Prof. Dr. Philipp Richter
On the importance of UV spectroscopy
for our understanding of the Universe
Over the last century, our general view on the structure and composition of the Universe has changed dramatically, not least because of the advent of powerful observatories that give access to the electromagnetic spectrum beyond the optical regime. Among the various spectral ranges, the ultraviolet (UV) range stands out in modern astrophysics because of it extraordinary relevance to study the abundance and distribution of atoms and molecules in large range of astrophysical environments by way of spectroscopic methods. Most of the electronic transitions in atoms and molecules are located in the UV between 90 and 300 nanometers, resulting in a very high information density of spectral features that can be used to explore planets, stars, gaseous nebulae, galaxies, active galactic nuclei, intergalactic matter, and cosmological scales at very high precision.
The extraordinary importance of previous UV spectroscopic missions, such as Copernicus, IUE, ORFEUS, FUSE, and the various UV instruments installed on the Hubble Space Telescope for our understanding of the Universe and its components is evident. UV observations require space-based observatories, as the earth’s atmosphere is opaque for radiation with wavelengths <300 nm. However, despite the importance of the UV range for modern astrophysics, there are no plans at ESA or NASA for future UV spectroscopic observatories after HST (with its powerful UV spectrograph COS) will have ceased operating around 2020. Its successor, JWST, is designed to primarily observe highly redshifted objects in the early Universe at infrared wavelengths and thus has no UV capabilities. In the following, the importance of UV observations for our understanding of the Universe is sketched for various important astrophysical environments.
Circumstellar disks & Planets
Finding and studying planets beyond the solar system (i.e., exoplanets) has become one of the most thrilling fields in modern astrophysics. The extended atmospheres of exoplanets can be probed during planetary transits by the method of transition spectroscopy, when the stellar radiation is transmitted through the exoplanet’s atmosphere. In this context, the use of the UV range is crucial for our understanding of exoplanets, as the UV band provide unique access to the important upper part of planetary atmospheres, the so-called exosphere. The low density gas of the exosphere absorbs in the resonance lines of many elements at different ionisation states. UV measurements thus provide fundamental information on the chemical composition and physical conditions in the atmospheres of distant planets. Moreover, UV spectroscopy is highly desired to supplement future infrared observations of molecules at planet-forming radii around stars.
Studying the properties of stars in the Milky Way and beyond still represents one of the most fundamental tasks in modern astrophysics. Clearly, the UV domain is of prime importance in stellar physics, as it covers the region in which the intrinsic spectral energy distributions of hot stars peak. In addition, UV lines are the least influenced by non-local thermodynamical equilibrium effects in stellar photospheres. They thus are most useful, e.g., for quantitative determinations of chemical abundances in stars. Another advantage of observing stars in the UV is the extreme sensitivity of the Planck function to the presence of small amounts of hot gas in dominantly cool environments. This allows the detection and monitoring of various phenomena that would otherwise be difficult to observe, such as accretion continua in young stars, magnetic activity, chromospheric heating, coronae, plages and faculae on cool stars, and intrinsically faint, but hot, companions of cool stars.
Massive stars are important agents driving feedback and energetic processes within galaxies. Massive stars drive extended stellar winds over most of the stellar lifetime, but there are large uncertainties concerning the nature of these winds (amount of mass-loss, clumping properties, outflow geometry). The best diagnostics of these outflows, which are also crucial for our understanding of stellar evolution, are provided by UV resonance lines that form over large distances inside the stellar winds.
Indeed, UV spectra provide fundamental information about the wind physics, in particular at the interface between the stellar surface and the wind base, and the complex mechanisms that cause such large-scale wind structures.
Interstellar medium and
the galactic matter cycle
The interstellar medium (ISM) represents diffuse gaseous matter between stars. The ISM contains free ions, atoms and molecules and hosts the material from which stars do form. Absorption spectroscopy in the UV range gives access to a huge number of transitions from neutral and ionized atoms and molecules and thus is a key method the explore the physical conditions in the ISM, it relation to stars and their energetic environment, the distribution of heavy elements that are produced in stars, and the properties of its host galaxy. Among the many fascinating aspects of the ISM, which is a key ingredient of Milky Way-type galaxies, the transformation from gas into stars is a particularly important process that basically alone has governed the overall evolution and shape of baryonic (ordinary) matter in the Universe from the first clumping of gaseous matter shortly after the Big Bang until today. Only UV observations give access to the absorption lines of diffuse molecular hydrogen, whose formation represents the “bottleneck” for the build-up of dense molecular clouds in which stars are forming.
It nowadays believed that most galaxies host a super-massive black hole (SMBH) in their center. Many billion years ago, these SMBHs possibly represented the seed for the continuous formation of the galaxies as we observe them today. SMBHs accrete gaseous material from their accretions disks and transform gravitational energy into radiation that spans a huge range in energies. Despite their small linear size, such active galactic nuclei (AGN) belong to the most energetic and dramatic environments in the Universe and their radiation and outflowing material influence the properties of individual galaxies as well as the physical conditions of baryonic matter on large scales far outside of galaxies. Because of the high energies, UV observations of AGN are crucial for our knowledge of the innermost regions of galaxies and their evolution through cosmic timescales.
While only the inner regions of galaxies contain interstellar gas at densities sufficiently high to allow star formation to occur, the outer regions of galaxies nevertheless are filled with diffuse gaseous matter that represents the so-called circumgalactic medium (CGM). The CGM is fed by outflowing gas that is ejected by supernovae explosions, stellar winds, active galactic nuclei as well as by material that is infalling from the intergalactic medium. Only UV observations give access to the important UV transitions of heavy elements that can be used to trace the matter cycle on circumgalactic scales, the above mentioned large-scale gas circulation processes, and their role for the formation and evolution of galaxies.
Intergalactic medium and large-scale structure
Most of the gas in-between galaxies, a gas phase that is commonly referred to as intergalactic medium (IGM), is very diffuse and highly ionized. However, because the IGM fills the entire Universe as part of cosmological filaments, it carries the majority of the baryonic matter in the Universe. Studying atomic transitions in the UV from highly-ionized metals in IGM is crucial to explore the mass and distribution of diffuse matter outside of galaxies in a cosmological context and obtain insight into the distribution of matter on the very largest scales in the Universe.
In summary, observations in the UV spectral range give access to many astrophysical phenomena that cannot be studied by other means. With the dismount of HST in 2020 the astrophysical community needs a new vision for UV science to continue the successful observations of the energetic Universe at small and large scales.
A new UV spectroscopic mission that includes state-to-the art optical and electronic components and that enables us to observe even faint objects at a relatively high spectral resolution holds out the prospect to continue the legacy of previous UV instruments and to push forward our understanding of atomic and molecular process in a large range of astrophysical environments.
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