The JCMT Transient Survey: Stochastic and Secular Variability of Protostars and Disks In the Submillimeter Region Observed over 18 Months

Abstract

We analyze results from the first eighteen months of monthly sub-mm monitoring of eight star-forming regions in the JCMT Transient Survey. In our search for stochastic variability in 1643 bright peaks, only the previously identified source, EC 53, shows behavior well above the expected measurement uncertainty. Another four sources, two disks and two protostars, show moderately-enhanced standard deviations in brightness, as expected for stochastic variables. For the two protostars, this apparent variability is the result of single epochs that are much brighter than the mean. In our search for secular brightness variations that are linear in time, we measure the fractional brightness change per year for 150 bright peaks, fifty of which are protostellar. The ensemble distribution of slopes is well fit by a normal distribution with σ ∼ 0.023. Most sources are not rapidly brightening or fading in the sub-mm. Comparison against time-randomized realizations shows that the width of the distribution is dominated by the uncertainty in the individual brightness measurements of the sources. A toy model for secular variability reveals that an underlying Gaussian distribution of linear fractional brightness change σ = 0.005 would be unobservable in the present sample, whereas an underlying distribution with σ = 0.02 is ruled out. Five protostellar sources, 10% of the protostellar sample, are found to have robust secular measures deviating from a constant flux. The sensitivity to secular brightness variations will improve significantly with a larger time sample, with a factor of two improvement expected by the conclusion of our 36-month survey.

Submillimeter Continuum Variability in Planck Galactic Cold Clumps

“…When accretion variability is detected, therefore, the flux can be diluted if the event originates in a single protostar as the beam contains many protostars. Nevertheless, as shown in previous studies by Mairs et al (2017b) and Johnstone et al (2018), we expect to uncover flux variability at about the 10%-level or larger.…”

“…Simulations (e.g. Bae et al 2014;Vorobyov & Basu 2015) as well as JCMT Transient Survey observations (Mairs et al 2017a;Johnstone et al 2018), however, suggest that less dramatic flux variations are more common. In order to increase detection reliability, it is advantageous to calibrate the flux in a relative sense using the method presented by Mairs et al (2017b).…”

“…From these relative flux calibrated images, we remeasured the peak fluxes in each epoch and compared the standard deviation, σ std,meas (i), of the clump fluxes with a fiducial standard deviation model, σ std,fid (i). The fiducial standard deviation model characterizes the uncertainty in a relative flux calibrated image based on the RMS noise (σ rms (i)) and the relative flux calibration uncertainty itself (u cal ; see Johnstone et al 2018 for further details). σ std,fid (i) is calculated as follows:…”

Probing the cold magnetised Universe with SPICA-POL (B-BOP)

“…Recently, significant variability has been detected in the submillimeter continuum emission of several nearby protostars (see, e.g., Mairs et al, 2017b;Yoo et al, 2017;Johnstone et al, 2018a), through an ongoing multi-year monitoring survey of eight nearby star-forming regions with JCMT at 850 µm (Herczeg et al, 2017). While uncertain due to small number statistics, it appears that roughly 10% of deeply embedded protostars vary over year timescales by around 10% at submillimeter wavelengths (Johnstone et al, 2018a).…”

“…Recently, significant variability has been detected in the submillimeter continuum emission of several nearby protostars (see, e.g., Mairs et al, 2017b;Yoo et al, 2017;Johnstone et al, 2018a), through an ongoing multi-year monitoring survey of eight nearby star-forming regions with JCMT at 850 µm (Herczeg et al, 2017). While uncertain due to small number statistics, it appears that roughly 10% of deeply embedded protostars vary over year timescales by around 10% at submillimeter wavelengths (Johnstone et al, 2018a). The dominant mode of variability uncovered by the survey is quasi-secular, with the protostellar brightness increasing or decreasing for extended -multi-year -periods (Mairs et al, 2017b;Johnstone et al, 2018a) and suggesting a link to non-steady accretion processes taking place within the circumstellar disk where the orbital timescales match those of the observed variability.…”