254 research outputs found

    On the masses of neutron stars

    Get PDF
    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

    Get PDF
    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

    Get PDF
    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

    Get PDF
    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

    Get PDF
    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

    Get PDF
    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 (\ell) 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 Hβ\beta, Hγ\gamma, and Hδ\delta 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 \ell identifications and add four new values, including an additional \ell=2 and a possible \ell=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

    Full text link
    We present near infrared (NIR) spectroscopy of the nearby supernova 2014J obtained \sim450 d after explosion. We detect the [Ni II] 1.939 μ\mum line in the spectra indicating the presence of stable 58^{58}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 AVA_V and low RVR_V found in earlier studies from data near maximum light. Using a 56^{56}Ni mass prior from near maximum light γ\gamma-ray observations, we find \sim0.05 M_\odot 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 \sim600 km s1^{-1}.Comment: 6 pages, 4 figures, submitted to A&

    The masses of the millisecond pulsar J1012+5307 and its white-dwarf companion

    Get PDF
    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 Teff=8550±25T_{\rm{}eff}=8550\pm25\,K and a surface gravity logg=6.75±0.07\log{}g=6.75\pm0.07 (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. Needs aas2pp4.sty, epsf.st
    corecore