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Home page of Floris van der TakSubmillimeter Scientist in theLow Energy Astrophysics division of theNetherlands Institute for Space Research (SRON)
Research InterestsPhysics of the interstellar medium and star formation
Stars with masses greater than 8 solar masses spend a significant fraction of their lifetime, at least 10%, embedded in molecular clouds. 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 dish observations at far-infrared and (sub)millimeter wavelengths, I derive the global temperature, density and velocity structure of proto-cluster envelopes. Then I use millimeter-wave interferometers 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, but studying larger source samples will have to await the sensitivity of future telescopes, in particular ALMA. AstrochemistryStudies 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: see this press release. The initial conditions of low-mass star formation
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. Very 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 (see press release). Follow-up studies are underway, using the CSO, IRAM 30m, Effelsberg and Arecibo telescopes. Molecular spectroscopy and radiative transferThe 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 an extensive benchmark campaign which resulted from a workshop at the Lorentz Center in Leiden in May 1999. 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. The next 5-10 years will bring a strong interest in interstellar H2O, triggered by the HIFI instrument onboard ESA's Herschel space observatory. Anticipating this trend, I am modeling the line spectrum of H2O for protostellar objects as well as normal galaxies and starbursts. Such models are vital to planning the observations as they reveal which lines are sensitive to which physical and chemical parameters. Observations of H2O isotopes with the IRAM 30m telescope and Plateau de Bure interferometer are underway to test these models. In March 2004, I organized another workshop at the Lorentz Center with the world experts on interstellar H2O. The aims were to develop benchmark tests and explore the diagnostic power of the H2O spectrum. A related effort is my modeling of atmospheric H2O emission, as part of the design of the 3-channel 183 GHz radiometer which is used to monitor atmospheric transmission at the APEX telescopes. Such a system offers higher sensitivity and higher time resolution than continuum skydips which older telescopes use. Moreover, the data refer to the same direction as the astronomical observations. |
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The
stars that we see in the night sky have not always been there,
and will not remain forever. The formation of stars and planets
is one of the central questions in modern astrophysics, and one
of the major drivers of the evolution of galaxies. Observational
and theoretical work in the 1990's has led to a fairly clear and
detailed picture of the formation of isolated low-mass stars. At
the same time, however, it has become clear that the majority of
stars forms in a clustered environment, whereas high-mass stars
exclusively form at the centers of clusters. High-mass stars
enrich and perturb their surroundings by synthesizing heavy
elements and by their strong ultraviolet radiation and mass loss.
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 is particularly
important for the study of starburst galaxies and for the
formation of the first stars, that re-ionized the Universe.
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.