254 research outputs found
On the masses of neutron stars
We analyze the currently available observations of X-ray binaries in a
consistent way, to re-determine the masses of the neutron stars in these
systems. In particular, our attention is focussed on a realistic and consistent
assessment of observational uncertainties and sources of systematic error.
Confidence limits for these new mass estimates are generally less optimistic
than previously assumed. The available observations, including data on six
radio pulsars, do not firmly constrain the equation of state of neutron star
matter. In particular, a firm upper mass limit cannot yet be established. An
improvement of the accuracy of optical data holds the key to further progress.Comment: 5 pages of compressed, uuencoded postscript (text+figure). Accepted
for publication in A&A. In case of problems, contact [email protected]
Mode identification from time-resolved spectroscopy of the pulsating white dwarf G 29-38
We have used time-resolved spectroscopy to measure the colour dependence of
pulsation amplitudes in the DAV white dwarf G 29-38. Model atmospheres predict
that mode amplitudes should change with wavelength in a manner that depends on
the spherical harmonic degree l of the mode. This dependence arises from the
convolution of mode geometry with wavelength-dependent limb darkening. Our
analysis of the six largest normal modes detected in Keck observations of G
29-38 reveals one mode with a colour dependence different from the other five,
permitting us to identify the l value of all six modes and to test the model
predictions. The Keck observations also show pulsation amplitudes that are
unexpectedly asymmetric within absorption lines. We show that these asymmetries
arise from surface motions associated with the non-radial pulsations (which are
discussed in detail in a companion paper). By incorporating surface velocity
fields into line profile calculations, we are able to produce models that more
closely resemble the observations.Comment: 10 pages, 9 figures, mn.sty. Accepted for publication in MNRA
Surface motion in the pulsating DA white dwarf G 29-38
We present time-resolved spectrophotometry of the pulsating DA white dwarf G
29-38. As in previous broad-band photometry, the light curve shows the presence
of a large number of periodicities. Many of these are combination frequencies,
i.e., periodicities occurring at frequencies that are sums or differences of
frequencies of stronger, real modes. We identify at least six real modes, and
at least five combination frequencies. We measure line-of-sight velocities for
our spectra and detect periodic variations at the frequencies of five of the
six real modes, with amplitudes of up to 5 km/s. We argue that these variations
reflect the horizontal surface motion associated with the g-mode pulsations. No
velocity signals are detected at any of the combination frequencies, confirming
that the flux variations at these frequencies do not reflect physical
pulsation, but rather mixing of frequencies due to a non-linear transformation
in the outer layers of the star. We discuss the amplitude ratios and phase
differences found for the velocity and light variations, as well as those found
for the real modes and their combination frequencies, both in a
model-independent way and in the context of models based on the
convective-driving mechanism. In a companion paper, we use the wavelength
dependence of the amplitudes of the modes to infer their spherical degree.Comment: 12 pages, 5 figures, mn.sty. Accepted for publication in MNRA
Low Mass Neutron Stars and the Equation of State of Dense Matter
Neutron-star radii provide useful information on the equation of state of
neutron rich matter. Particularly interesting is the density dependence of the
equation of state (EOS). For example, the softening of the EOS at high density,
where the pressure rises slower than anticipated, could signal a transition to
an exotic phase. However, extracting the density dependence of the EOS requires
measuring the radii of neutron stars for a broad range of masses. A ``normal''
1.4 solar mass neutron star has a central density of a few times nuclear-matter
saturation density. In contrast, low mass (of the order of 0.5 solar masses)
neutron stars have central densities near nuclear-matter saturation density so
its radius provides information on the EOS at low density. Unfortunately,
low-mass stars are rare because they may be hard to form. Instead, a precision
measurement of nuclear radii on atomic nuclei may contain similar information.
Indeed, we find a strong correlation between the neutron radius of 208Pb and
the radius of a 0.5 solar-mass neutron star. Thus, the radius of such a neutron
star can be inferred from a measurement of the the neutron radius of 208Pb.
Comparing this value to the measured radius of a 1.4 solar-mass neutron star
should provide the strongest constraint to date on the density dependence of
the equation of state.Comment: 9 pages and 5 eps. figures (included
The Wolf-Rayet counterpart of Cygnus X-3
We present orbital-phase resolved I and K-band spectroscopy of Cygnus X-3.
All spectra show emission lines characteristic of Wolf-Rayet stars of the WN
subclass. On time scales longer than about one day, the line strengths show
large changes, both in flux and in equivalent width. In addition, the line
ratios change, corresponding to a variation in spectral subtype of WN6/7 to
WN4/5. We confirm the finding that at times when the emission lines are weak,
they shift in wavelength as a function of orbital phase, with maximum blueshift
coinciding with infrared and X-ray minimum, and maximum redshift half an orbit
later. Furthermore, we confirm the prediction -- made on the basis of previous
observations -- that at times when the emission lines are strong, no clear
wavelength shifts are observed. We describe a simplified, but detailed model
for the system, in which the companion of the X-ray source is a Wolf-Rayet star
whose wind is at times ionised by the X-ray source, except for the part in the
star's shadow. With this model, the observed spectral variations can be
reproduced with only a small number of free parameters. We discuss and verify
the ramifications of this model, and find that, in general, the observed
properties can be understood. We conclude that Cyg X-3 is a Wolf-Rayet/X-ray
binary.Comment: 22 pages, 13 figures. Accepted by A&A. Uses l-aa.sty. A complete
20-page postscript file using the correct postscript fonts is available at
http://astro.caltech.edu/~mhvk/bigcyg.ps.
Deciphering the Pulsations of G 29-38 with Optical Time Series Spectroscopy
We present optical time series spectroscopy of the pulsating white dwarf star
G29-38 taken at the Very Large Telescope (VLT). By measuring the variations in
brightness, Doppler shift, and line shape of each spectrum, we explore the
physics of pulsation and measure the spherical degree () of each stellar
pulsation mode. We measure the physical motion of the g-modes correlated with
the brightness variations for three of the eight pulsation modes in this data
set. The varying line shape reveals the spherical degree of the pulsations, an
important quantity for properly modeling the interior of the star with
asteroseismology. Performing fits to the H, H, and H
lines, we quantify the changing shape of the line and compare them to models
and previous time series spectroscopy of G~29-38. These VLT data confirm
several identifications and add four new values, including an additional
=2 and a possible =4. In total from both sets of spectroscopy of
G29-38, eleven modes now have known spherical degrees.Comment: Accepted for publication by MNRA
Nebular spectroscopy of SN 2014J: Detection of stable nickel in near infrared spectra
We present near infrared (NIR) spectroscopy of the nearby supernova 2014J
obtained 450 d after explosion. We detect the [Ni II] 1.939 m line
in the spectra indicating the presence of stable Ni in the ejecta. The
stable nickel is not centrally concentrated but rather distributed as the iron.
The spectra are dominated by forbidden [Fe II] and [Co II] lines. We use lines,
in the NIR spectra, arising from the same upper energy levels to place
constraints on the extinction from host galaxy dust. We find that that our data
are in agreement with the high and low found in earlier studies
from data near maximum light. Using a Ni mass prior from near maximum
light -ray observations, we find 0.05 M of stable nickel
to be present in the ejecta. We find that the iron group features are
redshifted from the host galaxy rest frame by 600 km s.Comment: 6 pages, 4 figures, submitted to A&
The masses of the millisecond pulsar J1012+5307 and its white-dwarf companion
We report on spectroscopy of the white-dwarf companion of the millisecond
radio pulsar PSR J1012+5307. We find strong Balmer absorption lines, as would
be expected for a cool DA white dwarf. The profiles are much narrower than
usual, however, and lines are seen up to H12, indicating that the companion has
a low gravity and hence a low mass. This is consistent with the
expectation---based on evolutionary considerations and on the mass
function---that it is a low-mass white dwarf with a helium core. By comparing
the spectra to model atmospheres, we derive an effective temperature
K and a surface gravity (cgs
units). Using the Hamada-Salpeter mass-radius relation for helium white dwarfs,
with an approximate correction for finite-temperature effects, we infer a mass
\mwd=0.16\pm0.02\,\msun. This is the lowest mass among all spectroscopically
identified white dwarfs. We determine radial velocities from our spectra, and
find a radial-velocity amplitude of 280\pm15\,\kms. With the pulsar's
radial-velocity amplitude, the mass ratio \mpsr/\mwd=13.3\pm0.7. From all
constraints, we find that with 95\% confidence 1.5<\mpsr/\msun<3.2.Comment: 6 pages of text and figures. Refereed version, resubmitted to ApJL.
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