YOUNG STELLAR OBJECT VARIABILITY (YSOVAR): LONG TIMESCALE VARIATIONS IN THE MID-INFRARED

Abstract

The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope\nobserving program obtained the first extensive mid-infrared (3.6 & 4.5 um)\ntime-series photometry of the Orion Nebula Cluster plus smaller footprints in\neleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264,\nL1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C).\nThere are ~29,000 unique objects with light curves in either or both IRAC\nchannels in the YSOVAR data set. We present the data collection and reduction\nfor the Spitzer and ancillary data, and define the "standard sample" on which\nwe calculate statistics, consisting of fast cadence data, with epochs about\ntwice per day for ~40d. We also define a "standard sample of members",\nconsisting of all the IR-selected members and X-ray selected members. We\ncharacterize the standard sample in terms of other properties, such as spectral\nenergy distribution shape. We use three mechanisms to identify variables in the\nfast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and\nsignificant periodicity. We also identified variables on the longest timescales\npossible of ~6 years, by comparing measurements taken early in the Spitzer\nmission with the mean from our YSOVAR campaign. The fraction of members in each\ncluster that are variable on these longest timescales is a function of the\nratio of Class I/total members in each cluster, such that clusters with a\nhigher fraction of Class I objects also have a higher fraction of long-term\nvariables. For objects with a YSOVAR-determined period and a [3.6]-[8] color,\nwe find that a star with a longer period is more likely than those with shorter\nperiods to have an IR excess. We do not find any evidence for variability that\ncauses [3.6]-[4.5] excesses to appear or vanish within our data; out of members\nand field objects combined, at most 0.02% may have transient IR excesses.\n