A Mid‐Infrared Survey of L1641N with ISOCAM

Abstract

We present the results from our analysis of the Infrared Space Observatory (ISO) data on the L1641N outflow region located in the Orion A molecular cloud. The data were obtained with the array camera (ISOCAM) using two broadband filters, LW2 (6.7 μm) and LW3 (14 μm), and the narrowband circular variable filter (CVF), which provides low-resolution (R = λ/δλ ~ 40) spectra in the 5-17 μm region. We detect a total of 34 sources in the 765 × 840 region covered by the broadband filters. Four of these sources have no reported detection in previous studies of the region. The CVF data are available for only the central 32 × 32 portion of the total region, providing spectra for the brightest seven sources in that region. We find that the source previously identified as the near-IR counterpart to the IRAS-detected point source (IRAS 05338-0624) is not the brightest source in the wavelength region of the IRAS 12 μm filter. We find instead that a nearby object (within the beam of IRAS and not detected at near-IR wavelengths) outshines all other sources in the area by a factor of ~2. Despite its brightness, we conclude that this second object is unlikely to be the source detected by IRAS because that would imply a positional error of ~1' in the IRAS position. However, we also consider it unlikely that IRAS would not have spatially resolved and detected the second source. This implies that the second source must have brightened by at least 200 mJy in the decade between IRAS and ISO observations. A comparison of the near-IR (J - H vs. H - Ks) and mid-IR (H - Ks vs. [6.7 μm]-[14 μm]) color-color plots shows that while at near-IR wavelengths only four of the sources show evidence for emission above the values predicted from photospheric emission alone (hereafter referred to as excess emission), at least 85% of all sources show evidence for excess emission at mid-IR wavelengths. This result supports similar conclusions from L-band surveys. The CVF spectra suggest a range of evolutionary status in the program stars ranging from embedded young stellar objects to young disks. When combined with optical and near-IR age estimates, these results show active current star formation in the region that has been ongoing for at least 2 Myr.