8,243 research outputs found
The role of low-mass star clusters in massive star formation. The Orion Case
To distinguish between the different theories proposed to explain massive
star formation, it is crucial to establish the distribution, the extinction,
and the density of low-mass stars in massive star-forming regions. We analyze
deep X-ray observations of the Orion massive star-forming region using the
Chandra Orion Ultradeep Project (COUP) catalog. We studied the stellar
distribution as a function of extinction, with cells of 0.03 pc x 0.03 pc, the
typical size of protostellar cores. We derived stellar density maps and
calculated cluster stellar densities. We found that low-mass stars cluster
toward the three massive star-forming regions: the Trapezium Cluster (TC), the
Orion Hot Core (OHC), and OMC1-S. We derived low-mass stellar densities of
10^{5} stars pc^{-3} in the TC and OMC1-S, and of 10^{6} stars pc^{-3} in the
OHC. The close association between the low-mass star clusters with massive star
cradles supports the role of these clusters in the formation of massive stars.
The X-ray observations show for the first time in the TC that low-mass stars
with intermediate extinction are clustered toward the position of the most
massive star, which is surrounded by a ring of non-extincted low-mass stars.
This 'envelope-core' structure is also supported by infrared and optical
observations. Our analysis suggests that at least two basic ingredients are
needed in massive star formation: the presence of dense gas and a cluster of
low-mass stars. The scenario that better explains our findings assumes high
fragmentation in the parental core, accretion at subcore scales that forms a
low-mass stellar cluster, and subsequent competitive accretion. Finally,
although coalescence does not seem a common mechanism for building up massive
stars, we show that a single stellar merger may have occurred in the evolution
of the OHC cluster, favored by the presence of disks, binaries, and gas
accretion.Comment: 17 pages, 11 figures, 3 Tables. Accepted for publication in A&
Rashba interaction in quantum wires with in-plane magnetic fields
We analyze the spectral and transport properties of ballistic quasi
one-dimensional systems in the presence of spin-orbit coupling and in-plane
agnetic fields. Our results demonstrate that Rashba precession and intersubband
coupling must be treated on equal footing for wavevectors near the magnetic
field induced gaps. We find that intersubband coupling limits the occurrence of
negative effective masses at the gap edges and modifies the linear conductance
curves in the strong coupling limit. The effect of the magnetic field on the
spin textured orientation of the wire magnetization is discussed.Comment: 6 pages, 6 figures; new figures, discussion extende
The magnetic precursor of L1448-mm: Excitation differences between ion and neutral fluids
Shock modelling predicts an electron density enhancement within the magnetic
precursor of C-shocks. Previous observations of SiO, H13CO+, HN13C and H13CN
toward the young L1448-mm outflow showed an over-excitation of the ion fluid
that was attributed to an electron density enhancement in the precursor. We
re-visit this interpretation and test if it still holds when we consider
different source morphologies and kinetic temperatures for the observed
molecules, and also give some insight on the spatial extent of the electron
density enhancement around L1448-mm.
We estimate the opacities of H13CO+ and HN13C by observing the J=3\to2 lines
of rarer isotopologues to confirm that the emission is optically thin. To model
the excitation of the molecules, we use the large velocity gradient (LVG)
approximation with updated collisional coefficients to i) re- analyse the
observations toward the positions where the over-excitation of H13CO+ has
previously been observed [i.e. toward L1448- mm at offsets (0,0) and (0,-10)],
and ii) to investigate if the electron density enhancement is still required
for the cases of extended and compact emission, and for kinetic temperatures of
up to 400 K. We also report several lines of SiO, HN13C and H13CO+ toward new
positions around this outflow, to investigate the spatial extent of the
over-excitation of the ions in L1448-mm. From the isotopologue observations, we
find that the emission of H13CO+ and HN13C from the precursor is optically thin
if this emission is extended. Using the new collisional coefficients, an
electron density enhancement is still needed to explain the excitation of
H13CO+ for extended emission and for gas temperatures of\le 400 K toward
L1448-mm (0,-10), and possibly also toward L1448-mm (0,0). For compact emission
the data cannot be fitted. We do not find any evidence for the over-excitation
of the ion fluid toward the newly observed positions around L1448-mm.
The observed line emission of SiO, H13CO+ and HN13C toward L1448-mm (0,0) and
(0,-10) is consistent with an electron density enhancement in the precursor
component, if this emission is spatially extended. This is also true for the
case of high gas temperatures (\le400 K) toward the (0,-10) offset. The
electron density enhancement seems to be restricted to the southern, redshifted
lobe of the L1448-mm outflow. Interferometric images of the line emission of
these molecules are needed to confirm the spatial extent of the over-excitation
of the ions and thus, of the electron density enhancement in the magnetic
precursor of L1448-mm.Comment: Accepted for publication in A&A; 9 pages, 3 figure
A new intermediate mass protostar in the Cepheus A HW2 region
We present the discovery of the first molecular hot core associated with an
intermediate mass protostar in the CepA HW2 region. The hot condensation was
detected from single dish and interferometric observations of several high
excitation rotational lines (from 100 to 880K above the ground state) of SO2 in
the ground vibrational state and of HC3N in the vibrationally excited states
v7=1 and v7=2. The kinetic temperature derived from both molecules is 160K. The
high-angular resolution observations (1.25'' x 0.99'') of the SO2
J=28(7,21)-29(6,24) line (488K above the ground state) show that the hot gas is
concentrated in a compact condensation with a size of 0.6''(430AU), located
0.4'' (300AU) east from the radio-jet HW2. The total SO2 column density in the
hot condensation is 10E18cm-2, with a H2 column density ranging from 10E23 to 6
x 10E24cm-2. The H2 density and the SO2 fractional abundance must be larger
than 10E7cm-3 and 2 x 10E-7 respectively. The most likely alternatives for the
nature of the hot and very dense condensation are discussed. From the large
column densities of hot gas, the detection of the HC3N vibrationally excited
lines and the large SO2 abundance, we favor the interpretation of a hot core
heated by an intermediate mass protostar of 10E3 Lo. This indicates that the
CepA HW2 region contains a cluster of very young stars
On the evolution of the molecular line profiles induced by the propagation of C-shock waves
We present the first results of the expected variations of the molecular line
emission arising from material recently affected by C-shocks (shock
precursors). Our parametric model of the structure of C-shocks has been coupled
with a radiative transfer code to calculate the molecular excitation and line
profiles of shock tracers such as SiO, and of ion and neutral molecules such as
H13CO+ and HN13C, as the shock propagates through the unperturbed medium. Our
results show that the SiO emission arising from the early stage of the magnetic
precursor typically has very narrow line profiles slightly shifted in velocity
with respect to the ambient cloud. This narrow emission is generated in the
region where the bulk of the ion fluid has already slipped to larger velocities
in the precursor as observed toward the young L1448-mm outflow. This strongly
suggests that the detection of narrow SiO emission and of an ion enhancement in
young shocks, is produced by the magnetic precursor of C-shocks. In addition,
our model shows that the different velocity components observed toward this
outflow can be explained by the coexistence of different shocks at different
evolutionary stages, within the same beam of the single-dish observations.Comment: 7 pages, 4 figures, accepted for publication in Ap
Deep spectroscopic luminosity function of Abell 85: no evidence for a steep upturn of the faint-end slope
We present a new deep determination of the spectroscopic LF within the virial
radius of the nearby and massive Abell\,85 (A85) cluster down to the dwarf
regime (M* + 6) using VLT/VIMOS spectra for galaxies with m mag and mag arcsec. The
resulting LF from 438 cluster members is best modelled by a double Schechter
function due to the presence of a statistically significant upturn at the
faint-end. The amplitude of this upturn (),
however, is much smaller than that of the SDSS composite photometric cluster LF
by Popesso et al. 2006, -2. The faint-end slope of the LF in
A85 is consistent, within the uncertainties, with that of the field. The red
galaxy population dominates the LF at low luminosities, and is the main
responsible for the upturn. The fact that the slopes of the spectroscopic LFs
in the field and in a cluster as massive as A85 are similar suggests that the
cluster environment does not play a major role in determining the abundance of
low-mass galaxies.Comment: 6 pages, 4 figures, accepted at MNRAS lette
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