February 19, 2014
Plotting The Destruction Of A Star As It Falls Into A Black Hole
April Flowers for redOrbit.com - Your Universe Online
Exploring the universe's most violent events using computer simulations is what Enrico Ramirez-Ruiz does. So in 2012, when the first detailed observations of a star being ripped apart by a black hole were reported in Nature, Ramirez-Ruiz was eager to compare the data to his simulations. This was especially true because he doubted one of the published conclusions: that the disrupted star was a rare helium star.
"I was sure it was a normal hydrogen star and we were just not understanding what's going on," Ramirez-Ruiz, a professor of astronomy and astrophysics at the University of California, Santa Cruz, told Tim Stephens in a recent statement.
The findings of Ramirez-Ruiz's team, currently online at arXiv.org and appearing in an upcoming issue of the Astrophysical Journal, explain what happens during the disruption of a normal sun-like star by a supermassive black hole. The study also reveals why observers might fail to see evidence of hydrogen in the star. Ramirez-Ruiz collaborated with USCS graduate student James Guillochon, who is now an Einstein Fellow at Harvard University, and undergraduate student Haik Manukian to run a series of detailed computer simulations of encounters between stars and black holes.
Scientists believe that supermassive black holes lurk at the center of most galaxies. Some are very bright, emitting intense radiation from superheated gas falling into the black hole. These are known as active galactic nuclei. These are rare, however, as the central black holes of most galaxies have run out of gas and are quiescent. The galactic center of these galaxies only emits a bright flare when some unlucky star approaches too close and is shredded by the powerful tidal force of the black hole, in what is known as a "tidal disruption event" (TDE). In a typical galaxy, TDEs happen about once every 10,000 years.
"That means you have to survey the nearest 10,000 galaxies in order to see one event, so for many years this was very much a theoretical field," Ramirez-Ruiz said.
That was true until the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) began surveying the sky on a continual basis. Pan-STARRS has begun detecting and recording observations of the rare TDEs—the first of which, PS1-10jh, was observed in 2010 and published in 2012. Astronomers recorded the rise and fall in brightness over time, called the light curve, and took a spectrum at peak brightness to examine the different wavelengths of light.
Characteristic "emission lines" at specific wavelengths are shown in the spectrum of an active galactic nucleus (AGN). These emission lines correspond to the most common elements such as hydrogen and helium, and appear as spikes of increased intensity in a continuous spectrum. In PS1-10jh, however, the scientists were shocked by the absence of a hydrogen line in the spectrum.
"It's very unusual to have seen helium and not hydrogen. Stars are mainly made of hydrogen, and stars made only of helium are extremely rare, so this was a huge issue," Guillochon said. "People said maybe it was a giant star with a helium core and a hydrogen envelope, and the black hole removed the hydrogen first and then the helium core in a second pass."
Using computer simulations performed on the UCSC Pleiades, Hyades, and Laozi computer clusters and the NASA Pleiades computer cluster, Guillochon began to explore the possibilities. The results of his investigation provide a new understanding of the origin of the emission lines in a TDE, showing that the flare of light from a tidal disruption contains information about the type of star and the size of the black hole. The findings also show that PS1-10jh involved the most common type of star (a main-sequence star much like our sun) and a relatively small supermassive black hole.
As a supermassive black hole disrupts a star, the tidal forces stretch the star into an elongated blob before shredding it. About half the star's mass is ejected in a full disruption, and half remains bound in elliptical trajectories. The trapped mass eventually forms an "accretion disk" of material that spirals into the black hole.
Prior to this study, researchers thought the unbound material formed a wide "fan" of ejected material, which was the main source of emission lines. In the new computer simulations, however, the unbound material is confined by self-gravity into a narrow band that doesn't have enough surface area to be the source of the emission lines. The accretion disk, therefore, must be the source of the emission lines. The simulations demonstrate how such a disk forms over time, beginning with the inner part and growing outward.
Ramirez-Ruiz likens this process to watching the birth of an AGN, as the emission lines in a tidal disruption event correspond to the well-studied "broad line region" of AGNs. The emission lines of different elements in an AGN are produced at different distances from the central black hole. For example, helium lines are generated deep in, while hydrogen lines are produced farther out where the intensity of ionizing radiation is slightly lower. When the Pan-STARRS astronomers took the spectrum of PS1-10jh, the accretion disk simply had not grown big enough to reach the distance where hydrogen starts to produce an emission line.
"The hydrogen is there, you just don't see it because it is so highly ionized. The way to understand the spectrum of a TDE is to think of it as an AGN with a truncated disk, because the disk is still growing," Guillochon said. "In an AGN, the emission is steady because the disk is established. In our model of tidal disruption, you are seeing the broad line region being built."
Another TDE, PS1-11af, was recently detected. The spectrum of this TDE had neither hydrogen nor helium emission lines. "Our model tells us that this would have to be a smaller black hole, and when the spectrum was taken the disk was so small you would not expect to see either hydrogen or helium," Guillochon said.
The new study, which was supported by the David and Lucille Packard Foundation, the National Science Foundation (NSF), and NASA, also explains how the light curve of a TDE can yield information about the masses of both the star and the black hole. The observed light curves match remarkably well with those derived from the simulations. "With this simple model, we get a perfect fit to the data, and we're able to explain the light curve in multiple color bands," Ramirez-Ruiz said. "The type of star and the size of the black hole are imprinted in the light curve."
Ramirez-Ruiz says that Pan-STARRS is expected to detect dozens of tidal disruptions, while the planned Large Synoptic Survey Telescope (LSST) could detect thousands a year. This would allow astronomers to study quiescent black holes at the centers of local galaxies that would otherwise be difficult if not impossible to detect. If a supermassive black hole is not emitting light, it reveals its presence only through its effects on the motions of stars. The smaller the relative size of the black hole, the harder it becomes to observe those effects.
"We are now detecting black holes that are very close to the detection limit," Ramirez-Ruiz said. "Tidal disruption is elucidating a local population of low-mass quiescent black holes, and it is going to enable us to test the idea that every galaxy has a black hole at its center, even the tiny ones."