Condon: Dark Energy and the Hubble Constant
3. Dark Energy (DE) accounts for 74\% of the mass-energy of the universe, dominates its present expansion, and determines its ultimate fate. The best single complement to cosmic microwave background data for constraining the nature of DE is a measurement of the Hubble constant within 3\%. The Megamaser Cosmology Project (MCP) is using the GBT to discover and monitor 22 GHz water megamasers in the nuclei of Seyfert galaxies will into the Hubble flow (up to 200 Mpc distant) and the HSA (GBT, VLBA, and Effelsberg) for imaging and angular-size distance measurements. In the past year we refined our search strategy, developed a more sensitive self-calibration method, improved our measurement of the distance to UGC 3789, imaged four new maser disk systems, and derived a preliminary estimate of $H_0 = 71 \pm 10$ km s$^{-1}$ Mpc${-1}.
5. Accreting stellar-mass black holes spend most of their time in a quiescent state whose radio emission arises from unresolved, compact jets. Such pointlike, persistent radio sources make good astrometric targets. We will present recent HSA observations of one such system, yielding both its proper motion and the most accurate distance yet measured to a stellar-mass black hole, and constraining the black hole formation mechanism. With improved sensitivity and better calibration, the VLBA could be used to observe a larger sample of such systems, providing important observational constraints on the formation of black holes.
6. Microarcsecond astrometry in the optical represents a new window onto the formation and propagation of jets in blazars. Optical astrometry, combined with VLBA astrometry and imaging, provides powerful diagnostics. The ability to study jets in a waveband that is far from the radio, but on size scales of the formation of a jet, is an entirely new kind of measurement.
SIM Lite is an observatory-class mission dedicated to precison astrometry, which will offer this capability. As a flexibly pointed instrument, it can be scheduled to perform astrometric measurements as part of multi-instrument campaigns, which provide time and spectrally resolved data on the most variable of quasars. SIM Lite can deliver an astrometric precision of 4 microarcseconds in bright targets, an 10 microarcseconds on faint quasars (to around V=19), with observing time being the only fundamental limitation. Quasar astrometry is a SIM Lite Key Project (PI: Ann Wehrle), measuring position shifts that can be correlated with photometric variations, and compared with the structure and orientation of the blazar jet seen in VLBA imaging. We can also observe differential color shifts across the optical band, providing information of the propagation of disturbances down the jet, or hotspots in the accretion disk.
SIM Lite has a wide range of aaplications in astrophysics. With an accuracy of 1 microarcsecond in a single measurement accuracy and a noise floor below 0.035 microarcsecond it will have the capability to do an extencsive search for Earth-mass planets in the "habitable zone" around several dozen of the nearest stars. Its wide-angle mode opens up a wide range of problems in stellar and Galactic astrophysics, in addition to the blazar-monitoring project. SIM Lite will have a General Observer (GO) program, for all categories of astrometric observations - for instance, astrometry of blazars. The SIM Lite project successfully completed in 2005 a series of technology milestones that were independently verified and is currently under study by NASA as a flight mission.
The research described in this talk was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
8. Optical & Near-IR (OIR) Interferometry as a discipline has been contributing astrophysically-significant results since the work of Michelson et al in the 20s, and Hanbury-Brown et al in the 60s. Starting with the MarkIII interferometer at Mt Wilson in the 80s and 90s, OIR interferometry has made astrometrically-relevant contributions. In this talk I will give a brief overview of OIR interferometry as a technique, summarize its astrometric scientific contributions, and discuss examples synthesizing OIR and radio interferometry, including the analysis of the PMS binary system V773 Tau A.
9. The Australian LBA (Long Baseline Array) has been used to observe a list of candidate flat spectrum radio sources with declination < -50° to determine their position with milliarcsec accuracy and determine their suitability as calibrators for phase referencing observations and as target for astrometry and geodesy observations. Currently more than 316 new millisecond positions have been determined with a final goal of around 1000 astrometric positions for a high density phase reference grid. The candidate list is based on the AT20G survey, an all-sky 20 GHz survey using an 8 GHz analog correlator for the ATCA. So far a detection rate of candidate source of 97% has been achieved.
10. With a bandwidth of 500 MHz/polarization, the upgraded VLBA (10 stations) will match the point-source sensitivity of the original 2x50 MHz/polarization VLA (27 stations), despite having less than 40% of the collecting area. Given the identical primary beam size, the VLBA is thus theoretically capable of duplicating, in comparable observing time, such hugely influential efforts as the VLA FIRST survey, at VLBI resolution. Such a VLBI survey will be made feasible by the VLBA's new software correlator, which will enable correlation of multiple phase centers in a single correlator pass. I will briefly describe the motivations and likely outcomes of such a survey, focusing on 1) the attainable absolute astrometric accuracy, and 2) the logistical constraints which will determine the astrometric accuracy.
14. The VLBA Sensitivity Upgrade Project primarily targets a nearly sixfold increase in continuum sensitivity, expanding the entire data path, downstream from the IFs, to 4-Gbps bandwidth. The upgrade includes a new sampler-filter "digital backend" that replaces the existing analog baseband conversion and sampling scheme, a wider-band version of the Mark 5 disk-based recording system, and a flexible software correlator. The new equipment could be operational by years end, with throughput then limited only by available recording and correlation processing capacity, allowing operation at a 2-4 Gbps peak rate earlier than the 2011 project goal, for the most scientifically compelling observations.
15. HST imaging surveys reveal that the vast majority of Planetary Nebulae (PNe) deviate dramatically from spherical symmetry, and show a dazzling array of morphologies - a major problem in our understanding of the late stages of stellar evolution. Hydrodynamic sculpting of the progenitor AGB envelope by high-velocity jets early during the pre-planetary nebulae (PPN) phase, has been proposed as a primary mechanism for shaping PNs. Results from ongoing astrometric studies to carry out parallax and proper motion measurements of water and/or OH masers in young PPNe demonstrate the unique role of the VLBA in determining physical parameters such as the stellar luminosity, ejecta masses and mass-loss rates, and expansion ages, which are fundamental to a full understanding of their evolution.
17. The phase referencing technique is commonly used by the VLBA (and other arrays) in order to image weak sources that cannot be detected in a coherence time, and to determine astrometric information of both strong and weak sources. The basic technique alternates short observations between a calibrator (compact source which is easily detactable at all spacing) and a target in order to derive its structure and positional properties. This talk will discuss several aspects of phase referencing in order to optimize the results: (1) Temporal coherence; (2) Angular coherence; (3) Using weak calibrator sources; (4) Phase versus group delay; (5) Additional troposphere and ionosphere calibrations; (6) Multi-source phase referencing; (7) Editing criteria. Examples and suggested guidelines will be given.
18. Since 1997, we have been observing the S5 polar-cap sample (13 quasars and BL LAc objects) with the VLBA at three different frequencies (8.4, 15, and 43 GHz) using phase-referencing techniques. The astrometric analysis have been carried out over the three frequencies with precision in the relative position determination ranging from 100 to 20 microarcseconds. This multi-epoch/multi-frequency approach allows to obtain absolute kinematics and spectral information of all sources in the sample. We will also report on the new algorithms developped to analize the differential phase-delay observable in wide-field astrometric observations.
19. In this talk, I will present multi-epoch VLBA observations of two multiple young stars: T Tau and V773 Tau, and show that such observations place valuable constraints on their orbits. This, in turn, allow us to measure the mass of young stars very accurately, and provide very important constraints for pre-main sequence evolutionary models. V773 Tau is a binary with an orbital period of only 50 days. In this case our observations allowed us to fully characterize the orbit. Combining our observations with optical and near infrared data, we measured the mass of the individual stars and placed them on theoretical isochrones. The orbital period of T Tau, on the other hand, is at least few decades, so we have not been able to fully constrain the orbit yet. However, our VLBA data are already sufficient to put important constraints on the mass of the stars and the nature of the orbit.
22. Microarcsecond VLBA astrometry is possible if and only if there are suitable calibrators with coordinates known at sub-mas level within several degrees from targets. Significant efforts were undertaken for the last 15 years to build a catalogue of ~4000 calibrators. The technology of conducting calibrator survey observing experiments is presented. This includes selection of candidates, evaluation of the probability of their detection, schedule optimization, and data analysis. Optimization of source selection and observing schedules significantly (a factor of 2-4) increases the number of detected sources and reduces systematic errors of their position estimates. The current status of the VLBI calibrator list is presented. Further improvement of the VLBI calibrator catalogue may follow three different avenues. The first way is to observe a list of candidates uniformly distributed over the sky in order to achieve higher calibrator density over the entire sky area visible with the VLBA. The second strategy is to restrict observations to certain zones of the sky of special interest where the higher calibrator density is desirable (for instance, the Galactic plane, the ecliptic) and to observe candidate sources only in that area. The third strategy is to form the list of potential targets and then to observe candidate sources in the disk of 3 deg from each potential target. Arguments pro and contra and programmatic consequences of these strategies are discussed.
23. The SIM Lite Key Project on Active Galactic Nuclei, led by Ann Wehrle, will measure relative positions in bright quasars and other active galactic nuclei. Measurements accurate to a few microarcseconds can be achieved on ~ 50 quasars brighter than 18th magnitude. We will compare the offset between red and blue continuum features, stemming from the hot corona, the inner accretion disk and base of the relativistic jets. In addition, we will be able to track the orbital motion of binary black holes with periods comparable to the lifetime of SIM Lite (ten years or more), such as may exist in OJ 287. We will also compare the radio and optical positions, which are expected to be offset from one another due to opacity effects and different radiation emission mechanisms, in flat spectrum radio loud quasars such as those the SIM LIte Project plans to use for the optical/radio reference frame tie.
24. Multi-epoch observations with Very Long Baseline Interferometers (particularly the VLBA) can be used to trace the parallactic motion of young stars within several hundred parsecs with a precision of about 1%. This is more than one order of magnitude more accurate that the best existing alternatives. As a consequence, the VLBA has the potential to make a significant and lasting contribution to the study of star-formation, by pinpointing the position and determining the structure and kinematics of any northern star-forming within ~ 500 pc of the Sun. In this talk, I will show some example of existing observations of young stars in Taurus, Ophiuchus and Serpens. I will also present what could become possible as a result of a large key project after the sensitivity of the VLBA is improved by a factor of 10.
25. The relative positions of M87 and M84 were monitored at 43 GHz with the VLBA during a project whose primary goal was to make a properly sampled movie of the M87 jet collimation region. The RMS scatter of 12 observations constrains core wander to be less than about 3 by 10 times the Schwarzschild radius of M87. Comparison with reanalyzed 2001 phase referencing data suggests, barring long term systematic effects, a 5 sigma detection of a relative proper motion of about 800 km/s. This is consistent with the range of radial velocities seen in the Virgo Cluster.
26. Precision astrometry on neutron stars can yield model-independent distances and velocities to neutron stars. I will review how such measurements are being exploited, for example, to locate neutron star birth sites, establish reference frame ties, model the Galactic electron density distribution, and constrain the astrophysics of supernova explosions. In the short term, systematic surveys and high sensitivity on very long baselines will produce ongoing science dividends from precision astrometry. In the longer term, new technology such as focal plane arrays, new telescopes such as the Square Kilometre Array, and synergy with new instruments such as Gaia, LSST, and GLAST, all hold great promise in an upcoming era of microarcsecond astrometry.
28. Astrometry enters microlensing by two quite distinct but inter-related paths. In the "traditional path", which dates back to Eddington, microlensed sources suffer astrometric deviations, which can be measured to give information about the lens (its mass, distance, etc). But there is another path, in which purely photometric observations yield astrometric quantities, including the trigonometric parallax and proper motion. I show how these measurements are being used today to measure the masses of planets and of an old-brown dwarf in the thick disk. In the future, micro-arcsec astrometry with the Space Interferometry Mission will enable the first unbiased survey of compact objects, including brown-dwarfs, white dwarfs, neutron stars, and black holes.
29. Very Long Baseline Interferometry (VLBI) is essential to the development and maintenance of fundamental reference frames for astronomy and geodesy. We discuss the technique of global VLBI astrometry and its application to the celestial reference frame (CRF), the terrestrial reference frame (TRF) and the determination of orientation of the Earth in space. Particular emphasis will be placed on the key role that the VLBA plays in astrometry and geodesy, and the relevance of global astrometric results to users of the VLBA for astronomical science. New developments related to the International Celestial Reference Frame, in particular ICRF2, will be considered along with prospects for future reference frames at higher radio frequencies.
31. The Radio Interferometric Planet Search (RIPL) is a three year survey of 29 low mass stars with the goal of discovering extrasolar planets. Using the VLBA and the Green Bank 100 m telescope, we can routinely achieve 100 micro arc second resolution, allowing Jupiter mass sensitivity at 1 AU. Here, we present selected images and fits from observations between October 2007 and July 2009. We have observed each star about 4 times to date and achieve 6 sigma detections in approximately 65% of epochs. We observe changes in flux by a factor of 3 between epochs, indicating the presence of strong flaring.
33. VERITAS is an array of four imaging atmospheric Cherenkov telescopes (IACTs) detecting gamma-rays in the energy range of 100 GeV up to several 10's of TeV. Each telescope consists of a 12m diameter reflector and a photomultiplier camera (499 pixels) located in the focal plane. A point-source at the 1% level of the Crab nebula flux is detected in less than 50h. The observations with the full array started in 2007 and several new TeV gamma-ray sources were discovered. The interpretation of the TeV observations can benefit from high-resolution VLBA radio observations in order to understand the location and mechanisms of the TeV emission (i.e. relativistic jets of AGN, unidentified sources, etc.). This approach has already been shown to be successful in the case of the radio galaxy M87. The results of the VERITAS observations of different source classes and the prospects/potential of the radio/TeV connection will be discussed.
36. The International Celestial Reference Frame (ICRF) was adopted by the IAU in 1997 based on VLBI at S/X-band (2.3/8.4 GHz). At that time, the IAU encouraged the extension of the ICRF to other frequencies. In response, VLBI measurements have been made at 24, 32, and 43 GHz. We will discuss the motivations for extending the ICRF to these higher frequencies. A summary of results to date will be presented including evidence that these new high frequency frames are rapidly approaching the accuracy of the S/X-band ICRF. Finally, prospects for the future improvements of high frequency radio reference frames will be discussed.
37. I will show the VERA project status and future plan, which is a phase referencing VLBI array in Japan. We have started astrometry observations for the galactic maser sources to measure the annual parallaxes and proper motions. And also I will present the status of East Asia VLBI network.
38. The Gaia space astrometric mission, to be launched by 2012, will survey 500,000 QSOs brighter than magnitude 20, with an astrometric accuracy of a few tens of microarcseconds, hence realizing the extragalactic reference frame for the first time directly at optical wavelengths. For consistency, the Gaia frame will have to be linked to the International Celestial Reference Frame built from VLBI data, based on the extragalactic objects common to the two frames. To this end, we have initiated a VLBI observing program with the VLBA and the EVN to identify the most suitable candidates for this link. We report on the status of this project, including results of imaging of the first 100 candidate sources observed as part of the progranmme.
40. We will present the results of 6.7GHz methanol masers observations with Japanese VLBI Network (JVN), which consists of Japanese universities telescopes and VERA. Among several methanol maser sources monitored with JVN, massive star forming region W3(OH) was observed at three times, July 2007, May 2008 and Oct 2008. We obtained expanding internal proper motions. The internal motion is about 5 km/s, when we assume a distance of 1.95 kpc. The expanding motions basically resemble OH maser expanding motions, suggesting that the methanol masers are expanding with HII region. We also discussed about absolute positions and motions of W3(OH) methanol maser.
41. I summarize the current status of Galactic structure studies based on VLBI astrometry. Recently several astrometric programs have been conducted with VERA/VLBA/EVN etc. to measure the distances and proper motions of Galactic objects, mostly maser sources such as star forming regions and late-type stars. I review recent outputs of these programs and the current view of our Galaxy revealed by VLBI astrometry, and also discuss the future directions of VLBI astrometry for better understanding of the Galaxy.
42. Orion-KL is most studied massive star forming region. However, the driving sources of the high- and low-velocity outflow in this region are still uncertain. Water masers are associated with the outflows, thus, the proper motions of water masers are a key to reveal the driving sources of the outflows. We measured the proper motions of individual water maser features in Orion-KL using the phase-referencing observation data taken in 2005,2006 with VERA. Here, we discuss the driving sources of the outflows based on the measured proper motions of water masers.
43. We are pursuing a project of installing the K4/VSOP terminal to some antennas in Korea and China in order to connect between the VLBI networks (KVN, CVN, and JVN) of Korea, China, and Japan, and perform astronomical observations by the East Asia VLBI network (EAVN). We already sent and installed the terminal system to the KVN 3 antennas in Korea and the Sheshen antenna in Shanghai, China. In May 2009, we were successful in fringe check experiments for the EAVN in the K-band among Sheshen, KVN Yonsei, and VERA antennas. The EAVN will be powerful tool for astrometry observation.
44. The NGC7538 region is one of the active massive star-forming regions in Perseus arm. There are at least three regions IRS1-3, IRS9, and IRS11 in a molecular cloud core south of the HII region NGC7538. Multi-epoch 2 beam VLBI observations have been performed by using VERA in order to find new distribution of water vapor masers in three regions. Some interesting results are shown, and relationships between the distribution/motion of them and the other activities (IR sources, outflows, Ultra-compact HII regions, etc ) in three regions are discussed.
45. We report on astrometric results of H2O and SiO masers in the circumstellar envelopes of VY Canis Majoris (VY CMa) carried out with VERA for 2 years. Absolute positions and proper motions of 3 different frequencies of masers were measured with phase-referencing analyses. Using the positions and the 3-dimensional velocities of the masers, we considered the 3-dimensional structures and kinematics of the circumstellar envelopes around VY CMa. The H2O masers show bipolar outflow along the line of sight, and the SiO masers have both expanding and contracting motions with less than 5 km/s.
46. We present the result of a series of phase-referenced VLBA-HSA observations of Algol spanning 1995 to 2008, with which we have calculated higher-precision values of the annual parallax, proper motion and fiducial position of Algol than have yet been obtained, enabling us to make a highly plausible assumption about the position of the radio emission with respect to the optical position of the star. Our observations show that the radio emission moves in a way that is only consistent with origin on the KIV subgiant of the system (Algol B).
49. VERA telescopes are uniquely equipped with a dual-beam system, which enable us to observe simultaneously a target and a reference source within 2.2 degree. To verify the feasibility of VERA system, test observations of QSO pairs were conducted between 2004 and 2009. We will report the results of test observations and a briefly future upgrade plan and demonstration for the joint VERA and the OCTAVE (Optical fiber Connected real Time Array for VLBI Exploration) observations.
55. A series of 10 short VLBA observations, over ~20 days, tracked the Phoenix spacecraft as it approached and landed on Mars. Phase referencing to quasars and other Mars orbiters was demonstrated to achieve a precision of ~20 microarcseconds.
56. Application of the 2-D Fourier Transform to single-dish dynamic spectrum of certain pulsars results in a vivid parabolic arc image. A novel technique involving Very Long Baseline Interferometry extracts spatial information of the scattering process and provides new insight into the scattering phenominon. 100 microarcsecond resolution images of the scattering screen are produced via astrometric techniques at 327 MHz where the effects of scattering are very strong.
57. Planned and proposed upgrades to the VLBA will vastly increase its capabilities. This annotated block diagram will highlight the new components that will modernize the instrument. The new electronics include a digital "Back End" that will supercede the functionality of the baseband converters, samplers, and formatter, a new recording system (Mark5C), two new receivers (4 to 8 GHz and 26 to 40 GHz), new frequency synthesizers, and an EVLA-style control system. Interested parties should feel free to discuss with VLBA staff members.
This page last updated on 2009 July 14 by Walter Brisken.