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 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/* . 74 Individual files of catalog star positions sky path
tables/* . 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