The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16 × 25 pixels, each, and two filled silicon bolometer arrays with 16 × 32 and 32 × 64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60−210 μm wavelength regime. In photometry mode, it simultaneously images two bands, 60−85 μm or 85−125 μm and 125−210 μm, over a field of view of ∼1.75 × 3.5 , with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47 × 47 , resolved into 5 × 5 pixels, with an instantaneous spectral coverage of ∼ 1500 km s −1 and a spectral resolution of ∼175 km s −1 . We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the performance verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions. Key words. space vehicles: instruments -instrumentation: photometers -instrumentation: spectrographsHerschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Abstract. We present the results of the first extensive mid-infrared (IR) imaging survey of the ρ Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main ρ Ophiuchi molecular cloud L1688, as well as the two secondary clouds L1689N and L1689S, have been completely surveyed for point sources at 6.7 µm and 14.3 µm. A total of 425 sources are detected in ∼0.7 deg 2 , including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fν ∼ 10-15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the ρ Ophiuchi central region. There are, however, reasons to believe that several tens of Class III YSOs remain to be identified below L ∼ 0.2 L . The mid-IR luminosities of most (∼65%) Class II objects are consistent with emission from purely passive circumstellar disks. The stellar luminosity function of the complete sample of Class II YSOs is derived with good accuracy down to L ∼ 0.03 L . It is basically flat (in logarithmic units) below L ∼ 2 L , exhibits a possible local maximum at L ∼ 1.5 L , and sharply falls off at higher luminosities. A modeling of the luminosity function, using available pre-main sequence tracks and plausible star formation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the initial mass function (IMF) of the embedded cluster. After correction for the presence of unresolved binary systems, we estimate that the IMF in ρ Ophiuchi is well described by a two-component power law with a low-mass index of −0.35 ± 0.25, a high-mass index of −1.7 (to be compared with the Salpeter value of −1.35), and a break occurring at M flat = 0.55 ± 0.25 M . This IMF is flat with no evidence for a low-mass cutoff down to at least ∼0.06 M .

No abstract

The Trifid nebula is a young (10(5) years) galactic HII region where several protostellar sources have been detected with the infrared space observatory. The sources are massive (17 to 60 solar masses) and are associated with molecular gas condensations at the edges or inside the nebula. They appear to be in an early evolutionary stage and may represent the most recent generation of stars in the Trifid. These sources range from dense, apparently still inactive cores to more evolved sources, undergoing violent mass ejection episodes, including a source that powers an optical jet. These observations suggest that the protostellar sources may have evolved by induced star formation in the Trifid nebula.