The Circumstellar Matter Around OH44.8-2.3

4 Pages·0.07 MB·

by  M. Bobrowsky

Asymmetrical Planetary Nebulae II: From Origins to Microstructures
ASP Conference Series, Vol. 199, 2000
J.H. Kastner, N. Soker, & S. Rappaport, eds.
The Circumstellar Matter Around OH44.8–2.3
M. Bobrowsky
Challenger Center for Space Science Education, 1250 North Pitt Street,
Alexandria, VA 22314
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0 B.W. Greeley
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2 Orbital Sciences Corporation, 7500 Greenway Center Drive, Greenbelt,
MD 20770
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a
J T. Ueta, M.M. Meixner

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Department of Astronomy, University of Illinois, Urbana, IL 61801

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v
Abstract. I-band images of the OH maser source OH44.8–2.3 are pre-
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1 sented. Having a well-determined distance, this object is shown to be
1 very young and physically small, with an asymmetric compact bipolar
1 nebula already present. In the morphological terminology of Meixner et
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al. (2000) and Ueta et al. (2000), this object would be classified as a
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DUPLEX nebula because of its high optical depth and pole-to-equator
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/ density contrast.
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s 1. Introduction
a
:
v OH/IR stars are important for understanding mass loss and stellar evolution
Xi at the end of the AGB phase. Mass loss rates have been derived in a variety
of ways: from the IRAS 60 µm flux (Jura 1987), the ratio of the 25 µm to 12
r
a µm flux (Van der Veen 1989), the OH maser flux (Baud & Habing 1983), and
from an OH photodissociation model combined with the spatial extent of the
OH maser emission. Excluding the last of these, there is reasonable agreement
about the mass loss rate of OH44.8–2.3: M ∼10−5 M⊙ yr−1. Indeed, Heske
et al. (1990) found that in the less extreme OH/IR stars, like OH44.8–2.3, the
mass loss rate derived from infrared properties agrees reasonably well with that
estimated from the CO emission.
OH44.8ishighlyvariable(IRASvariability index9)andhasavisualextinc-
tion of 1.9 magnitudes. Thetotal flux at Earth, corrected for extinction, is 8.9 x
10−11 W m−2, and the luminosity is 3950 L⊙ (Groenewegen 1994). Groenewe-
gen used a dust radiative transfer model to simultaneously solve the radiative
transfer equation and the equation of thermal equilibrium for the dust assum-
ing a grain radius of 0.05 µm, a grain density of 2 g cm−3, and a condensation
temperature of 1000 K. Parameters derived from the model included the optical
depth at 5 µm (τ = 1.22), the inner radius of the dust shell (rinner = 6.28 R∗),
and the gas-to-dust ratio (ψ = 0.0070).
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2 M. Bobrowsky, B.W. Greeley, T. Ueta, & M.M. Meixner
Figure 1. HST WFPC2 PC image of OH44.8–2.3 acquired with the
I (F814W) filter.
2. Observations
HST observations were obtained with the WFPC2 Planetary Camera in June
and July 1999. Dithered images were acquired using the V (F555W) and I
(F814W) filters. The dithered images were interlaced according to the sta-
tistical significance of each pixel using the “drizzling” algorithm. During the
drizzling process, the dithered frames were subpixelized resulting in a final im-
age with a higher resolution and smaller pixel size (0.′′0228/pixel). Bad pixels
(due to cosmic-rays) were removed by replacing the pixels with a median of the
neighboring pixels in each of the dithered frames. Deconvolution was carried
out using the Lucy algorithm.
3. Results
OH44.8–2.3 is not visible in the V-band images, but it can be clearly seen in the
I-band images shown both before (Fig. 1) and after (Fig. 2) deconvolution.
It is extremely compact, which is not surprising since it was unresolved in
themid-infraredsurveyofMeixner etal.(1999). Thespatial extentofOH44.8 is
∼0.′′4x0.′′6,withthelongaxisorientedapproximatelynorth-south. Thedecon-
volved image shows a toroidal structure surrounding apparent bipolar outflows.
At a distance of 1.2 kpc (van Langevelde et al. 1990), the radius of the torus
is 2.3 x 1015 cm. An expansion velocity of 16 km s−1 (te Lintel Hekkert 1990,
Meyer et al. 1998) leads to an age of only 45 yr!
OH44.8wouldbeclassifiedasaDUPLEXobjectintheclassificationscheme
of Meixner et al. (2000) and Ueta et al. (2000). With a totally obscured central
APS Conf. Ser. Style 3
Figure 2. I-band (F814W) image of OH44.8–2.3 with Lucy deconvo-
lution.
star, this object is similar to IRAS 16342–3814, an extreme AGB star, also
known to have H2O and OH maser emission (Likkel & Morris 1988, Sahai et
al. 1999). Also like IRAS 16342–3814, OH44.8 has unequal bipolar lobes in
which some inner structure can be seen. DUPLEX nebulae like that seen in
OH44.8 might be expected to be younger than SOLE nebulae. This would be
consistent with both the young age of OH44.8 and its late spectral type. (The
late spectral type also probably accounts for why OH44.8 was not seen in the V-
band images.) Finally, it appears that bipolar nebulae are preferentially found
close to theGalactic plane(Corradi& Schwarz (1995), consistent withOH44.8’s
Galactic height of 100 pc. Thus, besides being an OH maser source, OH44.8 is
a very young and very dusty bipolar nebula.
4. Acknowledgments
This research is based on observations with the NASA/ESA Hubble Space
Telescope, obtained at the Space Telescope Science Institute, which is oper-
ated by the Association of Universities for Research in Astronomy, Inc. Under
NASAcontract No.NAS5-26555. Bobrowsky is supportedbyNASAGrant GO-
06364.01-94A, and Meixner is supported by NASA Grant GO-06364.02-95A.
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