Sub-Milliarcsecond Astrometry with Phase-Referenced VLBI
Astrometric VLBI observations of radio stars are being made to connect the.
Jones, D. L.↗
Engineering topics
Phillips, R. B.
Publications and source records attributed to Phillips, R. B..
Sub-Milliarcsecond Astrometry with Phase-Referenced VLBI
Astrometric VLBI observations of radio stars are being made to connect the Hipparcos optical reference frame and the extragalactic radio reference frame. The antennas are switched between a star and a strong, compact reference source a few degrees away every 2-3 minutes. After correlation, the strong reference source fringes are used to determine the exact delay and rate of the weak radio star fringes, allowing the visibilities to be measured and coherently integrated for the duration of the experiment. As a result accurate astrometry has been possible on radio stars with flux densities as low as 2 mJy. All five astrometric parameters (two positions, two proper motions, and one parallax) have been obtained for several stars, with formal errors and epoch-to-epoch residuals less than 1 milliarcsecond.
Jones, D. L.↗
Prospects for VLBI Detection of Planets
The precision of astrometric VLBI measurements of radio-emitting stars has improved significantly during the past several years. For some stars whose radio flux density is only a few mJy the formal error and epoch-to-epoch scatter in position is approximately 200 - 600 micro-arcsec. The intrinsic signal/noise ratio in the data should allow a further order of magnitude improvement in astrometric precision as sources of systematic error are better understood and corrected. Wider bandwidth VLBI data recording systems currently under development will allow precisions of approximately 3 microarcsec if the measurements are limited by thermal noise. At this level planets with mass greater than or equal to 0.1 x M_(JUPITER) could be detected for the nearest radio-emitting stars.
Jones, D. L.↗
Prospects for VLBI Detection of Planets
Astrometric VLBI observations of radio stars are being made to connect the Hipparcos star catalog to the extragalactic radio reference frame. Typically, the antennas are moved between a star and a strong, compact reference source a few degrees away every 2-3 minutes. After correlation, the strong reference source fringes are used to determine the exact delay and rate of the weak radio star fringes, allowing the visibilities to be measured and coherently integrated for the duration of the experiment. As a result accurate astrometry has been possible on radio stars with flux densities as low as 2 mJy...
Jones, D. L.↗
High-Precision VLBI Astrometric Observations of Radio-Emitting Stars for Detection of Extra-Solar Planets
Measurement of the displacement of a radio-emitting star around the barycenter of a possible planetary system can be carried out by astrometric.
radio-emitting stars astrometric monitoring progra↗
Stronger Source As Reference For Weaker Source In VLBI
Observation of weak astronomical radio source by very-long-baseline interferometry (VLBI) enhanced by using relatively strong astronomical radio source within small angular distance from weak source. Strong source and weak source observed alternately during measurement session. Direct detection of strong source provides reference delays and rates of change of delays used to integrate radio visibilities of weak source. Integrated visibilities added coherently over entire measurement session, using observed phases of strong source as references.
Lestrade, Jean-Francois↗
Milliarcsecond radio structure of weak-lined T Tauri stars
VLBI and VLA observations of six radio-bright weak-lined T Taur (WTT) stars are reported, as well as direct measurements of the sizes of the emitting regions. VLBI measurements established that essentially all the radio emission from these premain-sequence stars originates in regions 15 stellar radii or less in size. Corresponding brightness temperatures ranged from 10 exp 7.5 to not less than 10 exp 9 K, ruling out a thermal process such as free-free bremsstrahlung radiation from a circumstellar wind. The radio luminosity and structure of several stars changed significantly between measurements separated by 1 day. HD 283447 showed intraday radio variability on time scales as short as 1 hr. Corresponding VLBI measurements show a new unresolved component appearing after an increase in flux density, possibly indicating that the driving agent for larger radio flares originates close to the star. The high conformation rate of nonthermal radio emission from this initial sample of radio-bright WTT stars show that these solar-type premain-sequence stars alter their immediate environments via magnetic processes to an extent comparable to that shown by RS CVn or Algol close binaries.
Phillips, R. B.↗
VLBI astrometry and the Hipparcos link to the extragalactic reference frame
Intermediate results are reported from a program of VLBI radio observations designed to establish a link between the rotating reference frame of the ESA Hipparcos astrometric satellite and the extragalactic VLBI frame being developed by the International Earth Rotation Service. A group of 12 link stars have been observed at various epochs since 1982, and more observations are being undertaken during the 3-yr Hipparcos mission (1989-1992). Analysis of data on Algol indicates that phase-reference VLBI can determine an expected sky displacement of 4 marcsec with an uncertainty of 0.5 marcsec, even when the activity is only a few mJy.
Lestrade, J.-F.↗
Phase-referenced VLBI observations of weak radio sources - Milliarcsecond position of Algol
The Fourier inversion of all the delay/rate referenced visibilities of Algol was used to form a 'dirty' map which has dramatically improved VLBI array sensitivity. The Algol system, in a very low state of activity with a total flux density of only 3 mJy, was observed alternatively with the reference source 0309 + 411, about 1 deg away, throughout the 7 hr VLBI experiment. By combining all the individual VLBI observations, the resulting coherent integration time for Algol is 4.2 hr and the corresponding SNR is 12. Internal consistency checks and an analysis of possible systematic astrometric errors indicate that the accuracy of the relative VLBI position of Algol is + or - 0.0005 arcsec in both coordinates and that unknown systematic effects are presently limiting this astrometric measurement.
Lestrade, J.-F.↗
VLBI and optical astrometry of southern extragalactic radio sources and radio stars
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White, Graeme L.↗
Compact non-thermal radio emission from B-peculiar stars
Some stars hotter than 10,000 K show propensity for unusual surface abundances and excessive magnetic fields. These peculiar stars, Ap and Bp in the spectral nomenclature, show unusually prominent absorption lines of heavier elements. Rapid rotation and strong magnetic fields are revealed by line shapes and Zeeman splitting. VLBI is here used to directly probe the source size and brightness temperature of weak radio emission recently discovered from two isolated Bp stars, sigma Orionis E and HD37017. The emitting zone for each star is no more than 6 stellar diameters in extent, reflecting brightness temperatures of more than one billion K. Such high surface brightness resembles gyrosynchrotron radiation from mildly relativistic electrons trapped in the strong magnetic fields surrounding these stars. Compact radio radiation from these two stars presents new opportunities for probing the physical environments of early-type stars and for precise radio astrometry.
Phillips, R. B.↗
VLBI positions of eight stellar systems
Radio positions of eight stellar systems have been derived with accuracies of 3 to 300 milliarcseconds from MkIII VLBI observations conducted with multistation arrays. The best accuracy was obtained during a relatively strong outburst of the RS CVn system HR 5110. The epoch J2000.0 positions are obtained in the Jet Propulsion Laboratory VLBI reference frame of extragalactic radio sources.
Neill, A. E.↗
Dual polarization VLBI observations of stellar binary systems at 5 GHz
The binary systems Sigma CrB, SZ Psc, HR 1099, UX Ari, HR 5110, Algol, and Cyg X-1 are characterized in terms of marcsec-scale radio structure on the basis of dual-polarization observations at 4.969-4.997 GHz, obtained with an international VLBI array comprising the 100-m Effelsberg, 36-m Haystack, 42-m Green Bank, 26-m Fort Davis, 130-m-equivalent VLA, and 40-m Owens Valley telescopes (system temperatures 40-70 K) at 22-14 h UT on July 26-27, 1983. The data are presented in graphs and tables and discussed in detail. Single Gaussian brightness distributions with theta(FWHM) = 0.5-2.0 marcsec and brightness temperatures (2-200) x 10 to the 8th K are determined for four sources; core-halo structures are found for UX Ari and Algol; and an upper limit of 50 microarcsec is established for the angular separation of the regions of left and right circular polarization. The gross features seen in RS CVn systems are explained in terms of a simple expanding-coronal-loop model of flare events.
Mutel, R. L.↗
High-angular resolution observations of stellar binary systems
A long-term program designed to investigate the spatial structure of centimetric radio emission from close binary systems using multistation VLBI array is discussed. Eleven binaries, including eight RS CVn systems, Algol, LSI 61 deg 303, and Cyg X-1, have been detected. The measured brightness temperatures vary from T(B) about 10 exp 8.5 K during periods of low activity to T(B) about 10 exp 10.5 K during flares. Extensive observation of a few sources has shown that the spatial structure is 'core-halo' with linear dimensions of about a stellar radius and the binary system, respectively. The observations are consistent with gyrosynchrotron emission of mildly relativistic electrons in magnetic fields of about 10 exp 1.5 + or - 0.5 gauss. The core sources appear to be optically thick, while the halo component is optically thin.
Lestrade, J.-F.↗
Detection of a sub-milliarcsecond radio component in the RS CVn system HR 1099
The RS CVn system HR 1099 was observed with a five station VLBI array at a frequency of 8.4 GHz during a strong radio outburst of approximately 400 mJy. The data are consistent with a circular Gaussian source of 0.8 + or - 0.12 milli-arcsec (FWHM), corresponding to a linear size of 4 + or - 0.6 x 10 to the 11th cm. This is comparable to the distance between the surfaces of the two stars, or to 75 percent of the diameter of the chromospherically active K star. Extrapolation of published photometric data shows that the starspot formation of HR 1099 was facing toward the observers at the times of observations. The high equivalent brightness temperature, approximately 10 billion K, is consistent with gyrosynchrotron emission from a power-law energy distribution of electrons in a magnetic field of strength about 30 gauss.
Mutel, R. L.↗
Very long baseline interferometry observations of the RS Canum Venaticorum system HR 5110 at 8.4 GHz
The RS CVn binary system HR 5110 was observed with a well-calibrated four-element VLBI array at a frequency of 8.4 GHz. The total flux density, which was nearly constant at 32 + or - 2 mJy, was monitored throughout the experiment with a 20 km baseline interferometer allowing accurate visibilities to be calculated. The observed visibilities are consistent with a source unresolved on all the baselines. The maximum size of the equivalent Gaussian source is 1.4 milli-arcsec (FWHM), corresponding to a linear size of 1.1 x 10 to the 12th cm at 52 pc. This maximum size is comparable with the overall size of the binary system. The inferred lower limit of the brightness temperature is 4 x 10 to the 8th K, which is higher than the coronal temperature of 10 to the 7th K measured in a soft X-ray survey. This is consistent with a nonthermal mechanism for the radiation process of the moderate radio outburst observed.
Mutel, R. L.↗
VLBI observations of the RS Canum Venaticorum binary systems UX Arietis and HR 1099 at 1.65 GHz
VLBI observations of the RS CVn binaries UX Arietis and HR 1099 have been made at 1.65 GHz using a three-element array with a minimum fringe spacing of 11.5 milli-arcsec. Both sources were found to be unresolved within measurement uncertainties. In both cases, the derived upper limit to the source size was comparable to the overall size of each binary system. The lower limits to the brightness temperature were 1.4 x 10 to the 10th K for UX Arietis and 2.9 x 10 to the 10th K for HR 1099. Simultaneous polarization measurements at the VLA showed 4-8 percent circular polarization and less than 2 percent linear polarization. It is found that the data are consistent with gyrosynchrotron emission from a power-law energy distribution of electrons in a magnetic field B less than or approximately equal to 6 gauss.
Mutel, R. L.↗