Nebulae and Interstellar Medium

This category actually contains a number of related, and yet very different, programs to understand the interstellar medium (ISM), the tenuous regions of gas and dust between the stars.

Programs H10, H11, and G14 are closely related programs of observe shock waves in a range of astrophysical situations. The bulk of programs H10 and G14 deal with FUV observations of selected filaments in older galactic supernova remnants (SNRs) to understand the physics of shock waves and their interaction with the ISM. (Look here for an image.) Certain kinds of shock waves have been found that emit very little optical light but can be very bright at UV wavelengths. These so- called non-radiative shocks are of great interest because they are thought to occur right at the interface between the primary blast wave and the ISM. Only one such shock was observed on Astro-1. HUT scientists hope to observe several other shocks with higher velocity on Astro-2 to understand how the characteristics of these shock waves change with the speed of the shock wave.

HUT will also be used to observe optically-bright filaments in several SNRs to better understand what happens when a shock wave encounters "density enhancements" (sometimes called "clouds") in the ISM. Observations of the Cygnus Loop, Vela, and Puppis A SNRs and the objects N49 and N63a in the nearby galaxy called the Large Magellanic Cloud will study this phenomenon in detail. The observations will seek to understand the range of shock velocities present, the density of the material, and whether or not the shock waves are able to destroy the grains of interstellar dust they encounter. This affects the perceived abundances that are derived for interstellar gas.

Some young SNRs are being observed for a totally different reason. (Any age less than about 1000 - 2000 years is considered "young" for a SNR.) When a star explodes as a supernova, it tears the aging star apart and sends the star's outer layers expanding off into the surrounding space at velocities of thousands of kilometers per second. Until this material (called "ejecta") encounters sufficient interstellar material to slow it down, it maintains the chemical composition of the precursor star. Hence, by observing this material spectroscopically, scientists can investigate the chemical abundances of the material processed by the star during its lifetime. Understanding these processes is an important facet of astrophysics; since the Big Bang is thought to have produced mainly hydrogen and helium, nearly all of the heavier elements in the Universe, including the material in the earth and all of its inhabitants, have been generated by nuclear fusion in the cores of stars. This process is known as nucleosynthesis.

Ultraviolet observations are important in addition to optical and other observations because some important elements (such as carbon and silicon for example) only have strong spectral features in the UV. Other elements (such as oxygen and nitrogen) show spectral features arising from higher energy ionization states than are available in the optical. Hence, UV spectra provide unique information about these fascinating objects.

One particular observation is made possible by a coincidental alignment of a young SNR with an appropriate background object. A young SNR, called SN 1006 (because the supernova was observed in 1006 A.D. by Chinese astronomers), is seen today as an expanding circular bubble of X-ray and radio emission, with only a few very faint optical emission filaments. Seen in projection near its center is a hot, so-called subdwarf star (which is an evolved type of star that is burning helium in its core instead of hydrogen). This star was originally thought to be associated with the SNR, but today we know that it lies beyond the SNR; hence, to reach us the star's light must shine directly through the middle of the young remnant. The star is very faint at optical wavelengths, but because its temperature is about 38,000 K its spectrum gets brighter at UV wavelengths. This is the only known example of a hot star so closely aligned with a young SNR. This star is sometimes called the Schweizer-Middleditch star (or S-M star for short) after the two astronomers who discovered it.

Observations with the IUE satellite and with HST have detected a number of absorption lines in the spectrum of the S-M star that are believed to arise due to the expanding supernova ejecta. Of particular interest are some broad absorption lines of once-ionized iron (designated Fe II). Iron is predicted to be the end product of nucleosynthesis in stars, and supernovae of Type 1a (such as SN 1006 was purported to be) should produce up to half a solar mass of iron! This iron should be ejected at high velocities in the supernova explosion. Iron does not appear to be over-abundant in X-ray analyses of SN 1006, meaning it can't be hot. No strong neutral iron lines (Fe I) are detected in the optical spectrum, so the iron can't be completely cold. The amount of iron inferred from the observations of Fe II absorption in HST spectra is significant, but falls at least a factor of 20 below expectations. Where is the rest of the iron?

An obvious possible answer is that there is "warm" iron, more highly ionized than Fe II, but not heated to X-ray temperatures. A key absorption line of twice-ionized iron (Fe III) is expected at 1123 in the far-UV spectrum. This line is at too short a wavelength to be observed with Hubble, but is directly in HUT's prime wavelength range. Hence, a HUT observation of the S-M star will either go a long way toward settling the mystery of the missing iron or deepen the mystery even further. The star is faint, but the absorption should be strong if it is present near the expected level, making the HUT observation feasible.

An interesting aside: Astronomers would like to be able to use type 1a supernovae as cosmological "standard candles" because they are luminous and visible at great distances. However, it is difficult to trust the predictions that all type 1a supernovae have the same intrinsic luminosity (and hence can be used to judge relative distances) if one of the other basic predictions of the models is incorrect. Hence, confirming that the models of type 1a supernovae are correct in their predictions about iron is of more than passing interest.

Program H11 deals with Herbig-Haro objects, which are dense clumps of material that are heated by shock waves in regions near newly forming stars. Because the conditions are very different than seen in supernova remnants, observations of HH objects test a whole different astrophysical regime. Unfortunately, these objects are often found in dusty regions, making them difficult objects to observe in the UV (since dust absorbs and scatters UV light very effectively). The brightest HH objects are not well placed for night observation in March, and only one such object is in the premission observation timeline.

In program H12, HUT scientists are attempting to understand the interactions between young stars and the cocoons of dust and gas that still surround them. Much of this gas is in the form of molecular hydrogen, which has important transitions in the prime HUT wavelength range. While molecular hydrogen is also observable in the infrared spectrum, the UV lines arise from the dominant "ground- state connected" transitions of the molecule and the observations can be interpreted in a more straightforward manner. Observations of the nebulae will permit a better understanding of the effects of the stars on their surroundings, while observations of the stars in the nebulae will provide information on the characteristics of the over-lying interstellar dust, which absorbs some of the star's radiation.

Finally, program H08 investigates a very special portion of the ISM, the halo of our Galaxy. The galactic halo is a tenuous region of gas that extends both above and below the plane of our Milky Way galaxy. Supernovae and stellar winds from regions of star formation are thought to drive material out of the plane of the galaxy and into the halo. This material subsequently should cool down, and fall back onto the galactic plane in a process called the "galactic fountain." One of the main methods of probing the conditions in the galactic halo is to measure it's absorption of light coming from more distant sources. HUT offers the opportunity to observe absorption by five-times ionized oxygen (O VI), if it is present in a sufficient amount, which arises in hotter gas than any other UV line available for study.

During Astro-1, scientists were able to obtain data of sufficient quality along only one line-of-sight through the halo to constrain the presence of O VI. While the anticipated absorption was apparently detected, there are reasons to believe that the line-of-sight studied may be peculiar or unrepresentative of the halo as a whole. With HUT's increased sensitivity for Astro-2, astronomers hope to study several additional directions through the halo to confirm and/or expand the Astro-1 result. Interestingly, this program is done "piggy back," using observations primarily motivated by other scientific programs.

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