An Appulse event is when an asteroid passes nearby to a star. This
kind of events are is epecially interesting for high angular observations
of faint asteroids using Adaptive Optics system.
We offer in this web
page the results of appulse calculations between several Centaurs or
Trans-Neptunian Objects (TNOs) and Tycho-2 stars. For that purpose we
have considered in the ASTORB database
(Asteroid Orbital Element Database) of the Lowell observatory the targets with
a semi-major axis greater than 6 AU. We find 582 objects which are listed here. Their osculating orbital elements
have been used to generate their position using DE403 ephemeris at the
IMCCE. Assuming a geocentric position
and using the Tycho-2
catalog, appulses with a star have been determined. Complete and
detailed description of the calculation method is available in Berthier
(Ph.D Thesis, Observatoire de Paris,
1997). The criterion for an appulse to be considered includes a
minimal angular separation less then 65 arcsec between the star and
Since these bodies are too faint (mv > 19) for the AO systems, an
appulse event should be used to observe them taking advantage of the
high resolution and high S/N provided by this technics. The Tycho-2
catalog is limited to stars with a magnitude < 14. This is also the
limit of the wavefront sensing source for the actual AO systems
(ESO-ADONIS, Lick AO, Keck AO, etc.). In the future, we will run some
new calculations using a more sensitive catalogue, such as the
USNO-2.0 or Guide Star Catalogue (GSC). The maximal angular separation
of 65 arcsec corresponds roughly to the field of view of the majority
of AO systems. It should be made clear that the closer an appulse
event is, the better the effect of the AO corrections will be (see Fig. 1 and animation).
Application for Adaptive Optics
Our first observations performed at the Lick Shane and 3.6m-ESO
telescopes has shown that the main limit for this kind of observations
is the precision of the ephemeris. To better estimate the accuracy of
the Appulse calculation, we introduced the AEU (Appulse
Ephemeris Uncertainty) which is calculated through the CEU (Current 1-\sigma
Ephemeris Uncertainty) of the ASTORB table. The AEU gives an
estimation of the uncertainty in position of the asteroid at the date
of the appulse. It is estimated through this basic calculation:AEU = CEU + CEU_Rate * ( T(appulse) - T(CEU date)
The precision of the ephemeris is related with the
number of points used to fit the orbital parameters, the duration of
the observations and the date of the last observations. The success of
these techniques of observation is directly dependent of classical
observations performed by other teams to better constrain the
ephemeris of these minor bodies.
High-resolution images of TNOs provide better sampled lightcurves
in J,H and K bands, giving us direct information on the shape,
rotational state and albedo distribution of the body. Analysis of the
profile should also reveal the presence of a coma and/or a moonlet
companion. Quite recently three double TNOs have been discovered by
classical observations 1998 WW31
(Veillet et al., 2001), 2001
Fig. 1: Simulation of TNO/Centaurs observation with AO system
Click here to start animation
The result of the calculations is summarized in several ASCII files. Appulse
events have been calculated for the second semester of 2001(old version of the table) and
for the year 2002 and 2003. Calculations will be redone regularly
to include newly discovered bodies and more precise osculting orbital elements.
We will also extend the calculating period for the next 5 years.
Description and used of the generated files
The ASCII files is formatted and contains the following data columns:
|date time (TT)
||Date and time of the minimal angular distance of the appulse.
Time is expressed in Terrestrial time (TT) which is linked, at that time, to the Universal time
by the relation: TT-UTC = 64.184 sec.
||Elongation from the Sun in degree
||Minimal angular distance between the star and the object achieve during the appulse, in arcsec
||Asteroid's position angle at the time of the appulse,
defined as the angle between the star-asteroid direction and the celestial north pole direction,
counted positive toward the east. The codes N, E, S, O stand for, respectively,
North, East, South and West
||Name of the Centaurs or Trans-Neptunian object involved in the appulse event
||Visible magnitude of the asteroid calculated with the classical law from its absolute magnitude
and its slope parameter
||Angular motion of the object during the event, in arcsec per hour
||Star number in the Tycho-2 catalogue
||Visible magnitude (approximate Johnson photometry) of the star from the
Tycho-2 VT and BT magnitudes [V = VT - 0.09*(BT-VT)]
||Approximate J2000.0 equatorial coordinates of the star
|AEU in arcsec
||Estimated uncertainty of the position of the asteroid in arcsec at the date of the event
Hereafter, a short step-by-step instructions to observe one of these targets:
This calculation, based on the IMCCE ephemeris and appulse generator, has been provided to the
community for the use of anyone interested in observing TNOs and Centaurs
with Adaptive Optics systems. Many Adaptive Optics systems are now
available or will be opened to the community soon. The most recent ones mounted
on 8-m class telescopes (VLT, Subaru, Gemini) will be commissioned at the
end of the year 2001 and beginning of 2002. Such appulse observations may
be used to validate the AO systems, estimate the quality of correction
inside the anisoplanetism angle and gain sensitivity provided on
these faint objects. Scientific benefits can be also expected in the detection
of coma activity, moonlet companion and/or bifurcated shape. We encourage
you to contact us (email@example.com
and firstname.lastname@example.org) if you have additional comments or
technical questions or if you use these data.
|- Is this event observable from your site?
Note that we did not removed the events close to the Sun in the large tables, but removed all the event with a solar elongation lower than 60 degrees in the month-to-month tables. (check ephemeris)
|- Is this event observable with your AO system?
Check if the reference star is bright enough for your wavefront sensor and if the
minimal angular distance is less than your WFS field of view
|- What is the accuracy of the ephemeris position?
Check the value of the AEU (last column) if you get am uncertainty superior to 5 arcsec, it should be better to observe this TNO by classical observation a few weeks before your observation to get a more accurate position.
|- You must now regenerate the ephemeris of the targets.
You can use either the IMCCE
or the JPL ephemeris generator. In
both case you will use the astrometric J2000.0 equatorial coordinates (RA and DEC).
The "Time Span" can be estimated using the "Apparent motion" of the body
during the event and the minimal separation compared with the Field of
View of your AO system. Output interval of the ephemeris should be around 10 min.
|- If needed, retrieve the precise coordinates of the star in the Tycho-2 catalog.
It could be done using the VizieR Service
from CDS. Enter Tycho-2 in the catalogue name field and choose "Tycho-2 main catalogue". In the
"Query position", enter the position of the star from our table (the last two columns), reduce the
"Target dimension" to 1-2 arcmin and press "Submit". The star should appear (compare the name with
the column "Star No")... If it does not, contact us!! :-) You can also get an optical map of the region,
which should be useful later
|- So now you have everything ready to calculate the offset between the reference star and the target.
Soon we shall provide a short IDL program to generate and display an appulse event based
on this file
Please, refer to this web page in proposals and publications if you have used these calculations
to prepare the observations. You should also visit this web page regularly since it will be upgraded often.
During this period 1614 appulse events have been detected, amoung them
882 will have an angular separation < 30 arcsec and 758 have a solar elongation lower than 60 degrees. A description of all
these events is available:
Calculation for Period June, 1 - December, 31 2001
A month-to-month list of the 758 events observable is also offered:
||Files and size
||TNOs_2001_date06.txt (10 Kb)
||TNOs_2001_date07.txt (12 Kb)
||96 events, Appulse with 1998WW31 on Jul., 28
||TNOs_2001_date08.txt (15 Kb)
||TNOs_2001_date09.txt (11 Kb)
||TNOs_2001_date10.txt (12 Kb)
ESO AO obs. grant (Hainaut et al.) on the Oct. 26 (1996TO66 & 1998WH24)
||TNOs_2001_date11.txt (16 Kb)
||122 events, Appulse with 1998WW31 on Nov., 11
Lick AO obs. grant (Marchis et al.) on the Nov., 2 (1999UG5) and 3 (1998VG44)
Monthly appulse events for second semester of 2001
||TNOs_2001_date12.txt (21 Kb)
Lick AO obs. grant (Marchis et al.) on the Dec., 22 (1996TO66)
The calculations is now available, 4418 events have been detected, amoung them 1803 have been selected in the month-to-month tables.
Calculation for Year 2002
A month-to-month list is also proposed:
The table are proposed in the same format.
5014 events have been detected, amoung them 2422 have been selected in the month-to-month tables since their solar elongation is larger than 60 degrees.
Calculation for Year 2003
A month-to-month list is also proposed:
The table are proposed in the same format. We suppressed in these table the Trojan with an AEU greater than 30.0 arcsec.
3682 events have been detected, amoung them 1752 have been selected in the month-to-month tables since their solar elongation is larger than 60 degrees.
Calculation for Year 2004
A month-to-month list is also proposed:
We have performed the same kind of calculations for the Pluto-Charon system on the year 2002 and 2003. 10 events with a Solar elongation > 55 \deg are detected. Since the accuracy of the "planet" is known with a precision of about 0.3 arcsec, classical observations of these appulses are also interesting to better constrain the orbital elements of the ephemeris.
Calculations for Pluto-Charon system
Note that there is a possible occulation of the double system by a 11.5 magnitude star on the July, 1st, 2002.
Finally, combining the AO appulse observations with spectroscopic capabilities will allow to better characterize the surface component of the two separated bodies.
Here is the file in the same format than the previous calculation, but without the AEU column.
File: Rap_Pluto.nys-0203 (2.6 Kb)
The list of Appulses for 2004 is available.
File: Rap_Pluto.nys-04 (3.1 Kb)<
Observations of 1999 TC36 (mv=19.7) were performed on Oct. 4, 2001 with the Lick Shane 3m Telescope and its AO system equipped with the NIRCAL camera. This target was 8" away of its expected position (see Fig. 2), but it was closer to the Tycho-2 star. Its CEU (absolute value of the current 1-\sigma ephemeris uncertainty) indicating in the ephemeris was only 0.38". By the way, the variation was expected to be 4.7E-2 arcsec/day and the date of our observation was 180 days after the last measurement. The shift of 8" is then compatible with the accuracy of the ephemeris (the expecting AEU is 11.2").
First Observation of 1999 TC36
Direct JHK broadband images were acquiered with and without AO system. The improvement in resolution and S/N is obvious (see Fig. 3). After 300 s of integration time, the S/N obtained is 3 times better than the one expected using the ISAAC camera on the 8m-VLT telescope. The FWHM on the TNO is 0.24 arcsec in K band.
Quite recently, Trujillo et al. (IAU 7787, 2002) reported the detection of a companion around this plutino at 0.365" and with a difference of magnitude of 2.21 using the STIS camera on board the HST telescope. Such companion cannot be detected by classical (or uncorrected) ground-based observation as he shows in the simulation available on his web page.
The PSF profile of our AO observations indicates that we can detected the companion if its position do not vary a lot. We applied a mean profile subtraction on our data (see Fig. 4) which reveal the presence of a bright feature at a separation of 0.246" and a difference of magnitude of 1.90 (Marchis et al., IAUC 7807, 2002).
Brown et al. (IAU 7807, 2002) reported the presence of another closeby double TNO system on 1998 SM 165. High angular resolution through new generation of AO systems will allow to constrain the orbit of these systems which seems to be numerous.
Fig. 2: Observed and expected position of 1999 TC36 from the camera control of the telescope
Fig. 3: JHK band observations of the TNO and their criteria of quality.
Fig. 4: K band observation and same frame after applying the mean profile subtraction.