[Paper Review] Characterization of the gaseous companion {\kappa} Andromedae b: New Keck and LBTI high-contrast observations

M. Bonnefoy, Thayne Currie+65 more|arXiv (Cornell University)|Aug 18, 2013

Stellar, planetary, and galactic studies34 citations

TL;DR

This study presents new high-contrast imaging observations of the wide-orbit gas giant companion κ Andromedae b using Keck/NIRC2 and LBTI/LMIRCam, deriving its near-infrared photometry and spectral energy distribution. The analysis combines adaptive optics data and atmospheric models to constrain its temperature (1900+100−200 K), luminosity (log10(L/L⊙) = −3.76 ± 0.06), and mass (14+25−2 MJup under 'hot-start' models), suggesting it lies in the brown dwarf regime, with disk instability as a plausible formation mechanism.

ABSTRACT

We previously reported the direct detection of a low mass companion at a projected separation of 55+-2 AU around the B9 type star {\kappa} Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for the understanding of the formation and evolution of gas giant planets and brown dwarfs on wide-orbits. We present new angular differential imaging (ADI) images of the Kappa Andromedae system at 2.146 (Ks), 3.776 (L'), 4.052 (NB 4.05) and 4.78 {\mu}m (M') obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We derive a more accurate J = 15.86 +- 0.21, H = 14.95 +- 0.13, Ks = 14.32 +- 0.09 mag for {\kappa} And b. We redetect the companion in all our high contrast observations. We confirm previous contrasts obtained at Ks and L' band. We derive NB 4.05 = 13.0 +- 0.2 and M' = 13.3 +- 0.3 mag and estimate Log10(L/Lsun) = -3.76 +- 0.06. We build the 1-5 microns spectral energy distribution of the companion and compare it to seven PHOENIX-based atmospheric models in order to derive Teff = 1900+100-200 K. Models do not set constrains on the surface gravity. ``Hot-start" evolutionary models predict masses of 14+25-2 MJup based on the luminosity and temperature estimates, and considering a conservative age range for the system (30+120-10 Myr). ``warm-start" evolutionary tracks constrain the mass to M >= 11 MJup. Therefore, the mass of {\kappa} Andromedae b mostly falls in the brown-dwarf regime, due to remaining uncertainties in age and mass-luminosity models. According to the formation models, disk instability in a primordial disk could account for the position and a wide range of plausible masses of {\kappa} And b.

Motivation & Objective

• To improve the photometric and spectroscopic characterization of the directly imaged companion κ Andromedae b using high-contrast imaging.
• To determine the near-infrared spectral energy distribution (SED) of the companion from new observations at Ks, L′, NB 4.05, and M′ bands.
• To constrain the atmospheric and evolutionary properties of κ Andromedae b using PHOENIX-based atmospheric models and evolutionary tracks.
• To evaluate the formation mechanism of the companion, particularly disk instability versus core accretion, based on its mass, separation, and system age.
• To refine the system's age and mass estimates by accounting for uncertainties in initial conditions and evolutionary models.

Proposed method

• Acquired high-contrast angular differential imaging (ADI) data using Keck/NIRC2 and LBTI/LMIRCam at Ks (2.146 µm), L′ (3.776 µm), NB 4.05 (4.052 µm), and M′ (4.78 µm) bands.
• Performed flux extraction from ADI data using specialized analysis techniques tailored to suppress residual speckle noise.
• Compared the derived photometry to reference young and old objects and to a set of 7 PHOENIX-based atmospheric models including dust formation.
• Used 'hot-start' and 'warm-start' evolutionary models to estimate mass, incorporating a conservative age range of 30+120−10 Myr.
• Applied dedicated disk-instability formation models to assess the plausibility of in-situ formation at large separations.
• Corrected surface gravity estimates for the rapidly rotating primary star using the method of Huang & Gies (2006) to improve atmospheric parameter accuracy.

Experimental results

Research questions

• RQ1 What is the near-infrared spectral energy distribution (SED) of κ Andromedae b based on new high-contrast observations?
• RQ2 What are the best-fitting atmospheric parameters (effective temperature, surface gravity) for the companion?
• RQ3 What is the estimated mass of κ Andromedae b, and how does it depend on evolutionary model assumptions?
• RQ4 Can disk instability in a primordial disk explain the observed position and mass of the companion?
• RQ5 How do uncertainties in system age and mass-luminosity relations affect the classification of the companion as a planet or brown dwarf?

Key findings

• The companion was successfully redetected in all new high-contrast observations at Ks, L′, NB 4.05, and M′ bands with high signal-to-noise.
• The derived J, H, Ks, NB 4.05, and M′ magnitudes are J = 15.86 ± 0.21, H = 14.95 ± 0.13, Ks = 14.32 ± 0.09, NB 4.05 = 13.0 ± 0.2, and M′ = 13.3 ± 0.3 mag.
• The companion's luminosity is log10(L/L⊙) = −3.76 ± 0.06, and its effective temperature is 1900+100−200 K based on atmospheric models.
• Evolutionary models indicate a mass of 14+25−2 MJup under 'hot-start' assumptions and M ≥ 11 MJup under 'warm-start' models, with age uncertainty being a key driver of mass uncertainty.
• The object's mass lies predominantly in the brown dwarf regime due to uncertainties in age and mass-luminosity relations, though it may be a massive gas giant.
• Disk instability in a primordial disk is a viable formation mechanism for κ Andromedae b, consistent with its wide separation and observed mass range.