Appulse Events between
Centaurs/TNOs and TYCHO-2 stars
for Adaptive Optics Observations

F. Marchis 1, J. Berthier 2

1/ UC-Berkeley & Center for Adaptive Optics, Berkeley, California
2/ Institut de mécanique céleste, Observatoire de Paris, France


This page summarizes predictions of appulse events between TNOs/Centaurs and bright stars (mv < 14) for the second semester of 2001 and the year 2002 and 2003 which should be observable with Adaptive Optics systems.


  • Introduction
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 the target.
  • Application for Adaptive Optics observations
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).

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 QT297, and 2001 QW322.

Fig. 1: Simulation of TNO/Centaurs observation with AO system
Click here to start animation

  • Description and used of the generated files
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.

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.
Solar Elong. Elongation from the Sun in degree
min. dist Minimal angular distance between the star and the object achieve during the appulse, in arcsec
V. Pos 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 Name of the Centaurs or Trans-Neptunian object involved in the appulse event
V. mag. Visible magnitude of the asteroid calculated with the classical law from its absolute magnitude and its slope parameter
Motion Angular motion of the object during the event, in arcsec per hour
Star No Star number in the Tycho-2 catalogue
Mag. Visible magnitude (approximate Johnson photometry) of the star from the Tycho-2 VT and BT magnitudes [V = VT - 0.09*(BT-VT)]
(RA;Dec) 2000.0 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:

- 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
  • Comments and Copyright
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 ( and if you have additional comments or technical questions or if you use these data.

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.

  • Calculation for Period June, 1 - December, 31 2001
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:

   - Sorted by name    File: TNOs_2001_name.txt (202 Kb)
   - Sorted by date    File: TNOs_2001_date.txt (202 Kb)
   - Sorted by angular separation    File: TNOs_2001_dist.txt (202 Kb)

A month-to-month list of the 758 events observable is also offered: 

Month Files and size Comments
June 2001 TNOs_2001_date06.txt (10 Kb) 79 events
July 2001 TNOs_2001_date07.txt (12 Kb) 96 events, Appulse with 1998WW31 on Jul., 28
August 2001 TNOs_2001_date08.txt (15 Kb) 116 events
September 2001 TNOs_2001_date09.txt (11 Kb) 86 events
October 2001 TNOs_2001_date10.txt (12 Kb) 96 events
ESO AO obs. grant (Hainaut et al.) on the Oct. 26 (1996TO66 & 1998WH24)
November 2001 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)
December 2001 TNOs_2001_date12.txt (21 Kb) 163 events
Lick AO obs. grant (Marchis et al.) on the Dec., 22 (1996TO66)
Monthly appulse events for second semester of 2001
  • Calculation for Year 2002
The calculations is now available, 4418 events have been detected, amoung them 1803 have been selected in the month-to-month tables.

   - Sorted by name    File: TNOs_2002_name.txt (605 Kb)
   - Sorted by date    File: TNOs_2002_date.txt (605 Kb)
   - Sorted by date and with a AEU lower than 5 arcsec    File: TNOs_2002_date_lowAEU.txt (291 Kb)
   - Sorted by angular separation    File: TNOs_2002_dist.txt (605 Kb)

A month-to-month list is also proposed:

Month Files and size Comments
January 2002 TNOs_2002_date01.txt (27 Kb) 191 events, Appulse with 1999TC36 on Jan., 1
February 2002 TNOs_2002_date02.txt (24 Kb) 169 events
March 2002 TNOs_2002_date03.txt (24 Kb) 173 events
April 2002 TNOs_2002_date04.txt (19 Kb) 138 events
May 2002 TNOs_2002_date05.txt (19 Kb) 135 events
June 2002 TNOs_2002_date06.txt (21 Kb) 151 events
July 2002 TNOs_2002_date07.txt (19 Kb) 136 events
August 2002 TNOs_2002_date08.txt (25 Kb) 177 events
September 2002 TNOs_2002_date09.txt (24 Kb) 168 events
October 2002 TNOs_2002_date10.txt (25 Kb) 180 events, Appulse with 1998SM165 on Oct., 1; Appulse with 1999TC36 on Oct., 12
November 2002 TNOs_2002_date11.txt (26 Kb) 185 events,Appulse with 1998SM165 on Nov., 19
December 2002 TNOs_2002_date12.txt (26 Kb) 186 events, Appulse with 1998WW31 on Dec., 27
Monthly appulse events for the year 2002
  • Calculation for Year 2003
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.

   - Sorted by name    File: TNOs_2003_name.txt (686 Kb)
   - Sorted by date    File: TNOs_2003_date.txt (686 Kb)
   - Sorted by date and with a AEU lower than 5 arcsec    File: TNOs_2003_date.txt (340 Kb)
   - Sorted by angular separation    File: TNOs_2003_dist.txt (686 Kb)

A month-to-month list is also proposed:

Month Files and size Comments
January 2003 TNOs_2003_date01.txt (36 Kb) 256 events
February 2003 TNOs_2003_date02.txt (29 Kb) 208 events
March 2003 TNOs_2003_date03.txt (32 Kb) 230 events
April 2003 TNOs_2003_date04.txt (30 Kb) 215 events
May 2003 TNOs_2003_date05.txt (27 Kb) 194 events
June 2003 TNOs_2003_date06.txt (25 Kb) 182 events
July 2003 TNOs_2003_date07.txt (25 Kb) 182 events, Appulse with 1998WW31 on Jul., 27
August 2003 TNOs_2003_date08.txt (29 Kb) 207 events
September 2003 TNOs_2003_date09.txt (27 Kb) 194 events
October 2003 TNOs_2003_date10.txt (24 Kb) 170 events
November 2003 TNOs_2003_date11.txt (27 Kb) 197 events, Appulse with 1998WW31 on Nov., 3,6 and 18
December 2003 TNOs_2003_date12.txt (26 Kb) 187 events
Monthly appulse events for the year 2003
  • Calculation for Year 2004
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.

   - Sorted by name    File: TNOs_2004_name.txt (516 Kb)
   - Sorted by date    File: TNOs_2004_date.txt (516 Kb)
   - Sorted by date and with a AEU lower than 5 arcsec    File: TNOs_2004_date.txt (437 Kb)
   - Sorted by angular separation    File: TNOs_2004_dist.txt (516 Kb)

A month-to-month list is also proposed:

Month Files and size Comments
January 2004 TNOs_2004_date01.txt (27 Kb) 193 events
February 2004 TNOs_2004_date02.txt (25 Kb) 178 events
March 2004 TNOs_2004_date03.txt (25 Kb) 179 events
April 2004 TNOs_2004_date04.txt (15 Kb) 109 events
May 2004 TNOs_2004_date05.txt (19 Kb) 137 events
June 2004 TNOs_2004_date06.txt (15 Kb) 106 events
July 2004 TNOs_2004_date07.txt (20 Kb) 141 events
August 2004 TNOs_2004_date08.txt (20 Kb) 143 events
September 2004 TNOs_2004_date09.txt (20 Kb) 141 events
October 2004 TNOs_2004_date10.txt (19 Kb) 135 events
November 2004 TNOs_2004_date11.txt (20 Kb) 139 events
December 2004 TNOs_2004_date12.txt (21 Kb) 151 events
Monthly appulse events for the year 2004
  • Calculations for Pluto-Charon system
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.
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)<

  • First Observation of 1999 TC36
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").
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.

Offset of TC36
Fig. 2: Observed and expected position of 1999 TC36 from the camera control of the telescope

JHK band observations
Fig. 3: JHK band observations of the TNO and their criteria of quality.

Detection of the companion of 1999 TC36
Fig. 4: K band observation and same frame after applying the mean profile subtraction.

Acknowledgements: We are very grateful to the visitors of this web page who showed an interest in our work and gave us some precious advice to make this page more handy for observer-astronomers. Special thanks to H. Boehnhardt, C. Veillet, C. Trujillo and J. Parker. We have taken into account a part of your comments, but we also plan to propose soon an iteractive version with a small search tool.

© 2002-2003, F. Marchis & J. Berthier - CfAO-UC Berkeley/IMCCE
Last update: March 12th, 2003

Contact : F. Marchis,, UC-Berkeley/CfAO, California, USA
J. Berthier,, IMCCE, Observatoire de Paris, France

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