N. Morales

A terrestrial planet candidate in a temperate orbit around Proxima Centauri

At a distance of 1.295 parsecs, the red dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890 or simply Proxima) is the Sun’s closest stellar neighbour and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun and a mass of about 12 per cent of the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is about 83 days (ref. 3) and its quiescent activity levels and X-ray luminosity are comparable to those of the Sun. Here we report observations that reveal the presence of a small planet with a minimum mass of about 1.3 Earth masses orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 astronomical units. Its equilibrium temperature is within the range where water could be liquid on its surface.

A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation

The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto. It resides at present at 95.7 astronomical units (1 au is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 ut. The event is consistent with a spherical shape for Eris, with radius 1,163 ± 6 kilometres, density 2.52 ± 0.05 grams per cm3 and a high visible geometric albedo, . No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ∼1 nanobar, about 10,000 times more tenuous than Plutos present atmosphere. As Plutos radius is estimated to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.

A ring system detected around the Centaur (10199) Chariklo

Hitherto, rings have been found exclusively around the four giant planets in the Solar System. Rings are natural laboratories in which to study dynamical processes analogous to those that take place during the formation of planetary systems and galaxies. Their presence also tells us about the origin and evolution of the body they encircle. Here we report observations of a multichord stellar occultation that revealed the presence of a ring system around (10199) Chariklo, which is a Centaur—that is, one of a class of small objects orbiting primarily between Jupiter and Neptune—with an equivalent radius of 124  9 kilometres (ref. 2). There are two dense rings, with respective widths of about 7 and 3 kilometres, optical depths of 0.4 and 0.06, and orbital radii of 391 and 405 kilometres. The present orientation of the ring is consistent with an edge-on geometry in 2008, which provides a simple explanation for the dimming of the Chariklo system between 1997 and 2008, and for the gradual disappearance of ice and other absorption features in its spectrum over the same period. This implies that the rings are partly composed of water ice. They may be the remnants of a debris disk, possibly confined by embedded, kilometre-sized satellites.

Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation

Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies. Pluto possesses an atmosphere whereas Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos. Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto, and is currently at a distance to the Sun intermediate between the two. Although Makemake’s size (1,420 ± 60 km) and albedo are roughly known, there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere. Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo pV = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4–12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm−3 is inferred from the data.

Short-term variability of a sample of 29 trans-Neptunian objects and Centaurs

Aims. We attempt to increase the number of trans-Neptunian objects (TNOs) whose short-term variability has been studied and compile a high quality database with the least possible biases, which may be used to perform statistical analyses. Methods. We performed broadband CCD photometric observations using several telescopes (the 1.5 m telescope at Sierra Nevada Observatory, the 2.2 m Calar Alto telescope and the 2.5 m INT on La Palma). Results. We present the results of 6 years of observations, reduced and analyzed with the same tools in a systematic way. We report completely new data on 15 objects, for 5 objects we present a new analysis of previously published results plus additional data and for 9 objects we present a new analysis of data already published. Lightcurves, possible rotation periods, and photometric amplitudes are reported for all of them. The photometric variability is smaller than previously thought: the mean amplitude of our sample is 0.1 mag and only around 15% of our sample has a larger variability than 0.15 mag. The smaller variability seems to be caused by a bias of previous observations. We find a very weak trend of faster spinning objects towards smaller sizes, which appears to be consistent with the smaller objects being more collisionally evolved, but may also be a specific feature of the Centaurs, the smallest objects in our sample. We also find that the smaller the objects, the larger their amplitude, which is also consistent with the idea that small objects are more collisionally evolved and thus more deformed. Average rotation rates from our work are 7.5 h for the whole sample, 7.6 h for the TNOs alone and 7.3 h for the Centaurs. Maxwellian fits to the period distribution yield similar results.

Transneptunian objects and Centaurs from light curves

Aims. We compile and analyze an extended database of light curve parameters scattered in the literature to search for correlations and study physical properties, including internal structure constraints. Methods. We analyze a vast light curve database by obtaining mean rotational properties of the entire sample, determining the spin frequency distribution and comparing those data with a simple model based on hydrostatic equilibrium. Results. For the rotation periods, the mean value obtained is 6.95 h for the whole sample, 6.88 h for the Trans-neptunian objects (TNOs) alone and 6.75 h for the Centaurs. From Maxwellian fits to the rotational frequencies distribution the mean rotation rates are 7.35 h for the entire sample, 7.71 h for the TNOs alone and 8.95 h for the Centaurs. These results are obtained by taking into account the criteria of considering a single-peak light curve for objects with amplitudes lower than 0.15 mag and a double-peak light curve for objects with variability >0.15 mag. We investigate the effect of using different values other than 0.15 mag for the transition threshold from albedo-caused light curves to shape-caused light curves. The best Maxwellian fits were obtained with the threshold between 0.10 and 0.15 mag. The mean light-curve amplitude for the entire sample is 0.26 mag, 0.25 mag for TNOs only, and 0.26 mag for the Centaurs. The Period versus B −V color shows a correlation that suggests that objects with shorter rotation periods may have suffered more collisions than objects with larger ones. The amplitude versus Hv correlation clearly indicates that the smaller (and collisionally evolved) objects are more elongated than the bigger ones. Conclusions. From the model results, it appears that hydrostatic equilibrium can explain the statistical results of almost the entire sample, which means hydrostatic equilibrium is probably reached by almost all TNOs in the H range [−1,7]. This implies that for plausible albedos of 0.04 to 0.20, objects with diameters from 300 km to even 100 km would likely be in equilibrium. Thus, the great majority of objects would qualify as being dwarf planets because they would meet the hydrostatic equilibrium condition. The best model density corresponds to 1100 kg/m 3 .

THE SIZE, SHAPE, ALBEDO, DENSITY, AND ATMOSPHERIC LIMIT OF TRANSNEPTUNIAN OBJECT (50000) QUAOAR FROM MULTI-CHORD STELLAR OCCULTATIONS

We present results derived from the first multi-chord stellar occultations by the transneptunian object (50000) Quaoar, observed on 2011 May 4 and 2012 February 17, and from a single-chord occultation observed on 2012 October 15. If the timing of the five chords obtained in 2011 were correct, then Quaoar would possess topographic features (crater or mountain) that would be too large for a body of this mass. An alternative model consists in applying time shifts to some chords to account for possible timing errors. Satisfactory elliptical fits to the chords are then possible, yielding an equivalent radius Requiv = 555±2.5 km and geometric visual albedo pV = 0.109±0.007. Assuming that Quaoar is a Maclaurin spheroid with an indeterminate polar aspect angle, we derive a true oblateness of � = 0.087 +0.0268 −0.0175 , an equatorial radius of 569 +2417 km, and a density of 1.99 ± 0.46 g cm −3 . The orientation of our preferred solution in the plane of the sky implies that Quaoar’s satellite Weywot cannot have an equatorial orbit. Finally, we detect no global atmosphere around Quaoar, considering a pressure upper limit of about 20 nbar for a pure methane atmosphere.

Possible ring material around centaur (2060) Chiron

We propose that several short duration events observed in past stellar occultations by Chiron were produced by rings material. From a reanalysis of the stellar occultation data in the literature we determined two possible orientations of the pole of Chirons rings, with ecliptic coordinates l=(352+/-10) deg, b=(37+/-10) deg or l=(144+/-10) deg, b=(24+/-10) deg . The mean radius of the rings is (324 +/- 10) km. One can use the rotational lightcurve amplitude of Chiron at different epochs to distinguish between the two solutions for the pole. Both imply lower lightcurve amplitude in 2013 than in 1988, when the rotational lightcurve was first determined. We derived Chirons rotational lightcurve in 2013 from observations at the 1.23-m CAHA telescope and indeed its amplitude is smaller than in 1988. We also present a rotational lightcurve in 2000 from images taken at CASLEO 2.15-m telescope that is consistent with our predictions. Out of the two poles the l=(144+/-10) deg, b=(24+/-10) deg solution provides a better match to a compilation of rotational lightcurve amplitudes from the literature and those presented here. We also show that using this preferred pole, Chirons long term brightness variations are compatible with a simple model that incorporates the changing brightness of the rings as the tilt angle with respect to the Earth changes with time. Also, the variability of the water ice band in Chirons spectra in the literature can be explained to a large degree by an icy ring system whose tilt angle changes with time and whose composition includes water ice, analogously to the case of Chariklo. We present several possible formation scenarios for the rings from qualitative points of view and speculate on the reasons why rings might be common in centaurs. We speculate on whether the known bimodal color distribution of centaurs could be due to presence of rings and lack of them.

Rotational fission of trans-Neptunian objects: the case of Haumea

This research was partially supported by Spanish grants AYA2008-06202-C03-01, AYA-06202-C03-02, AYA2008-06202-C03-03, P07-FQM-02998 and European FEDER funds. RD acknowledges financial support from the MICINN (contract Ramon y Cajal). DCR acknowledges support from the National Aeronautics and Space Administration under grant No. NNX08AM39G issued through the Office of Space Science.

Robotic Systems for Meteor Observing and Moon Impact Flashes Detection in Spain

A robotic observatory has been setup in the south-west of Spain with the aim to study meteoroids interacting with the Earths atmosphere and meteoroids impacting on the Moons surface. This is achieved by using an array of high-sensitivity CCD video cameras and three automated Schmidt-Cassegrain telescopes. We summarize here the main characteristics of this new facility.