IRAS survey coadds for a 8.7 deg x 4.3 deg field near the Pleiades provide evidence for dynamical interaction between the cluster and the surrounding interstellar medium. The far-infrared images show large region of faint emission with bright rims east of the cluster, suggestive of a wake. Images of the far-infrared color temperature and 100 micron optical depth reveal temperature maxima and optical depth minima near the bright cluster stars, as well as a strong optical depth peak at the core of the adjacent CO cloud. Models for thermal dust emission near the stars indicate that most of the apparent optical depth minima near stars are illusory, but also provide indirect evidence for small interaction between the stars and the encroaching dust cloud

White, Richard E.

English

NASA Technical Reports Server

N9 1 - 1498
THERMAL EMISSION FROM INTERSTELLAR DUST IN AND NEAR THE
PLEIADES
Richard E. White*
*Five College Astronomy Department, Smith College, Northampton,
Massachusetts 01063 USA and
Department of Physics and Astronomy, University of Wyoming,
Laramie, Wyoming 82071 USA
ABSTRACT
IRAS survey coadds for a 8.7 ° x4.3 ° field near the Pleiades provide evidence for
dynamical interaction between the cluster and the surrounding interstellar medium.
The far-infrared images show large region of faint emission with bright rims east
of the cluster, suggestive of a wake. Images of the far-infrared color temperature
and 100 #m optical depth reveal temperature maxima and optical depth minima
near the bright cluster stars, as well as a strong optical depth peak at the core of
the adjacent CO cloud. Models for thermal dust emission near the stars indicate
that most of the apparent optical depth minima near stars are illusory, but also
and provide indirect evidence for small scale interaction between the stars and the
encroaching dust cloud.
INTRODUCTION
The encounter of the Pleiades star cluster with a small interstellar cloud (Gor-
don and Arny 1984; Federman and Willson 1984; White 1984a,b; Breger 1986, 1987)
has created a laboratory for the investigation of interstellar processes. The Infrared
Astronomical Satellite (IRAS) opened new windows to that laboratory, revealing
for the first time the distribution of long wavelength radiation emitted by the dust
in the familiar optical reflection nebulae. Castelaz, Sellgren, and Werner (1987,
CSW) reported on an initial examination of IRAS data for a 2.25°x2.25 ° region
centered on the cluster. They i:oncluded that the 60 #m and 100/_m data could be
understood as thermal emission from typical interstellar grains, whereas the 12 #m
and 25 #m data required the action of single-photon excitation of small grains or
large molecules.
This paper presents preliminary results from an investigation of new IRAS sur-
vey coadds. The data set is a mosaic of eight 2.25° x 2.25 ° fields, covering 8.7 ° x4.3 °,
a much larger region than studied by CSW.
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https://ntrs.nasa.gov/search.jsp?R=19910005668 2020-03-19T19:33:36+00:00Z
The data have received processing to remove some of the artifacts present in
the earlier data and the slope of the zodiacal background, but detectable signatures
of the satellite and the data processing remain.
The analysis follows the interpretation of CSW that the 60 #m and 100 #m
images primarily represent optically thin thermal emission, and yields images of
color temperature, To, and 100 #m optical depth, r, for an assumed dust emissivity
varying as A-1. [Although the derived values of Tc and r depend on the emissivity
law, qualitative characteristics of the images do not.]
LARGE SCALE STRUCTURE
Figure 1 is a contour diagram of the 60 #m surface brightness mosaic. The 100
#m mosaic exhibits the same features. The dominant features of both images are:
(1) bright emission to the right of center, at the position of the Pleiades; and (2)
the elongated region to the left (east) of the cluster, which exhibits faint emission
and a brighter rim. The orientation of this feature is consistent with the east-west
alignment of streamers in the Pleiades reflection nebulae (Amy 1977), and appears
to be the wake left by the passage of the cluster through the interstellar medium.
The individual subimages in the mosaic received entirely separate treatment
in the data processing. Consequently, although the surface brightness mosaics ap-
pear nearly seamless, images of far- infrared color temperature and optical depth
clearly reveal the subimage boundaries. This circumstance renders difficult further
interpretation of the large scale structure at the present time. The bright emission
near the cluster, however, is insensitive to small differences in background between
subimages.
THE CLUSTER ENVIRONMENT
The maximum surface brightness at both 60 #m and 100 #m occurs at a posi-
tion south of 23 Tau (Merope), although there are secondary peaks near positions of
luminous cluster stars. The images of color temperature and 100 #m optical depth
clarify the physical situation.
Figure 2 shows prominent peaks in To, up to 46 K, at or near the positions of
bright stars, above a broad plateau emission with Tc ~ 30 K. This pattern fulfills the
expectation that a star cluster embedded in an interstellar dust cloud would yield
local temperature maxima within a larger region heated by the collective ultraviolet
emission of the cluster stars.
Figure 3 exhibits a strong r peak that coincides with the core of the small CO
cloud southwest of 23 Tau (Federman and Willson 1984; Bally and White 1989),
fainter structure to the southwest of the peak that matches the morphology of the
CO cloud, and a ridge that details the dust distribution that is roughly outlined by
optical polarization data (Breger 1986, 198"/).
The figure also exhibits local minima near the positions of the brightest stars,
318
and a minimum that extends eastward from the cluster core. The latter feature
corresponds to the 21-cm "hole" discussed by Gordon and Arny (1984) and con-
firmed by Breger (1986). Its larger structure is confused by mosaic boundaries, but
it suggests that the large scale "hole" present in Figure 1 results primarily from a
dearth of dust, not from unusually low dust temperatures.
SMALL SCALE STRUCTURE
Preliminary analysis of these images has included construction of simple models
for the thermal emission near several stars, under the assumption that the dust is
optically thin both in the infrared and in the ultraviolet, with parameters adjusted
to give reasonable representations of the Te- and r-profiles within 15' of the star.
Results to date include the following.
1. Apparent minima in r occur at the positions of luminous stars even for uni-
form layers, as a consequence of the single temperature analysis of the surface
brightness ratios.
2. The most luminous Pleiades star, rt Tan, lies in a real cavity in the dust dis-
tribution that is evident both in Tc and r.
3. Unless the 100 #m dust emissivity is unexpectedly large, >0.005, all the stars
examined require unresolved cavities to account for the relatively modest color
temperatures observed.
Thus, although apparent r-minima near the stars must be regarded with cau-
tion, the analysis indicates that small scale interactions between the stars and the
surrounding interstellar medium do occur. Possible mechanisms for interaction
between the stars and the surrounding interstellar medium include ram pressure
from stellar winds, the direct influence of radiation pressure, and thermal pressure
resulting as the interstellar .gas is heated indirectly by starlight.
CONCLUSIONS
1. Far-infrared images of an 8.7 ° x4.3 ° region near the Pleiades provide evidence
for a "wake" produced by passage of the cluster through the ambient interstellar
medium.
2. Images of the far-infrared color temperature and 100 /_m optical depth in a
2.6 ° x2.6 ° region near the cluster show:
a. temperature peaks and optical depth minima near the positions of lumi-
nous stars;
b. an optical depth peak and fainter structure corresponding to that seen in
CO emission, plus more extended features previously inferred from polar-
ization data.
319
3. Models for the thermal emission near the stars provide indirect evidence for
unresolved cavities resulting from dynamical interaction between the stars and
the surrounding interstellar medium.
4. The optical depth distribution therefore suggests the presence of both large
scale and small scale dynamical interactions between the Pleiades and an in-
terstellar cloud that appears to be encroaching from the western side of the
cluster (Arny 1977; Gordon and Arny 1984).
REFERENCES
Arny, T. 1977, Ap. J., 217, 83.
Bally, J. and White, R. E. 1989, in preparation.
Breger, M. 1986, Ap. J., 309, 311.
Breger, M. 1987, _, 319, 754.
Castelaz, M., Sellgren, K., and Werner, M. W. 1987, Ap. J., 313, 509.
Federman, S. R., and Willson, R. F. 1984, Ap. J., 283, 626.
Gordon, K. J., and Arny, T. T. 1984, A. J_____=.,89, 672.
White, R. E. 1984a, _, 284, 685.
White, R. E. 1984b, _, 684,695.
260 O' 0"
25 ° O" 0"
Dec.
(1950}24 ° O' 0"
23 o O" 0"
,,..2" " y _ .... _ ""_--:"....... ':-
5 m 4 h 0 m 55m 50 m 45m 40rn 35 m 30 m
3 h
R.A. [19501
Figure 1. Contour diagram of 60 #m surface brightness mosaic, which covers
8.7°x 4.3 ° near the Pleiades. The contour interval is 0.25 dex. The brightest contour
represents log S60 (Jy sr -1) = 8.0. The peak value is 8.15.
320
25° O'
30'
Dec .
240 O'
119501
30'
230 O'
C:_ . - 0
0"
0" C)
0"
0"
0"
45 m 40m
3 h
R.A. 11950]
Figure 2. Color temperatures in the immediate vicinity of the Pleiades, derived
from the ratio of 60/_m and 100 #m surface brightnesses assuming that the opacity
varies as A-1.
The contour interval is 4 K. The brightest contour represents 44 K, and the
peak value is 45.3 K. The crosses represent positions of the most luminous cluster
stars.
25° O' O" '_ 0 r__o _ J
11950}24° O' 0"
30' 0"
230 O' 0"
45m 40m
3 h
R.A. 11950l
Figure 3. Optical depths derived from the 100 #m surface brightnesses and
the color temperatures in Figure 3. The contour interval is 5×10 -5 . The highest
contour represents 6×10 -4 . The peak value near the cluster is 5.6×10 -4 . The
crosses represent positions of the most luminous cluster stars.
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Thermal emission from interstellar dust in and near the Pleiades

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