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The abundance ratios of O,Si and Fe and abundance pattern of SN Ia

The abundance ratios of O,Si and Fe and abundance pattern of SN Ia

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78

c o n ~ t a i i t , ~ for

~ , ' a~ first

,

attempt we have assumed (Fe/Si)sNla, (Si/O)SNII,

and (Fe/O)sNII t o be constants. Here, (Fe/si)sNI, is the Fe/Si ratio of

ejecta of SN Ia, and (Si/O)sNII and (Fe/O)yNrr are the Si/O ratio and the

Fe/O ratio of the ejecta of SN 11, respectively.

The classical deflagration model of SN Ia, W719, expects the Fe/Si ratio

of 2.6 solar ratio. When we adopt the abundance pattern of the Galactic

metal poor stars by Clementini et al.s as that of SN 11, (Fe/Si)sNIa of M 87

is determined t o be 1 solar. That of the center of the Centaurus cluster

is slightly larger than M 87, but still smaller than the W7 ratio. Thus,

these values are much smaller than W? modellg, and in the range of the

ratio derived from the WDD r n o d e l ~ ' ~which

,

considers slow deflagration.

The light curves of observed SN Ia are riot identical but display a considerable variatio~i'~.

In SN Ia, the mass of synthesized Ni56 determines the

luminosity of each SN. Since the mass of the progenitor should be constant

at 1.4 Mo, the ratio of mass of intermediate group elements from Si to Ca,

t o the mass of Fe and Ni, should depend on the luminosity of SN Ia. The

observed luminosity of SN Ia correlates with the type of the host galaxy,

and is suggested t o be related t o the age of the system; SNe Ia in old stellar

system may have smaller luminosities'" and hence are suggested t o yield

a smaller Fe/Si ratio".

We note that the observed abundance patterns of the ICM are located

at a extension of that of Galactic stars, although the observed [Fe/O] range

of the ICM is systematically larger (Figure 4). The metal poor Galactic stars, i.e. those with lower [Fe/O], tend to be located around larger

(Fe/Si)sNra values. In contrast, the O/Si/Fe pattern of metal rich Galactic

stars favor lower (Fe/Si)SNIa values. The difference of metal rich and metal

poor Galactic stars are difference of age of the system when the stars were

born. This result suggests that the SN Ia products trapped in the metal

poor Galactic stars were dominated by those with larger (Fe/Si)sNra when

the Galaxy was a young stellar system.

In conclusion, as discussed in Finoguenov et a1.l2,Matsushita et a1.92,93

the smaller Fe/Si ratio observed for the ICM around M 87 and the Centaurus cluster niay reflect the fact that M 87 and the cD galaxy of the

Centaurus cluster are old stellar systems.



-



7. The failure of the cooling flow model



The Fe abundance profiles of M 87 and the Centaurus cluster contradicts

the standard cooling flow model, consistent with the recent finding of the



79

From the standard cooling flow model,

missing of the cooling

the mass deposition rates of the Centaurus cluster and M 87 within 10

kpc and 27 kpc are deterniiiied t o be 15Ma and 4 Ma,respectively’,”’,

using a Hubble constant of 70 M pc/km/s. These values are much larger

than the stellar mass loss rate within the radii, -0.4Ma and

lMa for

M 87 and NGC 4696, respectively. Even from the whole galaxy, the stellar

inass loss rate of M 87 and NGC 4696 are only lMaand 2Ma, respectively.

Therefore, if a cooling flow with this mass deposition rate exists, the fraction

of gas from the central galaxy must be low, and the central abundance of

Mg and Fe of the Centaurus cluster should be smaller than that of M 87.

N



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RECENT ADVANCES IN THE STUDY OF EXTREMELY

METAL-POOR STARS



F. PRIMAS

European Southern Observatory

KarlSchwarzschil~str.2,

0-85748 Garching bei Munehen, Germany

E-mail: fprimasOeso.org

The advent of 8-10m class telescopes equipped with very efficient and high resolution spectrographs has strongly boosted the study of chemical patterns in the

Galactic halo, allowing us t o derive very accurate abundances in the most metalpoor stars. Here, the tremendous progress recently achieved in this field of research

is presented and critically reviewed.



1. Introduction: The Role of Metal-Poor Stars



The history of the chemical composition of the Galaxy is dominated by the

nucleosynthesis occurring in many generations of stars. In each generation,

a fraction of the gas will be transformed into metals and returned to the

interstellar medium (ISM). What is important to know is how the environment dictated the kind of stars that formed and enriched the Galactic

gas, and how the enriched gas mixed with the interstellar medium to form

subsequent stellar generations.

A first step is to derive the halo metallicity distribution function which

provides direct information about the initial stages of galaxy formation,

being sensitive to the bulk chemical propert,ies of the interstellar gas from

which the earliest generations of stars were born. Comparisons between the

relative numbers of low metallicity stars in the halo and models for Galactic

chemical evolution can then be used to place constraints on the primordial

rate of Type I1 supernovae (SNe), the star formation rate (SFR), and the

timescale for the re-distribution of elements in the early Galaxy. Another

key ingredient is to assemble a large, representative sample of the early

Galactic halo, 2.e. a large sample of the most metal-poora and oldest stars.

awith a metal content [m/H] lower than 1/1000 solar (where [ m / H ]= log(m/H)*



80



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81

Very metal-poor stars have now become one of the main diagnostic tools

as they exhibit the products of nucleosynthesis from the first high-mass,

zero-metallicity objects to evolve and pollute the proto-galaxy. Elemental

abundance ratios observed in these low-mass stars allow us to probe the

ejecta of the earliest SNe and determine the nature of the stars and sites

of nucleosynthesis that existed during the first epochs of star formation in

the Galaxy. Any variation in the elemental abundance ratios observed at

different metallicites can then be compared with the yields derived from

SNe of different masses to determine which ones have contributed to the

Galactic chemical enrichment and when.

2. Metal-Poor Stars: Searches and Findings



Since the 50s, when the first two metal-poor stars were analysed (HD 19445

and HD 140283, Chamberlain & Aller, 1951) more than 8,000 metal-poor

stars have been identified bhrough a variety of techniques, the most successful of which have been proper-motion (e.g. Ryan & Norris 1991 arid

Carney et al. 1994) and objective-prism surveys (e.g. the HK survey of

Beers et al. 1985, and the exploitation of the stellar content, by Christlieb

et al. 1999, of the more recent HES survey).

The HK survey takes its name from the Ca I1 H and K lines falling in

the 150 A bandpass selected via an interference filter near the focal plane

of the 61cm Curtis Schmidt telescope a t CTIO. Because of its earlier start

(compared to the HES), it has had a very strong impact on this field: it has

produced a list of 10,000 candidate metal-poor stars, the identification of

which is based on a calibration of the Ca I1 K line at 3933 A as a function of

metallicity and broad-band B-V colour. It is complete in the 11-15 B mag

interval and the sky coverage is on the order of 7000 deg2 (including also

the Northern hemisphere fields, surveyed at the Burrell Schmidt telescope

at KPNO). Approximately 100 stars with a metal content less than 1/1000

solar have been identified.

The other very successful survey is the more recent Hamburg-ESO Survey (Wisotzki et al. 2000), an objective-prism survey targeting primarily bright QSOs and covering the full southern extra-galactic sky (2.e. at

Galactic latitudes Ibl >30°, -10,000 deg2). It is based on plates taken at the

ESO Schmidt telescope, using a 4" prism. Because it is based on un-widened

prism spectra it gains two magnitudes with respect to the HK survey (down

l o g ( m / H ) a , and rn usually refers to the iron content)



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