Obscured objects in the Magellanic Clouds
Coordinator: Cioni M.-R., Kaeufl H.-U., Loup C.
Bright infrared objects were discovered in the Magellanic Clouds by IRAS and followed up with ISO (improved resolution). Many of these objects are asymtotic giant branch (AGB) and red super-giant stars obscured by a dust envelope. Though they appear isolated, in galaxies like the Magellanic Clouds a contamination is expected from similar or other dusty objects (i.e. young stellar objects, compact HII regions, Planetary Nebulae).
Our goal is to use VISIR to observe a sufficient area of each Cloud to provide information of unprecedented spatial resolution, a great advantage over space observations at comparable wavelengths, about different kind of objects and in particular on AGB stars. Because most stellar parameters require the distance to the objects to be known, the Magellanic Clouds represent an ideal place to study stellar populations. The final product will be a catalogue useful as well for the entire ESO astronomy community. In this area about several sources were detected in JHK_s by 2MASS and among them are AGB stars above the tip of the red giant branch also observed by ISO at 7 and 12 micron.
Two filters are used: SiC and PAH1 narrow band filters. The SiC filter (10-12 micron) encompasses the SiC absorption feature in carbon-rich AGB stars while the PAH1 filter (8-9 micron) contains C_3 absorption also present in carbon stars. Oxygen-rich AGB stars will partly contribute at these wavelenghts with the emission and/or absorption of silicate at 9.7 micron. Combining these wavebands we will be able to study the properties of the detected objects in a colour-magnitude diagram and perhaps discriminate between AGB stars of a different nature.
In order to provide a link with existing dat-sets (i.e. DENIS, 2MASS, ISO and MSX) we propose to obtain almost simultaneous Ks band imaging observations of the same region. This will allow us to derive accurate bolometric magnitudes. Obscured AGB stars have a spectral energy distribution that peaks at the Ks band or at longer wavelenghts therefore obtaining "N-band" photometry is extremely valuable to recover a considerable amount of the flux that is often extrapolated or interpolated. Ks band data will also be used to estimate source variability. Note that an "N-band" survey, though of a small area in each Cloud, is a unique result and because the transmission in the N-band is almost 100% it will be a valuable information for many different stellar objects. As immediate result of this survey one will have a mid-IR luminosity function and a color-magnitude diagram for selected regions in the Magellanic Clouds.
Program is available and data products can be downloaded
This program makes use of the imaging mode of VISIR in two of its most sensitive bandwidths: SiC and PAH1 filters.
In the Small Magellanic Cloud we are going to mosaic an area of about 100"x100" (to start with, though a larger area would benefit this survey-program if time allows) using 4 different pointings. Details are listed above. Each pointing is observed twice, for a total integration time per position of 30min, with and without a small offset in both RA (1s) and DEC (1") to account for bad pixels. Moreover the second pointing is located on the first off-set chopping postion. Therefore in order to recover the same field as the previous observation the position angle equals 0 in the in the first observation and 90 in the second. This technique will allow us to re-construct positive and negative sources resulting from the A-B frame in the whole area. We expect to reach a sensitivity of approximately 18 and 22 mJy for a 5 sigma detection, based on the numbers obtained from the VISIR performance during the commitioning runs. The total integration time is of 16h+overheads.
Similar observations are planned in the LMC.