Category: Results

We report the discovery of a planetary-mass companion, HD 106906 b, with the new Magellan Adaptive Optics (MagAO) + Clio2 system. The companion is detected with Clio2 in three bands: J, KS, and L′, and lies at a projected separation of 7.1” (650 AU). It is confirmed to be comoving with its 13±2 Myr-old F5 host using Hubble Space Telescope/Advanced Camera for Surveys astrometry over a time baseline of 8.3 yr. DUSTY and COND evolutionary models predict the companion’s luminosity corresponds to a mass of 11±2MJup, making it one of the most widely separated planetary-mass companions known. We classify its Magellan/Folded-Port InfraRed Echellette J/H/K spectrum as L2.5±1; the triangular H-band morphology suggests an intermediate surface gravity. HD 106906 A, a pre-main-sequence Lower Centaurus Crux member, was initially targeted because it hosts a massive debris disk detected via infrared excess emission in unresolved Spitzer imaging and spectroscopy. The disk emission is best fit by a single component at 95 K, corresponding to an inner edge of 15-20 AU and an outer edge of up to 120 AU. If the companion is on an eccentric (e>0.65) orbit, it could be interacting with the outer edge of the disk. Close-in, planet-like formation followed by scattering to the current location would likely disrupt the disk and is disfavored. Furthermore, we find no additional companions, though we could detect similar-mass objects at projected separations >35 AU. In situ formation in a binary-star-like process is more probable, although the companion-to-primary mass ratio, at <1%, is unusually small. For more on HD 106906 b see: Bailey, V., et al. "HD 106906 b: A planetary-mass companion outside a massive debris disk". ApJL, 780, L4, 2013 ADS   preprint [pdf] arxiv preprint

We utilized the new high-order (250-378 mode) Magellan Adaptive Optics system
(MagAO) to obtain very high spatial resolution observations in “visible light”
with MagAO’s VisAO CCD camera. In the good-median seeing conditions of Magellan
(0.5-0.7″) we find MagAO delivers individual short exposure images as good as
19 mas optical resolution. Due to telescope vibrations, long exposure (60s) r’
(0.63 micron) images are slightly coarser at FWHM=23-29 mas (Strehl ~28%) with
bright (R<9 mag) guide stars. These are the highest resolution filled-aperture images published to date. Images of the young (~1 Myr) Orion Trapezium Theta 1 Ori A, B, and C cluster members were obtained with VisAO. In particular, the 32 mas binary Theta 1 Ori C1/C2 was easily resolved in non-interferometric images for the first time. Relative positions of the bright trapezium binary stars were measured with ~0.6-5 mas accuracy. We now are sensitive to relative proper motions of just ~0.2 mas/yr (~0.4 km/s at 414 pc) - this is a ~2-10x improvement in orbital velocity accuracy compared to previous efforts. For the first time, we see clear motion of the barycenter of Theta 1 Ori B2/B3 about Theta 1 Ori B1. All five members of the Theta 1 Ori B system appear likely a gravitationally bound "mini-cluster", but we find that not all the orbits can be both circular and co-planar. The lowest mass member of the Theta 1 Ori B system (B4; mass ~0.2 Msun) has a very clearly detected motion (at 4.1+/-1.3 km/s; correlation=99.9%) w.r.t B1 and will likely be ejected in the future. This "ejection" process of the lowest mass member of a "mini-cluster" could play a major role in the formation of low mass stars and brown dwarfs [caption id="attachment_4889" align="aligncenter" width="1585"] The power of visible light adaptive optics. Here we show (on the left) a “normal” photo of the theta 1 Ori C binary star in red light (in the r’ filter, 630 nm). It just looks an unresolved star. Then the middle image shows how if we remove (in real time) the blurring of the atmosphere with MagAO’s adaptive optics’ the resulting photo becomes ~17 times sharper (corrected resolutions range from 0.019-0.029 arcseconds on theta 1 Ori C). Both photos are 60 seconds long, and no post-detection image enhancement has been applied. These are the highest resolution photos taken by a telescope. Photo credit Laird Close, University of Arizona. [/caption]

For more on high resolution filled-aperture imaging of Trapezium see:
Close, L. M., et al. “Diffraction-limited Visible Light Images of Orion Trapezium Cluster With the Magellan Adaptive Secondary AO System (MagAO)”.
ApJ, 774, 94, 2013    preprint [pdf]    arxiv preprint

Disks of gas and dust around newly born stars, dubbed protoplanetary disks (proplyds), are believed to evolve into planetary systems. Understanding how these disks are affected by strong light/wind from nearby massive stars is crucial to theories of planet formation and evolution. We used MagAO to image the binary proplyd (called LV 1) in the Orion Nebula (M42) at the 653nm hydrogen emission line (H alpha) during the first commissioning run. The spatial resolution is comparable to that of the Hubble Space Telescope (HST), demonstrating the power of AO in visible wavelengths. Our H alpha image clearly shows that the gas of the binary is heavily distorted into “teardrop” shape due to strong UV light and wind from a nearby star, Theta 1 Ori C (See image below). The lack of orbital motion over ~18 years between the stars further suggests that they are of low masses, plausibly a double brown dwarf (the so-called “failed star” since it is not massive enough to initiate hydrogen fusion). This makes LV 1 a rare example of a low mass pair of young disks. Finally, we show that the macroscopic features of these disks may help infer the embedded magnetic field intensity and profile.

For more on LV 1 see:
Wu, Y. L., et al. “High Resolution H alpha Images of the Binary Low-mass Proplyd LV 1 with the Magellan AO System”.
ApJ, 775, 45, 2013    preprint [pdf]    arxiv preprint

Abstract: We utilize the new Magellan adaptive optics system (MagAO) to image the binary proplyd LV 1 in the Orion Trapezium at H alpha. This is among the first AO results in visible wavelengths. The H alpha image clearly shows the ionization fronts, the interproplyd shell, and the cometary tails. Our astrometric measurements find no significant relative motion between components over ~18 yr, implying that LV 1 is a low-mass system. We also analyze Large Binocular Telescope AO observations, and find a point source which may be the embedded protostar’s photosphere in the continuum. Converting the H magnitudes to mass, we show that the LV 1 binary may consist of one very-low-mass star with a likely brown dwarf secondary, or even plausibly a double brown dwarf. Finally, the magnetopause of the minor proplyd is estimated to have a radius of 110 AU, consistent with the location of the bow shock seen in H alpha.

The flat pancake-like disks of gas and dust that surround young stars (so-called “circumstellar disks”) are of interest to astronomers because we believe that planets are made from their material. In the 1990s, the Hubble Space Telescope completed groundbreaking observations of the Great Orion Nebula at a red wavelength of light termed “Hydrogen-alpha” (the brightest light emitted by glowing hydrogen gas), revealing a number of circumstellar disks seen in silhouette against the bright background nebula. At the time, these observations provided the most conclusive direct evidence of the existence of circumstellar disks around young stars, and providing some of the first measurements of their size and geometry.

These “silhouette disks” occur because the stars that host them lie between Earth and the glowing hydrogen gas of the nebula. The light from the nebula passes through the disk on its way to Earth and some of it is absorbed by the material in the disk so that less total light reaches us from the regions of the nebula that lie behind it. In other words, the disk casts a shadow that we see as a dark patch amid the glowing gas of the nebula.

The amount of light blocked varies depending on how much material lies in the disk. More specifically, the densest regions of the disk will absorb most or even all of the background light, while thinner regions will absorb less. Imagine a butterfly flying in front of a spotlight. While its body will block enough light to make it opaque, it’s wings will be partially translucent – blocking some, but not all, of the background light. The thicker the wings, the more light they will block. Analogously, the amount of nebular light passing through each point in a silhouette disk is a direct measurement of the thickness (amount of material) in the disk at that point.

Until now, these disks have not been observed in silhouette from the ground. Although the theoretical resolution limit of a large ground-based telescope is higher than that of the small Hubble Space Telescope, the earth’s atmosphere makes it difficult to make images at the theoretical resolution limit of the telescope. This conundrum was the impetus for the development of adaptive optics (AO) technology, which corrects for the blurring effect of the atmosphere.

However, most modern AO systems operate at infrared wavelengths because the Earth’s atmosphere is a little better behaved (more “coherent”) there. It is only with the advent of next generation AO systems like Magellan AO that we are able to correct atmospheric turbulence at the level needed to achieve AO correction in visible light. Since nebulae like Orion emit most of their light in the visible, it wasn’t possible to image these disks in silhouette from the ground until now.

The MagAO system gives us several advantages over HST, the primary being that we are able to take images where the starlight is concentrated in just a small number of pixels (the star is “unsaturated”, so to speak). This allows us to probe the silhouette disks much farther inward than was possible with HST.

We observed the silhouette disk called Orion 218-354 during our December 2012 commissioning run. We were able to probe the amount of light blocked by the large disk all the way from its outer regions at a radius of ~500AU inward to just ~20AU (about the orbit of Uranus in our solar system). Surprisingly, we found that the amount of background light from the nebula that made it through the disk decreased slowly towards the star, and never reached an opaque point. This was unexpected because previous observations of the disk suggested that there was probably more material in the disk, and that it should reach an opaque point very quickly.

There are several possible explanations for why there is so little material in this disk, but one of the more interesting and plausible explanations is that the small dust grains that block Hydrogen-alpha emission from the nebula most effectively have been depleted. Although we believe disks are born with mostly small grains, as they age these grains may collide and coagulate, forming larger and larger grains and eventually pebbles, boulders and planets. The small amount of material that we found in this disk may indicate that this process (called “grain growth”) is well underway in the Orion 218-354 disk.

For more on Orion 218-354 see:
Follette, K. B., et al. “The First Circumstellar Disk Imaged in Silhouette at Visible Wavelengths with Adaptive Optics : MagAO Imaging of Orion 218-534”.
ApJ, 775, L13, 2013    preprint [pdf]    arxiv preprint

Abstract: We present high resolution adaptive optics (AO) corrected images of the silhouette disk Orion 218-354 taken with Magellan AO (MagAO) and its visible light camera, VisAO, in simultaneous differential imaging (SDI) mode at H-alpha. This is the first image of a circumstellar disk seen in silhouette with adaptive optics and is among the first visible light adaptive optics results in the literature. We derive the disk extent, geometry, intensity and extinction profiles and find, in contrast with previous work, that the disk is likely optically-thin at H-alpha. Our data provide an estimate of the column density in primitive, ISM-like grains as a function of radius in the disk. We estimate that only ~10% of the total sub-mm derived disk mass lies in primitive, unprocessed grains. We use our data, Monte Carlo radiative transfer modeling and previous results from the literature to make the first self-consistent multiwavelength model of Orion 218-354. We find that we are able to reproduce the 1-1000micron SED with a ~2-540AU disk of the size, geometry, small vs. large grain proportion and radial mass profile indicated by our data. This inner radius is a factor of ~15 larger than the sublimation radius of the disk, suggesting that it is likely cleared in the very interior.