In my previous post on the Zooniverse Project IX I’m involved in, I talked about the importance of star formation in the Universe and some of the difficulties we face in studying it. Some big unanswered question particularly remain in our understanding of how massive stars form. Fittingly, the latest edition of Nature has a paper on a nice result in the study of massive star formation: a detection by direct imaging of an accretion disk around a massive young star.
Massive stars form in far smaller numbers than the regular run-of-the-mill stars, like our Sun. They also have far shorter lifetimes, exploding in cataclysmic supernovae after just millions, rather than billions, of years. Another difference between high and low mass stars lies in the sequence of events in their earliest years: low mass stars stop accreting new material once the stars have “turned on”, and the radiation from the new star blows away the surrounding envelope. High mass stars, however, manage somehow to continue accreting new material even after they’ve started emitting the ionising radiation that will slowly dissipate their natal clouds.
How large amounts of gas can find their way onto the star despite the outward force of the emitted radiation is tough to understand. A paper by Krumholz et al (2009) showed via modelling how non-spherical accretion scenarios, like filaments or disks, could play a role in channeling the accretion flow into the star. Accretion disks are ubiquitous around young low-mass stars, but around massive forming stars they had, despite ample circumstantial evidence, never been directly imaged.
In their Nature paper, Stefan Kraus of UMichigan at Ann Arbor and collaborators from Germany, France and Italy, describe how they combined observations at different wavelengths covering different spatial scales to build a model of the massive young stellar object (YSO) IRAS 13481-6124, which includes a massive central star, a dust disk heated by the central star, and a bipolar collimated outflow of molecular material, which is strongly indicative of ongoing accretion via a disk.
Using the near-infrared imager AMBER on ESO’s VLTI telescope – the interferometric mode offered by the array of 4 VLT unit telescopes and a number of smaller 1.8-m auxiliary telescopes – and the 3.5-m New Technology Telescope, they produced resolved images of 2 distinct components to the source. Combing their data processing and modelling, they identify these as a compact hot dusty disk that is being heated and slowly evaporated by the stellar radiation, surrounded by the extended remainder of the dense envelope from which the new star formed.
This is the second time in a few months that obervations with near-IR interferometry result in a Nature paper (see also the eps Aurigae paper in April) – a welcome sign perhaps that this technique is slowly coming into maturity and starting to produce good science. Interferometry, while long a staple in the radio astronomer’s toolbox, is really tough at shorter optical or infrared wavelengths, often requiring control of aberrations and deformations in the instruments to sub-micrometer scales. Converting the output from interferometric instruments to images or spectra also requires additional processing steps, complicating the data reduction procedures. If you’re interested in how that works, the gory details are described in the Supplementary information at Nature.
Is this the first ever detection of an accretion disk around a massive YSO? I don’t know everything that’s been published on the subject but I don’t think so. Several papers have been published in recent years with other kinds of disk detections that seem pretty conclusive to me, e.g. Davies et al’s paper of last year using integral field spectroscopy, showing clear signs of a rotating disk-like structure around the massive YSO W33A – so we had a good idea that they were there. But as for an actual image of such a disk, well, I guess Kraus et al may now take that prize.
Interestingly, Kraus et al fitted the observed characteristics of the disk with the analytical relations that low-mass YSO disks are observed to follow. They find that the disk of IRAS 13481-6124 is consistent with these same relations. This result suggests that the process of massive star formation is perhaps not as fundamentally different from “regular” low-mass star birth as astronomers thought, and give a nice vote of confidence to our currently accepted models of star formation.
A model that works: isn’t that nice for a change?
Kraus S, Hofmann KH, Menten KM, Schertl D, Weigelt G, Wyrowski F, Meilland A, Perraut K, Petrov R, Robbe-Dubois S, Schilke P, & Testi L (2010). A hot compact dust disk around a massive young stellar object. Nature, 466 (7304), 339-42 PMID: 20631793
Krumholz MR, Klein RI, McKee CF, Offner SS, & Cunningham AJ (2009). The formation of massive star systems by accretion. Science (New York, N.Y.), 323 (5915), 754-7 PMID: 19150809
Davies B et al (2009). The circumstellar disk, envelope, and bi-polar outflow of the Massive Young Stellar Object W33A. MNRAS, Vol. 402, Issue 3, pp. 1504-1515. (arxiv)
Image: Kraus et al (2010)