SNAPSHOT: Connections between Internal and Surface Properties of Massive Stars

Abstract

We introduce SNAPSHOT, a technique to systematically compute stellar structure models in hydrostatic and thermal equilibrium based on 3 structural properties - core mass $M_{\mathrm{core}}$, envelope mass $M_{\mathrm{env}}$ and core composition. This approach allows us to connect these properties of stellar interiors to the luminosity and effective temperature $T_{\mathrm{eff}}$ in a more systematic way than with stellar evolution models. For MS models, we derive an analytical relationship between $M_{\mathrm{core}}$, $M_{\mathrm{total}}$ and central H abundance that can be used in rapid stellar evolution algorithms. Core-He burning models with $M_{\mathrm{core}}/M_{\mathrm{total}}$ from 0.2 to 0.8 have convective envelopes, low $T_{\mathrm{eff}}$ and will appear as red supergiants. For a given $M_{\mathrm{core}}$, they exhibit a small variation in luminosity (0.02 dex) and $T_{\mathrm{eff}}$ ($\sim 400\mathrm{K}$) over a wide range of $M_{\mathrm{env}}$ ($\sim 2-20{\mathrm{M}}_{\odot }$). This means that it is not possible to derive red supergiant masses from luminosities and $T_{\mathrm{eff}}$ alone. We derive the following relationship between $M_{\mathrm{core}}$ and the total luminosity of a red supergiant during core He burning: $\log M_{\mathrm{core}}\simeq 0.44\log L/L_{\odot }-1.38$. At $M_{\mathrm{core}}$/$M_{\mathrm{total}}\approx 0.2$, our models exhibit a bi-stability and jump from a RSG to a BSG structure. Our models with $M_{\mathrm{core}}/M_{\mathrm{total}}>0.8$, which correspond to stripped stars produced by mass loss or binary interaction, show that $T_{\mathrm{eff}}$ has a strong dependence on $M_{\mathrm{env}}$, $M_{\mathrm{core}}$ and the core composition. We find the mass of one of these stripped stars in a binary system, HD 45166, to be less than its dynamical mass. When a large observational sample of stripped stars becomes available, our results can be used to constrain their masses and the physics of binary interaction.

Accepted for publication in MNRAS