14486748
The Distance to the Perseus Spiral Arm in the Milky Way
We have measured the distance to the massive star–forming region W3OH in the Perseus spiral arm of the Milky Way to be 1.95 ± 0.04 kiloparsecs (5.86 ×1016 km). This distance was determined by triangulation, with Earth's orbit as one segment of a triangle, using the Very Long Baseline Array. This resolves the long-standing problem that there is a discrepancy of a factor of 2 between different techniques used to determine distances. The reason for the discrepancy is that this portion of the Perseus arm has anomalous motions. The orientation of the anomalous motion agrees with spiral density-wave theory, but the magnitude of the motion is somewhat larger than most models predict.
119186799
TRIGONOMETRIC PARALLAXES OF HIGH MASS STAR FORMING REGIONS: THE STRUCTURE AND KINEMATICS OF THE MILKY WAY
Over 100 trigonometric parallaxes and proper motions for masers associated with young, high- mass stars have been measured with the Bar and Spiral Structure Legacy Survey, a Very Long Baseline Array key science project, the European VLBI Network, and the Japanese VLBI Exploration of Radio Astrometry project. These measurements provide strong evidence for the existence of spiral arms in the MilkyWay, accurately locating many arm segments and yielding spiral pitch angles ranging from about 7 degrees to 20 degrees. The widths of spiral arms increase with distance from the Galactic center. Fitting axially symmetric models of the MilkyWay with the three- dimensional position and velocity information and conservative priors for the solar and average source peculiar motions, we estimate the distance to the Galactic center, R-0, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, Theta(0), to be 240 +/- 8 km s(-1), and a rotation curve that is nearly flat ( i. e., a slope of -0.2 +/- 0.4 km s(-1) kpc(-1)) between Galactocentric radii of approximate to 5 and 16 kpc. Assuming a " universal" spiral galaxy form for the rotation curve, we estimate the thin disk scale length to be 2.44 +/- 0.16 kpc. With this large data set, the parameters R-0 and Theta(0) are no longer highly correlated and are relatively insensitive to different forms of the rotation curve. If one adopts a theoretically motivated prior that high- mass star forming regions are in nearly circular Galactic orbits, we estimate a global solar motion component in the direction of Galactic rotation, V-circle dot = 14.6 +/- 5.0 km s(-1). While Theta(0) and V-circle dot are significantly correlated, the sum of these parameters is well constrained, Theta(0) + V circle dot = 255.2 +/- 5.1 km s(-1), as is the angular speed of the Sun in its orbit about the Galactic center, ( Theta(0) + V-circle dot)/R-0 = 30.57 +/- 0.43 km s(-1) kpc(-1). These parameters improve the accuracy of estimates of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their Galactic orbits, significantly reducing the uncertainty in tests of gravitational radiation predicted by general relativity.
118538473
Constraints on the dark matter annihilation from Fermi-LAT observation of M31
Gamma-ray is a good probe of dark matter (DM) particles in the Universe. We search for the DM annihilation signals in the direction of the Andromeda galaxy (M31) using 7.5 year Fermi-LAT pass 8 data. Similar to Pshirkov et al. (2016), we find that there is residual excess emission from the direction of M31 if only the galactic disk as traced by the far infrared emission is considered. Adding a point-like source will improve the fitting effectively, although additional slight improvements can be found if an extended component such as a uniform disk or two bubbles is added instead. Taking the far infrared disk plus a point source as the background model, we search for the DM annihilation signals in the data. We find that there is strong degeneracy between the emission from the galaxy and that from 10s GeV mass DM annihilation in the main halo with quark final state. However, the required DM annihilation cross section is about 10−25–10−24 cm3s−1, orders of magnitude larger than the constraints from observations of dwarf spheroidal galaxies, indicating a non-DM origin of the emission. If DM subhalos are taken into account, the degeneracy is broken. When considering the enhancement from DM subhalos, the constraints on DM model parameters are comparable to (or slightly weaker than) those from the population of dwarf spheroidal galaxies. We also discuss the inverse Compton scattering component from DM annihilation induced electrons/positrons. For the first time we include an energy dependent template of the inverse Compton emission (i.e., a template cube) in the data analysis to take into account the effect of diffusion of charged particles. We find a significant improvement of the constraints in the high mass range of DM particles after considering the inverse Compton emission.
119233618
The K giant stars from the LAMOST survey data I: identification, metallicity, and distance
We present a support vector machine classifier to identify the K giant stars from the LAMOST survey directly using their spectral line features. The completeness of the identification is about 75% for tests based on LAMOST stellar parameters. The contamination in the identified K giant sample is lower than 2.5%. Applying the classification method to about two million LAMOST spectra observed during the pilot survey and the first year survey, we select 298,036Kgiant candidates. The metallicities of the sample are also estimated with an uncertainty of 0.13 similar to 0.29 dex based on the equivalent widths of Mgb and iron lines. A Bayesian method is then developed to estimate the posterior probability of the distance for the K giant stars, based on the estimated metallicity and 2MASS photometry. The synthetic isochrone-based distance estimates have been calibrated using 7 globular clusters with a wide range of metallicities. The uncertainty of the estimated distance modulus at K = 11 mag, which is the median brightness of the K giant sample, is about 0.6 mag, corresponding to similar to 30% in distance. As a scientific verification case, the trailing arm of the Sagittarius stream is clearly identified with the selected K giant sample. Moreover, at about 80 kpc from the Sun, we use our K giant stars to confirm a detection of stream members near the apo-center of the trailing tail. These rediscoveries of the features of the Sagittarius stream illustrate the potential of the LAMOST survey for detecting substructures in the halo of the Milky Way.
117818525
Proton irradiation effect on SCDs
The Low Energy X-ray Telescope is one of the main payloads on the Hard X-ray Modulation Telescope satellite. Swept charge devices (SCDs) are selected as detectors for the Low Energy X-ray Telescope. As SCDs are sensitive to proton irradiation, irradiation tests were carried out on the HI-13 accelerator at the China Institute of Atomic Energy. The beam energy was measured to be 10 MeV at the SCD. The proton fluence delivered to the SCD was 3x10(8)protons/cm(2) over two hours. By comparing the performance before and after irradiation, it is concluded that proton irradiation affects both the dark current and the charge transfer inefficiency of the SCD. The energy resolution of the proton-irradiated SCD is 212 eV@5.9 keV at -60 degrees C, while it before irradiated is 134 eV. Moreover, better performance can be reached by lowering the operating temperature of the SCD in orbit.
118659723
Search for TeV-antiprotons in space from the shadowing of cosmic rays by the moon with the L3+C detector
A search for antiprotons in the primary cosmic ray flux has been performed by observing the Moon shadow with the muon data collected by the L3+C experiment at CERN during 1999 and 2000. The angular resolution ofthe detector and its dependences on the muon energy and zenith angle are obtained at the event level with a Monte Carlo simulation and the analysis of real double muon data using a maximum likelihood method. The Moon shadow effect is observed in three muon momentum bands > 100 GeV/c, 65 100 GeV/c and 30 65 GeV/c with a significance of 7.0 0",5.80" and 5.2 0" respectively. Two dimensional maximum likelihood analyses are performed, both with binned data and unbinned data. The unbinned method gives a smaller uncertainty on the measurement of the antiproton to proton ratio. For a muon momentum cut at 70 GeV/c, the antiproton to proton ratio is measured to be -0.12 ±O.ll and an upper limit ofthis ratio is estimated to be 0.08 (at 90% confidence level) for primary energies in the range 0.8 TeV to 2.4 TeV. In addition, the above mentioned angular resolution is confirmed by the Moon shadow observation and the pointing precision of the detector is determined to be better than 0.1 0 •
91177423
THE ORBIT OF THE ORPHAN STREAM
We use recent Sloan Extension for Galactic Understanding and Exploration (SEGUE) spectroscopy and the Sloan Digital Sky Survey (SDSS) and SEGUE imaging data to measure the sky position, distance, and radial velocities of stars in the tidal debris stream that is commonly referred to as the "Orphan Stream." We fit orbital parameters to the data and find a prograde orbit with an apogalacticon, perigalacticon, and eccentricity of 90 kpc, 16.4 kpc, and e = 0.7, respectively. Neither the dwarf galaxy UMa II nor the Complex A gas cloud has velocities consistent with a kinematic association with the Orphan Stream. It is possible that Segue-1 is associated with the Orphan Stream, but no other known Galactic clusters or dwarf galaxies in the Milky Way lie along its orbit. The detected portion of the stream ranges from 19 to 47 kpc from the Sun and is an indicator of the mass interior to these distances. There is a marked increase in the density of Orphan Stream stars near (l, b) = (253°, 49°), which could indicate the presence of the progenitor at the edge of the SDSS data. If this is the progenitor, then the detected portion of the Orphan Stream is a leading tidal tail. We find blue horizontal branch (BHB) stars and F turnoff stars associated with the Orphan Stream. The turnoff color is (g – r)0 = 0.22. The BHB stars have a low metallicity of [Fe/H]WBG = –2.1. The orbit is best fit to a halo potential with a halo plus disk mass of about 2.6 × 1011 M ☉, integrated to 60 kpc from the Galactic center. Our fits are done to orbits rather than full N-body simulations; we show that if N-body simulations are used, the inferred mass of the galaxy would be slightly smaller. Our best fit is found with a logarithmic halo speed of v halo = 73 ± 24 km s–1, a disk+bulge mass of M(R < 60 kpc) = 1.3 × 1011 M ☉, and a halo mass of M(R < 60 kpc) = 1.4 × 1011 M ☉. However, we can find similar fits to the data that use a Navarro-Frenk-White halo profile or that have smaller disk masses and correspondingly larger halo masses. Distinguishing between different classes of models requires data over a larger range of distances. The Orphan Stream is projected to extend to 90 kpc from the Galactic center, and measurements of these distant parts of the stream would be a powerful probe of the mass of the Milky Way.