HST spectra reveal accretion in MY Lupi

Abstract

The mass accretion rate is a crucial parameter for the study of the evolution of accretion discs around young lowmass stellar and substellar objects (YSOs). We revisit the case of MY Lup, an object where VLT/X-Shooter data suggested a negligible mass accretion rate, and show it to be accreting on a level similar to other Class II YSOs in Lupus based on Hubble Space Telescope (HST) observations. In our HST-Cosmic Origins Spectrograph (HST-COS) and -Space Telescope Imaging Spectrograph (HST-STIS) spectra, we find many emission lines, as well as substantial far-ultraviolet (FUV) continuum excess emission, which can be ascribed to active accretion. The total luminosity of the C iv λ1549Å doublet is 4.1×10 −4 L . Using scalings between accretion luminosity, Lacc, and C iv luminosity from the literature, we derive Lacc∼2×10 −1 L , which is more than an order of magnitude higher than the upper limit estimated from the X-Shooter observations. We discuss possible reasons for the X-Shooter-HST discrepancy, the most plausible being that the low contrast between the continuum excess emission and the photospheric+chromospheric emission at optical wavelengths in MY Lup hampered detection of excess emission. The luminosity of the FUV continuum and C iv lines, strong H2 fluorescence, and a "1600 A Bump" place MY Lup in the class of accreting objects with gas-rich discs. So far, MY Lup is the only peculiar case in which a significant difference between the HST and X-ShooterṀacc estimates exists that is not ascribable to variability. The mass accretion rate inferred from the revisited Lacc estimate isṀacc ∼ 1( +1.5 −0.5 )×10 −8 M yr −1 . This value is consistent with the typical value derived for accreting YSOs of similar mass in Lupus and points to less clearing of the inner disc than indicated by near-and mid-infrared observations. This is confirmed by Atacama Large Millimeter Array (ALMA) data, which show that the gaps and rings seen in the sub-millimetre are relatively shallow.

A dusty origin for the correlation between protoplanetary disc accretion rates and dust masses

“…Some sources (plotted as downward pointing triangles) in the right mass range are classed as potentially non-accreting -as they have accretion luminosity consistent with chromospheric noise (Alcalá et al 2017;Manara et al 2020)) -and hence their measurements could be considered upper limits. However at least one of these, MY Lup, appears to be more strongly accreting when line luminosities rather than continuum luminosities are used (Alcalá et al 2019). It is likewise possible that some of the discs that we consider too massive to agree with the basic dust evolution models could also be explained as initially large discs shortly before their dispersal by photoevaporation.…”

“…One such process, the accretion of the material onto the central star, has been detected in most of these protoplanetary discs by measuring either the UV continuum luminosity excess (Alcalá et al 2014(Alcalá et al , 2017, or line luminosities from Hα (Fedele et al 2010) or CIV (Alcalá et al 2019). This not only shows that protoplanetary discs act as accretion discs (a suggestion that goes back to Lynden-Bell & Pringle 1974), but the measurements have been used for many years to provide important insights into the evolution of the circumstellar environment, for example constraining the initial E-mail: ads79@cam.ac.uk conditions of disc formation (Dullemond et al 2006;Alexander & Armitage 2006) or the rates of disc dispersal (Ercolano et al 2014).…”

ISO-ChaI 52: a weakly accreting young stellar object with a dipper light curve

“…NUV flux excess was observed in older M-type stars and was ascribed to stellar magnetic activity (e.g., Stelzer et al 2013), but it might be also indicative of mild accretion onto the central star. If this were the case, ISO-ChaI 52 would be somewhat similar to MY Lup, for which accretion is clearly displayed only by UV line and continuum emission revealed by the Hubble Space Telescope (HST; Alcalá et al 2019).…”

“…A further category of objects has recently emerged with apparently very little or no evidence for accretion in optical (λ > 3400 Å) spectra, but with the near-IR (NIR) emission characteristic of optically thick dust in the inner (few AU) regions of the disk, so that their spectral energy distribution resembles that of class II sources (e.g., Wahhaj et al 2010;Alcalá et al 2019;Thanathibodee et al 2019). The existence of such objects might be explained by slightly different timescales for the decline of disk and accretion processes in young stars (e.g., Fedele et al 2010).…”

“…Weak accretion is in general difficult to detect in the region of the Balmer jump. Other diagnostics such as modeling of the Hα line profile (e.g., Espaillat et al 2008;Thanathibodee et al 2019) and/or measures of excess emission in near-UV or far-UV spectra (e.g., Alcalá et al 2019, and references therein) are therefore necessary in these cases of very low accretion rates.…”