Quantifying Variability of Young Stellar Objects in the Mid-infrared Over 6 Years with the Near-Earth Object Wide-field Infrared Survey Explorer

Abstract

Variability in young stellar objects (YSOs) can be caused by various time-dependent phenomena associated with star formation, including accretion rates, geometric changes in the circumstellar disks, stochastic hydromagnetic interactions between stellar surfaces and inner-disk edges, reconnections within the stellar magnetosphere, and hot/cold spots on stellar surfaces. We uncover and characterize ∼1700 variables from a sample of ∼5400 YSOs in nearby low-mass star-forming regions using mid-IR light curves obtained from the 6.5 yr Near-Earth Object Wide-field Infrared Survey Explorer All Sky Survey. The mid-IR variability traces a wide range of dynamical, physical, and geometrical phenomenon. We classify six types of YSO mid-IR variability based on their light curves: secular variability (linear, curved, and periodic) and stochastic variability (burst, drop, and irregular). YSOs in earlier evolutionary stages have higher fractions of variables and higher amplitudes for the variability, with the recurrence timescale of FUor-type outbursts (defined here as ΔW1 or ΔW2 > 1 mag followed by inspection of candidates) of ∼1000 yr in the early embedded protostellar phase. Known eruptive young stars and subluminous objects show fractions of variables similar to the fraction (∼55%) found in typical protostars, suggesting that these two distinct types are not distinct in variability over the 6.5 yr timescale. Along with brightness variability, we also find a diverse range of secular color variations, which can be attributed to a competitive interplay between the variable accretion luminosity of the central source and the variable extinction by material associated with the accretion process.

Dissecting the Different Components of the Modest Accretion Bursts of the Very Young Protostar HOPS 373

“…Searches for variability in the K band (e.g., VVV Survey; Contreras Lucas et al 2017) or in W2 (Park et al 2021) may miss variability in sources like HOPS 373. Many spectra of outbursts identified in the VVV survey show strong H 2 emission (Guo et al 2020), although none are nearly as extreme as HOPS 373 because they would not have been identified as candidate outbursts.…”

“…Over the past few years, several surveys at longer wavelengths have been designed to statistically evaluate accretion variability at earlier stages of protostellar evolution (e.g., Scholz et al 2013;Antoniucci et al 2014;Rebull et al 2014;Lucas et al 2017;Johnstone et al 2018;Fischer et al 2019;Lee et al 2021;Park et al 2021;Zakri et al 2022), when the star is still in its main growth phase and the disk is accreting from the envelope. For protostars, submillimeter (submm) emission is produced by warm dust in the envelope (and disk), serving as a bolometer that can be converted into a luminosity (e.g., Johnstone et al 2013;MacFarlane et al 2019aMacFarlane et al , 2019bBaek et al 2020).…”

“…The NGC 2068 star-forming region, including HOPS 373, has been monitored in the submm by the James Clerk Maxwell Telescope (JCMT) Transient Survey since 2015 December (Herczeg et al 2017). Beginning in 2016, HOPS 373 had gotten fainter by 10% (Johnstone et al 2018) and stayed steady in this lower luminosity state until a brief increase of 25% in late 2019 (Lee et al 2021), a pattern also seen at 4.5 μm with NEOWISE (Contreras Peña et al 2020;Park et al 2021). We report that HOPS 373 has since returned to the bright state in the submm, and analyze in detail the location of emission across the spectrum.…”

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): Deriving Inclination Angle and Velocity of the Protostellar Jets from Their SiO Knots

“…It is therefore useful to compare these jet knot results against long-term, many-year monitoring observations of the mid-IR and submillimeter continuum emission from similar young stellar objects across the Gould Belt. Park et al (2021) analyzed 7 yr mid-IR light curves of 735 protostars, classifying 140 (20%) as having dominant brightness variation timescales longer than three years. Similarly, Lee et al (2021) classified 4 yr submillimeter light curves of 43 protostars and recovered 15 (35%) with dominant periods longer than three years.…”

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): Deriving Inclination Angle and Velocity of the Protostellar Jets from their SiO Knots

“…Park et al (2021) analyzed 7-year mid-IR light curves of 735 protostars, classifying 140 (20%) as having dominant brightness variation timescales longer than three years. Similarly,Lee et al (2021) classified 4-year sub-mm light curves of 43 protostars, and recovered 15 (35%) with dominant periods longer than three years.…”