33 research outputs found
PAH Formation in O-rich Planetary Nebulae
Polycyclic aromatic hydrocarbons (PAHs) have been observed in O-rich
planetary nebulae towards the Galactic Bulge. This combination of oxygen-rich
and carbon-rich material, known as dual-dust or mixed chemistry, is not
expected to be seen around such objects. We recently proposed that PAHs could
be formed from the photodissociation of CO in dense tori. In this work, using
VISIR/VLT, we spatially resolved the emission of the PAH bands and ionised
emission from the [SIV] line, confirming the presence of dense central tori in
all the observed O-rich objects. Furthermore, we show that for most of the
objects, PAHs are located at the outer edge of these dense/compact tori, while
the ionised material is mostly present in the inner parts of these tori,
consistent with our hypothesis for the formation of PAHs in these systems. The
presence of a dense torus has been strongly associated with the action of a
central binary star and, as such, the rich chemistry seen in these regions may
also be related to the formation of exoplanets in post-common-envelope binary
systems.Comment: 14, accepted for publication in the MNRAS Journa
Disk evaporation in a planetary nebula
We study the Galactic bulge planetary nebula M 2-29 (for which a 3-year
eclipse event of the central star has been attributed to a dust disk) using HST
imaging and VLT spectroscopy, both long-slit and integral field. The central
cavity of M 2-29 is filled with a decreasing, slow wind. An inner high density
core is detected, with radius less than 250 AU, interpreted as a rotating
gas/dust disk with a bipolar disk wind. The evaporating disk is argued to be
the source of the slow wind. The central star is a source of a very fast wind
(1000 km/s). An outer, partial ring is seen in the equatorial plane, expanding
at 12 km/s. The azimuthal asymmetry is attributed to mass-loss modulation by an
eccentric binary. M 2-29 presents a crucial point in disk evolution, where
ionization causes the gas to be lost, leaving a low-mass dust disk behind.Comment: 11 pages, accepted for publication in "Astronomy and Astrophysics
The Evolution of NGC 7027 at Radio Frequencies: A New Determination of the Distance and Core Mass
We present the results of a 25-year program to monitor the radio flux
evolution of the planetary nebula NGC7027. We find significant evolution of the
spectral flux densities. The flux density at 1465 MHz, where the nebula is
optically thick, is increasing at a rate of 0.251+-0.015 % per year, caused by
the expansion of the ionized nebula. At frequencies where the emission is
optically thin, the spectral flux density is changing at a rate of
-0.145+-0.005 % per year, caused by a decrease in the number of ionizing
photons coming from the central star. A distance of 980+-100 pc is derived. By
fitting interpolated models of post-AGB evolution to the observed changes, we
find that over the 25-yr monitoring period, the stellar temperature has
increased by 3900+-900 K and the stellar bolometric luminosity has decreased by
1.75+-0.38 %. We derive a distance-independent stellar mass of 0.655+-0.01
solar masses adopting the Bloecker stellar evolution models, or about 0.04
solar masses higher when using models of Vassiliadis & Wood which may provide a
better fit. A Cloudy photoionization model is used to fit all epochs at all
frequencies simultaneously. The differences between the radio flux density
predictions and the observed values show some time-independent residuals of
typically 1 %. A possible explanation is inaccuracies in the radio flux scale
of Baars et al. We propose an adjustment to the flux density scale of the
primary radio flux calibrator 3C286, based on the Cloudy model of NGC7027. We
also calculate precise flux densities for NGC7027 for all standard continuum
bands used at the VLA, as well as for some new 30GHz experiments.Comment: submitted to the Astrophysical Journa
A self-consistent stellar and 3D nebular model for Planetary Nebula IC418
We present a coherent stellar and nebular model reproducing the observations
of the Planetary Nebula IC418. We want to test whether a stellar model obtained
by fitting the stellar observations is able to satisfactory ionize the nebula
and reproduce the nebular observations, which is by no mean evident. This
allows us to determine all the physical parameters of both the star and the
nebula, including the abundances and the distance. We used all the
observational material available (FUSE, IUE, STIS and optical spectra) to
constrain the stellar atmosphere model performed using the CMFGEN code. The
photoionization model is done with Cloudy_3D, and is based on CTIO, Lick, SPM,
IUE and ISO spectra as well as HST images. More than 140 nebular emission lines
are compared to the observed intensities. We reproduce all the observations for
the star and the nebula. The 3D morphology of the gas distribution is
determined. The effective temperature of the star is 36.7kK. Its luminosity is
7700 solar luminosity. We describe an original method to determine the distance
of the nebula using evolutionary tracks. No clumping factor is need to
reproduce the age-luminosity relation. The distance of 1.25 kpc is found in
very good agreement with recent determination using parallax method. The
chemical composition of both the star and the nebula are determined. Both are
Carbon-rich. The nebula presents evidence of depletion of elements Mg, Si, S,
Cl (0.5 dex lower than solar) and Fe (2.9 dex lower than solar). This is the
first self-consistent stellar and nebular model for a Planetary Nebula that
reproduces all the available observations ranging from IR to UV, showing that
the combined approach for the modeling process leads to more restrictive
constraints and, in principle, more trustworthy results.Comment: Accepted for publication in Astronomy and Astrophysics. V2: after
corrections language edito
Carbon chemistry in Galactic Bulge Planetary Nebulae
Galactic Bulge Planetary Nebulae show evidence of mixed chemistry with
emission from both silicate dust and PAHs. This mixed chemistry is unlikely to
be related to carbon dredge up, as third dredge-up is not expected to occur in
the low mass Bulge stars. We show that the phenomenon is widespread, and is
seen in 30 nebulae out of 40 of our sample, selected on the basis of their
infrared flux. HST images and UVES spectra show that the mixed chemistry is not
related to the presence of emission-line stars, as it is in the Galactic disk
population. We also rule out interaction with the ISM as origin of the PAHs.
Instead, a strong correlation is found with morphology, and the presence of a
dense torus. A chemical model is presented which shows that hydrocarbon chains
can form within oxygen-rich gas through gas-phase chemical reactions. The model
predicts two layers, one at where small hydrocarbons form from
reactions with C, and one at , where larger chains (and by
implication, PAHs) form from reactions with neutral, atomic carbon. These
reactions take place in a mini-PDR. We conclude that the mixed chemistry
phenomenon occurring in the Galactic Bulge Planetary Nebulae is best explained
through hydrocarbon chemistry in an UV-irradiated, dense torus.Comment: 13 pages, 10 figue
A CO Survey of Young Planetary Nebulae
We report the results of a sensitive survey of young planetary nebulae in the
CO J=2-1 line that significantly increases the available data on warm, dense,
molecular gas in the early phases of planetary nebula formation. The
observations were made using the IRAM 30 m telescope with the 3 by 3 pixel
Heterodyne Receiver Array (HERA). The array provides an effective means of
discriminating the CO emission of planetary nebulae in the galactic plane from
contaminating emission of interstellar clouds along the line of sight. 110
planetary nebulae were observed in the survey and 40 were detected. The results
increase the number of young planetary nebulae with known CO emission by
approximately a factor of two. The CO spectra yield radial velocities for the
detected nebulae, about half of which have uncertain or no velocity
measurements at optical wavelengths. The CO profiles range from parabolic to
double-peaked, tracing the evolution of structure in the molecular gas. The
line widths are significantly larger than on the Asymptotic Giant Branch, and
many of the lines show extended wings, which probably result from the effects
on the envelopes of high velocity jets.Comment: 29 pages, 2 figures (with multiple panels), to be published in
Astrophysical Journal Supplement Serie
Far-UV Spectroscopic Analyses of Four Central Stars of Planetary Nebulae
We analyze the Far-UV/UV spectra of four central stars of planetary nebulae
with strong wind features -- NGC 2371, Abell 78, IC 4776 and NGC 1535, and
derive their photospheric and wind parameters by modeling high-resolution FUSE
(Far-Ultraviolet Spectroscopic Explorer) data in the Far-UV and HST-STIS and
IUE data in the UV with spherical non-LTE line-blanketed model atmospheres.
Abell 78 is a hydrogen-deficient transitional [WR]-PG 1159 object, and we find
NGC 2371 to be in the same stage, both migrating from the constant-luminosity
phase to the white dwarf cooling sequence with Teff ~= 120 kK, Mdot ~= 5x10^-8
Msun/yr. NGC 1535 is a ``hydrogen-rich'' O(H) CSPN, and the exact nature of IC
4776 is ambiguous, although it appears to be helium burning. Both objects lie
on the constant-luminosity branch of post-AGB evolution and have Teff ~= 65 kK,
Mdot ~= 1x10^-8 Msun/yr. Thus, both the H-rich and H-deficient channels of PN
evolution are represented in our sample. We also investigate the effects of
including higher ionization stages of iron (up to FeX) in the model atmosphere
calculations of these hot objects (usually neglected in previous analyses), and
find iron to be a useful diagnostic of the stellar parameters in some cases.
The Far-UV spectra of all four objects show evidence of hot (T ~ 300 K)
molecular hydrogen in their circumstellar environments.Comment: 38 pages, 8 figures (6 color). Accepted for publication in Ap
Expansion velocities and core masses of bright planetary nebulae in the Virgo cluster
The line-of-sight velocities and [OIII] 5007 AA expansion velocities are
measured for 11 planetary nebulae (PNs) in the Virgo cluster core, at 15 Mpc
distance, with the FLAMES spectrograph on the ESO VLT. These PNs are located
about halfway between the two giant ellipticals M87 and M86. From the [OIII]
5007 AA line profile widths, the average half-width at half maximum expansion
velocity for this sample of 11 PNs is v_HWHM = 16.5 km/s (RMS=2.6 km/s). We use
the PN subsample bound to M87 to remove the distance uncertainties, and the
resulting [OIII] 5007 AA luminosities to derive the central star masses. We
find these masses to be at least 0.6 M_sun and obtain PN observable life times
t_PN < 2000 yrs, which imply that the bright PNs detected in the Virgo cluster
core are compact, high density nebulae. We finally discuss several scenarios
for explaining the high central star masses in these bright M87 halo PNs.Comment: In press on Astrophysical Journal Letters, 12 pages, 3 figure
Excitation of emission lines by fluorescence and recombination in IC 418
We predict intensities of lines of CII, NI, NII, OI and OII and compare them
with a deep spectroscopic survey of IC 418 to test the effect of excitation of
nebular emission lines by continuum fluorescence of starlight. Our calculations
use a nebular model and a synthetic spectrum of its central star to take into
account excitation of the lines by continuum fluorescence and recombination.
The NII spectrum is mostly produced by fluorescence due to the low excitation
conditions of the nebula, but many CII and OII lines have more excitation by
fluorescence than recombination. In the neutral envelope, the NI permitted
lines are excited by fluorescence, and almost all the OI lines are excited by
recombination. Electron excitation produces the forbidden optical lines of OI,
but continuum fluorescence excites most of the NI forbidden line intensities.
Lines excited by fluorescence of light below the Lyman limit thus suggest a new
diagnostic to explore the photodissociation region of a nebula.Comment: 2 pages, 4 figures, to appear in proceedings of the IAU Symposium
283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L.
Stanghellini and D. Schoenberne
Evolution of Planetary Nebulae I. An improved synthetic model
We present a new synthetic model to follow the evolution of a planetary
nebula (PN) and its central star, starting from the onset of AGB phase up to
the white dwarf cooling sequence. The model suitably combines various
analytical prescriptions to account for different (but inter-related) aspects
of planetary nebulae, such as: the dynamical evolution of the primary shell and
surrounding ejecta, the photoionisation of H and He by the central star, the
nebular emission of a few relevant optical lines (e.g. Hbeta; HeII4686;
[OIII]5007). Predictions of the synthetic model are tested by comparison with
both findings of hydrodynamical calculations, and observations of Galactic PNe.
The sensitiveness of the results to the models parameters (e.g. transition
time, mass of the central star, H-/He-burning tracks, etc.) is also discussed.
We briefly illustrate the systematic differences that are expected in the
luminosities and lifetimes of PNe with either H- or He-burning central stars,
which result in different ``detection probabilities'' across the H-R diagram,
in both Hbeta and [OIII]5007 lines. Adopting reasonable values of the model
parameters, we are able to reproduce, in a satisfactory way, many general
properties of PNe, like the ionised mass--nebular radius relationship, the
trends of a few main nebular line ratios, and the observed ranges of nebular
shell thicknesses, electron densities, and expansion velocities. The models
naturally predict also the possible transitions from optically-thick to
optically-thin configurations (and vice versa). In this context, the origin of
the Zanstra discrepancy is also analysed. (abridged)Comment: 28 pages, 23 postscript figures, to appear in A&