857 research outputs found
Space and velocity distributions of Galactic isolated old Neutron stars
I present the results of Monte-Carlo orbital simulations of Galactic Neutron
Stars (NSs). The simulations take into account the up-to-date observed NS space
and velocity distributions at birth, and account for their formation rate. I
simulate two populations of NSs. Objects in the first population were born in
the Galactic disk at a constant rate, in the past 12 Gyr. Those in the second
population were formed simultaneously 12 Gyr ago in the Galactic bulge. I
assume that the NSs born in the Galactic disk comprise 40% of the total NS
population. Since the initial velocity distribution of NSs is not well known, I
run two sets of simulations, each containing 3x10^6 simulated NSs. One set
utilizes a bimodal initial velocity distribution and the other a unimodal
initial velocity distribution, both are advocated based on pulsars
observations. In light of recent observational results, I discuss the effect of
dynamical heating by Galactic structure on NS space and velocity distributions
and show it can be neglected. I present catalogue of simulated NS space and
velocity vectors in the current epoch, and catalogue of positions, distances
and proper motions of simulated NSs, relative to the Sun. Assuming there are
10^9 NSs in the Galaxy, I find that in the solar neighborhood the density of
NSs is about 2-4x10^-4 pc^-3 and their scale height is about 0.3-0.6 kpc
(depending on the adopted initial velocity distribution). These catalogue can
be used to test the hypothesis that some radio transients are related to these
objects.Comment: 11 pages, 10 figure
Calibrated griz magnitudes of Tycho stars: All-sky photometric calibration using bright stars
Photometric calibration to 5% accuracy is frequently needed at arbitrary
celestial locations; however, existing all-sky astronomical catalogs do not
reach this accuracy and time consuming photometric calibration procedures are
required. I fit the Hipparcos B_T and V_T magnitudes along with the 2MASS J, H,
and K magnitudes of Tycho-2 catalog-stars with stellar spectral templates. From
the best fit spectral template derived for each star, I calculate the synthetic
SDSS griz magnitudes and constructed an all-sky catalog of griz magnitudes for
bright stars (V<12). Testing this method on SDSS photometric telescope
observations, I find that the photometric accuracy for a single star is usually
about 0.12, 0.12, 0.10 and 0.08 mag (1 sigma), for the g, r, i, and z-bands,
respectively. However, by using ~10 such stars, the typical errors per
calibrated field (systematic + statistical) can be reduced to about 0.04, 0.03,
0.02, and 0.02,mag, in the g, r, i, and z-bands, respectively. Therefore, in
cases for which several calibration stars can be observed in the field of view
of an instrument, accurate photometric calibration is possible.Comment: 3 pages, PASP, in pres
A Sub-Saturn Mass Planet, MOA-2009-BLG-319Lb
We report the gravitational microlensing discovery of a sub-Saturn mass planet, MOA-2009-BLG-319Lb, orbiting a K- or M-dwarf star in the inner Galactic disk or Galactic bulge. The high-cadence observations of the MOA-II survey discovered this microlensing event and enabled its identification as a high-magnification event approximately 24 hr prior to peak magnification. As a result, the planetary signal at the peak of this light curve was observed by 20 different telescopes, which is the largest number of telescopes to contribute to a planetary discovery to date. The microlensing model for this event indicates a planet-star mass ratio of q = (3.95 ± 0.02) Ă 10^(â4) and a separation of d = 0.97537 ± 0.00007 in units of the Einstein radius. A Bayesian analysis based on the measured Einstein radius crossing time, t_E, and angular Einstein radius, Ξ_E, along with a standard Galactic model indicates a host star mass of M_L = 0.38^(+0.34)_(â0.18) M_â and a planet mass of M_p = 50^(+44)_(â24) M_â, which is half the mass of Saturn. This analysis also yields a planet-star three-dimensional separation of a = 2.4^(+1.2)_(â0.6) AU and a distance to the planetary system of D_L = 6.1^(+1.1)_(â1.2) kpc. This separation is ~2 times the distance of the snow line, a separation similar to most of the other planets discovered by microlensing
Massive stars exploding in a He-rich circumstellar medium - I. Type Ibn (SN 2006jc-like) events
We present new spectroscopic and photometric data of the Type Ibn supernovae 2006jc, 2000er and 2002ao. We discuss the general properties of this recently proposed supernova family, which also includes SN 1999cq. The early-time monitoring of SN 2000er traces the evolution of this class of objects during the first few days after the shock breakout. An overall similarity in the photometric and spectroscopic evolution is found among the members of this group, which would be unexpected if the energy in these core-collapse events was dominated by the interaction between supernova ejecta and circumstellar medium. Type Ibn supernovae appear to be rather normal Type Ib/c supernova explosions which occur within a He-rich circumstellar environment. SNe Ibn are therefore likely produced by the explosion of WolfâRayet progenitors still embedded in the He-rich material lost by the star in recent mass-loss
episodes, which resemble known luminous blue variable eruptions. The evolved WolfâRayet star could either result from the evolution of a very massive star or be the more evolved member of a massive binary system.We also suggest that there are a number of arguments in favour of a Type Ibn classification for the historical SN 1885A (S-Andromedae), previously considered as an anomalous Type Ia event with some resemblance to SN 1991bg
SDSS J131339.98+515128.3: A new gravitationally lensed quasar selected based on near-infrared excess
We report the discovery of a new gravitationally lensed quasar, SDSS
J131339.98+515128.3, at a redshift of 1.875 with an image separation of 1.24".
The lensing galaxy is clearly detected in visible-light follow-up observations.
We also identify three absorption-line doublets in the spectra of the lensed
quasar images, from which we measure the lens redshift to be 0.194. Like
several other known lenses, the lensed quasar images have different continuum
slopes. This difference is probably the result of reddening and microlensing in
the lensing galaxy. The lensed quasar was selected by correlating Sloan Digital
Sky Survey (SDSS) spectroscopic quasars with Two Micron All Sky Survey (2MASS)
sources and choosing quasars that show near-infrared (IR) excess. The near-IR
excess can originate, for example, from the contribution of the lensing galaxy
at near-IR wavelengths. We show that the near-IR excess technique is indeed an
efficient method to identify lensed systems from a large sample of quasars.Comment: Accepted to MNRAS, 8 pages, 7 figure
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