X-shooter spectroscopy of young stars with disks

Abstract

Context. Measurements of the fraction of disk-bearing stars in clusters as a function of age indicate protoplanetary disk lifetimes ≲10 Myr. However, our knowledge of the time evolution of mass accretion in young stars over the disk lifespans is subject to many uncertainties, especially at the lowest stellar masses (M⋆). Aims. We investigate ongoing accretion activity in young stars in the TW Hydrae association (TWA). The age of the association (∼8–10 Myr) renders it an ideal target for probing the final stages of disk accretion, and its proximity (∼50 pc) enables a detailed assessment of stellar and accretion properties down to brown dwarf masses. Methods. Our sample comprises eleven TWA members with infrared excess, amounting to 85% of the total TWA population with disks. Our targets span spectral types between M0 and M9, and masses between 0.58 M⊙ and 0.02 M⊙. We employed homogeneous spectroscopic data from 300 nm to 2500 nm, obtained synoptically with the X-shooter spectrograph, to derive the individual extinction, stellar parameters, and accretion parameters for each object simultaneously. We then examined the luminosity of Balmer lines and forbidden emission lines to probe the physics of the star–disk interaction environment. Results. Disk-bearing stars represent around 24% of the total TWA population. We detected signatures of ongoing accretion for 70% of our TWA targets for which accurate measurements of the stellar parameters could be derived. This implies a fraction of accretors between 13–17% across the entire TWA (that accounts for the disk-bearing and potentially accreting members not included in our survey). The spectral emission associated with these stars reveals a more evolved stage of these accretors compared to younger PMS populations studied with the same instrument and analysis techniques (e.g., Lupus): first, a large fraction (∼50%) exhibit nearly symmetric, narrow Hα line profiles; second, over 80% of them exhibit Balmer decrements that are consistent with moderate accretion activity and optically thin emission; third, less than a third exhibit forbidden line emission in [O I] 6300 Å, which is indicative of winds and outflows activity; and fourth, only one sixth exhibit signatures of collimated jets. However, the distribution in accretion rates (Ṁacc) derived for the TWA sample closely follows that of younger regions (Lupus, Chamaeleon I, σ Orionis) over the mass range of overlap (M⋆ ∼ 0.1–0.3 M⊙). An overall correlation between Ṁacc and M⋆ is detected and best reproduced by the function Ṁacc ∝ M∝2.1±0.5. Conclusion. At least in the lowest M⋆ regimes, stars that still retain a disk at ages ∼8–10 Myr are found to exhibit statistically similar, albeit moderate, accretion levels as those measured around younger objects. This “slow” Ṁacc evolution that is apparent at the lowest masses may be associated with longer evolutionary timescales of disks around low-mass stars, which is suggested by the mass-dependent disk fractions reported in the literature within individual clusters.

PENELLOPE: The ESO data legacy program to complement the Hubble UV Legacy Library of Young Stars (ULLYSES)

“…Overall, we do not see noticeable differences between the accretion rates measured in the 13 targets in the Orion OB1 and σ-Ori regions with respect to what is observed in other young star-forming regions with similar techniques. This result is in agreement with the findings of Manara et al (2020), Venuti et al (2019), Rugel et al (2018), andIngleby et al (2014), among others, who found that the accretion rates of still accreting objects at ages of >3−5 Myr are still comparable to the accretion rates of younger stars. However, this result is still puzzling as these high accretion rates are difficult to reconcile with the current framework to describe disk evolution (e.g., Ingleby et al 2014;Manara et al 2020).…”

“…It is only by observing large samples of young stars and probing wide ranges of age and mass that we can gain access to a sufficient set of statistics to aid in the understanding of both processes. Spectroscopic surveys of young stars in different star-forming regions have shown that the mass accretion rates onto the central star (Ṁ acc ) slowly decrease with isochronal ages, possibly in accordance with viscous evolution models (e.g., Hartmann et al 1998;Sicilia-Aguilar et al 2010;Antoniucci et al 2014), although high accretion rates are still observed at ages of >5−10 Myr A&A 650, A196 (2021) (e.g., Ingleby et al 2014;Frasca et al 2015;Rugel et al 2018;Venuti et al 2019;Manara et al 2020). The empirical measurement of the steep dependence ofṀ acc on the stellar mass (M ) (Hillenbrand et al 1992;Muzerolle et al 2003;Calvet et al 2004;Mohanty et al 2005;Natta et al 2006;Manara et al 2012Manara et al , 2016aManara et al , 2017aAlcalá et al 2014Alcalá et al , 2017Venuti et al 2014) may be the consequence of a single dominant phenomenon or a mix of many, such as, initial conditions followed by viscous evolution (e.g., Alexander et al 2006;Dullemond et al 2006), an imprint of internal photoevaporation (e.g., Ercolano et al 2014), different accretion regimes at different stellar masses (e.g., Mohanty et al 2005;Hartmann et al 2006), environmental effects (e.g., Padoan et al 2005), or self-gravity in the disk (e.g., Vorobyov & Basu 2009;DeSouza & Basu 2017).…”

“…The analysis of the X-shooter spectra to derive stellar and accretion properties is carried out with the method originally described in Manara et al (2013a) and later applied to a number of X-shooter studies of young stars (e.g., Alcalá et al 2014Alcalá et al , 2017Manara et al 2016aManara et al , 2017aVenuti et al 2019). In short, the observed spectrum is dereddened and fit with the sum of a photospheric template spectrum and a hydrogen slab model with uniform density and temperature gas to reproduce the continuum excess emission due to accretion.…”

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): A close low-mass companion to ET Cha

“…We note that very recently Manara et al (2020) found similarly high accretion rates reported for ET Cha around several members of the ∼5 Myr Upper Scorpius region (see also Ingleby et al 2014;Venuti et al 2019 for Orion OB1 and TWA). The mass estimate in this case was based on the dust.…”

X-shooter survey of disk accretion in Upper Scorpius

“…The detection of strong accretors at later ages, and the connected hint of a lack of a general decrease of accretion rates with time when the targeted stars are still hosting a disk, has already been observed in different older star-forming regions: the nearby loose associations TWA (Venuti et al 2019) and η-Cha (Rugel et al 2018), the more distant γ-Velorum cluster (Frasca et al 2015), Orion OB1b and Orion OB1a associations (Ingleby et al 2014), and even the very massive regions like NGC3603 or 30 Doradus (De Marchi et al 2017). Individual targets have also been found to be still accreting at age >20 Myr (e.g., Mamajek et al 2002;Zuckerman et al 2014;Murphy et al 2018;Lee et al 2020).…”