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J/A+A/541/A142    Prediction of stellar occultations 2008-2015  (Assafin+, 2012)

Candidate stellar occultations by large TNOs up to 2015. Assafin M., Camargo J.I.B., Vieira Martins R., Braga-Ribas F., Sicardy B., Andrei A.H., da Silva Neto D.N. <Astron. Astrophys. 541, A142 (2012)> =2012A&A...541A.142A
ADC_Keywords: Positional data ; Occultations ; Minor planets Keywords: astrometry - occultations - Kuiper belt: general Abstract: The prediction tables of stellar occultations by Eris, Haumea, Makemake, Quaoar, Orcus, Sedna, Varuna, Ixion, 2002TX300 and 2003AZ84 for 2008-2015 were built to support the investigation of the physical properties of large transneptunian objects - keystones in the study of structure, origin and evolution of the Solar System. Our goal was to derive precise, astrometric predictions. With this aim, we constructed astrometric star catalogs in the UCAC2 system covering their sky paths. For that, we have carried out in 2007-2009 an observational program at the ESO2p2/WFI instrument covering the sky path of these 10 large TNOs for the 2008-2015. We made the astrometry of 316 GB 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 50 mas precision. The catalog star positions were obtained in the UCAC2 frame with errors of 40mas for stars up to magnitude completeness (about R=19). New stellar proper motions were also determined with 2MASS and the USNO B1.0 catalog positions as first epoch. The catalogs of all TNOs contain in all more than 5.35 million stars with proper motions, covering the sky paths of the objects with 30 arcmin width. The magnitude completeness is about R=19 with a limit about R=21. Ephemeris offsets with about 50mas to 100mas precision were applied for each TNO orbit to improve the predictions. They were obtained during 2007-2010 from a parallel observational campaign carried out with 0.6m to 2.2m size telescopes. The 2718 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 a body with Pluto's size (50mas) plus the apparent Earth radius (285mas) as projected in the sky plane at 31AU (about the Pluto-Earth distance for 2008-2015). No threshold in R magnitude was used in the search for candidates, as 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 paths 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 all, for R=19 stars (catalog magnitude completeness) and 40mas errors in the WFI positions, we may assume a bulk error of about 80mas for C/A, dominated by the ephemeris offsets errors of about 70mas. For about 40AU, this implies a shadow path uncertainty over the Earth of the order of 2300km. If the ephemeris offsets can be well determined to within 30mas precision, then a bulk error of 50mas in C/A can be achieved, leading to a precision of about 1400km for the WFI occultation path predictions. Thus, the probability of actually observing the occultation 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: 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 ICRS (J2000) geocentric right ascension and declination of the TNO at the occultation, 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, ephemerides offsets in (RA, DEC) for the central instant (see JPL ephemeris version in Table 7 of the paper), 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 of a body at Pluto's distance at opposition). Magnormalized=mag+2.5log10(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. This situation is further favored as the brightness of TNOs are usually smaller than that of the stars. In all, the overall contribution to the total recorded flux must be evaluated and a case by case estimation of the SNR must be done for those candidates. The prediction tables are divided by body and year, with entries in chronological order. The reference ephemerides used are listed in Table 7 of the paper. Besides star positions, the ICRS (J2000) geocentric right ascension and declination of the TNO at the occultation are also given (they are corrected by ephemeris offsets). This makes it easy for the reader to evaluate his/her own updates on the star position or on the ephemeris offsets of the TNOs. Notice that some tables present zero entries. This is not in error and only means that for some TNOs and years no occultation is foreseen. Abstract for additional star catalogs: The catalogs of star positions for 10 large transneptunian objects (TNOs) covering their sky paths for 2008-2015 were built to support precise astrometric predictions for stellar occultations by Eris, Haumea, Makemake, Quaoar, Orcus, Sedna, Varuna, Ixion, 2002TX300 and 2003AZ84 for this period. For that, we have carried out during 2007-2009 an observational program at the ESO2p2/WFI instrument covering their 2008-2015 sky paths. We made the astrometry of 316 GB 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 40mas for stars up to the magnitude completeness (about R=19). New stellar proper motions were also determined with 2MASS and the USNO B1.0 catalog positions as first epoch. Astrometric catalogs with proper motions were produced for each TNO, containing more than 5.35 million stars covering the sky paths with 30 arcmin width. The magnitude completeness is about R=19 with a limit about R=21. We highlight the usefulness of this catalog as a reference frame for star/body astrometric follow-up before and after future stellar occultations involving these TNOs. 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 catalogs also furnish useful photometric information for field and candidate stars in the preparation of campaigns and flux calibrations of observed light curves. Description for additional star catalogs: 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 of the related 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. 3 of the related 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. For each TNO, we give separate catalogs labelled per year. Each year corresponds to the observed sky path. For some TNOs, there are small gaps without catalog stars between the years, corresponding to sky regions where, as seen from Earth, the Sun is too close to the TNO direction at date. The catalog entries are ordered by crescent declination. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file list.dat 66 74 List of objects with catalog star positions sky path and prediction stellar occultations catal/* 120 74 Individual files of catalog star positions sky path tables/* 195 67 Individual files for prediction stellar occultations
Byte-by-byte Description of file: list.dat
Bytes Format Units Label Explanations
1- 4 I4 yr Year Year of occultation 6- 15 A10 --- Object Object name 17- 28 A12 --- CatFile Name of the file with catalog star positions sky path, in "catal" subdirectory 30- 41 A12 --- TabFile Name of the file with prediction stellar occultations, in "tables" subdirectory 43- 66 A24 --- Com Comment
Byte-by-byte Description of file: catal/*
Bytes Format Units Label Explanations
2- 14 F13.9 deg RAdeg Right ascension in decimal degrees (J2000) at mean epoch 16- 28 F13.9 deg DEdeg Declination in decimal degrees (J2000) at mean epoch 30- 45 F16.8 d JD Mean epoch (Julian Date) of coordinates 47- 52 I6 mas e_RAs ?=999999 Right ascension error at mean epoch (1) 54- 59 I6 mas e_DEs ?=999999 Declination error at mean epoch (1) 61- 66 I6 mas/yr pmRA ?=999999 Proper motion in Right Ascension (RA*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 Magnitude (apparent) in J band (3) 89- 94 F6.3 mag Hmag Magnitude (apparent) in H band (3) 96-101 F6.3 mag Kmag Magnitude (apparent) in K band (3) 103-107 I5 mas e_x Measured error in CCD frames (x) coordinates 109-113 I5 mas e_y Measured error in CCD frames (y) coordinates 115-116 I2 --- o_Obs Number of observations per position 118 I1 --- f_cat Flag on catalog cross-identification (4) 120 I1 --- f_Pos Astrometric multiplicity flag (5)
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 Note (5): Astrometric multiplicity flags range from 0 to 6 as follows: 0 = final WFI mosaic star position came from all contributing individual CCD positions within 0.2arcsec from each other, after the final WFI mosaic solutions; no nearby individual CCD position was identified within 1.5arcsec of the final catalog star position, indicating good astrometry 1 = a number of individual CCD positions was identified within 1.5arcsec of each other; more than one of these individual positions was labeled as a UCAC2 star; the final catalog position assigned for this star came from the average over these individual CCD positions labeled 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 1.5arcsec of each other; but only one single of these positions was labeled 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 0.2arcsec from each other after the final WFI mosaic solutions, but the resulting positions were nearby others within 1.5arcsec; none of these positions were labeled 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
Byte-by-byte Description of file: tables/*
Bytes Format Units Label Explanations
2- 5 I4 yr App.Y UTC instant at closest approach (year) 7- 8 I2 "month" App.M UTC instant at closest approach (month) 10- 11 I2 d App.D UTC instant at closest approach (day) 14- 15 I2 h App.h UTC instant at closest approach (hours) 17- 18 I2 min App.m UTC instant at closest approach (minutes) 20- 22 F3.0 s App.s UTC instant at closest approach (seconds) 26- 27 I2 h RAh Right ascension (J2000) of star at epoch 29- 30 I2 min RAm Right ascension (J2000) of star at epoch 32- 38 F7.4 s RAs Right ascension (J2000) of star at epoch 40 A1 --- DE- Declination sign (J2000) of star at epoch 41- 42 I2 deg DEd Declination (J2000) of star at epoch 44- 45 I2 arcmin DEm Declination (J2000) of star at epoch 48- 53 F6.3 arcsec DEs Declination (J2000) of star at epoch 57- 58 I2 h RAoh TNO geocentric right ascension (J2000) at occultation 60- 61 I2 min RAom TNO geocentric right ascension (J2000) at occultation 63- 69 F7.4 s RAos TNO geocentric right ascension (J2000) at occultation 71 A1 --- DEo- TNO declination sign (J2000) at occultation 72- 73 I2 deg DEod TNO declination (J2000) at occultation 75- 76 I2 arcmin DEom TNO declination (J2000) at occultation 79- 84 F6.3 arcsec DEos TNO declination (J2000) at occultation 88- 92 F5.3 arcsec Sep Minimum Separation at closest approach (CA) 95-100 F6.2 deg PA Position angle (PA) at closest approach (1) 102-108 F7.2 km/s Vshad Occultation shadow velocity (2) 110-115 F6.2 AU Dist Apparent geocentric distance at occultation 117-120 F4.1 mag Rmag Normalized magnitude (apparent) in R band 122-125 F4.1 mag Jmag Normalized magnitude (apparent) in J band (3) 127-130 F4.1 mag Hmag Normalized magnitude (apparent) in H band (3) 132-135 F4.1 mag Kmag Normalized magnitude (apparent) in K band (3) 139-142 F4.0 deg Long Longitude of the sub-solar point at occultation 144-145 I2 h LST.h Local solar time at closest approach (hours) 147-148 I2 min LST.m Local solar time at closest approach (minutes) 152-158 F7.1 mas oRA Right ascension ephemeris offset correction 161-167 F7.1 mas oDE Declination ephemeris offset correction 169-170 A2 --- f_pm [ok/no] Proper motion flag (4) 172-173 A2 --- f_cat Catalog cross-identification flag (5) 175 I1 --- f_Pos Astrometric multiplicity flag (6) 177-180 I4 mas e_RAs ?=9999 Right ascension error at catalog mean epoch (7) 182-185 I4 mas e_DEs ?=9999 Declination error at catalog mean epoch (7) 187-190 I4 mas/yr pmRA ?=9999 Proper motion in Right Ascension (RA*cosDE) (8) 192-195 I4 mas/yr pmDE ?=9999 Proper motion in Declination (8)
Note (1): 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 (2): 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 the TNO's geocentric right ascension is respectively increasing/decreasing Note (3): J, H or K magnitudes marked "50.0" mean no infrared magnitudes available; this means that it is not a 2MASS star Note (4): Star has proper motion? If yes, flag = "ok", otherwise flag = "no" Note (5): Flag as follows: uc = UCAC2 star 2m = 2MASS star fs = field star (not a UCAC2 or 2MASS catalog star) Note (6): Astrometric multiplicity flags range from 0 to 6 as follows: 0 = final WFI mosaic star position ok; all contributing individual CCD positions were within 0.2arcsec from each other after the final WFI mosaic solutions; no nearby individual CCD position was identified within 1.5arcsec of the final catalog star position, indicating good astrometry 1 = a number of individual CCD positions was identified within 1.5arcsec of each other; more than one of these individual positions was labeled as a UCAC2 star; the final catalog position assigned for this star came from the average over these individual CCD positions labeled 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 1.5arcsec of each other; but only one single of these positions was labeled 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 0.2arcsec from each other after the final WFI mosaic solutions, but the resulting positions were nearby others within 1.5arcsec; none of these positions were labeled 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 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.
Acknowledgements: Marcelo Assafin, massaf(at)astro.ufrj.br
(End) Marcelo Assafin [UFRJ/OV, Brazil], Patricia Vannier [CDS] 22-Mar-2012
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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