10 research outputs found
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
WFPC2 Observations of Leo A: A Predominantly Young Galaxy within the Local Group
The unprecedented detail of the WFPC2 colour-magnitude diagrams of the
resolved stellar population of Leo A presented here allows us to determine a
new distance and an accurate star formation history for this extremely
metal-poor Local Group dwarf irregular galaxy. From the position of the red
clump, the helium-burning blue loops and the tip of the red giant branch, we
obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo
A firmly within the Local Group. Our interpretation of these features in the
WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004)
theoretical stellar evolution models suggests that this galaxy is predominantly
young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago
can explain the red clump luminosity and also fits in with our interpretation
of the number of anomalous Cepheid variable stars seen in this galaxy. We
cannot rule out the presence of an older, underlying globular cluster age
stellar population with these data. However, using the currently available
stellar evolution models, it would appear that such an older population is
limited to no more than 10% of the total star formation to have occured in this
galaxy. Leo A provides a nearby laboratory for studying young metal poor stars
and investigations of metal-poor galaxy evolution, such as is supposed to occur
for larger systems at intermediate and high redshifts.Comment: 64 pages, 18 figures, 4 tables accepted for publication in the
Astronomical Journal (Sept. 98