Formation of caustics in Dirac-Born-Infeld type scalar field systems

We investigate the formation of caustics in Dirac-Born-Infeld type scalar field systems for generic classes of potentials, viz., massive rolling scalar with potential, $V(\phi)=V_0e^{\pm \frac{1}{2} M^2 \phi^2}$ and inverse power-law potentials with $V(\phi)=V_0/\phi^n,~0<n<2$. We find that in the case of\texttt{} exponentially decreasing rolling massive scalar field potential, there are multi-valued regions and regions of likely to be caustics in the field configuration. However there are no caustics in the case of exponentially increasing potential. We show that the formation of caustics is inevitable for the inverse power-law potentials under consideration in Minkowski space time whereas caustics do not form in this case in the FRW universe.Comment: 16 pages, 14 figures, major revision, conclusions strengthen, to appear in PR

Finding cool subdwarfs using a V-J reduced proper-motion diagram: Stellar parameters for 91 candidates

We present the results of a search for cool subdwarfs for which our candidates were drawn from a V-J reduced proper-motion diagram constructed by Salim & Gould (2002). Kinematic (U, V, and W) and self-consistent stellar parameters (Teff, log g, [Fe/H], and V_t) are derived for 91 candidate subdwarfs based on high resolution spectra. The observed stars span 3900K < Teff < 6200K and -2.63 < [Fe/H] < 0.25 including only 3 giants (log g < 4.0). Of the sample, 77 stars have MgH lines present in their spectra. With more than 56% of our candidate subdwarfs having [Fe/H] < -1.5, we show that the V-J reduced proper-motion diagram readily identifies metal-poor stars.Comment: PASP (in press

The Temporal Modulation Structure of Infant-Directed Speech

The temporal modulation structure of adult-directed speech (ADS) is thought to be encoded by neuronal oscillations in the auditory cortex that fluctuate at different temporal rates. Oscillatory activity is thought to phase-align to amplitude modulations in speech at corresponding rates, thereby supporting parsing of the signal into linguistically relevant units. The temporal modulation structure of infant-directed speech (IDS) is unexplored. Here we compare the amplitude modulation (AM) structure of IDS recorded from mothers speaking, over three occasions, to their 7-, 9-, and 11-month-old infants, and the same mothers speaking ADS. Analysis of the modulation spectrum in each case revealed that modulation energy in the theta band was significantly greater in ADS than in IDS, whereas in the delta band, modulation energy was significantly greater for IDS than ADS. Furthermore, phase alignment between delta- and theta-band AMs was stronger in IDS compared to ADS. This remained the case when IDS and ADS were rate-normalized to control for differences in speech rate. These data indicate stronger rhythmic synchronization and acoustic temporal regularity in IDS compared to ADS, structural acoustic differences that may be important for early language learningWe thank Maria Christou-Ergos for preparation of data for analysis. This work was supported by funding from the Australian Research Council (DP110105123) to DB and UG