Small-amplitude Red Giants Elucidate the Nature of the Tip of the Red Giant Branch as a Standard Candle
Abstract
The tip of the red giant branch (TRGB) is an important standard candle for determining luminosity distances. Although several 105 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 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 and B. The B sequence is best suited for measuring distances to old RG populations, with M F814W,0 = ‑4.025 ± 0.014(stat.) ± 0.033(syst.) mag assuming the LMC's geometric distance. Control of systematics is demonstrated using detailed simulations. Population diversity affects m TRGB at a level exceeding the stated precision: the SARG and A-sequence samples yield 0.039 and 0.085 mag fainter (at 5σ significance) m TRGB values, respectively. Ensuring equivalent RG populations is crucial to measuring accurate TRGB distances. Additionally, luminosity function smoothing (∼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 RGs will enable further improvements to the TRGB as a standard candle.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 2024
- DOI:
- 10.3847/2041-8213/ad284d
- arXiv:
- arXiv:2303.04790
- Bibcode:
- 2024ApJ...963L..43A
- Keywords:
-
- Standard candles;
- Stellar distance;
- Red giant tip;
- Pulsating variable stars;
- Giant stars;
- Hubble constant;
- 1563;
- 1595;
- 1371;
- 1307;
- 655;
- 758;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Instrumentation and Methods for Astrophysics;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in ApJ Letters (9 Feb 2024). Manuscript and title have significantly changed, and much more information on SARGs and new results based on simulations are included. However, the measurements of TRGB magnitudes in the LMC are the same as reported in version 1