Circumstellar disc survival in Milky Way starburst clusters

The figures show the infrared colour-colour diagram and the proper motion plane of stars in the central field of the Quintuplet cluster. About 20 sources show thermal emission at L-band (3.8 micrometers), which is evidenced by their locations (diamonds) to the right side of the main sequence cluster population. With one exception, all disc candidates are proper members of the Quintuplet cluster, as can be seen from their location inside the member selection circle in the proper motion plane (right two figures). A detailed description of the figures can be found in Stolte et al. 2015.

The maps below display the sources with thermal infrared emission evidencing circumstellar material on the 2.2 micrometer (K-band) image. All circles denote disc candidates. Most of the sources are ver faint and hard to see in the 2-micron image. In the Arches cluster, some discs were known from one of our previous investigations with the Keck telescope on Mauna Kea, Hawai'i (Stolte et al. 2010). The discovery of discs in the Arches cluster center led us to cover the extended areas of both clusters out to their approximated tidal radius, such that most of the discs around intermediate-mass could be detected in the survey.

The figure below shows the disc fraction in nearby young star clusters and in the starburst clusters investigated in the survey. The black circles indicate young, rich clusters where only the high- and intermediate-mass stars of types O, B, or A could be included in the analysis. These disc host stars all have masses of more than 2 to 3 times the Sun. Blue circles, on the other hand, include nearby young clusters where the disc fraction was predominantly measured from lower-mass stars, similar to our Sun or even smaller (T Tauri stars). The three starburst clusters where we were able to measure the disc fraction from thermal emission are shown in red. NGC 3603 YC has the highest disc fraction, which is expected at its very young age. All starburst clusters display much lower disc fractions than their lower-mass counterparts in the solar neighbourhood. It is not yet clear if the low disc fraction is a selection effect because only high-mass stars could be investigated in the distant starburst clusters, for which a low disc fraction is expected. How much the dense environment with its strong stellar winds, UV radiation field and interactions between stars in starburst clusters affects the disc fraction remains to be investigated with simulations.

In contrast to the low disc fractions of the Arches and Quintuplet clusters, a disc fraction of about 30% is found in NGC 3603 (Stolte et al. 2004, Harayama et al. 2008) from mid-infrared L-band emission and Halpha as disc indicators. A possible increase in the disc fraction from 20% in the cluster core to 40% in the cluster outskirts is observed in seeing-limited VLT/ISAAC observations (Stolte et al. 2004). These disc fractions include both pre-main sequence stars down to 2 solar masses and high-mass stars up to 20 solar masses. If only the OB stars are considered, the disc fraction decreases to 12% in the cluster centre and 25% at larger radii (Stolte et al. 2004). Especially the central disc fraction from L-band excess sources of only 12% is similar to the Arches fraction of circumstellar disc candidates. With NGC 3603 being slightly younger than the Arches (Kudryavtseva et al. 2012, Martins et al. 2008), a higher fraction of primordial circumstellar discs is expected. The low disc fraction around OB stars in both cluster cores suggests that discs around the mass segregated high-mass stars are depleted rapidly either by their own UV radiation field or in the presence of external radiation and stellar encounters in the dense cluster cores. If, however, the discs in the Arches and the Quintuplet are of a secondary origin, e.g. from mass transfer in tight binary systems, the discs would not originate from the survival of primordial circumstellar material and the disc fraction would be a function of the fraction of high-mass mass-transfer binaries in each cluster core. In view of a secondary disc origin from mass transfer (Stolte et al. 2015), it is interesting to note that approximately 30% of the pre-main sequence stars in NGC 3603 are located on a secondary sequence in the colour-magnitude diagram, indicating that they are near-equal mass binaries (Stolte et al. 2004). Although the densest of these systems are good candidates for mass transfer, it is currently not clear whether the mass stream would be sufficient to build up a dusty disc with strong infrared emission.