Professor Suri Bhagavantam Centenary Celebrations

Fourteenth October 2009 wasthe 100th birth anniversary ofProf Suri Bhagavantam, the eminent Scientist, able Administrator, andone of the Principal Architects of Defence R&D Organisation. Hesucceeded Prof DS Kothari, the founder of the DRDO who servedas Scientific Adviser (SA) to Defence Minister and Director General, DRDO for eleven years.In the first fifteen years after independence, Indian security threat perception was quite low and this was reflected in the allocations for Defence by the Government of India. The only perceived threat was from another nascent and much smallernation Pakistan. The Himalayas were considered an impregnable natural barrier and in any case India firmly believed that China would never invade India. If the Defence allocations were low,the finances allotted to DRDO were even lower (percentage-wise). Apart from this low funding Dr Kothari felt in hibited to some extentby the advice given by Prof PMS Blacket that DRDO should not attempt to design and develop such major systems as aircraft, missiles, tanks, radars, etc., in view of the very poor industrial base in the country which did not hold much promise of expansionin the for eseeable future.Defence Science Journal, 2009, 59(6), pp.553-554, DOI:http://dx.doi.org/10.14429/dsj.59.157

The Lithium-Rotation Correlation in the Pleiades Revisited

The dispersion in lithium abundance at fixed effective temperature in young cool stars like the Pleiades has proved a difficult challenge for stellar evolution theory. We propose that Li abundances relative to a mean temperature trend, rather than the absolute abundances, should be used to analyze the spread in abundance. We present evidence that the dispersion in Li equivalent widths at fixed color in cool single Pleiades stars is at least partially caused by stellar atmosphere effects (most likely departures from ionization predictions of model photospheres) rather than being completely explained by genuine abundance differences. We find that effective temperature estimates from different colors yield systematically different values for active stars. There is also a strong correlation between stellar activity and Li excess, but not a one-to-one mapping between unprojected stellar rotation (from photometric periods) and Li excess. Thus, it is unlikely that rotation is the main cause for the dispersion in the Li abundances. Finally, there is a strong correlation between detrended Li excess and potassium excess but not calcium-- perhaps supporting incomplete radiative transfer calculations (and overionization effects in particular) as an important source of the Li scatter. Other mechanisms, such as very small metallicity variations and magnetic fields, which influence PMS Li burning may also play a role. Finally, we find no statistical evidence for a decrease in dispersion in the coolest Pleiades stars, contrary to some previous work

Optical characterisation of germanium optical fibres

Semiconductor core optical fibres are currently generating great interest as they promise to be a platform for the seamless incorporation of optoelectronic functionality into a new generation of all-fibre networks [1,2]. Although recent attentions have primarily focused on silicon as the material of choice for semiconductor photonics applications, germanium has some advantages over its counterpart. For example, it has higher nonlinearity, extended infrared transparency and has recently been demonstrated as a direct band gap laser medium [3]. Here we present the first optical characterisation of a germanium core optical fibre. The fibre was fabricated using a chemical micro fluidic deposition process [1] that uses GeH4 (germane) as a precursor to deposit amorphous germanium into the hole of a silica capillary. Figure 1 (a) shows an optical microscope image of the polished end face of a germanium fibre, with a 5.6 µm core diameter, which has been completely filled with the semiconductor material. Optical transmission measurements have been conducted over the wavelength range 2 µm to 11 µm, to confirm the broad mid-infrared operational window, and the guided output at 2.4 µm, imaged using a Spiricon Pyrocam III pyroelectric array camera, is shown in Figure 1 (b). At this wavelength the optical loss has been measured to be 20 dB/cm, which is comparable to losses measured for amorphous silicon fibres in the infrared. The potential for these germanium optical fibres to be used as optical modulators and infrared detectors will be discussed

A Search for Photometric Rotation Periods in Low-Mass Stars and Brown Dwarfs in the Pleiades

We have photometrically monitored (Cousins Ic) eight low mass stars and brown dwarfs which are probable members of the Pleiades. We derived rotation periods for two of the stars - HHJ409 and CFHT-PL8 - to be 0.258 d and 0.401 d, respectively. The masses of these stars are near 0.4 and 0.08 Msun, respectively; the latter is the second such object near the hydrogen-burning boundary for which a rotation period has been measured. We also observed HHJ409 in V; the relative amplitude in the two bands shows that the spots in that star are about 200 K cooler than the stellar effective temperature of 3560 K and have a filling factor on the order of 13%. With one possible exception, the remaining stars in the sample do not show photometric variations larger than the mean error of measurement. We also examined the M9.5V disk star 2MASSJ0149, which had previously exhibited a strong flare event, but did not detect any photometric variation.Comment: 13 pages, four figures. Accepted for publication in A

A further 'degree of freedom' in the rotational evolution of stars

Observational and theoretical investigations provide evidence for non-uniform spot and magnetic flux distributions on rapidly rotating stars, which have a significant impact on their angular momentum loss rate through magnetised winds. Supplementing the formalism of MacGregor & Brenner (1991) with a latitude-dependent magnetised wind model, we analyse the effect of analytically prescribed surface distributions of open magnetic flux with different shapes and degrees of non-uniformity on the rotational evolution of a solar-like star. The angular momentum redistribution inside the star is treated in a qualitative way, assuming an angular momentum transfer between the rigidly-rotating radiative and convective zones on a constant coupling timescale of 15 Myr; for the sake of simplicity we disregard interactions with circumstellar disks. We find that non-uniform flux distributions entail rotational histories which differ significantly from those of classical approaches, with differences cumulating up to 200% during the main sequence phase. Their impact is able to mimic deviations of the dynamo efficiency from linearity of up to 40% and nominal dynamo saturation limits at about 35 times the solar rotation rate. Concentrations of open magnetic flux at high latitudes thus assist in the formation of very rapidly rotating stars in young open clusters, and ease the necessity for a dynamo saturation at small rotation rates. However, since our results show that even minor amounts of open flux at intermediate latitudes, as observed with Zeeman-Doppler imaging techniques, are sufficient to moderate this reduction of the AM loss rate, we suggest that non-uniform flux distributions are a complementary rather than an alternative explanation for very rapid stellar rotation.Comment: 12 pages, 13 figures, accepted for publication by A&

New Rotation Periods in the Pleiades: Interpreting Activity Indicators

We present results of photometric monitoring campaigns of G, K and M dwarfs in the Pleiades carried out in 1994, 1995 and 1996. We have determined rotation periods for 18 stars in this cluster. In this paper, we examine the validity of using observables such as X-ray activity and amplitude of photometric variations as indicators of angular momentum loss. We report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. We show that the X-ray data can be used as observational indicators of mass-dependent saturation in the angular momentum loss proposed on theoretical grounds

Kepler Cycle 1 Observations of Low Mass Stars: New Eclipsing Binaries, Single Star Rotation Rates, and the Nature and Frequency of Starspots

We have analyzed Kepler light curves for 849 stars with T_eff < 5200 K from our Cycle 1 Guest Observer program. We identify six new eclipsing binaries, one of which has an orbital period of 29.91 d, and two of which are probably W UMa variables. In addition, we identify a candidate "warm Jupiter" exoplanet. We further examine a subset of 670 sources for variability. Of these objects, 265 stars clearly show periodic variability that we assign to rotation of the low-mass star. At the photometric precision level provided by Kepler, 251 of our objects showed no evidence for variability. We were unable to determine periods for 154 variable objects. We find that 79% of stars with T_eff < 5200 K are variable. The rotation periods we derive for the periodic variables span the range 0.31 < P_rot < 126.5 d. A considerable number of stars with rotation periods similar to the solar value show activity levels that are 100 times higher than the Sun. This is consistent with results for solar-like field stars. As has been found in previous studies, stars with shorter rotation periods generally exhibit larger modulations. This trend flattens beyond P_rot = 25 d, demonstrating that even long period binaries may still have components with high levels of activity and investigating whether the masses and radii of the stellar components in these systems are consistent with stellar models could remain problematic. Surprisingly, our modeling of the light curves suggests that the active regions on these cool stars are either preferentially located near the rotational poles, or that there are two spot groups located at lower latitudes, but in opposing hemispheres.Comment: 48 pages, 11 figure

Reduced magnetic braking and the magnetic capture model for the formation of ultra-compact binaries

A binary in which a slightly evolved star starts mass transfer to a neutron star can evolve towards ultra-short orbital periods under the influence of magnetic braking. This is called magnetic capture. In a previous paper we showed that ultra-short periods are only reached for an extremely small range of initial binary parameters, in particular orbital period and donor mass. Our conclusion was based on one specific choice for the law of magnetic braking, and for the loss of mass and angular momentum during mass transfer. In this paper we show that for less efficient magnetic braking it is impossible to evolve to ultra-short periods, independent of the amount of mass and associated angular momentum lost from the binary.Comment: 7 pages, 7 figures, accepted for publication in Astronomy and Astrophysics. See http://www.astro.uu.nl/~sluys/PhD