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Register a new userGemini Planet Imager Spectroscopy of the Dusty Substellar Companion HD 206893 B
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Kimberly Ward-Duong
Publication date
2020
fields
Physics
and research's language is
English
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We present new near-infrared Gemini Planet Imager (GPI) spectroscopy of HD 206893 B, a substellar companion orbiting within the debris disk of its F5V star. The $J$, $H$, $K1$, and $K2$ spectra from GPI demonstrate the extraordinarily red colors of the object, confirming it as the reddest substellar object observed to date. The significant flux increase throughout the infrared presents a challenging atmosphere to model with existing grids. Best-fit values vary from 1200 K to 1800 K for effective temperature and from 3.0 to 5.0 for log($g$), depending on which individual wavelength band is fit and which model suite is applied. The extreme redness of the companion can be partially reconciled by invoking a high-altitude layer of sub-micron dust particles, similar to dereddening approaches applied to the peculiar red field L-dwarf population. However, reconciling the HD 206893 B spectra with even those of the reddest low-gravity L-dwarf spectra still requires the contribution of additional atmospheric dust, potentially due to the debris disk environment in which the companion resides. Orbit fitting from four years of astrometric monitoring is consistent with a $sim$30-year period, orbital inclination of 147$^{circ}$, and semimajor axis of 10 au, well within the estimated disk inner radius of $sim$50 au. As one of very few substellar companions imaged interior to a circumstellar disk, the properties of this system offer important dynamical constraints on companion-disk interaction and provide a benchmark for substellar and planetary atmospheric study.
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We present new $H$ (1.5-1.8 $mu$m) photometric and $K_1$ (1.9-2.2 $mu$m) spectroscopic observations of the young exoplanet HD 95086 b obtained with the Gemini Planet Imager. The $H$-band magnitude has been significantly improved relative to previous measurements, whereas the low resolution $K_1$ ($lambda/deltalambda approx 66$) spectrum is featureless within the measurement uncertainties, and presents a monotonically increasing pseudo-continuum consistent with a cloudy atmosphere. By combining these new measurements with literature $L^{prime}$ photometry, we compare the spectral energy distribution of the planet to other young planetary-mass companions, field brown dwarfs, and to the predictions of grids of model atmospheres. HD 95086 b is over a magnitude redder in $K_1-L^{prime}$ color than 2MASS J12073346-3932539 b and HR 8799 c and d, despite having a similar $L^{prime}$ magnitude. Considering only the near-infrared measurements, HD 95086 b is most analogous to the brown dwarfs 2MASS J2244316+204343 and 2MASS J21481633+4003594, both of which are thought to have dusty atmospheres. Morphologically, the spectral energy distribution of HD 95086 b is best fit by low temperature ($T_{rm eff} =$ 800-1300 K), low surface gravity spectra from models which simulate high photospheric dust content. This range of effective temperatures is consistent with field L/T transition objects, but the spectral type of HD 95086 b is poorly constrained between early L and late T due to its unusual position the color-magnitude diagram, demonstrating the difficulty in spectral typing young, low surface gravity substellar objects. As one of the reddest such objects, HD 95086 b represents an important empirical benchmark against which our current understanding of the atmospheric properties of young extrasolar planets can be tested.
Detecting polarized light from self-luminous exoplanets has the potential to provide key information about rotation, surface gravity, cloud grain size, and cloud coverage. While field brown dwarfs with detected polarized emission are common, no exoplanet or substellar companion has yet been detected in polarized light. With the advent of high contrast imaging spectro-polarimeters such as GPI and SPHERE, such a detection may now be possible with careful treatment of instrumental polarization. In this paper, we present 28 minutes of $H$-band GPI polarimetric observations of the benchmark T5.5 companion HD 19467 B. We detect no polarization signal from the target, and place an upper limit on the degree of linear polarization of $p_{text{CL}99.73%} leq 2.4%$. We discuss our results in the context of T dwarf cloud models and photometric variability.
We aim to reveal the nature of the reddest known substellar companion HD 206893 B by studying its near-infrared colors and spectral morphology and by investigating its orbital motion. We fit atmospheric models for giant planets and brown dwarfs and perform spectral retrievals with petitRADTRANS and ATMO on the observed GRAVITY, SPHERE, and GPI spectra of HD 206893 B. To recover its unusual spectral features, we include additional extinction by high-altitude dust clouds made of enstatite grains in the atmospheric model fits. We also infer the orbital parameters of HD 206893 B by combining the $sim 100~mutext{as}$ precision astrometry from GRAVITY with data from the literature and constrain the mass and position of HD 206893 C based on the Gaia proper motion anomaly of the system. The extremely red color and the very shallow $1.4~mutext{m}$ water absorption feature of HD 206893 B can be fit well with the adapted atmospheric models and spectral retrievals. Altogether, our analysis suggests an age of $sim 3$-$300~text{Myr}$ and a mass of $sim 5$-$30~text{M}_text{Jup}$ for HD 206893 B, which is consistent with previous estimates but extends the parameter space to younger and lower-mass objects. The GRAVITY astrometry points to an eccentric orbit ($e = 0.29^{+0.06}_{-0.11}$) with a mutual inclination of $< 34.4~text{deg}$ with respect to the debris disk of the system. While HD 206893 B could in principle be a planetary-mass companion, this possibility hinges on the unknown influence of the inner companion on the mass estimate of $10^{+5}_{-4}~text{M}_text{Jup}$ from radial velocity and Gaia as well as a relatively small but significant Argus moving group membership probability of $sim 61%$. However, we find that if the mass of HD 206893 B is $< 30~text{M}_text{Jup}$, then the inner companion HD 206893 C should have a mass between $sim 8$-$15~text{M}_text{Jup}$.
We present new observations of the low-mass companion to HD 984 taken with the Gemini Planet Imager as a part of the Gemini Planet Imager Exoplanet Survey campaign. Images of HD 984 B were obtained in the J (1.12--1.3 micron) and H (1.50--1.80 micron) bands. Combined with archival epochs from 2012 and 2014, we fit the first orbit to the companion to find an 18 AU (70 year) orbit with a 68% confidence interval between 14 and 28 AU, an eccentricity of 0.18 with a 68% confidence interval between 0.05 and 0.47, and an inclination of 119 degrees with a 68% confidence interval between 114 degrees and 125 degrees. To address considerable spectral covariance in both spectra, we present a method of splitting the spectra into low and high frequencies to analyze the spectral structure at different spatial frequencies with the proper spectral noise correlation. Using the split spectra, we compare to known spectral types using field brown dwarf and low-mass star spectra and find a best fit match of a field gravity M6.5+/-1.5 spectral type with a corresponding temperature of 2730+120 K. Photometry of the companion yields a luminosity of log(L_bol/L_sun) = -2.88+/-0.07 dex, using DUSTY models. Mass estimates, again from DUSTY models, find an age-dependent mass of 34+/-1 to 95+/-4 M_Jup. These results are consistent with previous measurements of the object.
We present moderate-resolution ($Rsim4000$) $K$ band spectra of the super-Jupiter, $kappa$ Andromedae b. The data were taken with the OSIRIS integral field spectrograph at Keck Observatory. The spectra reveal resolved molecular lines from H$_{2}$O and CO. The spectra are compared to a custom $PHOENIX$ atmosphere model grid appropriate for young planetary-mass objects. We fit the data using a Markov Chain Monte Carlo forward modeling method. Using a combination of our moderate-resolution spectrum and low-resolution, broadband data from the literature, we derive an effective temperature of $T_mathrm{eff}$ = 1950 - 2150 K, a surface gravity of $log g=3.5 - 4.5$, and a metallicity of [M/H] = $-0.2 - 0.0$. These values are consistent with previous estimates from atmospheric modeling and the currently favored young age of the system ($<$50 Myr). We derive a C/O ratio of 0.70$_{-0.24}^{+0.09}$ for the source, broadly consistent with the solar C/O ratio. This, coupled with the slightly subsolar metallicity, implies a composition consistent with that of the host star, and is suggestive of formation by a rapid process. The subsolar metallicity of $kappa$ Andromedae b is also consistent with predictions of formation via gravitational instability. Further constraints on formation of the companion will require measurement of the C/O ratio of $kappa$ Andromedae A. We also measure the radial velocity of $kappa$ Andromedae b for the first time, with a value of $-1.4pm0.9,mathrm{km},mathrm{s}^{-1}$ relative to the host star. We find that the derived radial velocity is consistent with the estimated high eccentricity of $kappa$ Andromedae b.
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