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1. Tidal Disruption Events from Eccentric Orbits and Lessons Learned from the Noteworthy ASASSN-14ko

   https://doi.org/10.3847/1538-4357/acafe1  

   Liu, Chang; Mockler, Brenna; Ramirez-Ruiz, Enrico; Yarza, Ricardo; Law-Smith, Jamie A.; Naoz, Smadar; Melchor, Denyz; Rose, Sanaea (February 2023, The Astrophysical Journal)

   Abstract Stars grazing supermassive black holes (SMBHs) on bound orbits may survive tidal disruption, causing periodic flares. Inspired by the recent discovery of the periodic nuclear transient ASASSN-14ko, a promising candidate for a repeating tidal disruption event (TDE), we study the tidal deformation of stars approaching SMBHs on eccentric orbits. With both analytical and hydrodynamic methods, we show the overall tidal deformation of a star is similar to that in a parabolic orbit provided that the eccentricity is above a critical value. This allows one to make use of existing simulation libraries from parabolic encounters to calculate the mass fallback rate in eccentric TDEs. We find the flare structures of eccentric TDEs show a complicated dependence on both the SMBH mass and the orbital period. For stars orbiting SMBHs with relatively short periods, we predict significantly shorter-lived duration flares than those in parabolic TDEs, which can be used to predict repeating events if the mass of the SMBH can be independently measured. Using an adiabatic mass-loss model, we study the flare evolution over multiple passages, and show the evolved stars can survive many more passages than main-sequence stars. We apply this theoretical framework to the repeating TDE candidate ASASSN-14ko and suggest that its recurrent flares originate from a moderately massive ( M ≳ 1 M ⊙ ), extended (likely ≈10 R ⊙ ), evolved star on a grazing, bound orbit around the SMBH. Future hydrodynamic simulations of multiple tidal interactions will enable realistic models on the individual flare structure and the evolution over multiple flares. 

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2. Searching for Intermediate-mass Black Holes in Globular Clusters through Tidal Disruption Events

   https://doi.org/10.3847/1538-4357/ad1dd9  

   Tang, Vivian L; Madau, Piero; Bortolas, Elisa; Peng, Eric W; Feng, Yuting; Guhathakurta, Puragra (March 2024, The Astrophysical Journal)

   Abstract Intermediate-mass black holes (IMBHs) may be the link between stellar mass holes and the supermassive variety in the nuclei of galaxies, and globular clusters (GCs) may be one of the most promising environments for their formation. Here, we carry out a pilot study of the observability of tidal disruption events (TDEs) from 10³M_⊙<M_•< 10⁵M_⊙IMBHs embedded in stellar cusps at the center of GCs. We model the long super-Eddington accretion phase and ensuing optical flare, and derive the disruption rate of main-sequence stars as a function of black hole mass and GC properties with the help of a 1D Fokker–Planck approach. The photospheric emission of the adiabatically expanding outflow dominates the observable radiation and peaks in the near-ultraviolet/optical bands, outshining the brightness of the (old) stellar population of GCs in Virgo for a period of months to years. A search for TDE events in a sample of nearly 4000 GCs observed at multiple epochs by the Next Generation Virgo Cluster Survey yields null results. Given our model predictions, this sample is too small to set stringent constraints on the present-day occupation fraction of GCs hosting IMBHs. Naturally, better simulations of the properties of the cluster central stellar distribution, TDE light curves, and rates, together with larger surveys of GCs are all needed to gain deeper insights into the presence of IMBHs in GCs. 

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3. The Peak of the Fallback Rate from Tidal Disruption Events: Dependence on Stellar Type

   https://doi.org/10.3847/2041-8213/ad0388  

   Bandopadhyay, Ananya; Fancher, Julia; Athian, Aluel; Indelicato, Valentino; Kapalanga, Sarah; Kumah, Angela; Paradiso, Daniel A.; Todd, Matthew; Coughlin, Eric R.; Nixon, C. J. (January 2024, The Astrophysical Journal Letters)

   Abstract A star completely destroyed in a tidal disruption event (TDE) ignites a luminous flare that is powered by the fallback of tidally stripped debris to a supermassive black hole (SMBH) of massM_•. We analyze two estimates for the peak fallback rate in a TDE, one being the “frozen-in” model, which predicts a strong dependence of the time to peak fallback rate,t_peak, on both stellar mass and age, with 15 days ≲t_peak≲ 10 yr for main sequence stars with masses 0.2 ≤M_⋆/M_⊙≤ 5 andM_•= 10⁶M_⊙. The second estimate, which postulates that the star is completely destroyed when tides dominate the maximum stellar self-gravity, predicts thatt_peakis very weakly dependent on stellar type, with $t_{\mathrm{peak}}=\left(23.2\pm 4.0\mathrm{days}\right){\left(M_{•}/{10}^6{M}_{\odot }\right)}^{1/2}$for 0.2 ≤M_⋆/M_⊙≤ 5, while $t_{\mathrm{peak}}=\left(29.8\pm 3.6\mathrm{days}\right){\left(M_{•}/{10}^6{M}_{\odot }\right)}^{1/2}$for a Kroupa initial mass function truncated at 1.5M_⊙. This second estimate also agrees closely with hydrodynamical simulations, while the frozen-in model is discrepant by orders of magnitude. We conclude that (1) the time to peak luminosity in complete TDEs is almost exclusively determined by SMBH mass, and (2) massive-star TDEs power the largest accretion luminosities. Consequently, (a) decades-long extra-galactic outbursts cannot be powered by complete TDEs, including massive-star disruptions, and (b) the most highly super-Eddington TDEs are powered by the complete disruption of massive stars, which—if responsible for producing jetted TDEs—would explain the rarity of jetted TDEs and their preference for young and star-forming host galaxies. 

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4. Repeating Partial Tidal Encounters of Sun-like Stars Leading to Their Complete Disruption

   https://doi.org/10.3847/1538-4357/ad9b0b  

   Liu_刘, Chang 畅; Yarza, Ricardo; Ramirez-Ruiz, Enrico (January 2025, The Astrophysical Journal)

   Abstract Stars grazing supermassive black holes on bound orbits may produce periodic flares over many passages, known as repeating partial tidal disruption events (TDEs). Here, we present 3D hydrodynamic simulations of Sun-like stars over multiple tidal encounters. The star is significantly restructured and becomes less concentrated as a result of mass loss and tidal heating. The vulnerability to mass loss depends sensitively on the stellar density structure, and the strong correlation between the fractional mass loss ΔM/M_*and the ratio of the central and average density ${\rho }_{\mathrm{c}}/\overline{\rho }$, which was initially derived in disruption simulations of main-sequence stars, also applies for stars strongly reshaped by tides. Over multiple orbits, the star loses progressively more mass in each encounter and is doomed to a complete disruption. Throughout its lifetime, the star may produce numerous weak flares (depending on the initial impact parameter), followed by a couple of luminous flares whose brightness increases exponentially. Flux-limited surveys are heavily biased toward the brightest flares, which may appear similar to the flare produced by the same star undergoing a full disruption on its first tidal encounter. This places new challenges on constraining the intrinsic TDE rates, which need to take repeating TDEs into account. Other types of stars with different initial density structures (e.g., evolved stars with massive cores) follow distinct evolution tracks, which might explain the diversity of the long-term luminosity evolution seen in recently uncovered repeaters. 

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5. Rare Occasions: Tidal Disruption Events Rarely Power the AGNs Observed in Dwarf Galaxies

   https://doi.org/10.3847/1538-4357/ad8341  

   Tan, Joanne; Yang_杨, Guang 光; Walsh, Jonelle L; Brandt, W N; Luo, Bin; Bauer, Franz E; Chen, Chien-Ting; Sun, Mouyuan; Xue, Yongquan (January 2025, The Astrophysical Journal)

   Abstract Tidal disruption events (TDEs) could be an important growth channel for massive black holes in dwarf galaxies. Theoretical work suggests that the observed active galactic nuclei (AGNs) in dwarf galaxies are predominantly TDE-powered. To assess this claim, we perform variability analyses on the dwarf-hosted AGNs detected in the 7 Ms Chandra Deep Field-South survey, with observations spanning ≈16 yr. Based on the spectral energy distribution modeling withx-cigale, we select AGNs hosted by dwarf galaxies (stellar mass below 10¹⁰M_⊙). We focus on X-ray sources with full-band detections, leading to a sample of 78 AGNs (0.122 ≤z≤ 3.515). We fit the X-ray light curves with a canonical TDE model oft^−5/3and a constant model. If the former outperforms the latter in fitting quality for a source, we consider the source as a potential TDE. We identify five potential TDEs, constituting a small fraction of our sample. Using true- and false-positive rates obtained from fitting models to simulated light curves, we perform Bayesian analysis to obtain the posterior of the TDE fraction for our sample. The posterior peaks close to zero (2.56%), and we obtain a 2σupper limit of 9.80%. Therefore, our result indicates that the observed AGNs in dwarf galaxies are not predominantly powered by TDEs. 

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