Ed Fomalont

Flows of gas through a protoplanetary gap.

The formation of gaseous giant planets is thought to occur in the first few million years after stellar birth. Models predict that the process produces a deep gap in the dust component (shallower in the gas). Infrared observations of the disk around the young star HDu2009142527 (at a distance of about 140 parsecs from Earth) found an inner disk about 10 astronomical units (au) in radius (1u2009au is the Earth–Sun distance), surrounded by a particularly large gap and a disrupted outer disk beyond 140u2009au. This disruption is indicative of a perturbing planetary-mass body at about 90u2009au. Radio observations indicate that the bulk mass is molecular and lies in the outer disk, whose continuum emission has a horseshoe morphology. The high stellar accretion rate would deplete the inner disk in less than one year, and to sustain the observed accretion matter must therefore flow from the outer disk and cross the gap. In dynamical models, the putative protoplanets channel outer-disk material into gap-crossing bridges that feed stellar accretion through the inner disk. Here we report observations of diffuse CO gas inside the gap, with denser HCO+ gas along gap-crossing filaments. The estimated flow rate of the gas is in the range of 7u2009×u200910−9 to 2u2009×u200910−7 solar masses per year, which is sufficient to maintain accretion onto the star at the present rate.1. Departamento de Astronomı́a, Universidad de Chile, Casilla 36-D, Santiago, Chile 2. Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763-0355, Santiago Chile 3. European Southern Observatory (ESO), Casilla 19001, Vitacura, Santiago, Chile 4. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA 5. Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290, Versoix, Switzerland 6. Departamento de Astronomı́a y Astrofı́sica, Pontificia Universidad Católica de Chile, Santiago, Chile 7. UMI-FCA, CNRS / INSU France (UMI 3386) , and Departamento de Astronomı́a, Universidad de Chile, Santiago, Chile. 8. CNRS / UJF Grenoble 1, UMR 5274, Institut de Planétologie et dAstrophysique de Grenoble (IPAG), France 9. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 USA 10. Department of Astronomy, U. C. Berkeley, 601 Campbell Hall, Berkeley, CA 94720 11. Departamento de Fı́sica y Astronomı́a, Universidad Valparaiso, Av. Gran Bretana 111, Valparaiso, Chile. 12. University Observatory, Ludwig-Maximillians University, Munich.

Precise geodesy with the Very Long Baseline Array

We report on a program of geodetic measurements between 1994 and 2007 which used the Very Long Baseline Array (VLBA) and up to ten globally distributed antennas. One of the goals of this program was to monitor positions of the array at a 1xa0mm level of accuracy and to tie the VLBA into the international terrestrial reference frame. We describe the analysis of these data and report several interesting geophysical results including measured station displacements due to crustal motion, earthquakes, and antenna tilt. In terms of both formal errors and observed scatter, these sessions are among the very best geodetic very long baseline interferometry experiments.

Very Long Baseline Array Astrometric Observations of the Cassini Spacecraft at Saturn

The planetary ephemeris is an essential tool for interplanetary spacecraft navigation, studies of solar system dynamics (including, for example, barycenter corrections for pulsar timing ephemerides), the prediction of occultations, and tests of general relativity. We are carrying out a series of astrometric very long baseline interferometry observations of the Cassini spacecraft currently in orbit around Saturn, using the Very Long Baseline Array (VLBA). These observations provide positions for the center of mass of Saturn in the International Celestial Reference Frame (ICRF) with accuracies {approx}0.3 mas (1.5 nrad) or about 2 km at the average distance of Saturn. This paper reports results from eight observing epochs between 2006 October and 2009 April. These data are combined with two VLBA observations by other investigators in 2004 and a Cassini-based gravitational deflection measurement by Fomalont et al. in 2009 to constrain a new ephemeris (DE 422). The DE 422 post-fit residuals for Saturn with respect to the VLBA data are generally 0.2 mas, but additional observations are needed to improve the positions of all of our phase reference sources to this level. Over time we expect to be able to improve the accuracy of all three coordinates in the Saturn ephemeris (latitude, longitude,morexa0» and range) by a factor of at least three. This will represent a significant improvement not just in the Saturn ephemeris but also in the link between the inner and outer solar system ephemerides and in the link to the inertial ICRF.«xa0less

ASTROMETRY OF CASSINI WITH THE VLBA TO IMPROVE THE SATURN EPHEMERIS

Planetary ephemerides have been developed and improved over centuries. They are a fundamental tool for understanding solar system dynamics, and essential for planetary and small body mass determinations, occultation predictions, high-precision tests of general relativity, pulsar timing, and interplanetary spacecraft navigation. This paper presents recent results from a continuing program of high-precision astrometric very-long-baseline interferometry (VLBI) observations of the Cassini spacecraft orbiting Saturn, using the Very Long Baseline Array (VLBA). We have previously shown that VLBA measurements can be combined with spacecraft orbit determinations from Doppler and range tracking and VLBI links to the inertial extragalactic reference frame (ICRF) to provide the most accurate barycentric positions currently available for Saturn. Here we report an additional five years of VLBA observations along with improved phase reference source positions, resulting in an improvement in residuals with respect to the Jet Propulsion Laboratorys dynamical ephemeris.

THE POSITION/STRUCTURE STABILITY OF FOUR ICRF2 SOURCES

Four compact radio sources in the International Celestial Reference Frame (ICRF2) catalog were observed using phase referencing with the VLBA at 43, 23, and 8.6-GHz, and with VERA at 23-GHz over a one-year period. The goal was to determine the stability of the radio cores and to assess structure effects associated with positions in the ICRF2. Conclusions are: (1) 43-GHz VLBI high-resolution observations are often needed to determine the location of the radio core. (2) Over the observing period, the relative positions among the four radio cores were constant to 0.02 mas, suggesting that once the true radio core is identified, it remains stationary in the sky to this accuracy. (3) The emission in 0556+238, one of the four sources investigated and one of the 295 ICRF2 defining sources, was dominated by a strong component near the core and moved 0.1 mas during the year. (4) Comparison of the VLBA images at 43, 23, and 8.6-GHz with the ICRF2 positions suggests that the 8-GHz structure is often dominated by a bright non-core component. The measured ICRF2 position can be displaced more than 0.5 mas from the radio core and partake in the motion of the bright jet component.

A decade of astrometric observations of Cassini: Past results and future prospects

The Cassini spacecraft has been in orbit about Saturn since 2004. During this time, regular astrometric measurements of Cassinis sky position have been made with the Very Long Baseline Array (VLBA). These are high precision differential measurements that determine the position of Cassini with respect to angularly nearby extragalactic radio sources. Differential, narrow-angle astrometry reduces many error sources, particularly those associated with signal propagation effects in the ionosphere and troposphere. The background radio sources positions are tied to the inertial International Celestial Reference Frame (ICRF) by other international VLBI observations. Thus, we obtain a series of ICRF positions for Cassini, which can be combined with spacecraft orbit solutions from Deep Space Network Doppler tracking to get ICRF positions for the center of mass of the Saturn system. These positions have typical accuracies at the nano-radian level. For some epochs uncertainties in the background source positions are a major component of the total error, but these positions are being constantly improved as additional VLBI observations are incorporated into radio source catalogs. The planetary ephemeris group at the Jet Propulsion Laboratory uses our position measurements to fit improved orbital solutions for Saturn. As a result the orientation of the plane of Saturns orbit is now known to approximately 0.25 milli-arcseconds (1.25 nrad), nearly an order of magnitude improvement over its pre-VLBA uncertainty. We will continue this observing program until the end of the Cassini mission in late 2017. By that time we will have covered about 1/3 of Saturns orbital longitude range. Future improvements to this technique will include the use of higher spacecraft downlink frequencies (Ka band instead of X band) and higher ground array sensitivity to permit the use of weaker but angularly closer reference sources. In addition, the continuing international campaigns to enhance the accuracy of radio source catalogs will be extended to weaker sources, improving their ties to the ICRF.

ALMA fast switching phase calibration on long baselines

We present results of feasibility studies of Atacama Large Millimeter/submillimeter Array (ALMA) interferom- eter phase calibration scheme combined with the Fast Switching (FS) phase referencing and the Water Vapor Radiometer (WVR) phase correction (FS+WVR phase correction). With FS scheme, ALMA antennas observe a scientific target source and a nearby calibrator by turn very quickly. Because interferometer phase errors of the target due to the water vapor contents commonly exist in those of the calibrator, the target phase is corrected with the calibrator phase. We have demonstrated the FS+WVR phase correction for ALMA with baselines up to 2.7 km for various switching cycle times and separations between sources. For instance, in the case of sources with the 1° separation, root-mean-square phases of the target were reduced from 300 to 40 microns in path length for 1 km baselines, and the target interferometer phases could be stabilized to an ALMA specification requirement level for the interferometer phase stability. We also analytically evaluated the root-mean-square phase corrected with the FS+WVR phase correction to predict the performance as a function of the separation and switching cycle time.

Radio astrometry of the Cassini spacecraft with the very long baseline array

The planetary ephemeris is a fundamental tool of astronomy that is essential for dynamical studies of the solar system, pulsar timing, tests of general relativity, occultation and eclipse predictions, and interplanetary spacecraft navigation. Since Jupiter and Saturn dominate the dynamics of our solar system, improved knowledge of their orbits will result in a global improvement in the accuracy of the ephemeris. The Cassini spacecraft has been orbiting Saturn for over a decade, a third of Saturns orbital period. This has provided an unprecedented opportunity to improve all components of Saturns orbit by combining periodic very long baseline inferterometry (VLBI) measurements of Cassinis sky position with respect to background radio sources, which in turn can be tied to the inertial International Celestial Reference Frame (ICRF). The orbit of Cassini about the center of mass of Saturn is determined from Doppler tracking by the Deep Space Network. Combining these observations, we obtain the barycenter position of the Saturn system in an inertial frame at multiple epochs, with typical uncertainties of 0.3 milli-arcseconds in right ascension and 0.4 milli-arcseconds in declination. These results are then provided to JPLs ephemeris group for inclusion in future ephemeris solutions. At most epochs the largest component of the error budget is uncertainty in the ICRF position of the phase reference radio source used. These source positions are being continuously improved through additional VLBI observations. These VLBA observations have improved our knowledge of Saturns orbit by nearly an order of magnitude. This technique will be expanded to include astrometric observations of the Juno spacecraft as soon as it enters Jupiter orbit in mid-2016. Although the orbital phase of the Juno mission is expected to last only a bit over one year, it will still allow a significant improvement in Jupiters orbit. Previous missions to Jupiter have been single-epoch flybys with the exception of Galileo, for which the accuracy of VLBI position measurements was severely limited by failure of the high gain antenna. The Juno mission is scheduled to end in February 2018, five months after the scheduled end of the Cassini mission. At the ends of their missions the Juno and Cassini spacecraft will be destroyed in the atmospheres of Jupiter and Saturn to eliminate the possibility of a future crash onto any of the liquid-containing moons of these planets that may be habitats of life.

Analysis of antenna position measurements and weather station network data during the ALMA long baseline campaign of 2015

In a radio interferometer, the geometrical antenna positions are determined from measurements of the observed delay to each antenna from observations across the sky of many point sources whose positions are known to high accuracy. The determination of accurate antenna positions relies on accurate calibration of the dry and wet delay of the atmosphere above each antenna. For the Atacama Large Millimeter/Submillimeter Array (ALMA), with baseline lengths up to 15 kilometers, the geography of the site forces the height above mean sea level of the more distant antenna pads to be significantly lower than the central array. Thus, both the ground level meteorological values and the total water column can be quite different between antennas in the extended configurations. During 2015, a network of six additional weather stations was installed to monitor pressure, temperature, relative humidity and wind velocity, in order to test whether inclusion of these parameters could improve the repeatability of antenna position determinations in these configurations. We present an analysis of the data obtained during the ALMA Long Baseline Campaign of October through November 2015. The repeatability of antenna position measurements typically degrades as a function of antenna distance. Also, the scatter is more than three times worse in the vertical direction than in the local tangent plane, suggesting that a systematic effect is limiting the measurements. So far we have explored correcting the delay model for deviations from hydrostatic equilibrium in the measured air pressure and separating the partial pressure of water from the total pressure using water vapor radiometer (WVR) data. Correcting for these combined effects still does not provide a good match to the residual position errors in the vertical direction. One hypothesis is that the current model of water vapor may be too simple to fully remove the day-to-day variations in the wet delay. We describe possible new avenues of improvement, which include recalibrating the baseline measurement datasets using the contemporaneous measurements of the water vapor scale height and temperature lapse rate from the oxygen sounder, and applying more accurate measurements of the sky coupling of the WVRs.

Phase characteristics of the ALMA 3-km baseline data

We present the phase characteristics study of the Atacama Large Millimeter / submillimeter Array (ALMA) long (up to 3 km) baseline, which is the longest baseline tested so far using ALMA. The data consist of long time-scale (10 20 minutes) measurements on a strong point source (i.e., bright quasar) at various frequency bands (bands 3, 6, and 7, which correspond to the frequencies of about 88 GHz, 232 GHz, and 336 GHz) Water vapor radiometer (WVR) phase correction works well even at long baselines, and the efficiency is better at higher PWV (< 1mm) condition, consistent with the past studies. We calculate the spatial structure function of phase fluctuation, and display that the phase fluctuation (i.e., rms phase) increases as a function of baseline length, and some data sets show turn-over around several hundred meters to km and being almost constant at longer baselines. This is the first millimeter / submillimeter structure function at this long baseline length, and to show the turn-over of the structure function. Furthermore, the observation of the turn-over indicates that even if the ALMA baseline length extends to the planned longest baseline of 15 km, fringes will be detected at a similar rms phase fluctuation as that at a few km baseline lengths. We also calculate the coherence time using the 3 km baseline data, and the results indicate that the coherence time for band 3 is longer than 400 seconds in most of the data (both in the raw and WVR-corrected data) For bands 6 and 7, WVR-corrected data have about twice longer coherence time, but it is better to use fast switching method to avoid the coherence loss.