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— The metal rich globular cluster NGC 6441 observed through a 10 ′′ telescope (left) and the Hubble Space
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Preliminary evidence is presented reaffirming that SX Phe, RR Lyrae, and Type II Cepheid variables may be characterized by a common Wesenheit period-magnitude relation, to first order. Reliable distance estimates to RR Lyrae variables and Type II Cepheids are ascertained from a single VI-based reddening-free relation derived recently from OGLE phot...
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A review is presented of much of our current observational knowledge of
classical Cepheids. Outlined are the basic observational parameters of
Galactic Cepheids derived over the past 228 years, with emphasis on
current trends and ongoing problems. Although the calibration of the
Cepheid period-luminosity (PL) relation has normally made use of
variables in the Magellanic Clouds, presently that can be accomplished
using only Galactic Cepheids. It is also possible to calibrate the PL
relation without recourse to observations, given that their fundamental
properties are well enough understood.
We present new absolute trigonometric parallaxes and proper motions for seven Population II variable stars—five RR Lyr variables: RZ Cep, XZ Cyg, SU Dra, RR Lyr, and UV Oct; and two type 2 Cepheids: VY Pyx and κ Pav. We obtained these results with astrometric data from Fine Guidance Sensors, white-light interferometers on Hubble Space Telescope. We find absolute parallaxes in milliseconds of arc: RZ Cep, 2.12 ± 0.16 mas; XZ Cyg, 1.67 ± 0.17 mas; SU Dra, 1.42 ± 0.16 mas; RR Lyr, 3.77 ± 0.13 mas; UV Oct, 1.71 ± 0.10 mas; VY Pyx, 6.44 ± 0.23 mas; and κ Pav, 5.57 ± 0.28 mas; an average σπ/π = 5.4%. With these parallaxes, we compute absolute magnitudes in V and K bandpasses corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Using these RR Lyrae variable star absolute magnitudes, we then derive zero points for MV -[Fe/H] and MK -[Fe/H]-log P relations. The technique of reduced parallaxes corroborates these results. We employ our new results to determine distances and ages of several Galactic globular clusters and the distance of the Large Magellanic Cloud. The latter is close to that previously derived from Classical Cepheids uncorrected for any metallicity effect, indicating that any such effect is small. We also discuss the somewhat puzzling results obtained for our two type 2 Cepheids.
Evidence is presented which supports findings that the classical Cepheid VI
C
period Wesenheit function is relatively insensitive to metallicity. The viability of a recently advocated strong metallicity dependence was evaluated by applying the proposed correction (γ = –0.8 mag dex–1) to distances established for the Magellanic Clouds via a Galactic VI
C
Wesenheit calibration, which is anchored to 10 nearby classical Cepheids with measured Hubble Space Telescope (HST) parallaxes. The resulting γ-corrected distances for the Magellanic Clouds (e.g., Small Magellanic Cloud, μ0, γ ~ 18.3) are in significant disagreement with that established from a mean of >300 published estimates (NED-D), and a universal Wesenheit template featuring 11 δ Scuti, SX Phe, RR Lyrae, and Type II Cepheid variables with HST/Hipparcos parallaxes. Conversely, adopting a null correction (i.e., γ = 0 mag dex–1) consolidates the estimates. In tandem with existing evidence, the results imply that variations in chemical composition among Cepheids are a comparatively negligible source of uncertainty for -based extragalactic distances and determinations of H
0. A new approach is described which aims to provide additional Galactic Cepheid calibrators to facilitate subsequent assessments of the VI
C
Wesenheit function's relative (in) sensitivity to abundance changes. VVV/UKIDSS/Two Micron All Sky Survey JHKs photometry for clusters in spiral arms shall be employed to establish a precise galactic longitude-distance relation, which can be applied in certain cases to determine the absolute Wesenheit magnitudes for younger Cepheids.
We have analyzed archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations of the central region of the globular cluster M54. The 12 irregularly timed observations (6 F555W, 6 F814W) were reduced using the HSTphot stellar photometry package. Our statistical analysis of the HSTphot photometry yielded 50 variable stars that are likely RR Lyrae candidates. These candidates were missed by previous ground-based searches for RR Lyraes due to the extreme crowding in the core of M54. We performed two checks on our sample of RR Lyrae candidates to determine how different they were from the other variable candidates. The first test was a numerical analysis comparing the ratio of variable candidates to the total number of stars within the blue horizontal branch, the RR Lyrae region, and the red giant branch. We found ratios of 0.61%, 13%, and 1.1% in these regions, respectively. Our next test was a two-sample Kolmogorov-Smirnov (K-S) test, which allowed us to find out how different our 50 RR Lyrae candidates were from the 50 brightest other variable candidates. According to the K-S test, there is only a 0.0004% probability that these two populations are similar, indicating that our RR Lyrae candidates do indeed exhibit some real fluctuation in their brightness. We sampled the light curves of confirmed RR Lyrae light curves to simulate the HST observing conditions and found that a signature of variability in 2911 out of the 4000 (72%) generated "stars." We estimate that there maybe as many as 19 more RR Lyrae candidates that were undetected in the WFPC2 field of view. The presence of additional RR Lyraes has implications into the current Oosterhoff classification for the cluster.
This research aims to secure precise distances for cluster delta Scutis in
order to investigate their properties via a VI Wesenheit framework. Deep JHKs
colour-colour and ZAMS relations derived from ~700 unreddened stars featuring
2MASS photometry and precise HIP parallaxes (d<~25 pc) are applied to establish
distances to several benchmark open clusters that host delta Scutis: Hyades,
Pleiades, Praesepe, alpha Per, and M67 (d=47+-2,138+-6,183+-8,171+-8,815+-40
pc). That analysis provided constraints on the delta Sct sample's absolute
Wesenheit magnitudes (W_VI,0), evolutionary status, and pulsation modes (order,
n). The reliability of JHKs established cluster parameters is demonstrated via
a comparison with van Leeuwen 2009 revised HIP results. Distances for 7 of 9
nearby (d<250 pc) clusters agree, and the discrepant cases (Pleiades & Blanco
1) are unrelated to (insignificant) Te/J-Ks variations with cluster age or
[Fe/H]. JHKs photometry is tabulated for ~3x10^3 probable cluster members on
the basis of proper motions (NOMAD). The deep JHKs photometry extends into the
low mass regime (~0.4 Msun) and ensures precise (<5%) ZAMS fits. Pulsation
modes inferred for the cluster delta Scutis from VI Wesenheit and independent
analyses are comparable (+-n), and the methods are consistent in identifying
higher order pulsators. Most small-amplitude cluster delta Scutis lie on VI
Wesenheit loci characterizing n>~1 pulsators. A distance established to NGC
1817 from delta Scutis (d~1.7 kpc) via a universal VI Wesenheit template agrees
with estimates in the literature, assuming the variables delineate the n>~1
boundary. Small statistics in tandem with other factors presently encumber the
use of mmag delta Scutis as viable distance indicators to intermediate-age open
clusters, yet a VI Wesenheit approach is a pertinent means for studying delta
Scutis in harmony with other methods.
A VI Wesenheit diagram featuring SX Phoenicis, delta Scuti, RR Lyrae, type II and classical Cepheid variables is calibrated by means of geometric-based distances inferred from HST, Hipparcos, and VLBA observations (n=30). The distance to a target population follows from the offset between the observed Wesenheit magnitudes and the calibrated template. The method is evaluated by ascertaining the distance moduli for the LMC (mu_0=18.43+-0.03 se) and the globular clusters omega Cen, M54, M13, M3, and M15. The results agree with estimates cited in the literature, although a nearer distance to M13 is favoured (pending confirmation of the data's photometric zero-point) and observations of variables near the core of M15 suffer from photometric contamination. The calibrated LMC data is subsequently added to the Wesenheit template since that galaxy exhibits precise OGLE photometry for innumerable variables of differing classes, that includes recent observations for delta Scuti variables indicating the stars follow a steeper VI Wesenheit function than classical Cepheids pulsating in the fundamental mode. VI photometry for the calibrators is tabulated to facilitate further research, and includes new observations acquired via the AAVSO's robotic telescope network (e.g., VY Pyx: =7.25 and - =0.67). The approach outlined here supersedes the lead author's prior first-order effort to unify variables of the instability strip in order to establish reliable distances. Comment: Accepted for Publication (JAAVSO)
An analysis based on new OGLE observations reaffirms Ferrarese et al. discovery of 5 Type II Cepheids in NGC 5128. The distance to that comparatively unreddened population is d=3.8+-0.4(se) Mpc. The classical Cepheids in NGC 5128 are the most obscured in the extragalactic sample (n=30) surveyed, whereas groups of Cepheids tied to several SNe host galaxies feature negative reddenings. Adopting an anomalous extinction law for Cepheids in NGC 5128 owing to observations of SN 1986G (Rv~2.4) is not favoured, granted SNe Ia may follow small Rv. The distances to classical Cepheids in NGC 5128 exhibit a dependence on colour and CCD chip, which may arise in part from photometric contamination. Applying a colour cut to mitigate contamination yields d~3.5 Mpc (V-I<1.3), while the entire sample's mean is d~3.1 Mpc. The distance was established via the latest VI Galactic Wesenheit functions that include the 10 HST calibrators, and which imply a shorter distance scale than Sandage et al.2004 by ~>10% at P~25d. HST monitored classical Cepheids in NGC 5128, and the SNe hosts NGC 3021 & NGC 1309, follow a shallower VI Wesenheit slope than ground-based calibrations of the Milky Way, LMC, NGC 6822, SMC, and IC 1613. The discrepancy is unrelated to metallicity since the latter group share a common slope over a sizeable abundance baseline (a=-3.34+-0.08,d[Fe/H]~1). A negligible distance offset between OGLE Cepheids and RR Lyr variables in the LMC, SMC, and IC 1613 bolsters assertions that VI-based Wesenheit functions are relatively insensitive to chemical abundance. In sum, a metallicity effect (VI) is not the chief source of uncertainty associated with the Cepheid distance to NGC 5128 or the establishment of Hubble's constant, but rather it may be the admittedly challenging task of obtaining precise, commonly standardized, multiepoch, multiband, comparatively uncontaminated extragalactic Cepheid photometry. Comment: Accepted for Publication (Acta A.)
