Long baseline interferometric observations of long period variable stars

Recent observations of long-period variable stars at spatial resolutions from approximately 1 arcsec to several milli-arcsecs have provided new insights into pulsation, dust formation, and mass-loss of AGB stars. These insights have come from long baseline interferometric observations obtained across a wide range of wavelengths, from the optical, through the infrared, to wavelengths as long as several millimeters. The present status and recent results from long baseline interferometry, particularly at optical and infrared wavelengths, are discussed. Such results include diameters and limb-darkening, surface features, mode of pulsation, location of SiO masers, inner radii of dust shells, physical conditions in the dust formation zone and of the inner regions of the dust shells. The results are interpreted in terms of present models of dust formation and mass-loss.     

Results of the first two years of VLBI observations at the Svetloe Observatory within the framework of international geodynamical programs

We present the results of processing the VLBI observations performed at the Svetloe Observatory of the Institute of Applied Astronomy (IAA), Russian Academy of Sciences, in the period 2003–2005 within the framework of geodynamical programs of the International VLBI Service (IVS) for geodesy and astrometry. We analyzed the observations at the Svetloe Observatory, together with the observations at other stations of the global IVS network, at the IAA using a modified OCCAM package. The package uses new reduction models that decrease the systematic errors of the results. The motion of the stations, primarily of the Svetloe Observatory, is investigated to study the global geotectonic processes. Highly accurate estimates of the coordinate and baseline length variations have been obtained for the first time in Russia from observations at a Russian VLBI station. We determined the coordinates and velocity of the Svetloe VLBI station with errors of ～2 mm and 3 mm yr^?1, respectively, and the baseline lengths between the stations with a sufficiently long observational history with an accuracy of 1–3 mm. The results are shown to be in good agreement with currently available models for the motion of tectonic plates.     

Vlbi study of the sio maser emission at 43 GHZ in the circumstellar envelope of R cas

We discuss the European effort in the study of millimeter-wave circumstellar masers with very long baseline interferometry. We show some results of the first observations, and discuss present and future work.     

Analysis of λ = 6 cm VLBI polarization observations of a complete sample of northern BL Lacertae objects

The results of very long baseline interferometry (VLBI) total intensity ( I ) and linear polarization ( P ) observations at     are presented for 10 radio bright BL Lacertae objects. These images complete first-epoch polarization observations for the 1-Jy sample of northern BL Lacertae objects defined by Kühr & Schmidt. Estimates of superluminal speeds are presented for several sources, bringing the total number of sources in the sample for which such estimates are available to 16. Second-epoch observations currently being reduced should yield speed estimates for VLBI features in essentially all the sources in the sample. The jet magnetic fields of these BL Lacertae objects are usually transverse to the local jet direction, but a sizeable minority (about 30 per cent) have VLBI jet components with longitudinal magnetic fields. This may suggest that the conditions in the VLBI jets of BL Lacertae objects are favourable for the formation of relativistic shocks; alternatively, it may be that the toroidal component of the intrinsic jet magnetic field is characteristically dominant in these sources.     

EVN observations of low-luminosity flat-spectrum active galactic nuclei

We present and discuss the results of very-long baseline interferometry (VLBI, EVN) observations of three low-luminosity     broad emission line active galactic nuclei (AGNs) carefully selected from a sample of flat-spectrum radio sources (CLASS). Based on the total and the extended radio power at 5 and at 1.4 GHz respectively, these objects should be technically classified as radio-quiet AGN and thus the origin of their radio emission is not clearly understood. The VLBI observations presented in this paper have revealed compact radio cores which imply a lower limit on the brightness temperature of about     . This result rules out a thermal origin for the radio emission and strongly suggests an emission mechanism similar to that observed in more powerful radio-loud AGNs. Since, by definition, the three objects show a flat (or inverted) radio spectrum between 1.4 and 8.4 GHz, the observed radio emission could be relativistically beamed. Multi-epoch VLBI observations can confirm this possibility in two years' time.     

Space very long baseline interferometry observations of polarization in the jet of 3C 380

A comparison between low-frequency space very long baseline interferometry (VLBI) and high-frequency ground-based VLBI images can, in principle, be used to detect small variations in rotation measure (RM) on fine angular scales inaccessible to ground arrays alone. This paper reports an attempt to perform such a comparison using the jet in the quasar 3C 380. Observations made with the VSOP antenna HALCA together with a ground array at wavelength 1.6 GHz provide total intensity and polarization images of comparable resolution to those from the ground array alone at 5 GHz. The results provide an image showing derotated magnetic vector position angle of somewhat higher resolution than that available earlier. The results show variations in an RM around component A of the order of 10 rad m⁻² that could not have been detected with the ground array alone. It is concluded that satellite VLBI observations provide a promising means to study the distribution of matter and magnetic fields around parsec-scale jets.
The ground observations used here follow the steady outward drift of component A, which has approximately doubled its distance from the core since the first observations in 1982. They also reveal total intensity and polarization structure associated with a bright knot 0.7 arcsec from the core which is reminiscent of that expected for a conical shock wave.     

All you ever wanted to know about optical long baseline stellar interferometry, but were too shy to ask your adviser

I try to present a small view of the properties and issues related to astronomical interferometry observations. I recall a bit of history of the technique, give some basic assessments to the principle of interferometry, and finally, describe physical processes and limitations that affect optical long baseline interferometry and which are, in general, very useful for everyday work. Therefore, this text is not intended to perform strong demonstrations and show accurate results, but rather to transmit the general “feeling” one needs to have to not be destabilised by the first contact to real world interferometry.     

北斗亚欧全视时间比对试验

基于卫星导航双频时间传递型接收机的伪码观测量,利用国际全球卫星导航系统服务组织(International Global Navigation Satellite System (GNSS) Service, IGS)提供的高精度卫星轨道和钟差产品,实现了北斗全视法时间比对.以IGS提供的时间尺度为两个待比对站的公共参考时间,首先使用双频组合法消除电离层对伪距观测的影响,然后将对流层和地球自转效应带来的时延利用理论模型在伪码观测量中进行扣除,分别获得两个比对站时间与公共参考时间之差后,将2者再做差,便得到了北斗全视时间比对结果.以中国科学院国家授时中心(NTSC)、德国物理技术研究院(PTB)和西班牙海军天文台(ROA)所保持的国家标准时间作为比对对象,开展了长基线北斗全视时间比对试验,获得北斗全视时间传递结果,最后利用阿伦方差和时间方差两项关键性能指标以及卫星双向时间比对对其进行性能评估.结果表明:北斗全视时间比对的天稳为10-14量级,可以满足国际时间比对需求.     

The diffuse ultraviolet foreground

Ultraviolet observations from low Earth orbit (LEO) have to deal with a foreground comprised of airglow and zodiacal light which depend on the look direction and on the date and time of the observation. We have used all-sky observations from the GALEX spacecraft to find that the airglow may be divided into a baseline dependent on the sun angle and a component dependent only on the time from local midnight. The zodiacal light is observable only in the near ultraviolet band (2321 Å) of GALEX and is proportional to the zodiacal light in the visible but with a color of 0.65 indicating that the dust grains are less reflective in the UV.     

Present and Future Cometary Science with the IRAM Plateau de Bure Interferometer

Interferometric observations are essential to probe the molecular emission in the inner cometary atmospheres and study the outgassing from the nucleus. Mapping the continuum emission can provide information about the dust and/or nucleus properties. We present here a summary of the observations of the dust and gas coma of comet 17P/Holmes and nuclear observations of 8P/Tuttle, both carried out with the IRAM interferometer at Plateau de Bure (PdBI) in 2007–2008. The observations of these two comets demonstrate the ability of the PdBI in terms of cometary science. In the near future, several improvements will be made (new receivers at 0.8 mm, a new wide-band correlator) allowing more frequent and more detailed studies of comets. On the long term, NOEMA, an expansion project, may add up to six antennas to the Plateau de Bure Interferometer, and increase the baseline lengths. Such an instrument would offer a complement to ALMA to track comets of the northern hemisphere with about half the sensitivity of ALMA for continuum studies.     

Radio interferometric observations of solar bursts at 3.7 and 11.1 cm wavelengths

Several solar active regions were observed during the 1976 vernal equinox with the 3-element interferometer of the National Radio Astronomy Observatory. The element spacings for these observations were 600, 2100, and 2700 m, resulting in maximum angular resolutions of about 3 arc sec at 3.7 cm and 8.5 arc sec at 11.1 cm. We fitted the fringe visibility for each baseline pair as a function of projected baseline with a single gaussian component, and calculated the extrapolated flux at zero baseline, F ₀, the FWHM source size, a, and the peak brightness temperature T _b. We present physical parameters derived for the observed bursts.     

A model of geometric delay in Space VLBI

A model which includes the relativistic effect is derived that can be applied to space very long baseline interferometry(SVLBI) while taking observations of sources at infinite distance. In SVLBI,where one station is on a spacecraft,the length of the baseline and the orbiting station's maximum speed in an elliptical orbit around the Earth is much larger than the ground-based VLBI,which leads to a larger delay and higher delay rate. The delay models inside VLBI correlators are usually expressed as fifth-order polynomials during a limited time interval,which are evaluated by firmware in the correlator and track delays in the interferometer over the limited time interval. The higher SVLBI delay rate requires more accurate polynomial fitting and evaluation,as well as more frequent model updates.     

Measurements of radio emission from the compact source in the Crab nebula with the uran-1 interferometer at 16.7, 20 and 25 MHz

This paper presents the results of measurements of the Crab nebula in the decametre range with an interferometer whose baseline is 2.4–3.5×10³ of the wavelength. Visibility function values, which in these observations determine the contribution by the compact source to the total nebula flux, have been measured at frequencies 16.7, 20 and 25 MHz to be 0.64±0.07, 0.43±0.04 and 0.31±0.03, respectively. The spectral index of the spectrum obtained for the compact source in the range 16.7–122 MHz is 2.09±0.04. Flattening of the nebula spectrum without the compact source has been confirmed for the decametre range.     

Submillimetre detection of the z=2.83 Lyman-break galaxy, Westphal–MM8, and implications for SCUBA2

We present confusion-limited submillimetre (submm) observations with the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope of the   z = 2.83  Lyman-break galaxy (LBG), Westphal–MM8, reaching an 850 μm sensitivity even greater than that achieved in the SCUBA map of the Hubble Deep Field region. The detection of MM8  ( S _850 μm= 1.98 ± 0.48 mJy)  , along with the literature submm detections of lensed LBGs, suggests that the LBG population may contribute significantly to the source counts of submm-selected galaxies in the 1–2 mJy regime. Additionally, submm-luminous LBGs are a viable progenitor population for the recently discovered evolved galaxies at   z ∼ 2–2.5  . These observations represent an important baseline for SCUBA2 observations which will regularly map large regions of the sky to this depth.     

Fast radio burst search: cross spectrum vs. auto spectrum method

The search for fast radio bursts(FRBs) is a hot topic in current radio astronomy studies. In this work, we carry out a single pulse search with a very long baseline interferometry(VLBI) pulsar observation data set using both auto spectrum and cross spectrum search methods. The cross spectrum method,first proposed in Liu et al., maximizes the signal power by fully utilizing the fringe phase information of the baseline cross spectrum. The auto spectrum search method is based on the popular pulsar software package PRESTO, which extracts single pulses from the auto spectrum of each station. According to our comparison, the cross spectrum method is able to enhance the signal power and therefore extract single pulses from data contaminated by high levels of radio frequency interference(RFI), which makes it possible to carry out a search for FRBs in regular VLBI observations when RFI is present.     

Long-term atmospheric variability on Uranus and Neptune

Long-term photometric measurements of Uranus and Neptune through 2005 show variations in brightness. For Uranus, much of the variation can be interpreted as seasonal, i.e., caused by viewing angle changes of an oblate planet. The photometry suggests that if seasonal variations on Uranus are north-south symmetric, then the northern pole should begin to brighten in 2006. However, seasonal “aspect” changes cannot explain all the variation; the Uranus observations require intrinsic atmospheric change. Furthermore, Uranus observations spanning many scale heights in the atmosphere may show similar change. For Neptune, variations in sub-solar latitude may explain the general shape of the long-term light curve, but significant deviations occur that have no explanation at present. Observations are needed over a longer temporal baseline than currently exists to fully characterize both atmospheres.     

SiO maser emission in Miras

We describe a combined dynamic atmosphere and maser propagation model of SiO maser emission in Mira variables. This model rectifies many of the defects of an earlier model of this type, particularly in relation to the infrared (IR) radiation field generated by dust and various wavelength-dependent, optically thick layers. Modelled masers form in rings with radii consistent with those found in very long baseline interferometry (VLBI) observations and with earlier models. This agreement requires the adoption of a radio photosphere of radius approximately twice that of the stellar photosphere, in agreement with observations. A radio photosphere of this size renders invisible certain maser sites with high amplification at low radii, and conceals high-velocity shocks, which are absent in radio continuum observations. The SiO masers are brightest at an optical phase of 0.1–0.25, which is consistent with observed phase lags. Dust can have both mild and profound effects on the maser emission. Maser rings, a shock and the optically thick layer in the SiO pumping band at 8.13 μm appear to be closely associated in three out of four phase samples.     

First VLTI observations of Mira stars

We present first observations of Mira stars obtained at the Very Large Telescope Interferometer (VLTI) at 2 microns, equipped with the VINCI instrument, using different baselines with both the test siderostats and the 8.2 m unit telescopes. These observations, collected in the course of the VLTI commissining program, have targeted so far about sixty cool giant stars. In this talk, we list and describe the measurements obtained for 14 Mira stars, many of them representing first-time determinations of the visibility. In particular, we devote special attention to a study of Mira itself, for which very accurate determinations of the visibility at several baseline lenghts and orientations could be obtained. We find that a two-component model is more consistent with the data than a single stellar disc. Further observations are needed for a better understanding of this source. The VLTI will constitute an ideal instrument for such studies in the future. In particular it will permit detailed investigations of southern AGB stars, such as accurate measurements of surface structureparameters (diameters, diameter variations, asymmetries, center-to-limbvariations, special features like hot spots) and of circumstellar envelopes. This revised version was published online in July 2006 with corrections to the Cover Date.     

The orbital motion of the Arches cluster: clues on cluster formation near the Galactic Center

We have measured the proper motion of the Arches cluster near the Galactic Center (GC) with respect to the ambient field, using Keck/NIRC2 LGS-AO and VLT/NAOS-CONICA NGS-AO observations spanning a baseline of 4.3 yr. Combined with the radial velocity, we derive a 3D space motion of 232±30 km?s^?1 for the Arches cluster. This motion is exceptionally large compared to molecular cloud orbits at the GC, and places stringent constraints on the formation scenarios for starburst clusters in dense, nuclear environments.     

Conjugate and closely-spaced observations of auroral radio absorption—IV The movement of simple features

Although the time structure of auroral radio absorption, measured with riometers at high-latitude stations, is generally complex, some features are seen which have a relatively simple structure. Multiple riometer data from the magnetically-conjugate stations Great Whale River and Byrd demonstrate that many of these features show movements over a 250 km baseline. Both the velocities and the directions of movement are consistent with the substorm movements that have been detected in other auroral work, for example by visual observations or by auroral radar. The velocities range between 80 m/sec and 3.3 km/sec, the directions being predominantly westward in the evening sector and eastward in the morning. The observed behaviour is essentially the same in both hemispheres, though event-by-event comparisons show some differences of velocity. The data are applied to determine the instantaneous corresponding points in the two hemispheres; it is found that the corresponding point wanders by 130 km about its mean position. Some implications of the results are discussed.