UW Astronomer: Chris Anderson
Kappa Cas is a B1 Iae supergiant, a type of star quite
prone to intrinsic polarigenic effects. Gehrels and Silvester (1965)
noted significant position angle rotation with wavelength; Coyne and
Gehrels (1966) call it possibly polarization variable. Between
1989 and 1994 the star was observed with the PBO spectropolarimeter 20
times. The mean P of the whole set is 1.466% +/- 0.055% and the full
range is 1.39% to 1.57%. The typical errors of individual
measurements is +/- 0.003%. The object was observed in the UV
on both Astro-1 (Taylor et al. 1991b) and Astro-2(WUP96). The average difference
between the two is 0.148% in polarization, and the two P(lambda) spectra are
parallel. The position angle rotation continues into the ultraviolet.
Low level intrinsic polarization thus seems likely. On the other hand, the shape of the optical polarization curve is well represented by the traditional Serkowski formula. The polarization (1.5% at V) is typical of its color excess [E(B-V) = 0.33] (SMF) and much larger than typical intrinsic effects in emission line B supergiants. Furthermore, the position angle is consistent with those of stars in its general direction (MF70) and distance. When the HPOL data are plotted in the QU-plane the 20 PBO points scatter randomly in time about a mean location displaced from zero by an amount roughly a factor of ten larger than the total size of the pattern. The vector from the origin of the QU-plane to the centroid of the observed points is identified as the interstellar component of the polarization. The variations from the centroid to the individual observations are probably indicative of variations in the mass loss from the star. This behavior is characteristic of hot supergiants such as P Cyg (Taylor et al. 1991a).
HD24263 = 31 Tau
31 Tau is a completely unremarkable B5V star - an ideal polarization probe. It is at a rather high galactic latitude (35 degrees south of the Milky Way plane). Its interstellar polarization curve P(lambda) is well described by the Serkowski formula throughout the optical and UV.
Four stars in and along and near the line of sight to NGC1502 were observed with
WUPPE on Astro-1 and Astro-2. HD25638 (HR1260 = NGC1502 01) is the brightest
star in the cluster (although there is a similar star, SZ Cam = NGC1502 02 =
HD25639, 20" west of it), and the other three stars, HD25443, HD25090, HD24992AB,
are within about 40' of the cluster.
HPOL visual spectropolarimetry is available for all of these stars. The four stars are all at distances greater than about 850 pc and their polarizations are similar, being about 5% - 6.5% in V. Four other stars in this general direction but at distances between about 100 pc and 250 pc have also been observed at PBO, and they show polarization less than about .3%. This suggests that most of the polarization of the distant stars takes place somewhere in the interval between 250 pc and 850 pc, with the upper bound on this interval being about the assumed distance of HD25443. Inside about 250 pc and outside about 850 pc there appears to be less polarizing material. Or, alternately, at distances greater than about 850 pc the polarization position angle is different from that inside 850 pc (so as to not add linearly to polarization acquired inside 850pc)(Weitenbeck et al 1996). We use for HD25638 1100pc = a distance intermediate between those of Tapia et al (1991) and Reimann & Pfau (1987) for the cluster, 1100 pc for HD24992, 1150 for HD25090, and 830 pc for HD25443. These are spectroscopic parallaxes, with the usual uncertainties.
We have continued to make spectroscopic and polarimetric observations of stars in this region.
To facilitate this, a list has been made cross identifying stars in the cluster itself and the region surrounding it with GSC numbers.
HD24992 is a V = 8.03, B3 III type star located roughly 42 arc
minutes west and slightly north of the center of the cluster.
Its visual polarization is about 0.7% less than that of
central star of the cluster, HD25638. The polarization curve is reasonably well
represented by the extrapolation of the WLR Serkowski formula based solely on
the visual lambdamax.
This observation was of the two stars called ADS2924AB which are about 2" apart. The third star in the system, component C, which is about 9" south of the others, was outside the WUPPE and HPOL apertures [though we plan to make a separate observation with HPOL as part of the continuing study of the region].
This star is a V=7.34, B0.5III star 35' west of the cluster and perhaps slightly more distant. It has roughly the same polarization as HD24992 and is also well represented by the visual WLR Serkowski formula.
HD25443 is 18' southwest of the cluster and is about .6 mag brighter
than HD25090. Both stars have the same spectral type and reddening, so
HD25443 is probably somewhat closer (and since it has the same reddening
but is brighter, it is further evidence that the ism is less dense beyond
HD25443 than inside it). Its peak polarization is less by about
0.5% than HD25090 and HD24992.
Nonetheless, all three of these stars have roughly the same
The ultraviolet polarization of this object is not well represented by the WLR Serkowski formula. The visual and UV polarization curves do not join smoothly: the UV curve is parallel to the Serkowski extrapolation but above it by roughly 0.5%. This is reminiscent of the behavior of B-supergiants noted for HD2905. HD25443 may have weak, variable intrinsic polarization.
This star is also used as a WUPPE and HPOL polarization standard star.
HD25638 = HR1260
HD25638 is the brightest star in the cluster. It has a peak polarization about 0.5% greater than, and a lambdamax shorter by several hundred angstroms than HD25090, HD25443, and HD24992; and its position angle differs by several degrees from the others. DH find the extinction of the cluster is larger than 30' on either side, and Tapia et al. (1991) from extinction variations suggested that the distribution of dust grain sizes along the line of sight to the cluster at optical depth greater than E(B-V) = 0.35 must be biased toward smaller sizes than the general ISM.
If we assume that the foreground polarization of the cluster can be
represented by a Serkowski curve with parameters corresponding to the
visual polarization of HD25443 (lambdamax = 5000Å,
5.17 at Theta = 135 deg), then we may subtract it from HD25638,
(lambdamax = 4500Å,
Pmax = 6.61 at Theta = 141°) and
obtain an estimate of the polarization in the immediate vicinity of the
cluster. Using a WLR Serkowski curve with the above values for HD25443,
we find the residual polarization in HD25638 to be
3926Å +/-22Å, Pmax= 1.67 +/-0.01
at Theta =
157.6°+/-0.2. This significantly shorter wavelength of peak polarization is in
good general agreement with the Tapia et al. (1991) suggestion.
The lower lambdamax,
along with the distinctly different position angle
found in this subtraction, are seen in a slow but distinct position angle
HD25940 = 48 Per
HD25940 is a nearby B3Ve star, on the
WUPPE Be stars target list; however, with E(B-V) = 0.16, it might have
significant ISM polarization. The emission
lines indicate circumstellar matter which could cause polarization,
but Coyne & Gehrels (1967) say
the star is not polarimetrically variable. CGS (1974)
found lambdamax = 5700Å +/- 200Å
0.94%. Using a combination of polarimetry and interferometry Quirrenbach et
al. (1996) find an upper limit of 0.05% for any intrinsic
polarization and suggest a nearly pole-on geometry. [See also
HD34078.] Fits to more
recent spectropolarimetry from PBO are in excellent agreement with filter
polarimetry results from 25 years ago, so if 48 Per has an intrinsic
polarization, it is small and very stable. The ultraviolet polarization found
during Astro-2 appears to lie above the extrapolation of the Serkowski
formula shortward of about 2500Å.
HD30614 = Alpha Cam
Alpha Camelopardalis is a
a late O supergiant with weak Halpha emission.
However, the peak polarization is large, 1.58%, relative to the known
intrinsic polarimetric effects in such stars, and the shape of the polarization
spectrum in the optical is at least roughly that given by the Serkowski formula
with a rather short
lambdamax = 4600Å according to CGS.
This star was observed on both Astro flights, and the data
are the same within the errors. Similarly, 9 observations at PBO between 1989
and 1995 show no convincing variations greater than about +/- 0.1%.
The polarization spectrum appears to show a discontinuity between
the WUPPE and PBO data, with the UV elevated by about 0.2% with
respect to the extrapolated Serkowski formula. PBO data taken just
24 hours before the WUPPE observation during the Astro-2 mission also show
the 0.2% step relative to the Astro-2 data so polarimetric variability appears
likely. But we note that the WLR version of the Serkowski formula also fails to
match the polarization in the red where the very high signal-to-noise results of
the new HPOL system are significantly below its prediction.
This star may be a runaway from the cluster NGC1502.
HD34078 = AE Aurigae
AE Aurigae is an O9.5Ve star thought to be a runaway star
from the Orion Nebula complex (Blaauw and Morgan 1954). This
object was selected for observation by another Astro instrument
(HUT). The observation was a rather short
integration so the polarimetric signal-to-noise is not particularly good. The
emission star classification and reports of variable line profiles (Slettebak 1956)
are a concern. However, the star has sharp absorption lines indicating a low
vrot sin i (Uesugi and Fukuda 1982).
This suggests a nearly
pole-on geometry which for this type of star usually leads to little or no
detectable intrinsic polarization (Poeckert and Marlborough 1976)
[see also HD25940],
furthermore, the Serkowski curve based on parameters given by CGS
(1974) appear to fit the data reasonably well.
HD37903 is a B1.5V star which illuminates a five arc minute diameter
reflection nebula located roughly 12' northeast of the Horsehead Nebula in
Orion. Values of
lambdamax between 6800Å and 7500Å
are given for this star by SMF and CGS.
We use 6800Å which seems to match the data well.
The sight line to this B3V star, which appears to pass through a dense, shocked
filament of the Gum Nebula, has been studied by Cardelli et al.
(1990). The column densities of CH and CN are quite high considering the
modest reddening and extinction, but it is strange that CH+ is absent
because this species is thought to be produced in abundance in shocked regions such as
this. The 2175Å extinction bump is shallow, flat bottomed, and appears
displaced shortward of its normal position; shortward of the bump the extinction
rises very rapidly. Only filter polarimetry and a short segment of AAT
spectropolarimetry are available in the optical for HD62542, but a Serkowski
formula with lambdamax = 5900Å
fits these data well. The star
was observed on both Astro missions, and the two data sets agree well.
The combined data are shown in the plot. The polarimetric
signal-to-noise is disappointingly small in the UV. Nonetheless, it
seems that this sight line is one in which the UV polarization is elevated
relative to the extrapolation of the visual Serkowski fit. This would be contrary
to the correlation of this behavior with small
lambdamax suggested by CWAL.
HD73882 is an O9 III star on a well studied interstellar sight line
(Joseph, Snow, and Seab 1989) and near the edge of a small dark cloud. The
lambdamax value given by SMF is 7500Å,
but it has a normal value
of R (Massa, Savage, Fitzpatrick 1983). These characteristics occasioned its
inclusion in the WUPPE program, and we have obtained
spectropolarimetry for this southern object with HPOL on the WIYN telescope. The WIYN/HPOL
calibration is still preliminary and a slight mismatch between the data with the
blue and red gratings is present at about 6000Å. A fit of the WLR form of the Serkowski
relation to these data yield a
lambdamax of 6500Å. The
extrapolation of the Serkowski formula with this value fits the UV polarization
poorly. The observed polarization is slightly low at both UV and red
wavelengths. Reanalysis of the Serkowski fit will be done when the HPOL
final calibration is available.
HD77581 = Vela X-1
A strong X-ray source would seem a dangerous choice for an interstellar probe
because of the probable presence of circumstellar material, and material flowing
between the components (one star is a B star the other is a compact object),
and indeed it was selected for Astro-2 observation as part of the program
to observe x-ray binaries. [See also the
interacting binary stars program].
UBVRI polarimetry by Dolan and Tapia (1988) shows a classic double loop variation in the Stokes Q-U plane phase locked to the 8.96 day binary period, but the amplitude is only 0.1%. Furthermore, their mean curve is well represented by a Serkowski curve with lambdamax = 5300Å and Pmax= 3.82%. Other observers have also reported variations at the 0.1 to 0.3% level, phase locked with the orbit (van Paradijs 1980, Ostreicher and Schulte-Ladbeck 1982). An object of this sort might potentially exhibit circular polarization, but neither Angel, McGraw, Stockman (1973) nor Avery, Stokes, Michalsky, Ekstrom (1975) found a statistically significant effect.
Spectropolarimetry of this southern object was done with HPOL on the WIYN telescope. The Serkowski curve which best fits the new visual data has lambdamax = 5500Å and Pmax = 3.99%. The UV data shown are the average of two observations taken at roughly opposite quadratures, phases 0.85 and 0.22, and the two agree within the errors. Likewise, the WIYN observations occurred at phase 0.32. Thus all three polarization spectra are likely to be affected by about the same, small, amount of variable intrinsic polarization.
The visual data connect smoothly with the WUPPE observation, but the latter are distinctly higher than the extrapolation of the Serkowski curve. But, although the object would seem to be predominantly polarized by interstellar effects, given the deviation from Serkowski-like behavior this conclusion should be treated with caution.
The O6 III star HD94963 was chosen for Astro-2 observation by HUT. There is no indication of peculiarity and its interstellar extinction is quite normal (The, DeWinter, Arens, Heijblok, & Nieuwland 1989). Magalhães (private comm) obtained new BVRI imaging polarimetry (Magalhães et al. 1996). Unfortunately the object is rather faint, the polarization small, and thus the polarization errors in both the WUPPE and ground based observations are too high to justify a formal Serkowski curve fit. We use the mean of the two available V-band measurements for Pmax and use lambdamax 5500Å. Within the errors the object is as well represented by this WLR Serkowski curve as can be expected.
HD99264 is an unremarkable B2 IV-V star observed on
Astro-1. Its lambdamax = 5500Å
and Pmax = 2.65% are from
Serkowski and Robertson (1969). These are augmented by an
AAT spectropolarimetric scan by Schulte-Ladbeck (private comm).
The UV data are in adequate agreement with the extrapolation of the visual
HD113904 = Theta Mus
The WC6 + O9.5I Wolf-Rayet star Theta Muscae, on the
WUPPE Wolf-Rayet target list,
has turned out to be a disappointment in that otherwise
polarimetrically interesting class of stars
(Schulte-Ladbeck et al. 1992). Schulte-Ladbeck's AAT spectropolarimetry
along with data from SMF indicate
lambdamax = 5400Å and
This Astro-1 observation was obtained when the stars
were in conjunction with the WC6 star behind the O star, a situation which
minimizes intrinsic effects. The extrapolation of the visual Serkowski curve
represents the UV data quite well.
HD147084 = Omicron Sco
HD147084 was the latest type star (A4 II-III) in the WUPPE sample.
Its selection was prompted by a rather large lambdamax
(6800Å), substantial polarization in the visible
and a rich history of
PBO and near IR (Jones 1990, Nagata 1990, Martin et al. 1992)
polarimetry. Due to the late spectral type and substantial
reddening, even the reasonably long WUPPE observation yielded UV
spectropolarimetry of lower than desirable signal-to-noise. Nonetheless, the
extrapolation of the visual Serkowski curve is a reasonable representation of the
UV data although there may be a small excess of polarization in the 2000Å
to 3000Å range.
HD147888 = Rho Oph D
HD147888 is a B5V star with no characteristics which would lead one
to suspect intrinsic polarization. SMF give lambdamax of
6900Å while the inclusion of near IR data by WLR suggests an even redder
value of 7200Å. A fit to new PBO data suggests 6800Å, and the
extrapolation of that curve matches the WUPPE data at the shortest wavelengths.
Two separate observations were accomplished during Astro-2, and both seem to
suggest a slight excess of polarization between 2000Å and 3000Å.
HD147933 = Rho Oph AB
Just a few arc minutes from HD147888 and presumably affected by
nearly the same polarization and extinction-causing material, Rho Oph AB
was observed with very good signal-to-noise both by WUPPE and at
PBO. The SMF and WLR polarization parameters are in
reasonable agreement with the new data yielding lambdamax =
6900Å. The Serkowski curve based on this value represents the PBO and
WUPPE data at the shortest and longest wavelengths, but as with Rho Oph
D, there is a suggestion of excess polarization between 2000Å and
This B1.5 V star is a commonly used polarized standard star for
ground based observers and is the one object in common between the WUPPE
and the HST FOS data bases (Sommerville et al. 1994). The data from the two
instruments are in generally good agreement although the HST
data may show some position angle variation at longer wavelengths
not seen in the WUPPE data. On the other hand, the HPOL visual
data show an upturn in the position angle longward of 6500Å.
A Serkowski curve with lambdamax = 5900Å and
Pmax= 4.01% reproduces the UV
polarization shape quite well.
HD161961 is a B0III star fainter than optimum for WUPPE
observation, however, CGS (1974) quoted an
intriguingly low value for lambdamax 4700Å.
is better fit by a
lambdamax 5300Å. This larger value is
matches the UV data adequately.
This moderately high galactic latitude, slightly reddened, and rather faint A0V
star was observed by WUPPE in parallel with a HUT program. A
WLR Serkowski curve with a lambdamax of 5800Å,
Pmax= 2.37% fit the data adequately.
This O9.5 III star was observed on Astro-2 for a HUT program. It is faint and only weakly polarized. If the WUPPE data are taken at face value, the object would seem to have a remarkably short lambdamax. Some combination of regions with different polarization characteristics along the line of sight might be able to produce such an effect, but intrinsic polarization caused by circumstellar material is a more plausible explanation for a hot giant star (the spectrum shows possible partial filling in of Halpha).
HD193237 = P Cyg
P Cygni brightened by several magnitudes for some years about 300 years ago before returning about to its previous brightness of about magnitude 6. It is thought that this was a result of the star throwing off a large amount of mass from its outer atmosphere. It is still blowing off material; the material is evident in the spectrum of the star (and emission lines of a particular kind are said to have 'P Cygni profiles' when seen in other stars), and, just as for Be stars, should cause polarization of the starlight.
In fact, P Cygni is a known polarimetric variable (Taylor et al. 1991b and references therein), with many observations from PBO, and so was put on the WUPPE target list of supergiants. However, it has substantial interstellar reddening and a well developed 2175Å bump so it is also likely to have substantial interstellar polarization. WUPPE and new PBO/HPOL data were taken almost simultaneously on 1995 March 12, HJD2449788.95.
This faint, heavily reddened O5e star has a very deep 2175 Angstrom extinction bump (Savage et al. 1985). Unfortunately it was observed too soon after a bright target (Kappa Cas), so the observation was severely affected by afterglow of the detector phosphor. Recent PBO spectropolarimetry shows a polarization spectrum which continues to rise into the near UV. These data are presented here to prompt further investigation of this star and this sight line.
HD197770 is a B2III star which in the Astro-1 WUPPE data appeared to have a polarization enhancement in the vicinity of the 2175Å extinction bump (Clayton et al. 1992). It was reobserved on Astro-2 for more than 2.5 times the Astro-1 integration time and has also been extensively reobserved with HPOL at PBO. The visible polarization between 4000-8000Å is reasonably well represented by a WLR Serkowski curve with lambdamax = 5100Å and Pmax = 3.83%. When data from both missions are combined it is seen that the UV polarization deviates from the extrapolation of the Serkowski formula even at 3000Å and remains high well short of 2000Å. As with many other objects, the new PBO system data show polarizations well below the Serkowski extrapolations longward of 8000Å. Clayton (1996) confirms that it is a spectroscopic binary with 99.8 day period. Two nearly equal, very short eclipses suggest almost equal components and near zero inclination. Though this is favorable to the production of intrinsic polarization by scattering processes in the orbital plane, electron scattering (the process most likely to do so in a system of hot stars) is wavelength independent and not likely to produce a UV lump.
This O9 IIe star represents an extremely well studied interstellar sight line with the UV extinction increasing rapidly shortward of the extinction bump (Massa, Savage, Fitzpatrick 1983). A fit of the WLR Serkowski curve to the PBO data yields an exceptionally low lambdamax, 4600Å. The extrapolation of this fit falls well below the observed UV polarization in accordance with the relation suggested by CWAL. This star shows a strong position angle rotation through the visual and continuing into the UV. The object is at roughly 1 kpc and has an E(B-V) of 0.62. SMF showed that the upper envelope of the polarization reddening relation is given by P(V) = 9E(B-V). Thus the observed low polarization of HD207198, Pmax= 0.98%, and the position angle rotation may be indicative of a multiple cloud sight line effect. However, polarimetric variability is also possible. Visual spectropolarimetry with PBO/HPOL on three different nights over the course of more than a year shows no significant variation, but this object will be reobserved and monitored.
This highly polarized B0V star is assigned lambdamax =
5600Å and Pmax= 5.61%
Due to its faintness and high extinction the WUPPE data is of rather poor
quality, but the extrapolation of the WLR Serkowski curve based on these
parameters is an adequate fit. The visual
spectropolarimetry with PBO/HPOL shows distinct position angle rotation,
but the WUPPE data are not of sufficient quality to confirm its continuation into
the UV. The object will be monitored for polarimetric variations.
Is a member of the association III Ceph; it is about 2' from a similar star, HD216629 = IL Ceph.
CGS (1974) quote lambdamax = 5400Å
and Pmax= 4.2% for
this O8 star, but a fit to new PBO/HPOL data
is better represented by a peak polarization at 5100Å. Using this
value for extrapolation of the Serkowski curve results in an adequate match to
the UV. This would seem contrary to the inverse relationship between
lambdamax and the degree of
excess UV polarization suggested by CWAL.
Is a member of association III Ceph.
This rather heavily reddened B0.5Ia star is along a well-studied sight line and was assigned a rather low value of lambdamax 4500Å, by CGS(1974). New PBO/HPOL data are better represented by 4700Å, which is still quite low. The data also indicate small position angle rotation. The WUPPE data are not of the highest quality, but the polarization would seem to be greater than expected by extrapolation of the WLR Serkowski curve for either of these values of lambdamax .