Wolverine code

A new MHD code using moving non-structured cells.

Traditional codes in astrophysics are based in grids (e.g., FLASH; Fryxell et al.,2000) or particles (e.g., Gadget; Springel et al. 2001) , however, it is well known that these techniques present some disadvantages (see, e.g., Goldbaum, 2017, for a recent discussion). The AREPO code (Springel, 2010) uses a novel approach based ona moving unstructured mesh (Voronoi tessellation) of a set of discrete points, taking thus advantage of the strengths in grid/particles codes and eliminating their weaknesses. AREPO uses a MPI parallelization. However, this code is private. Although AREPO is the state of the art in their hydrodynamical treatment, its parallelization does not take advantage of GPU acceleration, which can considerably speed up the calculation of any code (e.g., Lukat and Banerjee, 2016). Thus, we propose to build a new moving mesh code designed to run in GPUs. We chose this approach to take full advantage of the architecture of modern hybrid supercomputers. The main characteristics of the proposed code are:

• • The code has been writen in C++.
• • We use the Voro++ library to build and manage the Voronoi tessellation. This library is written in C++, and Voronoi cell properties, such as volume, are called as classes.
• • To solve the hydrodynamics (fluxes between the cells) we will use the finite-volume approach.
• • It will be an openly developed community code.

So far, we have implemented a serial Python/C++ routine that build arbitrary initial conditions with particles, building at the same time the unstructured cells(Voronoi tessellation), and a first serial version of the hidrodynamics module. Although a lot of work still ahead, the hardest step is done.

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Simulating spiral arms

Studying the origin of the most massive Molecular Clouds in the Galaxy

Usually, giant molecular clouds (GMCs) are modeled as spheres. However, observations show that GMCs present a wide variety of morphologies (e.g., Lin et al., 2016). This project is aimed at studying the gravitational effect of spiral arms in the formation and further evolution of GMCs.

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Protoplanetary disks

Study of the conditions for planet formation

Recent ALMA observations have revealed that protoplanetary disks (PPDs) present common features: rings and gaps (e.g., ALMA Partnership, 2015). Numerical simulations have shown that these features can be produced by planets (e.g., Zhu et al., 2015). However, the question about the formation of planets remains. This study is aimed at studying the effects of inhomogeneities in the dust phase.

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Manuel Zamora-Aviles

Full CV and references under request.

Education

• Ph.D. - Astrophysics, 2015. Thesis: Reformulation of the concept of Star Formation Efficiency in evolving Molecular Clouds. Advisor: Dr. Enrique Vázquez-Semadeni. Centro de Radioastronomía y Astrofísica (UNAM), Morelia, Michoacán.

Present position

• Cátedra CONACyT at “Instituto Nacional de Astrofisica, Optica y Electronica” (INAOE); Puebla, Mexico.

Previous research positions

• Postdoctoral Researcher, UNAM (2018). Key colaborators: Enrique Vazquez-Semadeni and Javier Ballesteros-Paredes.
• Postdoctoral Researcher, University of Michigan (2016-2017). Key colaborator: Lee Hartmann.

Refereed Publications

1. Simultaneous Evolution of the Virial Parameter and Star Formation Rate in Molecular Clumps Undergoing Global Hierarchical Collapse; Camacho, Vianey; Vázquez-Semadeni, Enrique; Palau, Aina; Busquet, Gemma; Manuel Zamora-Avilés, 2020, ApJ, 903, 46 (arXiv)
2. Global Hierarchical Collapse In Molecular Clouds. Towards a Comprehensive Scenario; Vázquez-Semadeni, Enrique; Palau, Aina; Ballesteros-Paredes, Javier; Gómez, G.; Zamora-Avilés, M; 2019, MNRAS, 490, 3061 (arXiv)
3. What is the physics behind the Larson mass-size relation?; Ballesteros-Paredes, J.; Román-Zúñiga, C.; Salomé, Q.; Zamora-Avilés, M.; Jiménez-Donaire, M. J. 2019, MNRAS, 490, 2648 (arXiv)
4. Flipping-up the field: gravitational feedback as a mechanism for young clusters dispersal; Manuel Zamora-Avilés, Javier Ballesteros-Paredes, Jesús Hernández, Carlos Román-Zúñiga, Verónica Lora, and Marina Kounkel, 2019, MNRAS, 488, 3406 (arXiv)
5. Structure and Expansion Law of H II Regions in structured Molecular Clouds; Zamora-Avilés, Manuel; Vázquez-Semadeni, E., González, R. F., Franco, J.; Shore, Steven N.; Hartmann, L. W.; Ballesteros-Paredes, J.; Banerjee, R.; Körtgen, B., 2019, MNRAS, 487, 2200 (arXiv)
6. Magnetic suppression of turbulence and the star formation activity of molecular clouds; Zamora-Avilés, Manuel; Enrique Vázquez-Semadeni; Bastian Kortgen, Robi Banerjee, and Lee Hartmann; 2018, MNRAS 474, pp. 4824–4836.
7. On estimating angular momenta of infalling protostellar cores from observations; Zhang, Shangjia; Hartmann, Lee; Zamora-Avilés, Manuel; Kuznetsova, Aleksandra, 2018, MNRAS, 480, 5495
8. The magnetic field structure in molecular cloud filaments; Gómez, Gilberto C.; Vázquez-Semadeni, Enrique; Zamora-Avilés, Manuel, 2018, MNRAS, 480, 2939
9. Molecular cloud evolution - VI. Measuring cloud ages; Vázquez-Semadeni, Enrique; Zamora-Avilés, Manuel; Galván-Madrid, Roberto; Forbrich, Jan, 2018, MNRAS, 479, 3254
10. Magnetic suppression of turbulence and the star formation activity of molecular clouds; Zamora-Avilés, Manuel; Vázquez-Semadeni, Enrique; Körtgen, Bastian; Banerjee, Robi; Hartmann, Lee, 2018, MNRAS, 474, 4824
11. Are fibres in molecular cloud filaments real objects?; Zamora-Avilés, Manuel; Ballesteros-Paredes, Javier; Hartmann, Lee W., 2017, MNRAS, 472, 647
12. Magnetized Converging Flows toward the Hot Core in the Intermediate/High-mass Star-forming Region NGC 6334 V; Juárez, Carmen; Girart, Josep M.; Zamora-Avilés, Manuel, etal., 2017, ApJ, 844, 44
13. Supernova Feedback in Molecular Clouds: Global Evolution and Dynamics; Bastian Kortgen, Daniel Seifried, Robi Banerjee, Enrique Vázquez–Semadeni, and Manuel Zamora–Avilés, 2015, MNRAS, 459, 3460.
14. An Evolutionary Model for Collapsing Molecular Clouds and Their Star Formation Activity. II. Mass Dependence of the Star Formation Rate; Zamora-Aviles, Manuel; Vázquez-Semadeni, Enrique; 2014, ApJ, 793, 84
15. An evolutionary model for collapsing Molecular Clouds and their Star Formation Activity; Zamora-Aviles, Manuel; Vázquez-Semadeni, Enrique; Colín, Pedro; 2012, ApJ, 751, 77
16. Gravity or turbulence? Velocity dispersion-size relation; Ballesteros-Paredes, Javier; Hartmann, Lee W.; Vázquez-Semadeni, Enrique; Heitsch, Fabian; Zamora-Aviles, Manuel A.; 2011, MNRAS, 411, 65

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Evolution of Molecular Clouds

Simultaneous Evolution of the Virial Parameter and Star Formation Rate in Molecular Clumps Undergoing Global Hierarchical Collapse

We compare dense clumps and cores in a numerical simulation of molecular clouds (MCs) undergoing global hierarchical collapse (GHC) to observations in two MCs at different evolutionary stages, the Pipe and the G14.225 clouds, to test the ability of the GHC scenario to follow the early evolution of the energy budget and star formation activity of these structures. In the simulation, we select a region that contains cores of sizes and densities similar to the Pipe cores and find that it evolves through accretion, developing substructure similar to that of the G14.225 cloud after ∼1.6 Myr. Within this region, we follow the evolution of the Larson ratio L≡σ/R^1/2, where σ is the velocity dispersion and R is the size; the virial parameter α; and the star formation activity of the cores/clumps. In the simulation, we find that as the region evolves, (i) its clumps have L and α values first consistent with those of the Pipe substructures and later with those of G14.225; (ii) the individual cores first exhibit a decrease in α followed by an increase when star formation begins; (iii) collectively, the ensemble of cores/clumps reproduces the observed trend of lower α for higher-mass objects; and (iv) the star formation rate and star formation efficiency increase monotonically. We suggest that this evolution is due to the simultaneous loss of externally driven compressive kinetic energy and increase of the self-gravity-driven motions. We conclude that the GHC scenario provides a realistic description of the evolution of the energy budget of the cloud's substructure at early times, which occurs simultaneously with an evolution of the star formation activity.

• ADS link.
• Bibcode: 2020ApJ...903...46C
• Publication date: November 2020

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A new Star Formation scenario

Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario.

We present a unified description of the scenario of global hierarchical collapse (GHC). GHC constitutes a flow regime of (non-homologous) collapses within collapses, in which all scales accrete from their parent structures, and small, dense regions begin to contract at later times, but on shorter time-scales than large, diffuse ones. The different time-scales allow for most of the clouds' mass to be dispersed by the feedback from the first massive stars, maintaining the cloud-scale star formation rate low. Molecular clouds (MCs), clumps, and cores are not in equilibrium, but rather are either undergoing contraction or dispersal. The main features of GHC are as follows: (1) The gravitational contraction is initially very slow, and begins when the cloud still consists of mostly atomic gas. (2) Star-forming MCs are in an essentially pressureless regime, causing filamentary accretion flows from the cloud to the core scale to arise spontaneously. (3) Accreting objects have longer lifetimes than their own free-fall time, due to the continuous replenishment of material. (4) The clouds' total mass and its molecular and dense mass fractions increase over time. (5) The clouds' masses stop growing when feedback becomes important. (6) The first stars appear several megayears after global contraction began, and are of low mass; massive stars appear a few megayears later, in massive hubs. (7) The minimum fragment mass may well extend into the brown-dwarf regime. (8) Bondi-Hoyle-Lyttleton-like accretion occurs at both the protostellar and the core scales, accounting for an IMF with slope dN/dM ∝ M^-2. (9) The extreme anisotropy of the filamentary network explains the difficulty in detecting large-scale infall signatures. (10) The balance between inertial and gravitationally driven motions in clumps evolves during the contraction, explaining the approach to apparent virial equilibrium, from supervirial states in low-column density clumps and from subvirial states in dense cores. (11) Prestellar cores adopt Bonnor-Ebert-like profiles, but are contracting ever since when they may appear to be unbound. (12) Stellar clusters develop radial age and mass segregation gradients. We also discuss the incompatibility between supersonic turbulence and the observed scalings in the molecular hierarchy. Since gravitationally formed filaments do not develop shocks at their axes, we suggest that a diagnostic for the GHC scenario should be the absence of strong shocks in them. Finally, we critically discuss some recent objections to the GHC mechanism.

• ADS link.
• Bibcode: 2019MNRAS.490.3061V
• Publication date: December 2019

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Radiation-MHD

Structure and Expansion Law of H II Regions in structured Molecular Clouds

We present radiation-magnetohydrodynamic simulations aimed at studying evolutionary properties of H II regions in turbulent, magnetized, and collapsing molecular clouds formed by converging flows in the warm neutral medium. We focus on the structure, dynamics, and expansion laws of these regions. Once a massive star forms in our highly structured clouds, its ionizing radiation eventually stops the accretion (through filaments) towards the massive star-forming regions. The new overpressured H II regions push away the dense gas, thus disrupting the more massive collapse centres. Also, because of the complex density structure in the cloud, the H II regions expand in a hybrid manner: they virtually do not expand towards the densest regions (cores), while they expand according to the classical analytical result towards the rest of the cloud, and in an accelerated way, as a blister region, towards the diffuse medium. Thus, the ionized regions grow anisotropically, and the ionizing stars generally appear off-centre of the regions. Finally, we find that the hypotheses assumed in standard H II-region expansion models (fully embedded region, blister-type, or expansion in a density gradient) apply simultaneously in different parts of our simulated H II regions, producing a net expansion law ( R ~ t^a, with a in the range of 0.93-1.47 and a mean value of 1.2 +/- 0.17) that differs from any of those of the standard models.

• ADS link.
• Bibcode: 2019MNRAS.487.2200Z
• Publication date: August 2019

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Clusters dynamics

Flipping-up the field: gravitational feedback as a mechanism for young clusters dispersal

Flipping-up the field: gravitational feedback as a mechanism for young clusters dispersal Recent analyses of Gaia data have provided direct evidence that most young stellar clusters (YSCs) are in a state of expansion, with velocities of the order of ~0.5 km/s. Traditionally, expanding YSC have been pictured as entities that became unbound due to the lack of gravitational binding once the gas from the parental cloud that formed the YSC has been expelled by the stellar radiation of the massive stars in the cluster. In the present contribution we used radiation-magnetohydrodynamic numerical simulations of molecular cloud formation and evolution to understand how stellar clusters form and disperse. We found that the ionising feedback from the newborn massive stars expels the gas from the collapse centre, flipping-up the gravitational potential as a consequence of the mass removal from the inside-out. Since neither the parental clouds, nor the formed shells are distributed symmetrically around the HII region, net forces pulling out the stars are present, accelerating them towards the edges of the cavity. We call this mechanism ``gravitational feedback", in which the gravity from the expelled gas appears to be the crucial mechanism producing unbound clusters that expand away from their formation centre in an accelerated way in YSC. This mechanism naturally explains the "Hubble flow-like" expansion observed in several YSC.

• ADS link.
• Bibcode: 2019MNRAS.488.3406Z
• Publication date: September 2019

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