Adaptive Optics Infrared Imaging Polarimetry and Optical HST Imaging of Hubble's Variable Nebula (R Monocerotis/NGC 2261): A Close Look at a Very Young Active Herbig Ae/Be Star

We present high-resolution (FWHM = 02) near-IR (J, H, and K') adaptive optics images of the Herbig Ae/Be star R Monocerotis. Optical Hubble Space Telescope (HST) WFPC2 PC camera archival images are also presented. For the first time, adaptive optics was utilized to make high-resolution (FWHM = 02) IR-imaging polarimetry maps of R Mon. In addition, the first mid-IR array images (at 11.7 and 20.8 μm) of R Mon have been obtained. We also present new 3.16, 3.93, and 4.67 μm images.

We have found that R Mon is a 069 binary star with a companion that dereddens onto the classical T Tauri locus. Based on the near-infrared photometry of this companion we believe it is a 1.5 M_☉, very young (<3 × 10⁵ yr) classical T Tauri star. The close presence of a young companion suggests that R Mon itself is a rare example of a very young isolated massive star.

At the highest resolutions, R Mon is revealed to be extended by ~01 east-west, and ~005 north-south in the visible. The young R Mon star is not directly visible in the optical but appears as a resolved conical reflection nebula in scattered light. At infrared wavelengths, the dense circumstellar dust is penetrated and R Mon appears to be an unresolved point source located at 006 ± 002 south of the peak optical flux.

The large-scale optical-IR morphology of R Mon and its large reflection nebula (NGC 2261) suggests a thin bipolar parabolic shell of dust. The appearance of the parabolic shell is consistent with an inclination of 20° ± 10° from the plane of the sky. This inclination implies that R Mon is located 760^{+ 800}₋₂₈₀ pc distant based on previous proper-motion and radial velocity measurements of R Mon's jet. Our high-resolution (FWHM ~ 02) adaptive optics infrared polarimetry maps agree with the current interpretation that NGC 2261 is a reflection nebula illuminated by R Mon.

Interior to the parabolic shell there is a complex of twisted filaments along the eastern edge. These filaments resemble a double-helical structure which is well described by a power law from ~10³ to 10⁵ AU from R Mon. This double helix may trace a twisted magnetic field above R Mon.

Based on H I emission-line ratios, we find the direct extinction toward R Mon to be A_V = 13.1 mag in the infrared (λ > 1.28 μm), falling to a lower value of A_V = 3.6 mag in the optical (λ < 1.28 μm), where scattered light increasingly lowers the effective extinction in the line ratios. The large A_V = 13.1 extinction is likely due to the dusty atmosphere of an inclined R ~ 100 AU optically thick accretion disk surrounding R Mon. A simple model of such an accretion disk + star system (with M_acc ~ 8 × 10^-5 M_☉ yr^-1, M_* = 10.4 M_☉, R_* = 2 R_☉, and T_* ~ 3.5 × 10⁴ K) reproduces the observed dereddened R Mon spectral energy distribution (SED) from the optical (0.4 μm) to the millimeter region. Consideration of the lower extinction (A_V = 3.6) on the path followed by the scattered visible light eliminated any need for an inner "gap" in the accretion disk model to reproduce the SED.

In general, young stellar objects (YSOs) that are obscured in the optical but directly visible in the infrared will have different effective optical and infrared extinctions. Infrared extinctions derived from optical observations dominated by scattered light will be underestimates of the true IR extinction along the direct path. The use of an independent estimator of both the optical and infrared extinctions such as common upper-level H I recombination lines is highly desirable. The utilization of the correct optical and infrared extinctions may relieve the need for optically thin inner-disk gaps to explain YSO near-IR SEDs.