102 research outputs found
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
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
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
Studies of Variability in Proto-Planetary Nebulae: II. Light and Velocity Curve Analyses of Iras 22272+5435 and 22223+4327
We have carried out a detailed observational study of the light, color, and
velocity variations of two bright, carbon-rich proto-planetary nebulae, IRAS
22223+4327 and 22272+5435. The light curves are based upon our observations
from 1994 to 2011, together with published data by Arkhipova and collaborators.
They each display four significant periods, with primary periods for IRAS
22223+4327 and 22272+5435 being 90 and 132 days, respectively. For each of
them, the ratio of secondary to primary period is 0.95, a value much different
from that found in Cepheids, but which may be characteristic of post-AGB stars.
Fewer significant periods are found in the smaller radial velocity data sets,
but they agree with those of the light curves. The color curves generally mimic
the light curves, with the objects reddest when faintest. A comparison in
seasons when there exist contemporaneous light, color, and velocity curves
reveals that the light and color curves are in phase, while the radial velocity
curves are 0.25 out of phase with the light curves. Thus they differ from what
is seen in Cepheids, in which the radial velocity curve is 0.50 P out of phase
with the light curve. Comparison of the observed periods and amplitudes with
those of post-AGB pulsation models shows poor agreement, especially for the
periods, which are much longer than predicted. These observational data,
particularly the contemporaneous light, color, and velocity curves, provide an
excellent benchmark for new pulsation models of cool stars in the post-AGB,
proto-planetary nebula phase.Comment: 15 Figures plus Erratu
Spectroscopic Variability of IRAS 22272+5435
A time series of high-resolution spectra was observed in the optical wavelength region for the bright proto-planetary nebula IRAS 22272+5435 (HD 235858), along with a simultaneous monitoring of its radial velocity and BV R C magnitudes. The object is known to vary in light, color, and velocity owing to pulsation with a period of 132 days. The light and color variations are accompanied by significant changes in spectral features, most of which are identified as lines of carbon-bearing molecules. According to the observations, the C 2 Swan system and CN Red system lines are stronger near the light minimum. A photospheric spectrum of the central star was calculated using new self-consistent atmospheric models. The observed intensity variations in the C 2 Swan system and CN Red system lines were found to be much larger than expected if due solely to the temperature variation in the atmosphere of the pulsating star. In addition, the molecular lines are blueshifted relative to the photospheric velocity. The site of formation of the strong molecular features appears to be a cool outflow triggered by the pulsation. The variability in atomic lines seems to be mostly due variations of the effective temperature during the pulsation cycle. The profiles of strong atomic lines are split, and some of them are variable in a timescale of a week or so, probably because of shock waves in the outer atmosphere
Where Are the Binaries? Results of a Long-Term Search for Radial Velocity Binaries in Proto-Planetary Nebulae
We present the results of an expanded, long-term radial velocity search (25
yrs) for evidence of binarity in a sample of seven bright proto-planetary
nebulae (PPNe). The goal is to investigate the widely-held view that the
bipolar or point-symmetric shapes of planetary nebulae (PNe) and PPNe are due
to binary interactions. Observations from three observatories were combined
from 2007-2015 to search for variations on the order of a few years and then
combined with earlier observations from 1991-1995 to search for variations on
the order of decades. All seven show velocity variations due to periodic
pulsation in the range of 35-135 days. However, in only one PPN, IRAS
22272+5435, did we find even marginal evidence found for multi-year variations
that might be due to a binary companion. This object shows
marginally-significant evidence of a two-year period of low semi-amplitude
which could be due to a low-mass companion, and it also displays some evidence
of a much longer period of >30 years. The absence of evidence in the other six
objects for long-period radial velocity variations due to a binary companion
sets significant constraints on the properties of any undetected binary
companions: they must be of low mass, 30 years.
Thus the present observations do not provide direct support for the binary
hypothesis to explain the shapes of PNe and PPNe and severely constrains the
properties of any such undetected companions.Comment: 28 pages, 5 figure
Variability in Protoplanetary Nebulae: IX. Evidence for Evolution in a Decade
We have carried out a new photometric V,Rc study of 12 protoplanetary
nebulae, objects in the short-lived transition between the AGB and PN phases of
stellar evolution. These had been the subjects of an earlier study, using data
from 1994-2007, that found that all 12 varied periodically, with pulsation
periods in the range of ~38 to ~150 days. They are all carbon-rich, with F-G
spectral types. We combined our new (2008-2018) data with publicly-available
ASAS-SN data and determined new periods for their variability. The older and
newer period values were compared to investigate evidence of period change, for
which there is theoretical support that it might be detectable in a decade or
two in some cases. Such a detection is challenging since the light curves are
complicated, with multiple periods, changing amplitudes, and evidence of
shocks. Nevertheless, we found one, and possibly two, such cases, which are
associated with the higher temperature stars in the sample (7250 and 8000 K).
These results are most consistent with the evolution of stars at the lower end
of the mass range of carbon stars, ~1.5-2 M(sun). Several of the stars show
longer-term trends of increasing (six cases) or decreasing (one case)
brightness, which we think most likely due to changes in the circumstellar dust
opacity. There is one case of a possible ~1.8 yr period in addition to the
shorter pulsation. This is interpreted as possible evidence of an orbiting
companion.Comment: 26 pages, 9 figures. Accepted September 16, 2022 for publication in
the Ap
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