Stellar hydrodynamical modeling of dwarf galaxies: simulation methodology, tests, and first results

Cosmological simulations still lack numerical resolution or physical processes to simulate dwarf galaxies in sufficient details. Accurate numerical simulations of individual dwarf galaxies are thus still in demand. We aim at (i) studying in detail the coupling between stars and gas in a galaxy, exploiting the so-called stellar hydrodynamical approach, and (ii) studying the chemo-dynamical evolution of individual galaxies starting from self-consistently calculated initial gas distributions. We present a novel chemo-dynamical code in which the dynamics of gas is computed using the usual hydrodynamics equations, while the dynamics of stars is described by the stellar hydrodynamics approach, which solves for the first three moments of the collisionless Boltzmann equation. The feedback from stellar winds and dying stars is followed in detail. In particular, a novel and detailed approach has been developed to trace the aging of various stellar populations, which enables an accurate calculation of the stellar feedback depending on the stellar age. We build initial equilibrium models of dwarf galaxies that take gas self-gravity into account and present different levels of rotational support. Models with high rotational support develop prominent bipolar outflows; a newly-born stellar population in these models is preferentially concentrated to the galactic midplane. Models with little rotational support blow away a large fraction of the gas and the resulting stellar distribution is extended and diffuse. The stellar dynamics turns out to be a crucial aspect of galaxy evolution. If we artificially suppress stellar dynamics, supernova explosions occur in a medium heated and diluted by the previous activity of stellar winds, thus artificially enhancing the stellar feedback (abridged).

Variable protostellar accretion with episodic bursts

We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur in the partly embedded Class I phase, with a smaller fraction taking place in the deeply embedded Class 0 phase and a few possible bursts in the optically visible Class II phase. The average burst duration and mean luminosity are found to be in good agreement with those inferred from observations of FU-Orionis-type eruptions. The model predicts the existence of two types of bursts: the isolated ones, showing well-defined luminosity peaks separated with prolonged periods (~ 10^4 yr) of quiescent accretion, and clustered ones, demonstrating several bursts occurring one after another during just a few hundred years. Finally, we estimate that 40%--70% of the star-forming cores can display bursts after forming a star-disk system.

The effect of external environment on the evolution of protostellar disks

Using numerical hydrodynamics simulations we studied the gravitational collapse of pre-stellar cores of sub-solar mass embedded into a low-density external environment. Four models with different magnitude and direction of rotation of the external environment with respect to the central core were studied and compared with an isolated model. We found that the infall of matter from the external environment can significantly alter the disk properties as compared to those seen in the isolated model. Depending on the magnitude and direction of rotation of the external environment, a variety of disks can form including compact (<= 200 AU) ones shrinking in size due to infall of external matter with low angular momentum, as well as extended disks forming due to infall of external matter with high angular momentum. The former are usually stable against gravitational fragmentation, while the latter are prone to fragmentation and formation of stellar systems with sub-stellar/very-low-mass companions. In the case of counterrotating external environment, very compact (< 5 AU) and short-lived (<= a few * 10^5 yr) disks can form when infalling material has low angular momentum. The most interesting case is found for the infall of counterrotating external material with high angular momentum, leading to the formation of counterrotating inner and outer disks separated by a deep gap at a few tens AU. The gap migrates inward due to accretion of the inner disk onto the protostar, turns into a central hole, and finally disappears giving way to the outer strongly gravitationally unstable disk. This model may lead to the emergence of a transient stellar system with sub-stellar/very-low-mass components counterrotating with respect to that of the star.

The effect of episodic accretion on the phase transition of CO and CO_2 in low-mass star formation

We study the evaporation and condensation of CO and CO_2 during the embedded stages of low-mass star formation by using numerical simulations. We focus on the effect of luminosity bursts, similar in magnitude to FUors and EXors, on the gas-phase abundance of CO and CO_2 in the protostellar disk and infalling envelope. The evolution of a young protostar and its environment is followed based on hydrodynamical models using the thin-disk approximation, coupled with a stellar evolution code and phase transformations of CO and CO_2. The accretion and associated luminosity bursts in our model are caused by disk gravitational fragmentation followed by quick migration of the fragments onto the forming protostar. We found that bursts with luminosity on the order of 100-200 L_sun can evaporate CO ices in part of the envelope. The typical freeze-out time of the gas-phase CO onto dust grains in the envelope (a few kyr) is much longer than the burst duration (100-200 yr). This results in an increased abundance of the gas-phase CO in the envelope long after the system has returned into a quiescent stage. In contrast, luminosity bursts can evaporate CO_2 ices only in the disk, where the freeze-out time of the gas-phase CO_2 is comparable to the burst duration. We thus confirm that luminosity bursts can leave long-lasting traces in the abundance of gas-phase CO in the infalling envelope, enabling the detection of recent bursts as suggested by previous semi-analytical studies.

Formation of giant planets and brown dwarfs on wide orbits

Context: We studied numerically the formation of giant planet (GP) and brown dwarf (BD) embryos in gravitationally unstable protostellar disks and compared our findings with directly-imaged, wide-orbit (>= 50 AU) companions known to-date. The viability of the disk fragmentation scenario for the formation of wide-orbit companions in protostellar disks around (sub-)solar mass stars was investigated. Methods: We used numerical hydrodynamics simulations of disk formation and evolution with an accurate treatment of disk thermodynamics. The use of the thin-disk limit allowed us to probe the long-term evolution of protostellar disks. We focused on models that produced wide-orbit GP/BD embryos, which opened a gap in the disk and showed radial migration timescales similar to or longer than the typical disk lifetime. Results: While disk fragmentation was seen in the majority of our models, only 6 models out of 60 revealed the formation of quasi-stable, wide-orbit GP/BD embryos. Disk fragmentation produced GP/BD embryos with masses in the 3.5-43 M_J range, covering the whole mass spectrum of directly-imaged, wide-orbit companions to (sub-)solar mass stars. On the other hand, our modelling failed to produce embryos on orbital distances <= 170 AU, whereas several directly-imaged companions were found at smaller orbits down to a few AU. Disk fragmentation also failed to produce wide-orbit companions around stars with mass <= 0.7 Msun, in disagreement with observations. Conclusions: Disk fragmentation is unlikely to explain the whole observed spectrum of wide-orbit companions to (sub-)solar-mass stars and other formation mechanisms, e.g., dynamical scattering of closely-packed companions onto wide orbits, should be invoked to account for companions at orbital distance from a few tens to approx 150 AU and wide-orbit companions with masses of the host star <= 0.7 Msun. (abridged)

Self-gravitating equilibrium models of dwarf galaxies and the minimum mass for star formation

We construct a series of model galaxies in rotational equilibrium consisting of gas, stars, and a fixed dark matter (DM) halo and study how these equilibrium systems depend on the mass and form of the DM halo, gas temperature, non-thermal and rotation support against gravity, and also on the redshift of galaxy formation. For every model galaxy we find the minimum gas mass M_g^min required to achieve a state in which star formation (SF) is allowed according to contemporary SF criteria. The obtained M_g^min--M_DM relations are compared against the baryon-to-DM mass relation M_b--M_DM inferred from the LambdaCDM theory and WMAP4 data. Our aim is to construct realistic initial models of dwarf galaxies (DGs), which take into account the gas self-gravity and can be used as a basis to study the dynamical and chemical evolution of DGs. Rotating equilibria are found by solving numerically the steady-state momentum equation for the gas component in the combined gravitational potential of gas, stars, and DM halo using a forward substitution procedure. We find that for a given M_DM the value of M_g^min depends crucially on the gas temperature T_g, gas spin parameter alpha, degree of non-thermal support sigma_eff, and somewhat on the redshift for galaxy formation z_gf. Depending on the actual values of T_g, alpha, sigma_eff, and z_gf, model galaxies may have M_g^min that are either greater or smaller than M_b. Galaxies with M_DM ga 10^9 M_sun are usually characterized by M_g^min la M_b, implying that SF in such objects is a natural outcome as the required gas mass is consistent with what is available according to the LambdaCDM theory. On the other hand, models with M_DM la 10^9 M_sun are often characterized by M_g^min >> M_b, implying that they need much more gas than available to achieve a state in which SF is allowed. Abridged.

Destruction of massive fragments in protostellar disks and crystalline silicate production

We present a mechanism for the crystalline silicate production associated with the formation and subsequent destruction of massive fragments in young protostellar disks. The fragments form in the embedded phase of star formation via disk fragmentation at radial distances ga 50-100 AU and anneal small amorphous grains in their interior when the gas temperature exceeds the crystallization threshold of ~ 800 K. We demonstrate that fragments that form in the early embedded phase can be destroyed before they either form solid cores or vaporize dust grains, thus releasing the processed crystalline dust into various radial distances from sub-AU to hundred-AU scales. Two possible mechanisms for the destruction of fragments are the tidal disruption and photoevaporation as fragments migrate radially inward and approach the central star and also dispersal by tidal torques exerted by spiral arms. As a result, most of the crystalline dust concentrates to the disk inner regions and spiral arms, which are the likely sites of fragment destruction.

Embedded protostellar disks around (sub-)solar stars. II. Disk masses, sizes, densities, temperatures and the planet formation perspective

We present basic properties of protostellar disks in the embedded phase of star formation (EPSF), which is difficult to probe observationally using available observational facilities. We use numerical hydrodynamics simulations of cloud core collapse and focus on disks formed around stars in the 0.03-1.0 Msun mass range. Our obtained disk masses scale near-linearly with the stellar mass. The mean and median disk masses in the Class 0 and I phases (M_{d,C0}^{mean}=0.12 Msun, M_{d,C0}^{mdn}=0.09 Msun and M_{d,CI}^{mean}=0.18 Msun, M_{d,CI}^{mdn}=0.15 Msun, respectively) are greater than those inferred from observations by (at least) a factor of 2--3. We demonstrate that this disagreement may (in part) be caused by the optically thick inner regions of protostellar disks, which do not contribute to millimeter dust flux. We find that disk masses and surface densities start to systematically exceed that of the minimum mass solar nebular for objects with stellar mass as low as M_st=0.05-0.1 Msun. Concurrently, disk radii start to grow beyond 100 AU, making gravitational fragmentation in the disk outer regions possible. Large disk masses, surface densities, and sizes suggest that giant planets may start forming as early as in the EPSF, either by means of core accretion (inner disk regions) or direct gravitational instability (outer disk regions), thus breaking a longstanding stereotype that the planet formation process begins in the Class II phase.

Embedded protostellar disks around (sub-)solar protostars. I. Disk structure and evolution

We perform a comparative numerical hydrodynamics study of embedded protostellar disks formed as a result of the gravitational collapse of cloud cores of distinct mass (M_cl=0.2--1.7 M_sun) and ratio of rotational to gravitational energy (beta=0.0028--0.023). An increase in M_cl and/or beta leads to the formation of protostellar disks that are more susceptible to gravitational instability. Disk fragmentation occurs in most models but its effect is often limited to the very early stage, with the fragments being either dispersed or driven onto the forming star during tens of orbital periods. Only cloud cores with high enough M_cl or beta may eventually form wide-separation binary/multiple systems with low mass ratios and brown dwarf or sub-solar mass companions. It is feasible that such systems may eventually break up, giving birth to rogue brown dwarfs. Protostellar disks of {it equal} age formed from cloud cores of greater mass (but equal beta) are generally denser, hotter, larger, and more massive. On the other hand, protostellar disks formed from cloud cores of higher beta (but equal M_cl) are generally thinner and colder but larger and more massive. In all models, the difference between the irradiation temperature and midplane temperature triangle T is small, except for the innermost regions of young disks, dense fragments, and disks outer edge where triangle T is negative and may reach a factor of two or even more. Gravitationally unstable, embedded disks show radial pulsations, the amplitude of which increases along the line of increasing M_cl and beta but tends to diminish as the envelope clears. We find that single stars with a disk-to-star mass ratio of order unity can be formed only from high-beta cloud cores, but such massive disks are unstable and quickly fragment into binary/multiple systems.