First results of an on-going program to determine the wavelength dependence of the interstellar optical polarization of reddened stars in the Small Magellanic Cloud (SMC) are presented. IUE observations of reddened stars in the SMC (Bouchet et al. 1985) generally show marked differences in the extinction law as compared to both the Galaxy and the Large Megallanic Cloud. The aim here is to determine the wavelength dependence of the optical linear polarization in the direction of several such stars in the SMC in order to further constrain the dust composition and size distribution in that galaxy

Coyne, G. V.

Magalhaes, Antonio M.

Piirola, Valero

Rodrigues, C. V.

English

NASA Technical Reports Server

N91-I 98 
MULTICOLOR OPTICAL POLARIMETRY OF REDDENED STARS IN THE SMALL
MAGELLANIC CLOUD
A.M. Magalhaes,* G.V. Coyne,** V. Piirola,*** and C.V.
Rodrigues*
*Instituto Astronomico e Geof{sico, Universidade de Sao Paulo,
Caixa Postal 30.627, Sao Paulo 01051, Brazil
**Vatican Observatory, V-00120 Citta del Vaticano
***Observatory and Astrophysics Laboratory, University of
Helsinki, Tahtitorninmaki, SF-00130 Helsinki 13, Finland
SUMMARY
First results of an on-going program to determine the wavelength
dependence of the interstellar optical polarization of reddened stars
in the Small Magellanic Cloud are presented.
I. INTRODUCTION
IUE observations of reddened stars in the Small Magellanic Cloud
(SMC) (Bouchet et al. 1985) generally show marked differences in the
extinction law as compared to both the Galaxy and the Large Magellanic
Cloud. Our aim is to determine the wavelength dependence of the optical
linear polarization in the direction of several such stars in the SMC
in order to further constrain the dust composition and size distribution
in that galaxy.
II. INSTRUMENTATION
The observations reported here were mostly gathered with the
VATPOL polarimeter (Magalhaes et al. 1984) of the Vatican Observatory
at the 2.15m argentinan national telescope in San Juan and with the
MINIPOL polarimeter (Frecker and Serkowski 1976) at CTIO. A few
observations were also obtained with PISCO polarimeter (Stahl et al.
1986) at ESO/La Silla. Later in this season, observations are scheduled
with both VATPOL and the University of Sao Paulo IAGPOL polarimeter
(Magalhaes and Velloso 1988) at the 1.60m brazilian national telescope.
III. FOREGROUND GALACTIC POLARIZATION
The observed optical linear polarization in the SMC in our sample
is typically fairly small (section IV below), which makes the correction
due to foreground galactic dust fairly critical, despite the low (0.m02)
foreground reddening (McNamara and Feltz 1980). Schmidt (1976) presents
a rather detailed study of the foreground polarization in the SMC
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direction, dividing it into five fields and suggesting the necessary
foreground corrections in each section.
We also considered separately, from Schmidt's sample, stars in
each field at distances larger than 400 pc; most of the foreground
reddening occurs within this distance (McNamara and Feltz 1980). We
also considered averages of SMC stars from the sample of Magalhaes et
al. (1987) and from the present sample with visual polarization smaller
than 0.4%.
These estimates are compared in Table 1 below. We then corrected
the SMC observation in each filter employing Serkowski's (Coyne et al.
1974) relation for the galactic interstellar polarization,
P(%) = Pmax * exp {-K in2(% max/ % )},
chosing to use for Pmax the P value from galactic stars with r > 400pc
and the relation between K and %max as given by Wilking et al. (1982).
We used 0.55um for %max, consistent with own multicolor observations
of foreground stars.
IV. RESULTS
With one exception, our sample contains stars which have been
observed for ultraviolet extinction by Bouchet et al (1985) and we
refer to them according to their number in the catalogue of Azzopardi
and Vigneau (1982). The exception is AV 211 which (as far as we know)
has not had its UV extinction determined. We employ colour excesses
from Bouchet et al. For AV 211, we have estimated the colour excess
using photometric and spectroscopic data in Azzopardi and Vigneau and
the calibration by Brunet (1975).
Fig. i presents the correlation we obtain between Pmax and
EB-V. Pmax was obtained by fitting the data with Serkowski's relation,
using the galactic relation between K and %max. As can be seen, the
intrinsic polarization for most of the stars is fairly small, albeit
consistent with their colour excesses. The galactic envelope is also
indicated for comparison.
For the three more polarized objects, AV 211, AV 398 and AV 456,
we present details of the fits in Figs. 2 through 4 and in Table 2.
The derived values of Pmax and %max stay the same within the errors
even if a allowing the fit to include the parameter K as a free
parameter as well as rather insensitive to the foreground polarization
choice. We include in Table 2 the ratio between R (=Av/EB_v) and %max,
assuming R=2.7 ± 0.2 for the SMC as given by Bouchet et al. (1985).
These values should be compared to the galactic one of about 6.7
(Clayton and Mathis 1988) and are meant to be taken as indicative only,
since many more stars will have to be studied. More observations should
allow us to check more closely the relation between K and _max for the
SMC.
AV 456 has an extinction law (Lequeux et al. 1982) and gas-to-dust
ratio (Bouchet et al. 1985) close to galactic values and its %max
(Table 2) seems to reflect that the grains responsible for the optical
interstellar polarization in that direction are indeed similar to those
in the Galaxy. AV 398, the most heavily reddened object, is also the
most polarized one and presents UV extinction law typical of the SMC
(Bouchet et al. 1985), with a steep rise into the UV and absence of a
348
strong 2200 _ peak. Its %max, although somewhat smaller, is still
quite comparable to galactic values. AV 211 presents
the smaller %max of the three objects.
Even values of Xma x close to "normal" should have an important
bearing on grain modelling for the SMC. For instance, if the details
mentioned above in the SMC extinction law are interpreted, in the
context of the Mathis et al. (1977) model, as the result of a smaller
role of graphite grains in the visible, the size distribution of
silicate grains would have to be shifted in the direction of larger
grains (Bouchet et al. 1985) in order to account properly for the
visual extinction. If, as expected, variations in %max reflect distinct
size distributions of dust particles (as for the Galaxy - Clayton and
Mathis 1988), the preliminary results of Table 2 point either against
that conclusion or to a size distribution of aligned grains (probably
still silicates) in the SMC distinct from the one producing the
extinction. Clearly, the forthcoming observations of this program will
be of importance regarding the grain population in the SMC.
Finally, Table 2 shows a comparison between the optical (this
work) and radio polarization position angles (from Loiseau et al.
1987). Since the latter is of non-thermal origin and would indicate a
magnetic field direction orthogonal to that of a similarly oriented
optical polarization vector, it is quite probable that the radio and
optical polarixations originate from distinct regions in and around
the SMC (Loiseau et al. 1987; Magalhaes et al 1987).
ACKNOWLEDGEMENTS
A.M.M. and C.V.R. would like to acknowledge support from Fundacao
de Amparo a pesquisa do Estado de Sao Paulo (FAPESP) for this project.
A.M.M. also acknowledges support for his travel from FAPESP through
grant nQ 88/1415-4 and the IAU through the Local Organizing Committee.
REFERENCES
Azzopardi, M. and Vigneau, J.: 1982, Astron. Astrophys. Suppl. Ser.
50, 291.
Bouchet, P., Lequeux, J., Maurice, E., Prevot, L.,and Prevot-Burnichon,
M.L.: 1985, Astron. Astrophys. 149, 330.
Brunet, J.P.: 1975, Astron. Astrophys. 43, 345.
Clayton, G.C. and Mathis, J.S.: 1988, Ap. J. 327, 911.
Coyne, G.V., Gehrels, T. and Serkowski, K.: 1974, A.J. __79' 581.
Frecker, J. and Serkowski, K.: 1976, Appl. Opt. 15, 605.
Lequeux, J., Maurice, E., Prevot, L. and Prevot-Burnichon, M.L.: 1982,
Astron. Astrophys. 113, L15.
Loiseau, N., Klein, U., Greybe, A., Wielebinski, R. and Haynes, R.F.:
1987, Astron. Astrophys. 178, 62.
349
Magalhaes, A. M., Benedetti, E. and Roland, E. H.: 1984, Publ. Astron.
Soc. Pac. 96, 383.
Magalhaes, A. M., Loiseau, N. and Piirola, V.: 1987, Workshop in
"Magnetic Fields and Extragalatic Objets", Corsega, p. 185.
Magalhaes, A.M. and Velloso, W.F.: 1988, in "Instrumentation for
Ground-Based Optical Astronomy: Present and Future" (ed. L.B.
Robinson), Springer-Verlag (N. York), p. 638.
Mathis, J.S., Rumpl, W. and Nordsiek, K.H.: 1977, Ap. J. 217, 425.
McNamara, D.H. and Feltz Jr., K.A.: 1980, Publ. Astron. Soc. Pac.
92, 587.
Schmidt, Th.: 1976, Astron. Astrophys. Suppl. 24, 357.
Stahl, O., Buzzoni, B., Kraus, G., Schwarz, H., Metz, K. and Roth, M.:
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Wilking, B.A., Lebofsky, M.J. and Rieke, G.H.: 1982, A.J. 87, 695.
350
TABLE 1. FOREGROUND POLARIZATION IN THE DIRECTION OF THE SMC
Field
Schmidt (1976) r > 400 pc SMC stars*
P i o @ P±o _ PiO e
(7o) (o) (70) (o) (7o) (o)
I .37 ± .15 111 .36 ± .05 110 .22 ± .01 129
II .27 i .15 123 .27 ± .03 125 .32 i .01 118
III .06 i .09 139 .18 ± .03 132 .09 ± .01 139
IV .14 ± .12 125 .25 i .02 127 .09 ± .03 159
V .16 ± .12 93 .35 ± .04 102 .16 i .04 117
* w/P v < 0.4%
TABLE 2. FITTED PARAMETERS FOR THE MORE HIGHLY POLARIZED OBJECTS
Star Pmax % max R/% max @ opt Brad
(%) (A) (o) (o)
AV 211 1.01± .05 .41± .05 6.6± .9 128 135
AV 398 1.70± .05 .49± .04 5.5± .6 135 135
AV 456 1.19 ± .05 .55 ± .05 4.9± .6 166 140
Fig.
3
Pmax(%)
2
_" Av 4s6
• AV 456
"_ • .s. , , ,
0 OI 02 03 04 05
EB.v
i - Maximum polarization as a function of colour excess for
the observed SMC stars.
351
I I I I I I
AV 211
14
I0
P {%)
O6
02
03 014 015 0 i6 017 0_8 OL9 J0
Wavelength ( IJm )
Fig. 2 - Wavelength dependence of the linear polarization for the
SMC star AV 211.
25
P (%)
20
1.5
I0
05
I I I I I I
AV 398
I Io3 0'4 0'5 o16 o!7 08 o9
Wavelength ( i_ m )
Fig. 3 - Same as Fig.2 for AV 398.
18
P(%) I0
I I I I
06
I I
AV 456
o.'4 o15 o.'6 o17 o18
Wovelength ( pm )
Fig. 4 - Same as Fig.2 for AV 456.
02
03 019
352
I0


Multicolor optical polarimetry of reddened stars in the small Magellanic cloud

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