Electronic Telegram No. 4340 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET C/2017 A1 (PANSTARRS) Richard Wainscoat, Eva Lilly, Rob Weryk, and Marco Micheli report the discovery of another apparent comet in w-band exposures obtained with the 1.8-m Ritchey-Chretien Pan-STARRS1 telescope at Haleakala on Jan. 2 (discovery observations tabulated below). While the comet has significant interference from background objects in the discovery images, it shows a brighter, more diffuse nuclear condensation with a tail extending to the north for approximately 8". 2017 UT R.A. (2000) Decl. Mag. Jan. 2.33300 5 22 43.32 -24 40 38.4 19.6 2.34334 5 22 42.77 -24 40 27.0 19.6 The comet was also detected at mag 18.9-19.3 on images obtained on Jan. 2.38- 2.43 UT with the ATLAS 0.5-m f/2.0 Schmidt telescope at Haleakala. Three 60-s follow-up exposures (with g-, r-, and i-band filters) were obtained by R. J. Wainscoat, M. Micheli, and P. Forshay with the 3.6-m Canada-France- Hawaii Telescope on Mauna Kea on Jan. 3.4 UT, showing the new object to be clearly cometary with an obvious broad tail to the north, approximately 20" long; the g-band magnitude was measured as 19.5. After the comet was posted on the Minor Planet Center's PCCP webpage, other CCD astrometrists have also commented on the object's cometary appearance. Six stacked exposures taken on Jan. 3.6 UT by L. Buzzi, P. Concari, C. Cremaschini, S. Foglia, G. Galli, and M. Tombelli with an iTelescope 0.15-m f/7 refractor at Siding Spring reveal an object of size 10" x 15", elongated in p.a. 310, with red mag 17.9-18.0; they have posted their image at website URL http://asteroidi.uai.it/neocp/P10yz57/P10yz57.htm. Buzzi et al. obtained additional exposures in the same manner on Jan. 5.45- 5.47 that show similar details (but with red mag 19.0-19.1). E. Bryssinck, Kruibeke, Belgium, writes that seven co-added images taken remotely on Jan. 3.62 with a 0.7-m f/6.6 telescope (+ Luminance filter) at Siding Spring, NSW, reveals a starlike object with a condensed coma of size 12" and a hint of a tail 19" long towards p.a. 8 deg. Fifteen stacked 15-s images taken by T. Linder and R. Holmes with an 0.61-m f/6.5 Dall-Kirkham Astrograph at Cerro Tololo on Jan. 5.2 (measured with S. Foglia) show a large coma of size 10" with a tail 10" long in position angle 350 deg. J.-F. Soulier, Dauban, Banon, France, note that unfiltered stacked 60-s exposures taken on Jan. 3.9 with a 0.20-m f/4 Newtonian reflector at the Observatoire Chante-Perdrix shows a coma of size about 20" and a tail about 22" long in p.a. 350 deg; the red magnitude was measured as 18.8-19.4 in an aperture of radius 6".6. Unfiltered exposures taken by A. Maury, J.-B. de Vanssay, and J.-F. Soulier on Jan. 4.03-4.12 with a 0.4-m f/5.4 Ritchey-Chretien reflector at San Pedro de Atacama, Chile, show a coma of size about 17" and a tail about 23" long in p.a. about 10 degrees, the red magnitude being 18.6-18.9 in an aperture of radius 6".2. Soulier also obtained unfiltered stacked images on Jan. 5.9 with a 0.30-m f/3.8 Newtonian reflector at Maisoncelles, France, that show a coma of size about 17" and a tail about 20" long in p.a. about 20 deg; the red magnitude was measured as 19.0-19.3 in an aperture of radius 4".9. H. Sato, Tokyo, Japan, reports that ten stacked 60-s exposures taken on Jan. 5.45-5.46 with a 0.51-m f/6.8 astrograph (+ luminance filter) at Siding Spring show the comet to be strongly condensed with a coma 12" in diameter and a tail 25" long toward p.a. 355 degrees; the w-band magnitude was 17.7 as measured within a circular aperture of radius 6".5. The available astrometry, the following preliminary parabolic orbital elements by G. V. Williams (from 53 observations spanning Jan. 2-6), and an ephemeris appear on MPEC 2017-A31. T = 2017 May 17.6770 TT Peri. = 1.7923 Node = 120.1429 2000.0 q = 2.329691 AU Incl. = 50.6445 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2017 CBAT 2017 January 6 (CBET 4340) Daniel W. E. Green