164 research outputs found
Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula
HH 399 is one of the first Herbig Haro flows recognized to be irradiated by
the UV radiation of the massive O7.5 star in the Trifid nebula. We present the
proper motion of the first irradiated jet based on two epochs of HST
observations of HH 399 separated nearly by five years using H and [SII]
line filters. High proper motion with continuous velocities between 20055
and 528 \kms are detected in both lines along the 18 extent of the
jet axis. The irradiated fully-ionized jet consists of numerous knots along the
jet but also shows the evidence for a number of isolated blob-like structures
running immediately outside the jet with lower transverse velocities. The
transverse velocities combined with radial velocity measurements indicate that
the jet axis lies away from the plane of the sky by only few degrees. We argue
that the jet is fully ionized based on [SII]/H line ratio as well as
radio continuum emission detected from the full extent of the jet at 3.6cm
wavelength. The stellar mass-loss rate producing HH 399 is estimated to be
\approx 2\times10^{-6} \msol yr.Comment: 14 pages, 6 figures, ApJ (in press
X-ray variability in M87
We present the evidence for X-ray variability from the core and from knot A
in the M87 jet based on data from two observations with the Einstein
Observatory High Resolution Imager (HRI) and three observations with the ROSAT
HRI. The core intensity showed a 16% increase in 17 months ('79-'80); a 12%
increase in the 3 years '92 to '95; and a 17% drop in the last half of 1995.
The intensity of knot A appears to have decreased by 16% between 92Jun and
95Dec. Although the core variability is consistent with general expectations
for AGN nuclei, the changes in knot A provide constraints on the x-ray emission
process and geometry. Thus we predict that the x-ray morphology of knot A will
differ significantly from the radio and optical structure.Comment: 9 pages latex plus 6 ps figs. To appear in Monthly Notices of the RA
Collimation of extragalactic radio jets in compact steep spectrum and larger sources
We study the collimation of radio jets in the high-luminosity Fanaroff-Riley
class II sources by examining the dependence of the sizes of hotspots and knots
in the radio jets on the overall size of the objects for a sample of compact
steep-spectrum or CSS and larger-sized objects. The objects span a wide range
in overall size from about 50 pc to nearly 1 Mpc. The mean size of the hotspots
increases with the source size during the CSS phase, which is typically taken
to be about 20 kpc, and the relationship flattens for the larger sources. The
sizes of the knots in the compact as well as the larger sources are consistent
with this trend. We discuss possible implications of these trends. We find that
the hotspot closer to the nucleus or core component tends to be more compact
for the most asymmetric objects where the ratio of separations of the hotspots
from the nucleus, r_d > 2. These highly asymmetric sources are invariably CSS
objects, and their location in the hotspot size ratio - separation ratio
diagram is possibly due to their evolution in an asymmetric environment. We
also suggest that some soures, especially of lower luminosity, exhibit an
asymmetry in the collimation of the oppositely-directed radio jets.Comment: MNRAS in press, 9 pages and 3 figures, MNRAS LaTe
Morphology of the Nuclear Disk in M87
A deep, fuly sampled diffraction limited (FWHM ~ 70 mas) narrow-band image of
the central region in M87 was obtained with the Wide Filed and Planetary Camera
2 of the Hubble Space Telescope using the dithering technique. The
H-alpha+[NII] continuum subtracted image reveals a wealth of details in the
gaseous disk structure described earlier by Ford et al. (1994). The disk
morphology is dominated by a well defined three-arm spiral pattern. In
addition, the major spiral arms contain a large number of small "arclets"
covering a range of sizes (0.1-0.3 arcsec = 10-30 pc). The overall surface
brightness profile inside a radius ~1.5" (100 pc) is well represented by a
power-law I(mu) ~ mu^(-1.75), but when the central ~40 pc are excluded it can
be equally well fit by an exponential disk. The major axis position angle
remains constant at about PA_disk ~ 6 deg for the innermost ~1", implying the
disk is oriented nearly perpendicular to the synchrotron jet (PA_jet ~ 291
deg). At larger radial distances the isophotes twist, reflecting the gas
distribution in the filaments connecting to the disk outskirts. The ellipticity
within the same radial range is e = 0.2-0.4, which implies an inclination angle
of i~35 deg. The sense of rotation combined with the dust obscuration pattern
indicate that the spiral arms are trailing.Comment: 5 pages, 3 postscript figures, to appear in the Proceedings of the
M87 Workshop, Ringberg castle, Germany, 15-19 Sep 1997, also available from
http://jhufos.pha.jhu.edu/~zlatan/papers.htm
Proper Motions of Ionized Gas at the Galactic Center: Evidence for Unbound Orbiting Gas
We present radio continuum observations of the spiral-shaped ionized feature
(Sgr A West) within the inner pc of the Galactic center at three epochs
spanning 1986 to 1995. The VLA A-configuration was used at 2cm
(resolution of 0\dasec10\dasec2). We detect proper motions of a number
of features in the Northern and Eastern Arms of Sgr A West including the
ionized gas associated with IRS 13 with V(RA)= 113 \pm 10, V(Dec)=150 \pm15
km/s, IRS 2 with V(RA)= 122 \pm 11, V(Dec)=24 \pm 34 km/s and the Norther Arm
V(RA)= 126 \pm 30, V(Dec)=--207 \pm 58 km/s. We also report the detection of
features having transverse velocities > 1000 km/s including a head-tail radio
structure, the ``Bullet'', northwest of Sgr A with V(RA)= 722
\pm 156, V(Dec)=832 \pm 203 km/s, exceeding the escape velocity at the Galactic
center.
The proper motion measurements when combined with previous H92 radio
recombination line data suggest an unambiguous direction of the flow of ionized
gas orbiting the Galactic center. The measured velocity distribution suggests
that the ionized gas in the Northern Arm is not bound to the Galactic center
assuming a 2.5 million solar mass of dark matter residing at the Galactic
center. This implies that the stellar and ionized gas systems are not
dynamically coupled, thus, supporting a picture in which the gas features in
the Northern Arm and its extensions are the result of an energetic phenomenon
that has externally driven a cloud of gas cloud into the Galactic center.Comment: 11 pages, three figures (one color) and one table. Astrophysical
Journal Letters in pres
Magnetic collimation of the relativistic jet in M87
We apply a two-zone MHD model to the jet of M87. The model consists of an
inner relativistic outflow, which is surrounded by a non-relativistic outer
disk-wind. The outer disk-wind collimates very well through magnetic
self-collimation and confines the inner relativistic jet into a narrow region
around the rotation axis. Further, we show by example, that such models
reproduce very accurately the observed opening angle of the M87 jet over a
large range from the kiloparsec scale down to the sub-parsec scale.Comment: 4 pages, 2 figures, accepted by A&A Letter
What Fraction of the Young Clusters in the Antennae Galaxies are "Missing"?
A reexamination of the correspondence between 6 cm radio continuum sources
and young star clusters in the Antennae galaxies indicates that 85 % of the
strong thermal sources have optical counterparts, once the optical image is
shifted 1.2 arcsec to the southwest. A sample of 37 radio-optical matches are
studied in detail showing correlations between radio properties and a variety
of optical characteristics. There is a strong correlation between the radio
flux and the intrinsic optical brightness. In particular, the brightest radio
source is also the intrinsically brightest optical cluster (WS80). It is also
the most extincted cluster in the sample, the strongest CO source and the
strongest 15 micron source . Furthermore, the brightest ten radio sources are
all amongst the youngest clusters with ages in the range 0 - 4 Myr and
extinctions from A_V = 0.5 to 7.6 mag (with a median value of 2.6 mag). Only a
few of the very red clusters originally discovered by Whitmore & Schweizer are
radio sources, contrary to earlier suggestions. Finally, a new hybrid method of
determining cluster ages has been developed using both UBVI colors and H_alpha
equivalent widths to break the age-reddening degeneracy.Comment: 51 pages, 13 postscript figures, LaTex. To appear in the Astronomical
Journal, 124, 2002, Septembe
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