Strong irradiation of protostellar cores in Corona Australis

The importance of the physical environment in the evolution of newly formed low-mass stars remains an open question. In particular, radiation from nearby more massive stars may affect both the physical and chemical structure of these kinds of young stars. Aims: To constrain the physical characteristics of a group of embedded low-mass protostars in Corona Australis in the vicinity of the young luminous Herbig Be star R CrA. Methods: Millimetre wavelength maps of molecular line and continuum emission towards the low-mass star forming region IRS7 near R CrA from the SMA and APEX are presented. The maps show the distribution of 18 lines from 7 species (H2CO, CH3OH, HC3N, c-C3H2, HCN, CN and SiO) on scales from 3" to 60" (400-8000 AU). Using a set of H2CO lines, we estimate the temperatures and column densities in the region using LTE and non-LTE methods. The results are compared with 1-D radiative transfer modelling of the protostellar cores. These models constrain which properties of the central source, envelope, and environment can give rise to the observed line and continuum emission. Results: Most of the H2CO emission from the regions emerges from two elongated narrow ridges dominating the emission picked up in both interferometric and single-dish measurements. The temperatures inferred from the H2CO lines are no less than ~30 K and more likely 50-60 K, and the line emission peaks are offset by ~2500 AU from the location of the embedded protostars. The temperatures can not be explained by the heating from the young stellar objects themselves. Irradiation by the nearby Herbig Be star R CrA could, however, explain the high temperatures. The elevated temperatures can in turn impact the physical and chemical characteristics of protostars and lead to enhanced abundances of typical tracers of photon dominated regions seen in single-dish line surveys of embedded protostars in the region.Comment: Accepted for publication in A&A; 21 pages, 28 figures; Added footnote in Section 2.

Critical Current 0-π\pi Transition in Designed Josephson Quantum Dot Junctions

We report on quantum dot based Josephson junctions designed specifically for measuring the supercurrent. From high-accuracy fitting of the current-voltage characteristics we determine the full magnitude of the supercurrent (critical current). Strong gate modulation of the critical current is observed through several consecutive Coulomb blockade oscillations. The critical current crosses zero close to, but not at, resonance due to the so-called 0-$\pi$ transition in agreement with a simple theoretical model.Comment: 5 pages, 4 figures, (Supplementary information available at http://www.fys.ku.dk/~hij/public/nl_supp.pdf

The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars

(Abridged) The water deuterium fractionation (HDO/H$_2$O abundance ratio) has traditionally been used to infer the amount of water brought to Earth by comets. Measuring this ratio in deeply-embedded low-mass protostars makes it possible to probe the critical stage when water is transported from clouds to disks in which icy bodies are formed. We present sub-arcsecond resolution observations of HDO in combination with H$_2^{18}$O from the PdBI toward the three low-mass protostars NGC 1333-IRAS 2A, IRAS 4A-NW, and IRAS 4B. The resulting HDO/H$_2$O ratio is $7.4\pm2.1\times10^{-4}$ for IRAS 2A, $19.1\pm5.4\times10^{-4}$ for IRAS 4A-NW, and $5.9\pm1.7\times10^{-4}$ for IRAS 4B. Derived ratios agree with radiative transfer models within a factor of 2-4 depending on the source. Our HDO/H$_2$O ratios for the inner regions (where $T>100$ K) of four young protostars are only a factor of 2 higher than those found for pristine, solar system comets. These small differences suggest that little processing of water occurs between the deeply embedded stage and the formation of planetesimals and comets.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

On C*-algebras generated by pairs of q-commuting isometries

We consider the C*-algebras O_2^q and A_2^q generated, respectively, by isometries s_1, s_2 satisfying the relation s_1^* s_2 = q s_2 s_1^* with |q| < 1 (the deformed Cuntz relation), and by isometries s_1, s_2 satisfying the relation s_2 s_1 = q s_1 s_2 with |q| = 1. We show that O_2^q is isomorphic to the Cuntz-Toeplitz C*-algebra O_2^0 for any |q| < 1. We further prove that A_2^{q_1} is isomorphic to A_2^{q_2} if and only if either q_1 = q_2 or q_1 = complex conjugate of q_2. In the second part of our paper, we discuss the complexity of the representation theory of A_2^q. We show that A_2^q is *-wild for any q in the circle |q| = 1, and hence that A_2^q is not nuclear for any q in the circle.Comment: 18 pages, LaTeX2e "article" document class; submitted. V2 clarifies the relationships between the various deformation systems treate

Single wall carbon nanotube double quantum dot

We report on two top-gate defined, coupled quantum dots in a semiconducting single wall carbon nanotube, constituting a tunable double quantum dot system. The single wall carbon nanotubes are contacted by titanium electrodes, and gated by three narrow top-gate electrodes as well as a back-gate. We show that a bias spectroscopy plot on just one of the two quantum dots can be used to extract the addition energy of both quantum dots. Furthermore, honeycomb charge stability diagrams are analyzed by an electrostatic capacitor model that includes cross capacitances, and we extract the coupling energy of the double quantum dot.Comment: Published in Applied Physics Letters 4 December 2006. http://link.aip.org/link/?APL/89/23211

Atomic jet from SMM1 (FIRS1) in Serpens uncovers non-coeval binary companion

We report on the detection of an atomic jet associated with the protostellar source SMM1 (FIRS1) in Serpens. The jet is revealed in [FeII] and [NeII] line maps observed with Spitzer/IRS, and further confirmed in HiRes IRAC and MIPS images. It is traced very close to SMM1 and peaks at ~5 arcsec" from the source at a position angle of $\sim 125 degrees. In contrast, molecular hydrogen emission becomes prominent at distances > 5" from the protostar and extends at a position angle of 160 degrees. The morphological differences suggest that the atomic emission arises from a companion source, lying in the foreground of the envelope surrounding the embedded protostar SMM1. In addition the molecular and atomic Spitzer maps disentangle the large scale CO (3-2) emission observed in the region into two distinct bipolar outflows, giving further support to a proto-binary source setup. Analysis at the peaks of the [FeII] jet show that emission arises from warm and dense gas (T ~1000 K, n(electron) 10^5 - 10^6 cm^-3). The mass flux of the jet derived independently for the [FeII] and [NeII] lines is 10^7 M(sun)/yr, pointing to a more evolved Class~I/II protostar as the driving source. All existing evidence converge to the conclusion that SMM1 is a non-coeval proto-binary source.Comment: 10 pages, 7 figures, 1 table. Accepted for publication in Astronomy \& Astrophysic

On the origin of H_2CO abundance enhancements in low-mass protostars

High angular resolution H_2CO 218 GHz line observations have been carried out toward the low-mass protostars IRAS 16293-2422 and L1448-C using the Owens Valley Millimeter Array at ~2" resolution. Simultaneous 1.37 mm continuum data reveal extended emission which is compared with that predicted by model envelopes constrained from single-dish data. For L1448-C the model density structure works well down to the 400 AU scale to which the interferometer is sensitive. For IRAS 16293-2422 , a known proto-binary object, the interferometer observations indicate that the binary has cleared much of the material in the inner part of the envelope, out to the binary separation of ~800 AU. For both sources there is excess unresolved compact emission centered on the sources, most likely due to accretion disks ≾200 AU in size with masses of ≳0.02 M_☉ (L1448-C) and ≳0.1 M_☉ (IRAS 16293-2422). The H_2CO data for both sources are dominated by emission from gas close to the positions of the continuum peaks. The morphology and velocity structure of the H_2CO array data have been used to investigate whether the abundance enhancements inferred from single-dish modelling are due to thermal evaporation of ices or due to liberation of the ice mantles by shocks in the inner envelope. For IRAS 16293-2422 the H_2CO interferometer observations indicate the presence of rotation roughly perpendicular to the large scale CO outflow. The H_2CO distribution differs from that of C^(18)O, with C^(18)O emission peaking near MM1 and H_2CO stronger near MM2. For L1448-C, the region of enhanced H_2CO emission extends over a much larger scale >1" than the radius of 50-100 K (0."6-0".15) where thermal evaporation can occur. The red-blue asymmetry of the emission is consistent with the outflow; however the velocities are significantly lower. The H_2CO 3_(22)-2_(21)/3_(03)-2_(02) flux ratio derived from the interferometer data is significantly higher than that found from single-dish observations for both objects, suggesting that the compact emission arises from warmer gas. Detailed radiative transfer modeling shows, however, that the ratio is affected by abundance gradients and optical depth in the 3_(03)-2_(02) line. It is concluded that a constant H_2CO abundance throughout the envelope cannot fit the interferometer data of the two H_2CO lines simultaneously on the longest and shortest baselines. A scenario in which the H_2CO abundance drops in the cold dense part of the envelope where CO is frozen out but is undepleted in the outermost region provides good fits to the single-dish and interferometer data on short baselines for both sources. Emission on the longer baselines is best reproduced if the H_2CO abundance is increased by about an order of magnitude from ~ 10^(-10) to ~ 10^(-9) in the inner parts of the envelope due to thermal evaporation when the temperature exceeds ~50 K. The presence of additional H_2CO abundance jumps in the innermost hot core region or in the disk cannot be firmly established, however, with the present sensitivity and resolution. Other scenarios, including weak outflow-envelope interactions and photon heating of the envelope, are discussed and predictions for future generation interferometers are presented, illustrating their potential in distinguishing these competing scenarios

Externally heated protostellar cores in the Ophiuchus star-forming region

We present APEX 218 GHz observations of molecular emission in a complete sample of embedded protostars in the Ophiuchus star-forming region. To study the physical properties of the cores, we calculate H$_2$CO and c-C$_3$H$_2$ rotational temperatures, both of which are good tracers of the kinetic temperature of the molecular gas. We find that the H$_2$CO temperatures range between 16 K and 124 K, with the highest H$_2$CO temperatures toward the hot corino source IRAS 16293-2422 (69-124 K) and the sources in the $\rho$ Oph A cloud (23-49 K) located close to the luminous Herbig Be star S 1, which externally irradiates the $\rho$ Oph A cores. On the other hand, the c-C$_3$H$_2$ rotational temperature is consistently low (7-17 K) in all sources. Our results indicate that the c-C$_3$H$_2$ emission is primarily tracing more shielded parts of the envelope whereas the H$_2$CO emission (at the angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the outer irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino emission dominates. In some sources, a secondary velocity component is also seen, possibly tracing the molecular outflow.Comment: 19 pages, 9 figures, accepted for publication in Ap

Singlet-Triplet Physics and Shell Filling in Carbon Nanotube Double Quantum Dots

An artifcial two-atomic molecule, also called a double quantum dot (DQD), is an ideal system for exploring few electron physics. Spin-entanglement between just two electrons can be explored in such systems where singlet and triplet states are accessible. These two spin-states can be regarded as the two states in a quantum two-state system, a so-called singlet-triplet qubit. A very attractive material for realizing spin based qubits is the carbon nanotube (CNT), because it is expected to have a very long spin coherence time. Here we show the existence of a gate-tunable singlet-triplet qubit in a CNT DQD. We show that the CNT DQD has clear shell structures of both four and eight electrons, with the singlet-triplet qubit present in the four-electron shells. We furthermore observe inelastic cotunneling via the singlet and triplet states, which we use to probe the splitting between singlet and triplet, in good agreement with theory.Comment: Supplement available at: http://www.fys.ku.dk/~hij/public/singlet-triple_supp.pd