On the 3–5 μm variability of young variables in Vela-D through Spitzer-WISE observations

Flux variability is a common feature of Young Stellar Objects (YSOs), which is often related to intermittent events of disk accretion (EXors events in case of 3–4 magnitudes variations). Recently, thanks to the surveys carried out by the space missions Spitzer and WISE, it has become possible to perform statistical studies on the mid-IR variability on large samples of YSOs. As a follow-up of our recent statistical study on five star forming regions (Antoniucci et al., Astrophys. J. 782:51, 2014), we present the 3–5 µm variability study of the YSOs population of the Vela-D star forming region. We have compared the 3.6 µm and 4.5 µm Spitzer-IRAC fluxes of 181 YSOs in Vela-D with their WISE fluxes at 3.4 µm and 4.6 µm and selected those objects simultaneously varying in both bands. We have iden- tified a robust sample of 34 variables. On the base of the in- frared excess of the Spectral Energy Distribution (SED) and the magnitude vs. color variations, we select 5 EXors can- didates, which will be systematically monitored to firmly ascertain their nature. The selected 34 variables represent ∼18 % of the YSOs detected with Spitzer and WISE, a per- centage higher than that of other young star forming regions. Conversely, the percentage of candidate EXors (2.7 %) is quite similar to that measured in Perseus, Ophiuchus and Serpens, and also equals that found in Vela-D on the base of Spitzer variability (Giannini et al., Astrophys. J. 704:606, 2009). Consistently with our finding presented in Antoni- ucci et al. (2014), this fraction equals the probability of ob- serving the source once in burst and once in quiescence, un- der the hypothesis that the time elapsed between the two events is of about 0.5–1 year. Of the 5 selected EXors candi- dates, 3 are Class I sources, and 2 are flat-spectrum sources, a circumstance that suggests that accretion-driven variabil- ity is a common phenomenon during the earlier phases of the protostellar evolution. In the light of the new WISE data, we also re-examine a sample of 10 variables, which we had already selected in Giannini et al. (2009). From the inspec- tion of their light curves, we select two flat-spectrum sources as the best EXors candidates.