221 research outputs found
UV (IUE) spectra of the central stars of high latitude planetary nebulae Hb7 and Sp3
We present an analysis of the UV (IUE) spectra of the central stars of Hb7
and Sp3. Comparison with the IUE spectrum of the standard star HD 93205 leads
to a spectral classification of O3V for these stars, with an effective
temperature of 50,000 K. From the P-Cygni profiles of CIV (1550 A), we derive
stellar wind velocities and mass loss rates of -1317 km/s +/- 300 km/s and
2.9X10^{-8} solar mass yr^{-1} and -1603 km/s +/- 400 km/s and 7X10^{-9} solar
mass yr^{-1} for Hb7 and Sp3 respectively. From all the available data, we
reconstruct the spectral energy distribution of Hb7 and Sp3.Comment: 4 pages, 3 figures, latex, accepted for publication in Astronomy &
Astrophysic
Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)
We present a spectral analysis of the sdO central star K 648 based on
medium-resolution optical and high-resolution UV spectra. The photospheric
parameters are determined by means of state-of-the-art NLTE model atmosphere
techniques.
We found Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08)
and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a
factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-6), [N/H] = -2.0) appears at a
sub-solar value. However, these metal abundances are much higher than the
cluster's metallicity M 15: [Fe/H] = -2.25).
The surface composition appears to be a mixture of the original hydrogen-rich
material and products of helium burning (3 alpha process) which have been mixed
up to the surface. The abundances of He, C, and N are consistent with the
nebular abundance, while O is considerably more abundant in the photosphere
than in the nebula.
From a comparison of its position in the log Teff - log g plane with
evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity
of 3810 +/- 1200 Lsun are deduced.
Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the
distance of M 15 as determined by Alves et al. (2000).
From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.Comment: 8 pages, 13 figure
Post-AGB Stars in Globular Clusters and Galactic Halos
We discuss three aspects of post-AGB (PAGB) stars in old populations. (1) HST
photometry of the nucleus of the planetary nebula (PN) K 648 in the globular
cluster (GC) M15 implies a mass of 0.60 Msun, in contrast to the mean masses of
white dwarfs in GCs of ~0.5 Msun. This suggests that K 648 is descended from a
merged binary, and we infer that single Pop II stars do not produce visible
PNe. (2) Yellow PAGB stars are the visually brightest stars in old populations
(Mv ~ -3.3) and are easily recognizable because of their large Balmer jumps;
thus they show great promise as a Pop II standard candle. Two yellow PAGB stars
in the GC NGC 5986 have the same V magnitudes to within +/-0.05 mag, supporting
an expected narrow luminosity function. (3) Using CCD photometry and a u filter
lying below the Balmer jump, we have detected yellow PAGB stars in the halo of
M31 and in its dwarf elliptical companion NGC 205. With the Milky Way zero
point, we reproduce the Cepheid distance to M31, and find that NGC 205 is ~100
kpc further away than M31. The star counts imply a yellow PAGB lifetime of
about 25,000 yr, and their luminosities imply masses near 0.53 Msun.Comment: 6 pages, 2 figures. To appear in proceedings of Torun, Poland,
workshop on "Post-AGB Objects (Proto-Planetary Nebulae) as a Phase of Stellar
Evolution," ed. S.K. Gorn
Birth and early evolution of a planetary nebula
The final expulsion of gas by a star as it forms a planetary nebula --- the
ionized shell of gas often observed surrounding a young white dwarf --- is one
of the most poorly understood stages of stellar evolution. Such nebulae form
extremely rapidly (about 100 years for the ionization) and so the formation
process is inherently difficult to observe. Particularly puzzling is how a
spherical star can produce a highly asymmetric nebula with collimated outflows.
Here we report optical observations of the Stingray Nebula which has become an
ionized planetary nebula within the past few decades. We find that the
collimated outflows are already evident, and we have identified the nebular
structure that focuses the outflows. We have also found a companion star,
reinforcing previous suspicions that binary companions play an important role
in shaping planetary nebulae and changing the direction of successive outflows.Comment: 9 pages + 3 figures. To appear in Nature, 2 April 199
Chandra and FUSE spectroscopy of the hot bare stellar core H1504+65
H1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective
temperature close to 200,000 K. We present new FUV and soft X-ray spectra
obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere
primarily composed of carbon and oxygen. The Chandra LETG spectrum (60-160
Angstroem) shows a wealth of photospheric absorption lines from highly ionized
oxygen, neon, and - for the first time identified in this star - magnesium and
suggests relatively high Ne and Mg abundances. This corroborates an earlier
suggestion that H1504+65 represents a naked C/O stellar core or even the C/O
envelope of an O-Ne-Mg white dwarf.Comment: 15 pages, 10 figures, accepted for publication in A&
Luminosities of AGB Variables
The prevailing evidence suggests that most large-amplitude AGB variables
follow the period luminosity (PL) relation that has been established for Miras
in the LMC and galactic globular clusters. Hipparcos observations indicate that
most Miras in the solar neighbourhood are consistent with such a relation.
There are two groups of stars with luminosities that are apparently greater
than the PL relation would predict: (1) in the LMC and SMC there are large
amplitude variables, with long periods, P> 420 days, which are probably
undergoing hot bottom burning, but which are very clearly more luminous than
the PL relation (these are visually bright and are likely to be among the first
stars discovered in more distant intermediate age populations); (2) in the
solar neighbourhood there are short period, P<235 days, red stars which are
probably more luminous than the PL relation. Similar short-period red stars,
with high luminosities, have not been identified in the Magellanic Clouds.Comment: 8 pages, 2 figure, to be published in Mass-Losing Pulsating Stars and
their Circumstellar Matter, Y. Nakada & M. Honma (eds) Kluwer ASSL serie
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