Radial velocity variations in the young eruptive star EX Lupi

Abstract

Context. EX Lup-type objects (EXors) are low-mass pre-main sequence objects characterized by optical and near-infrared outbursts attributed to highly enhanced accretion from the circumstellar disk onto the star. Aims. The trigger mechanism of EXor outbursts is still debated. One type of theory requires a close (sub)stellar companion that perturbs the inner part of the disk and triggers the onset of the enhanced accretion. Here, we study the radial velocity (RV) variations of EX Lup, the prototype of the EXor class, and test whether they can be related to a close companion. Methods. We conducted a five-year RV survey, collecting 54 observations with HARPS and FEROS. We analyzed the activity of EX Lup by checking the bisector, the equivalent width of the Ca 8662 Å line, the asymmetry of the Ca II K line, the activity indicator S FEROS , the asymmetry of the cross-correlation function, the line depth ratio of the VI/FeI lines, and the TiO, CaH 2, CaH 3, CaOH, and Hα indices. We complemented the RV measurements with a 14-day optical/infrared photometric monitoring to look for signatures of activity or varying accretion. Results. We found that the RV of EX Lup is periodic (P = 7.417 d), with stable period, semi-amplitude (2.2 km s −1 ), and phase over at least four years of observations. This period is not present in any of the above-mentioned activity indicators. However, the RVs of narrow metallic emission lines suggest the same period, but with an anti-correlating phase. The observed absorption line RVs can be fitted with a Keplerian solution around a 0.6 M central star with m sin i = (14.7 ± 0.7) M Jup and eccentricity of e = 0.24. Alternatively, we attempted to model the observations with a cold or hot stellar spot as well. We found that in our simple model, the spot parameters needed to reproduce the RV semi-amplitude are in contradiction with the photometric variability, making the spot scenario unlikely. Conclusions. We qualitatively discuss two possibilities to explain the RV data: a geometry with two accretion columns rotating with the star, and a single accretion flow synchronized with the orbital motion of the hypothetical companion; the second scenario is more consistent with the observed properties of EX Lup. In this scenario, the companion's mass would fall into the brown dwarf desert, which, together with the unusually small separation of 0.06 au would make EX Lup a unique binary system. The companion also has interesting implications on the physical mechanisms responsible for triggering the outburst.

Accretion dynamics of EX Lupi in quiescence

“…Weaker narrow emission lines are noisier, although they are consistent with the same modulation. Kóspál et al (2014) noted that the NC of the emission lines shows RV patterns that are off-phase with respect to Fig. 3.…”

“…Such a spot would also produce a luminosity modulation that is not observed in the photometry records. Kóspál et al (2014) assumed that the spot would suffer occultations as the star rotates, which would increase the luminosity contrast, but a high-latitude spot visible at all times would not induce dramatic variations in the luminosity. In addition, if most or a large fraction of the energy released in the shock structure is emitted in lines (including many metallic UV lines) and not in the optical continuum, photometric rotational modulations may not be evident, while RV modulations would be significantly enhanced with respect to the continuumonly scenario.…”

“…The photospheric absorption lines during quiescence show a strong radial velocity (RV) modulation, which has been interpreted as being caused by a brown dwarf (BD) companion, although hot-spot-induced RV signatures could not be excluded (Kóspál et al 2014;hereafter K14). There are several precedents of objects where the RV initially suggested the presence of a companion, which was later ruled out as a signal caused by accretion and/or strong activity, such as RW Auriga (Gahm et al 1999;Petrov et al 2001;) and RU Lupi (Stempels et al 2007;Gahm et al 2013).…”

The Circumstellar Disk and Asymmetric Outflow of the EX Lup Outburst System

“…Many efforts have been made to identify a possible binary companion around EX Lup (Ghez et al 1997;Bailey 1998;Kóspál et al 2014). Kóspál et al (2014) detect a radial velocity signal that can be described by a companion with an m sin i = 14.7 ± 0.7 M Jup (0.018 M assuming an inclination of 50 • located 0.06 au from the primary. Sicilia-Aguilar et al (2015) however, show that the same signal can also be reproduced by a rotating, line-dependent veiling from an accretion column, without the need of a stellar companion.…”

A Candidate Young Massive Planet in Orbit Around the Classical T Tauri Star Ci Tau*

“…Bright accretion spots on classical T Tauri stars can in principle create the same apparent RV signal as dark spots (e.g., Kóspál et al 2014;Sicilia-Aguilar et al 2015). Accretion hot spots which produce veiling on CTTSs typically have temperatures ∼ 10, 000 K and produce a largely featureless continuum (Hartigan et al 1989;Basri & Batalha 1990;Hartigan et al 1991;Valenti et al 1993).…”