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Circumstellar Discs

Classical theories of star formation assume that a rotating, collapsing cloud will form a disc, through which the star will accrete matter. This has been confirmed observationally for low-mass stars but remains more elusive for their higher-mass couterparts. At Leeds we aim to explore the difference between the two regimes in a number of ways.

Around Massive Young Stellar Objects

In one approach we focus on Massive Young Stellar Objects (MYSOs), young stars still embedded in their natal cloud and undergoing accretion. We use observations at near-infrared wavelengths (λ~2 microns) to understand the environments of these objects.

Through studying certain spectral lines (such as Fe or CO lines) we can understand the properties of the accretion process, as such lines are expected to be formed in an accretion disc. In addition, looking at hydrogen recombination lines allows us to derive the rates at which the accretion process takes place. The results can be compared with values for lower-mass sources giving hints about the similarities between high- and low-mass young stellar objects.

As well as studying MYSOs at near-infrared wavelengths, we also use high-resolution observations at longer wavelengths to trace the colder material in the disc further from the young star. Millimetre-wavelength facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) can detect the dust particles which are mixed with gas in the disc, and also detect spectral lines emitted from the molecules in the gas. The doppler shift of these lines from their expected observed wavelength can then inform us about the kinematics of the gas in the disc. For instance, they can tell us whether the disc spins up as the gas gets closer to the forming star.

Using the high-resolution interferometry capabilities of ALMA, and the Northern Extended Millimeter Array (NOEMA), Leeds aims to find discs around the hot, massive O-type stars and to parametrise themi, comparing their properties to those around low-mass forming stars. A recent success of these efforts was the recent discovery of a Keplerian disc around O-type star AFGL 4176 (see image) [1].

The full press releases for the discovery are available here and here.

[1] Johnstone, K.G. et al. 2015,ApJL, 813, L19

Around Herbig Stars

Disc around O Star
An artist's impression of the disk around the forming high-mass star AFGL 4176. The disk is 50 times larger than the size of Pluto's orbit, but it rotates around its star in a similar way to disks around forming low-mass stars. Image credit: K G Johnston.
Another way to study the difference between high- and low- mass regimes is to focus on Herbig Ae/Be (HAeBe) objects: young stars with masses between ~ 2 and 10 times the mass of the Sun that are surrounded by gas and dust in disc-shaped structures where planets eventually form. The importance of HAeBes relies on their "intermediate" mass, with physical properties that link those of low- and high-mass young stars. Moreover, HAeBes could be more efficient forming planets in their discs, when compared to other types of young stars. A main focus at Leeds is to understand how the disc's material falls on to the growing stars, facilitating its formation, constraining the "protoplanetary" disc's lifetime, and therefore setting the time-scale of planet formation. This research is carried out from two complementary fronts. On the one hand, the group is involved on observational campaigns of wide samples of stars, which serves to derive statistically-based conclusions on the general properties of HAeBes. On the other, we make use of state-of-the-art, high-spatial resolution techniques to observe some of the most interesting HAeBes. The use of these techniques allows us to investigate the properties of the discs with unprecedented detail, even revealing the presence of forming planets in some cases. Recent examples of results obtained from the two previous approaches are [1] and [2], respectively. The latter has been considered one of the six scientific highlights of 2015 in the last annual report of the European Southern Observatory; see also the related research note in "Nature Physics"). [1] Fairlamb, J.R. et al. 2015, MNRAS,453, 976 [2] Mendigutía, I. et al. 2015, MNRAS,453, 212