A Simultaneous Optical and X‐Ray Variability Study of the Orion Nebula Cluster. I. Incidence of Time‐correlated X‐Ray/Optical Variations

Abstract

We present a database of BV RI time-series photometry of the Orion Nebula Cluster obtained with two ground-based telescopes at different longitudes to provide simultaneous coverage with the 13-d Chandra observation of the cluster. The resulting database of simultaneous optical and X-ray light curves for some 800 pre-main-sequence (PMS) stars represents, by a factor of hundreds, the largest synoptic, multi-wavelength-regime, time-series study of young stars to date. This database will permit detailed analyses of the relationship between optical and X-ray variability among a statistically significant ensemble of PMS stars, with the goal of elucidating the origins of PMS X-ray production. In this first paper, we present the optical observations, describe the combined X-ray/optical database, and perform an analysis of time-correlated variability in the optical and X-ray light curves. We identify 40 stars (representing 5% of our study sample) with possible time-correlated optical and X-ray variability. Examples of both positive and negative time-correlations are found, possibly representing X-ray flares and persistent coronal features associated with both cool and hot surface spots (i.e. magnetically active regions and accretion shocks). We also find two possible examples of "white-light" flares coincident with X-ray flares; these may correspond to the impulsive heating phase in solar-analog flares. However, though interesting, these represent unusual cases. More generally, we find very little evidence to suggest a direct causal link between the sources of optical and X-ray variability in PMS stars. The conclusion that accretion is a primary driver of X-ray production in PMS stars is not supported by our findings.

Correlated optical and X-ray variability in CTTS

“…The observation of correlated optical and soft X-ray variability for CTTSs in NGC 2264 is at odds with the negative results reported by Grosso et al (2007), for AA Tau, and Stassun et al (2006) for a sample of Orion stars. The different quality of the optical lightcurves and different timescales probed by these studies might explain the apparent contradiction.…”

“…Their failure to detect X-ray eclipses was interpreted as evidence that the X-ray emitting plasma is at high latitudes. Stassun et al (2006Stassun et al ( , 2007 examined ground-based optical photometry of ∼800 young stars in the Orion Nebula Cluster, overlapping, for ∼1 week, with the Chandra Orion Ultradeep Project (COUP) observation of the same region. They found "very little evidence to suggest a direct causal link between the sources of optical and X-ray variability in PMS stars".…”

ChandraX-ray observation of the H ii region Gum 31 in the Carina nebula complex

“…Three of these sources show flare-like variability, i.e., a fast increase in the count rate followed by a slow exponential decay, as typical for solar-like magnetic reconnection flares (see, e.g., Wolk et al 2005). The other variable sources show more slowly increasing or decreasing count rates, as also often found for young stellar objects (see, e.g., Stassun et al 2006). …”

Long-term stellar variability

“…Unambiguous detections of rotational modulation have been reported by Güdel (1995) for the young solar analogue EK Dra, by Marino et al (2003) for VXR45, a G9 V star in IC 2391 (cf. Figure 4), by Hussain et al (2005) for the K0 V AB Dor, and by Stassun et al (2006) for a number of young stars in the Orion Nebula Cluster. AB Dor as well as many of the stars in ONC showing rotational modulation are fast rotators with an X-ray emission in the saturation regime.…”