Illustration of a two-member constellation of small spacecraft at Venus with crosslinks for radio occultations that can lead to global coverage with high spatial and temporal resolutions. SummaryFrom Mercury to the outer reaches of the solar system, the past six decades have witnessed a vast set of discoveries utilizing radio science (RS) methods.For example, based on key gravitational evidence, sub-surface oceans have been inferred at Titan, Enceladus, and Europa, where potential future missions may search for life.

We report new radio interferometric observations of the quadruple pre-main-sequence (PMS) system ABD oradus. From these observations, combined with existing VLT near-infrared relative astrometry, we have refined the estimates of the dynamical masses of the system. In particular, we find component masses of 0.86 ± 0.09M⊙ and 0.090 ± 0.003M⊙ for ABD or A and ABD or C, respectively. These dynamical masses, coupled with temperatures and luminosities, allow for comparison with theoretical stellar models. The case of ABDorC, in terms of calibration of evolutionary models of low-mass young stars has been widely reported in previous studies. In this contribution, we compare the measured properties of ABDorA with several solar-composition models for PMS stars. The models used in this comparison predict the dynamical mass to within the quoted uncertainties.

Context. ABDoradus is the main system of the ABDoradus moving group. It is a quadruple system formed by two widely separated binaries of pre-main-sequence (PMS) stars: ABDor A/C and ABDor Ba/Bb. The pair ABDor A/C has been extensively studied and its dynamical masses have been determined with high precision, thus making of ABDor C a benchmark for calibrating PMS stellar models. If the orbit and dynamical masses of the pair ABDor Ba/Bb can be determined, they could not only play a similar role to that of ABDor C in calibrating PMS models, but would also help to better understand the dynamics of the whole ABDoradus system. Aims. We aim to determine the individual masses of the pair ABDor Ba/Bb using VLBI observations and archive infrared data, as part of a larger program directed to monitor binary systems in the ABDoradus moving group. Methods. We observed the system ABDor B between 2007 and 2013 with the Australian Long Baseline Array (LBA), at a frequency of 8.4 GHz in phase-reference mode. Results. We detected, for the first time, compact radio emission from both stars in the binary, ABDor Ba and ABDor Bb. This result allowed us to determine the orbital parameters of both the relative and absolute orbits and, consequently, their individual dynamical masses: 0.28+/-0.05 Msun and 0.25+/-0.05 Msun, respectively. Conclusions. Comparisons of the dynamical masses with the prediction of PMS evolutionary models show that the models underpredict the dynamical masses of the binary components Ba and Bb by 10-30% and 10-40%, respectively, although they all still agree at the 2-sigma level. Some of stellar models considered favour an age between 50 and 100 Myr for this system, meanwhile others predict older ages. We also discuss the evolutionary status of ABDor Ba/Bb in terms of an earlier double-double star scenario that might explain the strong radio emission detected in both components.

We are developing a purely commensal survey experiment for fast (<5s) transient radio sources. Short-timescale transients are associated with the most energetic and brightest single events in the Universe. Our objective is to cover the enormous volume of transients parameter space made available by ASKAP, with an unprecedented combination of sensitivity and field of view. Fast timescale transients open new vistas on the physics of high brightness temperature emission, extreme states of matter and the physics of strong gravitational fields. In addition, the detection of extragalactic objects affords us an entirely new and extremely sensitive probe on the huge reservoir of baryons present in the IGM. We outline here our approach to the considerable challenge involved in detecting fast transients, particularly the development of hardware fast enough to dedisperse and search the ASKAP data stream at or near real-time rates. Through CRAFT, ASKAP will provide the testbed of many of the key technologies and survey modes proposed for high time resolution science with the SKA.

Submitted by Alan Marscher & Svetlana Jorstad (Boston U.), David Murphy, David Meier, Robert Preston, & Stephen Unwin (JPL), Kenneth Kellermann, Joan Wrobel, & Jonathan Romney (NRAO), Daniel Homan (Denison U.), Matthew Lister (Purdue U.), Glenn Piner (Whittier College), Lincoln Greenhill & Mark Reid (SAO), Gregory Taylor (U. New Mexico), Ann Wehrle (Space Science Institute), David Roberts (Brandeis U.), Anthony Readhead (Caltech), Markus Bottcher (Ohio U.), Markos Georganopoulos (U. Maryland), Steven Bloom (HampdenSydney Col.), Kenneth Johnston (US Naval Obs.), C.C. Cheung (NASA/GSFC), Thomas Krichbaum (Max-Planck-Institut fur Radioastronomie), M. Tsuboi (ISAS/JAXA, Japan), M. Inoue (NAOJ, Japan), S. Kameno (Kagoshima U., Japan)

We have studied the growth of the shell-like radio structure of supernova SN1993J in M81 from September 1993 through October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18cm. For this purpose, we have developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure like SN1993J. The source structure of SN1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J through approximately day 1500 after explosion with an expansion parameter $m= 0.845\pm0.005$ ($R \propto t^{m}$). However, from that day onwards the expansion is different when observed at 6 and 18cm. Indeed, at 18cm, the expansion can be well characterized by the same $m$ as before day 1500, while at 6cm the expansion appears more decelerated, and is characterized by another expansion parameter, $m_{6}= 0.788\pm0.015$. Therefore, since about day 1500 on, the radio source size has been progressively smaller at 6cm than at 18cm. These findings are in stark contrast to previous reports by other authors on the details of the expansion. In our interpretation the supernova expands with a single expansion parameter, $m= 0.845\pm0.005$, and the 6cm results beyond day 1500 are due to physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6cm radiation, and (b) a radial decrease of the magnetic field in the emitting region. (Long abstract cut. Please, read full abstract in manuscript).

Large radio telescopes on Earth tracked the radio signal transmitted by the Huygens probe during its mission at Titan. Frequency measurements were conducted as a part of the Huygens Doppler Wind Experiment (DWE) in order to derive the velocity of the probe in the direction to Earth. The DWE instrumentation on board Huygens consisted of an ultrastable oscillator which maintained the high Doppler stability required for a determination of probe horizontal motion during the atmospheric descent. A vertical profile of the zonal wind velocity in Titan's atmosphere was constructed from the Doppler data under the plausible assumption of generally small meridional wind, as validated by tracked images from the Huygens Descent Imager/Spectral Radiometer (DISR). We report here on improved results based on data with higher temporal resolution than that presented in the preliminary analysis by Bird et al. (2005), corroborating the first in situ measurement of Titan's atmospheric superrotation and a region of strong vertical shear reversal within the lower stratosphere. We also present the first high‐resolution display and interpretation of the winds near the surface and planetary boundary layer.

We present an astrometric analysis of the binary systems ABDorA/ABDorC and ABDorBa/ABDorBb. These two systems of well-known late-type stars are gravitationally associated and they constitute the quadruple ABDoradus system. From the astrometric data available at different wavelengths, we report: (i) a determination of the orbit of ABDorC, the very low mass companion to ABDorA, which confirms the mass estimate of 0.090M⊙ reported in previous works; (ii) a measurement of the parallax of ABDorBa, which unambiguously confirms the long-suspected physical association between this star and ABDorA; and (iii) evi- dence of orbital motion of ABDorBa around ABDorA, which places an upper bound of 0.4M⊙ on the mass of the pair ABDorBa/ABDorBb (50% probability). Further astrometric monitoring of the system at all possible wavelengths would determine with extraordinary precision the dynamical mass of its four components.

We present a brief overview of the two-spacecraft iARISE Space VLBI mission concept.

New views of Titan The Huygens probe landed on Titan on 14 January this year, and seven papers published in this issue record the encounter. They describe a world that resembles a primitive Earth, complete with weather systems and geological activity. The ‘Huygens on Titan’ section opens with an overview of the descent and landing and a News and Views piece. Tomasko et al . describe the dry riverbed and drainage channels seen during Huygens' descent, evidence that liquid methane falls as rain or erupts from cryovolcanoes, periodically flooding the surface. This paper includes the images used on the cover to the Huygens section. Niemann et al . measured the abundances of isotopes of argon, nitrogen and carbon in the atmosphere, and conclude that there is no evidence that Titan's methane comes from biological activity. Fulchignoni et al . obtained precise measurements of temperature and pressure from the upper atmosphere right down to the surface. On the way down Huygens recorded evidence for lightning. Zarnecki et al . report that the probe landed on a relatively smooth surface of icy grains with the consistency of wet clay or sand. Isräl et al . report that the aerosols in Titan's clouds have solid cores made from complex organic molecules containing carbon and nitrogen. And Bird et al . found that on average Titan's winds blow in the same direction as the moon rotates, and that close to the surface these winds are very weak, travelling at around walking speed.

We are conducting an X-ray survey of flat spectrum radio quasars (FSRQs) with extended radio structures. We summarize our results from the first stage of our survey, then we present findings from its continuation. We have discovered jet X-ray emission from 12 of our first 20 Chandra targets, establishing that strong 0.5-7.0 keV emission is a common feature of FSRQ jets. The X-ray morphology is varied, but in general closely matches the radio structure until the first sharp radio bend. In the sources with optical data as well as X-ray detections we rule out simple synchrotron models for X-ray emission, suggesting these systems may instead be dominated by inverse Compton (IC) scattering. Fitting models of IC scattering of cosmic microwave background photons suggests that these jets are aligned within a few degrees of our line of sight, with bulk Lorentz factors of a few to ten and magnetic fields a bit stronger than 10^-5 G. In the weeks prior to this meeting, we have discovered two new X-ray jets at z > 1. One (PKS B1055+201) has a dramatic, 20”-long jet. The other (PKS B1421-490) appears unremarkable at radio frequencies, but at higher frequencies the jet is uniquely powerful: its optically-dominated, with jet/core flux ratios of 3.7 at 1 keV and 380 at 480 nm.

We are conducting a Chandra survey of a large, radio-selected sample of flat-spectrum radio quasars with extended radio structure. X-ray emission has now been discovered from 60% of our jet systems, with over half of the sample now observed. In support of this survey we have an ongoing program to obtain both spectroscopic and imaging data with the Magellan 6.5m telescopes. We provide an update on this optical program below, following a summary of our X-ray findings to date.