Chemical Compositions of Post-AGB Stars

1991 ◽  
Vol 145 ◽  
pp. 341-349 ◽  
Author(s):  
Howard E. Bond
Keyword(s):  
Normal Population ◽  
Chemical Compositions ◽  
Evolutionary Stage ◽  
Iron Group ◽  
New Class ◽  
Abundance Patterns ◽  
Peculiar Stars ◽  
Low Mass ◽  
Chemically Peculiar Stars ◽  
Process Elements

Objects that may be in a post-AGB evolutionary stage include high-latitude supergiants, UU Her and RV Tau variables, and IRAS sources identified with A- to G-type supergiants. Photospheric abundance analyses of such objects typically reveal moderate to extreme iron-group deficiencies, consistent with membership in the thick-disk or halo populations, and with having arisen from low-mass progenitors. The photospheric CNO abundance patterns in such stars are distinctly atypical of normal Population I supergiants, and appear to indicate the presence of helium-burning products at the stellar surface. However, enhancements of s-process elements are typically not seen, suggesting that most of the stars have avoided the AGB dredge-up. A subset of the stars, typified by HR 4049, show ultra-low iron-group abundances and extreme enhancements of C, N, O, and S. They appear to constitute a new class of chemically peculiar stars, with severely depleted photospheric metals. Grain formation is proposed as the depletion mechanism.

scholarly journals The SDSS/APOGEE catalogue of HgMn stars

2020 ◽  
Vol 496 (1) ◽  
pp. 832-850 ◽  
Author(s):  
S Drew Chojnowski ◽  
Swetlana Hubrig ◽  
Sten Hasselquist ◽  
Rachael L Beaton ◽  
Steven R Majewski ◽  
...  
Keyword(s):  
Rotational Velocity ◽  
Satisfactory Explanation ◽  
Strong Magnetic Fields ◽  
Multiple Systems ◽  
Abundance Patterns ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Band Spectra ◽  
Chemically Peculiar Stars ◽  
Narrow Emission

ABSTRACT We report on H-band spectra of chemically peculiar Mercury–Manganese (HgMn) stars obtained via the SDSS/APOGEE survey. As opposed to other varieties of chemically peculiar stars such as classical Ap/Bp stars, HgMn stars lack strong magnetic fields and are defined by extreme overabundances of Mn, Hg, and other heavy elements. A satisfactory explanation for the abundance patterns remains to be determined, but low rotational velocity is a requirement and involvement in binary/multiple systems may be as well. The APOGEE HgMn sample currently consists of 269 stars that were identified among the telluric standard stars as those whose metallic absorption content is limited to or dominated by the H-band Mn ii lines. Due to the fainter magnitudes probed by the APOGEE survey as compared to past studies, only 9/269 stars in the sample were previously known as HgMn types. The 260 newly identified HgMn stars represents a more than doubling of the known sample. At least 32 per cent of the APOGEE sample are found to be binary or multiple systems, and from multi-epoch spectroscopy, we were able to determine orbital solutions for at least one component in 32 binaries. Many of the multilined systems include chemically peculiar companions, with noteworthy examples being the HgMn+Ap/Bp binary HD 5429, the HgMn+HgMn binary HD 298641, and the HgMn+Bp Mn + Am triple system HD 231263. As a further peculiarity, roughly half of the sample produces narrow emission in the C i 16895 Å line, with widths and radial velocities that match those of the Mn ii lines.

scholarly journals Discovery of Secular Evolution of the Atmospheric Abundances of Ap Stars

2014 ◽  
Vol 9 (S307) ◽  
pp. 365-366
Author(s):  
J. D. Bailey ◽  
J. D. Landstreet ◽  
S. Bagnulo
Keyword(s):  
Main Sequence ◽  
Chemical Abundance ◽  
Mixing Processes ◽  
Chemical Tracers ◽  
Secular Evolution ◽  
Abundance Patterns ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Chemically Peculiar Stars ◽  
Ap Stars

AbstractThe stars of the middle main-sequence have relatively quiescent outer layers, and unusual chemical abundance patterns may develop in their atmospheres, revealing the action of such subsurface phenomena as gravitational settling and radiatively driven levitation of trace elements, and their competition with mixing processes such as turbulent diffusion. We report the discovery of the time evolution of such chemical tracers through the main-sequence lifetime of magnetic chemically peculiar stars.

scholarly journals Some observable indicators of s-process environments?

1984 ◽  
Vol 105 ◽  
pp. 195-198
Author(s):  
Keith H. Despain
Keyword(s):  
Element Abundances ◽  
Chemically Peculiar ◽  
Peculiar Stars ◽  
Chemically Peculiar Stars ◽  
Process Elements

By definition, the chemically peculiar stars have element abundances that are non-solar. Many of these stars show peculiarities in s-process elements. This paper discusses observable indicators of the neutron environment for the s-process in these stars.

Element Identifications in Chemically Peculiar Stars of the Upper Main Sequence

1976 ◽  
Vol 32 ◽  
pp. 275-303
Author(s):  
Charles R. Cowley
Keyword(s):  
Rare Earth ◽  
Slow Rotation ◽  
List Type ◽  
Abundance Patterns ◽  
Peculiar Stars ◽  
Statistical Support ◽  
The Rich ◽  
Chemically Peculiar Stars ◽  
Two Stages ◽  
Ap Stars

SummaryThe element identification procedures introduced by Hartoog et al. (1973) have now been applied to some 55 stars including 9 “normal” stars. We have searched for 92 elements, usually in two stages of ionization using, almost exclusively 2.4 Å/mm plates. The results that we wish to report are summarized below:1.A search for ten actinides, using some unpublished material generously provided by Drs. Worden, Fred and Crosswhite was entirely negative. Only uranium and probably thorium in HR 465 at rare earth (RE) maximum have statistical support for their presence, although uranium may be present in other stars.2.We have found no credible evidence for any rare earth in any manganese star. This is a rather basic difference between the spectra of the manganese stars and the Am stars. Calculations show that the rare earth abundances could be rather similar in the Am and Mn stars.3.We confirm the result of Jones, Wolff and Bonsack (1974), that Ho is present in HD 51418. HR 465, (at RE max) is the only other star for which we have statistical support for a Ho identification. We have not analyzed HD 101065. The heaviest lanthanide that we have identified is Er, again in HR 465 at RE maximum.4.Spectral similarities of some of the cool magnetic Ap stars and certain Am stars are remarkable, and deserve more consideration. For example, 32 Aqr an extreme Am, has about the same Strömgren 4-colors as γ Equ. With the exception of Call, the line spectra of the two stars are extraordinarily alike, and make it difficult to believe that the two objects are unrelated.5.We have published (or have in press) an examination of the odd - Z anomalies which appear in the manganese stars at P, Ga, and Y. These peculiarities are extremely difficult to explain with conventional nucleosynthesis. However, we note that slow rotation alone does not appear capable of producing the anomalies.6.The abundance variations among the lanthanides among the magnetic Ap stars are large and definite.The rich variety in the abundance patterns that we have observed is not readily accounted for by any of the current theories of Ap star abundances.

Chemical Composition and Evolution of Post-AGB Stars

2000 ◽  
Vol 177 ◽  
pp. 225-236
Author(s):  
M. Parthasarathy
Keyword(s):  
Carbon Star ◽  
Circumstellar Dust ◽  
Chemical Compositions ◽  
Agb Stars ◽  
Evolutionary Sequence ◽  
Convective Envelope ◽  
The Third ◽  
Refractory Elements ◽  
Abundance Patterns ◽  
Process Elements

Analysis of the chemical compositions of post-AGB stars reveals the following abundance patterns: (i) Post-AGB stars which are extremely underabundant in Fe and other refractory elements, but which have nearly normal abundances of C, N, O, S, and Zn. The depleted refractory elements are locked up in circumstellar dust grains. Formation of dust close to the star, and dust-gas separation and dust-driven mass loss driving out mostly the dust may explain the abundances of these stars. (ii) High-latitude hot post-AGB stars which show an underabundance of carbon, indicating that they left the AGB before the third dredge-up occurred. (iii) Post-AGB stars with overabundances of carbon and s-process elements, indicating that they have gone through the third dredge-up and carbon-star phase on the AGB. The overabundance of Li, Al, C and s-process elements in some post-AGB stars indicate that they have gone through the dredge-up and Hot Bottom Burning nucleosynthesis at the base of the convective envelope. The observed characteristics of post-AGB stars indicate an evolutionary sequence in the transition region from the tip of the AGB into the young planetary nebula stage.

scholarly journals Chemical Compositions of Population II Mid- and Late-Type Stars

1988 ◽  
Vol 132 ◽  
pp. 477-484
Author(s):  
Howard E. Bond ◽  
R. Earle Luck
Keyword(s):  
Chemical Elements ◽  
Chemical Compositions ◽  
Red Giants ◽  
Galactic Chemical Evolution ◽  
Preferential Sampling ◽  
Abundance Patterns ◽  
New Information ◽  
Process Elements ◽  
Heaviest Elements ◽  
Highly Evolved

The abundance patterns in Population II red giants provide information about galactic chemical evolution and nucleosynthesis sites for various chemical elements. These patterns were first revealed by moderate-S/N photographic spectra and are now being refined and extended to additional chemical elements and fainter stars with high-S/N data. The well-established features include overabundances of O and Ca, underabundances of s-process elements, and solar ratios of heavy r-process elements. An excess of nickel found by us on moderate-S/N spectra is less certain, and should be checked with high-S/N data. New information on oxygen abundances in Population II red giants is becoming available from high-S/N observations of weak [O I] lines, and consistently indicates an oxygen overabundance due to the preferential sampling in Population II stars of ejecta from massive stars of previous generations. Recent studies of highly evolved, post-AGB Population II stars are revealing remarkable overabundances of light elements (C, N, O, and S) and deficiencies of the heaviest elements; these are probably signatures of internal nucleosynthesis followed by mixing up to, or exposure by mass loss at, the stellar surface.

scholarly journals Observations of Complex Molecules in Low-Mass Protostars

2011 ◽  
Vol 7 (S280) ◽  
pp. 43-52 ◽  
Author(s):  
Nami Sakai ◽  
Satoshi Yamamoto
Keyword(s):  
Organic Molecules ◽  
Carbon Chain ◽  
Chemical Diversity ◽  
Chemical Compositions ◽  
Evolutionary Stage ◽  
Mass Star ◽  
Star Forming ◽  
Key Factor ◽  
Low Mass ◽  
Complex Organic

AbstractLow-mass star forming regions are rich inventories of complex organic molecules. Furthermore, they show significant chemical diversity even among sources in a similar physical evolutionary stage (i.e. Class 0 sources). One distinct case is the hot corino chemistry characterized by rich existence of saturated complex organic molecules such as HCOOCH3 and C2H5CN, whereas the other is the warm carbon-chain chemistry (WCCC) characterized by extraordinary richness of unsaturated complex organic molecules such as carbon-chain molecules. We here summarize these observational achievements during the last decade, and present a unified picture of carbon chemistry in low-mass protostellar cores. The chemical diversity most likely originates from the source-to-source difference in chemical compositions of grain mantles. In particular, the gas-phase abundance of CH4 evaporated from grain mantles is thought to be a key factor for appearance of WCCC. The origin of the diversity and its evolution to protopranetary disks are discussed.

scholarly journals The Chemical Composition of Asymptotic Giant Branch Stars

1989 ◽  
Vol 106 ◽  
pp. 101-130 ◽  
Author(s):  
David L. Lambert
Keyword(s):  
Chemical Composition ◽  
Asymptotic Giant Branch ◽  
Chemical Compositions ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Low Resolution ◽  
The Third ◽  
Low Mass ◽  
Process Elements ◽  
Giant Branch Stars

AbstractLow resolution spectroscopic and photometric studies of Magellanic Cloud AGB stars have shown that the M → S → C sequence is the result of the third dredge-up of 12C and s-process elements in AGB stars of low mass. In this paper, data on the chemical compositions of normal Galactic M → S → C stars are reviewed and shown to be broadly consistent with expectations for the third dredge-up on the AGB.

scholarly journals Heavy-metal enrichment of intermediate He-sdOB stars: the pulsators Feige 46 and LS IV–14°116 revisited

2020 ◽  
Vol 643 ◽  
pp. A22 ◽  
Author(s):  
M. Dorsch ◽  
M. Latour ◽  
U. Heber ◽  
A. Irrgang ◽  
S. Charpinet ◽  
...  
Keyword(s):  
Diffusion Processes ◽  
Chemical Compositions ◽  
Thermodynamical Equilibrium ◽  
Low Mass Stars ◽  
Atmospheric Diffusion ◽  
Near Ultraviolet ◽  
Abundance Patterns ◽  
Metal Abundance ◽  
Hot Subdwarfs ◽  
Chemically Peculiar Stars

Hot subdwarf stars of spectral types O and B represent a poorly understood phase in the evolution of low-mass stars, in particular of close compact binaries. A variety of phenomena are observed, which make them important tools for several astronomical disciplines. For instance, the richness of oscillations of many subdwarfs are important for asteroseismology. Furthermore, hot subdwarfs are among the most chemically peculiar stars known. Two intermediate He-rich hot subdwarf stars, LS IV–14°116 and Feige 46, are particularly interesting, because they show extreme enrichments of heavy elements such as Ge, Sr, Y, and Zr, which are strikingly similar in both stars. In addition, both stars show light oscillations at periods incompatible with standard pulsation theory and form the class of V366 Aqr variables. We investigated whether the similar chemical compositions extend to more complete abundance patterns in both stars and validate the pulsations in Feige 46 using its recent TESS light curve. High-resolution optical and near-ultraviolet spectroscopy are combined with non-local thermodynamical-equilibrium model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC to consistently determine detailed metal abundance patterns in both stars. Many previously unidentified lines were identified for the first time with transitions originating from Ga III, Ge III-IV, Se III, Kr III, Sr II-III, Y III, Zr III-IV, and Sn IV, most of which have not yet been observed in any star. The abundance patterns of 19 metals in both stars are almost identical, light metals being only slightly more abundant in Feige 46, while Zr, Sn, and Pb are slightly less enhanced compared to LS IV–14°116. Both abundance patterns are distinctively different from those of normal He-poor hot subdwarfs of a similar temperature. The extreme enrichment in heavy metals of more than 4 dex compared to the Sun is likely the result of strong atmospheric diffusion processes that operate similarly in both stars while their similar patterns of C, N, O, and Ne abundances might provide clues to their as yet unclear evolutionary history. Finally, we find that the periods of the pulsation modes in Feige 46 are stable to better than Ṗ ≲ 10−8 s s−1. This is not compatible with Ṗ predicted for pulsations driven by the ɛ-mechanism and excited by helium-shell flashes in a star that is evolving, for example, onto the extended horizontal branch.

Distribution of R- and S-Process Elements in the Galactic Halo

1988 ◽  
Vol 132 ◽  
pp. 501-506
Author(s):  
C. Sneden ◽  
C. A. Pilachowski ◽  
K. K. Gilroy ◽  
J. J. Cowan
Keyword(s):  
Heavy Element ◽  
Galactic Halo ◽  
Low Noise ◽  
Heavy Elements ◽  
Process Synthesis ◽  
Element Abundances ◽  
Halo Stars ◽  
Abundance Patterns ◽  
R Process ◽  
Process Elements

Current observational results for the abundances of the very heavy elements (Z>30) in Population II halo stars are reviewed. New high resolution, low noise spectra of many of these extremely metal-poor stars reveal general consistency in their overall abundance patterns. Below Galactic metallicities of [Fe/H] Ã −2, all of the very heavy elements were manufactured almost exclusively in r-process synthesis events. However, there is considerable star-to-star scatter in the overall level of very heavy element abundances, indicating the influence of local supernovas on element production in the very early, unmixed Galactic halo. The s-process appears to contribute substantially to stellar abundances only in stars more metal-rich than [Fe/H] Ã −2.

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