218 research outputs found
The origin of the molecular emission around the southern hemisphere Re 4 IRS - HH 188 region
We present SEST observations of the molecular environment ahead of the
southern Herbig-Haro object 188 (HH188), associated with the low-mass protostar
Re4 IRS. We have also used the SuperCosmos Halpha survey to search for Halpha
emission associated with the Re4 IRS - HH188 region. The aim of the present
work is to study the properties of the molecular gas and to better characterize
this southern star forming region. We mapped the HCO+ 3-2 and H13CO+ 1-0
emission around the YSO and took spectra of the CH3OH 2(0)-1(0)A+ and
2(-1)-1(-1)E and SO 6(5)-5(4) towards the central source. Column densities are
derived and different scenarios are considered to explain the origin of the
molecular emission. HCO+ arises from a relatively compact region around the
YSO; however, its peak emission is displaced to the south following the outflow
direction. Our chemical analysis indicates that a plausible scenario is that
most of the emission arises from the cold, illuminated dense gas ahead of the
HH188 object. We have also found that HH188, a high excitation object, seems to
be part of a parsec scale and highly collimated HH system. Re4 IRS is probably
a binary protostellar system, in the late Class 0 or Class I phase. One of the
protostars, invisible in the near-IR, seems to power the HH188 system.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Chemistry of dense clumps near moving Herbig-Haro objects
Localised regions of enhanced emission from HCO+, NH3 and other species near
Herbig-Haro objects (HHOs) have been interpreted as arising in a photochemistry
stimulated by the HHO radiation on high density quiescent clumps in molecular
clouds. Static models of this process have been successful in accounting for
the variety of molecular species arising ahead of the jet; however recent
observations show that the enhanced molecular emission is widespread along the
jet as well as ahead. Hence, a realistic model must take into account the
movement of the radiation field past the clump. It was previously unclear as to
whether the short interaction time between the clump and the HHO in a moving
source model would allow molecules such as HCO+ to reach high enough levels,
and to survive for long enough to be observed. In this work we model a moving
radiation source that approaches and passes a clump. The chemical picture is
qualitatively unchanged by the addition of the moving source, strengthening the
idea that enhancements are due to evaporation of molecules from dust grains. In
addition, in the case of several molecules, the enhanced emission regions are
longer-lived. Some photochemically-induced species, including methanol, are
expected to maintain high abundances for ~10,000 years.Comment: 7 pages, 3 figure
Multitransitional observations of the CS core of L673
A multitransitional study with the BIMA interferometric array was carried out
toward the starless core found in the L673 region, in order to study the
small-size structure of the cores detected with previous single--dish
observations, which provides us with a test of the predictions of the chemical
model of Taylor et al. (1996; 1998). We detected emission in the CS (2-1), N2H+
(1-0), and HCO+ (1-0) lines. Several clumps of size ~0.08 pc were found for
each line distributed all over the region where previous single-dish emission
was found (Morata et al. 1997). Each molecular transition traces differently
the clump distribution, although in some cases the detected clumps are
coincident. The distribution of the N2H+ emission and the single-dish NH3
emission are coincident and compatible with an origin in the same gas. The
large fraction of missing flux measured for the CS (2-1) transition can be
explained if the cloud is formed by a clumpy and heterogeneous medium. Four
positions were selected to derive the abundance ratios [N2H+/CS] and [HCO+/CS]
from the molecular column density determinations, and to compare them with the
values predicted by the chemical model. The model was able to explain the
interferometric observations, and, in particular, the chemical differentiation
of the detected clumps and the coincidence of the NH3 and N2H+ emissions. The
lack of HCO+ towards the two selected positions that trace the more evolved
clumps cannot be accounted for by the model, but it is possibly due to strong
self-absorption. We propose a classification of the studied clumps according to
the stage of chemical evolution indicated by the molecular abundances.Comment: 10 pages, 9 figures, accepted for publication in A&
Modeling the magnetic field in the protostellar source NGC 1333 IRAS 4A
Magnetic fields are believed to play a crucial role in the process of star
formation. We compare high-angular resolution observations of the submillimeter
polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a
low-mass protostar, with models of the collapse of magnetized molecular cloud
cores. Assuming a uniform dust alignment efficiency, we computed the Stokes
parameters and synthetic polarization maps from the model density and magnetic
field distribution by integrations along the line-of-sight and convolution with
the interferometric response. The synthetic maps are in good agreement with the
data. The best-fitting models were obtained for a protostellar mass of 0.8
solar masses, of age 9e4 yr, formed in a cloud with an initial mass-to-flux
ratio ~2 times the critical value. The magnetic field morphology in NGC 1333
IRAS 4A is consistent with the standard theoretical scenario for the formation
of solar-type stars, where well-ordered, large-scale, rather than turbulent,
magnetic fields control the evolution and collapse of the molecular cloud cores
from which stars form.Comment: 4 pages, 5 figures. Accepted by Astronomy and Astrophysic
The L723 low mass star forming protostellar system: resolving a double core
We present 1.35 mm SMA observations around the low-mass Class 0 source IRAS
19156+1906, at the the center of the L723 dark cloud. We detected emission from
dust as well as emission from H2CO, DCN and CN, which arise from two cores, SMA
1 and SMA 2, separated by 2.9" (880 AU). SMA 2 is associated with VLA 2. SiO
5-4 emission is detected, possibly tracing a region of interaction between the
dense envelope and the outflow. We modeled the dust and the H2CO emission from
the two cores: they have similar physical properties but SMA 2 has a larger
p-H2CO abundance than SMA 1. The p-H2CO abundances found are compatible with
the value of the outer part of the circumstellar envelopes associated with
Class 0 sources. SMA 2 is likely more evolved than SMA 1. The kinematics of the
two sources show marginal evidence of infall and rotation motions. The mass
detected by the SMA observation, which trace scales of ~1000 AU, is only a
small fraction of the mass contained in the large scale molecular envelope,
which suggests that L723 is still in a very early phase of star formation.
Despite the apparent quiescent nature of the L723, fragmentation is occurring
at the center of the cloud at different scales. Thus, at 1000 AU the cloud has
fragmented in two cores, SMA 1 and SMA 2. At the same time, at least one of
these cores, SMA 2, has undergone additional fragmentation at scales of 150 AU,
forming a multiple stellar system.Comment: 35 pages, 15 figures. Accepted to the Astrophysical Journa
Evidence for transient clumps and gas chemical evolution in the CS core of L673
We present FCRAO maps as well as combined BIMA and FCRAO maps of the high
density molecular emission towards the CS core in the L673 region. With the
FCRAO telescope, we mapped the emission in the CS(2-1), C34S(2-1), HCO+(1-0),
and H13CO+(1-0) lines. The high density molecular emission, which arises from a
filamentary structure oriented in the NW-SE direction, shows clear
morphological differences for each molecule. We find that HCO+ has an extremely
high optical depth, and that the H13CO+ emission is well correlated with submm
sources. The BIMA and FCRAO combined maps recover emission from a lot of other
structure which was previously undetected or only marginally detected, and show
an overall aspect of a filamentary structure connecting several intense clumps.
We found a total 15 clumps in our combined data cube, all of them resolved by
our angular resolution, with diameters in the 0.03-0.09 pc range. We find a
clear segregation between the northern and southern region of the map: the
northern section shows the less chemically evolved gas and less massive but
more numerous clumps, while the southern region is dominated by the largest and
most massive clump, and contains the more evolved gas, as traced by emission of
late-time molecules. We find that the derived clump masses are below the virial
mass, and that the clumps masses become closer to the virial mass when they get
bigger and more massive. This supports the idea that these clumps must be
transient, and only the more massive ones have a chance to last long enough to
form stars. The clumps we detect are probably in an earlier evolutionary stage
than the ``starless cores'' reported recently in the literature. Only the most
massive one has properties similar to a ``starless core''.Comment: 12 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics; minor revisions after language editin
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
An observational survey of molecular emission ahead of Herbig-Haro objects
Context. A molecular survey recently performed ahead of HH~2 supports the
idea that the observed molecular enhancement is due to UV radiation from the HH
object. Aims. The aim of the present work is to determine whether all HH
objects with enhanced HCO emission ahead of them also exhibit the same
enhanced chemistry as HH~2. We thus observed several molecular lines at several
positions ahead of five Herbig-Haro objects where enhanced HCO emission was
previously observed. Methods. We mapped the five Herbig-Haro objects using the
IRAM-30 m. For each position we searched for more than one molecular species,
and where possible for more than one transition per species. We then estimated
the averaged beam column densities for all species observed and also performed
LVG analyses to constrain the physical properties of the gas. Results. The
chemically richest quiescent gas is found ahead of the HH~7-11 complex, in
particular at the HH~7-11 A position. In some regions we also detected a high
velocity gas component. We find that the gas densities are always higher than
those typical of a molecular cloud while the derived temperatures are always
quite low, ranging from 10 to 25 K. The emission of most species seems to be
enhanced with respect to that of a typical dense clump, probably due to the
exposure to a high UV radiation from the HH objects. Chemical differentiation
among the positions is also observed. We attempt a very simple chemical
analysis to explain such differentiation.Comment: Accepted by Astronomy and Astrophysics; 17 pages, 8 figure
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