Subject: The first 2MASS parallax

From: Dave Monet

Submitted: Sun, 27 Jun 2004 07:01:14 -0700

Message number: 249 (previous: 246, next: 264 up: Index)

It is very rare for me to get as excited (professionally or otherwise)
than I have been this weekend.  The occasion is the 2MASS Extended
Mission's production of the first stellar parallax from 2MASS data.
The Extended Mission (Skrutskie, Cutri, et al.) is the effort to wring
yet more science out of the areas where 2MASS had extra coverage, either
from longer exposure times or from repetitions.  LHS191 lies in one
of the 2MASS photometric calibration patches, and was observed 2086
times on 130 nights.  Their astrometric reduction produced

   Proper motion    1.0228 +/- 0.0014  arcsec/year
   Parallax         0.0562 +/- 0.0020  arcsec

with a sigma per axis per observation of about 45mas.  The values presented
by Monet, Dahn, et al. (1992) are

   Proper Motion    1.0223 +/- 0.0017  arcsec/year
   Parallax         0.0585 +/- 0.0018  arcsec

To me, the 2MASS result stands as one of the Defining Moments of modern
astrometry, right up there with success of the Hipparcos mission.  Talk about
a survey not designed for astrometry: 2 arcsec pixels, 1.7sec integrations,
a stepping secondary mirror, a Level 1 Requirement for 500mas astrometric
accuracy for the final catalog, the HgCdTe sensor with zero astrometric
heritage and known sub-pixel structure, and a Project Manager determined
to keep costs under control.  I can do nothing more than express my
congratulations and profound appreciation to the 2MASS Project, and
in particular to Skrutskie, Cutri, and McCallon for their efforts to
optimize the astrometric accuracy.

Yes, this might just be beginner's luck since it is the first result and
there are thousands of stars (with known and unknown parallaxes) to be
processed, but it gives me a profound hope that astrometry really can
be done with surveys designed for other purposes (LSST and Pan-STARRS
come to mind but there are others).  Maybe the BS I have been spreading
in recent years might be true after all?

-Dave Monet is dgm@nofs.navy.mil

PS
No, I don't mean that a single parallax from 2MASS outshines all of
Hipparcos.  What I mean is these are both great steps forward, and that
they set examples for how we can do astrometry in the coming years.

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