Rachel Baker
East Tennessee State University

REU program-Summer 2004
University of Wisconsin-Madison
Madison, WI 53706

Mentor: Matt Haffner

baker@astro.wisc.edu

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WHAM Multiwavelength Observations of the zeta Oph H II Region

Introduction

Tremendous progress over the past decade has been made in revolutionizing our understanding of the distribution and kinematics of the diffuse interstellar medium (ISM). Warm ionized hydrogen has been observed in nearly all directions of the Milky Way Galaxy including regions above the galactic plane and in the halo. Detailed structure of supernova remnants in the form of giant bubbles and super-shells as well as filaments of warm pervasive gas are seen far from classical H II regions. The Wisconsin H-alpha Mapper (WHAM) was the first spectral survey to map the spatial and kinematic structure of hydrogen alpha emission throughout the Galaxy (above declination -30 degrees).

Though much progress in view of the ISM has been made, the physical conditions and the source of the warm ionized gas that is seen throughout interstellar space is not understood very well. By studying particular regions of diffuse interstellar gas (DIG) as seen with WHAM and other H-alpha surveys, we can gain a better understanding of interstellar matter and the critical role it plays in star formation and galactic evolution.

Particular areas of interest are classical H II regions around hot O and early B stars. Current data reveals that the classical Stromgren Sphere argument (i.e, H II regions only exist around O stars), is in defect since WIM has been discovered in regions void of O (and B) stars. While O stars are believed to be the primary source of the permeated ionized gas, significant differences in temperature and ionization conditions have been observed in the diffuse WIM (e.g., Haffner, et al. 1999). From spectral data analysis, the physical properties of classical H II regions can be compared with other studies of the WIM to help determine the source of ionization and how interstellar H II is related to the ISM. We would like to know if their systems are closed as once thought or if the ionizing photons somehow escape their Stromgren Sphere and contribute to the WIM.

This research focuses on the emission nebula surrounding the well-known O9.5V star zeta Ophiuchi, a runaway main-sequence star that is rapidly moving. The zeta Oph H II region lies above the galactic mid-plane and can thus provide important clues to the nature of the WIM. Using WHAM observations of the red Balmer-alpha line (6563 Å) and the forbidden optical emission lines [S II] and [N II], wavelengths 6716 and 6583 Å, respectively, I am conducting a study of the physical state of the H II region surrounding zeta Oph, better known as S27. Recent images taken with WHAM and SHASSA reveal an extended 10 X 10 degree region of bright, red emission gas (see figure 1 and 2 below). The angular region that I will be surveying spans a 25 X 25 degree area. Thus, I will be probing into regions beyond the apparent Stromgren sphere to gain clues about the nature of the ionizing gas.




Fig. 1 . The Wisconsin H-alpha Mapper Northern Sky Survey (WHAM-NSS) total intensity view of the ionized gas in the Milky Way. The h-alpha is integrated over all velocities (ranging from V(LSR)=-80 to +80 km/s). The map is centered at galactic longitude, l=120 deg and latitude, b=0 deg. zeta Oph lies at l=6.28, b=+23.59, thus, the H II region, S27, is located to the far right directly above the visible galactic plane. It appears as a solid bright feature; to the right of S27 is another emission nebulae that looks more diffuse. Notice the fainter emission extending beyond the bright knot and filling most of the Galaxy; known as the Warm Ionized Medium (WIM), this gas has an effective temperature close to 10,000 K and a density of 0.1 cm -3.


Fig. 2 . The Southern H-alpha Sky Survey Atlas (SHASSA) close-up view of zeta Oph's H II region. The massive star (twenty times the mass of Sun and eight times the diameter) is embedded in a huge cloud of gas, which is ionized by the high temperature of the class O star.

Observations and Data

All observations were obtained with WHAM, a large-aperture (15-cm) dual etalon Fabry-Perot spectrometer. WHAM has high spectral resolution (12 km/s) and can detect emission as faint as 0.05 Rayleighs (1 R = 106/4*Pi photons*cm-2s-1sr-1), corresponding to an emission measure of 0.1 cm-6pc. With this unprecedented sensitivity, WHAM detects emission ten million times fainter than the Orion Nebula. WHAM beams are limited to a one-degree diameter view on the sky, but is more than compensated by its sensitive velocity resolution of 12 km/s over a 200 km/s spectral range. The WHAM instrument is stationed at Kitt Peak National Observatory, but is fully operational, remotely, from Wisconsin headquarters.

The h-alpha emission in the entire northern sky was mapped over a two year period from November 1996 until 1998, and the full set of data (37,565 individual spectra) from the WHAM survey is made available to download at www.astro.wisc.edu/wham. With WHAM's wide spectral window, any wavelength between 4800 Å and 7300 Å can be centered, which allows WHAM to detect emission from other fainter optical lines, such as [S II], [N II], H-beta, He I, [O I], and [O III]. In addition to an H-alpha map of the region l = -11 to 28 deg, b = +11 to +42 deg, we will be looking at an [S II] and [N II] map along our line of sight. Observations of the H-alpha spectra were mainly taken March thru May 1997. The [S II] data was obtained in mid-1999 and the [N II] data in May 2001. There are 873 individual observations in [S II] and 826 observations in [N II]. The exposure integration time was 30s for H-alpha and 60s for [S II] and [N II].

Several standard steps are taken to convert the WHAM observations into spectra. A singe observation of the line of interest is outputted from the WHAM CCD as an image of narrow concentric rings. The rings are converted to spectra by summing each ring azimuthally, where the longest wavelength of the emission corresponds to the center and the shortest wavelength corresponds to the outer most ring. There are approximately 100 rings (i.e. data points) per one-degree pointing in the sky. A more thorough description of WHAM data reduction procedures, including flat-fielding and bias subtraction can be found in Haffner, et al. 2003.

Most of my time this summer was spent producing intensity maps of the [S II] and [N II] data. A Gaussian fitting and subtraction procedure is used to extract the galactic emission from the spectra. Similar to H-alpha observations, the background continuum and fainter atmospheric lines had to be subtracted from each of the [S II] and [N II] spectra. The [S II] spectra contained additional contamination due to the scattered city light seen from Kitt Peak. As a result, a Gaussian component was fit to the narrow neon 6717 Å emission line and then removed from each of the [S II] spectra. Although the strong geocoronal H-alpha line is not seen in [S II] and [N II], atmospheric contamination due to other factors had to be removed. The method used to to remove the fainter atmospheric contamination and background continuum can be found in Haffner et al. 2003.

Since WHAM contains velocity information, each spectra had to be calibrated to obtain a stable zero-point with respect to the local standard of rest. For [S II], velocity calibration was accomplished using the terrestrial neon line measured in the band of interest and then compared to the rest wavelength measured in a lamp. Both showed considerable agreement. A thorium argon (Th -Ar) lamp was used as the wavelength calibrator for the [N II] observations in addition to the atmospheric template.

Semiautomated software was used to fit Gaussian components to the galactic and atmospheric emission present in each individual pointing. The process of fitting Gaussians to each spectra is sped up by the surveying strategies that are incorporated into WHAM. The observations are grouped into blocks consisting of 49 one-degree pointings on the sky which form tractable observational units. Once the [S II] and [N II] observations were processed, each block was averaged. The average spectrum was fit and then used to estimate the parameters for each of the components in the individual observations of a single block. Below we present samples of the cleaned spectra and the resulting intensity maps.

Results and Discussion

The figures below are total intensity maps of the region under discussion. The intensity is integrated over all velocities ranging from (+-) 100 km/s of the local standard of rest. The bright white region in the maps is the apparent 10 X 10 degree Stromgren Sphere shown in the picture above. The darker regions are areas of the WIM, which is much fainter than the S27 region. Each of the three maps look qualitatively similar with the brightest emission being near zeta Oph.

Fig. 3 . H-alpha intensity map of the ionized region surrounding zeta Oph as shown by the WHAM-NSS. Emission is integrated over all velocities ranging from V(LSR)=-100 km/s to +100 km/s.


Fig. 4 . [S II] intensity map of the same area integrated over all velocities ranging from V(LSR)=-100 km/s to +100 km/s.


Fig. 5 . [N II] intensity integrated over the same velocities. The region
surveyed for [N II] contains 826 one-degree pointings instead of 873.
Notice the missing "block" in the lower left region. Pointings taken with
WHAM are grouped into tractable observational units.

Below are samples of individual spectra in each of the three lines. The first one is centered on the star, and the second one is approximately five degrees out from the center of the H II region. The spectra are plotted as intensity versus LSR velocity. The total integrated intensity of the emission is also labeled in the upper left corner of each plot. This is the value that was used to produce the total-intensity maps shown above. Something that is apparent in these plots is that intensity of [S II] and [N II] increases relative to H-alpha for the spectrum that is centered off the star.


Fig. 6 . Sample spectra centered on zeta Oph. The solid line is H-alpha emission. The dotted line is [N II], and the dashed line is [S II].


Fig. 7 . Sample spectra centered five degrees out from the center of the H II region. The solid line is H-alpha emission. The dotted line is [N II], and the dashed line is [S II].

To study the physical conditions of S27, I looked only at bright spectra that were fit well by a single Gaussian. The subset contained 126 pointings and spanned a 12 X 12 degree region, corresponding to a diameter of about 30 parsecs. To investigate the ionization state and temperature of the gas I produced ratio maps that compared [S II] and [N II] intensities to that of H-alpha and [S II] to that of [N II].

Below are the resulting ratio maps. H-alpha intensity is mapped to show for comparison. The gradient is apparent in each of these maps. In H-alpha there is a decrease in intensity by a factor of twenty as you move out from the center of the H II region. The [S II]/H-alpha ratio is typically 0.1 and the [N II]/H-alpha ratio is typically 0.2 in the center of S27, and both ratios increase by a factor of two as you move towards the edge of the nebula. The [S II]/[N II] ratio map is fairly constant as shown by figure eleven.

These findings are similar to what is seen in the WIM and may be the effect of several factors, including changes in the ionization states of the elements and a change in the temperature of the gas. To investigate the variations in the line ratios, I plotted the widths (FWHM) of the individual spectra and pulled out the temperature and non-thermal velocities of the surrounding gas.


Fig. 8 . H-alpha emission extending a radius of
approximately 14.5 pc (2.5 pc beyond the estimated
radius of zeta Oph's H II region).



Fig. 9 . [S II] to H-alpha emission line ratio map


Fig. 10 . [N II] to H-alpha emission line ratio map


Fig. 11 . [S II] to [N II] emission line ratio map

Below is a plot of the widths (FWHM) of the [S II] components versus the widths (FWHM) of the corresponding H-alpha components. It can be concluded from the figure that the [S II] line widths are correlated with the H-alpha line widths, and the [S II] width is much smaller than the corresponding H-alpha width (due to the heavier mass of [S II]). The correlation between the [S II] and H-alpha line widths, along with other factors found in the analyses of these data, such as the correlation between the [S II] and H-alpha line intensities, the close correspondence between the LSR velocities of the two lines, and the similarity in line profiles provide strong evidence that the two lines are produced in the same region of the diffuse nebula. If the ions in the H II region are well mixed and have approximately the same temperature then each pair of line widths is a measure of the temperatures and non-thermal velocities of the gas (Reynolds, 1985). Using two established equations, I was able to decompose the total line widths into their thermal and non-thermal components. A grid of these values is over plotted in figure twelve below.


Fig. 12 . Plot of widths (FWHM) of [S II] and over plot of temperatures and non-thermal velocities that were extracted from the widths

From the grid lines you can see that most of the data points lie between 7000 and 8000 K and between 4 and 8 km/s. The scatter that can be seen in the plot above is real, and is due to spatial correspondence within the nebula. Below are maps of the temperature and non-thermal velocities throughout the entire nebula. This is the first time ever that distribution maps of both these physical parameters have been produced throughout an entire diffuse nebula. The temperature map varies from about 6000 K at the center to about 8000 K at the rim of the H II region. In the non-thermal velocity map, there is a decrease by a factor of about two as you move radially from the center of S27. A variation like this could be due to a mild expansion within the nebula.


Fig. 13 . Temperature map throughout the entire diffuse nebula.


Fig. 14 . Non-thermal velocity map throughout entire diffuse nebula.

Conclusions

We've obtained multiwavelength spectral observations of the H II region, S27. From these spectra, we have produced velocity resolved maps. The line ratio maps that compare [S II] and [N II] to that of H-alpha reveal an increasing trend that is dependent on radius from the center of the H II region. Also for the first time ever we've produced a distribution map of the temperature and non-thermal velocities throughout an entire diffuse nebula.

Future Work

I am currently continuing the research on the zeta Oph H II region as my Senior Honors Thesis. I will continue the work with WHAM researches remotely from my local school at East Tennessee State University. My faculty thesis advisers for this project are Bev Smith, Mark Giroux and Matt Haffner. Some of the work that I am focusing on now includes estimating the Lyman Continuum and density of the region, as well as getting another temperature estimate using the [N II]/H-alpha ratios and comparing them to what I got with the line width decompositions.

I will be presenting the results of my summer research project at the 205th AAS meeting in San Diego in January 2005.

References

  • Haffner, L. M., Reynolds, R. J., & Tufte, S. L. 1999, ApJ, 523, 223 Full Text

  • Haffner, L. M., Reynolds, R. J., Tufte, S. L., Madsen, G.J., Jaehnig, K.P., & Percival, J.W. 2003, ApJS, 149, 405 Full Text

  • Reynolds, R.J. 1985, ApJ, 294, 256