Title: Possible Origin of the G2 Cloud From the Tidal Distruption of a Known Giant Star by Sgr A*
Authors: James Guillochon, Abraham Loeb, Morgan MacLeod, Enrico Ramirez-Ruiz
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics, the Institute for Theory and Computation, Cambridge, MA
One of the more publicized astronomical discoveries over the past couple of years has been the G2 gas cloud in orbit around our galaxy’s central black hole, Sgr A*. This topic has been brought up in several astrobites, discussing its original discovery, observational evidence of its possible origin, discussion of what will happen to the cloud as it comes near Sgr A*, and an update on its interaction with Sgr A* based on new observations. The G2 cloud entered its path towards closest approach last April, reaching its closest approach to Sgr A* this winter. This gas cloud provides a once-in-a-lifetime opportunity to observe the accretion, or feeding, process of our galaxy’s black hole. In order to better explain the many observations of G2, and their significance on past and future accretion, it is important to know the origin of the G2 cloud. The authors examine the possibility that the G2 cloud is one of many clouds that have fallen into Sgr A* over time, all originating from tidal disruptions of a giant star near the galactic center.
Tidal Disruptions of a Giant Star
The authors suggest that the G2 cloud is part of a large stream of gas in orbit around Sgr A*. To test this scenario, the authors use a FLASH simulation to slowly feed material in orbit around the black hole to mimic the tidal disruption of a star in orbit around Sgr A*. Once gas is stripped from the star, it begins to form into an optically thin stream at a temperature of around 104 K. Though the stream does not initially contain clumps, as the gas pressure of the stream reaches equilibrium with its surroundings the Kelvin-Helmholtz instability begins to fragment the stream. The newly-formed fragments cool faster than their surroundings, growing in size and spreading farther apart from one another. This ultimately forms several G2-like clumps of gas in orbit around Sgr A*. Snapshots of the simulation are shown in Fig. 1, labelled as A,B, and C, with the top panels showing surface density of the gas, and the bottom panels showing the flux of hydrogen recombination lines (which cool the gas). The right hand side gives a projection of the entire system, showing the gas via flux from the Brγ line of Hydrogen (the n=7 orbital to n=4 orbital transition). This emission line traces the presence of ionized hydrogen, or HII regions.
Which Star did it Come From?
Not only do the authors examine the possibility that G2 originated from a giant star, but they also find the most likely candidate in orbit around Sgr A* that could have spawned G2. For each of the 1,727 stars around Sgr A*, they use a model to calculate the statistical likelihood that any one of them could be the tidally disrupted star. This model accounts for a whopping 22 free parameters, which includes parameters describing the orbit of G2, position, velocity, and mass of Sgr A*, and the position of the possible tidally stripped star relative to Sgr A*. Determining the likelihood for each star would be a daunting task if not for the utilization of Bayesian inference (see “intermission” below) to develop a probability density function for each star.
(Intermission: I highly recommend reading this astrobite for an introduction to Bayesian statistics in an astronomical context. It includes a discussion of a MCMC, used in this paper in the form of a program called emcee, which I mention mainly to bring up its catch-phrase “The MCMC Hammer”.).
Using this analyses, the authors obtain only one likely candidate star that matches the distance of closest approach of the G2 cloud: S1-34. This is a giant star with Teff ~ 5,000 K and R ~ 1 AU. They mention, however, that a better measurement of S1-34’s position around Sgr A* is necessary before making a conclusive determination. The authors also constrain the not-yet-measured radial velocity of S1-34 to be between -400 and -30 km/s; a radial velocity outside this range would mean S1-34 cannot be the source of the G2 cloud.
What could this suggest?
Based upon S1-34’s luminosity and distance of closest approach, the authors assume it to be a 10 solar mass giant of radius 1 AU. The authors argue that this star would need to have lost 30 times the mass of the earth (denoted as M⊕) when it was disrupted by Sgr A*. For comparison, the G2 cloud itself is roughly 3 M⊕. Since this is only a small fraction of the total mass of the star, they argue that this process could have occurred many times over the course of millions of years. G2 would then be just one of many clouds that have accreted and will continue to accrete onto the black hole at the center of our galaxy. This could explain the origin of a significant fraction of the gas orbiting Sgr A*, and could indicate that a significant fraction of the activity in low-level supermassive black holes (low-level compared to the activity seen in the very luminous and energetic AGN) comes from gas stripped from giant stars.