3,018 research outputs found
Measurements of the 12C/13C ratio in Planetary Nebulae and implications for stellar evolution
We present the results of a study aimed at determining the 12C/13C ratio in
two samples of planetary nebuale (PNe) by means of mm-wave observations of 12CO
and 13CO. The first group includes six PNe which have been observed in the 3He+
hyperfine transition; the other group consists of 23 nebulae with rich
molecular envelopes. We have determined the isotopic ratio in 14 objects and
the results indicate a range of values between 9 and 23. In particular, three
PNe have ratios well below the value predicted by standard evolutionary models
(about 20), indicating that some extra-mixing process has occurred in these
stars. We briefly discuss the implications of our results for standard and
nonstandard stellar nucleosynthesis.Comment: 8 pages, LaTeX, 1 Postscript figure. to appear in The Primordial
Nuclei and Their Galactic Evolution, eds. N. Prantzos, M. Tosi, R. von
Steiger (Kluwer: Dordrecht
Neutral atomic carbon in the globules of the Helix
We report detection of the 609u line of neutral atomic carbon in globules of
the Helix nebula. The measurements were made towards the position of peak CO
emission. At the same position, we obtained high-quality CO(2-1) and 13CO(2-1)
spectra and a 135" x 135" map in CO(2-1). The velocity distribution of CI shows
six narrow (1 -> 2 km/sec) components which are associated with individual
globules traced in CO. The CI column densities are 0.5 -> 1.2 x 10^16/cm^2. CI
is found to be a factor of ~6 more abundant than CO. Our estimate for the mass
of the neutral envelope is an order of magnitude larger than previous
estimates. The large abundance of CI in the Helix can be understood as a result
of the gradual photoionisation of the molecular envelope by the central star's
radiation field.Comment: 5 pages, Latex, AAS macros, 3 EPS figures, to appear in Astrophysical
Journal Letter
Shells, jets, and internal working surfaces in the molecular outflow from IRAS 04166+2706
Context: IRAS 04166+2706 in Taurus is one of the most nearby young stellar
objects whose molecular outflow contains a highly collimated fast component.
Methods: We have observed the IRAS 04166+2706 outflow with the IRAM Plateau
de Bure interferometer in CO(J=2-1) and SiO(J=2-1) achieving angular
resolutions between 2'' and 4''. To improve the quality of the CO(2-1) images,
we have added single dish data to the interferometer visibilities.
Results: The outflow consists of two distinct components. At velocities <10
km/s, the gas forms two opposed, approximately conical shells that have the YSO
at their vertex. These shells coincide with the walls of evacuated cavities and
seem to result from the acceleration of the ambient gas by a wide-angle wind.
At velocities >30 km/s, the gas forms two opposed jets that travel along the
center of the cavities and whose emission is dominated by a symmetric
collection of at least 7 pairs of peaks. The velocity field of this component
presents a sawtooth pattern with the gas in the tail of each peak moving faster
than the gas in the head. This pattern, together with a systematic widening of
the peaks with distance to the central source, is consistent with the emission
arising from internal working surfaces traveling along the jet and resulting
from variations in the velocity field of ejection. We interpret this component
as the true protostellar wind, and we find its composition consistent with a
chemical model of such type of wind.
Conclusions: Our results support outflow wind models that have simultaneously
wide-angle and narrow components, and suggest that the EHV peaks seen in a
number of outflows consist of internally-shocked wind material.Comment: 13 pages, 10 figures. To appear in A&
High spectral resolution observations of HNC3 and HCCNC in the L1544 prestellar core
HCCNC and HNC3 are less commonly found isomers of cyanoacetylene, HC3N, a
molecule that is widely found in diverse astronomical sources. We want to know
if HNC3 is present in sources other than the dark cloud TMC-1 and how its
abundance is relative to that of related molecules. We used the ASAI unbiased
spectral survey at IRAM 30m towards the prototypical prestellar core L1544 to
search for HNC3 and HCCNC which are by-product of the HC3NH+ recombination,
previously detected in this source. We performed a combined analysis of
published HNC3 microwave rest frequencies with thus far unpublished millimeter
data because of issues with available rest frequency predictions. We determined
new spectroscopic parameters for HNC3, produced new predictions and detected it
towards L1544. We used a gas-grain chemical modelling to predict the abundances
of N-species and compare with the observations. The modelled abundances are
consistent with the observations, considering a late stage of the evolution of
the prestellar core. However the calculated abundance of HNC3 was found 5-10
times higher than the observed one. The HC3N, HNC3 and HCCNC versus HC3NH+
ratios are compared in the TMC-1 dark cloud and the L1544 prestellar core.Comment: Accepted in MNRAS letters. 5 pages plus 2 additional pages for the
on-line materia
W40 region in the Gould Belt : An embedded cluster and H II region at the junction of filaments
We present a multiwavelength study of W40 star-forming region using IR
observations in UKIRT JHK bands, Spitzer IRAC bands & Herschel PACS bands; 2.12
micron H2 narrow-band imaging; & radio observations from GMRT (610 & 1280 MHz),
in a FoV of ~34'x40'. Spitzer observations along with NIR observations are used
to identify 1162 Class II/III & 40 Class I sources in the FoV. The NN stellar
surface density analysis shows that majority of these YSOs constitute the
embedded cluster centered on the source IRS1A South. Some YSOs, predominantly
younger population, are distributed along & trace the filamentary structures at
lower stellar surface density. The cluster radius is obtained as 0.44pc -
matching well with the extent of radio emission - with a peak density of
650pc^-2. The JHK data is used to map the extinction which is subsequently used
to compute the cloud mass. It has resulted in 126 Msun & 71 Msun for the
central cluster & the northern IRS5 region, respectively. H2 narrow-band
imaging displays significant emission, which prominently resembles fluorescent
emission arising at the borders of dense regions. Radio analysis shows this
region as having blister morphology, with the radio peak coinciding with a
protostellar source. Free-free emission SED analysis is used to obtain physical
parameters of the overall region & the IRS5 sub-region. This multiwavelength
scenario is suggestive of star formation having resulted from merging of
multiple filaments to form a hub. Star formation seems to have taken place in
two successive epochs, with the first epoch traced by the central cluster & the
high-mass star(s) - followed by a second epoch which is spreading into the
filaments as uncovered by the Class I sources & even younger protostellar
sources along the filaments. The IRS5 HII region displays indications of
swept-up material which has possibly led to the formation of protostars.Comment: 17 pages, 12 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Molecular hydrogen jets and outflows in the Serpens south filamentary cloud
We aimed to map the jets and outflows from the Serpens South star forming
region and find an empirical relationship between the magnetic field and
outflow orientation. Near-infrared H2 v=1-0 S(1) 2.122{\mu}m -line imaging of
the \sim 30'-long filamentary shaped Serpens South star forming region was
carried out. K s broadband imaging of the same region was used for continuum
subraction. Candidate driving sources of the mapped jets/outflows are
identified from the list of known protostars and young stars in this region,
which was derived from studies using recent Spitzer and Herschel telescope
observations. 14 Molecular Hydrogen emission-line objects(MHOs) are identified
using our continuum-subtracted images. They are found to constitute ten
individual flows. Out of these, nine flows are located in the
lower-half(southern) part of the Serpens South filament, and one flow is
located at the northern tip of the filament. Four flows are driven by
well-identified Class 0 protostars, while the remaining six flows are driven by
candidate protostars mostly in the Class I stage, based on the Spitzer and
Herschel observations. The orientation of the outflows is systematically
perpendicular to the direction of the near-infrared polarization vector,
recently published in the literature. No significant correlation was observed
between the orientation of the flows and the axis of the filamentary cloud.Comment: Accepted by A&A for publication. 7 pages, 5 figure
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
A highly collimated, extremely high velocity outflow in Taurus
We present the first case of a highly collimated, extremely high velocity
bipolar outflow in Taurus. It is powered by the low-luminosity (0.4 L_sun)
source IRAS 04166+2706 and contains gas accelerated up to 50 km/s with respect
to the ambient cloud both toward the blue and the red (uncorrected for
projection). At the highest velocities, the outflow collimation factor exceeds
20, and the gas displays a very high degree of spatial symmetry. This very fast
gas presents multiple maxima, and most likely arises from the acceleration of
ambient material by a time-variable jet-like stellar wind. When scaled for
luminosity, the outflow parameters of IRAS 04166 are comparable to those of
other extremely high velocity outflows like L1448, indicating that even the
very quiescent star-formation mode of Taurus can produce objects powering very
high energy flows (L_mec/L_* > 0.15).Comment: 5pages, 3 figures. Accepted by Astronomy and Astrophysics Letters. v2
clarfies relation with HH390 thanks to private communication from John Bally
and Josh Walawende
First evidence for molecular interfaces between outflows and ambient cloud in high-mass star forming regions?
We present new observations of the CepA-East region of massive star formation and describe an extended and dynamically distinct feature not previously recognised. This feature is present in emission from H2CS, OCS, CH3OH, and HDO at -5.5 km/s, but is not traced by conventional tracers of star forming regions H2S, SO2, SO, CS. The feature is extended up to at least 0.1 pc. We show that the feature is neither a hot core nor a shocked outflow. However, the chemistry of the feature is consistent with predictions of a model of an eroding interface between a fast wind and a dense core; mixing between the two media occurs in the interface on a timescale of 10-50 years. If these observations are confirmed by detailed maps and by detections in species also predicted to be abundant (e.g. HCO+, H2CO, and NH3) this feature would be the first detection of such an interface in regions of massive star formation. An important implication of the model is that a significant reservoir of sulfur in grain mantles is required to be in the form of OCS
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