CSI 2264: Simultaneous optical and infrared light curves of young disk-bearing stars in NGC 2264 with CoRoT and Spitzer-- evidence for multiple origins of variability

Abstract: We present the Coordinated Synoptic Investigation of NGC 2264, a continuous 30-day multi-wavelength photometric monitoring campaign on more than 1000 young cluster members using 16 telescopes. The unprecedented combination of multi-wavelength, high-precision, high-cadence, and long-duration data opens a new window into the time domain behavior of young stellar objects. Here we provide an overview of the observations, focusing on results from Spitzer and CoRoT. The highlight of this work is detailed analysis of 162 classical T Tauri stars for which we can probe optical and mid-infrared flux variations to 1% amplitudes and sub-hour timescales. We present a morphological variability census and then use metrics of periodicity, stochasticity, and symmetry to statistically separate the light curves into seven distinct classes, which we suggest represent different physical processes and geometric effects. We provide distributions of the characteristic timescales and amplitudes, and assess the fractional representation within each class. The largest category (>20%) are optical "dippers" having discrete fading events lasting ~1-5 days. The degree of correlation between the optical and infrared light curves is positive but weak; notably, the independently assigned optical and infrared morphology classes tend to be different for the same object. Assessment of flux variation behavior with respect to (circum)stellar properties reveals correlations of variability parameters with H$\alpha$ emission and with effective temperature. Overall, our results point to multiple origins of young star variability, including circumstellar obscuration events, hot spots on the star and/or disk, accretion bursts, and rapid structural changes in the inner disk.

• The paper demonstrates that simultaneous optical and infrared observations reveal multifaceted variability in disk-bearing young stars.
• Using a 30-day continuous photometric campaign, the study classifies variability into seven distinct morphology classes with sub-hour precision.
• The findings underscore a weak optical-infrared correlation, suggesting different physical mechanisms operate at various wavelengths.

Analysis of Photometric Variability in Young Disk-Bearing Stars in NGC 2264

The research presented in the paper examines the photometric variability of young stellar objects (YSOs) within the NGC 2264 star-forming region using simultaneous optical and infrared observations from the CoRoT and Spitzer space telescopes. The focus of the study is on stars with circumstellar disks, specifically classical T Tauri stars (CTTSs), which are known for their complex variability patterns due to interactions with their surrounding environments.

Observations and Methodology

The study utilizes a comprehensive dataset acquired during a 30-day continuous photometric campaign, which monitored over 1000 young cluster members across multiple wavelengths. The observations achieved high precision and cadence, enabling the resolution of light curve variations on sub-hour timescales with amplitudes as small as 1%.

The authors employ both visual and statistical approaches to classify the variability observed. They identified seven distinct light curve morphology classes: periodic, aperiodic, quasi-periodic symmetric, quasi-periodic dipper, aperiodic dipper, burster, and long timescale variants. These classes are characterized based on the parameters of periodicity, stochasticity, and symmetry.

Key Findings

1. Prevalence of Variability Mechanisms: The study finds a range of variability mechanisms influencing the YSOs, including circumstellar obscuration, hot spots, accretion bursts, and dynamical changes in the inner disk. The prevalence of these mechanisms suggests complex interactions between the stars and their disks.
2. Optical vs. Infrared Correlation: The correlation between optical and infrared variability is generally positive but weak, indicating that different mechanisms may dominate at different wavelengths. Notably, the same YSO often exhibits divergent variability classifications in the optical and infrared.
3. Classification and Statistics: The largest group identified comprises "dippers," which experience discrete fading events likely attributed to circumstellar disk material. The distribution and characteristics of variability types provide insights into the potential geometries and physical processes at play in these systems.

Implications

This comprehensive analysis of variability among disk-bearing stars in NGC 2264 contributes significantly to our understanding of YSO behavior. The variability patterns observed offer clues to the underlying physical mechanisms, such as accretion processes, disk instabilities, and magnetic field interactions. These findings can inform theoretical models of star-disk interactions and the accretion processes that occur in young stellar systems.

Future Directions

The study's results highlight the need for extended multi-wavelength and spectroscopic observations to further disentangle the contributions of different physical processes to the observed variability. Enhanced characterization of YSOs, including detailed analysis of morphology classes, can improve models of young star evolution and the role of circumstellar disks.

The ongoing development of comprehensive, simultaneous observation campaigns will be crucial for advancing our understanding of the variability mechanisms in YSOs. Future studies might build on this work by exploring the evolutionary implications of variability on star formation and disk dissipation timescales.