Turnoffs and Ages of Globular Clusters

The way to arrive at (even relative) ages for globular clusters involves the determination of their (relative) distances. We would like to see a theory which would fit the absolute magnitudes of RR Lyrae stars as determined from observations (Sandage effect). We have examined a sample of 17 CM diagrams of galactic globular clusters, 11 of which were observed at ESO and reduced with the program, ROMAFOT and 6 of which were taken from the literature. In Fig. 1 the difference in bolometric magnitude between the turnoff point and the location of the zero-age horizontal branch (ZAHB), δV RRTO (bo1) is plotted versus [Fe/H]. It turns out that δV RRTO ≃ δ RRTO (bo1) + 0.1 = 3.56 ±0.15. We are faced with the problem of determining how the horizontal branch scales with metallicity in order to understand the constant value of 3.56 in this relation.

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The Luminosities of Horizontal Branches and RR Lyrae Stars in Globular Clusters

International Astronomical Union Colloquium ◽

10.1017/s025292110001126x ◽

2004 ◽

Vol 193 ◽

pp. 525-529 ◽

Cited By ~ 5

Author(s):

D.H. McNamara ◽

M.B. Rose ◽

P.J. Brown ◽

D.I. Ketcheson ◽

J.E. Maxwell ◽

...

Keyword(s):

Globular Clusters ◽

Absolute Magnitude ◽

Apparent Magnitude ◽

Rr Lyrae Stars ◽

Blue Stragglers ◽

Rr Lyrae ◽

The Absolute ◽

The Difference ◽

Absolute Magnitudes ◽

Lyrae Stars

AbstractWe have utilized the latest stellar models of the Y2 (Yonsei-Yale) collaborators and color-magnitude diagrams of globular clusters to infer ages and absolute magnitudes of their horizontal branches (HB). The intrinsic (B – V) color indices of the turn-offs, of the globular clusters were used to find ages. For 47 clusters that appear to be coeval (within ±0.7 Gyr), we find an average age of 12.5 Gyr. We adopt this age and infer the absolute magnitudes of the turn-offs, from the clusters [Fe/H] values. The absolute magnitude of the horizontal branches or RR Lyrae stars are then determined from the difference between the apparent magnitudes of the horizontal branches (or RR Lyrae stars) and the apparent magnitude of the turn-offs, VTO. We conclude: 1) The slope of the MV(HB), [Fe/H] relation is ~0.3 for clusters with [Fe/H] values between —0.5 to —1.5. The relation has zero slope for [Fe/H] values smaller than −1.5. 2) For [Fe/H] < -1.3, the MV(HB) or MV values of RRLyrae stars are not only a function of [Fe/H], but the horizontal-branch type in the sense that the clusters with the blue horizontal branches have more luminous horizontal branches than clusters with red horizontal branches. The same results are found by inferring the luminosities of the HBs from pulsating blue stragglers.

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The vertical height of the horizontal branch - The range in the absolute magnitudes of RR Lyrae stars in a given globular cluster

The Astrophysical Journal ◽

10.1086/168415 ◽

1990 ◽

Vol 350 ◽

pp. 603 ◽

Cited By ~ 58

Author(s):

Allan Sandage

Keyword(s):

Globular Cluster ◽

Horizontal Branch ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

The Absolute ◽

Absolute Magnitudes ◽

Lyrae Stars ◽

Vertical Height

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The Absolute Magnitudes of the RR Lyrae Stars

Highlights of Astronomy ◽

10.1007/978-94-010-3102-8_86 ◽

1971 ◽

pp. 771-776 ◽

Cited By ~ 1

Author(s):

Sir Richard Woolley

Keyword(s):

Rr Lyrae Stars ◽

Rr Lyrae ◽

The Absolute ◽

Absolute Magnitudes ◽

Lyrae Stars

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On the Two Oosterhoff Groups of Globular Clusters

International Astronomical Union Colloquium ◽

10.1017/s0252921100033522 ◽

1973 ◽

Vol 21 ◽

pp. 196-196

Author(s):

T. S. Van Albada ◽

Norman Baker

Keyword(s):

Stellar Evolution ◽

Globular Clusters ◽

Transition Point ◽

Transition Period ◽

Observational Evidence ◽

Horizontal Branch ◽

Average Difference ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

Lyrae Stars

AbstractThe observational evidence leading to the classification, following Oosterhoff, of globular clusters containing RR Lyrae stars into two distinct groups, is summarized and discussed in the light of results of stellar evolution theory and pulsation theory. The dichotomy is caused, at least in part, by a dichotomy in the ‘transition period’ between the type-ab and type-c stars which reflects a difference in effective temperature at the transition point. When this difference is accounted for, there remains a smaller average difference between the groups, though no longer a clear dichotomy, that is probably a mass and luminosity effect. If this remaining difference is interpreted as a luminosity effect the average difference in luminosity between the two Oosterhoff groups is at most 0.1 mag. It is suggested that Christy’s theoretical relationship between transition period and luminosity cannot be valid, at least not for clusters of different Oosterhoff groups. It is conjectured that the transition-temperature dichotomy may be a reflection of different predominant directions of evolution along the horizontal branch, accompanied by a hysteresis effect in the pulsations.

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Search for Variables in the Galactic Globular Clusters NGC 6544 and NGC 6642

International Astronomical Union Colloquium ◽

10.1017/s0252921100117774 ◽

1993 ◽

Vol 139 ◽

pp. 337-337

Author(s):

Martha L. Hazen

Keyword(s):

Globular Clusters ◽

Slow Variable ◽

Variable Stars ◽

Horizontal Branch ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

A Value ◽

Short Period ◽

Period Variable ◽

Lyrae Stars

A search for variable stars in the globular cluster NGC 6544 has revealed only one possible short period variable within the tidal radius of the cluster. A search in NGC 6642 yielded 16 new RR Lyrae stars within the tidal radius and 5 new field RRs. The previously discovered (Hoffleit 1972) V1 is a slow variable, and V2 is an RR Lyrae star. Photometry of the variables within the tidal radius gives a mean B for the horizontal branch of < B > = 17.0 mag. With E(B – V) = 0.37 mag and (B – V) = 0.35 mag for RR Lyraes, a value for V(HB) = 16.3 mag is derived. This is about one mag fainter than previous estimates (Webbink 1985), and places NGC 6642 at a distance of approximately 7.9 kpc.

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