Search for Dissociating Stars

CGPS Science Project
Search for Dissociating Stars

People involved

Peter Dewdney:
Box 248
Penticton BC V2A 6K3
Mark Heyer:
University of Massachusetts
Lederle Research Tower
Amherst, MA 01003
United States
Charles Kerton:
Box 248
Penticton BC V2A 6K3
Bill McCutcheon:
Department of Physics and Astronomy
5224 Agricultural Road
Vancouver, British Columbia V6T 1Z1
Magdalen Normandeau:
Astronomy Department
601 Campbell Hall
University of California
Berkeley CA 94720-3411
United States
Chris Purton:
Box 248
Penticton BC V2A 6K3
Rob Roger:
Box 248
Penticton BC V2A 6K3
Brad Wallace:
60 Keawe Street, RM 204
Hilo, Hawai'i 96720

Outline of the project

Dissociating stars are those late enough in the spectral sequence that the UV flux is insufficient to ionize appreciable amounts of HI, but sufficient to dissociate molecular hydrogen. They should be recognized as

We will use the DRAO, FCRAO and IRAS surveys in an attempt to discover more dissociating stars, and to study their frequency of occurence.

Suggested approaches and results

Using IRAS colours as a starting point (Magdalen & Brad)

The prototype dissociating star, IRAS 23545+6508 (Dewdney et al 1991), is a strong infrared source with IRAS colours falling within the Hughes & MacLoed (1989) criteria for HII regions. It had no continuum counterpart on the DRAO images and most importantly it had an associated smudge of HI emission. It should be noted however that the possible PDR in the Dragon's Breath (see below) does not fit their 89% criteria, its S12/S25 is too high. These sources would both be excluded from the 77% confidence lists because of confusion flags at more than one wavelength (other than that, the Dragon's Breath source would fit the 77% criteria).

In 1989, Hughes & MacLoed suggested the following criteria for identifying HII regions:

S12/S25 =< 0.3981,
S25/S60 =< 0.5623,
S100 > 80 Jy,
where an upper limit (fqual =1) is acceptable for the 12 micron flux but not at the other wavelengths (fqual = 2 or 3).

They estimated the confidence level of HII region selection using these criteria to be 89%. They also proposed a less stringent set of criteria which had an estimated selection confidence level of 77%. Considering only sources within 10° of the Galactic plane and for which there was a confusion flag for at most one wavelength, the criteria are:

fqual >= 1,2,2,1 at 12, 25, 60, and 100 microns,
S100 >= 50 Jy for fqual = 2 or 3,
S100 >= 150 Jy for fqual = 1,
S12/S25 =< 1.0000
S25/S60 =< 1.0000
S60/S100 =< 0.6310

A possible approach is to look at the position of all qualifying IRAS compact sources. There should be no 1420 MHz radio continuum to speak of and a knot in the HI and CO images in order to qualify as a dissociating star candidate.

Investigations along this line have begun but have yielded somewhat disappointing results. If you want to see the details and the numbers, go ahead and click here.

Using a catalogue of B type stars as a starting point

Brad has suggested this approach, in great part because we don't know if the colour selection criteria which are valid for HII regions are also valid for dissociating stars. The idea would be to try to find PDRs around some isolated B stars and try to establish criteria from there.

Individual objects

Candidate in the Dragon's Breath (Chris)

Brad Wallace pointed out the existence of a possible dissocating star at the apex of the so-called Dragon's Breath in the CGPS A fields. In his words: "it appears that the Dragon's Breath has a PDR in it. Nice unresolved HI peak, molecular material, IR source and even an optically visible star, all in the same place." (29June98)

The Dragon's Breath: HI in green, CO in blue and IR in red.
Image courtesy Brad Wallace.

His interests in this area lie elsewhere so he passed on the information to the dissociating star team. Bill, Chris and Rob are looking into it.

Chris has volunteered to put together a poster about this beasty for the 1999 CGPS meeting: "Dissociation Region the Two".

Brad is working on other aspects of this area. Please do not let this pretty picture inspire you to go off and work on something other than the possible PDR without first consulting Brad.

Follow-up observations

No follow-up observations have been carried out yet. The suggestion of using BIMA via Magdalen has been retracted. Getting time on the array would mean having to be telescope operator for another two weeks. This is not a fate to wish even on one's worst enemy.


Dewdney, P.E., Roger, R.S., Purton, C.R., McCutcheon, W.H. 1991, ApJ 370, 243.
Hughes, V.A., MacLoed, G.C. 1989, AJ 97, 786

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