X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus

Abstract: We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion as compared to previous relationships in the literature. Our measurements extend the Pab and Brg relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies to measure Lacc and Macc yield significantly different results: Ha line profile modelling may underestimate Macc by 0.6 to 0.8dex with respect to Macc derived from continuum-excess measures. Such differences may explain the likely spurious bi-modal relationships between Macc and other YSOs properties reported in the literature. We derive Macc in the range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Macc.

• The paper presents a robust investigation of accretion using continuum excess and emission-line diagnostics to derive L_acc and mass accretion rates in 36 Lupus young stellar objects.
• It establishes low-dispersion empirical relationships between accretion luminosity and 39 emission lines, extending previous correlations to L_acc values two orders of magnitude lower.
• The study identifies discrepancies in Hα modeling, updates the mass accretion rate-mass relationship, and underscores the key role of accretion flow geometry in stellar evolution.

Accretion in Low-Mass Stars and Sub-Stellar Objects in Lupus

The paper "X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus" presents a comprehensive study of young stellar objects (YSOs) within the Lupus star-forming region using the X-Shooter instrument at the Very Large Telescope (VLT). The analysis focuses on 36 accreting YSOs, characterized by masses ranging from approximately 0.03 to 1.2 solar masses, predominantly in the range of 0.1 to 0.5 solar masses.

Key Findings and Numerical Results

1. Accretion Diagnostics: The study employs excess-continuum and line emission diagnostics to assess accretion properties. The accretion luminosities ($L_{acc}$) and rates ($\dot{M}_{acc}$) are derived by modeling the excess emission as continuum emission originating from a slab of hydrogen.
2. Empirical Relationships: The research establishes empirical relationships between $L_{acc}$ and the luminosity of 39 emission lines (e.g., H, He, Ca II) observed across the spectral range from 330 nm to 2500 nm. The empirical relationships demonstrate lower dispersion compared to previous studies.
3. Accretion Luminosity Correlations: The relationship between $L_{acc}$ and the luminosity of the emission lines is robust, with a reduced variability in accretion diagnostics. Notably, these relationships extend previous correlations to $L_{acc}$ values two orders of magnitude lower than those reported in earlier studies.
4. Discrepancies in Accretion Rates: The paper addresses inconsistencies between different methodologies for measuring $L_{acc}$ and $\dot{M}_{acc}$, particularly the underestimation of accretion rates derived from H$\alpha$ line profile modeling compared to continuum-excess measures. The study concludes that H$\alpha$ line profile modeling may underestimate accretion rates by 0.6 to 0.8 dex.
5. Mass Accretion Rate and Mass Relationship: An updated relationship is presented, where the mass accretion rate is proportional to the mass of the central object raised to the power of 1.8±0.2, with a dispersion factor approximately two times smaller than in previous studies.
6. Implications for Accretion Flow Geometry: The research underscores the significance of the geometry of accretion flows in controlling accretion rates. The physical conditions of accreting gas are consistent across diverse accretion rates, suggesting that it is the flow geometry rather than gas conditions that determine accretion dynamics.

Implications and Future Research

The findings have significant implications for understanding PMS stellar evolution and the structure of accretion disks. They provide critical benchmarks for theoretical models of accretion physics and offer insights into the magnetic accretion processes at play in these systems.

The implications of this research extend to theoretical models of disc evolution and star formation, providing constraints on hypothesis models and serving as a reference for future empirical and theoretical studies. The results encourage further investigation into the geometry of accretion flows and their interaction with stellar magnetic fields, as well as continued improvement in the methodologies for measuring accretion rates across different spectral classes.

The study contributes valuable data to the field, fostering a detailed understanding of the accretion phenomena in low-mass stars and enhancing predictive models of star and planet formation. Future research could benefit from expanding the sample size and incorporating different star-forming regions to validate the universality of the empirical relations and patterns observed in the Lupus star-forming cloud.