34 research outputs found
Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main-Sequence Stars
We present deep VI photometry of stars in the globular cluster M5 (NGC 5904) based on images taken with the Hubble Space Telescope. The resulting color-magnitude diagram reaches below V ≈27 mag, revealing the upper 2–3 mag of the white dwarf cooling sequence and main-sequence stars 8 mag and more below the turnoff. We fit the main sequence to subdwarfs of known parallax to obtain a true distance modulus of (m ̶ M )0 = 14:45 ± 0:11 mag. A second distance estimate based on fitting the cluster white dwarf sequence to field white dwarfs with known parallax yielded (m _ M)0 = 14:67 ± 0:18 mag. We discuss the nature of the difference between the two distance estimates and suggest approaches for reducing the uncertainty in white dwarf fitting estimates for future studies. We couple our distance estimates with extensive photometry of the cluster’s RR Lyrae variables to provide a calibration of the RR Lyrae absolute magnitude yielding MV (RR) = 0:42 ± 0:10 mag at [Fe/H] = ̶ 1:11 dex. We provide another luminosity calibration in the form of reddening-free Wasenheit functions. Comparison of our calibrations with predictions based on recent models combining stellar evolution and pulsation theories shows encouraging agreement, and the existing differences may provide useful feedback to the models
HST Photometry for the Halo Stars in the Leo Elliptical NGC 3377
We have used the ACS camera on HST to obtain (V,I) photometry for 57,000
red-giant stars in the halo of the Leo elliptical NGC 3377. We use this sample
of stars to derive the metallicity distribution function (MDF) for its halo
field stars, and comment on its chemical evolution history compared with both
larger and smaller E galaxies. Our ACS/WFC field spans a radial range extending
from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers
a significant portion of this galaxy's halo. We find that the MDF is broad,
reaching a peak at [m/H] ~ -0.6. It may, in addition, have relatively few
stars more metal-rich than [m/H] = -0.3$, although interpretation of the
high-metallicity end of the MDF is limited by photometric completeness that
affects the detection of the reddest, most metal-rich stars. NGC 3377 appears
to have an enrichment history intermediate between those of normal dwarf
ellipticals and the much larger giants. As yet, we find no clear evidence that
the halo of NGC 3377 contains a significant population of ``young'' (< 3 Gy)
stars.Comment: 40 pages, 17 figure
The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy
We present a deep color-magnitude diagram in the VI passbands of the globular
cluster M54, a member of the Sagittarius dwarf galaxy. The data extend below
the cluster's main sequence turn-off, allowing us to estimate the cluster's
age. We find that M54 is 0.5--1.5 gigayears older than the Galactic globulars
M68 and M5. In absolute terms, the age is comparable to the published age
estimates of the other member clusters Arp 2 and Terzan 8, but is significantly
older than the member cluster Terzan 7. An age estimate of the Sagittarius
field population relative to M54 suggests that M54 is \gtrsim 3 Gyr older than
the field. We discuss briefly the star formation history of the Sagittarius
dwarf galaxy.Comment: 11 pages, AASTeX, 3 postscript figures, accepted for publication in
ApJ Letter
The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441
We present time-series VI photometry of the metal-rich ([Fe/H] = -0.53)
globular cluster NGC 6441. Our color-magnitude diagram shows that the extended
blue horizontal branch seen in Hubble Space Telescope data exists in the
outermost reaches of the cluster. The red clump slopes nearly parallel to the
reddening vector. A component of this slope is due to differential reddening,
but part is intrinsic. The blue horizontal branch stars are more centrally
concentrated than the red clump stars. We have discovered about 50 new variable
stars near NGC 6441, among them eight or more RR Lyrae stars which are very
probably cluster members. Comprehensive period searches over the range 0.2-1.0
days yielded unusually long periods (0.5-0.9 days) for the fundamental
pulsators compared with field RR Lyrae of the same metallicity. Three similar
long-period RR Lyrae are known in other metal-rich globulars. With over ten
examples in hand, it seems that a distinct sub-class of RR Lyrae is emerging.
The observed properties of the horizontal branch stars are in reasonable
agreement with recent models which invoke deep mixing to enhance the
atmospheric helium abundance, while they conflict with models which assume high
initial helium abundance. The light curves of the c-type RR Lyrae seem to have
unusually long rise times and sharp minima. Reproducing these light curves in
stellar pulsation models may provide another means of constraining the physical
variables responsible for the anomalous blue horizontal branch extension and
sloped red clump observed in NGC 6441.Comment: 30 pages plus 6 EPS and 6 JPEG figures; uses AAS TeX. Accepted by the
Astronomical Journal. Minor changes include computing He abundance,
modifications to Figs 1 and 8, and expansion on idea that blue HB stars may
be produced in binarie
Systematics of RR Lyrae Statistical Parallax III: Apparent Magnitudes and Extinctions
We sing the praises of the central limit theorem. Having previously removed
all other possible causes of significant systematic error in the statistical
parallax determination of RR Lyrae absolute magnitudes, we investigate
systematic errors from two final sources of input data: apparent magnitudes and
extinctions. We find corrections due to each of ~0.05 mag, i.e., ~1/2 the
statistical error. However, these are of opposite sign and so roughly cancel.
The apparent magnitude system that we previously adopted from Layden et al. was
calibrated to the photometry of Clube & Dawe. Using Hipparcos photometry we
show that the Clube & Dawe system is ~0.06 mag too bright. Extinctions were
previously pinned to the HI-based map of Burstein & Heiles. We argue that A_V
should rather be based on new COBE/IRAS dust-emission map of Schlegel,
Finkbeiner & Davis. This change increases the mean A_V by ~0.05 mag. We find
M_V=0.77 +/- 0.13 at [Fe/H]=-1.60 for a pure sample of 147 halo RR Lyraes, or
M_V=0.80 +/- 0.11 at [Fe/H]=-1.71 if we incorporate kinematic information from
716 non-kinematically selected non-RR Lyrae stars from Beers & Sommer-Larsen.
These are 2 and 3 sigma fainter than recent determinations of M_V from main
sequence fitting of clusters using Hipparcos measurements of subdwarfs by Reid
and Gratton et al. Since statistical parallax is being cleared of systematic
errors and since the chance of a >2 sigma statistical fluctuation is <1/20, we
conclude that these brighter determinations may be in error. In the course of
three papers, we have corrected 6 systematic errors whose absolute values total
0.20 mag. Had these, contrary to the expectation of the central limit theorem,
all lined up one way, they could have resolved the conflict in favor of the
brighter determinations. In fact, the net change was only 0.06 mag.Comment: submitted to ApJ, 21 pages, 2 tables, 4 figure
Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main Sequence Stars
We present deep VI photometry of stars in the globular cluster M5 (NGC 5904)
based on images taken with the Hubble Space Telescope. The resulting
color-magnitude diagram reaches below V ~ 27 mag, revealing the upper 2-3
magnitudes of the white dwarf cooling sequence, and main sequence stars eight
magnitudes and more below the turn-off. We fit the main sequence to subdwarfs
of known parallax to obtain a true distance modulus of (m-M)_0 = 14.45 +/- 0.11
mag. A second distance estimate based on fitting the cluster white dwarf
sequence to field white dwarfs with known parallax yielded (m-M)_0 = 14.67 +/-
0.18 mag. We couple our distance estimates with extensive photometry of the
cluster's RR Lyrae variables to provide a calibration of the RR Lyrae absolute
magnitude yielding M_V(RR) = 0.42 +/- 0.10 mag at [Fe/H] = -1.11 dex. We
provide another luminosity calibration in the form of reddening-free Wasenheit
functions. Comparison of our calibrations with predictions based on recent
models combining stellar evolution and pulsation theories shows encouraging
agreement. (Abridged)Comment: AASTeX, 29 pages including 5 figures. Complete photometry data and
FITS-format images are available at
http://physics.bgsu.edu/~layden/ASTRO/PUBL/published.html . Accepted for
publication in the Astrophysical Journal, 2005 October 20. Replaced errant
wording in last sentence of paragraph 4 of conclusion
The RR Lyrae variables and Horizontal Branch of NGC6656 (M22)
The first calibrated broadband UBVI time-series photometry is presented for
the RR Lyrae variable stars in NGC6656 (M22), with observations spanning a
range of twenty-two years. We have also redetermined the variability types and
periods for the RR Lyrae stars identified previously by photographic
observations, revising the number of fundamental-mode RR Lyrae variables (RR0)
to 10 and the number of first-overtone variables (RR1) to 16. The mean periods
of the RR0 and RR1 variables are _{RR0}=0.66 \pm 0.02 d and _{RR1}=0.33
\pm 0.01 d, respectively, supporting an Oosterhoff II classification for the
cluster. The number ratio of RR1- to all RR-type variables is N_1/N_{RR}=0.61,
also consistent with an Oosterhoff II designation. Both the RR Lyrae stars'
minimum light colors and the blue edge of the RR Lyrae instability strip
suggest E(B-V)=0.36 \pm 0.02 mag toward M22. Regarding the HB morphology of
M22, we find (B-R)/(B+V+R)=+0.97 \pm 0.1 and at least one "gap" located in an
unusual part of the blue HB, in the middle of the so-called hot HB stars.Comment: accepted to A