Probing Episodic Accretion in Very Low Luminosity Objects

Abstract

Episodic accretion has been proposed as a solution to the long-standing luminosity problem in star formation; however, the process remains poorly understood. We present observations of line emission from N 2 H + and CO isotopologues using the Atacama Large Millimeter/submillimeter Array (ALMA) in the envelopes of eight very low luminosity objects (VeLLOs). In five of the sources the spatial distribution of emission from N 2 H + and CO isotopologues shows a clear anticorrelation. It is proposed that this is tracing the CO snow line in the envelopes: N 2 H + emission is depleted toward the center of these sources, in contrast to the CO isotopologue emission, which exhibits a peak. The positions of the CO snow lines traced by the N 2 H + emission are located at much larger radii than those calculated using the current luminosities of the central sources. This implies that these five sources have experienced a recent accretion burst because the CO snow line would have been pushed outward during the burst because of the increased luminosity of the central star. The N 2 H + and CO isotopologue emission from DCE161, one of the other three sources, is most likely tracing a transition disk at a later evolutionary stage. Excluding DCE161, five out of seven sources (i.e., ∼70%) show signatures of a recent accretion burst. This fraction is larger than that of the Class 0/I sources studied by Jørgensen et al. and Frimann et al., suggesting that the interval between accretion episodes in VeLLOs is shorter than that in Class 0/I sources.

Chronology of Episodic Accretion in Protostars—An ALMA Survey of the CO and H2O Snowlines

“…We observed the N 2 H + (1 − 0) line emission toward 36 out of the 39 targets from 2018 March to 2018 April using ALMA (Cycle 5 project, 2017.1.01693.S, PI: T. Hsieh). The N 2 H + (1−0) data for the remaining three targets are taken from an earlier ALMA project (2015.1.01576.S), and the results of which were reported in Hsieh et al (2018). With the C43-4 configuration, the resulting beam size was ∼ 2.…”

“…Figure 6 (bottom) shows the modeled curve from a disk model in comparison with the no-disk model that is used for the HCO + and 44 have been classified as sources undergoing an accretion burst and are plotted in black. We note that the peak position in Hsieh et al (2018) is measured using abundance profiles, which could result in a slightly higher value. The modeled peak radius as a function of luminosity is shown with the blue lines for an inclination angle θ inc = 45 • .…”

“…From an evolutionary point of view, Vorobyov & Basu (2015) find that the outburst should preferentially occur during the Class I stage after the disk has accreted sufficient material to fragment. However, Hsieh et al (2018) found that accretion bursts with a few to a few tens of L have occurred in Very Low Luminosity Objects (VeLLOs), which are extremely young or very-low mass protostars and thus unlikely hosts of massive disks.…”

ALMA Observations of the Protostellar Disk around the VeLLO IRAS 16253–2429

“…It is also noteworthy that IRAS 16253 has experienced a past accretion burst that temporally enhanced the protostellar luminosity and sublimated CO within a radius of ∼1250 au (Hsieh et al 2018). The CO freeze-out timescale is a function of dust temperature, T g , and gas density, n H2 , τ fr = 1 × 10 4 T g 10K 10 6 cm −3 n H2 yr (11) (Visser & Bergin 2012).…”

Evolution of Hubble wedges in episodic protostellar outflows

“…However, the observed luminosities of such protostars, and therefore by implication also their accretion rates, are typically much lower than one would expect, given their masses and formation time scales, and assuming steady accretion; this mismatch is called 'the luminosity problem' (Kenyon et al 1990(Kenyon et al , 1994Kenyon & Hartmann 1995). The luminosity problem can be mitigated if the accretion from the disc onto the protostar is episodic, with a large fraction of the accretion occuring in short intense events called outbursts (Offner & Mc-Kee 2011;Dunham & Vorobyov 2012;Cesaroni et al 2018;Hsieh et al 2018;Ibryamov et al 2018;Kuffmeier et al E-mail: rohde@ph1.uni-koeln.de 2018). Such outbursts are observed in FU Orionis type stars (FUors), which undergo a rapid increase in accretion rate, from ∼ 10 −7 M yr −1 to ∼ 10 −4 M yr −1 , for a period of order 10 years (Herbig 1966;Hartmann & Kenyon 1985;Hartmann et al 1989;Audard et al 2014;Safron et al 2015;Fehér et al 2017).…”