The Optical Gravitational Microlensing Experiment (OGLE III) has monitored
both Magellanic Clouds in the V and I band. Using the public stellar
photometry database of OGLE III in order to derive spatially resolved
reddening maps, we subdivide the OGLE III observations of the Large
Magellanic Cloud (LMC) in 3174 subfields. The size of the fields varies from
4.5 x 4.5 square arcmin to 36 x 36 square arcmin in order to ensure that each
field contains at least 300 stars and no more than 4800 stars. I.e., in dense
regions our reddening resolution is higher.
In each of these subfields the mean (V-I) colour of the red clump is
determined. A theoretical colour of (V-I):sub:0 = 0.92 mag is adopted for the LMC
and subtracted from the measured value. The difference is defined as
reddening E(V-I). To provide you directly useable data we calculated the
reddening for different filters by adopting Schlegels list of conversion
factors (Table 6 in Schlegel et al. 1998, AJ, 500, 525). To use this table
the extinction has to be recalculated from E(B-V) to E(V-I). For this purpose
we adopted E(V-I) = 1.38 × E(B-V)
(Tammann et al. 2003, A&A, 404, 423).
For the Small Magellanic Cloud (SMC) the same procedure is used. The OGLE III
data are subdivided into 693 contiguous subfields. The reddening of each
subfield is determined adopting a theoretical colour of
(V-I)0 = 0.89 mag.
Crossmatching with your tables
If you want to add extinctions to your own tables, use TAP. Our TAP
server has the ivo-id ivo://org.gavo.dc/__system__/tap/run. You will need
a TAP client to do this. Recent versions of
TOPCAT, for example, allow you
use TAP. To add a column with the reddening, load the table into TOPCAT
and note its id (that's the number in front of the colon in the Table List in
TOPCAT's main window; if you've just started TOPCAT, that's 1).
Then, select TAP from the TOPCAT's VO menu, at the bottom ("TAP URL") enter
http://dc.g-vo.org/tap and click "Enter Query".
In the resulting dialog, enter:
SELECT ev_i, mine.* FROM
mcextinct.exts AS exts
JOIN tap_upload.t1 AS mine
ON (1=CONTAINS(POINT('ICRS', mine.raj2000, mine.dej2000),exts.bbox))
If your table has id 15, say, change tap_upload.t1 to
tap_upload.t15. You will also need to fix the column names for RA
and Dec (written as raj2000 and dej2000 here).
To compute extinctions in specific bands, use the factors given in the A/E(V-I)
column in the table above above. To, e.g., add the extinctions in Sloan u' and
Sloan z', say:
SELECT ev_i*3.74 as A_sloan_u, ev_i*1.07 as A_sloan_z, mine.* FROM
mcextinct.exts AS exts
JOIN tap_upload.t1 AS mine
ON (1=CONTAINS(POINT('ICRS', mine.raj2000, mine.dej2000),exts.bbox))
For batch processing, similar operations can be done with
STILTS or GAVO's
tapsh. For the latter,
you could use a shell script like the following to add an ev_i column
to the file named in the first command line argument argument
(note the backup); again, you'll need to fix the column names for RA and Dec:
#!/bin/sh
cp $1 $1.bak
tapsh <<EOF
server ivo://org.gavo.dc/__system__/tap/run
SELECT ev_i, mine.* FROM mcextinct.exts AS exts JOIN tap_upload.mytable AS mine ON (1=CONTAINS(POINT('ICRS', mine.raj2000, mine.dej2000),exts.bbox))
upload "$1" as mytable
run
save "$1"
EOF
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