99 research outputs found
Hubble Space Telescope Imaging of the Circumstellar Nebulosity of T Tauri
Short-exposure Planetary Camera images of T Tauri have been obtained using broadband filters spanning the wavelength range 0.55-0.80 μm. The optically visible star lies very close to an arc of reflection nebulosity. The arc's northern arm extends approximately 5" from the star, while its southwestern arm appears brighter and extends only 2". The arc shows an approximate symmetry along an axis toward the west-northwest, the direction of Hind's Nebula and the blueshifted molecular outflow. The morphology of the reflected light is similar to models of scattered light within an illuminated, axisymmetric outflow cavity in a circumbinary envelope, viewed ≈ 45° from the outflow axis. However, our model images do not successfully account for the amount of limb brightening that is seen. No optical counterpart to the infrared companion is seen to a limiting magnitude of V = 19.6, which suggests A_V > 7 mag toward this source. There is no evidence for an optical tertiary, to a limiting ΔV = 5.1 mag fainter than the primary, at the position where such an object has been previously reported
Stellar Populations at the Center of IC 1613
We have observed the center of the Local Group dwarf irregular galaxy IC 1613
with WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W
filters. We find a dominant old stellar population (aged ~7 Gyr), identifiable
by the strong red giant branch (RGB) and red clump populations. From the (V-I)
color of the RGB, we estimate a mean metallicity of the intermediate-age
stellar population [Fe/H] = -1.38 +/- 0.31. We confirm a distance of 715 +/- 40
kpc using the I-magnitude of the RGB tip. The main-sequence luminosity function
down to I ~25 provides evidence for a roughly constant SFR of approximately
0.00035 solar masses per year across the WFPC2 field of view (0.22 square kpc)
during the past 250-350 Myr. Structure in the blue loop luminosity function
implies that the SFR was ~50% higher 400-900 Myr ago than today. The mean heavy
element abundance of these young stars is 1/10th solar. The best explanation
for a red spur on the main-sequence at I = 24.7 is the blue horizontal branch
component of a very old stellar population at the center of IC 1613. We have
also imaged a broader area of IC 1613 using the 3.5-meter WIYN telescope under
excellent seeing conditions. The AGB-star luminosity function is consistent
with a period of continuous star formation over at least the age range 2-10
Gyr. We present an approximate age-metallicity relation for IC 1613, which
appears similar to that of the Small Magellanic Cloud. We compare the Hess
diagram of IC 1613 to similar data for three other Local Group dwarf galaxies,
and find that it most closely resembles the nearby, transition-type dwarf
galaxy Pegasus (DDO 216).Comment: To appear in the September 1999 Astronomical Journal. LaTeX, uses
AASTeX v4.0, emulateapj style file, 19 pages, 12 postscript figures, 2
tables. 5 of the figures available separately via the WW
Hubble Space Telescope Observations of the Draco Dwarf Spheroidal
We present an F606W-F814W color-magnitude diagram for the Draco dwarf
spheroidal galaxy based on Hubble Space Telescope WFPC2 images. The luminosity
function is well-sampled to 3 magnitudes below the turn-off. We see no evidence
for multiple turnoffs and conclude that, at least over the field of the view of
the WFPC2, star formation was primarily single-epoch. If the observed number of
blue stragglers is due to extended star formation, then roughly 6% (upper
limit) of the stars could be half as old as the bulk of the galaxy. The color
difference between the red giant branch and the turnoff is consistent with an
old population and is very similar to that observed in the old, metal-poor
Galactic globular clusters M68 and M92. Despite its red horizontal branch,
Draco appears to be older than M68 and M92 by 1.6 +/- 2.5 Gyrs, lending support
to the argument that the ``second parameter'' which governs horizontal branch
morphology must be something other than age. Draco's observed luminosity
function is very similar to that of M68, and the derived initial mass function
is consistent with that of the solar neighborhood.Comment: 16 pages, AASTeX, 9 postscript figures, figures 1 and 2 available at
ftp://bb3.jpl.nasa.gov/pub/draco/. Accepted for publication in the
Astronomical Journa
The HST Key Project on the Extragalactic Distance Scale XXVI. The Calibration of Population II Secondary Distance Indicators and the Value of the Hubble Constant
A Cepheid-based calibration is derived for four distance indicators that
utilize stars in the old stellar populations: the tip of the red giant branch
(TRGB), the planetary nebula luminosity function (PNLF), the globular cluster
luminosity function (GCLF) and the surface brightness fluctuation method (SBF).
The calibration is largely based on the Cepheid distances to 18 spiral galaxies
within cz =1500 km/s obtained as part of the HST Key Project on the
Extragalactic Distance Scale, but relies also on Cepheid distances from
separate HST and ground-based efforts. The newly derived calibration of the SBF
method is applied to obtain distances to four Abell clusters in the velocity
range between 3800 and 5000 km/s, observed by Lauer et al. (1998) using the
HST/WFPC2. Combined with cluster velocities corrected for a cosmological flow
model, these distances imply a value of the Hubble constant of H0 = 69 +/- 4
(random) +/- 6 (systematic) km/s/Mpc. This result assumes that the Cepheid PL
relation is independent of the metallicity of the variable stars; adopting a
metallicity correction as in Kennicutt et al. (1998), would produce a (5 +/-
3)% decrease in H0. Finally, the newly derived calibration allows us to
investigate systematics in the Cepheid, PNLF, SBF, GCLF and TRGB distance
scales.Comment: Accepted for publication in the Astrophysical Journal. 48 pages
(including 13 figures and 4 tables), plus two additional tables in landscape
format. Also available at http://astro.caltech.edu/~lff/pub.htm K' SBF
magnitudes have been update
Observations and Implications of the Star Formation History of the LMC
We present derivations of star formation histories based on color-magnitude
diagrams of three fields in the LMC from HST/WFPC2 observations. A significant
component of stars older than 4 Gyr is required to match the observed
color-magnitude diagrams. Models with a dispersion-free age-metallicity
relation are unable to reproduce the width of the observed main sequence;
models with a range of metallicity at a given age provide a much better fit.
Such models allow us to construct complete ``population boxes'' for the LMC
based entirely on color-magnitude diagrams; remarkably, these qualitatively
reproduce the age-metallicity relation observed in LMC clusters. We discuss
some of the uncertainties in deriving star formation histories. We find,
independently of the models, that the LMC bar field has a larger relative
component of older stars than the outer fields. The main implications suggested
by this study are: 1) the star formation history of field stars appears to
differ from the age distribution of clusters, 2) there is no obvious evidence
for bursty star formation, but our ability to measure bursts shorter in
duration than 25% of any given age is limited by the statistics of the
observed number of stars, 3) there may be some correlation of the star
formation rate with the last close passage of the LMC/SMC/Milky Way, but there
is no dramatic effect, and 4) the derived star formation history is probably
consistent with observed abundances, based on recent chemical evolution models.Comment: Accepted by AJ, 36 pages including 12 figure
The HST Key Project on the Extragalactic Distance Scale. XV. A Cepheid Distance to the Fornax Cluster and Its Implications
Using the Hubble Space Telescope (HST) 37 long-period Cepheid variables have
been discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V
and I period-luminosity relations yield a true distance modulus of 31.35 +/-
0.07 mag, which corresponds to a distance of 18.6 +/- 0.6 Mpc. This measurement
provides several routes for estimating the Hubble Constant. (1) Assuming this
distance for the Fornax Cluster as a whole yields a local Hubble Constant of 70
+/-18_{random} [+/-7]_{systematic} km/s/Mpc. (2) Nine Cepheid-based distances
to groups of galaxies out to and including the Fornax and Virgo clusters yield
Ho = 73 (+/-16)_r [+/-7]_s km/s/Mpc. (3) Recalibrating the I-band Tully-Fisher
relation using NGC 1365 and six nearby spiral galaxies, and applying it to 15
galaxy clusters out to 100 Mpc gives Ho = 76 (+/-3)_r [+/-8]_s km/s/Mpc. (4)
Using a broad-based set of differential cluster distance moduli ranging from
Fornax to Abell 2147 gives Ho = 72 (+/-)_r [+/-6]_s km/s/Mpc. And finally, (5)
Assuming the NGC 1365 distance for the two additional Type Ia supernovae in
Fornax and adding them to the SnIa calibration (correcting for light curve
shape) gives Ho = 67 (+/-6)_r [+/-7]_s km/s/Mpc out to a distance in excess of
500 Mpc. All five of these Ho determinations agree to within their statistical
errors. The resulting estimate of the Hubble Constant combining all these
determinations is Ho = 72 (+/-5)_r [+/-12]_s km/s/Mpc.Comment: Accepted for publication in the Astrophysical Journal, Apr. 10 issue
28 pages, 3 tables, 12 figures (Correct figures and abstract
WFPC2 Observations of the Cooling Flow Elliptical in Abell 1795
We present WFPC2 images of the core of the cooling flow cD galaxy in Abell
1795. An irregular, asymmetric dust lane extends 7 \h75 kpc in projection to
the north-northwest. The dust shares the morphology observed in the H
and excess UV emission. We see both diffuse and knotty blue emission around the
dust lane, especially at the ends. The dust and emission features lie on the
edge of the radio lobes, suggesting star formation induced by the radio source
or the deflection of the radio jets off of pre-existing dust and gas. We
measure an apparent R significantly less than 3.1, implying that the
extinction law is not Galactic in the dust lane, or the presence of line
emission which is proportional to the extinction. The dust mass is at least
2 M\solar\ and is more likely to be 6.5 M\solar.Comment: 14 pages, LaTeX, Figure 4 included, Postscript Figs. 1-3 available at
ftp://astro.nmsu.edu/pub/JASON/A1795/, accepted for publication in ApJ
Letter
The HST Key Project on the Extragalactic Distance Scale. XXII. The Discovery of Cepheids in NGC 1326-A
We report on the detection of Cepheids and the first distance measurement to
the spiral galaxy NGC 1326-A, a member of the Fornax cluster of galaxies. We
have employed data obtained with the Wide Field and Planetary Camera 2 on board
the Hubble Space Telescope. Over a 49 day interval, a total of twelve V-band
(F555W) and eight I-band (F814W) epochs of observation were obtained. Two
photometric reduction packages, ALLFRAME and DoPHOT, have been employed to
obtain photometry measures from the three Wide Field CCDs. Variability analysis
yields a total of 17 Cepheids in common with both photometry datasets, with
periods ranging between 10 and 50 days. Of these 14 Cepheids with high-quality
lightcurves are used to fit the V and I period-luminosity relations and derive
apparent distance moduli, assuming a Large Magellanic Cloud distance modulus
(m-M) (LMC) = 18.50 +- 0.10 mag and color excess E(B-V) = 0.10 mag. Assuming
A(V)/E(V-I) = 2.45, the DoPHOT data yield a true distance modulus to NGC 1326-A
of (m-M)_0 = 31.36 +- 0.17 (random) +- 0.13 (systematic) mag, corresponding to
a distance of 18.7 \pm 1.5 (random) \pm 1.2 (systematic) Mpc. The derived
distance to NGC 1326-A is in good agreement with the distance derived
previously to NGC 1365, another spiral galaxy member of the Fornax cluster.
However the distances to both galaxies are significantly lower than to NGC
1425, a third Cepheid calibrator in the outer parts of the cluster.Comment: 33 pages A gzipped tar file containing 12 figures can be obtained
from http://www.ipac.caltech.edu/H0kp/n1326a/n1326a.htm
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