Researchers have combined 27 years of infrared data to investigate how young stars change in brightness. Chinmay S Kulkarni, Thomas Behling, and Elisabeth E Banks, all from the Ritter Astrophysical Research Center at the University of Toledo, alongside Jason Jones, Tyler Robbins, and Nathanael Burns-Watson et al, analysed observations from the ISO, Spitzer and WISE space telescopes to identify large-amplitude variability in young stellar objects. This extensive study reveals previously unseen patterns of stellar bursts and fades, suggesting that internal processes, rather than dust extinction, are the primary drivers of these dramatic changes in luminosity. Understanding these variations is crucial for refining models of star and planet formation, offering new insights into the early lives of stars and the environments in which planets are born.
Despite gaps in coverage, these light curves demonstrate variability occurring over timescales ranging from days to decades, providing a unique long-term view of stellar activity. Furthermore, six YSOs were found to be undergoing declines, potentially representing the concluding stages of bursts that commenced before 1997. The most prevalent type of variability, observed in 26 YSOs, involved fluctuations over periods of months to years, without the presence of sustained bursts or fades. This comprehensive dataset allows for a detailed examination of episodic accretion, a process where stars experience periods of rapid mass gain, potentially dominating the overall star formation process. The research suggests that large-amplitude variability may be particularly important for star formation, potentially driven by changes in accretion rates where luminosity is reprocessed into infrared radiation. By directly measuring time-varying accretion, the study aims to constrain the frequency, amplitudes, and durations of these accretion bursts.
Experiments show that current estimates of burst frequency range from one event per 1000 years for protostars to one per over 10,000 years for more evolved pre-main sequence stars, though comparisons are complicated by differing criteria and data. The work opens new avenues for understanding the contribution of episodic accretion to the total mass of stars, a parameter that remains poorly constrained. This work identified seven YSOs displaying bursts of sustained high flux lasting over five years, with four showing steep decay at the burst’s end and three remaining active at the time of the final observation. Initial source detection within the ISOCAM images employed DAOStarFinder, a routine adapted from DAOFIND, modified to calculate local background noise within an 11 × 11 pixel square, searching for sources exceeding 3-5σ above the noise. Following automated detection, the team implemented a rigorous visual inspection process, removing artifacts, unresolved multiple stars, and sources near image edges or bright nebulosity. To refine astrometry, the study employed 2D quadratic polynomial fitting to the point spread functions using the centroid quadratic function within the Python PhotUtils package, utilizing fit and search box sizes of 5 and 9 pixels, respectively.
Aperture photometry was then performed with a 3-pixel radius circular aperture, with background subtraction calculated using the MMM statistic in an annulus extending from 4 to 10 pixels. The resulting ISO fluxes were converted to Vega magnitudes using a zero-point of 90.2 Jy/pixel. This meticulous approach enabled the identification of six YSOs undergoing declines, potentially representing the end of pre-1997 bursts, and 26 YSOs exhibiting variations over years to months without sustained bursts or fades, providing a detailed picture of variability timescales within young stellar populations.
Luminosity Variations Indicate Protostellar Accretion Dynamics and ongoing mass growth
Young Stars Reveal Dramatic Fluctuations in Brightness Recent observations utilizing infrared data have revealed a fascinating diversity in the behavior of young stellar objects within the star-forming regions of NGC 2264 and Ophiuchus. Researchers have identified a significant number of protostars exhibiting substantial variations in their luminosity over time, providing crucial insights into the early stages of star formation and planet formation processes. Detailed analysis of these objects demonstrates that many undergo dramatic bursts of energy, while others display more gradual fluctuations in brightness. These changes are likely driven by episodic accretion events, where material from the surrounding circumstellar disk falls onto the growing star, releasing gravitational potential energy as light. These bursts are particularly prominent in the Orion Nebula and the Ophiuchus star-forming region, suggesting that episodic accretion is a common phenomenon during the earliest phases of stellar evolution. The data indicates that these bursts are not random occurrences, but rather represent a fundamental aspect of how young stars gain mass and evolve. Furthermore, the observed burst durations and amplitudes provide constraints on the properties of the circumstellar disks and the accretion processes themselves. ## Diverse Variability Patterns Indicate Complex Disk-Star Interactions Beyond the dramatic bursts, the research also uncovered a population of young stars exhibiting more subtle, yet significant, fluctuations in brightness.
These stars show variations that are less extreme but still indicative of ongoing interactions between the star and its surrounding disk. The observed variability patterns are diverse, ranging from quasi-periodic oscillations to irregular, aperiodic changes. This diversity suggests that the disk-star interaction is a complex process, influenced by factors such as disk mass, viscosity, and the presence of companions. Classifying these stars into distinct variability types, burst, fluctuating, and relatively stable, allows for a more nuanced understanding of the different evolutionary pathways available to young stars. The bursts of energy can heat the disk, triggering chemical reactions and affecting the distribution of dust and gas. Future research will focus on combining these photometric observations with high-resolution imaging and spectroscopic data to characterize the physical properties of the circumstellar disks and to unravel the underlying mechanisms driving the observed variability. The research focused on YSOs exhibiting variations of at least twofold in brightness within the 3-8 micron range, revealing diverse patterns of photometric variability. Seven YSOs displayed bursts of sustained high flux lasting over five years, with some showing steep decays, while six others demonstrated ongoing declines potentially representing the end of earlier bursts. Furthermore, the study identified 26 YSOs exhibiting fluctuating brightness levels over periods ranging from months to years, without sustained bursts or fades.
Analysis of colour changes indicated that dust extinction is unlikely to be the primary driver of these large-amplitude variations, suggesting changes in mass accretion rates are more influential. The observed outburst durations ranged from at least six to over 20 years, with most lasting between 10 and 20 years, and fluctuators showed a broader range of behaviours and slopes than bursting or fading YSOs. The authors acknowledge that the uneven sampling of the light curves introduces limitations to the analysis, potentially affecting the precise determination of outburst durations and variability timescales. Future research could benefit from more densely sampled, long-term infrared monitoring to better characterise these phenomena and refine our understanding of the physical processes driving variability in young stars. These findings contribute to a more nuanced understanding of accretion processes and the diverse behaviours exhibited by YSOs, moving beyond simplified models of quiescent accretion punctuated by occasional outbursts.
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🗞 27 years of Spaceborne IR Astronomy: An ISO, Spitzer, WISE and NEOWISE Survey for Large-Amplitude Variability in Young Stellar Objects
🧠ArXiv: https://arxiv.org/abs/2601.21013