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Register a new userDifferential Proper-Motion Measurements of The Cygnus Egg Nebula: The Presence of Equatorial Outflow
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We present the results of differential proper-motion analyses of the Egg Nebula (RAFGL 2688, V1610 Cyg) based on the archived two-epoch optical data taken with the Hubble Space Telescope. First, we determined that the polarization characteristics of the Egg Nebula is influenced by the higher optical depth of the central regions of the nebula (i.e., the dustsphere of about 1000 AU radius), causing the nebula illuminated in two steps -- the direct starlight is first channeled into bipolar cavities and then scattered off to the rest of the nebula. We then measured the amount of motion of local structures and the signature concentric arcs by determining their relative shifts over the 7.25 yr interval. Based on our analysis, which does not rely on the single-scattering assumption, we concluded that the lobes have been excavated by a linear expansion along the bipolar axis for the past 400 yr, while the concentric arcs have been generated continuously and moving out radially at about 10 km/s for the past 5,500 yr, and there appears to be a colatitudinally-increasing trend in the radial expansion velocity field of the concentric arcs. There exist numerical investigations into the mass-loss modulation by the central binary system, which predict such a colatitudinally-increasing expansion velocity field in the spiral-shock trails of the mass-loss ejecta. Therefore, the Egg Nebula may represent a rare edge-on case of the binary-modulated circumstellar environs, corroborating the previous theoretical predictions.
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We present analysis of 25 years worth of archival VLA, VLBA and EVN observations of the X-ray binary Cygnus X-3. From this, we deduce the source proper motion, allowing us to predict the location of the central binary system at any given time. However, the line of sight is too scatter-broadened for us to measure a parallactic distance to the source. The measured proper motion allows us to constrain the three-dimensional space velocity of the system, implying a minimum peculiar velocity of 9 km/s. Reinterpreting VLBI images from the literature using accurate core positions shows the jet orientation to vary with time, implying that the jets are oriented close to the line of sight and are likely to be precessing.
HD179821 is an enigmatic evolved star that possesses characteristics of both a post-asymptotic giant branch star and a yellow hyper-giant, and there has been no evidence that unambiguously defines its nature. These two hypotheses are products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. We have obtained the two-epoch Hubble Space Telescope WFPC2 data of its circumstellar shell, which shows multiple concentric arcs extending out to about 8 arcsec. We have performed differential proper-motion measurements on distinct structures within the circumstellar shell of this mysterious star in hopes of determining the distance to the object, and thereby distinguishing the nature of this enigmatic stellar source. Upon investigation, rather than azimuthal radially symmetric expansion, we discovered a bulk motion of the circumstellar shell of (2.41+-0.43, 2.97+-0.32) mas/yr. This corresponded to a translational ISM flow of (1.28+-0.95, 7.27+-0.75) mas/yr local to the star. This finding implies that the distance to HD 179821 should be rather small in order for its circumstellar shell to preserve its highly intact spherical structure in the presence of the distorting ISM flow, therefore favoring the proposition that HD 179821 is a post-AGB object.
We present single-dish and VLBI observations of an outburst of water maser emission from the young binary system Haro 6-10. Haro 6-10 lies in the Taurus molecular cloud and contains a visible T Tauri star with an infrared companion 1.3 north. Using the Very Long Baseline Array, we obtained five observations spanning 3 months and derived absolute positions for 20 distinct maser spots. Three of the masers can be traced over 3 or more epochs, enabling us to extract absolute proper motions and tangential velocities. We deduce that the masers represent one side of a bipolar outflow that lies nearly in the plane of the sky with an opening angle of ~45deg. They are located within 50 mas of the southern component of the binary, the visible T Tauri star Haro 6-10S. The mean position angle on the sky of the maser proper motions (~220deg) suggests they are related to the previously observed giant Herbig-Haro (HH) flow which includes HH410, HH411, HH412, and HH184A-E. A previously observed HH jet and extended radio continuum emission (mean position angle of ~190deg) must also originate in the vicinity of Haro6-10S and represent a second, distinct outflow in this region. We propose that a yet unobserved companion within 150 mas of Haro6-10S is responsible for the giant HH/maser outflow while the visible star is associated with the HH jet. Despite the presence of H_2 emission in the spectrum of the northern component of the binary, Haro6-10N, none of outflows/jets can be tied directly to this young stellar object.
We present measurements of internal proper motions at more than five hundred positions of NGC 2392, the Eskimo Nebula, based on images acquired with WFPC2 on board the Hubble Space Telescope at two epochs separated by 7.695 years. Comparison of the two observations shows clearly the expansion of the nebula. We measured the amplitude and direction of the motion of local structures in the nebula by determining their relative shift during that interval. In order to assess the potential uncertainties in the determination of proper motions in this object, and in general, the measurements were performed using two different methods, used previously in the literature. We compare the results from the two methods, and to perform the scientific analysis of the results we choose one, the cross-correlation method, as the more reliable. We go on to perform a criss-cross mapping analysis on the proper motion vectors which helps in the interpretation of the velocity pattern. Combining our results on the proper motions with radial velocity measurements obtained from high resolution spectroscopic observations, and employing an existing 3D model, we estimate the distance to the nebula as 1300 pc.
The galactic globular cluster Omega Centauri is the most massive of its kind, with a complex mix of multiple stellar populations and several kinematic and dynamical peculiarities. Different mean proper motions have been detected among the three main sub-populations, implying that the most metal-rich one is of accreted origin. This particular piece of evidence has been a matter of debate because the available data have either not been sufficiently precise or limited to a small region of the cluster to ultimately confirm or refute the result. Using astrometry from the second Gaia data release and recent high-quality, multi-band photometry, we are now in a position to resolve the controversy. We reproduced the original analysis using the Gaia data and found that the three populations have the same mean proper motion. Thus, there is no need to invoke an accreted origin for the most metal-rich sub-population.
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