Ultracold collisions between two light indistinguishable diatomic molecules: elastic and rotational energy transfer in HD+HD

A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: $T \sim 10^{-8}$ K. A global six-dimensional H$_2$-H$_2$ potential energy surface is adopted from a previous work [Boothroyd {\it et al.}, J. Chem. Phys., {\bf 116}, 666 (2002).] State-resolved integral cross sections $\sigma_{ij\rightarrow i'j'}(\varepsilon_{kin})$ of different quantum-mechanical rotational transitions $ij\rightarrow i'j'$ in the HD molecules and corresponding state-resolved thermal rate coefficients $k_{ij\rightarrow i'j'}(T)$ have been computed. Additionally, for comparison, H$_2$+H$_2$ calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures.Comment: 9 pages, 9 figures. Accepted for publication in Physical Review

A Pre-Protostellar Core in L1551

Large field surveys of NH3, C2S, 13CO and C18O in the L1551 dark cloud have revealed a prolate, pre-protostellar molecular core (L1551-MC) in a relatively quiescent region to the northwest of the well-known IRS 5 source. The kinetic temperature is measured to be 9K, the total mass is ~2Msun, and the average particle density is 10^4-10^5 cm^(-3). L1551-MC is 2.25' x 1.11' in projection oriented at a position angle of 133deg. The turbulent motions are on the order of the sound speed in the medium and contain 4% of the gravitational energy, E_{grav}, of the core. The angular momentum vector is projected along the major axis of L1551-MC corresponding to a rotational energy of 2.5E-3(sin i)^(-2)|E_{grav}|. The thermal energy constitutes about a third of |E_{grav}| and the virial mass is approximately equal to the total mass. L1551-MC is gravitationally bound and in the absence of strong, ~160 microgauss, magnetic fields will likely contract on a ~0.3 Myr time scale. The line profiles of many molecular species suggest that the cold quiescent interior is surrounded by a dynamic, perhaps infalling envelope which is embedded within the ambient molecular gas of L1551.Comment: 27 pages, 7 figures, ApJ accepte

Optimized coupling of cold atoms into a fiber using a blue-detuned hollow-beam funnel

We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow-beam can confine cold atoms to the darkest regions of the beam thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically-oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-{\mu}m radius core hollow-fiber it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When MOT is positioned further away, coupling efficiencies over 50% can be achieved with larger core fibers.Comment: 11 pages, 12 figures, 1 tabl

Phase-space distribution of unbound dark matter near the Sun

We resolve discrepancies in previous analyses of the flow of collisionless dark matter particles in the Sun's gravitational field. We determine the phase-space distribution of the flow both numerically, tracing particle trajectories back in time, and analytically, providing a simple correct relation between the velocity of particles at infinity and at the Earth. We use our results to produce sky maps of the distribution of arrival directions of dark matter particles on Earth at various times of the year. We assume various Maxwellian velocity distributions at infinity describing the standard dark halo and streams of dark matter. We illustrate the formation of a ring, analogous to the Einstein ring, when the Earth is directly downstream of the Sun.Comment: 17 pages, 10 figures (better rendered in ps than pdf

Local dark matter searches with LISA

The drag-free satellites of LISA will maintain the test masses in geodesic motion over many years with residual accelerations at unprecedented small levels and time delay interferometry (TDI) will keep track of their differential positions at level of picometers. This may allow investigations of fine details of the gravitational field in the Solar System previously inaccessible. In this spirit, we present the concept of a method to measure directly the gravitational effect of the density of diffuse Local Dark Matter (LDM) with a constellation of a few drag-free satellites, by exploiting how peculiarly it would affect their relative motion. Using as test bed an idealized LISA with rigid arms, we find that the separation in time between the test masses is uniquely perturbed by the LDM, so that they acquire a differential breathing mode. Such a LDM signal is related to the LDM density within the orbits and has characteristic spectral components, with amplitudes increasing in time, at various frequencies of the dynamics of the constellation. This is the relevant result, in that the LDM signal is brought to non-zero frequencies.Comment: 8 pages, 1 figure; v2: minor changes to match the version in press on Classical and Quantum Gravity (special issue for the 7th International LISA Symposium proceedings

The Thermal Structure of Gas in Pre-Stellar Cores: A Case Study of Barnard 68

We present a direct comparison of a chemical/physical model to multitransitional observations of C18O and 13CO towards the Barnard 68 pre-stellar core. These observations provide a sensitive test for models of low UV field photodissociation regions and offer the best constraint on the gas temperature of a pre-stellar core. We find that the gas temperature of this object is surprisingly low (~7-8 K), and significantly below the dust temperature, in the outer layers (Av < 5 mag) that are traced by C18O and 13CO emission. As shown previously, the inner layers (Av > 5 mag) exhibit significant freeze-out of CO onto grain surfaces. Because the dust and gas are not fully coupled, depletion of key coolants in the densest layers raises the core (gas) temperature, but only by ~1 K. The gas temperature in layers not traced by C18O and 13CO emission can be probed by NH3 emission, with a previously estimated temperature of ~10-11 K. To reach these temperatures in the inner core requires an order of magnitude reduction in the gas to dust coupling rate. This potentially argues for a lack of small grains in the densest gas, presumably due to grain coagulation.Comment: 33 pages, 11 figures, accepted by Astrophysical Journa

High-spatial-resolution observations of NH3 and CH3OH towards the massive twin cores NGC6334 I & I(N)

Molecular line observations of NH3 (J,K)=(1,1), (2,2) and CH3OH at 24.93GHz taken with the Australian Telescope Compact Array (ATCA) toward the massive twin cores NGC6334 I & I(N) reveal significant variations in the line emission between the two massive cores. The UCHII region/hot core NGC6334 I exhibits strong thermal NH3 and CH3OH emission adjacent to the UCHII region and coincident with two mm continuum peaks observed by Hunter et al. (in prep.). In contrast, we find neither compact NH3 nor thermal CH3OH line emission toward NGC6334 I(N). There, the NH3 emission is distributed over a broad region (>1') without a clear peak, and we find Class I CH3OH maser emission with peak brightness temperatures up to 7000K. The maser emission peaks appear to be spatially associated with the interfaces between the molecular outflows and the ambient dense gas. Peak NH3(1,1) line brightness temperatures >= 70K in both regions indicate gas temperatures of the same order. NH3 emission is also detected toward the outflow in NGC6334 I resulting in an estimated rotational temperature of Trot~19K. Furthermore, we observe CH3OH and NH3 absorption toward the UCHII region, the velocity structure is consistent with expanding molecular gas around the UCHII region. Thermal and kinematic effects possibly imposed from the UCHII region on the molecular core are also discussed.Comment: Accepted for the Astrophysical Journa

Infalling Gas Towards the Galactic Center

VLA maps of ammonia emission were made for the Galactic Center region. The NH3(1,1) and NH3(2,2) transitions were observed in three 2' x 2' fields covering Sgr A* and the region 3' immediately south of it. In the central 3 parsecs surrounding Sgr A* we find emission which appears to be associated with the circumnuclear disk (CND), both morphologically and kinematically. This central emission is connected to a long, narrow 2 pc x 10 pc streamer of clumpy molecular gas located towards the south, which appears to be carrying gas from the nearby 20 km/s giant molecular cloud (GMC) to the circumnuclear region. We find a velocity gradient along the streamer, with progressively higher velocities as the gas approaches Sgr A*. The streamer stops at the location of the CND, where the line width of the NH3 emission increases dramatically. This may be the kinematic signature of accretion onto the CND. The ratio of the NH3(2,2)/NH3(1,1) emission indicates that the gas is heated at the northern tip of the streamer, located inside the eastern edge of the CND. The morphology, kinematics and temperature gradients of the gas all indicate that the southern streamer is located at the Galactic Center and is interacting with the circumnuclear region.Comment: 11 pages, 10 figures, accepted by The Astrophysical Journal (figure 1 contours have been corrected

NH3 in the Central 10 pc of the Galaxy I: General Morphology and Kinematic Connections Between the CND and GMCs

New VLA images of NH3 (1,1), (2,2), and (3,3) emission in the central 10 parsecs of the Galaxy trace filamentary streams of gas, several of which appear to feed the circumnuclear disk (CND). The NH3 images have a spatial resolution of 16.5''x14.5'' and have better spatial sampling than previous NH3 observations. The images show the ``southern streamer,'' ``50 km/s cloud,'' and new features including a ``western streamer'', 6 parsecs in length, and a ``northern ridge'' which connects to the CND. NH3(3,3) emission is very similar to 1.2 mm dust emission indicating that NH3 traces column density well. Ratios of the NH3(2,2) to (1,1) line intensities give an estimate of the temperature of the gas and indicate high temperatures close to the nucleus and CND. The new data cover a velocity range of 270 km/s, including all velocities observed in the CND, with a resolution of 9.8 km/s. Previous NH3 observations with higher resolution did not cover the entire range of velocities seen in the CND. The large-scale kinematics of the CND do not resemble a coherent ring or disk. We see evidence for a high velocity cloud within a projected distance of 50'' (2 pc) which is only seen in NH3(3,3) and is likely to be hot. Comparison to 6 cm continuum emission reveals that much of the NH3 emission traces the outer edges of Sgr A East and was probably pushed outward by this expanding shell. The connection between the northern ridge (which appears to be swept up by Sgr A East) and the CND indicates that Sgr A East and the CND are in close proximity to each other. Kinematic evidence for these connections is presented in this paper, while the full kinematic analysis of the central 10 pc will be presented in Paper II.Comment: 16 pages (containing 6 figures), 8 additional JPEG figures. Accepted for publication in ApJ. For full resolution images, see http://cfa-www.harvard.edu/~rmcgary/SGRA/nh3_figures.htm