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, 2015arxiv preprint
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Combining our new scattered light data with archival Hubble Space Telescope images at ~0.5-2 mum, along with previously unpublished Spitzer/MIPS thermal emission data and all other literature thermal data, we set out to constrain the chemical composition of the dust grains.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.
We utilized MagAO to obtain very high-resolution visible light images of HD142527 with MagAO’s VisAO science camera. In the median seeing conditions of the 6.5m Magellan telescope (0.5 − 0.7′′), we find MagAO delivers 24-19% Strehl at Ha (0.656 mm). We detect a faint companion (HD142527B) embedded in this young transitional disk system at just 86.3±1.9 mas (~12 AU) from the star. The companion is detected in both Halpha and a continuum filter (Dmag=6.33±0.20 mag at Ha and 7.50±0.25 mag in the continuum filter). This provides confirmation of the tentative companion discovered by Biller and co-workers (a past graduate of Dr. Close, who is now a professor at the University of Edinburgh) with sparse aperture masking at the 8m VLT.
MagAO found that the Halpha emission from the ~0.25 solar mass companion (EW=180 Angstroms) implies a mass accretion rate of ~5.9×10-10 Msun/yr, and a total accretion luminosity of 1.2% Lsun. Assuming a similar accretion rate, we estimate that a 1 Jupiter mass gas giant could have considerably better (50-1000x) planet/star contrasts at Halpha than at H band (COND models) for a range of optical extinctions (3.4-0 mag). We suggest that 0.5-5 Mjup extrasolar planets in their gas accretion phase should be much more luminous at Halpha than in the NIR. This is the motivation for our new MagAO GAPplanetS survey for extrasolar planets which is a key project for the MagAO team, and in particular senior PhD student Kate Follette who has lead most of the GAPplanets work to date.
Here is the Astrophysical Journal abstract:
“We utilized the new high-order 585 actuator Magellan Adaptive Optics system (MagAO) to obtain very high-resolution visible light images of HD 142527 with MagAO’s VisAO science camera. In the median seeing conditions of the 6.5 m Magellan telescope (0.”5-0.”7), we find MagAO delivers 24%-19% Strehl at Halpha (0.656 mum). We detect a faint companion (HD 142527B) embedded in this young transitional disk system at just 86.3 ± 1.9 mas (~12 AU) from the star. The companion is detected in both Halpha and a continuum filter (Deltamag = 6.33 ± 0.20 mag at Halpha and 7.50 ± 0.25 mag in the continuum filter). This provides confirmation of the tentative companion discovered by Biller and co-workers with sparse aperture masking at the 8 m Very Large Telescope. The Halpha emission from the ~0.25 solar mass companion (EW = 180 Å) implies a mass accretion rate of ~5.9 × 10-10 M sun yr-1 and a total accretion luminosity of 1.2% L sun. Assuming a similar accretion rate, we estimate that a 1 Jupiter mass gas giant could have considerably better (50-1000×) planet/star contrasts at Halpha than at the H band (COND models) for a range of optical extinctions (3.4-0 mag). We suggest that ~0.5-5 M jup extrasolar planets in their gas accretion phase could be much more luminous at Halpha than in the NIR. This is the motivation for our new MagAO GAPplanetS survey for extrasolar planets.”
There are a number of factors that have pushed extrasolar planet imagers to work in the infrared. The first is that young planets are very hot, so they are brighter in the infrared. In fact, they become very faint at shorter “optical” wavelengths. The second factor is that adaptive optics (AO), the technology we use to image exoplanets, has normally worked better in the infrared. You could say that the stars twinkle less there. But to know as much as we can about exoplanets and their atmospheres, we want to image them at as many wavelengths as we can. So we used MagAO’s VisAO camera to image the extrasolar planet beta Pictoris b with our CCD. This is the first time that has been done from the ground, and shows that we are pushing the capabilities of AO to ever shorter (and more difficult) wavelengths.
Abstract: We present the first ground-based CCD (λ<1μm) image of an extrasolar planet. Using MagAO's VisAO camera we detected the extrasolar giant planet (EGP) β Pictoris b in Y-short (YS, 0.985 μm), at a separation of 0.470±0.010′′ and a contrast of (1.63±0.49)×10−5. This detection has a signal-to-noise ratio of 4.1, with an empirically estimated upper-limit on false alarm probability of 1.0%. We also present new photometry from the NICI instrument on the Gemini-South telescope, in CH4S,1% (1.58 μm), KS (2.18μm), and Kcont (2.27 μm). A thorough analysis of our photometry combined with previous measurements yields an estimated near-IR spectral type of L2.5±1.5, consistent with previous estimates. We estimate log(Lbol/LSun) = −3.86±0.04, which is consistent with prior estimates for β Pic b and with field early-L brown dwarfs. This yields a hot-start mass estimate of 11.9±0.7 MJup for an age of 21±4 Myr, with an upper limit below the deuterium burning mass. Our Lbol based hot-start estimate for temperature is Teff=1643±32 K (not including model dependent uncertainty). Due to the large corresponding model-derived radius of R=1.43±0.02 RJup, this Teff is ∼250 K cooler than would be expected for a field L2.5 brown dwarf. Other young, low-gravity (large radius), ultracool dwarfs and directly-imaged EGPs also have lower effective temperatures than are implied by their spectral types. However, such objects tend to be anomalously red in the near-IR compared to field brown dwarfs. In contrast, β Pic b has near-IR colors more typical of an early-L dwarf despite its lower inferred temperature.
For more on our result see:
Males, J. R., et al. "Magellan Adaptive Optics first-light observations of the exoplanet β Pic b. I. Direct imaging in the far-red optical with MagAO+VisAO and in the near-IR with NICI"
ApJ, 786, 32, 2014ADSarxiv preprint
2M1207 b (pronounced two-mass-twelve-oh-seven-bee) is often considered the first directly imaged extrasolar planet. Though its primary star, 2M1207 A, is actually a brown dwarf, b shares many properties with the HR 8799 planets. Andy Skemer analyzed images taken with MagAO+Clio2 and compared the results with images of the HR 8799 planets taken with the LBT. The interesting thing about 2M1207 b is that it doesn’t seem to have methane in its atmosphere — otherwise we wouldn’t have been able to see it since methane should absorb all the light at the wavelength we used.
Imaging 2M1207 b is extremely challenging from a technical standpoint. 2M1207 A is a faint brown dwarf, which doesn’t emit enough visible photons for our wavefront sensor. Instead, we locked on an off-axis (and still faint) star 40″ away from the science target. The result demonstrates that MagAO can produce reasonable Strehl ratio images on targets that are too faint to serve as their own guide-star.
Abstract: Gas-giant planets emit a large fraction of their light in the mid-infrared (≳3μm), where photometry and spectroscopy are critical to our understanding of the bulk properties of extrasolar planets. Of particular importance are the L and M-band atmospheric windows (3-5μm), which are the longest wavelengths currently accessible to ground-based, high-contrast imagers. We present binocular LBT AO images of the HR 8799 planetary system in six narrow-band filters from 3-4μm, and a Magellan AO image of the 2M1207 planetary system in a broader 3.3μm band. These systems encompass the five known exoplanets with luminosities consistent with L→T transition brown dwarfs. Our results show that the exoplanets are brighter and have shallower spectral slopes than equivalent temperature brown dwarfs in a wavelength range that contains the methane fundamental absorption feature. For 2M1207 b, we find that thick clouds and non-equilibrium chemistry caused by vertical mixing can explain the object’s appearance. For the HR 8799 planets, we find that the atmospheres must have patchy clouds, along with non-equilibrium chemistry. Together, the presence of a heterogeneous surface and vertical mixing presents a picture of dynamic planetary atmospheres in which both horizontal and vertical motions influence the chemical and condensate profiles.