77 research outputs found
Multiwavelength study of Cygnus A IV. Proper motion and location of the nucleus
Context. Cygnus A, as the nearest powerful FR II radio galaxy, plays an
important role in understanding jets and their impact on the surrounding
intracluster medium. Aims. To explain why the nucleus is observed superposed
onto the eastern lobe rather than in between the two lobes, and why the jet and
counterjet are non-colinear. Methods. We made a comparative study of the radio
images at different frequencies of Cygnus A, in combination with the published
results on the radial velocities in the Cygnus A cluster. Results. From the
morphology of the inner lobes we conclude that the lobes are not interacting
with one another, but are well separated, even at low radio frequencies. We
explain the location of the nucleus as the result of the proper motion of the
galaxy through the cluster. The required proper motion is of the same order of
magnitude as the radial velocity offset of Cygnus A with the sub-cluster it
belongs to. The proper motion of the galaxy through the cluster likely also
explains the non-co-linearity of the jet and counterjet.Comment: Accepted for publication in A&A, 8 pages, 4 figure
Physical properties of eclipsing white dwarf binaries
Close binaries containing compact objects come in many different forms, but have one thing
in common: their evolution involves at least one common envelope phase and angular momentum
losses. However, many aspects of these two fundamental processes are still poorly understood. Ample
observational input holds the key to improving our understanding, as only then can theoretical
models be properly calibrated and tested. Close binaries containing a white dwarf are perhaps the
best-suited class of objects to provide such input, due to their ubiquity. White dwarf binaries that additionally
display eclipses are of added interest, as accurate and model-independent determinations
of fundamental stellar parameters, such as the masses and radii of the binary components, can only
be obtained in such systems.
In this thesis, I present a study of eclipsing white dwarf binaries. I identify SDSSJ 0110+1326,
SDSSJ 0303+0054, SDSSJ 1210+3347, SDSSJ 1435+3733 and SDSSJ 1548+4057 as new eclipsing,
detached, post-common-envelope, white dwarf +M-dwarf binaries. I use follow-up photometric and
spectroscopic observations, as well as a light curve fitting technique to measure their orbital periods,
and derive the masses, radii and radial velocities of the binary components. These five systems have
been identified as part of the first dedicated search for eclipsing post-common-envelope binaries
and almost double the existing population. The measurements of the stellar parameters, and others
obtained from similar systems, are of great value both for the calibration of the common envelope
equations and for testing the mass-radius relations of white dwarfs and low-mass main sequence
stars. I also identify HS 2325+8205 as a new eclipsing and very frequently outbursting dwarf nova.
Combined constraints from photometric and spectroscopic observations are used to infer the binary
and stellar parameters. The combination of eclipses, frequent outbursts, brightness range and
high declination make HS 2325+8205 an ideal laboratory for detailed studies of accretion discs and
accretion processes in close binaries. Finally I study the cataclysmic variable V455And, in an attempt
to verify the presence of non-radial pulsations in the white dwarf primary. This is achieved
by analysing ten-years worth of photometric observations using time-series analysis techniques and
Fourier transforms. The results are indeed consistent with white dwarf pulsations, although a very
complex behaviour of the power spectra is revealed, most likely a result of the rapid rotation of the
accreting white dwarf primary
Mass ratio from Doppler beaming and R{\o}mer delay versus ellipsoidal modulation in the Kepler data of KOI-74
We present a light curve analysis and radial velocity study of KOI-74, an
eclipsing A star + white dwarf binary with a 5.2 day orbit. Aside from new
spectroscopy covering the orbit of the system, we used 212 days of publicly
available Kepler observations and present the first complete light curve
fitting to these data, modelling the eclipses and transits, ellipsoidal
modulation, reflection, and Doppler beaming. Markov Chain Monte Carlo
simulations are used to determine the system parameters and uncertainty
estimates. Our results are in agreement with earlier studies, except that we
find an inclination of 87.0 \pm 0.4\degree, which is significantly lower than
the previously published value. We find that the mass ratio derived from the
radial velocity amplitude (q=0.104 \pm 0.004) disagrees with that derived from
the ellipsoidal modulation (q=0.052 \pm 0.004} assuming corotation). This was
found before, but with our smaller inclination, the discrepancy is even larger
than previously reported. Accounting for the rapid rotation of the A-star is
found to increase the discrepancy even further by lowering the mass ratio to
q=0.047 \pm 0.004. These results indicate that one has to be extremely careful
in using the amplitude of an ellipsoidal modulation signal in a close binary to
determine the mass ratio, when a proof of corotation is not firmly established.
The radial velocities that can be inferred from the detected Doppler beaming in
the light curve are found to be in agreement with our spectroscopic radial
velocity determination. We also report the first measurement of R{\o}mer delay
in a light curve of a compact binary. This delay amounts to -56 \pm 17 s and is
consistent with the mass ratio derived from the radial velocity amplitude. The
firm establishment of this mass ratio at q=0.104 \pm 0.004 leaves little doubt
that the companion of KOI-74 is a low mass white dwarf.Comment: 9 pages, 7 figures, 2 tables; accepted for publication in MNRA
A magnetic white dwarf in a detached eclipsing binary
SDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disc. In this paper, we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths, we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths, we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman-split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8âMG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disc. We also detect magnetically confined material located roughly midway between the two stars. Using measurements of the radial velocity amplitude and rotational broadening of the M star, we constrain the physical parameters of the system, a first for a magnetic white dwarf, and the location of the poles on the surface of the white dwarf. SDSS J030308.35+005444.1 is a pre-cataclysmic variable that will likely evolve into an intermediate polar in âź1âGyr
HS 2325+8205 - an ideal laboratory for accretion disk physics
We identify HS 2325+8205 as an eclipsing, frequently outbursting dwarf nova
with an orbital period of 279.841731(5) min. Spectroscopic observations are
used to derive the radial velocity curve of the secondary star from absorption
features and also from the H-alpha emission lines, originating from the
accretion disc, yielding K_secondary = K_abs = 237 +- 28 km/s and K_emn = 145
+- 9 km/s respectively. The distance to the system is calculated to be 400
(+200, -140) pc. A photometric monitoring campaign reveals an outburst
recurrence time of 12-14 d, The combination of magnitude range (17-14 mag),
high declination, eclipsing nature and frequency of outbursts makes HS
2325+8205 the ideal system for "real-time" studies of the accretion disc
evolution and behavior in dwarf nova outbursts.Comment: 20 pages, 7 figures. Accepted for Publications of the Astronomical
Society of the Pacifi
CONSTRAINING THE ANGULAR MOMENTUM EVOLUTION OF V455 ANDROMEDAE
Time-series photometry on the cataclysmic variable V455 Andromedae (hereafter V455 And, HS 2331+3905) reveals a rotation period shorter than the orbital period, implying the presence of a magnetic field. We expect that this magnetic field channels the accreted matter from the disk toward the white dwarf poles, classifying it as an Intermediate Polar. The two polar spinning emission areas are visible in the lightcurves at the rotation period of 67.61970396 Âą 0.00000072 s, and its harmonic. Using photometric observations of V455 And obtained from 2007 October to 2015, we derive 3Ď upper limits to the rate of change of the spin harmonic (SH) with time to be dPSH/dt ⤠â7.5 Ă 10â15 s sâ1 employing the OâC method, and â5.4 Ă 10â15 s sâ1 with a direct nonlinear least squares fit. There is no significant detection of a changing spin period for the duration of 2007 Octoberâ2015. The 3Ď upper limit for the rate of change of spin period with time is dPspin/dt ⤠â10.8 Ă 10â15 s sâ1 or â0.34 Îźs yrâ1. V455 And underwent a large-amplitude dwarf nova outburst in 2007 September. The pre-outburst data reflect a period 4.8 Âą 2.2 Îźs longer than the best-fit post-outburst spin period. The angular momentum gained by the white dwarf from matter accreted during outburst and its slight subsequent shrinking should both cause the star to spin slightly faster after the outburst. We estimate that the change in spin period due to the outburst should be 5 Îźs, consistent with the empirical determination of 4.8 Âą 2.2 Îźs (3Ď upper limit of 11.4 Îźs)
IPHAS J062746.41+014811.3: a deeply eclipsing intermediate polar
We present time-resolved photometry of a cataclysmic variable discovered in
the Isaac Newton Telescope Photometric Halpha Survey of the northern galactic
plane, IPHAS J062746.41+014811.3 and classify the system as the fourth deeply
eclipsing intermediate polar known with an orbital period of Porb=8.16 h, and
spin period of Pspin=2210 s. The system shows mild variations of its
brightness, that appear to be accompanied by a change in the amplitude of the
spin modulation at optical wavelengths, and a change in the morphology of the
eclipse profile. The inferred magnetic moment of the white dwarf is mu_wd = 6-7
x 10^33 Gcm^3, and in this case IPHAS J0627 will either evolve into a
short-period EX Hya-like intermediate polar with a large Pspin\Porb ratio, or,
perhaps more likely, into a synchronised polar. Swift observations show that
the system is an ultraviolet and X-ray source, with a hard X-ray spectrum that
is consistent with those seen in other intermediate polars. The ultraviolet
light curve shows orbital modulation and an eclipse, while the low
signal-to-noise ratio X-ray light curve does not show a significant modulation
on the spin period. The measured X-ray flux is about an order of magnitude
lower than would be expected from scaling by the optical fluxes of well-known
X-ray selected intermediate polars.Comment: 34 pages, 9 figures, accepted for publication in Ap
Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis
We derive precise system parameters for the pre-cataclysmic binary, NN Ser.
From light curve fitting we find an orbital inclination of i = 89.6 +/- 0.2
deg. From the HeII absorption line we find K_{WD}= 62.3 +/- 1.9 km/s. The
irradiation-induced emission lines from the surface of the secondary star give
a range of observed radial velocities. The corrected values give a radial
velocity of K_{sec}= 301 +/- 3 km/s, with an error dominated by the systematic
effects of the model. This leads to a binary separation of a = 0.934 +/- 0.009
R_{sun}, radii of R_{WD} = 0.0211 +/- 0.0002 R_{sun} and R_{sec} = 0.149 +/-
0.002 R_{sun} and masses of M_{WD} = 0.535 +/- 0.012 M_{sun} and M_{sec} =
0.111 +/- 0.004 M_{sun}. The masses and radii of both components of NN Ser were
measured independently of any mass-radius relation. For the white dwarf, the
measured mass, radius and temperature show excellent agreement with a `thick'
hydrogen layer of fractional mass M_{H}/{M}_{WD} = 10^{-4}. The measured radius
of the secondary star is 10% larger than predicted by models, however,
correcting for irradiation accounts for most of this inconsistency, hence the
secondary star in NN Ser is one of the first precisely measured very low mass
objects to show good agreement with models. ULTRACAM r', i' and z' photometry
taken during the primary eclipse determines the colours of the secondary star
as (r'-i')_{sec}= 1.4 +/- 0.1 and (i'-z')_{sec} = 0.8 +/- 0.1 which corresponds
to a spectral type of M4 +/- 0.5. This is consistent with the derived mass,
demonstrating that there is no detectable heating of the unirradiated face,
despite intercepting radiative energy from the white dwarf which exceeds its
own luminosity by over a factor of 20.Comment: 20 pages, 17 figures, 8 tables, minor changes, accepted for
publication in MNRA
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