X-shooter study of accretion in Chamaeleon I

II. A steeper increase of accretion with stellar mass for very low-mass stars?^⋆

¹ Science Support Office, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
e-mail: cmanara@cosmos.esa.int
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
⁵ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Haidian Qu, 100871 Beijing, PR China
⁶ Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA
⁷ INAF–Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁸ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliams Place, Dublin 2, Ireland
⁹ INAF–Osservatorio Astronomico di Roma, via di Frascati 33, 00078 Monte Porzio Catone, Italy
¹⁰ Earths in Other Solar Systems Team, NASA Nexus for Exoplanet System Science, USA
¹¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹² Imperial College London, 1010 Blackett Lab, Prince Consort Rd., London SW7 2AZ, UK
¹³ INAF–Osservatorio Astornomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 28 November 2016
Accepted: 6 April 2017

Abstract

The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M_⋆) ~0.1 M_⊙ for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 ± 0.1 and 2.3 ± 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than ~0.45 L_⊙ and for stellar masses lower than ~0.3 M_⊙ is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M_⋆ ~ 0.3 − 0.4 M_⊙. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are ~10^-10M_⊙/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk.