arXiv subjects

Gerhard Hensler

Publications and source records attributed to Gerhard Hensler.

49 records · Page 3Linked to original sources

arXiv2007

Internal chemo-dynamical Modeling of Gas Exchange within Galaxies and with their Environment

By a few but important examples as models of combined radiative and wind-driven HII regions and galactic winds we demonstrate the importance of refined small-to-medium scale studies of chemo-dynamical effects. These processes determine the internal dynamics and energetic of the ISM and affect its observational signatures, e.g. by abundance contributions, but are not yet reliably and satisfactorily explored.

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arXiv2007

Massive stars: their contribution to energy and element budget in chemo-dynamical galaxy evolution

Here results of numerical radiation hydrodynamical simulations are presented which explore the energetic impact of massive stars on the interstellar medium. We study the evolution of the ambient gas around isolated massive stars in the mass range between 15 and 85 Msun in order to analyze the formation of structures and further the transfer and deposit of the stellar wind and radiation energy into the circumstellar medium until the stars explode as a supernovae. The derived energy transfer efficiencies are much smaller than analytically estimated and should be inserted into chemo-dynamical evolutionary models of galaxies as appropriate parameter values. As an additional issue the element release in the Wolf-Rayet phases and its detectability have been investigated for comparison with observations.

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arXiv2007

The observable Metal-enrichment of Radiation-driven+Wind-blown HII Regions in the Wolf-Rayet Stage

From stellar evolution models and from observations of Wolf-Rayet stars it is known that massive stars are releasing metal-enriched gas in their Wolf-Rayet phase by means of strong stellar winds. Although HII region spectra serve as diagnostics to determine the present-day chemical composition of the interstellar medium, it is not yet reliably explored to what extent the diagnostic HII gas is already contaminated by chemically processed stellar wind matter. In a recent paper, we therefore analyzed our models of radiation-driven and wind-blown HII bubbles around an isolated 85 Msun star with originally solar metallicity with respect to its chemical abundances. Although the hot stellar wind bubble (SWB) is enriched with 14N during the WN phase and even much higher with 12C and 16O during the WC phase of the star, we found that at the end of the stellar lifetime the mass ratios of the traced lements N and O in the warm ionized gas are insignificantly higher than solar, whereas an enrichment of 22% above solar is found for C. The transport of enriched elements from the hot SWB to the cool gas occurs mainly by means of mixing of hot gas with cooler at the backside of the SWB shell.

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arXiv2006

Chemical Self-Enrichment of HII Regions by the Wolf-Rayet Phase of an 85 Msun star

It is clear from stellar evolution and from observations of WR stars that massive stars are releasing metal-enriched gas through their stellar winds in the Wolf-Rayet phase. Although HII region spectra serve as diagnostics to determine the present-day chemical composition of the interstellar medium, it is far from being understood to what extent the HII gas is already contaminated by chemically processed stellar wind. Therefore, we analyzed our models of radiative and wind bubbles of an isolated 85 Msun star with solar metallicity (Kröger et al. 2006) with respect to the chemical enrichment of the circumstellar HII region. Plausibly, the hot stellar wind bubble (SWB) is enriched with 14N during the WN phase and even much higher with 12C and 16O during the WC phase of the star. During the short period that the 85 Msun star spends in the WC stage enriched SWB material mixes with warm HII gas of solar abundances and thus enhances the metallicity in the HII region. However, at the end of the stellar lifetime the mass ratios of the traced elements N and O in the warm ionized gas are insignificantly higher than solar, whereas an enrichment of 22 % above solar is found for C. Important issues from the presented study comprise a steeper radial gradient of C than O and a decreasing effect of self-enrichment for metal-poor galaxies.

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arXiv2005

Massive stars and the energy balance of the interstellar medium. II. The 35 solar mass star and a solution to the "missing wind problem"

We continue our numerical analysis of the morphological and energetic influence of massive stars on their ambient interstellar medium for a 35 solar mass star that evolves from the main sequence through red supergiant and Wolf-Rayet phases, until it ultimately explodes as a supernova. We find that structure formation in the circumstellar gas during the early main-sequence evolution occurs as in the 60 solar mass case but is much less pronounced because of the lower mechanical wind luminosity of the star. Since on the other hand the shell-like structure of the HII region is largely preserved, effects that rely on this symmetry become more important. At the end of the stellar lifetime 1% of the energy released as Lyman continuum radiation and stellar wind has been transferred to the circumstellar gas. From this fraction 10% is kinetic energy of bulk motion, 36% is thermal energy, and the remaining 54% is ionization energy of hydrogen. The sweeping up of the slow red supergiant wind by the fast Wolf-Rayet wind produces remarkable morphological structures and emission signatures, which are compared with existing observations of the Wolf-Rayet bubble S308. Our model reproduces the correct order of magnitude of observed X-ray luminosity, the temperature of the emitting plasma as well as the limb brightening of the intensity profile. This is remarkable, because current analytical and numerical models of Wolf-Rayet bubbles fail to consistently explain these features. A key result is that almost the entire X-ray emission in this stage comes from the shell of red supergiant wind swept up by the shocked Wolf-Rayet wind rather than from the shocked Wolf-Rayet wind itself as hitherto assumed and modeled. This offers a possible solution to what is called the ``missing wind problem'' of Wolf-Rayet bubbles.

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arXiv2005

Modelling Galaxies with a 3d Multi-Phase ISM

We present a new particle code for modelling the evolution of galaxies. The code is based on a multi-phase description for the interstellar medium (ISM). We included star formation (SF), stellar feedback by massive stars and planetary nebulae, phase transitions and interactions between gas clouds and ambient diffuse gas, namely condensation, evaporation, drag and energy dissipation. The latter is realised by radiative cooling and inelastic cloud-cloud collisions. We present new schemes for SF and stellar feedback. They include a consistent calculation of the star formation efficiency (SFE) based on ISM properties as well as a detailed redistribution of the feedback energy into the different ISM phases. As a first test example we show a model of the evolution of a present day Milky-Way-type galaxy. Though the model exhibits a quasi-stationary behaviour in global properties like mass fractions or surface densities, the evolution of the ISM is locally strongly variable depending on the local SF and stellar feedback. We start only with two distinct phases, but a three-phase ISM is formed soon consisting of cold molecular clouds, a warm gas disk and a hot gaseous halo. Hot gas is also found in bubbles in the disk accompanied by type II supernovae explosions. The star formation rate (SFR) is ~1.6 M_sun/year on average decreasing slowly with time due to gas consumption. In order to maintain a constant SFR gas replenishment, e.g. by infall, of the order 1 M_sun/year is required. Our model is in fair agreement with Kennicutt's (1998) SF law including the cut-off at ~10 M_sun/pc^2. Models with a constant SFE, i.e. no feedback on the SF, fail to reproduce Kennicutt's law.

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arXiv2005

The refill of superbubble cavities

In this paper we study the late evolution of model galaxies after a single episode of star formation of different durations. The aim of the paper is to discover the timescale needed to refill with cold gas the center of the galaxy. This timescale strongly depends on the amount of gas initially present inside the galaxy and ranges between 125 and 600 Myr. A HI hole can therefore survive several hundred Myrs after the last SNII has exploded. If, as a consequence of the refill of the center of the galaxy, a second episode of star formation occurs, it pollutes the surrounding medium in a very short timescale (of the order of 10--15 Myr), at variance with what happens if the center of the galaxy is still occupied by hot and tenuous gas.

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arXiv2003

Massive stars and the energy balance of the ISM. I. The impact of an isolated 60 Msun star

We present results of numerical simulations carried out with a 2D radiation hydrodynamics code in order to study the impact of massive stars on their surrounding interstellar medium. This first paper deals with the evolution of the circumstellar gas around an isolated 60 Msun star. The interaction of the photoionized HII region with the stellar wind bubble forms a variety of interesting structures like shells, clouds, fingers, and spokes. These results demonstrate that complex structures found in HII regions are not necessarily relics from the time before the gas became ionized but may result from dynamical processes during the course of the HII region evolution. We have also analyzed the transfer and deposit of the stellar wind and radiation energy into the circumstellar medium until the star explodes as a supernova. Although the total mechanical wind energy supplied by the star is negligible compared to the accumulated energy of the Lyman continuum photons, the kinetic energy imparted to the circumstellar gas over the star's lifetime is 4 times higher than for a comparable windless simulation. Furthermore, the thermal energy of warm photoionized gas is lower by some 55%. Our results document the necessity to consider both ionizing radiation and stellar winds for an appropriate description of the interaction of OB stars with their circumstellar environment.

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arXiv2003

Chemodynamical gas flow cycles and their influence on the chemical evolution of dwarf irregular galaxies

Here we investigate an exemplary chemodynamical evolutionary simulation of a dwarf irregular galaxy. By means of this model we demonstrate the existence of three gas mixing cycles: 1) An inner "local cycle" mixing the metals produced in stars locally, and 2) an outer "galactic cycle" on which hot gas is driven out of the galaxy by multiple supernovae type II and mixes on a short timescale with the available cold gas. 3) Only a small fraction of the metals leaves the galactic gravitational field and follows the "global cycle" with the intergalactic matter. The large-scale mixing results in a temporary depletion of supernova ejected metals. We will discuss this "delayed recycling" and its influence on the chemical evolution, especially on the nitrogen over oxygen ratio which is increased temporarily. These results presented her are also relevant for less sophisticated analytical approaches and chemical evolutionary models of galaxies which have to parameterize the metal loss through outflow.

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arXiv2001

Exchange Processes in a Multi-Phase ISM

We present a new particle based code with a multi-phase description of the ISM implemented in order to follow the chemo-dynamical evolution of galaxies. The multi-phase ISM consists of clouds (sticky particles) and diffuse gas (SPH): Exchange of matter, energy and momentum is achieved by drag (due to ram pressure) and condensation or evaporation. Based on time scales we show that in Milky-Way-like galaxies the drag force is for molecular clouds only important, if their relative velocities exceed 100 km/s. For the mass exchange we find that clouds evaporate only if the temperature of the ambient gas is higher than one million Kelvin. At lower temperatures condensation takes place at time scales of the order of 1-10 Gyr.

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arXiv2000

Dynamical processes, element mixing and chemodynamical cycles in dwarf galaxies

Since the chemical evolution of galaxies seems to differ between morphological types and deviates in many details from the standard scenario the question has to be addressed when, how and to what amount metal-enriched ejecta from Supernovae and Planetary Nebulae polute their environment. Since recent observations of dwarf galaxies show no significant metal abundance gradients throughout the galaxies while enhancement of metals happens in isolated HII regions, an effective mixing process has to be assumed. Chemodynamical evolution models can provide a possible explanation by demonstrating that strong evaporation of gas clouds by hot gas and following condensation leads to an almost perfect mixing of the gas. We focus on the different phases of chemodynamical evolution that are experienced by a representative dwarf irregular galaxy model and present a quantitative analysis of the chemodynamical gas flow cycles.

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arXiv2000

Evaporation and Condensation Processes of Giant Molecular Clouds in a Hot Plasma

2d hydrodynamical simulations are performed to examine the evaporation and condensation processes of giant molecular clouds in the hot phase of the interstellar medium. The evolution of cold and dense clouds is calculated in the subsonic stream of a hot tenuous plasma. Our code includes self-gravity, heating and cooling processes and heat conduction by electrons. Significant differences occur between simulations with and without heat conduction.

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arXiv1999

Chemical Evolution of Dwarf Irregular Galaxies - chemodynamical models and the effect of gas infall

Because of their low gravitational energies dwarf galaxies are greatly exposed to energetical influences by the interstellar medium, like e.g. stellar radiation, winds or explosions, or by their environment. While the metallicity depletion in dwarf galaxies can be explained in general by supernova-driven galactic winds, the reason for their low N/O ratios at low O abundance is not yet completely understood. Stellar yields enrich the different gas phases with elements that are characteristic for their stellar progenitors. Gas-phase transitions are necessary for a mixing of elements, but depend sensitively on the thermal and dynamical state of the interstellar medium. Models of chemical evolution start usually with a high N/O ratio at low O abundance according to a metal enrichment by ancient stellar populations with traditional yields, but can only reproduce the N/O-O peculiarity by the stepwise element release and mostly by the application of multiple starbursts in order to account also for a selective element reduction by galactic winds. Chemodynamical models of dwarf galaxies, however, demonstrate that strong evaporation of clouds by the hot supernova gas leads to an almost perfect mixing of the interstellar gas and a large-scale homogenization of abundance ratios. In addition, even with star-formation self-regulation these models can successfully account for the observed N/O-O values in a self-consistent way. Although new stellar yields have been taken into account which provide mainly secondary N production from massive stars, a significant discrepancy remains between the chemodynamical dwarf galaxy models and closed-box models that take the chemodynamical treatment into account. In this paper we therefore discuss to what amount gas infall is responsible to affect the N/O ratio.

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