arXiv subjects

Hsien Shang

Publications and source records attributed to Hsien Shang.

At least 55 records · Page 3Linked to original sources

arXiv2022

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): Evidence for a Molecular Jet Launched at an Unprecedented Early Phase of Protostellar evolution

Protostellar outflows and jets play a vital role in star formation as they carry away excess angular momentum from the inner disk surface, allowing the material to be transferred toward the central protostar. Theoretically, low velocity and poorly collimated outflows appear from the beginning of the collapse, at the first hydrostatic core (FHSC) stage. With growing protostellar core mass, high-density jets are launched which entrain an outflow from the infalling envelope. Until now, molecular jets have been observed at high velocity ($\gtrsim$ 100 km/s) in early Class\,0 protostars. We, for the first time, detect a dense molecular jet in SiO emission with small-velocity ($\sim$ 4.2 km\,s$^{-1}$, deprojected $\sim$ 24 km\,s$^{-1}$) from source G208.89-20.04Walma (hereafter, G208Walma) using ALMA Band\,6 observations. This object has some characteristics of FHSCs, such as a small outflow/jet velocity, extended 1.3\,mm continuum emission, and N$_2$D$^+$ line emission. Additional characteristics, however, are typical of early protostars: collimated outflow and SiO jet. The full extent of the outflow corresponds to a dynamical time scale of $\sim$ 930$^{+200}_{-100}$ years. The spectral energy distribution also suggests a very young source having an upper limit of T$_{bol}$ $\sim$ 31 K and L$_{bol}$ $\sim$ 0.8 L$_\sun$. We conclude that G208Walma is likely in the transition phase from FHSC to protostar, and the molecular jet has been launched within a few hundred years of initial collapse. Therefore, G208Walma may be the earliest object discovered in the protostellar phase with a molecular jet.

astro-ph.GA↗

arXiv2022

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): A Hot Corino Survey toward Protostellar Cores in the Orion Cloud

The presence of complex organic molecules (COMs) in the interstellar medium (ISM) is of great interest since it may link to the origin and prevalence of life in the universe. Aiming to investigate the occurrence of COMs and their possible origins, we conducted a chemical census toward a sample of protostellar cores as part of the ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP) project. We report the detection of 11 hot corino sources, which exhibit compact emissions from warm and abundant COMs, among 56 Class 0/I protostellar cores. All the hot corino sources discovered are likely Class 0 and their sizes of the warm region ($>$ 100 K) are comparable to 100 au. The luminosity of the hot corino sources exhibits positive correlations with the total number of methanol and the extent of its emissions. Such correlations are consistent with the thermal desorption picture for the presence of hot corino and suggest that the lower luminosity (Class 0) sources likely have a smaller region with COMs emissions. With the same sample selection method and detection criteria being applied, the detection rates of the warm methanol in the Orion cloud (15/37) and the Perseus cloud (28/50) are statistically similar when the cloud distances and the limited sample size are considered. Observing the same set of COM transitions will bring a more informative comparison between the cloud properties.

astro-ph.GA↗

arXiv2022

Magneto-Centrifugal Origin for Protostellar Jets Validated through Detection of Radial Flow at the Jet Base

Jets can facilitate the mass accretion onto the protostars in star formation. They are believed to be launched from accretion disks around the protostars by magneto-centrifugal force, as supported by the detections of rotation and magnetic field in some of them. Here we report a radial flow of the textbook-case protostellar jet HH 212 at the base to further support this jet-launching scenario. This radial flow validates a central prediction of the magneto-centrifugal theory of jet formation and collimation, namely, the jet is the densest part of a wide-angle wind that flows radially outward at distances far from the (small, sub-au) launching region. Additional evidence for the radially flowing wide-angle component comes from its ability to reproduce the structure and kinematics of the shells detected around the HH 212 jet. This component, which can transport material from inner to outer disk, could account for the chondrules and Ca-Al-rich inclusions detected in the Solar System at large distances.

astro-ph.GA↗

arXiv2022

Magnetic Spirals in Accretion Flows Originated from Misaligned Magnetic Field

Misalignment between rotation and magnetic field has been suggested to be one type of physical mechanisms which can easen the effects of magnetic braking during collapse of cloud cores leading to formation of protostellar disks. However, its essential factors are poorly understood. Therefore, we perform a more detailed analysis of the physics involved. We analyze existing simulation data to measure the system torques, mass accretion rates and Toomre Q parameters. We also examine the presence of shocks in the system. While advective torques are generally the strongest, we find that magnetic and gravitational torques can play substantial roles in how angular momentum is transferred during the disk formation process. Magnetic torques can shape the accretion flows, creating two-armed magnetized inflow spirals aligned with the magnetic field. We find evidence of an accretion shock that is aligned according to the spiral structure of the system. Inclusion of ambipolar diffusion as explored in this work has shown a slight influence in the small scale structures but not in the main morphology. We discuss potential candidate systems where some of these phenomena could be present.

astro-ph.SR↗

arXiv2021

Detection of a dense SiO jet in the evolved protostellar phase

Jets and outflows trace the accretion history of protostars. High-velocity molecular jets have been observed from several protostars in the early Class\,0 phase of star formation, detected with the high-density tracer SiO. Until now, no clear jet has been detected with SiO emission from isolated evolved Class\,I protostellar systems. We report a prominent dense SiO jet from a Class\,I source G205S3 (HOPS\,315: T$_{bol}$ $\sim$ 180 K, spectral index $\sim$ 0.417), with a moderately high mass-loss rate ($\sim$ 0.59 $\times$ 10$^{-6}$ M$_\odot$ yr$^{-1}$) estimated from CO emission. Together, these features suggest that G205S3 is still in a high accretion phase, similar to that expected of Class\,0 objects. We compare G205S3 to a representative Class\,0 system G206W2 (HOPS\,399) and literature Class\,0/I sources to explore the possible explanations behind the SiO emission seen at the later phase. We estimate a high inclination angle ($\sim$ 40$^\circ$) for G205S3 from CO emission, which may expose the infrared emission from the central core and mislead the spectral classification. However, the compact 1.3\,mm continuum, C$^{18}$O emission, location in the bolometric luminosity to sub-millimeter fluxes diagram, outflow force ($\sim$ 3.26 $\times$ 10$^{-5}$ M$_\odot$km s$^{-1}$/yr) are also analogous to that of Class\,I systems. We thus consider G205S3 to be at the very early phase of Class\,I, and in the late phase of ``high-accretion". The episodic ejection could be due to the presence of an unknown binary, a planetary companion, or dense clumps, where the required mass for such high accretion could be supplied by a massive circumbinary disk.

astro-ph.GA↗

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.

astro-ph.SR↗

arXiv2021

Characterization of dense Planck clumps observed with Herschel and SCUBA-2

We aim to characterize a diverse selection of dense, potentially star-forming cores, clumps, and clouds within the Milky Way in terms of their dust emission and SF activity. We studied 53 fields that have been observed in the JCMT SCUBA-2 continuum survey SCOPE and have been mapped with Herschel. We estimated dust properties by fitting Herschel observations with modified blackbody functions, studied the relationship between dust temperature and dust opacity spectral index $β$, and estimated column densities. We extracted clumps from the SCUBA-2 850 $μ$m maps with the FellWalker algorithm and examined their masses and sizes. Clumps are associated with young stellar objects found in several catalogs. We estimated the gravitational stability of the clumps with virial analysis. The clumps are categorized as unbound starless, prestellar, or protostellar. We find 529 dense clumps, typically with high column densities from (0.3-4.8)$\times 10^{22}$ cm$^{-2}$, with a mean of (1.5$\pm$0.04)$\times10^{22}$ cm$^{-2}$, low temperatures ($T\sim $10-20 K), and estimated submillimeter $β$ =1.7$\pm$0.1. We detect a slight increase in opacity spectral index toward millimeter wavelengths. Masses of the sources range from 0.04 $M_\odot$ to 4259 $M_\odot$. Mass, linear size, and temperature are correlated with distance. Furthermore, the estimated gravitational stability is dependent on distance, and more distant clumps appear more virially bound. Finally, we present a catalog of properties of the clumps.Our sources present a large array of SF regions, from high-latitude, nearby diffuse clouds to large SF complexes near the Galactic center. Analysis of these regions will continue with the addition of molecular line data, which will allow us to study the densest regions of the clumps in more detail.

astro-ph.GA↗

arXiv2020

Revealing Ionization Conditions of Sz 102 with Spatially Resolved [Ne III] Microjets

Forbidden neon emission lines from small-scale microjets can probe high-energy processes in low-mass young stellar systems. We obtained spatially resolved [Ne III] spectra of the microjets from the classical T Tauri Star Sz 102 using the Hubble Space Telescope Imaging Spectrograph (HST/STIS) at a spatial resolution of ~0".1. The blueshifted and redshifted [Ne III] emission both peak in intensity within ~0".1 of the star and gradually decay along the flow outward to ~0".24. The spatial distribution and extent of the [Ne III] microjet is consistent with a jet that is ionized close to the base and subsequently recombines on a longer timescale than the flow time. Ca II H and K lines are also detected from the redshifted microjet with a line full-width at half-maximum of ~170 km/s, consistent with those of other forbidden emission lines, atop a 300-km/s wide stellar component. The launching radius of the Sz 102 jet, inferred from the observed line centroids and the range of inclination angles and stellar masses from the literature, is on the order of ~0.03 au. The possible proximity of the launching region to the star allows immediate ionization without distance dilution from the circumstellar ionization sources, most likely keV X-ray flares generated by magnetic reconnection events in the star-disk system, to sustain the observed [Ne III] flux.

astro-ph.SR↗

arXiv2020

Interaction of large- and small-scale dynamos in isotropic turbulent flows from GPU-accelerated simulations

Magnetohydrodynamical (MHD) dynamos emerge in many different astrophysical situations where turbulence is present, but the interaction between large-scale (LSD) and small-scale dynamos (SSD) is not fully understood. We performed a systematic study of turbulent dynamos driven by isotropic forcing in isothermal MHD with magnetic Prandtl number of unity, focusing on the exponential growth stage. Both helical and non-helical forcing was employed to separate the effects of LSD and SSD in a periodic domain. Reynolds numbers (Rm) up to $\approx 250$ were examined and multiple resolutions used for convergence checks. We ran our simulations with the Astaroth code, designed to accelerate 3D stencil computations on graphics processing units (GPUs) and to employ multiple GPUs with peer-to-peer communication. We observed a speedup of $\approx 35$ in single-node performance compared to the widely used multi-CPU MHD solver Pencil Code. We estimated the growth rates both from the averaged magnetic fields and their power spectra. At low Rm, LSD growth dominates, but at high Rm SSD appears to dominate in both helically and non-helically forced cases. Pure SSD growth rates follow a logarithmic scaling as a function of Rm. Probability density functions of the magnetic field from the growth stage exhibit SSD behaviour in helically forced cases even at intermediate Rm. We estimated mean-field turbulence transport coefficients using closures like the second-order correlation approximation (SOCA). They yield growth rates similar to the directly measured ones and provide evidence of $α$ quenching. Our results are consistent with the SSD inhibiting the growth of the LSD at moderate Rm, while the dynamo growth is enhanced at higher Rm.

physics.flu-dyn↗

arXiv2020

Two-dimensional Particle-in-Cell simulations of axisymmetric black hole magnetospheres

We investigate the temporal evolution of an axisymmetric magnetosphere around a rapidly rotating, stellar-mass black hole, applying a two-dimensional particle-in-cell simulation scheme. Adopting a homogeneous pair production, and assuming that the mass accretion rate is much less than the Eddington limit, we find that the black hole's rotational energy is preferentially extracted from the middle latitudes, and that this outward energy flux exhibits an enhancement that lasts approximately 160 dynamical time scales. It is demonstrated that the magnetohydrodynamic approximations cannot be justified in such a magnetically-dominated magnetosphere, because the Ohm's law completely breaks down, and because the charge-separated electron-positron plasmas are highly non-neutral. An implication is given regarding the collimation of relativistic jets.

astro-ph.HE↗

arXiv2020

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP) II. Survey overview: a first look at 1.3 mm continuum maps and molecular outflows

Planck Galactic Cold Clumps (PGCCs) are contemplated to be the ideal targets to probe the early phases of star formation. We have conducted a survey of 72 young dense cores inside PGCCs in the Orion complex with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3\,mm (band 6) using three different configurations (resolutions $\sim$ 0$\farcs$35, 1$\farcs$0, and 7$\farcs$0) to statistically investigate their evolutionary stages and sub-structures. We have obtained images of the 1.3\,mm continuum and molecular line emission ($^{12}$CO, and SiO) at an angular resolution of $\sim$ 0$\farcs$35 ($\sim$ 140\,au) with the combined arrays. We find 70 substructures within 48 detected dense cores with median dust-mass $\sim$ 0.093\,M$_{\sun}$ and deconvolved size $\sim$ 0$\farcs$27. Dense substructures are clearly detected within the central 1000\,au of four candidate prestellar cores. The sizes and masses of the substructures in continuum emission are found to be significantly reduced with protostellar evolution from Class\,0 to Class\,I. We also study the evolutionary change in the outflow characteristics through the course of protostellar mass accretion. A total of 37 sources exhibit CO outflows, and 20 ($>$50\%) show high-velocity jets in SiO. The CO velocity-extents ($Δ$Vs) span from 4 to 110 km/s with outflow cavity opening angle width at 400\,au ranging from $[Θ_{obs}]_{400}$ $\sim$ 0$\farcs$6 to 3$\farcs$9, which corresponds to 33$\fdg$4$-$125$\fdg$7. For the majority of the outflow sources, the $Δ$Vs show a positive correlation with $[Θ_{obs}]_{400}$, suggesting that as protostars undergo gravitational collapse, the cavity opening of a protostellar outflow widens and the protostars possibly generate more energetic outflows.

astro-ph.GA↗

arXiv2020

Hall Effect in Protostellar Disc Formation and Evolution

The Hall effect is recently shown to be efficient in magnetized dense molecular cores, and could lead to a bimodal formation of rotationally supported discs (RSDs) in the first core phase. However, how such Hall dominated systems evolve in the protostellar accretion phase remains unclear. We carry out 2D axisymmetric simulations including Hall effect and Ohmic dissipation, with realistic magnetic diffusivities computed from our equilibrium chemical network. We find that Hall effect only becomes efficient when the large population of very small grains (VSGs: $\lesssim$10 nm) is removed from the standard MRN size distribution. With such an enhanced Hall effect, however, the bimodality of disc formation does not continue into the main accretion phase. The outer part of the initial $\sim$40 AU disc formed in the anti-aligned configuration (${\bf Ω\cdot B}<0$) flattens into a thin rotationally supported Hall current sheet as Hall effect moves the poloidal magnetic field radially inward relative to matter, leaving only the inner $\lesssim$10--20 AU RSD. In the aligned configuration (${\bf Ω\cdot B}>0$), disc formation is suppressed initially but a counter-rotating disc forms subsequently due to efficient azimuthal Hall drift. The counter-rotating disc first grows to $\sim$30 AU as Hall effect moves the magnetic field radially outward, but only the inner $\lesssim$10 AU RSD is long-lived like in the anti-aligned case. Besides removing VSGs, cosmic ray ionization rate should be below a few 10$^{-16}$ s$^{-1}$ for Hall effect to be efficient in disc formation. We conclude that Hall effect produces small $\lesssim$10--20 AU discs regardless of the polarity of the magnetic field, and that radially outward diffusion of magnetic fields remains crucial for disc formation and growth.

astro-ph.SR↗

arXiv2020

The Interplay between Ambipolar Diffusion and Hall Effect on Magnetic Field Decoupling and Protostellar Disc Formation

Non-ideal MHD effects have been shown recently as a robust mechanism of averting the magnetic braking "catastrophe" and promoting protostellar disc formation. However, the magnetic diffusivities that determine the efficiency of non-ideal MHD effects are highly sensitive to microphysics. We carry out non-ideal MHD simulations to explore the role of microphysics on disc formation and the interplay between ambipolar diffusion (AD) and Hall effect during the protostellar collapse. We find that removing the smallest grain population ($\lesssim$10 nm) from the standard MRN size distribution is sufficient for enabling disc formation. Further varying the grain sizes can result in either a Hall-dominated or an AD-dominated collapse; both form discs of tens of AU in size regardless of the magnetic field polarity. The direction of disc rotation is bimodal in the Hall dominated collapse but unimodal in the AD-dominated collapse. We also find that AD and Hall effect can operate either with or against each other in both radial and azimuthal directions, yet the combined effect of AD and Hall is to move the magnetic field radially outward relative to the infalling envelope matter. In addition, microphysics and magnetic field polarity can leave profound imprints both on observables (e.g., outflow morphology, disc to stellar mass ratio) and on the magnetic field characteristics of protoplanetary discs. Including Hall effect relaxes the requirements on microphysics for disc formation, so that prestellar cores with cosmic-ray ionization rate of $\lesssim$2--3$\times10^{-16}$ s$^{-1}$ can still form small discs of $\lesssim$10 AU radius. We conclude that disc formation should be relatively common for typical prestellar core conditions, and that microphysics in the protostellar envelope is essential to not only disc formation, but also protoplanetary disc evolution.

astro-ph.SR↗

arXiv2020

Possible Time Correlation Between Jet Ejection and Mass Accretion for RW Aur A

For the active T-Taur star RW Aur A we have performed long-term (~10 yr) monitoring observations of (1) jet imaging in the [Fe II] 1.644-micron emission line using Gemini-NIFS and VLT-SINFONI; (2) optical high-resolution spectroscopy using CFHT-ESPaDOnS; and (3) V-band photometry using the CrAO 1.25-m telescope and AAVSO. The latter two observations confirm the correlation of time variabilities between (A) the Ca II 8542 A and O I 7772 A line profiles associated with magnetospheric accretion, and (B) optical continuum fluxes. The jet images and their proper motions show that four knot ejections occurred at the star over the past ~15 years with an irregular interval of 2-6 years. The time scale and irregularity of these intervals are similar to those of the dimming events seen in the optical photometry data. Our observations show a possible link between remarkable (Delta_V < -1 mag.) photometric rises and jet knot ejections. Observations over another few years may confirm or reject this trend. If confirmed, this would imply that the location of the jet launching region is very close to the star (r <<0.1 au) as predicted by some jet launching models. Such a conclusion would be crucial for understanding disk evolution within a few au of the star, and therefore possible ongoing planet formation at these radii.

astro-ph.SR↗

arXiv2020

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): I. Detection of New Hot Corinos with ACA

We report the detection of four new hot corino sources, G211.47-19.27S, G208.68-19.20N1, G210.49-19.79W and G192.12-11.10 from a survey study of Planck Galactic Cold Clumps in the Orion Molecular Cloud Complex with the Atacama Compact Array (ACA). Three sources had been identified as low mass Class 0 protostars in the Herschel Orion Protostar Survey (HOPS). One source in the lambda Orionis region is firstly reported as a protostellar core. We have observed abundant complex organic molecules (COMs), primarily methanol but also other oxygen-bearing COMs (in G211.47-19.27S and G208.68-19.20N1) and the molecule of prebiotic interest NH2CHO (in G211.47-19.27S), signifying the presence of hot corinos. While our spatial resolution is not sufficient for resolving most of the molecular emission structure, the large linewidth and high rotational temperature of COMs suggest that they likely reside in the hotter and innermost region immediately surrounding the protostar. In G211.47-19.27S, the D/H ratio of methanol ([CH2DOH]/[CH3OH]) and the 12C/13C ratio of methanol ([CH3OH]/[13CH3OH]) are comparable to those of other hot corinos. Hydrocarbons and long carbon-chain molecules such as c-C3H2 and HCCCN are also detected in the four sources, likely tracing the outer and cooler molecular envelopes.

astro-ph.SR↗

arXiv2020

Transition from ordered pinched to warped magnetic field on a 100 au scale in the Class 0 protostar B335

We present our observational results of the 0.87 mm polarized dust emission in the Class 0 protostar B335 obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) at a 0.2" (20 au) resolution. We compared our data at 0.87 mm with those at 1.3 mm from the ALMA archive. The observed polarization orientations at the two wavelengths are consistent within the uncertainty, and the polarization percentages are systematically higher at 1.3 mm than 0.87 mm by a factor of ~1.7, suggesting that the polarized emission originates from magnetically aligned dust grains. We inferred the magnetic field orientations from the observed polarization orientations. We found that the magnetic field changes from ordered and highly pinched to more complicated and asymmetric structures within the inner 100 au scale of B335, and the magnetic field connects to the center along the equatorial plane as well as along the directions which are ~40-60 degrees from the equatorial plane. We performed non-ideal MHD simulations of collapsing dense cores. We found that similar magnetic field structures appear in our simulations of dense cores with the magnetic field and rotational axis slightly misaligned by 15 degrees but not in those with the aligned magnetic field and rotational axis. Our results suggest that the midplane of the inner envelope within the inner 100 au scale of B335 could be warped because of the misaligned magnetic field and rotational axis, and the magnetic field could be dragged by the warped accretion flows.

astro-ph.SR↗

arXiv2020

Direct Calculation of Self-Gravitational Force for Infinitesimally Thin Gaseous Disks Using Adaptive Mesh Refinement

Yen et al. (2012) advanced a direct approach for the calculation of self-gravitational force to second order accuracy based on uniform grid discretization. This method improves the accuracy of N-body calculation by using exact integration of kernel functions and employing the Fast Fourier Transform (FFT) to reduce complexity of computation to nearly linear. This direct approach is free of artificial boundary conditions, however, the applicability is limited by the uniform discretization of grids. We report here an advancement in the direct method with the implementation of adaptive mesh refinement (AMR) and maintaining second-order accuracy, which breaks the barrier set by uniform grid discretization. The adoption of graphic process units (GPUs) can significantly speed up the computation and make application of this method possible to astrophysical systems of gaseous disk galaxies and protoplanetary disks.

physics.comp-ph↗

arXiv2019

Planet formation: The case for large efforts on the computational side

Modern astronomy has finally been able to observe protoplanetary disks in reasonable resolution and detail, unveiling the processes happening during planet formation. These observed processes are understood under the framework of disk-planet interaction, a process studied analytically and modeled numerically for over 40 years. Long a theoreticians' game, the wealth of observational data has been allowing for increasingly stringent tests of the theoretical models. Modeling efforts are crucial to support the interpretation of direct imaging analyses, not just for potential detections but also to put meaningful upper limits on mass accretion rates and other physical quantities in current and future large-scale surveys. This white paper addresses the questions of what efforts on the computational side are required in the next decade to advance our theoretical understanding, explain the observational data, and guide new observations. We identified the nature of accretion, ab initio planet formation, early evolution, and circumplanetary disks as major fields of interest in computational planet formation. We recommend that modelers relax the approximations of alpha-viscosity and isothermal equations of state, on the grounds that these models use flawed assumptions, even if they give good visual qualitative agreement with observations. We similarly recommend that population synthesis move away from 1D hydrodynamics. The computational resources to reach these goals should be developed during the next decade, through improvements in algorithms and the hardware for hybrid CPU/GPU clusters. Coupled with high angular resolution and great line sensitivity in ground based interferometers, ELTs and JWST, these advances in computational efforts should allow for large strides in the field in the next decade.

astro-ph.EP↗