We present a study of 15 new brown dwarfs belonging to the $sim7$ Myr old 25 Orionis group and Orion OB1a sub-association with spectral types between M6 and M9 and estimated masses between $sim0.07$M$_odot$ and $sim0.01$ M$_odot$. By comparing them t
hrough a Bayesian method with low mass stars ($0.8lesssim$ M/M$_odotlesssim0.1$) from previous works in the 25 Orionis group, we found statistically significant differences in the number fraction of classical T Tauri stars, weak T Tauri stars, class II, evolved discs and purely photospheric emitters at both sides of the sub-stellar mass limit. Particularly we found a fraction of $3.9^{+2.4}_{-1.6}~%$ low mass stars classified as CTTS and class II or evolved discs, against a fraction of $33.3^{+10.8}_{-9.8}~%$ in the sub-stellar mass domain. Our results support the suggested scenario in which the dissipation of discs is less efficient for decreasing mass of the central object.
In studies of star-forming regions, near-infrared excess (NIRX) sources--objects with intrinsic colors redder than normal stars--constitute both signal (young stars) and noise (e.g. background galaxies). We hunt down (identify) galaxies using near-in
frared observations in the Perseus star-forming region by combining structural information, colors, and number density estimates. Galaxies at moderate redshifts (z = 0.1 - 0.5) have colors similar to young stellar objects (YSOs) at both near- and mid-infrared (e.g. Spitzer) wavelengths, which limits our ability to identify YSOs from colors alone. Structural information from high-quality near-infrared observations allows us to better separate YSOs from galaxies, rejecting 2/5 of the YSO candidates identified from Spitzer observations of our regions and potentially extending the YSO luminosity function below K of 15 magnitudes where galaxy contamination dominates. Once they are identified we use galaxies as valuable extra signal for making extinction maps of molecular clouds. Our new iterative procedure: the Galaxies Near Infrared Color Excess method Revisited (GNICER), uses the mean colors of galaxies as a function of magnitude to include them in extinction maps in an unbiased way. GNICER increases the number of background sources used to probe the structure of a cloud, decreasing the noise and increasing the resolution of extinction maps made far from the galactic plane.
