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Register a new userPSR J1840-1419: A very cool neutron star
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We present upper limits on the X-ray emission for three neutron stars. For PSR J1840$-$1419, with a characteristic age of 16.5 Myr, we calculate a blackbody temperature upper limit (at 99% confidence) of $kT_{mathrm{bb}}^{infty}<24^{+17}_{-10}$ eV, making this one of the coolest neutron stars known. PSRs J1814$-$1744 and J1847$-$0130 are both high magnetic field pulsars, with inferred surface dipole magnetic field strengths of $5.5times10^{13}$ and $9.4times10^{13}$ G, respectively. Our temperature upper limits for these stars are $kT_{mathrm{bb}}^{infty}<123^{+20}_{-33}$ eV and $kT_{mathrm{bb}}^{infty}<115^{+16}_{-33}$ eV, showing that these high magnetic field pulsars are not significantly hotter than those with lower magnetic fields. Finally, we put these limits into context by summarizing all temperature measurements and limits for rotation-driven neutron stars.
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The pulsar PSR J1756$-$2251 resides in a relativistic double neutron star (DNS) binary system with a 7.67-hr orbit. We have conducted long-term precision timing on more than 9 years of data acquired from five telescopes, measuring five post-Keplerian parameters. This has led to several independent tests of general relativity (GR), the most constraining of which shows agreement with the prediction of GR at the 4% level. Our measurement of the orbital decay rate disagrees with that predicted by GR, likely due to systematic observational biases. We have derived the pulsar distance from parallax and orbital decay measurements to be 0.73$_{-0.24}^{+0.60}$ kpc (68%) and < 1.2 kpc (95% upper limit), respectively; these are significantly discrepant from the distance estimated using Galactic electron density models. We have found the pulsar mass to be 1.341$pm$0.007 M$_odot$, and a low neutron star (NS) companion mass of 1.230$pm$0.007 M$_odot$. We also determined an upper limit to the spin-orbit misalignment angle of 34{deg} (95%) based on a system geometry fit to long-term profile width measurements. These and other observed properties have led us to hypothesize an evolution involving a low mass loss, symmetric supernova progenitor to the second-formed NS companion, as is thought to be the case for the double pulsar system PSR J0737$-$3039A/B. This would make PSR J1756$-$2251 the second compact binary system providing concrete evidence for this type of NS formation channel.
The rapid-neutron-capture (r) process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far fail to account for the observed over-enhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger and explore if varying levels of neutron richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, beta-decay, and mass model. For most cases, the actinides are over-produced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of a neutron star merger is a likely production site for the formation of actinides, a significant contribution from another site or sites (e.g., the neutron star merger accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.
We report the discovery of a very cool, isolated brown dwarf, UGPS 0722-05, with the UKIDSS Galactic Plane Survey. The near-infrared spectrum displays deeper H2O and CH4 troughs than the coolest known T dwarfs and an unidentified absorption feature at 1.275 um. We provisionally classify the object as a T10 dwarf but note that it may in future come to be regarded as the first example of a new spectral type. The distance is measured by trigonometric parallax as d=4.1{-0.5}{+0.6} pc, making it the closest known isolated brown dwarf. With the aid of Spitzer/IRAC we measure H-[4.5] = 4.71. It is the coolest brown dwarf presently known -- the only known T dwarf that is redder in H-[4.5] is the peculiar T7.5 dwarf SDSS J1416+13B, which is thought to be warmer and more luminous than UGPS 0722-05. Our measurement of the luminosity, aided by Gemini/T-ReCS N band photometry, is L = 9.2 +/- 3.1x10^{-7} Lsun. Using a comparison with well studied T8.5 and T9 dwarfs we deduce Teff=520 +/- 40 K. This is supported by predictions of the Saumon & Marley models. With apparent magnitude J=16.52, UGPS 0722-05 is the brightest T dwarf discovered by UKIDSS so far. It offers opportunities for future study via high resolution near-infrared spectroscopy and spectroscopy in the thermal infrared.
We study the evolution of close binary systems in order to account for the existence of the recently observed binary system containing the most massive millisecond pulsar ever detected, PSR J0740+6620, and its ultra-cool helium white dwarf companion. In order to find a progenitor for this object we compute the evolution of several binary systems composed by a neutron star and a normal donor star employing our stellar code. We assume conservative mass transfer. We also explore the effects of irradiation feedback on the system. We find that irradiated models also provide adequate models for the millisecond pulsar and its companion, so both irradiated and non irradiated systems are good progenitors for PSR J0740+6620. Finally, we obtain a binary system that evolves and accounts for the observational data of the system composed by PSR J0740+6620 (i.e. orbital period, mass, effective temperature and inferred metallicity of the companion, and mass of the neutron star) in a time scale smaller than the age of the Universe. In order to reach an effective temperature as low as observed, the donor star should have an helium envelope as demanded by observations.
Two low mass neutron stars, J0737-3039B and the companion to J1756-2251, show strong evidence of being formed from the collapse of an ONeMg core in an electron capture supernova (ECSN) or in an ultra-stripped iron core collapse supernova (FeCCSN). Using three different systematically generated sets of equations of state we explore the relationship between the moment of inertia of J0737-3039A and the binding energy of the two low mass neutron stars. We find this relationship, a less strict variant of the recently discovered I-Love-Q relations, is nevertheless more robust than a previously explored correlation between the binding energy and the slope of the nuclear symmetry energy L. We find that, if either J0737-3039B or the J1756-2251 companion were formed in an ECSN, no more than 0.06 solar masses could have been lost from the progenitor core, more than four times the mass loss predicted by current supernova modeling. A measurement of the moment of inertia of J0737-3039A to within 10% accuracy from pulsar timing, possible within a decade, can discriminate between formation scenarios such as ECSN or ultra-stripped FeCCSN and, given current constraints on the predicted core mass loss, potentially rule them out. Using the I-Love-Q relations we find that an Advanced LIGO can potentially measure the neutron star tidal polarizability to equivalent accuracy in a neutron star-neutron star merger at a distance of 200 Mpc, thus obtaining similar constraints on the formation scenarios. Such information on the occurrence of ECSNe is important for population synthesis calculations, especially for estimating the rate of binary neutron star mergers and resulting electromagnetic and gravitational wave signals. Further progress needs to be made modeling the core collapse process that leads to low-mass neutron stars, particularly in making robust predictions for the mass loss from the progenitor core.
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