The Carbon and Nitrogen Abundance Ratio in the Broad Line Region of Tidal Disruption Events

Yang, Chenwei; Wang, Tinggui; Ferland, Gary J.; Dou, Liming; Zhou, Hongyan; Jiang, Ning; Sheng, Zhenfeng

doi:10.3847/1538-4357/aa8598

Cited by 31 publications

(41 citation statements)

References 59 publications

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“…Furthermore, C iii] λ1908, having a similar ionization potential to that of N iii] (47.9 eV vs. 47.5 eV), is also absent. Since the ratio C iii]/N iii] has only a moderate dependence on the physical conditions of the gas (under local thermodynamic equilibrium conditions), the lack of C in the spectra of TDEs has been attributed to the unusually high abundance ratio of N to C in the debris of the disrupted star (Kochanek 2016a;Yang et al 2017).…”

Section: Broad Linesmentioning

confidence: 99%

“…While the semiforbidden N iii] lines are clearly detected with broad velocity spread, the C iii] is lacking in all three TDEs having FUV spectra. The origin for this discrepancy has been associated with the nitrogen-enhanced composition of the stellar debris for super-solar-mass stars (Kochanek 2016b;Yang et al 2017). The carbonnitrogen-oxygen cycle would have substantially altered the star's composition with enhanced nitrogen at the expenses of carbon, translated into the observed line ratios.…”

Section: Ionmentioning

confidence: 99%

“…To date, UV spectra of only four TDEs have been obtained, including the object discussed herein. While the optical spectra of these events are usually marked by He ii, He i, and occasionally H broad emis-sion lines, UV observations of ASASSN14li (Cenko et al 2016), iPTF16fnl , PS16dtm (Blanchard et al 2017), and iPTF15af (also published in Yang et al 2017) have shown the presence of collisionally excited lines of Lyα, C iv, N v, and Si iv. UV wavelengths are especially relevant for characterizing the geometry, density, and kinematics of the stellar debris and preexisting circumnuclear material.…”

Section: Introductionmentioning

confidence: 99%

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The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution

Blagorodnova

Cenko

Kulkarni

et al. 2019

ApJ

102

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We present multiwavelength observations of the tidal disruption event (TDE) iPTF15af, discovered by the intermediate Palomar Transient Factory (iPTF) survey at redshift z = 0.07897. The optical and ultraviolet (UV) light curves of the transient show a slow decay over five months, in agreement with previous optically discovered TDEs. It also has a comparable black-body peak luminosity of L peak ≈ 1.2 × 10 44 erg. The inferred temperature from the optical and UV data shows a constant value of (3−5) ×10 4 K. The transient is not detected in X-rays up to L X < 3 × 10 42 erg s −1 within the first five months after discovery. The optical spectra exhibit two distinct broad emission lines in the He ii region, and at later times also Hα emission. Additionally, emission from [N iii] and [O iii] is detected, likely produced by the Bowen fluorescence effect. UV spectra reveal broad emission and absorption lines associated with high-ionization states of N v, C iv, Si iv, and possibly P v. These features, analogous to those of broad absorption line quasars (BAL QSOs), require an absorber with column densities N H > 10 23 cm −2 . This optically thick gas would also explain the nondetection in soft X-rays. The profile of the absorption lines with the highest column density material at the largest velocity is opposite that of BAL QSOs. We suggest that radiation pressure generated by the TDE flare at early times could have provided the initial acceleration mechanism for this gas. Spectral line monitoring of future TDEs could test this proposal.

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Section: Broad Linesmentioning

confidence: 99%

Section: Ionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution

Blagorodnova

Cenko

Kulkarni

et al. 2019

ApJ

102

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“…On the other hand, the prominent N III] λ1750 relative to C III] λ1909 in TDEs may imply abundance anomalies due to CNO cycle in the pre-disrupted star (Cenko et al 2016;Kochanek 2016). Simulations have shown that the anomalous abundance features should be present after the time of peak fallback rate (t peak ) and are more significant in higher mass stars (Kochanek 2016;Yang et al 2017;Gallegos-Garcia et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event

Hung

Cenko

Roth

et al. 2019

ApJ

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We report the discovery of non-stellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05c) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr (z = 0.071). In the HST UV spectra, absorption of high-and low-ionization lines are also present at this velocity, making AT2018zr resemble a low-ionization broad absorption line (LoBAL) QSO. We conclude that these transient absorption features are more likely to arise in fast outflows produced by the TDE than absorbed by the unbound debris. In accordance with the outflow picture, we are able to reproduce the flat-topped Hα emission in a spherically expanding medium, without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow (∼1000 km s −1 ) N III λ4640Å, He II λ4686, Hα, and Hβ emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late time as observed by Swift. Including AT2018zr, we find a high association rate (3 out of 4) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and may be less sensitive to viewing angle effects compared to QSO outflows.

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“…The disruption of even a single star could be enough to temporarily pollute the broad-line region (BLR) gas, assuming the BLR gas mass is on the order of 10 −3 M ⊙ 1 (Peterson 1997). Although significant differences exist between the rest-frame UV spec-tra of low-redshift TDEs and high-redshift N-rich quasars, unusually strong nitrogen emission is seen in all of the three optical TDEs that have UV spectra in low-redshift galaxies (Cenko et al 2016;Yang et al 2017;Brown et al 2018).…”

Section: Introductionmentioning

confidence: 99%

A Candidate Tidal Disruption Event in a Quasar at z = 2.359 from Abundance Ratio Variability

Liu

Dittmann

Shen

et al. 2018

ApJ

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A small fraction of quasars show an unusually high nitrogen-to-carbon ratio (N/C) in their spectra. These "nitrogen-rich" (N-rich) quasars are a long-standing puzzle because their interstellar medium implies stellar populations with abnormally high metallicities. It has recently been proposed that N-rich quasars may result from tidal disruption events (TDEs) of stars by supermassive black holes. The rapid enhancement of nitrogen and the depletion of carbon due to the carbon-nitrogen-oxygen cycle in supersolar mass stars could naturally produce high N/C. However, the TDE hypothesis predicts that the N/C should change with time, which has never hitherto been observed. Here we report the discovery of the first N-rich quasar with rapid N/C variability that could be caused by a TDE. Two spectra separated by 1.7 years (rest-frame) show that the N III] λ1750/C III] λ1909 intensity ratio decayed by ∼ 86%±14% (1σ). Optical (rest-frame UV) lightcurve and X-ray observations are qualitatively consistent with the TDE hypothesis; though, the time baseline falls short of a definitive proof. Putting the single-object discovery into context, statistical analyses of the ∼ 80 known N-rich quasars with high-quality archival spectra show evidence (at a 5σ significance level) of a decrease in N/C on timescales of > 1 year (rest-frame) and a constant level of ionization (indicated by the C III] λ1909/C IV λ1549 intensity ratio). If confirmed, our results demonstrate the method of identifying TDE candidates in quasars via abundance ratio variability, opening a new window of TDE observations at high redshift (z > 2) with upcoming large-scale time-domain spectroscopic surveys. 1 The BLR gas mass is luminosity dependent and is likely higher in luminous quasars (Baldwin et al. 2003a).

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The Carbon and Nitrogen Abundance Ratio in the Broad Line Region of Tidal Disruption Events

Cited by 31 publications

References 59 publications

The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution

The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution

Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event

A Candidate Tidal Disruption Event in a Quasar at z = 2.359 from Abundance Ratio Variability

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