115 research outputs found
HST NICMOS Images of the HH 7/11 Outflow in NGC1333
We present near infrared images in H2 at 2.12um of the HH 7/11 outflow and
its driving source SVS 13 taken with HST NICMOS 2 camera, as well as archival
Ha and [SII] optical images obtained with the WFPC2 camera. The NICMOS high
angular resolution observations confirm the nature of a small scale jet arising
from SVS 13, and resolve a structure in the HH 7 working surface that could
correspond to Mach disk H2 emission. The H2 jet has a length of 430 AU (at a
distance of 350 pc), an aspect ratio of 2.2 and morphologically resembles the
well known DG Tau optical micro-jet. The kinematical age of the jet (approx. 10
yr) coincides with the time since the last outburst from SVS 13. If we
interpret the observed H2 flux density with molecular shock models of 20-30
km/s, then the jet has a density as high as 1.e+5 cc. The presence of this
small jet warns that contamination by H2 emission from an outflow in studies
searching for H2 in circumstellar disks is possible. At the working surface,
the smooth H2 morphology of the HH 7 bowshock indicates that the magnetic field
is strong, playing a major role in stabilizing this structure. The H2 flux
density of the Mach disk, when compared with that of the bowshock, suggests
that its emission is produced by molecular shocks of less than 20 km/s. The
WFPC2 optical images display several of the global features already inferred
from groundbased observations, like the filamentary structure in HH 8 and HH
10, which suggests a strong interaction of the outflow with its cavity. The H2
jet is not detected in {SII] or Ha, however, there is a small clump at approx.
5'' NE of SVS 13 that could be depicting the presence either of a different
outburst event or the north edge of the outflow cavity.Comment: 13 pages, 5 figures (JPEGs
Optical and Near Infrared Study of the Cepheus E outflow, a very low excitation object
We present images and spectra of the Cepheus E (Cep E) region at both optical
and infrared wavelengths. Only the brightest region of the southern lobe of the
Cep E outflow reveals optical emission, suggesting that the extinction close to
the outflow source plays an important r\^ole in the observed difference between
the optical and IR morphologies. Cep E is a unique object since it provides a
link between the spectroscopic properties of the optical Herbig-Haro (HH)
objects and those of deeply embedded outflows.Comment: Accepted Astron. J., 8 files: paper, tables plus 6 figure
Hydrodynamical Simulations of Jet- and Wind-driven Protostellar Outflows
We present two-dimensional hydrodynamical simulations of both jet- and
wind-driven models for protostellar outflows in order to make detailed
comparisons to the kinematics of observed molecular outflows.
Comparing the different simulations with observations, we find that some
outflows, e.g., HH 212, show features consistent with the jet-driven model,
while others, e.g., VLA 05487, are consistent with the wind-driven model.Comment: 38 pages, 14 figures, accepted for publication in Ap
Highly Collimated Molecular Hydrogen Jets Near IRAS 05487+0255: NIR Imaging and Spectroscopy
We present new narrow-band near-infrared images together with K band spectra
of highly collimated bipolar jets close to the IRAS 05487+0255 source. The jets
are located at 50" West of the Herbig-Haro 110 outflow. The jets are not
visible at optical wavelengths, and therefore, do not fall into the `standard'
Herbig-Haro object classification scheme. Nevertheless, they belong to an ever
growing group of molecular hydrogen jets associated with YSOs which are
optically undetected. The jets are very well collimated, with a length-to-width
ratio of 10-20. The spectra of the jet and counter-jet in the K-band show a
limited number of molecular hydrogen emission lines which makes it difficult to
obtain an accurate excitation temperature. We estimate Tex = 1104+/-67 K and
Tex = 920+/- 156 K for the red and blue jet components respectively. The radial
velocities of the jet and counter-jet, based on the shift of the (1,0) S(1)
2.121 micron line, are -275+/- 50 km/s and 180+/- 50 km/s respectively,
suggesting an angle of 30 to 45 degrees between the jet and the line of sight.
The molecular hydrogen emission of the entire jet extends for at least 40" or
0.1 pc at the distance of Orion. If the flow velocity is comparable to that of
the radial velocities, then the dynamical age of the system is quite short
(about 500 yrs), consistent with a young jet arising from an embedded source.
Entrainment in a turbulent mixing layer may explain this morphology and
spectral character.Comment: 15 pages, 5 postscript figures, Accepted to the Ap
Improving the Efficiency of Reasoning Through Structure-Based Reformulation
Abstract. We investigate the possibility of improving the efficiency of reasoning through structure-based partitioning of logical theories, combined with partitionbased logical reasoning strategies. To this end, we provide algorithms for reasoning with partitions of axioms in first-order and propositional logic. We analyze the computational benefit of our algorithms and detect those parameters of a partitioning that influence the efficiency of computation. These parameters are the number of symbols shared by a pair of partitions, the size of each partition, and the topology of the partitioning. Finally, we provide a greedy algorithm that automatically reformulates a given theory into partitions, exploiting the parameters that influence the efficiency of computation.
Proper Motions and Variability of the H Emission in the HH~46/47system
We report here on the first proper motion measurements of molecular hydrogen
emission features in the Herbig-Haro 46/47 outflow. Assuming a distance of 350
pc to this flow, the inferred tangential velocities range from a few tens to
almost 500 km/s . The highest velocities are observed for H2 knots either in,
or close to, the jet/counterjet axes. Knots constituting the wings of the large
scale H2 bow (see, for example, Eisl\"offel et al. 1994) are found to move much
more slowly. These results appear to be in agreement with recent numerical
simulations of H2 emission from pulsed jets. We also report the first detection
of variability in H2 features for a young stellar object (YSO) outflow. It was
found that several H2 knots significantly changed their luminosity over the 4
year timebase used to conduct our study. This is in line with current estimates
for the cooling time of gas radiating shocked H2 emission in YSO environments.Comment: 2 figure
Infrared and Millimetric Study of the Young Outflow Cepheus E
The Cepheus E outflow has been studied in the mid and far infrared using the
ISO CAM and LWS instruments, and at millimetric wavelengths using OVRO. In the
near and mid-IR, its morphology is similar to that expected for a jet driven
outflow, where the leading bow shocks entrain and accelerate the surrounding
molecular gas. As expected, fine structure atomic/ionic emission lines arise
from the bow shocks, at both the Mach Disk and the stagnation tip, where
J-shocks are dominant. The H2, H2O and CO molecular emission could arise
further `downstream' at the bow shock wings where the shocks (v = 8-35 km/s)
are oblique and more likely to be C-type. The 13CO emission arises from
entrained molecular gas and a compact high velocity emission is observed,
together with an extended low velocity component that almost coincides
spatially with the H2 near-IR emission. The millimetric continuum emission
shows two sources. We identify one of them with IRAS 23011+6126, postulating is
the driver of the Cepheus E outflow; the other, also an embedded source, is
likely to be driving one of other outflows observed in the region.Comment: 47 pages, 13 figure
Multi-wavelength spectroscopy of the bipolar outflow from Cepheus E
Cepheus E is the site of an exceptional example of a protostellar outflow
with a very young dynamical age and extremely high near infrared luminosity. We
combine molecular spectroscopic data from the submillimeter to the near
infrared in order to interpret the rotational excitation of CO and the
ro-vibrational excitation of H2. We conclude that C-type shocks with a
paraboloidal bow shock geometry can simultaneously explain all the molecular
excitations. Extinction accounts for the deviation of the column densities from
local thermodynamic equilibrium. A difference in the extinction between the red
and blue-shifted outflow lobes may account for the measured flux difference.
The outflow is deeply embedded in a clump of density 10^5cm^-3, yet a good
fraction of atomic hydrogen, about 40%, is required to explain the excitation
and statistical equilibrium. We propose that this atomic component arises,
self-consistently, from the dissociated gas at the apex of the leading bow
shocks and the relatively long molecule reformation time. At least 20 bow
shocks are required in each lobe, although these may be sub-divided into
smaller bows and turbulent shocked regions. The total outflow mechanical power
and cooling amounts to over 30L_\odot, almost half the source's bolometric
luminosity. Nevertheless, only about 6% of the clump mass has been set in
outward motion by the outflow, allowing a collapse to continue.Comment: 14 pages, 8 figures, accepted for publication in Ap
Bow shocks, Wiggling Jets, and Wide-Angle Winds: A High Resolution Study of the Entrainment Mechanism of the PV Ceph Molecular (CO) Outflow
We present a new set of high-resolution molecular line maps of the gas
immediately surrounding various Herbig-Haro (HH) knots of the giant HH flow HH
315, from the young star PV Cephei. The observations, aimed at studying the
entrainment mechanism of the 2.6 pc-long HH 315 flow, include IRAM 30m maps of
the 12CO(2-1), 12CO(1-0), and 13CO(1-0) lines, with beam sizes of 11'', 21'',
and 22'', respectively. We compare the morphology and the kinematics of the
outflow gas, as well as the temperature and momentum distribution of the
molecular outflow with those predicted by different entrainment models. With
our detailed study we are able to conclude that jet bow shock entrainment by an
episodic stellar wind, with a time-varying axis, produces most of the
high-velocity molecular outflow observed far from the source. In addition, near
PV Cephei we find evidence for a poorly collimated, wide-angle, molecular
outflow and a collimated wiggling jet-like molecular outflow. We propose that
the poorly collimated component is entrained by a wide-angle wind, and the
collimated component is entrained by a variable jet with internal working
surfaces. If this picture is true, then a stellar wind model which allows for
the coexistence of a wide-angle component and a collimated (jet-like) stellar
wind component is needed to explain the observed properties of the PV Ceph
outflow. The wiggling axis of the redshifted molecular outflow lobe indicates
that the outflow ejection axis is changing over time. We find that the
time-scale of the axis variation shown by the molecular outflow lobe is about a
factor of 10 less than that shown by the large-scale optical HH knots.Comment: 41 pages, including 7 tables. 18 figures included separately. Version
with embedded full-resolution figures available at
http://www.astro.caltech.edu/~harce/papers . Accepted by The Astrophysical
Journa
Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
We study the formation of H2 in the ISM, using a modified version of the
astrophysical magnetohydrodynamical code ZEUS-MP that includes a
non-equilibrium treatment of the formation and destruction of H2. We examine
two different approximations to treat the shielding of H2 against
photodissociation: a local approximation, which gives us a solid lower bound on
the amount of shielding, and a method based on ray-tracing that is considerably
more accurate in some circumstances but that produces results that are harder
to clearly interpret. Either approximation allows one to perform
three-dimensional high-resolution simulations of cloud formation with only
modest computational resources. We also include a detailed treatment of the
thermal behaviour of the gas.
In this paper, we focus on the problem of molecular cloud formation in
gravitationally unstable, initially static gas. We show that in these
conditions, and for initial densities consistent with those observed in the
cold, neutral atomic phase of the interstellar medium, H2 formation occurs on a
timescale t > 10 Myr, comparable to or longer than the gravitational free-fall
timescale of the cloud. We also show that the collapsing gas very quickly
reaches thermal equilibrium and that the equation of state of the gas is
generally softer than isothermal.
Finally, we demonstrate that although these results show little sensitivity
to variations in most of our simulation parameters, they are highly sensitive
to the assumed initial density n_i. Reducing n_i significantly increases the
cloud formation timescale and decreases the amount of hydrogen ultimately
converted to H2. (Abridged).Comment: 89 pages, 40 figures, AASTex. Results section significantly revised
and extended. Includes results from a large number of new simulations
performed using a treatment of H2 photodissociation based on ray-tracing.
This version matches that accepted by ApJ
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