Keywords: DataAccWG Hi All,
I am responsible for the large number of stars which Jacek has in the current sizing estimate. I have had a look at various stellar density studies. A recent theoretical model of the Milky Way is given in Robin etal (astroph/0401052) which also presents a number of recent observational results for comparison (and more-or-less all of the observational results have higher stellar densities). I also think Hall etal (ApJS 140:185, 1996) is relevant. I am assuming a depth of a single exposure is about 24.5 in r. I have estimated the galactic latitude and longitude coverage of the WL survey and other surveys (NEA, KBO, SN) from my experience with the LSST operations simualtor. I estimate about 6,000 square degrees in the galactic latitude range of +/-10-20, about 6,000 degrees in the range -20-40, and about 10,000 degrees in the range -40-90. This is conservative since I have no fields within 10 degrees of the galactic plane. From Robin, I estimate about 300,000 stars per square degree at 10-20 degrees, 5,000 stars per square degree for 20-40 and 3,000 per square degree for 40-90. These are some sort of geometric mean for the rapidly declining density as we move away from the plane. For a sanity check, Hall etal get densities of 25,000 per square degree averaged over a range of latitudes from 35 to 63--clearly higher than my Robin etal model estimates. This sums to 2,000,000,000 stars in the database for single exposure depths. I conservatively assume that with 10 years of co-additions, we will see about twice as many stars. I also assume that the mean properties of all detected stars for a data releaese are part of the deep database. Now, there is a choice which needs to be made. We can release light curves for all detected stars, or we can only release light curves for stars which are detected in the difference images (ie variable stars). If we only release variable objects, this will decrease the number by about a factor of 20 (5% variability). This would be the cheap way to do things, but might impose some serious loading on the archive site by at least two sets of users: low amplitude pulsation studies and planetary transit studies. We will have to set some limit (3 sigma? 5 sigma?) for detections in the difference images which will be greater than a detectable signal in the folded light curve of a periodic variable. So, either we release light curves of all detected objects and let people analyze them as they will, or we face the prospect of having people generate light curves of all detectable objects in order to do this analysis. This sizing has not explicitly accounted for AGN and QSO variabliity, but this is well included in the 5% variability assumption for the 'stellar' objects. I think we may actually detect more stars in single exposures than I have estimated based upon the fact that we will likely have nearby, resolved galaxies in some of our fields and we will be observing the ecliptic, probably as it passes through the galactic plane. I also think that a factor of two is conservative going from single exposures to DR20 depths. cheers, Kem _______________________________________________ LSST-data mailing list [email protected] http://www.lsstmail.org/mailman/listinfo/lsst-data
