摘要:

This review is divided into two parts: first some of the mechanisms altering the surface composition of small mass single stars are considered, including pre‐main sequence nuclear burning, dilution during the subgiant phase, first dredge‐up and further mixing mechanisms along the RGB. A fairly good agreement exists between theoretical predictions and observations for stars in not too dense environments; departures from theoretical predictions observed for stars in the field and in open clusters might be attributed to the effects of some additional parameters (e.g., rotation), which is not included in standard models.The behavior of stars in globular clusters is still not understood. On the whole, it may be safely concluded that surface abundances of most elements remain unchanged during the evolution of small mass stars up to the AGB‐phase. The aftermaths of some of the observed element‐to‐element abundance ratio runs in metal‐poor stars are then examined. &agr;‐elements show a similar behavior: there is a small but significant trend with [Fe/H] in the halo ([Fe/H]<−1) that can be attributed to a variation of the yields as a function of the mass for type‐II SNe; the location of the change of slope (related to the onset of the contribution by type Ia SNe, and to the timescale of the halo collapse) is still not well established, being in the range −1.7<[Fe/H]<−1 (corresponding to a timescale for the halo collapse between 3 and 1 Gyr). An early onset of the contribution by type Ia SNe is suggested by the analysis of the Cu/Fe trend. Recent results for the n‐capture elements supports the predictions that the bulk of ther‐process contribution is due to the less massive type II SNe, and that thes‐process contribution is a primary‐like process in intermediate and small mass star. Finally, in metal‐poor stars there is a small excess of those elements which solar abundance is mainly attributed to thes‐process, with respect to the predictions of a solar‐scaledr‐process distribution. This excess can be explained by a small contribution by thes‐process, by too large solars‐fractions, or by a dependence of ther‐process nucleosynthesis on metallicity. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47372

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This talk reviews the measurements of isotopic abundances in the galactic and extragalactic interstellar medium and in the circumstellar envelopes, based on radio molecular lines observations. The derivation of isotopic abundance ratios from molecular lines is discussed in terms of accuracy and reliability. The observational data are compared with theoretical results on galactic chemical evolution and stellar processing. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47395

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

LUNA is a pilot project initially focused on the3He(3He,2p)4He cross section measurement within the thermal energy region of the Sun (15–27 KeV). A compact high current 50 KV ion accelerator facility including a windowless gas target system, a beam calorimeter, and four detector telescopes has been built, tested, and installed underground at the Laboratori Nazionali del Gran Sasso. The sensitivity has been improved by more than four orders of magnitude, as compared to the previous experiment. In particular, thanks to the cosmic ray suppression, we could attain a background level of less than 1 event per week, a rate similar to the one expected from3He(3He,2p)4He at the lower edge of the Sun thermal energy region. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47362

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The southern Galactic halo star CS 22892‐052 is extremely metal‐poor, but has very large enhancements of all elements formed by neutron capture nucleosynthesis. In this paper, we discuss a high resolution spectroscopic analysis of this star, showing that r‐process neutron capture syntheses may account fully for the relative abundances of elements Ba through Er, and that probably a combination of the r‐process and the weak‐component s‐process may match the abundances of the lighter elements Sr‐Y‐Zr. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47423

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

On the basis of high‐resolution spectrometry and NLTE analysis we investigate abundance patterns of non‐nuclear origin in normal main‐sequence A stars and in their metal‐deficient counterparts, the Lambda Bootis stars. We try to identify and separate the effects of diffusion (gravitational settling, radiative levitation) and accretion (separation of gas and dust) on the surface composition. Two blue stragglers in the old open cluster M67 are compared with the normal A stars in an attempt to trace the signature of anomalous stellar evolution. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47376

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The so‐called PG 1159 stars form a new spectroscopic class of extremely hot hydrogen‐deficient post‐AGB stars. Our spectral analyses with model atmospheres show that their chemical surface composition is dominated by carbon, helium, and oxygen. We suggest that these peculiar stars have suffered a late helium‐shell flash which has caused the removal of the hydrogen‐rich envelope and even most of the helium‐rich intershell matter. The idea that the former helium‐burning region is now exposed at the surface of the PG 1159 stars is supported by the recent detection of a very high amount of neon in some objects. The most extreme object is H1504+65 which is one of the brightest X‐ray sources in the sky and the hottest star ever analyzed with model atmosphere techniques (Teff=170 000 K). The photosphere is devoid of hydrogen and helium and composed of oxygen and carbon by equal amounts! This means that we look at the naked core of the former Red Giant. This gives us the unique possibility to confine empirically the still uncertainty known12C(&agr;,&ggr;)16O nuclear reaction rate. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47385

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Elemental abundances are derived for a sample of southern galactic planetary nebulae. We define Type I PN as those which have experienced envelope‐burning conversion to nitrogen of dredged‐up primary carbon. Such nebulae have nitrogen abundances which exceed the total (C+N) abundance of H II regions in the same galaxy. In the current galactic sample, 11 nebulae are classified as Type I, having N/O≳0.8 For these PN, no evidence is found for oxygen depletion, compared to non‐Type I PN, hence no evidence for the ON‐cycle (predicted to operate during the 2nd dredge‐up) significantly altering the surface abundances of the progenitor stars is found. A comparison between the N abundances in the PN and the (C+N) abundances in galactic H II regions indicates that ∼36% of the initial C is converted into N in the non‐Type I PN, consistent with model predictions for the 1st dredge‐up (Becker & Iben 1980). However, the high nitrogen abundances found in the Type I PN require envelope‐burning of dredged‐up carbon into nitrogen, during the 3rd dredge‐up phase. Total C+N+O abundances are correlated with C/H for the entire sample; carbon has been enhanced by He‐burnt material brought up by the 3rd dredge‐up. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47330

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Some experimental data on the energy spectrum and composition of high energy cosmic rays are discussed. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47338

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Data of the first 30 solar neutrino runs of the GALLEX detector, covering the time period from May 1991 to October 1993, are presented along with a discussion of their implications. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47345

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The recent development of bi‐dimensional detectors operating in the mid‐infrared has opened new possibilities for the observation and analysis of the high mass loss processes in the last stages of stellar evolution. Radio (CO) and maser (OH) observations show that an important roˆle is played by intermediate and low mass stars (1–8 M⊙) in the TP‐AGB phase, and by their circumstellar envelopes of gas and dust. We analyze the problem of characterizing the chemical nature of the AGB envelopes (C‐rich or O‐rich) through mid‐infrared observations by means of a suitable photometric system. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.47347

出版商:AIP    

年代:1995

数据来源: AIP