Insights into the properties of the Local (Orion) spiral arm. NGC 2302: First results and description of the program

Context. The spiral structure of the Milky Way (MW) is highly uncertain and is the subject of much discussion nowadays. Even the spiral structure close to the Sun and the real nature of the so-called Local or Orion arm are poorly known. Aims. We present the first result from a program that determines the properties of the Local (Orion) spiral arm (LOA), together with a full description of the program. In this context we have made a comprehensive study of the young LOA open cluster NGC 2302, which includes a UBVRI photometric analysis and determination of its kinematic properties - proper motion (PM) and radial velocity (RV) - and of its orbital parameters. Methods. Making a geometric registration of our ad-hoc first- and second-epoch CCD frames (12-year timeframe), we determined the mean PM of NGC 2302 relative to the local field of disk stars, and, through a comparison with the UCAC4 catalog, we transformed this relative PM into an absolute one. Using medium-resolution spectroscopy of 26 stars in the field of NGC 2302, we derived its mean RV.We determined the cluster's structure, center, and radius by means of a density analysis of star counts. Photometric diagrams for several color combinations were built using our data, which allowed us to identify the stellar populations present in the field of NGC 2302 and to carry out our photometric membership analysis. Isochrone fits to the photometric diagrams allowed us to determine the fundamental parameters of NGC 2302, including reddening, distance, and age. The kinematic data and derived distance allowed us to determine the space motion of NGC 2302. This was done by adopting a time-independent, axisymmetric, and fully analytic gravitational potential for the MW. Results. We obtained an absolute PM for NGC 2302 of (μα cos δ μδ) = (-2.09; -2.11) mas yr-1, with standard errors of 0.410 and 0.400 mas yr-1. The mean RV of NGC 2302 turned out to be 31.2 km s-1 with a standard error of 0.7 km s-1. The density analysis revealed a remarkably spherical concentration of stars centered at α2000 = 06:51:51:820, δ2000 = -07:05:10:68 with a radius of 2:50′ Although densely contaminated by field stars, all our photometric diagrams show a recognizable cluster sequence of bright stars (V ≤ 18). The color-color diagrams show the existence of more than one population, each a ected by distinct reddening with the cluster sequence at E(B - V) = 0.23. Isochrone fits displaced for this reddening and for a distance modulus of (m - M)0 = 10.69 (distance, d = 1:40 kpc) indicate an age of log(t) = 7:90-8.00 with a slight tendency toward the younger age. Inspection of the shape of the orbit of NGC 2302 and the resulting orbital parameters indicate that it is a typical population I object.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Insights into the properties of the Local (Orion) spiral arm. NGC 2302: First results and description of the program

Context. The spiral structure of the Milky Way (MW) is highly uncertain and is the subject of much discussion nowadays. Even the spiral structure close to the Sun and the real nature of the so-called Local or Orion arm are poorly known. Aims. We present the first result from a program that determines the properties of the Local (Orion) spiral arm (LOA), together with a full description of the program. In this context we have made a comprehensive study of the young LOA open cluster NGC 2302, which includes a UBVRI photometric analysis and determination of its kinematic properties - proper motion (PM) and radial velocity (RV) - and of its orbital parameters. Methods. Making a geometric registration of our ad-hoc first- and second-epoch CCD frames (12-year timeframe), we determined the mean PM of NGC 2302 relative to the local field of disk stars, and, through a comparison with the UCAC4 catalog, we transformed this relative PM into an absolute one. Using medium-resolution spectroscopy of 26 stars in the field of NGC 2302, we derived its mean RV.We determined the cluster's structure, center, and radius by means of a density analysis of star counts. Photometric diagrams for several color combinations were built using our data, which allowed us to identify the stellar populations present in the field of NGC 2302 and to carry out our photometric membership analysis. Isochrone fits to the photometric diagrams allowed us to determine the fundamental parameters of NGC 2302, including reddening, distance, and age. The kinematic data and derived distance allowed us to determine the space motion of NGC 2302. This was done by adopting a time-independent, axisymmetric, and fully analytic gravitational potential for the MW. Results. We obtained an absolute PM for NGC 2302 of (μα cos δ μδ) = (-2.09; -2.11) mas yr-1, with standard errors of 0.410 and 0.400 mas yr-1. The mean RV of NGC 2302 turned out to be 31.2 km s-1 with a standard error of 0.7 km s-1. The density analysis revealed a remarkably spherical concentration of stars centered at α2000 = 06:51:51:820, δ2000 = -07:05:10:68 with a radius of 2:50′ Although densely contaminated by field stars, all our photometric diagrams show a recognizable cluster sequence of bright stars (V ≤ 18). The color-color diagrams show the existence of more than one population, each a ected by distinct reddening with the cluster sequence at E(B - V) = 0.23. Isochrone fits displaced for this reddening and for a distance modulus of (m - M)0 = 10.69 (distance, d = 1:40 kpc) indicate an age of log(t) = 7:90-8.00 with a slight tendency toward the younger age. Inspection of the shape of the orbit of NGC 2302 and the resulting orbital parameters indicate that it is a typical population I object.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

TRIO OF STELLAR OCCULTATIONS BY PLUTO ONE YEAR PRIOR TO NEW HORIZONS' ARRIVAL

We observed occultations by Pluto during a predicted series of events in 2014 July with the 1 m telescope of the Mt. John Observatory in New Zealand. The predictions were based on updated astrometry obtained in the previous months at the USNO, CTIO, and Lowell Observatories. We successfully detected occultations by Pluto of an R = 18 mag star on July 23 (14:23:32 ± 00:00:04 UTC to 14:25:30 ± 00:00:04 UTC), with a drop of 75% of the unocculted stellar signal, and of an R = 17 star on July 24 (11:41:30 ± 00:00:08 UTC to 11:43:28 ± 00:00:08 UTC), with a drop of 80% of the unocculted stellar signal, both with 20 s exposures with our frame-transfer Portable Occultation, Eclipse, and Transit System. Since Pluto had a geocentric velocity of 22.51 km s[superscript −1] on July 23 and 22.35 km s[superscript −1] on July 24, these intervals yield limits on the chord lengths (surface and lower atmosphere) of 2700 ± 130 km and 2640 ± 250 km, respectively, indicating that the events were near central, and therefore provide astrometric constraints on the prediction method. Our coordinated observations with the 4 m AAT in Australia on July 23 and the 6.5 m Magellan/Clay on Las Campanas, the 4.1 m Southern Astrophysical Research Telescope on Cerro Pachön, the 2.5 m DuPont on Las Campanas (LCO), the 0.6 m SARA-South on Cerro Tololo of the Southeastern Association for Research in Astronomy (SARA), the MPI/ESO 2.2 m on La Silla, and the 0.45 m Cerro Calán telescope and 0.36 telescope in Constitución in Chile on July 27 and 31, which would have provided higher-cadence observations for studies of Pluto's atmosphere, were largely foiled by clouds, but led to detection with the LCO Magellan/Clay and DuPont Telescopes on July 31 of the grazing occultation of a previously unknown 15th-magnitude star, completing the trio of occultations successfully observed and reported in this paper.National Research Foundation (South Africa