AN AUGMENTED STARS DATABASE FOR CELESTIA

Pascal Hartmann (baten.kaitos@wanadoo.fr)
Date of release : May 08, 2003
Version : 2.1 A (1016385 stars)

Many thanks to Alexis Brandeker for his help in the development of the discrimination procedure for Tycho stars.


1) STATISTICS

The new stars database V 2.1 A contains :

Hipparcos stars :        117053
Sun :                                      1
Tycho stars :              887643
Companions stars :      11688

Total stars :             1016385


2) HOW TO INSTALL THE NEW DATABASE IN CELESTIA

To install the new database in Celestia, simply replace the previous file stars.dat with the new one.


3) REVISION HISTORY

Version 2.1

 Imprecision on data that comes from the Tycho catalogs and the current limits of the interpolation procedure are more noticeable for Suns neighborhood. To avoid any conflict with well known nearby stars, no Tycho stars have been added in this new release below 25 pc. A next step could be the integration of non-Hipparcos stars that come from the Gliese catalog.

 DIFFERENCES BETWEEN V 2.1 A and V 2.1 C

I have made two versions of the V 2.1 new stars database.

In the version 2.1 A of 1016385 stars, the extraction parameters are set to :
parallax relative error < 35% and B-V error < 0.15.

In the version 2.1 C of 2072873 stars, the extraction parameters are set to :
parallax relative error < 45% and B-V error < 0.35.

Thus V 2.1 C contains more stars than V 2.1 A but with lower precision on distances.


Version 2.0

 New Tycho stars in this release have been extracted from the All Sky Compiled Catalog of 2.5 million stars (I/280 Kharchenko 2001). This catalog of 2501968 stars includes Hipparcos, Tycho-1 and Tycho-2 catalogs and some ground-based ones.

Thus the amount of stars in this new release is bigger than in V1.x.

 The extraction procedure has been improved, so stars with very small parallaxes (< 0.0002) are no more systematically rejected.

  New fitting functions Mv = f(B-V) have been applied to correct Hipparcos stars with negative or strange parallaxes values (4519 on 5064 stars have been corrected).
These new fitting functions are derived from Hipparcos data for luminosity class III and V stars, and from theoretical values (Astrophysical quantities) for all other luminosity classes.  These new fitting functions should lead to better estimated distances for those corrected stars.

 DIFFERENCE BETWEEN V 2.0 A and V 2.0 C

I have made two versions of the V 2.0 new stars database.

In the version 2.0 A of 1044025 stars, the extraction parameters are set to :
parallax relative error < 35% and B-V error < 0.15.

In the version 2.0 C of 2102323 stars, the extraction parameters are set to :
parallax relative error < 45% and B-V error < 0.35.

Thus V 2.0 C contains more stars than V 2.0 A but with lower precision on distances.


Version 1.2

 Chi2 Ori is back among other Hipparcos stars that were rejected in the release 1.1, due to their strange parallax values (negative or lesser than 0.0002)

I applied the same procedure as I did on Hipparcos stars with negative parallax values, to Hipparcos stars with parallax values lesser than 0.0002 to have a better distance estimation for them. Thus 4356 among 5064 Hipparcos stars with bad parallax values have been corrected.

 The database should be more precise for stars that lie within 20 ly. But there are still new Tycho stars in this range, accepted with good parallax values (relative error < 25%)

 Rigel Kentaurus B is back. It was rejected in V1.1 and replaced by HIP 33626113 due to big error values in RA and declination.


Version 1.1

This version should be more accurate than V1.0.

Whats new ?

 All stars of parallax values lesser than 0.0002 (d > 5000 pc) have been rejected, either from the Hipparcos or from the Tycho catalog, because their parallaxes are undoubtedly false.

 All Tycho stars with strange B-V (< -0.5 or > 2.0) color index values were rejected.

 There is no 8154 ly limit anymore. Hipparcos stars which have negative parallax values (4196 stars), and were therefore set at an arbitrary distance of about 8154 ly in the initial database have been either completely rejected when correction was not possible (332 stars), or set at a new computed distance (3864 stars).

To compute an estimated distance for a Hipparcos star with an initial negative parallax value, I used - when possible - the B-V color index of the star and its luminosity class.

example : HIP 251 has a parallax value of -0.53 in the Hipparcos catalog. BV = 1.349, m = 8.65 and it is a star of luminosity class III (K2 III).

With our fitting polynomial function for class III, we then calculate an estimated absolute magnitude : M = P3(BV). The result is here : M = 0.2965.
With M and m, we can now compute an estimated distance for the star : d = 468.49 pc ~ 1527 ly.


4) THE ASCC CATALOG

The All-Sky Compiled Catalogue of 2501968 stars (ASCC-2.5) with the limiting magnitude V=12-14 is a result of a merging of star lists from present day large high-precision catalogues from space (Hipparcos-Tycho family catalogues: Hipparcos main catalogue including Multiple System Annex <I/239>, Tycho-1 <I/239>, Tycho-2 <I/259>, ACT-RC <I/246>, TRC <I/250>) and ground-based (PPM-N <I/146>, PPM-S <I/193>,PPM-add <I/208>, CMC11 <I/256>) observations and reduction to standard systems of corresponding stellar data. Main stellar data presented in the ASCC-2.5 are the equatorial coordinates (J2000, epoch 1991.25), proper motions in the Hipparcos system, B and/or V stellar magnitudes in the Johnson system. Additionally for some stars are given trigonometric parallaxes, spectral classes in the MK or HD system, multiplicity and variability flags, Hipparcos, Tycho-2, HD, DM designations.


5) HOW THE NEW STARS WERE EXTRACTED FROM THE ASCC CATALOG

To build a 3D stars database, you need at least the following parameters for a star : Right Ascension, Declination, Apparent Magnitude, the Parallax value which is transformed in distance to the star and a color index.

The ASCC catalog provides all these fields for each star, but the parallax values are often unusable, because negative or lesser than their corresponding parallax errors.

Thus, a good approximation procedure is needed to compute the distances of stars only from their B-V color index and apparent magnitude.

- when a star has a good parallax value, its distance is computed directly.
- when it is not the case, an estimated parallax value is computed with its B-V color index and the help of polynomial fitting functions (computed with the stars of the Hipparcos catalog), assuming this star is either a dwarf of class V (main sequence stars : about 44% of Hipparcos stars) or a giant of class III (about 39% of Hipparcos stars).

Thus all new Tycho stars are either of class III or class V due to this simplification procedure.

It is also difficult to estimate the error on the distances of these additional stars : from 1% to 50% depending on input parameters.

Here is the extraction procedure output (V2.1 A) :

Number of Tycho stars analysed : 2454555
Number of Tycho stars accepted : 887643
Number of Tycho stars rejected : 1566912

Number of Tycho stars with Hip numbers rejected : 124235
Number of Tycho stars rejected through BV out of range : 30674
Number of Tycho stars accepted through acceptable parallax value : 5604
Number of Tycho stars accepted through acceptable B-V error value : 882039
Number of Tycho stars accepted through acceptable B-V error value with initial neg. parallax : 494514

Number of Tycho stars accepted of Luminosity Class III : 185036
Number of Tycho stars accepted of Luminosity Class V : 702607

Number of Tycho stars with appMag < 6.0 : 6
Number of Tycho stars with 6.0 <= appMag < 7.0 : 98
Number of Tycho stars with 7.0 <= appMag < 8.0 : 4405
Number of Tycho stars with 8.0 <= appMag < 9.0 : 39644
Number of Tycho stars with 9.0 <= appMag < 10.0 : 188776
Number of Tycho stars with 10.0 <= appMag < 11.0 : 444736
Number of Tycho stars with 11.0 <= appMag < 12.0 : 209690
Number of Tycho stars with 12.0 <= appMag < 13.0 : 288
Number of Tycho stars with 13.0 <= appMag : 0

Number of Tycho stars with distance <= 25 pc : 0
Number of Tycho stars with 25 < distance < 50 pc : 44033
Number of Tycho stars with 50 <= distance < 100 pc : 43500
Number of Tycho stars with 100 <= distance < 500 pc : 520551
Number of Tycho stars with 500 <= distance < 1000 pc : 250659
Number of Tycho stars with 1000 <= distance < 2000 pc : 27177
Number of Tycho stars with 2000 pc <= distance : 1723

Number of Hipparcos stars dropped : 903
Number of Hipparcos stars with parallax set to limit : 53

Stars processed: 1017366   Number dropped: 981  number dubious: 1036 .

Total stars count in stars.dat : 1016385


6) HOW THE TYCHO STARS ARE LABELLED IN CELESTIA

A certain amount of Tycho stars have a HD number and can be searched in Celestia with this number (ex : HD 169236)
This HD number comes either from the original HD catalog (range 1-225300) or from its extensions HDE (range 225301-272150) and HDEC (range 272151-359083).

A Tycho star number has the form : TYC Tyc1-Tyc2-Tyc3. Ex : TYC 8745-250-1

To avoid conflicts with real and existing Hipparcos numbers in Celestia, and to allow searching for Tyc stars in Celestia, I have used the following formula :

Fictious HIP number = Tyc1 + 10^4 * Tyc2 + 10^9 * Tyc 3       (or abbbbbcccc where a = Tyc3; bbbbb = Tyc2 and cccc = Tyc1)

Thus our example TYC 8745-250-1 will have the following fictious HIP identificator in Celestia : HIP 1002508745.

All fictious HIP numbers for new Tycho stars are therefore coded on 10 digits and greater than 1 000 000 000.


7) ACKNOWLEDGEMENTS

To Chris Laurel, the creator of this wonderfull software.

To Alexis Brandeker for his help in astronomy and his kindness.

To Bob Ippolito who ported Celestia to OS X.

To all the users who make new textures, new models, ..., so Celestia is going every day more beautiful...


8) LINKS

To learn more on the Hipparcos space astrometry mission, visit : http://astro.estec.esa.nl/Hipparcos/hipparcos.html

To query yourself the Hipparcos or Tycho catalogs on the web : http://astro.estec.esa.nl/hipparcos_scripts/hipMultiSearch.pl
 