Extreme asteroids in the Pan-STARRS 1 Survey

Using the first 18 months of the Pan-STARRS 1 survey we have identified 33 candidate high-amplitude objects for follow-up observations and carried out observations of 22 asteroids. 4 of the observed objects were found to have observed amplitude $A_{obs}\geq 1.0$ mag. We find that these high amplitude objects are most simply explained by single rubble pile objects with some density-dependent internal strength, allowing them to resist mass shedding even at their highly elongated shapes. 3 further objects although below the cut-off for 'high-amplitude' had a combination of elongation and rotation period which also may require internal cohesive strength, depending on the density of the body. We find that none of the 'high-amplitude asteroids' identified here require any unusual cohesive strengths to resist rotational fission. 3 asteroids were sufficiently observed to allow for shape and spin pole models to be determined through light curve inversion. 45864 was determined to have retrograde rotation with spin pole axes $\lambda=218\pm 10^{\circ}, \beta=-82\pm 5^{\circ}$ and asteroid 206167 was found to have best fit spin pole axes $\lambda= 57 \pm 5^{\circ}$, $\beta=-67 \pm 5^{\circ}$. An additional object not initially measured with $A_{obs}>1.0$ mag, 49257, was determined to have a shape model which does suggest a high-amplitude object. Its spin pole axes were best fit for values $\lambda=112\pm 6^{\circ}, \beta=6\pm 5^{\circ}$. In the course of this project to date no large super-fast rotators ($P_{rot} < 2.2$ h) have been identified.Comment: 31 pages; accepted by A

Equilibrium and stability of supermassive stars in binary systems

We investigate the equilibrium and stability of supermassive stars of mass M \agt 10^5M_{\odot} in binary systems. We find that corotating binaries are secularly unstable for close, circular orbits with r \alt 4R(M/10^6M_{\odot})^{1/6} where $r$ is the orbital separation and $R$ the stellar radius. We also show that corotation cannot be achieved for distant orbits with r \agt 12 R (M/10^6M_{\odot})^{-11/24}, since the timescale for viscous angular momentum transfer associated with tidal torques is longer than the evolution timescale due to emission of thermal radiation. These facts suggest that the allowed mass range and orbital separation for corotating supermassive binary stars is severely restricted. In particular, for supermassive binary stars of large mass M \agt 6\times 10^6M_{\odot}, corotation cannot be achieved, as viscosity is not adequate to mediate the transfer between orbital and spin angular momentum. One possible outcome for binary supermassive stars is the onset of quasi-radial, relativistic instability which drives each star to collapse prior to merger: We discuss alternative outcomes of collapse and possible spin states of the resulting black holes. We estimate the frequency and amplitude of gravitational waves emitted during several inspiral and collapse scenarios.Comment: 20 pages, to be published in PR

A Change in the Lightcurve of Kuiper Belt Contact Binary (139775) 2001 QG298

New observations show that the lightcurve of Kuiper belt contact binary (139775) 2001 QG298 has changed substantially since the first observations in 2003. The 2010 lightcurve has a peak-to-peak photometric of range \Deltam{2010}=0.7\pm0.1 mag, significantly lower than in 2003, \Deltam{2003}=1.14\pm0.04 mag. This change is most simply interpreted if 2001 QG298 has an obliquity near 90 deg. The observed decrease in \Deltam is caused by a change in viewing geometry, from equator-on in 2003 to nearly 16 deg (the orbital angular distance covered by the object between the observations) off the equator in 2010. The 2003 and 2010 lightcurves have the same rotation period and appear in phase when shifted by an integer number of full rotations, also consistent with high obliquity. Based on the new 2010 lightcurve data, we find that 2001 QG298 has an obliquity {\epsilon}=90\pm30 deg. Current estimates of the intrinsic fraction of contact binaries in the Kuiper belt are debiased assuming that these objects have randomly oriented spins. If, as 2001 QG298, KBO contact binaries tend to have large obliquities, a larger correction is required. As a result, the abundance of contact binaries may be larger than previously believed.Comment: 28 pages, 10 figures, 5 tables. Accepted for publication in The Astronomical Journal (2011 July 18

Equilibrium Configurations of Synchronous Binaries: Numerical Solutions and Application to Kuiper-Belt Binary 2001 QG298

We present numerical computations of the equilibrium configurations of tidally-locked homogeneous binaries, rotating in circular orbits. Unlike the classical Roche approximations, we self-consistently account for the tidal and rotational deformations of both components, and relax the assumptions of ellipsoidal configurations and Keplerian rotation. We find numerical solutions for mass ratios q between 1e-3 and 1, starting at a small angular velocity for which tidal and rotational deformations are small, and following a sequence of increasing angular velocities. Each series terminates at an appropriate ``Roche limit'', above which no equilibrium solution can be found. Even though the Roche limit is crossed before the ``Roche lobe'' is filled, any further increase in the angular velocity will result in mass-loss. For close, comparable-mass binaries, we find that local deviations from ellipsoidal forms may be as large as 10-20%, and departures from Keplerian rotation are significant. We compute the light curves that arise from our equilibrium configurations, assuming their distance is >>1 AU (e.g. in the Kuiper Belt). We consider both backscatter (proportional to the projected area) and diffuse (Lambert) reflections. Backscatter reflection always yields two minima of equal depths. Diffuse reflection, which is sensitive to the surface curvature, generally gives rise to unequal minima. We find detectable intensity differences of up to 10% between our light curves and those arising from the Roche approximations. Finally, we apply our models to Kuiper Belt binary 2001 QG298, and find a nearly edge-on binary with a mass ratio q = 0.93 ^{+0.07}_{-0.03}, angular velocity Omega^2/G rho = 0.333+/-0.001 (statistical errors only), and pure diffuse reflection. For the observed period of 2001 QG298, these parameters imply a bulk density, rho = 0.72 +/- 0.04 g cm^-3.Comment: Accepted to Ap

The unusual Kuiper belt object 2003 SQ317

We report photometric observations of Kuiper belt object 2013 SQ317 obtained between 2011 August 21 and 2011 November 1 at the 3.58 m New Technology Telescope, La Silla. We obtained a rotational lightcurve for 2013 SQ317 with a large peak-to-peak photometric range 0.85+/-0.05 mag, and a periodicity, P=7.210+/-0.001 hr. We also measure a nearly neutral broadband colour B-R=1.05+/-0.18 mag and a phase function with slope beta=0.95+/-0.41 mag/deg. The large lightcurve range implies an extremely elongated shape for 2013 SQ317, possibly as a single elongated object but most simply explained as a compact binary. If modelled as a compact binary near hydrostatic equilibrium, the bulk density of 2013 SQ317 is near 2670 kg m^(-3). If 2003 SQ317 is instead a single, elongated object, then its equilibrium density is about 860 kg m^(-3). These density estimates become uncertain at the 30% level if we relax the hydrostatic assumption and account for solid, "rubble pile"-type configurations. 2013 SQ317 has been associated with the Haumea family based on its orbital parameters and near-infrared colour; we discuss our findings in this context. If confirmed as a close binary, 2013 SQ317 will be the second object of its kind identified in the Kuiper belt.Comment: 8 pages, 10 figures, 4 tables. Submitted to MNRA

Densities of Solar System Objects from their Rotational Lightcurves

We present models of the shapes of four Kuiper belt objects (KBOs) and Jovian Trojan (624) Hektor as ellipsoidal figures of equilibrium and Roche binaries. Our simulations select those figures of equilibrium whose lightcurves best match the measured rotational data. The best fit shapes, combined with the knowledge of the spin period of the objects provide estimates of the bulk densities of these objects. We find that the lightcurves of KBOs (20000) Varuna and 2003 EL61 are well matched by Jacobi triaxial ellipsoid models with bulk densities 992 (-15,+86) kg/m^3 and 2551 (-10,+115) kg/m^3, respectively. The lightcurves of (624) Hektor and KBO 2001 QG298 are well-described by Roche contact binary models with densities 2480 (-80,+292) kg/m^3 and 590 (-47,+143) kg/m^3, respectively. The nature of 2000 GN171 remains unclear: Roche binary and Jacobi ellipsoid fits to this KBO are equivalent, but predict different densities, ~2000 kg/m^3 and ~650 kg/m^3, respectively. Our density estimates suggest a trend of increasing density with size.Comment: 13 pages (emulateapj), 20 figures, AJ, accepted 2006/12/

Detection of Contact Binaries Using Sparse High Phase Angle Lightcurves

We show that candidate contact binary asteroids can be efficiently identified from sparsely sampled photometry taken at phase angles >60deg. At high phase angle, close/contact binary systems produce distinctive lightcurves that spend most of the time at maximum or minimum (typically >1mag apart) brightness with relatively fast transitions between the two. This means that a few (~5) sparse observations will suffice to measure the large range of variation and identify candidate contact binary systems. This finding can be used in the context of all-sky surveys to constrain the fraction of contact binary near-Earth objects. High phase angle lightcurve data can also reveal the absolute sense of the spin.Comment: 4 pages, 4 figures, 1 table. Accepted for publication in ApJ

Rotational properties of the Haumea family members and candidates: Short-term variability

Haumea is one of the most interesting and intriguing transneptunian objects (TNOs). It is a large, bright, fast rotator, and its spectrum indicates nearly pure water ice on the surface. It has at least two satellites and a dynamically related family of more than ten TNOs with very similar proper orbital parameters and similar surface properties. The Haumean family is the only one currently known in the transneptunian belt. Various models have been proposed but the formation of the family remains poorly understood. In this work, we have investigated the rotational properties of the family members and unconfirmed family candidates with short-term variability studies, and report the most complete review to date. We present results based on five years of observations and report the short-term variability of five family members, and seven candidates. The mean rotational periods, from Maxwellian fits to the frequency distributions, are 6.27+/-1.19 h for the confirmed family members, 6.44+/-1.16 h for the candidates, and 7.65+/-0.54 h for other TNOs (without relation to the family). According to our study, there is a suggestion that Haumea family members rotate faster than other TNOs, however, the sample of family member is still too limited for a secure conclusion. We also highlight the fast rotation of 2002 GH32. This object has a 0.36+/-0.02 mag amplitude lightcurve and a rotational period of about 3.98 h. Assuming 2002 GH32 is a triaxial object in hydrostatic equilibrium, we derive a lower limit to the density of 2.56 g cm^-3. This density is similar to Haumea's and much more dense than other small TNO densities.Comment: Accepted for publication, A

Properties of the redback millisecond pulsar binary 3FGL J0212.1+5320

Linares et al. (2016) obtained quasi-simultaneous g', r' and i-band light curves and an absorption line radial velocity curve of the secondary star in the redback system 3FGL J0212.1+5320. The light curves showed two maxima and minima primarily due to the secondary star's ellipsoidal modulation, but with unequal maxima and minima. We fit these light curves and radial velocities with our X-ray binary model including either a dark solar-type star spot or a hot spot due to off-centre heating from an intrabinary shock, to account for the unequal maxima. Both models give a radial velocity semi-amplitude and rotational broadening that agree with the observations. The observed secondary star's effective temperature is best matched with the value obtained using the hot spot model, which gives a neutron star and secondary star mass of $M_{\rm 1}$=1.85$^{+0.32}_{-0.26}$ $M_{\odot}$and $M_{\rm 2}$=0.50$^{+0.22}_{-0.19}$ $M_{\odot}$, respectively.Comment: 10 pages, 8 figues, accepted by MNRA