PENELLOPE

VII. Revisiting empirical relations to measure accretion luminosity^★

¹ INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
² Instituto de Astrofísica de Canarias, IAC, Vía Láctea s/n, 38205 La Laguna (S.C.Tenerife), Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna (S.C.Tenerife), Spain
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁶ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁷ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
⁸ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁹ Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
¹⁰ SETI Institute, 339 Bernardo Ave., Suite 200, Mountain View, CA 94043, USA
¹¹ Observatoire de Paris – PSL University, Sorbonne Université, LERMA, CNRS, Paris, France
¹² Univ. grenoble Alpes, CNRS, IPAG, Grenoble, France
¹³ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, PR China
¹⁴ Max-Planck-Insitut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
¹⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300RA, Leiden, The Netherlands

^★★ Corresponding author: eleonora.fiorellino@inaf.it

Received: 25 July 2025
Accepted: 22 September 2025

Abstract

Context. The accretion luminosity (L_acc) in young, low-mass stars is crucial for understanding stellar formation. However, obtaining direct measurements is often hindered by limited spectral coverage and challenges in UV-excess modeling. Empirical relations linking L_acc to various accretion tracers are widely used to overcome these limitations.

Aims. This work revisits these empirical relations using the PENELLOPE dataset, evaluating their applicability across different star-forming regions as well as accreting young objects other than Classical T Tauri Stars (CTTSs; Class II sources).

Methods. We analyzed the PENELLOPE VLT/X-shooter dataset of 64 CTTSs, measuring fluxes of several accretion tracers and adopting the stellar and accretion parameters derived from studies based on PENELLOPE. For 61 sources, we supplemented our analysis with the ODYSSEUS HST data set, which covers a wider spectral range in NUV bands.

Results. We compared the L_acc values obtained in the PENELLOPE and ODYSSEUS surveys, which employed a single hydrogen slab model (XS-fit) and a multi-column accretion shock model (HST-fit), respectively, and found statistically consistent results. Our analysis confirms that existing empirical relations, previously derived for the Lupus sample, provide reliable L_acc estimates for CTTSs in several other star-forming regions. We revisit empirical relations for accretion tracers in our dataset, based on HST-fit, with coefficients which are consistent within 1σ with XS-fit results for most lines. We also propose a method to estimate extinction using these relations and investigate the empirical relations for Brackett lines (Br8 to Br21).

Conclusions. The L_acc − L_line empirical relations can be successfully used for statistical studies of accretion on young forming objects in different star-forming regions. These relations also offer a promising approach to independently estimate extinction in CTTSs, provided a sufficient number of flux-calibrated tracers are available across a broad spectral range. We confirm that near-infrared lines (Paβ and Brγ) serve as reliable tracers of L_acc in high accretors, making them valuable tools for probing accretion properties of high accreting young stars not accessible in the UVB.