125 research outputs found
The clumpiness of molecular clouds: HCO+ (3--2) survey near Herbig-Haro objects
Some well-studied Herbig Haro objects have associated with them one or more
cold, dense, and quiescent clumps of gas. We propose that such clumps near an
HH object can be used as a general measure of clumpiness in the molecular cloud
that contains that HH object. Our aim is to make a survey of clumps around a
sample of HH objects, and to use the results to make an estimate of the
clumpiness in molecular clouds. All known cold, dense, and quiescent clumps
near HH objects are anomalously strong HCO+ emitters. Our method is, therefore,
to search for strong HCO+ emission as an indicator of a clump near to an HH
object. The searches were made using JCMT and SEST in the HCO+ 3-2 and also
H13CO+ 1-0 lines, with some additional searches for methanol and sulphur
monoxide lines. The sources selected were a sample of 22 HH objects in which no
previous HCO+ emission had been detected. We find that half of the HH objects
have clumps detected in the HCO+ 3-2 line and that all searches in H13CO$+ 1-0
lines show evidence of clumpiness. All condensations have narrow linewidths and
are evidently unaffected dynamically by the HH jet shock. We conclude that the
molecular clouds in which these HH objects are found must be highly
heterogeneous on scales of less than 0.1 pc. An approximate calculation based
on these results suggests that the area filling factor of clumps affected by HH
objects is on the order of 10%. These clumps have gas number densities larger
than 3e4 cm-2.Comment: 11 pages, 14 figures. Accepted for publication in Astronomy and
Astrophysic
Analysis and test of the central-blue-spot infall hallmark
The infall of material onto a protostar, in the case of optically thick line
emission, produces an asymmetry in the blue- and red-wing line emission. For an
angularly resolved emission, this translates in a blue central spot in the
first-order moment (intensity weighted velocity) map. An analytical expression
for the first-order moment intensity as a function of the projected distance
was derived, for the cases of infinite and finite infall radius. The effect of
a finite angular resolution, which requires the numerical convolution with the
beam, was also studied. This method was applied to existing data of several
star-forming regions, namely G31.41+0.31 HMC, B335, and LDN 1287, obtaining
good fits to the first-order moment intensity maps, and deriving values of the
central masses onto which the infall is taking place (G31.41+0.31 HMC: 70-120
; B335: 0.1 ; Guitar Core of LDN 1287: 4.8 ). The
central-blue-spot infall hallmark appears to be a robust and reliable indicator
of infall.Comment: Accepted for publication in A&
The high-velocity outflow in the proto-planetary nebula Hen 3-1475
The proto-planetary nebula Hen 3-1475 shows a remarkable highly collimated
optical jet with an S-shaped string of three pairs of knots and extremely high
velocities. We present here a detailed analysis of the overall morphology,
kinematic structure and the excitation conditions of these knots based on deep
ground-based high dispersion spectroscopy complemented with high spatial
resolution spectroscopy obtained with STIS onboard HST, and WFPC2 [N II]
images. The spectra obtained show double-peaked, extremely wide emission line
profiles, and a decrease of the radial velocities with distance to the source
in a step-like fashion. We find that the emission line ratios observed in the
intermediate knots are consistent with a spectrum arising from the
recombination region of a shock wave with shock velocities ranging from 100 to
150 km/s. We propose that the ejection velocity is varying as a function of
time with a quasi-periodic variability (with timescale of the order of 100
years) and the direction of ejection is also varying with a precession period
of the order of 1500 years.Comment: 19 pages, 8 figures, accepted for publication in A&
Thermal Jeans fragmentation within 1000 AU in OMC-1S
We present subarcsecond 1.3 mm continuum ALMA observations towards the Orion
Molecular Cloud 1 South (OMC-1S) region, down to a spatial resolution of 74 AU,
which reveal a total of 31 continuum sources. We also present subarcsecond 7 mm
continuum VLA observations of the same region, which allow to further study
fragmentation down to a spatial resolution of 40 AU. By applying a Mean Surface
Density of Companions method we find a characteristic spatial scale at ~560 AU,
and we use this spatial scale to define the boundary of 19 `cores' in OMC-1S as
groupings of millimeter sources. We find an additional characteristic spatial
scale at ~2900 AU, which is the typical scale of the filaments in OMC-1S,
suggesting a two-level fragmentation process. We measured the fragmentation
level within each core and find a higher fragmentation towards the southern
filament. In addition, the cores of the southern filament are also the densest
(within 1100 AU) cores in OMC-1S. This is fully consistent with previous
studies of fragmentation at spatial scales one order of magnitude larger, and
suggests that fragmentation down to 40 AU seems to be governed by thermal Jeans
processes in OMC-1S.Comment: Accepted to Ap
The nature of HH 223 from long-slit spectroscopy
HH 223 is a knotty, wiggling nebular emission of ~30" length found in the
L723 star-forming region. It lies projected onto the largest blueshifted lobe
of the cuadrupolar CO outflow powered by a low-mass YSO system embedded in the
core of L723. We analysed the physical conditions and kinematics along HH 223
with the aim of disentangling whether the emission arises from shock-excited,
supersonic gas characteristic of a stellar jet, or is only tracing the wall
cavity excavated by the CO outflow. We performed long-slit optical spectroscopy
along HH 223, crossing all the bright knots (A to E) and part of the
low-brightness emission nebula (F filament). One spectrum of each knot,
suitable to characterize the nature of its emission, was obtained. The physical
conditions and the radial velocity of the HH 223 emission along the slits were
also sampled at smaller scale (0.6") than the knot sizes. {The spectra of all
the HH 223 knots appear as those of the intermediate/high excitation
Herbig-Haro objects. The emission is supersonic, with blueshifted peak
velocities ranging from -60 to -130 km/s. Reliable variations in the kinematics
and physical conditions at smaller scale that the knot sizes are also found.
The properties of the HH 223 emission derived from the spectroscopy confirm the
HH nature of the object, the supersonic optical outflow most probably also
being powered by the YSOs embedded in the L723 core.Comment: A&A accepte
- …