1,127 research outputs found
The kinematics of the most oxygen-poor planetary nebula PN G135.9+55.9
PN G135.9+55.9 is a compact, high excitation nebula that has been identified
recently as the most oxygen-poor halo planetary nebula. Given its very peculiar
characteristics and potential implications in the realms of stellar and
Galactic evolution, additional data are needed to firmly establish its true
nature and evolutionary history. Here we present the first long-slit, high
spectral resolution observations of this object in the lines of H and
He II 4686. The position-velocity data are shown to be compatible with the
interpretation of PN G135.9+55.9 being a halo planetary nebula. In both
emission lines, we find the same two velocity components that characterize the
kinematics as that of an expanding elliptical envelope. The kinematics is
consistent with a prolate ellipsoidal model with axis ratio about 2:1, a
radially decreasing emissivity distribution, a velocity distribution that is
radial, and an expansion velocity of 30 km/s for the bulk of the material. To
fit the observed line profiles, this model requires an asymmetric matter
distribution, with the blue-shifted emission considerably stronger than the
red-shifted emission. We find that the widths of the two velocity components
are substantially wider than those expected due to thermal motions, but
kinematic structure in the projected area covered by the slit appears to be
sufficient to explain the line widths. The present data also rule out the
possible presence of an accretion disk in the system that could have been
responsible for a fraction of the H flux, further supporting the
planetary nebula nature of PN G135.9+55.9.Comment: accepted by Astronomy & Astrophysic
A Cluster of Compact Radio Sources in NGC 2024 (Orion B)
We present deep 3.6 cm radio continuum observations of the H II region NGC
2024 in Orion B obtained using the Very Large Array in its A-configuration,
with angular resolution. We detect a total of 25 compact radio
sources in a region of . We discuss the nature of these sources
and its relation with the infrared and X-ray objects in the region. At least
two of the radio sources are obscured proplyds whose morphology can be used to
restrict the location of the main ionizing source of the region. This cluster
of radio sources is compared with others that have been found in regions of
recent star formation.Comment: 21 pages, 7 figure
NGC 2419, M92, and the Age Gradient in the Galactic Halo
The WFPC2 camera on HST has been used to obtain deep main sequence photometry
of the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. A
differential fit of the NGC 2419 CMD to that of the similarly metal-poor \
standard cluster M92 shows that they have virtually identical principal
sequences and thus the same age to well within 1 Gyr. Since other
low-metallicity clusters throughout the Milky Way halo have this same age to
within the 1-Gyr precision of the differential age technique, we conclude that
the earliest star (or globular cluster) formation began at essentially the same
time everywhere in the Galactic halo throughout a region now almost 200 kpc in
diameter. Thus for the metal-poorest clusters in the halo there is no
detectable age gradient with Galactocentric distance. To estimate the absolute
age of NGC 2419 and M92, we fit newly computed isochrones transformed through
model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance
scales that represent the range currently debated in the literature.
Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting age
of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would
further reduce this estimate by about 1 Gyr. A distance scale as bright as
M_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leave
several serious problems which have no obvious solution in the context of
current stellar models.Comment: 32 pages, aastex, 9 postscript figures; accepted for publication in
AJ, September 1997. Also available by e-mail from [email protected]
Star counts in NGC 6397
I-band CCD images of a large area of the nearby globular cluster NGC~6397
have been used to construct a surface density profile and two luminosity and
mass functions. The surface density profile extends out to 14\arcm from the
cluster center and shows no sign of a tidal cutoff. The inner profile is a
power-law with slope -0.8 steepening to -1.7 outside of 1\arcm. The mass
functions are for fields at 4\arcm\ and 11\arcm from the cluster center and
confirm the upturn in the mass function for stars less massive than about 0.4
M\solar. There appears to be an excess of low-mass stars over higher-mass stars
in the outer field with respect to the inner, in qualitative agreement with
expectations for mass segregation.Comment: 16 pages + 7 pages of tables, LaTeX using AASTeX macros, 11 figures
available by request, IoA preprin
The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping At Work In The Virgo Cluster
We present the results of optical spectroscopy of a newly discovered H II
region residing in the H I gas cloud located between the dwarf irregular galaxy
UGC 7636 and the giant elliptical galaxy NGC 4472 in the Virgo Cluster. By
comparing UGC 7636 with dwarf irregular galaxies in the field, we show that the
H I cloud must have originated from UGC 7636 because (1) the oxygen abundance
of the cloud agrees with that expected for a galaxy with the blue luminosity of
UGC 7636, and (2) M_{H I}/L_B for UGC 7636 becomes consistent with the measured
oxygen abundance of the cloud if the H I mass of the cloud is added back into
UGC 7636. It is likely that tides from NGC 4472 first loosened the H I gas,
after which ram-pressure stripping removed the gas from UGC 7636.Comment: 12 pages, 2 eps figures (AASTeX 5.0); accepted for publication in ApJ
Letter
Deep HST Imaging in NGC 6397: Stellar Dynamics
Multi-epoch observations with ACS on HST provide a unique and comprehensive
probe of stellar dynamics within NGC 6397. We are able to confront analytic
models of the globular cluster with the observed stellar proper motions. The
measured proper motions probe well along the main sequence from 0.8 to below
0.1 M as well as white dwarfs younger than one gigayear. The observed
field lies just beyond the half-light radius where standard models of globular
cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution)
make very robust predictions for the stellar proper motions as a function of
mass. The observed proper motions show no evidence for anisotropy in the
velocity distribution; furthermore, the observations agree in detail with a
straightforward model of the stellar distribution function. We do not find any
evidence that the young white dwarfs have received a natal kick in
contradiction with earlier results. Using the observed proper motions of the
main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397
of kpc and a mass of the cluster of at the photometric distance of 2.53 kpc. One of the
main-sequence stars appears to travel on a trajectory that will escape the
cluster, yielding an estimate of the evaporation timescale, over which the
number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The
proper motions of the youngest white dwarfs appear to resemble those of the
most massive main-sequence stars, providing the first direct constraint on the
relaxation time of the stars in a globular cluster of greater than or about 0.7
Gyr.Comment: 25 pages, 20 figures, accepted for publication in Astrophysical
Journa
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