I. Pelupessy

Diagnostics of the molecular component of photon-dominated regions with mechanical heating

Context. Multitransition CO observations of galaxy centers have revealed that significant fractions of the dense circumnuclear gas have high kinetic temperatures, which are hard to explain by pure photon excitation, but may be caused by dissipation of turbulent energy. Aims: We aim to determine to what extent mechanical heating should be taken into account while modeling PDRs. To this end, the effect of dissipated turbulence on the thermal and chemical properties of PDRs is explored. Methods: Clouds are modeled as 1D semi-infinite slabs whose thermal and chemical equilibrium is solved for using the Leiden PDR-XDR code, where mechanical heating is added as a constant term throughout the cloud. An extensive parameter space in hydrogen gas density, FUV radiation field and mechanical heating rate is considered, covering almost all possible cases for the ISM relevant to the conditions that are encountered in galaxies. Effects of mechanical heating on the temperature profiles, column densities of CO and H2O and column density ratios of HNC, HCN and HCO+ are discussed. Results: In a steady-state treatment, mechanical heating seems to play an important role in determining the kinetic temperature of the gas in molecular clouds. Particularly in high-energy environments such as starburst galaxies and galaxy centers, model gas temperatures are underestimated by at least a factor of two if mechanical heating is ignored. The models also show that CO, HCN and H2O column densities increase as a function of mechanical heating. The HNC/HCN integrated column density ratio shows a decrease by a factor of at least two in high density regions with n ~ 105 cm-3, whereas that of HCN/HCO+ shows a strong dependence on mechanical heating for this same density range, with boosts of up to three orders of magnitude. Conclusions: The effects of mechanical heating cannot be ignored in studies of the molecular gas excitation whenever the ratio of the star formation rate to the gas density (SFR/n3/2) is close to, or exceeds, 7 × 10-6 M⊙ yr-1 cm4.5. If mechanical heating is not included, predicted column densities (such as those of CO) are underestimated, sometimes (as in the case of HCN and HCO+) even by a few orders of magnitude. As a lower bound to its importance, we determined that it has non-negligible effects already when mechanical heating is as little as 1% of the UV heating in a PDR. Appendix A is available in electronic form at http://www.aanda.org

Diagnostics of the molecular component of photon-dominated regions with mechanical heating - II. Line intensities and ratios

CO observations in active galactic nuclei and starbursts reveal high kinetic temperatures. Those environments are thought to be very turbulent due to dynamic phenomena, such as outflows and high supernova rates. We investigate the effect of mechanical heating on atomic fine-structure and molecular lines and on their ratios. We try to use those ratios as a diagnostic to constrain the amount of mechanical heating in an object and also study its significance on estimating the H2 mass. Equilibrium photodissociation models (PDRs hereafter) were used to compute the thermal and chemical balance for the clouds. The equilibria were solved for numerically using the optimized version of the Leiden PDR-XDR code. Large velocity-gradient calculations were done as post-processing on the output of the PDR models using RADEX. High-J CO line ratios are very sensitive to mechanical heating (Γmech hereafter). These emission becomes at least one order of magnitude brighter in clouds with n ~ 105 cm-3 and a star formation rate of 1 M⊙ yr-1 (corresponding to Γmech = 2 × 10-19 erg cm-3 s-1). The emission of low-J CO lines is not as sensitive to Γmech, but they do become brighter in response to Γmech. Generally, for all of the lines we considered, Γmech increases excitation temperatures and decreases the optical depth at the line centre. Hence line ratios are also affected, strongly in some cases. Ratios involving HCN are a good diagnostic for Γmech, where the HCN(1-0)/CO(1-0) increases from 0.06 to 0.25, and the HCN(1-0)/HCO+(1-0) increase from 0.15 to 0.5 for amounts of Γmech that are equivalent to 5% of the surface heating rate. Both ratios increase to more than 1 for higher Γmech, as opposed to being much less than unity in pure PDRs. The first major conclusion is that low-J to high-J intensity ratios will yield a good estimate of the mechanical heating rate (as opposed to only low-J ratios). The second one is that the mechanical heating rate should be taken into account when determing AV or, equivalently, NH, and consequently the cloud mass. Ignoring Γmech will also lead to large errors in density and radiation field estimates. Appendices are available in electronic form at http://www.aanda.orgThe data used to generate all the grids are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/574/A127

Multi-scale and multi-domain computational astrophysics

Astronomical phenomena are governed by processes on all spatial and temporal scales, ranging from days to the age of the Universe (13.8 Gyr) as well as from kilometre size up to the size of the Universe. This enormous range in scales is contrived, but as long as there is a physical connection between the smallest and largest scales it is important to be able to resolve them all, and for the study of many astronomical phenomena this governance is present. Although covering all these scales is a challenge for numerical modellers, the most challenging aspect is the equally broad and complex range in physics, and the way in which these processes propagate through all scales. In our recent effort to cover all scales and all relevant physical processes on these scales, we have designed the Astrophysics Multipurpose Software Environment (AMUSE). AMUSE is a Python-based framework with production quality community codes and provides a specialized environment to connect this plethora of solvers to a homogeneous problem-solving environment.

Constraining cloud parameters using high density gas tracers in galaxies

Far-infrared molecular emission is an important tool used to understand the excitation mechanisms of the gas in the inter-stellar medium of star-forming galaxies. In the present work, we model the emission from rotational transitions with critical densities n >~ 10^4 cm-3. We include 4-3 < J <= 15-14 transitions of CO and 13CO, in addition to J <= 7-6 transitions of HCN, HNC, and HCO+ on galactic scales. We do this by re-sampling high density gas in a hydrodynamic model of a gas-rich disk galaxy, assuming that the density field of the interstellar medium of the model galaxy follows the probability density function (PDF) inferred from the resolved low density scales. We find that in a narrow gas density PDF, with a mean density of ~10 cm-3 and a dispersion \sigma = 2.1 in the log of the density, most of the emission of molecular lines, emanates from the 10-1000 cm-3 part of the PDF. We construct synthetic emission maps for the central 2 kpc of the galaxy and fit the line ratios of CO and 13CO up to J = 15-14, as well as HCN, HNC, and HCO+ up to J = 7-6, using one photo-dissociation region (PDR) model. We attribute the goodness of the one component fits for our model galaxy to the fact that the distribution of the luminosity, as a function of density, is peaked at gas densities between 10 and 1000 cm-3. We explore the impact of different log-normal density PDFs on the distribution of the line-luminosity as a function of density, and we show that it is necessary to have a broad dispersion, corresponding to Mach numbers >~ 30 in order to obtain significant emission from n > 10^4 cm-3 gas. Such Mach numbers are expected in star-forming galaxies, LIRGS, and ULIRGS. By fitting line ratios of HCN(1-0), HNC(1-0), and HCO+(1-0) for a sample of LIRGS and ULIRGS using mechanically heated PDRs, we constrain the Mach number of these galaxies to 29 < M < 77.

Intermediate-mass black holes in globular clusters: observations and simulations

The study of intermediate-mass black holes (IMBHs) is a young and promising field of research. If IMBHs exist, they could explain the rapid growth of supermassive black holes by acting as seeds in the early stage of galaxy formation. Formed by runaway collisions of massive stars in young and dense stellar clusters, intermediate-mass black holes could still be present in the centers of globular clusters, today. Our group investigated the presence of intermediate-mass black holes for a sample of 10 Galactic globular clusters. We measured the inner kinematic profiles with integral-field spectroscopy and determined masses or upper limits of central black holes in each cluster. In combination with literature data we further studied the positions of our results on known black-hole scaling relations (such as M - σ) and found a similar but flatter correlation for IMBHs. Applying cluster evolution codes, the change in the slope could be explained with the stellar mass loss occurring in clusters in a tidal field over its life time. Furthermore, we present results from several numerical simulations on the topic of IMBHs and integral field units (IFUs). We ran N-body simulations of globular clusters containing IMBHs in a tidal field and studied their effects on mass-loss rates and remnant fractions and showed that an IMBH in the center prevents core collapse and ejects massive objects more rapidly. These simulations were further used to simulate IFU data cubes. For the specific case of NGC 6388 we simulated two different IFU techniques and found that velocity dispersion measurements from individual velocities are strongly biased towards lower values due to blends of neighboring stars and background light. In addition, we use the Astrophysical Multipurpose Software Environment (AMUSE) to combine gravitational physics, stellar evolution and hydrodynamics to simulate the accretion of stellar winds onto a black hole.

Gone with the wind: the impact of wind mass transfer on the orbital evolution of AGB binary systems

In low-mass binary systems, mass transfer is likely to occur via a slow and dense stellar wind when one of the stars is in the AGB phase. Observations show that many binaries that have undergone AGB mass transfer have orbital periods of 1-10 yr, at odds with the predictions of binary population synthesis models. We investigate the mass-accretion efficiency and angular-momentum loss via wind mass transfer in AGB binary systems. We use these quantities to predict the evolution of the orbit. We perform 3D hydrodynamical simulations of the stellar wind lost by an AGB star using the AMUSE framework. We approximate the thermal evolution of the gas by imposing a simple effective cooling balance and we vary the orbital separation and the velocity of the stellar wind. We find that for wind velocities

A connected component-based method for efficiently integrating multi-scale N-body systems

v_{\infty}

The consequences of a nearby supernova on the early solar system

larger than the relative orbital velocity of the system

Modelling mechanical heating in star-forming galaxies: CO and 13CO Line ratios as sensitive probes

v_\mathrm{orb}

The Oceanographic Multipurpose Software Environment (OMUSE v1.0)

the flow is described by the Bondi-Hoyle-Lyttleton approximation and the angular-momentum loss is modest, leading to an expansion of the orbit. For low wind velocities an accretion disk is formed around the companion and the accretion efficiency as well as the angular-momentum loss are enhanced, implying that the orbit will shrink. We find that the transfer of angular momentum from the orbit to the outflowing gas occurs within a few orbital separations from the center of mass of the binary. Our results suggest that the orbital evolution of AGB binaries can be predicted as a function of the ratio