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A Re-Evaluation of the Evolved Stars in the Globular Cluster M13
Solar and Stellar Astrophysics · Submitted 30 Mar 2010
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim We find support for the idea that the population of HB stars redder than the primary group are noticeably evolved, which resolves a disagreement between distance moduli derived from the tip of the RGB and from stars near the instability strip.
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We present photometry for all bright red giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB) stars within 10' of the center of M13. We find support for the idea that the population of HB stars redder than the primary group are noticeably evolved, which resolves a disagreement between distance moduli derived from the tip of the RGB and from stars near the instability strip. The sharp cut at the red end of the HB provides strong evidence that stars from the dominant HB group must still be undergoing blue loops, implying that diffusion is being inhibited. We argue that M13's HB is a somewhat pathological case - the dominant HB population occurs very near the "knee" in optical CMDs, and evolved stars exclusively appear redward of that peak, leading to the incorrect appearance of a continuation of the unevolved HB. M13 has a distinct group of HB stars previously identified with the second U jump, which may be examples of early hot flashers that ignite core helium fusion shortly after leaving the RGB. However, there is not convincing evidence that a large fraction of stars leave the RGB before helium flash. We revisited the helium-sensitive R ratio, and find that M13's ratio is in agreement with theoretical values for primordial helium abundance Y_P = 0.245 and inconsistent with a helium enhancement DY = 0.04. The brightness of the HB (both in comparison to the end of the canonical HB and to the tip of the RGB) also appears to rule out the idea that the envelopes of the reddest HB stars have been significantly enriched in helium. The absolute colors of the turnoffs of M3 and M13 may potentially be used to look for differences in their mean helium abundances.(ABRIDGED)
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We present photometry for all bright red giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB) stars within 10' of the center of M13. | contribution 0.90 | 0.97 | 0.03 | asserted 0.99 |
| s1 | We find support for the idea that the population of HB stars redder than the primary group are noticeably evolved, which resolves a disagreement between distance moduli derived from the tip of the RGB and from stars near the instability strip. | result 0.86 | 0.94 | 0.48 | asserted 0.93 |
| s2 | The sharp cut at the red end of the HB provides strong evidence that stars from the dominant HB group must still be undergoing blue loops, implying that diffusion is being inhibited. | interpretation 0.92 | 0.80 | 0.18 | asserted 0.98 |
| s3 | We argue that M13's HB is a somewhat pathological case - the dominant HB population occurs very near the "knee" in optical CMDs, and evolved stars exclusively appear redward of that peak, leading to the incorrect appearance of a continuation of the unevolved HB. | interpretation 1.00 | 0.66 | 0.21 | asserted 0.95 |
| s4 | M13 has a distinct group of HB stars previously identified with the second U jump, which may be examples of early hot flashers that ignite core helium fusion shortly after leaving the RGB. | result 0.65 | 0.35 | 0.05 | hedged possibility 1.00 |
| s5 | However, there is not convincing evidence that a large fraction of stars leave the RGB before helium flash. | result 0.84 | 0.68 | 0.08 | asserted 0.85 |
| s6 | We revisited the helium-sensitive R ratio, and find that M13's ratio is in agreement with theoretical values for primordial helium abundance Y_P = 0.245 and inconsistent with a helium enhancement DY = 0.04. | result 0.98 | 0.96 | 0.17 | asserted 1.00 |
| s7 | The brightness of the HB (both in comparison to the end of the canonical HB and to the tip of the RGB) also appears to rule out the idea that the envelopes of the reddest HB stars have been significantly enriched in helium. | interpretation 0.65 | 0.74 | 0.09 | hedged possibility 0.71 |
| s8 | The absolute colors of the turnoffs of M3 and M13 may potentially be used to look for differences in their mean helium abundances.(ABRIDGED) | future work 0.95 | 0.19 | 0.06 | hedged possibility 1.00 |
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Original abstract
We present photometry for all bright red giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB) stars within 10' of the center of M13. We find support for the idea that the population of HB stars redder than the primary group are noticeably evolved, which resolves a disagreement between distance moduli derived from the tip of the RGB and from stars near the instability strip. The sharp cut at the red end of the HB provides strong evidence that stars from the dominant HB group must still be undergoing blue loops, implying that diffusion is being inhibited. We argue that M13's HB is a somewhat pathological case - the dominant HB population occurs very near the "knee" in optical CMDs, and evolved stars exclusively appear redward of that peak, leading to the incorrect appearance of a continuation of the unevolved HB. M13 has a distinct group of HB stars previously identified with the second U jump, which may be examples of early hot flashers that ignite core helium fusion shortly after leaving the RGB. However, there is not convincing evidence that a large fraction of stars leave the RGB before helium flash. We revisited the helium-sensitive R ratio, and find that M13's ratio is in agreement with theoretical values for primordial helium abundance Y_P = 0.245 and inconsistent with a helium enhancement DY = 0.04. The brightness of the HB (both in comparison to the end of the canonical HB and to the tip of the RGB) also appears to rule out the idea that the envelopes of the reddest HB stars have been significantly enriched in helium. The absolute colors of the turnoffs of M3 and M13 may potentially be used to look for differences in their mean helium abundances.(ABRIDGED)
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For perspective, [ref] summarize literature data for the well-known anticorrelation between [O/Fe] and [Na/Fe] for stars in 20 clusters, and [ref] present new spectroscopic data for 19 clusters.
Properties of second generation stars in Globular Clusters · The Bimodal Horizontal Branch · 0.89
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