FitteR for Accretion ProPErties of T Tauri stars (FRAPPE): A new approach to use Class III spectra to derive stellar and accretion properties

TL;DR

This paper introduces FRAPPE, a new method using an interpolated grid of non-accreting star spectra to improve the measurement of stellar and accretion properties in T Tauri stars, especially in estimating lower limits of accretion luminosities.

Contribution

The paper develops a continuous spectral template grid for non-accreting stars and applies it to derive more reliable stellar and accretion properties in T Tauri stars.

Findings

Lower limit on accretion luminosity ranges from 2.7 dex below stellar luminosity in M5.5 stars to 1.3 dex in G8 stars.

Estimated mass accretion rate limit is around 10^{-10} solar masses per year.

Improved disentangling of degenerate solutions leads to more accurate accretion rate estimates.

Abstract

Studies of the stellar and accretion properties of classical T Tauri stars (CTTS) require comparison with photospheric spectral templates. Here we aim at expanding the currently available grid of wide-wavelength coverage observed spectra of non-accreting stars with additional new spectra and an interpolation method that allows us to obtain a continuous grid of low resolution spectra ranging from spectral type G8 to M9.5, while also mitigating observational uncertainties. This interpolated grid is then implemented in the self-consistent method to derive stellar and accretion properties of CTTS. With the new templates, we aim to estimate a lower limit on the accretion luminosities that can be obtained through a study of the UV excess emission using observed templates. We analyse the molecular photospheric features present in the VLT/X-Shooter spectra of the targets to perform a spectral classification, including estimates of their extinction. We apply a non-parametric fitting method to the full grid of observed templates to obtain an interpolated grid of templates. We use the uncertainties on our interpolated grid to estimate a lower limit on the accretion luminosity that we can measure with this method. We find that the measurable accretion luminosities ranges from $\sim 2.7$ dex lower than the stellar luminosity in M5.5 stars to $\sim 1.3$ dex lower for G8 stars. For young stars with masses of $\sim 1M_{\odot}$ and ages of 3-6 Myr this limit translates into an observational limit of mass accretion rate on the order of $10^{-10} \rm M_{\odot}/yr$. The implementation of an interpolated grid of observed templates allows us to better disentangle degenerate solutions, leading to a more reliable estimate of accretion rates in young accreting stars.