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J/A+A/515/A32   Pluto, Charon, Nix, Hydra occultations 2008-15  (Assafin+, 2010)

Precise predictions of stellar occultations by Pluto, Charon, Nix and Hydra for 2008-2015. Assafin M., Camargo J.I.B., Vieira Martins R., Andrei A.H., Sicardy B., Young L., da Silva Neto D.N., Braga-Ribas F. <Astron. Astrophys. 515, A32 (2010)> =2010A&A...515A..32A
ADC_Keywords: Positional data ; Occultations ; Planets Keywords: astrometry - occultations - planets and satellites: individual: Pluto, Charon, Nix, Hydra Abstract: We made the astrometry of 110GB of images with the Platform for Reduction of Astronomical Images Automatically (PRAIA). By relatively simple astrometric techniques, we treated the overlapping observations and derived a field distortion pattern for the WFI mosaic of CCDs to within 50mas precision. Positions were obtained in the UCAC2 frame with errors of 50mas for stars up to magnitude R=19, and 25mas up to R=17. New stellar proper motions were also determined with 2MASS and the USNO B1.0 catalog positions as first epoch. The catalog contains 2.24 million stars with proper motions, covering Pluto's sky path with 30 arcmin width. Its magnitude completeness is about R=18 with a limit about R=21. The catalog of star positions along Pluto's 2008-2015 sky path was built in the context of the investigation of Pluto and its satellites, aiming to support precise astrometric predictions for stellar occultations by Pluto, Charon, Hydra and Nix for the period 2008-2015. For that, we have carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. We highlight the usefulness of this catalog as a reference frame for star/body astrometric follow-up before and after future stellar occultations involving the Pluto system. In this way, updates for the ephemeris offsets of these bodies and for the position of star candidates for occultations can be enhanced. Besides, the catalog also furnishes useful photometric information for field stars in the flux calibration of observed light curves. The prediction tables of stellar occultations by Pluto, Charon, Nix and Hydra for 2008-2015 were built for investigating the physical properties of Pluto and its satellites, for understanding in a broader sense transneptunian objects - keystones in the study of structure, origin and evolution of the Solar System. Our aim was to derive precise, astrometric predictions. We then constructed an astrometric star catalog in the UCAC2 system covering Pluto's sky path. For that, we have carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. Based on the past 2005-8 occultations successfully predicted, recorded and fitted, a linear drift with time in declination with regard to DE418/plu017 ephemerides was determined for Pluto and used in the current predictions. For offset (mas)=A(t(yr)-2005.0)+B, we find A=+30.5±4.3mas/yr and B=-31.5±11.3mas, with standard deviation of 14.4mas for the offsets. For these past occultations, predictions and follow-up observations were made with the 0.6m and 1.6m telescopes at the Laboratorio Nacional de Astrofisica/Brazil. No ephemeris correction was applied for right ascension. Although an oscillation pattern related to an error in Pluto's heliocentric distance (geocentric parallax error) cannot be ruled out, none of the attempted models for this scenario could fit the more dispersed right ascension ephemeris offsets derived from the studied past occultations, at least not well below 50mas, in contrast with the standard deviation of only 14.4mas achieved for declination. The candidate stars listed in the prediction tables were searched using a proximity radius of 335mas with the geocentric apparent orbit (corrected by ephemeris offsets) of the body considered. This radius is about the apparent radius of Pluto (50mas) plus the apparent Earth radius (285mas) as projected in the sky plane at 31AU (Pluto-Earth distance for 2008-2015). No threshold in R magnitude was used in the search for candidates. Pluto is crossing interstellar clouds, so relatively faint R objects may turn out to be bright infrared stars, perfect targets for the SOFIA observatory and for ground-based instruments well equipped with H, J or K band detectors (H, J and K magnitudes are promptly available in the tables if the star belongs to the 2MASS). Besides, events may be also favored by slow shadow speeds of less than 20km/s. Also, no constraint on a geographic place was applied, as in principle SOFIA observations can be done from any sub-solar point on Earth. Events in daylight at sub-planet point were not excluded either, as they could yet be observable in the dark, right above the horizon, from places near the Earth terminator. We furnish here prediction tables for future and also for past stellar occultations covering the sky path by Pluto between 2008-2015. The importance of predictions for occultations still to come is obvious. But the predictions of past occultations are also useful for at least three reasons. First, they can be used by anyone as reference for ongoing fittings of light curves of recent past observed events. Second, they serve to derive ephemeris drifts by comparing expected and observed central instants and C/A values. Finally, they can be used as an external check for the accuracy and precision of our prediction tables. In a general sense, assuming a bulk error of 30mas for C/A from the estimated errors of the catalog star positions and from the errors of the derived ephemeris offsets, we can state that the shadow path uncertainties over Earth are on the order of less than 800km for the stellar occultation predictions of Pluto and Charon. Considering orbital issues and the smaller sizes of Nix and Hydra, the probability of success quoted for these two satellites are about 10-25%. This is not as high as hoped, but not despairingly small, especially if the event occurs above a dense, populated region in terms of astronomers, including amateurs. Description: The catalog consists of (RA, DEC) star positions in the ICRS (J2000) at mean epoch of observation, proper motions and R magnitudes (J, H and K magnitudes are also furnished in the case of 2MASS stars). The mean epoch of observation is given as Modified Julian Date. The position error at mean epoch of observation, the estimated (x,y) error from CCD frame measurements and the number of contributing positions in the WFI mosaics are also furnished. Two flags are included. One indicates if the star belongs to the UCAC2 or 2MASS catalogs. The other flag indicates astrometric position multiplicity in the WFI mosaic solutions (see details in Sect. 4.4 of the paper). For refined astrometry, the user should only use stars with this flag equal to zero. The position error is estimated from repeatability, by the standard deviation (mean error) of contributing individual CCD positions about the final catalog star positions (last iteration in global mosaic solution - see Sect. 4.3 of the paper). By default, multiple entry flagged stars have no position error estimates. Complementary to these position errors, the (x,y) errors from CCD frame measurements (2-D Gaussian fits to the star image profiles) and the number of contributing positions per star in the WFI mosaics may also be used to weight the positions. Proper motions were computed using the 2MASS as first epoch. If it was not a 2MASS star, the USNO B1.0 catalog was used instead. If it was neither a 2MASS nor a USNO B1.0 star, no proper motion was computed. The second epoch position came from the WFI astrometry. Since only two epochs/positions were used in computations, no proper motion error estimates could be derived. For UCAC2 stars, proper motions were directly extracted from that catalog. The catalog flag allows for the identification of the star catalog used as first epoch. The observations were made with a broad-band R filter (ESO#844 with lambdacentral=651.725nm and deltalambda=162.184nm at FWHM). The magnitudes were derived from PSF photometry, but since they were calibrated in the UCAC2 system, magnitude zero-point errors up to 0.3 might be expected for R>17. The infrared magnitudes were extracted from the 2MASS catalog. Their typical errors are less than 0.6, 0.040 and 0.040 for magnitudes J, H and K, respectively. The catalog is divided by year. The sky coverage of the catalog for each year is displayed in Table 1 and in Fig. 1 of the paper (for more details, see Sect. 3). There are small gaps without catalog stars between the years (see Fig. 1), corresponding to sky regions where, as seen from Earth, the Sun is too close to Pluto's direction at date. For each year, the catalog entries are ordered by crescent declination. Each prediction table contains the date and instant of stellar occultation (UTC), the ICRS (J2000) star coordinates at the event date (that is, corrected by proper motion), the closest apparent geocentric distance between star and body, the position angle of the shadow across the Earth (clockwise, zero at North), the velocity in km/s, the distance to the Earth (AU), longitude of the sub-solar point, local solar time, DE418 and plu017 ephemerides offsets in (RA, DEC) for the central instant, the catalog proper motion, the catalog and multiplicity flags, the estimated star catalog position errors, the proper motions and the magnitudes R*, J*, H* and K*. Magnitudes are normalized to a reference shadow velocity v of 20km/s (the typical shadow velocity around the Pluto opposition). Magnormalized=mag+2.5log_10(v/20). These normalized magnitudes may bring forward faint stars involved in slow events, thus allowing for longer integration time, and consequently reasonably good signal-to-noise ratios (SNRs) without loss of spatial resolution in diameter measurements and in probing atmosphere altitudes in the light curves, in spite of the faintness of the targets. Note however that Pluto's important contribution to the total recorded flux will then be an issue in those situations, so that a case by case estimation of the SNR must be conducted for those candidates. The prediction tables are divided by body and year, with entries in chronological order. The reference ephemerides used for Pluto and its satellites were DE418 and plu017 ephemerides. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 17 150 *The (RA, DE) ESO2p2/WFI mosaic centers for Pluto sky path from 2008 to 2015 catalog.dat 118 2242275 Catalog of star positions: Pluto sky path 2008-2015 pluto.dat 164 1190 Prediction table of stellar occ. Pluto 2008-2015 charon.dat 164 1207 Prediction table of stellar occ. Charon 2008-2015 nix.dat 164 1276 Prediction table of stellar occ. Nix 2008-2015 hydra.dat 164 1209 Prediction table of stellar occ. Hydra 2008-2015 files/* 0 32 *Individual files by planet and year
Note on table1.dat: Each overlapping WFI mosaic encompass a 30'x30' area in the sky. Sky paths 2008-2010 and 2011-2015 were observed in September and October 2007 runs, respectively. Note on files/*: the subdirectory contains the yearly files (8 per planet or star positions).
Byte-by-byte Description of file: table1.dat
Bytes Format Units Label Explanations
1- 4 I4 yr Year Year 6- 7 I2 h RAh Mosaic center right ascension (J2000) 9- 10 I2 min RAm Mosaic center right ascension (J2000) 12 A1 --- DE- Mosaic center declination sign (J2000) 13- 14 I2 deg DEd Mosaic center declination (J2000) 16- 17 I2 arcmin DEm Mosaic center declination (J2000)
Byte-by-byte Description of file: catalog.dat
Bytes Format Units Label Explanations
2- 14 F13.9 deg RAdeg Right ascension in decimal degrees (J2000) at mean epoch "Epoch" 16- 28 F13.9 deg DEdeg Declination in decimal degrees (J2000) at mean epoch "Epoch" 30- 45 F16.8 d Epoch Mean epoch of coordinates (JD) 47- 52 I6 mas e_RAdeg ?=999999 Right ascension error at mean epoch (1) 54- 59 I6 mas e_DEdeg ?=999999 Declination error at mean epoch (1) 61- 66 I6 mas/yr pmRA ?=999999 Proper motion in Right Ascension (pmRA*cosDE) (2) 68- 73 I6 mas/yr pmDE ?=999999 Proper motion in Declination (2) 75- 80 F6.3 mag Rmag Magnitude (apparent) in R band 82- 87 F6.3 mag Jmag ?=99.999 Magnitude (apparent) in J band (3) 89- 94 F6.3 mag Hmag ?=99.999 Magnitude (apparent) in H band (3) 96-101 F6.3 mag Kmag ?=99.999 Magnitude (apparent) in K band (3) 103-106 I4 mas erx Measured error in CCD frames (x) coordinates 108-111 I4 mas ery Measured error in CCD frames (y) coordinates 113-114 I2 --- No Number of observations per position 116 A1 --- Cat [129] Flag on catalog cross-identification (4) 118 I1 --- Amf [0/6] Astrometric multiplicity flag (G1)
Note (1): Error values marked "999999" mean no estimated errors; this regards to astrometric flagged positions or to positions from one observation Note (2): proper motion values marked "999999" mean no computed proper motions; this is because no first epoch position counterpart could be found in the 2MASS or USNO B1.0 catalogs Note (3): J, H or K magnitudes marked "99.999" mean no infrared magnitudes available; this means that it is not a 2MASS star Note (4): Flag on catalog cross-identification as follows: 1 = UCAC2 star 2 = 2MASS star 9 = not a UCAC2/2MASS star
Byte-by-byte Description of file: pluto.dat charon.dat nix.dat hydra.dat
Bytes Format Units Label Explanations
2- 11 A10 "YYYY/MM/DD" Ep.date UTC instant at closest approach 14- 21 A8 "h:m:s" Ep.time UTC instant at closest approach 26- 27 I2 h RAh Right ascension (J2000) of star (1) 29- 30 I2 min RAm Right ascension (J2000) of star (1) 32- 38 F7.4 s RAs Right ascension (J2000) of star (1) 40 A1 --- DE- Declination sign (J2000) of star (1) 41- 42 I2 deg DEd Declination (J2000) of star (1) 44- 45 I2 arcmin DEm Declination (J2000) of star (1) 48- 53 F6.3 arcsec DEs Declination (J2000) of star (1) 57- 61 F5.3 arcsec Sep Minimum Separation at closest approach (CA) 64- 69 F6.2 deg PA Position angle (PA) at closest approach (2) 71- 77 F7.2 km/s Vshad Occultation shadow velocity (3) 79- 84 F6.2 AU Dist Apparent geocentric distance at occultation 86- 89 F4.1 mag Rmag Normalized apparent magnitude in R band 91- 94 F4.1 mag Jmag ?=50.0 Normalized apparent J magnitude (4) 96- 99 F4.1 mag Hmag ?=50.0 Normalized apparent H magnitude (4) 101-104 F4.1 mag Kmag ?=50.0 Normalized apparent K magnitude (4) 108-111 F4.0 deg Long Longitude of the sub-solar point at occultation 113-114 I2 h LST.h Local solar time at closest approach 116-117 I2 m LST.m Local solar time at closest approach 121-127 F7.1 mas oRA Right ascension ephemeris offset correction 130-136 F7.1 mas oDE Declination ephemeris offset correction 138-139 A2 --- f_pm [ok no] Star proper motion flag (5) 141-142 A2 --- Cat Catalog cross-identification flag (6) 144 I1 --- Amf [0/6] Astrometric multiplicity flag (G1) 146-149 I4 mas e_RAs ?=9999 Right ascension error at catalog mean epoch (7) 151-154 I4 mas e_DEs ?=9999 Declination error at catalog mean epoch (7) 156-159 I4 mas/yr pmRA ?=9999 Proper motion in Right Ascension (pmRA*cosDE) (8) 161-164 I4 mas/yr pmDE ?=9999 Proper motion in Declination (8)
Note (1): at epoch Ep.date Ep.time Note (2): This is the position angle of the body with respect to the star at closest approach; PA is zero when the body is north of the star and is counted clockwise. Note (3): This is the occultation shadow velocity across the Earth at the apparent geocentric distance of the body; it has a positive/negative sign for prograde/retrograde velocities, that is, when Pluto's geocentric right ascension is respectively increasing/decreasing. Note (4): J, H or K magnitudes marked "50.0" mean no infrared magnitudes available; this means that it is not a 2MASS star. Note (5): Proper motion flag is: ok = proper motion no = no proper motion Note (6): Catalog cross-identification flag as follows: uc = UCAC2 star 2m = 2MASS star fs = field star (not a UCAC2 or 2MASS catalog star) Note (7): Error values marked "9999" mean no estimated errors; this regards to astrometric flagged positions or to positions from one observation Note (8): proper motion values marked "9999" mean no computed proper motions; this is because no first epoch position counterpart could be found in the 2MASS or USNO B1.0 catalogs.
Global notes: Note (G1): Astrometric multiplicity flags as follows: 0 = final WFI mosaic star position ok; all contributing individual CCD positions were within 0.2" from each other after the final WFI mosaic solutions; no nearby individual CCD position was identified within 1.5" of the final catalog star position, indicating good astrometry 1 = a number of individual CCD positions was identified within 1.5" of each other; more than one of these individual positions was labelled as a UCAC2 star; the final catalog position assigned for this star came from the average over these individual CCD positions labelled as UCAC2; the same flag is assigned if it refers to the 2MASS catalog (one can distinguish between both cases looking at the catalog flag) 2 = a number of individual CCD positions was identified within 1.5" of each other; but only one single of these positions was labelled UCAC2/2MASS; this single position was assigned as the final catalog star position (the catalog flag indicates which catalog is involved - UCAC2 or 2MASS) 3 = some contributing individual CCD positions were within 0.2" from each other after the final WFI mosaic solutions, but the resulting positions were nearby others within 1.5"; none of these positions were labelled UCAC2 or 2MASS; here, the final catalog star position assigned for this star came from the entry with highest number (N not equal to 1) of individual position contributions from the WFI mosaic solution 4 = same situation as in flag "3", but more than one entry presented the same highest number of individual position contributions; in this case, the final catalog star position assigned for this star came from the single entry with least (x,y) measured errors 5 = same situation as in flag "4", but more than one entry presented the same best (x,y) measurement errors; in this case, the final catalog star position assigned for this star came from the single entry with brightest R magnitude 6 = same situation as in flag "5", but more than one entry presented the same brightest R magnitudes; in this case, the final catalog star position assigned for this star came from the average over all detected multiple entries
Acknowledgements: Marcelo Assafin, massaf(at)astro.ufrj.br History: * 04-Jun-2010 : Original version * 01-Aug-2011 : Corrected files, from author
(End) Marcelo Assafin [UFRJ/OV, Brazil], Patricia Vannier [CDS] 27-Feb-2010
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