This section should be read with Section 3.6 on calibration. Standard
stars exist for many purposes and it is essential to select those that
your programme requires; lack of calibration is fatal, but too
much is wasteful. There is very little that can really serve as
standard in the practice of spectropolarimetry; be wary of narrow-band
polarization effects when reducing standard star measurements.
Basically, for spectropolarimetry we shall have to create our own set
of standard stars; if you are interested in starting a cooperative
long-range programme, contact TINBERGEN@HRDKSW5.
Useful references, which between them cover wavelength dependence,
zero-point, degree and angle, are listed below. Figs 13 to 19 have
been copied from some of these papers for ease of use at the
telescope, but we advise you to scan through the original papers. A
very recent paper, covering a wide range in wavelength, is:
Whittet D C B, Martin P G, Hough J H, Rouse M F, Bailey J
A and Axon D J, 1992,
Astrophysical Journal 386, 562
`Systematic variations in the wavelength dependence of
interstellar linear polarization'
U to K on 105 stars; use with Bailey and Hough
1982, Hsu and Breger 1982
Bailey and Hough, 1982, Pub Astr
Soc Pacific 94, 618 U to K on 11 stars
Tinbergen, 1979, Astron & Astrophys
Suppl 35, 325 zero-point
Hsu and Breger, 1982, Astrophysical
Journal 262, 732 degree, angle; U to I
Dolan and Tapia, 1986, Pub Astr
Soc Pacific 98, 792 time variations
Bastien et al., 1988, Astronomical Journal 95, 900
time variations
Turnshek et al., 1990, Astronomical Journal 99, 1243
rough and ready single reference
Some years ago one of us constructed a bibliography of all polarimetry
thorough enough to serve as standards. Though no longer up-to-date, it
will serve as a guide for a literature search and is reproduced here,
with later additions, as Fig 20.
Finally we quote two recent workers in the subject, whose opinion on
standards we value and to whom we specifically put the question of
spectropolarimetric standards in connection with this manual. Note
their caution.
From: bastien@PHYSCN.UMontreal.CA (Bastien Pierre)
To: TINBERGEN@HRDKSW5.BITNET
Subject: poln stds
For their suitability as spectropolarimetric standards, I
think that the work has not been done properly yet on this subject.
I think that an observing campaign is needed. What do you
think?
Best regards, Pierre
- - - - - - - - - - - - - - - - - - - - - - - -
Dear Jaap, believing in what I published, my suggestion would be to
use as best candidates the ones which have the lowest ratio of sigma1
to sigma2, as defined in the 1988 AJ paper you referred to earlier.
Fig. 1 in my Vatican paper on poln standards - reproduced here
as part of Fig 16 - presents them according to the value of this
ratio. I would stay away as much as possible from the ones with the
largest values of this ratio. So this covers the variability concerns
for the standards.
Conclusion. To sum up the present situation: for high-accuracy
spectropolarimetry, it is probably best to use stars for zeropoint,
twin-tilted-plate for degree of polarization, and suspended polaroid
or Dolan & Tapia's method (or possibly asteroid or zenith blue sky)
for angle. Standard stars are probably satisfactory for less demanding
programmes.
[ TIFF ]
Figure 13: Extract from Tinbergen 1979: zero-polarization stars.
[ TIFF ]
Figure 14: Extract from Hsu and Breger 1982: standard stars for degree
and angle of polarization, in 5 broad bands. Use with Bailey and Hough
1982, Whittet et al 1992.
[ TIFF ]
Figure 15: Extract from Dolan and Tapia 1986: polarization variations in
`standard stars'.
[ TIFF ]
Figure 16: Extract from Bastien et al. 1988: polarization variations in
`standard stars'.
[ TIFF ]
Figure 17: Extract from Turnshek et al. 1990: HST polarization
standards.
[ TIFF ]
Figure 18: Extract from Turnshek et al. 1990: HST high-polarization
extended objects.
[ TIFF ]
Figure 19: Extract from Bailey and Hough 1982: standard stars for degree
and angle of polarization, in 9 broad bands (U to K). Use with Whittet
et al 1992, Hsu and Breger 1982.
[ TIFF ]
Figure 20: Bibliography of polarization standards up to 1985.