Magellan Adaptive Optics first-light observations of the exoplanet ${\hat{I}}^2$ Pic b. I. Direct imaging in the far-red optical with MagAO+VisAO and in the near-IR with NICI

Abstract

We present the first ground-based CCD ($\hat{I}»<1\hat{I}¼$m) image of an extrasolar planet. Using MagAO's VisAO camera we detected the extrasolar giant planet (EGP) ${\hat{I}}^2$ Pictoris b in $Y$-short ($Y_S$, 0.985 $\hat{I}¼$m), at a separation of $0.470\pm 0.010^{\prime \prime }$ and a contrast of $(1.63\pm 0.49)\times 10^{-5}$. 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 $C{H}_{4S,1\%}$ ($1.58$ $\hat{I}¼m$), $K_S$ ($2.18\hat{I}¼m$), and $K_{cont}$ (2.27 $\hat{I}¼m$). A thorough analysis of our photometry combined with previous measurements yields an estimated near-IR spectral type of L$2.5\pm 1.5$, consistent with previous estimates. We estimate log$(L_{bol}/L_{Sun})$ = $-3.86\pm 0.04$, which is consistent with prior estimates for ${\hat{I}}^2$ Pic b and with field early-L brown dwarfs. This yields a hot-start mass estimate of $11.9\pm 0.7$ $M_{Jup}$ for an age of $21\pm 4$ Myr, with an upper limit below the deuterium burning mass. Our $L_{bol}$ based hot-start estimate for temperature is $T_{eff}=1643\pm 32$ K (not including model dependent uncertainty). Due to the large corresponding model-derived radius of $R=1.43\pm 0.02$ $R_{Jup}$, this $T_{eff}$ is $\sim$$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, ${\hat{I}}^2$ Pic b has near-IR colors more typical of an early-L dwarf despite its lower inferred temperature.

40 pages, 20 figures. Accepted to ApJ