N.R. Trams

Disappearance of stellar debris disks around main-sequence stars after 400 million years

Almost 5 billion years ago, the Sun formed in a local contraction of a cloud of molecular gas. A rotating disk of gas and dust is believed to have fed material onto the proto-Sun for the first few million years of its life, and to have formed the planets, comets and other Solar System objects. Similar disks, but with less mass, have been observed around a few main-sequence stars such as Vega. The dust particles orbiting stars like Vega will be removed on timescales of the order of 1 Myr (Vega is about 350 Myr old), and therefore must be resupplied, at least for a time. But earlier surveys lacked the sensitivity to determine how many nearby stars have dust disks, and to investigate how long such disks survive. Here we report infrared observations indicating that most stars younger than 300 Myr have dust disks, while most older than 400 Myr do not: ninety per cent of the disks disappear when the star is between 300 and 400 Myr old. Several events that are related to the ‘clean up’ of debris in the early history of our Solar System have a similar timescale.

The Moon and Extra-Solar Planets

We have used ISO, the Infrared Space Observatory, to perform a systematic search for protoplanetary discs around nearby main-sequence stars. We find a strong correlation with stellar age: more than half of the stars younger than 400 Myr do have such a disc, whereas less than 10% of the older stars exhibit disc emission. Such discs seem thus to decay on a timescale of a few hundred Myr. Both the stability of the discs during this period and their decay can be explained by collisions of planetesimals. Such collisions produce the dust which is necessary to replenish the disc. The process stops as soon as the planetesimals run out, either because they are all destroyed, have escaped, or merged into planets, which implies the decay of the disc. The timescale for the dissipation of protoplanetary discs is remarkably similar to the timescale of the heavy bombardment in our Solar System. The cratering of the Moon is a clear signature of the heavy bombardment which may provide further fundamental clues to the formation of planetary systems.

Evolution and Mass Loss of AGB Stars in the Small Magellanic Cloud

To study the final stages of the evolution of stars in the range of 1–8M ⊙ we have started a project on AGB stars in the Small Magellanic Cloud (SMC). Based on the IRAS Faint Source Catalog we have selected 29 objects with IRAS 12 and 25 micron colours indicative of red AGB stars. Ground-based near-infrared (NIR) JHK images were obtained of these fields. With ISO1 (CAM and PHT) a follow-up study is being made to get the spectral energy distribution (SED) between 3.6 and 60 microns of the AGB stars detected by IRAS. We are analysing the SEDs using a radiative transfer model which provides accurate determinations of the luminosities and mass loss rates of the objects. Combining the data on the SMC, LMC and existing data on the Galaxy will answer the open question of the metallicity dependence of mass loss in late-type stars. This in turn is important in the ejection of matter by AGB stars to the interstellar medium over the age of the Galaxy.

Dust in LBV Nebulae

The upper part of the HR diagram is populated by massive, luminous stars that show strong stellar winds. These stellar winds have a profound effect on the evolution of the most massive stars, e.g. they lead to the formation of He-rich Wolf-Rayet (WR) stars. It was shown by Humphreys & Davidson (1979) that no red supergiants (RSG) exist with luminosity above Mbol ≈ -9.7, while there are many blue supergiants with luminosities that exceed this limit. This lack of luminous RSG can be explained if very massive stars go through a short-lived phase of extensive post-main-sequence mass loss, during which a significant fraction of the H-rich envelope is removed. The Luminous Blue Variables (LBVs) may represent this short-lived phase of very high mass loss. It has been suggested that a very brief (~ 103 yrs) phase as yellow or red supergiant precedes the LBV phase (Stothers & Chin 1996). For recent reviews see Luminous Blue Variables: massive stars in transition, eds. A. Nota & H.J.G.L.M. Lamers.

Infrared observations of high-mass X-ray binaries

We present the first results of our ISO program on High-Mass X-ray Binaries (HMXBs). Infrared photometry, obtained at different aspect angles of the systems, is used to investigate the massive star’s disrupted stellar wind and the accretion flow towards the compact X-ray source. Scenarios for massive binary evolution predict that HMXBs receive a large kick velocity during the supernova explosion of the compact star’s progenitor. One might, therefore, expect to observe wind bow shocks around these systems such as observed around many OB-runaway stars. Such a wind bow shock has recently been discovered around the system Vela X-1.