3,905 research outputs found
Laser Guide Star Adaptive Optics Integral Field Spectroscopy of a Tightly Collimated Bipolar Jet from the Herbig Ae star LkHa 233
We have used the integral field spectrograph OSIRIS and laser guide star
adaptive optics at Keck Observatory to obtain high angular resolution (0.06"),
moderate spectral resolution (R ~ 3800) images of the bipolar jet from the
Herbig Ae star LkHa 233, seen in near-IR [Fe II] emission at 1.600 & 1.644
microns. This jet is narrow and tightly collimated, with an opening angle of
only 9 degrees, and has an average radial velocity of ~ 100 km/s. The jet and
counterjet are asymmetric, with the red-shifted jet much clumpier than its
counterpart at the angular resolution of our observations. The observed
properties are in general similar to jets seen around T Tauri stars, though it
has a relatively large mass flux of (1.2e-7 +- 0.3e-7) M_sun/year, near the
high end of the observed mass flux range around T Tauri stars. We also
spatially resolve an inclined circumstellar disk around LkHa 233, which
obscures the star from direct view. By comparison with numerical radiative
transfer disk models, we estimate the disk midplane to be inclined i = 65 +- 5
degrees relative to the plane of the sky. Since the star is seen only in
scattered light at near-infrared wavelengths, we detect only a small fraction
of its intrinsic flux. Because previous estimates of its stellar properties did
not account for this, either LkHa 233 must be located closer than the
previously believed, or its true luminosity must be greater than previously
supposed, consistent with its being a ~4 M_sun star near the stellar birthline.Comment: Accepted for publication in the Ap
Fast, low-ionization emission regions of the planetary nebula M2-42
Spatially resolved observations of the planetary nebula M2-42 (PN
G008.2-04.8) obtained with the Wide Field Spectrograph on the Australian
National University 2.3 m telescope have revealed the remarkable features of
bipolar collimated jets emerging from its main structure. Velocity-resolved
channel maps derived from the [N II] 6584 emission line disentangle
different morphological components of the nebula. This information is used to
develop a three-dimensional morpho-kinematic model, which consists of an
equatorial dense torus and a pair of asymmetric bipolar outflows. The expansion
velocity of about 20 km s is measured from the spectrum integrated over
the main shell. However, the deprojected velocities of the jets are found to be
in the range of 80-160 km s with respect to the nebular center. It is
found that the mean density of the collimated outflows, 595 125
cm, is five times lower than that of the main shell, 3150 cm,
whereas their singly ionized nitrogen and sulfur abundances are about three
times higher than those determined from the dense shell. The results indicate
that the features of the collimated jets are typical of fast, low-ionization
emission regions.Comment: 6 pages, 4 figures, 4 tables, accepted for publication in The
Astronomical Journa
Birth and early evolution of a planetary nebula
The final expulsion of gas by a star as it forms a planetary nebula --- the
ionized shell of gas often observed surrounding a young white dwarf --- is one
of the most poorly understood stages of stellar evolution. Such nebulae form
extremely rapidly (about 100 years for the ionization) and so the formation
process is inherently difficult to observe. Particularly puzzling is how a
spherical star can produce a highly asymmetric nebula with collimated outflows.
Here we report optical observations of the Stingray Nebula which has become an
ionized planetary nebula within the past few decades. We find that the
collimated outflows are already evident, and we have identified the nebular
structure that focuses the outflows. We have also found a companion star,
reinforcing previous suspicions that binary companions play an important role
in shaping planetary nebulae and changing the direction of successive outflows.Comment: 9 pages + 3 figures. To appear in Nature, 2 April 199
Protostellar Jets: The Best Laboratories for Investigating Astrophysical Jets
Highly collimated supersonic jets are observed to emerge from a wide variety
of astrophysical objects, ranging from Active Nuclei of Galaxies (AGN's) to
Young Stellar Objects (YSOs) within our own Galaxy. Despite their different
physical scales (in size, velocity, and amount of energy transported), they
have strong morphological similarities. Thanks to the proximity and relatively
small timescales, which permit direct observations of evolutionary changes, YSO
jets are, perhaps, the best laboratories for cosmic jet investigation. In this
lecture, the formation, structure, and evolution of the YSO jets are reviewed
with the help of observational information, MHD and purely hydrodynamical
modeling, and numerical simulations. Possible applications of the models to AGN
jets are also addressed.Comment: 19 pages, PostScript (9 figures upon request). Invited review for
proceedings of the International Conference on Plasma Physics (Foz do
Iguassu, November 1994) eds. P. Sakanaka et al
MHD Stellar and Disk Winds: Application to Planetary Nebulae
MHD winds can emanate from both stars and surrounding accretion disks. It is
of interest to know how much wind power is available and which (if either) of
the two rotators dominates that power. We investigate this in the context of
multi-polar planetary nebulae (PNe) and proto-planetary nebulae (PPNe), for
which recent observations have revealed the need for a wind power source in
excess of that available from radiation driving, and a possible need for
magnetic shaping. We calculate the MHD wind power from a coupled disk and star,
where the former results from binary disruption. The resulting wind powers
depend only on the accretion rate and stellar properties. We find that if the
stellar envelope were initially slowly rotating, the disk wind would dominate
throughout the evolution. If the envelope of the star were rapidly rotating,
the stellar wind could initially be of comparable power to the disk wind until
the stellar wind carries away the star's angular momentum. Since an initially
rapidly rotating star can have its spin and magnetic axes misaligned to the
disk, multi-polar outflows can result from this disk wind system. For times
greater than a spin-down time, the post-AGB stellar wind is slaved to the disk
for both slow and rapid initial spin cases and the disk wind luminosity
dominates. We find a reasonably large parameter space where a hybrid star+disk
MHD driven wind is plausible and where both or either can account for PPNe and
PNe powers. We also speculate on the morphologies which may emerge from the
coupled system. The coupled winds might help explain the shapes of a number of
remarkable multi-shell or multi-polar nebulae. Magnetic activity such as X-ray
flares may be associated with the both central star and the disk and would be a
valuable diagnostic for the dynamical role of MHD processes in PNe.Comment: ApJ accepted version, incorporating some important revisions. 25
Pages, LaTex, + 5 fig
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