A&A 489, L9-L13 (2008)
DOI: 10.1051/0004-6361:200810596

Letter

TW Hydrae: evidence of stellar spots instead of a Hot Jupiter

N. Huélamo¹, P. Figueira², X. Bonfils^{3, 4}, N.C. Santos³, F. Pepe², M. Gillon², R. Azevedo³, T. Barman⁵, M. Fernández⁶, E. di Folco², E. W. Guenther⁷, C. Lovis², C. H. F. Melo⁸, D. Queloz², and S. Udry<sup>2

1</sup>  Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), Apdo. 78, 28691 Villanueva de la Cañada, Spain
    e-mail: nhuelamo@laeff.inta.es
²  Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
³  Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁴  Centro de Astronomia e Astrofísica da Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁵  Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA
⁶  Instituto de Astrofísica de Andalucía, CSIC, Apdo. 3004, 18080 Granada, Spain
⁷  Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, Sternwarte 5, 07778 Tautenburg, Germany
⁸  European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received 14 July 2008 / Accepted 30 July 2008

Abstract
Context. TW Hya is a classical T Tauri star that shows significant radial-velocity variations in the optical regime. These variations have been attributed to a 10  M_</I>Jup planet orbiting the star at 0.04 AU.
Aims. The aim of this letter is to confirm the presence of the giant planet around TW Hya by (i) testing whether the observed RV variations can be caused by stellar spots and (ii) analyzing new optical and infrared data to detect the signal of the planet companion.
Methods. We fitted the RV variations of TW Hya using a cool spot model. In addition, we obtained new high-resolution optical & infrared spectra, together with optical photometry of TW Hya and compared them with previous data.
Results. Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54° can reproduce the reported RV variations. The model also predicts a bisector semi-amplitude variation <10 m s^-1, which is less than the errors of the RV measurements discussed in Setiawan et al. (2008, Nature, 451, 38). The analysis of our new optical RV data, with typical errors of 10 m s^-1, shows a larger RV amplitude that varies depending on the correlation mask used. A slight correlation between the RV variation and the bisector is also observed although not at a very significant level. The infrared H-band RV curve is almost flat, showing a small variation (<35 m s^-1) that is not consistent with the published optical orbit. All these results support the spot scenario rather than the presence of a hot Jupiter. Finally, the photometric data shows a 20% (peak to peak) variability, which is much larger than the 4% variation expected for the modeled cool spot. The fact that the optical data are correlated with the surface of the cross-correlation function points towards hot spots as being responsible for the photometric variability.
Conclusions. We conclude that the best explanation for the RV signal observed in TW Hya is the presence of a cool stellar spot and not an orbiting hot Jupiter.

Key words: stars: pre-main sequence -- stars: planetary systems -- stars: individual: TW Hya

© ESO 2008