The JCMT Transient Survey: Detection of Submillimeter Variability in a Class I Protostar EC 53 in Serpens Main

Abstract

During the protostellar phase of stellar evolution, accretion onto the star is expected to be variable, but this suspected variability has been difficult to detect because protostars are deeply embedded. In this paper, we describe a sub-mm luminosity burst of the Class I protostar EC 53 in Serpens Main, the first variable found during our dedicated JCMT/SCUBA-2 monitoring program of eight nearby star-forming regions. EC 53 remained quiescent for the first 6 months of our survey, from February to August 2016. The sub-mm emission began to brighten in September 2016, reached a peak brightness of 1.5 times the faint state, and has been decaying slowly since February 2017. The change in submm brightness is interpreted as dust heating in the envelope, generated by a luminosity increase of the protostar of a factor of ≥ 4. The 850 µm lightcurve resembles the historical K-band lightcurve, which varies by a factor of ∼ 6 with a 543 period and is interpreted as accretion variability excited by interactions between the accretion disk and a close binary system. The predictable detections of accretion variability observed at both near-infrared and sub-mm wavelengths make the system a unique test-bed, enabling us to capture the moment of the accretion burst and to study the consequences of the outburst on the protostellar disk and envelope.

The Circumstellar Environment around the Embedded Protostar EC 53

“…However, the freeze-out timescale could be longer than a few years when the maximum grain size is larger than 1 cm (see Section 4.1), since the total surface area of grains would then be reduced by a factor of ∼300, assuming that the total grain mass is conserved, and the minimum size of grain and the power-law index of grain size distribution are 0.1 µm and -3.5, respectively. This freeze-out timescale is longer than the periodicity of 1.5 years in the accretion onto EC 53 (Yoo et al 2017). The CH 3 OH emission could trace the water snow line constructed during the burst phase of the period, when the luminosity is four times brighter (Yoo et al 2017) and the water snow line would then be two times larger, or ∼0.06 ′′ (Min et al 2011;Jørgensen et al 2015;Visser et al 2015).…”

Observational signatures of outbursting protostars - I: From hydrodynamic simulations to observations

“…An embedded FU Ori-type object is difficult to observe directly as radiation is absorbed very close to the protostar and re-emitted at longer wavelengths (e.g. OO Serpentis, Kóspál et al (2007); NGC 6334l: MM1 Hunter et al (2017); HOPS 383, Safron et al (2015); EC 53, Yoo et al (2017)). Johnstone et al (2013) suggested that variations in the far IR through (sub-)mm emission from protostars might be observed as a proxy for episodic accretion, although they found that there may be a delay in the (sub-)mm response of weeks to months after the outburst has started.…”

Observational signatures of outbursting protostars – II. Exploring a wide range of eruptive protostars

“…Furthermore, as young protostars are deeply embedded in their parent envelopes the effect of an outburst may be difficult to observe, at least at optical wavelengths. Therefore, longer wavelength, observations may be more appropriate (e.g Kóspál et al 2007;Hunter et al 2017;Safron et al 2015;Yoo et al 2017).…”