RR LYRAE VARIABLE STARS: PULSATIONAL CONSTRAINTS RELEVANT TO THE OOSTERHOFF CONTROVERSY

A solution to the old Oosterhoff controversy is proposed on the basis of a new theoretical pulsational scenario concerning RR Lyrae cluster variables (Bono and coworkers). We show that the observed constancy of the lowest pulsation period in both Oosterhoff type I (OoI) and Oosterhoff type II (OoII) prototypes (M3, M15) can be easily reproduced only by assuming the canonical evolutionary horizontal-branch luminosity levels of these Galactic globular clusters and therefore by rejecting the Sandage period shift effect (SPSE).Comment: postscript file of 7 pages and 2 figures; one non postcript figure is available upon request; for any problem please write to [email protected]

Is There a Difference in Luminosity between Field and Cluster RR Lyrae Variables?

Recent Hipparcos results have lent support to the idea that RR Lyrae variables in the halo field and in globular clusters differ in luminosity by ~0.2mag. In this Letter, we study the pulsation properties of RR Lyraes in clusters with distances determined via main-sequence fitting to Hipparcos parallaxes for field subdwarfs, and compare them with the properties of field variables also analyzed with Hipparcos. We show that the period-temperature distributions for field and cluster variables are essentially indistinguishable, thus suggesting that there is no significant difference in luminosity between them.Comment: 11 pages, including three embedded figures and one table. ApJ (Letters), in pres

RR Lyrae Stars in the Bootes dSph

We present a catalog of 15 RR Lyrae variable stars in the recently discovered Bootes galaxy -- the most metal-poor simple stellar population with measured RR Lyrae stars. The pulsational properties of the RR Lyrae conform closely to period-abundance trends extrapolated from more metal-rich populations and we estimate the distance of Bootes to be (m-M)_0=18.96+-0.12. The average period (0.69 days), the ratio of type c to type ab pulsators (0.53) and the RRab period shift (-0.07) indicate an Oosterhoff II classification for Bootes, a marked contrast to the other dSph galaxies, which are Oosterhoff intermediate. This supports the contention that the Oosterhoff dichotomy is a continuum -- that RR Lyrae properties, to first order, vary smoothly with abundance. The dSph galaxies are not distinct from the Galactic globular clusters, but bridge the Oosterhoff gap. The absence of any anomalous Cepheids in Bootes could indicate the lack of an intermediate age population.Comment: 16 pages, submitted to Astrophysical Journal Letter

Structure of S. aureus HPPK and discovery of a new inhibitor

The first structural and biophysical data on the folate pathway enzyme and drug target, 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), from the pathogenic bacterium Staphylococcus aureus is presented. HPPK is the second essential enzyme in the folate biosynthesis pathway, responsible for catalysing pyrophosphoryl transfer from cofactor (ATP) to the substrate (6-hydroxymethyl- 7,8-dihydropterin, HMDP). In-silico screening led to the discovery of a substrate competitive inhibitor, San1, which was subsequently co-crystallised with HPPK. A 1.65 Å resolution x-ray structure showed this to bind at the pterin site sharing many of the key intermolecular interactions of the substrate. ITC and SPR measurements yielded an equilibrium binding constant, Kd, of ~13 μM for San1. An IC50 of ~12 μM was determined by means of a new convenient tri-enzyme-coupled spectrophotometric assay. ITC and SPR further showed that the San1 inhibitor has no requirement for magnesium or ATP cofactor for competitive binding to the substrate site. According to 15N heteronuclear NMR measurements, the fast motion of the pterin loop (L2) is partially dampened in the ternary complex between SaHPPK, HMDP and , -methylene adenosine 5-triphosphate (AMPCPP), but the ATP loop (L3) remains mobile on the μs timescale. In contrast, for the SaHPPK/San1/AMPCPP ternary complex, loop L2 becomes rigid on the fast timescale and loop L3 becomes more ordered which are supported by a large entropic penalty associated with San1 binding as revealed by ITC. Backbone assignments and chemical shift perturbations implicate the sulphur in San1 as a likely important loop L2/L3 stabilizing mediato

CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes

The variable star population of the galactic globular cluster NGC 1851 (C0512-400) has been studied by CCD photometry, from observations made in the B, V, and I bands during 1993-4. Light curves are presented for 29 variables, seven of which are new discoveries. The behavior of the RR lyraes in the period-temperature diagram appears normal when compared to clusters which bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated, with no compelling evidence to change from accepted values. Photometry for stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02 mag in all colors with extensive earlier photometry, to at least V = 18.5. Instability strip boundary positions for several clusters shows a trend for the red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.

The RR Lyrae Distance Scale

We review seven methods of measuring the absolute magnitude M_V of RR Lyrae stars in light of the Hipparcos mission and other recent developments. We focus on identifying possible systematic errors and rank the methods by relative immunity to such errors. For the three most robust methods, statistical parallax, trigonometric parallax, and cluster kinematics, we find M_V (at [Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and theoretical models both yield a broad range of possible values (0.45-0.70 and 0.45-0.65) due to systematic uncertainties in the temperature scale and input physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but this may be due to a difference in the metallicity scales of the cluster giants and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67 +/- 0.13 and is potentially very robust, but at present is too new to be fully tested for systematics. If the three most robust methods are combined with Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at [Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21 pages including 1 table; uses Kluwer's crckapb.sty LaTeX style file, enclose

The slope of the RR Lyrae MV-[Fe/H] relation

We review the available observational data to show that the slope of the RR Lyrae MV–[Fe/H] relation is 0.18¹0.03. The recent claim by Feast that, because of biases, the true slope is much steeper is not justified

CU Comae: a new field double-mode RR Lyrae, the most metal poor discovered to date

We report the discovery of a new double-mode RR Lyrae variable (RRd) in the field of our Galaxy: CU Comae. CU Comae is the sixth such RRd identified to date and is the most metal-poor RRd ever detected. Based on BVI CCD photometry spanning eleven years of observations, we find that CU Comae has periods P0=0.5441641 +/-0.0000049d and P1=0.4057605 +/-0.0000018d. The amplitude of the primary (first-overtone) period of CU Comae is about twice the amplitude of the secondary (fundamental) period. The combination of the fundamental period of pulsation P0 and the period ratio of P1/P0=0.7457 places the variable on the metal-poor side of the Petersen diagram, in the region occupied by M68 and M15 RRd's. A mass of 0.83 solar masses is estimated for CU Comae using an updated theoretical calibration of the Petersen diagram. High resolution spectroscopy (R=30,000) covering the full pulsation cycle of CU Comae was obtained with the 2.7 m telescope of the Mc Donald Observatory, and has been used to build up the radial velocity curve of the variable. Abundance analysis done on the four spectra taken near minimum light (phase: 0.54 -- 0.71) confirms the metal poor nature of CU Comae, for which we derive [Fe/H]=-2.38 +/-0.20. This value places this new RRd at the extreme metal-poor edge of the metallicity distribution of the RR Lyrae variables in our Galaxy.Comment: 21 pages including 8 Tables, Latex, 11 Figures. Accepted for publication in The Astronomical Journal, October 2000 issu

Distances, ages, and epoch of formation of globular clusters

We review the results on distances and absolute ages of galactic globular clusters (GCs) obtained after the release of the Hipparcos catalogue. Several methods for the Population II local distance scale are discussed, exploiting NEW RESULTS for RR Lyraes in the Large Magellanic Cloud (LMC). We find that the so-called Short and Long Distance Scales may be reconciled whether a consistent reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC. Distances and ages for the 9 clusters discussed in Paper I are re-derived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (Delta p/p < 0.12) in the whole Hipparcos catalogue. On average, our revised distance moduli are decreased by 0.04 mag with respect to Paper I. The corresponding age of the GCs is t=11.5+-2.6 Gyr (95% confidence range). The relation between Mv(ZAHB) and metallicity for the nine programme clusters turns out to be Mv(ZAHB)=(0.18+-0.09)([Fe/H]+1.5)+(0.53+-0.12).Thanks to Hipparcos the major contribution to the total error budget associated with the subdwarf fitting technique has been moved from parallaxes to photometric calibrations, reddening and metallicity scale. This total uncertainty still amounts to about +-0.12 mag. Comparing the corresponding (true) LMC distance modulus 18.64+-0.12 mag with other existing determinations, we conclude that at present the best estimate for the distance of the LMC is: 18.54+-0.03+-0.06, suggesting that distances from the subdwarf fitting method are 1 sigma too long. Consequently, our best estimate for the age of the GCs is revised to: Age = 12.9+-2.9 Gyr (95% confidence range). The best relation between Mv(ZAHB) and [Fe/H] is: Mv(ZAHB) =(0.18+-0.09)([Fe/H]+1.5)+(0.63+-0.07).Comment: 76 pages, 6 encapsulated figures and 6 tables. Latex, uses aasms4.sty. Revised and improved version, with new data on field RR Lyraes in LMC. Accepted in the Astrophysical Journa

Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs

We discuss the impact of Population II and Globular Cluster (GCs) stars on the derivation of the age of the Universe, and on the study of the formation and early evolution of galaxies, our own in particular. The long-standing problem of the actual distance scale to Population II stars and GCs is addressed, and a variety of different methods commonly used to derive distances to Population II stars are briefly reviewed. Emphasis is given to the discussion of distances and ages for GCs derived using Hipparcos parallaxes of local subdwarfs. Results obtained by different authors are slightly different, depending on different assumptions about metallicity scale, reddenings, and corrections for undetected binaries. These and other uncertainties present in the method are discussed. Finally, we outline progress expected in the near future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22 pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty LaTeX style file, enclose