27 years of Spaceborne IR Astronomy: An ISO, Spitzer, WISE and NEOWISE Survey for Large-Amplitude Variability in Young Stellar Objects

TL;DR

We address how large-amplitude mid-infrared variability in young stellar objects traces episodic accretion across evolutionary stages by constructing 27-year $3-8\,\mu$m light curves from ISO, Spitzer, and WISE/NEOWISE. The study introduces two composite light curves and analyzes 221 YSOs with disks or envelopes, identifying 39 with $\geq 2\times$ flux changes and classifying them as completed bursts, ongoing bursts, fades, or fluctuators; it uses $\Delta t-\Delta m$ diagrams and color-magnitude plots to diagnose the underlying physics. The results reveal burst durations of $6-17$ years, fades of $7-18$ years, and a prevalent fluctuator population with multi-year and shorter timescale variability; color trends indicate accretion-driven luminosity changes rather than extinction as the primary driver for bursts/fades, with extinction playing a larger role for fluctuators. This long-baseline IR variability study implies episodic accretion is common in nearby star-forming regions and provides constraints on burst durations and their relation to disk accretion processes, informing models of stellar mass assembly. The work demonstrates how cross-instrument calibration and long temporal baselines are essential to uncover the full phenomenology of YSO variability in the mid-IR.

Abstract

Infrared observations can probe photometric variability across the full evolutionary range of young stellar objects (YSOs), from deeply embedded protostars to pre-main-sequence stars with dusty disks. We present 3-8 micron light curves extending 27 years from 1997 to 2024 obtained with three space-based IR telescopes: ISO, Spitzer and WISE. Although unevenly sampled with large gaps in coverage, these light curves show variability on time scales ranging from days to decades. We focus on the Spitzer-identified YSOs with disks and envelopes that exhibit variations of a factor of two or more in this wavelength range. We identified seven YSOs where the light curves are dominated by bursts of sustained (> 5 yr) high flux, including four that show a steep decay ending the burst and three that are ongoing as of the final observation. We find six YSOs that are undergoing declines, which may be the end of bursts that began before 1997. The most common form of variability, exhibited by 26 YSOs in our sample, show variations over time intervals of years to months but do not exhibit sustained bursts or fades. The Spitzer [3.6]-[4.5] and WISE [3.5]-[4.6] colors either increase or remain constant with increasing brightness, inconsistent with dust extinction as being the primary source of the large-amplitude variability.

Figures (60)

• Figure 1: Transmission profiles of the six filters discussed in this work. While the WISE W1 and W2 filters significantly overlap with the IRAC I1 and I2 filters, the LW2 filter used for most of the ISO data overlaps with the I3 and I4 filters, with only minor overlap with the I2 filter. Thus, we use cryogenic Spitzer observations made in the I2, I3 and I4 filters to determine a typical offset of the LW2 filter to the I2 filter (\ref{['sec:ISOCAM']}). The bandpasses are sourced from https://svo2.cab.inta-csic.es/theory/fps/2012ivoa.rept.1015R.
• Figure 2: ISOCAM - IRAC magnitude differences for each of the clouds. The red dots are YSOs classified as protostars and the green dots are those classified as pre-main sequence stars with disks. The median differences between the ISOCAM and IRAC data presented in Table \ref{['table:median']} have been subtracted so that the points have a median value of 0, as shown by the solid blue line. The values of the median absolute deviations (MADs) are given by the dotted blue lines. Sources toward the top of the plots show large increases in magnitudes between the ISO and Spitzer epochs, while points near the bottom show large decreases. In the LW2-I$_{\rm comb}$ plot (3rd panel), the error bars extends between the LW2-I3 and the LW2-I4 points. If that value is less than the MAD value for the combined plot, we set the length of each of the error bars equal to the MAD. Note that some of the data points showing large differences are not in our sample of large-amplitude variables due to the lack of the necessary Spitzer and (NEO)WISE data.
• Figure 3: Difference between the NEOWISE photometry found in the IRSA and unTimely archives, with each point representing a matched set of unTimely and IRSA photometry for a specific source and epoch. These data are collected from all of the sources examined for variability in this work.
• Figure 4: Offsets between concurrent Spitzer and unTimely photometry due to differences between the W1 and I1 bandpasses and between the W2 and I2 bandpasses as a function of the Spitzer 3.6 $\mu$m - 4.5 $\mu$m color. In total, 256 sources with ISOCAM photometry are used. Each epoch is considered independently, and distinct clusters of points are typically from a single source. In particular, the W1 and I1 magnitudes diverge for sources with large I1-I2 colors. In addition, there is significant scatter; this may be due to both the range of spectral shapes of the sources and variability between the WISE and Spitzer epochs.
• Figure 5: The 3.6 and 4.5 $\mu$m light curves of the completed bursts. We label the sources of the photometry in legend. Data that may be affected by saturation are shown with a grey color. The legends also contain the Spitzer magnitudes corresponding to $\Delta m = 0$, and the offsets added to the (NEO)WISE magnitudes to align those better to the Spitzer photometry.