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ABSTRACT Standard stellar evolution theory poorly predicts the surface abundances of chemical species in low-mass, red giant branch (RGB) stars. Observations show an enhancement of p–p chain and CNO cycle products in red giant envelopes, which suggests the existence of non-canonical mixing that brings interior burning products to the surface of these stars. The 12C/13C ratio is a highly sensitive abundance metric used to probe this mixing. We investigate extra RGB mixing by examining: (1) how 12C/13C is altered along the RGB, and (2) how 12C/13C changes for stars of varying age and mass. Our sample consists of 43 red giants, spread over 15 open clusters from the Sloan Digital Sky Survey’s APOGEE DR17, that have reliable 12C/13C ratios derived from their APOGEE spectra. We vetted these 12C/13C ratios and compared them as a function of evolution and age/mass to the standard mixing model of stellar evolution, and to a model that includes prescriptions for RGB thermohaline mixing and stellar rotation. We find that the observations deviate from standard mixing models, implying the need for extra mixing. Additionally, some of the abundance patterns depart from the thermohaline model, and it is unclear whether these differences are due to incomplete observations, issues inherent to the model, our assumption of the cause of extra mixing, or any combination of these factors. Nevertheless, the surface abundances across our age/mass range clearly deviate from the standard model, agreeing with the notion of a universal mechanism for RGB extra mixing in low-mass stars.
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The 3-Helium Problem
The light element 3He is produced in copious amounts during the first three minutes after the Big Bang. The 3He abundance is then modified primarily by nucleosynthesis in stars, whereby low-mass stars (< 2 solar masses) are expected to produce 3He due to the astration of deuterium. The higher temperatues in more massive stars fuse 3He completely into 4He thus destroying 3He. Measurements of 3He are made via observations of the hyperfine transition of 3He+ at 3.46 cm. Observations of 3He+ in HII regions located throughout the Milky Way disk reveal very little variation in the 3He/H abundance ratio — the "3He Plateau", indicating that the net effect of 3He production in stars is negligible. This is in contrast to much higher 3He/H abundance ratios found in some planetary nebula (PNe). This discrepancy is known as the "3He Problem." One solution to this problem is that thermohaline mixing occurs just above the hydrogen-burning shell to process 3-Helium: 3He(3He, 2p)4He. Thermohaline mixing is a double-diffusive instability that occurs in oceans and is also called thermohaline convection. We discuss how more accurate observations of the 3He/H abundance ratio can constrain stellar evolution models that include thermohaline mixing.
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- Award ID(s):
- 1714688
- PAR ID:
- 10342821
- Date Published:
- Journal Name:
- American Astronomical Society meeting #235
- Volume:
- 52
- Issue:
- 1
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The hydrogen isotope value (δD) of water indigenous to the mantle is masked by the early degassing and recycling of surface water through Earth's history. High 3He/4He ratios in some ocean island basalts, however, provide a clear geochemical signature of deep, primordial mantle that has been isolated within the Earth's interior from melting, degassing, and convective mixing with the upper mantle. Hydrogen isotopes were measured in high 3He/4He submarine basalt glasses from the Southeast Indian Ridge (SEIR) at the Amsterdam–St. Paul (ASP) Plateau (δD = −51 to −90‰, 3He/4He = 7.6 to 14.1 RA) and in submarine glasses from Loihi seamount south of the island of Hawaii (δD = −70 to −90‰, 3He/4He = 22.5 to 27.8 RA). These results highlight two contrasting patterns of δD for high 3He/4He lavas: one trend toward high δD of approximately −50‰, and another converging at δD = −75‰. These same patterns are evident in a global compilation of previously reported δD and 3He/4He results. We suggest that the high δD values result from water recycled during subduction that is carried into the source region of mantle plumes at the core–mantle boundary where it is mixed with primordial mantle, resulting in high δD and moderately high 3He/4He. Conversely, lower δD values of −75‰, in basalts from Loihi seamount and also trace element depleted mid-ocean ridge basalts, imply a primordial Earth hydrogen isotopic value of −75‰ or lower. δD values down to −100‰ also occur in the most trace element-depleted mid-ocean ridge basalts, typically in association with 87Sr/86Sr ratios near 0.703. These lower δD values may be a result of multi-stage melting history of the upper mantle where minor D/H fractionation could be associated with hydrogen retention in nominally anhydrous residual minerals. Collectively, the predominance of δD around −75‰ in the majority of mid-ocean ridge basalts and in high 3He/4He Loihi basalts is consistent with an origin of water on Earth that was dominated by accretion of chondritic material.more » « less
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The hydrogen isotope value (δD) of water indigenous to the mantle is masked by the early degassing and recycling of surface water through Earth’s history. High 3He/4He ratios in some ocean island basalts, however, provide a clear geochemical signature of deep, primordial mantle that has been isolated within the Earth’s interior from melting, degassing, and convective mixing with the upper mantle. Hydrogen isotopes were measured in high 3He/4He submarine basalt glasses from the Southeast Indian Ridge (SEIR) at the Amsterdam–St. Paul (ASP) Plateau (δD =−51 to −90, 3He/4He =7.6 to 14.1 RA) and in submarine glasses from Loihi seamount south of the island of Hawaii (δD =−70 to −90, 3He/4He =22.5 to 27.8 RA). These results highlight two contrasting patterns of δD for high 3He/4He lavas: one trend toward high δD of approximately −50, and another converging at δD =−75. These same patterns are evident in a global compilation of previously reported δD and 3He/4He results. We suggest that the high δD values result from water recycled during subduction that is carried into the source region of mantle plumes at the core–mantle boundary where it is mixed with primordial mantle, resulting in high δD and moderately high 3He/4He. Conversely, lower δD values of −75, in basalts from Loihi seamount and also trace element depleted mid-ocean ridge basalts, imply a primordial Earth hydrogen isotopic value of −75or lower. δD values down to −100also occur in the most trace element-depleted mid-ocean ridge basalts, typically in association with 87Sr/86Sr ratios near 0.703. These lower δD values may be a result of multi-stage melting history of the upper mantle where minor D/H fractionation could be associated with hydrogen retention in nominally anhydrous residual minerals. Collectively, the predominance of δD around −75in the majority of mid-ocean ridge basalts and in high 3He/4He Loihi basalts is consistent with an origin of water on Earth that was dominated by accretion of chondritic material.more » « less
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Abstract Observations show an almost ubiquitous presence of extra mixing in low-mass upper giant branch stars. The most commonly invoked explanation for this is thermohaline mixing. One-dimensional stellar evolution models include various prescriptions for thermohaline mixing, but the use of observational data directly to discriminate between thermohaline prescriptions has thus far been limited. Here, we propose a new framework to facilitate direct comparison: using carbon-to-nitrogen measurements from the Sloan Digital Sky Survey-IV APOGEE survey as a probe of mixing and a fluid parameter known as the reduced density ratio from one-dimensional stellar evolution programs, we compare the observed amount of extra mixing on the upper giant branch to predicted trends from three-dimensional fluid dynamics simulations. Using this method, we are able to empirically constrain how mixing efficiency should vary with the reduced density ratio. We find the observed amount of extra mixing is strongly correlated with the reduced density ratio and that trends between reduced density ratio and fundamental stellar parameters are robust across choices for modeling prescription. We show that stars with available mixing data tend to have relatively low density ratios, which should inform the regimes selected for future simulation efforts. Finally, we show that there is increased mixing at low reduced density ratios, which is consistent with current hydrodynamical models of thermohaline mixing. The introduction of this framework sets a new standard for theoretical modeling efforts, as validation for not only the amount of extra mixing, but trends between the degree of extra mixing and fundamental stellar parameters is now possible.more » « less
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Abstract Consistent 3He/4He ratios have been measured for >25 years in geothermal fluids and gases from Cumbre Vieja, La Palma (9.4 ± 0.1RA, where RA is the 3He/4He of air), and Teide, Tenerife (6.8 ± 0.3RA), Canary Islands. Both locations are characterized by similar CO2/3He (∼2 to 4 × 109), mantle-like δ13C (−3.3‰ to −4.4‰) and CO2 output (0.1–0.2 × 1010 mol yr–1). Helium isotopic differences between the islands cannot be explained by differential aging and 4He ingrowth in their mantle sources. Instead, distinct He reservoirs exist, with a high-μ (HIMU)–type mantle source for La Palma and a more enriched mantle, with possible lithospheric mantle influence, for Tenerife. Geothermal samples from the Canary Islands record a present-day He distribution distinct from higher 3He/4He in olivine from older eastern Canary Island lavas, indicating temporal variability in sources. Comparison of geothermal sample data versus olivine, pyroxene, and glass He isotope data for the Canary Islands, Azores, Cape Verde, Hawaiian islands, and Iceland reveals generally good correspondence, even across >1 m.y. of stratigraphy. However, in addition to the Canary Islands, there are examples of inter-island heterogeneity for He isotopes at Hawaii, the Azores, and within Iceland, preserved in hydrothermal samples, minerals, and glasses. In particular, in northwest Iceland, olivine separates from older lavas preserve higher 3He/4He than present-day geothermal samples from the same region. This difference likely reflects a reduced mantle-derived 3He input to Icelandic magmatism since the Miocene. Temporal variability in 3He/4He, assessed using geothermal and geological materials in conjunction, offers a powerful tool for examining heterogeneity and temporal evolution of mantle sources at intraplate volcanoes.more » « less
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