The stars that we see in the night sky have not always been there, and will not remain forever. Galaxies like the Milky Way contain not just stars, but also clouds of gas and dust where new stars are born. Astronomers would like to know how exactly this happens. For example, how many stars are born with planets, and why do not all stars have the same mass? In normal galaxies like our own Milky Way, most stars have about the mass of the Sun (1 M0 = 2 x 1033 g), and only ~1% of the stars is more massive than 10 M0. Despite this rarity, high-mass stars have a strong influence on their surroundings. They are very bright, and their ultraviolet radiation dissociates and ionizes atoms and molecules in nearby gas clouds. Their interiors are hot enough for the synthesis of heavy elements such as iron, which is key to making DNA. They also have strong stellar winds which blow holes and bubbles in the surrounding gas. And at the ends of their lives, they explode as supernovae which shock and perturb gas clouds out to large distances.
This strong environmental impact makes unraveling the origin of high-mass stars essential for understanding the formation of planets and of life on one hand, and for the evolution of galaxies on the other. High-mass star formation also important for our understanding of the early Universe, when the fraction of high-mass stars was much larger than it is today. Stars like the Sun form by accretion of gas and dust from their parent clouds via a circumstellar disk. When this accretion ends, planets may form out of the remnant disk. The formation of high-mass stars may proceed in a similar way, and the difference in stellar mass may be due to a larger accretion rate or a higher temperature in the parent cloud. Alternatively, newly-formed low-mass stars may merge to form young high-mass stars, as suggested by the observation that high-mass stars are always found in the centers of young star clusters. Understanding high-mass star formation requires big telescopes because the objects are rare and lie on average at large distances from the Earth, and requires telescopes for long-wavelength radiation (infrared and radio waves) which penetrates even the densest and darkest clouds of gas and dust.
Stars with masses greater than about 10 solar masses spend a significant fraction of their lifetime, 10-20%, hidden in interstellar clouds of gas and dust. My work aims to characterize this so-called "embedded" phase of high-mass star formation, to understand what type of cloud produces what type of stars. From single-antenna observations (for example with JCMT and APEX) at far-infrared and (sub)millimeter wavelengths, I derive the global temperature, density and velocity structure of the environments of young high-mass stars. Then I use millimeter-wave interferometers (such as IRAM and SMA) to zoom in on the accretion disks and bipolar outflows of individual stars. I also use mid-infrared images to determine the luminosities and temperatures of the stars. A case study of an intermediate-mass object, combining single-dish and interferometry data, has revealed a massive circumstellar disk, as well as several companion stars which drive their own bipolar outflows. Such a mix of properties from high-mass and low-mass objects may well be common, and studying larger source samples is now becoming possible with the ALMA telescope.
Studies of star formation use observations of cold dust and molecular gas, the interpretation of which requires understanding of chemical processes. My favourite astrochemical laboratories are the envelopes of embedded massive stars, whose large masses and high temperatures create a rich line spectrum, allowing to measure many trace species. The chemistry of these regions also depends on their temperature history, which is useful to order sources chronologically.
My chemical studies focus on `families' of molecules, and identify tracers of chemical processes, such as adsorption on dust grains, ice evaporation, high-temperature reactions, and grain sputtering in shocks. Of particular interest is the ionization fraction of molecular clouds, which controls the influence of magnetic fields on their dynamics, and sets the chemical time scale. My estimates of the cosmic-ray ionization rate combine H3+ absorption lines in the mid-infrared with submillimeter HCO+ and H3O+ line emission and with dust continuum maps. The main conclusion from this work is that the cosmic-ray ionization rate varies significantly within the Galaxy. Part of the reason is that the cosmic-ray flux increases by a factor of 10 when going from the solar neighbourhood to the galactic center, but there is also a propagation effect: cosmic-ray particles appear to penetrate more deeply into diffuse clouds than in dense clouds.
In certain external galaxies with active nuclei, the ionization rate is even higher than in the center of our Galaxy!
The study of low mass star formation has led to a clear and detailed picture of the stages where an embedded star has formed, thanks to a flurry of recent observations. The front line of this field lies now at the `pre-stellar cores', which are the initial conditions of star formation. Although not the major mode of star formation, pre-stellar cores are ideal laboratories for the earliest stages of low-mass star formation, because of their isolated location and near-spherical shape. The temperatures and densities of these systems can be probed by dust continuum observations, but kinematic information only comes from molecular lines. However, several recent studies have shown that `standard' kinematical probes for more advanced stages (e.g., CO) fail to trace pre-stellar cores, because they freeze out onto dust grains.
The only molecule that does not suffer from this depletion effect is H2D+, and my work in the field of low-mass star formation has centered around this special molecule. I have been involved in the first detections of H2D+ and in the breakthrough observation of strong H2D+ emission of pre-stellar cores. Follow-up observations of a larger source sample and comparison with models of kinematics are underway.
Recently, astrochemists have found that under extremely high densities and low temperatures, molecules with several H atoms substituted with D are produced in detectable amounts. Such observations are important to characterize the physical conditions before star formation, and to explore the extremes of interstellar chemistry. The current `deuterium record' is our detection of interstellar triply deuterated ammonia, set in 2002 and unsurpassed to this day!
The dense and cool material in the interstellar medium of galaxies can only be probed by observations of dust continuum and molecular lines. The proper interpretation of such observations depends on the availability of radiative transfer tools and of basic molecular data. The program by Michiel Hogerheijde and myself for two-dimensional molecular radiative transfer, based on the Monte Carlo method, is a state-of-the-art tool to analyze observations. Being publicly available over the Internet, the program is used by people all over the world to model observations of interstellar molecules. The code has been part of several benchmark campaigns which resulted from workshops at the Lorentz Center in Leiden.
The Monte Carlo code is useful if extensive, detailed observations of a source are available, but this is not always the case. For the analysis of more limited data sets, I have developed a simpler program based on the escape probability method. This program is very fast and has only a few input parameters, which makes it very suitable for extensive searches of parameter space and for the analysis of large source samples.
The results of radiative transfer models depend critically on the input molecular data. I am critically reviewing spectroscopic and collisional parameters of molecules. The results of these reviews are collected in a data base which is accessible over the Internet. Like the Monte Carlo code, this database plays an important role in the analysis of data from submillimeter telescopes.