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Abstract: NaSt1 is a peculiar emission-line star embedded in an extended nebula of optical [N II] emission with a compact dusty core. This object has been characterized as a Wolf-Rayet (WR) star cloaked in an opaque nebula of material that shows signs of advanced nuclear processing in the core of a massive star. To discern the morphology of the [N II] nebula we performed narrowband imaging using the Hubble Space Telescope. The images reveal that the nebula has a disk-like geometry, composed of a bright central ellipsoid surrounded by a larger clumpy ring. Near-infrared imaging with Magellan AO resolved a compact ellipsoid of Ks-band emission aligned with the larger [N II] nebula. The source is more compact at L’ and M’, so we suspect the Ks-band emission result of neutral helium emission from the inner stellar wind scattered outward by reflection off of cool dust in the nebula. Observations with the Chandra X-ray Observatory have revealed a hard X-ray point source at the core of the nebula that is consistent with WR stars and massive binaries where powerful stellar winds collide and make X-rays. We suggest that NaSt1 is a WR binary embedded in an equatorial outflow that formed as the result of non-conservative mass transfer (the transfer is not completely efficient, and some material is lost from the binary system through the outer Lagrange points). NaSt1 thus appears to be a rare and important example of a stripped-envelope WR forming through binary interaction.

Mauerhan, J., et al. “Multiwavelength Observations of NaSt1 (WR 122): Equatorial Mass Loss and X-rays from an Interacting Wolf-Rayet Binary”

MNRAS 450, 3, 2551; arxiv preprint

Abstract: We present multi-epoch non-redundant masking observations of the T Cha transition disk, taken at the VLT and Magellan in H, Ks, and L’ bands. T Cha is one of a small number of transition disks that host companion candidates discovered by high-resolution imaging techniques, with a putative companion at a position angle of 78 degrees, separation of 62 mas, and contrast at L’ of 5.1 mag. We find comparable binary parameters in our re-reduction of the initial detection images, and similar parameters in the 2011 L’, 2013 NaCo L’, and 2013 NaCo Ks data sets. We find a close-in companion signal in the 2012 NaCo L’ dataset that cannot be explained by orbital motion, and a non-detection in the 2013 MagAO/Clio2 L’ data. However, Monte-carlo simulations show that the best fits to the 2012 NaCo and 2013 MagAO/Clio2 followup data may be consistent with noise. There is also a significant probability of false non-detections in both of these data sets. We discuss physical scenarios that could cause the best fits, and argue that previous companion and scattering explanations are inconsistent with the results of the much larger dataset presented here.

Sallum, S., et al. “New Spatially Resolved Observations of the T Cha Transition Disk and Constraints on the Previously Claimed Substellar Companion”
ApJ, 801, 85, 2015     arxiv preprint

Abstract:
We used the Magellan adaptive optics (MagAO) system and its VisAO CCD camera to image the young low mass brown dwarf companion CT Chamaeleontis B for the first time at visible wavelengths. We detect it at r’, i’, z’, and Ys. With our new photometry and Teff~2500 K derived from the shape of its K-band spectrum, we find that CT Cha B has Av = 3.4+/-1.1 mag, and a mass of 14-24 Mj according to the DUSTY evolutionary tracks and its 1-5 Myr age. The overluminosity of our r’ detection indicates that the companion has significant Halpha emission and a mass accretion rate ~6*10^-10 Msun/yr, similar to some substellar companions. Proper motion analysis shows that another point source within 2″ of CT Cha A is not physical. This paper demonstrates how visible wavelength AO photometry (r’, i’, z’, Ys) allows for a better estimate of extinction, luminosity, and mass accretion rate of young substellar companions.

Wu, Y.-L., et al. “New Extinction and Mass Estimates from Optical Photometry of the Very Low Mass Brown Dwarf Companion CT Chamaeleontis B with the Magellan AO System”
ApJ, 801, 4, 2015      arxiv preprint

Abstract:
We present resolved images of the HR 4796A debris disk using the Magellan adaptive optics system paired with Clio-2 and VisAO. We detect the disk at 0.77 microns, 0.91 microns, 0.99 microns, 2.15 microns, 3.1 microns, 3.3 microns, and 3.8 microns. We find that the deprojected center of the ring is offset from the star by 4.76 ± 1.6 AU and that the deprojected eccentricity is 0.06 ± 0.02, in general agreement with previous studies. We find that the average width of the ring is 14+3-2% (11.1+2.4-1.6 AU), also comparable to previous measurements. Combining our new scattered light data with archival Hubble Space Telescope images at ~0.5-2 mum, along with previously unpublished Spitzer/MIPS thermal emission data and all other literature thermal data, we set out to constrain the chemical composition of the dust grains. After testing 19 individual root compositions and more than 8400 unique mixtures of these compositions, we find that good fits to the scattered light alone and thermal emission alone are discrepant, suggesting that caution should be exercised if fitting to only one or the other. When we fit to both data sets simultaneously, we find that silicates and organics are generally the most favored, while large abundances of water ice are usually not favored. These results suggest the HR 4796A dust grains are similar to interstellar dust and solar system comets, though improved modeling is necessary to place better constraints on the exact chemical composition of the dust.

Rodigas, T. J., et al. “On the Morphology and Chemical Composition of the HR 4796A Debris Disk”
ApJ, 798, 96, 2015     ADS    arxiv preprint