mountain profile Institute for Astronomy University of Hawaii

Stars and interstellar matter

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 The Sun is one of about 200 billion stars in the Milky Way galaxy. Between the stars are vast clouds of interstellar dust and gas -- the material out of which new stars are made. The conversion of interstellar matter into stars is one of the most fundamental topics in modern astronomy, not only for what it tells us about the birth of the Solar System, but because so many of the large-scale properties of galaxies are the direct result of the star-formation process

Star Formation

Image of HL Tau by UH adaptive optics groupThe earliest stages of star formation take place deep within molecular clouds, from which little or no light emerges. Infrared radiation passes much more freely through interstellar clouds than does light, so several astronomers at UH, including Klaus Hodapp, John Rayner and Alan Tokunaga use infrared telescopes to study the star formation processes. Broadly, the goals of these programs are to understand the sequence of events that leads from an interstellar cloud via a protostar to a mature star. The evolution of a single object through these stages takes millions of years, so astronomers need to collect data from different objects at different ages and try to place them in the correct sequence. Some - perhaps all - newly formed stars go through a turbulent adolescence in which they eject streams of gas that collide with the surrounding molecular cloud material. Klaus Hodapp uses infrared cameras to detect the 2.2 micron emission from hydrogen molecules that are excited by these collisions, while Alan Tokunaga uses infrared cameras and spectrographs to study the faintest members of dark clouds. Some of these objects may be substellar-- brown dwarfs or massive planets.

The dust and molecules in these star forming clouds produce strong emission at sub-millimeter wavelengths. The high altitude of Mauna Kea makes observations at these frequencies feasible despite significant atmospheric absorption and the mountain hosts three sub-millimeter telescopes (CSO, JCMT, and SMA) including the only interferometer in the world to operate at these wavelengths. Jonathan Williams uses these telescopes to study molecular clouds so as to learn about how they form stars, and also to observe disks around young stars in order to learn about the processes of planet formation.

After a few million years, new stars start to emerge from the molecular clouds in which they were born. George Herbig is engaged in studies of very faint very young stars in several Galactic clusters that are still partially embedded in the dense molecular clouds from which they formed a few million years ago. These stars can be detected spectroscopically from their characteristic signature of a bright H-alpha spectral line, which is believed to be produced either in an expanding wind or a deep chromosphere. These new detections reveal, among other things, the relative numbers of stars of different masses formed in a molecular cloud.

Bo Reipurth leads the Center for Star and Planet Formation at the Institute for Astronomy, a group of faculty, postdocs and students who share a common interest in the origins of stars and planets. He studies the highly collimated Herbig-Haro jets that emerge from newborn stars. These HH jets plow through the ambient medium, thus helping to excavate cavities around the young stars and make them emerge from their placental gas and dust clouds. HH flows can attain gigantic proportions, stretching over tens of lightyears. They consist of luminous shocks, each of which represent an explosive event in the newborn star. HH jets can thus be read as a fossil record of activity of young stars.

Young star clusters

LkHalpha 198Young galactic clusters and stellar associations provide ideal locations for star formation research because they contain large numbers of coeval stars evolving under similar conditions and with identical chemical compositions. By using young clusters of different ages as snapshots in time, George Herbig and graduate student Scott Dahm are able to study large populations of solar-like stars as they emerge from their parental molecular clouds as T Tauri stars and ultimately evolve into Zero-Age Main Sequence stars (ZAMS). These young, low-mass stars often exhibit hydrogen line emission and enhanced X-ray luminosities resulting from accretion processes and chromospheric activity. Excess infrared emission suggests the presence of circumstellar dust and gas which is thought to dissipate over the first ten million years of a star's lifetime. During this critical period, it is believed that planet formation occurs in solar-like stars. By studying these young stellar populations we are given insight into the early history of the Sun and our own solar system.

Brown Dwarfs and Extrasolar Planets

AuMicMichael Liu  focuses on understanding the nature and origin of substellar objects, i.e. brown dwarfs and extrasolar planets. The last decade has witnessed a revolution in astronomy with the discovery of these long-sought objects. Observations of dusty disks around nearby young stars have helped to illuminate how planets form. Infrared studies of brown dwarfs, objects too low in mass to steadily produce their own energy, have revealed the properties of very low-temperature objects, found both free-floating in the solar neighborhood and as companions to other nearby stars. Finally, direct detection and characterization of extrasolar planets is becoming possible through the use of high-contrast adaptive optics imaging on the largest ground-based telescopes.

Jeffrey Kuhn is using new techniques for extending the dynamic range of existing Mauna Kea telescopes (like UKIRT) to look for evidence of dust or planetary systems around nearby bright stars.

Hot Massive Stars

ngc3621.jpg (344616 bytes)Rolf Kudritzki  and Fabio Bresolin study hot massive stars. Such stars, which are so luminous that they are easily detected in distantg alaxies, have enormous stellar winds which provide the surrounding interstellar medium with mechanical energy and momentum and recycledn uclear burned material. Theory predicts a tight relationship between the momentum rate of the wind and the luminosity of the star,called the Wind momentum - Luminosity Relation (WLR). A second relationship, the Flux-weighted gravity - Luminosity Relationship(FGLR) is also predicted between the fundamental stellar parameters (gravity and temperature) and luminosity.

Observations and diagnostics of the winds from hot stars, performed in our own Milky Way and Local Group galaxies, have confirmed these theoretical predictions. Drs Kudritzki and Bresolin, together with their colleagues, are presently carrying out a vigorous observing project (which includes observations of additional distance indicators such as Cepheids, RR Lyrae stars, the Tip of the Red Giant Branch and planetary nebulae) to calibrate the WLR and FGLR as function of spectral type and metallicity. After the calibration phase is finished, these relationships can be used as new and independent primary distance indicators, allowing for the measurement of extragalactic distances out to the Virgo and Fornax galaxy clusters, thus helping to further constrain the Hubble constant. Hot stars are also a good way of studying chemical compositions in distant galaxies (H, He, Mg, Si, C, N, O, Fe, Ti, Cr, Ba etc.), providing a unique way of understanding the evolution of spirals and the first step in this direction beyond our Milky Way.

Extragalactic HII regions

NGC 300Fabio Bresolin investigates the massive stellar populations embedded in giant extragalactic HII regions with optical and near-IR spectroscopy. The direct spectral signatures of massive objects, such as the Wolf-Rayet stars, together with emission-line diagnostics from the ionized gas, help us to contrain the stellar mass function, especially in high-metallicity environments. Information on the nebular abundances of elements like oxygen, sulphur and nitrogen in spiral and dwarf irregular galaxies can be derived and compared with abundance determinations from blue supergiants. Such data are a necessary observational ingredient for the modeling of galactic
chemical evolution.

 

 

Planetary Nebulae

Ant Nebula - HSTRoberto Mendez and Rolf Kudritzki study planetary nebulae in our Galaxy and in other galaxies. Planetary nebulae (PNs) are a brief phase (a few tens of thousands of years) in the late evolution of stars with initial masses below about 8 or 10 solar masses, immediately before they run out of nuclear fuel and become white dwarfs. Our Sun will probably produce a PN, but we will have to wait some 5 billion years to witness this.

The PN evolutionary phase is characterized by severe mass loss; the dying star has a dense core (which is about to become a white dwarf) and a low-density envelope around the core. The envelope is progressively lost in the surrounding space. At the beginning of this process we call such stars "asymptotic giant branch stars". They are red giants of enormous size and low surface temperature. As the envelope is lost, the surface temperature of the remaining core increases until the core becomes so hot that it emits most of its radiation in the far ultraviolet. This UV radiation is able to ionize the hydrogen atoms in the ejected envelope, which shines in visible light by fluorescence. Thus the envelope, which now we call a PN, becomes much brighter, in visible light, than its central star. The name "PN" is used for historical reasons. The ejected envelope does not form planets; on the contrary, it may cause substantial damage to any preexisting planets around the former red giant star.

PNs can be counted among the most easily detectable individual objects in any galaxy. They have emission-line spectra, easy to recognize even if the PN is very faint. Mendez and Kudritzki search for PNs in elliptical galaxies at distances up to 30 Mpc from us. These PNs are very useful because they can provide an accurate measurement of the distance to the galaxy where they are found; and their radial velocities can be used to study the angular momentum distribution and to test for the existence and distribution of dark matter in their galaxies. In the case of the nearest elliptical galaxies (closer than 15 Mpc) it may be possible also to get some information about the chemical abundances of the PNs, which may provide important clues to test current ideas about elliptical galaxy formation.

Element Abundances in stars

Ann Boesgaardand her graduate students use a high-resolution spectrographs on the Keck telescope to measure the concentrations of particular elements in the atmospheres of stars. The three elements lithium, beryllium and boron are particularly interesting in that they are formed by the action of cosmic rays in the interstellar medium rather than by nuclear reactions in the centers of stars. Since these elements are destroyed by nuclear reactions inside stars at the relatively cool temperatures of a few million Kelvins, their abundances in the atmospheres of stars can provide us with valuable information about what is going on far below the stellar surface. For example, Dr Boesgaard has found evidence for rotationally-induced mixing -- motions deep inside the star that are presumably caused by shear flow instabilities below the convection zone -- in stars warmer than the Sun.

Another useful element is oxygen, whose abundance in stars of different masses and ages can tell us much about the history of our Galaxy. Boesgaard's studies with the Keck telescope have found oxygen enhancements in stars high above the galactic plane, a result which indicates that there must have been many high-mass stars formed when the Galaxy was young.

Other clues to the chemical history of the Galaxy are to be found the faint, unevolved stars in globular clusters. These faint stars reveal the original composition of the gas that they were made of billions of years ago. Among the elements that have been studied are lithium, sodium, iron, nickel, chromium, magnesium, calcium, silicon, titanium, yttrium and barium. Of these, only lithium shows star-to-star variations: factors of 4 - a challenge for the Big Bang nucleosynthesis theories

Infrared Stellar Classification

Stars are usually classified by the types of absorption lines found in their visible spectra; the relative strength of different absorption lines provide clues to a star's temperature, surface gravity, luminosity and mass. Some stars, however, are hidden in dust clouds so thick that they can be seen only by the infrared radiation they emit. John Rayner is methodically using the IRTF to build a reference collection of infrared spectra of stars that can be used to identify them when they are obscured. They have observed several hundred stars of all spectral types between 0.8 and 2.4 microns, and a smaller number of them out to 5 microns. Some of these reference stars are also being observed at ultraviolet wavelengths using the Hubble Space Telescope. The combined spectra make a unique tool for modelling composite stellar systems and for testing and improving atmosphere models over a huge wavelength range at all relevant metallicities, temperatures and luminosities.

Rayner and Cushing are also collecting and studying infrared spectra of faint stars which are not obscured, but which have temperatures below about 2000 K. Such objects, which are so cool that almost of their power is emitted at infrared wavelengths, include the Brown Dwarfs -- objects which have too little mass to support nuclear fusion in their cores, but which are too big to be a planet.

 

Colliding Stars

ßStars are so far apart that they do not collide often. The rare collisions that do occur may result in some exotic astronomical objects. Joshua Barnes has been throwing virtual stars at each other using a computer and examining the results in the form of spectacular movies. 

 

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