90 research outputs found
Deuterium toward the WD0621-376 sight line: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
Far Ultraviolet Spectroscopic Explorer observations are presented for
WD0621-376, a DA white dwarf star in the local interstellar medium (LISM) at a
distance of about 78 pc. The data have a signal-to-noise ratio of about 20-40
per 20 km/s resolution element and cover the wavelength range 905-1187 \AA.
LISM absorption is detected in the lines of D I, C II, C II*, C III, N I, N II,
N III, O I, Ar I, and Fe II. This sight line is partially ionized, with an
ionized nitrogen fraction of > 0.23. We determine the ratio (2). Assuming a standard interstellar
oxygen abundance, we derive . Using the
value of N(H I) derived from EUVE data gives a similar D/H ratio. The D I/N I
ratio is (2).Comment: accepted for publication in the ApJ
High-resolution FUSE and HST ultraviolet spectroscopy of the white dwarf central star of Sh 2-216
LS V +4621 is the DAO-type central star of the planetary nebula Sh 2-216. We
perform a comprehensive spectral analysis of high-resolution, high-S/N
ultraviolet observations obtained with FUSE and STIS aboard the HST as well as
the optical spectrum of LS V +4621 by means of state-of-the-art NLTE
model-atmosphere techniques in order to compare its photospheric properties to
theoretical predictions from stellar evolution theory as well as from diffusion
calculations.
From the N IV - NV, O IV - O VI, Si IV - Si V, and Fe V - Fe VII ionization
equilibria, we determined an effective temperature of 95 +/- 2 kK with high
precision. The surface gravity is log g = 6.9 +/- 0.2. An unexplained
discrepancy appears between the spectroscopic distance d = 224 +46/-58 pc and
the parallax distance d = 129 +6/-5 pc of LS V +4621. For the first time, we
have identified Mg IV and Ar VI absorption lines in the spectrum of a
hydrogen-rich central star and determined the Mg and Ar abundances as well as
the individual abundances of iron-group elements (Cr, Mn, Fe, Co, and Ni). With
the realistic treatment of metal opacities up to the iron group in the
model-atmosphere calculations, the so-called Balmer-line problem (found in
models that neglect metal-line blanketing) vanishes.
Spectral analysis by means of NLTE model atmospheres has presently arrived at
a high level of sophistication, which is now hampered largely by the lack of
reliable atomic data and accurate line-broadening tables. Strong efforts should
be made to improve upon this situation.Comment: 13 pages, 20 figure
Slowing down atomic diffusion in subdwarf B stars: mass loss or turbulence?
Subdwarf B stars show chemical peculiarities that cannot be explained by
diffusion theory alone. Both mass loss and turbulence have been invoked to slow
down atomic diffusion in order to match observed abundances. The fact that some
sdB stars show pulsations gives upper limits on the amount of mass loss and
turbulent mixing allowed. Consequently, non-adiabatic asteroseismology has the
potential to decide which process is responsible for the abundance anomalies.
We compute for the first time seismic properties of sdB models with atomic
diffusion included consistently during the stellar evolution. The diffusion
equations with radiative forces are solved for H, He, C, N, O, Ne, Mg, Fe and
Ni. We examine the effects of various mass-loss rates and mixed surface masses
on the abundances and mode stability. It is shown that the mass-loss rates
needed to simulate the observed He abundances (10^{-14}<=Mdot
[Msun/yr]<=10^{-13}) are not consistent with observed pulsations. We find that
for pulsations to be driven the rates should be Mdot<=10^{-15} Msun/yr. On the
other hand, weak turbulent mixing of the outer 10^{-6} Msun can explain the He
abundance anomalies while still allowing pulsations to be driven. The origin of
the turbulence remains unknown but the presence of pulsations gives tight
constraints on the underlying turbulence model.Comment: 12 pages, 8 figures, 1 table, accepted for publication in MNRA
Far Ultraviolet Spectroscopic Explorer Observations of the Supernova Remnant N49 in the Large Magellanic Cloud
We report a Far Ultraviolet Spectroscopic Explorer satellite observation of the supernova remnant N49 in the Large Magellanic Cloud, covering the 905 -- 1187 A spectral region. A 30'' square aperture was used, resulting in a velocity resolution of ~100 km/s. The purpose of the observation was to examine several bright emission lines expected from earlier work and to demonstrate diffuse source sensitivity by searching for faint lines never seen previously in extragalactic supernova remnant UV spectra. Both goals were accomplished. Strong emission lines of O VI 1031.9 A, 1037.6 A and C III 977.0 A were seen, Doppler broadened to +/- 225 km/s and with centroids red-shifted to 350 km/s, consistent with the LMC. Superimposed on the emission lines are absorptions by C III and O VI 1031.9 at +260 km/s, which are attributed to warm and hot gas (respectively) in the LMC. The O VI 1037.6 A line is more severely affected by overlying interstellar and H2 absorption from both the LMC and our galaxy. N III 989.8 A is not seen, but models indicate overlying absorption severely attenuates this line. A number of faint lines from hot gas have also been detected, many of which have never been seen in an extragalactic supernova remnant spectrum
Modeling the System Parameters of 2M1533+3759: A New Longer-Period Low-Mass Eclipsing sdB+dM Binary
We present new photometric and spectroscopic observations for 2M 1533+3759 (=
NSVS 07826147). It has an orbital period of 0.16177042 day, significantly
longer than the 2.3--3.0 hour periods of the other known eclipsing sdB+dM
systems. Spectroscopic analysis of the hot primary yields Teff = 29230 +/- 125
K, log g = 5.58 +/- 0.03 and log N(He)/N(H) = -2.37 +/- 0.05. The sdB velocity
amplitude is K1 = 71.1 +/- 1.0 km/s. The only detectable light contribution
from the secondary is due to the surprisingly strong reflection effect. Light
curve modeling produced several solutions corresponding to different values of
the system mass ratio, q(M2/M1), but only one is consistent with a core helium
burning star, q=0.301. The orbital inclination is 86.6 degree. The sdB primary
mass is M1 = 0.376 +/- 0.055 Msun and its radius is R1 = 0.166 +/- 0.007 Rsun.
2M1533+3759 joins PG0911+456 (and possibly also HS2333+3927) in having an
unusually low mass for an sdB star. SdB stars with masses significantly lower
than the canonical value of 0.48 Msun, down to as low as 0.30 Msun, were
theoretically predicted by Han et al. (2002, 2003), but observational evidence
has only recently begun to confirm the existence of such stars. The existence
of core helium burning stars with masses lower than 0.40--0.43 Msun implies
that at least some sdB progenitors have initial main sequence masses of
1.8--2.0 Msun or more, i.e. they are at least main sequence A stars. The
secondary is a main sequence M5 star.Comment: 47 pages, 7 figure
Overview of the Far Ultraviolet Spectroscopic Explorer Mission
The Far Ultraviolet Spectroscopic Explorer satellite observes light in the
far-ultraviolet spectral region, 905 - 1187 A with high spectral resolution.
The instrument consists of four coaligned prime-focus telescopes and Rowland
spectrographs with microchannel plate detectors. Two of the telescope channels
use Al:LiF coatings for optimum reflectivity from approximately 1000 to 1187 A
and the other two use SiC coatings for optimized throughput between 905 and
1105 A. The gratings are holographically ruled to largely correct for
astigmatism and to minimize scattered light. The microchannel plate detectors
have KBr photocathodes and use photon counting to achieve good quantum
efficiency with low background signal. The sensitivity is sufficient to examine
reddened lines of sight within the Milky Way as well as active galactic nuclei
and QSOs for absorption line studies of both Milky Way and extra-galactic gas
clouds. This spectral region contains a number of key scientific diagnostics,
including O VI, H I, D I and the strong electronic transitions of H2 and HD.Comment: To appear in FUSE special issue of the Astrophysical Journal Letters.
6 pages + 4 figure
FUSE Survey of the Local Interstellar Medium within 200 Parsec
We present FUSE observations of the interstellar gas toward 30 white dwarf
and 1 subdwarf (SdO) stars. These sightlines probe the Local Bubble (LB) and
the local interstellar medium (LISM) near the LB. Our survey detected only
diffuse H_2 molecular clouds along six sightlines. There is no evidence from
this study that H_2 exists well inside the perimeter of the LB. The kinematical
temperature for H_2 is less than the usual temperature observed in the local
interstellar clouds, implying different gas phases in the LISM. The relative
abundance ratios of Si II, P II, and Fe II give insight about the dust content.
These ratios vary, but are similar to the depletion patterns observed in warm
and halo diffuse clouds in more distant sightlines in the Galaxy. The N I/O I
and Ar I/O I ratios are significantly subsolar within the LB. Outside the LB a
larger scatter is observed from subsolar to solar. Because Ar and N are only
weakly depleted into dust grains if at all, the deficiencies of their neutral
forms are likely due to photoionization. The evidence for significant
ionization of N (and hence Ar) is strengthened by the detection and measurement
of N II, which is a dominant ion for this element toward many sightlines. These
observations imply that photoionization is the main ionization mechanism in the
LISM and do not support the existence of a highly ionized condition in the
past. In view of the variations observed in the different atomic and ionic
ratios, the photoionization conditions vary significantly in the LB and the
LISM. [Abridged.]Comment: Accepted for publication in the Ap
Circumstellar features in hot DA white dwarfs
We present a phenomenological study of highly ionised, non-photospheric
absorption features in high spectral resolution vacuum ultraviolet spectra of
23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige
24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these
features. We find four new objects with multiple components in one or more of
the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD
2218+706. A fifth object, REJ 2156-546 also shows some evidence of multiple
components, though further observations are required to confirm the detection.
We discuss possible origins for these features including ionisation of the
local interstellar environment, the presence of material inside the
gravitational well of the white dwarf, mass loss in a stellar wind, and the
existence of material in an ancient planetary nebula around the star. We
propose ionisation of the local interstellar medium as the origin of these
features in G191-B2B and REJ 1738+665, and demonstrate the need for higher
resolution spectroscopy of the sample, to detect multiple ISM velocity
components and to identify circumstellar features which may lie close to the
photospheric velocity.Comment: 20 figures, 21 page
Abundances of Deuterium, Nitrogen, and Oxygen toward HZ 43A: Results from the FUSE Mission
We present an analysis of interstellar absorption along the line of sight to
the nearby white dwarf star HZ43A. The distance to this star is 68+/-13 pc, and
the line of sight extends toward the north Galactic pole. Column densities of
OI, NI, and NII were derived from spectra obtained by the Far Ultraviolet
Spectroscopic Explorer (FUSE), the column density of DI was derived from a
combination of our FUSE spectra and an archival HST GHRS spectrum, and the
column density of HI was derived from a combination of the GHRS spectrum and
values derived from EUVE data obtained from the literature. We find the
following abundance ratios (with 2-sigma uncertainties): DI/HI = (1.66 +/-
0.28) x 10^-5, OI/HI = (3.63 +/- 0.84) x 10^-4, and NI/HI = (3.80 +/- 0.74) x
10^-5. The NII column density was slightly greater than that of NI, indicating
that ionization corrections are important when deriving nitrogen abundances.
Other interstellar species detected along the line of sight were CII, CIII,
OVI, SiII, ArI, MgII, and FeII; an upper limit was determined for NIII. No
elements other than HI were detected in the stellar photosphere.Comment: 52 pages, 12 figures. To be published in the May ApJS 200
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