HD 172147: MIII Giant with A Debris Disk

1
HD 172147 MIII Giant with A Debris Disk?

• S. D. Bloom (Hampden-Sydney), D. E. Backman
  (NASA/Ames) R. O. Gray (Appalachian State)

Abstract The
faint IRAS object, F183425913, is one of many
which have not been well studied. It is shown
here that this IRAS source is identified with the
star HD 172147 (V8.28) and additional data
indicate there is possibly a debris disk around
this star. The dust in this disk may originate
from sublimation of comets heated by the
increasing luminosity of the evolving star.
Though originally classified as a K5 star, we
also conclude from the most recent visual and
infrared data (including spectroscopy) that this
star was spectrally misclassified in the HD
catalogue, and is a M0 IIIab giant. As such, it
is only one of few giants with a known debris
disk. We further conclude that the star must be
nearly nine times more distant than indicated by
the Tycho parallax (60.6 pc).
HD 172147 (IRAS F183425913)
Stellar Spectrum
Palomar Sky Survey image (negative) of HD 172147
Observations and
Methods We have used archived visual and
infrared photometric data to construct a spectral
energy distribution (Figure 2). The near infrared
data are from 2MASS, and the far-infrared data
are from IRAS. The upper limits are from COBEs
DIRBE instrument. We have also obtained a recent
spectrum at Appalachian States Deep Sky
Observatory (Figure 1). The Gray/Miller
Spectrograph with 600 g/mm was used in the first
order. The 2 pixel resolution of the spectrum is
3.6 angstroms and the spectral range is 3800-5600
angstroms. The spectral type has been determined
by comparison to MK standards. In particular,
the relative strengths of the Ca I line (4227
angstroms) and TiO bands are compared. This
process indicates an M0 IIIab spectral type for
HD 172147.
Dust Spectrum
Figure 2 Spectral Energy Distribution of HD
172147
Discussion and
Conclusions Modeling of the broadband spectrum
indicates the possibility of a far-infrared
excess, particularly at 60 microns (Figure 2).
Such an excess can be explained by a
circumstellar dust disk . Though we can only set
limits to model values, we determine 40 K lt T lt
80 K with an area of 3.5 x 1028 cm2 lt A lt 8.8 x
1028 cm2 (smaller area corresponds to higher
temperature). Various ranges of distance from
the star to the dust are acceptable, depending on
other model assumptions, but , for example, a
belt extending from 100 500 A. U. with a mass
of about 100 ME is consistent with the
observations.
Debris Disk (Sublimating Comets?)
HD 172147 (MIII Giant)
100 A.U.
500 A. U.
Sun (GV Dwarf)
Neptune
30 A. U.
Kuiper Belt
Figure 1 Spectrum of HD 172147. The spectrum of
this star is shown between two standards with
similar spectral features.
HD 172147 System Compared to Solar System
(not to scale)