Protostellar accretion traced with chemistry

Abstract

Context. Understanding how accretion proceeds is a key question of star formation, with important implications for both the physical and chemical evolution of young stellar objects. In particular, very little is known about the accretion variability in the earliest stages of star formation. Aims. To characterise protostellar accretion histories towards individual sources by utilising sublimation and freeze-out chemistry of CO. Methods. A sample of 24 embedded protostars are observed with the Submillimeter Array (SMA) in context of the large program "Mass Assembly of Stellar Systems and their Evolution with the SMA" (MASSES). The size of the C 18 O emitting region, where CO has sublimated into the gas-phase, is measured towards each source and compared to the expected size of the region given the current luminosity. The SMA observations also include 1.3 mm continuum data, which are used to investigate whether a link can be established between accretion bursts and massive circumstellar disks. Results. Depending on the adopted sublimation temperature of the CO ice, between 20 % and 50 % of the sources in the sample show extended C 18 O emission indicating that the gas was warm enough in the past that CO sublimated and is currently in the process of refreezing; something which we attribute to a recent accretion burst. Given the fraction of sources with extended C 18 O emission, we estimate an average interval between bursts of 20 000 yr-50 000 yr, which is consistent with previous estimates. No clear link can be established between the presence of circumstellar disks and accretion bursts, however the three closest known binaries in the sample (projected separations <20 AU) all show evidence of a past accretion burst, indicating that close binary interactions may also play a role in inducing accretion variability.

Probing Episodic Accretion in Very Low Luminosity Objects

“…The outward shift of the water snow line in IRAS 15398-3359 is further confirmed by HDO observations (Bjerkeli et al 2016). Similarly, the CO snow line can be used to probe episodic accretion Anderl et al 2016;Frimann et al 2017). The radii of several CO snow lines reported by Jørgensen et al (2015) and Frimann et al (2017) are larger than those predicted from the current luminosities.…”

“…Similarly, the CO snow line can be used to probe episodic accretion Anderl et al 2016;Frimann et al 2017). The radii of several CO snow lines reported by Jørgensen et al (2015) and Frimann et al (2017) are larger than those predicted from the current luminosities. They concluded that 20%-50% of their sample sources have experienced recent accretion bursts.…”

“…The radii of the red points correspond to R burst in Table 4. The black points and blue points are taken from Jørgensen et al (2015) and Frimann et al (2017), respectively. The radii from the literature are measured through the CO extended emission, and for comparison, the orange points indicate the radii calculated through CO observations of VeLLOs.…”

“…Figure 10 also shows the data points from Jørgensen et al (2015) and Frimann et al (2017) that plot the half-width at half-maximum (HWHM) of the C 18 O (2-1) emission as a function of the bolometric luminosity. For comparison, we plot in orange the HWHM of the CO isotopologue emission from our targets ( Table 3).…”

How Do Stars Gain Their Mass? A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions

“…Only a few outbursts have been detected on a deeply embedded Class 0 star (Kóspál et al 2007;Safron et al 2015;Hunter et al 2017), the stage when the star should accrete much of its mass-although many FUor objects retain some envelopes and are classified as Class I objects (e.g., Zhu et al 2008;Caratti o Garatti et al 2011, Caratti O Garatti et al 2016Fischer et al 2012;Green et al 2013;Kóspál et al 2017). Indirect evidence for outbursts includes chemical signatures of past epochs with high luminosity (e.g., Kim et al 2012;Vorobyov et al 2013;Jørgensen et al 2015;Frimann et al 2017) and periodic shocks/bullets along protostellar jets, which may offer a historical record of accretion events (e.g., Reipurth 1989;Raga et al 2002;Plunkett et al 2015). In addition to these large events, instabilities in the inner disk likely lead to more frequent but smaller bursts of accretion, as seen in more evolved disks (e.g., Costigan et al 2014;Venuti et al 2014;Cody et al 2017).…”