KMOS study of the mass accretion rate from Class I to Class II in NGC 1333^⋆,⋆⋆

¹ Dipartimento di Fisica, Università di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1, 00133 Roma, Italy
e-mail: eleonora.fiorellino@inaf.it
² INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
³ INAF-IAPS, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
⁶ INAF-Osservatorio Astrofisico di Arcetri, L.go E.Fermi 5, 50126 Firenze, Italy
⁷ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy

Received: 26 August 2020
Accepted: 24 February 2021

Abstract

Context. The mass accretion rate (Ṁ_acc) is the fundamental parameter to understand the process of mass assembly that results in the formation of a low-mass star. This parameter has been largely studied in Classical T Tauri stars in star-forming regions with ages of ∼1 − 10 Myr. However, little is known about the accretion properties of young stellar objects (YSOs) in younger regions and early stages of star formation, such as in the Class 0/I phases.

Aims. We present new near-infrared spectra of 17 Class I/Flat and 35 Class II sources located in the young (< 1 Myr) NGC 1333 cluster, acquired with the KMOS instrument at the Very Large Telescope. Our goal is to study whether the mass accretion rate evolves with age, as suggested by the widely adopted viscous evolution model, by comparing the properties of the NGC 1333 members with samples of older regions.

Methods. For the Class II sources in our sample, we measured the stellar parameters (SpT, A_V, and L_⋆) through a comparison of the IR spectra with a grid of non-accreting Class III stellar templates. We then computed the accretion luminosity by using the known correlation between L_acc and the luminosity of HI lines (Paβ and Brγ). For the Class I sample, where the presence of a large IR excess makes it impossible to use the same spectral typing method, we applied a procedure that allowed us to measure the stellar and accretion luminosity in a self-consistent way. Mass accretion rates Ṁ_acc were then measured once masses and radii were estimated adopting suitable evolutionary tracks.

Results. The NGC 1333 Class II sources of our sample have L_acc ∼ 10⁻⁴ − 1 L_⊙ and Ṁ_acc ∼ 10⁻¹¹ − 10⁻⁷ M_⊙ yr⁻¹. We find a correlation between accretion and stellar luminosity in the form of log L_acc = (1.5 ± 0.2)log L_⋆ + ( − 1.0 ± 0.1), and a correlation between the mass accretion rate and stellar mass in the form of log Ṁ_acc = (2.6 ± 0.9) log M_⋆ + (−7.3 ± 0.7). Both correlations are compatible within the errors with the older Lupus star-forming region, while only the latter is consistent with results from Chamaeleon I. The Class I sample shows larger accretion luminosities (∼10⁻² − 10² L_⊙) and mass accretion rates (∼10⁻⁹ − 10⁻⁶ M_⊙ yr⁻¹) with respect to the Class II stars of the same cloud. However, the derived mass accretion rates are not sufficiently high to build up the inferred stellar masses, assuming steady accretion during the Class I lifetime. This suggests that the sources are not in their main accretion phase and that most of their mass has already been accumulated during a previous stage and/or that the accretion is an episodic phenomenon. We show that some of the targets originally classified as Class I through Spitzer photometry are in fact evolved or low accreting objects. This evidence can have implications for the estimated protostellar phase lifetimes.

Conclusions. The accretion rates of our sample are larger in more embedded and early stage YSOs. Further observations of larger samples in young star-forming regions are needed to determine if this is a general result. In addition, we highlight the importance of spectroscopic surveys of YSOs to confirm their classification and perform a more correct estimate of their lifetime.