Hostname: page-component-cd9895bd7-gvvz8 Total loading time: 0 Render date: 2024-12-25T15:36:06.214Z Has data issue: false hasContentIssue false
  • English
  • Français

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

Published online by Cambridge University Press:  12 April 2016

Allan Sandage
Allan Sandage*
Affiliation:
Mount Wilson and Las Campanas Observatories of the Carnegie Institution of Washington

Abstract

Core share and HTML view are not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

It 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.

Type
2. Morphology and History of the Galaxy
Information
International Astronomical Union Colloquium , Volume 111: The Use of Pulsating Stars in Fundamental Problems of Astronomy , 1989 , pp. 121 - 140
Copyright
Copyright © Cambridge University Press 1989

References

Barnes, T.G. & Hawley, S.L. (1986). Ap.J., 307, L9.CrossRefGoogle Scholar
Buonanno, R., Corsi, C.E. & Fusi Pecci, F. (1988). Astron. & Astrophys., submitted, (ESO preprint 594).Google Scholar
Butler, D., Dickens, R.J. & Epps, E. (1978). Ap.J., 225, 148.Google Scholar
Cacciari, C., Clementini, G. & Buser, R. (1988). STScI preprint 268.Google Scholar
Cameron, L.M. (1985). Astron. & Astrophys., 146, 59.Google Scholar
Ciardullo, R.B. & Demarque, P. (1977). Yale Trans., 33.Google Scholar
Clement, CM., Nemec, J.M., Wells, N.R., Wells, T., Dickens, R.J. & Bingham, E.A. (1986). A.J., 92, 825.Google Scholar
Cox, A.N. (1987). In Second Conference on Faint Blue Stars,IAU Colloquium No.95, eds. Philip, A.G. Davis, Hayes, D.S. & Liebert, J.W., p. 161. Schenectady, NY: L. Davis Press.Google Scholar
Cox, A.N., Hodson, S.W. & Clancy, S.P. (1983). Ap.J., 266, 94.Google Scholar
Eggen, O.J., Lynden-Bell, D. & Sandage, A. (1962). Ap.J., 136, 748.Google Scholar
Eggen, O.J. & Sandage, A. (1962). Ap.J., 136, 735.CrossRefGoogle Scholar
Gautschy, A. (1987). Vistas in Astronomy, 30, 197.Google Scholar
Gratton, R.G., Tornambé, A. & Ortolani, S. (1986). Astron. & Astrophys., 169, 111.Google Scholar
Hawley, S.L., Jeffreys, W.H., Barnes, T.G. & Lai, W. (1986). Ap.J., 302, 626.CrossRefGoogle Scholar
Iben, I. (1971). P.A.S.P., 83, 697.Google Scholar
Iben, I. & Rood, R.T. (1970). Ap.J., 159, 605.CrossRefGoogle Scholar
Jameson, R.F., Fernley, J.A. & Longmore, A.J. (1987). M.N.R.A.S., in press.Google Scholar
Jones, R.V., Carney, B.W. & Latham, D.W. (1988). Ap.J., 332, 206.CrossRefGoogle Scholar
Liu, T. & Janes, K.A. (1988). In Calibrating Stellar Ages, IAU Colloquium, ed. Philip, A.G. Davis. Schenctady, NY: L. Davis Press.Google Scholar
Lub, J. (1977). Thesis, University of Leiden.Google Scholar
Lub, J. (1987). In Stellar Pulsation, Lecture Notes in Physics, No. 274, eds. Cox, A.N., Sparks, W.M. & Starrfield, S.G., p. 218. Berlin: Springer-Verlag.Google Scholar
Noble, R. & Dickens, R.J. (1988). In New Ideas in Astronomy, ed. Bertola, F., Sulentic, J.W. & Madore, B.F., p. 59. Cambridge: Cambridge University Press.Google Scholar
Peterson, J.O. (1978). Astron. & Astrophys., 62, 205.Google Scholar
Peterson, J.O. (1979). Astron. & Astrophys., 80, 53.Google Scholar
Sandage, A. (1970). Ap.J., 162, 841.Google Scholar
Sandage, A. (1987). In Second Conference on Faint Blue Stars, IAU Colloq. 95, eds. Philip, A.G. Davis, Hayes, D.S. & Liebert, J.W., p. 41. Schenectady, NY: L. Davis Press.Google Scholar
Sandage, A. (1989a). Ap.J., in press.Google Scholar
Sandage, A. (1989b). Ap.J., in press.Google Scholar
Sandage, A. & Cacciari, C. (1989). Ap.J., in press.Google Scholar
Sandage, A. & Eggen, O.J. (1959). M.N.R.A.S., 119, 278.Google Scholar
Sandage, A., Katem, B.N. & Sandage, M. (1981). Ap.J. Suppl., 46, 41.Google Scholar
Simoda, J. & Iben, I. (1968). Ap.J., 152, 509.Google Scholar
Simoda, J. & Iben, I. (1970). Ap.J. Suppl., 22, 81.Google Scholar
Sneden, C. (1985). In Production and Distribution of C,N,O Elements, eds. Danziger, I.J., Matteucci, F. & Kjar, K., p. 1. Garching: ESO.Google Scholar
Stellingwerf, R.F. (1975). Ap.J., 199, 705.Google Scholar
Sweigart, A. V. & Gross, P. G. (1976). Ap.J. Suppl., 32, 367.Google Scholar
Sweigart, A.V., Renzini, A. & Tornambé, A. (1987). Ap.J., 312, 762.Google Scholar
van Albada, T.S. & Baker, N. (1971). Ap.J., 169, 311.CrossRefGoogle Scholar
VandenBerg, D.A. (1983). Ap.J. Suppl., 51, 29.Google Scholar
VandenBerg, D.A. (1987). Colloquium at Space Science Science Institute, Baltimore.Google Scholar
VandenBerg, D.A. & Bell, R.A. (1985). Ap.J. Suppl., 58, 561.Google Scholar
VandenBerg, D.A. & Bridges, T. (1984). Ap.J, 278, 679.Google Scholar