70 research outputs found
Accelerated post-AGB evolution, initial-final mass relations, and the star-formation history of the Galactic bulge
We study the star-formation history of the Galactic bulge, as derived from
the age distribution of the central stars of planetary nebulae that belong to
this stellar population. The high resolution imaging and spectroscopic
observations of 31 compact planetary nebulae are used to derive their central
star masses. The Bloecker tracks with the cluster IFMR result in ages, which
are unexpectedly young. We find that the Bloecker post-AGB tracks need to be
accelerated by a factor of three to fit the local white dwarf masses. This
acceleration extends the age distribution. We adjust the IFMR as a free
parameter to map the central star ages on the full age range of bulge stellar
populations. This fit requires a steeper IFMR than the cluster relation. We
find a star-formation rate in the Galactic bulge, which is approximately
constant between 3 and 10 Gyr ago. The result indicates that planetary nebulae
are mainly associated with the younger and more metal-rich bulge populations.
The constant rate of star-formation between 3 and 10 Gyr agrees with
suggestions that the metal-rich component of the bulge is formed during an
extended process, such as a bar interaction.Comment: accepted for publication in A&
Kinematics, turbulence and evolution of planetary nebulae
This paper discusses the location of a sample of planetary nebulae on the HR diagram. We determine the internal velocity fields of 14 planetary nebulae from high-resolution echelle spectroscopy, with the help of photoionization models. The mass averaged velocity is shown to be a robust, simple parameter describing the outflow. The expansion velocity and radius are used to define the dynamical age; together with the stellar temperature, this gives a measurement of the luminosity and core mass of the central star. The same technique is applied to other planetary nebulae with previously measured expansion velocities, giving a total sample of 73 objects. The objects cluster closely around the Schoenberner track of 0.61 M_sun, with a very narrow distribution of core masses. The masses are higher than found for local white dwarfs. The luminosities determined in this way tend to be higher by a factor of a few than those derived from the nebular luminosities. The discrepancy is highest for the hottest (most evolved) stars. We suggest photon leakage as the likely cause. The innermost regions of the non-[WC] nebulae tend to show strong acceleration. Together with the acceleration at the ionization front, the velocity field becomes 'U'-shaped. The presence of strong turbulent motions in [WC] nebulae is confirmed. Except for this, we find that the [WC] stars evolve on the same tracks as non-[WC] stars
White dwarf masses derived from planetary nebulae modelling
We compare the mass distribution of central stars of planetary nebulae (CSPN)
with those of their progeny, white dwarfs (WD). We use a dynamical method to
measure masses with an uncertainty of 0.02 M. The CSPN mass
distribution is sharply peaked at . The WD distribution peaks
at lower masses () and shows a much broader range of masses.
Some of the difference can be explained if the early post-AGB evolution is
faster than predicted by the Bl\"ocker tracks. Between 30 and 50 per cent of WD
may avoid the PN phase because of too low mass. However, the discrepancy cannot
be fully resolved and WD mass distributions may have been broadened by
observational or model uncertainties.Comment: 4 pages, accepted for A&A Letter
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
The evolving spectrum of the planetary nebula Hen 2-260
We analysed the planetary nebula Hen 2-260 using optical spectroscopy and
photometry. We compared our observations with the data from literature to
search for evolutionary changes. The nebular line fluxes were modelled with the
Cloudy photoionization code to derive the stellar and nebular parameters. The
planetary nebula shows a complex structure and possibly a bipolar outflow. The
nebula is relatively dense and young. The central star is just starting ionization (). Comparison of our
observations with literature data indicates a 50% increase of the [OIII] 5007
\AA\ line flux between 2001 and 2012. We interpret it as the result of the
progression of the ionization of . The central star evolves to
higher temperatures at a rate of . The heating rate
is consistent with a final mass of
or for two different sets of
post-AGB evolutionary tracks from literature. The photometric monitoring of Hen
2-260 revealed variations on a timescale of hours or days. The variability may
be caused by pulsations of the star. The temperature evolution of the central
star can be traced using spectroscopic observations of the surrounding
planetary nebula spanning a timescale of roughly a decade. This allows us to
precisely determine the stellar mass, since the pace of the temperature
evolution depends critically on the core mass. The kinematical age of the
nebula is consistent with the age obtained from the evolutionary track. The
final mass of the central star is close to the mass distribution peak for
central stars of planetary nebulae found in other studies. The object belongs
to a group of young central stars of planetary nebulae showing photometric
variability.Comment: accepted for publication in A&
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
The history of the Galactic bulge
Planetary nebulae form in stellar populations with ages from 1 to 10 Gyr, and can be used to trace their star formation histories. Here we apply this to the Galactic bulge, where there are indications both for an old origin and for younger stars. We use new stellar models, which have significant different evolutionary speeds during the post-AGB phase. We apply these new models to a sample of 32 planetary nebulae with HST imaging and VLT spectroscopy. The results show evidence for an old starburst, followed by continuous star formation until at least 2Gyr ago. This agrees very well with recent analysis of colour-magnitude diagrams of the bulge. We show that the new models can also explain the [OIII] luminosity functions, and predict the uniform luminosity cut-off both in spiral galaxies and old elliptical galaxies.Facultad de Ciencias Astronómicas y Geofísica
Planetary nebulae with emission-line central stars
The kinematic structure of a sample of planetary nebulae, consisting of 23
[WR] central stars, 21 weak emission line stars (wels) and 57 non-emission line
central stars, is studied. The [WR] stars are shown to be surrounded by
turbulent nebulae, a characteristic shared by some wels but almost completely
absent from the non-emission line stars. The fraction of objects showing
turbulence for non-emission-line stars, wels and [WR] stars is 7%, 24% and 91%,
respectively. The [WR] stars show a distinct IRAS 12-micron excess, indicative
of small dust grains, which is not found for wels. The [WR]-star nebulae are on
average more centrally condensed than those of other stars. On the
age-temperature diagram, the wels are located on tracks of both high and low
stellar mass, while [WR] stars trace a narrow range of intermediate masses.
Emission-line stars are not found on the cooling track. One group of wels may
form a sequence wels--[WO] stars with increasing temperature. For the other
groups both the wels and the [WR] stars appear to represent several,
independent evolutionary tracks. We find a discontinuity in the [WR] stellar
temperature distribution and suggest different evolutionary sequences above and
below the temperature gap. One group of cool [WR] stars has no counterpart
among any other group of PNe and may represent binary evolution. A prime factor
distinguishing wels and [WR] stars appears to be stellar luminosity. We find no
evidence for an increase of nebular expansion velocity with time.Comment: 14 pages, 9 figures, accepted to A&
- …