[vsnet-chat 1686] CW Mon

[Ron Webbink <webbink@astro.uiuc.edu>]

In regard to the discrepancy in coordinates for CW Mon between DWS (Downes,
Webbink & Shara) and USNO-A2.0, these can be laid to differences in the
reference frames.

Except as noted below, the coordinates listed by DWS were measured on the
digitized Guide Star Survey plates, and are in the astrometric reference 
system of GSC1.0 and GSC1.1.  The source plates are identified in the column
labeled "Coord Ref" in Table 1 of DWS, by a four-character ID (beginning 
with 0); details (plate centers, exposure dates, emulsions, etc.) of the 
Guide Star Survey plates can be found in Lasker, et al. 1990, AJ, 90, 2019.
Notations in "Coord Ref" which do not conform to this four-character format
identify other sources from the literature (Table 4); we did not remeasure
these stars.

The GSC reference frame is known to have significant local systematic errors.
These are reflected in the difference between the positions for this star
in the USNO-A1.0 catalog (which is tied to the GSC reference frame) and the 
USNO-A2.0 catalog (which is derived, I believe, from the same plate digitiza-
tion as the A1.0 catalog, but tied to the Hipparcos frame):

DWS				06 36 54.526	+00 02 16.32
USNO-A1.0 0900-03187401		06 36 54.477	+00 02 16.63
USNO-A2.0 0900-03178341		06 36 54.547	+00 02 17.69

Note that applying the difference between USNO-A2.0 and A1.0 positions to the
DWS coordinates gives almost precisely those measured by Arne Henden (vsnet-
chat 1684):

Henden				06 36 54.59	+00 02 17.4

In fact, if one arranges the AC2000 coordinates (which are tied to the
Hipparcos frame), USNO-A2.0, corrected DWS, and Henden coordinates in
chronological order, it appears that the remaining discrepancies (to the
extent that they are statistically significant) can be laid to a small, but
nonzero proper motion:
				  RA(J2000)	 Dec(J2000)	 Epoch

AC2000 48063			06 36 54.465	+00 02 17.42	1909.08
USNO-A2.0 0900-03178341		06 36 54.547	+00 02 17.69	1955.88
DWS				06 36 54.596	+00 02 17.38	1984.18
Henden				06 36 54.59	+00 02 17.4	1999.2

At a distance of ~300 pc, this would make the transverse velocity of CW Mon 
relative to the Sun a mundane ~35 km/s.

Ron Webbink

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