Young Stellar Object Variability (Ysovar): Long Timescale Variations in the Mid-Infrared

Abstract

The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope observing program obtained the first extensive mid-infrared (3.6 and 4.5 µm) time-series photometry of the Orion Nebula Cluster plus smaller footprints in eleven other star-forming cores (AFGL 490, NGC 1333, (CCDs) revolutionized our ability to discern precisely optical variability trends and 2MASS did the same for near-infrared variability, the Spitzer Space Telescope (Werner et al. 2004) has allowed us to probe even small variations in the mid-infrared; Spitzer is a photometrically stable (better than 1%), sensitive, wide-field (5 ×5 ), Earth-trailing (avoiding orbital day/night aliasing) platform. Spitzer, specifically the Infrared Array Camera (IRAC; Fazio et al. 2004), observes at bands sensitive to both YSO photospheres and circumstellar dust.Cycle 6 was the first post-cryogen Spitzer cycle, using just IRAC's first two channels (3.6 and 4.5 µm, often abbreviated IRAC-1 and IRAC-2, or I1 and I2; when reporting measurements in magnitudes, the bands are written with brackets, e.g., [3.6]=16.38 mag). The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope Cycle-6 Exploration Science (ES) Program was approved for 550 hours of observations, with the goal of obtaining the first extensive mid-infrared time-series photometry of the central ∼ 1 • of the Orion Nebula Cluster plus smaller footprints in eleven other star-forming cores; see Table 1 for a list of the clusters. There are several other ES and smaller programs exploring YSOs in the time domain with Spitzer, many of which are affiliated (to varying degrees) with the larger YSOVAR effort. We have incorporated under the YSOVAR umbrella some additional strongly related programs pre-dating and arising from YSOVAR, resulting in a total of 786 hours of Spitzer time. About 130 hours of that is dedicated observations of NGC 2264 (Coordinated Synoptic Investigation: NGC 2264, or CSI 2264), which is discussed by Cody et al. (2014), among other papers (e.g., Stauffer et al. 2014Stauffer et al. , 2015. CSI 2264 is not discussed in the same way as the rest of the data here, in no small part because the observations are generally different and because it involves coordination of more telescopes. A list of core YSOVAR programs and affiliated programs is presented in Table 2 and discussed in Section 2.2. We sometimes refer to the components of the original YSOVAR program as 'YSOVAR-classic' to distinguish them from the smaller affiliated observations obtained over the same time period. There are ∼29,000 unique objects with light curves from either (or both) of the IRAC channels in the YSOVAR data set, matched to ∼39,000 individual light curves. These light curve counts include light curves from both cluster members and a significant number of non-member stars, and also likely include extragalactic objects. There are more light curves than objects because most objects have light curves at just I1 or I2, but many have light curves at both I1 and I2.d Ratio of Class II to Class I sources, usi...

Chemical tracers of episodic accretion in low-mass protostars

“…The spacings between periodic shocks in jets and outflows support quiescent intervals anywhere from 10 4 yr down to 10 yr, separating bursts with an unknown range of accretion rates (Devine et al 1997;Raga et al 2002;Arce et al 2013). Highcadence photometric surveys show luminosity variations at the 5-50% level on timescales of hours to weeks (Billot et al 2012;Stauffer et al 2014), though only some of that may be related to accretion variability (Morales-Calderón et al 2011;Rebull et al 2014). Despite the incomplete statistics, it appears that stronger accretion bursts occur less frequently than weaker ones.…”

Spitzer Observations of Long-Term Infrared Variability Among Young Stellar Objects in Chamaeleon I

“…Recent surveys have found that the vast majority of YSOs are variable in the mid-infrared on timescales of days to weeks Rebull et al 2014), expanding on earlier ground-based near-infrared studies (e.g., Carpenter et al 2001). These surveys have also revealed additional clues as to the structure of the inner disk.…”

The Bochum Survey of the Southern Galactic Disk: II. Follow‐up measurements and multi‐filter photometry for 1323 square degrees monitored in 2010 – 2015

“…The For evenly sampled data other techniques like the autocorrelation of the light curves may be used to identify variable sources (e.g., Rebull et al 2014). However, our light curves are sampled fairly inhomogeneously and any kind of interpolation of gaps will introduce additional uncertainties; therefore we did not apply the autocorrelation technique here.…”