arXiv subjects

R. Castro

Publications and source records attributed to R. Castro.

4 recordsLinked to original sources

arXiv2020

The ELFIN Mission

The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (~93deg inclination), nearly circular, low-Earth (~450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (~90min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50keV to 5MeV electrons with deltaE/E<40% and a fluxgate magnetometer (FGM) on a ~72cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5Hz Nyquist (nominally) with <0.3nT/sqrt(Hz) noise at 1Hz. The spinning satellites (T_spin~3s) are equipped with magnetorquers that permit spin-up/down and reorientation maneuvers. The spin axis is placed normal to the orbit plane, allowing full pitch-angle resolution twice per spin. An energetic particle detector for ions (EPDI) measures 250keV-5MeV ions, addressing secondary science. Funded initially by CalSpace and the University Nanosat Program, ELFIN was selected for flight with joint support from NSF and NASA between 2014 and 2018.

physics.space-ph↗

arXiv2002

On the nature of a secondary main sequence turn-off in the rich LMC cluster NGC 1868

Evidence for a second main-sequence turn-off in a deep colour-magnitude diagram of NGC 1868 is presented. The data were obtained with HST/WFPC2 and reach down to m_{555} ~ 25. Besides the usual t ~ 0.8 Gyr turn-off found in previous analyses, another possible turn-off is seen at m_{555} ~ 21 (M_V ~ 2.5), which is consistent with an age of t ~ 3 Gyrs. This CMD feature stands out clearly especially when contaminating field LMC stars are statistically removed. The background subtracted CMD also visibly displays a red subgiant branch extending about 1.5 mag below the younger turn-off and the clump of red giants. The significance of the secondary turn-off in NGC 1868 was confirmed with Monte-Carlo simulations and bootstrapping techniques. Star-counts in selected regions in the cluster CMD indicate a mass ratio of old population/young population in the range 5% <= M_{old} / M_{young} <= 12%, depending on the mass function slope. The existence of such a subpopulation in NGC 1868 is significant even in the presence of uncertainties in background subtraction. The possibility that the secondary turn-off is associated with the field star population was examined by searching for similar features in CMDs of field stars. Statistically significant excesses of stars redwards of the main-sequence were found in all such fields in the range 20 <= m_{555} <= 22. These however are much broader features that do not resemble the main-sequence termination of a single population. We also discuss other alternative explanations for the feature at m_{555} ~ 21, such as unresolved binarism, peculiar stars or CMD discontinuities associated with the Bohm-Vitense gap.

astro-ph↗

arXiv2002

Analysis of colour-magnitude diagrams of rich LMC clusters: NGC 1831

We present the analysis of a deep colour-magnitude diagram (CMD) of NGC 1831, a rich star cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W (~V) and F814W (~I) filters, reaching m_555 ~ 25. We discuss and apply a method of correcting the CMD for sampling incompleteness and field star contamination. Efficient use of the CMD data was made by means of direct comparisons of the observed to model CMDs. The model CMDs are built by an algorithm that generates artificial stars from a single stellar population, characterized by an age, a metallicity, a distance, a reddening value, a present day mass function and a fraction of unresolved binaries. Photometric uncertainties are empirically determined from the data and incorporated into the models as well. Statistical techniques are presented and applied as an objective method to assess the compatibility between the model and data CMDs. By modelling the CMD of the central region in NGC 1831 we infer a metallicity Z = 0.012, 8.75 < log(tau) < 8.80, 18.54 < (m-M)_0 < 18.68 and 0.00 < E(B-V) < 0.03. For the position dependent PDMF slope (alpha = -dlog(Phi(M))/dlog(M)), we clearly observe the effect of mass segregation in the system: for projected distances R < 30 arcsec, alpha ~ 1.7, whereas 2.2 < alpha < 2.5 in the outer regions of NGC 1831.

astro-ph↗

arXiv1996

Noise-enhanced reconstruction of attractors

In principle, the state space of a chaotic attractor can be partially or wholly reconstructed from interspike intervals recorded from experiment. Under certain conditions, the quality of a partial reconstruction, as measured by the spike train prediction error, can be increased by adding noise to the spike creation process. This phenomenon for chaotic systems is an analogue of stochastic resonance.

chao-dyn↗