YSO Variability in the W51 Star-Forming Region

TL;DR

We analyze a decade-long NEOWISE time series to quantify mid-infrared variability of YSOs in the distant W51 region, focusing on intermediate-to-high-mass objects due to distance and extinction biases. By classifying variability into secular and stochastic types in the W1 and W2 bands and separating Protostar, Disk, and PMS+E classes, we show that protostars exhibit higher-amplitude, stochastic fluctuations while disks display more varied secular patterns, coupled with distinct color-magnitude behaviors. The CMD analysis indicates protostars redden as they brighten, whereas disks often become bluer or show mixed trends, reflecting different dust emission and extinction processes. Compared with nearby regions, W51 displays higher variability fractions and amplitudes, highlighting the influence of environment and mass on YSO variability and offering constraints on accretion and disk evolution in massive star-forming environments.

Abstract

Time-domain studies of mid-infrared and submillimeter variability have shown that at least half of protostars are variable. We present a statistical analysis of mid-infrared variability among young stellar objects (YSOs) in the distant, massive star-forming region W51 using NEOWISE data. From a catalog of 81 protostars, 527 disk objects, and 37,687 other sources including diskless pre-main sequence and evolved contaminants, we identified significant variability in the 3.4 um (W1) and 4.6 um (W2) bands. Because of W51's distance (~5.4 kpc) and extinction, the sample mainly includes intermediate- to high-mass YSOs (>2 Msun), unlike nearby regions dominated by low-mass stars. This mass bias may affect the observed variability. In W2, 11.1% of protostars, 7.6% of disk objects, and 0.6% of PMS+E sources showed secular variability, while 8.6%, 2.3%, and 0.5% showed stochastic variability; similar fractions were found in W1. The variability fraction and amplitude increase toward earlier stages. Protostars exhibit high-amplitude stochastic changes likely driven by dynamic accretion and extinction, whereas disk objects show more secular patterns-linear, curved, or periodic-possibly due to moderate accretion variations or disk geometry. Color-magnitude analysis shows that protostars generally redden as they brighten, consistent with enhanced dust emission or variable extinction, while disk objects show mixed trends: roughly balanced in W1 but more often bluer in W2, suggesting reduced extinction or hotspot modulation. These results highlight distinct mechanisms of variability across evolutionary stages and demonstrate that mid-infrared monitoring offers key insight into accretion and disk evolution in young stars.

Figures (9)

• Figure 1: RGB image of the W51 made from WISE W1 (3.4 $\mu$m) in blue, W2 (4.6 $\mu$m) in green, and W4 (22 $\mu$m) in red.
• Figure 2: AllWISE color-magnitude diagram. Colored circle symbols indicate the evolutionary stages. Black dots represent sources rejected as contaminants based on the criteria of Koenig.2014, including AGB stars, AGNs, and galaxies. The dashed lines indicate the threshold above which sources are excluded as probable AGB stars.
• Figure 3: (a) Histograms of the W1 (blue) and W2 (green) mean magnitudes for protostars (left), disk objects (middle), and PMS+E sources (right). (b) Histograms of the W1–W2 color for the same evolutionary classes.
• Figure 4: The flux change ($\Delta$W1 and $\Delta$W2) between the maximum and minimum phases as a function of stochasticity (SD/$\sigma$) for all YSOs classified as Protostars, Disk objects, and PMS+E sources. SD represents the standard deviation of fluxes from the light curve, and $\sigma$ denotes the mean flux uncertainty. The vertical dashed line at SD/$\sigma$=3 marks the commonly used threshold for identifying significant variability. Top and right panels show the normalized distributions of SD/$\sigma$ and $\Delta$W, respectively, for each evolutionary stage.
• Figure 5: The distribution of linear ($\rm FAP_{Lin}$) and periodic ($\rm FAP_{LSP}$) FAPs for our sample in the W1 (left) and W2 (right). The vertical dashed lines at $\rm FAP_{Lin}$ = $10^{-4}$ separate sources with linear variability, while the horizontal dashed lines at $\rm FAP_{LSP}$ = $10^{-2}$ indicate sources with periodic variability. Small filled symbols represent all sources colored by evolutionary class, while large open markers indicate variability types (linear, curved, periodic, burst, drop, irregular), shown only for classified variable sources among protostars and disks. PMS+E sources, which vastly outnumber the other classes, are plotted in yellow regardless of variability status.