Stable accretion in young stars: the cases of EX Lupi and TW Hya

Abstract

We examine the long-term spectroscopic and photometric variability of EX Lupi and TW Hya, studying the presence of stable accretion and the role it plays in the observed variability. Analysing the velocity modulations of the emission lines with STAR-MELT, we obtain information on the structure of the accretion columns and the disk-star connection. The emission line radial velocities reveal that TW Hya, like EX Lupi, has a remarkably stable or slow-varying accretion column footprint, locked to the star for several years. The line-emitting regions are non-polar for both EX Lupi and TW Hya, and species with different energies differ in position. In contrast, the continuum emission as observed in the photometry is very variable and can be modelled by hot spot(s) that change over time in phase, shape, temperature, size, and location with respect to the emission line region. The continuum emission region may not be limited to the stellar surface, especially during episodes of high accretion. The broad line emission observed in EX Lupi during episodes of increased accretion reveals a further structure, which can be fitted by non-axisymmetric disk in Keplerian rotation inwards of the corotation radius. Since the radial velocity modulation due to accretion footprints is so stable, we used it to search for further velocity modulations. While no residual modulation (other than caused by stellar rotation) is found in these objects, a similar analysis could help to identify young planets/companions. Therefore, determining whether stable accretion footprints are common among young stars is a key to detect young planets.

A Multiwavelength, Multiepoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. II. Photometric Light Curves

“…Also seen in ugriz are broad signals near 6.1 and 8.2 days. The periodicities near 3.54 and 4.26 days in E1 and E2 are roughly in line with previous studies that found a range of periodicities between 1.4 and 4.7 days (Huélamo et al 2008;Siwak et al 2011Siwak et al , 2014Siwak et al , 2018Herczeg et al 2023;Sicilia-Aguilar et al 2023), with 3.57 days being the assumed rotational period for TW Hya. While some statistically significant, jagged peaks are seen between 1.0 and 3.0 days, they are not consistent between bands or between epochs and should not be considered physical.…”

“…For TW Hya, in E2 we recover a robust period near 3.54 days and see evidence for this period in E1, consistent with its 3.57-day rotational period (Huélamo et al 2008;Sicilia-Aguilar et al 2023). This signal is consistent throughout both epochs and may additionally be stable over a timescale of at least 1 yr.…”

A Multiwavelength, Multiepoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. I. HST Far-UV and Near-UV Spectra

“…Despite its nearly pole-on geometry, TW Hya has shown photometric periodicity, though at a wide and inconsistent range of periods (0.16-4.7 days; Siwak et al 2011Siwak et al , 2014Siwak et al , 2018. Radial velocity studies (Huélamo et al 2008;Sicilia-Aguilar et al 2023) reveal a strong, stable, sinusoidal rotational modulation at about 3.57 days.…”

“…The range of measured accretion rates in TW Hya is (0.05-0.6) × 10 −8 M e yr −1 (Ingleby et al 2013;Manara et al 2014;Herczeg et al 2023;Sousa et al 2023). This accretion is thought to primarily occur through a single, large, poleward flow (Donati et al 2011;Sicilia-Aguilar et al 2023), along with many smaller, more sporadic accretion tongues near the equator (Siwak et al 2018).…”

Accretion Funnel Reconfiguration during an Outburst in a Young Stellar Object: EX Lupi

“…This CO isotopologue modeling assumed a smaller value of a stellar mass of 0.5 M e for EX Lupi. Recent work by Sicilia-Aguilar et al (2023) used an intermediate inclination angle of 20°-40°t o explain the color modulation in lightcurves with a single hotspot model. The authors further modeled the width of Ca II 8662 Å lines and found that an inclination angle of around 45°b est matches the observation.…”