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

J. M. Alcalá, A. Natta+12 more|arXiv (Cornell University)|Oct 8, 2013

Astrophysics and Star Formation StudiesPhysics and Astronomy109 references184 citations

TL;DR

This study presents X-Shooter/VLT spectroscopy of 36 low-mass and sub-stellar young stellar objects in Lupus, deriving accretion rates ($\dot{M}_{\rm acc}$) via continuum excess modeling and emission line luminosities. It finds $\dot{M}_{\rm acc} \propto M_\star^{1.8\pm0.2}$ with lower scatter than prior work, confirms line luminosities scale linearly with $\dot{M}_{\rm acc}$ over 5 orders of magnitude, and shows that H$\alpha$ profile modeling underestimates $\dot{M}_{\rm acc}$ by 0.6–0.8 dex compared to continuum methods.

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.

Motivation & Objective

• To analyze the relationship between continuum excess and emission line diagnostics of accretion in low-mass and sub-stellar YSOs.
• To investigate how accretion properties correlate with the physical parameters of the central star, particularly mass.
• To resolve discrepancies in accretion rate measurements arising from different methodologies.
• To extend empirical relationships between accretion luminosity ($L_{\rm acc}$) and line luminosities ($L_{\rm line}$) to lower $L_{\rm acc}$ values, including the very low-mass regime.
• To test whether physical conditions in accretion flows remain consistent across a wide range of accretion rates.

Proposed method

• Modeling the UV-to-near-IR continuum excess as emission from a hydrogen slab in local thermodynamic equilibrium (LTE) to derive $L_{\rm acc}$.
• Measuring the luminosity of 39 emission lines (H, He, Ca ii, etc.) across 330–2500 nm from high-resolution X-Shooter spectra.
• Deriving $\dot{M}_{\rm acc}$ from $L_{\rm acc}$ using stellar mass and radius estimates, assuming standard magnetospheric accretion physics.
• Comparing $\dot{M}_{\rm acc}$ derived from H$\alpha$ line profile fitting with that from continuum excess modeling to assess systematic biases.
• Establishing empirical $L_{\rm acc}$–$L_{\rm line}$ relationships for 39 lines, with uncertainty quantification and scatter analysis.
• Analyzing line ratios and luminosity distributions to assess consistency of accretion physics across $\dot{M}_{\rm acc}$ ranges.

Experimental results

Research questions

• RQ1 How do $L_{\rm acc}$ and $\dot{M}_{\rm acc}$ derived from continuum excess compare with those from H$\alpha$ line profile modeling?
• RQ2 What is the empirical relationship between $L_{\rm acc}$ and the luminosity of individual emission lines across a wide range of accretion rates?
• RQ3 Is the $\dot{M}_{\rm acc}$–$M_\star$ relationship consistent across the Lupus YSO sample, and how does its scatter compare to previous studies?
• RQ4 Are the physical conditions in accretion flows (e.g., line ratios, energy distribution) independent of $\dot{M}_{\rm acc}$ over five orders of magnitude?
• RQ5 To what extent do methodological differences in $\dot{M}_{\rm acc}$ derivation explain reported bi-modality in accretion rate relationships?

Key findings

• H$\alpha$ line profile modeling underestimates $\dot{M}_{\rm acc}$ by 0.6 to 0.8 dex compared to continuum excess-based measurements.
• $\dot{M}_{\rm acc}$ is found to scale with stellar mass as $\dot{M}_{\rm acc} \propto M_\star^{1.8\pm0.2}$, with a scatter reduced by a factor of about two compared to previous studies.
• The Pa$\beta$ and Br$\gamma$ lines show the most reliable $L_{\rm acc}$–$L_{\rm line}$ relationships, with lower scatter and less sensitivity to chromospheric activity than optical lines.
• Line luminosities increase almost linearly with $\dot{M}_{\rm acc}$ over more than five orders of magnitude, indicating consistent physical conditions in the accretion flow.
• The fraction of $L_{\rm acc}$ emitted in each line and hydrogen line ratios remain roughly constant across the $\dot{M}_{\rm acc}$ range, supporting a common accretion geometry.
• The previously reported bi-modality in $\dot{M}_{\rm acc}$ relationships is likely an artifact of mixing accretion rates derived from inconsistent methodologies.