X-shooter spectroscopy of young stellar objects in Lupus

Abstract

The mass accretion rate,Ṁ acc , is a key quantity for the understanding of the physical processes governing the evolution of accretion discs around young low-mass (M 2.0M ) stars and substellar objects (YSOs). We present here the results of a study of the stellar and accretion properties of the (almost) complete sample of class II and transitional YSOs in the Lupus I, II, III and IV clouds, based on spectroscopic data acquired with the VLT/X-Shooter spectrograph. Our study combines the dataset from our previous work with new observations of 55 additional objects. We have investigated 92 YSO candidates in total, 11 of which have been definitely identified with giant stars unrelated to Lupus. The stellar and accretion properties of the 81 bona fide YSOs, which represent more than 90% of the whole class II and transition disc YSO population in the aforementioned Lupus clouds, have been homogeneously and self-consistently derived, allowing for an unbiased study of accretion and its relationship with stellar parameters. The accretion luminosity, L acc , increases with the stellar luminosity, L , with an overall slope of ∼1.6, similar but with a smaller scatter than in previous studies. There is a significant lack of strong accretors below L ≈0.1L , where L acc is always lower than 0.01 L . We argue that the L acc -L slope is not due to observational biases, but is a true property of the Lupus YSOs. The logṀ acclogM correlation shows a statistically significant evidence of a break, with a steeper relation for M 0.2 M and a flatter slope for higher masses. The bimodality of theṀ acc -M relation is confirmed with four different evolutionary models used to derive the stellar mass. The bimodal behaviour of the observed relationship supports the importance of modelling self-gravity in the early evolution of the more massive discs, but other processes, such as photo-evaporation and planet formation during the YSO's lifetime, may also lead to disc dispersal on different timescales depending on the stellar mass. The sample studied here more than doubles the number of YSOs with homogeneously and simultaneously determined L acc and luminosity, L line , of many permitted emission lines. Hence, we also refined the empirical relationships between L acc and L line on a more solid statistical basis.

X-shooter study of accretion in Chamaeleon I

“…When fitting the logarithmic relations between accretion and stellar parameters with a single linear fit, we derive L acc ∝L 1.9±0.1 andṀ acc ∝M 2.3±0.3 , which is consistent with previous studies (e.g., Muzerolle et al 2003;Natta et al 2006;Mohanty et al 2005;Herczeg & Hillenbrand 2008;Rigliaco et al 2011a;Natta et al 2014;Alcalá et al 2014Alcalá et al , 2017Venuti et al Article number, page 8 of 28…”

“…These four objects, together with the target 2MASS J11082570-7716396, which was not observed with XShooter, are the same five underluminous objects in the ALMA sample as were reported by Pascucci et al (2016). The fact that stars seen through an edge-on disk have L corresponding to a position on the HRD below the 30 Myr isochrone is consistent with what is found in the Lupus star-forming region by Alcalá et al ( , 2017.…”

“…Stars with higher M have generally higheṙ M acc . Similarly to the findings by Manara et al (2014, as well as others (e.g., Sicilia-Aguilar et al 2009;Fang et al 2013;Keane et al 2014;Alcalá et al 2017), transition disks are well mixed with objects with a full disk. Non-parametric analyses (see Appendix B) suggest that a break in the distribution ofṀ acc for increasing M is present, with a steeper slope for log(M /M ) −0.5 than above this value, whereṀ acc is almost constant.…”

“…This same procedure was used by MFH16, , and Alcalá et al (2014Alcalá et al ( , 2017, among others. Briefly, the observed spectrum is modeled as a sum of a photospheric template and of a slab model to reproduce the excess emission due to accretion (as in, e.g., Valenti et al 1993;Herczeg & Hillenbrand 2008), which are reddened assuming a typical extinction law with R V =3.1 (Cardelli et al 1989).…”

A search for passive protoplanetary discs in the Taurus–Auriga star-forming region

“…First of all, some of our spectra do not include the He I 5876 nor the Ca triplet, which are among the strongest lines in accreting TTS. Furthermore, even these lines remain undetected in objects whose accretion status is established by UV/blue excess emission (e.g., Alcalá et al 2017). Similarly, when the Hα line profile is relatively narrow, modest-to-weak profile asymmetries may remain undetected at our spectral resolution.…”