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Thermohaline Mixing in Extremely Metal-poor Stars

Henkel, Kate ; Karakas, Amanda I. ; Casey, Andrew R. ; Church, Ross P. LU orcid and Lattanzio, John C. (2018) In Astrophysical Journal Letters 863(1).
Abstract

Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well studied, they make up less than half of all EMP stars with [Fe/H] ∼ -3.0. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offsets are derived) is very important; however, the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot... (More)

Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well studied, they make up less than half of all EMP stars with [Fe/H] ∼ -3.0. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offsets are derived) is very important; however, the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coefficient. Our stellar evolution models utilize a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at [Fe/H] = -3 and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.

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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
stars: abundances, stars: evolution, stars: interiors, stars: low-mass
in
Astrophysical Journal Letters
volume
863
issue
1
article number
L5
publisher
IOP Publishing
external identifiers
  • scopus:85051457669
ISSN
2041-8205
DOI
10.3847/2041-8213/aad552
language
English
LU publication?
yes
id
1d5479e6-3a7a-49ef-a69a-2bfeffd2da98
date added to LUP
2018-09-10 11:05:46
date last changed
2021-10-06 01:28:11
@article{1d5479e6-3a7a-49ef-a69a-2bfeffd2da98,
  abstract     = {<p>Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well studied, they make up less than half of all EMP stars with [Fe/H] ∼ -3.0. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offsets are derived) is very important; however, the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coefficient. Our stellar evolution models utilize a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at [Fe/H] = -3 and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.</p>},
  author       = {Henkel, Kate and Karakas, Amanda I. and Casey, Andrew R. and Church, Ross P. and Lattanzio, John C.},
  issn         = {2041-8205},
  language     = {eng},
  month        = {08},
  number       = {1},
  publisher    = {IOP Publishing},
  series       = {Astrophysical Journal Letters},
  title        = {Thermohaline Mixing in Extremely Metal-poor Stars},
  url          = {http://dx.doi.org/10.3847/2041-8213/aad552},
  doi          = {10.3847/2041-8213/aad552},
  volume       = {863},
  year         = {2018},
}