Yeshe Fenner

The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way

We modeled the evolution of the Milky Way Galaxy to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution, and an environment free of life-extinguishing supernovae. We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun.

The chemical evolution of helium in globular clusters : Implications for the self-pollution scenario

We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y ~ 0.28-0.40) compared to the primordial value. We assume that a previous generation of massive asymptotic giant branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.

Deriving the Metallicity Distribution Function of Galactic Systems

The chemical evolution of the Milky Way is investigated using a dual-phase metal-enriched infall model in which primordial gas fuels the earliest epoch of star formation, followed by the ongoing formation of stars from newly accreted gas. The latest metallicity distribution of local K-dwarfs is reproduced by this model, which allows the Galactic thin disk to form from slightly metal-enriched gas with α-element enhancement. Our model predicts ages for the stellar halo and thin disk of 12.5 and 7.4 Gyr respectively, in agreement with empirically determined values. The model presented in this paper is compared with a similar dual-phase infall model from Chiappini et al. (2001). We discuss a degeneracy that enables both models to recover the K-dwarf metallicity distribution while yielding different star formation histories. The metallicity distribution function (MDF) of K-dwarfs is proposed to be more directly comparable to chemical evolution model results than the G-dwarf distribution because lower mass K-dwarfs are less susceptible to stellar evolutionary effects. The K-dwarf MDF should consequently be a better probe of star formation history and provide a stronger constraint to chemical evolution models than the widely used G-dwarf MDF. The corrections that should be applied to a G-dwarf MDF are quantified for the case of the outer halo of NGC 5128.

Cosmological Implications of Dwarf Spheroidal Chemical Evolution

The chemical properties of dwarf spheroidals in the local group are shown to be inconsistent with star formation being truncated after the reionization epoch (z �8). Enhanced levels of [Ba/Y] in stars in dwarf spheroidals like Sculptor indicate strong s-process production from low-mass stars whose lifetimes are comparable with the duration of the pre-reionization epoch. The chemical evolution of Sculptor is followed using a model with SNe II and SNe Ia feedback and mass- and metallicitydependent nucleosynthetic yields for elements from H to Pb. We are unable to reproduce the Ba/Y ratio unless stars formed over an interval long enough for the low-mass stars to pollute the interstellar medium with s-elements. This robust result challenges the suggestion that most of the local group dwarf spheroidals are fossils of reionization and supports the case for large initial dark matter halos. Subject headings: galaxies: abundance ratios — galaxies: dwarf — galaxies: Local Group — galaxies: evolution — stars: abundances — stars: AGB — nucleosynthesis

On the origin of fluorine in the Milky Way

The main astrophysical factories of fluorine ( 19 F) are thought to be Type II supernovae, Wolf‐ Rayet stars, and the asymptotic giant branch (AGB) of intermediate-mass stars. We present a model for the chemical evolution of fluorine in the Milky Way using a semi-analytic multizone chemical evolution model. For the first time, we demonstrate quantitatively the impact of fluorine nucleosynthesis in Wolf‐Rayet and AGB stars. The inclusion of these latter two fluorine production sites provides a possible solution to the long-standing discrepancy between model predictions and the fluorine abundances observed in Milky Way giants. Finally, fluorine is discussed as a possible probe of the role of supernovae and intermediate-mass stars in the chemical evolution history of the globular cluster ω Centauri. Ke yw ords: stars: abundances ‐ stars: evolution ‐ galaxies: evolution.

The Chemical Evolution of Magnesium Isotopic Abundances in the Solar Neighbourhood

The abundance of the neutron-rich magnesium isotopes observed in metal-poor stars is explained quantitatively with a chemical evolution model of the local Galaxy that considers — for the first time — the metallicity-dependent contribution from intermediate mass stars. Previous models that simulate the variation of Mg isotopic ratios with metallicity in the solar neighbourhood have attributed the production of 25 Mg and 26 Mg exclusively to hydrostatic burning in massive stars. These models match the data well for (Fe/H) > −1.0 but severely underestimate 25,26 Mg/ 24 Mg at lower metallicities. Earlier studies have noted that this discrepancy may indicate a significant role played by intermediate mass stars. Only recently have detailed calculations of intermediate mass stellar yields of 25 Mg and 26 Mg become available with which to test this hypothesis. In an extension of previous work, we present a model that successfully matches the Mg isotopic abundances in nearby Galactic disk stars through the incorporation of nucleosynthesis predictions of Mg isotopic production in asymptotic giant branch stars.

Contrasting the chemical evolution of the Milky Way and Andromeda

The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass.

An Evolutionary Model for Submillimeter Galaxies

We calculate multi-wavelength spectral energy distributions (SEDs) (spanning optical to millimeter wavelengths) from simulations of major galaxy mergers with black hole feedback which produce submillimeter bright galaxies (SMGs), using a self-consistent three-dimensional radiative transfer code. These calculations allow us to predict multiwavelength correlations for this important class of galaxies. We review star formation rates, the time evolution of the 850 � m fluxes, along with the time evolution of the MBH Mstar relation of the SMGs formed in the mergers. We reproduce correlations for local AGN observed in Spitzer Space Telescope’s IRAC bands, and make definitive predictions for infrared X-ray correlations that should be testable by combining observations by Spitzer and the upcoming Herschel mission with X-ray surveys. To aid observational studies, we quantify the far-infrared X-ray correlations. Our dynamical approach allows us to directly correlate observed clustering in the data as seen in IRAC color-color plots with the relative amount of time the system spends in a region of color-color space. We also find that this clustering is positively correlated with the stars dominating in their contribution to the total bolometric luminosity. The merger simulations also allow us to directly correlate the 850 � m flux with the ratio of the black hole luminosity to the total luminosity, which is an inherent and testable feature of our model. We present photo albums spanning the lifetime of SMGs, from their infancy in the pre-merger phase to the final stage as an elliptical galaxy, as seen in the observed 3.6 � m and 450 � m band to visually illustrate some of the morphological differences between mergers of differing orbital inclination and progenitor redshift. We compare our SEDs from the simulations to observations of SMGs and find good agreement. We find that SMGs are a broader class of systems than starbursts or quasars. We introduce a simple, heuristic classification scheme on the basis of the LIR/Lx ratios of these galaxies, which may be interpreted qualitatively as an evolutionary scheme, as these galaxies evolve in LIR/Lx while transiting from the pre-merger stage, through the quasar phase, to a merger remnant. Subject headings: galaxies: formation—galaxies: AGN—infrared: galaxies—radiative transfer—stars: formation

A Limit on the Metallicity of Compact High-Velocity Clouds

There is a fortuitous coincidence in the positions of the quasar Ton S210 and the compact H I high-velocity cloud CHVC 224.0-83.4-197 on the sky. Using Far Ultraviolet Spectroscopic Explorer observations of the metal line absorption in this cloud and sensitive H I 21 cm emission observations obtained with the multibeam system at Parkes Observatory, we determine a metallicity of (O/H) < 0.46 solar at a confidence of 3 σ. The metallicity of the high-velocity gas is consistent with either an extragalactic or Magellanic Cloud origin but is not consistent with a location inside the Milky Way unless the chemical history of the gas is considerably different from that of the interstellar medium in the Galactic disk and halo. Combined with measurements of highly ionized species (C III and O VI) at high velocities, this metallicity limit indicates that the cloud has a substantial halo of ionized gas; there is as much ionized gas as neutral gas directly along the Ton S210 sight line. We suggest several observational tests that would improve the metallicity determination substantially and help to distinguish between possible origins for the high-velocity gas. Additional observations of this sight line would be valuable, since the number of compact HVCs positioned in front of background sources bright enough for high-resolution absorption-line studies is extremely limited.

Galactic chemical evolution redux: atomic numbers

Motivated by the inability of Galactic chemical evolution models to reproduce some of the observed solar neighbourhood distribution of elements (and isotopes) with atomic numbers 6 ⩽ Z ⩽ 15 , we have revisited the relevant stellar and Galactic models as part of an ambitious new program aimed at resolving these long-standing discrepancies. Avoiding the use of (traditional) parametric models for low- and intermediate-mass stellar evolution, we have generated a new, physically self-consistent, suite of stellar models and integrated the nucleosynthetic outputs into GEtool , our semi-analytical galactic chemical evolution software package. The predicted temporal evolution of several light- and intermediate- mass elements (and their isotopes) in the solar neighbourhood - from carbon to phosphorus - demonstrate the efficacy of the new yields in reconciling theory and observation.