Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. I. An Accurate Look at the L-to-T Transition at $\sim$300 Myr from Optical through Mid-infrared Spectrophotometry

Abstract

We present $Spitzer$ IRS 5--14 $\mu$m spectra and 16 $\mu$m and 22 $\mu$m photometry of the T2.5 companion to the $\sim$300 Myr-old G0V star HN Peg. We incorporate previous 0.8--5 $\mu$m observations to obtain the most comprehensive spectral energy distribution of an intermediate-gravity L/T-transition dwarf which, together with an accurate Gaia EDR3 parallax of the primary, enable us to derive precise fundamental parameters. We find that young ($\approx$0.1--0.3 Gyr) early-T dwarfs on average have $\approx$140 K lower effective temperatures, $\approx$20% larger radii, and similar bolometric luminosities compared to $\gtrsim$1 Gyr-old field dwarfs with similar spectral types. Our accurate infrared spectrophotometry offers new detail at wavelengths where the dominant carbon-bearing molecules have their strongest transitions: at 3.4 $\mu$m for methane and at 4.6 $\mu$m for carbon monoxide. We assess the performance of various widely available photospheric models and find that models with condensates and/or clouds better reproduce the full SED of this moderately young early-T dwarf. However, cloud-free models incorporating a more general convective instability treatment reproduce at least the low-resolution near-IR spectrum similarly well. Our analysis of $R\approx 2300$ $J$-band spectra shows that the near-infrared potassium absorption lines in HN Peg B have similar strengths to those seen in both younger and older T2-T3 dwarfs. We conclude that while alkali lines are well-established as surface gravity indicators for L-type or warmer stars, they are insensitive to surface gravity in early-T dwarfs

arXiv commentAccepted for publication in ApJ

References in corpus (61)

• The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
• Direct Imaging of Multiple Planets Orbiting the Star HR 8799
• Cosmic-Ray Rejection by Laplacian Edge Detection
• Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 20945
• The Infrared Spectrograph on the Spitzer Space Telescope
• A Non-Gray Theory of Extrasolar Giant Planets and Brown Dwarfs
• Evolutionary models for very-low-mass stars and brown dwarfs with dusty atmospheres
• Images of a fourth planet orbiting HR 8799
• Models of Stars, Brown Dwarfs and Exoplanets
• Solar Models with Helium and Heavy Element Diffusion
• Ages for illustrative field stars using gyrochronology: viability, limitations and errors
• The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
• The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T Transition
• Discovery and spectroscopy of the young Jovian planet 51 Eri b with the Gemini Planet Imager
• Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: the Effects of Temperature, Clouds, and Gravity
• A Unified Near Infrared Spectral Classification Scheme for T Dwarfs
• Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis
• Preliminary Parallaxes of 40 L and T Dwarfs from the U.S. Naval Observatory Infrared Astrometry Program
• The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification
• The 0.8-14.5 micron Spectra of Mid-L to Mid-T Dwarfs: Diagnostics of Effective Temperature, Grain Sedimentation, Gas Transport, and Surface Gravity
• Fundamental Parameters and Spectral Energy Distributions of Young and Field Age Objects with Masses Spanning the Stellar to Planetary Regime
• A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs
• Atmospheric Parameters of Field L and T Dwarfs
• The NIRSPEC Brown Dwarf Spectroscopic Survey. I. Low-Resolution Near-Infrared Spectra
• A new set of atmosphere and evolution models for cool T-Y brown dwarfs and giant exoplanets
• The Stellar Population of the Chamaeleon I Star-Forming Region
• Masses, Radii, and Cloud Properties of the HR 8799 Planets
• Weather on Other Worlds. II. Survey Results: Spots Are Ubiquitous on L and T Dwarfs
• Population Properties of Brown Dwarf Analogs to Exoplanets
• Discovery of Two T Dwarf Companions with the Spitzer Space Telescope
• A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf "Lithium Test" at Early Ages
• Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs
• The Missing Link: Early Methane ("T") Dwarfs in the Sloan Digital Sky Survey
• Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition
• Discovery of a probable 4-5 Jupiter-mass exoplanet to HD 95086 by direct-imaging
• Individual Dynamical Masses of Ultracool Dwarfs
• Cloudless atmospheres for L/T dwarfs and extra-solar giant planets
• Calculations of the Far-Wing Line Profiles of Sodium and Potassium in the Atmospheres of Substellar-Mass Objects
• Identifying Young Brown Dwarfs Using Gravity-Sensitive Spectral Features
• The Spectra of T Dwarfs. II. Red Optical Data
• Discovery of 11 New T Dwarfs in the Two Micron All-Sky Survey, Including a Possible L/T Transition Binary
• HD 203030B: an Unusually Cool Young Sub-Stellar Companion near the L/T Transition
• Confirmation of the planet around HD 95086 by direct imaging
• Infrared Observations and Modeling of one of the Coolest T Dwarfs, Gl 570D
• The Carina-Near Moving Group
• Discovery of the brightest T dwarf in the northern hemisphere
• Discovery of a T Dwarf Binary with the Largest Known J-Band Flux Reversal
• The Hawaii Infrared Parallax Program. IV. A Comprehensive Parallax Survey of L0-T8 dwarfs with UKIRT
• A Survey for H-alpha Emission from Late L dwarfs and T dwarfs
• SIMP J013656.5+093347 is Likely a Planetary-Mass Object in the Carina-Near Moving Group
• Thermo-compositional diabatic convection in the atmospheres of brown dwarfs and in Earth's atmosphere and oceans
• Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey
• Cloudless atmospheres for young low-gravity substellar objects
• HN Peg B: A Test of Models of the L to T Dwarf Transition
• The Hawaii Infrared Parallax Program. V. New T-Dwarf Members and Candidate Members of Nearby Young Moving Groups
• Discovery of a directly imaged planet to the young solar analog YSES 2
• COol Companions ON Ultrawide orbiTS (COCONUTS). I. A High-Gravity T4 Benchmark around an Old White Dwarf and A Re-Examination of the Surface-Gravity Dependence of the L/T Transition
• The Minimum Jeans Mass, Brown Dwarf Companion IMF, and Predictions for Detection of Y-type Dwarfs
• Cloud Atlas: Rotational Modulations in the L/T Transition Brown Dwarf Companion HN Peg B
• 2MASS J13243553+6358281 is an Early T-Type Planetary-Mass Object in the AB Doradus Moving Group
• The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass

Cited by in corpus (6)

• Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. II. Emergence and Sedimentation of Silicate Clouds in L Dwarfs, and Analysis of the Full M5--T9 Field Dwarf Spectroscopic Sample
• Four-of-a-kind? Comprehensive atmospheric characterisation of the HR 8799 planets with VLTI/GRAVITY
• Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph -- III. Dust Grains in Young L Dwarf Atmospheres Are Heavier
• A 1.46-2.48 $\mu$m Spectroscopic Atlas of a T6 Dwarf (1060 K) Atmosphere with IGRINS: First Detections of H${}_2$S and H${}_2$, and Verification of H${}_2$O, CH${}_4$, and NH${}_3$ Line Lists
• Diversity of Cold Worlds: A Near Complete Spectral Energy Distribution for 2MASS J04151954-0935066 using JWST
• Silicate Clouds, SiO, and Molecular Trends across the L-T Sequence: A JWST/MIRI LRS Spectral Library of Brown Dwarfs