Research column

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2015

Dr. PoShin Chiang: We report the discovery of two methane dwarfs in the dark cloud L 1688 of the ρ Oph star-forming region. The two objects were among the T dwarf candidates with possible methane absorption and cool atmospheres, as diagnosed by infrared colors using deep Wide-field InfraRed Camera/Canada–France–Hawaii Telescope HK plus CH4ON images, and IRAC/Spitzer c2d data. Follow-up spectroscopic observations with the FLAMINGOS-2/Gemini South confirmed the methane absorption at 1.6 μm. Compared with spectral templates of known T dwarfs in the field, i.e., of the old populations, Oph J162738−245240 (Oph-T3) is a T0/T1 type, whereas Oph J162645−241949 (Oph-T17) is consistent with a T3/T4 type in the H band but an L8/T1 in the K band. Compared with the BT-Settl model, both Oph-T3 and Oph-T17 are consistent with being cool, ~1000 K and ~900 K, respectively, and of low surface gravity, $\mathrm{log}(g)=3.5.$ With an age no more than a couple Myr, these two methane dwarfs thereby represent the youngest T dwarfs ever confirmed. A young late L dwarf, Oph J162651−242110, was found serendipitously in our spectroscopic observations.

Vickie Lee: We identify 3546 boulders larger than 7 m on the imaged surface (36.4 km2), with a global number density of nearly 100/km2 and a cumulative size-frequency distribution represented by a power-law with index of −3.6 +0.2/−0.3. The two lobes of 67P appear to have slightly different distributions, with an index of −3.5 +0.2/−0.3 for the main lobe (body) and −4.0 +0.3/−0.2 for the small lobe (head). The steeper distribution of the small lobe might be due to a more pervasive fracturing. The difference of the distribution for the connecting region (neck) is much more significant, with an index value of −2.2 +0.2/−0.2. We propose that the boulder field located in the neck area is the result of blocks falling from the contiguous Hathor cliff. The lower slope of the size-frequency distribution we see today in the neck area might be due to the concurrent processes acting on the smallest boulders, such as i) disintegration or fragmentation and vanishing through sublimation; ii) uplifting by gas drag and consequent redistribution; and iii) burial beneath a debris blanket. We also derived the cumulative size-frequency distribution per km2 of localized areas on 67P. By comparing the cumulative size-frequency distributions of similar geomorphological settings, we derived similar power-law index values. This suggests that despite the selected locations on different and often opposite sides of the comet, similar sublimation or activity processes, pit formation or collapses, as well as thermal stresses or fracturing events occurred on multiple areas of the comet, shaping its surface into the appearance we see today.

Dr. Ngeow, Chow-Choong: We present observational details and first results of a near-infrared (JHKs) synoptic survey of the central region of the Large Magellanic Cloud (LMC) using the CPAPIR camera at the CTIO 1.5 m telescope. We covered 18 square degrees to a depth of {{K}s}∼ 16.5 mag and obtained an average of 16 epochs in each band at any given location. Our catalog contains more than 3.5× {{10}6} sources, including 1417 Cepheid variables previously studied at optical wavelengths by the OGLE survey. Our sample of fundamental-mode pulsators represents a nine-fold increase in the number of these variables with time-resolved, multi-band near-infrared photometry. We combine our large Cepheid sample and a recent precise determination of the distance to the LMC to derive a robust absolute calibration of the near-infrared Leavitt Law for fundamental-mode and first-overtone Cepheids with 10× better constraints on the slopes relative to previous work. We also obtain calibrations for the tip of the red giant branch and the red clump based on our ensemble photometry which are in good agreement with previous determinations.

Prpf. Ip, Wing-Huen: Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of >3 astronomical units is redominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss.