Small amplitude red giants elucidate the nature of the Tip of the Red Giant Branch as a standard candle

The tip of the red giant branch (TRGB) is an important standard candle for determining luminosity distances. Although several \(10^5\) small amplitude red giant stars (SARGs) have been discovered, variability was previously considered irrelevant for the TRGB as a standard candle. Here, we show that...

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Bibliographic Details
Published in:arXiv.org 2024-03
Main Authors: Anderson, Richard I, Koblischke, Nolan W, Eyer, Laurent
Format: Article
Language:English
Subjects:
Accuracy
Astronomical models
Big Bang theory
Calibration
Celestial bodies
Cepheid variables
Cosmic microwave background
Cosmology
Luminosity
Magellanic clouds
Red giant stars
Stars & galaxies
Universe
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Summary:The tip of the red giant branch (TRGB) is an important standard candle for determining luminosity distances. Although several \(10^5\) small amplitude red giant stars (SARGs) have been discovered, variability was previously considered irrelevant for the TRGB as a standard candle. Here, we show that all stars near the TRGB are SARGs that follow several period-luminosity sequences, of which sequence A is younger than sequence B as predicted by stellar evolution. We measure apparent TRGB magnitudes, m\(_{\mathrm{TRGB}}\), in the Large Magellanic Cloud (LMC), using Sobel filters applied to photometry from the Optical Gravitational Lensing Experiment and the ESA Gaia mission, and we identify several weaknesses in a recent LMC-based TRGB calibration used to measure the Hubble constant. We consider four samples: all Red Giants (RGs), SARGs, and sequences A & B. The B-sequence is best suited for measuring distances to old RG populations, with M\(_{\mathrm{F814W,0}}\) = -4.025 \(\pm\) 0.014(stat.) \(\pm\) 0.033(syst.) mag assuming the LMC's geometric distance. Control of systematics is demonstrated using detailed simulations. Population diversity affects m\(_{\mathrm{TRGB}}\) at a level exceeding the stated precision: the SARG and A-sequence samples yield 0.039 mag and 0.085 mag fainter (at 5{\sigma} significance) m\(_{\mathrm{TRGB}}\) values, respectively. Ensuring equivalent RG populations is crucial to measuring accurate TRGB distances. Additionally, luminosity function smoothing (\(\sim\) 0.02 mag) and edge detection response weighting (as much as -0.06 mag) can further bias TRGB measurements, with the latter introducing a tip-contrast relation. We are optimistic that variable red giants will enable further improvements to the TRGB as a standard candle.
ISSN:2331-8422