351 research outputs found
Observations of pre-stellar cores
Our understanding of the physical and chemical structure of pre-stellar
cores, the simplest star-forming sites, has significantly improved since the
last IAU Symposium on Astrochemistry (South Korea, 1999). Research done over
these years has revealed that major molecular species like CO and CS
systematically deplete onto dust grains at the interior of pre-stellar cores,
while species like N2H+ and NH3 survive in the gas phase and can usually be
detected towards the core centers. Such a selective behaviour of molecular
species gives rise to a differentiated (onion-like) chemical composition, and
manifests itself in molecular maps as a dichotomy between centrally peaked and
ring-shaped distributions. From the point of view of star-formation studies,
the identification of molecular inhomogeneities in cores helps to resolve past
discrepancies between observations made using different tracers, and brings the
possibility of self-consistent modelling of the core internal structure. Here I
present recent work on determining the physical and chemical structure of two
pre-stellar cores, L1498 and L1517B, using observations in a large number of
molecules and Monte Carlo radiative transfer analysis. These two cores are
typical examples of the pre-stellar core population, and their chemical
composition is characterized by the presence of large freeze out holes in most
molecular species. In contrast with these chemically processed objects, a new
population of chemically young cores has started to emerge. The characteristics
of its most extreme representative, L1521E, are briefly reviewed.Comment: 10 pages, 5 figures. To appear in IAU 231 conf. proc.
"Astrochemistry: Recent Successes and Current Challenges," eds. D.C. Lis,
G.A. Blake, and E. Herbs
Nitrogen chemistry and depletion in starless cores
We investigated the chemistry of nitrogen--containing species, principally
isotopomers of CN, HCN, and HNC, in a sample of pre-protostellar cores. We used
the IRAM 30 m telescope to measure the emission in rotational and hyperfine
transitions of CN, HCN, 13CN, H13CN, HN13C, and HC15N, in L 1544, L 183, Oph D,
L 1517B, L 310. The observations were made along axial cuts through the dust
emission peak, at a number of regularly--spaced offset positions. The
observations were reduced and analyzed to obtain the column densities, using
the measurements of the less abundant isotopic variants in order to minimize
the consequences of finite optical depths in the lines. The observations were
compared with the predictions of a free--fall gravitational collapse model,
which incorporates a non-equilibrium treatment of the relevant chemistry. We
found that CN, HCN, and HNC remain present in the gas phase at densities well
above that at which CO depletes on to grains. The CN:HCN and the HNC:HCN
abundance ratios are larger than unity in all the objects of our sample.
Furthermore, there is no observational evidence for large variations of these
ratios with increasing offset from the dust emission peak and hence with
density. Whilst the differential freeze--out of CN and CO can be understood in
terms of the current chemistry, the behaviour of the CN:HCN ratio is more
difficult to explain. Models suggest that most nitrogen is not in the gas phase
but may be locked in ices. Unambiguous conclusions require measurements of the
rate coefficients of the key neutral--neutral reactions at low temperatures
The chemical structure of the very young starless core L1521E
L1521E is a dense starless core in Taurus that was found to have relatively
low molecular depletion by earlier studies, thus suggesting a recent formation.
We aim to characterize the chemical structure of L1521E and compare it to the
more evolved L1544 pre-stellar core. We have obtained 2.52.5
arcminute maps toward L1521E using the IRAM-30m telescope in transitions of
various species. We derived abundances for the species and compared them to
those obtained toward L1544. We estimated CO depletion factors. Similarly to
L1544, -CH and CHOH peak at different positions. Most species
peak toward the -CH peak. The CO depletion factor derived toward the
dust peak is 4.31.6, which is about a factor of three lower
than that toward L1544. The abundances of sulfur-bearing molecules are higher
toward L1521E than toward L1544 by factors of 2-20. The abundance of
methanol is similar toward the two cores. The higher abundances of
sulfur-bearing species toward L1521E than toward L1544 suggest that significant
sulfur depletion takes place during the dynamical evolution of dense cores,
from the starless to pre-stellar stage. The CO depletion factor measured toward
L1521E suggests that CO is more depleted than previously found. Similar
CHOH abundances between L1521E and L1544 hint that methanol is forming at
specific physical conditions in Taurus, characterized by densities of a few
10 cm and (H)10 cm, when CO
starts to catastrophically freeze-out, while water can still be significantly
photodissociated, so that the surfaces of dust grains become rich in solid CO
and CHOH, as already found toward L1544. Methanol can thus provide
selective crucial information about the transition region between dense cores
and the surrounding parent cloud.Comment: Accepted for publication in A&A, abstract abridge
CCH in prestellar cores
We study the abundance of CCH in prestellar cores both because of its role in
the chemistry and because it is a potential probe of the magnetic field. We
also consider the non-LTE behaviour of the N=1-0 and N=2-1 transitions of CCH
and improve current estimates of the spectroscopic constants of CCH. We used
the IRAM 30m radiotelescope to map the N=1-0 and N=2-1 transitions of CCH
towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the
1.3 mm dust emission, the J=1-0 transition of N2H+ and the J=2-1 transition of
C18O. We used a Monte Carlo radiative transfer program to analyse the CCH
observations of L1498. We derived the distribution of CCH column densities and
compared with the H2 column densities inferred from dust emission. We find that
while non-LTE intensity ratios of different components of the N=1-0 and N=2-1
lines are present, they are of minor importance and do not impede CCH column
density determinations based upon LTE analysis. Moreover, the comparison of our
Monte-Carlo calculations with observations suggest that the non-LTE deviations
can be qualitatively understood. For L1498, our observations in conjunction
with the Monte Carlo code imply a CCH depletion hole of radius 9 x 10^{16} cm
similar to that found for other C-containing species. We briefly discuss the
significance of the observed CCH abundance distribution. Finally, we used our
observations to provide improved estimates for the rest frequencies of all six
components of the CCH(1-0) line and seven components of CCH(2-1). Based on
these results, we compute improved spectroscopic constants for CCH. We also
give a brief discussion of the prospects for measuring magnetic field strengths
using CCH.Comment: 14 pages, 13 figures, to be published in Astronomy and Astrophysic
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
CN in prestellar cores
Determining the structure of and the velocity field in prestellar cores is
essential to understanding protostellar evolution.} {We have observed the dense
prestellar cores L 1544 and L 183 in the rotational transition of
CN and \thcn in order to test whether CN is depleted in the high--density
nuclei of these cores.} {We have used the IRAM 30 m telescope to observe along
the major and minor axes of these cores. We compare these observations with the
1 mm dust emission, which serves as a proxy for the hydrogen column
density.}{We find that while CN\jone is optically thick, the distribution of
\thcn\jone intensity follows the dust emission well, implying that the CN
abundance does not vary greatly with density. We derive an abundance ratio of
\rm [CN]/[\hh]=\dix{-9} in L 183 and 1-3\tdix{-9} in L 1544, which, in the
case of L 183, is similar to previous estimates obtained by sampling
lower--density regions of the core.}{We conclude that CN is not depleted
towards the high--density peaks of these cores and thus behaves like the
N-containing molecules \nnhp and \nhhh. CN is, to our knowledge, the first
C--containing molecule to exhibit this characteristic.Comment: Accepted for publication in A&A Letter
Dense Cores in Dark Clouds. XIV. N2H+(1-0) maps of dense cloud cores
We present results of an extensive mapping survey of N2H+(1-0) in about 60
low mass cloud cores already mapped in the NH3(1,1) inversion transition line.
The survey has been carried out at the FCRAO antenna with an angular resolution
about 1.5 times finer than the previous ammonia observations. Cores with stars
typically have map sizes about a factor of two smaller for N2H+ than for NH3,
indicating the presence of denser and more centrally concentrated gas compared
to starless cores. Significant correlations are found between NH3 and N2H+
column densities and excitation temperatures in starless cores, but not in
cores with stars, suggesting a different chemical evolution of the two species.
Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been
found with NH3 data. ``Local'' velocity gradients show significant variation in
both magnitude and direction, suggesting the presence of complexmotions not
interpretable as simple solid body rotation. Integrated intensity profiles of
starless cores present a ``central flattening'' and are consistent with a
spherically symmetric density law n ~ r^{-1.2} for r < ~0.03 pc and n ~ r^{-2}
at larger r. Cores with stars are better modelled with single density power
laws with n ~ r^{-2}. Line widths change across the core but we did not find a
general trend. The deviation in line width correlates with the mean line width,
suggesting that the line of sight contains ~ 10 coherence lengths. The
corresponding value of the coherence length, ~ 0.01 pc, is similar to the
expected cutoff wavelength for MHD waves. This similarity may account for the
increased ``coherence'' of line widths on small scales. Despite of the finer
angular resolution, the majority of N2H+ and NH3 maps show a similar ``simple''
structure, with single peaks and no elongation.Comment: 62 pages, 11 figures, ApJ, in pres
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&
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