arXiv subjects

Philipp Girichidis

Publications and source records attributed to Philipp Girichidis.

63 records · Page 4Linked to original sources

arXiv2015

Impact of supernova and cosmic-ray driving on the surface brightness of the galactic halo in soft X-rays

The halo of the Milky Way contains a hot plasma with a surface brightness in soft X-rays of the order $10^{-12}$erg cm$^{-2}$ s$^{-1}$ deg$^{-2}$. The origin of this gas is unclear, but so far numerical models of galactic star formation have failed to reproduce such a large surface brightness by several orders of magnitude. In this paper, we analyze simulations of the turbulent, magnetized, multi-phase interstellar medium including thermal feedback by supernova explosions as well as cosmic-ray feedback. We include a time-dependent chemical network, self-shielding by gas and dust, and self-gravity. Pure thermal feedback alone is sufficient to produce the observed surface brightness, although it is very sensitive to the supernova rate. Cosmic rays suppress this sensitivity and reduce the surface brightness because they drive cooler outflows. Self-gravity has by far the largest effect because it accumulates the diffuse gas in the disk in dense clumps and filaments, so that supernovae exploding in voids can eject a large amount of hot gas into the halo. This can boost the surface brightness by several orders of magnitude. Although our simulations do not reach a steady state, all simulations produce surface brightness values of the same order of magnitude as the observations, with the exact value depending sensitively on the simulation parameters. We conclude that star formation feedback alone is sufficient to explain the origin of the hot halo gas, but measurements of the surface brightness alone do not provide useful diagnostics for the study of galactic star formation.

astro-ph.GA↗

arXiv2015

Mach number study of supersonic turbulence: The properties of the density field

We model driven, compressible, isothermal, turbulence with Mach numbers ranging from the subsonic ($\mathcal{M} \approx 0.65$) to the highly supersonic regime ($\mathcal{M}\approx 16 $). The forcing scheme consists both solenoidal (transverse) and compressive (longitudinal) modes in equal parts. We find a relation $σ_{s}^2 = \mathrm{b}\log{(1+\mathrm{b}^2\mathcal{M}^2)}$ between the Mach number and the standard deviation of the logarithmic density with $\mathrm{b} = 0.457 \pm 0.007$. The density spectra follow $\mathcal{D}(k,\,\mathcal{M}) \propto k^{ζ(\mathcal{M})}$ with scaling exponents depending on the Mach number. We find $ζ(\mathcal{M}) = α\mathcal{M}^β$ with a coefficient $α$ that varies slightly with resolution, whereas $β$ changes systematically. We extrapolate to the limit of infinite resolution and find $α= -1.91 \pm 0.01,\, β=-0.30\pm 0.03$. The dependence of the scaling exponent on the Mach number implies a fractal dimension $D=2+0.96 \mathcal{M}^{-0.30}$. We determine how the scaling parameters depend on the wavenumber and find that the density spectra are slightly curved. This curvature gets more pronounced with increasing Mach number. We propose a physically motivated fitting formula $\mathcal{D}(k) = \mathcal{D}_0 k^{ζk^η}$ by using simple scaling arguments. The fit reproduces the spectral behaviour down to scales $k\approx 80$. The density spectrum follows a single power-law $η= -0.005 \pm 0.01$ in the low Mach number regime and the strongest curvature $η= -0.04 \pm 0.02$ for the highest Mach number. These values of $η$ represent a lower limit, as the curvature increases with resolution.

astro-ph.SR↗

arXiv2014

Hierarchical Bayesian analysis of the velocity power spectrum in supersonic turbulence

Turbulence is a dominant feature operating in gaseous flows across nearly all scales in astrophysical environments. Accordingly, accurately estimating the statistical properties of such flows is necessary for developing a comprehensive understanding of turbulence. We develop and employ a hierarchical Bayesian fitting method to estimate the parameters describing the scaling relationships of the velocity power spectra of supersonic turbulence. We demonstrate the accuracy and other advantages of this technique compared with ordinary linear regression methods. Using synthetic power spectra, we show that the Bayesian method provides accurate parameter and error estimates. Commonly used normal linear regression methods can provide estimates that fail to recover the underlying slopes, up to 70% of the instances, even when considering the 2std.dev. uncertainties. Additionally, we apply the Bayesian methods to analyse the statistical properties of compressible turbulence in 3D numerical simulations. We model driven, isothermal, turbulence with rms Mach numbers in the highly supersonic regime M~15. We study the influence of purely solenoidal (divergence-free) and purely compressive (curl-free) forcing on the scaling exponent of the power spectrum. In simulations with solenoidal forcing and 1024^3 resolution, our results indicate that there is no extended inertial range with a constant scaling exponent. The bottleneck effect results in a curved power spectrum at all wave numbers and is more pronounced in the transversal modes compared with the longitudinal modes. Therefore, this effect is stronger in stationary turbulent flows driven by solenoidal forcing compared to the compressive one. The longitudinal spectrum driven with compressive forcing is the only spectrum with constant scaling exponent z=-1.94 +- 0.01, corresponding to slightly shallower slopes than the Burger's prediction.

astro-ph.SR↗

arXiv2014

Anisotropic transport and early dynamical impact of Cosmic Rays around Supernova remnants

We present a novel implementation of cosmic rays (CR) in the magneto-hydrodynamic code FLASH. CRs are described as separate fluids with different energies. CR advection, energy dependent anisotropic diffusion with respect to the magnetic field and adiabatic losses to follow the evolution of spectra are taken into account. We present a first study of the transport and immediate (~150 kyr) dynamical impact of CRs on the turbulent magnetised interstellar medium around supernova remnants on scales up to 80 pc. CR diffusion quickly leads to an efficient acceleration of low-density gas (mainly perpendicular to the magnetic field) with accelerations up to two orders of magnitude above the thermal values. Peaked (at 1 GeV) CR injection spectra have a stronger impact on the dynamics than power-law spectra. For realistic magnetic field configurations low energy CRs (with smaller diffusion coefficients) distribute anisotropically with large spatial variations of a factor of ten and more. Adiabatic losses can change the local spectra perceptibly but do not have an integral effect on the dynamics at the spatial and temporal scales considered here. We discuss the potential global impact of CRs and anisotropic transport on the dynamical structure of the ISM and also detail the limitations of the model.

astro-ph.HE↗

arXiv2014

On the evolution of the density pdf in strongly self-gravitating systems

The time evolution of the probability density function (PDF) of the mass density is formulated and solved for systems in free-fall using a simple appoximate function for the collapse of a sphere. We demonstrate that a pressure-free collapse results in a power-law tail on the high-density side of the PDF. The slope quickly asymptotes to the functional form $\mathrm{P}_v(ρ)\proptoρ^{-1.54}$ for the (volume-weighted) PDF and $\mathrm{P}_m(ρ)\proptoρ^{-0.54}$ for the corresponding mass-weighted distribution. From the simple approximation of the PDF we derive analytic descriptions for mass accretion, finding that dynamically quiet systems with narrow density PDFs lead to retarded star formation and low star formation rates. Conversely, strong turbulent motions that broaden the PDF accelerate the collapse causing a bursting mode of star formation. Finally, we compare our theoretical work with observations. The measured star formation rates are consistent with our model during the early phases of the collapse. Comparison of observed column density PDFs with those derived from our model suggests that observed star-forming cores are roughly in free-fall.

astro-ph.GA↗

arXiv2012

A new density variance - Mach number relation for subsonic and supersonic, isothermal turbulence

The probability density function (PDF) of the gas density in subsonic and supersonic, isothermal, driven turbulence is analyzed with a systematic set of hydrodynamical grid simulations with resolutions up to 1024^3 cells. We performed a series of numerical experiments with root mean square (r.m.s.) Mach number M ranging from the nearly incompressible, subsonic (M=0.1) to the highly compressible, supersonic (M=15) regime. We study the influence of two extreme cases for the driving mechanism by applying a purely solenoidal (divergence-free) and a purely compressive (curl-free) forcing field to drive the turbulence. We find that our measurements fit the linear relation between the r.m.s. Mach number and the standard deviation of the density distribution in a wide range of Mach numbers, where the proportionality constant depends on the type of the forcing. In addition, we propose a new linear relation between the standard deviation of the density distribution and the standard deviation of the velocity in compressible modes, i.e. the compressible component of the r.m.s. Mach number. In this relation the influence of the forcing is significantly reduced, suggesting a linear relation between the standard deviation of the density distribution and the standard deviation of the velocity in compressible modes, independent of the forcing, ranging from the subsonic to the supersonic regime.

astro-ph.SR↗

arXiv2011

Importance of the Initial Conditions for Star Formation - III: Statistical Properties of Embedded Protostellar Clusters

We investigate the formation of protostellar clusters during the collapse of dense molecular cloud cores with a focus on the evolution of potential and kinetic energy, the degree of substructure, and the early phase of mass segregation. Our study is based on a series of hydrodynamic simulations of dense cores, where we vary the initial density profile and the initial turbulent velocity. In the three-dimensional adaptive mesh refinement simulations, we follow the dynamical formation of filaments and protostars until a star formation efficiency of 20%. Despite the different initial configurations, the global ensemble of all protostars in a setup shows a similar energy evolution and forms sub-virial clusters with an energy ratio $E_\mathrm{kin}/|E_\mathrm{pot}|\sim0.2$. Concentrating on the innermost central region, the clusters show a roughly virialised energy balance. However, the region of virial balance only covers the innermost $\sim10-30%$ of all the protostars. In all simulations with multiple protostars, the total kinetic energy of the protostars is higher than the kinetic energy of the gas cloud, although the protostars only contain 20% of the total mass. The clusters vary significantly in size, mass, and number of protostars, and show different degrees of substructure and mass segregation. Flat density profiles and compressive turbulent modes produce more subclusters then centrally concentrated profiles and solenoidal turbulence. We find that dynamical relaxation and hence dynamical mass segregation is very efficient in all cases from the very beginning of the nascent cluster, i.e., during a phase when protostars are constantly forming and accreting.

astro-ph.SR↗

arXiv2011

Importance of the Initial Conditions for Star Formation - II. Fragmentation Induced Starvation and Accretion Shielding

We investigate the impact of different initial conditions for the initial density profile and the initial turbulence on the formation process of protostellar clusters. We study the collapse of dense molecula r cloud cores with three-dimensional adaptive mesh refinement simulations. We focus our discussion on the distribution of the gas among the protostellar objects in the turbulent dynamical cores. Despite the large variations in the initial configurations and the resulting gas and cluster morphology we find that all stellar clusters follow a very similar gas accretion behaviour. Once secondary protostars begin to form, the central region of a cluster is efficiently shielded from further accretion. Hence, objects located close to the centre are starved of material, as indicated by a strong decrease of the central acc retion rate. This Fragmentation Induced Starvation occurs not only in rotationally supported discs and filaments, but also in more spherically symmetric clusters with complex chaotic motions.

astro-ph.SR↗

arXiv2011

Importance of the Initial Conditions for Star Formation - I. Cloud Evolution and Morphology

We present a detailed parameter study of collapsing turbulent cloud cores, varying the initial density profile and the initial turbulent velocity field. We systematically investigate the influence of different initial conditions on the star formation process, mainly focusing on the fragmentation, the number of formed stars, and the resulting mass distributions. Our study compares four different density profiles (uniform, Bonnor-Ebert type, $ρ\propto r^{-1.5}$, and $ρ\propto r^{-2}$), combined with six different supersonic turbulent velocity fields (compressive, mixed, and solenoidal, initialised with two different random seeds each) in three-dimensional simulations using the adaptive-mesh refinement, hydrodynamics code FLASH. The simulations show that density profiles with flat cores produce hundreds of low-mass stars, either distributed throughout the entire cloud or found in subclusters, depending on the initial turbulence. Concentrated density profiles always lead to the formation of one high-mass star in the centre of the cloud and, if at all, low-mass stars surrounding the central one. In uniform and Bonnor-Ebert type density distributions, compressive initial turbulence leads to local collapse about 25% earlier than solenoidal turbulence. However, central collapse in the steep power-law profiles is too fast for the turbulence to have any significant influence. We conclude that (I) the initial density profile and turbulence mainly determine the cloud evolution and the formation of clusters, (II) the initial mass function (IMF) is not universal for all setups, and (III) that massive stars are much less likely to form in flat density distributions. The IMFs obtained in the uniform and Bonnor-Ebert type density profiles are more consistent with the observed IMF, but shifted to lower masses.

astro-ph.SR↗