Abundance Anomalies Research Articles

AmFm Stars as a Test of Rotational Mixing Models

Stellar evolution models have been calculated for stars of 1.7-2.5 M☉ with both the Geneva-Toulouse and Montreal codes. In the Geneva-Toulouse code, the internal evolution of angular momentum is calculated self-consistently along with the transport of a few species. In the Montreal code, the transport of 24 species is treated in detail, taking into account radiative accelerations, thermal diffusion, and gravitational settling, along with the turbulent diffusion coefficients calculated in the Geneva-Toulouse code. It is verified that the two codes lead to very similar internal structure for a given mass. The calculated surface abundances are compared to abundance anomalies observed on AmFm stars. It is found that with approximately the same parameters as used for other types of stars, the Geneva-Toulouse code leads to turbulent transport coefficients that produce abundance anomalies consistent with the observed ones for HD 73045, HD 23610, and Sirius. Taking into account the effect of the anisotropy of turbulence on vertical transport plays an important role, although the level of anisotropy in stellar envelopes is very uncertain; this effect is usually neglected in calculations. The stabilizing effect of the mean molecular weight gradient can also be important. The current level of accuracy of observed abundances only permits to choose within a one-parameter family of models. To distinguish between turbulent models with different interior profiles, an accuracy of 0.03 dex is required of abundance determinations, a level of accuracy that is not currently achieved. It is also shown that taking into account the pre-main-sequence evolution of the rotation profile leads to an important reduction in the Ω dependence of turbulent transport for slow rotators.

Discovery of a λ Bootis Like Abundance Pattern in the Pulsating Algol-Type System RZ Cassiopeiae

The abundances of eight elements (C through Fe) were determined in the primary star of the short-period Algol-type binary RZ Cas relative to the A3 V spectroscopic standard star $\alpha$ PsA. Definite under-abundances (by $-0.45 \,\mathrm{dex}$ or more) of Mg, Si, Ti, Cr, and Fe were established. On the other hand, light elements C and O show solar abundances, and Ca is slightly under-abundant ($\sim-0.3 \,\mathrm{dex}$). This abundance pattern demonstrates that RZ Cas shows a characteristic of mild $\lambda$ Boo-type stars. This finding raises a question on the physical relations between the metallicity (abundance anomaly), the pulsational characteristics, and the mass accretion processes.

Coronal Emission Measures and Abundances for Moderately Active K Dwarfs Observed byChandra

We have used Chandra to resolve the nearby 70 Oph (K0 V+K5 V) and 36 Oph (K1 V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-S spectra of all four of these stars are presented and compared with an archival LETG spectrum of another moderately active K dwarf, Epsilon Eri. Coronal densities are estimated from O VII line ratios and emission measure distributions are computed for all five of these stars. We see no substantial differences in coronal density or temperature among these stars, which is not surprising considering that they are all early K dwarfs with similar activity levels. However, we do see significant differences in coronal abundance patterns. Coronal abundance anomalies are generally associated with the first ionization potential (FIP) of the elements. On the Sun, low-FIP elements are enhanced in the corona relative to high-FIP elements, the so-called Different levels of FIP effect are seen for our stellar sample, ranging from 70 Oph A, which shows a prominent solar-like FIP effect, to 70 Oph B, which has no FIP bias at all or possibly even a weak inverse FIP effect. The strong abundance difference exhibited by the two 70 Oph stars is unexpected considering how similar these stars are in all other respects (spectral type, age, rotation period, X-ray flux). It will be difficult for any theoretical explanation for the FIP effect to explain how two stars so similar in all other respects can have coronae with different degrees of FIP bias. Finally, for the stars in our sample exhibiting a FIP effect, a curious difference from the solar version of the phenomenon is that the data seem to be more consistent with the high-FIP elements being depleted in the corona rather than a with a low-FIP enhancement

On the Primordial Scenario for Abundance Variations within Globular Clusters: The Isochrone Test

Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globulars.We have tested, with fully consistent stellar evolution calculations, if theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the Color-Magnitude-Diagram (CMD). To this purpose, we have computed for the first time low-mass stellar models and isochrones with an initial metal mixture that includes the extreme values of the observed abundance anticorrelations, and varying initial He mass fractions. Comparisons with 'normal' alpha-enhanced isochrones and suitable Monte Carlo simulations that include photometric errors show that a significant broadening of the CMD sequences occurs only if the helium enhancement is extremely large (in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar luminosity functions up to the Red Giant Branch tip are also very weakly affected, apart from - depending on the He content of the polluting material - the Red Giant Branch bump region. We also study the distribution of stars along the Zero Age Horizontal Branch, and derive general constraints on the relative location of objects with and without abundance anomalies along the observed horizontal branches of globular clusters.

The Sulfur Abundance Anomaly in Planetary Nebulae

The failure of S and O abundances in most planetary nebulae to display the same strong direct correlation that is observed in extragalactic H II regions represents one of the most perplexing problems in the area of PN abundances today. Galactic chemical evolution models as well as large amounts of observational evidence from H II region studies support the contention that cosmic abundances of alpha elements such as O, Ne, S, Cl, and Ar increase together in lockstep. Yet abundance results from the Henry, Kwitter, & Balick (2004) database show a strong tendency for most PNe to have S abundances that are significantly less than expected from the observed level of O. One reasonable hypothesis for the sulfur anomaly is the past failure to properly measure the abundances of unseen ionization stages above S^+2. Future observations with Spitzer will allow us to test this hypothesis.

The sodium abundance in ${\lambda}$ Bootis stars

Context. Numerous observational studies have shown that the atmospheres of λ Boo type stars are strongly deficient in such elements as Ti, Cr, Fe, while some of the light elements (e.g. carbon, oxygen and sulphur) do not show any remarkable abundance anomalies. Such a specific abundance pattern is regarded to be the direct consequence of the selective accretion of stellar envelope material after its depletion of refractory elements. At the same time, sodium with its rather high condensation temperature shows almost normal content in λ Boo star atmospheres in apparent contradiction to a prediction of the accretion hypothesis. Aims. We aim to show that the normal sodium abundance seen in some λ Boo type stars is connected with the specific ionization regime of the sodium atoms which is formed in the circumstellar envelope. Methods. The analytical expression connecting the fraction of ionized atoms and circumstellar envelope characteristics (electron temperature, optical depth, etc.) is used. Results. Under the special assumptions about the circumstellar envelope characteristics, the sodium remains essentially ionized over the whole envelope volume, while elements such as iron are ionized only in the vicinity of the central star. This favours iron dust grain formation in the envelope, and prevents effective adhesion of the sodium charged particles. Thus, the sodium remains in the gaseous medium which is accreted by the star, while such elements as Ti, Fe and some other refractory elements, which are locked in the dust grains, are effectively swept out of the envelope. This results in a specific abundance pattern which is seen in some λ Boo stars whose atmospheres appear to be deficient in Ti, Fe, Ni, but possess almost normal sodium content.

Rubidium and Lead Abundances in Giant Stars of the Globular Clusters M13 and NGC 6752

We present measurements of the neutron-capture elements Rb and Pb in five giant stars of the globular cluster NGC 6752 and Pb measurements in four giants of the globular cluster M13. The abundances were derived by comparing synthetic spectra with high-resolution, high signal-to-noise ratio spectra obtained using HDS on the Subaru telescope and MIKE on the Magellan telescope. The program stars span the range of the O-Al abundance variation. In NGC 6752, the mean abundances are [Rb/Fe] = -0.17 ± 0.06 (σ = 0.14), [Rb/Zr] = -0.12 ± 0.06 (σ = 0.13), and [Pb/Fe] = -0.17 ± 0.04 (σ = 0.08). In M13 the mean abundance is [Pb/Fe] = -0.28 ± 0.03 (σ = 0.06). Within the measurement uncertainties, we find no evidence for star-to-star variation for either Rb or Pb within these clusters. None of the abundance ratios [Rb/Fe], [Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 may have slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to the small sample of field stars at the same metallicity. For M13 and NGC 6752 the Pb abundances are in accord with predictions from a Galactic chemical evolution model. If metal-poor intermediate-mass asymptotic giant branch stars did produce the globular cluster abundance anomalies, then such stars do not synthesize significant quantities of Rb or Pb. Alternatively, if such stars do synthesize large amounts of Rb or Pb, then they are not responsible for the abundance anomalies seen in globular clusters.

A Comparative Study on Lithium Abundances in Solar-Type Stars With and Without Planets

We have investigated the abundance anomalies of lithium for stars with planets in the temperature range of 5600--5900 K reported by Israelian and coworkers, as compared to 20 normal stars at the same temperature and metallicity ranges. Our result indicates a higher probability of lithium depletion for stars with planets in the main--sequence stage. It seems that stellar photospheric abundances of lithium in stars with planets may be somewhat affected by the presence of planets. Two possible mechanisms are considered to account for the lower Li abundances of stars with planets. One is related to the rotation-induced mixing due to the conservation of angular momentum by the protoplanetary disk, and the other is a shear instability triggered by planet migration. These results provide new information on stellar evolution and the lithium evolution of the Galaxy.

Long-term vegetation response to mesquite removal in Desert Grassland

Forty-six years of vegetation response to mesquite removal at dry, low elevation sites on the Santa Rita Experimental Range in southern Arizona was only slightly different than the vegetation dynamics where mesquite trees were left intact. Only the density of threeawn grass species ( Aristida spp.) was greater in the mesquite removal areas: and that difference persisted even after the cover of mesquite was no longer different between treatment and control areas. Cover of shrubs and perennial grasses, and density of all other perennial grasses did not differ between mesquite treatments throughout the study period. Mesquite cover on treated areas was not different than untreated areas 40 years after tree removal. The long-term results support the interpretation that vegetation dynamics at these dry locations, are not limited by the abundance of neighbouring mesquite. Alternatively, mesquite abundance is self-limiting at levels less than would influence grass abundance and precipitation anomalies may override any effects of neighbouring mesquite. Practically, these results suggest that areas with ⩽350 mm year −1 of annual precipitation and <20% mesquite cover may have very little potential for increasing grass abundance through the removal of mesquite trees.

Trace element analysis in pre-solar stardust grains via full-field imaging XPS (Nano-ESCA)

An acid-resistant, SiC-rich, residue from the Murchison meteorite was investigated by means of a novel imaging XPS instrument. The micrometer-sized grains were deposited on a Si wafer from an aqueous suspension. Energy filtered ESCA images have been taken in the kinetic energy range from the threshold up to about 400eV for various photon energies. A lateral resolution of the order of 120nm along with a high energy resolution in the range of 100meV provides the basis for chemical trace element analysis with maximum sensitivity. Apart from major (Si, C) and minor (N, Mg, Al, Fe) elements, the energy filtered images and local microspectra revealed the presence of a variety of heavy trace elements: Sr, Y, Zr, Nb, Mo, Ba and, interestingly the rare earth elements Dy, Er and possibly Tm. Nano-ESCA thus provides a powerful non-destructive screening method for a later analysis of isotopic abundance anomalies using, e.g. Nano-SIMS.

Chemical Composition of the Planet-harboring Star TrES-1

We present a detailed chemical abundance analysis of the parent star of the transiting extrasolar planet TrES-1. Based on high-resolution Keck HIRES and Hobby-Eberly Telescope HRS spectra, we have determined abundances relative to the Sun for 16 elements (Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, and Ba). The resulting average abundance of = -0.02 ± 0.06 is in good agreement with initial estimates of solar metallicity based on iron. We compare the elemental abundances of TrES-1 with those of the sample of stars with planets, searching for possible chemical abundance anomalies. TrES-1 appears not to be chemically peculiar in any measurable way. We investigate possible signs of selective accretion of refractory elements in TrES-1 and other stars with planets and find no statistically significant trends of metallicity [X/H] with condensation temperature Tc. We use published abundances and kinematic information for the sample of planet-hosting stars (including TrES-1) and several statistical indicators to provide an updated classification in terms of their likelihood to belong to either the thin disk or the thick disk of the Milky Way. TrES-1 is found to be very likely a member of the thin-disk population. By comparing α-element abundances of planet hosts and a large control sample of field stars, we also find that metal-rich ([Fe/H] 0.0) stars with planets appear to be systematically underabundant in [α/Fe] by ≈0.1 dex with respect to comparison field stars. The reason for this signature is unclear, but systematic differences in the analysis procedures adopted by different groups cannot be ruled out.

Toward a Working Model for the Abundance Variations in Stars within Globular Clusters

A popular self-enrichment scenario for the formation of globular clusters assumes that the abundance anomalies seen in the stars in many clusters are due to a second stage of star formation occurring from the matter lost by the winds of massive asymptotic giant branch (AGB) stars. Until today, the modelings of the AGB evolution by several different groups has failed, for different reasons, to account for the patterns of chemical anomalies. Here we show that our own modeling can provide a consistent picture if we constrain the three main parameters that regulate AGB evolution, by (1) adopting a high efficiency convection model, (2) adopting rates of mass loss with a high dependence on the stellar luminosity, and (3) assuming a very small overshooting below the formal convective regions during the thermal pulse (TP) phase. The first assumption is needed to obtain an efficient oxygen depletion in the AGB envelopes, and the second is needed to lose the whole stellar envelope within few thermal pulses, so that the sum of CNO elements does not increase too much, consistent with observations. The third assumption is needed to fully understand the sodium production. We also show that the Mg-Al anticorrelation can be explained by adopting the higher limit of the NACRE rates for proton captures by 25Mg and 26Mg; the models are consistent with the recently discovered F-Al correlation. Problems remain in fully explaining the observed Mg isotope ratios.

Abundance Anomalies and Rotational Evolution of Low‐Mass Red Giants: A Maximal Mixing Approach

(ABRIDGED) We use a fully self-consistent evolutionary code to follow the rotational evolution of red giants, making a comprehensive attempt to assess the role of rotationally induced mixing in the development of abundance anomalies in giants with a range of masses and metallicities in stellar clusters and the field. Unlike most previous work, we do not focus on the determination of combinations of mixing rate and depth that reproduce the data on a particular stellar type. Instead, we concentrate on the more fundamental problem of the simultaneous reproduction of the CNO surface patterns in both Population I and Population II giants using the same physics and models. A general result of all our models is that rotational mixing, although present in small amounts, is inefficient on the lower RGB independently of any inhibiting effect of composition barriers. Models with differentially rotating envelopes are able to reproduce the carbon isotope data on M67 giants with initial rotation rates adequate to their progenitors, but fail to do so for open clusters of larger turnoff mass as well as for metal-poor giants in the field and globular clusters. Possible solutions are discussed. Our favored scenario is one in which the overall strength of canonical extra mixing has been underestimated by existent derivations, but which additionally needs to be coupled with a much lower efficiency for rotational mixing among the rapidly rotating open cluster giants than in the slowly rotating ones in the field and globular clusters. We hypothesize that this last requirement is provided by the interaction between convection and rotation in the envelopes of giants, in the sense that rapidly rotating stars would develop much shallower angular velocity profiles in their envelopes than do slowly rotating stars.

Li in NGC 6752 and the formation of globular clusters

Li abundances for 9 Turn-off (TO) stars of the intermediate metallicity cluster ([Fe/H] =-1.4) NGC 6752 are presented. The cluster is known to show abundance anomalies and anticorrelations observed in both evolved and main sequence stars. We find that Li abundance anticorrelates with Na (and N) and correlates with O in these Turn-Off stars. For the first time we observe Pop II hot dwarfs systematically departing from the Spite plateau. The observed anticorrelations are in qualitative agreement with what is expected if the original gas were contaminated by Intermediate Mass AGB - processed material. However, a quantitative comparison shows that none of the existing models can reproduce all the observations at once. The very large amount of processed gas present in the cluster does not imply a pollution, but rather that the whole protocluster cloud was enriched by a previous generation of stars. We finally note that the different abundance patterns in NGC 6397 and NGC 6752 imply different composition of the pre-enrichment ejecta for the two clusters.

The sub-damped Ly α system toward HE 0001-2340: galaxy formation at z ≈ 2

We present a detailed analysis of chemical abundances in a sub-damped Ly α absorber (sub-DLA) at z = 2.187 towards the quasar HE 0001−2340 (zem = 2.28). Our study is based on high-resolution (R ≈ 45 000) spectroscopic data from the UVES instrument installed on the ESO Very Large Telescope (VLT). This sub-DLA system consists of at least 25 individual subcomponents spanning a restframe velocity range of ∼400 km s −1 . The total neutral hydrogen column density is log N(H ) ≈ 19.7. Detected species include C  ,C  ,N  ,N  ,O  ,M g ,A l ,A l ,S i ,S i ,P  ,F e, and possibly D  .F or the dominating neutral gas component at vrel =+ 13 km s −1 (relative to z = 2.187) we derive an oxygen abundance of (O/H) = −1.81 ± 0.07 (1/65 solar). With its extremely low nitrogen content ((N/H) < −3. 3a nd (N/O) −1.5) the absorber exhibits a classic massive star abundance pattern. Our measurements place the z = 2.187 absorber towards HE 0001−2340 among the systems with the lowest ever measured (N/α) ratios. The low (N/O) value is consistent with the idea that primary nitrogen production by the very first stars have enriched the intergalactic gas to a level of (N/O) ≈− 1.5. Peculiar abundances are found in the outermost blue components near −350 km s −1 (in the z = 2.187 restframe) where we observe significant overabundances of phosphorus ((P/C) ≈ +1.5), silicon ((Si/C) ≈ +0.6), and aluminum ((Al/C) ≈ +0.5) after correcting for the effects of ionization. Our study suggests that the line of sight passes through the gaseous environment of one or more stellar clusters that locally enriched their interstellar neighbourhood by supernova ejecta generating the observed abundance anomalies. The large velocity spread of the entire absorption system points to a merger that triggers the formation of these clusters. We thus may be observing a young galaxy at z ≈ 2 that is forming out of a merger event.

Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and oxygen anoxic event 2

Rapid eruption of submarine lava flows during formation of the Caribbean plateau correlates closely with ocean anoxic event 2 (OAE2), which bracketed the Cenomanian/Turonian boundary (∼93.5 Ma). These events also correspond with a positive excursion in carbon isotopic composition of seawater. Hydrothermal activity associated with large‐scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry. We determined the distribution of major, minor, and trace element abundances in the Rock Canyon marine sedimentary section (Pueblo, Colorado). After normalizing element concentrations to Zr to remove the variable contribution of terrigenous material to these sediments, we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive δ13C isotope excursion. The metal abundance anomalies indicate that intermittent hydrothermal activity, in the form of both water/rock exchange and magmatic degassing, introduced large concentrations of trace metals into the Cretaceous ocean at the same time that extinctions of benthic species, turnover in plankton communities, and increases in isotopically light organic carbon burial occurred. The stratigraphic position of this interval of trace metal anomalies matches events associated with OAE2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents.

High‐Resolution Spectroscopy of Ursa Major Moving Group Stars1,1This paper includes data taken at the McDonald Observatory of the University of Texas at Austin. 22Based on observations obtained at Kitt Peak National Observatory, a division of the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science

ABSTRACTWe use new and extant literature spectroscopy to address abundances and membership for UMa moving group stars. We first compare the UMa, Coma, and Hyades H‐R diagrams via a homogeneous set of isochrones and find that these three aggregates are essentially coeval; this (near) coevality can explain the indistinguishable distributions of UMa and Hyades dwarfs in the chromospheric emission versus color plane. Our spectroscopy of cool UMa dwarfs reveals striking abundance anomalies—trends with Teff, ionization state, and excitation potential—like those recently seen in young, cool M34, Pleaides, and Hyades dwarfs. In particular, the trend of rising λ7774‐based O i abundance with declining Teff is markedly subdued in UMa compared to the Pleiades, suggesting a dependence on age or metallicity. Recent photometric metallicity estimates for several UMa dwarfs are markedly low compared to the group’s canonical metallicity, and similar deviants are seen among cool Hyads as well. Our spectroscopy does not confirm these curious photometric estimates, which seem to be called into question for cool dwarfs. Despite disparate sources of Li data, our homogeneous analysis indicates that UMa members evince remarkably small scatter in the Li‐ Teff plane for Teff≥5200 K. Significant star‐to‐star scatter suggested by previous studies is seen for cooler stars. Comparison with the consistently determined Hyades Li‐ Teff trend reveals differences that are qualitatively consistent with this cluster’s larger [Fe/H] (and perhaps slightly greater age). However, quantitative comparison with standard stellar models indicates the differences are smaller than expected, suggesting the action of a fourth parameter beyond age, mass, and [Fe/H] controlling Li depletion. The UMa‐Coma cool star Li abundances may show a slight 0.2 dex difference; however, this may be mass‐independent and thus more consistent with a modest initial Li abundance difference.

Abundances in giant stars of the globular cluster NGC 6752

Recent theoretical yields and chemical evolution models demonstrate that intermediate-mass AGB stars cannot reproduce the observed abundance distributions of O, Na, Mg, and Al. As a further observational test of this finding, we present elemental abundance ratios [X/Fe] for 20 elements in 38 bright giants of the globular cluster NGC 6752 based on high-resolution, high signal-to-noise spectra obtained with UVES on the VLT. This is the most complete spectroscopic analysis of this cluster in terms of the number of elements considered and the number of stars in the sample. The stars span more than 1000 K in effective temperature and more than 3 visual magnitudes along the red giant branch. None of the abundance ratios [X/Fe] show a correlation with evolutionary status. For Si and heavier elements, the small scatter in [X/Fe] may be attributable to the measurement uncertainties. Our mean abundance ratios [X/Fe] are in good agreement with previous studies of this cluster and are also consistent with other globular clusters and field stars at the same metallicity. The mean abundance ratios [Ba/Eu] and [La/Eu] exhibit values, in agreement with field stars at the same metallicity, that lie approximately midway between the pure r -process and the solar (s -process + r -process) mix, indicating that AGB stars have played a role in the chemical evolution of the proto-cluster gas. For the first time, we find possible evidence for an abundance variation for elements heavier than Al in this cluster. We find a correlation between [Si/Fe] and [Al/Fe] which is consistent with the abundance anomalies being synthesized via proton captures at high temperatures. Leakage from the Mg-Al chain into 28 Si may explain the Si excess in stars with the highest [Al/Fe]. We identify correlations between [Y/Fe] and [Al/Fe], [Zr/Fe] and [Al/Fe], and [Ba/Fe] and [Al/Fe] suggesting that Y, Zr, and Ba abundances may increase by about 0.1 dex as Al increases by about 1.3 dex. While the correlations are statistically significant, the amplitudes of the variations are small. If the small variations in Y, Zr, and Ba are indeed real, then the synthesis of the Al anomalies must have taken place within an unknown class of stars that also ran the s -process.

Abundance Anomalies in NGC6752 - Do AGB Stars Have a Role?

We are in the process of testing a popular theory that the observed abundance anomalies in the Globular Cluster NGC 6752 are due to ‘internal pollution’ from intermediate mass asymptotic giant branch stars. To this end we are using a chemical evolution model incorporating custom-made stellar evolution yields calculated using a detailed stellar evolution code. By tracing the chemical evolution of the intracluster gas, which is polluted by two generations of stars, we are able to test the internal pollution scenario in which the Na- and Al-enhanced ejecta from intermediate mass stars is either accreted onto the surfaces of other stars, or goes toward forming new stars. In this paper we focus mainly on the nucleosynthetic yields of the AGB stars and discuss whether these stars are the source of the observed Na-O anticorrelation. Comparing our preliminary results with observational data suggests that the qualitative theory is not supported by this quantitative study. This study has recently been completed and published in [Fenner, Y., Campbell, S.W., Karakas, A.I., Lattanzio, J.C, Gibson, B.K., 2004, MNRAS, 353, 789]. Details of the stellar models will be in a forthcoming paper [Campbell, S. W., et al. 2004, in prep.].

Thermal Evolution of Supernova Iron in Elliptical Galaxies

Interpretations of the spatial distribution, abundance ratios, and global masses of metals in the hot gas of galaxy clusters in terms of supernova enrichment have been problematical. For example, the abundance of iron and other elements occasionally declines toward the center just where the stellar and supernova densities are highest. Also, the mass of gas-phase iron per unit stellar mass or light is lower in elliptical galaxies and groups than in rich galaxy clusters. We discuss hypothetical scenarios in which these abundance anomalies can result from the preferential buoyant separation of metals. However, in this and all previous attempts to explain these metallicity observations it has been assumed that all metals created by supernovae are present in either visible stars or the hot gas. We discuss here the possibility that some of the iron expelled into the hot gas by Type Ia supernovae may have radiatively cooled, avoiding detection by X-ray and optical observers. Hydrodynamic models of Type Ia explosions in the hot gas inside elliptical galaxies create a gas of nearly pure iron that is several times hotter than the local interstellar gas. We describe the subsequent thermal evolution of the iron-rich gas as it radiates and thermally mixes with the surrounding gas. There is a critical time by which the iron ions must mix into the ambient gas to avoid rapid radiative cooling. We find that successful mixing is possible if the iron ions diffuse with large mean free paths, as in an unmagnetized plasma. However, the Larmor radii of the iron ions are exceptionally small in microgauss fields, so the field geometry must be highly tangled or radial to allow the iron to mix by diffusion faster than it cools by radiative losses. The possibility that some of the supernova iron cools cannot be easily discounted.