265 research outputs found
Variability in Proto-Planetary Nebulae: I. Light Curve Studies of 12 Carbon-Rich Objects
We have carried out long-term (14 years) V and R photometric monitoring of 12
carbon-rich proto-planetary nebulae. The light and color curves display
variability in all of them. The light curves are complex and suggest multiple
periods, changing periods, and/or changing amplitudes, which are attributed to
pulsation. A dominant period has been determined for each and found to be in
the range of ~150 d for the coolest (G8) to 35-40 d for the warmest (F3). A
clear, linear inverse relationship has been found in the sample between the
pulsation period and the effective temperature and also an inverse linear
relationship between the amplitude of light variation and the effective
temperature. These are consistent with the expectation for a pulsating post-AGB
star evolving toward higher temperature at constant luminosity. The published
spectral energy distributions and mid-infrared images show these objects to
have cool (200 K), detached dust shells and published models imply that
intensive mass loss ended a few thousand years ago. The detection of periods as
long as 150 d in these requires a revision in the published post-AGB evolution
models that couple the pulsation period to the mass loss rate and that assume
that intensive mass loss ended when the pulsation period had decreased to 100
d. This revision will have the effect of extending the time scale for the early
phases of post-AGB evolution. It appears that real time evolution in the
pulsation periods of individual objects may be detectable on the time scale of
two decades
Variability in Proto-Planetary Nebulae: IV. Light Curve Analyses of Four Oxygen-Rich, F Spectral-Type Objects
We present new light curves covering 14 to 19 years of observations of four
bright proto-planetary nebulae (PPNs), all O-rich and of F spectral type. They
each display cyclical light curves with significant variations in amplitude.
All four were previously known to vary in light. Our data were combined with
published data and searched for periodicity. The results are as follows: IRAS
19475+3119 (HD 331319; 41.0 days), 17436+5003 (HD 161796; 45.2 days),
19386+0155 (101.8 days), and 18095+2704 (113.3 days). The two longer periods
are in agreement with previous studies while the two shorter periods each
reveal for the first time reveal a dominant period over these long observing
intervals. Multiple periods were also found for each object. The secondary
periods were all close to the dominant periods, with P2/P1 ranging from 0.86 to
1.06. The variations in color reveal maximum variations in T(eff) of 400 to 770
K. These variations are due to pulsations in these post-AGB objects. Maximum
seasonal light variations are all less than 0.23 mag (V), consistent for their
temperatures and periods with the results of Hrivnak et al. (2010) for 12
C-rich PPNs. For all of these PPNs, there is an inverse relationship between
period and temperature; however, there is a suggestion that the
period-temperature relationship may be somewhat steeper for the O-rich than for
the C-rich PPNs.Comment: 28 pages, 13 figures, to appear in the Astronomical Journa
Are Proto-Planetary Nebulae Shaped by a Binary? Results of a Long-Term Radial Velocity Study
The shaping of the nebula is currently one of the outstanding unsolved
problems in planetary nebula (PN) research. Several mechanisms have been
proposed, most of which require a binary companion. However, direct evidence
for a binary companion is lacking in most PNs. We have addressed this problem
by obtaining precise radial velocities of seven bright proto-planetary nebulae
(PPNs), objects in transition from the asymptotic giant branch to the PN phases
of stellar evolution. These have F-G spectral types and have the advantage over
PNs of having more and sharper spectral lines, leading to better precision. Our
observations were made in two observing intervals, 1991-1995 and 2007-2010, and
we have included in our analysis some additional published and unpublished
data. Only one of the PPNs, IRAS 22272+5435, shows a long-term variation that
might tentatively be attributed to a binary companion, with P 22 years, and
from this, limiting binary parameters are calculated. Selection effects are
also discussed. These results set significant restrictions on the range of
possible physical and orbital properties of any binary companions: they have
periods greater than 25 years or masses of brown dwarfs or super-Jupiters.
While not ruling out the binary hypothesis, it seems fair to say that these
results do not support it.Comment: 14 pages, 3 figures, to appear Astrophys J, 734 (2011 June 10
A Study of H2 Emission in Three Bipolar Proto-Planetary Nebulae: IRAS 16594-4656, Hen 3-401, and Rob 22
We have carried out a spatial-kinematical study of three proto-planetary
nebulae, IRAS 16594-4656, Hen 3-401, and Rob 22. High-resolution H2 images were
obtained with NICMOS on the HST and high-resolution spectra were obtained with
the Phoenix spectrograph on Gemini-South. IRAS 16594-4656 shows a
"peanut-shaped" bipolar structure with H2 emission from the walls and from two
pairs of more distant, point-symmetric faint blobs. The velocity structure
shows the polar axis to be in the plane of the sky, contrary to the impression
given by the more complex visual image and the visibility of the central star,
with an ellipsoidal velocity structure. Hen 3-401 shows the H2 emission coming
from the walls of the very elongated, open-ended lobes seen in visible light,
along with a possible small disk around the star. The bipolar lobes appear to
be tilted 10-15 deg with respect to the plane of the sky and their kinematics
display a Hubble-like flow. In Rob 22, the H2 appears in the form of an "S"
shape, approximately tracing out the similar pattern seen in the visible. H2 is
especially seen at the ends of the lobes and at two opposite regions close to
the unseen central star. The axis of the lobes is nearly in the plane of the
sky. Expansion ages of the lobes are calculated to be approximately 1600 yr
(IRAS 16594-4656), 1100 yr (Hen 3-401), and 640 yr (Rob 22), based upon
approximate distances
Spitzer IRS Spectra of Luminous 8 micron Sources in the Large Magellanic Cloud: Testing color-based classifications
We present archival Spitzer IRS spectra of 19 luminous 8 micron selected
sources in the Large Magellanic Cloud (LMC). The object classes derived from
these spectra and from an additional 24 spectra in the literature are compared
with classifications based on 2MASS/MSX (J, H, K, and 8 micron) colors in order
to test the "JHK8" classification scheme (Kastner et al. 2008). The IRS spectra
confirm the classifications of 22 of the 31 sources that can be classified
under the JHK8 system. The spectroscopic classification of 12 objects that were
unclassifiable in the JHK8 scheme allow us to characterize regions of the
color-color diagrams that previously lacked spectroscopic verification,
enabling refinements to the JHK8 classification system. The results of these
new classifications are consistent with previous results concerning the
identification of the most infrared-luminous objects in the LMC. In particular,
while the IRS spectra reveal several new examples of asymptotic giant branch
(AGB) stars with O-rich envelopes, such objects are still far outnumbered by
carbon stars (C-rich AGB stars). We show that Spitzer IRAC/MIPS color-color
diagrams provide improved discrimination between red supergiants and
oxygen-rich and carbon-rich asymptotic giant branch stars relative to those
based on 2MASS/MSX colors. These diagrams will enable the most luminous IR
sources in Local Group galaxies to be classified with high confidence based on
their Spitzer colors. Such characterizations of stellar populations will
continue to be possible during Spitzer's warm mission, through the use of IRAC
[3.6]-[4.5] and 2MASS colors.Comment: 31 pages, 10 figures, to be published in A
A Spitzer/IRS spectral study of a sample of galactic carbon-rich proto-planetary nebulae
Recent infrared spectroscopic observations have shown that proto-planetary
nebulae (PPNs) are sites of active synthesis of organic compounds in the late
stages of stellar evolution. This paper presents a study of Spitzer/IRS spectra
for a sample of carbon-rich PPNs, all except one of which show the unidentified
21 micron emission feature. The strengths of the aromatic infrared band (AIB),
21 micron, and 30 micron features are obtained by decomposition of the spectra.
The observed variations in the strengths and peak wavelengths of the features
support the model that the newly synthesized organic compounds gradually change
from aliphatic to aromatic characteristics as stars evolve from PPNs to
planetary nebulae.Comment: 32 pages, 11 figures, accepted for publication in Ap
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