Search

APOGEE is a large-scale, NIR, high-resolution (R~20,000) spectroscopic survey of Galactic stars. It is one of the four experiments in SDSS-III. Because APOGEE will observe in the H band, it will be the first survey to pierce through Galactic dust and provide a vast, uniform database of chemical abundances and radial velocities for stars across all Galactic populations (bulge, disk, and halo). The survey will be conducted with a dedicated, 300-fiber, cryogenic, spectrograph that is being built at the University of Virginia, coupled to the ARC 2.5m telescope at Apache Point Observatory. APOGEE will use a significant fraction of the SDSS-III bright time during a three-year period to observe, at high signal-to-noise ratio (S/N>100), about 100,000 giant stars selected directly from 2MASS down to a typical flux limit of H<13. The main scientific objectives of APOGEE are: (1) measuring unbiased metallicity distributions and abundance patterns for the different Galactic stellar populations, (2) studying the processes of star formation, feedback, and chemical mixing in the Milky Way, (3) surveying the dynamics of the bulge and disk, placing constraints on the nature and influence of the Galactic bar and spiral arms, and (4) using extensive chemodynamical data, particularly in the inner Galaxy, to unravel its formation and evolution., Comment: 4 pages, 2 figures; proceedings of the conference on Galactic & Stellar Dynamics in Strasbourg 16-20 March 2008

The discovery of extra-solar planets is one of the greatest achievements of modern astronomy. The detection of planets with a wide range of masses demonstrates that extra-solar planets of low mass exist. In this paper we describe a mission, called Darwin, whose primary goal is the search for, and characterization of, terrestrial extrasolar planets and the search for life. Accomplishing the mission objectives will require collaborative science across disciplines including astrophysics, planetary sciences, chemistry and microbiology. Darwin is designed to detect and perform spectroscopic analysis of rocky planets similar to the Earth at mid-infrared wavelengths (6 - 20 micron), where an advantageous contrast ratio between star and planet occurs. The baseline mission lasts 5 years and consists of approximately 200 individual target stars. Among these, 25 to 50 planetary systems can be studied spectroscopically, searching for gases such as CO2, H2O, CH4 and O3. Many of the key technologies required for the construction of Darwin have already been demonstrated and the remainder are estimated to be mature in the near future. Darwin is a mission that will ignite intense interest in both the research community and the wider public.

NGC 205, a close satellite of the M31 galaxy, is our nearest example of a dwarf elliptical galaxy. Photometric and kinematic observations suggest that NGC 205 is undergoing tidal distortion from its interaction with M31. Despite earlier attempts, the orbit and progenitor properties of NGC 205 are not well known. We perform an optimized search for these unknowns by combining a genetic algorithm with restricted N-body simulations of the interaction. This approach, coupled with photometric and kinematic observations as constraints, allows for an effective exploration of the parameter space. We represent NGC 205 as a static Hernquist potential with embedded massless test particles that serve as tracers of surface brightness. We explore 3 distinct, initially stable configurations of test particles: cold rotating disk, warm rotating disk, and hot, pressure-supported spheroid. Each model reproduces some, but not all, of the observed features of NGC 205, leading us to speculate that a rotating progenitor with substantial pressure support could match all of the observables. Furthermore, plausible combinations of mass and scale length for the pressure-supported spheroid progenitor model reproduce the observed velocity dispersion profile. For all 3 models, orbits that best match the observables place the satellite 11+/-9 kpc behind M31 moving at very large velocities: 300-500 km/s on primarily radial orbits. Given that the observed radial component is only 54 km/s, this implies a large tangential motion for NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC 205's velocity appears to be near or greater than its escape velocity, signifying that the satellite is likely on its first M31 passage., Comment: 34 pages, 20 figures, accepted for publication in the Astrophysical Journal, A pdf version with high-resolution figures may be obtained from http://www.ucolick.org/~kirsten/ms.pdf

We present the largest and most detailed survey to date of the stellar populations in the outskirts of M31 based on the analysis of 14 deep HST/ACS pointings spanning the range 11.5-45.0 kpc. We conduct a quantitative comparison of the resolved stellar populations in these fields and identify several striking trends. The color-magnitude diagrams (CMDs), which reach ~3 magnitudes below the red clump, can be classified into two main categories based on their morphologies. `Stream-like' fields, so named for their similarity to the CMD of the giant stellar stream, are characterized by a red clump that slants bluewards at fainter magnitudes and an extended horizontal branch. On the other hand, `disk-like' fields exhibit rounder red clumps with significant luminosity width, lack an obvious horizontal branch and show evidence for recent star formation (~0.25 - 2.0 Gyr ago). We compare the spatial and line-of-sight distribution of stream-like fields with a recent simulation of the giant stream progenitor orbit and find an excellent agreement. These fields, found across the face of M31, attest to the high degree of pollution caused by this event. Disk-like material resides in the extended disk structure of M31 and is detected out to 44 kpc (projected); the uniform populations in these fields, including the ubiquitous presence of young populations, and the strong rotation reported elsewhere are most consistent with a scenario in which this structure has formed through heating and disruption of the existing thin disk, perhaps due to the impact of the giant stream progenitor. Our comparative analysis sheds new light on the likely composition of two of the ultra-deep pointings formerly presented as pure outer disk and pure halo in the literature., Comment: Accepted to AJ. ftp://ftp.roe.ac.uk/pub/jcr/richardson2008.pdf contains high resolution figures. 16 pages, 10 figures; best seen in color on above link

We present the results of an experiment to image the interacting binary star beta Lyrae with data from the Navy Prototype Optical Interferometer (NPOI), using a differential phase technique to correct for the effects of the instrument and atmosphere on the interferometer phases. We take advantage of the fact that the visual primary of beta Lyrae and the visibility calibrator we used are both nearly unresolved and nearly centrally symmetric, and consequently have interferometric phases near zero. We used this property to detect and correct for the effects of the instrument and atmosphere on the phases of beta Lyrae and to obtain differential phases in the channel containing the Halpha emission line. Combining the Halpha-channel phases with information about the line strength, we recovered complex visibilities and imaged the Halpha emission using standard radio interferometry methods. We find that the results from our differential phase technique are consistent with those obtained from a more-standard analysis using squared visibilities (V^2's). Our images show the position of the Halpha emitting regions relative to the continuum photocenter as a function of orbital phase and indicate that the major axis of the orbit is oriented along p.a.=248.8+/-1.7 deg. The orbit is smaller than previously predicted, a discrepancy that can be alleviated if we assume that the system is at a larger distance from us, or that the contribution of the stellar continuum to the Halpha channel is larger than estimated. Finally, we also detected a differential phase signal in the channels containing HeI emission lines at 587.6 and 706.5nm, with orbital behavior different from that of the Halpha, indicating that it originates from a different part of this interacting system., Comment: Submitted to ApJ

During the last few years, considerable effort has been directed towards large-scale (>> $1 Billion US) missions to detect and characterize earth-like planets around nearby stars, such as the Terrestrial Planet Finder Interferometer (TPF-I) and Darwin missions. However, technological and budgetary issues as well as shifting science priorities will likely prevent these missions from entering Phase A until the next decade. The secondary eclipse technique using the Spitzer Space Telescope has been used to directly measure the temperature and emission spectrum of extrasolar planets. However, only a small fraction of known extrasolar planets are in transiting orbits. Thus, a simplified nulling interferometer, which produces an artificial eclipse or occultation, and operates in the near- to mid-infrared (e.g. ~ 3 to 8 or 10 microns), can characterize the atmospheres of this much larger sample of the known but non-transiting exoplanets. Many other scientific problems can be addressed with a system like this, including imaging debris disks, active galactic nuclei, and low mass companions around nearby stars. We discuss the rationale for a probe-scale mission in the $600-800 Million range, which we name here as the Small Prototype Planet Finding Interferometer (SPPFI)., Comment: 8 pages, 4 figures, white paper for Exoplanet Task Force, March 2007

We report on interferometric and radial-velocity observations of the double-lined 51-d period binary (A) component of the quadruple pre-main sequence (PMS) system V773 Tau. With these observations we have estimated preliminary visual and physical orbits of the V773 Tau A subsystem. Among other parameters, our orbit model includes an inclination of 66.0 $\pm$ 2.4 deg, and allows us to infer the component dynamical masses and system distance. In particular we find component masses of 1.54 $\pm$ 0.14 and 1.332 $\pm$ 0.097 M$_{\sun}$ for the Aa (primary) and Ab (secondary) components respectively. Our modeling of the subsystem component spectral energy distributions finds temperatures and luminosities consistent with previous studies, and coupled with the component mass estimates allows for comparison with PMS stellar models in the intermediate-mass range. We compare V773 Tau A component properties with several popular solar-composition models for intermediate-mass PMS stars. All models predict masses consistent to within 2-sigma of the dynamically determined values, though some models predict values that are more consistent than others., Comment: ApJ in press; 25 pages, 6 figures; data tables available in journal version

The past two Decadal Surveys in Astronomy and Astrophysics recommended the completion of a space-based interferometry mission, known today as SIM PlanetQuest, for its unique ability to detect and characterize nearby rocky planets (Bahcall 1991, McKee & Taylor 2001), as well as contributions to a broad range of problems in astrophysics. Numerous committees of the National Research Council as well as NASA Roadmaps have similarly highlighted SIM as the one technology that offers detection and characterization of rocky planets around nearby stars and which is technically ready. To date, SIM remains the only program with the capability of detecting and confirming rocky planets in the habitable zones of nearby solar-type stars. Moreover, SIM measures masses and three-dimensional orbits of habitable planets around nearby stars (within 25 pc); these are the only stars for which follow-up by other techniques is feasible, such as space-based spectroscopy, ground-based interferometry, and of course TPF., Comment: ExoPTF white paper 8 pages, 2 figures

We have used the Very Large Array (VLA), linked with the Pie Town Very Long Baseline Array antenna, to determine astrometric positions of 46 radio stars in the International Celestial Reference Frame (ICRF). Positions were obtained in the ICRF directly through phase referencing of the stars to nearby ICRF quasars whose positions are accurate at the 0.25 mas level. Radio star positions are estimated to be accurate at the 10 mas level, with position errors approaching a few milli-arcseconds for some of the stars observed. Our measured positions were combined with previous measurements taken from as early as 1978 to obtain proper motion estimates for all 46 stars with average uncertainties of ~1.7 mas/yr. We compared our radio star positions and proper motions with the Hipparcos Catalogue data, and find consistency in the reference frames produced by each data set on the 1-sigma level, with errors of ~2.7 mas per axis for the reference frame orientation angles at our mean epoch of 2003.78. No significant spin is found between our radio data frame and the Hipparcos Celestial Reference Frame (HCRF) with largest rotation rates of +0.55 and -0.41 mas/yr around the x and z axes, respectively, with 1-sigma errors of 0.36 mas/yr. Thus, our results are consistent with a non-rotating Hipparcos frame with respect to the ICRF., Comment: 29 pages, 8 figures, accepted for publication in the Astronomical Journal

Observations of the Mercury-Manganese star Phi Herculis with the Navy Prototype Optical Interferometer (NPOI) conclusively reveal the previously unseen companion in this single-lined binary system. The NPOI data were used to predict a spectral type of A8V for the secondary star Phi Her B. This prediction was subsequently confirmed by spectroscopic observations obtained at the Dominion Astrophysical Observatory. Phi Her B is rotating at 50 +/-3 km/sec, in contrast to the 8 km/sec lines of Phi Her A. Recognizing the lines from the secondary permits one to separate them from those of the primary. The abundance analysis of Phi Her A shows an abundance pattern similar to those of other HgMn stars with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr, Y, Zr, Ba, Ce, and Hg being very overabundant., Comment: Accepted to ApJ, 45 pages, 11 figures

We present a wide-field (4.5 deg^2) photometric and spectroscopic survey of the Leo I dwarf spheroidal (dSph) galaxy to explore its extended morphology and dynamics. As in previous papers in this series, we take advantage of photometry in the M, T_2, and DDO51 filter system to select LeoI red giant branch star candidates, and, so far, this selection technique has proven 100% reliable in selecting actual Leo I members among more than 100 M < 21.5 Leo I giant candidates having previous or new Keck DEIMOS spectroscopy to a radius >1.3 times the limiting radius of the fitted, central King profile. The two-dimensional distribution of all similarly-selected Leo I giant candidates is well fitted by a central single-component King profile of limiting radius 13.3 arcmin, but many giant stars are found outside this newly derived King limiting radius. The density profile thus shows a break at a major axis radial distance of ~10 arcmin produced by an excess of stars at and beyond the King limiting radius (spectroscopically confirmed to be made of true Leo I members), and primarily along the major axis of the main body of the rather elongated satellite. This spatial configuration, a rather flat velocity dispersion profile and an asymmetric radial velocity (RV) distribution among the Leo I members at large radii together support a picture where Leo I has been tidally disrupted on at least one, but at most two, perigalactic passages of a massive Local Group member. (abridged), Comment: Accepted by Astrophysical Journal on March 15, 2007; 49 pages, 26 figures, uses emulateapj.cls

Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard. While its spectrum is dominated by broad hydrogen lines, the narrower lines of the heavy elements suggested slow to moderate rotation, giving confidence that the ground-based calibration of its visibile spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers and it has unusually shaped absorption line profiles, leading some to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations of Vega which detect the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93% of breakup speed. In addition to explaining the unusual brightness and line shape pecularities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations. The large temperature differences predicted across its surface call into question composition determinations, adding uncertainty to Vega's age and opening the possibility that its debris disk could be substantially older than previously thought., Comment: 10 pages, 2 figures, a Letter to Nature, tentatively scheduled for April 13

We report successful fitting of a Roche model, with a surface temperature gradient following the von Zeipel gravity darkening law, to observations of Altair made with the Navy Prototype Optical Interferometer. We confirm the claim by Ohishi, Nordgren, & Hutter that Altair displays an asymmetric intensity distribution due to rotation, the first such detection in an isolated star. Instrumental effects due to the high visible flux of this first magnitude star appear to be the limiting factor in the accuracy of this fit, which nevertheless indicates that Altair is rotating at 0.90+/-0.02 of its breakup (angular) velocity. Our results are consistent with the apparent oblateness found by van Belle et al. and show that the true oblateness is significantly larger owing to an inclination of the rotational axis of ~64 degrees to the line of sight. Of particular interest, we conclude that instead of being substantially evolved as indicated by its classification, A7 VI-V, Altair is only barely off the ZAMS and represents a good example of the difficulties rotation can introduce in the interpretation of this part of the HR diagram., Comment: Accepted by the Astrophysical Journal

We present the results of multisite observations spanning two years on the pre--main-sequence (PMS) star V351 Ori. A total of around 180 hours of observations over 29 nights have been collected, allowing us to measure five different periodicities, most likely related to the delta Scuti variability of V351 Ori. Comparison with the predictions of linear nonadiabatic radial pulsation models put stringent constraints on the stellar parameters and indicate that the distance to V351 Ori is intermediate between the lower limit measured by Hipparcos (210 pc) and that of the Orion Nebula (450 pc). However, radial pulsation models are unable to reproduce all of the observed frequencies with a single choice of (M, L, and Te), suggesting the presence of additional nonradial modes., Comment: 10 pages, including 7 postscript figures, accepted for publication on A&A

We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine astrometric positions of 19 radio stars in the International Celestial Reference Frame (ICRF). The positions of these stars were directly linked to the positions of distant quasars through phase referencing observations. The positions of the ICRF quasars are known to 0.25 mas, thus providing an absolute reference at the angular resolution of our radio observations. Average values for the errors in our derived positions for all sources were 13 mas and 16 mas in R.A. and declination respectively, with accuracies approaching 1-2 mas for some of the stars observed. Differences between the ICRF positions of the 38 quasars, and those measured from our observations showed no systematic offsets, with mean values of -0.3 mas in R.A. and -1.0 mas in declination. Standard deviations of the quasar position differences of 17 mas and 11 mas in R.A. and declination respectively, are consistent with the mean position errors determined for the stars. Our measured positions were combined with previous Very Large Array measurements taken from 1978-1995 to determine the proper motions of 15 of the stars in our list. With mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions approach those derived from Hipparcos, and for a few of the stars in our program, are better than the Hipparcos values. Comparing the positions of our radio stars with the Hipparcos catalog, we find that at the epoch of our observations, the two frames are aligned to within formal errors of approximately 3 mas. This result confirms that the Hipparcos frame is inertial at the expected level., Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March 18

We describe first results of a spectroscopic probe of selected fields from the Grid Giant Star Survey. Multifiber spectroscopy of several hundred stars in a strip of eleven fields along delta approximately -17^{circ}, in the range 12 <~ alpha <~ 17 hours, reveals a group of 8 giants that have kinematical characteristics differing from the main field population, but that as a group maintain coherent, smoothly varying distances and radial velocities with position across the fields. Moreover, these stars have roughly the same abundance, according to their MgH+Mgb absorption line strengths. Photometric parallaxes place these stars in a semi-loop structure, arcing in a contiguous distribution between 5.7 and 7.9 kpc from the Galactic center. The spatial, kinematical, and abundance coherence of these stars suggests that they are part of a diffuse stream of tidal debris, and one roughly consistent with a wrapped, leading tidal arm of the Sagittarius dwarf spheroidal galaxy., Comment: 8 pages including 4 figures. Accepted for publication in ApJL

We present direct measurements of the limb-darkened intensity profiles of the late-type giant stars HR5299, HR7635, and HR8621 obtained with the Navy Prototype Optical Interferometer (NPOI) at the Lowell Observatory. A triangle of baselines with lengths of 18.9 m, 22.2 m, and 37.5 m was used. We utilized squared visibility amplitudes beyond the first minimum, as well as triple amplitudes and phases in up to 10 spectral channels covering a wavelength range of ~650 nm to ~850 nm. We find that our data can best be described by featureless symmetric limb-darkened disk models while uniform disk and fully darkened disk models can be rejected. We derive high-precision angular limb-darkened diameters for the three stars of 7.44 mas +/- 0.11 mas, 6.18 mas +/- 0.07 mas, and 6.94 mas +/- 0.12 mas, respectively. Using the HIPPARCOS parallaxes, we determine linear limb-darkened radii of 114 R$_\odot \pm $13 R$_\odot$, 56 R$_\odot \pm $4 R$_\odot$, and 98 R$_\odot \pm $9 R$_\odot$, respectively. We compare our data to a grid of Kurucz stellar model atmospheres, with them derive the effective temperatures and surface gravities without additional information, and find agreement with independent estimates derived from empirical calibrations and bolometric fluxes. This confirms the consistency of model predictions and direct observations of the limb-darkening effect., Comment: 13 pages, accepted for publication in A&A

We present light curves for 149 sources monitored with the Green Bank Interferometer. The light curves are at two radio frequencies (approximately 2.5 and 8.2 GHz) and range from 3 to 15 yrs in length, covering the interval 1979--1996, and have a typical sampling of one flux density measurement every 2 days. We have used these light curves to conduct various variability analysis (rms flux density variations and autoregressive, integrated, moving average modeling) of these sources. We find suggestive, though not unambiguous evidence, that these sources have a common, broadband mechanism for intrinsic variations, in agreement with previous studies of a subset of these source. We also find that the sources generally display a short-term variability (~ 10 d) that arises from radio-wave scattering in an extended medium. These conclusions extend those of Fiedler et al. (1987) who used a sub-sample of these data. The primary motivation for this monitoring program was the identification of extreme scattering events. In an effort to identify ESEs in a systematic manner, we have taken the wavelet transform of the light curves. We find 15 events in the light curves of 12 sources that we classify as probable ESEs. However, we also find that five ESEs previously identified from these data do not survive our wavelet selection criteria. Future identification of ESEs will probably continue to rely on both visual and systematic methods. Instructions for obtaining the data are also presented., Comment: 34 pages, LaTeX with AASTeX-5, 7 figures in 20 PostScript files; because of their length, Table 1 and Figures 8 and 9 can be obtained from http://ese.nrl.navy.mil/GBI/GBI.html

As part of our survey for substructure in the Milky Way halo as traced by giant stars, and to look for tidal stellar debris in the halo viable for measurement of the Galactic mass potential with SIM, we explore the distribution of stars beyond the nominal tidal radius of, but still associated with, the Carina dwarf spheroidal galaxy. We make use of a photometric technique described in Majewski et al. (1999) to identify giant star candidates at the distance and metallicity of Carina across the entire extent of the photometric survey covering some 2.2 deg^2 on and around Carina. These Carina-associated giant candidates are identified by a combination of (1) their M-DDO51 colors, and (2) by locations in the color-magnitude diagram commensurate with the Carina RGB in the core of the galaxy. The density distribution of the extratidal giant candidates is similar to that found from previous statistical star-counting approaches. However, we are now able to pinpoint actual, remotely-situated Carina stars individually. Because we can exclude foreground dwarf stars, our approach allows greater sensitivity and the ability to map the detailed two-dimensional distribution of extended Carina populations to much larger radii, with smaller aperture telescopes, than other techniques. Moreover, we identify candidate lists of widely displaced Carina-associated stars bright enough for spectroscopic studies of large-scale dynamical and metallicity properties of the system, and for astrometric study by SIM. We obtained spectroscopy for three such ``extratidal'' stars and from their radial velocities conclude that all three are associated with Carina., Comment: 30 pages, plus 15 figures

We present a comparison of semi-analytic models of the phase-space structure of tidal debris with observations of stars associated with the Sagittarius dwarf galaxy (Sgr). We find that many features in the data can be explained by these models. The properties of stars 10-15 degrees away from the center of Sgr --- in particular, the orientation of material perpendicular to Sgr's orbit (c.f. Alard 1996) and the kink in the velocity gradient (Ibata et al 1997) --- are consistent with those expected for unbound material stripped during the most recent pericentric passage ~50 Myrs ago. The break in the slope of the surface density seen by Mateo, Olszewski & Morrison (1998) at ~ b=-35 can be understood as marking the end of this material. However, the detections beyond this point are unlikely to represent debris in a trailing streamer, torn from Sgr during the immediately preceding passage ~0.7 Gyrs ago, but are more plausibly explained by a leading streamer of material that was lost more that 1 Gyr ago and has wrapped all the way around the Galaxy. The observations reported in Majewski et al (1999) also support this hypothesis. We determine debris models with these properties on orbits that are consistent with the currently known positions and velocities of Sgr in Galactic potentials with halo components that have circular velocities v_circ=140-200 km/s. The best match to the data is obtained in models where Sgr currently has a mass of ~10^9 M_sun and has orbited the Galaxy for at least the last 1 Gyr, during which time it has reduced its mass by a factor of 2-3, or luminosity by an amount equivalent to ~10% of the total luminosity of the Galactic halo. These numbers suggest that Sgr is rapidly disrupting and unlikely to survive beyond a few more pericentric passages., Comment: 19 pages, 5 figures, accepted to Astronomical Journal

As part of the Selected Areas Starcount Survey (SASS), a CCD survey to V>21, we have obtained VI photometry of two fields at b=+-40 aligned roughly with an extrapolation of the major axis of the Sagittarius (Sgr) dwarf spheroidal galaxy. Comparison of the color-magnitude diagram (CMD) for some of these fields to the CMDs of fields reflected about the Galactic l=0 meridian reveals an excess of stars at V_o=17.85 and 0.9 < (V-I)_o < 1.1 in the (l,b) = (11,-40) field. The excess stars have colors consistent with the Sgr red clump, and deeper CMD imaging in these locations shows evidence of a main-sequence turnoff at V=21 with the main-sequence extending to the limit of our data (V=24). The surface brightnesses we derive from either the potential excess of red clump stars or the apparent excess of MSTO stars are consistent with each other and with the results of other surveys at this latitude. No similar excess appears in our northern Galactic hemisphere fields near l=353, b=+41 field. We have obtained spectroscopy of all 30 candidate red clump stars in the range 0.9 < (V-I)_o < 1.1 and 17.75 < V_o < 17.95. The radial velocity distribution of the stars, while dissimilar from expectations of Galactic structure models, does not show a contribution by stars near the Galactocentric radial velocity seen in other studies near the Sgr core. It is difficult to reconcile a photometric result that is consistent with other explorations of the Sagittarius stream with a radial velocity distribution that is apparently inconsistent. In a companion paper (Johnston et al. 1999), we discuss how some of the discrepancies are resolved if ur potential Sgr detection corresponds to a different Sgr tidal streamer than that detected by most other surveys., Comment: 26 pages, including 8 figures, accepted to Astronomical Journal

The luminous high-redshift (z=2.17) quasar S5 0836+710 has been observed at 5GHz with the VSOP. We compare the properties of three images obtained from the observation: a low-resolution ground array image (dynamic range 4600:1), a full-resolution VSOP image (900:1), and an image made with only the space baselines (200:1). The space baselines alone are sufficient for a reliable recovery of the source structure, within the limits of the achieved spatial sampling of the visibility data. The curved jet ridge line observed in the images can be described by Kelvin-Helmholtz instabilities developing in a relativistic outflow with the Mach number of about 6. This description holds on the scales of up to 700h^-1 pc, and is shown to be consistent with variable apparent speeds observed in the jet., Comment: 5 pages, 5 PostScript figures, LaTeX2e, A&A macros, A&A, accepted

Using CGRO/BATSE hard X-ray (HXR) data and GHz radio monitoring data from the Green Bank Interferometer (GBI), we have performed a long term study ($\sim$ 1800 days) of the unusual X-ray binary Cyg X-3 resulting in the discovery of a remarkable relationship between these two wavelength bands. We find that, during quiescent radio states, the radio flux is strongly anticorrelated with the intensity of the HXR emission. The relationship switches to a correlation with the onset of major radio flaring activity. During major radio flaring activity the HXR drops to a very low intensity during quenching in the radio and recovers during the radio flare. Injection of plasma into the radio jets of Cyg X-3 occurs during changes in the HXR emission and suggests that disk-related and jet-related components are responsible for the high energy emission., Comment: 19 pages, Latex, 4 figures, 2 tables, submitted to Ap.J

We report multifrequency observations of the $\gamma$-ray blazar 0528+134 with the Effelsberg 100-m telescope, the IRAM 30-m telescope, and the NRL Green Bank Interferometer. 0528+134 underwent a major radio and mm outburst in 1993 a few month after a very strong outburst in high energy $\gamma$-rays. A similar behaviour is indicated for the weaker $\gamma$-ray outburst in May 1991 and the mm activity in the beginning of 1992. This result fits nicely to the general finding that blazars are bright in $\gamma$-rays preferrentially at the beginning of a radio outburst. 0528+134 was observed with a global VLBI array at 22$\,$GHz in November 1992. Within the IRIS-S and EUROPE geodetic VLBI observing campaigns the source was also observed regularly at 8.4$\,$GHz. A new VLBI component which is not present in data at 22 $\,$GHz from 1991 also exhibits superluminal motion. Backextra\-polating in time we find that the new component was expelled from the core in the first half of 1991, near the first $\gamma$-ray outburst in Mai 1991. Newest VLBI data indicate the appearance of another new component in 1994. Although this new component has yet been found only at two epochs, it appears to move superluminally and may have been expelled from the core between fall 1992 and summer 1993, i.e. near the time of the second gamma ray outburst and at the beginning of a strong radio outburst., Comment: 3 pages, Postscript, Proceedings of IAU Symposium 175 on 'Extragalactic Radio Sources'

In this pilot investigation, precise optical positions in the FK5 system are presented for a set of 16 compact extragalactic radio sources, which will be part of the future radio--optical reference frame. The 0.9 m KPNO and CTIO telescopes equipped with 2K CCD's have been used for this project. The astrometric properties of these instruments are investigated in detail. New techniques of using wide field CCD observations for astrometry in general are developed. An internal precision of 5 to 31 mas in position per single exposure is found, depending on the brightness of the object. The tie to the primary optical reference system is established by photographic astrometry using dedicated astrographs on both hemispheres. An accuracy of $\approx 30$ mas per source is estimated for the multi--step reduction procedure when based on the future Hipparcos catalog, while the FK5--based positions suffer from system errors of 100 to 200 mas as compared to the radio positions. This work provides a contribution to the international effort to link the Hipparcos instrumental coordinate system to the quasi--inertial VLBI radio reference frame. Precise radio and optical astrometry of a large sample of compact extragalactic sources will also contribute to the astrophysics of these objects by comparing the respective centers of emission at the optical and radio wavelengths., Comment: AAS v.4 LaTeX, 2 parts on 1 file (main text + deluxetable), accepted by AJ, Dec.95, fig. with reprints

We report on multifrequency radio observations made with the Effelsberg 100-m telescope, the IRAM 30-m telescope and the Green Bank Interferometer between 1992 and 1994 of the $\gamma$-ray quasar 0528+134. We present a new VLBI based map of 0518+134 at 22 GHz with sub-mas angular resolution observed in November 1992, which shows a one-sided core jet structure of $\sim$ 5 mas length. One component, seen also previously at 8.4 GHz & 22 GHz, shows apparent superluminal motion with $\beta_{app}$=4.4$\pm$1.7. In summer 1993 we observed a major outburst at all frequencies higher than a few GHz, which peaked a few months after a strong outburst in high energy $\gamma$-rays and showed the canonical time evolution of a cooling and/or expanding electron distribution. Our data indicate that the outburst in the EGRET range originated very close to the central object of the AGN and that a remnant of this outburst moved further outward in the jet until it became optically thin at radio frequencies after a few months. During the flare in July 1993 we observed with the Effelsberg 100-m telescope an unusually strong decrease of the flux density by about 50% at 4.75 GHz and 10.55 GHz and slightly less at 2.695 GHz. This behaviour is also seen in the monitoring data at 2.25 GHz and 8.3 GHz taken with the Green Bank Interferometer (NRL-GBI). An extreme scattering event by a small dense plasma cloud in the line of sight is able to match the observed time lag in the lightcurves if we take into account the mas-structure of the source and different spectra of the components on the basis of their brightness in the VLBI maps., Comment: 16 pages, compressed and uuencoded postscript-file

The radio observations are based on dual frequency Mark--III VLBI data from 1979 until the end of 1993. Currently 436 sources define the celestial inertial frame. A comparison to the JPL94R01 catalog is made. Position vs. time plots of selected sources show significant apparent non linear proper motions. The status of the project and recent results from optical observations are presented., Comment: 1 page (LaTeX), IAU Symp.166, The radio catalog is available at ftp://maia.usno.navy.mil/rorf , and complete poster paper is available as hardcopy from nz@pyxis.usno.navy.mil

The proposed design and construction of the Fizeau astrometric mapping explorer (FAME), a small astrometric instrument for use on an artificial satellite, is reported on. The instrument and spacecraft are designed to slowly spin and will repeatedly scan great circles on the sky so that, over a period of time, it will cover the complete sky and repeat in a manner similar to that of the Hipparcos satellite. The instrument will use the two fixed dilute aperture telescopes to measure a fixed angle between stars and detect the positions, magnitude and color of all stars crossing its field of view to a visual magnitude of approximately 15 mag. The aim of the instrument is to obtain a catalog of positions, proper motions and parallaxes of all stars down to about 15 mag, with a magnitude dependent accuracy of positions of 20 to 800 micro arcsec, proper motions of 20 to 800 micro arcsec per year and parallaxes of 20 to 800 micro arcsec.

We present 18 cm wavelength Very Long Base Interferometry (VLBI) observations of Cyg X-3 obtained 1989 June 6 along with 18 and 20 cm Very Large Array (VLA) A-array data obtained 1985 February 8. In both data sets, the flux density was relatively high, and the intrinsic source size small compared to the scattering, ideal conditions for a detailed study of interstellar scattering. We find that the scattering disk is anisotropic, with an axial ratio 1.31 +/- 0.02 along position angle 52.0 deg +/- 1.5 deg. This is the first unambiguous measurement of anisotropic scattering in the interstellar medium. We also find a significant break in the phase structure function, the first measurement of a break in any source. The data exclude a steep turbulent spectrum, but are consistent with a shallow turbulent spectrum with an inner scale of 300 km, with a net uncertainty of approximately 50%. We tentatively identify the scattering plasma with the H II region DR 11, for which we estimate a visual extinction of 4 mag, and an emission measure toward Cyg X-3 of 6000 cm(exp -6)pc. We make the case that DR 11 is part of a shell of H II regions centered on the Cyg OB2 association (which is approximately 1.8 kpc distant) that is 2 deg (60 pc) in radius and expanding at 10 km/s. Our inner scale is consistent with that expected for an H II region, and the amount of scattering is consistent with the emission measure. We discuss how the anisotropy can be used to confirm the identification with DR 11 and to distinguish between different models of turbulence by comparison with measurements of the polarization of starlight along the line of sight.

We report new accurate radio position measurements for 30 sources, preliminary positions for two sources, improved radio postions for nine additional sources which had limited previous observations, and optical positions and optical-radio differences for six of the radio sources. The Very Long Baseline Interferometry (VLBI) observations are part of the continuing effort to establish a global radio reference frame of about 400 compact, flat spectrum sources, which are evenly distributed across the sky. The observations were made using Mark III data format in four separate sessions in 1988-89 with radio telescopes at Tidbinbilla, Australia, Kauai, USA, and Kashima, Japan. We observed a total of 54 sources, including ten calibrators and three which were undetected. The 32 new source positions bring the total number in the radio reference frame catalog to 319 (172 northern and 147 southern) and fill in the zone -25 deg greater than delta greater than -45 deg which, prior to this list, had the lowest source density. The VLBI positions have an average formal precision of less than 1 mas, although unknown radio structure effects of about 1-2 mas may be present. The six new optical postion measurements are part of the program to obtain positions of the optical counterparts of the radio reference frame source and to map accurately the optical on to the radio reference frames. The optical measurements were obtained from United States Naval Observatory (USNO) Black Birch astrograph plates and source plates from the AAT, and Kitt Peak National Observatory (KPNO) 4 m, and the European Southern Observatory (ESO) Schmidt. The optical positions have an average precision of 0.07 sec, mostly due to the zero point error when adjusted to the FK5 optical frame using the IRS catalog. To date we have measured optical positions for 46 sources.

Radio and optical positions for northern hemisphere extragalactic sources are reported. Milliarcsecond (mas) accurate radio positions of 106 sources north of -2 deg declination are derived from Mark III Very Long Baseline Interferometry (VLBI) observations taken during ten experiments from 1990 January through 1990 October. The results presented supplement an ongoing project to define and maintain an all-sky radio/optical reference frame of 400 or more extragalactic sources with mas accurate radio and optical positions. Radio positions for 34 new sources are presented along with improved radio positions for 72 sources already in the reference frame catalog. An additional nine sources have been determined to be unsuitable reference frame objects. Radio observations of nine calibration sources tie the new positions to the existing catalogue. The radio positions of the new sources have formal mean errors of approximately 0.7 mas in right ascension and approximately 1.0 mas in declination. Sources for which we report improved radio positions now have formal mean errors of approximately 0.5 mas in both coordinates, an improvement in some cases by as much as 75%. Positions in the FK5 system have also been obtained for the optical counterparts of an additional five northern hemisphere radio sources using prime focus plates from the Kitt Peak National Observatory's 4 m telescope and a Ritchey-Chretien focus plate from the Calar Alto 2.2 m telescope. The optical positions have internal accuracies of about 0.03 sec and differ from the radio positions by about 0.07 sec on the average.

The apparent orbit of the double-lined spectroscopic binary Alpha Equulei was measured using observations, from June 13, 1989 to September 15, 1990, with the Mark III Optical Interferometer. The results, combined with the spectroscopic results of Rosvick and Scarfe (1991), were used to obtain estimates of the masses of the components, their absolute magnitudes, and the distance to the system. In addition, the magnitude differences between the components were determined at four wavelengths; these were combined with the colors reported by Stickland (1976) to derive colors for the two Alpha Equulei components and to estimate their spectral types.

Radio and optical positions are presented for southern hemisphere extragalactic sources from the Parkes 2.7 GHz survey. Sixty-one sources were observed with Mark III VLBI at 8.4 GHz between Tidbinbilla, Australia, and Hartebeesthoek, South Africa. The results presented are part of the effort to establish a global reference frame of 400 extragalactic radio sources. Radio positions with about 10 milliarcsec errors have been estimated for 39 sources not previously in the present radio reference frame catalog, and provisional positions were obtained for two additional sources, bringing the total number of catalog sources to 276. The principal source of error is the uncalibrated ionosphere. Of the remaining sources five were completely undetected, six were either too faint or too resolved, and nine had previous catalog positions. Optical positions on the FK5 system have also been measured for four southern sources using prime focus plates from the Anglo-Australian 4 m telescope with an accuracy of 0.06 arcsec. This raises to 40 the number of radio sources with accurately measured positions for their optical counterparts.

The long-period variable Mira Omicron Ceti has been observed at 800 nm wavelength with the Mk III Optical Interferometer at photometric phases 0 = 0.96, 0.05, and 0.14 in 1990; some additional data were taken in 1989. The star is not spherically symmetric, and temporal variations of the size and the position angle of the asymmetry are detected. The visibility data can be represented by two uniform elliptical disks, which could correspond to different layers in Mira's atmosphere. If the size variations are interpreted in terms of physical motion of the emitting material, an infall velocity of about 15 km/s around phi = 0 is derived.

VLBI spectral line observations made at the frequency of the z = 0.437 H I absorption line toward the extended radio QSO 3C 196 demonstrate that the absorption does not occur against the more compact continuum structure in this object. Apparently the H I absorption line results from absorption of the lower brightness continuum emission by an intervening cloud with an angular diameter which must be greater than 0.63 arcsec. For a standard cosmology, this corresponds to a linear extent greater than 2.25 kpc. For a spherical cloud, the mass of absorbing gas is near 7 x 10 to the 7th solar masses. Thus this particular QSO absorbing cloud is surprisingly large and massive.

This paper presents new measurements of the microwave flux of Ceres obtained at wavelengths between 3.3 mm and 20 cm using the 12 m Kitt Peak antenna and the Very Large Array. These new measurements are combined with previous measurements to confirm a substantial decrease in flux density at centimeter wavelengths compared to millimeter wavelength. Using a statistical technique adapted from terrestrial microwave remote sensing, this spectrum has been compared with that of various candidate materials and models for the subsurface structure of Ceres. It is concluded that Ceres is largely covered with a 3-cm-thick layer whose dielectric properties resemble dry terrestrial clay. This layer may have formed by micrometeorite impact on hydrothermally altered basic or ultrabasic rock or on carbonaceous chondrite material.

The nucleus and jet of M87 was mapped with an eight-station very long baseline interferometric array at 18 cm wavelength with high dynamic range. It was found that the nucleus of M87 consists of a core-jet structure with a peak brightness temperature greater than 10 to the 10th K. Emission is shown to extend for more than 50 milli-arcsec with a brightness temperature exceeding 10 to the 8th K along a position angle of 288 degrees, which precisely matches the position angle of the 20'' radio/optical/X-ray jet. In addition, the nucleus contains a significant structure of lower brightness at approximately the same position angle, although no counterjet is observed. By invoking relativistic beaming in order to enhance the jet and diminish the counterjet, it is shown that the jet must be aligned within about 60 degrees to our line of sight, and its flow velocity must exceed about 60% of the speed of light. The knots embedded in the 20'' jet contain no bright compact structures, and the sizes of the innermost knot (knot D) is between 0.1-0.3''.

The relationship between the radio and optical emissions from the nucleus of the Seyfert galaxy NGC 4151 is investigated by mapping the radio radiation from this source at wavelengths of 20 and 6 cm using the Very Large Array of the National Radio Astronomy Observatory. Results show that the radio emission at wavelengths from 20 to 6 cm extend 10'' (950 pc) along a position angle of 72-84 degrees. This nonthermal emission is found to consist of at least six components and is similar to jets observed in other compact extragalactic radio sources. These radio jets appear to be coincident with the optical line emission region in NGC 4151 and are aligned with the position angle of the linearly polarized optical continuum emission.

Results of VLBI observations of the nucleus and jet of M87 at 1666.6 MHz in right circular polarization are presented. A hybrid map of the nucleus was made revealing the presence of a one-sided jet, whose position angle is 290.5 (+ or - 1) deg. Assuming that no counter-jet exists because of the effects of relativistic beaming, limits can be placed on the flow velocity of the jet, and the resulting ratio of the observed intensities of the jet to the counter-jet explains the absence of the counter-jet. Another explanation is that jets are intrinsically one-sided, or that counter-jet observed emissions are delayed. Finally, the possibility of existing small wiggles is considered, but further observations are required to verify their existence in M87.

Nine transitions of the possible parent molecules H2O, NH3, CH3OH and N2O as well as the OH radical were searched for in Comet Kohoutek (1973f) in the frequency range 22.2-25.2 GHz. These molecules were not detected, but the upper limits for the optical depth, mean column density and the production rate are derived for each of the molecules. These results are discussed and compared with the reported detections of HCN and CH3CN emission and OH absorption.