Emission line variability of young 10–30 M_Jup companions

Abstract

Context. Emission lines that are indicative of active accretion have been observed for a number of low-mass companions (M < 30 MJup) to stars. Line variability is ubiquitous on stellar accretors, but it has never been characterized in detail for low-mass companions. Such characterizations can offer insights into the accretion mechanism at play. Aims. We aim to characterize the short-to-long-term H I Paschen β emission line variability of two 10 to 30 MJup companions on wide orbits: GQ Lup b and GSC 06214-00210 b. We also aim to clarify their accretion mechanisms. Methods. We used J-band observations at R = 1800–2360, obtained with VLT/SINFONI in 2017, to record time-series investigations of the hours-to-weeks variability of the H I Paschen β emission line (1.282 µm). Contrary to H∝, it should be less affected by chromospheric activity contamination. The photospheric emission was analyzed at each epoch and removed with the ForMoSA forward-modeling tool, using new grids of ATMO models exploring different C/O and [M/H] values. The time series of line profiles and intensities were compared to those of more massive accretors and to predictions from the latest magnetospheric accretion and shock models. To complement these results, we also re-investigated archival spectroscopic observations at near-infrared wavelengths of each target to increase the time frame up to a decade and to build a more comprehensive understanding of the variability processes at play. Results. For GQ Lup b, we find line variability on timescales of several months to decades, whereas it is within the acceptable noise levels on shorter timescales. For GSC 06214-00210 b, we find line variability on timescales of tens of minutes all the way up to a decade. The line profiles of GSC 06214-00210 b are partially resolved in at least one epoch. Both objects show H I Paschen β flux variability that is moderate (<50%), on timescales that are below their rotation period, and that is more significant on longer timescales (up to ~1000% on decade-long timescales). This behavior resembles that of classical T Tauri stars. The line profiles of GQ Lup b are blue-shifted and can only be reproduced by magnetospheric accretion models, while those of GSC 06214-00210 b are fairly well reproduced by both magnetospheric accretion and shock models, except for one epoch for which the shock model is highly favored. The companions have C/O values broadly consistent with solar values. Conclusions. While magnetospheric accretion is favored for GQ Lup b, higher resolution (R > 10 000) observations are required to disentangle the two (non-exclusive) emitting mechanisms. The similar variability behavior observed in these low mass companions and in classical T Tauri stars may support similar accretion mechanisms. The high amplitude of variability on timescales of over a month and longer that is found for both objects could be key to explaining the low yield of H∝ imaging campaigns.

Monitoring Hα Emission from the Wide-orbit Brown-dwarf Companion FU Tau B

“…Along with the asymmetric line profiles nicely fit with multiple velocity shifts and widths, these features are consistent with the magnetospheric accretion models. A <1% filling factor has also been inferred for the two 10-30 M Jup wide companions GQ Lup B and GSC 06214-00210 B (Demars et al 2023).…”

“…Nonetheless, accretion variability has been found to increase on longer timescales for young stars (e.g., Nguyen et al 2009;Mendigutía et al 2011;Pogodin et al 2012;Costigan et al 2014;Zsidi et al 2022), brown dwarfs (e.g., Stelzer et al 2007), and even for substellar companions (Demars et al 2023). Stelzer et al (2007) found that accretion onto the 20-30 M Jup brown dwarf 2MASS J12073346-3932539 mildly varied between 10 −7.1 and 10 −6.8 M Jup yr −1 (a factor of two) for two consecutive nights, but changed by at least a factor of 10 on monthly to yearly timescales.…”

“…Wolff et al (2017) also found that the equivalent width of the 1.282 μm Paβ emission from the 10-15 M Jup DH Tau B could vary by a factor of a few in weeks. Recently, Demars et al (2023) show that the variability amplitude of the Paβ line stays low (<50%) on hourly timescales but becomes significant (100% and up to ∼1000%) on monthly to yearly timescales for GQ Lup B and GSC 06214-00210 B. To examine if FU Tau B exhibits the same feature, we follow Demars et al (2023) to calculate the flux variation as (fluxmax flux min )/flux min for each individual pair of measurements in Table 3, where fluxmax and flux min are the higher and the lower values of each pair, respectively.…”

“…Recently, Demars et al (2023) show that the variability amplitude of the Paβ line stays low (<50%) on hourly timescales but becomes significant (100% and up to ∼1000%) on monthly to yearly timescales for GQ Lup B and GSC 06214-00210 B. To examine if FU Tau B exhibits the same feature, we follow Demars et al (2023) to calculate the flux variation as (fluxmax flux min )/flux min for each individual pair of measurements in Table 3, where fluxmax and flux min are the higher and the lower values of each pair, respectively. Due to our large photometric uncertainties, however, we find a similar flux variations of 10%-20% (median) from hourly to daily timescales.…”

“…While there has been accretion monitoring for T Tauri stars (e.g., Nguyen et al 2009;Biazzo et al 2012;Pouilly et al 2020;Sousa et al 2021;Zsidi et al 2022) and young isolated brown dwarfs (e.g., Natta et al 2004;Scholz & Jayawardhana 2006;Stelzer et al 2007;Herczeg et al 2009), such efforts near the planetary regime have been rare. Among the dozens of young substellar companions and protoplanets discovered in direct-imaging surveys, very few of them have multiepoch accretion-rate measurements (e.g., GQ Lup B and GSC 06214-00210 B; Demars et al 2023;PDS 70 b, Zhou et al 2021). Moreover, many of these studies were carried out at different observatories, further complicating the interpretations.…”