56 research outputs found
The analysis of spectra of novae taken near maximum
A project to analyze ultraviolet spectra of novae obtained at or near maximum optical light is presented. These spectra are characterized by a relatively cool continuum with superimposed permitted emission lines from ions such as Fe II, Mg II, and Si II. Spectra obtained late in the outburst show only emission lines from highly ionized species and in many cases these are forbidden lines. The ultraviolet data will be used with calculations of spherical, expanding, stellar atmospheres for novae to determine elemental abundances by spectral line synthesis. This method is extremely sensitive to the abundances and completely independent of the nebular analyses usually used to obtain novae abundances
UV Spectral Synthesis of Vega
We show that the UV spectrum (1280-3200 A) of the "superficially normal"
A-star Vega, as observed by the IUE satellite at a resolution comparable to the
star's rotational broadening width, can be fit remarkably well by a
single-temperature synthetic spectrum based on LTE atmosphere models and a
newly constructed UV line list. If Vega were a normal, equator-on,
slow-rotating star, then its spectrum and our analysis would indicate a
temperature of Teff ~ 9550 K, surface gravity of log g ~ 3.7, general surface
metallicity of [m/H] ~ -0.5, and a microturbulence velocity of v(turb) ~ 2.0
km/s. Given its rapid rotation and nearly pole-on orientation, however, these
parameters must be regarded as representing averages across the observed
hemisphere. Modeling the complex UV line spectrum has allowed us to determine
the specific surface abundances for 17 different chemical elements, including
CNO, the light metals, and the iron group elements. The resultant abundance
pattern agrees in general with previous results, although there is considerable
scatter in the literature. Despite its peculiarities, Vega has turned out to
provide a powerful test of the extent of our abilities to model the atmospheric
properties of the early A-stars, particularly the detailed UV line spectrum.
The value of the measurements from this pilot study will increase as this
analysis is extended to more objects in the rich high-dispersion IUE data
archive, including both normal and peculiar objects.Comment: To appear in the Astrophysical Journa
Spatial Separation of the 3.29 micron Emission Feature and Associated 2 micron Continuum in NGC 7023
We present a new 0.9" resolution 3.29 micron narrowband image of the
reflection nebula NGC 7023. We find that the 3.29 micron IEF in NGC 7023 is
brightest in narrow filaments NW of the illuminating star. These filaments have
been seen in images of K', molecular hydrogen emission lines, the 6.2 and 11.3
micron IEFs, and HCO+. We also detect 3.29 micron emission faintly but
distinctly between the filaments and the star. The 3.29 micron image is in
contrast to narrowband images at 2.09, 2.14, and 2.18 micron, which show an
extended emission peak midway between the filaments and the star, and much
fainter emission near the filaments. The [2.18]-[3.29] color shows a wide
variation, ranging from 3.4-3.6 mag at the 2 micron continuum peak to 5.5 mag
in the filaments. We observe [2.18]-[3.29] to increase smoothly with increasing
distance from the star, up until the filament, suggesting that the main
difference between the spatial distributions of the 2 micron continuum and the
the 3.29 micron emission is related to the incident stellar flux. Our result
suggests that the 3.29 micron IEF carriers are likely to be distinct from, but
related to, the 2 micron continuum emitters. Our finding also imply that, in
NGC 7023, the 2 micron continuum emitters are mainly associated with HI, while
the 3.29 micron IEF carriers are primarily found in warm molecular hydrogen,
but that both can survive in HI or molecular hydrogen. (abridged)Comment: to appear in ApJ, including 1 table and 8 figures, high resolution
figures available at http://www.ast.cam.ac.uk/~jin/n7023
The Chemical Composition of Cernis 52 (BD+31 640)
We present an abundance analysis of the star Cernis 52 in whose spectrum we
recently reported the napthalene cation in absorption at 6707.4 {\AA}. This
star is on a line of sight to the Perseus molecular complex. The analysis of
high-resolution spectra using a chi^2-minimization procedure and a grid of
synthetic spectra provides the stellar parameters and the abundances of O, Mg,
Si, S, Ca, and Fe. The stellar parameters of this star are found to be T_{eff}
= 8350 +- 200 K, logg= 4.2 +- 0.4 dex. We derived a metallicity of [Fe/H] =
-0.01 +- 0.15. These stellar parameters are consistent with a star of
\Msun in a pre-main-sequence evolutionary stage. The stellar spectrum is
significantly veiled in the spectral range 5150-6730 {\AA} up to almost 55 per
cent of the total flux at 5150 {\AA} and decreasing towards longer wavelengths.
Using Johnson-Cousins and 2MASS photometric data, we determine a distance to
Cernis 52 of 231 pc considering the error bars of the stellar
parameters. This determination places the star at a similar distance to the
young cluster IC 348. This together with its radial velocity, v_r=13.7+-1 km/s,
its proper motion and probable young age support Cernis 52 as a likely member
of IC 348. We determine a rotational velocity of v\sin i=65 +- 5 km/s for this
star. We confirm that the stellar resonance line of \ion{Li}{1} at 6707.8 {\AA}
is unable to fit the broad feature at 6707.4 {\AA}. This feature should have a
interstellar origin and could possibly form in the dark cloud L1470 surrounding
all the cluster IC 348 at about the same distance.Comment: Accepted for publication in The Astrophysical Journa
Radiation pressure and pulsation effects on the Roche lobe
Several observational pieces of evidence indicate that specific evolutionary
channels which involve Roche lobe overflow are not correctly accounted for by
the classical Roche model. We generalize the concept of Roche lobe in the
presence of extra forces (caused by radiation pressure or pulsations). By
computing the distortion of the equipotential surfaces, we are able to evaluate
the impact of these perturbing forces on the stability of Roche-lobe overflow
(RLOF). Radiative forces are parametrized through the constant reduction factor
that they impose on the gravitational force from the radiating star (neglecting
any shielding in case of large optical thickness). Forces imparted by
pulsations are derived from the velocity profile of the wind that they trigger.
We provide analytical expressions to compute the generalized Roche radius.
Depending on the extra force, the Roche-lobe radius may either stay unchanged,
become smaller, or even become meaningless (in the presence of a radiatively-
or pulsation-driven wind). There is little impact on the RLOF stability.Comment: 11 pages, 13 Postscript figure
The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni
The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg with the Hobby-Eberly Telescope (HET). Fits of synthetic spectra for Roche-lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio: K_{K} = 162.5 +/- 1.0 km/s and q= M_{K} /M_{wd} = 0.685 +/- 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 +/- 0.075 of the total flux at 5500 angstroms. Taking the weighted average of our results with previously published results obtained using similar techniques, we find = 163.7 +/- 0.7 km/s and = 0.683 +/- 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations we limit the orbital inclination to the range 45 deg. <= i <= 56 deg. The derived masses of the K star and white dwarf are M_{K} = 0.55 +/- 0.13 M_sun and M_{wd} = 0.81 +/- 0.19 M_sun, where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10% to 50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for Zero-Age Main-Sequence stars; nor does it follow the ZAMS mass/spectral-type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted
The accuracy of stellar atmospheric parameter determinations: a case study with HD 32115 and HD 37594
We present detailed parameter determinations of two chemically normal late
A-type stars, HD 32115 and HD 37594, to uncover the reasons behind large
discrepancies between two previous analyses of these stars performed with a
semi-automatic procedure and a "classical" analysis. Our study is based on high
resolution, high signal-to-noise spectra obtained at the McDonald Observatory.
Our method is based on the simultaneous use of all available observables:
multicolor photometry, pressure-sensitive magnesium lines, metallic lines and
Balmer line profiles. Our final set of fundamental parameters fits, within the
error bars, all available observables. It differs from the published results
obtained with a semi-automatic procedure. A direct comparison between our new
observational material and the spectra previously used by other authors shows
that the quality of the data is not the origin of the discrepancies. As the two
stars require a substantial macroturbulence velocity to fit the line profiles,
we concluded that neglecting this additional broadening in the semi-automatic
analysis is one origin of discrepancy. The use of FeI excitation equilibrium
and of the Fe ionisation equilibrium, to derive effective temperature and
surface gravity, respectively, neglecting all other indicators leads to a
systematically erroneously high effective temperature. We deduce that the
results obtained using only one parameter indicator might be biased and that
those results need to be cautiously taken when performing further detailed
analyses, such as modelling of the asteroseismic frequencies or characterising
transiting exoplanets.Comment: Accepted for publication by MNRA
On the Limb Darkening, Spectral Energy Distribution, and Temperature Structure of Procyon
We have fit synthetic visibilities from 3-D (CO5BOLD + PHOENIX) and 1-D
(PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to
high-precision interferometric data from the VLTI Interferometer (K-band) and
from the Mark III interferometer (500 nm and 800 nm). These data sets provide a
test of theoretical wavelength dependent limb-darkening predictions. The work
of Allende Prieto et al. has shown that the temperature structure from a
spatially and temporally averaged 3-D hydrodynamical model produces
significantly less limb darkening at 500 nm relative to the temperature
structure of a 1-D MARCS model atmosphere with a standard mixing-length
approximation for convection. Our direct fits to the interferometric data
confirm this prediction. A 1-D ATLAS 12 model with ``approximate overshooting''
provides the required temperature gradient. We show, however, that 1-D models
cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective
temperatures in the range constrained by the measured bolometric flux and
angular diameter. We find that a good match to the full spectral energy
distribution can be obtained with a composite model consisting of a weighted
average of twelve 1-D model atmospheres based on the surface intensity
distribution of a 3-D granulation simulation. We emphasize that 1-D models with
overshooting may realistically represent the mean temperature structure of
F-type stars like Procyon, but the same models will predict redder colors than
observed because they lack the multicomponent temperature distribution expected
for the surfaces of these stars.Comment: 24 pages, 8 figures, accepted for publication in the Astrophysical
Journa
The Spectral Energy Distribution and Mass-loss Rate of the A-Type Supergiant Deneb
A stellar wind module has been developed for the PHOENIX stellar atmosphere
code for the purpose of computing non-LTE, line-blanketed, expanding
atmospheric structures and detailed synthetic spectra of hot luminous stars
with winds. We apply the code to observations of Deneb, for which we report the
first positive detections of mm and cm emission (obtained using the SCUBA and
the VLA), as well a strong upper limit on the 850 micron flux (using the HHT).
The slope of the radio spectrum shows that the stellar wind is partially
ionized. We report a uniform-disk angular diameter measurement, 2.40 +/- 0.06
mas, from the Navy Prototype Optical Interferometer (NPOI). The measured
bolometric flux and corrected NPOI angular diameter yield an effective
temperature of 8600 +/- 500 K. Least-squares comparisons of synthetic spectral
energy distributions from 1220 A to 3.6 cm with the observations provide
estimates for the effective temperature and the mass-loss rate of 8400 +/- 100
K and 8 +/- 3 E-7 M_sun/yr, respectively. This range of mass-loss rates is
consistent with that derived from high dispersion UV spectra when non-LTE
metal-line blanketing is considered. We are unable achieve a reasonable fit to
a typical Halpha P-Cygni profile with any model parameters over a reasonable
range. This is troubling because the \ha profile is the observational basis for
Wind Momentum-Luminosity Relationship.Comment: Accepted by the Astrophysical Journal, 43 pages, 23 figure
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