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The origin and evolution of the X-rays in very young stellar objects (YSOs)are not yet well understood since it is very hard to observe YSOs in theprotostellar phase. We study the X-ray properties of Class 0-I objects in theOrion Nebula Cluster (ONC) and compare them with those of the more evolvedClass II and III members. Using Chandra Orion Ultradeep Project (COUP) data, westudy the X-ray properties of stars in different evolutionary classes:luminosities, NH, temperatures and time variability are compared in order tounderstand if the interaction between the circumstellar material and thecentral object can influence the X-ray emission. We have assembled the deepestand most complete photometric catalog of objects in the ONC region from the UVto 8 microns using data from HST, WFI@2.2m ESO and ISPI@4m CTIO telescopes, andSpitzer IRAC. We select high probability candidate Class 0-I protostars,distinguishing between those having a spectral energy distribution which risesfrom K up to 8 microns (Class 0-Ia) from those where the SED rises from K up to4.5 microns and decreasing afterwards (Class 0-Ib). We select a sample of bonafide Class II stars and a set of Class III stars with IR emission consistentwith normal photospheres. Our principal result is that Class 0-Ia objects aresignificantly less luminous in X-rays, both in the total and hard bands, thanthe more evolved Class II stars with mass larger than 0.5 Msun; these lattershow X-ray luminosities similar to those of Class 0-Ib stars. This resultsupports the hypothesis that the onset of X-ray emission occurs at a very earlystage of star formation. Temporal variability and spectral properties of Class0-I stars are similar to those of the more evolved Class II and III objects,except for a larger absorption likely due to gas in the circumstellar material.

X-ray properties of protostars in the Orion Nebula