2003 March: New Pre-Flames Survey Data Release

The Stellar Preparatory Survey

The high multiplex capabilities of FLAMES makes it an ideal toolfor many stellar astronomy studies. It also poses the problem of having stellar fields dense enough to feed the 130 fibers of GIRAFFE in the MEDUSA mode: stellar densities of selected objects should be >400 objects/sqdeg. Moreover, the small diameter of the fibres (1.2 arcsec)imply that the astrometric positions of the objects have to be known with high precision: mean astrometric error <0.3 arcsec, in order tolimit fiber off centering losses.

Available catalogs rarely addresses these requisites for observing in MEDUSA mode in an adequate way. Therefore ESO started a public survey of fields of high potential interest for stellar astronomy studies as a preparation for FLAMES (see the ESO Imaging Survey).

In a typical night, MEDUSA can deliver around 1000 stellar spectra from up to 5-10 different fields, i.e., approximately 500stellar fields per year will be needed to feed FLAMES. Considering that half of the fields can be provided by private preparatory survey,a working group has selected 160 obvious fields to be observed at the Wide Field Imager of the 2.2m in La Silla in at least 3 bands.

During the target selection, the EIS Working Group has outlined a few scientific programs that are nicely fitted for the characteristics of GIRAFFE+MEDUSA and that will benefit enormously from the preparatory survey. These programs are listed below.

- Bulge of the Galaxy and its chemical and dynamical evolution history.

- Chemical evolution and dynamics of the stellar populations of dwarf spheroidal galaxies.

- Chemical abundances of stars in clusters and in selected extra-galactic stellar regions. Possible targets are: low-mass stars,RR Lyrae stars, Wolf-Rayet stars and their associated stellar winds,white dwarfs in open clusters, Blue Horizontal Branch stars, AsymptoticGiant Branch stars, Carbon stars.

- Stellar kinematics and structure of stellar clusters.

- Circumstellar activity in Young Stellar Objects.

- Very low mass stars and brown dwarfs in star forming regions.

In the observing Period 64, the EIS working group asked for 24nights of WFI time over the period of two years to complete the survey on the selected 160 target fields. The table below shows the actual number of fields to be observed per each class of stellar object.

The present status of the ongoing survey can be checked on the EIS webpages. The latest list of observed targets can be viewed at this page. The list of released reduced images can be found here.

The observing strategy

The EIS strategy for the WFI observations has been decided considering two major constraints:

- astrometry better than 0.2 arcsec rms

- photometric accuracy of 0.03 mag at V=20

A misplacement of a fiber of ~0.5 arcsec lowers the collected flux by~50% with a seeing of 0.7 arcsec. The above precision can be easily reached with WFI data since the EIS astrometric pipeline can calibrate the data to an internal (relative) accuracy of ~0.03 arcsec rms.A good photometric accuracy down to the limit foreseen limit of GIRAFFE combined with multiband photometry is needed in order to efficiently select interesting targets for the observations. Therefore, it has been decided to observe each targetin:

- 3 passbands, B, V, I

- 2 long exposures: 4 minutes each, plus

- 1 short exposure: 30 seconds.

The two long exposures have been dithered by 30 arcsec in RA and Decto fill at the 99.91% level the gaps between the WFI CCDs and toefficiently remove cosmic ray hits and hot columns. The single short exposure will be used to recover the magnitude of the brightest unsaturated objects necessary as reference stars for each FLAMES field. The same exposure time has been adopted for all 3 bands.