43 research outputs found
WFPC2 Observations of the Carina Dwarf Spheroidal Galaxy
We present our analysis of Hubble Space Telescope Wide Field Planetary Camera
2 observations in F555W (~V) and F814W (~I) of the Carina dwarf spheroidal
galaxy. The resulting V vs (V-I) color-magnitude diagrams reach V ~ 27.1 mag.
The reddening of Carina is estimated to be E(V-I) = 0.08 +- 0.02 mag. A new
estimate of the distance modulus of Carina, (m-M)_0 = 19.87 +- 0.11 mag, has
been derived primarily from existing photometry in the literature. The apparent
distance moduli in V and I were determined to be (m-M)_V = 20.05 +- 0.11 mag
and (m-M)_I = 19.98 +- 0.12 mag, respectively. These determinations assumed
that Carina has a metallicity of [Fe/H] = -1.9 +- 0.2 dex. This space-based
observation, when combined with previous ground-based observations, is
consistent with (but does not necessarily prove) the following star formation
scenario. The Carina dwarf spheroidal galaxy formed its old stellar population
in a short burst (=< 3 Gyr) at about the same time the Milky Way formed its
globular clusters. The dominant burst of intermediate-age star formation then
began in the central region of the galaxy where stars formed for several
billion years before the process of star formation became efficient enough in
the outer regions of the galaxy to allow for the formation of large numbers of
stars. There has been negligible star formation during the last few billion
years. This observation provides evidence that at least some dwarf galaxies can
have complex global star formation histories with local variations of the rate
of star formation as a function of time and position within the galaxy.Comment: 23 pages (LaTeX+aaspp4.sty), 4 tables and 9 figures (Postscript,
gzipped tar file). Postscript version of paper, tables, and full-resolution
figures available at http://www.noao.edu/noao/staff/mighell/carina.html To
appear in the Astronomical Journa
Astrometry of the Omega Centauri Hubble Space Telescope Calibration Field
Astrometry, on the International Celestial Reference Frame (epoch J2000.0),
is presented for the Walker (1994, PASP, 106, 828) stars in the Omega Centauri
(= NGC 5139 = C1323-1472) Hubble Space Telescope Wide Field/Planetary Camera
(WF/PC) calibration field of Harris et al. (1993, AJ, 105, 1196). Harris et al.
stars were first identified on a WFPC2 observation of the omega Cen HST
calibration field. Relative astrometry of the Walker stars in this field was
then obtained using Walker's CCD positions and astrometry derived using the
STSDAS METRIC task on the positions of the Harris et al. stars on the WFPC2
observation. Finally, the relative astrometry, which was based on the HST Guide
Star Catalog, is placed on the International Celestial Reference Frame with
astrometry from the USNO-A2.0 catalog. An ASCII text version of the astrometric
data of the Walker stars in the omega Cen HST calibration field is available
electronically in the online version of the article.Comment: 9 pages (LaTeX+aaspp4.sty), 6 tables (LaTeX+apjpt4.sty) and 1 figure
(PostScript format). The PostScript version of the paper, the full-resolution
color figure, and tables are available at
http://www.noao.edu/staff/mighell/wcen/ To appear in the August 2000 issue of
the Publications of the Astronomical Society of the Pacifi
Stellar Photometry and Astrometry with Discrete Point Spread Functions
The key features of the MATPHOT algorithm for precise and accurate stellar
photometry and astrometry using discrete Point Spread Functions are described.
A discrete Point Spread Function (PSF) is a sampled version of a continuous PSF
which describes the two-dimensional probability distribution of photons from a
point source (star) just above the detector. The shape information about the
photon scattering pattern of a discrete PSF is typically encoded using a
numerical table (matrix) or a FITS image file. Discrete PSFs are shifted within
an observational model using a 21-pixel-wide damped sinc function and position
partial derivatives are computed using a five-point numerical differentiation
formula. Precise and accurate stellar photometry and astrometry is achieved
with undersampled CCD observations by using supersampled discrete PSFs that are
sampled 2, 3, or more times more finely than the observational data. The
precision and accuracy of the MATPHOT algorithm is demonstrated by using the
C-language MPD code to analyze simulated CCD stellar observations; measured
performance is compared with a theoretical performance model. Detailed analysis
of simulated Next Generation Space Telescope observations demonstrate that
millipixel relative astrometry and millimag photometric precision is achievable
with complicated space-based discrete PSFs. For further information about
MATPHOT and MPD, including source code and documentation, see
http://www.noao.edu/staff/mighell/matphotComment: 19 pages, 22 figures, accepted for publication in MNRA
Parameter Estimation in Astronomy with Poisson-Distributed Data
Applying the standard weighted mean formula, [Sigma (sub i)n(sub i)ssigma(sub i, sup -2)], to determine the weighted mean of data, n(sub i), drawn from a Poisson distribution, will, on average, underestimate the true mean by approx. 1 for all true mean values larger than approx.3 when the common assumption is made that the error of the i th observation is sigma(sub i) = max square root of n(sub i), 1).This small, but statistically significant offset, explains the long-known observation that chi-square minimization techniques which use the modified Neyman'chi(sub 2) statistic, chi(sup 2, sub N) equivalent Sigma(sub i)((n(sub i) - y(sub i)(exp 2)) / max(n(sub i), 1), to compare Poisson - distributed data with model values, y(sub i), will typically predict a total number of counts that underestimates the true total by about 1 count per bin. Based on my finding that weighted mean of data drawn from a Poisson distribution can be determined using the formula [Sigma(sub i)[n(sub i) + min(n(sub i), 1)](n(sub i) + 1)(exp -1)] / [Sigma(sub i)(n(sub i) + 1)(exp -1))], I propose that a new chi(sub 2) statistic, chi(sup 2, sub gamma) equivalent, should always be used to analyze Poisson- distributed data in preference to the modified Neyman's chi(exp 2) statistic. I demonstrated the power and usefulness of,chi(sub gamma, sup 2) minimization by using two statistical fitting techniques and five chi(exp 2) statistics to analyze simulated X-ray power - low 15 - channel spectra with large and small counts per bin. I show that chi(sub gamma, sup 2) minimization with the Levenberg - Marquardt or Powell's method can produce excellent results (mean slope errors approx. less than 3%) with spectra having as few as 25 total counts
Period Error Estimation for the Kepler Eclipsing Binary Catalog
The Kepler Eclipsing Binary Catalog (KEBC) describes 2165 eclipsing binaries identified in the 115 deg^2 Kepler Field based on observations from Kepler quarters Q0, Q1, and Q2. The periods in the KEBC are given in units of days out to six decimal places but no period errors are provided. We present the PEC (Period Error Calculator) algorithm, which can be used to estimate the period errors of strictly periodic variables observed by the Kepler Mission. The PEC algorithm is based on propagation of error theory and assumes that observation of every light curve peak/minimum in a long time-series observation can be unambiguously identified. The PEC algorithm can be efficiently programmed using just a few lines of C computer language code. The PEC algorithm was used to develop a simple model that provides period error estimates for eclipsing binaries in the KEBC with periods less than 62.5 days: log σ P ≈ – 5.8908 + 1.4425(1 + log P), where P is the period of an eclipsing binary in the KEBC in units of days. KEBC systems with periods ≥62.5 days have KEBC period errors of ~0.0144 days. Periods and period errors of seven eclipsing binary systems in the KEBC were measured using the NASA Exoplanet Archive Periodogram Service and compared to period errors estimated using the PEC algorithm
Improving the photometric precision of IRAC Channel 1
Planning is underway for a possible post-cryogenic mission with the Spitzer Space Telescope. Only Channels 1 and 2 (3.6 and 4.5 μm) of the Infrared Array Camera (IRAC) will be operational; they will have unmatched sensitivity from 3 to 5 microns until the James Webb Space Telescope is launched. At SPIE Orlando, Mighell described his NASA-funded MATPHOT algorithm for precision stellar photometry and astrometry and presented MATPHOT-based simulations that suggested Channel 1 stellar photometry may be significantly improved by modeling the nonuniform RQE within each pixel, which, when not taken into account in aperture photometry, causes the derived flux to vary according to where the centroid falls within a single pixel (the pixel-phase effect). We analyze archival observations of calibration stars and compare the precision of stellar aperture photometry, with the recommended 1-dimensional and a new 2-dimensional pixel-phase aperture-flux correction, and MATPHOT-based PSF-fitting photometry which accounts for the observed loss of stellar flux due to the nonuniform intrapixel quantum efficiency. We show how the precision of aperture photometry of bright isolated stars corrected with the new 2-dimensional aperture-flux correction function can yield photometry that is almost as precise as that produced by PSF-fitting procedures. This timely research effort is intended to enhance the science return not only of observations already in Spitzer data archive but also those that would be made during the Spitzer Warm Mission