Interstellar medium

Paul P. van der Werf

Principal collaborators: Miller Goss, Paul Vanden Bout, Lloyd Higgs, Peter Dewdney, Amiel Sternberg, Jürgen Stutzki

Overview:

The interstellar medium (ISM) of our Galaxy is key to understanding the lifecycle of stars and gas in the universe. Stars form from molecular clouds, but provide radiative and mechanical feedback on these clouds that makes the ISM a life, dynamical entity. The radiative feedback from massive stars on molecular clouds is observed directly in socalled Photon Dominated Regions (PDRs), where the stellar ultraviolet (UV) radiation heats the ambient gas, affects its thermal balance and chemistry, and partly dissociates the H₂ gas. The resulting atomic hydrogen (HI) can be observed directly as HI zones around HII regions. Molecular clouds without associated HII regions also contain atomic hydrogen, which is observed as HI in dark clouds.

Highlights:

The Orion bar PDR: The Orion bar is a perfect example of an edge-on Photon Dominated Region or PDR, offering, at its distance of approximately 450 pc, excellent spatial resolution of its stratified structure. The bar is an edge-on ionization front at the South-East edge of the Orion Nebula (M42). The neutral gas outside the bar reveals a number of successive layers with increasing distance from the exciting stars (see image below). Closest to the ionization front, the emission of photo-excited polycyclic aromatic hydrocarbons (PAHs) peaks. Somewhat further from the ionization front, vibrationally excited H₂ is found, which radiates through fluorescence following the absorption of ultraviolet photons. Still farther from the ionization front, the emission from CO peaks.

The stratified structure in the Orion bar PDR is shown by the different locations of PAH emission at 3.3 µm (blue), H2 vibrational line emission (green) and CO emission (red). The exciting stars are towards the North-West.

The stellar wind shell HII region BG2107+49: The HII region BG2107+49 is remarkable for its morphology: a moderately compact source combined with a more diffuse curved tail or ring segment and an additional patch of diffuse emission. In order to study the origin of the remarkable shape of this complex, we observed the region in the HI 21 cm line with the DRAO synthesis telescope. A movie of the HI datacube, shows at the velocity of the HII region a remarkable, almost circular ring structure with a radius of 75 pc. This structure is seen better in the picture below.

Grayscale image of the HI 21 cm emission at the systemic velocity of BG2107+49, with contours of the 21 cm (1400 MHz) continuum overlaid. These data were obtained with the DRAO synthesis telescope and are complete in all spatial frequencies, and therefore give a reliable representation of even the most extended emission.

The ring seen in HI, ionized on the inside, is interpreted as a stellar wind shell. The radio continuum requires the presence of an O4 star inside the ring. Since Galactic extinction in this region is very large, this star cannot actually be seen. Careful inspection of the HI datacube shows that the ring is expanding at a velocity of 24 km/s. Together with its size, this gives an age for the ring of about 2 million years (which is shorter than the lifetime of an O4 star). The more compact "head" component is possibly a second generation HII region, resulting from star formation triggered by the passing stellar wind shock. Its age of 3.6×10⁵ years is consistent with this interpretation. The BG2107+49 complex thus presents a unique view of the combined effects of ionizing radiation, stellar winds and several generations of massive stars.

HI in the molecular cloud L134: In molecular clouds, most the available hydrogen is in the form of H₂. Yet, trace abundances of atomic hydrogen (HI) exist and in fact in typical molecular clouds HI is the most abundant species after H₂ and He. The presence of this HI component results from cosmic-ray initiated chemistry, and/or incomplete HI-H₂ conversion. The HI in molecular clouds can be observed in the 21 cm line, and the HI is either observed in absorption towards the warmer HI background, if the HI is cold, or in emission, if it is warm. An excellent case study is presented by the dark cloud L134, where cold HI from the cloud core is observed in absorption, while the warmer edge of the externally heated cloud is seen in emission, as seen in the channel map below.

HI channel map of the dark cloud L134. Note the absorption signal in the dark cloud core, and the emission signal from the cloud envelope. The cold hydrogen in the cloud core is seen in absorption towards the warmer general Galactic background HI. However, since the latter is smooth at this high Galactic latitude, it is not detected by the interferometer, so that the absorption signal dips below zero.

These results show that the cloud is externally heated by the interstellar radiation field. In the opaque cloud core, where photodissociation does not play a role, HI can only be produced by cosmic rays. The derived HI abundance in this region is however much higher than can be explained by cosmic rays alone. This probably points to a time-dependent effect: conversion of HI to H₂ is simply not complete yet.

Principal publications:

1. High resolution HI observations of dark clouds: I. L134
   Van der Werf, Paul P., Goss, W.M., & Vanden Bout, P.A.
   A&A, 201, 311 (1988)
   [ ADS entry ]
2. An HI study of HII regions and dark clouds
   Van der Werf, Paul P.
   Ph.D. thesis, University of Groningen, The Netherlands (1989)
   [ ADS entry | ]
3. High resolution HI observations of HII regions: I. Orion A
   Van der Werf, Paul P., & Goss, W.M.
   A&A, 224, 209 (1989)
   [ ADS entry ]
4. Radio and infrared observations of the HII complex BG2107+49
   Van der Werf, Paul P., & Higgs, L.A.
   A&A, 235, 407 (1990)
   [ ADS entry ]
5. Sub-parsec size cloudlets associated with Orion A
   Van der Werf, Paul P., & Goss, W.M.
   ApJ, 364, 157 (1990)
   [ ADS entry ]
6. Anatomy of the photodissociation region in the Orion bar
   Tielens, A.G.G.M., Meixner, M.M., Van der Werf, Paul P., Bregman, J., Tauber, J.A., Stutzki, J., & Rank, D.
   Sci, 262, 86 (1993)
   [ ADS entry ]
7. Structure and chemistry of the Orion bar photon-dominated region
   Van der Werf, Paul P., Stutzki, J., Sternberg, A., & Krabbe, A.
   A&A, 313, 633 (1996)
   [ ADS entry ]

See also:

Extragalactic interstellar medium

Galactic centre

Interstellar medium links