Monthly Roundup (Halloween Edition): Graveyards of Shredded Stars

There are many ways for a star to meet its end: suddenly and spectacularly, as a supernova; gradually and gracefully, as a white dwarf wreathed in a planetary nebula; and gruesomely, torn apart by a black hole. Tidal disruption events, in which a star gets too close to a black hole and is ripped apart by tidal forces, play out over the course of months or years in galaxies across the universe.

First predicted by theorists in the 1970s, the first tidal disruption event candidate was discovered in X-ray data collected in 1990–1991 by the ROSAT All Sky Survey. While early candidates were identified by their bright, transient X-ray emission, the majority of candidates today are found by optical surveys. As the number of known tidal disruption events grows, so does the diversity of these events, creating new sub-classes like partial tidal disruption events, in which just a fraction of a star’s atmosphere is torn away, or repeating tidal disruption events, in which a star is progressively peeled apart each time it draws near a black hole.

Researchers have analyzed observations of dozens of tidal disruption events, and the number of known events is growing rapidly thanks to wide-field searches for astrophysical transients. The upcoming Rubin Observatory’s Legacy Survey of Space and Time alone is predicted to discover 1,000 tidal disruption events per year. Today, we’ll explore three research articles that probe various aspects of tidal disruption events, seeking to understand these untimely ends of unsuspecting stars.

Signature of a Tilted Disk

When a supermassive black hole rips a star apart, some of the star’s gas is cast off into the galaxy, and some of it collects in an accretion disk around the black hole. This searingly hot disk is the source of the ultraviolet and X-ray emission we see from afar, and variations in the disk’s emission can tell us about the particular characteristics of a tidal disruption event.

plot of X-ray counts per second and ultraviolet flux density for AT2020ocn

Demonstration of the event’s different behavior at X-ray (top) and ultraviolet (bottom) wavelengths. Click to enlarge. [Adapted from Cao et al. 2024]

Zheng Cao (SRON Netherlands Institute for Space Research; Radboud University) and collaborators used models of a tilted accretion disk to interpret observations of a highly variable tidal disruption event candidate discovered in 2020. In the first hundred days after its discovery, AT 2020ocn churned out multiple X-ray flares before settling down into calmer behavior. Some observations showed semi-regular X-ray brightness changes with a cadence of about 17 days, which the team suspected were due to precession of the accretion disk. Curiously, AT 2020ocn showed no sign of flares in the ultraviolet, instead fading gradually from its initial brightness at those wavelengths.

To learn more, Cao’s team analyzed and modeled ultraviolet and X-ray observations of AT 2020ocn from Swift Observatory, XMM-Newton, and the Neutron star Interior Composition Explorer (NICER). The team found that AT 2020ocn’s behavior can be explained by a star that was initially traveling at an angle relative to the direction of the black hole’s spin, producing an accretion disk tilted in the same direction as the star’s motion. Over time, the inner part of the disk becomes aligned with the black hole’s spin, making the disk precess over time and changing its orientation from our point of view. The timeframe of these changes match what’s predicted as well: flares lasting 1–10 days, with the flaring behavior persisting for less than 200 days overall.

Cao and coauthors noted that their modeling has limitations. For example, the model handles a change in the disk’s tilt by changing the observer’s position rather than manipulating the actual tilt of the disk, which may not fully capture the dynamic behavior of a tilted, precessing disk. Similarly, it’s possible that the inner disk is also warped, creating a complicated scenario in which the disk periodically partially obscures itself.

In addition to enhancing our understanding of AT 2020ocn’s behavior, the team’s findings may apply to other tidal disruption events with flares that turn off after a few months and ultraviolet emission that doesn’t flare at all.

Modeling a Missing Structure

While the bright ultraviolet and X-ray emission of a tidal disruption event is readily explained by a super-heated accretion disk circling a black hole, the origins of these events’ optical emission is less understood. The optical emission from a tidal disruption event appears to arise from surprisingly cool, slow-moving gas tens to hundreds of astronomical units from the black hole. Where is this emission coming from?

plots showing the evolution of the gas column density during a tidal disruption event

The evolution of the gas column density over the course of a year during a tidal disruption event. You can see an animation of this figure here. Click to enlarge. [Price et al. 2024]

To answer this question, a team led by Daniel Price (Monash University) modeled the destruction of a solar-mass star by a million-solar-mass black hole, tracing the changes in the debris and its emission properties over a full year of simulation time. Using general relativistic smoothed particle hydrodynamics modeling, the team observed the star being stretched into a stellar spaghetti noodle, half of which collects around the black hole and half of which is cast away. As the narrow stream of gas circles the black hole, it collides with itself, causing some of the material to fall toward the black hole and some of it to form an outflow.

Over time, the outflow takes the shape of a thin expanding bubble. This structure closely resembles a hypothesized featured called an Eddington envelope, which is thought to capture high-energy emission from the disk and accretion stream, reprocessing it to generate the optical emission researchers observe. This is the first time such a structure has formed self-consistently in simulations, representing a breakthrough in the understanding of tidal disruption event emissions.

A Chemical Conundrum

In addition to illuminating the physics of accretion and outflows in general, tidal disruption events can also clue us in to the properties of individual stars. Tidal disruption events often show prominent spectral lines, allowing researchers to study the chemical compositions of individual stars in the cores of distant galaxies — a population that’s otherwise difficult to examine closely.

The chemical analyses of certain tidal disruption events have revealed surprisingly large ratios of nitrogen to carbon (N/C ratio) in the disrupted stars. As Brenna Mockler (The Observatories of the Carnegie Institution for Science) and collaborators explain, a high N/C ratio seems to suggest that the disrupted star was a relatively massive one (>2 solar masses), bulky enough for the CNO cycle to take place in its interior. The CNO cycle produces energy by fusing hydrogen into helium and shifts the proportions of carbon and nitrogen, gradually increasing the amount of nitrogen relative to carbon. However, recent research suggests that the N/C ratios of certain events are even larger than what’s expected for the disruption of a massive star.

calculated N/C ratios for tidally disrupted stars

Comparison of the N/C ratios relative to the solar value for stripped stars (red lines; the different line styles represent stars at different stages of their helium-burning lifetimes) and normal stars (blue lines; the different line styles represent results for different metallicities). Click to enlarge. [Mockler et al. 2024]

To explore possible reasons, Mockler and collaborators simulated the disruption of a star that has lost its outer envelope — a stripped star. These stars are thought to be created in interacting binary systems. Unlike typical main-sequence stars, stripped stars have outer layers that are poor in hydrogen and rich in helium and nitrogen, and thus have higher N/C ratios. Mockler’s team compared the results of the simulated tidal disruptions of two main-sequence stars with masses of 2 solar masses (one with the same abundance of metals as the Sun and one with three times the metal abundance) and a stripped star that was initially 3 solar masses but has dwindled to just 0.58 solar mass.

The team found that the 2-solar-mass main-sequence stars, regardless of metallicity, could not attain the extreme N/C ratios seen in some events. For example, the tidal disruption event ASASSN-14li appears to have an N/C ratio greater than 300 times the Sun’s ratio. This is similar to the predicted N/C ratio for stripped stars but 10 or more times larger than predictions for a 2-solar-mass main-sequence star. The N/C ratios of tidally disrupted stripped stars appear to decrease over time, while the observed N/C ratios of normal main-sequence stars were found to increase over time, providing another avenue to determine the identity of a star after its demise.

Citation

“Tidal Disruption Event AT2020ocn: Early Time X-Ray Flares Caused by a Possible Disk Alignment Process,” Z. Cao et al 2024 ApJ 970 89. doi:10.3847/1538-4357/ad496f

“Eddington Envelopes: The Fate of Stars on Parabolic Orbits Tidally Disrupted by Supermassive Black Holes,” Daniel J. Price et al 2024 ApJL 971 L46. doi:10.3847/2041-8213/ad6862

“Tidal Disruption Events from Stripped Stars,” Brenna Mockler et al 2024 ApJL 973 L9. doi:10.3847/2041-8213/ad6c34