Red Giants Have it All: Dark Matter Constraints from Stellar Evolution

Paper Title: Dark matter limits from the tip of the red giant branch of globular clusters

Authors: Haozhi Hong and Aaron C. Vincent

First-author institution: University of British Columbia

Status: Published in PRJ D [open access]

Globular clusters are an interesting playground for astronomers. Generally consisting of relatively densely-packed stars which are (for the most part) roughly the same age and metallicity, these locales provide unique insight into stellar dynamics and evolution. In particular, globular clusters are useful for population studies. A population study is simply an avenue of research which focuses on a large group of similar objects, allowing astronomers to ascertain more robust or different knowledge than can be obtained from studying single objects alone. For example, plotting a single-age population of stars on a Hertzsprung-Russell (HR) diagram can show how stars of different initial masses evolve differently over time. As part of this, we can identify patterns in where stars fall on the HR diagram, yielding new information about the underlying physics and providing new tools for research.

One such location is called the Tip of the Red Giant Branch (TRGB), the HR diagram location where helium core burning is triggered in red giant stars (also known as the “helium flash”). Generally, the TRGB occurs at the same intrinsic luminosity, allowing astronomers to use it as a standard candle for distance determinations. Today’s paper, however, considers how the luminosity of the TRGB may change for stellar populations which have captured significant amounts of dark matter (DM). In doing so, they develop a unique method for constraining some of the physics of WIMP-like particles. 
The Standard Model implies that WIMP-nucleon elastic scattering can occur. Simply put, this means that interactions between WIMPs and neutrons or protons can cause a scattering effect where energy is released in some way. In dense regions like the cores of massive stars, this loss of energy can slow the WIMPs to below the local escape velocity, causing them to be gravitationally captured. Over the very long lifetime of a star, this may cause a significant accumulation of DM in its core. When accumulated DM self-annihilates, it can release a significant amount of energy, heating up the center and prematurely kick-starting the ignition of the helium core, lowering the TRGB luminosity. The exact amount the luminosity changes, however, will depend on the rate of annihilation, which is predominantly dependent on the local DM density over the star’s lifetime (see Fig 1.), and the mass and WIMP-nucleon scattering cross-section (σ) of the DM particle .

Globular clusters serve as a useful test for this TRGB probe, as they contain large, relatively isolated populations of stars, and (in the Milky Way) a wide range of local DM environment histories, because they exist in a wide variety of locations within the galaxy. Today’s authors use Gaia data to model the long-term physical trajectories of 161 different Milky Way globular clusters through the galaxy. By tracing these trajectories through a handful of known Milky Way DM halo models (which estimate the local DM density at different locations in the galaxy), they are able to estimate the average lifetime local DM density for each cluster. 

From here, they make use of MESA, a common stellar evolution code. On top of the normal evolution models, they use an additional MESA-compatible code module which accounts for the energy input of DM annihilation. Using the modeled DM density histories of a subset of 22 clusters for which the metallicity is well known, they model the lifetime of a 0.8 solar mass star for a range of DM cross-sections and masses. As the speed at which stars evolve depends on their mass, 0.8 solar mass stars will be the ones approaching the TRGB in modern globular clusters. They additionally account for changes in DM capture rate due to the globular cluster velocity relative to the center of the Milky Way, as well as an individual star’s velocity within its host cluster. 

Finally, today’s authors compare the luminosities of the modeled stars’ TRGBs to the measured TRGB luminosity of the associated globular clusters. For each tested DM particle mass, they calculate the likelihood that any difference between the models and measured values is expected to be a result of DM energy input for the range of cross sections, as compared to typical standard model physics. From this, they derive 90% confidence level (CL) limits for each of the DM halo models used to derive the environment histories (Fig. 2).

Ultimately, they find that the limits on dark matter properties provided by this method are less constraining than those obtained by direct-detection, ground-based experiments like LUX-ZEPLIN. Here, the cross-sectional area is only constrained, at best, to σ < ~10^-27 cm², while ground-based work obtains σ < 10^-42 cm². They note, however, that this work is independent of systematic errors related to the local dark matter density distribution, or the construction of direct-detection tools. Finally, in good news for the TRGB-based distance ladder, they highlight that the expected change in TRGB luminosity due to DM annihilation in red giant cores is less than the standard observational error, meaning our distances remain as robust as previously believed.

Astrobite edited by: Skylar Grayson

Featured image credit: NASA’s Goddard Space Flight Center/Chris Smith (KBRwyle) via NASA SVS