284 research outputs found
A Survey for Infall Motions toward Starless Cores. II. and Mapping Observations
We present the results of an extensive mapping survey of 53 `starless' cores
in the optically thick line of CS 2-1 and the optically thin lines of N2H+ 1-0
and C18O 1-0. The purpose of this survey was to search for signatures of
extended inward motions.
This study finds 10 `strong' and 9 `probable' infall candidates, based on
analysis and on the spectral shapes of CS lines.
From our analysis of the blue-skewed CS spectra and the
parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple
two layer radiative transfer model to fit the profiles, we derive
one-dimensional infall speeds, half of whose values lie in the range of
0.05-0.09 km s. These values are similar to those found in L1544 by
Tafalla et al., and this result confirms that infall speeds in starless cores
are generally faster than expected from ambipolar diffusion in a strongly
sub-critical core. In addition, the observed infall regions are too extended to
be consistent with the `inside-out' collapse model applied to a very low-mass
star. In the largest cores, the spatial extent of the CS spectra with infall
asymmetry is larger than the extent of the core by a factor of
2-3. All these results suggest that extended inward motions are a common
feature in starless cores, and that they could represent a necessary stage in
the condensation of a star-forming dense core.Comment: Two tex files for manuscript and tables, and 38 figures. To appear in
ApJ
Transmission through a biased graphene bilayer barrier
We study the electronic transmission through a graphene bilayer in the
presence of an applied bias between layers. We consider different geometries
involving interfaces between both a monolayer and a bilayer and between two
bilayers. The applied bias opens a sizable gap in the spectrum inside the
bilayer barrier region, thus leading to large changes in the transmission
probability and electronic conductance that are controlled by the applied bias.Comment: 10 pages, 8 figures, extended versio
Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes
Monolayer graphene exhibits exceptional electronic and mechanical properties,
making it a very promising material for nanoelectromechanical (NEMS) devices.
Here, we conclusively demonstrate the piezoresistive effect in graphene in a
nano-electromechanical membrane configuration that provides direct electrical
readout of pressure to strain transduction. This makes it highly relevant for
an important class of nano-electromechanical system (NEMS) transducers. This
demonstration is consistent with our simulations and previously reported gauge
factors and simulation values. The membrane in our experiment acts as a strain
gauge independent of crystallographic orientation and allows for aggressive
size scalability. When compared with conventional pressure sensors, the sensors
have orders of magnitude higher sensitivity per unit area.Comment: 20 pages, 3 figure
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
DFT study of graphene doping due to metal contacts
The experimental results of Metal\u2013graphene (M\u2013G) contact resistance (RC) have been investigated in\u2013depth by means of Density Functional Theory (DFT). The simulations allowed us to build a consistent picture explaining the RC dependence on the metal contact materials employed in this work and on the applied back\u2013gate voltage. In this respect, the M\u2013G distance is paramount in determining the RC behavior
Dense Cores in Dark Clouds. XIV. N2H+(1-0) maps of dense cloud cores
We present results of an extensive mapping survey of N2H+(1-0) in about 60
low mass cloud cores already mapped in the NH3(1,1) inversion transition line.
The survey has been carried out at the FCRAO antenna with an angular resolution
about 1.5 times finer than the previous ammonia observations. Cores with stars
typically have map sizes about a factor of two smaller for N2H+ than for NH3,
indicating the presence of denser and more centrally concentrated gas compared
to starless cores. Significant correlations are found between NH3 and N2H+
column densities and excitation temperatures in starless cores, but not in
cores with stars, suggesting a different chemical evolution of the two species.
Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been
found with NH3 data. ``Local'' velocity gradients show significant variation in
both magnitude and direction, suggesting the presence of complexmotions not
interpretable as simple solid body rotation. Integrated intensity profiles of
starless cores present a ``central flattening'' and are consistent with a
spherically symmetric density law n ~ r^{-1.2} for r < ~0.03 pc and n ~ r^{-2}
at larger r. Cores with stars are better modelled with single density power
laws with n ~ r^{-2}. Line widths change across the core but we did not find a
general trend. The deviation in line width correlates with the mean line width,
suggesting that the line of sight contains ~ 10 coherence lengths. The
corresponding value of the coherence length, ~ 0.01 pc, is similar to the
expected cutoff wavelength for MHD waves. This similarity may account for the
increased ``coherence'' of line widths on small scales. Despite of the finer
angular resolution, the majority of N2H+ and NH3 maps show a similar ``simple''
structure, with single peaks and no elongation.Comment: 62 pages, 11 figures, ApJ, in pres
Systematic Molecular Differentiation in Starless Cores
(Abridged) We present evidence that low-mass starless cores, the simplest
units of star formation, are systematically differentiated in their chemical
composition. Molecules including CO and CS almost vanish near the core centers,
where the abundance decreases by one or two orders of magnitude. At the same
time, N2H+ has a constant abundance, and the fraction of NH3 increases toward
the core center. Our conclusions are based on a study of 5 mostly-round
starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have
mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and
(2,2), and the 1.2 mm continuum. For each core we have built a model that fits
simultaneously the radial profile of all observed emission and the central
spectrum for the molecular lines. The observed abundance drops of CO and CS are
naturally explained by the depletion of these molecules onto dust grains at
densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to
densities of several 10^5, while the NH3 abundance may be enhanced by reactions
triggered by the disappearance of CO from the gas phase. With the help of our
models, we show that chemical differentiation automatically explains the
discrepancy between the sizes of CS and NH3 maps, a problem which has remained
unexplained for more than a decade. Our models, in addition, show that a
combination of radiative transfer effects can give rise to the previously
observed discrepancy in the linewidth of these two tracers. Although this
discrepancy has been traditionally interpreted as resulting from a systematic
increase of the turbulent linewidth with radius, our models show that it can
arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap
A Catalogue of Optically Selected Cores
We present a new catalogue of 406 dense cores optically selected by using the
STScI Digitized Sky Survey (DSS). In this catalogue 306 cores have neither an
Embedded YSO (EYSO) nor a Pre-Main-Sequence (PMS) star, 94 cores have EYSOs (1
core has both an EYSO and a PMS star), and 6 cores have PMS star only. Our
sample of dense cores in the catalogue is fairly complete within a category of
northern Lynds class 5, 6 clouds, and southern Hartley et al. (1986)'s class A
clouds, providing a database useful for the systematic study of dense cores.
Most of the cores listed in the catalogue have diameters between
pc with a mean of pc. The sizes ( pc in the mean) of
cores with EYSOs are found to be usually larger than the sizes ( pc
in the mean) of starless cores. The typical mean gas density of the cores is
. Most of the cores are more likely elongated than
spherical (mean aspect ratio: ). The ratio of the number of cores
with EYSOs to the number of starless cores for our sample is about 0.3,
suggesting that the typical lifetime of starless cores is Myr, about
3 times longer than the duration of the Class 0 and Class I phases. This
lifetime is shorter than expected from models of ambipolar diffusion, by
factors of 2-44.Comment: 22 pages, 8 figures, 3 tables, and to appear in ApJS.
Harvard-Smithsonian Center for Astrophysic
CCH in prestellar cores
We study the abundance of CCH in prestellar cores both because of its role in
the chemistry and because it is a potential probe of the magnetic field. We
also consider the non-LTE behaviour of the N=1-0 and N=2-1 transitions of CCH
and improve current estimates of the spectroscopic constants of CCH. We used
the IRAM 30m radiotelescope to map the N=1-0 and N=2-1 transitions of CCH
towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the
1.3 mm dust emission, the J=1-0 transition of N2H+ and the J=2-1 transition of
C18O. We used a Monte Carlo radiative transfer program to analyse the CCH
observations of L1498. We derived the distribution of CCH column densities and
compared with the H2 column densities inferred from dust emission. We find that
while non-LTE intensity ratios of different components of the N=1-0 and N=2-1
lines are present, they are of minor importance and do not impede CCH column
density determinations based upon LTE analysis. Moreover, the comparison of our
Monte-Carlo calculations with observations suggest that the non-LTE deviations
can be qualitatively understood. For L1498, our observations in conjunction
with the Monte Carlo code imply a CCH depletion hole of radius 9 x 10^{16} cm
similar to that found for other C-containing species. We briefly discuss the
significance of the observed CCH abundance distribution. Finally, we used our
observations to provide improved estimates for the rest frequencies of all six
components of the CCH(1-0) line and seven components of CCH(2-1). Based on
these results, we compute improved spectroscopic constants for CCH. We also
give a brief discussion of the prospects for measuring magnetic field strengths
using CCH.Comment: 14 pages, 13 figures, to be published in Astronomy and Astrophysic
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