• Authors: M. Mapelli, T. Hayfield, L. Mayer, J. Wadsley
• First Author’s Institution: INAF-Osservatorio astronomico di Padova, Vicolo dell’Osservatorio
The Galactic Center
The Center of our Galaxy is one of the most extreme dynamical environments we can observe in detail because individual stars can actually be resolved using adaptive optics. Over time, monitoring individual stellar orbits has firmly established the presence of a supermassive black hole of about 4 x 106 M☉ (check out this video too). Further examination of these fast-moving stars’ properties (via infrared spectroscopy) revealed a surprising detail — many of these stars are young! To appreciate why this is unexpected, you must understand two important details. (1) When we say these stars are young, it means they are main sequence stars younger than 10 Myr or so. This is significant because it indicates that these stars must have formed near their current location in similar conditions to what we observe today (i.e. in close proximity to a supermassive black hole!) or have migrated from another location in this short period of time. (2) This creates the so-called “paradox of youth” because star formation this close to a supermassive black hole doesn’t fit in with our standard picture of how stars form. The authors of this paper have examined the formation of young massive stars in the Galactic Center by modeling an alternative star formation scenario, but before we go any further, we should review our standard picture of star formation.
Our standard picture of star formation
Our basic model of star formation begins with a molecular cloud, or a giant cold cloud of molecular gas and dust. If this cloud reaches the Jean’s mass it will collapse under its self-gravity. The Jean’s mass tells us when the gravity of the cloud can overcome outward (thermodynamic) pressure. After the initial collapse, it is expected that the cloud will continue to fragment, eventually forming protostars. Our understanding of star formation, even in “normal” conditions, is by no means complete, so if you are interested in reading more about star formation, check out these previous astrobites by Michelle and Elizabeth for starters.
The disagreement and reconciliation
So why doesn’t this picture of star formation work in the Galactic Center? It’s all about the parent molecular cloud — in the Galactic Center, the tidal forces from the central black hole would disrupt a “normal” cloud at the distance where the young stars are observed. This indicates that the stars either (1) formed further from the black hole and migrated inwards (the “in-falling cluster” scenario) or (2) formed in situ (in their current location), but via a mechanism which is physically consistent with a strong tidal field. Previous work on the “in-falling cluster” scenario has suggested this mechanism has difficulties reproducing the current observational properties of the young stars in the Galactic Center; therefore, Mapelli et al focus on the in situ formation scenario. Specifically, they model the infall of a turbulent molecular cloud which forms a gaseous disk orbiting the supermassive black hole using the N-body/Smooth Particle Hydrodynamics code GASOLINE.
Mapelli et al run five simulations which begin with a molecular cloud at 25 pc from the central black hole on a highly eccentric, marginally bound orbit (which the authors admit is a biased initial choice, but one made because it is likely to disrupt the cloud into a disk). They vary the initial conditions of the the cloud including the initial mass, the initial temperature, and the equation of state, which is either isothermal or includes radiative cooling. As in most star formation simulations, the authors must also choose a critical density at which stars are assumed to form. The background potential, which represents the old stellar population, is modeled to be clumpy. This is an important distinction from a smooth potential, because a clumpy potential can perturb the disk in the same manner that actual background stars can. A “sink particle”, or a particle for which the internal structure is not followed is used to represent the central black hole. Sink particles interact gravitationally and can gain mass, but do not lose mass. Each gas particle in the simulation is 0.04 M☉. One of the reasons these simulations are unique is because they do not use sink particles to represent collapsing stars, as some other work has done. This is important because the sink particles do not trace cloud fragmentation directly, and the uncertainties associated with the sink particle method are not well characterized.
Stars form in four of the simulations as shown in the figure to the right. The simulated stellar populations are consistent with the global properties (eccentricity distribution, initial mass function, total mass) of the observed young massive stars in the Galactic Center; therefore, this study indicates that the infall of a molecular cloud can account for the recent burst of star formation in the center of our Galaxy if the cloud is sufficiently massive (> 105 M☉) and has a high initial temperature (> 100 K).
Limitations and Uncertainties
There are a few limitations of this study that are worth noting. First of all, while the authors chose a physically motivated density to represent the density at which stars form, that does not mean it is correct since the collapse itself was not simulated. In reality, I should have used the phrase “star candidate” instead of star throughout this post. This chosen collapse density can vary by several orders of magnitude among star formation simulations, and can effect the initial mass function of the stars. Another limitation to this study is the size of the gas particles (0.04 M☉), which can barely resolve 1-2 M☉ stars given the collapse criterion. This could also have an effect on the derived initial mass function. The authors also note that they have only considered the infall of a single molecular cloud, yet some previous research suggests the collision of two molecular clouds could better address the properties of the young Galactic Center clouds.
The final word
This paper supports in situ star formation of young Galactic Center stars and provides constraints on the mass and temperature of a single infalling molecular cloud; however, this study does not rule out other in situ star formation scenarios (e.g. two clouds).