Proper Motion

The proper motions of 276 individual sunspots were observed with a triple-exposures method, and then analyzed. Their linear drifts give the mean motions (degree day(-1) ), which, depending on the heliographic latitude B, are the differential rotation, omega (B) = 14.5 - 2.5sin (2B) , and the equatorward meridional flow, v_B =-0.05sin 2B. The deviations of the linear drift from the mean motions have a small correlation between the longitudinal and latitudinal ones, which correspond to the equatorial acceleration. The deviations in longitude are clearly separated by the sunspot polarities. The average separation velocity between the preceding and following polarities is 52 m s(-1) . The drifts of individual sunspots corrected for the mean motion are the smallest for the Zurich GHJ class. However, they are fairly large and may influence the determination of the mean motion. Short-term variations appear as positional deviations from the linear-drift curve. These fast drifts are detected in the active Zurich classes, and are one order of magnitude larger than the linear drift.

Aims. A census of molecular hydrogen flows across the entire Orion A giant molecular cloud is sought. With this paper we aim to associate each flow with its progenitor and associated molecular core, so that the characteristics of the outflows and outflow sources can be established. Methods. We present wide-field near-infrared images of Orion A, obtained with the Wide Field Camera, WFCAM, on the United Kingdom Infrared Telescope. Broad-band K and narrow-band H 2 1-0S (1) images of a contiguous~ 8 square degree region are ...

Aims. A census of molecular hydrogen flows across the entire Orion A giant molecular cloud is sought. With this paper we aim to associate each flow with its progenitor and associated molecular core, so that the characteristics of the outflows and outflow sources can be established. Methods. We present wide-field near-infrared images of Orion A, obtained with the Wide Field Camera, WFCAM, on the United Kingdom Infrared Telescope. Broad-band K and narrow-band H 2 1-0S (1) images of a contiguous~ 8 square degree region are ...

Aims. A census of molecular hydrogen flows across the entire Orion A giant molecular cloud is sought. With this paper we aim to associate each flow with its progenitor and associated molecular core, so that the characteristics of the outflows and outflow sources can be established. Methods. We present wide-field near-infrared images of Orion A, obtained with the Wide Field Camera, WFCAM, on the United Kingdom Infrared Telescope. Broad-band K and narrow-band H 2 1-0S (1) images of a contiguous~ 8 square degree region are ...

We present a spectral atlas of the post-main-sequence population of the most massive Galactic globular cluster, omega Centauri. Spectra were obtained of more than 1500 stars selected as uniformly as possible from across the (B, B-V) colour-magnitude diagram of the proper motion cluster member candidates of van Leeuwen et al. (2000). The spectra were obtained with the 2dF multi-fibre spectrograph at the Anglo Australian Telescope, and cover the approximate range lambda~3840-4940 Angstroem. We measure the radial velocities, effective temperatures, metallicities and surface gravities by fitting ATLAS9 stellar atmosphere models. We analyse the cluster membership and stellar kinematics, interstellar absorption in the Ca II K line at 3933 Angstroem, the RR Lyrae instability strip and the extreme horizontal branch, the metallicity spread and bimodal CN abundance distribution of red giants, nitrogen and s-process enrichment, carbon stars, pulsation-induced Balmer line emission on the asymptotic giant branch (AGB), and the nature of the post-AGB and UV-bright stars. Membership is confirmed for the vast majority of stars, and the radial velocities clearly show the rotation of the cluster core. We identify long-period RR Lyrae-type variables with low gravity, and low-amplitude variables coinciding with warm RR Lyrae stars. A barium enhancement in the coolest red giants indicates that 3rd dredge-up operates in AGB stars in omega Cen. This is distinguished from the pre-enrichment by more massive AGB stars, which is also seen in our data. The properties of the AGB, post-AGB and UV-bright stars suggest that RGB mass loss may be less efficient at very low metallicity, [Fe/H]<<-1, increasing the importance of mass loss on the AGB. The catalogue and spectra are made available via CDS.

... We use as input the T eff and log g that correspond best to the spectral type derived according to the calibration of Morton and Adams (1968) for B-type main sequence stars and that of Humphreys and McElroy (1984) for one supergiant A-type star and two giant B-type stars ...

Shortly after birth, open clusters start dissolving; gradually losing stars into the surrounding star field. The time scale for complete disintegration depends both on their initial membership and location within the Galaxy. Open clusters undergoing the terminal phase of cluster disruption or open cluster remnants (OCRs) are notoriously difficult to identify. From an observational point, a combination of low number statistics and minimal contrast against the general stellar field conspire to turn them into very challenging objects. To make the situation even worst, random samples of field stars often display features that may induce to classify them erroneously as extremely evolved open clusters. In this paper, we provide a detailed study of the stellar content and kinematics of NGC 6863, a compact group of a few stars located in Aquila and described by the POSS as a non existent cluster. Nonetheless, this object has been recently classified as OCR. The aim of the present work is to either confirm or disprove its OCR status by a detailed star-by-star analysis. The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC3 catalogue, and high resolution spectroscopy as well as results from extensive $N$-body calculations. Our results show that the four brightest stars commonly associated to NGC 6863 form an asterism, a group of non-physically associated stars projected together, leading to the conclusion that NGC 6863 is not a real open cluster.

We have measured the mean three-dimensional kinematics of stars in Kapteyn's Selected Area (SA) 76 (l = 209fdg3, b = 26fdg4) that were selected to be Anticenter Stream (ACS) members on the basis of their radial velocities (RVs), proper motions (PMs), and location in the color-magnitude diagram. From a total of 31 stars ascertained to be ACS members primarily from its main-sequence turnoff, a mean ACS RV (derived from spectra obtained with the Hydra multi-object spectrograph on the WIYN 3.5 m telescope) of V helio = 97.0 ± 2.8 km s-1 was determined, with an intrinsic velocity dispersion sigma o = 12.8 ± 2.1 km s-1. The mean absolute PMs of these 31 ACS members are mualpha cos delta = -1.20 ± 0.34 mas yr-1 and mudelta = -0.78 ± 0.36 mas yr-1. At a distance to the ACS of 10 ± 3 kpc, these measured kinematical quantities produce an orbit that deviates by ~30° from the well-defined swath of stellar overdensity constituting the ACS in the western portion of the Sloan Digital Sky Survey footprint. We explore possible explanations for this and suggest that our data in SA 76 are measuring the motion of a kinematically cold sub-stream among the ACS debris that was likely a fragment of the same infalling structure that created the larger ACS system. The ACS is clearly separated spatially from the majority of claimed Monoceros ring detections in this region of the sky; however, with the data in hand, we are unable to either confirm or rule out an association between the ACS and the poorly understood Monoceros structure.

One of the main endeavors of fundamental astrometry is to establish a practical realization of a nonrotating, inertial reference frame anchored to celestial objects whose positions are defined in the barycentric coordinates of the solar system matching the current level of astrometric observational accuracy. The development of astrometric facilities operating from space at a microarcsecond level of precision makes the nonuniformity of the Galactic motion of the barycenter an observable and nonnegligible effect that violates the desired inertiality of the barycentric frame of the solar system. Most of the observable effect is caused by the nearly constant (secular) acceleration of the barycenter with respect to the center of the Galaxy. The acceleration results in a pattern of secular aberration that is observable astrometrically as a systematic vector field of the apparent proper motions of distant quasars.

We employ the classic approximations of planar epicycle and vertical harmonic oscillation for the Sun's Galactic motion to estimate the magnitude of secular acceleration components in the Galactic coordinates and to show that the peculiar accelerations are smaller than the main galactocentric component. We employ the vector spherical harmonic formalism to describe the predicted field of proper motions and evaluate the amplitude of this field at each point on the celestial sphere. We show that the pattern of secular aberration is fully represented by three low-order electric-type vector harmonics; hence, it is easily distinguishable from the residual rotations of the reference frame and other possible effects, such as the hypothetical long-period gravitational waves, which are described by other types of vector or tensor harmonics. Comprehensive numerical simulations of the grid astrometry with Space Interferometry Mission (SIM) PlanetQuest are conducted assuming that 110 optically bright quasars are included as grid objects and observed on the same schedule, but to lower precision due to their faintness, as regular grid stars. The full covariance matrix of the simulated grid solution is used to evaluate the covariances of the three electric harmonic coefficients, representing the secular aberration pattern of proper motions. This is the only reliable method to estimate such sample-based statistics in view of the considerable star-to-star correlations in SIM global astrometry. We conclude that the grid astrometry with SIM PlanetQuest will be sensitive to the main galactocentric component of secular acceleration, arising from the circular motion of the local standard of rest (LSR) around the Galactic center, while the peculiar acceleration of the Sun with respect to the LSR is expected to be too small to be detected with this astrometric space interferometer.