A Survey for Infall Motions toward Starless Cores. II. CS(2−1)CS (2-1) and N2H+(1−0)N_2H^+ (1-0) 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 $\delta V_{CS}$ analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the $\delta V_{CS}$ 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$^{-1}$. 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 $\rm N_2H^+$ 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 $M_\odot$) and low field strength (of order 10 $\mu$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 $0.05 - 0.36$ pc with a mean of $\sim 0.24$ pc. The sizes ($\sim 0.33$ pc in the mean) of cores with EYSOs are found to be usually larger than the sizes ($\sim 0.22$ pc in the mean) of starless cores. The typical mean gas density of the cores is $\sim7\times 10^3 cm^{-3}$. Most of the cores are more likely elongated than spherical (mean aspect ratio: $\sim 2.4$). 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 $0.3-1.6$ 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