33 research outputs found
Illumination in symbiotic binary stars: Non-LTE photoionization models. II. Wind case
We describe a non-LTE photoionization code to calculate the wind structure
and emergent spectrum of a red giant wind illuminated by the hot component of a
symbiotic binary system. We consider spherically symmetric winds with several
different velocity and temperature laws and derive predicted line fluxes as a
function of the red giant mass loss rate, \mdot. Our models generally match
observations of the symbiotic stars EG And and AG Peg for \mdot about 10^{-8}
\msunyr to 10^{-7} \msunyr. The optically thick cross- section of the red giant
wind as viewed from the hot component is a crucial parameter in these models.
Winds with cross-sections of 2--3 red giant radii reproduce the observed
fluxes, because the wind density is then high, about 10^9 cm^{-3}. Our models
favor winds with acceleration regions that either lie far from the red giant
photosphere or extend for 2--3 red giant radii.Comment: 51 pages, LaTeX including three tables, requires 15 Encapsulated
Postscript figures, to appear in Ap
Discovery of a Magnetic White Dwarf in the Symbiotic Binary Z Andromedae
We report the first result from our survey of rapid variability in symbiotic
binaries: the discovery of a persistent oscillation at P=1682.6 +- 0.6 s in the
optical emission from the prototype symbiotic, Z Andromedae. The oscillation
was detected on all 8 occasions on which the source was observed over a
timespan of nearly a year, making it the first such persistent periodic pulse
found in a symbiotic binary. The amplitude was typically 2 - 5 mmag, and it was
correlated with the optical brightness during a relatively small outburst of
the system. The most natural explanation is that the oscillation arises from
the rotation of an accreting, magnetic (B_S > 10^5 G) white dwarf. This
discovery constrains the outburst mechanisms, since the oscillation emission
region near the surface of the white dwarf was visible during the outburst.Comment: Accepted for publication in the Astrophysical Journal (6 pages,
including 4 figures), LaTe
Modeling RR Tel through the Evolution of the Spectra
We investigate the evolution of RR Tel after the outburst by fitting the
emission spectra in two epochs. The first one (1978) is characterized by large
fluctuations in the light curve and the second one (1993) by the slow fading
trend. In the frame of a colliding wind model two shocks are present: the
reverse shock propagates in the direction of the white dwarf and the other one
expands towards or beyond the giant. The results of our modeling show that in
1993 the expanding shock has overcome the system and is propagating in the
nearby ISM. The large fluctuations observed in the 1978 light curve result from
line intensity rather than from continuum variation. These variations are
explained by fragmentation of matter at the time of head-on collision of the
winds from the two stars. A high velocity (500 km/s) wind component is revealed
from the fit of the SED of the continuum in the X-ray range in 1978, but is
quite unobservable in the line profiles. The geometrical thickness of the
emitting clumps is the critical parameter which can explain the short time
scale variabilities of the spectrum and the trend of slow line intensity
decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To
appear in "The Astrophysical Journal
Synthetic Spectral Analysis of the Hot Component in the S-Type Symbiotic Variable EG Andromeda
We have applied grids of NLTE high gravity model atmospheres and optically
thick accretion disk models for the first time to archival IUE and FUSE spectra
of the S-type symbiotic variable EG And taken at superior spectroscopic
conjunction when Rayleigh scattering should be minimal and the hot component is
viewed in front of the red giant. For EG And's widely accepted, published hot
component mass, orbital inclination and distance from the Hipparcos parallax,
we find that hot, high gravity, NLTE photosphere model fits to the IUE spectra
yield distances from the best-fitting models which agree with the Hipparcos
parallax distance but at temperatures substantially lower than the modified
Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same
orbital phase as the IUE spectra yield the same temperature as the IUE
temperature (50,000K). However, for the same hot component mass, inclination
and parallax-derived distance, accretion disk models at moderately high
inclinations, with accretion rates to 1\times 10^{-9} M_{\sun}/yr for white dwarf masses M_{wd} = 0.4
M_{\sun} yield distances grossly smaller than the distance from the Hipparcos
parallax. Therefore, we rule out an accretion disk as the dominant source of
the FUV flux. Our findings support a hot bare white dwarf as the dominant
source of FUV flux.Comment: AJ, Oct. 200
Raman Scattered He II 6545 Line in the Symbiotic Star V1016 Cygni
We present a spectrum of the symbiotic star V1016 Cyg observed with the 3.6 m
Canada-France-Hawaii Telescope, in order to illustrate a method to measure the
covering factor of the neutral scattering region around the giant component
with respect to the hot emission region around the white dwarf component. In
the spectrum, we find broad wings around H and a broad emission feature
around 6545 that is blended with the [N II] 6548 line.
These two features are proposed to be formed by Raman scattering by atomic
hydrogen, where the incident radiation is proposed to be UV continuum radiation
around Ly in the former case and He II 1025 emission line
arising from transitions for the latter feature. We remove the
H wings by a template Raman scattering wing profile and subtract the [N
II] 6548 line using the 3 times stronger [N II] 6583
feature in order to isolate the He II Raman scattered 6545 \AA line. We obtain
the flux ratio of the He II 6560 emission
line and the 6545 \AA feature for V1016 Cyg. Under the assumption that the He
II emission from this object is isotropic, this ratio is converted to the ratio
of the number of the incident photons and that
of the scattered photons. This implies that the scattering region with H I
column density covers 17 per cent of the
emission region. By combining the presumed binary period yrs of this
system we infer that a significant fraction of the slow stellar wind from the
Mira component is ionized and that the scattering region around the Mira
extends a few tens of AU, which is closely associated with the mass loss
process of the Mira component.Comment: 12 pages, 6 figures, accepted for publication in Ap
The Continuing Slow Decline of AG Pegasi
We analyze optical and ultraviolet observations of the symbiotic binary AG
Pegasi acquired during 1992-97. The bolometric luminosity of the hot component
declined by a factor of 2-3 from 1980-1985 to 1997. Since 1992, the effective
temperature of the hot component may have declined by 10%-20%, but this decline
is comparable to the measurement errors. Optical observations of H-beta and He
I emission show a clear illumination effect, where high energy photons from the
hot component ionize the outer atmosphere of the red giant. Simple illumination
models generally account for the magnitude of the optical and ultraviolet
emission line fluxes. High ionization emission lines - [Ne V], [Mg V], and [Fe
VII] - suggest mechanical heating in the outer portions of the photoionized red
giant wind. This emission probably originates in a low density region
30-300 AU from the central binary.Comment: 17 pages, 7 pages, 5 tables; to be published in the Astronomical
Journal, July 200
A "Combination Nova" Outburst in Z Andromedae: Nuclear Shell Burning Triggered by a Disk Instability
We describe observational evidence for a new kind of interacting-binary-star
outburst that involves both an accretion instability and an increase in
thermonuclear shell burning on the surface of an accreting white dwarf. We
refer to this new type of eruption as a combination nova. In late 2000, the
prototypical symbiotic star Z Andromedae brightened by roughly two magnitudes
in the optical. We observed the outburst in the radio with the VLA and MERLIN,
in the optical both photometrically and spectroscopically, in the far
ultraviolet with FUSE, and in the X-rays with both Chandra and XMM. The
two-year-long event had three distinct stages. During the first stage, the
optical rise closely resembled an earlier, small outburst that was caused by an
accretion-disk instability. In the second stage, the hot component ejected an
optically thick shell of material. In the third stage, the shell cleared to
reveal a white dwarf whose luminosity remained on the order of 10^4 Lsun for
approximately one year. The eruption was thus too energetic to have been
powered by accretion alone. We propose that the initial burst of accretion was
large enough to trigger enhanced nuclear burning on the surface of the white
dwarf and the ejection of an optically thick shell of material. This outburst
therefore combined elements of both a dwarf nova and a classical nova. Our
results have implications for the long-standing problem of producing shell
flashes with short recurrence times on low-mass white dwarfs in symbiotic
stars.Comment: Accepted for publication in ApJ. 24 pages, 10 figure
The large-scale ionised outflow of CH Cygni
HST and ground-based [OII} and [NII] images obtained from 1996 to 1999 reveal
the existence of a ionised optical nebula around the symbiotic binary CH Cyg
extending out to 5000 A.U. from the central stars. The observed velocity range
of the nebula, derived from long-slit echelle spectra, is of 130 km/s. In spite
of its complex appearence, the velocity data show that the basic morphology of
the inner regions of the optical nebula is that of a bipolar (or conical)
outflow extending nearly along the plane of the sky out to some 2000 A.U. from
the centre. Even if the extension of this bipolar outflow and its position
angle are consistent with those of the radio jet produced in 1984 (extrapolated
to the time of our optical imagery), no obvious counterpart is visible of the
original, dense radio bullets ejected by the system. We speculate that the
optical bipolar outflow might be the remannt of the interaction of the bullets
with a relatively dense circumstellar medium.Comment: 8 text pages + 3 figures (jpeg). ApJ in press. For a full PostScript
version with figures inline see
ftp://ftp.ll.iac.es/pub/research/preprints/PP252001.ps.g
Two SMC Symbiotic stars undergoing steady hydrogen burning
Two symbiotic stars in the Small Magellanic Cloud (SMC), Lin 358 and SMC 3,
have been supersoft X-ray sources (SSS) for more than 10 years. We fit
atmospheric and nebular models to their X-ray, optical and UV spectra obtained
at different epochs. The X-ray spectra are extremely soft, and appear to be
emitted by the white dwarf atmosphere and not by the nebula like in some other
symbiotics. The white dwarf of SMC 3, the hottest of the two sources, had a
constant effective temperature ~500,000 K at various epochs during 12 years. No
nova-like outbursts of these systems have been recorded in the last 50 years,
despite continuous optical monitoring of the SMC, and there are no indications
of cooling of the white dwarf, expected after a thermonuclear flash. The
bolometric luminosity of this system in March of 2003 was more than an order of
magnitude lower than three years later, however the time of the observation is
consistent with a partial eclipse of the white dwarf, previously found in ROSAT
and optical observations. The red giant wind is either very asymmetric or very
clumpy. The conpact object of Lin 358 has been at T>~180,000 K since 1993,
perhaps with a moderate increase. Atmospheric fits are obtained with log(g)>=9,
appropriate only for WD mass >1.18 M(sol). The two systems are probably
accreting and burning hydrogen steadily at the high rate required for type Ia
supernova progenitors.Comment: In press on the Astrophysical Journa
Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic
We have computed, based on 17 infrared radial velocities, the first set of
orbital elements for the M giant in the symbiotic binary V2116 Ophiuchi. The
giant's companion is a neutron star, the bright X-ray source GX 1+4. We find an
orbital period of 1161 days by far the longest of any known X-ray binary. The
orbit has a modest eccentricity of 0.10 with an orbital circularization time of
less than 10^6 years. The large mass function of the orbit significantly
restricts the mass of the M giant. Adopting a neutron-star mass of 1.35M(Sun),
the maximum mass of the M giant is 1.22M(Sun), making it the less massive star.
Derived abundances indicate a slightly subsolar metallicity. Carbon and
nitrogen are in the expected ratio resulting from the red-giant first dredge-up
phase. The lack of O-17 suggests that the M-giant has a mass less than
1.3M(Sun), consistent with our maximum mass. The red giant radius is 103R(Sun),
much smaller than the estimated Roche lobe radius. Thus, the mass loss of the
red giant is via a stellar wind. Although the M giant companion to the neutron
star has a mass similar to the late-type star in low-mass X-ray binaries, its
near-solar abundances and apparent runaway velocity are not fully consistent
with the properties of this class of stars.Comment: In press to The Astrophysical Journal (10 April 2006 issue). 23 page