763 research outputs found
The structure of molecular clumps around high-mass young stellar objects
We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular
clumps associated with 12 ultracompact (UC) HII regions in the J=6-5, 8-7 and
13-12 rotational transitions of methyl-acetylene (CH3C2H). Under the assumption
of LTE and optically thin emission, we have derived temperature estimates
ranging from 30 to 56 K. We estimate that the clumps have diameters of 0.2-1.6
pc, H_2 densities of 10^5-10^6 {cm^{-3}}, and masses of 10^2-2 10^4 M_\odot. We
compare these values with those obtained by other authors from different
molecular tracers and find that the H_2 density and the temperature inside the
clumps vary respectively like n_{H_2} ~ R^{-2.6} and T ~ R^{-0.5}, with R
distance from the centre. We also find that the virial masses of the clumps are
~3 times less than those derived from the CH3C2H column densities: we show that
a plausible explanation is that magnetic fields play an important role to
stabilise the clumps, which are on the verge of gravitational collapse.
Finally, we show that the CH3C2H line width increases for decreasing distance
from the clump centre: this effect is consistent with infall in the inner
regions of the clumps. We conclude that the clumps around UC HII regions are
likely to be transient (~10^(5) yr) entities, remnants of isothermal spheres
currently undergoing gravitational collapse: the high mass accretion rates
(~10^{-2} M_\odot yr^{-1}) lead to massive star formation at the centre of such
clumps.Comment: 15 pages, 11 figures, A & A in pres
A spectral line survey of the starless and proto-stellar cores detected by BLAST toward the Vela-D molecular cloud
We present a 3-mm and 1.3-cm spectral line survey conducted with the Mopra
22-m and Parkes 64-m radio telescopes of a sample of 40 cold dust cores,
previously observed with BLAST, including both starless and proto-stellar
sources. 20 objects were also mapped using molecular tracers of dense gas. To
trace the dense gas we used the molecular species NH3, N2H+, HNC, HCO+, H13CO+,
HCN and H13CN, where some of them trace the more quiescent gas, while others
are sensitive to more dynamical processes. The selected cores have a wide
variety of morphological types and also show physical and chemical variations,
which may be associated to different evolutionary phases. We find evidence of
systematic motions in both starless and proto-stellar cores and we detect line
wings in many of the proto-stellar cores. Our observations probe linear
distances in the sources >~0.1pc, and are thus sensitive mainly to molecular
gas in the envelope of the cores. In this region we do find that, for example,
the radial profile of the N2H+(1-0) emission falls off more quickly than that
of C-bearing molecules such as HNC(1-0), HCO+(1-0) and HCN(1-0). We also
analyze the correlation between several physical and chemical parameters and
the dynamics of the cores. Depending on the assumptions made to estimate the
virial mass, we find that many starless cores have masses below the
self-gravitating threshold, whereas most of the proto-stellar cores have masses
which are near or above the self-gravitating critical value. An analysis of the
median properties of the starless and proto-stellar cores suggests that the
transition from the pre- to the proto-stellar phase is relatively fast, leaving
the core envelopes with almost unchanged physical parameters.Comment: Submitted for publication to Astronomy & Astrophysics on January
18th, 201
Reverberation of pulsar wind nebulae (III): Modelling of the plasma interface empowering a long term radiative evolution
The vast majority of Pulsar Wind Nebulae (PWNe) present in the Galaxy is
formed by middle-aged systems characterized by a strong interaction of the PWN
itself with the supernova remnant (SNR). Unfortunately, modelling these systems
can be quite complex and numerically expensive, due to the non-linearity of the
PWN-SNR evolution even in the simple 1D / one-zone case when the reverse shock
of the SNR reaches the PWN, and the two begin to interact (and reverberation
starts).
Here we introduce a new numerical technique that couples the numerical
efficiency of the one-zone thin shell approach with the reliability of a full
``lagrangian'' evolution, able to correctly reproduce the PWN-SNR interaction
during the reverberation and to consistently evolve the particle spectrum
beyond. Based on our previous findings, we show that our novel strategy
resolves many of the uncertainties present in previous approaches, as the
arbitrariness in the SNR structure, and ensure a robust evolution, compatible
with results that can be obtained with more complex 1D dynamical approaches.
Our approach enable us for the first time to provide reliable spectral models
of the later compression phases in the evolution of PWNe. While in general we
found that the compression is less extreme than that obtained without such
detailed dynamical considerations, leading to the formation of less structured
spectral energy distributions, we still find that a non negligible fraction of
PWNe might experience a super-efficient phase, with the optical and/or X-ray
luminosity exceeding the spin-down one.Comment: 12 pages, 2 tables, 5 figure
Spitzer-IRAC survey of molecular jets in Vela-D
We present a survey of H2 jets from young protostars in the Vela-D molecular
cloud (VMR-D), based on Spitzer -IRAC data between 3.6 and 8.0 micron. Our
search has led to the identification of 15 jets and about 70 well aligned knots
within 1.2 squared degree. We compare the IRAC maps with observations of the H2
1-0 S(1) line at 2.12 micron, with a Spitzer-MIPS map at 24 and 70 micron, and
with a map of the dust continuum emission at 1.2 mm. We find a association
between molecular jets and dust peaks. The jet candidate exciting sources have
been searched for in the published catalog of the Young Stellar Objects of
VMR-D. We selected all the sources of Class II or earlier which are located
close to the jet center and aligned with it.The association between jet and
exciting source was validated by estimating the differential extinction between
the jet opposite lobes. We are able to find a best-candidate exciting source in
all but two jets. Four exciting sources are not (or very barely) observed at
wavelengths shorter than 24 micron, suggesting they are very young protostars.
Three of them are also associated with the most compact jets. The exciting
source Spectral Energy Distributions have been modeled by means of the
photometric data between 1.2 micron and 1.2 mm. From SEDs fits we derive the
main source parameters, which indicate that most of them are low-mass
protostars. A significant correlation is found between the projected jet length
and the [24] - [70] color, which is consistent with an evolutionary scenario
according to which shorter jets are associated with younger sources. A rough
correlation is found between IRAC line cooling and exciting source bolometric
luminosity, in agreement with the previous literature. The emerging trend
suggests that mass loss and mass accretion are tightly related phenomena and
that both decrease with time.Comment: Accepted by The Astrophysical Journa
A New Galactic 6cm Formaldehyde Maser
We report the detection of a new H2CO maser in the massive star forming
region G23.71-0.20 (IRAS 18324-0820), i.e., the fifth region in the Galaxy
where H2CO maser emission has been found. The new H2CO maser is located toward
a compact HII region, and is coincident in velocity and position with 6.7 GHz
methanol masers and with an IR source as revealed by Spitzer/IRAC GLIMPSE data.
The coincidence with an IR source and 6.7 GHz methanol masers suggests that the
maser is in close proximity to an embedded massive protostar. Thus, the
detection of H2CO maser emission toward G23.71-0.20 supports the trend that
H2CO 6cm masers trace molecular material very near young massive stellar
objects.Comment: Accepted for publication in The Astrophysical Journal Letter
Reverberation of pulsar wind nebulae (I): Impact of the medium properties and other parameters upon the extent of the compression
The standard approach to the long term evolution of pulsar wind nebulae
(PWNe) is based on one-zone models treating the nebula as a uniform system. In
particular for the late phase of evolved systems, many of the generally used
prescriptions are based on educated guesses for which a proper assessment
lacks. Using an advanced radiative code we evaluate the systematic impact of
various parameters, like the properties of the supernova ejecta, of the inner
pulsar, as well of the ambient medium, upon the extent of the reverberation
phase of PWNe. We investigate how different prescriptions shift the starting
time of the reverberation phase, how this affects the amount of the
compression, and how much of this can be ascribable to the radiation processes.
Some critical aspects are the description of the reverse shock evolution, the
efficiency by which at later times material from the ejecta accretes onto the
swept-up shell around the PWN, and finally the density, velocity and pressure
profiles in the surrounding supernova remnant. We have explicitly treated the
cases of the Crab Nebula, and of J1834.9--0846, taken to be representatives of
the more and the less energetic pulsars, respectively. Especially for the
latter object the prediction of large compression factors is confirmed, even
larger in the presence of radiative losses, also confirming our former
prediction of periods of super-efficiency during the reverberation phase of
some PWNe.Comment: 12 pages, 7 figures, accepted for publication in MNRA
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