Molecular filaments in the Crab Nebula

2026-04-16T00:00:00Z

Organisateur : Philippe Salome (LERMA)

The Crab Nebula (SN1054) is a very famous and well studied Supernova Remnant with a
33ms pulsar at a distance of about 2 kpc (see Hester 2008, ARAA for a review). Some SN
remnants like IC 443, Cygnus Loop have been detected in CO emission lines (Scoville et al.,
1977, Dickman et al., 1992) or CTB87 (Cho et al., 1994). The well known cases of molecular
emission associated with SNRs are examples where a rapidly moving shell strikes
surrounding interstellar matter. The emission is formed in a shock-heated interstellar cloud.
In contrast, the Crab is a rare example of a young SNR which is not yet interacting with the
ISM. The molecules and dust observed within the Crab have been produced during, or after,
the supernova explosion, and so probe the physics of the end stages of stellar evolution and
processes within an environment permeated by relativistic particles and illuminated by an
exceptionally hard SED. The Crab is a uniquely clean case to study these processes. The type
II supernova explosions are thought to enrich the ISM in heavy elements produced by
nucleosynthesis in the stellar interiors. The SN ejecta are seen in expansion around the central
Crab pulsar with velocity up to 1000 km/s.
The crab nebula (Messier 1) is known for its bright optical emission line filaments. The nature
of those filaments is not well known. They are very likely mostly made of elements ejected
from the star and poorly mixed with the surrounding ISM. The optical spectra show low
ionisation emission lines that are different from PDR lines. The line emission of the filaments
material could be explained by cosmic rays rather than shocks excitation. These kind of
excitation processes, visible in the optical spectra are very similar to what we see inside
filaments around cooling flow clusters of galaxies like Perseus. The presence of molecular gas
in cluster centers is due to the filaments : a small fraction of this gas (probably the surface)
being excited and visible in the IR and optical. The Crab nebula is an exceptional laboratory
to study the microphysics that governs the line excitation in extreme environments like the
cooling flow filaments (Ferland et al., 2009).
So this kind of study is very important to probe the scenarios of dust and molecules formation
in very hostile environments. A deep study of this nearby laboratory would certainly help to
understand how the cold gas can survive extreme conditions (like it also happens in high
redshift dusty galaxies). Spitzer (Temim et al. 2006) pointed out the presence of small
amounts of warm dust (70 K) inside the Crab filaments (0.01 Msun). Following the work by
Graham et al, 1990, our group has found molecular gas, detected in H2 ro-vibrationnal
emission lines in two pointed regions (Loh et al 2010, 2011, 2012). We also have started a
mm-submm follow-up of these molecular filaments with the IRAM 30m-telescope and PdB
interferometer (Salomé et al, 2012 in prep). Our plan for the proposed meeting is to discuss
these new results, face-to-face all together (for the first time since the beginning of the
collaboration). We also intend to discuss an ALMA proposal (dead line in July 2012) to map
the regions where the molecular gas is associated with optical filaments. This is the next step
of a comprehensive project (multiwavelength observations and modelling), which aim is to
determine what dominates the filaments excitation (shocks/cosmic rays).

Centre de Conférences Jules Janssen

Molecular filaments in the Crab Nebula