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

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 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.

Evidence of a substellar companion around a very young T Tauri star

“…Spectra were collected using the 4096 × 512 pixel Aladdin III detectors and a 0.2" slit which delivered a R∼100000 around 1598.0 nm (CRIRES setup 36). This particular setting was chosen because we could benefit from the CO 2 telluric atmospheric lines as simultaneous wavelength calibrators (Huélamo et al 2008;Figueira et al 2010c) to derive precise RVs. Previous studies have indeed shown that telluric lines are steady RV zero-point tracers (Figueira et al 2010b) that display a long-term stability and, for this reason, can deliver RVs with a precision down to 5-10 ms −1 .…”

RAFT – I. Discovery of new planetary candidates and updated orbits from archival FEROS spectra

“…It has been shown that RV signals caused by rotational modulation of star spots can mimic Doppler signals, causing possible misinterpretation of the measurements (e.g. Huélamo et al 2008), particularly when the star is at low inclination since some activity indicators, like the bisector inverse slope (BIS), are insensitive in this regime (Desort et al 2007). In order to test this possibility, we first computed the spot filling factor from Hatzes (2002).…”

Chromospheric activity and rotation of FGK stars in the solar vicinity

“…Stellar RV variations are produced by different magneticactivity-related phenomena: convection (Saar 2009), starspots , magnetic plage/network (Saar 2003) and flares (Saar 2009;Reiners 2009). Although bisector analysis may sometimes lead to the confirmation of a planet orbiting a star even when RV jitter is present (Sozzetti et al 2006;Setiawan et al 2007Setiawan et al , 2008, the latter technique is not always successful (Huélamo et al 2008;Figueira et al 2010). Several authors have studied the impact of activity on RV jitter using R HK as a proxy (Saar et al 1998;Santos et al 2000;Paulson et al 2002;Saar et al 2003;Wright 2005;Paulson & Yelda 2006;Santos et al 2010).…”