296 research outputs found
Merging of globular clusters within inner galactic regions. II. The Nuclear Star Cluster formation
In this paper we present the results of two detailed N-body simulations of
the interaction of a sample of four massive globular clusters in the inner
region of a triaxial galaxy. A full merging of the clusters takes place,
leading to a slowly evolving cluster which is quite similar to observed Nuclear
Clusters. Actually, both the density and the velocity dispersion profiles match
qualitatively, and quantitatively after scaling, with observed features of many
nucleated galaxies. In the case of dense initial clusters, the merger remnant
shows a density profile more concentrated than that of the progenitors, with a
central density higher than the sum of the central progenitors central
densities. These findings support the idea that a massive Nuclear Cluster may
have formed in early phases of the mother galaxy evolution and lead to the
formation of a nucleus, which, in many galaxies, has indeed a luminosity
profile similar to that of an extended King model. A correlation with galactic
nuclear activity is suggested.Comment: 18 pages, 10 figures, 3 tables. Submitted to ApJ, main journa
From 10 Kelvin to 10 TeraKelvin: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts
Recent work has both illuminated and mystified our attempts to understand
cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring
how CRs interact with the ISM in starbursts. Molecular clouds provide targets
for CR protons to produce pionic gamma rays and ionization, but those same
losses may shield the cloud interiors. In the densest molecular clouds, gamma
rays and Al-26 decay can provide ionization, at rates up to those in Milky Way
molecular clouds. I then consider the free-free absorption of low frequency
radio emission from starbursts, which I argue arises from many small, discrete
H II regions rather than from a "uniform slab" of ionized gas, whereas
synchrotron emission arises outside them. Finally, noting that the hot
superwind gas phase fills most of the volume of starbursts, I suggest that it
has turbulent-driven magnetic fields powered by supernovae, and that this phase
is where most synchrotron emission arises. I show how such a scenario could
explain the far-infrared radio correlation, in context of my previous work. A
big issue is that radio and gamma-ray observations imply CRs also must interact
with dense gas. Understanding how this happens requires a more advanced
understanding of turbulence and CR propagation.Comment: Conference proceedings for "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012). 16 pages, 5 figure
KELT-20b: A Giant Planet With A Period Of P ~ 3.5 Days Transiting The V ~ 7.6 Early A Star HD 185603
We report the discovery of KELT-20b, a hot Jupiter transiting a early A star, HD 185603, with an orbital period of days. Archival and follow-up photometry, Gaia parallax, radial velocities, Doppler tomography, and AO imaging were used to confirm the planetary nature of KELT-20b and characterize the system. From global modeling we infer that KELT-20 is a rapidly rotating ( ) A2V star with an effective temperature of K, mass of , radius of , surface gravity of , and age of . The planetary companion has a radius of , a semimajor axis of au, and a linear ephemeris of . We place a upper limit of on the mass of the planet. Doppler tomographic measurements indicate that the planetary orbit normal is well aligned with the projected spin axis of the star ( ). The inclination of the star is constrained to , implying a three-dimensional spinâorbit alignment of . KELT-20b receives an insolation flux of , implying an equilibrium temperature of of âŒ2250 K, assuming zero albedo and complete heat redistribution. Due to the high stellar , KELT-20b also receives an ultraviolet (wavelength nm) insolation flux of , possibly indicating significant atmospheric ablation. Together with WASP-33, Kepler-13 A, HAT-P-57, KELT-17, and KELT-9, KELT-20 is the sixth A star host of a transiting giant planet, and the third-brightest host (in V ) of a transiting planet
First radial velocity results from the MINiature Exoplanet Radial Velocity Array (MINERVA)
The MINiature Exoplanet Radial Velocity Array (MINERVA) is a dedicated
observatory of four 0.7m robotic telescopes fiber-fed to a KiwiSpec
spectrograph. The MINERVA mission is to discover super-Earths in the habitable
zones of nearby stars. This can be accomplished with MINERVA's unique
combination of high precision and high cadence over long time periods. In this
work, we detail changes to the MINERVA facility that have occurred since our
previous paper. We then describe MINERVA's robotic control software, the
process by which we perform 1D spectral extraction, and our forward modeling
Doppler pipeline. In the process of improving our forward modeling procedure,
we found that our spectrograph's intrinsic instrumental profile is stable for
at least nine months. Because of that, we characterized our instrumental
profile with a time-independent, cubic spline function based on the profile in
the cross dispersion direction, with which we achieved a radial velocity
precision similar to using a conventional "sum-of-Gaussians" instrumental
profile: 1.8 m s over 1.5 months on the RV standard star HD 122064.
Therefore, we conclude that the instrumental profile need not be perfectly
accurate as long as it is stable. In addition, we observed 51 Peg and our
results are consistent with the literature, confirming our spectrograph and
Doppler pipeline are producing accurate and precise radial velocities.Comment: 22 pages, 9 figures, submitted to PASP, Peer-Reviewed and Accepte
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Active Damping of the E-P Instability at the Los Alamos Proton Storage Ring
A prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability has been developed and successfully tested at the Los Alamos Proton Storage Ring (PSR). This system was able to improve the instability threshold by approximately 30% (as measured by the change in RF buncher voltage at instability threshold). The feedback system configuration, setup procedures, and optimization of performance are described. Results of several experimental tests of system performance are presented including observations of instability threshold improvement and grow-damp experiments, which yield estimates of instability growth and damping rates. A major effort was undertaken to identify and study several factors limiting system performance. Evidence obtained from these tests suggests that performance of the prototype was limited by higher instability growth rates arising from beam leakage into the gap at lower RF buncher voltage and the onset of instability in the horizontal plane, which had no feedback
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