Very Low Mass Stars

Evolution of the gravitational contraction phase of stars having masses 0.3 < M/M⊙ < 1.5 were studied along with deuterium burning. The equation of state developed by Mihalas et al. (1988) and OPAL opacity tables were used in our investigation. The theoretical time lines were compared with observations.

Brown dwarfs and low-mass stars together comprise the group of galactic objects known as ultracool dwarfs. A number have been detected as radio sources – in some cases, emitting periodic radio pulses synchronised to the dwarf's known rotational period. Detections of optical variability in ultracool dwarfs have been primarily attributed to stellar rotation, with the modulations observed a consequence of either magnetic spots on the surface, the presence of atmospheric dust, or auroral emission. A combination of these mechanisms may be required to explain some dwarf lightcurves. Rotational estimates of ultracool dwarfs are typically obtained spectroscopically; however accurate estimates of true rotation velocities require knowledge of the dwarf's rotational inclination axes. Direct measurement of a rotational signature in photometric data however provides an unambiguous rotational period, and this information can be used to constrain dwarf inclination geometries. In this thesis, we report on over ~ 160 hrs monitoring in I-band of multiple epochs from four ultracool dwarfs, spanning M tight binary dwarf 2MASS J1314203+132001A and L tight binary dwarf 2MASS J0746425+200032AB, the M9.5 dwarf BRI 0021-0214, and the L3.5 dwarf 2MASS J00361617+18211. This photometric campaign was carried out using the Galway Ultra-Fast Imager (GUFI) on the 1.8m Vatican Advanced Technology Telescope (VATT), on Mt. Graham, Arizona. All selected dwarfs exhibit periodic optical variability, where periods of both secondary components for our binary samples were newly discovered. This thesis discusses the use of two photometric analysis tools with the explicit aim of improving the quality of ground-based photometric measurements. Each data set was used to test the performance of the two systems. We find the LuckyPhot technique has obvious benefits to high precision photometry by reducing photometric errors, where the mean RMS error was reduced by ~ 47% with respect to the errors produced by the more standard GUFI pipeline method. This thesis also outlines a novel tool, Light Curve Fitter, which we apply to the binaries to investigate the presence of periodic photometric modulation in both binary members: refining the dominant member variability parameters, and searching for an elusive period of the weaker member. Light Curve Fitter is a python-based program, capable of detecting superposition of two sinusoidal waves to untangle the weaker components variability signature from that of the dominant source variability. We identify a newly discovered optical variability in the primary and secondary components of ultracool dwarf binary 2MASS J1314203+132001AB and 2MASS J0746425+200032AB, respectively. The optical data presented for both systems shows strongly correlated emissions in terms of phase and temporal variability. We have also shown the A and B variability signals of both dwarf binaries 2MASS J0746425+200032AB and 2MASS J1314203+132001AB, respectively, to be extremely consistent and stable over multiple epochs. This stability had seen in both radio and spectroscopic data, and the mechanism driving these processes in different parts of the electromagnetic spectrum could perhaps be fundamentally linked. We also investigate the orbital coplanarity of both binary dwarfs. Here the ability to deconvolve the inclination angle from the spectroscopic radial velocities, using direct estimates of the dwarf rotational periods, allows us to constrain the spin-orbit coupling of the binary system. In the case of the L dwarf binary 2MASS J0746425+200032AB, we calculate the equatorial inclination angle of the binary rotation axes are in alignment with the orbital plane of the system to within 10 degrees, consistent with solar-type binary formation mechanisms. For the M7 dwarf binary 2MASS J1314203+132001AB, due to missing parameters for the primary component, we investigate a tentative alignment of the spin-orbital axes of the A component. We find that the equatorial inclination angle of the secondary member spin axes is largely consistent with being aligned perpendicularly to the orbital plane. Finally, we find the rotation axes of the two single dwarfs are not perpendicular to our line of sight.

We report the detection of eclipses in GJ 3236, a bright (I = 11.6) very low mass binary system with an orbital period of 0.77 days. Analysis of light- and radial velocity curves of the system yielded component masses of 0.38 +/- 0.02 and 0.28 +/- 0.02 Msol. The central values for the stellar radii are larger than the theoretical models predict for these masses, in agreement with the results for existing eclipsing binaries, although the present 5% observational uncertainties limit the significance of the larger radii to approximately 1 sigma. Degeneracies in the light curve models resulting from the unknown configuration of surface spots on the components of GJ 3236 currently dominate the uncertainties in the radii, and could be reduced by obtaining precise, multi-band photometry covering the full orbital period. The system appears to be tidally synchronized and shows signs of high activity levels as expected for such a short orbital period, evidenced by strong Halpha emission lines in the spectra of both components. These observations probe an important region of mass-radius parameter space around the predicted transition to fully-convective stellar interiors, where there are a limited number of precise measurements available in the literature.

We report the discovery of six new substellar companions of main-sequence stars, detected through multiple Doppler measurements with the instrument HARPS installed on the ESO 3.6m telescope, La Silla, Chile. These extrasolar planets are orbiting the stars BD -17 0063, HD 20868, HD 73267, HD 131664, HD 145377, HD 153950. The orbital characteristics which best fit the observed data are depicted in this paper, as well as the stellar and planetary parameters. Masses of the companions range from 2 to 18 Jupiter masses, and periods range from 100 to 2000 days. The observational data are carefully analysed for activity-induced effects and we conclude on the reliability of the observed radial-velocity variations as of exoplanetary origin. Of particular interest is the very massive planet (or brown-dwarf companion) around the metal-rich HD 131664 with M2sini= 18.15 MJup, and a 5.34-year orbital period. These new discoveries reinforces the observed statistical properties of the exoplanet sample as known so far.

... Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 DANA E. AND ANNA M. BACKMAN1 ANGELOVA ... Finally, we have obtained a moderate-resolution spectrum of one of the new brown dwarf candidates with the Keck LRIS spectrograph. ...

Time series spectrophotometry of the L5 dwarf 2MASS J01443536-0716142 showed strong H α emission that declined by nearly 75% in four consecutive exposures. The line was not detected in emission on a spectrum obtained 11 months later. This behavior contrasts with that of 2MASSI J1315309-264951, an L5 dwarf that has shown even stronger H α emission on four separate occasions. The observational database suggests that L dwarfs can be found in such strong flares only occasionally, with a duty cycle on the order ...

Light and intermediate nuclei as well as s-process elements have been detected in presolar grains and in evolved red giants. The abundances of some of these nuclei cannot be accounted for by canonical stellar models and require non-convective mixing below the envelope, occurring during the phases of the Red Giant Branch (RGB) and of the Asymptotic Giant Branch (AGB). Similar

... Received 2011 Feb 2, accepted 2011 Feb 7 Published online 2011 Mar 17 ... This hampers our ability to address the issue of planet formation, which depends on disk properties, such as the mass accretion rate ( ˙Macc), the ejection of jets and winds, and the growth and settling of ...

We present the current performances of the AMBER / VLTI instrument in terms of differential observables (differential phase and differential visibility) and show that we are already able to reach a sufficient precision for very low mass companions spectroscopy and mass characterization. We perform some extrapolations with the knowledge of the current limitations of the instrument facility. We show that with the current setup of the AMBER instrument, we can already reach $3\sigma = 10^{-3}$ radians and have the potential to some low mass companions characterization (Brown dwarves or hypothetical very hot Extra Solar Giant Planets). With some upgrades of the VLTI infrastructure, improvements of the instrument calibration and improvements of the observing strategy, we will be able to reach $3\sigma = 10^{-4}$ radians and will have the potential to perform Extra Solar Giant Planets spectroscopy and mass characterization.

We report the discovery of a high proper motion L subdwarf ($\mu$ =0.617arcsec/yr) in the Sloan Digital Sky Survey spectral database. The optical spectrum from the star SDSS J125637-022452 has mixed spectral features of both late-M spectral subtype (strong TiO and CaH at 7000A) and mid-L spectral subtype (strong wings of KI at 7700A, CrH and FeH), which is interpreted as the signature of a very low-mass, metal-poor star (ultra-cool subdwarf) of spectral type sdL. The near infrared (NIR) (J-Ks) colors from 2MASS shows the object to be significantly bluer compared to normal L dwarfs, which is probably due a strong collision induced absorption (CIA) due to H2 molecule. This is consistent with the idea that CIA from H2 is more pronounced at low metallicities. Proper motion and radial velocity measurements also indicate that the star is kinematically &quot;hot&quot; and probably associated with the Galactic halo population. Comment: 13 pages, 2 figures. Accepted for ApJL

We report the discovery of an optical counterpart to a quiescent neutron star in the globular cluster Omega Centauri (NGC 5139). The star was found as part of our wide-field imaging study of Omega Cen using the Advanced Camera for Surveys (ACS) on Hubble Space Telescope. Its magnitude and color (R_625 = 25.2, B_435 - R_625 = 1.5) place it more than 1.5 magnitudes to the blue side of the main sequence. Through an H-alpha filter it is ~ 1.3 magnitudes brighter than cluster stars of comparable M_625 magnitude. The blue color and H-alpha excess suggest the presence of an accretion disk, implying that the neutron star is accreting from a binary companion and is thus a quiescent low-mass X-ray binary. If the companion is a main-sequence star, then the faint absolute magnitude (M_625 ~ 11.6) constrains it to be of very low mass (M <~ 0.14 Msolar). The faintness of the disk (M_435 ~ 13) suggests a very low rate of accretion onto the neutron star. We also detect 13 probable white dwarfs and three possible BY Draconis stars in the 20" x 20" region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.