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Centro de Astrofísica da Universidade do Porto

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

N. Huélamo, P. Figuera, X. Bonfils, N. C. Santos, F. Pepe, M. Gillon, R. Azevedo, T. Barman, M. Fernández, E. di Folco, E. W. Guenther, C. Lovis, C. Melo, D. Queloz, S. Udry

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 MJup 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="" sup="">−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="" spot="" scenario="" rather="" than="" presence="" of="" a="" hot="" jupiter="" finally="" photometric="" data="" shows="" 20="" peak="" to="" variability="" which="" much="" larger="" 4="" variation="" expected="" for="" modeled="" cool="" fact="" are="" correlated="" surface="" cross-correlation="" function="" points="" towards="" spots="" as="" being="" responsible="" br="">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.

Astronomy and Astrophysics
Volume 489, Page L9
October 2008

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