While young stellar objects sometimes undergo bursts of accretion, these bursts usually occur sporadically, making them challenging to study observationally and to explain theoretically. We build a schematic description of cyclical bursts of the young stellar object EC 53 using near-IR and sub-mm monitoring obtained over six cycles, each lasting ⇡ 530 days. EC 53 brightens over 0.12 yr by 0.3 mag at 850 µm, 2 mag at 3.35 µm, and 1.5 mag at near-IR wavelengths, to a maximum luminosity consistent with an accretion rate of ⇠ 8 ⇥ 10 6 M yr 1 . The emission then decays with an e-folding timescale of ⇡ 0.74 yr until the accretion rate is ⇠ 1 ⇥ 10 6 M yr 1 . The next eruption then occurs, likely triggered by the buildup of ⇠ 5 ⇥ 10 6 M of mass in the inner disk, enough that it becomes unstable and drains onto the star. Just before outburst, when the disk is almost replenished, the near-IR colors become redder, indicating an increase in the geometrical height of the disk by this mass buildup. The reddening disappears soon after the initial burst, as much of the mass is drained from the disk. We quantify physical parameters related to the accretion process in EC 53 by assuming an a-disk formulation, constrained by the observed disk properties and accretion rate. While we can only speculate about the possible trigger for these faithful eruptions, we hope that our quantified schematic will motivate theorists to test the hypothesized mechanisms that could cause the cyclical buildup and draining of mass in the inner disk.

We present initial results of the first panoramic search for high-amplitude near-infrared variability in the Galactic Plane. We analyse the widely separated two-epoch K-band photometry in the 5th and 7th data releases of the UKIDSS Galactic Plane Survey. We find 45 stars with ∆K > 1 mag, including 2 previously known OH/IR stars and a Nova. Even though the mid-plane is not yet included in the dataset, we find the majority (66%) of our sample to be within known star forming regions (SFRs), with two large concentrations in the Serpens OB2 association (11 stars) and the Cygnus-X complex (12 stars). Sources in SFRs show spectral energy distributions (SEDs) that support classification as Young Stellar Objects (YSOs). This indicates that YSOs dominate the Galactic population of high amplitude infrared variable stars at low luminosities and therefore likely dominate the total high amplitude population. Spectroscopic follow up of the DR5 sample shows at least four stars with clear characteristics of eruptive pre-main-sequence variables, two of which are deeply embedded. Our results support the recent concept of eruptive variability comprising a continuum of outburst events with different timescales and luminosities, but triggered by a similar physical mechanism involving unsteady accretion. Also, we find what appears to be one of the most variable classical Be stars.

We present a catalogue of 618 high amplitude infrared variable stars (1 < ∆K < 5 mag) detected by the two widely separated epochs of 2.2 µm data in the UKIDSS Galactic plane survey, from searches covering ∼1470 deg 2 . Most were discovered by a search of all fields at 30 < l < 230 • . Sources include new dusty Mira variables, three new CV candidates, a blazar and a peculiar source that may be an interacting binary system. However, ∼60% are YSOs, based on spatial association with star forming regions at distances ranging from 300 pc to over 10 kpc. This confirms our initial result in Contreras Peña et al. (Paper I) that YSOs dominate the high amplitude infrared variable sky in the Galactic disc. It is also supported by recently published VVV results at 295 < l < 350 • . The spectral energy distributions of the YSOs indicate class I or flat spectrum systems in most cases, as in the VVV sample. A large number of variable YSOs are associated with the Cygnus X complex and other groups are associated with the North America/Pelican nebula, the Gemini OB1 molecular cloud, the Rosette complex, the Cone nebula, the W51 star forming region and the S86 and S236 HII regions. Most of the YSO variability is likely due to variable/episodic accretion on timescales of years, albeit usually less extreme than classical FUors and EXors. Luminosities at the 2010 WISE epoch range from ∼0.1 L to 10 3 L but only rarely exceed 10 2.5 L .

In the episodic accretion scenario, a large fraction of the protostellar mass accretes during repeated and large bursts of accretion. Since outbursts on protostars are typically identified at specific wavelengths, interpreting these outbursts requires converting this change in flux to a change in total luminosity. The Class I young stellar object EC 53 in the Serpens Main cloud has undergone repeated increases in brightness at 850 μm that are likely caused by bursts of accretion. In this study, we perform two- and three-dimensional continuum radiative transfer modeling to quantify the internal luminosity rise in EC 53 that corresponds to the factor of ∼1.5 enhancement in flux at 850 μm. We model the spectral energy distribution and radial intensity profile in both the quiescent and outburst phases. The internal luminosity in the outburst phase is ∼3.3 times brighter than the luminosity in the quiescent phase. The radial intensity profile analysis demonstrates that the detected submillimeter flux variation of EC 53 comes from the heated envelope by the accretion burst. We also find that the role of external heating of the EC 53 envelope by the interstellar radiation field is insignificant.

We report the discovery of a mid-infrared outburst in a Young Stellar Object (YSO) with an amplitude close to 8 mag at λ ≈ 4.6 μm. WISEA J142238.82-611553.7 is one of 23 highly variable Wide-field Infrared Survey Explorer (WISE) sources discovered in a search of Infrared Dark Clouds (IRDCs). It lies within the small IRDC G313.671-0.309 (d≈2.6 kpc), seen by the Herschel/Hi-Gal survey as a compact, massive cloud core that may have been measurably warmed by the event. Pre-outburst data from Spitzer in 2004 suggest it is a class I YSO, a view supported by observation of weak 2.12 μm H2 emission in an otherwise featureless red continuum spectrum in 2019 (6 mag below the peak in Ks). Spitzer, WISE and VISTA Variables in the Via Lactea (VVV) data show that the outburst began by 2006 and has a duration &gt;13 yr, with a fairly flat peak from 2010–2014. The low pre-outburst luminosity implies a low mass progenitor. The outburst luminosity of a few × 102 L⊙ is consistent with an accretion rate $\dot{M} \approx 10^{-4}$ M⊙yr−1, comparable to a classical FU Orionis event. The 4.6 μm peak in 2010 implies T = 800-1000 K and a disc radial location R ≈ 4.5 au for the emitting region. The colour evolution suggests subsequent progression outward. The apparent absence of the hotter matter expected in thermal instability or MRI models may be due to complete obscuration of the innermost disc, e.g. by an edge-on disc view. Alternatively, disc fragmentation/infalling fragment models might more naturally explain a mid-infrared peak, though this is not yet clear.