Molecular filaments in the Crab Nebula

Organisateur :

Date prévue : 27 juin - 06 juillet 2012
Date définitive : 2012-06-27

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).