4 Conclusions

We have discovered a new carbon rich white dwarf (DQ), which shows very strong C₂ Deslandres-d'Azambuja and Swan bands. To the best of our knowledge, no other object is known today which such a strong simultaneous evidence of the two molecular band systems associated with C₂.

Comparisons to published synthetic spectra suggest $6000 < T_{\rm eff} < 8000$ K, while a black-body fit to the observed fluxes at $\lambda > 7000$ Å, and to the peaks below $\sim$4100 Å supports the possibility that $T_{\rm BB}\sim 6000$ K. Therefore, it is evident that the reliable determination of temperature and chemical composition of GSCU J131147.2+292348 must await more detailed atmosphere model calculations. Anyhow, it is likely that the carbon abundance in the atmosphere of this WD is significantly enhanced compared to other known DQ stars of similar temperature.

A photometric distance of 70-90 parsecs has been estimated, which implies a relatively large spatial velocity and makes this new DQ white dwarf a likely member of the halo or thick disk population. Of course, a direct determination of the distance will be the only way to derive model independent absolute magnitude and kinematics for this object.

Acknowledgements

We are indebted to the referee, U. Heber, for his valuable comments and suggestions that were essential for the proper interpretation of our observations. The constant support of our GSC2 collaborators B. Bucciarelli, J. Garcia, V. Laidler, C. Loomis, and R. Morbidelli is acknowledged. And thanks go also to A. Boden and R. Cutri who reprocessed their 2MASS frames to look for this object. The GSC II is a joint project of the Space Telescope Science Institute and the Osservatorio Astronomico di Torino. Space Telescope Science Institute is operated by AURA for NASA under contract NAS5-26555. Current participation of the Osservatorio Astronomico di Torino is supported by the Italian National Institute for Astrophysics (INAF). Partial financial support to this research comes from the Italian CNAA and the Italian Ministry of Research (MIUR) through the COFIN-2001 program. STH and DJP acknowledge the financial support of the Particle Physics and Astronomy Reasearch Council of the United Kingdom. This research has made use of the SIMBAD database, operated at CDS, Strasbourg (France).