10 research outputs found
Evidence for a Massive Neutron Star from a Radial-Velocity Study of the Companion to the Black Widow Pulsar PSR B1957+20
The most massive neutron stars constrain the behavior of ultra-dense matter,
with larger masses possible only for increasingly stiff equations of state.
Here, we present evidence that the black widow pulsar, PSR B1957+20, has a high
mass. We took spectra of its strongly irradiated companion and found an
observed radial-velocity amplitude of K_obs=324+/-3 km/s. Correcting this for
the fact that, due to the irradiation, the center of light lies inward relative
to the center of mass, we infer a true radial-velocity amplitude of K_2=353+/-4
km/s and a mass ratio q=M_PSR/M_2=69.2+/-0.8. Combined with the inclination
i=65+/-2 deg inferred from models of the lightcurve, our best-fit pulsar mass
is M_PSR=2.40+/-0.12 M_sun. We discuss possible systematic uncertainties, in
particular in the lightcurve modeling. Taking an upper limit of i<85 deg based
on the absence of radio eclipses at high frequency, combined with a
conservative lower-limit to the motion of the center of mass, K_2>343 km/s
(q>67.3), we infer a lower limit to the pulsar mass of M_PSR>1.66 M_sun.Comment: 7 pages, 3 figures, 1 table, accepted for publication in ApJ;
revision includes more detail on the spectral classification and discussion
of other recent high neutron-star masse
Chemical abundances in M31 from HII regions
We have obtained multi-slit spectroscopic observations from 3700A to 9200A
with LRIS at the Keck I telescope for 31 HII regions in the disk of the
Andromeda galaxy (M31), spanning a range in galactocentric distance from 3.9
kpc to 16.1 kpc. In 9 HII regions we measure one or several auroral lines
([OIII]4363, [NII]5755, [SIII]6312, [OII]7325), from which we determine the
electron temperature (Te) of the gas and derive chemical abundances using the
'direct Te-based method'. We analyze, for the first time in M31, abundance
trends with galactocentric radius from the 'direct' method, and find that the
Ne/O, Ar/O, N/O and S/O abundance ratios are consistent with a constant value
across the M31 disc, while the O/H abundance ratio shows a weak gradient. We
have combined our data with all spectroscopic observations of HII regions in
M31 available in the literature, yielding a sample of 85 HII regions spanning
distances from 3.9 kpc to 24.7 kpc (0.19 - 1.2 R25) from the galaxy center. We
have tested a number of empirical calibrations of strong emission line ratios.
We find that the slope of the oxygen abundance gradient in M31 is
-0.023+/-0.002 dex/kpc, and that the central oxygen abundance is in the range
12+log(O/H) = 8.71 - 8.91 dex (i.e. between 1.05 and 1.66 times the solar
value, for 12+log(O/H)_solar=8.69), depending on the calibration adopted. The
HII region oxygen abundances are compared with the results from other
metallicity indicators (supergiant stars and planetary nebulae). The comparison
shows that HII region O/H abundances are systematically ~0.3 dex below the
stellar ones. This discrepancy is discussed in terms of oxygen depletion onto
dust grains and possible biases affecting Te-based oxygen abundances at high
metallicity.Comment: 21 pages and 11 figures. Accepted for publication in MNRA