Tree-based solvers for adaptive mesh refinement code FLASH -- III: a novel scheme for radiation pressure on dust and gas and radiative transfer from diffuse sources

Radiation is an important contributor to the energetics of the interstellar medium, yet its transport is difficult to solve numerically. We present a novel approach towards solving radiative transfer of diffuse sources via backwards ray tracing. Here we focus on the radiative transfer of infrared radiation and the radiation pressure on dust. The new module, \textsc{TreeRay/RadPressure}, is an extension to the novel radiative transfer method \textsc{TreeRay} implemented in the grid-based MHD code {\sc Flash}. In \textsc{TreeRay/RadPressure}, every cell and every star particle is a source of infrared radiation. We also describe how gas, dust and radiation are coupled via a chemical network. This allows us to compute the local dust temperature in thermal equilibrium, leading to a significantly improvement over the classical grey approximation. In several tests, we demonstrate that the scheme produces the correct radiative intensities as well as the correct momentum input by radiation pressure. Subsequently, we apply our new scheme to model massive star formation from a collapsing, turbulent core of 150 ${\rm M}_\odot$. We trace the effects of both, ionizing and infrared radiation on the dynamics of the core. We find that the newborn massive star(s) prevent fragmentation in their proximity through radiative heating. Over time, dust and radiation temperature equalize, while the gas temperature can be either warmer due to shock heating or colder due to insufficient dust-gas coupling. Compared to gravity, the effects of radiation pressure become significant on the core scale only at an evolved stage.Comment: 25 pages, 19 figures, submitted to MNRA

Tree-based solvers for adaptive mesh refinement code FLASH - I: gravity and optical depths

We describe an OctTree algorithm for the MPI parallel, adaptive mesh refinement code FLASH, which can be used to calculate the gas self-gravity, and also the angle-averaged local optical depth, for treating ambient diffuse radiation. The algorithm communicates to the different processors only those parts of the tree that are needed to perform the tree-walk locally. The advantage of this approach is a relatively low memory requirement, important in particular for the optical depth calculation, which needs to process information from many different directions. This feature also enables a general tree-based radiation transport algorithm that will be described in a subsequent paper, and delivers excellent scaling up to at least 1500 cores. Boundary conditions for gravity can be either isolated or periodic, and they can be specified in each direction independently, using a newly developed generalization of the Ewald method. The gravity calculation can be accelerated with the adaptive block update technique by partially re-using the solution from the previous time-step. Comparison with the FLASH internal multigrid gravity solver shows that tree-based methods provide a competitive alternative, particularly for problems with isolated or mixed boundary conditions. We evaluate several multipole acceptance criteria (MACs) and identify a relatively simple approximate partial error MAC which provides high accuracy at low computational cost. The optical depth estimates are found to agree very well with those of the RADMC-3D radiation transport code, with the tree-solver being much faster. Our algorithm is available in the standard release of the FLASH code in version 4.0 and later

The relative impact of photoionizing radiation and stellar winds on different environments

Photoionizing radiation and stellar winds from massive stars deposit energy and momentum into the interstellar medium (ISM). They might disperse the local ISM, change its turbulent multi-phase structure, and even regulate star formation. Ionizing radiation dominates the massive stars' energy output, but the relative effect of winds might change with stellar mass and the properties of the ambient ISM. We present simulations of the interaction of stellar winds and ionizing radiation of 12, 23, and 60 M-circle dot stars within a cold neutral (CNM, n(0) = 100 cm(-3)), warm neutral (WNM, n(0) = 1, 10 cm (-3)), or warm ionized (WIM, n(0) = 0.1 cm(-3)) medium. The FLASH simulations adopt the novel tree-based radiation transfer algorithm TREERAY. With the On-the-Spot approximation and a temperature-dependent recombination coefficient, it is coupled to a chemical network with radiative heating and cooling. In the homogeneous CNM, the total momentum injection ranges from 1.6 x 10(4) to 4 x 105 M-circle dot km s(-1) and is always dominated by the expansion of the ionized HII region. In the WIM, stellar winds dominate (2 x 10(2) to 5 x 10(3) M-circle dot km s(-1)), while the input from radiation is small (similar to 10(2) M-circle dot km s(-1)). The WNM (n(0) = 1 cm(-3)) is a transition regime. Energetically, stellar winds couple more efficiently to the ISM (similar to 0.1 percent of wind luminosity) than radiation (< 0.001 percent of ionizing luminosity). For estimating the impact of massive stars, the strongly mass-dependent ratios of wind to ionizing luminosity and the properties of the ambient medium have to be considered

A Novel Mechanism of Osmosensing, a Salt-dependent Protein-Nucleic Acid Interaction in the Cyanobacterium Synechocystis Species PCC 6803

The de novo synthesis of compatible solutes is an essential part of the cellular osmotic stress response. Upon an osmotic challenge, it is regulated by the immediate biochemical activation of preformed enzymes and by activation of gene expression. Whereas the transcriptional response has been investigated intensively, the mechanisms by which enzymes are activated in osmotic stress situations are still elusive. Here, we address this topic for the moderately halotolerant cyanobacterium Synechocystis sp. PCC 6803, which synthesizes glucosylglycerol as a compatible solute. The key enzyme of the glucosylglycerol pathway (GgpS) is inhibited by nucleic acids in a sequence- and length-independent manner. The protein binds DNA, RNA, and heparin via a salt-dependent electrostatic interaction with the negatively charged backbone of the polyanions. Mechanistically, DNA binding to the enzyme causes noncompetitive inhibition of GgpS activity. The interaction of the enzyme and nucleic acids under in vivo conditions is indicated by the co-purification of both after cross-linking in Synechocystis cells. We propose a novel mechanism of activity regulation by the nonspecific salt-dependent binding of an enzyme to nucleic acids