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Polarization standard stars

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)


Subject: poln stds

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.

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

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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.


Figure 13: Extract from Tinbergen 1979: zero-polarization stars.


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.


Figure 15: Extract from Dolan and Tapia 1986: polarization variations in `standard stars'.


Figure 16: Extract from Bastien et al. 1988: polarization variations in `standard stars'.


Figure 17: Extract from Turnshek et al. 1990: HST polarization standards.


Figure 18: Extract from Turnshek et al. 1990: HST high-polarization extended objects.


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.


Figure 20: Bibliography of polarization standards up to 1985.

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Tue Oct 7 17:34:45 BST 1997