The results of a combined astrometric, photometric, and spectroscopic program to identify members of the open cluster IC 4665 are presented. Numerous new proper motion/photometric candidate members and at least 23 M dwarfs with H-alpha emission have been identified. A reanalysis of IC 4665's age using different methods yields conflicting results ranging from approx. 3x10^7^ yr to the age of the Pleiades. This study provides a list of candidate cluster members in the intermediate and low-mass regime of this cluster. Future spectroscopic observations of these candidates should eventually identify true cluster members. The results of new echelle observations of some candidates and the photometric monitoring of one apparent cluster member are given in an appendix.
We investigate a sample of 3413 International Celestial Reference Frame (ICRF3) extragalactic radio-loud sources with accurate positions determined by very long baseline interferometry in the S/X band, mostly active galactic nuclei and quasars, which are cross-matched with optical sources in the second Gaia data release (Gaia DR2). The main goal of this study is to determine a core sample of astrometric objects that define the mutual orientation of the two fundamental reference frames, the Gaia (optical) and the ICRF3 (radio) frames. The distribution of normalized offsets between the VLBI sources and their optical counterparts is non-Rayleigh, with a deficit around the modal value and a tail extending beyond the 3{sigma} confidence level. A few filters are applied to the sample in order to discard double cross-matches, confusion sources, and Gaia astrometric solutions of doubtful quality. Panoramic Survey Telescope and Rapid Response System and Dark Energy Survey stacked multicolor images are used to further deselect objects that are less suitable for precision astrometry, such as extended galaxies, double and multiple sources, and obvious misidentifications. After this cleaning, 2643 quasars remain, of which 20% still have normalized offset magnitudes exceeding 3, or a 99% confidence level. We publish a list of 2119 radio-loud quasars of prime astrometric quality. The observed dependence of binned median offset on redshift shows the expected decline at small redshifts, but also an unexpected rise at z~1.6, which may be attributed to the emergence of the CIV emission line in the Gaia's G band. The Gaia DR2 parallax zero-point is found to be color-dependent, suggesting an uncorrected instrumental calibration effect.
We obtained improved optical positions for 300 ICRF2 sources - the Rio survey. We compared the Rio survey with 10 other selected optical astrometric surveys and studied the link between the Hipparcos Catalogue Reference Frame (HCRF) and the International Celestial Reference Frame, Second Realization (ICRF2). We investigated the possible causes for the observed non-coincidence between the optical and ICRF2 positions. The Rio survey positions were referred to the second version of the United States Naval Observatory CCD Astrograph Catalog (UCAC2), currently the best tested HCRF densification. The sources are between -90{deg}<{delta}<+30{deg}. We used two telescopes with suitable diameters and focal lengths to properly link the observed ICRF2 sources with the UCAC2, using intermediate brightness stars. We certified the astrometry done with many statistical tests. The average 'optical minus ICRF2' offsets and respective standard deviations in ({alpha}, {delta}) were -3mas (41mas) and +4 (45mas). The Rio survey represents well the zero-point offset of the other surveys. The standard error of 3.5mas found for the HCRF/ICRF2 link indicates an error excess that can be originated by a non-coincidence between the observed optical/VLBI positions. We thus discussed the influence of the errors from the UCAC2. Then, we searched for correlations with the source morphology, represented by structure indices defined in the radio and in the optical domain. Finally, we studied how the position offsets could originate from the perturbation of the optical point spread function (PSF) of the source's core, by a second source of flux. We found an analytical relation that describes the resulting centroid shift, as a function of the atmospheric seeing, the brightness ratio and the relative distance between the two contributing flux sources. Two scenarios, modelled by this relation, are discussed: an extinction window in the dust torus nearby the core, and a Galactic star near the line of sight.
The compiled catalogue of 231044 stars was created from photographic and CCD observations of position catalogues observed by ground based telescopes. The catalogue destine for optical observations VLBI-ICRS astrometrical extragalactic radiosources (ERS) with telescopes equiped with CCD-cameras. The main purpose of this catalogue is to ensure the reference stars in optics for reducing positions of faint astrometrical ERS 17-22mag relative to more bright reference stars 14-17mag. The catalogue contains 231044 star positions in fields 40'x40' near 240 ERS. More than nine catalogues of stars coordinates were used for compiling the Pul-ERS catalogue. For 69% of stars the proper motions were taken from UCAC3 catalogue to convert the position stars to the common epoch. The number of position measurements for each star varies from one to more than ten records. The inner precision of positions of stars in Pul-ERS is from 4 to 150mas. 159331 stars of the compiled catalogue were identified with stars of the UCAC3. The average internal accuracy of positions on both coordinates is not worse 0.11".
Accurate equatorial coordinates have been determined for 143 variable and suspected variable stars in the globular cluster M5=NGC 5904. For 28 stars, identifications with Kustner (1933VeUSB..26....1K) catalogue were found for the first time.
Equatorial coordinates have been determined for 61 variables in the northern globular cluster M 53 and for 12 variables in the southern globular cluster NGC 2808 with accuracy sufficient for reliable identifications. A number of errors have been revealed in publications on variable starts in M 53.
Equatorial coordinates of variable stars in the globular cluster M3 (NGC 5272) and its nearest surroundings, mainly measured on a plate taken with The Zeiss-1000 telescope, are presented. Positions in a homogeneous system are presented for the first time for objects from the Catalogue of variable stars in globular clusters as well as for GCVS and NSV catalogue stars.
The Ilkhani zij compiled by Nasir al-Din al-Tusi and his colleagues in the first period of the astronomical activities (the 1260s and early 1270s) in the Maragha observatory includes a star table collecting important observations of Islamic astronomers from the early ninth century through the third quarter of the thirteenth century, including the Mumtahan astronomers, Ibn al-A'lam, Ibn Yunus, as well as the Maragha astronomers themselves. This table gives the ecliptical coordinates of 18 bright stars in comparison with Ptolemy's corresponding values. This medieval bright star table is especially interesting for two reasons: first, it provides reliable evidence for the examination of the accuracy of the observations made and the instruments employed (notably, an armillary sphere) in the Maragha observatory. Second, it facilitates a comparative study of the accuracy of stellar observations in medieval Middle Eastern astronomy in the period in question. We have obtained the result that the Maragha astronomers observed more accurate star longitudes than did their predecessors, while for the latitudes, all Islamic observers appear to have gained about the same degree of accuracy. We also discuss two delicate matters raised by this table: first, the problem of the use of the two different values for the rate of precession by the Maragha astronomers in order to convert earlier star longitudes to the epoch of the Ilkhani zij (1{deg}/66 years for Ptolemy's longitudes and 1{deg}/70 years for those measured by their Islamic predecessors); second, the change in the star latitudes essentially related to the various values measured by Ptolemy and the Islamic astronomers represented side-by-side in it. Finally, we briefly discuss a small celestial globe designed by the son of Mu'ayyad al-Din al-'Urdi, the instrument-maker of the Maragha observatory.
UV stellar photometry is presented for 1563 stars within a 40' circular field in the LMC, excluding the 10'x10' field centered on R136 investigated earlier by Hill et al. (1993). Magnitudes are computed from images obtained by the Ultraviolet Imaging Telescope in bands centered at 1615A and 2558A. Stellar masses and extinctions are estimated for the stars in associations using the evolutionary models of Schaerer et al. (1993), assuming the age is 4Myr and that the local LMC extinction follows the Fitzpatrick (1985) 30 Dor extinction curve. The estimated slope of the initial mass function (IMF) for massive stars (>15M_{sun}_) within the Lucke and Hodge (LH) associations is {gamma}=-1.08+/-0.2. Initial masses and extinctions for stars not within LH associations are estimated assuming that the stellar age is either 4Myr or half the stellar lifetime, whichever is larger. The estimated slope of the IMF for massive stars not within LH associations is {gamma}=-1.74+/-0.3 (assuming continuous star formation), compared with {gamma}=-1.35, and {gamma}=-1.7+/-0.5, obtained for the Galaxy by Salpeter (1955) and Scalo (1986), respectively, and {gamma}=-1.6 obtained for massive stars in the Galaxy by Garmany, Conti, & Chiosi (1982). The shallower slope of the association IMF suggests that not only is the star formation rate higher in associations, but that the local conditions favor the formation of higher mass stars there. We make no corrections for binaries or incompleteness.
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