There exists a need to address and resolve the growing evidence for short-baseline neutrino oscillations and the possible existence of sterile neutrinos. Such non-standard particles require a mass of $sim 1$ eV/c$^2$, far above the mass scale associa
ted with active neutrinos, and were first invoked to explain the LSND $bar u_mu rightarrow bar u_e$ appearance signal. More recently, the MiniBooNE experiment has reported a $2.8 sigma$ excess of events in antineutrino mode consistent with neutrino oscillations and with the LSND antineutrino appearance signal. MiniBooNE also observed a $3.4 sigma$ excess of events in their neutrino mode data. Lower than expected neutrino-induced event rates using calibrated radioactive sources and nuclear reactors can also be explained by the existence of sterile neutrinos. Fits to the worlds neutrino and antineutrino data are consistent with sterile neutrinos at this $sim 1$ eV/c$^2$ mass scale, although there is some tension between measurements from disappearance and appearance experiments. In addition to resolving this potential major extension of the Standard Model, the existence of sterile neutrinos will impact design and planning for all future neutrino experiments. It should be an extremely high priority to conclusively establish if such unexpected light sterile neutrinos exist. The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, built to usher in a new era in neutron research, provides a unique opportunity for US science to perform a definitive world-class search for sterile neutrinos.
We report the first results of a multi-epoch search for wide (separations greater than a few tens of AU), low-mass tertiary companions of a volume-limited sample of 118 known spectroscopic binaries within 30 pc of the Sun, using the 2MASS Point Sourc
e Catalog and follow-up observations with the KPNO and CTIO 4m telescopes. Note that this sample is not volume-complete but volume-limited, and, thus, there is incompleteness in our reported companion rates. We are sensitive to common proper motion companions with separations from roughly 200 AU to 10,000 AU (~10 -> ~10). From 77 sources followed-up to date, we recover 11 previously known tertiaries, three previously known candidate tertiaries, of which two are spectroscopically confirmed and one rejected, and three new candidates, of which two are confirmed and one rejected. This yields an estimated wide tertiary fraction of 19.5^+5.2%_-3.7%. This observed fraction is consistent with predictions set out in star formation simulations where the fraction of wide, low-mass companions to spectroscopic binaries is >10%, and is roughly twice the wide companion rate of single stars.
We present the results of a survey of nearby, young M stars for wide low-mass companions with IRAC on the Spitzer Space Telescope. We observed 40 young M dwarfs within 20 pc of the Sun, selected through X-ray emission criteria. A total of 10 candidat
e companions were found with IRAC colors consistent with T dwarfs. Extensive ground-based NIR follow-up observations rejected all these candidates. Two additional candidates were discovered via common proper motion measurements, one of which was rejected as a background object and the other is a bona fide companion to GJ 2060, a member of the AB Doradus moving group.
I have undertaken a comprehensive statistical investigation of the ultracool dwarf companion distribution (spectral type M6 and later). Utilizing a Bayesian algorithm, I tested models of the companion distribution against data from an extensive set o
f space and ground-based imaging observations of nearby ultracool dwarfs. My main conclusions are fivefold: 1) Confirm that the concentration of high mass ratio ultracool binary systems is a fundamental feature of the companion distribution, not an observational or selection bias; 2) Determine that the wide (>~20 AU) binary frequency can be no more the 1-2%; 3) Show that the decreasing binary frequency with later spectral types is a real trend; 4) Demonstrate that a large population of currently undetected low mass ratio systems are not consistent with the current data; 5) Find that the population of spectroscopic binaries must be be at least 30% that of currently known ultracool binaries. The best fit value for the overall M6 and later binary frequency is ~20%-22%, of which only ~6% consists of currently undetected companions with separations less than 1 AU. If this is correct, then the upper limit of the ultracool binary population discovered to date is ~75%. I find that the numerical simulation results of the ejection formation method are inconsistent with the outcome of this analysis. However, dynamics do seem to play an important role as simulations of small-N clusters and triple system decays produce results similar to those of this work. The observational efforts required to improve these constraints are shown to be primarily large spectroscopic binary surveys and improved high-resolution imaging techniques.
