History of the creation of the file "pg_pos.dat" by Brian Skiff (Lowell Obs., 14 April 2009) ================================================================================ The list (file "pg_pos.dat") shows precise coordinates for the PG catalogue of uv-excess objects. Even though there is substantial follow-up work in the literature, their coordinates have never been improved except inter-alia via external identifications; many of the objects have positions several arcminutes in error. Five stars must have errors of a degree or more, and are lost for now. I was not able to find them making wide-area searches on the POSS-I prints (two lack finder charts in any case). As a result of the poor coordinates, SIMBAD has a modest mess with multiple entries for a significant portion of the stars. I worked from the VizieR file, apparently provided to the CDS by Heinz Andernach from a file supplied by Richard Green. It is described at the URL given below. Because of the diverse nature of the entries and the semi-accurate original positions, I had to search for each object one-at-a-time in VizieR using a 60" search radius. (The source paper shows the positions have nominal 2-sigma scatter of 23".) Hundreds of the objects were recovered by comparing the published charts against DSS/SDSS/2MASS cut-outs using the Goddard SkyView utility. It was also convenient that many PG stars were targeted in the Lick NPM survey, so the stars could often be recovered from the "pretty good" positions there. Often simply the very-blue color in USNO-A2.0 (b-r < -0.5) near the top of the VizieR report was enough to confirm the object. From the original file I preserve only the ID (N.B.: many names are at variance in SIMBAD and in the literature!) and the spectral classification. In NED I note that the PG objects are all shown as "blue star". In fact spectra were obtained for every object, and none is actually called merely a blue star, and the various sorts of galaxies are identified as such (Gal, Sey, QSO, BLL), and were not misidentified as blue stars. Thus the object-type should be changed in NED to reflect this. There are indeed a few objects given as stars (e.g. 'DC' spectral type) that turn out to be QSOs or BL Lac-type galaxies, and I have noted each of these in the remarks. The new coordinates were drawn from a larger than usual variety of sources. When available the UCAC2 or SDSS DR6 was preferred as long as the latter were not in the saturated regime. (I did not use SDSS DR7 since it is not in VizieR, and the SDSS query pages are inconvenient.) Nearly always the stars were too faint to have high S/N in 2MASS, and so the astrometry there is poor. Stars with larger motion were taken from the Lepine LSPM catalogue, USNO-B1.0 (after vetting), or the Bordeaux meridian circle catalogue. When the motion was less than several tens of mas/year, I took positions from what seemed to be the most accurate source. The source of the position is given in every case by a single-letter code in column 's' following the coordinates and described in the list below. Stars with motion greater than 0".15/year are flagged in the remarks ('lg pm' = large proper motion). To the extent possible the positions are for both equinox and epoch J2000. Just now the VizieR output does not include the SDSS proper motions. The UCAC2 positions should be good to < 0".1, while SDSS, 2MASS, and LSPM should have errors < 0".2. A few objects are ICRF calibrators, where the coordinates are known to the submilliarcsec level. The rest should be reliable at the ~0".3 level. Occasionally I have taken mid-points for double galaxies, as noted in the remarks. Original PG positions that are poor are also flagged ('PG coords wrong' etc). The original PG photographic magnitudes are rather poor, being much too bright at the bright end, noisy in the middle, and too bright again at the faint end (i.e. the list actually goes rather fainter than advertised). I replaced everything with Johnson V magnitudes if available from the Mermilliod UBV or Hauck uvby compilations. Other V magnitudes were taken from the ASAS-3 or TASS MkIV surveys, as well as other published sources not shown in the UBV/uvby compilations but present elsewhere in VizieR. I frequently relied on the Carlsberg meridian circle catalogue, which provides Sloan r' for its stars, and adopted V = r' - 0.25 for the blue stars as a result of continual comparison with 'legit' UBV/uvby throughout the list. For the many faint stars luckily the transformation from SDSS g and r to Johnson V is small, so the straight average (g+r/2) was taken. Often several of these sources were available for the stars, and the intercomparisons were generally quite gratifying (such as the mighty 18cm refractor of the Carlsberg meridian circle photometry matching the Sloan 2.5-m data to ~0.03 mag rms even at r mag 16!). For galaxies I tried to obtain something close to V-sub-T either directly from NED, again using the SDSS Petrosian magnitudes if necessary, or from the ASAS-3 aperture-photometry for some brighter objects (a mighty 200mm telephoto lens!). Everything is rounded to 0.1 mag, the intent being to get rid of the systematic problems in the original PG photometry rather than to obtain high precision. I should note that the new (in 1986) photoelectric UBV data listed in the PG catalogue is generally okay if there is no uncertainty flag (:), in which case the B magnitude can be 0.5 mag or more in error. Also, this photometry is overlooked in the Mermilliod UBV compilation of 1991. I matched the completed list again in VizieR (in big batches this time) against the GCVS, the WDS, and the Downes CV catalogue. I also looked up each of the extragalactic objects in NED, and have added to the remarks some 'classical' name if available. The extragalactic objects (only) were also sought in the three IRAS catalogues (IRAS point-source, faint-source, and the faint-source 'rejects' catalogues); the 'rejects' IDs are new and provide an additional 25- or 60-micron data-point for the SEDs. If I happened to notice that a star had a red color in 2MASS, and was not not obviously known as a hot/cool binary, I have flagged them in the remarks. This is something folks have worked on recently via SDSS/2MASS match-ups, so there may not be anything new there. As part of the identification process, many reasonably close pairs were noted, and where it makes a difference, these are also noted. The WDS match-up suggests most of these are also known, but one or two may be new common-motion binaries. I call them cpm pairs if this is clearly the case based on existing proper motions (sometimes by comparing the POSS-I and POSS-II images), otherwide 'optical' or non-committal. Two such white-dwarf/red-dwarf pairs are both resolved in SDSS and two spectral types given in the PG catalogue, so I have shown the stars on separate lines below. For most such pairs the red component was not noted with a separate spectral type in the PG list, but are flagged in the remarks nevertheless. I started making a list of aliases in SIMBAD---but gave up! There is a very large number of them, and this is probably more efficiently done by the folks at the CDS with the database tools they have. I'd suggest using a 3' search-radius since many of the aliases also have approximate coordinates in SIMBAD (e.g. the old Tonantzintla lists). All this (well, the stars at least) will be larded into my comprehensive spectral-types file (B/mk in VizieR), and the present list copied out to the Lowell ftp area, where corrections can be included. I thank Richard Green for noting the typo in the coordinates (and name!) for PG 2223+143. As usual, corrections and amendments are welcome. ------------------------------------------------------------------------------- Palomar-Green survey uv-excess stars and galaxies version: 2009 Apr 13, lost stars recovered by John Greaves and Richard Green; same-name stars PG 1536+097 and PG 1610+239 disambiguated 2009 Apr 3 source: 1986ApJS...61..305G Green R.F., Schmidt M., and Liebert J. Astrophys. J. Suppl. Ser. 61, 305-352 (1986) The Palomar-Green catalog of ultraviolet-excess stellar objects. VizieR files: http://cdsarc.u-strasbg.fr/viz-bin/Cat?II/207 lost PG stars: o r i g i n a l Name RA (1950) Dec PG 1638+076 16 38 36.2 +07 33 44 PG 2239+043 22 39 37.0 +04 19 44 position sources (column 's'): b USNO-B1.0 (2003AJ....125..984M, I/284) B Bordeaux meridian circle (2006A&A...448.1235D, I/300) C Carlsberg meridian circle, CMC14 (I/304) g GSC-2.3 (2006yCat.1305.....S, I/305) L Lepine LSPM (2005AJ....129.1483L, I/298) M 2MASS (2006AJ....131.1163S, II/246) S SDSS DR6 (2008ApJS..175..297A, II/292) s Skiff estimate +/- 1" T Tycho-2 (2000A&A...357..367H, I/259) U UCAC2 (2004AJ....127.3043Z, I/289) - "other", mostly VLBI ICRF sources (2004AJ....127.3587F, J/AJ/127/3587) ================================================================================