4,965 research outputs found
The Circumstellar Extinction of Planetary Nebulae
We analyze the dependence of circumstellar extinction on core mass for the
brightest planetary nebulae (PNe) in the Magellanic Clouds and M31. We show
that in all three galaxies, a statistically significant correlation exists
between the two quantities, such that high core mass objects have greater
extinction. We model this behavior, and show that the relation is a simple
consequence of the greater mass loss and faster evolution times of high mass
stars. The relation is important because it provides a natural explanation for
the invariance of the [O III] 5007 planetary nebula luminosity function (PNLF)
with population age: bright Population I PNe are extinguished below the cutoff
of the PNLF. It also explains the counter-intuitive observation that
intrinsically luminous Population I PNe often appear fainter than PNe from
older, low-mass progenitors.Comment: 12 pages, 2 figures, accepted for ApJ, April 10, 199
Chemical Abundances of Planetary Nebulae in the Sagittarius Dwarf Elliptical Galaxy
Spectrophotometry and imaging of the two planetary nebulae He2-436 and
Wray16-423, recently discovered to be in the Sagittarius dwarf elliptical
galaxy, are presented. Wray16-423 is a high excitation planetary nebula (PN)
with a hot central star. In contrast He2-436 is a high density PN with a cooler
central star and evidence of local dust, the extinction exceeding that for
Wray16-423 by E(B-V)=0.28. The extinction to Wray16-423, (E(B-V)=0.14), is
consistent with the extinction to the Sagittarius (Sgr) Dwarf. Both PN show
Wolf-Rayet features in their spectra, although the lines are weak in
Wray16-423. Images in [O III] and H-alpha+[N II], although affected by poor
seeing, yield a diameter of 1.2'' for Wray16-423 after deconvolution; He~2-436
was unresolved. He2-436 has a luminosity about twice that of Wray16-423 and its
size and high density suggest a younger PN. In order to reconcile the differing
luminosity and nebular properties of the two PN with similar age progenitor
stars, it is suggested that they are on He burning tracks
The abundance pattern is very similar in both nebulae and shows an oxygen
depletion of -0.4 dex with respect to the mean O abundance of Galactic PN and
[O/H] = -0.6. The Sgr PN progenitor stars are representative of the higher
metallicity tail of the Sgr population. The pattern of abundance depletion is
similar to that in the only other PN in a dwarf galaxy companion of the Milky
Way, that in Fornax, for which new spectra are presented. However the
abundances are larger than for Galactic halo PN suggesting a later formation
age. The O abundance of the Sgr galaxy deduced from its PN, shows similarities
with that of dwarf ellipticals around M31, suggesting that this galaxy was a
dwarf elliptical before its interaction with the Milky Way.Comment: 24 pages, Latex (aas2pp4.sty) including 5 postscript figures. To
appear in Ap
A New Way to Detect Massive Black Holes in Galaxies: The Stellar Remnants of Tidal Disruption
We point out that the tidal disruption of a giant may leave a luminous
(10^35-10^39 ergs/s), hot (10-100 eV) stellar core. The ``supersoft'' source
detected by Chandra at the center of M31 may be such a core; whether or not it
is, the observations have shown that such a core is detectable, even in the
center of a galaxy. We therefore explore the range of expected observational
signatures and how they may be used to (1) test the hypothesis that the M31
source is a remnant of tidal stripping and (2) discover evidence of black holes
and disruption events in other galaxies.Comment: Four pages with 1 figure. Appeared in ApJL (2001, 551, L37
Spitzer/IRS spectroscopy of high mass precursors to planetary nebulae
We present Spitzer/IRS observations of a small sample of heavily obscured
IRAS sources displaying both the infrared and OH maser emission characteristic
of OH/IR stars on the asymptotic giant branch (AGB), but also radio continuum
emission typical of ionized planetary nebulae (PNe), the so-called OHPNe. Our
observations show that their mid-infrared spectra are dominated by the
simultaneous presence of strong and broad amorphous silicate absorption
features together with crystalline silicate features, originated in their
O-rich circumstellar shells. Out of the five sources observed, three of them
are clearly non-variable at infrared wavelengths, confirming their post-AGB
status, while the remaining two still show strong photometric fluctuations, and
may still have not yet departed from the AGB. One of the non-variable sources
in the sample, IRAS 17393-2727, displays a strong [Ne II] nebular emission at
12.8 microns, indicating that the ionization of its central region has already
started. This suggests a rapid evolution from the AGB to the PN stage. We
propose that these heavily obscured OHPNe represent the population of high mass
precursors to PNe in our Galaxy.Comment: To appear in The Astrophysical Journal Letters (scheduled in the 2007
September 1 issue
Exploring Halo Substructure with Giant Stars. VI. Extended Distributions of Giant Stars Around the Carina Dwarf Spheroidal Galaxy -- How Reliable Are They?
The question of the existence of active tidal disruption around various dSph
galaxies remains controversial. That debate often centers on the nature (bound
vs. unbound) of extended populations of stars. However, the more fundamental
issue of the very existence of the extended populations is still contentious.
We present an evaluation of the debate centering on one particular dSph,
Carina, for which claims both for and against the existence of stars beyond the
King radius have been made. Our review includes an examination of all previous
studies bearing on the Carina radial profile and shows that the survey method
which achieves the highest detected dSph signal-to-background in the outer
parts of the galaxy is the Washington M, T2 + DDO51 (MTD) filter approach from
Paper II in this series. We then address statistical methods used to evaluate
the reliability of MTD surveys in the presence of photometric errors and for
which a new, a posteriori statistical analysis methodology is provided.
Finally, these statistical methods are tested by new spectroscopy of stars in
the MTD-selected Carina candidate sample. Of 74 candidate giants with follow-up
spectroscopy, the MTD technique identified 61 new Carina members, including 8
stars outside the King radius. From a sample of 29 stars not initially
identified as candidate Carina giants but that lie just outside of our
selection criteria, 12 have radial velocities consistent with membership,
including 5 extratidal stars. Carina is shown to have an extended population of
giant stars extending to a major axis radius of 40' (1.44x the nominal King
radius).Comment: 56 pages, 10 figures. Submitted to the Astronomical Journal, 2004 Sep
2
Confirmation of SBS 1150+599A As An Extremely Metal-Poor Planetary Nebula
SBS 1150+599A is a blue stellar object at high galactic latitude discovered
in the Second Byurakan Survey. New high-resolution images of SBS 1150+599A are
presented, demonstrating that it is very likely to be an old planetary nebula
in the galactic halo, as suggested by Tovmassian et al (2001). An H-alpha image
taken with the WIYN 3.5-m telescope and its "tip/tilt" module reveals the
diameter of the nebula to be 9.2", comparable to that estimated from spectra by
Tovmassian et al. Lower limits to the central star temperature were derived
using the Zanstra hydrogen and helium methods to determine that the star's
effective temperature must be > 68,000K and that the nebula is optically thin.
New spectra from the MMT and FLWO telescopes are presented, revealing the
presence of strong [Ne V] lambda 3425, indicating that the central star
temperature must be > 100,000K. With the revised diameter, new central star
temperature, and an improved central star luminosity, we can constrain
photoionization models for the nebula significantly better than before. Because
the emission-line data set is sparse, the models are still not conclusive.
Nevertheless, we confirm that this nebula is an extremely metal-poor planetary
nebula, having a value for O/H that is less than 1/100 solar, and possibly as
low as 1/500 solar.Comment: 19 pages, 6 figures. Accepted for publication in the Astronomical
Journa
The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB
We have investigated the dynamical interaction of low- and-intermediate mass
stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first
paper, we examine the structures generated by the stellar winds during the
Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind
history predicted by stellar evolution. The influence of the external ISM is
also taken into account. We find that the wind variations associated with the
thermal pulses lead to the formation of transient shells with an average
lifetime of 20,000 yr, and consequently do not remain recorded in the density
or velocity structure of the gas. The formation of shells that survive at the
end of the AGB occurs via two main processes: shocks between the shells formed
by two consecutive enhancements of the mass-loss or via continuous accumulation
of the material ejected by the star in the interaction region with the ISM. Our
models show that the mass of the circumstellar envelope increases appreciably
due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar
model). We also point out the importance of the ISM on the deceleration and
compression of the external shells. According to our simulations, large regions
(up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars
are expected. These large regions of gas are formed from the mass-loss
experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical
Journa
HST Snapshot Survey of Post-AGB Objects
The results from a Hubble Space Telescope (HST) snapshot survey of post-AGB
objects are shown. The aim of the survey is to complement existing HST images
of PPN and to connect various types of nebulosities with physical and chemical
properties of their central stars. Nebulosities are detected in 15 of 33
sources. Images and photometric and geometric measurements are presented. For
sources with nebulosities we see a morphological bifurcation into two groups,
DUPLEX and SOLE, as previous studies have found. We find further support to the
previous results suggesting that this dichotomy is caused by a difference in
optical thickness of the dust shell. The remaining 18 sources are classified as
stellar post-AGB objects, because our observations indicate a lack of
nebulosity. We show that some stellar sources may in fact be DUPLEX or SOLE
based on their infrared colors. The cause of the differences among the groups
are investigated. We discuss some evidence suggesting that high progenitor-mass
AGB stars tend to become DUPLEX post-AGB objects. Intermediate progenitor-mass
AGB stars tend to be SOLE post-AGB objects. Most of the stellar sources
probably have low mass progenitors and do not seem to develop nebulosities
during the post-AGB phase and therefore do not become planetary nebulae.Comment: 21 pages, 11 figure
Abundances of Planetary Nebula M1-42
The spectra of the planetary nebula M1-42 is reanalysed using spectral
measurements made in the mid-infrared with the Spitzer Space Telescope. The aim
is to determine the chemical composition of this object. We also make use of
ISO, IUE and ground based spectra. Abundances determined from the mid- and
far-infrared lines, which are insensitive to electron temperature, are used as
the basis for the determination of the composition, which are found to
substantially differ from earlier results. High values of neon, argon and
sulfur are found. They are higher than in other PN, with the exception of
NGC6153, a nebula of very similar abundances. The high values of helium and
nitrogen found indicate that the second dredge-up and hot bottom burning has
occurred in the course of evolution and that the central star was originally
more massive than 4Msun. The present temperature and luminosity of the central
star is determined and at first sight may be inconsistent with such a high
mass.Comment: 9 pages, 8 tables, 1 figure. Accepted for publication in A&
White Dwarfs in Globular Clusters: HST Observations of M4
Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258
white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial
distances from the cluster center were observed and sizeable WD populations
were found in all three. The location of these WDs in the color-magnitude
diagram, their mean mass of 0.51()M, and their luminosity
function confirm basic tenets of stellar evolution theory and support the
results from current WD cooling theory. The WDs are used to extend the cluster
main-sequence mass function upward to stars that have already completed their
nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and
found to be at most a few percent of all the main-sequence stars. The most
ancient WDs found are about 9 Gyr old, a level which is set solely by the
photometric limits of our data. Even though this is less than the age of M4, we
discuss how these cooling WDs can eventually be used to check the turnoff ages
of globular clusters and hence constrain the age of the Universe.Comment: 46 pages, latex, no figures included, figures available at
ftp://ftp.astro.ubc.ca/pub/richer/wdfig.uu size 2.7Mb. To be published in the
Astrophysical Journa
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