Radial Abundance Distribution Research Articles

Elemental Abundance Ratios in Stars of the Outer Galactic Disk. I. Open Clusters

We summarize radial velocity studies of selected stars in the old, distant clusters Berkeley 20, Berkeley 21, NGC 2141, Berkeley 29, and Berkeley 31. Cluster members are identified using optical and infrared color-magnitude diagrams, as well as radial velocities derived from high-resolution echelle spectra. Three members of M67 were observed similarly, and those velocities compare extremely well with prior measures. Mean cluster radial velocities are determined. We also employ the highest quality spectra to analyze the chemical compositions of all six clusters for [Fe/H], as well as abundances of α-elements, iron-peak elements, and those synthesized in either the s-process or the r-process. In Be 21 our observed star is found to be rotating rapidly and overabundant in lithium, the second Li-rich star found in this sparse cluster. We confirm the lack of correlation between abundance and age. For the outer disk, the abundance gradient for [Fe/H] deviates from the trend defined near the solar neighborhood. Rather than declining with increasing galactocentric distance, [Fe/H] appears to reach a basement at [Fe/H] ≈ -0.5 beyond RGC ≈ 10–12 kpc. Our radial abundance distribution for [Fe/H] is not inconsistent with the radial abundance discontinuity exhibited by Cepheids. We find enhanced [O/Fe], [α/Fe], and [Eu/Fe] in the outer disk, revealing a rapid star formation history. The outer disk also exhibits enhancements for s-process elements. We compare the open cluster compositions with those of the thin disk, thick disk, halo, bulge, and dwarf spheroidal galaxies. None of these stellar populations perfectly matches the abundance ratios of the outer disk open clusters. Several key points arise from these comparisons: (1) [O/Fe] and [α/Fe] resemble those of the thick disk. (2) [Na/Fe] and [Al/Fe] are enhanced relative to those of the thin disk. (3) [Ni/Fe] and [Mn/Fe] are in accord with those of the thin disk, while [Co/Fe] may be slightly enhanced. (4) The neutron-capture elements indicate different ratios of s-process to r-process material, with no cluster showing a pure r-process distribution. (5) An unusual pattern exists among the α-elements, with [Mg + Ti/Fe] enhanced while [Si + Ca/Fe] is normal. Similar abundance ratios have been reported for Galactic bulge giants and indicate a common but not necessarily shared nucleosynthetic history between the bulge and the outer disk. Enhanced ratios of [Al/Fe] and [Co/Fe] offer another possible similarity between the bulge and the outer disk. An intriguing but tentative conclusion is that the outer disk open cluster abundance ratios are consistent with the outer disk being formed via a merger event or series of merger events. The basement in [Fe/H] and enhanced [α/Fe] suggest that the outer disk formed from a reservoir of gas with a star formation history distinct from the solar neighborhood. That the open clusters may be associated with an accreted dwarf galaxy or galaxies is appealing, since the clusters are young and have [α/Fe] ratios indicating a rapid star formation history. However, the high [α/Fe] ratios are unlike those seen in any current dwarf galaxies at the same [Fe/H]. Therefore, the open clusters may have formed as a result of star formation triggered by a merger event or series of mergers in the outer disk. The ages of the outer disk open clusters would then be a measure of when the merger(s) occurred. However, Be 29 is a candidate merger member, while Be 31 is not. One problem with the merger scenario is that open clusters with presumably very different origins have similar and unusual compositions.

Old open clusters as key tracers of Galactic chemical evolution. First results: NGC 3960

In the context of a large observational project on old open clusters, we employed FLAMES + UVES on VLT UT2 to collect new high resolution spectra for giant stars in 7 clusters with the aim of investigating the radial abundance distributions in the Galactic disk. The gradients of Fe, α and Fe-peak elements and their evolution with Galactic age are indeed one of the most critical constraints to the star formation history in the Galactic disk and to Galactic chemical evolution models. We present here our preliminary results on the metallicity and abundance ratios for red clump stars in the ∼1 Gyr old cluster NGC 3960, located at a Galactocentric distance kpc.

A grid of chemical evolution models as a tool to interpret spiral and irregular galaxies data

We present a generalization of the multiphase chemical evolution model applied to a wide set of theoretical galaxies with different masses and evolutionary rates. This generalized set of models has been computed using the so-called Universal Rotation Curve from Persic et al (1996) to calculate the radial mass distribution of 44 theoretical protogalaxies. This distribution is a fundamental input which, besides its own effect on the galaxy evolution, defines the characteristic collapse time-scale or gas infall rate onto the disc.We have adopted 10 sets of values, between 0 and 1, for the molecular cloud and star formation efficiencies, as corresponding to their probability nature, for each one of the radial distributions of total mass. Thus, we have constructed a bi-parametric grid of models, depending on those efficiency sets and on the rotation velocity, whose results are valid in principle for any spiral or irregular galaxy. The model results provide the time evolution of different regions of the disc and the halo along galactocentric distance, measured by the gas (atomic and molecular) and stellar masses, the star formation rate and chemical abundances of 14 elements, for a total of 440 models. This grid may be used to estimate the evolution of a given galaxy for which only present time information -- such as radial distributions of elemental abundances, gas densities and/or star formation, which are the usual observational constraints of chemical evolution models -- is available.

High-Velocity Features: A Ubiquitous Property of Type Ia Supernovae

Evidence of high-velocity features (HVFs) such as those seen in the near-maximum spectra of some Type Ia supernovae (SNe Ia; e.g., SN 2000cx) has been searched for in the available SN Ia spectra observed earlier than 1 week before B maximum. Recent observational efforts have doubled the number of SNe Ia with very early spectra. Remarkably, all SNe Ia with early data (seven in our Research Training Network sample and 10 from other programs) show signs of such features, to a greater or lesser degree, in Ca II IR and some also in the Si II λ6355 line. HVFs may be interpreted as abundance or density enhancements. Abundance enhancements would imply an outer region dominated by Si and Ca. Density enhancements may result from the sweeping up of circumstellar material (CSM) by the highest velocity SN ejecta. In this scenario, the high incidence of HVFs suggests that a thick disk and/or a high-density companion wind surrounds the exploding white dwarf, as may be the case in single degenerate systems. Large-scale angular fluctuations in the radial density and abundance distribution may also be responsible: this could originate in the explosion and would suggest a deflagration as the more likely explosion mechanism. CSM interaction and surface fluctuations may coexist, possibly leaving different signatures on the spectrum. In some SNe, the HVFs are narrowly confined in velocity, suggesting the ejection of blobs of burned material.

Elemental abundances in the intracluster medium as observed withXMM-Newton

XMM-Newton observations of 19 galaxy clusters are used to measure the elemental abundances and their spatial distributions in the intracluster medium. The sample mainly consists of X-ray bright and relaxed clusters with a cD galaxy. Along with detailed Si, S and Fe radial abundance distributions within 300-700 kpc in radius, the O abundances are accurately derived in the central region of the clusters. The Fe abundance maxima towards the cluster center, possibly due to the metals from the cD galaxy, are spatially resolved. The Si and S abundances also exhibit central increases in general, resulting in uniform Fe-Si-S ratios within the cluster. In contrast, the O abundances are in general uniform over the cluster. The mean O to Fe ratio within the cluster core is sub-solar, while that of the cluster scale is larger than the solar ratio. These measurements indicate that most of the Fe-Si-S and O in the intracluster medium have different origins, presumably in supernovae Ia and II, respectively. The obtained Fe and O mass are also used to discuss the past star formation history in clusters.

On the oxygen abundance in our Galaxy

The compilation of published spectra of Galactic HII regions with available diagnostic [OIII]4363 line has been carried out. Our list contains 71 individual measurements of 13 HII regions in the range of galactocentric distances from 6.6 to 14.8 kpc. The oxygen abundances in all the HII regions were recomputed in the same way, using the classic Te - method. The oxygen abundance at the solar galactocentric distance traced by those HII regions is in agreement with the oxygen abundance in the interstellar medium in the solar vicinity derived with high precision from the interstellar absorption lines towards stars. The derived radial oxygen abundance distribution was compared with that for HII regions from the Shaver et al. (1983) sample which is the basis of many models for the chemical evolution of our Galaxy. It was found that the original Shaver et al.'s oxygen abundances are overestimated by 0.2-0.3 dex. Oxygen abundances in HII regions from the Shaver et al. sample have been redetermined with the recently suggested P - method. The radial distribution of oxygen abundances from the Shaver et al. sample redetermined with the P - method is in agreement with our radial distribution of (O/H)_Te abundances.

The Stellar Populations of Spiral Disks. III. Constraining Their Evolutionary Histories

We study the old problem of the uniqueness of chemical evolution models by analyzing a set of multiphase models for the galaxy NGC 4303 computed for a variety of plausible physical input parameters. Molla et al. showed previously that multiphase chemical evolution models for the three Virgo Cluster galaxies NGC 4303, 4321, and 4535, were able to reproduce the observed radial distributions of spectral indices Mg2 and Fe52. Chemical evolution models may, however, fit the present-epoch radial distributions with different star formation histories; thus we need to include time-dependent constraints in the problem. The two spectral indices above depend on the time-averaged history of star formation, but they are in turn affected by the well-known age-metallicity degeneracy, which prevents the disentangling of age and metallicity for stellar populations, another uniqueness problem we also discuss. Our aim is to determine whether the possible input parameters may be strongly constrained when both radial distributions (nebular and stellar) are used. In order to accomplish this, we run a large number of models (500) for NGC 4303, varying the input parameters. Less than 4% of the models (19) fit the present-day observational data within a region of 95% probability. The number of models reduces to ~1% (6) when we also ask them to reproduce the time-averaged abundances represented by the spectral indices. Thus, by demonstrating that only a small fraction of the models are favored in reproducing the present-day radial abundance distributions and the spectral indices data simultaneously, we show that these spectral indices provide strong time-dependent additional constraints on the possible star formation and chemical histories of spiral disks.

The oxygen abundance distribution in M 101

The well-observed spiral galaxy M 101 was considered. The radial distributions of oxygen abundances determined in three dierent ways (with the classic Te{method, with the R23{method, and with the P {method) were compared. It was found that the parameters (the central oxygen abundance and the gradient) of the radial (O/H)P abundance distribution are close to those of the (O/H)Te abundance distribution. The parameters of the (O/H)R23 abundance distribution dier signicantly from those of the (O/H)Te abundance distribution: the central (O/H)R23 oxygen abundance is higher by around 0.4 dex and the gradient is steeper by a factor of around 1.5 as compared to those values in the (O/H)Te abundance distribution. The dispersion in (O/H)P abundance at xed radius is rather small,0.08 dex, and is equal to that in (O/H)Te abundance. The dispersion in (O/H)R23 abundance at xed radius is appreciably larger,0.16 dex, compared to that in (O/H)Te abundance. It has been shown that the extra dispersion in (O/H)R23 abundances is an artifact and reflects scatter in excitation parameter P at xed radius.

XMM-Newton observations of M 87 and its X-ray halo

We report performance verification observations of the giant elliptical galaxy M 87 in the Virgo Cluster with the MOS, pn, and optical monitor instruments on board of XMM - Newton . With the energy sensitive imaging instruments MOS and pn we obtain the first spatially constrained X-ray spectra of the nucleus and the jet of the galaxy. The good photon statistics of the pn and MOS allow a detailed analysis of the radial temperature and abundance distribution of 6 elements. The data provide no indication of a multi-temperature structure for radii $\ge $2 ' . An apparent sharp metal abundance drop deduced for the regions inside this radius is probably due to resonant line scattering.

Modeling the Radial Abundance Distribution of the Transition Galaxy NGC 1313

NGC 1313 is the most massive disk galaxy showing a flat radial abundance distribution in its interstellar gas, a behavior generally observed in Magellanic and irregular galaxies. We have attempted to reproduce this flat abundance distribution using a multiphase chemical evolution model, which has been previously used successfully to depict other spiral galaxies along the Hubble morphological sequence. We found that it is not possible to reproduce the flat radial abundance distribution in NGC 1313 and at the same time be consistent with observed radial distributions of other key parameters, such as the surface gas density and star formation profiles. We conclude that a more complicated galactic evolution model, including radial flows and possibly mass loss due to supernova explosions and winds, is necessary to explain the apparent chemical uniformity of the disk of NGC 1313.

Evolution of Spiral Galaxies. VII. Time Evolution of the Radial Distributions of Abundances

This is an electronic version of an article published in The Astrophysical Journal. Molla, M., Ferrini, F. and A.I. Diaz. Evolution of spiral galaxies. VII. Time evolution of the radial distributions of abundances. The Astrophysical Journal 475 (1997): 519-533

The distribution of silicon sulfide in the envelope of IRC + 10216

The J = 5-4 and J = 6-5 transitions of SiS toward the carbon star IRC + 10216 have been mapped using the Berkely-Illinois-Maryland Array. The distribution of the emission is strongly peaked at the position of the star and shows no sign of shell structure. The radial SiS abundance distribution which best fits both the aperture synthesis maps and existing single-dish data has been derived for several rotational lines, and it is concluded that inside a radius of 3 x 10 to the 15th cm, the SiS abundance is 7.5 x 10 to the -6th with respect to H2, but falls to 6.5 x 10 to the -7th outside that radius. Beyond a radius of 4 x 10 to the 16th cm, the SiS molecule is photodissociated by ambient interstellar UV, with an outer cutoff of about 8 x 10 to the 16th cm. More than 98 percent of the SiS may be removed from the gas phase within about 60 yr from the time the gas leaves the star. The depletion time scale appears to be consistent with recent model calculations which include the effects of molecular adsorption onto dust grains. 19 refs.