
/10 au)
−1.27
K. The observed disk contains an
m
= 1 spiral substructure; we use our model to predict the spiral’s pitch angle, and the prediction is consistent with the observations. This agreement serves as both a test of our model and strong evidence of the gravitational nature of the spiral. Our model estimates a maximum grain size
a
max
∼
196
(
R
/
10
au
)
−
2.45
μ
m
, which is consistent with grain growth being capped by a fragmentation barrier with a threshold velocity of ∼1 m s
−1
. We further demonstrate that the observational properties of TMC1A are typical among the observed population of class 0/I disks, which hints that traditional methods of disk data analysis based on Gaussian fitting and the assumption of optically thin dust emission could have systematically underestimated the disk size and mass and overestimated the grain size.
Display omitted
Alkali-activated slag concrete (AASC) is a new green building material. The amount of CO2 produced by AASC is 1/5th of that produced by ordinary Portland cement concrete (OPCC). In ...addition, AASC promotes the reuse of slag and other wastes and saves resources. Furthermore, the scope of use of slag has been expanded. The progress of the research on the hydration characteristics, microstructure, interfacial transition zone, and pore structure of AASC based on the relevant literatures was analyzed and summarized in this study. The influences of the slag composition, the type and dosage of the alkali activator, and the curing conditions on the hydration characteristics and the microstructure of the AASC were discussed. Relatively few research results on the microstructure of AASC are available, and the relevant conclusions are not completely consistent. Moreover, there are many constraints on the development of AASC (e.g., complex composition of raw materials of slag, large shrinkage deformation, and low fluidity). Therefore, further research is required.
Road traffic noise calculation method behind buildings included in the ASJ RTN-Model 2018 is an empirical model formula based on experimental results of a scale model. Therefore, the scope of the ...calculation method is formally limited in terms of building height, density, and distance from the road, dependent on the range of the experimental conditions. However, the roadside conditions in Japanese urban and suburban areas are diverse, and it is necessary to quantitatively investigate the validity of the calculation in such various conditions in order to widely expand the applicability of the calculation method. In European countries, creating noise map has been obliged by the Environmental Noise Directive, and the noise maps are strategically utilized for environmental noise reduction. In Japan, such a utilization of noise map is highly expected, and it is necessary to investigate an accurate estimation method which is suitable for Japanese build-up areas. In this study, we examined the validity of the road traffic noise calculation method in building complex by comparing the actual measurement results in some build-up areas around arterial roads.
Abstract We present a new Submillimeter Array survey of 47 Class II sources in the Taurus–Auriga region. Our observations made 12 independent samples of flux densities over the 200–400 GHz frequency ...range. We tightly constrained the spectral indices of most sources to a narrow range of 2.0 ± 0.2; only a handful of spatially resolved (e.g., diameter >250 au) disks present larger spectral indices. The simplest interpretation for this result is that the (sub)millimeter luminosities of all of the observed target sources are dominated by very optically thick (e.g., τ ≳ 5) dust thermal emission. Some previous works that were based on the optically thin assumption thus might have underestimated optical depths by at least 1 order of magnitude. Assuming DSHARP dust opacities, this corresponds to underestimates of dust masses by a similar factor. For our specific selected sample, the lower limits of dust masses implied by the optically thick interpretation are 1–3 times higher than those previous estimates that were made based on the optically thin assumption. Moreover, some population synthesis models show that, to explain the observed, narrowly distributed spectral indices, the disks in our selected sample need to have very similar dust temperatures ( T dust ). Given a specific assumption of median T dust , the maximum grain sizes ( a max ) can also be constrained, which is a few times smaller than 0.1 mm for T dust ∼ 100 K and a few millimeters for T dust ∼ 24 K. The results may indicate that dust grain growth outside the water snow line is limited by the bouncing/fragmentation barriers. This is consistent with the recent laboratory experiments, which indicated that the coagulation of water-ice-coated dust is not efficient, and the water-ice-free dust is stickier and thus can coagulate more efficiently. In the Class II disks, the dust mass budget outside of the water snow line may be largely retained instead of being mostly consumed by planet formation. While Class II disks still possess sufficient dust masses to feed planet formation at a later time, it is unknown whether or not dust coagulation and planet formation can be efficient or natural outside of the water snow line.
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