1,025 research outputs found
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&
A submillimeter search for pre- and proto-brown dwarfs in Chamaeleon II
Context. Chamaeleon II molecular cloud is an active star forming region that
offers an excellent opportunity for studying the formation of brown dwarfs in
the southern hemisphere. Aims. Our aims are to identify a population of pre-
and proto- brown dwarfs (5 sigma mass limit threshold of ~0.015 Msun) and
provide information on the formation mechanisms of substellar objects. Methods.
We performed high sensitivity observations at 870 microns using the LABOCA
bolometer at the APEX telescope towards an active star forming region in
Chamaeleon II. The data are complemented with an extensive multiwavelength
catalogue of sources from the optical to the far-infrared to study the nature
of the LABOCA detections. Results. We detect fifteen cores at 870 microns, and
eleven of them show masses in the substellar regime. The most intense objects
in the surveyed field correspond to the submillimeter counterparts of the well
known young stellar objects DK Cha and IRAS 12500-7658. We identify a possible
proto-brown dwarf candidate (ChaII-APEX-L) with IRAC emission at 3.6 and 4.5
microns. Conclusions. Our analysis indicates that most of the spatially
resolved cores are transient, and that the point-like starless cores in the
sub-stellar regime (with masses between 0.016 Msun and 0.066 Msun) could be
pre-brown dwarfs cores gravitationally unstable if they have radii smaller than
220 AU to 907 AU (1.2" to 5" at 178 pc) respectively for different masses. ALMA
observations will be the key to reveal the energetic state of these pre-brown
dwarfs candidates.Comment: 11 pages, 6 figure
Molecular Line Observations of the Small Protostellar Group L1251B
We present molecular line observations of L1251B, a small group of pre- and
protostellar objects, and its immediate environment in the dense C18O core
L1251E. These data are complementary to near-infrared, submillimeter and
millimeter continuum observations reported by Lee et al. (2006, ApJ, 648, 491;
Paper I). The single-dish data of L1251B described here show very complex
kinematics including infall, rotation and outflow motions, and the
interferometer data reveal these in greater detail. Interferometer data of N2H+
1-0 suggest a very rapidly rotating flattened envelope between two young
stellar objects, IRS1 and IRS2. Also, interferometer data of CO 2-1 resolve the
outflow associated with L1251B seen in single-dish maps into a few narrow and
compact components. Furthermore, the high resolution data support recent
theoretical studies of molecular depletions and enhancements that accompany the
formation of protostars within dense cores. Beyond L1251B, single-dish data are
also presented of a dense core located ~150" to the east that, in Paper I, was
detected at 850 micron but has no associated point sources at near- and
mid-infrared wavelengths. The relative brightness between molecules, which have
different chemical timescales, suggests it is less chemically evolved than
L1251B. This core may be a site for future star formation, however, since line
profiles of HCO+, CS, and HCN show asymmetry with a stronger blue peak, which
is interpreted as an infall signature.Comment: 46 pages, 18 figures. Accepted for publication in Ap
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
A search for pre- and proto-brown dwarfs in the dark cloud Barnard 30 with ALMA
In this work we present ALMA continuum observations at 880 m of 30
sub-mm cores previously identified with APEX/LABOCA at 870m in the Barnard
30 cloud. The main goal is to characterize the youngest and lowest mass
population in the cloud. As a result, we report the detection of five (out of
30) spatially unresolved sources with ALMA, with estimated masses between 0.9
and 67 M. From these five sources, only two show gas emission. The
analysis of multi-wavelength photometry from these two objects, namely B30-LB14
and B30-LB19, is consistent with one Class II- and one Class I low-mass stellar
object, respectively. The gas emission is consistent with a rotating disk in
the case of B30-LB14, and with an oblate rotating envelope with infall
signatures in the case of LB19. The remaining three ALMA detections do not have
infrared counterparts and can be classified as either deeply embedded objects
or as starless cores if B30 members. In the former case, two of them (LB08 and
LB31) show internal luminosity upper limits consistent with Very Low Luminosity
objects, while we do not have enough information for LB10. In the starless core
scenario, and taking into account the estimated masses from ALMA and the
APEX/LABOCA cores, we estimate final masses for the central objects in the
substellar domain, so they could be classified as pre-BD core candidates.Comment: Published in A&
First detection of thermal radio jets in a sample of proto-brown dwarf candidates
We observed with the JVLA at 3.6 and 1.3 cm a sample of 11 proto-brown dwarf
candidates in Taurus in a search for thermal radio jets driven by the most
embedded brown dwarfs. We detected for the first time four thermal radio jets
in proto-brown dwarf candidates. We compiled data from UKIDSS, 2MASS, Spitzer,
WISE and Herschel to build the Spectral Energy Distribution (SED) of the
objects in our sample, which are similar to typical Class~I SEDs of Young
Stellar Objects (YSOs). The four proto-brown dwarf candidates driving thermal
radio jets also roughly follow the well-known trend of centimeter luminosity
against bolometric luminosity determined for YSOs, assuming they belong to
Taurus, although they present some excess of radio emission compared to the
known relation for YSOs. Nonetheless, we are able to reproduce the flux
densities of the radio jets modeling the centimeter emission of the thermal
radio jets using the same type of models applied to YSOs, but with
corresponding smaller stellar wind velocities and mass-loss rates, and
exploring different possible geometries of the wind or outflow from the star.
Moreover, we also find that the modeled mass outflow rates for the bolometric
luminosities of our objects agree reasonably well with the trends found between
the mass outflow rates and bolometric luminosities of YSOs, which indicates
that, despite the "excess" centimeter emission, the intrinsic properties of
proto-brown dwarfs are consistent with a continuation of those of very low mass
stars to a lower mass range. Overall, our study favors the formation of brown
dwarfs as a scaled-down version of low-mass stars.Comment: 18 pages, 8 figures, 14 tables, accepted by the Astrophysical Journa
Water maser detections in southern candidates to post-AGB stars and Planetary Nebulae
We intended to study the incidence and characteristics of water masers in the
envelopes of stars in the post-AGB and PN evolutionary stages.
We have used the 64-m antenna in Parkes (Australia) to search for water maser
emission at 22 GHz, towards a sample of 74 sources with IRAS colours
characteristic of post-AGB stars and PNe, at declination . In our
sample, 39% of the sources are PNe or PNe candidates, and 50% are post-AGB
stars or post-AGB candidates.
We have detected four new water masers, all of them in optically obscured
sources: three in PNe candidates (IRAS 12405-6219, IRAS 15103-5754, and IRAS
16333-4807); and one in a post-AGB candidate (IRAS 13500-6106). The PN
candidate IRAS 15103-5754 has water fountain characteristics, and it could be
the first PN of this class found.
We confirm the tendency suggested in Paper I that the presence of water
masers in the post-AGB phase is favoured in obscured sources with massive
envelopes. We propose an evolutionary scenario for water masers in the post-AGB
and PNe stages, in which ``water fountain'' masers could develop during
post-AGB and early PN stages. Later PNe would show lower velocity maser
emission, both along jets and close to the central objects, with only the
central masers remaining in more evolved PNe.Comment: 11 pages, 5 figures. Accepted by Astronomy and Astrophysic
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