Dissipative N - body code for galaxy evolution

The evolving galaxy is considered as a system of baryonic fragments embedded into the static dark nonbaryonic (DH) and baryonic (BH) halo and subjected to gravitational and viscous interactions. Although the chemical evolution of each separate fragment is treated in the frame of one -- zone close box model with instantaneous recycling, its star formation (SF) activity is a function of mean local gas density and, therefore, is strongly influenced by other interacting fragments. In spite of its simplicity this model provides a realistic description of the process of galaxy formation and evolution over the Hubble timescale.Comment: 11 pages, LaTeX, 7 figures, using the article.sty, expected in A&ApTr, 18, 83

UV (IUE) spectra of the central stars of high latitude planetary nebulae Hb7 and Sp3

We present an analysis of the UV (IUE) spectra of the central stars of Hb7 and Sp3. Comparison with the IUE spectrum of the standard star HD 93205 leads to a spectral classification of O3V for these stars, with an effective temperature of 50,000 K. From the P-Cygni profiles of CIV (1550 A), we derive stellar wind velocities and mass loss rates of -1317 km/s +/- 300 km/s and 2.9X10^{-8} solar mass yr^{-1} and -1603 km/s +/- 400 km/s and 7X10^{-9} solar mass yr^{-1} for Hb7 and Sp3 respectively. From all the available data, we reconstruct the spectral energy distribution of Hb7 and Sp3.Comment: 4 pages, 3 figures, latex, accepted for publication in Astronomy & Astrophysic

Physical Processes in Star-Gas Systems

First we present a recently developed 3D chemodynamical code for galaxy evolution from the K**2 collaboration. It follows the evolution of all components of a galaxy such as dark matter, stars, molecular clouds and diffuse interstellar matter (ISM). Dark matter and stars are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse (hot) gas and a sticky particle scheme for the (cool) molecular clouds. Physical processs such as star formation, stellar death or condensation and evaporation processes of clouds interacting with the ISM are described locally. An example application of the model to a star forming dwarf galaxy will be shown for comparison with other codes. Secondly we will discuss new kinds of exotic chemodynamical processes, as they occur in dense gas-star systems in galactic nuclei, such as non-standard ``drag''-force interactions, destructive and gas producing stellar collisions. Their implementation in 1D dynamical models of galactic nuclei is presented. Future prospects to generalize these to 3D are work in progress and will be discussed.Comment: 4 pages, 4 figures, "The 5th Workshop on Galactic Chemodynamics" - Swinburne University (9-11 July 2003). To be published in the Publications of the Astronomical Society of Australia in 2004 (B.K. Gibson and D. Kawata, eds.). Accepted version, minor changes relative to origina

The puzzling abundance pattern of HD134439 and HD134440

Abundances of 18 elements are determined for the common proper-motion pair, HD134439 and HD134440, which shows high [Mn/Fe] and low [\alpha/Fe] ratios as compared to normal halo stars. Moreover, puzzling abundances are indicated from elements whose origins are normally considered to be from the same nucleosynthesis history. Particularly, we have found that [Mg/Fe] and [Si/Fe] are lower than [Ca/Fe] and [Ti/Fe] by 0.1-0.3 dex. When elemental abundances are interpreted in term of their condensation temperatures (Tc), obvious trends of [X/Fe] vs. Tc for alpha elements and probably iron-peak elements as well are shown. The hypothesis that these stars have formed from a dusty environment in dSph galaxy provides a solution to the puzzling abundance pattern.Comment: 13 pages,5 figures, MN, in pres

Abundances for metal-poor stars with accurate parallaxes II. alpha-elements in the halo

Abundances for alpha-elements and Fe in about 150 field subdwarfs and early subgiants with accurate parallaxes and kinematic data are used to discuss the run of abundance ratios in metal-poor stars in the solar neighborhood. Based on kinematics, we separated stars into two populations: the first one has a positive velocity of rotation around the galactic center, and it is likely to be related to the dissipational collapse of the galaxy; the second one has either negligible or negative rotational velocity, and it is likely related to an accretion component. The two populations show a large overlap in metallicity. However, they show distinct chemical properties. For the first population we found that there are close correlations (with small scatters around) of the rotational velocity with metallicity and with the Fe/alpha abundance ratio: this might be a signature of a not very fast collapse of the progenitor clouds, with enough time for a significant contribution by SNe Ia, although this result needs to be confirmed by a 3-D/non-LTE study. On the other side, the second population exhibits a larger scatter in both the above mentioned relations, and on average, a larger Fe/alpha ratio at a given metallicity, suggesting a larger scatter in ages. We argue that the lack of stars with moderate rotational velocities and high Fe/alpha abundance ratios is due to the short merging time for protogalactic clouds with prograde motion, while the presence of a group of counter-rotating stars with this characteristics indicates a much longer typical lifetimes for protogalactic fragments having such a motion. Finally, we found that perigalactic distances correlate with the Fe/alpha abundance ratios better than the apogalactic distances.Comment: 10 pages, 6 encapsulated figures, accepted for publication in A&

Evaporation and condensation of spherical interstellar clouds. Self-consistent models with saturated heat conduction and cooling

Shortened version: The fate of IS clouds embedded in a hot tenuous medium depends on whether the clouds suffer from evaporation or whether material condensates onto them. Analytical solutions for the rate of evaporative mass loss from an isolated spherical cloud embedded in a hot tenuous gas are deduced by Cowie & McKee (1977). In order to test the validity of the analytical results for more realistic IS conditions the full hydrodynamical equations must be treated. Therefore, 2D numerical simulations of the evolution of IS clouds %are performed with different internal density structures and surrounded by a hot plasma reservoir. Self-gravity, interstellar heating and cooling effects and heat conduction by electrons are added. Classical thermal conductivity of a fully ionized hydrogen plasma and saturated heat flux are considered. Using pure hydrodynamics and classical heat flux we can reproduce the analytical results. Heat flux saturation reduces the evaporation rate by one order of magnitude below the analytical value. The evolution changes totally for more realistic conditions when interstellar heating and cooling effects stabilize the self-gravity. Evaporation then turns into condensation, because the additional energy by heat conduction can be transported away from the interface and radiated off efficiently from the cloud's inner parts. I.e. that the saturated heat flux consideration is inevitable for IS clouds embedded in hot tenuous gas. Various consequences are discussed in the paper.Comment: 16 pages, 24 figures, accepted in Astronomy and Astrophysic

MaGICC baryon cycle: the enrichment history of simulated disc galaxies

Using cosmological galaxy formation simulations from the MaGICC (Making Galaxies in a Cosmological Context) project, spanning stellar mass from ∼107 to 3 × 1010 M⊙, we trace the baryonic cycle of infalling gas from the virial radius through to its eventual participation in the star formation process. An emphasis is placed upon the temporal history of chemical enrichment during its passage through the corona and circumgalactic medium. We derive the distributions of time between gas crossing the virial radius and being accreted to the star-forming region (which allows for mixing within the corona), as well as the time between gas being accreted to the star-forming region and then ultimately forming stars (which allows for mixing within the disc). Significant numbers of stars are formed from gas that cycles back through the hot halo after first accreting to the star-forming region. Gas entering high-mass galaxies is pre-enriched in low-mass proto-galaxies prior to entering the virial radius of the central progenitor, with only small amounts of primordial gas accreted, even at high redshift (z ∼ 5). After entering the virial radius, significant further enrichment occurs prior to the accretion of the gas to the star-forming region, with gas that is feeding the star-forming region surpassing 0.1 Z⊙ by z = 0. Mixing with halo gas, itself enriched via galactic fountains, is thus crucial in determining the metallicity at which gas is accreted to the disc. The lowest mass simulated galaxy (Mvir ∼ 2 × 1010 M⊙, with M⋆ ∼ 107 M⊙), by contrast, accretes primordial gas through the virial radius and on to the disc, throughout its history. Much like the case for classical analytical solutions to the so-called ‘G-dwarf problem’, overproduction of low-metallicity stars is ameliorated by the interplay between the time of accretion on to the disc and the subsequent involvement in star formation – i.e. due to the inefficiency of star formation. Finally, gas outflow/metal removal rates from star-forming regions as a function of galactic mass are presented

C, N and O abundances in red clump stars of the Milky Way

The Hipparcos orbiting observatory has revealed a large number of helium-core-burning "clump" stars in the Galactic field. These low-mass stars exhibit signatures of extra-mixing processes that require modeling beyond the first dredge-up of standard models. The 12C/13C ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters. In this work we present 12C/13C determinations in a sample of 34 Galactic clump stars as well as abundances of nitrogen, carbon and oxygen. Abundances of carbon were studied using the C2 Swan (0,1) band head at 5635.5 A. The wavelength interval 7980-8130 A with strong CN features was analysed in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 A. Compared with the Sun and dwarf stars of the Galactic disk, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by 0.2 dex and oxygen is close to abundances in dwarfs. Comparisons to evolutionary models show that the stars fall into two groups: the one is of first ascent giants with carbon isotope ratios altered according to the first dredge-up prediction, and the other one is of helium-core-burning stars with carbon isotope ratios altered by extra mixing. The stars investigated fall to these groups in approximately equal numbers.Comment: 8 pages 6 figures Accepted for publication in MNRA

The evolution of interstellar clouds in a streaming hot plasma including heat conduction

To examine the evolution of giant molecular clouds in the stream of a hot plasma we performed two-dimensional hydrodynamical simulations that take full account of self-gravity, heating and cooling effects and heat conduction by electrons. We use the thermal conductivity of a fully ionized hydrogen plasma proposed by Spitzer and a saturated heat flux according to Cowie & McKee in regions where the mean free path of the electrons is large compared to the temperature scaleheight. Significant structural and evolutionary differences occur between simulations with and without heat conduction. Dense clouds in pure dynamical models experience dynamical destruction by Kelvin-Helmholtz (KH) instability. In static models heat conduction leads to evaporation of such clouds. Heat conduction acting on clouds in a gas stream smooths out steep temperature and density gradients at the edge of the cloud because the conduction timescale is shorter than the cooling timescale. This diminishes the velocity gradient between the streaming plasma and the cloud, so that the timescale for the onset of KH instabilities increases, and the surface of the cloud becomes less susceptible to KH instabilities. The stabilisation effect of heat conduction against KH instability is more pronounced for smaller and less massive clouds. As in the static case more realistic cloud conditions allow heat conduction to transfer hot material onto the cloud's surface and to mix the accreted gas deeper into the cloud.Comment: 19 pages, 12 figures, accepted in Astronomy and Astrophysic

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Aims. We investigate statistical equilibrium of Cr in the atmospheres of late-type stars to show whether the systematic abundance discrepancy between Cr I and Cr II lines, as often encountered in the literature, is due to deviations from LTE. Furthermore, we attempt to interpret the NLTE trend of [Cr/Fe] with [Fe/H] using chemical evolution models for the solar neighborhood. Methods. NLTE calculations are performed for the model of Cr atom, comprising 340 levels and 6806 transitions in total. We make use of the quantum-mechanical photoionization cross-sections of Nahar (2009) and investigate sensitivity of the model to uncertain cross-sections for H I collisions. NLTE line formation is performed for the MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with -3.2 < [Fe/H] < -0.5, and abundances of Cr are derived by comparison of the synthetic and observed flux spectra. Results. We achieve good ionization equilibrium of Cr for the models with different stellar parameters, if inelastic collisions with H I atoms are neglected. The solar NLTE abundance based on Cr I lines is 5.74 dex with {\sigma} = 0.05 dex; it is \sim 0.1 higher than the LTE abundance. For the metal-poor stars, the NLTE abundance corrections to Cr I lines range from +0.3 to +0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range of metallicities analyzed here, which is consistent with current views on production of these iron peak elements in supernovae. Conclusions. The tendency of Cr to become deficient with respect to Fe in metal-poor stars is an artifact due to neglect of NLTE effects in the line formation of Cr I, and it has no relation to peculiar physical conditions in the Galactic ISM or deficiencies of nucleosynthesis theory.Comment: 14 pages, 13 figures, to be published in A&