Dr. Andrea Stolte


Image: Hubble Space Telescope/WFC3,
Credits: A. M. Ghez, M. R. Morris, A. Stolte

Timescales for starburst cluster formation

One question in the starburst cluster project came as a surprise - we found we could answer a long-discussed cluster formation timescale question beyond our original expectation:

  • Do starburst clusters form in true "bursts"?
  • Is the phrase "starburst" justified? Or do massive, dense clusters form over extended periods of time?
  • A related question is: Are starburst clusters dynamically stable?
  • Are starburst clusters close to virial equilibrium or do they expand or collapse?
  • Together with our colleagues at the Max-Planck-Institute for Astronomy in Heidelberg, we could also analyse the rich data set covering the spiral arm clusters:

No age spread in starbursts

In a PhD thesis conducted under supervision of Dr. Wolfgang Brandner at MPIA, Natalia Kudryavtseva was able to show that the spiral-arm clusters NGC 3603 YC and Westerlund 1 formed almost instantaneously. The time span for formation of both clusters is estimated to be less than 10% of the cluster ages of 2 and 4 Myr, respectively. Using the proper motion membership samples of stars in each cluster, the young, pre-main sequence population is tightly constrained. Especially in NGC 3603, the population is so tight that the age spread is expected to be less than 100,000 years. This implies that the young star cluster with its tens of thousands of stars formed in about onehundred thousand years - universally, a very short timescale indeed.
Colour-magnitude diagrams of Westerlund 1 and NGC 3603 YC (Kudryavtseva et al. 2012, Fig. 1). The green asterisks of the young population in Westerlund 1 indicate proper motion-selected cluster members. In the colour-magnitude diagram of NGC 3603 YC, only cluster member candidates are shown. The dense, lower populations are the pre-main sequence stars, while the dense vertically aligned population is the hydrogen-burning main sequence in both clusters. The blue lines represent the best-fitting isochrones, suggesting an age of 5 Myr for Westerlund 1 and 2 Myr for NGC 3603. The transition region marked in both diagrams is a strong indicator for the cluster age. The width and location of the transition region changes quickly with time. If multiple generations of stars with various ages are present, the transition region becomes wider and broader with large scatter, because the different evolutionary stages of the young stars are mixed. In Kudryavtseva et al. (2012), Natalia used the shape of the transition region to derive limits on the age spread. The tight and sparse transition regions suggest that the age spread in both clusters has to be very small. A statistical analysis of these CMDs (inside the area of each red box) confirmed this impression. In both clusters, the maximum possible spread between the ages of the young cluster members is less than 10% of the cluster age.
Likelyhood distribution for stellar evolution models with different ages (Kudryavtseva et al. 2012, Figs. 2 and 3). The likelyhoods are derived from fitting isochrones to the red boxed regions in the CMDs (see Kudryavtseva et al. 2012 for details). From this set of stellar evolution models, the most likely age for Westerlund 1 is 5 Myr with a possible spread of 400,000 years, and 2 Myr with a spread of less than 100,000 years in NGC 3603. In both cases, the width in the age distribution is consistent with the uncertainties in the photometries and a possible binary contribution. This implies that no physical "real" age spread is needed to explain the width of each age distribution.
Here, the proper motion membership proved vital for constraining the age spread and hence the timescale of cluster birth. Without membership, the colour-magnitude diagrams might appear very wide and scattered, especially in the transition region between the pre-main sequence and the hydrogen-burning zero-age main sequence (see also Rochau et al. 2010 and Rochau 2011). With the capability to reject the majority of field stars and keep only the proper motion cluster members, this transition region became very clean and narrow in both clusters. From these colour-magnitude diagrams, Natalia was then able to derive the maximum spread in the age of each population using statistical methods.
Conclusion: The age spread in both spiral arm starburst clusters is found to be less than 10% of the cluster age. The statistical analysis showed that it was critical to employ proper-motion membership to derive clean transition regions and pre-main sequence distributions to constrain the age spreads. The observed 10% possible age variation can be explained by photometric errors and the contribution of binaries to the width of the pre-main sequence transition region. Hence, no true age spread is needed to explain the young cluster population. The two starbursts Westerlund 1 and NGC 3603 appear to have been born in "bursts of star formation" indeed.

Internal velocity dispersion & virial equilibrium

A second result that could only be obtained with a good cluster membership sample was the derivation of the internal velocity dispersion. This spread in orbital velocities is related to the virial mass of each cluster if the cluster is in virial equilibrium. For NGC 3603 YC, Boyke Rochau found a velocity disperion of 4.5 km/s, which suggests that the dynamical mass is in very good agreement with the photometric mass of the cluster, and hence that NGC 3603 YC is in virial equilibrium (Rochau et al. 2010 and Rochau 2011). A similar result was found for the Arches cluster with a central velocity dispersion of about 6 km/s ( Clarkson et al. 2012). If a cluster is in virial equilibrium, this implies that the cluster is neither rapidly expanding nor collapsing. The cluster can still dynamically evolve slowly with time, for instance because of the segregation of high-mass stars to the cluster core and the evaporation of low-mass stars near the cluster outskirts. These mechanisms also lead to changes in the cluster extent and the core radius, but this dynamical evolution is much slower than the revirialisation of a cluster out of virial equilibrium.

Conclusion: The proper-motion membership samples allowed the derivation of the central, internal velocity dispersion of starburst clusters for the first time. The clusters seem to be dynamically stable: neither the Arches nor NGC 3603 YC are rapidly expanding or collapsing. Because of mass segregation, where the high-mass stars sink to the center and loose orbital energy to the low-mass population, the central velocity dispersions are lower limits to the overall velocity dispersion in each starburst cluster. A mean, global velocity dispersion can only be derived when including the low-mass stars at larger radii, which is not feasible with current instruments. Nevertheless, the comparison with dynamical simulations shows that the central velocity dispersion is expected to be between 50% and 75% of the global, mean velocity dispersion in these clusters (Olczak et al., work in progress).

Kudryavtseva, N., Brandner, W., Gennaro, M., Rochau, B., Stolte, A., et al. 2012:
Instantaneous Starburst of the Massive Clusters Westerlund 1 and NGC 3603 YC
Astrophysical Journal Letters, 750, 44

Kudryavtseva, Natalia S. 2012:
Micro-arcsecond astrometry of exoplanet host stars and starburst clusters
PhD thesis, University of Heidelberg, Germany

Rochau, Boyke 2011:
Young massive star clusters as probes for stellar evolution, cluster dynamics and long term survival
PhD thesis, University of Heidelberg, Germany

Rochau, B., Brandner, W., Stolte, A., Gennaro, M., Gouliermis, D., Da Rio, N., Dzyurkevich, N., Henning, Th. 2010:
Internal Dynamics and Membership of the NGC 3603 Young Cluster from Microarcsecond Astrometry
Astrophysical Journal Letters, 716, 90

© Andrea Stolte -- April 2015