An ultraviolet-optical flare from the tidal disruption of a helium-rich stellar core

Abstract: The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies. Previous candidate flares have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two `relativistic' candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet. Here we report the discovery of a luminous ultraviolet-optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decline of its light curve follows the predicted mass accretion rate, and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about 2 million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.

• The paper reports the detection of an ultraviolet-optical flare from the tidal disruption of a helium-rich stellar core, determining the event timing to within two days.
• It employs a comprehensive light curve analysis that reveals a peak bolometric luminosity of ~2.1×10^51 ergs and an accreted mass of at least 0.012 solar masses.
• The study identifies spectral signatures indicating a hydrogen-poor, helium-rich environment and discusses reprocessing effects that yield a cooler continuum than standard models predict.

Ultraviolet-Optical Flares from Tidal Disruptions in Helium-Rich Stars

The paper reports the discovery and detailed analysis of an ultraviolet-optical flare resulting from the tidal disruption of a helium-rich stellar core by a supermassive black hole (SMBH). This event was observed in a distant, inactive galaxy at a redshift of 0.1696, providing valuable insights into stellar dynamics in extreme gravitational regimes and the nature of SMBHs in quiescent galaxies.

Tidal Disruption Events and Observational Evidence

Tidal disruption events (TDEs) occur when a star ventures within the tidal radius of an SMBH, leading to the star being torn apart by tidal forces. Approximately half of the stellar debris remains gravitationally bound, eventually accreting onto the black hole and producing a luminous flare. While previous observations of candidate TDEs have revealed declining light curves consistent with theoretical models, they often lacked constraints on the timing or nature of the disrupted star due to incomplete data during the flare's rise. This paper provides a rare, well-sampled light curve capturing both the rise and decline of a TDE.

The observed TDE, named PS1-10jh, was characterized by a continuum cooler than a simple accreting debris disk model predicts. Nevertheless, its light curve adhered closely to the anticipated mass accretion rate, allowing the time of disruption to be determined within a two-day precision. The SMBH responsible for this disruption is estimated to have a mass of about 2 million solar masses.

Stellar Properties and Implications

A notable aspect of the study is the determination of the disrupted star's identity as a helium-rich stellar core, informed by the spectroscopic detection of ionized helium and the absence of Balmer line emissions. This suggests a hydrogen-poor environment, which aligns with theoretical scenarios where a star has lost its hydrogen envelope through previous interactions or mass loss.

Key numerical results include the flare's peak bolometric luminosity, which implies a minimum total energy release of approximately 2.1 × 10^51 ergs, translating to an accreted mass of at least 0.012 solar masses, assuming a typical conversion efficiency. These metrics provide a reference for future studies examining the accretion dynamics and energy outputs of SMBHs interacting with compact stellar objects.

Challenges and Theoretical Consistency

The paper addresses a discrepancy between the observed continuum temperature and theoretical predictions, suggesting possible reprocessing mechanisms that affect the emission's spectral profile. This reprocessing, potentially through atomic line emissions and absorptions, could mask the expected blackbody continuum, leading to the cooler observed temperatures.

The black hole mass estimation and flare energetics further support the model of a tidally stripped red giant core, linked to scenarios of star formation and stripping in dense galactic centers. Such findings underscore the relevance of TDEs in understanding both SMBH properties and star evolution in high-density environments.

Future Directions

This study's methodologies and findings enhance our understanding of TDEs, providing a framework for future exploration of such phenomena in different galactic contexts. As observational technology progresses, capturing more complete spectroscopic signatures and finer temporal resolution will refine the dynamic models and challenge theoretical assumptions about matter accretion in SMBH environments. Moving forward, the detection and characterization of more such events will be crucial in unraveling the diverse behaviors of both stars and black holes under extreme physics conditions.