RR Lyrae Stars and Type Ia Supernovae: Discovery and Calibration of Astronomical Standard Candles

Abstract

Accurate apparent and intrinsic luminosities of objects are very important in astronomy, for without them we cannot calculate accurate distances or scale the energy budget of physical models. This dissertation deals with observations of two of the most widely used astronomical standard (or standardizable) candles. The Sloan Digital Sky Survey (SDSS) is an important survey presently being carried out, which will provide photometry of roughly 10 objects over a 10,000 square degree area of sky (D. G. York et al. 2000, AJ, 120, 1579). One million spectra of galaxies will be obtained, and many high-redshift quasars will be discovered (see, e.g., X. Fan et al. 1999, AJ, 118, 1). Many new candidate variable stars in our Galaxy will also be identified, amongst them RR Lyrae stars (roughly one per square degree). Distances and velocities of a large number of such objects will allow us to refine models of the Galaxy’s halo and to measure the Galaxy’s mass. We observed a number of types of objects of interest in SDSS filters (RR Lyrae stars, Cepheids, cataclysmic variables, carbon stars, and asteroids) and showed to what extent these objects can be identified solely on the basis of their SDSS colors. Details can be found in a paper already published in this journal (K. Krisciunas, B. Margon, & P. Szkody 1998, PASP, 110, 1342). In particular, Figure 12 of that paper shows the color locus of bona fide RR Lyrae stars. We have observed one dozen RR Lyrae candidates identified from SDSS commissioning data and proved beyond a shadow of a doubt that all are RR Lyrae stars. Light curves of six candidates identified by B. Willman and S. Anderson are to be found in Z. Ivezic et al. (2000, AJ, 120, 963). Five of the other confirmed RR Lyrae stars are objects 5, 9, 134, 141, and 144 from Table 2 of Ivezic et al. Figure 1 shows the folded light curve of star 144. Owing to their luminosities ( at maximum), Type M ≈ 19.4 V Ia supernovae (SNe) are the most important standardizable candle used in extragalactic astronomy because they can be seen halfway across the visible universe. We use the word “standardizable” because, unlike RR Lyrae stars of identical metallicity, Type Ia SNe do not all have the same intrinsic luminosity. Those which rise faster and subsequently decline faster are intrinsically fainter at maximum compared to those which rise more slowly and decline more slowly. See M. M. Phillips et al. (1999, AJ, 118, 1766). As to an explosion mechanism, the favorite scenario is the deflagration of a carbonoxygen white dwarf which reaches the Chandrasekhar limit due to mass infall from a companion star (M. Livio 2000, preprint, astro-ph/0005344). Deflagration is a type of rapid, but subsonic, combustion in which the flame front is driven by turbulent heat exchange. It is also possible that some Type Ia SNe are due to a sub–Chandrasekhar-mass detonation of an accreted surface helium layer on a C-O white dwarf (P. Pinto, R. G. Eastman, & T. Rogers 2000, preprint, astro-ph/0008330). One of the potentially significant sources of error in determining the distance to a Type Ia SN is the contribution of extinction by dust in the host galaxy. What we need is a “wellbehaved” color index with the largest possible wavelength baseline. Since V) for standard Galactic dust, the A ≈ 3.1 # E(B V uncertainties in the color excess and the scale factor can lead to significant uncertainties in the extinction correction. J. H. Elias et al. (1985, ApJ, 296, 379) already showed that Type Ia SNe exhibit some degree of uniform V K color evolution, while B H photometry such as that of B. Leibundgut (1988, Ph.D. dissertation, Univ. Basel) is not as well behaved. Since K), the potential for A ≈ 1.21 # E(V H) ≈ 1.13 # E(V V smaller uncertainties for using V minus near-infrared colors AV was encouraging. New optical and near-infrared (JHK) photometry of Type Ia SNe, especially data obtained before B-band maximum, was needed. Y. D. Mayya et al. (1998, IAU Circ. 6907), S. Jha et al. (1999, ApJS, 125, 73), and P. Meikle (2000, MNRAS, 314, 782) have published data for the spectroscopically normal SN 1998bu, the distance of the host of which has also been measured with HST using Cepheids. We have obtained optical and infrared data for the spectroscopically normal Type Ia SNe 1999cl, 1999cp, 2000bk, and 2000ce and for the overluminous, slowly declining, spectroscopically peculiar SNe 1999aa, 1999gp, and 2000cx. Data for three of these have already been published (K. Krisciunas et al. 2000, ApJ, 539, 658).