Context : Knowledge of the evolution of circumstellar accretion disks is pivotal to our understanding of star and planet formation ; and yet despite intensive theoretical and observational studies , the disk dissipation process is not well understood .
Infrared observations of large numbers of young stars , as performed by the Spitzer Space Telescope , may advance our knowledge of this inherently complex process .
While infrared data reveal the evolutionary status of the disk , they hold little information on the properties of the central star and the accretion characteristics .

Aims : Existing 2MASS and Spitzer archive data of the Lynds 1630N and 1641 clouds in the Orion GMC provide disk properties of a large number of young stars .
We wish to complement these data with optical data that provide the physical stellar parameters and accretion characteristics .

Methods : We performed a large optical spectroscopic and photometric survey of the aforementioned clouds .
Spectral types , as well as accretion and outflow characteristics , are derived from our VLT/VIMOS spectra .
Optical SDSS and CAHA/LAICA imaging was combined with 2MASS , Spitzer IRAC , and MIPS imaging to obtain spectral energy distributions from 0.4 to 24 \mu m. Reddened model atmospheres were fitted to the optical/NIR photometric data , keeping T { { } _ { eff } } fixed at the spectroscopic value .
Mass and age estimates of individual objects were made through placement in the HR diagram and comparison to several sets of pre-main sequence evolutionary tracks .

Results : We provide a catalog of 132 confirmed young stars in L1630N and 267 such objects in L1641 .
We identify 28 transition disk systems , 20 of which were previously unknown , as well as 42 new transition disk candidates for which we have broad-band photometry but no optical spectroscopy .
We give mass and age estimates for the individual stars , as well as equivalent widths of optical emission lines , the extinction , and measures of the evolutionary state of the circumstellar dusty disk .
We estimate mass accretion rates \dot { M } _ { acc } from the equivalent widths of the H \alpha , H \beta , and He I 5876Å emission lines , and find a dependence of \dot { M } _ { acc } \propto M _ { * } ^ { \alpha } , with \alpha \sim 3.1 in the subsolar mass range that we probe .
An investigation of a large literature sample of mass accretion rate estimates yields a similar slope of \alpha \sim 2.8 in the subsolar regime , but a shallower slope of \alpha \sim 2.0 if the whole mass range of 0.04 M _ { \odot } \leq M _ { * } \leq 5 M _ { \odot } is included .
The fraction of stars with transition disks that show significant accretion activity is relatively low compared to stars with still optically thick disks ( 26 \pm 11 % vs. 57 \pm 6 % , respectively ) .
However , those transition disks that do show significant accretion have the same median accretion rate as normal optically thick disks of 3-4 \times 10 ^ { -9 } M _ { \odot } yr ^ { -1 } .
Analyzing the age distribution of various populations , we find that the ages of the CTTSs and the WTTSs with disks are statistically indistinguishable , the WTTSs without disks are significantly older than the CTTSs , and the ages of the transition disks and the WTTSs without disks are statistically indistinguishable .
These results argue against disk-binary interaction or gravitational instability as mechanisms causing a transition disk appearance .
Our observations indicate that disk lifetimes in the clustered population are shorter than in the distributed population .
In addition to the spectroscopic sample analyzed in this paper , we provide a photometric catalog of sources detected in the optical and infrared , but without spectroscopic observations .
As judged by their infrared colors , many of these are YSO candidates .
In our survey we identify 2 new aggregates in L1641 .
We find 4 apparently subluminous objects with extremely high equivalent widths of H \alpha and other emission lines , and 1 previously unknown FU Orionis object .
We find that the low-density molecular cloud emission that surrounds the star-forming cores has significant substructure on scales of \lesssim 0.2 pc in L1641 but not in L1630 .
We propose refined H \alpha equivalent width criteria to distinguish WTTSs from CTTSs in which the boundary EW is lowered significantly for late M spectral types . Tables 7 , 8 , and 11-14 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/

Conclusions :