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

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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Turnoffs and Ages of Globular Clusters

Symposium - International Astronomical Union ◽

10.1017/s0074180900043412 ◽

1988 ◽

Vol 126 ◽

pp. 635-636

Author(s):

F. Buonanno ◽

C. E. Corsi ◽

F. Fusi Pecci

Keyword(s):

Globular Clusters ◽

Horizontal Branch ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

The Absolute ◽

The Difference ◽

Absolute Magnitudes ◽

Lyrae Stars ◽

The Way

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 Absolute Magnitudes of RR Lyrae Stars and the Age of the Galaxy

International Astronomical Union Colloquium ◽

10.1017/s0252921100011489 ◽

1989 ◽

Vol 111 ◽

pp. 121-140

Author(s):

Allan Sandage

Keyword(s):

Globular Cluster ◽

Globular Clusters ◽

Main Sequence ◽

Absolute Magnitude ◽

Cluster System ◽

Apparent Magnitude ◽

Rr Lyrae ◽

Globular Cluster System ◽

The Absolute ◽

Absolute Magnitudes

AbstractIt is shown that the intrinsic spread in the absolute magnitudes of the RR Lyrae variables in a given globular cluster can reach 0.5 magnitudes at a given period or at a given color, due to luminosity evolution away from the zero age horizontal (ZAHB). The size of this intrinsic luminosity spread is largest in clusters of the highest metallicity.The absolute magnitude of the ZAHB itself also differs from cluster to cluster as a function of metallicity, being brightest in clusters of the lowest metallicity. Three independent methods of calibrating the ZAHB RR Lyrae luminosities each show a strong variation of MV(RR) with [Fe/H]. The pulsation equation of P<ρ>0.5 = Q(M,Te, L) used with the observed periods, temperatures, and masses of field and of cluster RR Lyraes gives the very steep luminosity-metallicity dependence of dMv(RR)/d[Fe/H] = 0.42. Main sequence fitting of the color-magnitude diagrams of clusters which have modern main-sequence photometry gives a confirming steep slope of 0.39. A summary of Baade-Wesselink MV(RR) values for field stars determined in four independent recent studies also shows a luminosity-metallicity dependence, but less steep with a slope of dMV(RR)/d[Fe/H] = 0.21.Observations show that the magnitude difference between the main sequence turn-off point and the ZAHB in a number of well observed globular clusters is independent of [Fe/H], and has a stable value of dV = 3.54 with a disperion of only 0.1 magnitudes. Using this fact, the absolute magnitude of the main sequence turn-off is determined in any given globular cluster from the observed apparent magnitude of the ZAHB by adopting any particular MV(RR) = f([Fe/H]) calibration.Ages of the clusters are shown to vary with [Fe/H] by amounts that depend upon the slopes of the MV(RR) = f([Fe/H]) calibrations. The calibrations show that there would be a steep dependence of the age on [Fe/H] if MV(RR) does not depend on [Fe/H]. No dependence of age on metallicity exists if the RR Lyrae luminosities depend on [Fe/H] as dMV(RR)/d[Fe/H] = 0.37. If Oxygen is not enhanced as [Fe/H] decreases, the absolute average age of the globular cluster system is 16 Gyr, independent of [Fe/H], using the steep MV(RR)/[Fe/H] calibration that is favored. If Oxygen is enhanced by [O/Fe] = – 0.14 [Fe/H] + 0.40 for [Fe/H] < –1.0, as suggested from the observations of field subdwarfs, then the age of the globular cluster system decreases to 13 Gyr, again independent of [Fe/H], if the RR Lyrae ZAHB luminosities have a metallicity dependence of dMV(RR)/d[Fe/H] = 0.37.

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The Luminosity-Metallicity Relation for RR Lyrae Stars and its Implications for the Astronomical Distance Scale

International Astronomical Union Colloquium ◽

10.1017/s0252921100011635 ◽

1989 ◽

Vol 111 ◽

pp. 255-255

Author(s):

G. Clementini ◽

C. Cacciari

Keyword(s):

Globular Clusters ◽

Galactic Center ◽

Absolute Magnitude ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

Visual Magnitude ◽

Phase Range ◽

The Absolute ◽

Lyrae Stars ◽

Magnitude Difference

AbstractThe surface brightness version of the Baade-Wesselink method, has been applied to 7 field RR Lyrae stars with metallicity ranging from [Fe/H]= −0.2 to −1.5. V magnitudes, V-R and V-I colors and CORAVEL radial velocities were used, and the analysis was performed over a restricted phase range in order to avoid the complications caused by the pulsating atmospheres. The combination with previous results of the B-W method, which used comparable criteria (Jones, Carney, & Latham, 1988, preprint; Jameson, Fernley, & Longmore 1987, in press M.N.R.A.S; Cohen & Gordon 1987, Ap.J., 318, 215) leads to the following relation between the absolute luminosity and metallicity:Mv = (1.0 ± 0.05) + (0.17 ± 0.05) [Fe/H]This relation is in very good agreement with the preliminary results found by Liu and Janes (this volume). The application of the above relation to the RR Lyraes in M31 and the Magellanic Clouds leads to distance moduli of (m–M)o= 24.21 ± 0.20 for M31, (m–M)o = 18.26 ± 0.20 for the LMC, and (m–M)o =18.85 ± 0.20 for the SMC, and the distance to the galactic center turns out to be approximately 7.2 kpc. From the absolute magnitude of the RR Lyraes and adopting a constant visual magnitude difference between the RR Lyraes and the turn-off ΔV = 3.55 (Buonanno 1986, Mem.S.A.It., 57, 333), we estimate ages of 18.8 and 15.7 Gyr for globular clusters of metallicity [Fe/H] = −2.2 (e.g. M92) and [Fe/H] = −0.8 (e.g. 47 Tuc) respectively, using the age-turnoff luminosity relation derived by Sandage (1982, Ap.J., 252, 553) or 20.9 and 16.9 Gyr using Buonanno’s relation.

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