Horsehead NeBULA

 

Located in the Orion constellation, the Horsehead nebula is one of the most

famous and easily-recognizable shapes in the sky. The nebula, also known as

Barnard 33, was first discovered by Williamina Fleming in the late 1800s on

a photographic plate taken at the Harvard College Observatory. At visible

wavelengths, it appears as a 5-arcminute dark patch against the bright

Halpha emission from the HII region IC 434. At mid-infrared and radio

wavelengths, on the other hand, the nebula is bright due to dust and

molecular emission.















Fig 1: Maps of the Horsehead nebula in different tracers:  Halpha (courtesy of

Reipurth & Bally), 7.7 mum aromatic continuum (Abergel et al.  2003),

9.1--11.8 km/s C18O J=2-1 emission and 1.2 mm dust continuum (Hily-Blant et

al. 2005), 850 µm and 450 µm dust continuum (Courtesy of G.Sandell,

published by Ward-Thompson et al. 2006). Values of contour levels are shown

on each image lookup table.



The closest pillar to Earth

Named for its optical appearance, the shape of the nebula at radio

wavelengths is indeed more reminiscent of a seahorse (see top middle panel

of Fig. 1). A curved rim including the nose, ridge and mane of the horse

is located at the western end of the nebula (i.e., the top of the

head). This ridge is connected to the eastern L1630 molecular cloud by a

filament, the horse's neck. While optically thick tracers like the 12CO

lines show a low density halo surrounding the neck, optically thin tracers

like the dust emission reveal the presence of two dense condensations: the

first one associated with the western ridge, called B33-MM1, and the second

one in the middle of the neck, called B33-MM2.


Two massive stars are located at the same projected distance of the

Horsehead (0.5deg), the O9.6Ib star zetaOri north of the nebula and the

O9.5V star sigmaOri west of the nebula. However, Hipparcos estimates of

their distance (Perryman et al. 1997) indicate that zetaOri

(250±50 pc) is located further away from the Horsehead nebula than

sigmaOri (352±113 pc). Hence, the far UV illumination primarily

comes from sigmaOri. This incident radiation field shaped the molecular

cloud into the famous Horsehead.


A prototypical, low illumination Photon-Dominated Region


High density gas and far-UV stellar light interact at the illuminated

edge of the western condensation, forming a Photon-Dominated Region

(PDR) where the physics and chemistry is driven by the far-UV

radiation. PDRs play an important role in astrophysics as they are

found everywhere in the interstellar medium, e.g. in diffuse gas, star

forming regions, protoplanetary disks, and circumstellar envelopes

around evolved stars.  Moreover, PDRs dominate the IR and sub-mm

spectra in external galaxies.  Understanding PDRs thus sheds light on

these objects. Therefore, numerous models have been developed to study

the different physical and chemical processes involved in PDRs since

the early 1970's. This requires good modeling of many microscopic

processes (UV radiative transfer, heating, cooling, turbulent mixing,

gas phase and grain surface chemistry) with very different

timescales. It also relies on many different microphysical parameters

(e.g., chemical rates, adsorption and desorption coefficients, etc...)

calculated/measured by several theoretical and experimental

groups. However, the difficulty of this last effort implies that only

a few reactions (among the thousands used in chemical networks) can be

thoroughly studied.


In view of the intrinsic complexity of building reliable chemical

networks and models, there is an obvious need of well-constrained

observations that can serve as basic references. PDRs are particularly

well suited to serve as references because they make the link between

diffuse and dark clouds, thus enabling to probe a large variety of

physical and chemical processes. In this context, the Horsehead is

particularly interesting because it has the most simple geometry we

can hope to find in the interstellar medium, i.e., edge-on.