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Register a new user2MASS J13243553+6358281 is an Early T-Type Planetary-Mass Object in the AB Doradus Moving Group
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We present new radial velocity and trigonometric parallax measurements indicating that the unusually red and photometrically variable T2 dwarf 2MASS J13243553+6358281 is a member of the young (~150 Myr) AB Doradus moving group based on its space velocity. We estimate its model-dependent mass in the range 11-12 $M_{rm Jup}$ at the age of the AB Doradus moving group, and its trigonometric parallax distance of 12.7 $pm$ 1.5 pc makes it one of the nearest known isolated planetary-mass objects. The unusually red continuum of 2MASS J13243553+6358281 in the near-infrared was previously suspected to be caused by an unresolved L+T brown dwarf binary, although it was never observed with high-spatial resolution imaging. This new evidence of youth suggests that a low surface gravity may be sufficient to explain this peculiar feature. Using the new parallax we find that its absolute $J$-band magnitude is ~0.4 mag fainter than equivalent-type field brown dwarfs, suggesting that the binary hypothesis is unlikely. The fundamental properties of 2MASS J13243553+6358281 follow the spectral type sequence of other known high-likelihood members of the AB Doradus moving group. The effective temperature of 2MASS J13243553+6358281 provides the first precise constraint on the L/T transition at a known young age, and indicates that it happens at a temperature of ~1150 K at ~150 Myr, compared to ~1250 K for field brown dwarfs.
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Context. Precise determination of stellar masses is necessary to test the validity of pre-main-sequence (PMS) stellar evolutionary models, whose predictions are in disagreement with measurements for masses below 1.2 Msun. To improve such a test, and based on our previous studies, we selected the AB Doradus moving group (AB Dor-MG) as the best-suited association on which to apply radio-based high-precision astrometric techniques to study binary systems. Aims. We seek to determine precise estimates of the masses of a set of stars belonging to the AB Dor-MG using radio and infrared observations. Methods. We observed in phase-reference mode with the Very Large Array (VLA) at 5 GHz and with the European VLBI Network (EVN) at 8.4 GHz the stars HD 160934, EK Dra, PW And, and LO Peg. We also observed some of these stars with the near-infrared CCD AstraLux camera at the Calar Alto observatory to complement the radio observations. Results. We determine model-independent dynamical masses of both components of the star HD 160934, A and c, which are 0.70+/-0.07 Msun and 0.45+/-0.04 Msun , respectively. We revised the orbital parameters of EK Dra and we determine a sum of the masses of the system of 1.38+/-0.08 Msun. We also explored the binarity of the stars LO Peg and PW And. Conclusions. We found observational evidence that PMS evolutionary models underpredict the mass of PMS stars by 10%-40%, as previously reported by other authors. We also inferred that the origin of the radio emission must be similar in all observed stars, that is, extreme magnetic activity of the stellar corona that triggers gyrosynchrotron emission from non-thermal, accelerated electrons.
PSO J318.5338$-$22.8603 is an extremely-red planetary-mass object that has been identified as a candidate member of the $beta$ Pictoris moving group based on its spatial position and tangential velocity. We present a high resolution $K$-band spectrum of PSO J318.5338$-$22.8603. Using a forward-modeling Markov Chain Monte Carlo approach, we report the first measurement of the radial velocity and $v$ sin($i$) of PSO J318.5$-$22, $-$6.0$^{+0.8}_{-1.1}$ km s$^{-1}$ and 17.5$^{+2.3}_{-2.8}$ km s$^{-1}$, respectively. We calculate the space velocity and position of PSO J318.5$-$22 and confirm that it is a member of the $beta$ Pictoris moving group. Adopting an age of 23$pm$3 Myr for PSO J318.5$-$22, we determine a mass of $8.3pm0.5$ $M_{rm{Jup}}$ and effective temperature of $1127^{+24}_{-26}$ K using evolutionary models. PSO J318.5338$-$22.8603 is intermediate in mass and temperature to the directly-imaged planets $beta$ Pictoris b and 51 Eridani b, making it an important benchmark object in the sequence of planetary-mass members of the $beta$ Pictoris moving group. Combining our $v$ sin($i$) measurement with recent photometric variability data, we constrain the inclination of PSO J318.5$-$22 to $>29^{circ}$ and its rotational period to 5-10.2 hours. The equatorial velocity of PSO J318.5$-$22 indicates that its rotation is consistent with an extrapolation of the velocity-mass relationship for solar system planets.
Measurements of photometric variability at different wavelengths provide insights into the vertical cloud structure of brown dwarfs and planetary-mass objects. In seven Hubble Space Telescope consecutive orbits, spanning $sim$10 h of observing time}, we obtained time-resolved spectroscopy of the planetary-mass T8-dwarf Ross 458C using the near-infrared Wide Field Camera 3. We found spectrophotometric variability with a peak-to-peak signal of 2.62$pm$0.02 % (in the 1.10-1.60~$mu$m white light curve). Using three different methods, we estimated a rotational period of 6.75$pm$1.58~h for the white light curve, and similar periods for narrow $J$- and $H$- band light curves. Sine wave fits to the narrow $J$- and $H$-band light curves suggest a tentative phase shift between the light curves with wavelength when we allow different periods between both light curves. If confirmed, this phase shift may be similar to the phase shift detected earlier for the T6.5 spectral type 2MASS J22282889-310262. We find that, in contrast with 2M2228, the variability of Ross~458C shows evidence for a {color trend} within the narrow $J$-band, but gray variations in the narrow $H$-band. The spectral time-resolved variability of Ross 458C might be potentially due to heterogeneous sulfide clouds in the atmosphere of the object. Our discovery extends the study of spectral modulations of condensate clouds to the coolest T dwarfs, planetary-mass companions.
We present the discovery of WISEA J083011.95+283716.0, the first Y dwarf candidate identified through the Backyard Worlds: Planet 9 citizen science project. We identified this object as a red, fast-moving source with a faint $W2$ detection in multi-epoch textit{AllWISE} and unWISE images. We have characterized this object with Spitzer Space Telescope and textit{Hubble Space Telescope} follow-up imaging. With mid-infrared detections in textit{Spitzer}s emph{ch1} and emph{ch2} bands and flux upper limits in Hubble Space Telescope $F105W$ and $F125W$ filters, we find that this object is both very faint and has extremely red colors ($ch1-ch2 = 3.25pm0.23$ mag, $F125W-ch2 geq 9.36$ mag), consistent with a T$_{eff}sim300$ K source, as estimated from the known Y dwarf population. A preliminary parallax provides a distance of $11.1^{+2.0}_{-1.5}$ pc, leading to a slightly warmer temperature of $sim350$ K. The extreme faintness and red Hubble Space Telescope and Spitzer Space Telescope colors of this object suggest it may be a link between the broader Y dwarf population and the coldest known brown dwarf WISE J0855$-$0714, and highlight our limited knowledge of the true spread of Y dwarf colors. We also present four additional Backyard Worlds: Planet 9 late-T brown dwarf discoveries within 30 pc.
Substellar members of young ($lesssim$150 Myr) moving groups are valuable benchmarks to empirically define brown dwarf evolution with age and to study the low-mass end of the initial mass function. We have combined Pan-STARRS1 (PS1) proper motions with optical$-$IR photometry from PS1, 2MASS and $textit{WISE}$ to search for substellar members of the AB Dor Moving Group within $approx$50 pc and with spectral types of late-M to early-L, corresponding to masses down to $approx$30 M$_{Jup}$ at the age of the group ($approx$125 Myr). Including both photometry and proper motions allows us to better select candidates by excluding field dwarfs whose colors are similar to young AB~Dor Moving Group members. Our near-IR spectroscopy has identified six ultracool dwarfs (M6$-$L4; $approx$30$-$100 M$_{Jup}$) with intermediate surface gravities (INT-G) as candidate members of the AB Dor Moving Group. We find another two candidate members with spectra showing hints of youth but consistent with field gravities. We also find four field brown dwarfs unassociated with the AB Dor Moving Group, three of which have INT-G gravity classification. While signatures of youth are present in the spectra of our $approx$125 Myr objects, neither their $J-K$ nor $W1-W2$ colors are significantly redder than field dwarfs with the same spectral types, unlike younger ultracool dwarfs. We also determined PS1 parallaxes for eight of our candidates and one previously identified AB Dor Moving Group candidate. Although radial velocities (and parallaxes, for some) are still needed to fully assess membership, these new objects provide valuable insight into the spectral characteristics and evolution of young brown dwarfs.
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