2008A&A...489.1409G

We search for young stellar objects (YSOs) in the L1641N cluster and characterize the star formation activity through determination of the age distribution, mass function, spatial distribution, and the star formation history. Multi-wavelength broad band photometry both from space and the ground are used to look for IR excess in order to separate field stars from YSOs and to sample the spectral energy distributions. Space-based observations were obtained using the ISO satellite (ISOCAM) in two filters, centred at 6.7 and 14.3µm, and Spitzer (IRAC) at 3.6, 4.5, 5.8, and 8.0µm. Our ground-based observations were made with the Nordic Optical Telescope (NOT) using ALFOSC (I band), NOTCam (J, K_S and 2.12µm H₂), and SIRCA (L'). More than 50 of the brightest I-band sources were then studied with follow-up optical spectroscopy (5780-8340 Å) to check for signs of accretion (Hα in emission) and youth (LiIλ6707 in absorption) and to determine their effective temperatures. By comparing theoretical evolution tracks with our YSO sample in the H-R diagram, we calculated an age, luminosity, and mass distribution. We detect a total of 216 (Spitzer or I band) sources in L1641N, 89 of which are YSO candidates. Most of the spectra are of M-type with Hα strongly in emission, and many have Li6707 in absorption. The four brightest I band sources (F and G stars) are suggested as foreground stars, and the L1641N IRAS source is shown to be the combined flux of at least four sources. We find that the interstellar extinction is well-fit in the optical and near-IR by a power law with an exponent of 1.58, although in the mid-IR the Spitzer observations show a higher extinction than expected from theory. The median age of the YSO sample is ∼1Myr and the resulting MF has a flat distribution for low masses down to the completeness limit. There is evidence of a constant star formation rate of one star in 3.7x10⁴yr during the past few Myr. We find 11 sources older than 10Myr and a spatial separation between younger and older YSOs, suggesting that many of the older stars formed in L1641N could have left the cluster, giving the appearance of an increased star formation rate with time.