MY RESEARCH PROJECT: the [SII], [NII], and H-beta regions of Spica

Introduction

The University of Wisconsin-Madison's H-Alpha Mapper (WHAM) Sky Survey of Galactic Ionized Hydrogen began taking data in 1996, allowing astrophysicists to view the distribution and kinematics of the warm ionized medium for the first time. With its deep, velocity-resolved maps of the warm ionized medium (WIM), WHAM has allowed the first clear detection of H-alpha emission from high and intermediate velocity clouds in our galaxy. The detector itself uses large-aperture Fabry-Perot detection techniques, including two etalons and a 0.6 m lens. The resolution is limited by the one-degree beam of the device, but this is more than compensated for by the extreme sensitivity of the results. WHAM also has velocity resolution, a unique attribute in this field.

The first results, published in 1999, investigated the global physical properties of the WIM in the galaxy, finding that the abundance of H-alpha depends greatly on the distance from the galactic plane, possibly due to the change in the galactic temperature (the WIM itself varies from ~6,000-10,000 K).
Fig. 1 . This is an intensity-map of the H-alpha region around Spica. You can see the intensity greatly increases near the star (the bright region to the right).

Ionized regions are created around the hottest of stars by the interaction of released photons with nearby gasses. Until the late 1960's, it was believed that the great majority ionized gas was constrained around these hot, typically O-type stars, and thus constrained by their Stromgren Spheres. With pulsar research, however, a delay in signal times allowed researchers to infer that the pulses were traveling through a refractive medium - the interstellar medium - along a line of sight that included no such ionized nebula; from the delay, they were able to infer that the WIM exists in regions that contain no hot stars! Additionally, since O-type stars are almost always restricted to the galactic plane, they cannot possibly account for the large amounts of WIM discovered in the halo of the galaxy. There are a few O-type stars traveling across the galaxy at high speeds -- perhaps as a result of ejection from a binary system -- but the small number of these unusual stars that have been observed could not possibly account for the massive amounts of WIM that extends in all directions throughout the galaxy. If the Warm Interstellar Medium exists in great abundance large distances from any hot stars, the question arises of how it got there. Was it created by these stars and then somehow released from their Stromgren Spheres, or is there another mechanism at work? It is this question, along with many, many others, that WHAM seeks to answer.

WHAM is the deepest survey of the WIM ever undertaken. By knowing the location and density of the WIM throughout the galaxy, scientists can now begin to unravel the mystery of ionized gasses that exist far from any hot stars. We can now search for the causes of ionization, the mechanism behind the WIM's motion, and so forth. Possible explanations for the ionization currently include supernova blasts, the photon radiation from stars, and cosmic rays (which, although not plentiful enough to fully explain the WIM, are energetic enough to ionize hydrogen).

Much more information about WHAM can be found at http://www.astro.wisc.edu/wham/description.html

What I'm doing right now

While H-alpha has been mapped across the entire sky, other regions such as [NII], [SII], and H-beta have yet to be mapped. During the summer of 2003, i mapped these three gasses in the vicinity of Spica, a B-type star within the constellation of Virgo. In the past, B-type stars were thought of as not luminous enough to ionize nearby gasses. However, the WHAM survey generated initial indications to the contrary, finding [HII] regions around several B-type stars including Spica. This is important not only because it increases the number of stars that can produce WIM, thus explaining at least in part its abundance, but also because B-type stars are not restricted to the plane of the galaxy like O-type stars, explaining some of the WIM that lies outside the plane of the galaxy. It was my hope to conclude whether or not the ionization created by B-type stars could possibly escape the stars' Stromgren spheres and contribute to the HII regions in teh WIM. With this in mind, I created maps of the [NII], [SII], and H-beta around Spica to determine where exactly the HII region exists and if it contributes at all to the WIM. This research helped us to understand how B stars ionize, how similar this is to O-type stars, and whether B-type stars could be a significant contributor to the WIM.
Fig. 2 . This is an intensity-map of the SII region around Spica, the same region shown in figure 1.

In case you're totally confused, a short explaination of the terminology used on this page follows.

[SII],[NII]: These refer to the spectral lines corresponding to singly-ionized Sulfur and Nitrogen, respectively. Neutral Sulfer and Nitrogen would be abbreviated as SI and NI. The brackets around the lines indicate that they are forbidden lines -- not that it's impossible to observe these lines, but rather it's impossible to create them from Earth. In space, where the density is much, much lower than on Earth, if an atom is excited by a collision, it won't necessarily collide with another atom for quite a long time -- long enough for it to emit the extra energy gained in the collison via a photon. On Earth, the atom would certainly collide with another atom long before this photon would be emitted. Additionally, because the energy of this photon released in space was created in a collision, the phton has an unusual angular momentum, which allows it to travel all the way to our detector on Earth without being reabsorbed. This makes forbidden lines especially good indicators of physical conditions -- just what we want to know.

H-Alpha: This is the atomic transition from three to two in the Balmer series. As the electron drops energy states, a photon is emitted. This is what we observe.

H-Beta: This is the atomic transition from four to two in the Balmer series.

HII: This is the bright region around a star in which UV radiation emitted from the star ionizes the atoms nearby. So what you end up with is a sphere of ionized gas centered around the hot star. This sphere is also referred to as the Stromgren sphere.

Fabry Perot Spectrometer: This is the type of instrument we use to gather data. It consists of two mobile mirrors that reflect the photons into a 0.6 meter lens, which in turn focuses the light onto a third mirror. The photons then travel through two Fabry Perot etalons. These are two slabs of thick glass with a narrow gap between them. These both have highly reflective surfaces (as close to perfect mirrors as possible). If the gap is at just the right thickness, only certain wavelengths of light are able to make it through the glass to a CCD camera below. And what's more, they can only make it through at certain angles. So what we end up with is a circular image (an example is available in my powerpoint presentation), with each "ring" corresponding to a particular wavelength of light. During analysis, we can simply integrate azimuthally to recover a specific wavelenth: one data point. Thus the largest ring in our image corresponds to the smallest wavelength and the innermost circle corresponds to the largest. We can then graph the number of photons at each wavelength (intensity) against velocity (ascertainable from wavelength via the Doppler Shift formula).

Below is my final mapping of the [NII] region near and around Spica. Compare this to the mappings of H-Alpha and [SII] above. Nii_Intensity_Spica

Fig. 3 . This is the finished intensity-map of the [NII] region around Spica.

The dramatic conclusions of my research project

From the maps above, you can see the bright Stromgren sphere of Spica towards the right. In the [NII] and [SII] maps, you can also see Spica itself -- this is the single red pixel in the midst of the Stromgren sphere. One unexpected feature is the arm or bar extending above and to the left of this sphere. We're still unsure of what's going on here. The brightness indicates that the gas here is ionized. Could it be that this too is part of the Stromgren sphere and that the dark stream separating it from the rest of the sphere is due to an obstruction within our line of sight? Or perhaps could it be that there is no gas in this region, leaving nothing for the photons to ionize? This would allow the photons to continue on their merry way through this region until they reach a clump of gas on the other side -- the bright arm we see.

Unfortunately, we have very few quantitative results so far. But by knowing the line widths of the H-alpha and [SII] emission lines, we have been able to determine the temperature of the gas closest to Spica: between 8-9,000 K. This seems reasonable when compared to WIM estimates of 6-10,000 K. We're still working on estimating the density and volume of the ionized region. But there was one surprise in our analysis...

If you look at the above map around a longitude of 338 and a latitude of 47, you will see three pixels that are much brighter than the others. We've tentatively identified this [NII] region as HD 125924, another B-type star approximately 2.5 kpc away. For comparison, Spica lies about 80 pc from Earth. HD 125924 is a very strange star in that it is hurtling through the galaxy at the huge velocity of 250 km/s TOWARDS the plane (downward, in the map above). Lone stars traveling through the galaxy were generally either blown out of a binary system when the companion star went supernova or gravitationally ejected by a large group of bound stars. Yet, because the great majority of stars in our galaxy exist within the plane, nearly all "runaway" stars are traveling away from the plane of the galaxy. How did HD 125924 come to travel at such a high speed toward the plane? We don't know. But we do know that it's very spiffy, especially because it appears to be creating a bowshock due to its high speed. This means that the ram pressure of the free-streaming stellar wind has surpassed that of the interstellar wind (similar to a sonic boom), spewing ionization (photons) into the WIM in front of the star. We believe this to be the case because HD 125924 is moving into the [NII] region, not surrounded by it as is usually the case. On the map above, HD 125924 exists just above the [NII] region we see, pushing forward (downward) with such a force that it ionizes the WIM in front of it, creating the [NII] region you're looking at. All of this happens before the star even reaches the region where the [NII] is building! Although we have not done nearly enough research on this star to draw any conclusions, it is a very promising mechanism by which HII could get into the WIM. Although this type of star is probably not numerous enough to be able to explain the HII in the halo, it could contribute. By understanding the mechanism behind this star's HII region, we could possibly advance our understanding of the HII in the WIM in general.

Useful links

The following links proved extremely helpful either in the creation of this webpage or in understanding the science behind it.

Celestial background thanks to artie.com

Basic info about Spica

Check out Washburn Observatory if you're in Madison