The Charge Transfer Efficiency and Calibration of WFPC2

A new determination of WFPC2 photometric corrections is presented, using HSTphot reduction of the WFPC2 Omega Centauri and NGC 2419 observations from January 1994 through March 2000 and a comparison with ground-based photometry. No evidence is seen for any position-independent photometric offsets (the "long-short anomaly"); all systematic errors appear to be corrected with the CTE and zero point solution. The CTE loss time dependence is determined to be very significant in the Y direction, causing time-independent CTE solutions (Stetson 1998; Saha, Lambert, & Prosser 2000) to be valid only for a small range of times. On average, the present solution produces corrections similar to Whitmore, Heyer, & Casertano (1999), although with an improved functional form that produces less scatter in the residuals and determined with roughly a year of additional data. In addition to the CTE loss characterization, zero point corrections are also determined as functions of chip, gain, filter, and temperature. Of interest, there are chip-to-chip differences of order 0.01-0.02 magnitudes relative to the Holtzman et al. (1995) calibrations, and the present study provides empirical zero point determinations for the non-standard filters such as the frequently-used F450W, F606W, and F702W.Comment: 30 pages, 10 figures Accepted for publication in October 2000 PAS

A Revised Characterization of the WFPC2 CTE Loss

Charge-transfer loss on the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope is a primary source of uncertainty in stellar photometry obtained with this camera. This effect, discovered shortly after the camera was installed, has grown over time and can dim stars by several tenths of a magnitude (or even more, in particularly bad cases). The impact of CTE loss on WFPC2 stellar photometry was characterized by several studies between 1998 and 2000, but has received diminished attention since ACS became HST's primary imager. After the failure of ACS in January 2007, WFPC2 once again became the primary imaging instrument onboard HST, restoring the importance of ensuring accurate CTE corrections. This paper re-examines the CTE loss of WFPC2, with three significant changes over previous studies. First, the present study considers calibration data obtained through 2007, thus increasing the confidence in the reliability of the CTE corrections when applied to recent observations. Second, the change in CTE loss during readout is accounted for analytically. Finally, a reanalysis of the CTE dependencies on counts, background, and observation date was made. The resulting correction is significantly more accurate than that provided in the WFPC2 Instrument Handbook (Dolphin 2002 and updates through 2004), resulting in photometry that can be enhanced by over 5% in certain circumstances.Comment: 30 pages, 11 figures. PASP in pres

Numerical Methods of Star Formation History Measurement and Applications to Seven Dwarf Spheroidals

A comprehensive study of the measurement of star formation histories from colour-magnitude diagrams (CMDs) is presented, with an emphasis on a variety of subtle issues involved in the generation of model CMDs and maximum likelihood solution. Among these are the need for a complete sampling of the synthetic CMD, the use of of proper statistics for dealing with Poisson-distributed data (and a demonstration of why chi-square must not be used), measuring full uncertainties in all reported parameters, quantifying the goodness-of-fit, and questions of binning the CMD and incorporating outside information. Several example star formation history measurements are given. Two examples involve synthetic data, in which the input and recovered parameters can be compared to locate possible flaws in the methodology (none were apparent) and measure the accuracy with which ages, metallicities, and star formation rates can be recovered. Solutions of the histories of seven Galactic dwarf spheroidal companions (Carina, Draco, Leo I, Leo II, Sagittarius, Sculptor, and Ursa Minor) illustrate the ability to measure star formation histories given a variety conditions -- numbers of stars, complexity of star formation history, and amount of foreground contamination. Significant measurements of ancient >8 Gyr star formation are made in all seven galaxies. Sculptor, Draco, and Ursa Minor appear entirely ancient, while the other systems show varying amounts of younger stars.Comment: 19 pages accepted by MNRAS corrected typo in abstrac

On the Estimation of Systematic Uncertainties of Star Formation Histories

In most star formation history (SFH) measurements, the reported uncertainties are those due to effects whose sizes can be readily measured: Poisson noise, adopted distance and extinction, and binning choices in the solution itself. However, the largest source of error, systematics in the adopted isochrones, is usually ignored and very rarely explicitly incorporated into the uncertainties. I propose a process by which estimates of the uncertainties due to evolutionary models can be incorporated into the SFH uncertainties. This process relies on application of shifts in temperature and luminosity, the sizes of which must be calibrated for the data being analyzed. While there are inherent limitations, the ability to estimate the effect of systematic errors and include them in the overall uncertainty is significant. Effects of this are most notable in the case of shallow photometry, with which SFH measurements rely on evolved stars.Comment: 28 pages, 9 figures, ApJ in pres

Calibration of BVRI Photometry for the Wide Field Channel of the HST Advanced Camera for Surveys

We present new observations of two Galactic globular clusters, PAL4 and PAL14, using the Wide-Field Channel of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST), and reanalyze archival data from a third, NGC2419. We matched our photometry of hundreds of stars in these fields from the ACS images to existing, ground-based photometry of faint sequences which were calibrated on the standard BVRI system of Landolt. These stars are significantly fainter than those generally used for HST calibration purposes, and therefore are much better matched to supporting precision photometry of ACS science targets. We were able to derive more accurate photometric transformation coefficients for the commonly used ACS broad-band filters compared to those published by Sirianni, et al. (2005), owing to the use of a factor of several more calibration stars which span a greater range of color. We find that the inferred transformations from each cluster individually do not vary significantly from the average, except for a small offset of the photometric zeropoint in the F850LP filter. Our results suggest that the published prescriptions for the time-dependent correction of CCD charge-transfer efficiency appear to work very well over the ~3.5 yr interval that spans our observations of PAL4 and PAL14 and the archived images of NGC2419.Comment: 22 pages, 12 figures. Accepted for publication in PAS

Using the Tip of the Red Giant Branch as a Distance Indicator in the Near Infrared

The tip of the red giant branch (TRGB) is a well-established standard candle used to measure distances to nearby galaxies. The TRGB luminosity is typically measured in the I-band, where the luminosity has little dependency on stellar age or stellar metallicity. As the TRGB is brighter at wavelengths redder than the I-band, observational gains can be made if the TRGB luminosity can be robustly calibrated at longer wavelengths. This is of particular interest given the infrared capabilities that will be available with the James Webb Space Telescope and an important calibration consideration for using TRGB distances as part of an independent measurement of the Hubble constant. Here, we use simulated photometry to investigate the dependency of the TRGB luminosity on stellar age and metallicity as a function of wavelength (475 nm - 4.5 micron). We find intrinsic variations in the TRGB magnitude to increase from a few hundredths of a magnitude at 800-900 nm to ~0.6 mag by 1.5 micron. We show that variations at the longer infrared wavelengths can be reduced to 0.02-0.05 mag (1-2% accuracy in distance) with careful calibrations that account for changes in age and metal content. These represent the minimum uncertainties; observational uncertainties will be higher. Such calibration efforts may also provide independent constraints of the age and metallicity of stellar halos where TRGB distances are best measured. At 3.6 and 4.5 micron, the TRGB magnitude is predicted to vary up to ~0.15 mag even after corrections for stellar age and metallicity, making these wavelengths less suitable for precision distances.Comment: 11 pages, 7 figures, 1 table, Accepted to the Astrophysical Journa