A. C. Gilmore

Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing

In the favoured core-accretion model of formation of planetary systems, solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars (the most common stars in our Galaxy), this model favours the formation of Earth-mass (M⊕) to Neptune-mass planets with orbital radii of 1 to 10 astronomical units (au), which is consistent with the small number of gas giant planets known to orbit M-dwarf host stars. More than 170 extrasolar planets have been discovered with a wide range of masses and orbital periods, but planets of Neptunes mass or less have not hitherto been detected at separations of more than 0.15 au from normal stars. Here we report the discovery of a 5.5+5.5-2.7 M⊕ planetary companion at a separation of 2.6+1.5-0.6 au from a 0.22+0.21-0.11 M[circdot] M-dwarf star, where M[circdot] refers to a solar mass. (We propose to name it OGLE-2005-BLG-390Lb, indicating a planetary mass companion to the lens star of the microlensing event.) The mass is lower than that of GJ876d (ref. 5), although the error bars overlap. Our detection suggests that such cool, sub-Neptune-mass planets may be more common than gas giant planets, as predicted by the core accretion theory.

The host of GRB 030323 at z=3.372: A very high column density DLA system with a low metallicity

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lyα (DLA) absorption and low- and high-ionization lines at a redshift z = 3.3718 ± 0.0005. The inferred neutral hy- drogen column density, log N(Hi) = 21.90 ± 0.07, is larger than any (GRB- or QSO-) DLA H  column density inferred directly from Lyα in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: AV < 0.5 mag. The iron abundance is (Fe/H) = −1.47 ± 0.11, while the metallicity of the gas as measured from sulphur is (S/H) = −1.26 ± 0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2) + N(Hi)) < 10 −6 .I n the Lyα trough, a Lyα emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Myr −1 . All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, Si *, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the H  volume density to be nHi = 10 2 −10 4 cm −3 .H ST/ACS imaging 4 months after the burst shows an extended AB(F606W) = 28.0 ± 0.3 mag object at a distance of 0.

The host of GRB 030323 at z = 3.372: A very high column density DLA system with a low metallicity

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lyα (DLA) absorption and low- and high-ionization lines at a redshift z = 3.3718 ± 0.0005. The inferred neutral hy- drogen column density, log N(Hi) = 21.90 ± 0.07, is larger than any (GRB- or QSO-) DLA H  column density inferred directly from Lyα in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: AV < 0.5 mag. The iron abundance is (Fe/H) = −1.47 ± 0.11, while the metallicity of the gas as measured from sulphur is (S/H) = −1.26 ± 0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2) + N(Hi)) < 10 −6 .I n the Lyα trough, a Lyα emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Myr −1 . All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, Si *, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the H  volume density to be nHi = 10 2 −10 4 cm −3 .H ST/ACS imaging 4 months after the burst shows an extended AB(F606W) = 28.0 ± 0.3 mag object at a distance of 0.

MICROLENSING EVENT MOA-2007-BLG-400: EXHUMING THE BURIED SIGNATURE OF A COOL, JOVIAN-MASS PLANET

We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb. The planet was detected in a high-magnification microlensing event (with peak magnification A max = 628) in which the primary lens transited the source, resulting in a dramatic smoothing of the peak of the event. The angular extent of the region of perturbation due to the planet is significantly smaller than the angular size of the source, and as a result the planetary signature is also smoothed out by the finite source size. Thus, the deviation from a single-lens fit is broad and relatively weak (approximately few percent). Nevertheless, we demonstrate that the planetary nature of the deviation can be unambiguously ascertained from the gross features of the residuals, and detailed analysis yields a fairly precise planet/star mass ratio of , in accord with the large significance () of the detection. The planet/star projected separation is subject to a strong close/wide degeneracy, leading to two indistinguishable solutions that differ in separation by a factor of ~8.5. Upper limits on flux from the lens constrain its mass to be M < 0.75 M ? (assuming that it is a main-sequence star). A Bayesian analysis that includes all available observational constraints indicates a primary in the Galactic bulge with a mass of ~0.2-0.5 M ? and thus a planet mass of ~0.5-1.3 M Jup. The separation and equilibrium temperature are ~5.3-9.7 AU (~0.6-1.1 AU) and ~34 K (~103 K) for the wide (close) solution. If the primary is a main-sequence star, follow-up observations would enable the detection of its light and so a measurement of its mass and distance.

The line-of-sight towards GRB 030429 at z = 2.66: Probing the matter at stellar, galactic and intergalactic scales

We report the discovery of the optical afterglow (OA) of the long-duration gamma-ray burst GRB 030429, and present a comprehensive optical/near-infrared dataset used to probe the matter at different distance scales, i.e. in the burst environment, in the host galaxy and in an intervening absorber. A break in the afterglow light curve is seen approximately 1 day from the onset of the burst. The light curve displays a significant deviation from a simple broken power-law with a bright 1.5 mag bump with a duration of 2-3 days. The optical/near-infrared spectral energy distribution is best fit with a power-law with index beta = -0.36 ± 0.12 reddened by an SMC-like extinction law with (a modest) AV = 0.34 ± 0.04. In addition, we present deep spectroscopic observations obtained with the Very Large Telescope. The redshift measured via metal absorption lines in the OA is z = 2.658 ± 0.004. Based on the damped Lyalpha absorption line in the OA spectrum we measure the H I column density to be log N(H I) = 21.6 ± 0.2. This confirms the trend that GRBs tend to be located behind very large H I column densities. The resulting dust-to-gas ratio is consistent with that found in the SMC, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. We find that a neighbouring galaxy, at a separation of only 1.2 arcsec, has z = 0.841 ± 0.001, ruling it out as the host of GRB 030429. The small impact parameter of this nearby galaxy, which is responsible for Mg II absorption in the OA spectrum, is in contrast to previous identifications of most QSO absorption-selected galaxy counterparts. Finally, we demonstrate that the OA was not affected by strong gravitational lensing via the nearby galaxy.

Size and albedo of Kuiper belt object 55636 from a stellar occultation

The Kuiper belt is a collection of small bodies (Kuiper belt objects, KBOs) that lie beyond the orbit of Neptune and which are believed to have formed contemporaneously with the planets. Their small size and great distance make them difficult to study. KBO 55636 (2002 TX300) is a member of the water-ice-rich Haumea KBO collisional family. The Haumea family are among the most highly reflective objects in the Solar System. Dynamical calculations indicate that the collision that created KBO 55636 occurred at least 1 Gyr ago. Here we report observations of a multi-chord stellar occultation by KBO 55636, which occurred on 9 October 2009 ut. We find that it has a mean radius of 143 ± 5 km (assuming a circular solution). Allowing for possible elliptical shapes, we find a geometric albedo of in the V photometric band, which establishes that KBO 55636 is smaller than previously thought and that, like its parent body, it is highly reflective. The dynamical age implies either that KBO 55636 has an active resurfacing mechanism, or that fresh water-ice in the outer Solar System can persist for gigayear timescales.

The 1993–1994 Activity of EX Lupi

EX Lupi is a classical T Tauri star (and the EXor prototype) subject to sporadic outbursts. The historic record shows that it remains at about V = 13.2 (or mpg = 14.7:) for extended periods, from which it has been observed to brighten to as much as V = 8.4 (on one occasion in 1955-1956). During 1993-1994 the star remained slightly above normal minimum, at about V = 12.8, and from that level rose to three maxima at V = 11.4 and on other occasions to about V = 12.0. At minimum light an M0 V absorption spectrum is present. At outburst this spectrum is veiled by a hot continuum (well shown by an IUE exposure), the equivalent widths of the optical-region emission lines decrease, and reverse P Cygni absorption components appear at the higher Balmer lines. The outbursts are believed to be due to episodic infall onto the M0 star.

High precision radial-velocity measurements of late-type evolved stars

High dispersion spectra for 44 southern evolved stars of spectral type K or M have been obtained. From these observations relative radial velocities of 50 m/s precision have been obtained by the method of digital cross-correlation. This method of achieving precise relative radial velocities for late-type stars, and the problems encountered in its use, are discussed. Using this method, statistically significant radial-velocity variations have been found. Two of the observed stars have their precise radial velocities presented and the potential mechanisms of their variation examined.

Spectroscopic observations of SN1987a in the LMC

This paper describes the results of optical spectroscopy of SN1987a carried out at Mt John University Observatory on a regular basis since 1987 February 25. Typical spectra are presented in an evolutionary sequence, and velocities of Balmer and selected metallic lines are measured in either emission or absorption. The velocities are interpreted together with the results of photoelectric photometry. The Barnes-Evans relationship has been applied to the photometry to give an angular diameter of the photosphere. The expansion velocity of the photosphere is initially about 3700 km s-1 which is similar to the asymptotic value from weaker absorption lines such as the MgIb line. After 40 days the photosphere appears to expand more slowly and it reaches a maximum size of about 145 A.U. after about 100 days before receding inwards. The photometry and spectroscopy together result in a distance modulus of 18.3 ± 0.2.

Mt John extinction results 1988–1993

We summarize Mt John extinction measures over the past five years and show the considerable effect of recent volcanic activity on atmospheric clarity.