1,181 research outputs found
New detections of H2O masers in planetary nebulae and post-AGB stars using the Robledo-70m antenna
Aims: We investigated the possible relationship between the evolutionary
stage of post-AGB stars and planetary nebulae (PNe) and the presence of water
masers in their envelopes.
Methods: We have used NASA's 70-m antenna in Robledo de Chavela (Spain) to
search for the water maser transition at 22235.08 MHz, towards a sample of 105
sources with IRAS colour characteristic of post-AGB stars and PNe at
declination >-32 deg. 83% of the sources in the sample are post-AGB stars, 15%
PNe or PN candidates, while only 2% seem to be HII regions.
Results: We have detected five water masers, of which four are reported for
the first time: two in PNe (IRAS 17443-2949 and IRAS 18061-2505), a ``water
fountain'' in a post-AGB star (IRAS 16552-3050), and one in a source previously
catalogued as a PN, but whose classification is uncertain (IRAS 17580-3111).
Conclusions: The unexpected detections of water masers in two objects among
the small subset of PNe led us to suggest that the PNe harbouring water masers
are a special type of massive, rapidly evolving PNe.Comment: 15 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
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&
Monte Carlo simulations of H2 formation on stochastically heated grains
Continuous-time, random-walk Monte Carlo simulations of H2 formation on
grains have been performed for surfaces that are stochastically heated by
photons. We have assumed diffuse cloud conditions and used a variety of grains
of varying roughness and size based on olivine. The simulations were performed
at different optical depths. We confirmed that small grains (r <= 0.02 micron)
have low modal temperatures with strong fluctuations, which have a large effect
on the efficiency of the formation of molecular hydrogen. The grain size
distribution highly favours small grains and therefore H2 formation on these
particles makes a large contribution to the overall formation rate for all but
the roughest surfaces. We find that at A_V=0 only the roughest surfaces can
produce the required amount of molecular hydrogen, but by A_V=1, smoother
surfaces are possible alternatives. Use of a larger value for the evaporation
energy of atomic hydrogen, but one still consistent with experiment, allows
smoother surfaces to produce more H2.Comment: MNRAS LaTeX, 10 pages, 11 eps-figures to be published in MNRA
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
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
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