  We present a study of the infrared variability of young stellar objects by
means of two Spitzer-IRAC images of the Vela Molecular Cloud D (VMR-D) obtained
in observations separated in time by about six months. By using the same
space-born IR instrumentation, this study eliminates all the unwanted effects
usually unavoidable when comparing catalogs obtained from different
instruments. The VMR-D map covers about 1.5 square deg. of a site where star
formation is actively ongoing. We are interested in accreting pre-main sequence
variables whose luminosity variations are due to intermittent events of disk
accretion (i.e. active T Tauri stars and EXor type objects). The variable
objects have been selected from a catalog of more than 170,000 sources detected
at a S/N ratio > 5. We searched the sample of variables for ones whose
photometric properties are close to those of known EXor's. These latter are
monitored in a more systematic way than T Tauri stars and the mechanisms that
regulate the observed phenomenology are exactly the same. Hence the modalities
of the EXor behavior is adopted as driving criterium for selecting variables in
general. We selected 19 bona fide candidates that constitute a well-defined
sample of new variable targets for further investigation. Out of these, 10
sources present a Spitzer MIPS 24 micron counterpart, and have been classified
as 3 Class I, 5 flat spectrum and 2 Class II objects, while the other 9 sources
have spectral energy distribution compatible with phases older than Class I.
This is consistent with what is known about the small sample of known EXor's,
and suggests that the accretion flaring or EXor stage might come as a Class
I/II transition. We present also new prescriptions that can be useful in future
searches for accretion variables in large IR databases.
