Hi Kem,
Thanks, that is very helpful to have. If I understand what you've
done, the rough averaging of the stellar density gives about 80000 / sq
deg, vs the 10000 I assumed. This underscores the point that the
sizing of the time-dependent database will depend *critically* on
whether we have a galactic plane exclusion zone, and if so, how big it is!
So I agree with nearly everything, but do have one significant
disagreement as well. First, I am in complete agreement that we want
lightcurves for all stellar objects, not just those above some threshold
in a difference image. Where I disagree (I think) is about extending
this to objects that are detectable only in image stacks. This is
obviously what's needed for the static properties of galaxies (and stars
as well), but seems pretty useless for variability studies. I'm
inclined to say that unless an object has a useful SNR in a single
image, it will not have a lightcurve, but only static properties.
Obviously this is one extreme strategy, and yours is the other. In the
middle one can do photometry of sequences of stacks, each of which is
made by N individual exposures, where N = 4 or 10 or whatever. I
suppose there might be some interesting science there, but I do have my
doubts.
So, I think we have a factor of two or so uncertainty based on how we
answer that question, and probably at least another factor of two
arising from how far into the plane we choose to go.
Cheers,
Tim
Kem Cook wrote:
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
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