Observations were made in the IUE LWP camera, low dispersion mode, with alpha Ori being offset various distances from the center of the Long Wavelength Large Aperture along its major axis. Signal was acquired at all offset positions and is comprised of unequal components of background/dark counts, telescope-scattered light, and scattered light emanating from the extended circumstellar shell. The star is known from optical and infrared observations to possess an extended, arc-minute sized, shell of cool material. Attempts to observe this shell with the IUE are described, although the deconvolution of the stellar signal from the telescope scattered light requires further calibration effort

Brown, A.

Carpenter, K. G.

Judge, P.

Pesce, J. E.

Skinner, S.

Stencel, R. E.

English

NASA Technical Reports Server

/i,
N89-10730
249
DIRECT UV OBSERVATIONS OF THE
CIRCUMSTELLAR ENVELOPE OF _, ORIONIS
I/.E.Stem'el, I,:.G.Carpcm.cr, J.E.Pesce, S.Sklnner
Center for Ast,ophysics
and Space Astionomy
Uniw:rsity of Colorado
Bouhlel, Colorado 80309 USA
_lllt{
A.Brown, P.Judge
Joint Institute for
Laboratory Astrophysic_
Univc'isity of Colorado
Boulder, Colorado 80309 USA
ABSTRACT
(_ Orionis is a red supergiant st,ax with significant mass
loss (_ 10-%\'/c:)gr -I ). It. is known flora optical and inflated
o])sei viltlons to possess lilt extendc(1, arcqlllllllt, e sized, shell
of cool material. Attempts to observe this shell with the
IUE are described, alth(mgll t.},(.'deconvolution (,fthe stellar
signal fl'om the telescope sca ttei ed light will requn (.' fin ther
calibration effort.
I',:eywords. Cxrcumstellar shell; Seatteied light.; Chromo-
spheres, Ultraviolet Slmetroscopy.
1. INTF{ODUC']'I()N
There exists all. are-mimlte sized shell of luminous material
surrounding the red Sulwrgiant , (_ Ori. It is observed m the
light, of the low al)undance species, neutial potassiunl (at
7699,:{ - Honeycutt, et al. 1980), and in the far-infrared
by IRAS (StenceI, Bauer and Pebce 1988). This fact sug-
gested to us that it might be l)ossible to observe scattered
Mg II 2800,4 t)hotons over a comparable dm_ension, if the
chromospheric ionization fractmn is maintained in the out-
flow. Theoretical predictmns of the ionization flactmn in
the outer atmosphere, by Glassgold and Huggins (1986): as
well as direct measurement, usnlg microwave techniques,
(Drake and Lmsky 1986; Drake et al. 1987), appear to
support the expectalion that there are numerous Mg ions
scattered over large (hstances in the envelope of n Orl. IUE
as well matched to an investigation of the outer envelope,
given the 10 by 20 arc second apertures, which sample di-
mcnsums of roughly 0Xl()16Cl,_( _ ]():_R.) at the (hstanee of
(_ Ori
A series of observations wine made in the LWP camera,
h,w dispei.sion mode (IUE piogram CS.IRS), with (_ OIi
l,cing offset, various distances from the center of the Long
Wavelength Large Aperture (L\VLA), along its major axis.
The longest exl)osure time was 480 mimltes, with tim star
30 aicsecon(ls flora the ccilter of the LWLA. Delived si R-
hal levels arc provided ill Table 1. normahzed to an IUE
Flux Nu,nlml (EN) per mimtte scale. The peak FN levels
m e in the ,,'ent,er of the Mg II emissum feature and in th("
lleal'by continuum, around 2825A. The reported temI)oral
variations in the Mg II emission strength (of. Dul,iee et
al., tln,_ volume) were not considered in the present work
Signal was acquired at all offset pobit, lOll.S (Figures 1 and
2), and is comprised of unequal comi)onents of (1) back-
ground/dark counts, (2) teh:scope scattered light and (3)
_(',_t.teIc(1 hght _m,anatHtg fI(ml the extort(led cxicumslcll,tr
shell of a Ori. The chalh:nge is to succc.ssfully de('onvolve
these component signals and theIeby reveal the intrinsic
stellar signal, winch can then he used to estm_ate the den-
sity, ionization and velocity field of the outer envelope.
2. SCATTERED LIGHT CORRECTIONS
In order to remove the detector background (dark count),
we averaged the signal m imrt of the image, off the spec-
trum, near Mg II 2800A. We then subtx acted this dark level
from the data containing the spectrum data. The residual
e.mission line signal [net FN per minute in Mg II] exhibits
an approximate d -_l falloff, for offsets (d) greater than 5
aresec. The on-source exposure and the image with the
LWLA centered 13" away (peak flux at 3" fl'om the star),
seem to lie along a steeper slope. Although signal in nearby
continuum is weak, its falloff may he even slower If we use
the ironer 5 arcsec as a zero-order ai)proxnn_ion to the
telescope scattered light, it, would appear that the signal at
A Decade of UV Astronomy wtth IUE, Proc Celebrato 0, Sympo,_non: GSFC, Greenbelt, USA, 12-15 Aprtl 1988, ESA SP-281, Vol. I (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001359 2020-03-20T06:08:55+00:00Z
,°
J
250 R E STENCEL ET AL
10 and 20 arcsec from the star is relatively uncontaminated
by scattered light. The ratios of brightnesses, compared
to on-source values, are about 100 times that reported by
Mauron et al. (1984) for K I 7699]1 observations. However,
there is no a priom justification for this kind of signal cor-
rection, and there is reasonable concern that tile innermost
data could be contaminated by the stellar image (3 arcsec
FWHM).
The precise removal of telescope-scattered light from the
residual signal is less straightforward. Unfortunately, there
appears to be little surviving documentation of any pre-
flight scattered light cahbrations (e.g. Coleman et al. 1977).
The original on-orbit empirical studies were made by Witt
el al. (1982) who observed the early B dwarf star, 77 UMa
at positions along the major axis of the LWLA and derived
an approximate d -3 3 falloff. We have re-examined their
data and agree with their conclusion. In principle, this in-
formation could be used _o remove the telescope- scattered
light component from the residual signal, except for the fol-
lowing complications:
(a) Witt st al. reported scattered light levels in the
LWR camera, not the LWP camera we used. The extra
mirror feeding the LWR camera might be expected to in-
crease the absolute magnitude of scattered light. DeBoer
and Cassatella (1986) reported on measures of scattered
light in the LVV'P camera, but unfortunately placed r/ UMa
along the minor axis of the LWLA, which, as Carpenter
et al. (1987) demonstrated, is heavily contaminated by a
&ffraction spike. It is unclear whether the slope is affected,
but the magnitude of the scattered light could be enhanced.
We argued that new LWP observations of 7/UMa along the
major axis of the LWLA would be required, and three such
observations were obtained during 1988 February [program
OD38Y] as detailed in Table 2. The short time between re-
ceipt of the data tapes and this conference has not allowed
for in-depth analysis of the new information. However, pre-
liminary analysis suggests the magnitude of the scattered
light for _7UMa is slightly greater than that for a Ori, by 12
and 357o at 10 and 20 arcsec respectively. Given this ten-
tative result, we conclude that the blue star, 77 UMa, may
not be the most suitable object for establishing the level of
telescope scattered light contribution around red stars like
a Ori. Another problem with 77 gMa is that the on-source
observations must be trailed to avoid overexposure. Paren-
thetmally, we also note the non-symmetry m the LWR data
obtained on opposite sides of the L\VLA, at +20 and 20
arcsec. The peak signal strengths differ by a factor of 1.5,
despite the virtually identical exposures. Given the slightly
different optical path, the detailed mirror surface influence
apparently is non-negligible.
(b) Unfortunately, at the time of tills writing, there are
no data which would allow us to assess the significance of
stellar effective temperature as it relates to the intensity of
telescope scattered light, a On is a red supergiant with
a color temperature of about 3500K, while 77 UMa is a B
dwarf with a color temperature of about 20,000K. Signifi-
cant scattered light has been observed shortward of Ly a
(1215A) in tile SWP camera when observing F type stars,
although no analogous light has been seen shortward of C
II] 2325A even in deep LWR or LWP camera observations
of red stars. To investigate the magmtude of this effect,
we propose that off-source observations of bright late-type
inain sequence stars, with negligible mass loss [e.g. e Eri]
should be undertaken. Our eleventh year IUE program,
CMKRS, will pursue this.
3. DISCUSSION
Despite the present ambiguity regarding the correction for
scattered light, we can still examine the implications of de-
tection of Mg II light far from the star. Various authors
have derived mass loss rates for a Ori, typically around
10-6M®yr -1 . With the observed circumstellar shell expan-
sion of approximately 15 km/sec, this implies a matter den-
slty of '_, IOlcnz -3 at 10 3 radii. From this, we could esti-
mate an intrinsic efficiency for scattering of 2800A photons
by Mg ions, subject to uncertain assumptions about tile
degree of ionization in the outer envelope. Calculations of
the brightness distributions of resonant line photons have
been made by Natta and Beckwith (1986) but their results
are restricted to Sobolev regimes which cannot be applied to
tile Mg II line transfer problem m coot supergiants, namely,
low optical depths and generally higher ratios of flow speed
to microturbulence than are present in the expanding ehro
mosphere of a Ori. Judge intends to begin calculations
which remove the Natta and Beckwith iN&B] restriction
to aro much less than 1 (where a is the Voigt parameter),
and to include "diffusive" transfer in the line wings (e.g.
Basri 1979) which inhibit the direct escape processes as in
N&B's calculations. Tiffs will enhance the intensity of pho-
tons further out in the circnmstellar shell relative to the
N&B results. Drake (1985) found a large effective angular
dmmeter in Mg lI 2800A for a Boo using this approach.
A more complete analysis of these data is being prepared
for pubhcation elsewhere. We are pleased to acknowledge
the support of NASA grant NAG5-816 to the University
of Colorado, as well as the outstanding help of the Res-
ident Astronomers and Telescope Operators at both tile
Goddard and Vilspa ground station SOCs. We also thank
.James Neff for software assistance, Susan Conat Sfiencel for
echtorial improvements of the text, and acknowledge the
Boulder RDAF (NAS5-28731) for its helpful facilitation of
this work
4. REFERENCES
Basri, G. 1979 A_trophys. J. 230, 924.
Beckwith, S. and Natta, A. 1987 A,_tron. Aatrophys 181,
Ol.
Carpenter, K., Stencel, 12., Pesce, J., Brown, A. and Judge.
P. 1987 IUE NASA Newsletter aa. p. 2O.
Coleman, C. et al. 1977 "IUE Camera Users' Guide"
IUE Tech. Note al, UCL/Rutherford Appleton Lab, cited
in NASA IUE Newsletter 32 (1987).
DeBoer, I(. and Cassatella, A. 1986 New Insighta into Astro-
physzca ESA Spec. Pubh 263, p. 665.
_:C'_, : ,,,. F ,'.,..,_ "_
oF QUAUT 
DIRECT UV OBSERVATIONS OF THE C1RCUMSTELLAR ENVELOPE OF ALPHA ORIONIS 251
Drake, S. 1985 in Progre.q,s zn Stellar Spectral Lzne For-
rnatzon and Theory, e(E. J. Beckman and L. C,rlvcllniu
(Dordrecht; iC[eidel).
Drake, S. and Linsky, J. 1986 A,_tron. J. 91, G02.
Drake, S., Linsky, J. and Elitzur, M 1987 Aatron J. 94,
1280.
Glassgold, A. and Huggins, P. 1986 Ap.J. 306,605.
Honeycutt, R., Bernat, A., Kephart, J., Gow, C., Sandford,
M. and Lambert, D. 1980 Ap.J. 239, 565.
Mauron, N., Fort, B., Querci, F., Dreux, M , Fauconmer,
T. and Lamy, P. 1984 Aatron. A,_trophy._ 130,341
Natta, A. and Beckwith, S. 1986 Aatron. A,_trophy,< 158,
310.
Stencel, R., Bauer, W. and Pesee, J. 1988 A J 95 141.
Witt, A., Walker, G., Bohlin, R. and Stechm, T. 1982 Ap J
261,492.
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Flgurc 1: Positions of the LWLA with respect to o. Ori, for
the new LWP observations reported here (Table 1).
Table 1
_ Orx IUE Data Used. programs CSJRS and OD14Y
hnage OffSet ExI)osme Peak FN Bkgd FN Log Net. FN/mln Log Net FN/min
arcsec (in Mg II) (nc, ml)y c, mt )
LWPll600 0'" 5 sec. 8,224 _ 0 4.99 ( 0 00) 4 08 ( 0 IO())
LWP10239 +13" 5rain. 5,519 _0 3.04(--195) 223( I 85)
L\VP1035G +15" 30 rain 6,830 1,850 99o (_2.77) 156 (252)
L\VP11599 +20" 225 rain. 12,796 2,475 1.66 (333) 1.00 (3.08)
LWP11598 +30" 480 rain 8,032 3,328 0.99(400) 039( 3.691
Note: Offset is to center of LWLA, whih' peak signal occurq
in the al)erture at tlm posit:on closest to the star, goner
ally 10" tic,:::the nonnnal offset The Log Net FN/mm m
parcnthe.w.s are relative to the on source vahw. Prehmi
nary values
Tal)h. '2
_/ UMa IUE Data Used, progta,n* PttCAL and OD38Y
Ima,gc Offset Exposure Peak FN Bkgd FN Log Net FN/mln
a, (',(,'c [n('ar 2S00A]
L\\7122127 0" 0.29 s('c,'F 34,106 _ 0. 7.36 ( 0 00)
L\VRO576 + 12.5 5 sec 61,524 _ 0. 5 57 (1.79)
L\VR6575 + 15"' 25 sec 57,000 _ 0 5 14 ( 2 22)
L\VRC579 + 18" 22 sec 6,172 _ 0 4 22 ( 3 14)
L\VRO57-1 + 20" 08 sec 17,146 _ 0 4 17 (3.19)
L\VRO577 -20" 67 sec 10,810 _ 0. 3.99 ( 3 37)
L\V126578 40" 271 _ec 4,856 771 2 96 (4.40)
LWP 9700 0" 0.19 sec,T 7,556 _ 0. 6.85 / 0.00)
L\VP12653 + 20" 45 sec 2,827 _ 0. 3.58 (3.27)
L\VP12652 + 30"' 00 sec 949 _ 0. 2.98 (-3.87)
LWP12654 + 30" 300 sec 5,048 258. 2 98 (-3 87)
Note: offset is to center of LWLA, while peak signal occurs
m the aperture at the positlon closest to the star, generally
10" flora dm nominal offset. The T next to the exposure
time indicates a trailed exposure, where the effedive ex-
l,OSu:e :s 3.7 time_ the pointed exposure. No COllC('tions
f',)r cam(!ra scnstivity changes have 1)con made The Log
N('t FN/minute numbers in par(,nth(,._es aw r(:l_t:v(, to the
on source value. Pr('hminary values.
252 R E STENCEL ET AL
£
3o
LWP11600, on source alpha Orl, 2750 to 2850A
i
10 15 20 25
d_stence in orcseconds
A
t
&,
.,',,
LWP10239, alp Or, 13 orcsec offset, 5 rain
B
o , _°-_. _,__0 ,,","°
dlstonce m orcseconds
£
3o
LWP10356, otp Orb 15 orcsec offset, 30 mm
@
C_
i ;
,?,
L I
5
C
v
zoO4
_.,g. ,&,
o
CZ_
, 5-v
v
d,stonce m arcseconds
30
20 _7_
10
0
3O
LWP11599, alp On, 20 orcsec offset, 225 m*n
_z2
°
_-----o-o-o-o-o-o-o-_
c_ t
5
D
L A,
d*s_ance m orcsecond_
_0
5O
4O
3O
20
10
0
0
LWP11598,' olp Ori, 30 orcsec offset, 480 ram,
r ..... % ?_-'/? _.----'_
o .o:o 
5 10 ;@ 20 2@ 30
dlstonce *n orc_econdm
60,
40
30
1:!
f
½
J
L_
J
LWP9700, eta UMo traded across enbre LWLA
Tf
10 15 20 25
dl=tonce m o¢¢seconds
Figure 2: Montage of contour maps of spectral images of
a On, near Mg II at varying offset distances. The images
are: (a) LWP 11600; (b) LWP 10239; (c) LWP 10356; (d)
LWP 11599; (e) LWP 11598, and (f) LWP 9700 (,_ UMa,
on source, trailed exposure illuminating entire LWLA, for
scale). See Tables 1 and 2 for characteristics of each. Each
frame plots a spatial dimension (abscissa, 30 arcseconds full
range) versus a spectral dimension (2750 to 2850A). Note
the gradient in the signal toward the edge of the LWLA, in
the direction of the star (off to the right). Local maxima
and minima at marked, units are FN.
F
l


Direct UV observations of the circumstellar envelope of alpha Orionis

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