248 research outputs found
Superhumps: Confronting Theory with Observation
We review the theory and observations related to the ``superhump'' precession
of eccentric accretion discs in close binary sytems. We agree with earlier
work, although for different reasons, that the discrepancy between observation
and dynamical theory implies that the effect of pressure in the disc cannot be
neglected. We extend earlier work that investigates this effect to include the
correct expression for the radius at which resonant orbits occur. Using
analytic expressions for the accretion disc structure, we derive a relationship
between the period excess and mass-ratio with the pressure effects included.
This is compared to the observed data, recently derived results for detailed
integration of the disc equations and the equivalent empirically derived
relations and used to predict values for the mass ratio based on measured
values of the period excess for 88 systems.Comment: 11 pages, 7 figures, 4 tables, accepted for publication in MNRA
Theoretical Modeling of the Thermal State of Accreting White Dwarfs Undergoing Classical Novae
White dwarfs experience a thermal renaissance when they receive mass from a
stellar companion in a binary. For accretion rates < 10^-8 Msun/yr, the freshly
accumulated hydrogen/helium envelope ignites in a thermally unstable manner
that results in a classical novae (CN) outburst and ejection of material. We
have undertaken a theoretical study of the impact of the accumulating envelope
on the thermal state of the underlying white dwarf (WD). This has allowed us to
find the equilibrium WD core temperatures (T_c), the classical nova ignition
masses (M_ign) and the thermal luminosities for WDs accreting at rates of
10^-11 - 10^-8 Msun/yr. These accretion rates are most appropriate to WDs in
cataclysmic variables (CVs) of P_orb <~ 7 hr, many of which accrete
sporadically as dwarf novae. We have included ^3He in the accreted material at
levels appropriate for CVs and find that it significantly modifies the CN
ignition mass. We compare our results with several others from the CN
literature and find that the inclusion of ^3He leads to lower M_ign for
>~ 10^-10 Msun/yr, and that for below this the particular author's
assumption concerning T_c, which we calculate consistently, is a determining
factor. Initial comparisons of our CN ignition masses with measured ejected
masses find reasonable agreement and point to ejection of material comparable
to that accreted.Comment: 14 pages, 11 figures; uses emulateapj; accepted by the Astrophysical
Journal; revised for clarity, added short discussion of diffusio
V4332 Sagittarii revisited
The eruption of V4332 Sgr discovered in February 1994 shows striking
similarities to that of V838 Mon started in January 2002. The nature of these
eruptions is, however, enigmatic and unclear. We present new photometric and
spectroscopic data on V4332 Sgr obtained in April-May 2003 at the SAAO. The
obtained spectrum shows an unusual emission-line component superimposed on an
early M-type stellar spectrum. The emission-line spectrum is of very low
excitation and is dominated by lines from neutral elemets (NaI, FeI, CaI) and
molecular bands (TiO, ScO, AlO). We also analyse all the observational data,
mainly photometric measurements, available for V4332 Sgr. This allows us to
follow the evolution of the effective temperature, radius and luminosity of the
object since February 1994 till 2003. We show that the observed decline of
V4332 Sgr can be accounted for by a gravitational contraction of an inflated
stellar envelope. The combined optical and infrared photometry in 2003 shows
that apart from the M-type stellar component there is a strong infrared excess
in the KLM bands. This excess was absent in the 2MASS measurements done in 1998
but was probably starting to appear in K in 1999 when the object was observed
in the DENIS survey. We interpret the results in terms of a stellar merger
scenario proposed by Soker & Tylenda. The infrared excess is likely to be due
to a disc-like structure which is either of protostellar nature or has been
produced during the 1994 eruption and stores angular momentum from the merger
event.Comment: 11 pages, 5 figures, accepted in Astronomy & Astrophysic
A cyclical period variation detected in the updated orbital period analysis of TV Columbae
The two CCD photometries of the intermediate polar TV Columbae are made for
obtaining the two updated eclipse timings with high precision. There is an
interval time \sim 17yr since the last mid-eclipse time observed in 1991. Thus,
the new mid-eclipse times can offer an opportunity to check the previous
orbital ephemerides. A calculation indicates that the orbital ephemeris derived
by Augusteijn et al. (1994) should be corrected. Based on the proper linear
ephemeris (Hellier, 1993), the new orbital period analysis suggests a cyclical
period variation in the O-C diagram of TV Columbae. Using Applegate's mechanism
to explain the periodic oscillation in O-C diagram, the required energy is
larger than that a M0-type star can afford over a complete variation period
\sim 31.0(\pm 3.0)yr. Thus, the light travel-time effect indicates that the
tertiary component in TV Columbae may be a dwarf with a low mass, which is near
the mass lower limit \sim 0.08Msun as long as the inclination of the third body
high enough.Comment: 10 pages, 5 figure
The Extraordinary X-ray Light Curve of the Classical Nova V1494 Aquilae (1999 #2) in Outburst: The Discovery of Pulsations and a "Burst"
V1494 Aql (Nova Aql 1999 No. 2) was discovered on 2 December 1999. We
obtained Chandra ACIS-I spectra on 15 April and 7 June 2000 which appear to
show only emission lines. Our third observation, on 6 August, showed that its
spectrum had evolved to that characteristic of a Super Soft X-ray Source. We
then obtained Chandra LETG+HRC-S spectra on 28 September (8 ksec) and 1 October
(17 ksec). We analyzed the X-ray light curve of our grating observations and
found both a short time scale ``burst'' and oscillations. Neither of these
phenomena have previously been seen in the light curve of a nova in outburst.
The ``burst'' was a factor of 10 rise in X-ray counts near the middle of the
second observation, and which lasted about 1000 sec; it exhibited at least two
peaks, in addition to other structure. Our time series analysis of the combined
25 ksec observation shows a peak at 2500 s which is present in independent
analyses of both the zeroth order image and the dispersed spectrum and is not
present in similar analyses of grating data for HZ 43 and Sirius B. Further
analyses of the V1494 Aql data find other periods present which implies that we
are observing non-radial g+ modes from the pulsating, rekindled white dwarf.Comment: ApJ accepte
Nucleosynthesis in ONeMg Novae: Models versus Observations to Constrain the Masses of ONeMg White Dwarfs and Their Envelopes
Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of
three parameters, i.e., the white dwarf mass, the envelope mass at ignition,
and the initial composition. A quasi-analytic one-zone approach is used with an
up-to-date nuclear reaction network. The nucleosynthesis results show
correlation with the peak temperatures or the cooling timescales during
outbursts. Among the combinations of white dwarf and envelope masses which give
the same peak temperature, the explosion is more violent for a lower white
dwarf mass owing to its smaller gravitational potential. Comparison of the
nucleosynthesis results with observations implies that at least two-third of
the white dwarf masses for the observed ONeMg novae are ,
which are significantly lower than estimated by previous hydrodynamic studies
but consistent with the observations of V1974 Cyg. Moreover, the envelope
masses derived from the comparison are , which are in
good agreement with the ejecta masses estimated from observations but
significantly higher than in previous hydrodynamic studies. With such a low
mass white dwarf and a high mass envelope, the nova can produce interesting
amounts of -ray emitters Be, Na, and Al. We suggest
that V1974 Cyg has produced Na as high as the upper limit derived from
the COMPTEL survey. In addition, a non-negligible part of the Galactic
Al may originate from ONeMg novae, if not the major contributors. Both
the future INTEGRAL survey for these -ray emitters and abundance
estimates derived from ultraviolet, optical, and near infrared spectroscopies
will impose a severe constraint on the current nova models.Comment: 21 pages, 23 figures, to appear in the Astrophysical Journal, Vol.
523, No.1, September 20, 1999; preprint with embedded images can be obtained
from http://th.nao.ac.jp/~wanajo/journal/onenova.p
V344 Lyrae: A Touchstone SU UMa Cataclysmic Variable in the Kepler Field
We report on the analysis of the Kepler short-cadence (SC) light curve of
V344 Lyr obtained during 2009 June 20 through 2010 Mar 19 (Q2--Q4). The system
is an SU UMa star showing dwarf nova outbursts and superoutbursts, and promises
to be a touchstone for CV studies for the foreseeable future. The system
displays both positive and negative superhumps with periods of 2.20 and
2.06-hr, respectively, and we identify an orbital period of 2.11-hr. The
positive superhumps have a maximum amplitude of ~0.25-mag, the negative
superhumps a maximum amplitude of ~0.8 mag, and the orbital period at
quiescence has an amplitude of ~0.025 mag. The quality of the Kepler data is
such that we can test vigorously the models for accretion disk dynamics that
have been emerging in the past several years. The SC data for V344 Lyr are
consistent with the model that two physical sources yield positive superhumps:
early in the superoutburst, the superhump signal is generated by viscous
dissipation within the periodically flexing disk, but late in the
superoutburst, the signal is generated as the accretion stream bright spot
sweeps around the rim of the non-axisymmetric disk. The disk superhumps are
roughly anti-phased with the stream/late superhumps. The V344 Lyr data also
reveal negative superhumps arising from accretion onto a tilted disk precessing
in the retrograde direction, and suggest that negative superhumps may appear
during the decline of DN outbursts. The period of negative superhumps has a
positive dP/dt in between outbursts.Comment: ApJ, In Press (20 pages, 27 figures) A version with full-resolution
figures is available at http://www.astro.fit.edu/wood/WoodV344.pd
The Swift X-ray flaring afterglow of GRB 050607
The unique capability of the Swift satellite to perform a prompt and
autonomous slew to a newly detected Gamma-Ray Burst (GRB) has yielded the
discovery of interesting new properties of GRB X-ray afterglows, such as the
steep early lightcurve decay and the frequent presence of flares detected up to
a few hours after the GRB trigger. We present observations of GRB 050607, the
fourth case of a GRB discovered by Swift with flares superimposed on the
overall fading X-ray afterglow. The flares of GRB 050607 were not symmetric as
in previously reported cases, showing a very steep rise and a shallower decay,
similar to the Fast Rise, Exponential Decay that are frequently observed in the
gamma-ray prompt emission. The brighter flare had a flux increase by a factor
of approximately 25,peaking for 30 seconds at a count rate of approximately 30
counts s-1, and it presented hints of addition short time scale activity during
the decay phase. There is evidence of spectral evolution during the flares. In
particular, at the onset of the flares the observed emission was harder, with a
gradual softening as each flare decayed. The very short time scale and the
spectral variability during the flaring activity are indicators of possible
extended periods of energy emission by the GRB central engine. The flares were
followed by a phase of shallow decay, during which the forward shock was being
refreshed by a long-lived central engine or by shells of lower Lorentz factors,
and by a steepening after approximately 12 ks to a decay slope considered
typical of X-ray afterglows.Comment: 23 pages, 5 figures, Accepted by the Astrophysical Journa
Low- and Medium-Dispersion Spectropolarimetry of Nova V475 Sct (Nova Scuti 2003): Discovery of an Asymmetric High-Velocity Wind in a Moderately Fast Nova
We present low-resolution () and medium-resolution ()
spectropolarimetry of Nova V475 Sct with the HBS instrument, mounted on the
0.91-m telescope at the Okayama Astrophysical Observatory, and with FOCAS,
mounted on the 8.2-m Subaru telescope. We estimated the interstellar
polarization toward the nova from the steady continuum polarization components
and H line emission components. After subtracting the interstellar
polarization component from the observations, we found that the H
emission seen on 2003 October 7 was clearly polarized. In the polarized flux
spectrum, the H emission had a distinct red wing extending to km s and a shoulder around km s, showing a
constant position angle of linear polarization \theta_{\rm *}\simeq
155\arcdeg\pm 15\arcdeg. This suggests that the nova had an asymmetric outflow
with a velocity of km s or more, which is six
times higher than the expansion velocity of the ionized shell at the same
epoch. Such a high-velocity component has not previously been reported for a
nova in the `moderately fast' speed class. Our observations suggest the
occurrence of violent mass-loss activity in the nova binary system even during
the common-envelope phase. The position angle of the polarization in the
H wing is in good agreement with that of the continuum polarization
found on 2003 September 26 (--0.6 %), which disappeared
within the following 2 d. The uniformity of the PA between the continuum
polarization and the wing polarization on October 7 suggests that the axis of
the circumstellar asymmetry remained nearly constant during the period of our
observations.Comment: 27 pages, 7 figures, accepted for publication in A
Modeling Kepler Observations of Solar-like Oscillations in the Red-giant Star HD 186355
We have analysed oscillations of the red giant star HD 186355 observed by the
NASA Kepler satellite. The data consist of the first five quarters of science
operations of Kepler, which cover about 13 months. The high-precision
time-series data allow us to accurately extract the oscillation frequencies
from the power spectrum. We find the frequency of the maximum oscillation
power, {\nu}_max, and the mean large frequency separation, {\Delta}{\nu}, are
around 106 and 9.4 {\mu}Hz respectively. A regular pattern of radial and
non-radial oscillation modes is identified by stacking the power spectra in an
echelle diagram. We use the scaling relations of {\Delta}{\nu} and {\nu}_max to
estimate the preliminary asteroseismic mass, which is confirmed with the
modelling result (M = 1.45 \pm 0.05 M_sun) using the Yale Rotating stellar
Evolution Code (YREC7). In addition, we constrain the effective temperature,
luminosity and radius from comparisons between observational constraints and
models. A number of mixed l = 1 modes are also detected and taken into account
in our model comparisons. We find a mean observational period spacing for these
mixed modes of about 58 s, suggesting that this red giant branch star is in the
shell hydrogen-burning phase.Comment: 26 pages, 5 figures and 2 table
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