I demonstrate that the two unexpected results in the local Universe:
anomalous intrinsic (V-I)_0 colors of RR Lyrae stars and clump giants in the
Galactic center, and very short distances to Magellanic Clouds inferred from
clump giants, can be at least partially resolved with a modified coefficient of
selective extinction A_V/E(V-I). With this modification, I find a new
clump-giant distance modulus to the Large Magellanic Cloud, mu_{LMC} = 18.27
+/- 0.07, which is 0.09 larger than the Udalski (1998b) result. When distance
estimates from the red clump, RR Lyrae stars and the eclipsing binary HV2274
are combined, one obtains mu_{LMC} = 18.31 +/- 0.04 (internal).Comment: less detailed and more review-style version of ApJ Letter (528, L9),
  to be published in the ASP Conference Series Vol. 203, 1999: ``The Impact of
  Large Scale Surveys on Pulsating Star Research'', L. Szabados and D. Kurtz,
  ed

Popowski, Piotr

English

arXiv

The author demonstrates that the two unexpected results in the local Universe: anomalous intrinsic (V--I){sub 0} colors of RR Lyrae stars and clump giants in the Galactic center, and very short distances to Magellanic Clouds inferred from clump giants, can be at least partially resolved with a modified coefficient of selective extinction A{sub V}/E(V--I). With this modification, the author found a new clump-giant distance modulus to the Large Magellanic Cloud, {mu}{sub LMC} = 18.27 {+-} 0.07, which is 0.09 larger than the Udalski (1998b) result. When distance estimates from the red clump, RR Lyrae stars and the eclipsing binary HV2274 are combined, one obtains {mu}{sub LMC} = 18.31 {+-} 0.04 (internal)

Popowski, P

UNT Digital Library

U.S. Department of Energy Preprint UCRL-JC-137446 The Distance to the Large Magellanic Cloud P. Popowski This article was submitted to The Impact of Large-Scale Surveys on Pulsating Star Research Budapest, Hungary August 8-l 2, 1999 January 31,200O - Approved for public release; further dissemination unlimited DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. This report has been reproduced directly from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information P.O. Box 62, Oak Ridge, TN 37831 Prices available from (423) 576-8401 http://apollo.osti.gov/bridge/ Available to the public from the National Technical Information Service U.S. Department of Commerce 5285 Port Royal Rd., Springfield, VA 22161 http://www.ntis.gov/ OR Lawrence Livermore National Laboratory Technical Information Department’s Digital Library http://www.llnl.gov/tid/Library.html The Impact of Large Scale Surveys on Pulsating Stars Research ASP Conference Series, Vol. 203, 1999 L. Szabados, and D. Kurtz, eds. The Distance to the Large Magellanic Cloud Piotr Popowski Institute of Geophysics and Planetary Physics, L-413 Lawrence Livermore National Laboratory, University of California P. 0. Box 808, Livermore, CA 94551, USA Abstract. I demonstrate that the two unexpected results in the local Universe: anomalous intrinsic (V - 1)s colors of RR Lyrae stars and clump giants in the Galactic center, and very short distances to Mag- ellanic Clouds inferred from clump giants, can be at least partially re- solved with a modified coefficient of selective extinction Av/E(V - I). With this modification, I find a new clump-giant distance modulus to the Large Magellanic Cloud, PLMC = 18.27 f 0.07, which is 0.09 larger than the Udalski (1998b) result. When distance estimates from the red clump, RR Lyrae stars and the eclipsing binary HV2274 are combined, one obtains PLMC = 18.31 f 0.04 (internal). 1. Distance to the LMC - Controversy and New Determinations The Hubble constant, Ho, is one of the most important cosmological parameters. There are two major paths to determine Ho. The more elegant one is non-local and based on observations of the high-redshift Universe. Modeling of gravita- tional lensing of quasars and observations of the Cosmic Microwave Background both belong to this group of methods. The second path goes through a determi- nation of distances and recession velocities of objects with motions dominated by the Hubble linear expansion. The distances, which are harder to measure than recession velocities, are determined based on a distance-ladder approach. The Magellanic Clouds, and especially the Large Magellanic Cloud (LMC), play a major role in this treatment. Almost all of the extragalactic distance scale is known only relative to the LMC (Madore et al. 1999). Therefore, it is abso- lutely essential to establish a reliable distance to the LMC (&MC). This can be achieved only with an understanding of the systematics inherent in the standard candles used for the distance determinations. For many years now there has been a division between the so called “short” and “long” distance scales to the LMC. Currently, the measured values of &MC span a range of over 25% (see e.g., Feast & Catchpole 1997; Stanek, Zaritsky, & Harris 1998). Distances are measured based on a general formula PLMC=~-Mx-AX (1) where X is an apparent magnitude, J,!fx is an absolute magnitude, and Ax is an extinction in the band of the observations. The uncertainties in all ingredients 201 - 202 Popowski present in equation (1) may compromise the final answer. Here I am going to concentrate on three new or revised methods that cluster consistently around a short distance to the LMC: [l] Paczynski & Stanek (1998) pointed out that clump giants should consti- tute an accurate distance indicator. Udalski et al. (1998b) and Stanek et al. (1998) applied the clump method and found a very short distance to the LMC (CL LMCL 18.2). In response, Cole (1998) and Girardi et al. (1998) suggested that clump giants are not standard candles and that their absolute I magnitudes, MI(RC), depend on the metallicity and age of the population. Udalski (1998a, 199813) rejected this criticism by showing that the metallicity dependence is at a low level of about 0.1 mag/dex, and that A.JI(RC) is approximately constant for cluster ages between 2 and 10 Gyr. Recent developments (see e.g., Popowski 2000 for a short review) suggest that the absolute character of &f1(RC) is a major systematic uncertainty in this method. [2] Popowski & Gould (1999) determined the absolute magnitude of RR Lyrae stars, Mv(RR) = 0.71 f 0.07 at [Fe/H] = -1.6, from the statistical parallax, cluster kinematics and trigonometric parallax methods. When this result is cou- pled with the LMC RR Lyrae photometry of Udalski et al. (1999) and Walker (1992), one obtains PLMC w 18.30 f 0.08. The value of A&(RR) remains the main uncertainty of this determination. [3] Guinan et al. (1998) solved the eclipsing binary HV2274 and obtained vari- ous stellar parameters and the distance to the LMC. The spectra used for this purpose did not extend far enough toward long wavelengths, and the B and V photometry was needed to break the degeneracy between the reddening and the shape of the extinction curve. With Udalski et al. (1998c) photometry, Guinan et al. (1998) obtained PLMC = 18.30 f 0.07. Application of Nelson et al. (2000) photometry would result in ,&MC = 18.40 f 0.07. The reddening constitutes a major uncertainty. In two out of three cases the absolute magnitudes of distance indicators are under debate. Due to a huge number of possible environments, it is very hard to prove the standard character of a given candle. However, it should be possible to check whether other stellar characteristics of a candle behave in a predictable fashion. To follow this suggestion, I will concentrate on the stars in the Galactic bulge. 2. The Mystery of Anomalous Colors in the Galactic Bulge Paczynski (1998) tried to explain why the clump giants in the Baade’s Window have (V - 1)s colors which are approximately 0.2 mag redder than in the solar neighborhood. He could not find any satisfactory answer. Stutz, Popowski, & Gould (1999) f ound a corresponding effect for the Baade’s Window RR Lyrae stars, which have (V - 1)s redder by about 0.17 than their local counterparts. The similar size of the color shift in RR Lyrae stars and clump giants suggests common origin. The bulge RR Lyrae stars and clump giants both burn Helium in their cores, but similarities end there. RR Lyrae stars pulsate, clump giants do not. RR Lyrae stars are metal-poor, clump giants are metal-rich. RR Lyrae stars are a part of an axisymmetric stellar halo, whereas clump giants form a Distance to the LMC 203 bar. Stutz et al. (1999) suggested that the very red (V - I)0 of the bulge RR Lyrae stars might have resulted from an unusual abundance of a elements. Why should a clump population which emerged in a different formation process share the same property? The presence of the same type of color anomaly for different types of stars suggests that the effect might be unrelated to the physics of those stars. The investigated RR Lyrae and clump giants share two things in common. First, photometry of both types of stars comes from the OGLE, phase-I, project. In- deed, Paczynski et al. (1999) showed that OGLE-I V-magnitudes are 0.021 mag fainter, and I-magnitudes 0.035 mag brighter than better calibrated OGLE-II magnitudes. Therefore, the correct (V - 1) colors should be 0.056 bluer. Addi- tionally, the new (V - I)0 from the more homogeneous Baade’s Window clump is bluer than Paczynski’s & Stanek’s (1998) color, even when reduced to OGLE-I calibration. As a result, the (V - I)0 anomaly shrinks and the remaining shift amounts to - 0.11 for both the RR Lyrae stars and clump giants. Second, Paczynski (1998) and Stutz et al. (1999) use the same extinction map (Stanek 1996) and the same coefficient of conversion from visual extinction Av to a color excess E(V - 1). The absolute values of the Avs are probably correct, because the zero-point of the extinction map was determined from the (V - K) color, and Av/E(V - K) is very close to 1. However, Rvr = Av/E(V - I) is not as secure, and has a pronounced effect on the obtained color. The value and variation of Rvr was thoroughly investigated by Wozniak & Stanek (1996). The essence of the Wozniak & Stanek (1996) method to determine differential extinction is an assumption that regions of the sky with a lower surface density of stars have higher extinction. This is quite a natural expectation, as far as the density of the underlying true population of stars does not depend on location. However, it is not obvious a priori how to convert a certain density of stars to an amount of visual extinction. Therefore, Wozniak & Stanek (1996) used clump giants to calibrate their extinction. To make a calibration procedure completely unbiased would require, among other things, that the V-magnitudes of clump giants do not depend on their color [here (V - I)s], that reddened and unreddened clump giants be drawn from the same parent population, and that clump giants were selected without any assumption about Rvr. None of those is true (for details see Popowski 2000). Because the smaller selective extinction coefficient is not excluded by the current studies, I will assume Rvl = 2.1 to match the (V - I)0 colors of the bulge with the ones in the solar neighborhood. The color is a weak function of [Fe/H], so this procedure is justified because the metallicities of the bulge and solar neighborhood are similar. The change in RVI from 2.5 to 2.1 will decrease the I-mag extinction by 0.11 mag, and increase the clump-based distance to the Galactic center by the same amount. 3. Recalibration of Clump Giant Stars How do the bulge results bear on the distance to the LMC? The better pho- tometry from Paczynski et al. (1999) and the modification of Rvr influence the relative RR Lyrae and clump distances to the Galactic center. Thus, the MI(RC) - [Fe/H] relation for clump giants used in the LMC, which was calibrated with 204 Popowski respect to the baseline provided by RR Lyrae stars (Udalski 1998a), changes. Assuming linearity of Ml(RC) - [Fe/H] an making some small adjustments to d [Fe/H] used by Udalski (1998a), I find: MI(RC) = -0.23 + O.lS[Fe/H], (2) with a slope 0.10 mag/dex steeper than the original result. Such adjustment increases the best clump giant estimate from PLMC = 18.18 f 0.06 to PLMC = 18.27 f 0.07. When distance estimates from the red clump, RR Lyrae stars and eclipsing binary HV2274 are combined, one obtains /&MC = 18.31f0.04. The la uncertainty of this determination is only a formal error. The systematic errors are likely to dominate the true uncertainty. However, if the methods presented do not suffer from severe biases, then a distance to the LMC as long as 52-55 kpc (PLMC NN 18.6-18.7) is highly disfavored by the current results. References Cole, A. A. 1998, ApJ, 500, L137 Feast, M. W. & Catchpole, R. M. 1997, MNRAS, 286, Ll Girardi, L., Groenewegen, M. A. T., Weiss, A., & Salaris, M. 1998, MNRAS, 301, 149 Guinan, E. F., Fitzpatrick, E. L., Dewarf, L. E., et al. 1998, ApJ, 509, L21 Madore, B. F., Freedman, W. L., Silbermann, N., et al. 1999, ApJ, 515, 29 Nelson, C. A., Cook, K. H., Popowski, P., & Alves, D. A. 2000, AJ, to appear in March 2000 issue Castro-ph/9910172] Paczynski, B. 1998, Acta Astron., 48, 405 Paczynski, B. & Stanek, K. Z. 1998, ApJ, 494, L219 Paczynski, B., Udalski, A.,.Szymanski, M., Kubiak, M., Pietrzynski, G., Soszynski, I., Wozniak, P., & Zebrun, K., 1999, Acta Astron., 49, 319 Popowski, P. 2000, ApJ, 528, L9 Popowski, P. & Gould, A. 1999, in Post-Hipparcos Cosmic Candles, ed. A. Heck & F. Caputo (Dordrecht: Kluwer), 53 [astro-ph/9808006] Stanek, K. Z. 1996, ApJ, 460, L37 Stanek, K. Z., Zaritsky, D., & Harris, J. 1998, ApJ, 500, L141 Stutz, A., Popowski, P., & Gould, A. 1999, ApJ, 521, 206 Udalski, A. 1998a, Acta Astron., 48, 113 Udalski, A. 1998b, Acta Astron., 48, 383 Udalski, A., Pietrzynski, G., Wozniak, P., Szymanski, M., Kubiak, M., & Zebrun, K. 1998a, ApJ, 509, L25 Udalski, A., Szymanski, M., Kubiak, M., Pietrzynski, G., Wozniak, P., & Zebrun, K. 1998b, Acta Astron., 48, 1 Udalski, A., Szymanski, M., Kubiak, M., Pietrzynski, G., Soszynski, I., Wozniak, P., & Zebrun, K. 1999, Acta Astron., 49, 201 [astro-ph/9907236] Walker, A. R. 1992, ApJ, 390, L81 Wozniak, P. & Stanek, K. Z. 1996, ApJ, 464, 233 Distance to the LMC 205 Discussion Darragh O’Donoghue: Your presentation is a little more than a party political propaganda talk. It will help now if you were to investigate why advocates of the long distance scale such as Feast & Catchpole (1997) are wrong. Piotr Popowski: It is enough if I prove that I am right, and not that they are wrong. We have three reliable methods which consistently cluster around a short distance to the LMC of about 45 kpc, and there is another potentially reliable method or standard candle (Cepheids) which gives a different result. I think that we should investigate what is going on with Cepheids (it is the best if this is done by Cepheid people). David Laney: The statistical arguments by Feast & Catchpole have been demon- strated to be correct by Koen & Laney and, using Monte Carlo simulations, by Pont. The error bar from the Monte Carlo simulations is closer to 0.15 than 0.1, however. Piotr Popowski: This point has been raised by someone in the audience. No disagreement here, so I made no comment about it. Giuseppe Bono: Two comments: 1. In a recent investigation by Romaniello et al. (1999) based on HST data of red clump stars, they found a distance modulus for LMC which seems to support the long distance scale. 2. Evolutionary models suggest that the RR Lyrae luminosity decreases with increasing metallicity. G&a ltiovcics: When you talk about distance of the LMC based on RR Lyrae stars you should also consider RRd stars. Applying them as distance indicators for the LMC yields M 18.5 mag for the distance modulus (Kovacs & Walker 1998). This is in agreement with the Cepheid B-W distance scale. Piotr Popowski: The preliminary results of reanalysis of RRd stars from the MACHO group (David Alves and I are involved in this work) indicate that RRd stars’ distance to the LMC is short as well. *This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. 

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