Introduction and Latest Results from the Laval Wide Field
Survey of Old and Peculiar HII Regions

Steve Godbout, Gilles Joncas and, Laurent Drissen

Département de physique
Université Laval & Observatoire astronomique du mont-Mégantic
Electronic-mail: godbout@phy.ulaval.ca, joncas@phy.ulaval.ca, ldrissen@phy.ulaval.ca

Abstract:

The mysteries of the interaction of young stars with their environment are still far from being fully understood. But, with a fair amount of younger H II regions having been studied, our attention now turns to older and larger H II regions in the hopes of solving a few more riddles. The latter will be observed to establish their kinematical structure when almost no molecular material is left to produce photodissociated flows. We will then compare the kinematical behavior of old H II regions in M33 with that of such Galactic H II regions. The Fabry-Perot H$\alpha$ data cubes of M33 were obtained at the CFHT. For both data sets, a comparison of the neutral and ionized phases of the gas will be made. This, we hope, will help reveal if there is some kind of energy dumping between the phases of interstellar gas and if so, on what scales it takes place.

Introduction

  Up to now, our research has been centered on the study of younger H II regions ($< 1 \times 10^6$ years) such as Sh142, Sh158, Sh170, Sh212, Sh269, M42 and M17, to name a few. Three of these were observed at the CFHT. Kinematical information on these was obtained through the use of Fabry-Perot spectroscopy, usually centered on H$\alpha$ ($\lambda$6562.82 Å) but in two cases (Sh269 and M42) on the [SII] doublet ($\lambda\lambda$6716, 6731 Å), and in another (Sh212) centered on [OIII] ($\lambda$5007 Å). Since every element of resolution where emission is strong enough yields a spectrum to which a gaussian function can be fitted, detailed information can be gathered on radial velocity, radial velocity dispersion and also flux (if necessary) for the study.

With these studies, we have been able to test the assertions made by the Champagne model (Tenorio-Tagle 1979) and found a very good agreement between theoretical large scale velocity fields and observed ones. Only one study permitted determination of the density field structure and there, an agreement could not be found (Godbout et al. 1997). This is perhaps due to the rather high lower limit ($\sim\ 100\ cm^{-3}$) imposed by electron density determination through the [SII] doublet flux ratio.

Small scale velocity analysis was also done within these studies and we find remarkable agreement from one H II region to the next. Using statistical tools, such as the two point autocorrelation functions, small scale velocity fluctuations were scrutinized to reveal the presence of turbulence within these objects. From these statistical tools, a maximum size for turbulent cells has been inferred and is found to be, in every instance, of the order of $\sim$ 0.1 pc. Such an undertaking might be part of this newer project in the future.

Moving on...

  Having extensively studied younger H II regions, this project now focuses on larger and older ones and the associated local neutral gas. As this work goes forth, we will be observing HII regions where the ionizing star, or star association, has almost completely dissociated and ionized the gas from the parental molecular cloud. Regions where neutral gas observed by the CGPS is peculiar, either morphologically or by unusual association with ionized nebulae, will also be studied. The first observations were done on the W4 H II region, where a galactic chimney has been discovered by the CGPS pilot project (Normandeau, Taylor & Dewdney 1996). We also have in hand data from M33's central and southern arm regions which we will return to in §2.1. HI data on M33 will be provided by D. Thilker of the University of New Mexico. For a more detailed description of the project in general and an overview if its association with the Canadian Galactic Plane Survey (CGPS), we refer the reader to Godbout, Joncas and Drissen (1998).

With these data in hand, it will be interesting to find out what happens to the large and small scale flows (and associated turbulence) without the presence of the pressure gradient between cold melecular and ionized material. We may also ask ourselves whether or not the neutral gas seen on the edges of old H II regions retains an imprint of it former kinematical behavior. All of these interrogations will lead to a more fundamental one: Is there an energy exchange between older/larger H II regions and the colder, neutral interstellar medium in our Galaxy and if so, on what scales?

The CFHT data

  We cannot limit ourselves to H II regions within our own Galaxy. Because of their low surface brightness, there are few observable old H II regions in the northern Milky Way. To increase the number of objects and also build an extragalactic database for comparison, we will use H$\alpha$Fabry-Perot data cubes of the southern and central parts of M33. These cubes were obtained in October of 1992 at the CFHT. That data has already been through the process of reduction and is now being analyzed. Velocity fields for the oldest H II regions will be extracted with precautions taken to avoid giant extragalactic H II regions. With this data, not only can we study the kinematics of H II regions but an in-depth study can be made of Wolf-Rayet stars, supernova remnants and basically, of anything which is a strong emitter in H$\alpha$ and resolvable by the CFHT. To give the reader a preview of what can be extracted from such data cubes, we present in Figures 1 and 2 the H$\alpha$ velocity fields of the central and southern arm regions of M33.

 

Figure 1:   Velocity field of the central region of M33. The field is 7' 45'' across (2.6 kpc). North is up and east is to the left. The measured velocity range is between -220 km s^-1 (bluer shades) and -140 km s^-1 (redder shades).

 

Figure 2:   Velocity field of the southern arm region of M33. The field is 7' 45'' across (2.6 kpc). North is up and east is to the left. The measured velocity range is between -160 km s^-1 (bluer shades) and -120 km s^-1 (redder shades).