Modeling the Galactic Chemical Evolution of Helium
arxiv(2024)
摘要
We examine the galactic chemical evolution (GCE) of ^4He in one-zone and
multi-zone models, with particular attention to theoretical predictions and
empirical constraints on IMF-averaged yields. Published models of massive star
winds and core collapse supernovae span a factor of 2 – 3 in the IMF-averaged
^4He yield, y_He^CC. Published models of intermediate mass,
asymptotic giant branch (AGB) stars show better agreement on the IMF-averaged
yield, y_He^AGB, and they predict that more than half of this
yield comes from stars with M=4-8 M_⊙, making AGB ^4He enrichment rapid
compared to Fe enrichment from Type Ia supernovae. Although our GCE models
include many potentially complicating effects, the short enrichment time delay
and mild metallicity dependence of the predicted yields makes the results quite
simple: across a wide range of metallicity and age, the non-primordial ^4He
mass fraction Δ Y = Y-Y_P is proportional to the abundance of
promptly produced α-elements, like oxygen, with Δ Y/Z_O≈ (y_He^CC+y_He^AGB)/y_O^CC.
Reproducing solar abundances with our fiducial choice of the oxygen yield
y_O^CC=0.0071 implies
y_He^CC+y_He^AGB≈ 0.022, i.e.,
0.022M_⊙ of net ^4He production per solar mass of star formation. Our
GCE models with this yield normalization are consistent with most available
observations, though the implied y_He^CC is low compared to most
of the published massive star models. More precise measurements of Δ Y
in stars and gas across a wide range of metallicity and [α/Fe] ratio
could test our models more stringently, either confirming the simple picture
suggested by our calculations or revealing surprises in the evolution of the
second most abundant element.
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