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ABSOLUTE MAGNITUDES of RR LYRAE STARS J. Fernley1, T.G

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ABSOLUTE MAGNITUDES of RR LYRAE STARS J. Fernley1, T.G 635 ABSOLUTE MAGNITUDES OF RR LYRAE STARS 1 2 1 3 3 J. Fernley , T.G. Barnes , I. Skillen , S.L. Hawley , C. Hanley , 4 1 5 D.W. Evans , E. Solano , R. Garrido 1 IUE Observatory,P.O. Box 50727, 28080 Madrid, Spain 2 McDonald Observatory, Univ. of Texas, Austin, Texas 78712, USA 3 Dept. Physics and Astronomy, Michigan State Univ., E. Lansing, Michigan 48824, USA 4 Royal Greenwich Obs., Madingley Rd, Cambridge CB3 OHA, England 5 Inst. Astro sica de Andalucia, Ap do 3004, 18080 Granada, Spain 1. Of the 180 stars listed as RR Lyraes in the Hip- ABSTRACT parcos Input Catalogue we removed 36 stars, either b ecause they were not RR Lyraes or b ecause the data Using rstly, the Hipparcos prop er motions and the were missing or of p o or quality. metho d of Statistical Parallax and secondly, the Hip- parcos parallax of RR Lyrae itself and thirdly, the 2. In order that the V magnitudes were homogeneous Baade-Wesselink results from the literature we nd we used the Hipparcos photometry. For each star the zero-p oint of the RR Lyrae absolute magnitude - the raw V magnitudes were converted to uxes Hip metallicity relation to b e M = 0.720.10 at [Fe/H] v and then phased using the p erio d from the GCVS = 1:52. The small error on this zero-p oint re ects Kholop ov et al. 1985. The p erio d was then opti- the remarkably good agreement between the three mised and the resulting light curve tted to a Fourier indep endent metho ds. Taking a value of 0.180.03 Series. This analysis was done using the program for the slop e of the relation from the literature we ob- PULSAR Skillen 1985. The mean ux was then tain a distance mo dulus of the LMC of 18.31. This is converted back into a magnitude and transformed compared to other recent determinations of the dis- onto the Johnson system using the equations given tance to the LMC. by the Hipparcos pro ject. Comparing these intensity mean magnitudes with those listed by Liu and Janes 1990a shows, for 13 stars in common, a mean di er- ence of 0.003 mag and an rms scatter of 0.007 mag. 1. INTRODUCTION 3. Reddenings were taken from Burstein and Heiles 1982. The de-reddened stars were then used to de- termine p erio d-colour relations which were in turn RR Lyraes are one of the primary distance indicators, used to estimate the reddening for the stars at low b oth within the Galaxy and within the Lo cal Group, galactic latitudes. and in this article we use the recently released Hip- parcos data to estimate their absolute magnitudes. 4. The parallaxes are from Hipparcos. Only one In Section 3 we consider the trigonometric parallaxes star, RR Lyrae itself, has a well-determined paral- and in Section 4 the prop er motions and the metho d lax, 4.380.59 mas. For the remaining stars the par- of statistical parallax. In Section 5 we then take these allaxes are smaller mean value = 0.8 mas and the results and combine them with previous work to de- standard errors larger mean value = 2.6 mas. riveanM ,[Fe/H] calibration for RR Lyraes. Finally v in Section 6 we discuss the distance to the LMC us- 5. The prop er motions are also from Hipparcos. ing this calibration and compare it with other recent In Figure 1 we compare these prop er motions with determinations of the LMC distance mo dulus. We ground-based measurements as given in Layden et al. b egin in Section 2 with a brief discussion of the data. 1996. It can b e seen that overall the agreementis go o d, the main di erence is that the Hipparcos stan- dard errors are lower than the ground-based ones, 2. THE DATA = 2.2 mas/yr compared to 5.6 mas/yr. 6. The radial velo cities are from the literature. The For the purp oses of the present pap er the following radial velo cities typically have a standard error of data were required: intensity mean V magnitudes, 3 km/s which, at the mean distance of the RR Lyraes reddenings, parallaxes, prop er motions, radial velo c- of 1250 p cs = 0.8 mas in parallax, translates into ities and metallicities. These data were taken b oth an error of 0.55 mas/yr. from Hipparcos and previously published work and a full listing will app ear in Fernley et al. 1997a. Here 7. The metallicities are also taken from the literature. we make only a few comments: 636 3. TRIGONOMETRIC PARALLAXES 5. THE ABSOLUTE MAGNITUDE CALIBRATION As discussed in the previous section only one star, If we write: RR Lyrae itself, has a well-determined parallax, M = [Fe=H] + 1 v 4.380.59 mas. With V = 7.76 and EB-V = 0.06 J this gives M = 0.780.29. For the remaining stars v then we are concerned with determining the zero- the parallaxes are to o uncertain to give any useful p oint, , and slop e, , using the Hipparcos results information, either individually or collectively. given in this pap er and previously published work. No Lutz-Kelker correction Lutz & Kelker 1973, Han- Zero-Point : the Baade-Wesselink work on RR son 1979 was applied to the derived absolute magni- Lyraes Fernley 1994 and references therein gives tude of RR Lyrae since the selection criterion was not values for b oth the slop e and zero-p oint. The slop e the parallax there are 13 other RR Lyraes for which is still the sub ject of debate and so to derive a zero- Hipparcos gives a parallax greater than it but the p oint in the least controversial way we have rstly, standard error on the parallax RR Lyrae has = up dated the metallicityvalues in Fernley 1994 and 0.59 mas whereas the remaining stars all have then we have simply taken the mean values of the 0.90 mas. It should b e noted that the correction is in metallicity and magnitude for the 15 stars listed by any case small, from Hanson 1979 we estimate the Fernley that have 1:0 [Fe=H] 2:0 for reasons correction is 0.07 mag in the sense that the derived discussed in that pap er we have excluded SS Leo. magnitude would b e brighter. This gives M = 0.66 0.08 at [Fe/H] = 1:50. v The Baade-Wesselink work is sub ject to systematic errors from several sources and these are estimated as 0:12 mag Fernley et al. 1989, to give a nal error on the Baade-Wesselink zero-p ointof0:14. 4. STATISTICAL PARALLAX Combining the results from the Baade-Wesselink work with those from the Trigonometric Parallax of RR Lyrae M =0:780.29 at [Fe/H] = 1:39 and v the Statistical Parallax solution for the pure halo Using the program describ ed in Hawley et al. 1986 sample M = 0:77 0:17 at a mean metallicity v and the data describ ed in Section 2 we obtained the of [Fe/H] = 1:66 and inversely weighting by the solutions shown in Table 1. It is imp ortant in the square of the error we obtain M = 0:72 0:10 at v Statistical Parallax metho d to isolate a dynamically [Fe/H] = 1:52. The small error on M re ects the v homogeneous sample of stars. In the present context remarkably go o d agreementbetween the three inde- this means separating the Halo and Old Disk com- p endent metho ds used to determine the zero-p oint. p onents and we have done this by making a cut in metallicity. Based on previous work e.g. Layden et Slop e : this is a sub ject of some controversy and al. 1996, Figure 4 it is clear that b elow[Fe/H] = 1:3 has most recently b een discussed by Fernley et al. the stars are almost entirely Halo and ab ove[Fe/H] 1997b. Based on b oth the Baade-Wesselink results = 0:8 they are almost entirely Old Disk. Unfortu- referred to previously and the observations by Fusi nately there are insucient stars with [Fe/H]0:8 Pecci et al. 1996 of globular clusters in M31, they to obtain a useful solution and so we have run in- estimate a slop e of 0.180.03. Adopting this value stead a metal-rich solution which contains all stars we obtain: with [Fe/H] 1:3. This sample is therefore not dy- namically homogeneous in that it contains b oth Halo M =0:18 0:03[Fe=H]+1:52 + 0:72 0:04 2 and Old Disk stars as of course do es the solution for v all stars. 6. DISTANCE TO THE LMC Table 1. Absolute magnitudes from statistical paral laxes. There are observations of RR Lyraes in 5 LMC Clus- ters Walker 1992, Reid & Freedman 1994 and com- Sample No Stars [Fe/H] M v bining the data from the clusters gives a mean dered- dened magnitude m of 18.98 and a mean [Fe/H] of All RR Lyraes 144 -1.32 0.76 0.13 v Halo RR Lyraes 84 -1.66 0.77 0.17 1:8. From Equation 2 we obtain a distance mo du- Metal-Rich RR Lyraes 60 -0.85 0.69 0.21 lus m M of 18.31 and in Table 2 we compare this with other recent determinations.
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